Author Topic: Bench CC/CV PSU Based on Daves uSupply (Not Anymore)  (Read 22023 times)

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Offline KC0PPHTopic starter

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Bench CC/CV PSU Based on Daves uSupply (Not Anymore)
« on: March 10, 2019, 05:30:42 am »
I am in need of a CC/CV Bench PSU and decided to build one. I could get a decent unit for a cheaper price than all the parts, but then I would not learn anything.

Anyway the goal of the project is:

1.2V - 12V 1A CC/CV PSU
20mV accuracy
20mA accuracy
5V Fixed Output
3.3V Fixed Output
All Unused IO ran to headers for future fun

I plan on powering this from a 18V Laptop Charger or something similar.

A brief description of the system:

Page 1 has the 5V and 3.3V regulators. It also has a switching regulator that should keep its output 1.2V higher than the 3080's output.

Page 2 is the microcontroller. -- Note a display and rotary encoders are the last things to go on. Display I am still torn on I2C, SPI or Parallel. I have done them parallel in the past. Rotary encoders I have not finished researching them to make an informed decision.

Page 3 is the circuit I basically copied from Daves RevC Schematic. There are a few minor changes, but 99% of it is his work.

So my question is, are there any errors that can be spotted, and what improvements should I make? In the board layout I tried to keep the analog stuff as far from the switchmode PSU as possible.

Thanks in advance.

Also if anyone would like a PCB let me know. Ill have some spares as JLC requires a min order of 5.


*Sorry did not realize I could edit once I submitted*
« Last Edit: March 12, 2019, 01:36:22 am by KC0PPH »
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #1 on: March 10, 2019, 05:32:00 am »
Sorry it did not print out correctly.

Correct Version Below.
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #2 on: March 10, 2019, 05:33:37 am »
Here is the Initial Layout.

 

Offline iMo

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #3 on: March 10, 2019, 09:33:43 am »
I would remove the IC4A buffer.. I would use it for buffering the Vref..
Otherwise the Q is how stable with different loads and fast the CC CV loop will be.
« Last Edit: March 10, 2019, 09:42:56 am by imo »
Readers discretion is advised..
 

Offline Kleinstein

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #4 on: March 10, 2019, 12:47:11 pm »
Daves circuit is a really poor starting point for a lab supply. Usually starting with a voltage regulator is a poor idea, as  is it hard to add a variable current limit. The poor starting point was a reason the µ-supply never got finished because of problems under certain loads.

My best advice on this circuit is try to find the the errors and weak point and learn from it, but do not try to fix it, as it's starting with the wrong concept.  It's easier to start from scratch.

A much better starting point would be circuit from the start of this thread:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/msg2232978/#msg2232978

The task would be to get around the the negative supply. A simple way would be with 2 powerful series diodes to generate some -1.2 V. Not very power saving but with some 19 V to start with and a 12 V target the voltage is there to spare.
Something like 0-14 V at up to some 2 A should be relatively simple

However there is a downside of using a low side shunt: it is hard to add a second output. So the fixed 3.3 V and 5 V would need a different power source or fully isolated with a switched mode converter.
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #5 on: March 10, 2019, 10:53:30 pm »
I would remove the IC4A buffer.. I would use it for buffering the Vref..
Otherwise the Q is how stable with different loads and fast the CC CV loop will be.

I was curious as to why there was a buffer and my only thought was to use up that opamp so its not floating. I am not sure about what you are talking about with the Q and stability. As for putting the buffer on the Vref I would think (zero formal experience) that the less things you have in your VRef the better, as each component has its own temp co and drift.

Daves circuit is a really poor starting point for a lab supply. Usually starting with a voltage regulator is a poor idea, as  is it hard to add a variable current limit. The poor starting point was a reason the µ-supply never got finished because of problems under certain loads.

My best advice on this circuit is try to find the the errors and weak point and learn from it, but do not try to fix it, as it's starting with the wrong concept.  It's easier to start from scratch.

