Author Topic: how to design fast bench supply with CC and CV?  (Read 10260 times)

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Online exe

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how to design fast bench supply with CC and CV?
« on: April 07, 2020, 08:35:46 pm »
Hello!

This is a continuation of https://www.eevblog.com/forum/testgear/output-capacitance-of-higher-end-bench-power-supplies/ .

I would like to build a high-precision, high-performance power supply. How would I do that?

Some general specs to give direction:
1. Digital control and readback
2. Output voltage range 0..15V (min 12V)
3. Current ranges 0..100mA and 0..3A (min 2A) with 1mA step, and 0.25mA accuracy. Or, alternatively, I can simply build two channels with different current capabilities.
4. Precision current set and readback. Many power supplies have cannot do that because of different parasitic currents, although error can be pretty small
5. Remote sensing. Well, remote up to output terminals, I don't long external connectors.
6. Stability with minimum output capacitance
7. Stability with large output capacitance
8. Performance... Well, how about recovery time from 0 to 100% load and back? Say, in less than 20us. Also fast CV/CC switch without glitches. Not sure how to define that.

Bonus points if PSU can:
1. Sink current
1. Bipolar
1. Multiple voltage ranges, but I don't really need steps more than 1mV. But nice to have :).
1. True remote sensing.

I decided to split task into two. The first one is choosing power supply architecture. The second one is choosing components.

I'm more or less set on parts to be used. And, I think, it's less important than architecture. I need you help there. May be gurus of analog engineering want to share their designs and ideas? :)

Currently the best I can do is the following. I didn't figure out how to make a beefy, precise and fast power supply. So, I decided that I only need precision and performance for low-power and low-voltage ranges. So, I'd make a low-power channel that would consist of simple emmitter follower with a fast bjt (like 2n2222 or something, point is it has much lower collector-base capacitance than, say, 2STA1943).

To reduce recovery time, I'd add a current sink at the output. This will create an offset current as it goes through the shunt. So, this needs to be taken into account. But even if we resolve this problem, I still haven't figured out how to switch fast between CC and CV mode. I'm currently looking at opamps that have ability to limit output swing, but there are not many around. So, clearly, not the best design.

Now to start answering from that thread:

The TIP2955/TIP3055 are more like normal slow BJTs. For fast response I would consider 2SC5200 or similar transistors made for high end audio. For a smaller version D44H / D45H could be an option.

Thanks for suggestion, I'll try 2SC5200. As of D44H and TIP2955, I think the performance of these part varies a lot from manufacturer to manufacturer. My measurements show that original parts from Toshiba (okay, I have only one sample of 2ta1943) was much slower than 2STA1943 (ST). As of D44H, I think showed similar performance to TIP2955/TIP3055. I can't find curves for D44H8 the moment, but I'll do measurement again and share them here. Here is methodology I'm using for measurements: https://www.eevblog.com/forum/beginners/evaluating-bjts-for-a-linear-psu-am-i-doing-it-right/msg2950816/#msg2950816 .

As for other questions, I'll answer them next time. It's a lot of topics yet to discuss, and I want to start with something. So, here is the thread for that.

I'm not investigating push-pull stages, but I have zero previous experience with them, so it will take time how to figure out how they work, how to optimize, say, biasing, and how to sense current.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #1 on: April 07, 2020, 08:53:33 pm »
Ah, I just got to know that 2SC5200 is a brother of 2SA1943. In this case I'm fairly confident that modern TIP3055/2955 from onsemi and ST are no worse. In fact, it can be just the same die. I'll share curves later.
 

Offline Smokey

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Re: how to design fast bench supply with CC and CV?
« Reply #2 on: April 07, 2020, 10:52:30 pm »
Since I'm stuck at home in this stupid coronavirus quarantine, I've been working on my Mars rover for Mars again.  It sounds like your power supply would be a perfect addition to my project and it sounds like you are SUPER excited about electronics just like me!!!!!!!!!!111111

https://www.eevblog.com/forum/projects/i-would-like-to-build-a-mars-rover-for-mars-please-help/

Lets work together!!! 

Since the coronavirus will be over by Easter, that's my new deadline for my Mars Rover for Mars.  It sounds like your project should be done by then too!!!!!! I'm so Excited!!!
 
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Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #3 on: April 08, 2020, 12:07:27 am »
For fast response and low output capacitance, take a look at how class-AB audio power amplifier output stages are designed.  And there is a very small step from a class-AB output stage to 2 or 4 quadrant operation.

I do not necessarily recommend using them but Linear Technology, now Analog Devices, makes some power output stage controllers which might be useful.  Check out their datasheets and application notes for some ideas:

https://www.analog.com/en/products/lt1166.html
https://www.analog.com/en/products/lt1970.html
https://www.analog.com/en/products/lt1970a.html

Keep the voltage and current control loops as simple as possible, which almost always means one operational amplifier for each.  Do *not* cascade amplifier stages within the control loops unless it is absolutely necessary, and it almost never is.  Level shift control and read-back signals to your digital common outside of any control loops.

As a corollary, this also means that you will likely get maximum overall performance with relatively low bandwidth operational amplifiers because faster ones will just require external compensation for stability anyway.  And external compensation creates problems with integrator windup unless special steps are taken.
 
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Offline H713

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Re: how to design fast bench supply with CC and CV?
« Reply #4 on: April 08, 2020, 04:53:22 am »
Highly doubtful that the TIP3055 and 2SC5200 have the same die- the voltage ratings, power ratings and speeds are all different.

The 2SA1943 and 2SC5200 are extremely high performance transistors and are probably completely overkill. Also commonly counterfeit- so buy from a reputable supplier. Other overkill (and extremely high performance) transistors include the MJL3281 and MJL1302. These are ON-Semi reproductions off the 2SA1302 and 2SC3281. The original Toshiba parts have been out of production since the late 1990s. If you find "Toshiba" parts for sale, be careful. The TIP3055s and TIP2955s that I have tested were dog slow.

The MJ21194 is also a pretty nice choice. It is also available in plastic packages.


 

Online Radiohead

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Re: how to design fast bench supply with CC and CV?
« Reply #5 on: April 08, 2020, 06:42:57 am »
Hello exe,

Your requirements look a lot like an Source Measurement Unit (SMU), which often have very little output capacitance and very high programmable output precision. I haven't found any articles of someone designing a true SMU as a hobby project, but I have a few interesting resources you might want to read:
https://poormanssmu.wordpress.com/research/

The lt1970A is also an interesting chip. The current limiting isn't precise, but the application example in the datasheet to build a four quadrant power supply with the LT1970A is still worth a look.

Of course Blackdog has an (unfinished) design of a high-speed, low capacitance and low current power supply project: (Warning: it's in Dutch!)
https://www.circuitsonline.net/forum/view/135399
 
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Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #6 on: April 08, 2020, 09:57:20 am »
A first point to decide for the design is the type of output stage: One has to decide between a current controlled output stage
and a voltage controlling output stage with a low output impedance. The later may split the problem to the amplifier / control part and the output stage. Remember that even just a emitter follower can oscillate with capacitive load. 
With the low voltage aimed for both variants may be feasible.

For the transistors the TIP3055 and similar may be fast from some sources, but they usually should not be fast.
The main limiting factors for the control loop should be parasitic inductance (e.g. of the shunt resistors, connections to the output) and the
speed of the power transistors. At low current even the faster power transistors can become relatively slow. So one may have to operate them with some minimum current. It is tricky enough to cope with a variable load impedance.
 
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Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #7 on: April 08, 2020, 05:40:49 pm »
The 2SA1943 and 2SC5200 are extremely high performance transistors and are probably completely overkill. Also commonly counterfeit- so buy from a reputable supplier. Other overkill (and extremely high performance) transistors include the MJL3281 and MJL1302. These are ON-Semi reproductions off the 2SA1302 and 2SC3281. The original Toshiba parts have been out of production since the late 1990s. If you find "Toshiba" parts for sale, be careful. The TIP3055s and TIP2955s that I have tested were dog slow.

Ring and perforated emitter transistors are common in high performance regulators.  They used to show up in linear point of load regulators for Pentium processors.  Old timers may recognize the D44H11/D45H11 series of transistors which would be a good choice except for needing several in parallel for this application to handle the worst case power dissappation.  The MJL4302AG/MJL4281AG would be better choices both for economy and a simpler design and there are lots of similar parts in the same family which will work. 

Besides being fast, ring emitter transistors also have less hfe and ft droop at high current.  All three make them popular for high performance audio power amplifiers which provide enough demand to make them inexpensive.

If I used power MOSFETs for the output transistors, then I might use ring emitter transistors to provide gate drive.
 

Offline Vovk_Z

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Re: how to design fast bench supply with CC and CV?
« Reply #8 on: April 08, 2020, 08:52:50 pm »
Isn't it easy to achieve bipolar <=3 A output with something like TDA7293 at output? (It needs precision op amp to control it, of cause).
« Last Edit: April 09, 2020, 08:29:51 pm by Vovk_Z »
 

Offline duak

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Re: how to design fast bench supply with CC and CV?
« Reply #9 on: April 09, 2020, 03:16:46 am »
I've been out of bipolar power transistors for a while; the MJL4302AG/MJL4281AG Ft are quite impressive, especially considering the breakdown voltages.  I remember that in olden times 2N3055 were single diffused and slow.  Sometime along the way they became the epitaxial 2N3055E, about 3X better Ft, and more prone to oscillation in unmodified circuits.  The above devices appear to be an order of magnitude faster.

The D44H11/D45H11 also rings a bell.  I have a Tek FG503 that I got for almost nothing because there was no AC output but a very large DC offset.  I expected it to have a 2N3866/2N5109 combo but it had something from the D40 series, now apparently unobtainable.  I kludged in a 2N2219 & 2N2905 soldered on a copper heat spreader and let it go at that until I found something with more muscle.  I was perusing Digikey and found the 2SC6144SG/2SA2222SG.  They were cheap, had Ft and hfe to burn so I bought some but haven't tried them out yet.  They use something called the "MBIT" process.

I don't know much about ring or perforated emitter devices but apparently it's a transistor geometry used in the VHF 2N3866?  Any more info on what's being done in these devices to pull this off?
 

Offline profdc9

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Re: how to design fast bench supply with CC and CV?
« Reply #10 on: April 09, 2020, 05:59:54 am »
I designed a supply based on a popular op-amp design.  It also supports external programmability.

http://www.github.com/profdc9/LinearPS

The output capacitance is on the order of 10-47 uF.
 
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Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #11 on: April 09, 2020, 12:12:22 pm »
Switching between CC and CV modes is tricky/impossible to do without glitches, resulting in voltage spikes or current surges. Most power supplies will do one or the other, with the latter being more common. You might want to add a switch to allow the power supply to be changed between optimal voltage and current regulation modes, so it will work just as well for CC loads such as LEDs, as the usual CV loads like microcontrollers.
 

Offline mzzj

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Re: how to design fast bench supply with CC and CV?
« Reply #12 on: April 09, 2020, 12:32:06 pm »
For fast response and low output capacitance, take a look at how class-AB audio power amplifier output stages are designed.  And there is a very small step from a class-AB output stage to 2 or 4 quadrant operation.


HP 6632A or 6632B are pretty much like that.
Actually quite heavily biased, IIRC around 2A, possibly to lower output impedance.
Separate shunts for low and high range, 20mA range really accurate for a PSU.
Output capacitance selector switch, normal mode and fast more. IIRC also controls the feedback loop.

6632B’s are common on second hand market and often real bargain for what you gets. Going rate for these has been around 150usd and this was over 2 grand per piece from Keysight just a few years ago.
 
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Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #13 on: April 09, 2020, 01:00:43 pm »
Hello exe,

Your requirements look a lot like an Source Measurement Unit (SMU), which often have very little output capacitance and very high programmable output precision. I haven't found any articles of someone designing a true SMU as a hobby project, but I have a few interesting resources you might want to read:
https://poormanssmu.wordpress.com/research/

Oh, man, is that your blog? I read it all, too bad it somewhat abruptly stopped, so I'm trying to carry on :). Thanks for all the info! I'm now experimenting with push-pull too, have promising results already.

Switching between CC and CV modes is tricky/impossible to do without glitches

Yep, figured that the hard way :(. Blackdog's power supply offers an elegant solution with clamping diodes. See schematic here, for example: https://www.eevblog.com/forum/beginners/how-does-blackdog_s-psu-work/ . Very nice unit, really hard to beat.

Another approach: "anti-soar clamp" in AoE x-chapters. Afaik, essentially the same concept, but using an opamp, bjt, and a few passives to do the same. I'll post a picture of it.

6632B’s are common on second hand market and often real bargain for what you gets. Going rate for these has been around 150usd and this was over 2 grand per piece from Keysight just a few years ago.

Yeah, but I want to build mine :).


Sorry folks, can't reply to all of you, you gave me readings for weeks ahead. Nice to get to know new parts. I thought I knew all the bjts on the market, but I was wrong :).
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #14 on: April 09, 2020, 02:43:08 pm »
Switching between CC and CV modes is tricky/impossible to do without glitches

Yep, figured that the hard way :(. Blackdog's power supply offers an elegant solution with clamping diodes. See schematic here, for example: https://www.eevblog.com/forum/beginners/how-does-blackdog_s-psu-work/ . Very nice unit, really hard to beat.

Another approach: "anti-soar clamp" in AoE x-chapters. Afaik, essentially the same concept, but using an opamp, bjt, and a few passives to do the same. I'll post a picture of it.
The problem with the diodes is the voltage is fixed. The anti-soar clamp sounds like a good idea. I haven't seen the circuit but I can imagine what it looks like.

The output capacitors are responsible for the huge current circuit but it'll still happen without them. Here's a quick design I've done in LTSpice to simulate the transient response. It's not supposed to be a final design and isn't very good.
[attachimg=1]
[attachimg=2]
« Last Edit: April 09, 2020, 06:32:26 pm by Zero999 »
 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #15 on: April 09, 2020, 04:01:51 pm »
For the current spikes with an emitter follower configuration there are 2 problems:
One is that the amplifier for the voltage control has a limited slew rate. The other problem is wind of the current regulator. So in the case of a sudden short there has to be some additional fast limit. Often this limit may be fixed.

Bockdogs circuit has a limited anti-windup with the diodes in FB at the OPs. This is more like windup limiting, not fully preventing windup. Still this is a good idea if it works (depends on the configuration and needed control voltages).

One step closer anti-windup can be done with extra transistor(s) to couple the 2 regulators. There is a slight disadvantage the leakage can effect the precision. Attached is a simulation file for such a anti-windup for the voltage regulator. The simulation shows the CC to CV transition were the anti-windup part helps.  Beside the anti windup the circuit also has a nice simple constant current sink.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #16 on: April 09, 2020, 05:57:41 pm »
Bockdogs circuit has a limited anti-windup with the diodes in FB at the OPs. This is more like windup limiting, not fully preventing windup.

Could you please elaborate this? As far as my understanding goes, it works almost perfectly: it clamps both opamps just above the current output voltage (+1-2V above the output voltage). For CV this is done by raising voltage at output feedback pin. For CC this is done by clamping voltage to the highest side of high-side shunt. CV/CC switching works really well. Both opamps still track the output voltage.
 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #17 on: April 09, 2020, 07:00:15 pm »
In the circuit from Backdog the diodes limit the OPs output voltage to about +0.6 V, maybe +1 V for the current limit.
The active channel is at some -1 to -2 V. So on the transition the OPs have to slew from some +0.6 V to some -1 ot -2 V.  One has little room to reduce the range, as there has to be room for the regulator to operate.

