Why LLC needs an auxiliary power supply

The reason why LLC needs an auxiliary power supply is because switching losses reduce conversion efficiency. The LLC architecture is a two-tube half-bridge resonant, using a resonant inductor, an excitation inductor, and a resonant capacitor in series, hence the name LLC. Using zero-voltage switching (ZVS) soft-switching technology, it has the advantages of high operating frequency, low loss, high efficiency, and small size, which can increase the charger power density.

Resonant Operation of LLC

Resonant operation enables soft switching over the full load range and reduces switching losses, making it ideal for high-frequency and high-power-density designs, suitable for fixed-voltage outputs with better EMI characteristics. Conventional flyback architectures require RCD absorber circuits to absorb the resonant spikes generated by the transformer leakage inductance at the moment the switch is turned off, which can reduce conversion efficiency to a certain degree, as well as switching losses that can also reduce conversion efficiency, making them unsuitable for higher power outputs.

LLC contains inductive, capacitive and resistive components of the single-port network, at certain operating frequencies, the port voltage and current waveforms appear the same phase, said the circuit resonance. The circuit that can resonate is called a resonant circuit, the original side of the LLC converter is turned on, and the output diode ZCS is turned off, there is no reverse recovery problem, the switching loss is small, and it is suitable to be applied to high-frequency, high-power density design.