TMS320F280049C: Optocoupler and TL431 rejection of line frequency ripple

Part Number: TMS320F280049C
Other Parts Discussed in Thread: TL431

Hi team,

Here's an issue from the customer may need your help:

The front stage PFC regulates the bus voltage, the bus voltage of the PFC output is inherently 100 Hz twice the line frequency ripple, and the output voltage of the LLC is closed loop using optocoupler+TL431. A 100-Hz ripple is also found on the output voltage of the LLC. 

Is this due to the optocoupler and the slow response of the 431 loop? How to tune the loop to fix it? 

Could you help check this case? Thanks.

Best Regards,