The additional transistor Q6 in Blockdogs circuit adds an additional fast current limit, though at a fixed level. Ideally one would not need this, but it does not hurt to have this extra safety net.

With the transistor the step for the regulator to overcome is at some 1.1 V. Slightly smaller, though not much.
The other point that can help is to have some of the feedback from behind the diodes. However this can also sometimes lead to the current limit to engage a little earlier on voltage transients, when just at the edge. So this is a little 2 sided.

Both versions are quite a bit faster to react than the simple circuit where on transition between modes the regulator has to slew quite a bit, like some 5 v set by the supply.
 

Offline not1xor1

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Re: how to design fast bench supply with CC and CV?
« Reply #18 on: April 09, 2020, 07:28:29 pm »
Since I'm stuck at home in this stupid coronavirus quarantine, I've been working on my Mars rover for Mars again.  It sounds like your power supply would be a perfect addition to my project and it sounds like you are SUPER excited about electronics just like me!!!!!!!!!!111111

https://www.eevblog.com/forum/projects/i-would-like-to-build-a-mars-rover-for-mars-please-help/

Lets work together!!! 

Since the coronavirus will be over by Easter, that's my new deadline for my Mars Rover for Mars.  It sounds like your project should be done by then too!!!!!! I'm so Excited!!!

Easter? Yes... may be 2021 or 2022...
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #19 on: April 10, 2020, 12:05:06 pm »
Isn't it easy to achieve bipolar <=3 A output with something like TDA7293 at output? (It needs precision op amp to control it, of cause).

An integrated audio amplifier could be used but they are commonly decompensated for a minimum gain limiting their flexibility or performance.  I am sure it has been done though.

I've been out of bipolar power transistors for a while; the MJL4302AG/MJL4281AG Ft are quite impressive, especially considering the breakdown voltages.

What really matters are the secondary breakdown and power ratings.

Quote
I remember that in olden times 2N3055 were single diffused and slow.  Sometime along the way they became the epitaxial 2N3055E, about 3X better Ft, and more prone to oscillation in unmodified circuits.  The above devices appear to be an order of magnitude faster.

The original 2N3055s also had outstanding secondary breakdown characteristics making them very tough.  Does anybody have a link to the original 2N3055 datasheet with some characteristic curves?

Quote
The D44H11/D45H11 also rings a bell.  I have a Tek FG503 that I got for almost nothing because there was no AC output but a very large DC offset.  I expected it to have a 2N3866/2N5109 combo but it had something from the D40 series, now apparently unobtainable.  I kludged in a 2N2219 & 2N2905 soldered on a copper heat spreader and let it go at that until I found something with more muscle.  I was perusing Digikey and found the 2SC6144SG/2SA2222SG.  They were cheap, had Ft and hfe to burn so I bought some but haven't tried them out yet.  They use something called the "MBIT" process.

I don't know much about ring or perforated emitter devices but apparently it's a transistor geometry used in the VHF 2N3866?  Any more info on what's being done in these devices to pull this off?

The way I understand it, the D44H11/D45H11 and similar ring or perforated emitter transistors are distinct from RF transistors which commonly have multiple ballasted emitters.  From its description, MBIT seems like another variation of ring or perforated emitter.

Switching between CC and CV modes is tricky/impossible to do without glitches, resulting in voltage spikes or current surges. Most power supplies will do one or the other, with the latter being more common. You might want to add a switch to allow the power supply to be changed between optimal voltage and current regulation modes, so it will work just as well for CC loads such as LEDs, as the usual CV loads like microcontrollers.

Good frequency compensation and provisions to remove excessive charge from the output devices go a long ways toward minimizing glitches during mode changes.  That plus clamping the error amplifiers to prevent integrator windup can solve this problem.

The external compensation connections on some operational amplifiers, like the LM301A, can be used as clamp inputs.
« Last Edit: April 10, 2020, 12:13:27 pm by David Hess »
 
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Offline duak

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Re: how to design fast bench supply with CC and CV?
« Reply #20 on: April 10, 2020, 04:35:38 pm »
Here's a link to the 1966 RCA Transistor Manual:  http://www.rsp-italy.it/Electronics/Databooks/RCA/_contents/RCA%20Transistor%20Manual%20SC12%201966.pdf

The 2N3055 specs start on p 228.  I don't see an SOA curve however, some other power devices have a DC SOA only graph, but no pulse ratings.  I suppose it took a few more years before more complete SOA graphs were produced.  Motorola certainly had more complete graphs in the early 70s.  Does anyone remember the big, brown Motorola semiconductor data book?

Here's a link to a later RCA datasheet:  https://www.silicon-ark.co.uk/datasheets/2n3055-datasheet-rca.pdf but no SOA graph.  It appears to be the older, slower part.  I see it was sent from GE Solid State after RCA was folded back into GE.

https://en.wikipedia.org/wiki/2N3055 has some history and info but no data.

BTW, I think RCA were the first to come out with low threshold voltage power FETs in '86 or so.  I thought they might work well as pass transistors in a bench supply as they were touted as having Positive Temperature Coefficients and freedom from secondary breakdown.  In the end, I gave up and went back to bipolar darlingtons, as the LVFETs clearly suffered from Electro-thermal Instability.  If memory serves, they couldn't always handle a sudden short circuit load.
« Last Edit: April 10, 2020, 07:04:22 pm by duak »
 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #21 on: April 10, 2020, 04:53:59 pm »

BTW, I think RCA were the first to come out with low threshold voltage power FETs in '86 or so.  I thought they might work well as pass transistors in a bench supply as they were touted as having Positive Temperature Coefficients and freedom from secondary breakdown.  In the end, I gave up and went back to bipolar darlingtons, as the LVFETs clearly suffered from Electro-thermal Instability.  If memory serves, they couldn't always handle a sudden short circuit load.

The problem with using MOSFETs and a sudden short is especially there with a source follower configuration. In this case the current goes up very fast (with MOSFETs the current goes up even faster than linear with the gate voltage). In addition there is some danger to even exceed the maximum gate voltage.  With BJTs on contrast, the current gain goes down once on gets very high in current. So even if the base drive is not going down fast enough the current does not peak so bad.

The early MOSFETs had a rather high R_on and relatively large die. There SOA was reasonable good. Later generations got smaller dies and later a very limited SOA, especially for the lower voltage ones. In additions there are lots of unreliable data-sheets around ignoring the thermal instability SOA limit. So even if there is a DC SOA curve one has to check if one can depend on it.

The old, slow 2N3055 version is still available as 2N3055H.  Newer chips tend to get smaller and smaller dies for cost reasons.
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #22 on: April 10, 2020, 05:33:27 pm »
Adding a diode to limit the slew rate is a good idea. Ideally the stage needs to be unity gain, otherwise it ends up amplifying the diode forward voltage. If needed a gain stage added to the reference, but it would require another op-amp. I thought I'd try it with my circuit, leaving it with a gain of 10, just to see what would happen. In this case it needs to go on to the voltage, rather than current amplifier, because my circuit is a current source first and a voltage source second. Obviously it makes no difference to the current surge, but the voltage spike is reduced, as expected.

Yes, something to clamp Q2's base very quickly, could be used to reduce the current surge. Ideally it should be variable, but the sense resistor's value would need to be switchable anyway, to get the range desired by the original poster. I have an idea, but will post it later.
[attachimg=1]
[attachimg=2]
 

Offline duak

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Re: how to design fast bench supply with CC and CV?
« Reply #23 on: April 10, 2020, 08:39:29 pm »
Kleinstein points out the reasons why MOSFET pass transistors may not be the best without some extra considerations.  For my supply, I had considered a fast current limiter that reduced gate drive like that in the 723 regulator, ie., an NPN transistor driven from a current sense resistor in the pass transistor circuit.  (Q3 in the attached file)  But, the board was done and it was easier to go with a Darlington.

I'd suggest this type of fast current limiter in addition to the current control loop to protect the output devices in case of an accidental short circuit.
 
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Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #24 on: April 10, 2020, 10:36:21 pm »
Here's my shot at the fast current limit. The idea is to clamp the voltage on Q2's base to a low level, when the current limit is drastically exceeded. The main current limiting amplifier U2 starts to cut-off Q2's base drive, when the voltage across R1 exceeds one tenth of the current limit reference voltage: i.e V2/10. The fast current limiter is set at around 50% higher, than the main current limit, so V2/6.667. U3 is a non-inverting summing amplifier. It has two inputs, the output voltage of the power supply and the current limit reference voltage V2. R10 and R11 divide V2 by 62/3, which is added onto the output voltage. The output is connected to C1, via R7, which is needed for stability.

VOUT+VC1 = VOUT+ V2/6.667

So when V2 = 1
VOUT+VC1 = VOUT + V2/6.667 = 0.15V

When the output is shorted, the current through R5 will drastically increase, pushing the emitter and thus base voltage up. When the base voltage exceeds that on C2 by a diode drop, D3 will clamp the base voltage, pulling it down into C1, cutting off the drive. C1 will charge a bit, causing the current to slowly rise, but U2 starts to react before the current gets too high.

The advantage of this circuit is the higher current limit scales with the current setting. I will work better, if a greater voltage drop is allowed across R5, which will still need to be switched to different values, to meet the OP's specification.

I've changed the voltage amplifier to unity gain. The reference will have to have a separate amplifier, if some gain is required.

[attachimg=1]
[attachimg=2]
« Last Edit: April 10, 2020, 10:39:10 pm by Zero999 »
 
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Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #25 on: April 11, 2020, 02:23:22 am »
Does anyone remember the big, brown Motorola semiconductor data book?

I have a brown Motorola book sitting here in front of me but it dates from 1984.  It shows a 0.8MHz minimum 2N3055A with a straight SOA curve and no secondary breakdown and a 2.5MHz minimum 2N3055 with secondary breakdown starting at 40 volts.  What I have not seen is the SOA for the earlier 2N3055 which was even slower, 0.2 MHz?

My workstation is in pieces at the moment or I would scan them in and post them but I am sure the Motorola book is available online.  Somewhere I have an older one with a blue cover I think.
 

Offline not1xor1

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Re: how to design fast bench supply with CC and CV?
« Reply #26 on: April 11, 2020, 05:41:47 am »
[...]
8. Performance... Well, how about recovery time from 0 to 100% load and back? Say, in less than 20us. Also fast CV/CC switch without glitches. Not sure how to define that.

I might be wrong, but while I see a clear advantage in fast switching from CV to CC I can't see any usefulness in real life in a fast recovery from 100% load (i.e. overload).  :-//

I think the most important features are:
-1) fast CV->CC switch with as little over-current spike as possible
-2) CC-CV switch with no over-voltage spike

In various different circuits I've simulated in past I've got the feeling that the worst overload recovery (i.e. highest relative value voltage spikes) occur when output voltage is set at a few volts (1-2V and below).
Besides that anti-windup diodes (like in those blackdog circuits) do affect load regulation due to leakages (at least in simulation).
For instance simulate a 5-95% load variation and check output voltage with and without the windup diodes.
 

Offline udok

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Re: how to design fast bench supply with CC and CV?
« Reply #27 on: April 11, 2020, 06:56:28 am »
Just my thoughts on your requirement list:

6. Stability with minimum output capacitance

=> That should be easy as it is only shunting the internal capacitor.

7. Stability with large output capacitance

=> That could be very difficult.  Especially with modern low ESR types.
  Add inductors to the list and you have enough work for the next year.

I would not be surprised if many lab power supplies have serious overshoots or oscillate with some
combinations of modern Polymers with low ESR.

Maybe that is the real reason, why commercial supplies are not faster than 50 - 100 us,
or maybe they still continue to copying the old HP schematics.
The old HP and Harrison designs do not mention stability concerns or calculations.
The were build by experimenters and work very well with the components of the 60.
But they use a lot of positive feedback for bootstrapping and this could cause troubles
with faster designs.

8. Performance... Well, how about recovery time from 0 to 100% load and back? Say, in less than 20us. Also fast CV/CC switch without glitches. Not sure how to define that.

=> A awful lot of more work.


x. Bonus points if PSU can:
1. Sink current
1. Bipolar

=> This is a SMU and that greatly complicates the design and the price
     The usefulness  for a power supply is limited.


1. Multiple voltage ranges, but I don't really need steps more than 1mV. But nice to have :).

=> Not necessary with 16 bit DACs

1. True remote sensing.

This is necessary with rear connectors


Why not build a simple proven design like the NG304, schematic is on page 12:
https://www.mikrocontroller.net/attachment/412953/DBL_BEHA_NG304_MANUAL_DEUTSCH.PDF
(in German, but schematic is very simple)

I have the impression that you have never build even such a simple design?


y. I would like to build a high-precision, high-performance power supply. How would I do that?

=> Study the schematics of the older HP/Harrison/RohdeSchwarz/Kepco/TTi and understand them.
If you then still want to improve them, then simply do it.

Switching from CV/CC and back is only a minor detail in this venture.
First you need a clear understanding how stability in this supplies depends on the output load.


« Last Edit: April 11, 2020, 07:07:11 am by udok »
 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #28 on: April 11, 2020, 08:49:34 am »
Stability with large capacitive (and low ESR) load is the difficult part. For voltage regulation a inductive load is not a problem - it may though lead to some overshoot in designs with a large capacitor at the output. Aiming for low internal capacitance makes this point less of a point. An inductive load can be an issue in CC mode. However real world inductors tend to be less ideal than some capacitors especially in the lower frequency range (e.g. 10 kHz) where things could become tricky.

The large low ESR capacitors (e.g. polymer electrolytic) still tend to have quite some loss - so they may not be so bad as one may think. The tricky part could be more like some 1000 µF as C0G or PP film cap, but this is still a little on the rare side.

To get get good good stability one may even need at least some sinking (at least limited) capability. Other wise the transition towards the output stage all the way off can easily cause an instability problem. This can be fixed with sinking capability - but may need compromises in the way the negative current is limited (e.g. no fixed time independent limit there).
Stability in the linear / small signal range is still relatively simple - here the theoretical background is well founded. The tricky part comes with stability in the large signal area when some regulator parts hit limits (e.g. transistors turn all the way off). With the control going beyond all the way off will add delay when turning on again and this increases overshoot and can cause oscillations.
 
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Offline udok

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Re: how to design fast bench supply with CC and CV?
« Reply #29 on: April 11, 2020, 09:25:58 am »
FYI: Attached are the stability margins for CC and CV operation of the HP6624.

When the regulator loop is running out of gain, the output cap is providing the energy
and the ESR of the output cap is dampening the resonances.

With low value capacitors the regulator needs a larger bandwidth, and the
output cap is lower and often of ceramic type with low ESR.

This could be a problem because the remaining total ESR is not enough to dampen resonances.

A lot of PCBs today use only ceramic caps at the input, because they are small and cheap,
but this could lead to large overshoots.  Some engineers even put a ferrite bead in front,
and build a series resonance converter for the input voltage, maybe to test the voltage limits of the chips.



