In this article, a high-efficiency high-power-density wide-bandgap-based CLLC resonant converter with a low-stray-capacitance and well-heat-dissipated planar transformer is presented, which is used as the isolated dc-dc stage for an electric vehicle on-board charger. A generalized planar transformer design methodology is proposed and validated by practical designs and experimental tests. A novel and simple transformer configuration is proposed to reduce the winding stray capacitance and enhance the winding thermal dissipation. The proposed transformer configuration is compared with different planar transformer designs, and the tradeoffs of employing the proposed design are well analyzed. Moreover, the system design and optimization of the high-efficiency high-power-density CLLC resonant converter is studied. The proposed transformer design and the system optimization approach are employed in a 6.6-kW/500-kHz CLLC resonant converter prototype. The prototype achieves a peak efficiency of 97.85% and a power density of 114 W/in<inline-formula><tex-math notation="LaTeX">^\text {3}</tex-math></inline-formula>.