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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Mosfet body diodes vary widely. Some of them are actually kind of OK: usually in the newer low-voltage devices.
I've designed a few synchronous buck converters to step 12V down to 5 or 3.3, and I always put a Schottky diode across the low-side FET. In lab tests I could see that it improved the efficiency a little.
For high voltage converters, often it's not enough to just put a diode across the MOSFET. You need a low voltage schottky in series with it, and a fast diode across the whole lot. But low voltage converters can't tolerate the extra conduction losses of this series diode.
I think in this case the 0 reverse recovery time is more important. If you have any recovery time the mosfet body diode will be forward biased/turn on.
However something I don't quite understand is, schotty diodes have capacitance that has to be charged, dosn't that act as a small reverse recovery time? in effect being the same as a fast normal diode?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, the effect of the junction capacitance is the same as reverse recovery charge in a normal diode. It's particularly bad in high voltage SiC schottkys where it eats much of the advantage over a silicon diode.
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Mosfet body diode
