Author Topic: LM324 Power Supply with variable voltage and current (Read 153240 times)

iMo · « **Reply #75 on:** March 20, 2019, 02:38:37 pm »

Well done!!

Finally we see something working in HW, not only in LTspice

The 100ohm is there to create the "idle load current". It could be something different, it is up to you.

The pictures DS0478, DS0480 are almost "identical" with what I see in simulation, including the small negative glitch at the end of the pulse.

It starts with the "idle load current", then it goes into the FastCC peak, usually 20-50us long, then it goes to SlowCC for the rest of the 1ms shorting, then back to the "idle load current".

From by the o'scope measured voltages and the known resistor's values you can calculate the currents in idle, FastCC and the SlowCC.

100nF ceramic and the applied voltage - MMs 50V rating is 2x the 24V so it is ok, I think the most affected are the multilayers ceramics (up to 80% capacity loss at full DC rating bias).

The buzzing sound - it is the 20Hz repetition rate of the pulses. The ratio is 1ms:50ms so the stuff should not get hot, imho.

iMo · « **Reply #76 on:** March 20, 2019, 03:17:33 pm »

You may use the "PSU SHORTER" for measuring the PSU stability as well..

Simply put a second load resistor into the MOSFET's drain (collector), for example 15ohm, set Vout=15V for example, let the first load resistor 100ohm there and the PSU SHORTER will then switch between 150mA and 1.15A.

You may search at the various points in the wiring for ringing or oscillation while changing the Vout, SlowCC and load currents (by changing the mosfet's drain resistor).

You may also increase the 4k7 at 555 to something bigger to make the pulse wider for this measurement.

mike_mike · « **Reply #77 on:** March 20, 2019, 04:30:40 pm »

I made the following tests:
1. Vout=10.12V, Iout=3.25A (max current, the led does not light), R23=100R, Rdrain=1R
DS0510-DS0511.jpg

2. Vout=10.12V, Iout=3.25A (max current, the led does not light), R23=100R, Rdrain=10R
DS0512.jpg

3. Vout=25.5V, Iout=3.25A (max current, the led does not light), R23=100R, Rdrain = 10R
DS0513-DS0514.jpg

4. Vout=25.5V, Iout=half of 3.25A (the pot at half, the led lights up), R23=100R, Rdrain=10R
DS0515.jpg

The oscilloscope probe was put on the output of the power supply, with the crocodile clip at GND and the other end of the probe at +. I used a 8k2 resistor instead of 4k7 for the 555 "PSU SHORTER".

Please have a look and tell me what you think...

iMo · « **Reply #78 on:** March 20, 2019, 04:59:11 pm »

The pulse for the stability measurement is still too short, imho, thus you do not see all the ringing at rising ad falling edges.
Try to go with even higher resistor, say 100-220k, you may decrease the 330n capacitor to stay at 20Hz. It will never be 1:1 but that is ok.

For example 220k/220k/100n will be 22Hz 1:2 ratio - that is ok.

PS: your "Iout=" shall be "SlowCC=" in your above post.

Your Iout = Vout/R23 + Vout/Rdrain

when Iout is smaller than SlowCC, otherwise your Iout=SlowCC.

BTW - your max SlowCC (provided your measurements where with full pot) based on DS0480 is aprox 3.09A (see above table).

mike_mike · « **Reply #79 on:** March 20, 2019, 05:24:38 pm »

I used the combination 220k/220k/100n, but I found the attached waveform at the G of the mosfet. Is that normal ? Or it should be a perfect square wave ? I checked the probe and it is correctly compensated...

iMo · « **Reply #80 on:** March 20, 2019, 05:29:11 pm »

What is your "PSU Shorter" power source?
It seems it cannot source enough current.

