Another Noob PSU Design Needing Review, Please

[DavidAlfa]

I'd put a dc/dc buck module, then add a linear regulator after it to clean the ripple.
These linear designs have the efficiency of a rock!

[David Hess]

What does the D2 do on the pin8 of the LM301 in the original schematics?

D2 uses the 301A's compensation pin to clamp its output making recovery time faster.  The 301A is almost uniquely suited for this sort of circuit because it supports clamping.

D2 at the LM301 should speed up the recovery from saturation and thus the reaction the current limit somewhat. The higher speed of the tl071 would about make up for this.

External compensation through C5 limits speed, so a faster operational amplifier does not improve recovery time.

Extra: Actually only 2 important things are needed. the opamp needs to be able to safely run on 36V and the input common mode range needs to include V+.

The 301A has an input common mode range which includes its positive supply, and the better ones can operate up to 44 volts.

How is the pass transistor going to survive an output of 1V at 10A (specified in the original post).  It can't really handle 320W, it is only rated for 200W and that's probably when the moon is low on the horizon.

And the pass transistor is not protected from an instantaneous short in this circuit anyway.  The configuration shown below allows the regulator's current limit to protect the pass transistor or transistors by better controlling the current gain.

Parallel transistors are still a good idea at this power level but the configuration below handles that as well.

[t1d]

Below is a list of the Op Amps that I have on hand. Maybe someone will see one that would work. That failing, I will order a 301A, because Kleinstein seems to think it okay.

Here is a thread I started on the easy way to create the inventory.
https://www.eevblog.com/forum/projects/an-easy-method-of-making-a-inventory-of-your-components/msg3703468/#msg3703468

I will be replying to some of you recent posts, but I have to take a break and eat, presently.

[t1d]

The can-style LM301 is quite expensive. The On brand LM201AV appears to be more robust and way less expensive. $0.81USD. Data sheet below.

[t1d]

As drawn in the original sketch, there is a missing joint in the current adjust circuit.

Great catch! Corrected.

[t1d]

Any brave worms collector here?
Below the LTSpice file..

Thank you, imo, for this amazing contribution of effort!

[t1d]

Here for example.. The R2 and R10 1Meg resistors (at CC and CV potentiometers) will not affect the setting linearity "too much" and in case of an open wiper the resistors pull the opmap's inputs to gnd.
Your design is different, so you have to think twice where to wire the cold sides of those Rwipers to..

Ahh... This is a wonderful teaching point... Let's see if I understand it correctly... If the wiper of RV2, or RV3, losses contact, then the op amp will swing to its full output, which drives the BJT to its full output, which creates a meltdown. By adding high value resistors, when the wiper contact is lost, we shunt the controlling voltage through the protection resistor, which would cause the op amp and, therefore, the BJT to go low. The cost of adding this protection resistor is the small amount of current that it leaks continuously.

If adding the protection is desired, then the question becomes where to inject and terminate the protection resistor. This is where my knowledge is lacking...

But, I must admit that, for a stationary bench device, I wonder how great of a probability this type of failure would be. The consideration of it is well worth the education therefrom. However, will it actually be included in the final build? I am not sure.

It does occur to me that the issue could be resolved for the offset trimmer by testing the unit with a trimmer and, once the trimmer value is known, to replace it with individual resistors. Of course, there is the factors of drift to consider, with a fixed resistor approach.

I will think on it.

[t1d]

If you back off on the 10A requirement, maybe... 

Yes, the design goals were changed to 30V/3A, in the run of the text.

[iMo]

Before you spend a lot of effort and $, prototype the PSU with what you have handy and with lower voltages and currents. The stuff did not work in my simulation with the values as in your schematics (see above the LTSpice file with your PSU).

[t1d]

Before you spend a lot of effort and $, prototype the PSU with what you have handy and with lower voltages and currents.

Yep, that is always a good idea.

The stuff did not work in my simulation with the values as in your schematics (see above the LTSpice file with your PSU).

As mentioned earlier, I know nothing about LTspice. So, I would not know how to run the file that you were so extremely gracious to create and share. I say that with regret and embarrassment, because you worked so hard on it. Hopefully, it will help others that do know how to use it to give me tips, clues and instructions. My extreme apologies.

[t1d]

D2 uses the 301A's compensation pin to clamp its output making recovery time faster.  The 301A is almost uniquely suited for this sort of circuit because it supports clamping.

A few posts back, I added the data sheet for the LM201AH. I am hopeful that it will be suitable. I do need your instructions as to what to do with the two Balance pins (#1 and #5) and the Compensation pin (#8).

And the pass transistor is not protected from an instantaneous short in this circuit anyway.  The configuration shown below allows the regulator's current limit to protect the pass transistor or transistors by better controlling the current gain.

I very much want to include these improvements, but I lack the knowledge to be able to adapt my proposed circuit. See the next post.

Parallel transistors are still a good idea at this power level but the configuration below handles that as well.

