> As Bob Blick hinted at, the reverse body diodes in power mosfets
> definitely do NOT have the same ratings as the forward FET.
>
> They are also not great diodes.  They have fairly high forward
> voltage, they are typically medium speed or fast, but not ultra fast,
> and they can be destroyed by large reverse recovery currents
>
> I'm not saying you shouldn't use them.  Many applicaions use them very
> successfully.  I'm suggesting that you be careful and do your homework
> before relying on them.
>
> Don

I hate to disagree with you Don, but the power FETs I looked at
definitely DO have the same specs as the forward FETs, they are
not slow, and they are not bad diodes.

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.

To be honest I don't know what percentage of power mosfets have
"fully characterised" drain-source diodes. I just assumed most of
them are. Maybe you assumed most of them aren't. You are right
in saying be careful and do your homework on the device data
sheets.

The NDP7050 data sheet also says:- "Rugged internal source-drain
diode can eliminate the need for an external Zener diode transient
suppressor". I think this a reference to the diode actually behaving
like a zener, and they draw it so in their diagram. Anyway, I have
no problems about using these internal diodes as they spec them.
The 750A controller we made with these FETs has worked
flawlessly to date, even surviving a motor armature melt down.

As has already been stated by several posters, in a full bridge
configuration you have 4 FETs and 4 of these internal diodes
forming a natural protection bridge. Even better, with appropriate
control you have the ability to actually turn ON the FETs you want
to be diodes at the right time, lowering voltage drop and reducing
power dissipation.

Regeneration is also possible. If the motor goes into an over speed
condition it will generate back into the supply through the diodes.
Regeneration at lower speeds is more complex, but still possible I
think.

Regards, Brent.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile: 025 334 069
eMail:  brent.brown@clear.net.nz

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