LM324 Power Supply with variable voltage and current

[mike_mike]

The washers are made from mica, that's all that I know about them. I don't know the thickness or other properties of the washers. I bought them a few years ago, at a very low price...

[xavier60]

Mica is good.

[mike_mike]

I just tested the pcb that is installed into the case and I got the following screenshots.
The screenshots look a little bit different, is the screenshot on the R_Shunt still good (0067.png) ? it looks like the first negative spike is bigger in comparison with the other pcb. In this pcb I used another type of capacitor (for the CV capacitor), but it is also 1nF (it is not ceramic, it is polypropylene capacitor) and the main filter capacitor is 6800uF in this pcb, while in the first one was 4700uF.
The other screenshot is on the output with 32V output and 10R load.

LE: also in this circuit, a analog voltmeter was connected to the output and some of the wires were not the same length as the first circuit that I tested.

[mike_mike]

I changed the 1nF capacitor (CV loop), with a blue color capacitor, also 1nF, and the LM358 with a new one and now the wave form looks like the attached screenshots. Does they still look normal ?

[xavier60]

To prove that the first spike is being caused by the fast fall time of the output voltage, try a 1Ω in series with the shorter.
Extra: the probe's ground lead needs to be connected close the the right side of the CS resistor.

[mike_mike]

I tested with no resistor (0076.png) and with a 1 ohm resistor (0075.png) in series with the PSU shorter.
Please find the results attached. Please have a look and tell me what you think.

LE: I kept the gnd and CH1 probe in the same place on both screenshots.

LE2: Please also find attached the results when the CH1 and GND of the oscilloscope were connected right on the R_Shunt terminals: 0077.png - no resistor in series with the PSU_shorter and 0078.png - 1R resistor in series with the PSU_Shorter.

[xavier60]

It seems there is a combination of 2 reasons for the first spike, Miller capacitance of the output transistors and ground wiring inductance giving exaggerated DSO readings. All nothing to worry about.
Im not too sure why the shape of the second pulse changed if the new capacitor is the same value. Because it's physically large, it might be causing more parasitic capacitance to a nearby part of the circuit.
Because there is no sign of instability, there nothing to worry about.
Just to demonstrate to yourself the significance of wiring inductance, take a DSO reading across a few centimeters of ground wire between the PSU and shorter while doing shorting tests with and without the 1Ω resistor.

[mike_mike]

I made a test, with the CH1 probe connected to the right side of R_Shunt and the GND probe connected to the output of the PSU (about 2-3 cm distance).
With R=1 ohm: 0085
Without R=1 ohm: 0084.
I think that now is clear that the GND have some influence in readings.

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

[xavier60]

In those examples it would be mainly the current from the discharging output capacitor causing a voltage spike across the wire's impedance, inductance + resistance. Whatever I think is causing the spike across the CS resistor, it should not be related to the output capacitor's discharge current.

[mike_mike]

In the first part of the reply you are saying that the spike is from the discharge of the output capacitor, and in the second part of the reply you are saying that the spike on the CS resistor is not related to the output capacitor ?
If the spike on the CS resistor is not related to the output capacitor, then would it be related to other things ? is it still normal ?

[xavier60]

In the first part of the reply you are saying that the spike is from the discharge of the output capacitor, and in the second part of the reply you are saying that the spike on the CS resistor is not related to the output capacitor ?
If the spike on the CS resistor is not related to the output capacitor, then would it be related to other things ? is it still normal ?

Yes, capacitor discharge current only flows externally to the regulator PCB because it's at the output of the PCB.
None of this discharge current flows through the CS resistor.
Yes, the first spike across the CS resistor is caused by something else. One suspect is self turn on of the output transistors. I have not thought of an easy way to confirm this.
The main thing is that there is no ringing at all, meaning there are no stability problems.

[mike_mike]

Hello @xavier60 , when I attended your psu design, the current limit was set at about maximum 5A, but I changed it to maximum 3A.
After using the psu for a while I notified that I need a maximum of 5A, for some of the loads.
I know that I need to change the value of R8 and that I need a more powerful transformer in order to achieve the 5A output.
Also I know that I should make again some tests, with different loads at the output of the psu.
What other components do I need to change in order to have a maximum of 5A at the output ?

[xavier60]

Hello @xavier60 , when I attended your psu design, the current limit was set at about maximum 5A, but I changed it to maximum 3A.
After using the psu for a while I notified that I need a maximum of 5A, for some of the loads.
I know that I need to change the value of R8 and that I need a more powerful transformer in order to achieve the 5A output.
Also I know that I should make again some tests, with different loads at the output of the psu.
What other components do I need to change in order to have a maximum of 5A at the output ?

