I have an off line flyback converter power supply that keeps killing its clamp diodes and I'm not entirely sure what to attribute the issue to.
Initial symptom was a "ticking" sound and highly unstable output voltages. The ticking turned out to be the switching controller repeatedly turning on and off again. It's got a stable output to the fet when its on, but it quickly recognizes an issue and shuts off.
The FET checks out ok though I haven't taken it out of circuit to be sure. Feedback resistors are ok. Oscillator freq is about 48Khz and based on R12 and C3 that's about right.
The only problem component I could find was D6, a 100V zener diode. It's paired with a 400V avalanche rectifier D5, but that one seems to survive the issue. This happens even with D3, D4, and D7 out of circuit.
All measurements and observations taken under no load, I don't have a suitable synthetic load unfortunately.
Attached is the schematic and scope measured from the anode of D6 to the anode of D5, right before D5 dies. About 288Vpp, coming from a bridge rectifier fed by 120V. It starts around 200Vpp and quickly ramps to ~300 and that's when D5 dies and it settles there.
Any and all help is greatly appreciated.