Dear Lisa,
For setup VBIAS, is the output of the Aux winding of the Flyback Transforme, we need to consider two things for designing with PFC:
1. UVLO of UCC28600,
turn-on threshold=13V and turn-off threshold=8V
2. Start up voltage of the PFC controller
UCC28051: turn-on threshold=12.5V and turn-off threshold=9.7V
That means VBIAS need to large than 13V, and also need to add the forward voltage of rectifier. How about let VBIAS=19.4V?
BR