There should be no need to check the stability.
If the PSU still looks like this, https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg4109188/#msg4109188
I feel that it's able to safely handle well over the expected 60W of dissipation. It's difficult to know exactly. Measuring the heatsink temperature close to an output transistor might give us some idea. I'm still not certain by how much junction temperate degrades SOA.

No other changes should be needed,

[mike_mike]

I have some more TIP3055 power transistors at home, and I wanted to know if 4xTIP3055 are suitable for the 5A version of the psu (initially I used 3xTIP35C) ?
If using 4x TIP3055, then should I try to make again some tests ?

[xavier60]

I have some more TIP3055 power transistors at home, and I wanted to know if 4xTIP3055 are suitable for the 5A version of the psu (initially I used 3xTIP35C) ?
If using 4x TIP3055, then should I try to make again some tests ?

According to this data sheet, https://www.onsemi.com/pdf/datasheet/tip3055-d.pdf , one at TC = 25°C is just enough.
So 4 of them will allow plenty of derating for higher temperature.

[Kleinstein]

The TIP3055 is a bit weaker than the TIP35. So the change may not give much more power. The limiting point is likely the SOA curve. It allows for some 2 A at 40 V and close to 1 A at 50 V if at 25 C. The problem is that a higher temperature has a similar effect as a high voltage. At 60 C for the transistor may only work reliably up to 30-40 V. So 5 A from 4 x TIP3055 may be boarderline. It depends on the transformer voltage and heat sinking.
For high power like 5 A it is really worth considering transformer tap switching or similar. This mainly helps with the heat sink, but can still be a big plus.

Speed wise the transistors are about the same and thus no real need for extended tests of the stability. A short test, e.g. compare the transient response or a difficult scenario may still be useful.

[xavier60]

Im assuming that the PSU is still using tap switching. The data sheet does have the info for calculating the SOA/temperature derating.
Im still trying to make sense of it.

[xavier60]

1.25A per transistor X 15V = 18.75W
Data sheet says, Thermal Resistance Junction−to−Case RJC ~1.4 °C/W. Allowed junction temperature, 150°C.
150°C - 18.75w X 1.4 °C/W = 123.75°C, maximum allowable TC.
Assuming 1°C/W for the mica washer, maximum allowable heatsink temperature is 123.75°C - 18.75w x 1°C/W = 105°C.
This agrees closely with the 0.72W/°C above 25°C derating spec.
123.75°C - 25°C X 0.72W/°C = 71.1W derate from 90W leaves 18.9W

[mike_mike]

Yes, the PSU is still using tap switching.

Then the maximum allowed power dissipation for 1x TIP3055 is 18.9W ?

[xavier60]

Yes, the PSU is still using tap switching.

Then the maximum allowed power dissipation for 1x TIP3055 is 18.9W ?

The maximum expected dissipation is 18.9W for each.
This allows maximum heatsink temperature of 105°C, which is very unlikely while the fans are working.

[mike_mike]

This allows maximum heatsink temperature of 105°C, which is very unlikely while the fans are working.

This mean that if the heatsink is cooler than 105°C then the power dissipation could be higher than 18.9W ?
Yes, the fans will be controller by a sensor circuit, that will make them spin faster is the temperature is high.

[xavier60]

This allows maximum heatsink temperature of 105°C, which is very unlikely while the fans are working.

This mean that if the heatsink is cooler than 105°C then the power dissipation could be higher than 18.9W ?

Yes. keep in mind that there are variables that might not exactly match what I used in my calculations. For example, I used 15V worst case. It may be a bit higher with the new transformer. And temperature is very difficult to measure properly. For example, heatsink temperature should be measured by a probe buried in the metal just under a mica washer.
Im certain that it will all be safe.

[xavier60]

The stress on the driver transistor is more difficult to calculate because the current is unknown.
I automatically always use a D45H11.

[mike_mike]

I am also using D45H11.
I made a few tests, just to see the response.
1. 010-011.png
Vout=32.79V
Imax=5.2A
Rload=5.7R

2. 012-013.png
Vout=32.79V
Imax=5.2A
Rload=6.7R

3. 014.png
Vout=32.79V
Imax=5.2A
Rload=0R

4. 015-016.png (the probe was connected on the pins of R18)
Vout=32.79V
Imax=5.2A
Rload=0R

[Kleinstein]

With tap switching one would not have to worry that much about the heat sink temperature. Still 15 V are a bit optimistic: the older posts suggest 40 and 20 V for the input steps. So the worst case voltage drop is more like a little more than 20 V (like 25 V to have reserve for the drop on relay switching).
There is still the short time peak before the relay actually switches. So the transistors would also have to be good for 40 V for some 10 ms or so.