Hi

Working on 800W resonant mode converter with UCC25600 LLC controller.

Build up as the typical application diagram and by use of the design tool.

It work perfect in steady state (same output current, same load) and Eff. shows up to 95 % close to max load.

But with transient loads something strange happens.The Upper fet is blowing even at very light loads. So changing

from load to no load which means that the converter goes into burst mode damage the fet. (FCPF22N60)

Can this be caused by the high drain current when gate turns on again after the burst break?

I did'nt quite hit the Lr calculated in the spreadsheet 31uH but ended up with a

Leakage induction Lr at about 22UH.

Here is the input parameters used in the spreadsheet.

Vin Min: 375V

Vin Max: 410V

Fsw norm.: 134khz

Fsw Max.: 350khz

Fsw Min : 65khz

Max. Power Limit: 800W

Max. Output Power: 775W

Full load eff: .92

Vo: 155V

M: 9

Qr:0.5

Tss:25msec

Td: 200nsec

Br

Finn