The demand for high frequency, high power density and high efficiency on power converters has been increasing through the years. Solid state transformer (SST), which is typically formed by three stages, one being DC/AC inverter, which is controllable, one being medium-frequency transformer isolated converter and the other being AC/DC rectifier, has been developed quickly during past few decades to replace traditional 50Hz transformer. In SST, the use of planar transformer on print circuit boards (PCB) working at tens to hundreds of kilohertz can provide isolation and save space. In this thesis, research activities to develop a planar transformer isolated DC/DC converter for solid-state transformer are presented. To further increase efficiency, resonant converter with series LC and shunt LC filter is adopted to realize soft-switching, and to eliminate high order harmonics hence reducing unnecessary core loss and copper loss. For the designs of planar magnetics, PCB based litz wire air-core planar inductor and PCB based litz wire planar transformer are proposed, analyzed, simulated and tested, which shows better performance than traditional solid rectangular winding ones. By applying the novel litz wire inductor and transformer in the proposed DC/DC converter prototype, high order harmonics are eliminated and the efficiency of 90% is reached from experimental results, which can be further developed by an integrated system.