 
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Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #30 on: April 11, 2020, 11:46:20 am »
This is the post regulator in my 20 amp bench supply. For fast current limiting, the CC op-amp operates in open loop until it takes control of the MOSFET's Gate from the CV op-amp. Then Q2 turns on, closing the feedback loop via C1.
The precharge on C1 determines the amount of allowed current overshoot, about 50%, less at higher CC settings. The precharge is set by the supply's micro-controller according to the present CC setting.

Extra: In the design of a second smaller bench supply, the CV op-amp's compensation is taken from the ORing node ,suggested by Kleinstein.
This greatly reduces voltage overshoot caused by when the supply transitions from CV to CC and to CV again before the output voltage drops by much.
« Last Edit: April 12, 2020, 09:53:55 am by xavier60 »
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Offline udok

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Re: how to design fast bench supply with CC and CV?
« Reply #31 on: April 12, 2020, 10:56:31 am »
Xaviers and Kleinsteins circuits are clever improvements for a SMU.

But i do not quite understand why a quick transition from CV to CC mode is important for a power supply.
In a bench power supply the CC mode is mainly a safety feature to limit destructive energy.

And anyway the CC mode kicks in after the output capacitor has depleted, which is many microseconds
later.

And a transient increase of output current does not harm and is often a very desirable feature.
Rohde & Schwarz even advertises power supplies as "large current pulse capable".

In my opinion there is no need for a fast transition from CV to CC, and these
circuits may even severely limit the usefulness of the power supply in practice.

Normally the CV error amplifier is build 5-10x faster than the CC amplifier, and therefore
the voltage does not overshoot for a CC to CV transition.

This is the expected behavior of a bench power supply and there is no need to improve.

The one exception are absolute beginners, who mix up a power supply with a current source.

SMUs on the other hand have no output capacitors but are only unconditionally stable with no more than 20 nF load in CV mode.
They are designed as excellent CC sources.

But modern SMUs use fast CMOS switches for the range switching, controlled digitally by FPGA.
Some SMUs even implement the control loop in FPGA and there is no need for clever analog solutions  anymore.

Long ago HP build constant current power supplies for device breakdown testing (HP 6177, 6181, and there was a 300 Volt model too).
These devices used a current regulation amplifier which *never* goes into saturation.
If the output voltage limit is exceeded, a shunt regulator shunts current to ground until the voltage
stays in the CV limit.
They have two nested regulation loops, a fast one for CC and a slower one for CV.
These are excellent devices for LED testing or transistor breakdown testing.

And what is the use case for a fast response power supply?  And what is fast?
« Last Edit: April 12, 2020, 11:07:45 am by udok »
 
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Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #32 on: April 12, 2020, 12:04:18 pm »
I like fast limiting mainly to protect the power supply itself. My designs intentionally allow some current overshoot.
Curiously, my Agilent U8002A outputs virtually unlimited current for 100us or so when short circuited. It has no fast current limiting at all.
My 2 designs that I regularly use on the bench have a response that's unnecessarily fast at about 10us, just because.
All of the stuff I work on is normally powered by long power cables anyway so speed is not important at all.
My Agilent U8002A is about 30us.
I think minimizing voltage overshoots at low voltages is important as it could cause a load to be damaged without being certain why.
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Offline imo

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Re: how to design fast bench supply with CC and CV?
« Reply #33 on: April 12, 2020, 02:24:19 pm »
I want a 30V/3A PSU which will not destroy a red LED when set to 20mA and 30V.. :)
« Last Edit: April 12, 2020, 02:26:06 pm by imo »
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #34 on: April 12, 2020, 02:45:33 pm »
And what is the use case for a fast response power supply?  And what is fast?

Good question. I like playing with small circuits made of sensitive components. I often do wiring errors, or use damaged components (without knowing that), that often lead to high fault currents. For this reason I'd like to minimize damage. One practical situation I had is I damaged my ne5532 because I connected my power supply to it's input in wrong polarity. The opamp survided, but the input protection diode was damaged, this lead to excessive input current. I wonder if a faster power supply with less capacitance could prevent the damage.

For the cases when I don't need fast response, I'd like to use bigger output cap.

What is fast? A few tens of us I'd consider fast. I think what I really want is to figure out how to avoid long opamp recovery time and big output swings when changing between modes. This will make me feel good. Ah, no overshoots, as I mainly work with low-power ICs.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #35 on: April 12, 2020, 02:47:53 pm »
I want a 30V/3A PSU which will not destroy a red LED when set to 20mA and 30V.. :)

I can't speak for 30V, but for 17V I had hard times destroying a red led. I tried quite significant output capacitance (afaik some hundreds of uF), it survived. So, either this is not a big challenged, or I have an outstanding diode. I'll repeat the test tonight with a few diodes at hand and report here.
 

Offline imo

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Re: how to design fast bench supply with CC and CV?
« Reply #36 on: April 12, 2020, 03:09:23 pm »
I want a 30V/3A PSU which will not destroy a red LED when set to 20mA and 30V.. :)

I can't speak for 30V, but for 17V I had hard times destroying a red led. I tried quite significant output capacitance (afaik some hundreds of uF), it survived. So, either this is not a big challenged, or I have an outstanding diode. I'll repeat the test tonight with a few diodes at hand and report here.

Try it with a) wire a red LED to the PSU and switch the PSU on/off several times (PSU set 20mA/30V), b) while PSU on (PSU set 20mA/30V) connect the LED to the PSU few times (forward biased).
« Last Edit: April 12, 2020, 03:21:30 pm by imo »
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #37 on: April 12, 2020, 05:11:27 pm »
Xaviers and Kleinsteins circuits are clever improvements for a SMU.

But i do not quite understand why a quick transition from CV to CC mode is important for a power supply.
In a bench power supply the CC mode is mainly a safety feature to limit destructive energy.

And anyway the CC mode kicks in after the output capacitor has depleted, which is many microseconds
later.

And a transient increase of output current does not harm and is often a very desirable feature.
Rohde & Schwarz even advertises power supplies as "large current pulse capable".

In my opinion there is no need for a fast transition from CV to CC, and these
circuits may even severely limit the usefulness of the power supply in practice.

Normally the CV error amplifier is build 5-10x faster than the CC amplifier, and therefore
the voltage does not overshoot for a CC to CV transition.

This is the expected behavior of a bench power supply and there is no need to improve.

The one exception are absolute beginners, who mix up a power supply with a current source.
A current source is a type of power supply. It would be handy to have a general purpose power supply which is designed to be both a good current source for powering things such as LEDs, as well as a voltage source.

As long as it's stable with a large output capacitance and the voltage doesn't overshoot too much, I don't see why the transient response of the power supply is that important. Poor voltage transient response can easily be straightened up with a large capacitor. Any load, which requires a good transient response, such as a microcontroller or FPGA, should have lots of supply decoupling, to ride out any troughs.

Good constant current regulation can be achieve with a large inductor, but large, high quality inductors are expensive, compared to, large, high quality capacitors, which are cheap.

I appreciate it's not possible for a power supply to both have a good CC and CV modes. One fix would be to add a switch to select between optimal CV and CC operation.

By the way, please refrain from using a new line each time, allow the forum to word wrap: it's so much easier to read.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #38 on: April 12, 2020, 05:34:55 pm »
Try it with a) wire a red LED to the PSU and switch the PSU on/off several times (PSU set 20mA/30V), b) while PSU on (PSU set 20mA/30V) connect the LED to the PSU few times (forward biased).

Oh, just saw your message after I recorded the video. So, I didn't set to 20mA, but to ~10mA because I think 20mA is too much for leds. And it's not a big deal anyway, I don't think 20mA or 10mA make any difference in the test. I did power on/off test separately, all leds survived except (spoiler alert!) the one that was fried, it remained dead. So, here is how the led torture test went:



One led died even at 15V, although survived first test, but not subsequent. Rest were fine. So, the led does matter too :). Also shows there is a room for improvement in this power supply.

 
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Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #39 on: April 12, 2020, 08:57:33 pm »
Why not build a simple proven design like the NG304, schematic is on page 12:
https://www.mikrocontroller.net/attachment/412953/DBL_BEHA_NG304_MANUAL_DEUTSCH.PDF
(in German, but schematic is very simple)

(sorry, didn't notice your post).

Nice design, I've built a prototype of it (blackdog's version of it). The only concern is voltage set resistor creates parasitic current. It's small but I'd like not to have. I didn't figure how to solve this. May be the easiest option would be to have reference ground-referenced, and control it via an optocoupler or something. Or just make this resistor big so the error current is small...

Starting with a clean simple design and add features is that what I tried to do, but no luck there. Most "golden" designs use isolated auxilary voltages, pots, and panel meters. This greatly simplifies design.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #40 on: April 13, 2020, 10:11:43 am »
6. Stability with minimum output capacitance

=> That should be easy as it is only shunting the internal capacitor.

I think he means the internal output capacitance should be minimized.

Quote
7. Stability with large output capacitance

=> That could be very difficult.  Especially with modern low ESR types.

I would not be surprised if many lab power supplies have serious overshoots or oscillate with some
combinations of modern Polymers with low ESR.

There are at least two solutions for solving the problem of unconditional stability into a capacitive load:

1.  Conventional frequency compensation typically privides about 45 degrees of phase margin.  The open loop output series resistance combined with the output capacitance adds phase lag further lowering phase margin.  The output capacitor's ESR adds phase lead which increases phase margin so that is why low ESR output capacitors are especially troublesome and require an added series resistance for stability.

The solution which regulators use to allow operation with large low ESR output capacitors is to take AC feedback from before the open loop output series resistance.  On an integrated circuit, a special transistor structure can be used to do this however a separate fixed resistance also works. This resistance produces the same result as increasing the ESR and is in series with the capacitor but it located in a different place and the amplifier's low frequency DC gain removes its effect on regulation.

2. Another solution implied above is to use frequency compensation which has less than the common 90 degrees of phase lag.  For instance replace the capacitor in the commonly used Miller integrator with a series of RC networks which provide -3 dB/octave of roll-off instead of -6 dB.  The trade off here is poorer frequency response but the results are better under adverse conditions.  I have seen this method used with ATE (automated test equipment) pin drivers.
 
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Offline udok

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Re: how to design fast bench supply with CC and CV?
« Reply #41 on: April 13, 2020, 05:34:19 pm »
Why not build a simple proven design like the NG304, schematic is on page 12:
https://www.mikrocontroller.net/attachment/412953/DBL_BEHA_NG304_MANUAL_DEUTSCH.PDF
(in German, but schematic is very simple)

(sorry, didn't notice your post).

Nice design, I've built a prototype of it (blackdog's version of it). The only concern is voltage set resistor creates parasitic current. It's small but I'd like not to have. I didn't figure how to solve this. May be the easiest option would be to have reference ground-referenced, and control it via an optocoupler or something. Or just make this resistor big so the error current is small...

Starting with a clean simple design and add features is that what I tried to do, but no luck there. Most "golden" designs use isolated auxilary voltages, pots, and panel meters. This greatly simplifies design.

The current voltage set current is only a minor error compared to the error introduced by driving the power transistor.  The base current goes
through the current measurement shunt resistor too.
The cure to this problems is to pin the floating ground of the bias supply to the output of the power transistor *before* the shunt resistor.
In this way all the leakage currents are not measured by  the shunt resistor.
This is done in "precision" power supplies, but it is more work because if you do this, you cannot measure the output voltage without a true differential
amplifier.  Often it is easier to provide a current offset trimmer, and keep the leakage currents constant and down to a minimum.

Today you cannot build and sell a bench power supply which has less than 30 Volt and 3 Ampere output capability, because the
market is dictated by the cheap china HP replicas.
At the same time modern electronics does not need more than 100 mA @ 3.3 Volt.  This is a problem 100 mA are only 3% of the 3 Ampere range,
and the leakage currents scale with max current.
 

Offline udok

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Re: how to design fast bench supply with CC and CV?
« Reply #42 on: April 13, 2020, 05:55:15 pm »
Quote
7. Stability with large output capacitance

=> That could be very difficult.  Especially with modern low ESR types.

I would not be surprised if many lab power supplies have serious overshoots or oscillate with some
combinations of modern Polymers with low ESR.

There are at least two solutions for solving the problem of unconditional stability into a capacitive load:

1.  Conventional frequency compensation typically privides about 45 degrees of phase margin.  The open loop output series resistance combined with the output capacitance adds phase lag further lowering phase margin.  The output capacitor's ESR adds phase lead which increases phase margin so that is why low ESR output capacitors are especially troublesome and require an added series resistance for stability.

The solution which regulators use to allow operation with large low ESR output capacitors is to take AC feedback from before the open loop output series resistance.  On an integrated circuit, a special transistor structure can be used to do this however a separate fixed resistance also works. This resistance produces the same result as increasing the ESR and is in series with the capacitor but it located in a different place and the amplifier's low frequency DC gain removes its effect on regulation.

2. Another solution implied above is to use frequency compensation which has less than the common 90 degrees of phase lag.  For instance replace the capacitor in the commonly used Miller integrator with a series of RC networks which provide -3 dB/octave of roll-off instead of -6 dB.  The trade off here is poorer frequency response but the results are better under adverse conditions.  I have seen this method used with ATE (automated test equipment) pin drivers.

The problem is more complicated than it first seems, because the output impedance should be as low as possible *and* the power supply
should handle all loads well, including 3 meter of cables with 100 nF ceramic capacitor at the end. The 100 nF case needs a few ohms of damping.

The idea to take feedback before the current measurement shunt does not work as it increases the output impedance from <1mR to at least 0.1-1 Ohm.

Form the user viewpoint the most important feature of a power supply is to provide a constant voltage with any load as long as the current is
within the limits for 99% of the time.  Because there is no universal solution to this problem power supplies used to have over voltage protection circuits.

The second option is much work to implement and has in 100% of the test cases mediocre performance, which is not competitive.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #43 on: April 13, 2020, 06:39:56 pm »
Attaching the schematic of that anti-soar clamp that I advertised earlier. Hope posting it here falls under fair use.

Here's my shot at the fast current limit.

Thanks a lot for your effort and time, learning this design. Kleinstein's design will be next :).

The current voltage set current is only a minor error compared to the error introduced by driving the power transistor.  The base current goes
through the current measurement shunt resistor too.

I don't understand this part. If using an NPN (or darlington/sziklai equivalent) transistor as emitter follower, all current goes into the load. So, no problem there.

The error from voltage set current is... well, depends on the resistor values and control/output voltages, but the thing is, it varies with the output voltags. So, if power supply in CC mode, then output current varies with the load, this greatly reduces output impedance. How greatly?

For example, I have voltage set from DAC in the range of 0-2.5V. Say, I need maximum voltage of 15V, so I need 15V/2.5=6x gain. If I use 10k and 2k resistors, then the parasitic current is (I=U/R) (at maximum output voltage of 15V): 15/10k=1.5mA. Sort of a lot... Probably I could raise resistor values, I was just worried about noise. I can bypass the lower resistor, but then I won't be able to quickly set voltage, i.e., use my power supply as AWG :).


A solution could be to have ground-referenced voltage, and somehow with some level-shifting set current on a high side. Like on the included schematic. I discarded that idea. One of problem is that it didn't want to start, so I added this startup resistor across pass transistor. The second is I cannot use a clamping diode with this design (this is the only type of clamping I mastered so far, but I'm studying other proposed methods).
 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #44 on: April 13, 2020, 07:10:33 pm »
Wether the base current contributes to the output current and is measured correctly depends on how the ground of the floating supply is connected. The floating supply type regulators can cause quite some confusion there. It gets even more tricky with extra sense inputs.