The 220ohm resistors are there to drive even high gate capacitance mosfets. The currents are high.
You may go with higher resistor's values, provided your mosfet has got lower gate capacitance ("gate capacitance" is somehow simplified name for it, though).

mike_mike · « **Reply #81 on:** March 20, 2019, 05:30:15 pm »

It is a 2A, LM723 variable power supply from 3.5V to 30V.

iMo · « **Reply #82 on:** March 20, 2019, 05:38:49 pm »

I've done some edits in my above posts - read it plz.

Do you have a 100uF capacitor connected to the 555 Vcc and GND?
PS: the 0.3ohm resistors with 100uF caps are their internal ESR, so do not wire any resistors there.

mike_mike · « **Reply #83 on:** March 20, 2019, 05:43:32 pm »

Yes, I have a 100uF capacitor, but at the beginning of the tests I though that the capacitor from the power supply output will be sufficient and I did not used the 100uF capacitor...

I'll modify the values of the 220R resistors and I will come back with the results.

Edit: I modified the values of 220R resistors to 1k but the waveform at the G of mosfet is almost the same with the one from reply #79. I used only the 100uF capacitor without any resistor in series.

iMo · « **Reply #84 on:** March 20, 2019, 05:56:48 pm »

With 220ohm resistors and 5nF input capacitance of your mosfet the gate pulses shall be nice square.

PS: do you have AC or DC coupling of your o'scope probes???

It must be DC.

mike_mike · « **Reply #85 on:** March 20, 2019, 06:07:02 pm »

I have AC coupling on all tests ...

iMo · « **Reply #86 on:** March 20, 2019, 06:11:38 pm »

Set it to DC.
With short pulses AC was not critical, but with longer pulses it makes the sawtooth.

mike_mike · « **Reply #87 on:** March 20, 2019, 06:18:21 pm »

You can find attached the screenshot with the coupling set to DC.
If I begin the tests and I connect the oscilloscope probe to the output of the supply, the coupling should be set to DC ?

iMo · « **Reply #88 on:** March 20, 2019, 06:27:35 pm »

Set probe to 1:10 and DC, sure.

mike_mike · « **Reply #89 on:** March 20, 2019, 06:52:40 pm »

I am attaching the results:
1. Rdrain=10R, R23=100R, Vout=25.5V, SlowCC=3.25A
DS0520.jpg

2. Rdrain=10R, R23=100R, Vout=25.5V, SlowCC=1.625A (the current pot at half) - led flashes
DS0521.jpg

3. Rdrain=10R, R23=100R, Vout=12.12VV, SlowCC=3.25A
DS0522.jpg

4. Rdrain=10R, R23=100R, Vout=12.12V, SlowCC=1.10A - led flashes
DS0524.jpg

DC coupling, probe x10, oscilloscope set on x10 probe, measured with the alligator clip at gnd and the other end of the probe at + of the output of the power supply.

iMo · « **Reply #90 on:** March 20, 2019, 07:14:17 pm »

Quote

2. Rdrain=10R, R23=100R, Vout=25.5V, SlowCC=1.625A (the current pot at half) - led flashes
DS0521.jpg

While the SlowCC triggers, the Vout decreases by 11V only. Is this measurement ok?

mike_mike · « **Reply #91 on:** March 20, 2019, 07:18:49 pm »

Yes, if I make a test and directly connect the 10R resistor to the output of the power supply, while the output voltage is 25.5V and output current is at half (about 1.4A), the voltage drops to 14.1V.
There is the 10R resistor in the Drain of the mosfet which determines the power supply to enter in the CC mode.

iMo · « **Reply #92 on:** March 20, 2019, 07:34:23 pm »

Quote

..if I make a test and directly connect the 10R resistor to the output of the power supply, while the output voltage is 25.5V and SlowCC is at half (about 1.6A), the voltage drops to 14.1V.
There is the 10R resistor in the Drain of the mosfet which determines the power supply to enter in the CC mode.