A much better starting point would be circuit from the start of this thread:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/msg2232978/#msg2232978

The task would be to get around the the negative supply. A simple way would be with 2 powerful series diodes to generate some -1.2 V. Not very power saving but with some 19 V to start with and a 12 V target the voltage is there to spare.
Something like 0-14 V at up to some 2 A should be relatively simple

However there is a downside of using a low side shunt: it is hard to add a second output. So the fixed 3.3 V and 5 V would need a different power source or fully isolated with a switched mode converter.

I was initially thinking about doing something without using a regulator, but decided on using a regulator to expedite the process. It will be interesting as I go through this process what I learn. In the end I will most likely build something else for my true bench power supply as this will not fill all of my needs (I want multi channel, and much better regulation, and measurement)

Is there anything that is going to be disastrous with this design, or will it just have quirks that I can analyze and learn from as I build and test it?

In reality the projects that this will be used on will all be low current, and I foresee using the fixed regulators more than the CC/CV. My interest is in PIC, FPGA and possibly some RF stuff (I am a HAM after all) I could cover 99% of my use cases with 150ma fixed supplies in 5V, 3V3, 2V5, 1V8, 1V2, -5V. I do plan on adding the 2V5, 1V8 and 1V2 fixed supplies to this project before its finished. The CC/CV Adj is more of just something to play with as well. If you notice I also am sending all of the unused IO to a header and have lots of LED's to play around with, so it will also be a dev board.
 

Offline iMo

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #6 on: March 11, 2019, 07:28:29 am »
I would remove the IC4A buffer.. I would use it for buffering the Vref..
Otherwise the Q is how stable with different loads and fast the CC CV loop will be.
I was curious as to why there was a buffer and my only thought was to use up that opamp so its not floating. I am not sure about what you are talking about with the Q and stability. As for putting the buffer on the Vref I would think (zero formal experience) that the less things you have in your VRef the better, as each component has its own temp co and drift.

"Otherwise the question is how stable with different loads and fast the CC CV loop will be.."

The stability is an important factor as the whole system may oscillate, ring, etc. with various loads and operation conditions. There were several threads on PSUs with the simulations with switching the loads, capacitive loads, etc. recently..

The IC4A only adds a propagation delay, which may add to stability issues.

The Vref buffer - your targeted resolution is 20mV/20mA - the drift's tempco of the 358 will not add significant error under normal conditions, imho. Otherwise the ADC/DAC components like low impedance Vref source usually, therefore the buffer..
« Last Edit: March 11, 2019, 07:33:52 am by imo »
Readers discretion is advised..
 

Offline Kleinstein

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #7 on: March 11, 2019, 06:36:43 pm »
With daves circuit the adjustable current limit is very slow and chances are high it may oscillate with some (if not most) loads. So for the initial phase of a few ms there would be only the regulator internal current limit (e.g. 1.5 -2 A). So the current limit could offer protection to physical larger resistors, but not so much to semiconductors.  If one tries to speed up the response (reduce the 22 µF capacitor), there is a chance to get voltage overshoot during transients, going from CC to CV mode.

AFAIK there was also a slight problem during turn on/ turn off.

With a switched mode regulator before, chances are high to see quite some ripple - though it depends on the layout.

Another limitation is that the voltage regulation is not very accurate / stable. Due to heating of the chip the voltage can change a little and there is some series resistance (chip internal and from the layout).
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #8 on: March 11, 2019, 07:44:46 pm »
Honestly, skip the LT3080, and rather than multiple resistors for the current shunt, use a 0.1 ohm resistor in a 1206 package.

Use a topology something like this (attached). Configure the gain on I_SENSE and the resistive divider V_SENSE so you get 0-<fullscale DAC voltage> for 0-1A and 0-12V respectively. Feed I_SET and V_SET from your DAC, feed I_SENSE and V_SENSE to your ADC. You'll need to experiment to get the right compensation across U1 and U2.

This sort of topology is pretty much what you'll find in any power supply - usually the opamps are powered from a separate voltage rail, but for this low voltage you should be OK to power them as shown. Make sure you use a single supply rail opamp so that your outputs will go down to zero.
 

