LLC resonant converter has been used widely in DC-DC converters for constant output voltage with high efficiency. However, most of the design methods based on fundamental harmonic approximation (FHA) can't be used for the design of charging converter of Li-Ion batteries as the output voltage of Li-Ion batteries charger is wide. In this paper, an optimal design method for LLC resonant converter has been developed. The objective of this optimal design method is maximizing the magnetizing inductance. The relationship between the operation range and circuit parameters has been revealed based on the frequency response of the converter. Maximum conversion efficiency is achieved under the normal condition by using the maximum magnetizing inductance. A numerical method is developed based on the LLC converter's steady state equations to eliminate the limitation that the FHA method fails to describe voltage gain characteristic more precisely. A 3kW prototype LLC converter with output voltage ranging from 50V to 100V is built according to the proposed optimization method. Simulation results show that the proposed converter can achieve the higher efficiency than the conventional method. Furthermore, the full load efficiency of a 3kW prototype is about 94.5%.
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