Analysis of the Zero-Voltage Switching Condition in LLC Series Resonant Converter with Secondary Parasitic Capacitors

The LLC series resonant converter is widely used in many applications due to its high power density and efficiency. However, under applications with relatively small magnetizing current and resonant inductance such as DC transformer (DCX) for telecom power systems and PV optimizers, the resonance between the leakage inductance and the secondary parasitic capacitance during dead-time is unneglectable. It will affect the zero-voltage switching (ZVS) transition and the resonant current during the power delivery period. Therefore, in this paper, two different scenarios of the ZVS condition for an LLC series resonant converter with secondary parasitic capacitance considered will be presented. Moreover, the impact of the resonant to steady state operation under different ZVS timing will be discussed. Experimental results of a 140kHz prototype are provided for verification.