Mosfets cooled with Heatsink and fan?
These things actually exist.

http://www.ebay.com/bhp/mosfet-heatsink

And well, roughly you could say that the amount of surface area will allow better cooling.
In combination with the amount of air flowing through/against it.



So that it will have the most possible contact with cool air.
I'm not sure what your implementation is. But I'd suggest to try passive cooling.
It really helps a lot, won't cause any noise or 'waste' energy.
(Cooling with a fan might increase the current over the mosfet and thus backfire you?)
Also, if it gets too hot for passive cooling (probably) something is wrong with your implementation.



So yes, it will work, it's effectiveness will vary on the exact implementation. But it 'should' not be neccesary to use active cooling (with a fan). The mosfet heatsinks on ebay look pretty promising and are way easier to implement and cheaper than an extra fan+temperature sensor.