Zero-voltage-switching (ZVS) of dual active bridge (DAB) converter and its range extension have been studied intensively. However, this paper proves that the external inductance location, i.e. placing on which side of the transformer, does affect the ZVS operation due to transformers' parasitic capacitances, in particular at a high switching frequency of higher than 1 MHz. Thus, this paper, for the first time, gives the mathematical model of the resonant transition during the dead time and thereby analyzes partial hard switching under different modulation schemes with consideration of transformer's parasitic capacitances. From the analysis of ZVS operation, the solution of splitting the interfacing inductance and placing them on both sides of the transformer is proposed and its associated methodology of selecting these inductances is introduced. It extends the ZVS for all the switching devices, therefore, enhance the efficiency or move losses away from the critical semiconductor devices. Finally, the theoretical analysis is verified by the experimental results from a 1 MHz Gallium Nitride (GaN) based DAB converter prototype.

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