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[DPRG] Power MOSFET drive currents

Subject: [DPRG] Power MOSFET drive currents
From: Randy M. Dumse rmd at newmicros.com
Date: Sun Feb 9 12:22:01 CST 2003

Hey Guys,

I wonder if anyone could clarify something for me. I figure a
typical power fet might have a gate capacity of a few thousand
pF.

For reference I lookd at IRF1405 and found input capacitance to
be typ. 5480pF

So if a 20KHz drive signal were applied, a current through the
gate would see a 1.45Kohms reactance.

For reference I use the calculator at:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/accap.html#c
2

So with a 10V gate drive, the current through the gate would be
6.89 mA.

Am I thinking about this right? If so, why are the MOSFET
drivers like the TC4244 rated for multiple amps (yet have high
output resistance)? Can't see that much current capacity is
needed or desireable, unless this is to handle peak currents at
the gate is drained of a couple hundred nanocoulombs in a few
nanoseconds. Am I missing something?

Randy
www.newmicros.com


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