LM324 Power Supply with variable voltage and current

[xavier60]

This mean that I should use a lower value capacitor instead of 47uF ?
Or this behaviour of the power supply is normal ?

You could try lowing the total ESR by fitting an extra 47uF. The extra capacitance might also reduce the overshoot,

[mike_mike]

I will test the power supply using 2x 47uF capacitors on the output.
For C1, it is fine if I will use a 100pF in parallel with a 120pF , which will be in total 220pF ?

[xavier60]

I will test the power supply using 2x 47uF capacitors on the output.
For C1, it is fine if I will use a 100pF in parallel with a 120pF , which will be in total 220pF ?

Yes

[mike_mike]

I also want to apply the rule that @imo suggested me, at reply #118, page 5.
If the power supply oscillated at 100pF, and at 150pF it did not oscillated, then the safe value for C1 will be (2..5)x150pF=300pF...750pF, so I should use a 330pF to 1nF capacitor ?

[xavier60]

I don't really know. I mainly like to see little overshoot and no ringing if possible. The resistor might have to be decreased also,

[mike_mike]

I modified the output capacitor to 2x47uF, R30=3k3, C1=220pF, C5=2n2, C4 and R4 removed.
1. Vout=2.10V, SlowCC=3A, RLoad=10R
0934-0936
2. Vout=2.10V, SlowCC=3A, RLoad= 0R
0937-0941

Please have a look at the attached screenshots and tell me what you think.

[xavier60]

Everything looks fine. I didn't expect you to change R30 but it has caused the correct result.
Your 47uF capacitors might have more than the normal amount of ESR. Mine are 500mΩ.
Did you confirm the parasitic ringing?

[mike_mike]

Yellow = base of 2N5551
Blue = output of the power supply
Vout=2.10V, SlowCC=3A, RLoad=0R

This probably confirm that the ringing is also present at the base of Q1.
Please have a look at the attached screenshots and tell me what you think.

[xavier60]

I cant make sense of that.
Look at the output of the CC op-amp.

[mike_mike]

The output of the CC op amp (yellow). Same test condition as in my previous post. Output of the power supply with blue.

[xavier60]

That last test was done with the timebase too fast.
With no load, the output of the cc op-amp should be at its maximum voltage.
We need to see the output of the cc op-amp go from maximum  voltage to some lower voltage where it takes control of Q1 from the CV op-amp.

[mike_mike]

We need to see the output of the cc op-amp go from maximum  voltage to some lower voltage where it takes control of Q1 from the CV op-amp.

1. Can you please explain how to do this test. I did not understood completely how to do this test...
2. What will happen if I will let the power supply as it is in this moment, and I will only fine tune the C1 and C5, to eliminate the possible oscillations ?

[xavier60]

This is where you need to start understanding how it's supposed to work.
Q1 gets controlled by which ever op-amp has the lowest output voltage.
With no load, the cc op-amp should be high and the cv op-amp at some mid voltage.
Confirm this now.

[xavier60]

Im starting to wonder if the output transistors are being turned on enough to pass 3 Amps.

[mike_mike]

With no load, the cc op-amp should be high and the cv op-amp at some mid voltage.
Confirm this now.

With no load:
CC op amp = 10.90V output
CV op amp = 5.66V output
Supply voltage of the 324 = 12.15V

[iMo]

@MM:

1. imagine there are no diodes, the opamp's outputs are disconnected - the Q1 will be fully opened (because the base current through R1 from Vcc) and the power pack's base current is pulled down, therefore the Vout voltage is near 35V

2. now the diodes are connected - the diodes create a "wired OR" - whatever diode's cathode (CC or CV) is pulled below the Q1's base voltage (here the base is set to about 6-7V) the transistor starts to close itself (and the Vout voltage goes down to 0V), because the forward biased diodes start to conduct and the R1 current diverts and flows into the opamp's outputs (CC or CV or both). The opamp's outputs sink the base current to the ground.

3. so the Q1's base is pulled below 6V either by CV or CC during the operation and that closes the Q1 (as the emitter is fixed at 5.1V) and that pulls the Vout down to 0V.

