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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?

Thank you

EDIT: Load of the converter is only resistive (a heater).

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  • \$\begingroup\$ Check resonance with wiring L and use twisted pairs and consider snubber RC. Then include that in simulation with wiring L. \$\endgroup\$ – Sunnyskyguy EE75 Oct 3 '17 at 15:56
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    \$\begingroup\$ "Hyperfast recovery time" lol got to love sales people... However, if it was recovery time I'd still expect your diode to fry when using short PWM cycles since the switching events would be identical. Since it is happening on high loads I'd suspect the currents and voltages involved instead. BTW heaters can have significant inductance. \$\endgroup\$ – Trevor_G Oct 3 '17 at 15:57
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    \$\begingroup\$ hyper marketing people (lol) \$\endgroup\$ – Sunnyskyguy EE75 Oct 3 '17 at 15:58
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    \$\begingroup\$ @Trevor: It also happens on small loads (even less than 1/4 of maximum nominal load) if the PWM duty is high enough so that the converter operates in continuous mode. \$\endgroup\$ – tk_ Oct 3 '17 at 16:15
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    \$\begingroup\$ If it works fine in discontinuous mode and suddenly fries in continuous mode it might be a control loop issue. The control to output transfer function is different between the two conditions and you might be getting damaging oscillations in continuous mode. Is your controller using peak current mode? If so do you have slope compensation to avoid subharmonic oscillations? (Though they are usually not fatal to the converter.) \$\endgroup\$ – John D Oct 3 '17 at 17:00

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