The plan shown is actually not that different from the simulation version I showed, just with more details and a MOSFET power stage.

I don't like the way the output voltage is set - this is more like the old way from the 1970s, with a more or less constant current from the reference and than a variable resistor. This way the variable divider in the feedback path changes the compensation is the voltage is adjusted. It is usually better and with digital control also the logical way to adjust the reference voltage side and use a fixed divider in feedback.

Using part of the AC feedback from before the current shunt is viable option. This adds some resistance (e.g. 0.1-1 Ohms) to the output impedance, but only in the higher frequency area where it is wanted to dampen oscillations.
 
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Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #45 on: April 13, 2020, 08:10:04 pm »
I might be wrong, but while I see a clear advantage in fast switching from CV to CC I can't see any usefulness in real life in a fast recovery from 100% load (i.e. overload).  :-//

That's just a figure of merit, it doesn't come from a practical scenario. I'm open for suggestions, including those you mentioned below. The reason it starts not from 5% or 10% load is because in my designs I most of the do some sort of pre-loading, it's when there is a current sink from the output stage. This improves regulation under light load.

I think the most important features are:
-1) fast CV->CC switch with as little over-current spike as possible
-2) CC-CV switch with no over-voltage spike

In various different circuits I've simulated in past I've got the feeling that the worst overload recovery (i.e. highest relative value voltage spikes) occur when output voltage is set at a few volts (1-2V and below).
Besides that anti-windup diodes (like in those blackdog circuits) do affect load regulation due to leakages (at least in simulation).
For instance simulate a 5-95% load variation and check output voltage with and without the windup diodes.

Oh, wow, I see you are speaking from experience (at least with simulators) :). I had issues with diodes too, esp. with LEDs. Definitely rf schottky worked better in the simulator. I'm so happy that I have no shortage of high-performance parts (except some good parts from the past that extinct). It's not like 20 years ago I was limited what was in my local store.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #46 on: April 14, 2020, 05:45:16 pm »
Quote
7. Stability with large output capacitance

=> That could be very difficult.  Especially with modern low ESR types.

I would not be surprised if many lab power supplies have serious overshoots or oscillate with some
combinations of modern Polymers with low ESR.

There are at least two solutions for solving the problem of unconditional stability into a capacitive load:

1.  Conventional frequency compensation typically privides about 45 degrees of phase margin.  The open loop output series resistance combined with the output capacitance adds phase lag further lowering phase margin.  The output capacitor's ESR adds phase lead which increases phase margin so that is why low ESR output capacitors are especially troublesome and require an added series resistance for stability.

The solution which regulators use to allow operation with large low ESR output capacitors is to take AC feedback from before the open loop output series resistance.  On an integrated circuit, a special transistor structure can be used to do this however a separate fixed resistance also works. This resistance produces the same result as increasing the ESR and is in series with the capacitor but it located in a different place and the amplifier's low frequency DC gain removes its effect on regulation.

2. Another solution implied above is to use frequency compensation which has less than the common 90 degrees of phase lag.  For instance replace the capacitor in the commonly used Miller integrator with a series of RC networks which provide -3 dB/octave of roll-off instead of -6 dB.  The trade off here is poorer frequency response but the results are better under adverse conditions.  I have seen this method used with ATE (automated test equipment) pin drivers.

The problem is more complicated than it first seems, because the output impedance should be as low as possible *and* the power supply
should handle all loads well, including 3 meter of cables with 100 nF ceramic capacitor at the end. The 100 nF case needs a few ohms of damping.

The idea to take feedback before the current measurement shunt does not work as it increases the output impedance from <1mR to at least 0.1-1 Ohm.

No, DC voltage low frequency feedback is taken *after* the added series resistance so the output resistance is divided by the open loop gain.  The same thing happens to reduce the inherent output resistance of the output stage but in this case, the current shunt and any deliberately added series resistance is included as well.

The change in this case is to move the resistance which would be placed in series with the low ESR output capacitor to aid stability to a point where it is before low frequency feedback is taken so its effects on DC accuracy are removed but since AC feedback is taken before the resistor, it still serves to improve stability.  The result is that the high frequency output impedance is dominated by the low ESR output capacitor and the low frequency output impedance is dominated by the series resistance, up to the feedback point, divided by the open loop gain.

This is how integrated regulators designed to operate with ceramic output capacitors work but they can use a special output transistor structure to take AC feedback from before some of the output resistance of the output transistor.

Quote
The second option is much work to implement and has in 100% of the test cases mediocre performance, which is not competitive.

The second method is still higher performance than if the regulator relied on dominant pole compensation to make a low ESR output capacitor stable.  Dominant pole compensation would provide a -6dB/octave roll-off but would need an even lower cutoff frequency.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #47 on: April 15, 2020, 06:37:35 am »
Quote
No, DC voltage low frequency feedback is taken *after* the added series resistance so the output resistance is divided by the open loop gain.  The same thing happens to reduce the inherent output resistance of the output stage but in this case, the current shunt and any deliberately added series resistance is included as well.

Ok, i did not get it that you only take the AC feedback from the point before the shunt resistor.  This should work, but you have to design for
the worst case capacitor (largest one). 
If the largest cap is 100 mF and Rout is 0.1 Ohm, you pole is at 16 Hz.   This is not promising.

Quote
The second method is still higher performance than if the regulator relied on dominant pole compensation to make a low ESR output capacitor stable.  Dominant pole compensation would provide a -6dB/octave roll-off but would need an even lower cutoff frequency.

The -6dB/octave gives you a very clean impulse response.  This is an advantage, but i think that the method with the -3 dB/octave would work too.

If you have 100 dB open loop gain at 1 Hz, you have to get down to 0 dB at about 100 kHz. With the -3 dB/octave method you need a -12 dB/octave
region somewhere or you will not be down at at 0 dB@100kHz.  Could be complicated.

But anyway, it is getting too complicated for my brain here.
 

Offline udok

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Re: how to design fast bench supply with CC and CV?
« Reply #48 on: April 15, 2020, 06:44:40 am »
Quote
Quote from: udok on April 13, 2020, 05:34:19 pm

    The current voltage set current is only a minor error compared to the error introduced by driving the power transistor.  The base current goes
    through the current measurement shunt resistor too.


I don't understand this part. If using an NPN (or darlington/sziklai equivalent) transistor as emitter follower, all current goes into the load. So, no problem there.

If you need precise current measurement the base current is one of the main error sources in the classical circuit which blackdog uses.
The base current flows through the current shunt and depends on transistor beta which is dependent on temperature and collector-emitter voltage.
This circuit is not designed for precision current measurement, but this does not matter in most cases because a bench power supply is a voltage source.
« Last Edit: April 15, 2020, 06:49:35 am by udok »
 

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Re: how to design fast bench supply with CC and CV?
« Reply #49 on: April 15, 2020, 09:28:46 am »

Ok, i did not get it that you only take the AC feedback from the point before the shunt resistor.  This should work, but you have to design for
the worst case capacitor (largest one). 
If the largest cap is 100 mF and Rout is 0.1 Ohm, you pole is at 16 Hz.   This is not promising.


The -6dB/octave gives you a very clean impulse response.  This is an advantage, but i think that the method with the -3 dB/octave would work too.

If you have 100 dB open loop gain at 1 Hz, you have to get down to 0 dB at about 100 kHz. With the -3 dB/octave method you need a -12 dB/octave
region somewhere or you will not be down at at 0 dB@100kHz.  Could be complicated.
For largest considered capacitance one can compromise: there are 2 regions with a large external cap: one usually does not want the supply to oscillate with any reasonable external cap, but one does not need good performance with a really large cap. So it's more like not oscillating up to some 100 mF, and low ringing up to a lower limit like 100 µF low ESR or 1000 µF with normal ESR.

The simple dominant pole compensation with -6dB/octave gives an inductive output impedance. So with an external or internal low ESR cap, one has a resonant circuit and thus quite some ringing.
One may have the 100dB or similar loop gain not at 1 Hz but only well below that.  Still it gets better with a steeper part in between and than a little less than the -6dB for the higher frequencies / cross over region so that the output does not behave like an ideal inductor.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #50 on: April 15, 2020, 01:04:52 pm »
In practice dominant pole compensation is good enough, probably there is not much need to improve. 
Its the task of exe to make his requirements clear, he has not defined a test setup yet.  Without more information this is going in circles.
 
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Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #51 on: April 15, 2020, 01:41:47 pm »
Attaching the schematic of that anti-soar clamp that I advertised earlier. Hope posting it here falls under fair use.

Here's my shot at the fast current limit.

Thanks a lot for your effort and time, learning this design. Kleinstein's design will be next :).
It's not finished. As drawn it will oscillate with a capacitive load. The main weakness it has is the voltage control loop pulls down the current limit, so U2 is inside U1's loop, which isn't ideal. For completeness here's the the design again, with a frequency compensation capacitor on the voltage amplifier and a pre-amplifier for the reference. It will now be more stable for capacitive loads, but you'll need to build it to find out. The op-amp models I've used are fairly slow, with similar speed to the OP07. You can try faster op-amps, but it will be more difficult to stabilise.
[attachimg=2]

Your idea of pulling the output up with a current source and using the op-amps to pull it down should be more stable, because only one amplifier is in the loop. There's no reason why my design can't be modified to use the same technique.

Note how my design senses the current, without having to use a current mirror to move the reference voltage to the output? The current limiting is achieved using a Howland current pump. The downside is it needs closely matched resistors to achieve a high output impedance, but that's probably not a priority here.

Here's a TI application note which covers the circuit in great detail.
« Last Edit: April 15, 2020, 04:01:03 pm by Zero999 »
 
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Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #52 on: April 15, 2020, 05:29:23 pm »
Here's a TI application note which covers the circuit in great detail.

Sorry, what note? :) Thanks for the update!


The simple dominant pole compensation with -6dB/octave gives an inductive output impedance. So with an external or internal low ESR cap, one has a resonant circuit and thus quite some ringing.
One may have the 100dB or similar loop gain not at 1 Hz but only well below that.  Still it gets better with a steeper part in between and than a little less than the -6dB for the higher frequencies / cross over region so that the output does not behave like an ideal inductor.

You guys give really useful insights on compensation, thanks a lot! Hopefully I'll able to use this information.



PS I realized how much time I've spent already on this task. It's starts getting frustrating, so I'll relax requirements quite a bit:
1) voltage-controlled CC and CV modes (on the same ground), so I can use a two-channel DAC to do this (I can use a four-channel dac too if this helps)
2) precise current limit, that is: all current to the load goes through the shunt, no parasitic or quiescent current that is unaccounted
3) opamps don't go into hard saturation. This will ensure fast switching. How fast? Doesn't really matter, it's more about flexing brain muscules and feel good.


Ideally I want to use the same design for high-current (3A max), and low current (100-200mA) of power supply. The only different I expect is pass element (darlington output stage for high-current version), and different compensation. I believe low-current version will be inherently fast-enough for all my practical needs.
 

Offline not1xor1

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Re: how to design fast bench supply with CC and CV?
« Reply #53 on: April 15, 2020, 05:40:52 pm »
I think the most important features are:
-1) fast CV->CC switch with as little over-current spike as possible
-2) CC-CV switch with no over-voltage spike

In various different circuits I've simulated in past I've got the feeling that the worst overload recovery (i.e. highest relative value voltage spikes) occur when output voltage is set at a few volts (1-2V and below).
Besides that anti-windup diodes (like in those blackdog circuits) do affect load regulation due to leakages (at least in simulation).
For instance simulate a 5-95% load variation and check output voltage with and without the windup diodes.

Oh, wow, I see you are speaking from experience (at least with simulators) :). I had issues with diodes too, esp. with LEDs. Definitely rf schottky worked better in the simulator. I'm so happy that I have no shortage of high-performance parts (except some good parts from the past that extinct). It's not like 20 years ago I was limited what was in my local store.

sorry for my late reply, due to the COVID-19 quarantine I cannot hire anybody to take care of my large garden and so I'm just too busy at the moment   :)
I found that old floating supply (Harrison design) simulation of mine and realized that those poor load regulation problems caused by the anti-windup diodes disappear as soon as the CC/CV OR diodes are replaced by 3 diodes (3 x 1N4148 - in simulation) in series. So I guess that blackdog's circuit is unlikely to be affected by that problem since LED diodes (with higher Vf) are used instead of 1N4148.

In the attachment a working simulation with LT1056, 2N3055 and LED diodes (with 2SC5200 it is much better)
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #54 on: April 15, 2020, 05:45:07 pm »
Here's a TI application note which covers the circuit in great detail.

Sorry, what note? :) Thanks for the update!
Sorry, I thought I'd pasted the link into my previous post, obviously not. Here it is:
http://www.ti.com/lit/an/snoa474a/snoa474a.pdf
 

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Re: how to design fast bench supply with CC and CV?
« Reply #55 on: April 15, 2020, 06:10:47 pm »
those poor load regulation problems caused by the anti-windup diodes disappear as soon as the CC/CV OR diodes are replaced by 3 diodes (3 x 1N4148 - in simulation) in series. So I guess that blackdog's circuit is unlikely to be affected by that problem since LED diodes (with higher Vf) are used instead of 1N4148.

This is my experience as well. Only after re-designing it from scratch I realized how much effort it was put into the circuit. All those resistors and diodes are carefully arranged and have particular values ensuring good performance and smooth operation. I have the analog part built and quickly tested it, it worked really well, almost as good as lt3080 in terms of transient response (tested at 1A, with 2sta1943 as a pass transistor), which is considered a fast regulator.

Good luck with the garden!
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #56 on: April 16, 2020, 04:53:20 am »
Chapter 2 of "Analog Circuits - World Class Designs", written by Phil Perkins and edited by Robert Pease, describes the -3dB per octave compensation method.

Ok, i did not get it that you only take the AC feedback from the point before the shunt resistor.  This should work, but you have to design for the worst case capacitor (largest one). 
If the largest cap is 100 mF and Rout is 0.1 Ohm, you pole is at 16 Hz.   This is not promising.

The lead resistance and capacitor ESR add phase lead and if the capacitance is large enough, then dominant pole compensation applies so it is really not too difficult.  Well designed power supplies have no trouble with stability with practical large capacitance loads.  It gets trickier with power supplies designed for fast response.

In practice dominant pole compensation is good enough, probably there is not much need to improve. 
Its the task of exe to make his requirements clear, he has not defined a test setup yet.  Without more information this is going in circles.

A fast design is not going to use dominant mode compensation though.  Most "slow" designs do not either and rely on the output capacitor's ESR for phase lead.
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #57 on: April 16, 2020, 01:40:14 pm »
I think the most important features are:
-1) fast CV->CC switch with as little over-current spike as possible
-2) CC-CV switch with no over-voltage spike

In various different circuits I've simulated in past I've got the feeling that the worst overload recovery (i.e. highest relative value voltage spikes) occur when output voltage is set at a few volts (1-2V and below).
Besides that anti-windup diodes (like in those blackdog circuits) do affect load regulation due to leakages (at least in simulation).
For instance simulate a 5-95% load variation and check output voltage with and without the windup diodes.

