UCC256402 and UCC256404 uses a self bias start up scheme, thus eliminating the need for a separate auxiliary supply. When AC power is first plugged in, the PFC and LLC are both off. The internal JFET on the HV pin will be enabled and will start to deliver current from a source connected from the HV pin to the VCC capacitor. The charge current is small when VCC pin voltage is below VCCShort , and then becomes larger when VCC pin voltage is above VCCShort.Once the VCC pin voltage exceeds its VCCStartSwitching threshold, the current source will be turned off and RVCC will be enabled to turn on the PFC. When the PFC output voltage reaches a certain level, the LLC is turned on. When the LLC is operating and the output voltage is established, the bias winding will supply current for both the PFC and the LLC controller devices.
UCC256403 does not include a high voltage start-up feature. It requires an external auxiliary supply to power on the IC. The HV pin needs to be grounded when using the UCC256403.