[SOLVED] Double ended Forward Converter

[russ]

double ended forward converter

Hi
I need some thoughts or advice regarding a design which is almost done.

I have a PFC pre-regulator that puts out 387Vdc.
I am using the 387Vdc as an input for my double ended forward converter. The specs for the forward converter are as follows:
Output Power: 300W
Output Voltage: 0-12V
Output Current: 0-25A
Currently, my supply is set up such that I get 12V@25A (full power) at 40% duty cycle using UCC35702 from TI. I am having a problem where when I adjust my duty cycle to get a lower output voltage, my feedback has some sub-harmonic oscillation (not too bad), but sometimes destroys my FET. I am using a FET that has more than enough margin since I am using 600V part with 14A rating. I should never see more than 4A through the FET (depends on the duty cycle/output voltage). It runs fine from 9-12V. It is when I try going below 9V irrespective of the output load current, it takes my FET out. Since I am using double ended forward, any leakage would be taken care off by the diodes. It is a bench supply.

Any thoughts that would shed light on my problem would be appreciated.

Thanks in advance.
If a schematic would help, let me know.