4. when the CC or CV opamp's outputs voltages are higher than Q1's base voltage -> that does not affect the Q1 as the diodes are reverse biased and the opamps do not source the base of Q1. Therefore this situation is the same as the point 1. above.

[xavier60]

With no load, the cc op-amp should be high and the cv op-amp at some mid voltage.
Confirm this now.

With no load:
CC op amp = 10.90V output
CV op amp = 5.66V output
Supply voltage of the 324 = 12.15V

That looks right.
To transfer from cv to cc mode, there has to be enough output current to cause the cc op-amp to go low and take control of Q1. At this point the cv op-amp will sense the voltage drop and go high but it will have no control of Q1.
If you think it's safe, short the output for a second to see if the cc LED lights. If not, try  lower current settings.

[mike_mike]

@imo Thank you for the explanation. I understood how it works.

@xavier60 I shorted the output, at 2V and at max output voltage, at SlowCC=3A and the cc led lighted up.

[xavier60]

@imo Thank you for the explanation. I understood how it works.

@xavier60 I shorted the output, at 2V and at max output voltage, at SlowCC=3A and the cc led lighted up.

See if you can capture the drop of the cc op-amp when the output is shorted. Do at 1A for now.

[mike_mike]

I managed to catch the voltage drop at the CC op amp output using the PSU Shorter.
I set the Vout=26.1V, SlowCC=1A.
Please find attached the results.

Later Edit: I also made some simulations, and I found that it oscillates in simulation with 220pF.

I would also like to use a safer value for C1, in order to eliminate the possibility to oscillate.

[xavier60]

That looks right, Not sure where to go next.
Looks like the cc op-amp takes about 100µs to slew down and take control. during this time, the cv op-amp should be allowing the Base of Q1 to go higher and the fast limiting should be active.
Our concern was to find out how big the voltage overshoot is when the short is removed while the fast limiting is active.

[mike_mike]

It is possible to use a higher value for C1, in order to eliminate the possibility of oscillation ? (as I done with the other LM324 power supply).
I found in simulation that it oscillate with 220pF. I know that is only simulation - as you said - but I want, if possible, to be safe.

[Kleinstein]

The picture are still hiding much of the interesting part.

For the ringing in the CC to CV transition one has to distinguish 2 cases: one is the classical oscillation and ringing with 220 pF and likely 470 pF. The other case it the ringing that includes some windup and some stages to reach saturation. One likely has to live with some ringing of the second type unless one would use a really stringent anti-windup.

However a really good anti windup can effect the accuracy near transition and may not even be desirable for the CC part, as the limit could engage too fast. The CV mode regulator needs some current excursions to bring the voltage back. The plans of most commercial supplies do not include much anti-windup, if at all. So in this respect the current circuit is already on the better side.
Avoiding the 2 nd type of ringing may need adjustments to the CC loop part, not just the CV loop.

If looks like the regulator just starts oscillation at some 220 pF or a little more. So C1 at around 1 nF or a little more seems reasonable.

For the compensation it is not just the value of C1 that counts, there are also resistors that set the loop gain. E.g. reducing R30 has a similar effect to increasing C1.
There are also 2 principle types of regulators, that can used quite different compensation. The circuit here uses output stage that sets the current (high output impedance). These usually need the extra phase boost from C4 / R4 or similar (Xavier60 has it at the transistors emitter), an output capacitor and usually a relatively fast regulation.  The other regulator class has a low output impedance power stage (e.g. emitter follower). These can work essentially without an output capacitor and often use a rather slow regulation loop from the OP. This second class tends to be less good with current regulation and gets tricky with more than same 30 V, as usually the OP has to provide the full voltage swing.

[mike_mike]

I found the problem in the circuit. The transistors (TIP3055) were not working correctly. The current was not equally distributed on each power transistor.
I changed the entire circuit, and now it seems to work.
Please find attached the screenshots with the oscilloscope probe on the shunt (0.22R).
Before changing the transistor circuit: 0954
After changing the transistor circuit: 0959.jpg

Please have a look at the attached screenshots and tell me what to do next.

[mike_mike]

New tests:
Vout=1.90V, SlowCC=3A, Rload=0R, measured on the output of the power supply.

Please have a look at the attached screenshots and let me know what else should I test.