Theoretical analysis and optimal design of LLC resonant converter

A LLC resonant topology is analyzed to derive efficiency and cost optimal design for wide input ranges and load variations. In the LLC converter, a wide range of output power is controlled with only a narrow variation in operating frequency since this converter is capable of both step-up and step-down. In addition, ZVS turn-on and ZCS turn-off of MOSFETs and diode rectifiers can be achieved over the entire operating range. Finally, the inductance of a resonant tank in the primary side can be merged in the main power transformer by resonant inductance and the absence of the secondary filter inductor makes low voltage stress on secondary rectifier and cost-effective property. DC characteristics and input-output response in frequency domain are obtained with the equivalent circuit derived by first harmonic approximation (FHA) method. In addition, operational principles are explained to show the ZVS and ZCS conditions of primary switches and output diode rectifiers, respectively. Efficiency and cost optimal design rules of the LLC resonant converter are derived by a primary resonant network, operating frequency, and dead time duration. Proposed analysis and designation are proved by experimental results with a 400 W LLC resonant converter.