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Re: LF: MOSFET gate resistors question

To: "[email protected]" <[email protected]>
Subject: Re: LF: MOSFET gate resistors question
From: Andy Talbot <[email protected]>
Date: Sun, 2 Jun 2019 15:50:44 +0100
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Teh 4k7 on the gate is not a pull down.  I assume it is associated with gate drive via a capacitor and diode to give DC restoration.   A safe way of driving FETs to prevent them beoing switched permanantly on if a driver or logic circuit goes haywire.   All the resistor does is prevent the mean DC component on the gate from floatiing off to some arbitrarily high value.

The gate series resistor is there to prevent spurious ringing and high frequency oscillations.   Using a decent driver chip and short leads / tracks it shouldn't be necessary, but doesn't do any harm.

Just use a good driver chip capable of driving the gate cap of your FET,   If it doesn't, itself, have a Schmitt input, make sufre its drive with a sharp edged logic drive waveform.  Perhaps from a Schmitt gate.    

On Sun, 2 Jun 2019 at 14:46, Chris Wilson <[email protected]> wrote:
Hi all,

My pal G6ECH and myself are battling poor drain waveforms in Class D
470 kHz push pull amps. We have two main initial questions. Some
circuits show a gate pull down resistor used, some do not. Is one
needed? How do you ascertain a suitable value? Given a 475 kHz amp
would a pull down of say 47k on the gate be a ball park figure? Do
some FET drivers have this internally?

Similarly the series gate resistor. Current limiting and also a slow
down function? 10 Ohms seems the common value, how do you know if
higher or lower resistance might be better?

Maybe our issue is noise on the INPUT to the FET driver IC's, so would
a small pF cap to ground be prudent? Say 30pF at these frequencies, or
not required?

Finally  is  it  worth  delving  into  "exotic" FET's and drivers that
utilize  a  negative  gate  turn  off  voltage  and  hence need a dual
polarity supply? FET's like the Cree Wolfspeed C3M0065090D


Best regards,
 Chris                          mailto:[email protected]

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