Oh, wow, I see you are speaking from experience (at least with simulators) :). I had issues with diodes too, esp. with LEDs. Definitely rf schottky worked better in the simulator. I'm so happy that I have no shortage of high-performance parts (except some good parts from the past that extinct). It's not like 20 years ago I was limited what was in my local store.

sorry for my late reply, due to the COVID-19 quarantine I cannot hire anybody to take care of my large garden and so I'm just too busy at the moment   :)
I found that old floating supply (Harrison design) simulation of mine and realized that those poor load regulation problems caused by the anti-windup diodes disappear as soon as the CC/CV OR diodes are replaced by 3 diodes (3 x 1N4148 - in simulation) in series. So I guess that blackdog's circuit is unlikely to be affected by that problem since LED diodes (with higher Vf) are used instead of 1N4148.

In the attachment a working simulation with LT1056, 2N3055 and LED diodes (with 2SC5200 it is much better)
That schematic made my head hurt. Labels are good, but try to use wires as well, whenever practical, otherwise it means one has to keep searching for them. Try to avoid overlapping text with the symbols. It makes it hard to read. I've rearranged the schematic a bit to make it clearer. I know some of the changes are a bit petty and just personal preference and you and others might not like them, but everyone's different.
[attachimg=2]
[attachimg=1]

There were some models which aren't included in the standard LTSpice install, which I had to Google and import. Hopefully I've chosen the same ones as you have on your machine. I'd be nice if LTSpice included a feature to tell you which models aren't included by default and a way of importing symbols would be nice, so people don't have to use zip files.

You have some good ideas there: a current source to provide a minimum load and LEDs doubling as ORing diodes, so the user can tell whether it's in CV or CC mode.

The issue with this design is the fast current limit is defeated by C4 and C2, which will cause cause current surges, when short circuited and could easily toast an LED. Unfortunately removing them isn't an option as it oscillates, without them.

Which three 1N4148 diodes are you talking about in the simulation? Unless they're subject to high temperatures or reverse voltages, the 1N4148 has a fairly low leakage current and shouldn't affect the regulation that much, unless very high value resistors are used. The data sheet specifies a maximum leakage of 25nA, at 25oC and VR = 20V, which is a voltage drop of just 25mV across a 1M resistor.
https://www.vishay.com/docs/81857/1n4148.pdf

EDIT:
Note, that in the .asc file attached to this post, the load current pulse is set to 100A, to test the overcurrent protection circuit. The result is posted  here:
https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/msg3019102/#msg3019102
« Last Edit: April 16, 2020, 09:27:03 pm by Zero999 »
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #58 on: April 16, 2020, 02:07:38 pm »
C1 and R7 do not provide any compensation feedback for U2. The CV response must be very fast and possibly borderline unstable.
« Last Edit: April 16, 2020, 02:10:04 pm by xavier60 »
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Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #59 on: April 16, 2020, 02:57:11 pm »
C1 and R7 do not provide any compensation feedback for U2. The CV response must be very fast and possibly borderline unstable.
You're right and D10 will also short circuit U2, causing it to current limit. I can't believe I spent so long looking at the circuit and rearranging it, without noticing.  :palm:
 

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Re: how to design fast bench supply with CC and CV?
« Reply #60 on: April 16, 2020, 03:12:25 pm »
Which three 1N4148 diodes are you talking about in the simulation?

It's where you have LEDs.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #61 on: April 16, 2020, 03:44:49 pm »
Which three 1N4148 diodes are you talking about in the simulation?

It's where you have LEDs.
But there are only two LEDs and he mentioned three diodes.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #62 on: April 16, 2020, 04:28:39 pm »
Which three 1N4148 diodes are you talking about in the simulation?

It's where you have LEDs.
But there are only two LEDs and he mentioned three diodes.

Afaik he meant three diodes in series to create a needed voltage drop. Or this can be replaced with one LED, that creates voltage drop of 1.8V+.
 

Offline not1xor1

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Re: how to design fast bench supply with CC and CV?
« Reply #63 on: April 16, 2020, 07:26:34 pm »
That schematic made my head hurt. Labels are good, but try to use wires as well, whenever practical, otherwise it means one has to keep searching for them. Try to avoid overlapping text with the symbols. It makes it hard to read. I've rearranged the schematic a bit to make it clearer. I know some of the changes are a bit petty and just personal preference and you and others might not like them, but everyone's different.
[attachimg=2]
[attachimg=1]

There were some models which aren't included in the standard LTSpice install, which I had to Google and import. Hopefully I've chosen the same ones as you have on your machine. I'd be nice if LTSpice included a feature to tell you which models aren't included by default and a way of importing symbols would be nice, so people don't have to use zip files.

You have some good ideas there: a current source to provide a minimum load and LEDs doubling as ORing diodes, so the user can tell whether it's in CV or CC mode.

The issue with this design is the fast current limit is defeated by C4 and C2, which will cause cause current surges, when short circuited and could easily toast an LED. Unfortunately removing them isn't an option as it oscillates, without them.

Which three 1N4148 diodes are you talking about in the simulation? Unless they're subject to high temperatures or reverse voltages, the 1N4148 has a fairly low leakage current and shouldn't affect the regulation that much, unless very high value resistors are used. The data sheet specifies a maximum leakage of 25nA, at 25oC and VR = 20V, which is a voltage drop of just 25mV across a 1M resistor.
https://www.vishay.com/docs/81857/1n4148.pdf

I apologize, but when I wrote yesterday (and now too) I was physically exhausted so I just modified an old test circuit and tried to remove all non standard models but as you noticed I forgot some of them.

The original circuit diagram used other components. I just modified a bit the compensation network to avoid oscillations.
LEDs as ORing diodes are taken from Blackdog's circuit. I had noticed that with just 1N4148 as OR diodes, when diodes were used in the opamp feedback to prevent windup, load regulation failed misearbly, but the problem was solved with LEDs or 3 1N4148 diodes as OR devices.

Regarding current surge when using a LED as load, I think one should consider the energy stored in the capacitor. Let's make it 50µF, at 30V of output voltage there is a (50*30*30)/2e6 = 45000/2e6 = 22.5mJ.
I suspect that would not damage a LED.

 

Offline not1xor1

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Re: how to design fast bench supply with CC and CV?
« Reply #64 on: April 16, 2020, 07:46:51 pm »
C1 and R7 do not provide any compensation feedback for U2. The CV response must be very fast and possibly borderline unstable.
You're right and D10 will also short circuit U2, causing it to current limit. I can't believe I spent so long looking at the circuit and rearranging it, without noticing.  :palm:

You probably missed that the opamp supply is floating and positive output is ground. That is a simplified version of Blackdog's circuit.
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #65 on: April 16, 2020, 08:40:45 pm »
C1 and R7 do not provide any compensation feedback for U2. The CV response must be very fast and possibly borderline unstable.
You're right and D10 will also short circuit U2, causing it to current limit. I can't believe I spent so long looking at the circuit and rearranging it, without noticing.  :palm:

You probably missed that the opamp supply is floating and positive output is ground. That is a simplified version of Blackdog's circuit.
No, I get that. The problem is, when the current limit kicks in, U2's output tries to go high, but is short circuited to 0V, via D10. In the .asc file attached previous post, I set the load current to 100A, just to see what would happen. I should have posted the result and made it clear that I had done that. Here's the plot of the current through D10, whilst it's in CC mode.
[attachimg=1]

Regarding xavier60's remark about C1 and R7: he's right they can't provide any compensation or feedback, because the inverting input is just connected straight to 0V. To get negative feedback, the output needs to be able to affect the voltage on the non-inverting input.
Which three 1N4148 diodes are you talking about in the simulation?

It's where you have LEDs.
But there are only two LEDs and he mentioned three diodes.

Afaik he meant three diodes in series to create a needed voltage drop. Or this can be replaced with one LED, that creates voltage drop of 1.8V+.
How does having a higher forward voltage help with the transient response? I can't see how it makes any difference and would have thought, the lower the forward drop the better. Silicon diodes should be better than LEDs.
« Last Edit: April 16, 2020, 08:47:27 pm by Zero999 »
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #66 on: April 16, 2020, 09:12:49 pm »
After the U2"s compensation feedback is fix with a resistor between the Inverting input and ground, it should be tested with C1 connected from the ORing node. It will allow U2 to operate in open loop while the PSU is transitioning from CC back to CV witch should be fast with that op-amp.
I feel that C1 should be something larger for now.
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Re: how to design fast bench supply with CC and CV?
« Reply #67 on: April 16, 2020, 09:35:17 pm »
Regarding current surge when using a LED as load, I think one should consider the energy stored in the capacitor. Let's make it 50µF, at 30V of output voltage there is a (50*30*30)/2e6 = 45000/2e6 = 22.5mJ.
I suspect that would not damage a LED.
t depends on the LED. I think a 5mm LED might go pop, if connected to a 50µF capacitor, charged to 30V. The only way of knowing is to test a wide range of LEDs.

A capacitor only helps with a constant voltage source. If you want the power supply to be a good current source, then the output capacitance should be zero. In fact an inductor should be used, rather than a capacitor, for a current source.

Unfortunately in this case we can't have both. A relay is far too slow to switch the output between an inductor and capacitor. A simple workaround would be a manual switch to select between optimal current or voltage regulation.
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #68 on: April 16, 2020, 09:54:57 pm »
About C7 and R5. They help prevent overshoot after the PSU's output has been held low for some time but don't help at all when recovering from a  brief overload that doesn't cause a large drop in output voltage. I eventually omitted these parts from my design. The mod I mentioned earlier greatly reduces overshoot.
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Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #69 on: April 17, 2020, 12:09:24 am »
Windup problems can be minimized by not having the Vcc much higher than needed.
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Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #70 on: April 17, 2020, 07:57:46 am »
Windup problems can be minimized by not having the Vcc much higher than needed.

Interesting thought, came to me yesterday too :). Blackdog's version has a pre-regulator, so we can supply opamps from it. It maintains voltage ~2-3V above the current output (at least in the circuit I've built). So, actually, can be a solution.

A second approach I came with is to modify blackdog's circuit to eliminate sources of parasitic current. That is:
1) set voltage not with a voltage divider (not sure if "divider" is a correct term, more like a summing node?), but with a reference. Yes, this adds complexity, but it's gonna be complicated anyway.
2) buffer voltage sense pin, this way when CV opamp is clamped, it cannot pump current into the load. To avoid performance loss due to additional phase shift on the buffer, I'm willing to try, uhm, bypassing buffer with a small cap. I think this way we'll get two feedback paths: 1) for fast AC 2) slow for DC servo. This is inspired by earlier comments in the thread.

I'll start designing the circuit tonight.

As of why not using big resistors to minimize all those parasitic currents. I think they 1) add noise 2) add offset voltages because of opamp bias current 3) may introduce a pole because with parasitic capacitance they form an RC-filter. So, I'd like to limit resistor values to 20k or below. That's why I learned from "Art of Electronics", hope this is not complete nonsense. I'm yet to do noise simulation in LTSpice, may be I overestimate the noise from resistors.
 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #71 on: April 17, 2020, 08:04:29 am »
C7 and R5 not only help to avoid overshoot. They can also help with the general compensation with larger capacitive load. One may want to change the values though (e.g. like 10 time smaller values). It is more like that C6 and R27 have marginal effect and could be removed.

A reduced (not much higher than needed) supply to the OPs can be the simplest from to limit windup. It works kind of similar to the diodes by setting an upper limit to the OPs output.
 

Offline not1xor1

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Re: how to design fast bench supply with CC and CV?
« Reply #72 on: April 17, 2020, 05:19:01 pm »
OK I spotted a few mistakes. There was a missing resistor on the inverting input of the voltage regulation opamp. I also had to add a 1N4148 in series with the LEDs to avoid load regulation problems and slightly changed the compensation network.
I also rearranged a bit the schematic.

[attachimg=1]

I've not yet tested current regulation, loop stability, etc... I'll try to do that tomorrow.

I'll try to read the other messages (and reply) later.
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #73 on: April 17, 2020, 06:45:30 pm »
OK I spotted a few mistakes. There was a missing resistor on the inverting input of the voltage regulation opamp. I also had to add a 1N4148 in series with the LEDs to avoid load regulation problems and slightly changed the compensation network.
I also rearranged a bit the schematic.

I've not yet tested current regulation, loop stability, etc... I'll try to do that tomorrow.

I'll try to read the other messages (and reply) later.

You forgot to put the model for the 1N4002 in the .asc file.

Just one comment about the presentation: please put the trailing zeros or use R notation for component values <1, i.e. 0.22 or 0R22, rather than just .22. It's very easy to miss the fact you've got 0.22, especially with the grid setting enabled, it looks like 22, at a quick glance.

Are you using the default font settings? When I load that file, lots of the text overlaps with the lines. Is it just because I'm running it under WINE? Your file looks like this on my system. The only thing I've changed on my install is the resistor symbol to the European one because I prefer it to the American one, but the fonts are default.
[attachimg=1]
 

Offline not1xor1

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Re: how to design fast bench supply with CC and CV?
« Reply #74 on: April 18, 2020, 06:44:45 pm »
Are you using the default font settings? When I load that file, lots of the text overlaps with the lines. Is it just because I'm running it under WINE? Your file looks like this on my system. The only thing I've changed on my install is the resistor symbol to the European one because I prefer it to the American one, but the fonts are default.
[attachimg=1]

I'm sorry, my LTSpice installation (WINE running on kubuntu 18.04) is heavily customized and unfortunately I forgot that.
 

Offline temperance

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Re: how to design fast bench supply with CC and CV?
« Reply #75 on: April 20, 2020, 06:15:20 pm »
I wonder why people want to try to build a fast high current high voltage power supply without output capacitors. ...Just to be able to not burn out a small 25mA max LED's when you forgot to turn down the voltage?

Think about what will happen when there is no output cap and you connect some inductance or capacitance to the supply. How are you going to compensate for this? You can try to implement two feedback paths. A fast lane taken somewhere from a pre-driver and a DC lane taken from the output. Or just over compensate. But that's not what you really wanted to do.

Simply put, different applications require different power supplies.

If you happen to work a lot on low power applications, you will need a 15V precision 250mA power supply with a multi turn pot to adjust the output voltage. The output current limiting is not that critical, just to not damage anything.
 
For general work, 1A 25V supply is more then you'll need for every day work. The days of power hungry devices are long gone.

And for some things you might need a 40V 20A supply. In my case, I will just buy what I need because I don't have time to build one. Of course this beast will not be used to test 25mA LED's.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #76 on: April 20, 2020, 06:56:31 pm »
I plan on building building PSU with four channels: 2x2A and 2x125mA or thereabout. I want to use the same architecture, may be even the same pcb, the difference only in output stage, shunt resistor, and compensation network.
 

Offline temperance

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Re: how to design fast bench supply with CC and CV?
« Reply #77 on: April 20, 2020, 07:51:39 pm »
A simple op amp with an emitter follower will outperform a more complicated architectures with ease when it comes to speed. Try one in LT spice and you will see what I mean.

The often copied circuit with the floating op amps is actually a fancy common emitter circuit which requires a lot of compensation. This thing will for sure oscillate with some self inductance at the output with no output capacitor. A common emitter circuit with some self inductance in it's collector is a well known oscillator circuit.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #78 on: April 20, 2020, 08:22:34 pm »
I wonder why people want to try to build a fast high current high voltage power supply without output capacitors. ...Just to be able to not burn out a small 25mA max LED's when you forgot to turn down the voltage?

