This paper proposes a systematic approach to design the transformers used in LLC resonant converters, with a focus on selecting magnetic cores that satisfy system efficiency and power density requirements. The conventional area product method uses simple calculations to select transformer cores based on the power capacity of the system. However, the conventional approach preselects cores before evaluating their loss characteristics, limiting the flexibility of the design process. The proposed approach determines the combination of cores with the lowest losses without preselecting the cores. Moreover, the optimal flux density point for each of the commercially available planar cores is derived. Thus, the system efficiency with the proposed design is improved. The proposed approach is validated on an LLC resonant converter with an input voltage range of 320 V−400 V, an output voltage of 24 V, and an output power of 576 W.
