The medium frequency transformer is a key component for the design of input-output isolated converter design when the isolation and/or voltage matching is needed. These kinds of converters are used in different applications such as battery based energy storage systems, the high voltage DC conversion, grid interfaces of renewable energy sources, etc. Design at the high frequency significantly decreases the size and improves the efficiency of the power transformer. With the recent advancements of soft magnetic core materials and switching devices, the high frequency transformers become more interesting not only as a part of power converters but also as a replacement of conventional line frequency transformers. In this detailed review study, studies on the design of the power transformers used in power electronic converters are examined and their application areas, operating frequency values, core material types are investigated and classified. In addition, the design methodology is proposed with Finite Element Analysis (FEA) software and a power electronic transformer is designed with the different core materials.