I have a few weeks with a problem in the design of my bench power supply. First of all I have this design from this guy on YouTube https://youtu.be/_CFIovMkRyg that made a variable power supply using two of four op amp LM324 and the PSU behavior is really stable perhaps I don't find a way to get work that design in LTSpice.

Here's a snapshot of the video showing the design he used.

Lab power supply linear regulator

Here's I've tried so far in LTSpice.

Darlington PSU.asc

I think is a reference ground problem because he has two grounds. When I simulate all the current from the current source is sinked by a single OPAMP and doesn't matter the value of the pot wiper the control doesn't work. In any case Does this design really work? if it doesn't work, you know a way to make it work?


So yes, the design it's perfectly working, by now I tested on LTSpice and works very well the error that I had in my simulation was the reference Vo3(is the voltage out regulated) that is AGND of the control circuit without that reference the control circuit wouldn't work. Here's the updated circuit

And the schematic: enter image description here

I understanded the principle of CC/CV mode from this webpages:

  • \$\begingroup\$ Your circuit possibly has 8 mA drive current returning via the sense resistor to the opamp power supply.... which may or may not be relevant. Either way, it seems to me that this circuit needs a switching preregulator to reduce stress on the series pass and remain within SOA. \$\endgroup\$ – Indraneel Aug 18 '20 at 7:33
  • \$\begingroup\$ Thank you for your feedback. Do you recommend me any design for the pregulator? \$\endgroup\$ – R.Saavedra Aug 18 '20 at 13:19
  • \$\begingroup\$ I have no experience with such high current supplies, so can't really say. \$\endgroup\$ – Indraneel Aug 18 '20 at 18:08

With just 2 opamps, you will need an isolated power supply for the opamps with it's ground between the series pass and sense resistor like this:


simulate this circuit – Schematic created using CircuitLab

The current control is not very accurate since the voltage reference is drawing some current. Adding another 3 opamps (1.5 LM358) will allow you to buffer the output and use positive reference for V and I (if you want to set reference via microcontroller). That will be more accurate. The same thing can also be done on the return path. It is important not to have low impedance path between the current sense resistor and output.

  • \$\begingroup\$ Ok, yes I noticed that problem with current control in the simulation.Yes, the control by a microcontroller would be ideal, but for now I prefer only analog control. So are you recommend me to add a voltage follower for the return path (Vo3)? Can you please add more information about the ouput of those opamps? \$\endgroup\$ – R.Saavedra Aug 18 '20 at 14:17
  • \$\begingroup\$ @R.Saavedra The opamps simply go to the drive circuit for the darlington. Your circuit is fine for such high current because the 8mA is very little compared to the output. I guess your primary concern will be fans and heatsinks. It becomes a problem only if one is trying to avoid heatsinks. Then one will need a switching preregulator, and minimum voltage drop on the series pass, which will mean not using a darlington, which means more base current, and so shifting the control ground before the sense resistor, which leads to requirement of more opamps. \$\endgroup\$ – Indraneel Aug 18 '20 at 18:20
  • \$\begingroup\$ @ Indraneel Yes, my principal concern it's about power dissipated. I added on-off enable switch by putting a transistor directly on the Base_control net and simulated the short-circuit protection. So it's ok for transistor reach peaks of 150W for about ~10us ? It's any chance that can damage the transistor? Thanks in advance \$\endgroup\$ – R.Saavedra Aug 18 '20 at 23:02
  • \$\begingroup\$ @R.Saavedra See the Safe Operating Area curve for the transistors. TIP142 specifies max about 2.5A at 40V and 25C. Also, much better to stay at lower current the higher the voltage across the transistor (Vce) goes. BJT is definitely much better than mosfets regarding this, but at higher voltage, a badly manufactured bjt may fail at lower current than specified in datasheet. Most transistors will follow the datasheet correctly till 10V, but after that it is wise to leave a lot of margin. Otherwise an output short circuit may suddenly smoke the bjt. Make sure to have crowbar protection. \$\endgroup\$ – Indraneel Aug 19 '20 at 11:15
  • \$\begingroup\$ Thanks @Indraneel. I had a problem with the simulation because the initial conditions, but now problem is solved, so when I short-circuit the output with 1m resistor the peak power is about 30 W per transistor. \$\endgroup\$ – R.Saavedra Aug 19 '20 at 14:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.