So, I've just been reading the datasheet for an Infinion ProFET i'm using - and in the typical application description is this...
Freewheeling diode for commutation of load current. Depending on load current
and thermal boundary conditions, it may be necessary to use active freewheeling
by a MOSFET, instead of the diode.
Now, I've heard of active freewheeling before - is this simply, replacing the freewheel diode to ground, with a MOSFET - which you switch on immediately after turning off the load, then switch back off after a couple of uS?
Yes - you choose the minimum deadtime you can get away with - shoot-through current must be
prevented though. Another term for this is "synchronous rectification".
Thanks for the info - however, the ProFETs are high-side switching devices - so if my understanding of shoot-through is correct, there won't be any whatsoever as the supply is being switched off to the load, so the active MOSFET could be left on indefinitely without shoot-through.
I should imagine the primary concern is selecting a MOSFET with a high enough breakdown voltage to withstand the spikes? Or, actually - the back-emf should never have chance to build a high potential spike if there's a clear path to ground?
Remember, connect the MOSFET with the intrinsic body diode in mind. Fortunately, a MOSFET is really a voltage controlled resistor (very roughly), so it will conduct current in the reverse direction. The gate bias is still referenced to the Source.