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From: Mike Poulton
Subject: Re: Problems with MOSFET drivers
Date: 16 Nov 2002 10:20:05 GMT
Organization: MTP Technologies
On 15 Nov 2002, Winfield Hill said:
> Now examine the Supply-Current vs Frequency curves, and imagine
> a curve between the 22,000pF and 47,000pF curves. Their curves
> stop at 200 and 600kHz respectively, so our imaginary curve must
> stop near 400kHz. Clearly you're operating in a past-dangerous
> region for these driver ICs.
Interesting. I'm not sure why the supply current vs. frequency curves
stop at such low currents. The TC442xCAT (TO-220 device) is rated for
12.5W dissipation or 2A continuous output current. The other two
versions of this device (PDIP and CERDIP) are rated for less than a
watt. These curves do not approach the current or dissipation limits
for the TO-220 device. Is there some other factor at play here besides
current and power dissipation?
Keep in mind that these driver failures are occuring within a couple
seconds of power on, and are explosive in nature -- it's not a result of
moderately excessive power dissipation. Remember that they work fine
when driven with a function generator. The drivers do not overheat, and
the supply current is less than 2A for the pair. Even if 700kHz into
30000pf is beyond the device ratings, they work just fine. In fact, I
drove them up to 1MHz with the function generator with no problems. The
drivers are on a massive heatsink, and didn't even get warm.
> Hmm, why do you feel the need to drive the FET gates to 18 volts?
> The FETs are well on at 10V gate drive (IR specs Rds(on) at 10V).
> Also, why not use four transformers and drivers instead of one?
I suppose a lower gate voltage would work fine. Good idea. I just
wanted to make sure the switching time was a short as possible. Using a
single transformer and driver pair made the circuit physically simpler
-- but that's no help if it doesn't work. Four transformers and eight
drivers is getting a bit large and expensive. I suppose driving each
transformer single-ended might be okay, but that leaves the gates
subject to noise during their offtime -- negative gate voltage is good.
Perhaps two transformers with a pair of drivers each would be a
compromise. That would also allow for the insertion of dead-time, which
will be healthy for the mosfets.
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