The LLC resonant converter is widely used for consumer and industrial applications due to its high efficiency, small size and low noise. In a practical design, a trade-off is needed between the input voltage operation range and the transformer size for miniaturization. The transformer size is determined at the minimum input voltage with full load. It is very important to have an accurate design technique so that the transformer does not have any unnecessary size margin. In this paper, inductance (L)-matrix transformer model-based design is introduced and compared with the conventional transformer model such as T-type and L-type. A 200 kHz, 240W LLC resonant converter is implemented and verified that the inductance (L)-matrix method is accurate, and it can contribute to miniaturization.