Low output capacitance yields better constant current performance which is important in some applications.  But the most common reason to have a low output capacitance is for fast response in automated testing applications where time is money.
 

Offline temperance

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Re: how to design fast bench supply with CC and CV?
« Reply #79 on: April 20, 2020, 09:00:42 pm »
I wonder why people want to try to build a fast high current high voltage power supply without output capacitors. ...Just to be able to not burn out a small 25mA max LED's when you forgot to turn down the voltage?

Low output capacitance yields better constant current performance which is important in some applications.  But the most common reason to have a low output capacitance is for fast response in automated testing applications where time is money.


You didn't read what I wrote. I'm referring to what people want to have in their bench power supplies and how the no output cap idea conflicts with their requirements and the very often copied and misunderstood common emitter power supply design.

They are not building an in circuit tester able to measure 100 resistors in a second.
« Last Edit: April 20, 2020, 09:17:17 pm by temperance »
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #80 on: April 20, 2020, 10:09:16 pm »
My friends, just in case, the thread is started exactly for this reason: share my ideas, get other's ideas and get feedback for them. So, it's totally fine to provide feedback and question requirements and assumptions.

I'm discovering what is possible, and how to achieve that. If a few cheap parts can let me achieve "100 per second", then why not? Esp. if I can make a flexible design with multiple modes of operation (or build two versions: with slow response and fast response).
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #81 on: April 21, 2020, 12:31:26 am »
You didn't read what I wrote. I'm referring to what people want to have in their bench power supplies and how the no output cap idea conflicts with their requirements and the very often copied and misunderstood common emitter power supply design.

I just did not bother to address it.  It is a small step from an emitter follower to a class-AB output stage which has stability advantages whether the output capacitance is high or low.

If a class-AB output stage is not used, then it is often beneficial to add an active pull-down (or pull-up) so when adjusting the output voltage with no load, the output voltage does not linger.

Common emitter?  Earlier you wrote:

A simple op amp with an emitter follower will outperform a more complicated architectures with ease when it comes to speed.

An emitter follower is common collector and completely different from common emitter.  Common emitter output stages have a wider variation in output impedance which complicates frequency compensation.  But either can be used successfully.

Quote
They are not building an in circuit tester able to measure 100 resistors in a second.

But they might want a fast response current limit unhindered by high output capacitance.  Or some tests require high slew rate.

Where things really fall apart is poor power supply designs which have 1000s of microfarads of output capacitance which implies frequency compensation problems or a "more is better" attitude.  A good "high capacitance" design can get by with 22 to 100 microfarads per amp.
« Last Edit: April 21, 2020, 12:41:20 am by David Hess »
 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #82 on: April 21, 2020, 07:04:58 am »
The OP and emitter follower variant can be relatively easy for lower voltages up to about 20 V, maybe 25 V. It needs to OP to to cover the whole output voltage range + a little reserve.  Here the voltage regulation tends to be easy (at least for moderate speed), but the current regulation can turn out a little tricky. Even though there is no physical cap at the output, the limited slew rate of the OP can also cause a large current spike in case of a short. One kind of gets a simulated capacitance.

The common emitter type circuit with a current setting output stage is often used as a floating regulator. It needs some capacitance at the output, though not very much. It need a more complicated compensation even in the basic version.  If one wants fast regulation from the emitter-follower version one often ends up with a quite similar compensation. So at the higher performance level the difference is not that relevant anymore. The floating regulator is not so much limited in the voltage range and the same board (just different values, transistors) could be use for low voltages (e.g. 5 V) and high current or high voltages (e.g. 300 V) and lower current.

Some designs (even some commercial lab supplies) use a large capacitance in the 1000 µF range  as a brute force way to avoid overshoot in the CC to CV transition. However there are more intelligent ways to avoid this and avoid the overshoot without a large capacitor.
A large capacitance at the output does not really help the regulator. It is more making things more difficult and the regulation slower.
A large capacitor at the output can also make a fast current measurement more tricky.
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #83 on: April 21, 2020, 11:20:18 am »
I wonder why people want to try to build a fast high current high voltage power supply without output capacitors. ...Just to be able to not burn out a small 25mA max LED's when you forgot to turn down the voltage?

Low output capacitance yields better constant current performance which is important in some applications.  But the most common reason to have a low output capacitance is for fast response in automated testing applications where time is money.


You didn't read what I wrote. I'm referring to what people want to have in their bench power supplies and how the no output cap idea conflicts with their requirements and the very often copied and misunderstood common emitter power supply design.

They are not building an in circuit tester able to measure 100 resistors in a second.
You clearly didn't read the original post or much of this thread. No one is talking about building a high current, high current power supply. The specification in the original post is 15V, 3A maximum.

I don't see how not having a built-in output capacitor can cause any problems. As mentioned earlier, any circuit sensitive to voltage troughs and spikes, should already have good enough supply decoupling, that it shouldn't matter. Even if the power supply has a very low output impedance, up to hundreds of MHz, this will no longer be the case for a circuit connected to the output, via a 1m length of cable, which will have an impedance of the order of 100R, higher if it's two separate flying leads, at RF.

A solution could be to have ground-referenced voltage, and somehow with some level-shifting set current on a high side. Like on the included schematic. I discarded that idea. One of problem is that it didn't want to start, so I added this startup resistor across pass transistor. The second is I cannot use a clamping diode with this design (this is the only type of clamping I mastered so far, but I'm studying other proposed methods).
The main show stopper there is the minimum compliance voltage of the current sink will limit the minimum output voltage. It's better to go for an inverter output, like Blackdog's circuit or high side current sensing, such as the Howland topology, in the circuits I've posted. Low side current sensing is also a possibility, but that makes controlling the voltage reference more tricky, as it will float above the output's negative rail.
[attachimg=1]
 

Offline temperance

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Re: how to design fast bench supply with CC and CV?
« Reply #84 on: April 21, 2020, 07:21:27 pm »
Picture on the left: emitter follower
Picture middle: common emitter circuit. The usual floating op amp circuit.
Picture on the right: an oscillator. Especially with C2 installed

« Last Edit: April 21, 2020, 07:23:09 pm by temperance »
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #85 on: April 21, 2020, 07:38:20 pm »
My friends, where do you get the idea that I want to build a power supply with no output cap? I think mentioned "minimum" capacitance*. Even if I said so, all requirements are flexible.

Nonetheless, it's a very nice remark and a reminder that some output capacitance is needed for stability. Esp. if it's a high-speed design.

*I never defined what minimum is. As a benchmark, my current power supply has 4.7uF and outputs 1A. The output cap is mlcc, so it can discharge quite fast (and this is probably not a good thing).
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #86 on: April 21, 2020, 08:58:34 pm »
Picture on the left: emitter follower
Picture middle: common emitter circuit. The usual floating op amp circuit.
Picture on the right: an oscillator. Especially with C2 installed
Sorry, I don't know what you're on about. All of the circuits posted in this thread have an emitter follower output stage. The schematics you've posted there are nothing like any others in this thread. Only the left one makes any sense and bares some resemblance to a power supply and a basic one at that: just an emitter follower and voltage reference. The middle one has no 0V reference and the right one has no DC connection to 0V.  :palm:
My friends, where do you get the idea that I want to build a power supply with no output cap? I think mentioned "minimum" capacitance*. Even if I said so, all requirements are flexible.

Nonetheless, it's a very nice remark and a reminder that some output capacitance is needed for stability. Esp. if it's a high-speed design.

*I never defined what minimum is. As a benchmark, my current power supply has 4.7uF and outputs 1A. The output cap is mlcc, so it can discharge quite fast (and this is probably not a good thing).
It should be possible to make a voltage regulator which is stable, without any output capacitance. It just won't have so good transient response, in voltage mode. Lots of voltage regulator ICs, such as the LM317 will work without an output capacitor, with minimal oscillation. Try a simple LM317 circuit, set to say 5V, with on output capacitor. It might undershoot and overshoot a bit, when a load is applied and removed, but it won't go into full blown oscillation.

Whether there's an output capacitor or not, depends on how important the voltage regulation is, compared to the current regulation. Adding a capacitor will improve the former, at the cost of the latter, which ideally needs an inductor. A simple DPDT switch can be used to select between either a capacitor or inductor on the output, do change from either better voltage or current regulation.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #87 on: April 21, 2020, 09:08:09 pm »
Try a simple LM317 circuit, set to say 5V, with on output capacitor. It might undershoot and overshoot a bit, when a load is applied and removed, but it won't go into full blown oscillation.

How did they achieve that? I always thought it's because they are very slow (overcompensated) and because they use emitter follower output stage. It also has an internal shunt which, I guess, prevents fast oscillation of the output stage.
 

Offline temperance

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Re: how to design fast bench supply with CC and CV?
« Reply #88 on: April 21, 2020, 09:15:29 pm »
It's correct, you never said something like that. Under 6 you mention: minimum output capacitance. Others have taken this to the extreme in post 14.

The circuit in Reply 57 is a common emitter circuit which you can not simulate properly in spice without mortifying the voltage source.

The LM317 doesn't oscillate without output caps because the common emitter output stage is fed by a current source rolling of the gain of the output stage early on.

@ Zero999
The circuits as drawn don't need a reference. They only demonstrate that the circuit in post 57 is a common emitter amplifier. You can put R4 in the second schematic in the collector if you want. It's still the same circuit.
« Last Edit: April 21, 2020, 10:06:48 pm by temperance »
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #89 on: April 22, 2020, 07:03:26 pm »
There was a very good post here and it's just vanished. One of the points made was a emitter follower is better for voltage regulation, as it has a very low output impedance and a common emitter is better at current regulation, as it has a very high output impedance. I did design a simple CV/CC PSU circuit using three output transistors, so when in CV mode the output stage was a Darlington pair and when it CC mode, it switched to Sziklai pair configuration. I'll have to dig it out and finish it off.

It's correct, you never said something like that. Under 6 you mention: minimum output capacitance. Others have taken this to the extreme in post 14.

The circuit in Reply 57 is a common emitter circuit which you can not simulate properly in spice without mortifying the voltage source.

The LM317 doesn't oscillate without output caps because the common emitter output stage is fed by a current source rolling of the gain of the output stage early on.

@ Zero999
The circuits as drawn don't need a reference. They only demonstrate that the circuit in post 57 is a common emitter amplifier. You can put R4 in the second schematic in the collector if you want. It's still the same circuit.
What's your point? All of the circuits here have an emitter follower output stage. It isn't obvious to me what the circuits you've posted there are supposed to represent. Of course a power supply circuit needs a reference.  :palm:
 

Offline udok

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Re: how to design fast bench supply with CC and CV?
« Reply #90 on: April 22, 2020, 07:56:10 pm »
I deleted my post because i don't want to get into too long discussions.

But one main point was, that a voltage source and current source do not mix up well.

It starts with the output stage design, a voltage source needs a low impedance node.
Therefore choose an emitter follower and not a collector output.
Global feedback will lower the output impedance by the feedback factor,
but it is better to start with a low value if you want a voltage source.
If you want a current source, start with a collector output which has high impedance
right from the start.
The output impedance of an emitter follower is less than 1 Ohm right from the start,
and the second pole which comes into play with a capacitive loads is often at a much higher
frequency than the unity gain frequency of the regulator loop.
This is the case with the LM317, which often does not need a capacitor for stability.

With "fast" regulator loops you have to cope with the second pole introduced by the
open loop output resistance and the load capacitor.
Two poles together have a -12 db/octave Bode plot which is instable if the
unity gain frequency is in this region.
In this case you can use tricks like the -3 dB/octave Bode plot mentioned by David Hess,
or you could place a zero in the loop, or you can specify requirements for the output
capacitor and the ESR.  The ESR together with the output cap introduces a zero,
and the -12 dB/octave becomes a stable -6 dB/octave Bode plot.
Fast designs use almost always sense wires.  This creates more problems,
as the resistance and inductance of the power wiring does not isolate
the capacitor anymore from the regulator loop.

The problems with mixing up current and voltage sources does not end here.
The very efficient Harrison circuit is not a good current source because of the
bias and leakage currents which flow through the current measuring shunt.
Then the current shunt itself should be higher for a good current source
which means more wasted power.
Almost any modern power supply uses a preregulator to reduce power waste
and costs for cooling.
These designs have only one or two Volts of headroom.  This makes
them useless as a current source with dynamic loads.
 






 

Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #91 on: April 22, 2020, 08:08:04 pm »

What's your point? All of the circuits here have an emitter follower output stage.

Not all the circuits shown here have the emitter follower output stage. Especially the floating regulators are usually not. They may look like an emitter follower, but the control is relative to the positive side / emitter. So the control is actually common emitter and thus a current setting output stage.

Switching between the 2 types of regulator can be quite confusing. Both have there pros and cons and one kind of has to decide which way to go. In such a forum it gets very confusing of different circuit types are mixed in one thread.  Quite often the emitter follower type is easier to design and can be a little simpler, at least at low speed. However it is also usually limited to low output voltage, like 20-25V. Already 30 V may need special OPs that can work with a little more supply.

 

Offline temperance

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Re: how to design fast bench supply with CC and CV?
« Reply #92 on: April 22, 2020, 08:26:49 pm »
Thank you Kleinstein for confirming my input in here about common emitter stages.

@ Zero999 thank you for those multiple  :palm:

Some advise: learn the basics before you give other people advice and spice up power supplies without understanding what you're doing. Also, keep the  :palm: to yourself even if you disagree. It's not very polite and you look stupid if you turn out to be wrong. What does the person who opened this topic has to think about this?


I'm out of here.
« Last Edit: April 22, 2020, 08:34:45 pm by temperance »
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #93 on: April 22, 2020, 10:21:15 pm »
Thank you Kleinstein for confirming my input in here about common emitter stages.

@ Zero999 thank you for those multiple  :palm:

Some advise: learn the basics before you give other people advice and spice up power supplies without understanding what you're doing. Also, keep the  :palm: to yourself even if you disagree. It's not very polite and you look stupid if you turn out to be wrong. What does the person who opened this topic has to think about this?


I'm out of here.
There's no need to throw your toys out of the pram. A simple, succinct explanation of the confusing schematics you posted would have done. Post crap, incoherent, garbage and you will get plenty of :palm: emojis. I get that they weren't supposed to be complete designs, but come on, at least show where the load, voltage reference and 0V are supposed to be. I know this is the beginners section, but I thought you were supposed to be advising the original poster how to do it properly.


What's your point? All of the circuits here have an emitter follower output stage.

Not all the circuits shown here have the emitter follower output stage. Especially the floating regulators are usually not. They may look like an emitter follower, but the control is relative to the positive side / emitter. So the control is actually common emitter and thus a current setting output stage.

Switching between the 2 types of regulator can be quite confusing. Both have there pros and cons and one kind of has to decide which way to go. In such a forum it gets very confusing of different circuit types are mixed in one thread.  Quite often the emitter follower type is easier to design and can be a little simpler, at least at low speed. However it is also usually limited to low output voltage, like 20-25V. Already 30 V may need special OPs that can work with a little more supply.
Which circuits, posted in this thread don't have an emitter follower output stage? Could temperance be confusing this thread with another one, or am I mistaken? If it's the latter, please link to the ones which don't have an emitter follower output stage.

