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From: Winfield Hill
Subject: Re: 87% All that vector calculus paid off
Date: 22 Dec 2002 06:31:26 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> I rather doubt you are actually getting real spikes to 600-700V at the drain
> at MOSFET switch off. If we look at figure 12d from the IRF730a datasheet
> (http://www.irf.com/product-info/datasheets/data/irf730a.pdf), we can very
> roughly estimate that the anticipated avalanche voltage would be around 550V
> at an avalanche current of around 1.5A (whatever the maximum inductor
> current at switchoff is, but in your case in the vicinity of 1.5A).
> Assuming you are reallying using an IRF730a (by International Rectifier),
> and assuming you aren't dealing with a major fluke on your hands, it is most
> unlikely that you are actually getting 600-700V spikes.
It's not at all unusual for production MOSFETs to have breakdown voltages
several hundred volts above their rating. For example I have measured dc
breakdown voltages close to 1400V for some 900V-rated FETs. (I have also
measured 900V breakdowns with 1200V FETs, but that's another story. :-(
After having made them fine for 5 years, Motorola took them off the market
for a few years while they figured out how to make them again. A few years
later they discontinued them. And the entire line a few years later.)
> Chances are there is something wrong with your measuring technique. ...
An easy trap to fall into, calibrated setups are necessary.
> Even if your MOSFET is avalanching I would not worry about it. Some
> people may disagree with me on this, but it is perfectly acceptable to
> operate an avalanche rated MOSFET in avalanche provided you never exceed
> the maximum avalanche current rating (for any duration) or the maximum
> junction temperature. For the circuit conditions you describe it doesn't
> sound possible that you are ever exceeding either of those ratings so you
> are okay.
I agree with your points. Except to mention that if you carefully examine
the avalanche rating curves you'll see they momentarily take the junction
temperature up to its rated maximum. If one has decided to stay below the
maximum Operating Junction Temperature Range (150C for a IRF730a), derate
the acceptable avalanche condition accordingly. Note, repeated temperature
excursions with the resulting thermal expansion cycles are arguably worse
than a steady high-temperature condition.
One important point, it's necessary to use the Effective Transient Thermal
Impedance curves to evaluate avalanche energy. Generally speaking the peak
avalanche current is derated by the square root of the avalanche duration.
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