Author Topic: LM324 Power Supply with variable voltage and current (Read 152351 times)

mike_mike · « **Reply #100 on:** March 21, 2019, 09:06:33 pm »

Quote from: imo on March 21, 2019, 08:56:19 pm

Define "problems"..

Sorry, I was not sufficiently clear when I said problems...
Problems =
1. the power supply may be damaged by the ringing or overshoot.
2. the load may be damaged by the ringing or overshoot.

Later Edit: I would go for accepting the 1nF values for C2 and C5, if there are no oscillations.

iMo · « **Reply #101 on:** March 21, 2019, 09:23:48 pm »

What if your PSU starts to oscillate at 970pF?

Kleinstein · « **Reply #102 on:** March 21, 2019, 09:24:56 pm »

The power supply usually does not get damaged from ringing or overshoot. There might be theoretical a damage from oscillation, as some small transistors or caps may see more power than normal. However normally the supply should survive.

Too much overshoot (in the voltage) might damage a sensitive load.

For the CC to CV transition the usual way is to make the recovery relatively slow. There are ways to implement at least a partial anti windup to reduce the voltage overshoot.

Too much overshoot in the CV to CC transition could in theory damage the supply and maybe a circuit that relies on the limited current. This a reason for the extra fast current limit. However some extra current for a short time is quite often desired. The output transistors can also stand quite some current for a short time.

For the adjustment it is not just the capacitors but also often a series resistor to the compensation cap for the CV mode. I somewhat got lost in what circuit was actually measured.

mike_mike · « **Reply #103 on:** March 22, 2019, 06:13:26 am »

Quote from: imo on March 21, 2019, 09:23:48 pm

What if your PSU starts to oscillate at 970pF?

This is the reason I did not wanted to reduce the value below 1nF...

Kleinstein · « **Reply #104 on:** March 22, 2019, 07:27:08 am »

To check that the supply is sufficiently in the safe range one should also do test with smaller caps than one would later use. So to make sure 1 nF is OK one should have it not oscillating with 500 pF as well.

In the old days the method was to reduce the compensation cap so far that the circuit starts to oscillates and than increase it by something like a factor of 2 to 5.

The overshoot also depends on the size of the output capacitor - more capacitance usually gives less overshoot.

mike_mike · « **Reply #105 on:** March 22, 2019, 12:25:52 pm »

I made a few more tests using 470pF for C2 and 470pF for C5.
Oscilloscope: DC coupling, probe x10, scope setting x10.
The measurement point for the oscilloscope was on the output of the power supply.
R23 was always 100R/3W.
1. Rdrain=10R, Vout=25.5V, SlowCC=3.25A
DS0567-DS0571.jpg

2. Rdrain=1R, Vout=25.5V, SlowCC=3.25A
DS0572.BMP

3. Rdrain=10R, Vout=25.5V, SlowCC=1.63A
DS0573-0574.jpg

4. Rdrain=10R, Vout=12.1V, SlowCC=3.25A
DS0575-0578.jpg

5. Rdrain=1R, Vout=12.1V, SlowCC=3.25A
DS0579-0580.jpg

6. Rdrain=10R, Vout=12.1V, SlowCC=1.59A
DS0581-0588.jpg

7. Rdrain=1R, Vout=2.49V, SlowCC=3.25A
DS0589-0594.jpg

8. Rdrain=1R, Vout=2.49V, SlowCC=1.66A
DS0594-0598.jpg

mike_mike · « **Reply #106 on:** March 22, 2019, 12:28:28 pm »

continuation from the reply #105...

