All silicon MOSFETS have a substrate source-drain diode that is a result of the semiconductor structure used. In the case of the Fairchild FDMS7556S it looks pretty good - typical forward voltage at 2A of 0.37V, 0.74V @ 35A and 44nsec typical reverse recovery.

Many times in MOSFETS this diode is not real great, and with inductive loads can result in EMI or extra heating of the MOSFET as the inductor kicks back when the MOSFET turns off. To handle the kick back in these situations, a separate diode must be added across the MOSFET with a lower forward voltage to take the current. With the FDMS7556S you will not need the extra diode unless you're driving a really high inductance, or a transformer with a high leakage inductance.

I'd include an external one just in case. If it turns out you need it stuff it.

Keith Cress
kcress - http://www.flaminsystems.com

Note a couple of things. Since the body diode is integral to the MOSFET, any heating issues that you might have had with the MOSFET aren't necessarily mitigated with the body diode, since they're essentially the same device. Secondly, most MOSFET body diodes have relatively poor conductance, since high conductivity is bad for MOSFET performance, so high currents might not be as efficiently removed as with a separate, dedicated diode.

TTFN
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