LM324 Power Supply with variable voltage and current

[xavier60]

How have you wired the switch poles?

[mike_mike]

The first pole is wired like in the first schematic, and the second pole is connected as a switch that is connecting and disconnecting the wire between the 5V reference and the pot's pins (reply #997 schematic)

[xavier60]

Is one pole on while the other pole is off?

[mike_mike]

Both poles are on at the same time.

[xavier60]

Both poles are on at the same time.

That's right. It should be working.

[mike_mike]

I am always hearing the relay clicking when the psu is going from off to on state.
For example: Vout=32V, I out=3A, I am turning off the psu then I turn it on and while I am pressing the switch I hear the relay clicking. Even if the relay was initially on.

[xavier60]

Put the switch in the position where there is 5V on the pots and the PSU has output.
Then check that the pole in the tap change circuit is on.

[xavier60]

Maybe there is a big problem with the idea if the pole for the reference turns off before the other pole.

[xavier60]

With the way it originally was, the switch contacts timing avoided glitches. The pole in the tap changer opened before the pole on the Shutdown closed. This prevented the tap changer responding to the drop of PSU output voltage.

[mike_mike]

I don't think that I will find a perfect switch.
If I will use the power supply with the actual relay clicking, it could be a problem ? For example will the relay be damaged ?

[mike_mike]

No, it's fine the way it is. It was a suggested improvement.
The switch needs to be wired so that both poles are on at the same time.

Ok, then I would like to leave the psu as it is.

[xavier60]

No, it's fine the way it is. It was a suggested improvement.
The switch needs to be wired so that both poles are on at the same time.

Ok, then I would like to leave the psu as it is.

I fell asleep here. Yes, it will have to stay the way it is. I can't think of a simple solution for the relay clicking which we were wanting to avoid in the first place  in the design of the tap change circuit.

[xavier60]

No, it's fine the way it is. It was a suggested improvement.
The switch needs to be wired so that both poles are on at the same time.

Ok, then I would like to leave the psu as it is.

Actually I have just thought of something. Increasing C2 in the tap change circuit will increase the delay to change time.
There still could be unforeseeable complications.  So experiment only if you have plenty of spare time to use.

[mike_mike]

I will leave the psu as it was before the last suggested improvement.

[not1xor1]

I tried to simulate with LTspice that schematic with TLC072 (PS4s03.jpg - is that the last schematic?) but it oscillates at about 1-2kHz. The bode plot is scary... 

If anybody would like to check I can post a zip with the models I used.

[xavier60]

I tried to simulate with LTspice that schematic with TLC072 (PS4s03.jpg - is that the last schematic?) but it oscillates at about 1-2kHz. The bode plot is scary... 

If anybody would like to check I can post a zip with the models I used.

The latest is PS42 012.png on https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664
It's not significantly different.
The CV loop should oscillate over 100Khz with no compensation.

[mike_mike]

Is there anything that I should do if the psu oscillates ? I checked the psu, some time ago, when I finished the latest schematic, and it seems that it didn't oscillate (at that moment).
What else should I do ? is there anything that I should modify or test ?

[xavier60]

Is there anything that I should do if the psu oscillates ? I checked the psu, some time ago, when I finished the latest schematic, and it seems that it didn't oscillate (at that moment).
What else should I do ? is there anything that I should modify or test ?

You could test it again, it will be fine.  I use the same design in my main bench PSU and have not seen oscillation problems in almost 3 years.
There is very likely a mistake in the simulation.

[Kleinstein]

The ESR of C2 could make a difference for the stability and help the voltage loop.
There are other parts that are not modeled perfectly.  One such part is inductance of the shunt, that can have an effect, though often also to make a real world circuit oscillate if the simulation is stable.

[xavier60]

A sure way to make it oscillate is to apply type 1 compensation(C + no R) and an extremely low ESR cap across the output.
This is commonly known to apply in general to current sourcing designs such as Harrison topology and LDO regulators.

[not1xor1]

I tried to simulate with LTspice that schematic with TLC072 (PS4s03.jpg - is that the last schematic?) but it oscillates at about 1-2kHz. The bode plot is scary... 

If anybody would like to check I can post a zip with the models I used.

The latest is PS42 012.png on https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664
It's not significantly different.
The CV loop should oscillate over 100Khz with no compensation.

I downloaded the TLC072 model from TI and the TIP35 one from onsemi.
The bodeplot of a simple circuit with just TLC072 matches the datasheet one, so the library should be OK.
Unfortunately the test (control voltage only) yields quite wild results. (See image below)
I'm including the simulation in order to allow to check what is wrong with that.
Please let me know what is wrong ... or if it is just a simulation problem.



thanks

[xavier60]

Your signal injection point isn't in the usual spot. It should be between the + output and R16.
I have just been doing some testing on my actual PSU. Ill report back when I make sense of it all.

[xavier60]

Ok, with a 1A load, loop gain is 0dB at 115KHz. Allowing for the phase inversion, the output lags the waveform at the top of R16 by 120°.
So I guess the stability phase margin is 60°.

[mike_mike]

@xavier60 Did you made any other tests with the psu ? If yes, then could you please share with us the conclusions ?

[xavier60]

@xavier60 Did you made any other tests with the psu ? If yes, then could you please share with us the conclusions ?

No, just the stability phase margin test. 115Khz isn't all that fast but the stability is the most important thing which is proven to be  very good.