The VCC capacitor should be sized based on the total start-up charge required by the system. The start-up charge will mostly be consumed by the gate driver circuit. Thus the total start-up charge can be estimated by the start-up switching frequency, MOSFET gate charge, and the soft-start time.
Assume the total start-up charge required by the system is shown in Equation 80
During PFC and LLC startup phase, the maximum VCC voltage drop allowed is
The minimum VCC capacitor needed:
Choose at least 100 µF capacitor or combination of capacitors.