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From: John Popelish
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Subject: Re: Help - Power mosfets - difficult load
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Date: Fri, 25 Oct 2002 15:31:09 GMT
NNTP-Posting-Date: Fri, 25 Oct 2002 11:31:09 EDT
Bill Allison wrote:
> Would I be correct in thinking that in addition to the Schottkys and
> attention to short stout wiring to minimise inductance, there may be
> some scope for further tuning out of fast spikes e.g. by judicious
> application of RC snubbers?
At these low impedances (low voltage and high current) I think it is
impractical to build effective snubbers. The best you can do is drive
the gate hard and fast, and use some hefty bypass capacitors between
the non switched end of the schottky and the source of the fet. And
tie the motor lead that is unswitched to this cap also on its way back
to the battery. Your circuit is going to produce some whopping big
transients in the battery leads, and you need to shunt those with a
very local capacitor. If you do that, I think the drain voltage will
be well behaved, and the turn on and turn off transients so fast that
the total heat generated will be low compared to the high current
conduction losses. The big risk is that the wiring inductance back to
the battery will produce unintended multiple edges when you try to
turn the fet off while something like 100 amps is flowing in those
leads. Inductance in the motor leads is not only harmless, (if the
diode is right up against the fet) but actually beneficial, and you
may want to increase it by wrapping the motor leads through a
laminated core. This will also let your switch deal with slightly
lower current peaks at low switching frequencies.
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