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From: Mike Poulton
Subject: Re: Problems with MOSFET drivers
Date: 16 Nov 2002 17:07:54 GMT
Organization: MTP Technologies
On 16 Nov 2002, Winfield Hill said:
> It likely has to do with what's often called shoot-through, the
> fact that one FET turns on before the other turns off, causing
> destructive rail-rail supply currents. The effect gets worse
> with high load capacitance.
That does explain the sudden and complete device failure! I'm not sure why
shoot-through within the drivers never occured to me, since I was already
concerned about it in the big mosfet H-bridge due to the lack of dead time.
Perhaps there are transients in the feedback signal that I couldn't see on
my scope which are causing multiple state changes during the rise/fall
times. They would have to be 300mV transients, but that is certainly
possible. This would explain why a clean squarewave from a function
generator doesn't cause this problem.
> Again I suggest using a comparator to get fast-edge logic-swing
> input signals to your driver ICs, thereby allowing the internal
> circuitry a chance to safely do its tough job.
Alright, a comparator will be included in the next version -- along with
quite a few other new parts.
> I've been meaning to point out that a square wave isn't optimal
> drive for a resonant circuit, it's much better to use a shorter
> POS and NEG drive, say 20 to 25%, with an intervening zero-volt
> or floating period,
A bit of dead time is good to prevent shoot-through, but I was under the
impression that an essentially square voltage drive *was* optimal for a
resonant circuit since it provides zero-current switching. It also
prevents the free-wheeling diodes from ever having to conduct. If you do
the switching at any point on the primary current waveform other than zero
crossing, won't the switching losses and component stresses go way up?
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