LM324 Power Supply with variable voltage and current

[xavier60]

3A is the maximum output current, I do not have a resistor or a group of resistors which draw exactly 3A, can I use a load of 2.66A ?
Or should I test with a short circuit on the output ?

2.66A is fine. Don't allow it to go into current limiting.

[mike_mike]

Voltage recovery after an 2.66A at 26.6V output voltage:
172-174.jpg

Recovery after a short circuit:
175-176

On the R_Shunt during short circuit on output:
178-180.jpg

I made a few more tests, for my curiosity.
All the tests were made using the R32 = 100R.

[xavier60]

Im mainly curious to know what the voltage overshoot is when unloading from a normal load.

[mike_mike]

The voltage overshoot is not represented in 172-174 ?
I think I am missing something... how to check the voltage overshoot ?

[xavier60]

The voltage overshoot is not represented in 172-174 ?
I think I am missing something... how to check the voltage overshoot ?

Trigger your DSO at the high to low transition of the MOSFET's gate. This will be the time that the power supply unloads.
We are looking for how much the output voltage increases by.

[mike_mike]

With yellow color is the output of the power supply and with blue color is the gate of the mosfet.

[xavier60]

Do the test without allowing the power supply to current limit.
Choose a resistor and voltage setting that draws a bit under 3A.
Set the current limit to maximum so that it doesn't go into current limit mode.

[mike_mike]

Vout=26.6V
Rload=10R
Ilimit=3A

With yellow = output of the power supply
With blue = gate of the mosfet

With R32 (100R) connected: 196,295
Without R32 (100R): 198, 199.

[xavier60]

The 1V overshoot is one thing, maybe normal. I am puzzled by the way the voltage settles so quickly. What's causing the output capacitors to discharge so quickly.

[mike_mike]

Probably the 1k/5W resistor which is connected in parallel with the output ?

Are you saying that the transient response should be longer ?

[xavier60]

I had the reason worked out a while ago then forgot it. I did the same test on my bench supply.
After the MOSFET turns off, the only place the output current can go is through the output capacitance causing a voltage rise.
The CV loop takes a few µs to reduce the drive to the output transistors. The 5µs overshoot is actually the temporary voltage drop across the capacitors' ESR. The actual capacitance gets charged by only 100mV or so.
You might see some other activity further along caused by the 100mV overshoot being corrected.
Your power supply is working properly.

[mike_mike]

Do I need to test any other things ?

[xavier60]

Do I need to test any other things ?

One minor thing, check that the current can be adjusted to zero, Then I suggest that you  put it into service with the recent changes.

[mike_mike]

Yes, the current can be adjusted to zero. For example, with the 1k resistor on the output, I read 0.0mA on my multimeter, and the led lit when I rotate the current pot down to zero.

LE: and I also read 0.0mA without the 1k resistor.

LE2: ... the fine tuning of the CC and CV loop capacitors is necessary or the capacitors does not allow any tuning ?

[xavier60]

Yes, the current can be adjusted to zero. For example, with the 1k resistor on the output, I read 0.0mA on my multimeter, and the led lit when I rotate the current pot down to zero.

LE: and I also read 0.0mA without the 1k resistor.

LE2: ... the fine tuning of the CC and CV loop capacitors is necessary or the capacitors does not allow any tuning ?

It's not necessary for it to go any faster. If you want to experiment on your own, start with increasing the gm of the output path by reducing R25.
The fast limiting is likely to be affected.
You should just leave it as is.

[mike_mike]

One more question that I have probably previos asked but I am still blurred: if I use the power supply as it is in this moment, can it start oscillate ?

[xavier60]

One more question that I have probably previos asked but I am still blurred: if I use the power supply as it is in this moment, can it start oscillate ?

It shouldn't, not the CV loop anyway. To be more certain, do the load transient tests while adding very low ESR capacitors across the output, near to the PCB as possible.You can use either large MLCCs or POSCAPs, https://en.wikipedia.org/wiki/Polymer_capacitor
Be aware of the voltage ratings.
If it rings for one or two cycles, it's ok.

[mike_mike]

I tried to search for the mentioned capacitors but I did not found them at the local shop. Also I don't have them in my stock at home. An order on the internet only for a few components does not worth it.
Is there any other testing method ?

[xavier60]

I tried to search for the mentioned capacitors but I did not found them at the local shop. Also I don't have them in my stock at home. An order on the internet only for a few components does not worth it.
Is there any other testing method ?

No, Just don't bother with the test. I have done the tests on my bench supply which is very similar to yours.

[mike_mike]

 After all the tests, can I put the power suppy into service ?

[xavier60]

After all the tests, can I put the power suppy into service ?

Yes, it will fine so long as no serious mistakes are made. Also, dont make the wires to the heat sink any longer than they have been, shorter is usually better. No one can easily say how long is too long.

[mike_mike]

Will the power supply work correctly if I will use TIP35C instead of TIP3055 ?

[xavier60]

Will the power supply work correctly if I will use TIP35C instead of TIP3055 ?

That should be fine,

[mike_mike]

It is a good idea to use a multi turn potentiometer (U7) for fine current adjustment instead of the standard potentiometer that I have been used ?

[xavier60]

It is a good idea to use a multi turn potentiometer (U7) for fine current adjustment instead of the standard potentiometer that I have been used ?

I used 10 turn Pots for voltage and current control with my bench supply to give fine control.
Genuine branded Pots are too expensive and I have experienced some trouble with the cheap knock off Pots which ebay is flooded with.
I mentioned the problems in the other thread, https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2368746/#msg2368746