I have a classic buck converter configuration (http://en.wikipedia.org/wiki/Buck_converter) with IXFK120N65X2 power mosfet used as switch and VS-8ETL06PbF used as freewheeling diode. The input voltage is 300VDC, PWM frequency 50kHz, power mosfet driven by ADUM3123 isolated gate driver, DC/DC supply for the high side is MEJ1S0512SC. Max. continuous current is limited to 10A. It is a prototype and the only I have.
a) When the mosfet is fully switched on (PWM duty = 100%) the converter operates fine for hours - here it seems to be fine.
b) When the buck converter operates in discontinuous mode (i.e. bigger PWM duty and small load OR smaller PWM duty and bigger load) the converter operates fine for hours and all measured waveforms correspond to our SPICE simulations - here it seems to be fine as well.
c) But when the buck converter operates in continuous mode the diode burns within just seconds and in most cases the power mosfet burns as well, causing short circuit of the buck converter and overload protection of our input voltage source to become active. When inspecting the damaged parts after this event, the diode is then fully conductive with zero resistance in both directions and the mosfet sometimes as well (zero resistance between drain and source).
I've experienced 5 those "crashes" before I could isolate the event to occur only in the continuous mode. I think the cause here is a fairly long reverse recovery time of this diode, causing repetitive short circuits during each reverse recovery cycle which finally destroy the diode after few seconds. But I am not sure. Maybe also this diode is not suitable for this purpose. Because the operating time ad c) was always very short I was not able to measure anything. Now I don't want to destroy more diodes and FETs. I already ordered a SiC diode alternative (which has a slightly higher forward voltage however) and will try it again then.
Question: Did you experience such behavior and/or are my assumptions correct?
EDIT: Load of the converter is only resistive (a heater).