I plan to use dual IRF9540 MOSFETs, See the next post.

[t1d]

At this point, I think it would be good to summarize where we are and create a new schematic that incorporates the decisions that we have made. Once we have the new schematic in hand, we can continue to tweak it. It would be easier to use a new schematic to add David Hess's improvements for better controlling the current gain. So, I will be working on the schematic and I hope to have it ready soon.

I am having great fun working on this with all of you. You are teaching me lots. Thank you so much.

[t1d]

Here is a summation of where we are in the project, the updated schematic and the questions resulting therefrom.

Summation
- The design goals have been adjusted down to 30V/3A, due to power management considerations.
- A transformer with separate secondaries for the 33V and -5V rails will be used to lessen the total voltage spread. I am not sure that I know what changes this requires in the circuit. I think all it requires is tying the two grounds of the two secondaries.
- The Tant cap was changed to a 10uF e-cap; low ESR and a 100nF ceramic cap was added.
- The op amp considerations seem to be pointing toward the LM101/201/301 series of op amps. The LM201AH has been used as a place holder in the schematic. I terminated the two Balance pins and the Compensation pin, pending instructions.
- I have incorporated two MOSFETs for the BJT and added dual Schottky diodes, for balancing, because that is what I have on hand.
- I added fuses on the supply inputs.
- David Hess’s current limiting design changes are pending his instructions.
- imo’s voltage pot and trimmer design changes are pending his instructions.

What follows… I went back through the posts and tried to pick up any issues that might still need consideration...

@ Kleinstein
“As shown, at the max setting the voltage across the shunt would be 1.2 V and thus relatively high, but to limit the heat there it may be advisable to change to Pot more to 50 K or less, so that the max would be more like 0.6 V or maybe even 0.3 V.  The current limit is not high precision anyway. So from the precision the TL071 should be good enough, unless one uses a very low (< 100mV) drop on the shunt.”
Are you saying that I need to change the value of the Current Limiting pot? If so, to what value. Remember, the op amp choice is now narrowing to the LM201AH/LM301.

@ parieanuo
“not sure if D3 will ever work properly, but i haven't put the pen on paper to check if he can light correctly with current draw from adj pin, maybe it's safer to put that missing resistor
anyway i don't like that configuration with led entering conduction linear-like, i like the on/off approach, either the current protection (limit) is ON and LED is ON, either it's off. with your schematic i'm not sure it works that way”
Did you want to suggest a solution? A comparator, possibly?

@ Kleinstein
“If powered from a transformer, and this with some variations and hum on the main supply, it would be a good idea to have a seprate limit for the OPs supply. This could be a simple resistor +33 (or 35) V zener + emitter follower type circuit. So the OP would not see the variations and hum on top the main supply.”
A TL431 Adjustable Zenner has been added to the circuit. I will need help with determining the supporting component values; please and thank you. Should one of the resistors in the divider that sets the Zenner voltage be a trimmer pot?

@ Kleinstein
“If the OP is powered from an extra supply, it does not need to work all the way to it's upper supply at the input.
The OPs listed are pretty similar low noise BJTs and they work up to some 2-3 V to the positive rail only. With the positive supply at some 33 V they would work. They may have a lower limit to the supply and thus powering from the output does not work.”
Two question arise for me from your comments:
1) Excepting “Over-The-Top” op amps, I thought that op amps must be powered at least with the full output they are to drive (via the MOSFETs) plus any voltage drops.
2) Are you suggesting a third transformer secondary to power the op amp separately. If so, I think that is doable.

Consideration of the Back Voltage Diode; D2
D2 is for relieving back voltage. In addition to common 1N4007 @ 1000/1A rectifying diodes, I have two additional types of Schottkies, (in addition to the two MBRD10150CT dual Schottkies that I have already assigned to balance the input voltage to the MOSFETs.) They are
- 1N5818-G @ 30V/1A
- 1N5822-T @ 40V/3A

I am considering what to use for D2. I know that back flow voltage spikes can be very high in voltage amplitude. And, IIRC, that the amperage of those spikes is likely to be small. But, we have the 3 amps of the supply that we must prevent slipping by the MOSFETs. So, I need help in allocating the diodes. I guess D2 could be four 1N4007s in parallel. What’s the trick, here?

I would be glad to provide my KiCad files to anyone that would like them to work with. But, I think I will have to use DropBox to get them to you. The files are too big to post on this forum, IIRC. I will be sure to make them freely available, once we finalize the project.

Thanks for all of your great help. We are making fantastic progress!

[t1d]

Okay, imo, let’s see if I can meet your objectives, for protecting the circuit from a pot wiper failure. But, keep in mind that this is outside of my present knowledge, so this is just my attempt.

The op amp’s inverting pin is supplied -5V through the zero voltage trim pot and then the voltage adjustment pot. Let’s connect a 1M resistor from the -5V rail to the low side of the voltage adjustment pot. If either of the pots’ wipers fail, power passes through the complete resistor track of the voltage adjustment pot and continues on to the inverting pin. The power would remain uninterrupted.

