UCC28951: Duty cycle varies a lot at a give load above 50% full load.

Tool/software:

Hi Mike, 

I am running the PSFB topology with Diode full bridge on the secondary side of the converter with UCC28951 controller. As I increase load above 250W, the converter generates loud noise. From the waveform, I found that the effective duty cycle varies a lot with a fixed resistive load. Duty cycle changes form 1usec to 2usec. This phenomenon is observed above 50% of full load (250W). Does it have to do with the compensation network? If yes, then how to optimize the compensator components? 

Vin=400V, Vout=24V, Pout=500W, Fsw=200kHz

Here are the waveforms:

CH1: VDS of QA

CH2: VGS of QA

CH3: VDS of QC 

CH4: VGS of QC

CH6: Voltage across the transformer [Effective Duty cycle]

Thank you.