I have a circuit that is switching a transformer primary winding from 15 volts to ground, through an N-channel MOSFET at 50 kHz. I was seeing occasional field failures and found that every time the MOSFET was switched off, a 90 volt 100 ns flyback voltage spike was created. This circuit had no flyback diode. The Vds of the MOSFET is 80 volts.
I added a 1N4148 flyback diode and the spike shrank to about 50 volts and about 30 ns at half power. I expected more reduction and would like to remove more of the spike. Any ideas of other methods for suppression?
I thought about the zener clamp over the weekend. Many thanks for the input and the easy to read answer. My load in this case is a piezo transducer. The point of all this is to make a 50khz noise. My situation is a little more complicated in that I have two primaries run in opposite directions with two Mosfet switches in one SO8 package. I need to add two zieners to fix this. Its also complicated by the same PCB is asked to run either at 75khz or 50khz software dependent. My diode will have to work for 75khz and that will like also work for 50khz. This morning I discovered that my primary transformer windings are between pins 1 to 3 and 2 to 4 not 1 to 2 and 3 to 4 like I thought. When I put the fly back in correctly I saw exactly what you predicted, my output power was lowered to less then half. Time to hunt down some suitable parts for test. Thanks!