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MOSFET driving

Discussion in 'Electronic Design' started by Frederic, Jan 23, 2007.

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  1. Frederic

    Frederic Guest

    Hello everyone,

    I'm a bit confused about the use of a small series resistor at the gate of a
    power MOSFET. I've seen this in some schematics.
    I have the gate of a IRFD120 MOSFET pulled up to 13V (with respect to
    source) with a 100K resistor. This gate is then pulled down by the
    "discharge pin" of a TLC555 (CMOS version of a NE555), in few hundred
    nanoseconds at most.
    Is a small resistor needed in series with the gate? What's the purpose?
    Which value? How is it calculated???

  2. I doubt that you need the additional series resistance,
    since the discharge pin has considerable resistance,
    already. Such a series resistor is often needed when the
    gate is driven by a very low impedance source that doesn't
    damp the resonance created by the gate to drain capacitance
    and the lead and trace inductance. Such cases risk becoming

    A more important question might be, how fast do you need the
    MOSFET to turn on when the discharge pin goes high
    impedance, and the gate capacitance is being charged
    positive only by the 100k pull up resistor?
  3. Jamie

    Jamie Guest

    that's a common method used to remove parasitics
    sort of lowers the Q in the circuit among other
    things like, help saving some major components if an
    attached component fails and could cause over drive for
    which the R could take the heat instead.
    there are to many reasons popping in my head at the moment.
    it simply depends on the application.
    Another one is to reduce High freq noise..
    i'll stop now.
  4. Terry Given

    Terry Given Guest

    an even better question is do you care if Cgd takes on your 100k pullup
    resistor, and wins? I have seen a commercial gatedrive circuit using 47k
    pullups and BC547 pulldown transistor actually latch at Vth, causing
    enormous power loss :)

  5. Frederic

    Frederic Guest

    It doesn't need to be charged fast. In fact I count on it to delay the
    turn-on for roughly 50uS or so, not critical.
    Thanks for your insight!
  6. Robert Scott

    Robert Scott Guest

    We use a 560 Ohm resistor to drive the gate of a IRL3103 to soften the turn-on
    and turn-off switching a little to prevent EMI. It is still fast enough to
    drive the PWM solenoids we are driving without dissipating any significant heat.

    Robert Scott
    Ypsilanti, Michigan
  7. Frederic

    Frederic Guest

    I think I got the picture now.


  8. Ben Jackson

    Ben Jackson Guest

    In HF amps this is to avoid VHF oscillations. You can put a bead in
    instead, but I imagine they are less common because they are more
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