Puporse of schottky diode in a MOSFET - Page 2 - Electronic Circuits Projects Diagrams Free

mneary · 8th October 2009, 01:06 AM

Quote:

Originally Posted by smanches

The point I wanted to make is the difference between the body diode, which is the intrinsic diode that is formed as part of the mosfet structure, and a true reverse protection diode which is sometimes embedded into a mosfet, or just used externally.

I still missed the point. How can activating a MOSFET while its body diode is in reverse recovery mode damage anything?

Quote:

This diode is part of the mosfet structure and is "in use" while the mosfet is conducting. If it is trying to handle a reverse current when you turn the mosfet on, you can get some spectacular results ending with buying more mosfets.

I understand that the body diode ratings must be respected, but what is this reverse current problem? You're not referring to shoot-through on an H-Bridge due to timing problems?

Mr RB · 8th October 2009, 03:22 AM

Quote:

Originally Posted by mneary

I still missed the point. How can activating a MOSFET while its body diode is in reverse recovery mode damage anything?
...

It shouldn't. If the body diode is conducting reverse current and the FET is then turned on it just gets greater conductivity (in either current direction).

If they use a schottky in parallel with the FET body diode it's just to decrease the dissipation as the schottky is faster and lower Vf and of course dissipation is also external to the FET package.

RCinFLA · 8th October 2009, 04:07 AM

Also called substrate diode or bulk diodes. They are not schottky. They are pretty fast recovery but have relatively high Rs so should not be relied upon for reverse current suppression.

Insulated gate bipolar transistors are putting fast diodes into their package that can be used for reverse current suppression.

Schottky diodes are limited to about 60 v reverse breakdown.

smanches · 8th October 2009, 06:17 PM

Quote:

Originally Posted by mneary

I still missed the point. How can activating a MOSFET while its body diode is in reverse recovery mode damage anything?

I understand that the body diode ratings must be respected, but what is this reverse current problem? You're not referring to shoot-through on an H-Bridge due to timing problems?

I think I was confused. I just reread everything again, and realize they were talking about h-bridges in particular. The extra reverse current being handled by one of the mosfets can cause the di/dt of it's complementary mosfet to increase too quickly, causing the parasitic BJT to turn on. This is what kills the mosfet.

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8th October 2009, 04:07 AM	#18
RCinFLA	Also called substrate diode or bulk diodes. They are not schottky. They are pretty fast recovery but have relatively high Rs so should not be relied upon for reverse current suppression. Insulated gate bipolar transistors are putting fast diodes into their package that can be used for reverse current suppression. Schottky diodes are limited to about 60 v reverse breakdown.