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atypical MOSFET drive question

Discussion in 'Electronic Basics' started by Nick, Jul 25, 2006.

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  1. Nick

    Nick Guest

    I'm needing to switch up to a few 100mA to a device, must switch the
    source rather than the sink, and only have n-channel mosfets to hand,
    specifically 30A STP36NF06's. The supply is 12V and with logic level
    gate voltage. Taking into account the voltage drop through the FET in
    resistor calculations for the current, this appeasr to work and I'm
    wondering what the downside is to using the device to source current
    rather than sink it? For example, does the MOSFET need to be derated,
    is it likely to fail after a time because it simply shouldn't be used
    this way, or is it inefficient but in princple ok? Guidance
    appreciated.
     
  2. John Larkin

    John Larkin Guest

    How do you plan to drive the gate?

    John
     
  3. leeps

    leeps Guest

    The problem is the gate must be 10 volts above the source(5 volts for
    logic level fets) so if your sourcing 12 volts to a device you need 22
    volts to the gate, makes things a little more complicated. Other than
    that by all means source away.

    Joe
     
  4. John Fields

    John Fields Guest

    ---
    Since it's a logic-level device and you're using it for a high-side
    switch, the down side is you'll need to drive the gate to about 5V
    more positive than the source in order to get it to fully turn on.

    That means that if you want to get 12V into your load from a 12V
    supply connected to the drain, you'll need to hit the gate with at
    least 17V. A high-side driver will do what you want, and here's
    one:

    http://www.micrel.com/_PDF/mic5011.pdf

    Those parts seem to be way overkill for driving a couple of hundred
    milliamps into a load, but if that's what you've got they'll
    certainly work.
     
  5. Nick

    Nick Guest

    Thanks for the quick and informative replies. I thought that there had
    to be at least one good reason, and in this case it's the most
    fundamental one of being that it actually doesn't work for reasons that
    you all explained. Not having time today to test the mosfet in the full
    circuit where I've been using them as a sink up until now, I tried the
    modified driver side briefly with a gate drive of 12v where it did
    work, and also noticed that touching a floating gate that it switched
    on, and so assumed that it would switch when driven at 5v. However
    trying this now it obviously doesn't work.

    Is it correct that P channel devices would also need level shifting on
    the gate, and so if having to switch the source one might as well use
    an N channel device with suitable gate drive?
     
  6. With a P-channel device level shifting is not needed to source current, as
    long as the negative rail is considered ground or common, but it will
    require a negative drive voltage on the gate instead of a positive drive
    voltage.

    Level shifting on a MOSFET is only required if the load is attached to the
    source of the MOSFET instead of the drain. The gate-to-source voltage is the
    only voltage relevant to turning on a MOSFET. With a P-Channel device the
    source will be connected to the positive rail and the load will be on the
    drain. The difference in potential between ground and VCC is enough to fully
    turn on the MOSFET.

    This topic is complicated by the fact that the word "source" means two
    different things here.

    Dorian
     
  7. Rich Grise

    Rich Grise Guest

    The P-channel device doesn't need any level shifting. You either
    open the gate, and the resistor from it to the source, at +12, turns
    off the FET. Ground the gate through a suitable resistor or whatever,
    and the FET sources current at almost +12V.

    I guess you could call it "level shifting" when your driver turns
    off - you might need to use either the N logicFET or an NPN to switch
    the gate of the P, and invert the sense of the driving logic.

    Good Luck!
    Rich
     
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