Current limit for HV Power Supply


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  1. #1
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    Default Current limit for HV Power Supply

    I am designing a HV Power Supply controlled by a PWM signal (from a PIC).

    The problem is that either of the attached circuits for the current limit does not work. Even with no current load, the MOSFET is not allowed to work, as if the transistors are shorting the G-S of the MOSFET.

    Can anyone explain why is that happening? I though of leakage current but whatever transistor I tried the same happens.

    Ioannis

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  2. #2
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    Default Re: Current limit for HV Power Supply

    You need N-channel mosfet. Not P. When DS voltage on P type mosfet is just 2-3V it will conduct.
    Your design need voltage follower, so you need to use N channel mosfet.

    https://en.wikipedia.org/wiki/Buffer_amplifier
    https://en.wikipedia.org/wiki/File:N...e_follower.svg

  3. #3
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    Default Re: Current limit for HV Power Supply

    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?
    Last edited by pedja089; - 24th June 2018 at 00:19.

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    Default Re: Current limit for HV Power Supply

    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

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    Default Re: Current limit for HV Power Supply

    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...???

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    Default Re: Current limit for HV Power Supply

    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

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