Estimation of eddy current winding losses in soft-switching solid-state transformer

The high frequency (HF) transformer is a critical component of any solid state transformer. In the case of soft-switching solid-state transformer (S4T), the HF transformer is similar to a flyback transformer with the addition of transition stages, resulting in some atypical operating modes. Only one winding is active during the active energy transfer time. However, there is significant proximity effect induced eddy current losses in the inactive winding. During the transition times, both the windings are carrying very HF currents (10-20 times switching frequency) resulting in significant AC copper loss even though the transition period is small (5-10% of switching period). This paper explains the nature of these eddy currents in the open winding and during transitions stages, which are unique to S4T, and presents analysis to evaluate these losses. Mathematical derivations to calculate these unique AC losses are presented. FEM analysis to verify the analytical method is presented.