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    P-channel fet driver problem

    Hello,

    I am trying to figure it out why the following circuit is not working properly in reality while in theory should work. It is a P-CHANNEL FET driver shifting the gate voltage within the limits of the MOSFET.

    The input (R39) is a 5v control signal and in my simulation at R37 it should swing between 8v and 24v. However when I built the circuit, the voltage changed only between 9v and 10 volts (the MOSFET after R37 is removed). When the TIP122 and TIP127 are removed, voltages at the 2N2222A are correct, but when I place the TIP122 and TIP127 the output is only between 9 and 10v.

    I replaced all transistors with new ones and still got the same problem. Any help what am I missing and where could be/is the problem?

    Thank you.

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    Re: P-channel fet driver problem

    TIP122/127 are low gain devices, you probably are starving them of base current to get full switching. Also check reverse breakdown voltages on those devices.

    Brian.
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    Re: P-channel fet driver problem

    Hi,

    Why is R39 = 220R while R4 = 1200R?
    I'd rather expect it vice versa.

    Klaus
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    Re: P-channel fet driver problem

    With that much difference between the sim and real circuit, I suspect a problem with the real circuit wiring.
    Make sure both transistors are wired properly (especially the collector and emitters).
    Zapper
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    Re: P-channel fet driver problem

    Quote Originally Posted by crutschow View Post
    With that much difference between the sim and real circuit, I suspect a problem with the real circuit wiring.
    Make sure both transistors are wired properly (especially the collector and emitters).
    Thank you indeed that was the problem. I drew the PCB using the wrong model - 2N2222A instead of MMBT2N2222A - looks like their collector-emitter pins are switched.

    Anyway, the TIP122-TIP127 are darlington pair transistors and now the circuit works exactly as in the simulation and drives the gate between 8v and 24v to keep it within the 20v Vgs limits of the MOSFET.



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