From: Winfield Hill
Subject: Re: Problems with MOSFET drivers
Date: 16 Nov 2002 06:39:46 -0800
Organization: Rowland Institute
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> Winfield Hill said:
>> Now examine the Supply-Current vs Frequency curves...
>> Clearly you're operating in a past-dangerous region ...
> Interesting. I'm not sure why the supply current vs. frequency
> curves stop at such low currents.
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.
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.
I've been meaning to point out that a sqaure 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,
. ____| |____ ____
So you'll need a bit of circuitry to create this anyway, get the
phase right, etc., and you will then naturally have logic outputs
to feed the drivers.
> Four transformers and eight drivers is getting a bit large ...
You can use two inverting dual drivers and two non-inverting.
> I suppose driving each transformer single-ended might be okay,
> but that leaves the gates subject to noise during their offtime
Not really an issue, the driver clamps solidly to ground, so
that's a low-Z source impedance. A bigger issue is converting
the transformer's output from ac to dc, etc. Generally some
circuitry is used to optimize this step, several different
schemes are common. Nicely detailed in Unitrode's app notes.
In trying to avoid this simple circuitry, you are needlessly
doubling the required driver output voltage.