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can you offer more detail ?
1.what the input voltage is at no load condition?
2.when the Qc and Qd die, does the failure mos still has the "Diode" feature?
3.can you offerQC and QD VGS and VDS at no load condition?
As you know, UCC28950 has a burst mode at the no-load condition, and for the phase shifted topology, the mos will work at hard switching mode at the no load. so the VGS is easy to be mistriggered if the driving circuit has bad PCB layout.
You can have a look at the current through the QC and QD at no load condition,to see whether there is a short through current and make your mos die. If yes, try to optimize the driving circuit or you can change the mosfet(I saw your mosfet SIGH33N60E, the min VGS level is only 2V at turning on)
