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From: firstname.lastname@example.org (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 23 Oct 2002 14:30:06 -0700
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <3DB5EEF9.BF30CE32@rica.net> <firstname.lastname@example.org> <3DB69CBA.B6E79464@rica.net>
NNTP-Posting-Date: 23 Oct 2002 21:30:06 GMT
John Popelish wrote in message news:<3DB69CBA.B6E79464@rica.net>...
> 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?
Have to say thanks for the effort you're putting into this! I'm amazed
at the amount of interest shown by all. Yes - I had planned to use a
4.8v nicad trickle charged via hi value resistor from the 12v to
supply the RC receiver and control system. And a 9.6v nicad
(10.something fully charged on load) for the gate drive IC so that
stiff drive is guaranteed and the 12v rail is powering only the motor
/ fet / diode circuit.
I do have a 2 channel scope and sure, I can find a way of measuring V
and I in the fet. Easy with initial tests 'cos of course I'll put some
low value resistance (low inductance I hear you scream...) in series
with the motor.
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