At 11:55 AM 10/8/01 -0500, you wrote:

> You need to turn
>these mosfets on and off very fast to minimize power dissipation, and every
>time I've tried to get away without a mosfet driver I've met problems.

OTOH, be careful if your driver is *too* good, the source inductance can start
to cause major transients at the 40A level. It doesn't take many nanohenries
or inches of conductor to get major (many volts) transients happening.
Consider
slowing it down deliberately to reduce RFI and such problems. Unless your PWM
is very fast it may not make much difference. A series gate resistor of
~20R per
MOSFET is not uncommon. A PIC output has an output impedance of perhaps 100R
around the threshold, so the switching will be a bit softer. Easiest MOSFET
driver is two transistors in complimentary emitter follower configuration. You
can put a resistor from input to output to get a bit more drive at DC.

Try to visualize the current flow and look at each piece of wire or conductor
over about 1/4" as an inductor, particularly if the 40A is flowing through
it.

You need a fairly decent scope to see these things happening too.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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