Please have a look at the screenshots from reply #105 and #106 and tell me what you think.

iMo · « **Reply #107 on:** March 22, 2019, 12:38:51 pm »

You know how an oscillation looks like.
So the goal is to find the moment it starts to oscillate.
Mind the oscillation may start at certain Vout, Iout, SlowCC settings, so with the new caps values you have to run the PSU shorter while turning the Vout and the SLowCC pots up and down slowly (with 1ohm or 10ohm in the mosfet's drain).
Do not make a lot of screenshots, rather do decrease the caps values even more, do manipulate settings and do watch your o'scope.
Make one screenshot with nice oscillation.
A nice exercise for the upcoming weekend

PS: it could be your PSU will never oscillate..

mike_mike · « **Reply #108 on:** March 22, 2019, 01:28:07 pm »

I tested using the following resistors (connected in the Drain of the power mosfet):
10R - from 0V to 25.5V in steps of 5V, and at each step I varied the current from 0 to 3.25A.
1R - from 0V to about 3.3V, and the current from 0 to 3.25A.
5.7R (1R+4.7R) from 3.3V to 14.8V, in steps and the current from 0 to 3.25A.

I found the ringing and the overshoot at all tests as was in the previous screenshots, but at 0.01V output and 0A, I found the following waveform, and I do not know what is it...

Edit: I also modified R23 (100R) to 1.8k, because at 100R, it almost burned. I used 470pF for C2 and C5.
@Kleinstein I used the schematic from post #12 , v2.7.4 v2, with capacitors at the power rails of 324 and with single BD139 fast CC.

Please have a look and tell me what you think.

iMo · « **Reply #109 on:** March 22, 2019, 04:32:00 pm »

DS0603 - the transients there come from PSU_Shorter switching the loads (peaks are 45ms apart).
From the pictures I cannot say whether the noise is just a noise or it oscillates somehow.
Try with lower cap's values.

mike_mike · « **Reply #110 on:** March 22, 2019, 05:56:36 pm »

I changed the C2 and C5 values to 100pF, and I found the oscillation.
The oscillation happens when the output is set to 5.1V, when the current limitation occurs and the voltage goes down to about 4.2V. The current limitation is set by the current potentiometer.
100pF:
DS0616-0617.jpg
I also checked in the same conditions with 470pF, and I got the following screenshot:
DS0620.jpg

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

iMo · « **Reply #111 on:** March 22, 2019, 06:11:12 pm »

Exactly as my simulation above. Nice oscillation, indeed.

I would try with 220pF and/or 330pF and if there will be none oscillation (again you have to sweep through all possible settings) then use the rule Kleinstein has suggested above.

Quote from: Kleinstein on March 22, 2019, 07:27:08 am

.. In the old days the method was to reduce the compensation cap so far that the circuit starts to oscillates and than increase it by something like a factor of 2 to 5.

floobydust · « **Reply #112 on:** March 22, 2019, 06:15:12 pm »

Quote from: imo on March 22, 2019, 04:32:00 pm

DS0603 - the transients there come from PSU_Shorter switching the loads (peaks are 45ms apart).
From the pictures I cannot say whether the noise is just a noise or it oscillates somehow.
Try with lower cap's values.

OP might be observing effects of mains ripple on the -5V rail. It is a problem with the cheap chinese 0-30V PSU design, mainly when CC mode is activated and the current draw from the -5V rail is higher. I would scope the -5V rail, comparing to when the PSU is in CV and CC mode.

Based on Paul's Blog as he debugs that PSU design: http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html
It's why I suggested using a 79L05 or LM337 instead of the zener.

mike_mike · « **Reply #113 on:** March 22, 2019, 06:43:35 pm »

I tried with 330pF, and I found the following problems (I used as load only the 10R resistor):
1. Vout=15V, SlocCC=3.25A, Rdrain=10R
DS0621.jpg
but if I go to 100nS, it does not show that oscillation:
DS0622.jpg

2. Vout=4.8V, SlocCC=3.25A, Rdrain=10R
DS0623.jpg
but if I go to 50nS, it does not show that oscillation:
DS0624.jpg

I don't know if they are oscillations or other things ?

iMo · « **Reply #114 on:** March 22, 2019, 06:58:34 pm »

Those oscillation freqs are >100MHz - I doubt it comes from your PSU. Try to set your oscope's input filter to 20Mhz if applicable.
What is your GWInstek type?

mike_mike · « **Reply #115 on:** March 22, 2019, 07:04:39 pm »

It's a GDS-1052-U...

