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[DPRG] Power MOSFET drive currents

Subject: [DPRG] Power MOSFET drive currents
From: Earl Bollinger earlwbollinger at attbi.com
Date: Sun Feb 9 15:23:00 CST 2003

Using my favorite IRL3803

As I understand it, MOSFETS are voltage driven and not current driven.
I usually run a resistor on the gate to limit current as well.
Thus the current would be below 6ma anyway.
But what your seeing for current seems reasonable.

The IRL3803 has about 5000pf for Ciss and 1800pf for Coss and 880pf for
Crss, but this is at 1mhz using a 1us pulse width. That seems to be way
above the 2khz to 40khz PWM we normally use for motor controllers. Plus the
capacitance decreases sharply as the voltage gets higher see fig 5.

Hummm....Something interesting here going on.

Looking at my favorite super powerful dangerous IRF3415 which is a 43amp
150v MOSFET.
Ciss = 2400pf, Coss=640pf and Crss=340pf at 100v or so.
Same fig 5, capacitance goes way down at 100v or so, even at the same 1mhz
At lower voltages the capacitance goes up sharply.

Obviously, the MOSFETS behave their best at higher voltages and higher
frequencies per the type of Mosfet.

Another observation is as you increase the PWM rate the rounding off of the
output squarewaves aren't affected as much. But the squarewave rounding off
is more affected by the voltage level. the higher the voltage the less
rounding off you get. I was looking at a O'scope output being fed by a PWM
coprocessor to one of my H-bridges with a adjustable power supply.

Might help to explain why they are wanting to run higher voltages and higher
PWM rates on switching power supplies (besides smaller transformers or

But then this App note may help a lot too:
If nothing else it does help explain why I am running higher Rg values than
other people.
I guess I need to use shorter wires and shorter traces somehow. Dang always
something to mess it up.

-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org]On Behalf
Of Randy M. Dumse
Sent: Sunday, February 09, 2003 12:22 PM
To: Dprglist at Dprg. Org
Subject: [DPRG] Power MOSFET drive currents

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

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:

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?


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