In the normal (non-failed) working state of the voltage adjustment pot, the power flowing through the new 1M resistor would flow back to ground. The downside to this arrangement is that the low side of the voltage adjustment pot would always be offset by the voltage flowing through the 1M resistor. I am not sure that I know all of the ramifications of this voltage offset. So, what say you?

[t1d]

Look at the implementation of the LM301 op amp in the original data sheet version of this circuit. It shows how to connect the two Balance pins and the Compensation pin. Notice the connections and components for pins #1 and #8. If those are correct for our use, I can add them to our schematic. Pin #5 is not shown, so I take it that it is supposed to just be not connected?

[David Hess]

A few posts back, I added the data sheet for the LM201AH. I am hopeful that it will be suitable. I do need your instructions as to what to do with the two Balance pins (#1 and #5) and the Compensation pin (#8).

The LM301A, LM201A, and LM101A are all the same part with different temperature and supply voltage ranges.  They are functionally interchangeable.  The A suffix distinguishes them from the original and short lived LM301, LM201, and LM101 which can be ignored.  Any place an LM301 is called for, use an LM301A. (1)

The 75 picofarad capacitor between pins 1 and 8 implements the external frequency compensation which other operational amplifier have internally.  It *must* be included if an LM301A is used.

Check the datasheet for how to implement offset null, but I think you can leave it out.  The current regulation of this power supply circuit is not particularly good because it depends on a constant ratio of current through the regulator and pass transistor, which is optimistic at best.  The pass transistor suggestion I made improves this situation.

Add the clamping diode or not as you prefer.  You do not *have* to clamp the operational amplifier for the circuit to work, but it does give some performance benefit when switching to constant current mode which other operational amplifiers will have difficulty duplicating performance wise without help.

(1) The A series of these operational amplifiers use an improved internal biasing arrangement which leads to a lower and more consistent input bias and offset currents.  The A parts completely replaced the non-A parts so the non-A parts can be ignored.

[t1d]

The LM301A, LM201A, and LM101A are all the same part with different temperature and supply voltage ranges.  They are functionally interchangeable.  The A suffix distinguishes them from the original and short lived LM301, LM201, and LM101 which can be ignored.  Any place an LM301 is called for, use an LM301A. (1)

Really good info; thank you. I have ordered the two LM201AHs from Mouser.

The 75 picofarad capacitor between pins 1 and 8 implements the external frequency compensation which other operational amplifier have internally.  It *must* be included if an LM301A is used.

I will add it in.

Check the datasheet for how to implement offset null, but I think you can leave it out.

Agreed.

The current regulation of this power supply circuit is not particularly good because it depends on a constant ratio of current through the regulator and pass transistor, which is optimistic at best.  The pass transistor suggestion I made improves this situation.

As I mentioned in post numbers 60 and 62, I would very much like to include the pass transistor, but I lack the knowledge to incorporate it, even with the schematic that you were so kind to provide. What would be wonderful is either a drawing, or instructions as to what components to add and where they go, and an explanation of what they do and how they do it. As always, please and thank you.

Add the clamping diode or not as you prefer.  You do not *have* to clamp the operational amplifier for the circuit to work, but it does give some performance benefit when switching to constant current mode which other operational amplifiers will have difficulty duplicating performance wise without help.

I will add it.

[t1d]

Per the original circuit design, the Clamping Diode and the supporting Cap have been added. Pin5/Compensation has been terminated as floating. Please confirm that I have it all correctly.

On the PCB, should the cap be closer to pin 8, or pin 1? It appears to be pin 1.

[t1d]

Shunt Resistor Consideration...
I have a very nice 0.1R/16W/Precision/Aluminum Case resistor. Will it do for the 0.2R/5W resistor that is specified? What effect would using it have. I think it only changes the math calculation, when calculating the current? x10, instead of x20?
EDIT: I do have the 0.2R/7W on order.

[t1d]

Bump... I still need some help with post #62 and forward... Please and thank you.

[Kleinstein]

For the compensation cap it does not really matter if closer to pin 1 or 8 as the OP is relatively slow.  Pin 1 seems to be the more sensitive pin ( lower internal impedance), so I would put the cap closer to pin 1 to minimize the capacitance at that pin.

A smaller shunt resistor increased the current. As a compensation one can change the resistors at the OPs input.
As shown in the DS circuit the voltage limit there is quite high. The more normal shunt to use for 5 A would be even less than 0.1 Ohms.
The DS circuit is anyway only good for some 2.5 A with 35 V in. So they should change the current setting pot.

The 1 M resistor added for the voltage setting port would not save much if the wiper goes open - the voltage would still go up quite a lot. The main point would be getting a really good pot, so it would rarely fail. I see no easy fix for the problem in this circuit. Modern lab supplies use a different way to set the voltage, but this does not work with the LM317.