How to choose FPGA for converter control?

A friend once said "If you're not sure you need an FPGA you probably don't." If you want to make a case for using an FPGA, look at your controller in terms of how much better it would be if it was running faster than 400 kHz. You'd of course need to design the whole system at a higher freq, but the end result would be a faster acting and possibly more stable control loop. If you don't need the extra margin then there isn't really a case for using an FPGA over a "good enough" microcontroller.