The UCC28610 VDD can be charged through high-voltage MOSFET directly from the high-voltage bus at startup. Due to the nature of high loss, this charging path is intended for startup operation only. During normal operation, the VDD voltage should be high enough to avoid this high-loss charging path.

In order to achieve the lowest possible no-load power, select the number of turns in the bias winding so that VDD is higher than 16V-V_TH of the high voltage MOSFET. A bias winding voltage between 17 V and 20 V usually achieves minimum loss. The bias winding often tracks the primary leakage inductance turn-off voltage spike. Place a 20-V Zener diode between VDD and GND in applications where the heavy loads cause excessive VDD voltage.