Late edit: I used the BW limit option from the oscilloscope, and I got the following results:
1. Vout=14.9V, SlocCC=3.25A, Rdrain=10R
DS0625-DS0632.jpg

The waveform from 0625 is modifying over time and it transform in the waveform from 0629. Please have a look at the screenshots and tell me what you think ...

iMo · « **Reply #116 on:** March 22, 2019, 07:43:38 pm »

With 20MHz BW limit you can hardly see a square 1Vpp 125MHz pulses, afaik. That would be something with your o'scope settings. Try to set the trigger better..

mike_mike · « **Reply #117 on:** March 22, 2019, 08:17:46 pm »

1. I tried with the trigger in different positions, and the results are in the following screenshots:
Sometime like this DS0642 and sometime like this 0643.jpg

2. I continued the tests using different Vout, different Iout and different resistors in the Drain of the mosfet. The most "strange" waveforms are attached:
a. Vout=1.3V, led flashes, because I reduced the output current
DS0636jpg

b. Vout=2.3V, led flashes, because I reduced the output current
DS0639.jpg

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

iMo · « **Reply #118 on:** March 22, 2019, 08:37:32 pm »

The first one is the issue with the oscope, the last two are ok, I can see that in the simulation as well.

I would say, based on your measurements, it does oscillate with 100pF, does not oscillate with 330pF and 470pF, thus applying the 2-5x rule, the 470pF is optimal and the 1nF is safe..

mike_mike · « **Reply #119 on:** May 05, 2019, 05:32:47 am »

Hello,
I just finished the attached schematic, and I am trying to check if the power supply is oscillating. I have also used the fast CC as suggested by imo.
I used 1N4007 instead of HER508, and I used 3v3 zener diode instead of 13V zeneer diode. The resistor Imin, was modified from 300R to 10R and the Umax resistor was modified to 12K. The transformer is rated at 24Vac and 160VA. The bridge rectifier is 35A/1000V, the filter capacitor is 2x6800uF/80V.
I am using the attached power supply shorter, with a 4.7R/5W resistor. The power supply output voltage is set at 10V.
This is the signal at pin 3 of LM555: DS0000.jpg
This is the output of the power supply: DS0001.jpg

Are there any oscillations at the output of the power supply ? Basically, I don't know what I am seeing at the output. Is that a "slow" response of the power supply ?
Please have a look at the screenshots and tell me what you think.

iMo · « **Reply #120 on:** May 05, 2019, 07:28:01 am »

I think the 100n ser 100ohm compensation is causing the response is "slow".
Try with 470p-1n ser 10k.

Kleinstein · « **Reply #121 on:** May 05, 2019, 07:53:55 am »

The frequency of the test circuit is too high to see the important part of the curve, when the voltage really approaches the set value.

Especially with a DSO there is no need for so fast tests.

The low speed is due to the really slow compensation.

In the initial phase when the voltage recovers, it looks like there could be some ringing (hard to see on the picture - we just need the rising part once). Normally such a circuit (current controlling power stage) would need some capacitance (e.g. 10-100 µF) at the output to be stable.

mike_mike · « **Reply #122 on:** May 05, 2019, 08:03:10 am »

I tested again, with 470pF and 10K: DS0000.jpg.
If I will leave the circuit as it was originally (100nF and 100R) what problems could appear ? Could it oscillate, or the 100nF+100R will prevent the oscillation ?

iMo · « **Reply #123 on:** May 05, 2019, 08:04:01 am »

now I see your psu-shorter pulses freqs is 28kHz!!
I think my initial design was around 22Hz (the mosfet is 1ms "ON")..

Something is wrong with your "PSU shorter"..
You cannot get 28kHz with your above schematics. Double-check the values of the 220k/4k7/330nF parts.

mike_mike · « **Reply #124 on:** May 05, 2019, 08:39:02 am »

I corrected the values 220k/4k7/330nF.
The output of 555: DS0699.jpg
The output of power supply, using 100R in series with 100nF: DS0698, DS0697.

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


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Author Topic: LM324 Power Supply with variable voltage and current (Read 152351 times)

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