>The MOSFETs I used in a DC motor speed controller were
>Fairchild NDP7050 (formerly National NDP705). Reading from the
>data sheet, the N-channel enhancement mode FET is rated at 50V,
>75A max continuous, 225A peak, typical RdsON of 10 milliohms.
>
>The drain-source diode in this device is rated at 75A max
>continuous, 225A peak, typical forward voltage of 0.9V at 37.5A
>(note this spec is for a pulse <300us and <2.0% duty cycle) and a
>typical reverse recovery time of 80ns.

Hi Brent,

Let me add to this. Note that the diode has 0.9 volt drop at 37.5 Amps. How
about the FET at the same current? 37.5 Amps times 0.01 ohm = 0.375 volt.
The diode drops more than twice as much. I agree with Russell that you have
to expect FET and diode current to be about equal at 50% duty cycle, so
your diode losses are really double the FET losses.

I agree with you it pays to do the math - the juice is always right!

Cheerful regards,

Bob

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