Offline Kleinstein

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #9 on: March 11, 2019, 08:41:01 pm »
Honestly, skip the LT3080, and rather than multiple resistors for the current shunt, use a 0.1 ohm resistor in a 1206 package.

Use a topology something like this (attached). Configure the gain on I_SENSE and the resistive divider V_SENSE so you get 0-<fullscale DAC voltage> for 0-1A and 0-12V respectively. Feed I_SET and V_SET from your DAC, feed I_SENSE and V_SENSE to your ADC. You'll need to experiment to get the right compensation across U1 and U2.

This sort of topology is pretty much what you'll find in any power supply - usually the opamps are powered from a separate voltage rail, but for this low voltage you should be OK to power them as shown. Make sure you use a single supply rail opamp so that your outputs will go down to zero.

+1 with most of this, except for the 0.1 Ohms resistor.  For 1 A of maximum current and the differential amplifier the shunt should be a little larger. So more like 0.2 to 0.5 Ohms and also with higher power.  Some of the 0.22 - 0.47 power wire wound resistors are not that bad in TC.

The OPs might like to see a slight negative supply, e.g. from a diode from the negative power to ground, common to the whole circuit, including the fixed supplies.  No need to go for rather expensive LT1006. Something like TLE2021 or similar should be good enough.
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #10 on: March 11, 2019, 08:44:22 pm »
You could always put two 2016 packages in parallel and have a 0.05 ohm shunt.. then adjust the gain of the sense amp, if dissipation is an issue. There's also plenty of real shunt resistors out there which are not all that expensive.

I chose the LT1006 simply because it was the first single supply opamp I came across in LTSPICE. Im sure a great many parts would work

I've now simulated it and it does work fairly well. I'll post the LTSPICE file in a bit.
 

Offline Kleinstein

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #11 on: March 11, 2019, 09:33:07 pm »
The simulation has no problem with the difference amplifier, but in real world the common mode rejection is not that good unless very accurate resistors are used. So the tendency is to use a higher value shunt to start with a little higher voltage at the shunt. This also helps when a small current limit is set.

I know the problem of heating of the shunt. The resistor is a compromise between self heating and demand on the amplifier. For a simple circuit I wold prefer a high value and higher power resistor, e.g. vitrohm KH206 or KH208 (4/6 W cemented wire wound). A larger resistor needs less speed from the amplifier and thus makes the CC regulation easier.
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #12 on: March 11, 2019, 10:24:23 pm »
Let me pull my horns in for a bit and breadboard up that circuit and see what I get. I have some general Op Amps (LM324 I think) I might be shy on Cap values but should be good on resistors.

Thanks for the advice and I will update with how the circuit performs.
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #13 on: March 11, 2019, 10:48:52 pm »
Have a play around with this in LTSPICE :)
LM324 might work actually. I couldn't get LTSPICE to behave with the TLE2021 model... convergence problems
edit: Added an image for those who are LTSPICE impaired :)
« Last Edit: March 11, 2019, 10:50:46 pm by jaycee »
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #14 on: March 11, 2019, 11:50:08 pm »
I got it in LTSPICE and have been playing around with it.

In playing with the Voltage Divider for V-SENSE I am a bit confused why the 3 resistors and the capacitor C6 vs just 2 resistors.
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #15 on: March 12, 2019, 12:08:03 am »
C6 basically works as a "speedup" capacitor to improve the response time to voltage transients. Try removing it and you'll see the effect
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #16 on: March 12, 2019, 12:20:35 am »
To be honest I just downloaded LTSpice for the first time, so just getting started playing with it. I need to RTFM as I could not get the Load to work correctly. I replaced it with just a Resistor and made a few tweaks.

First I made a 1 Ohm Current Shunt -- I want a bit more of a voltage drop as I will be using a 13 bit ADC and just want a bit more flexibility to play a bit.

Second I tweaked the voltage dividers to achieve a 12V 1A @ 4.096. Some of the resistors are not a common value but I can figure something out. I need to now dig into some datasheets and figure out if I have transistors and diodes on hand that will work for this circuit to test out. I also have some learning to do as I have no clue about how a few parts of the circuit work.

