Output diode is burning

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I designed a power supply with TOP258 (sheme in attachment).
Vin = 85-285 VAC
Vout = 72 VDC
Iout = 1.3 A
Output diode is FR257 (D3 in scheme).
After a few seconds at load 0.8 A the output diode is very hot and Vout = 0, then it burns (FR257). What is the reason of that diode's burning?

top258.PNG14.62 KB

At such a high output voltage there reverse voltage across the diode is very large. This will result in a couple of things - high switching losses and high reverse recovery losses. Also you did not indicate whether your supply operates in continuous mode or not.
Also this diode is an axial diode and not best suited from a thermal management viewpoint. You probably need a diode in a bigger package like a TO-220.

I recommend the following
1) Design supply to operate in discontinuous mode
2) Select a better, faster output diode (at-least an ultrafast recovery type). Examples of such diodes are QH03TZ600 or LQA03TC600 from Qspeed. There are others also available from ST or Vishay.
3) You will need a RC snubber across the diode to limit its reverse peak voltage.
4) You will need an external heatsink with diode to manage thermals effectively.