Ok, I finally got a circuit I'm happy with to level shift the=20
mosfets....  Basically I'm finally able to slew the gate voltage on a P=20
channel mosfet between about Vcc and Vcc-15 in about 250ns from a=20
bipolar totem pole output stage.  Vcc is around 32V in this application.

Now comes the hard part - possibly slowing this back down with a gate=20
resistor.

I'm trying to figure out how to determine what, if any gate resistor is=20
needed.

The mosfet I'm using is a FQI22P10.   I understand that from a=20
reliability standpoint a gate resistor can both help with overcurrents=20
in the driver, and also resolve di/dt and dv/dt issues in the mosfet. =20
And possibly something else I've missed.

On the driver side, the driver transistors consist of a totem pole pair=20
of MMBTA06/MMBTA56 which are rated at 500mA continuous with no peak=20
rating in the datasheet.   Gate charge of 50nC discharged in 250nS seems=20
to indicate that we're around 200mA... is this the right way to do this=20
math?

The other issue is the di/dt and dv/dt ratings for the mosfet...  I=20
really don't even know how to approach this.

Is there a good guide to this somewhere?  Or some rules of thumb?  I'm=20
tempted just to throw a 10 ohm resistor in there since I can live with=20
the little bit of switching time which is added, but of course really=20
would like to have an engineered answer to this.

Thanks.

-forrest


--=20
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