I don't see the issue with using an emitter follower at higher voltages. There are plenty of ways of boosting the output voltage from an op-amp to drive an emitter follower at higher voltages, allowing a low voltage op-amp to be used. Granted, adding another stage of amplification will increase the risk of oscillation, but I don't see how it's as bad as having a common emitter output stage who's gain is heavily dependant on the load.
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #94 on: April 22, 2020, 11:32:26 pm »
Technically, the position of the CS resistor in the diagram in  Reply #57 makes the Darligton an Emitter follower.
The function of the Darlington in the design as a whole is of a common Emitter driven current source.
« Last Edit: April 23, 2020, 03:06:48 am by xavier60 »
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Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #95 on: April 28, 2020, 05:43:54 pm »
Hello again.

I tried my idea. There is no problem with setting voltage, but I have problem reading it back (see the picture). Any current drawn by the floating circuit goes through the shunt (because floating ground is connected to the output which is happen to be shunt). So, even if I buffer output voltage (in this case it will be virtual ground), the current drawn from the sensing circuit will go through the shunt (the return path).

There were several proposals using differential amplifier. My reasons to avoid howland/differential circuits were:
1) they have low input impedance
2) CMRR is poor and I need to match resistors
3) limited output swing (afaik)

Now I'm thinking that it might be not so bad idea, esp. because common mode shouldn't be too big across the shunt. So, I'll try circuits posted earlier.

There are ready instrumentation amplifiers, but they are quite slow, I'm worried this may affect performance. They are also mostly low-voltage (3-7V range typically, afaik).

PS emitter follower vs common emitter: it's an interesting detail that I missed originally. This explains why moving reference voltage from positive output to ground make the circuit unstable, especially when switching CC/CV mode. I spent a lot of time stabilizing it, and mostly failed. So, good to know.

PPS I also tried doing some analog switching with jfets and low-level mosfets, but this didn't get me far.
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #96 on: April 28, 2020, 09:16:10 pm »
Yes, the Howland circuits need well-matched resistors to work well, otherwise the output impedance will be low, when in CC mode. I don't see how having a relatively low input impedance is an issue, just add a unity gain stage. If an instrumentation amplifier IC, with a sense, as well as a reference input, can be used to make a Howland current source. Some examples include AMP02, INA103,  INA125.

I'm not sure what you're trying to achieve. The current is only significant because you chose very low resistor values.
[attachimg=1]
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Re: how to design fast bench supply with CC and CV?
« Reply #97 on: April 28, 2020, 09:51:15 pm »
The current is only significant because you chose very low resistor values.

I didn't use high-value resistors to avoid the noise. Perhaps, this is not so important for readback as I can heavily filter it, I only need may be a few kHz of bandwidth. In simulations I used small values (way smaller than I'd use) to exaggerate the effect to ease the analysis.

EDIT: also offset currents may come into play. So, I only use high-value resistors if I have to.
« Last Edit: April 28, 2020, 09:53:04 pm by exe »
 

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Re: how to design fast bench supply with CC and CV?
« Reply #98 on: April 29, 2020, 07:11:18 am »
Ah, silly me, I think I can buffer the output on the "floating" side. Stay tuned :)
 

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Re: how to design fast bench supply with CC and CV?
« Reply #99 on: May 01, 2020, 08:21:24 pm »
The idea didn't work. In this arrangement, in CC mode the reference voltage V7 goes below ground. The clamping diode would clamp it forever. So, time to try other solutions from the thread. I have thoughts to give up on this idea, but I still would like to figure out how other solutions work.
 

Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #100 on: May 01, 2020, 10:10:57 pm »
The idea didn't work. In this arrangement, in CC mode the reference voltage V7 goes below ground. The clamping diode would clamp it forever. So, time to try other solutions from the thread. I have thoughts to give up on this idea, but I still would like to figure out how other solutions work.
I'm still not sure what you're trying to achieve. Let me guess: a ground referenced current control input and high side current sense, after the pass transistor, so the base current doesn't affect the current limit and the output voltage can easily be measured using a potential divider?

Another way to shift the reference voltage to the high side is to use a non-inverting adder circuit, as per a circuit I posted earlier (note U3) and use another op-amp, for the current amplifier, rather than another one in Howland configuration. Unfortunately, as per the Howland, resistor matching will be important. Everything is a compromise. I've already done a quick draft in LTSpice, but will post it tomorrow, as it's needs a bit more work.
 
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Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #101 on: May 02, 2020, 03:36:18 pm »
Here's the sort of thing I was talking about. U3 divides V4 by 4 and adds it to the output voltage. R6, R7 and R8 divides it again by 1.25, so the voltage at the input of the current amplifier (U2) is V4/5. Q2 is the fast current limit, which clamps Q1's base at the voltage on Q1's base.
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Offline Marco

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Re: how to design fast bench supply with CC and CV?
« Reply #102 on: May 03, 2020, 08:47:25 am »
I think that if you really want to have a power supply functional as a good current source, you're going to need to go with PNP/P-MOSFET despite the stability problems. A circuit expecting CV will likely have some capacitance of its own any way, a circuit expecting CC won't generally have inductance. So between CC and CV, the former is more important. If you short out an emitter follower, you're shorting out the capacitance in front of it before regulation catches up, regardless of the capacitance behind it.
 
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Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #103 on: May 03, 2020, 09:19:24 am »
I think that if you really want to have a power supply functional as a good current source, you're going to need to go with PNP/P-MOSFET despite the stability problems. A circuit expecting CV will likely have some capacitance of its own any way, a circuit expecting CC won't generally have inductance. So between CC and CV, the former is more important. If you short out an emitter follower, you're shorting out the capacitance in front of it before regulation catches up, regardless of the capacitance behind it.
That's correct if the output stage is driven from the low-side via a level shifter. Designs of the Harrison topology typically use an NPN Darlington driven WRT the + output terminal. They are current sourcing, which is desirable.
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Re: how to design fast bench supply with CC and CV?
« Reply #104 on: May 03, 2020, 10:22:53 am »
Ah okay, I see your point. Now the common emitter vs. emitter follower discussion earlier in the thread make sense to me ... even though it abuses the normal meanings of the terms.
 
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Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #105 on: May 03, 2020, 11:15:34 am »
This is my attempt at a Harrison design. Using a MOSFET caused the voltage regulation to be a bit on the slow side.
The current limiting is very fast. The CC op-amp operates in open loop until it takes control.
The precharge on C1 sets the allowed current overshoot. It is controlled by the PSU's micro-controller according to the CC setting.
The amount of allowed current overshoot could be made selectable.
This fast limiting idea might not work as well with a BJT output stage.

Edit: Just realized I had already posted this earlier in the thread.
Using a MOSFET avoids the problem of drive current being measured by the CS resistor.
Or a MOSFET/BJT Darlington.
« Last Edit: May 04, 2020, 08:52:55 am by xavier60 »
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Re: how to design fast bench supply with CC and CV?
« Reply #106 on: May 14, 2020, 04:56:03 pm »
My friends, it's been a while since I touched this topic. I didn't forget, I just don't have enough time bandwidth to move it as fast as I would like to do.

I'm playing with Zero999 circuit. I like it. Took me forever to learn how it works. Then I realized that it's like a differential amplifier with a reference pin where reference is tight to the positive rail. There was a reason why I didn't want to mess up with differential amplifier (resistor matching, etc), but I think I can afford a set of 0.1% resistors and a trimpot :)


xavier60, thanks for your circuit, still figuring out how the overshoot circuit works.
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #107 on: May 14, 2020, 09:27:40 pm »

xavier60, thanks for your circuit, still figuring out how the overshoot circuit works.
To keep things simple, assume the MOSFET's Gate is always 4V and ignore diode drops.
In CV mode, the CV opamp is sinking Q4's current, controlling the Gate at 4V.
The CC opamp's output will be near its full + rail voltage.
Q2 acts as an analog switch which is off in CV mode.
When increasing PSU load current reaches the CC set threshold, because the CC opamp has no feedback path, it acts as a fast comparitor.
When its output slews down to 3.4V causing Q2's  B-E junction to conduct, two things happen at the same time.
The CC opamp is now sinking Q4's current  so controlling the Gate.
 Because Q2 is turned on, it has effectively enabled the feedback path via C1. The CC opamp becomes a slow Miller Integrator.
Q2 is not acting as an amplifier. It could be said to be in a saturated state and because of the relatively high drive current, the B-E junction has a low dynamic resistance.

I'm finding it difficult to explain how the precharge on C1 affects the allowed current overshoot.

Edit: I had to correct some of the transistor references.
« Last Edit: May 14, 2020, 11:21:47 pm by xavier60 »
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Offline Zero999

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Re: how to design fast bench supply with CC and CV?
« Reply #108 on: May 14, 2020, 10:26:20 pm »
My friends, it's been a while since I touched this topic. I didn't forget, I just don't have enough time bandwidth to move it as fast as I would like to do.

I'm playing with Zero999 circuit. I like it. Took me forever to learn how it works. Then I realized that it's like a differential amplifier with a reference pin where reference is tight to the positive rail. There was a reason why I didn't want to mess up with differential amplifier (resistor matching, etc), but I think I can afford a set of 0.1% resistors and a trimpot :)
Which circuit? I've posted two completely different topologies in this thread: the Howland and voltage reference shifter.

Have you got as far as prototyping it with real components?
 

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Re: how to design fast bench supply with CC and CV?
« Reply #109 on: May 15, 2020, 08:30:19 am »
Which circuit? I've posted two completely different topologies in this thread: the Howland and voltage reference shifter.

Have you got as far as prototyping it with real components?

The last one, with voltage shifter.

As of building the actual circuit: not yet, I'm still experimenting. I want to try a push-pull output stage, that is adding another bjt to suck the current (unless someone tells me it's a stupid idea). Realistically it will take me weeks, unless I give up on improving the cuircuit and just build a power supply with what we have already.
 

Offline Marco

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Re: how to design fast bench supply with CC and CV?
« Reply #110 on: May 16, 2020, 06:18:29 pm »
It's kind of boring, but the LT3081 looks like a pretty nice regulator which can work with no output capacitor.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #111 on: May 16, 2020, 07:26:48 pm »
It's kind of boring, but the LT3081 looks like a pretty nice regulator which can work with no output capacitor.
At first glance the LT3081 appears to be ideal, but it's unsuitable for the project the original poster is building because a resistor sets the current limit and the requirement is for it to be voltage controlled. The minimum load current is also a nuisance, but a current sink can fix that.

Which circuit? I've posted two completely different topologies in this thread: the Howland and voltage reference shifter.

Have you got as far as prototyping it with real components?

The last one, with voltage shifter.

As of building the actual circuit: not yet, I'm still experimenting. I want to try a push-pull output stage, that is adding another bjt to suck the current (unless someone tells me it's a stupid idea). Realistically it will take me weeks, unless I give up on improving the cuircuit and just build a power supply with what we have already.
Prototyping with real components is part of experimenting. Don't purely rely on simulation results, especially for a project such as this one.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #112 on: May 16, 2020, 08:19:05 pm »
At first glance the LT3081 appears to be ideal, but it's unsuitable for the project the original poster is building because a resistor sets the current limit

What happens if you use a 450 Ohm resistor on ILim, but then inject some extra current there? If internally there's just a high side current source using the resistor to the output to set an output referenced voltage for the inner current limiting circuit, then this should let you set a current limit with a voltage.

It works in LTSpice at any rate, it seems to pump a constant 6 uA out of ILim in their model and you can just add some extra current for the same effect as increasing the resistor.

Even if it works in practice it's not a very precise current limit, but you could wrap it in a digital control loop to walk it to the desired current limit.
« Last Edit: May 20, 2020, 07:42:14 pm by Marco »
 

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Re: how to design fast bench supply with CC and CV?
« Reply #113 on: May 28, 2020, 04:28:54 pm »
Hi there,

Just a quick update. I feel like the project is slowly duying, and additional research doesn't bring much value, and only furthers me away from finishing it. So, I'll take the circuit proposed by  Zero999 with the following modifications:

1. Output sensing will be buffered because CV and CC circuits sink some noticeable current from the output.
2. Instead of "discrete" diffamp that sets CC threshold I'll use ad8476 just not to bother with resistor matching (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8476.pdf)
3. To minimize errors introduced by the diffamp. the final divider (R1/R2 on https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/msg3045938/#msg3045938) will be at the output. This way it will attenuate offset voltage.
4. No pulling capability except for constant current sink at the bjt output because it seems designing a push-pull stage is beyond my skills and adds new problems, such as limiting pull current. But if one wants to change my mind I'm fine :). At the end, ad8476 provides two symmetrical output voltages: above and below reference. So, technically, it can be used to set the maximum sink current. But too much trouble for now and I'm not sure I'll ever use it.

Any suggestions for maximum voltage across shunt under full load? I was thinking of 0.5V to get a good precision, but now I'm thinking it's kinda too much. Or may be not. I want to build two versions of this power supply: one is 0..2A, and another one is 0..0.2A. So, for the later I think higher voltage is fine.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #114 on: May 29, 2020, 12:32:58 pm »
For the maximum drop at the shunt 0.5 V sounds reasonable, maybe at the upper limit for 2 A. It should be a relatively high power shunt - more like 5-10 W power rating to keep the temperature rise small.
It is a compromise between the temperature rise from self heating and the effect of offsets and low frequency noise when the CC limit is set relatively low.  A larger resistance gives better performance at low currents, but more drift near for high currents (like more than 50%).
 
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Re: how to design fast bench supply with CC and CV?
« Reply #115 on: June 01, 2020, 09:16:11 pm »
@Zero999  compensation for CV opamp includes the diode. Is this an error? (see pic). Interestingly that the circuit is stable.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #116 on: June 01, 2020, 09:29:49 pm »
@Zero999  compensation for CV opamp includes the diode. Is this an error? (see pic). Interestingly that the circuit is stable.

Doesn't that allow faster recovery from integrator windup?  I seem to recall seeing the same external compensation scheme used for integrated switching controllers which include a pair of error amplifiers driving the compensation node through diodes.

It should be stable because the compensation network is only disconnected when the other amplifier is controlling the output.
 
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Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #117 on: June 02, 2020, 12:06:05 am »
@Zero999  compensation for CV opamp includes the diode. Is this an error? (see pic). Interestingly that the circuit is stable.
A simulation showing the much reduced overshoot.
https://www.eevblog.com/forum/projects/oscillation-in-psu-simulation/msg3023772/#msg3023772
« Last Edit: June 02, 2020, 01:44:58 am by xavier60 »
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Offline schmitt trigger

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Re: how to design fast bench supply with CC and CV?
« Reply #118 on: June 02, 2020, 01:31:58 am »
Subscribing to very interesting thread
 

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Re: how to design fast bench supply with CC and CV?
« Reply #119 on: June 02, 2020, 12:35:05 pm »
Wow, I'm glad I asked. What looked like an error to me turned out to be that secret sauce that makes the circuit fast. I was wondering why it's so fast, now I know.

@Zero999  compensation for CV opamp includes the diode. Is this an error? (see pic). Interestingly that the circuit is stable.
A simulation showing the much reduced overshoot.
https://www.eevblog.com/forum/projects/oscillation-in-psu-simulation/msg3023772/#msg3023772


Thanks for the link, it has an interesting idea I haven't considered before: using a differential amp to sense current. This will solve multiple problems at once:
1. max current is set via voltage referenced to the ground
2. May be I don't need a separate current sense amplifier to feed ADC? Can I feed a sigma-delta adc directly? LTC2402 is my current choice.

