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From: firstname.lastname@example.org (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 27 Oct 2002 04:56:52 -0800
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <3DB81206.E28CA99F@rica.net> <firstname.lastname@example.org> <3DB963BE.939694C2@rica.net>
NNTP-Posting-Date: 27 Oct 2002 12:56:52 GMT
Thanks for that. FYI, I ordered the semis today. Had to go to Farnell
as RS, although much less pricey, were out of stock in the UK. One or
two responses below...
John Popelish wrote in message news:<3DB963BE.939694C2@rica.net>...
> 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.
Ok - that's that idea dead.
> The best you can do is drive the gate hard and fast,
I'd much appreciate your comments on the thread with Winfield re
switching speeds. My cheap little scope, by comparison to what you
guys have access to, has a usable response to 40Mhz-ish and a level
response only to 20Mhz, so maybe that sets a limit to how fast I
should attempt to drive the gate, if I'm to commission this by
assessing transient amplitude with scope whilst reducing safety
resistor initially in series with motor?
> and use some hefty bypass capacitors between
> the non switched end of the schottky and the source of the fet.
How hefty do you think? And low value low inductance caps in parallel?
> 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
Fortunately, the layout is such that battery leads can be very short
and I'll also oversize them massively (10mm dia copper). I've no idea
what the self-inductance of the battery is though.
>Inductance in the motor leads is not only harmless, (if the
> diode is right up against the fet)
It will be
> 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.
If I can get by without that - for the sake of uncluttered layout - I
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