This chapter presents a brief introduction focusing on the applications and classifications of unidirectional high frequency isolated semi-dual active bridge (semi-DAB) DC-DC converters which are widely used in many applications such as energy power conversions for renewable energies for PV or fuel cells, power supply for all electrical aircrafts and electrical vehicles, etc. High frequency isolated semi-DAB AC-DC converters can be used to charge EV batteries. To achieve high power conversion efficiency and high power density, the topology candidates and the corresponding power control methods has been a challenge in both industry and academy research fields. This chapter provides a basic foundation for the whole work, and it gives the goal of this book which is to provide valuable knowledge, advanced control methods and practical design guides for the high frequency isolated unidirectional semi dual active bridge DC-DC converters with wide voltage gain.