Next generation of electric vehicles will be equipped with high power density and high efficiency onboard charging systems with bi-directional power flow. These benefits can be achieved by utilizing the emerging Wide Bandgap devices, planar magnetic solutions, innovative circuit topologies, and advanced control methods to enable MHz switching frequencies without sacrificing efficiency. However, the advantage of higher switching speed is gained at the expense of higher switching losses in both the semiconductors and the magnetics. Conventional circuit topologies, operation modes and control algorithms would no longer be effective in such conditions. Furthermore, the practical implementation of the system has shown more stringent requirements on the controller speed, layout parasitic and the thermal management. In this Ph.D. dissertation work, aforementioned challenges have been addressed, and the proposed innovations have been validated through design and development of a new bi-directional onboard charger using Gallium Nitride devices. The first part of this work has been focused on a thorough characterization of the front-end AC-DC power factor correction and rectification stage of an onboard charger, utilizing advanced planar magnetics and newly proposed soft-switching control methods. The second part of this work is focused on developing a CLLC DC-DC converter, to interface the AC-DC stage and the high-voltage traction battery. Extended Harmonic Approximation method has been developed and a novel “f-φ” control scheme is proposed to enhance the efficiency at high switching speed. This system allows insights into the practical implementation and evaluation of utilizing Wide Bandgap semiconductors to achieve high power density and efficiency for the transportation industry.