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From: John Popelish
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Subject: Re: Help - Power mosfets - difficult load
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Date: Sun, 27 Oct 2002 18:07:31 GMT
NNTP-Posting-Date: Sun, 27 Oct 2002 13:07:31 EST
Bill Allison wrote:
> 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?
Your scope can very nicely show sub microsecond transitions, and this
is what you should shoot for.
> > 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?
This all depends on the length and inductance of the loop back to the
battery. The shorter these leads and the smaller the loop area, the
less capacitance you can get by with. Remember that during the time
that the battery lead inductance limits the current, this bank of
capacitors has to carry up to the full motor current. During the on
transient they keep the supply voltage from collapsing, but during the
turn off transient, they must absorb the motor current that is flowing
through the battery lead inductance. I would probably try a few
microfarads of low inductance film capacitors in parallel with a
couple high value electrolytic caps like the Panasonic T-UP series.
The 100,000 uf 16 v ECE-T1CP104FA is listed in the Digikey catalog as
having an 11.36 amp ripple current rating and a .02 ohm ESR (at 120
Hz). They don't list a an effective series inductance.
I would also probably make a multi conductor battery cable that had
several conductors for the positive and negative leads, with the whole
bundle braided to interweave the two currents to lower the inductance,
unless the battery were right next to the control circuits. Making
the control circuit to be bolted directly to the battery would be even
better, and eliminate most of the supply side bypass requirement. One
of those side bolted GM batteries would make this easier.
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