As a part of an electric vehicle (EV) onboard charger, a highly efficient, highly compact, lightweight and isolated DC-DC converter is required to enable battery charging through voltage/current regulation. In addition, a bidirectional on-board charger requires the DC-DC converter to achieve bidirectional power flow: grid-to-vehicle (G2V) and vehicle-to-grid (V2G). In this work, performance characteristics of two popular DC-DC topologies, CLLC and dual active bridge (DAB), are analyzed and compared for EV charging applications. The CLLC topology is selected due to its wide gain range, soft-switching capability over the full load range, and potential for a smaller and more compact size. This dissertation outlines the feasibility, analyses, and performance of a CLLC converter investigated and designed to operate at 1 MHz and 3.3 kW for EV onboard chargers. The proposed design utilizes the emerging wide bandgap (WBG) gallium nitride (GaN) based MOSFETs to enable high-frequency switching without sacrificing the conversion efficiency. One of the major challenges in MHz-level power converter design is to reduce the parasitic components of printed circuit boards (PCBs), which can cause faulty triggering of switches leading to circuit failure. An innovative gate driver is designed and optimized to minimize the effect of parasitic components, which includes a +6/-3 V driving logic enhancing the noise immunity of the system. Another challenge is the efficient design of magnetic components, which requires minimizing the impacts of skin and proximity effects on the transformer winding to reduce the conduction loss at high frequencies. A novel MHz-level planar transformer with adjustable leakage inductance is modeled, designed, and developed for the proposed converter. A comprehensive system level power loss analysis is completed and confirmed with the help of experimental results. This is the first prototype of a 3.3 kW power bidirectional CLLC converter operating at 1 MHz operating frequency with 400-450 V input voltage range, 250-420 V output voltage range. The experiment results have successfully validated the feasibility of the proposed converter conforming to the analysis carried out during the design phase. With an appropriate design of driving circuit and control signal, the prototype achieves a peak efficiency of 97.2% with 9.22 W/cm3 (151.1 W/in3) power density which is twice more power dense than other state-of-the-art isolated DC-DC converters.