I found ad8421 chip that looks very promising (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8421.pdf). Unlike current sense amplifiers, it has quite more bandwidth. The downside is CMRR is not as good. I wonder if can trim it somehow. I think by attaching a resistor to REF pin I can do that, but only one way...

I'll give it a try in the simulator.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #120 on: June 02, 2020, 02:35:55 pm »
Concerning ADC, I think I found the answer in the datasheet. They say the input of an ADC is a 2.5pF capacitor that they sample at ~153kHz frequency. So, I guess, this does add some disturbance. Fortunately, they provide some guidance how to implement the input filtering with an RC-filter.

I think I need to finish reading the datasheet before asking questions :).
 

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Re: how to design fast bench supply with CC and CV?
« Reply #121 on: June 02, 2020, 08:18:03 pm »
Thanks for the link, it has an interesting idea I haven't considered before: using a differential amp to sense current. This will solve multiple problems at once:
1. max current is set via voltage referenced to the ground
2. May be I don't need a separate current sense amplifier to feed ADC? Can I feed a sigma-delta adc directly? LTC2402 is my current choice.

Ah, silly me, it has to be on the high side so speed-up diodes clamp the output.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #122 on: June 03, 2020, 04:22:50 pm »
Wow, I'm glad I asked. What looked like an error to me turned out to be that secret sauce that makes the circuit fast. I was wondering why it's so fast, now I know.

That configuration is particularly suitable for transconductance error amplifiers (current output) commonly used in power supply controllers including the 723 because it completely disconnects the frequency compensation from the unused amplifier.

It works with conventional operational amplifiers but not to the same extent because the internal compensation still limits performance which neatly explains why you never see it when no external compensation is required; there is no external compensation to disconnect.  Clamping provides higher performance but almost no operational amplifiers support clamping.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #123 on: June 03, 2020, 07:39:42 pm »
Do I need to include the diode of CC opamp into the compensation too? It seems to make the transition from CC to CV a bit more stable.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #124 on: June 04, 2020, 09:51:28 pm »
Where would you point someone who's interested in designing op-amp circuits and feedback loops like whats in this post?
I'm not an absolute beginner but I don't have much experience with it  :) .
 

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Re: how to design fast bench supply with CC and CV?
« Reply #125 on: June 07, 2020, 10:34:03 am »
Some updates on the project: this week I spent quite some time on the project. It seemed close to be done, but I stunk with an unexpected problem: current readback.

To my surprise, most current sense amplifiers have one or more of those unwanted properties:
1) Cannot measure when voltage close to ground (common-mode voltage does not include ground)
2) imprecise (CMRR ~80db)
3) do not work high common-mode voltage (15V in my case)
4) exhibit increased error when measuring close to the ground
5) have high input current
6) confusing datasheet

My workaround is I'll use an instrumentation opamp (such as ad8422). Their CMRR is not that great, so I'll use two current ranges to mitigate that. I wanted to have two current ranges anyway.

Overall, I think for best precision the current-sensing should on the low side of the measurement circuit, otherwise there is a huge common-mode voltage that is hard to deal with if precision is needed.


Where would you point someone who's interested in designing op-amp circuits and feedback loops like whats in this post?
I'm not an absolute beginner but I don't have much experience with it  :) .

I'd start here: https://www.allaboutcircuits.com/technical-articles/negative-feedback-part-1-general-structure-and-essential-concepts/ . I think it's one of the best introductions into the subject.  I'll post more links once I found them. The topic is quite hard to grasp, expect to spend some time. I also suggest not just read it, but practice it in a spice simulator.
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #126 on: June 07, 2020, 10:54:00 am »
Overall, I think for best precision the current-sensing should on the low side of the measurement circuit, otherwise there is a huge common-mode voltage that is hard to deal with if precision is needed.
Problem then is the CS will measure the current drawn by the control and measurement circuitry unless an independent control rail is provided.
A work around I used is  powering the LED panel meter with a shunt regulator to prevent fluctuating CS current.
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Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #127 on: June 08, 2020, 05:06:28 am »
Overall, I think for best precision the current-sensing should on the low side of the measurement circuit, otherwise there is a huge common-mode voltage that is hard to deal with if precision is needed.

Instead of using an instrumentation amplifier, move the current limit amplifier to the high side and have it follow the output.  Now the common mode rejection is only limited by the amplifier itself.

The above means level shifting the control signal to the current limit amplifier's common mode level however this is a low frequency signal so this is easily accomplished with high precision.

 
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Re: how to design fast bench supply with CC and CV?
« Reply #128 on: July 09, 2020, 08:30:19 pm »
Quick update: I'm finalizing the schematic and revising for things that can be done better. Particularly layout. I want to sense on the terminals, which is in my case meaning that the "-" output should be a star ground. But I'm worried that it will be too many wires: reference ground, analog ground, digital ground, power ground... I can, say, simplify a little bit and separate only power ground and control ground, but that might compromise precision and noise.

Instead of doing a star ground on the "-" terminal, I consider using an instrumentation amplifier for voltage sensing. What do you think? My current candidate is AD8421.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #129 on: July 10, 2020, 09:44:03 am »
Instead of doing a star ground on the "-" terminal, I consider using an instrumentation amplifier for voltage sensing. What do you think? My current candidate is AD8421.

That is feasible but it complicates the frequency response and precision.
 

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Re: how to design fast bench supply with CC and CV?
« Reply #130 on: July 11, 2020, 04:47:10 pm »
That is feasible but it complicates the frequency response and precision.

Is there anything specific to in-amps? I understand why adding an opamp buffer makes it harder: it adds offset voltage, phase lag and may limit slew rate. For in-amps, in addition to that,  I only know their CMRR is about 80-90db, and gain accuracy can be a problem. Is there anything else I should know about them?
 

Offline Neomys Sapiens

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Re: how to design fast bench supply with CC and CV?
« Reply #131 on: July 12, 2020, 02:19:56 am »
I found AD's AN-539 quite enlightening about them.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #132 on: July 12, 2020, 02:42:17 am »
That is feasible but it complicates the frequency response and precision.

Is there anything specific to in-amps? I understand why adding an opamp buffer makes it harder: it adds offset voltage, phase lag and may limit slew rate. For in-amps, in addition to that,  I only know their CMRR is about 80-90db, and gain accuracy can be a problem. Is there anything else I should know about them?

No, you covered it; the added phase lag requires extra attention to frequency compensation and DC errors affect the accuracy.  Obviously it is feasible to do because it is a common way to implement Kelvin connections or remote sense, but often there is a way to implement this without a separate amplifier.

The fastest operational amplifiers which would add a minimum of phase lag tend to have poorer DC performance so that is no solution except where lower precision is acceptable.  Fortunately however in many regulators, the output stage is what limits frequency response so cascading a couple of amplifiers is not a problem, but this is application dependent.

« Last Edit: July 12, 2020, 03:00:00 am by David Hess »
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #133 on: August 01, 2020, 09:09:51 am »
Everyone would agree that the HP/Harrison topology allows the best performance because all of the important bits are ground  referenced to one point, the positive output terminal. But having to ground test equipment to the positive rail makes testing and debugging awkward.

I wonder if it would all be easier with the pass transistor in the negative rail allowing the negative output to be the ground reference point.
Up until a year ago, my only bench supply was something I built 40 years ago. Its 2N3055 was in the negative rail with its Collector bolted directly to the aluminum front panel. It works just fine.
« Last Edit: August 01, 2020, 09:44:49 am by xavier60 »
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Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #134 on: August 01, 2020, 10:02:30 am »
One can mirror the HP/Harrison topology, just use  PNP or P-MOS power devices. These are however often a little slower or high input capacitance. So the regulator floating on the positive side is a little more attractive.
 

Offline David Hess

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Re: how to design fast bench supply with CC and CV?
« Reply #135 on: August 01, 2020, 04:37:29 pm »
Everyone would agree that the HP/Harrison topology allows the best performance because all of the important bits are ground  referenced to one point, the positive output terminal. But having to ground test equipment to the positive rail makes testing and debugging awkward.

I wonder if it would all be easier with the pass transistor in the negative rail allowing the negative output to be the ground reference point.
Up until a year ago, my only bench supply was something I built 40 years ago. Its 2N3055 was in the negative rail with its Collector bolted directly to the aluminum front panel. It works just fine.

Sometimes that is done so that the uninsulated collector/source can be bolted directly to the heat sink.  Or maybe only NPN/N-Channel transistors are desired because of their higher performance and lower cost.  In the later case, you can find old designs were the NPN transistor is used on the high side as an emitter follower, and then its output is grounded to make a negative supply.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #136 on: August 02, 2020, 11:16:15 am »
One can mirror the HP/Harrison topology, just use  PNP or P-MOS power devices. These are however often a little slower or high input capacitance. So the regulator floating on the positive side is a little more attractive.

I wanted to do this way, but this means that the pass element on the ground. What if output is disabled and the pass element not conducting? Doesn't this mean there is no return path for current anymore? Or it depends if aux supply is floating or not?

I also wanted to "invert" the control signal so that opamps see the grounded load, but afaik it's less stable ("ldo topology") because output stage has gain (on the pic).
 

Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #137 on: August 02, 2020, 12:04:30 pm »

I also wanted to "invert" the control signal so that opamps see the grounded load, but afaik it's less stable ("ldo topology") because output stage has gain (on the pic).
This kind of level shifter?, https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/?action=dlattach;attach=1000379;image
It can be made to work very well.
« Last Edit: August 03, 2020, 12:38:41 am by xavier60 »
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Offline xavier60

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Re: how to design fast bench supply with CC and CV?
« Reply #138 on: August 02, 2020, 12:24:26 pm »
To avoid unwanted current flow through the CS resistor, two control rails are needed. One can be regulated from the main unreg  in.
The other has to be floating.
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Re: how to design fast bench supply with CC and CV?
« Reply #139 on: August 04, 2020, 07:31:58 pm »
Hi ,
I have found this article that describe the schematic of Error Amplifier with Forced Equilibrium Adaptor , it give a very good switch from CV to CC maybe it can help https://www.kepcopower.com/equibm2.htm , the schematic is given in figure 3
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #140 on: August 04, 2020, 07:39:10 pm »
I almost finished the design, but then I decided to add a current sinking capability*, which makes things more complicated... Meanwhile I found a few good opamps with good slew rate.

What if instead of fighting the problem I just take an opamp with fast recovery? Like opa189 which I wanted to use anyway. Usually people say auto-zero opamps go nuts when saturated, but this one seems to be an exception. This will greatly simplify the circuit and will let me use a ground shunt. Although, even with a ground shunt I think it still will be a challenge to route the pcb.

The topology i want to use is attached. It's Keithley 236/237 (pic is taken from the blog linked below).

* After re-reading https://poormanssmu.wordpress.com/research/ , which makes much more sense to me now.

UP:
Hi ,
I have found this article that describe the schematic of Error Amplifier with Forced Equilibrium Adaptor , it give a very good switch from CV to CC maybe it can help https://www.kepcopower.com/equibm2.htm , the schematic is given in figure 3

Oh, just saw your post. Thanks, I'll read it.
 

Offline bikeNomad

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Re: how to design fast bench supply with CC and CV?
« Reply #141 on: August 10, 2020, 04:16:31 pm »
How about something like the HP 6826a or 6827a? As I recall, they have a fast (~100µs) switchover between CV and CC modes, and have 4-quadrant outputs.
I'm an autodidact who believes in Sturgeon's Law and wants to continue contributing to the creation and improvement of the other 10% of everything.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #142 on: August 14, 2020, 11:28:35 am »
How about something like the HP 6826a or 6827a? As I recall, they have a fast (~100µs) switchover between CV and CC modes, and have 4-quadrant outputs.

Thanks, I'll have a look. Never heard of this models before.

100us is I'd say quite slow, with slew rate of even a few volts per us, the power supply can dump a lot of energy into DUT before switching before CV/CC. I did a simulation with OPA189 without anti-windup diodes, for example. It showed a 2V overshoot when switching from CC to CV mode in less than 3us. So, a lot can happen during these 100us.

Although I start to question how fast a power supply should really be. Would this ultra-fast switching really help when testing sensitive semiconductors?
 

Offline bikeNomad

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Re: how to design fast bench supply with CC and CV?
« Reply #143 on: September 19, 2020, 04:42:48 pm »
I just tested my (newly refurbished) HP 6826A going from 4V constant voltage to 20mA constant current (using a 100Ω resistor) and found that it took 400µs to switch to CC mode.
I'm an autodidact who believes in Sturgeon's Law and wants to continue contributing to the creation and improvement of the other 10% of everything.
 
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Online Kleinstein

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Re: how to design fast bench supply with CC and CV?
« Reply #144 on: September 19, 2020, 06:34:36 pm »
How about something like the HP 6826a or 6827a? As I recall, they have a fast (~100µs) switchover between CV and CC modes, and have 4-quadrant outputs.

Thanks, I'll have a look. Never heard of this models before.

100us is I'd say quite slow, with slew rate of even a few volts per us, the power supply can dump a lot of energy into DUT before switching before CV/CC. I did a simulation with OPA189 without anti-windup diodes, for example. It showed a 2V overshoot when switching from CC to CV mode in less than 3us. So, a lot can happen during these 100us.

Although I start to question how fast a power supply should really be. Would this ultra-fast switching really help when testing sensitive semiconductors?

A fast transition from CC to CV mode limits the overshoot. In designs with a slow switch over one may need additional capacitance at the output to limit the overshoot.  Relaying on the capacitor to limit the rise may need quite some extra capacitance. With 1 A into some 100 µF (usually plenty for small signal stability) and some 0.5 V accepted overshoot is would be some 50 µs to react. The other way around some 400 µs reaction time would require substantial capacitance to keep the overshoot in the voltage low.

The CV to CC more transition may not need to be so fast, as there always is the charge from the output capacitor. A slow reaction kind of looks like additional simulated capacitance. It can make a difference if there is an additional fast (but usually not accurate) current limit so to avoid a large current spike in case of a short. Especially a source follower output stage can be really bad in this respect. For many uses one can accept and may even want not too fast a reaction, so the short current spikes don't let the supply drop even though the average current is still well below the limit.

For really sensitive parts the actual capacitance at the output alone can be deadly. It would be still nice to protect things like To92 transistors or SOIC8 chips from a thermal damage.
 

Online exe

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Re: how to design fast bench supply with CC and CV?
« Reply #145 on: September 20, 2020, 09:32:36 am »
May it's worth posting a small update on the project. The project is not dead, but I took a break. So I investigated how much I need an active rectifier for the power supply. I think I can get away with this approach: https://github.com/kopchik/semi_active_rectifier . Basically, I replaced normal diodes with FERD diodes, and bottom diodes are "shunted" with n-fets.

I have pcb already, waiting for Mouser to deliver missing parts and I'll do evaluation. I have a nice DIY power supply which is currently limited to ~0.7A max output due to the diode bridge not mounted on a heat sink (yeah, stupid thermal design, I know). The surrounding plastic (PETG) cannot handle more than 70C. After upgrade I expect it to handle about 1.5A or so.
 


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