Your SlowCC max is 3.1A (based on the "shorting" measurement).
Half is 1.55A.
When triggered SlowCC the 1.55A flows into 10||100ohm.
Vout=1.55A * 9.091ohm = 14.091V

Looks ok.

iMo · « **Reply #93 on:** March 20, 2019, 07:49:57 pm »

Try to increase the value of the resistor from 555 into the npn base from 220ohm to 1k, plz.
With the 220 it is too much base current..

mike_mike · « **Reply #94 on:** March 20, 2019, 08:06:08 pm »

I increased the base resistor to 1k, and I made a few quick tests.
The results are shown below:
1. Rdrain=10R, R23=100R, Vout=25.5V, SlowCC=3.25A
DS0526.jpg

2. Rdrain=10R, R23=100R, Vout=12.18V, SlowCC=3.25A
DS0527-DS0528.jpg

DC coupling, probe x10, oscilloscope set on x10 probe, measured with the alligator clip at gnd and the other end of the probe at + of the output of the power supply.

Later Edit: I could not find the response of the power supply on 10mS/div, instead I found the response (the overshoot) of the power supply on 250 and 500nS/div. If I use the oscilloscope on 10mS/div, then nothing appears when I connect the power supply to the "PSU SHORTER".

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

iMo · « **Reply #95 on:** March 21, 2019, 05:53:33 am »

Quote

If I use the oscilloscope on 10mS/div, then nothing appears when I connect the power supply to the "PSU SHORTER".

You have to play with the trigger level, it should work, imho. You may ask GW_Instek experts here how to set up your o'scope for this task.

In the pictures there is a 20% overshoot and ringing.

When you change the capacitors values (slowly, say in 100pF increments) in the opamp's feedbacks the overshoot and the ringing should go lower, and vice versa. With lower capacities there is chance of oscillation, though.

The goal is to find such compensation level where the transitions are optimal (in above examples minimal overshoot and ringing) and the PSU is STABLE AT ALL Vout, Iout and SLowCC settings.

PS: You may start with Vset opamp compensation in above example, as the SlowCC is not in action.

Below is a simulation of the Vset opamp compensation from 100pF to 2000pF step 50pF. It is a simulation only, it will differ in reality.
Also mind with lower compensation caps values the PSU will most probably oscillate.

xavier60 · « **Reply #96 on:** March 21, 2019, 07:59:28 am »

There might not be enough minimum load.
39K is too high for R9.
Edit; and R8 can be a lower resistance. Clamping diodes across R9 will limit the drive current.

mike_mike · « **Reply #97 on:** March 21, 2019, 01:27:35 pm »

I increased the value of C2 (the Vset opamp) by 100pF and I got the following results:
1. Rdrain=1R, R23=100R, Vout=2.57V, SlowCC=3.25A
DS0558
2. Rdrain=10R, R23=100R, Vout=12.57V, SlowCC=3.25A
DS0559-DS0560
3. Rdrain=10R, R23=100R, Vout=25.5V, SlowCC=3.25A
DS0561-DS0564

Then, I increased the C2 value to 2nF (2x1nF in parallel):
4. Rdrain=10R, R23=100R, Vout=25.4V, SlowCC=3.25A
DS0565-0566

I see that the overshoot does not go under 4V, if the output voltage is 25.5V...

mike_mike · « **Reply #98 on:** March 21, 2019, 08:26:01 pm »

1. If the capacitors remain at the original value (C2=C5=1nF), and the overshoot and ringing will exist at the values from the screenshots, is there the possibility to appear problems at the power supply ?
I am afraid to reduce the value of those capacitors (C2 and C5) because the possibility to oscillate will increase...

2. Please have a look at the screenshots from reply #97 and tell me what you think.

iMo · « **Reply #99 on:** March 21, 2019, 08:56:19 pm »

Define "problems at the PSU"..

You have got basically two options:
1. to accept the values given in the schematics.
2. to make measurements and find the optimal or safe values.

Measurements - you have not decreased the capacitance values, so you do not know where the PSU starts to oscillate.
Therefore nobody can tell you what is the right capacitance value.


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Author Topic: LM324 Power Supply with variable voltage and current (Read 153240 times)

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