Thank you for your help, i think this is going to be even more enjoyable.

-Keith
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #17 on: March 12, 2019, 12:33:07 am »
I noticed I goofed something - Q6 should be a 2N3904, and R9 should be 68 ohms.

I would not adjust the gain of the current sense amp like that - it is best to keep R5 = R6 and R3 = R4. Look up "differential amplifier"

Try R5=R6=1K2 and R3=R4=47K. For the voltage sense, try R18 = 68K, R8 = 8K2 and R19=39K

For odd values of resistors, you can simply put resistors in series... for example the 1K2's you could use 1K + 220
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #18 on: March 12, 2019, 12:39:03 am »
Also be aware that the higher the value of the current shunt, the more power it will dissipate. I would stick to 0.1 ohms... this is not a hard value to get. The resolution of your ADC doesnt really matter here as the gain of the current sense amp can be adjusted to suit, and you can connect the ADC to I_SENSE.

What problems did you have with the I1 load ?

If you're not sure how anything works, ask ;) As for transistors... the 2N3904/2N3906's are not critical and can be e.g. BC847/BC857. If you're breadboarding, BC546/BC556 are fine. BCP56 can be a BD139 - I simply chose BCP56 because it's a good surface mount part for the job. ES1D's can be 1N4004s. TIP35C can likely be any power transistor, TIP3055/2N3055 for example. Make sure that is heatsinked - an old CPU cooler is ideal!

« Last Edit: March 12, 2019, 12:40:58 am by jaycee »
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #19 on: March 12, 2019, 01:04:17 am »
Thanks for all the replies..

Well I found 2 2n3055 that I can use as the pass transistor.  I have a bunch of 1n400x that I can use for the diodes. I also found some LM118 that claim they are good to use in A/D circuits. I found some LM324 and also LF147. Ill use the 324 to start with as they are cheap and I have a pile of them. I might have to wait to do this as I cant find ANY transistors. I though i had a pile of TO-92 2n390x...
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #20 on: March 12, 2019, 01:13:40 am »
The LM324 actually seems to work pretty well. The important thing is that this opamp can work near its negative rail... most opamps cannot go right down to 0v output unless they have a -ve supply.
Pretty much any small signal transistor should work in place of the 2N's...
 

Offline KC0PPHTopic starter

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Re: Bench CC/CV PSU Based on Daves uSupply
« Reply #21 on: March 12, 2019, 01:35:41 am »
Yeah I will see how they work out. Im not too concerned with getting down to zero for an output, although it would be nice.

Just placed an order for some parts. They will be on the slow boat from china. I ordered from Tayda and their faster shipping methods were a bit extreme on price.
 

Offline jaycee

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Re: Bench CC/CV PSU Based on Daves uSupply (Not Anymore)
« Reply #22 on: March 12, 2019, 03:26:18 am »
A little update...
 
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Offline xavier60

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Re: Bench CC/CV PSU Based on Daves uSupply (Not Anymore)
« Reply #23 on: March 12, 2019, 03:43:44 am »
If the current regulation loop is difficult to compensate, a transconductance output stage might work better rather than the voltage follower.
I have found that change in load impedance doesn't affect the transfer function as much with a transconductance output stage while in CC mode.
HP 54645A dso, Fluke 87V dmm,  Agilent U8002A psu,  FY6600 function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.
 

Offline Kleinstein

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Re: Bench CC/CV PSU Based on Daves uSupply (Not Anymore)
« Reply #24 on: March 12, 2019, 07:14:59 am »
The compensation for the CC mode gets easier when the shunt is a little higher in value.
Both for the CV and CC mode it help if there is some ESR with the main capacitor at the output. Just the normal ESR of an electrolytic cap already helps a lot. If the ESR in included the 100 µF should be well large enough that the load impedance should not effect the compensation of the CC mode very much.

The CC mode may have to be relatively slow (still much faster than in Daves version) and are additionally limited by the slew rate of the OP. So it may help to have an additional fast current limit (e.g. a transistor over the shunt).
 


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