Say, we are building a synchronous buck and we have some beefy 600V 10A MOSFET, but it is not really suited for high frequency application - we need to be aware of dv/dt limits and it is hard to achieve fast switching with FET huge capacitance. Also it may have up to 20V at gate.
What if we add a dirt cheap 20V 20A logic level MOSFET at source of our HV MOSFET.
It sound like this way we will be able to achieve insane switching speeds even with a bad driver in cost of small switching loss of bottom FET. In case of ZVS topology, our switching loss will even decrease because of smaller gate voltage.
I've never seen anything like this in practice however (except GaN and some other scientific stuff). I suppose there is a good reason for it (layout? diode recovery/freewheeling? some other severe problems?)? Why not use two different cascoded FETs as a power switch?