Electronics > Projects, Designs, and Technical Stuff
0-70V, 0-5A Lab Power Supply Design
H713:
Here's a version with the Vbe current limit circuit. R1 and R12 represent a 5K pot. Yes, there would be limiting resistors so that the operator can't disable current limiting. Diode D2 serves two purposes. Firstly, it protects the series pass transistors from an external high voltage. You laugh, but I've blown up multiple power supplies this way. It is also there so that I can get the full adjustment range without the need for a 6 ohm 200W resistor. If desired, I could add a "low current mode" by switching an additional diode in series with D2. That said, my current bench supply has a minimum current limit of 250 mA and I rarely wish it had a lower current limit range.
This won't be as accurate as the version with the op-amp, but it's quite rare that accurate current limiting is needed. Fast current limiting, on the other hand, is pretty handy.
H713:
Interestingly, the Vbe current limiter suffered from similar instability issues to the op-amp based one. This (combined with the fact that a series RC network to ground from the base of Q6 stabilized it) leads me to believe that it is a stability issue with the series pass transistors. I seem to have shut them up with a 470 pF cap from the base to collector of Q6.
In testing, using a BD139 for Q7 showed relatively minimal thermal drift and generally performed well. Thermal drift over 2 minutes was good enough, less than 15 mA. The main issue that showed up is very poor ripple rejection while current limiting, even at lower frequencies (120 Hz). This may not be an issue, however, as I intend to use a capacitance multiplier in the filtering circuit anyways to achieve a relatively quiet DC bus without the need for 30,000 uF of filter capacitance, which takes up valuable chassis space (remember, this is a dual power supply in a 2u enclosure). Huge filter caps also have a big inrush current, which would require efforts to mitigate in order to avoid welding relay contacts in the tap switching circuit.
I intend to do a bit more testing on the op-amp based current limiting circuit tomorrow, then make a final decision. Beyond that, things are looking good and I am planning to start my PCB layout in the next few days.
magic:
You will probably get better PSRR by replacing the Darlington's base pullup resistor with an active source.
Also with higher loop gain, did you add that resistor as David told you?
H713:
Yes, I added that quite a while ago, just forgot to include it in the Spice schematic.
b_force:
--- Quote from: H713 on July 20, 2020, 02:33:44 am ---@b_force, the problem is a lack of adjustable current limiting. When doing initial tests on a circuit that I don't necessarily trust, either because it is new or intermittently failing. Once I am confident that it is behaving well, I may turn that maximum current up to something like 3A- just so I don't vaporize PCB traces if something stupid happens.
--- End quote ---
Just add a current limiter?
That's not so hard, quite a straight forward circuit actually.
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