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Diode across gate resistor of MOSFET

Discussion in 'Electronic Design' started by [email protected], Jul 16, 2007.

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  1. Guest

    I've heard many people suggest to put a diode across gate resistor to
    make the MOSFET turn off faster, and I know the purpose of gate
    resistor is to slow down turn on and damp ringing due to tank circuit
    formed by miller capacitance and stray inductance.

    So my question is, does the ringing problem only affects the turn on
    moment and not during turn off? If we don't limit the speed of
    discharge of gate capacitance during turn off, wouldn't that also
    causes ringing/EMI since the current is falling too quickly?

    And also, does it make any difference of using a schottky diode or
    typical kind such as 1N4148?

    Thanks for any advice.
  2. D from BC

    D from BC Guest

    Diode??? On gate??..Then again I'm still learning smps.
    I think a bipolar transistor can discharge the gate the fastest.
    Look at the internal design of mosfet driver IC's.
    D from BC
  3. Guest

    Comments: It depends
    1) Sometimes you want to cut back on crossover shoot-through.
    2) Sometimes you want to cut back on the switching losses.
    3) Your right, the edges will generate more EMI, but I have found that
    good cabling will keep the significant radiation below 25 MHz and out
    of sight of the FCC.
    4) The reason why turn-off is safer than turn-on is that the gain is
    lower when the device is turned off vs. when it is turned on.
    5) When I use a diode ( I don't need it in my present design) I use a
    Schottky in order not to worry about speed or reverse recovery.
    Probably overkill.
    6) Since your concerned about stray inductance; remember you have to
    dump the inductive stored energy somewhere and sometime. Know where
    it's going; at least qualitatively, and quantitatively is you aren't
    really (5:1) safe.

  4. legg

    legg Guest

    The 'damped ringing' on a mosfet drain is much more likely to be
    associated with turn-off than turn-on.

    At mosfet turn on, 'damped ringing' may occur on other coupled nodes
    where other semiconductors are being forced to turn off. The active
    mosfet cannot damp these nodes, it can only crudely limit the initial
    rate of reverse current and its peak. The total removed charge is
    unaffected - damping has to be performed locally, where the
    disturbance occurs.
    Besides other posted advice, you've also got to remember that the
    mosfet turn on threshold is fixed and is not likely to be half of the
    drive peak voltage - this makes it unlikely that turn on and turn off
    speeds are equal - if that was your original intention.

    The high forward turn-on overvoltage and higher conductive impedance
    of regular diodes will reduce their effectiveness in more demanding
    situations. Remember that you are asking the part to perform at high
    pulse current for short intervals.

    Externally induced dVDS/dt can force a mosfet to turn on, if the gate
    resistor or drive impedance is too large.

  5. Guest

    Hi Ray, thanks for answering. when you mentioned the gain is lower
    when device is off, is it because the gain is proportional to (Vgs -
    Vth)^2, and when Vgs is small the d(ID)/d(Vgs) is also smaller?
  6. Guest

    For turnon/turnoff ringing you should refer to RL's comments.
    I spent about 10 minutes trying to find an excuse for my mistake.
    Didn't find a good one. The typicall effect is in fact the turnoff
    due to dumping of the L energy into various capacitances including
    Cdg , which can turn the fet back on if the gate impedance to ground
    isn't low enough. If you end up with turnoff ringing you should
    probably prevent overshoot with a catch diode; the diode to the PS
    (or in some cases a zener to ground) has to be rated for the full
    current. Typically very little power, but high peak current. I have
    seen several instances where the catch diode is rated for power but
    gets blown away because the current rating is exceeded. In any case
    you don't want the transient dv/dt turning the FET back on because a
    lot of power can be dumped into the Fet during the turnon event.
    Checking the drain voltage in the application is a really good idea;
    better to be surprised in the lab than in the field. In my last
    application I put a series RC network between the drain and gate to
    control the rise and fall times. This was possible because I had
    plenty of power margin in the design. You usually don't have this
    leeway available.

  7. Guest

    I made a mistake with respect to the turn-off/turn-on; RL is correct.
    I spent about 10 minutes trying to think of an excuse for making the
    error but couldn't find a decent one.
    Don't forget to check the drain output in the real application to make
    sure you don't need catch diodes. If you do make sure they are rated
    for the full current; not just the wattage.

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