Current limit for HV Power Supply

[pedja089]

EDIT: "DS voltage" on SHOULD BE: "GateSource voltage".
https://reviseomatic.org/help/s-foll...20Follower.php
And right schematic for t2 is correct. Emitter should be on higher potential than base, base should be higher than collector for PNP BJT.
http://www.wikiwand.com/en/Current_limiting
Resistor on base is god idea to limit base current.
Another edit: what is part number for t2?

[Ioannis]

Thanks for the reply.

Well, the power supply is correct with the P-channel fet. If you check the op-amp configuration the more Vout the less T1 drive. So the Mosfet will conduct less also having R1 at Vgs that is bringing mosfet closer in cutoff.

The regulation and in general the operation of the circuit is great.

You may think of the P-channel as a current source, regulated by T1.

Only problem is that when the protection is connected,, the mosfet is cutoff. If collector of the PNP or NPN is disconnected, everything works fine (except protection of course).

The base resistor will be added. Good point. But right now the problem is why the 10K does not stop the operation but connecting a small signal transistor that is OFF, does...

Ioannis

[amgen]

how about, the current limit is tripped when I=.4 amps...… instantaneous that is and that limit ckt would work for linear to hold to .4, but with pwm, the current levels can goes over that. Short out the I limit resistor for a test leaving the rest of ckt. And maybe sense the current and send that signaling to PIC to work the pwm to limit the current...???

[Ioannis]

Have not tested that idea. Will short the resistor, but I doubt this is the problem. OK, maybe when the circuit is first started, the output capacitor (10μF) may for a nanosecond overload the circuit. But after that, no current is running to the output. So the Mosfet should run free.

Thanks anyway.

Will report what I find.

Ioannis

[amgen]

just out of curiosity, what is the pwm freq. ?

[Ioannis]

It is set at 31,25KHz and it is filter out by 33K and 220nF.

In general the supply works great, adjusts from 0 to max Vout, regulates better that my requirements.

Just no protection so far.

Ioannis

[pedja089]

It was late when I replayed and didn't look in to schematic too much. And now it late again, so sorry in advanced if I miss something.
You are right, for P channel configuration is correct. Your design look like LDO. And that is inherently unstable. So watch out for that.
Left schematic for NPN will not work as expected. Collector base is reverse biased. And it act as diode.

So that is reason mosfet is cut of. Voltage drop across diode is much lower than Vgs threshold.
NPN current limit is tricky to implement here, because collector must be on higher potential than base...

PNP circuit should work. Or to be precise must work. I suspect that pin out is wrong or you didn't use PNP transistor. If pin out is wrong, T2 isn't correctly biased. I would start with transistor tester, or create simple circuit with PNP just to be sure is it what you think it is and pin out is correct. Try to disconnect base and see is it working. Measure voltage across 1.5R R8(current sense resistor, I don't see clearly part name) with and without base connected.

Also just to remind you that you are measuring output current and current thru R1. So take that in account. With 22K current is 12mA, so across R8 is only 20mV, not enough to bias BE junction, but check voltages current and resistor values. I'll also suggest that you move high side od R1 to 280Vin so it wont affect current limiting.