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MOSFET plus electromechanical switch

Discussion in 'Electronic Design' started by The Dude, Dec 7, 2006.

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  1. The Dude

    The Dude Guest

    I'm using an IRF540 MOSFET plus a chunky electromechanical slide switch and
    am encountering some problems. I have 12V connected to one contact of the
    switch, that when it is switched on, connects the 12V to the drain of the
    FET. The source of the FET is connected to ground. The gate is off (low)
    when the electromechanical switch is flipped on. I believe the FET is
    getting damaged by a high transient voltage on the drain when the e-m switch
    is flipped.

    The VDSS (max drain-source voltage) is 100V. This seems pretty high for
    such a transient, but in another circuit configuration (where the e-m switch
    was controlling the gate of the FET, without a series resistor) it was even
    more reliably and thoroughly destroying the FET, presumably by exceeding the
    even lower VGS (max gate-source voltage) of 20V. This was fixed by adding a
    1k series resistor between the switch and gate. In the current
    configuration, the series resistance of the load (between the 12V from the
    switch, and the drain of the FET) is only a few ohms.

    I have tried to view the switch-on transient on a scope, but it seems to be
    faster than the resolution of my scope.

    For a solution, I'm thinking of putting a small general-purpose diode across
    the e-m switch -- basically with the anode connected to the drain of the
    FET, and the cathode connected to the 12V on the other side of the switch --
    so that when the e-m switch closes, any resulting spike on the drain would
    be clamped and conducted back to the 12V supply rail.

    Does anyone have any other thoughts on what is happening, or if this is a
    good or bad solution for any reason? I'm also not sure how to size the
    diode -- maybe 50 or 100 mA?

    Thanks for any help...
     
  2. Andrew Holme

    Andrew Holme Guest

    What is the load? Is it inductive? If so, you need a diode across it to
    handle the back-e.m.f.

    You said you previously controlled the gate using a switch. You should
    never leave the gate floating / open-circuit. One solution would be to
    connect a high value resistor between the gate and ground.
     
  3. John Fields

    John Fields Guest

     
  4. Luhan

    Luhan Guest

    So your circuit is a mosfet connected from 12 volts directly to
    ground???

    Just what is that about... killing mosfets, crowbarring power
    supplies???

    Luhan
     
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