Design optimisation of an inductor-integrated MF transformer for a high-power isolated dual-active-bridge DC-DC converter

Medium-/high-frequency isolated power conversion systems are continuously evolving towards higher power densities. A comprehensive design strategy is required to find a medium-frequency (MF) transformer design that maximizes efficiency and power density while complying with material, electrical, and temperature limits. The proposed approach takes the optimum size of winding conductor conducting harmonic currents into account. The influence of the winding interleaving configuration on the leakage magnetic field is discussed and a method to tune the leakage inductance in a transformer is proposed. A novel semi-empirical expression providing an improved accuracy of an AC resistance factor is introduced. Two well-known configurations of transformers utilised in MF high-power applications are also investigated, namely shell type and core type. The design methodology is established based on the parameter sweep method, and many candidate solutions are enumerated and tested one by one to determine the solution set that satisfies various constraints. Two prototypes of 5 kHz/10 kW nanocrystalline-based transformers are designed and manufactured. The results show that the natural-cooled shell-type prototype can reach a power density of 17.6 MW/m(3) at an efficiency of 99.36%, and the natural-cooled core-type prototype can reach a power density of 14.2 MW/m(3) at an efficiency of 99.33%.