MOSFET as a two-way-switch?

Ubergeek63 · Jun 9, 2008

logicnibble said:
Hi!

The extract you've posted has the free-wheeling diode characteristics.
I believe the free-wheeling diode is put inside the chip along with the MOSFET to protect it when switching (dI/dt).

But from my testing results, it looks like its not this free-wheeling diode that conducts the reverse current but the MOSFET itself.
If the diode is conducting, then why the dropout voltage is different whether the MOSFET is ON or OFF?

The diode is a parasitic, it is ALWAYS there. Newer FETs have them optimized as switching or zener diodes, in the case of avalanche rated FETs, and are specified as such.

logicnibble · Jun 9, 2008

Ubergeek63 said:
The diode is a parasitic, it is ALWAYS there. Newer FETs have them optimized as switching or zener diodes, in the case of avalanche rated FETs, and are specified as such.

MOSFETs can have an integrated diode or not, as you can read in Wikipedia:
MOSFET - Wikipedia, the free encyclopedia
in the MOSFET analog switch section or in this article:
**broken link removed**

When you say the diode is a parasitic are you referring to this one or to the one that is formed by the PN junction of the MOSFET itself?

mneary · Jun 9, 2008

Every MOSFET I've ever tried worked great as a bidirectional switch. (But don't forget the diode). Check "synchronous rectification". You turn the MOSFET on when the diode would otherwise be conducting. People do it all the time. Unfortunately very few MOSFET data sheets show characteristics of all quadrants.

We've built tens of thousands of H-drive circuits using this method.

Ubergeek63 · Jun 10, 2008

logicnibble said:
MOSFETs can have an integrated diode or not, as you can read in Wikipedia:
MOSFET - Wikipedia, the free encyclopedia
in the MOSFET analog switch section or in this article:
**broken link removed**

When you say the diode is a parasitic are you referring to this one or to the one that is formed by the PN junction of the MOSFET itself?

The PN junction of the mosfet itself. It is ALWAYS there, it can not be helped. When they copak a mosfet with a schottky it is to cut down the drop voltage of the diode, not because there is not one there already.

IGBTs, on the other hand, do not have a substrate diode in their structure so they need a separate diode.

MOSFET as a two-way-switch?

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