I am working with a motor controller that runs a 3 phase switched reluctance motor. I am in the process of replacing the freewheel diodes
for the power mosfets. The originals are ST microelectronics number BYW98 200 fast recovery diodes. These are rated at 200 volts. I would like to upgrade these with 400volt units but some I am running across are ultra fast recovery. Would using the ultra fast recovery units cause a problem if I were to use them?
The mosfets I will be using are Fairchild FQP17N40.

Thanks

 

I wouldn't think so. Just make sure that the current rating and the power dissipation are higher. Ultra fast recovery diode should have lower trr than fast recovery one. trr is the reverse recovery time which is the time needed by the diode to transition from conducting to blocking. So the new diode should have lower trr.