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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Help - Power mosfets - difficult load
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Date: Wed, 23 Oct 2002 12:57:30 GMT
NNTP-Posting-Date: Wed, 23 Oct 2002 08:57:30 EDT
Bill Allison wrote:
> Re your latest - I understand that the low frequency will give time
> for the current to overshoot the average, but will the losses due to
> that be dissipated in the fet or in the schottky? My concern, because
> of their cost, is to minimise the risk of blowing the fets. I'm not
> that concerned about overall efficiency. And as I've said, 50Hz is
> conveniently to hand and I suspect won't be that terrible with my
> large motor.
During both turn on and turn off, the fet passes through a period of
both conducting current and dropping voltage. The schottky doesn't
conduct till the full battery voltage is across the fet, while it
conducts the peak motor current. At that point, the current trades
off from the fet to the diode. This is the big loss for the fet.
When it turns on, it has to drop the full supply voltage while it
carries both the motor current and the diode capacitive current as the
diode starts to charge up into reverse voltage (look at the data sheet
for the diode capacitance versus reverse voltage and you will find
that it acts as a capacitor that is several times larger at near zero
reverse volts than it does at 12 volts reverse). But with a low pulse
rate, the current will be essentially zero at this point, so this
phase does not stress the fet. The trick is to help the worst case
out (turn off, in your case) without increasing the conduction or turn
on losses too much. If you have access to a two channel scope and a
way to put a fet current signal into one channel and fet voltage into
he other, you can multiply the two and calculate the instantaneous
power at any point in the cycle. Then you can try a few different
pulse rates and pick the least stressful one.
My guess is something in the 200 to 1000 Hz will run the fets the
coolest. But is you have large enough devices, they will tolerate 50
Hz also. At that frequency, you may have trouble making the control
system work properly, though, because the battery voltage will be
swinging around pretty severely. You may need to provide a smaller
second battery to run the control stuff with its negative side
commoned at the fet sources. Filtering the ripple current drawn from
the motor battery gets easier with a higher pulse rate. You got any
of those 1 farad capacitors the audio fanatics use on their big amps?
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