LM324 Power Supply with variable voltage and current

[xavier60]

The 40A spike shouldn't be possible. If the spike is ignored, the rest looks as expected with the current increasing from zero and limiting at about 5A for 80µs.

[mike_mike]

I added the corrected schematic at page 23.
I also made some more experiments in LTSpice, but it looks like the output is always oscillating when the current is set to maximum. Even if I increase the value of C1 and C5 (C1 and C5 from the simulation schematic).

[xavier60]

Removing the fast limiting components from the high side would cause increased gain of the output stage. I should have asked you to check the stability of the CV loop first and change the compensation if necessary, decrease the resistor and increase the capacitor.
Even if the fast limiting is unstable, the instability should only be brief until the slow loop takes control.
Make certain that the fast limiting current is set higher than the slow CC maximum.

[xavier60]

I bread-boarded the design, not in an exact way.
The low side fast limiting does cause brief instability until the slow CC loop takes control.
This was solved by adding a 6.8nF compensation capacitor between C-B of the fast limiting  transistor.
An OR-ing diode was then also needed for the fast limiting transistor.
The compensating capacitor value can be experimented with. I used the particular value of 6.8nF because it is one of the values that my collection is over populated with.

[xavier60]

I also tested the diode type fast limiting method that I used in my bench supply. Using a 1N914, it worked just fine.
I am using a LM358 with is supposed to be the same or very similar to the LM324.

[mike_mike]

I also simulated using 1N914 diode (connected as it is in the other schematic - your bench power supply) and I have got the attached results. I removed the Fast CC NPN transistor because I used the 1N914 diode.
Please have a look at the schematic and at the results.

[xavier60]

The time scale doesn't show the important detail.
The photo shows the shunt waveform when a short was applied with the CC set to 2A.
There are factors that might change the size of the initial spike, but it's only 10µs.

[mike_mike]

I added that diode (1N914) into the LTSpice and Eagle schematic. I attached the LTSpice schematic. Please have a look at it.
Can you post the schematic that you tested ?
I want to build power supply and test it using the oscilloscope. I also want to finalize the PCB layout.

[xavier60]

I don't have a schematic. I tried to build it the same as your schematic with the parts that I have and leaving out unnecessary bits.
You should modify and test your existing PCB for now.
If you can do this, I would like to see the load transient response first.

R10, 1M, isn't needed.

[mike_mike]

Ok, I'll do that. I hope that my oscilloscope will arrive next week to start testing.
Before testing, I would like to know if D3 - 1N914 is correctly connected ?

[xavier60]

Ok, I'll do that. I hope that my oscilloscope will arrive next week to start testing.
Before testing, I would like to know if D3 - 1N914 is correctly connected ?

Yes, D3 is correctly connected.
Also it can be the same type as the 1N4148 diodes.

[mike_mike]

Today I feel ready to start testing.
I will test the FastCC using the attached PSU Shorter schematic.

Can you please let me know the start values for C5 (CC loop) and C1 (CV loop) ? Are them the same with the schematic from reply #582 ?

[xavier60]

Today I feel ready to start testing.
I will test the FastCC using the attached PSU Shorter schematic.

Can you please let me know the start values for C5 (CC loop) and C1 (CV loop) ? Are them the same with the schematic from reply #582 ?

Stay with those compensation values. Do the load transient recovery test first to confirm that the design is still stable.

[mike_mike]

I checked with the output voltage at about 26V, output current at maximum (3A) and I used the PSU shorter configuration from my previous reply.
From the LTSpice schematic, C5=2n2 (current loop) and C1=68p (voltage loop).

[xavier60]

That's looks like a good start.
Will you also do the load transient test? And why change C1 to 68pF ?
The diode fast limiting works best with the CV compensation set slightly to the slow side.

[mike_mike]

Yes, I will also do the load transient test. But first, I need to be sure that there are no oscillations and the FastCC is working good if there is a short circuit on the output.
In my mind, it remained 68p. I don't know why. I changed it now to 470p.

PS: Please find attached the results with C1=470p.


1. Why in the simulation the maximum current is aprox. 8A and in the real circuit is aprox. 14A ?
2. If I will use a Schottky diode (D3), then the results will be better ?

[xavier60]

Try a schottky diode, even a 1 amp one. Even a fast silicon diode just to see what happens. But leave the original diode in circuit as a back up.
You can even try 400mW and 1W zener diodes, but be certain to put it in circuit the same way as the original diode, don't reverse it.

When the output voltage drops due to the short circuit, the CV loop responds by increasing the drive to the output transistors.
Although the diode speeds up the response of the CC loop, it still takes some time for the output of the CC op-amp to slew down and take control of the output. The size of the current spike is difficult to simulate accurately.
If the current is well within the ratings of the output transistors, then they will be safe. Also the spike is so brief. Look for the Safe Operating Area spec in  the data sheet to see how time affects how much current a transistor can safely pass at given voltages.

The lowside transistor limiting idea will allow a spike to occur also.

[mike_mike]

I used a UF4007 in parallel with the 1N4148 and I've got the following screenshots: DS0014-0015.
And with BAT85S, also in parallel with 1N4148: DS0017-0018.

I have also checked only with BAT85S, and the screenshots were the same as with BAT85S and 1N4148.
I checked the SOA of TIP3055, and it looks like at 15ms, and 40V CE voltage, the Ic is less than 2A.

[xavier60]

I used a UF4007 in parallel with the 1N4148 and I've got the following screenshots: DS0014-0015.
And with BAT85S, also in parallel with 1N4148: DS0017-0018.

I have also checked only with BAT85S, and the screenshots were the same as with BAT85S and 1N4148.
I checked the SOA of TIP3055, and it looks like at 15ms, and 40V CE voltage, the Ic is less than 2A.

Your current spike is 15µs at its base. None of the SOA charts go this low. Even at a duration of 300µs, the TIP3055 is allowed to pass 15A at 40V.
https://www.onsemi.com/pub/Collateral/TIP3055-D.PDF
Do the load transient test next.

More: I prefer the result with the UF4007 . There is less under shoot after the spike.

[mike_mike]

It looks like my attention was not good when I checked the TIP3055 SOA chart.
I have got another problem...
In the previous message I sad UF4007, but I used, in reality, FR107 for D3. Sorry about my mistake.
Should I continue testing with FR107 for D3 ?

[xavier60]

Yes, keep using the FR107, it seems to work well.

[mike_mike]

I managed to make a few tests using the oscilloscope and the Switch load. I used for switch load 220k/220k/100nF.
Capacitor in CC loop = 2n2, capacitor in CV loop = 470p.
The oscilloscope probe connected on the output of the power supply.
1. Vo=26.6V, Io=3A, a wire as load at PSU Shorter: 29.jpg

2. Vo=26.6V, Io=3A, Rl=10R: 32, 33.jpg

3. V0=26.6V, Io=almost minimum, Rl=10R: 37, 38.jpg

4. Vo=13.9V, Io=3A, Rl=10R:44, 45.jpg

5. Vo=2.08V, Rl=1R, Io=3A: 46, 47.jpg

6. Vo=2.08V, Rl=1R, Io=1.5A: 49, 50, 51, 52.jpg

I added only the results that I considered important.

[xavier60]

I mainly want to see the voltage dip where the load turns on. Just a  detailed shot at 2 amps with no current limiting.
It doesn't matter much what the voltage is set to.

[mike_mike]

Can you please explain how to see this voltage dip ?

[xavier60]

Set the trigger polarity to negative going and place the trigger level just below the steady state voltage.