A Four-Degrees-of-Freedom Modulation Strategy for Dual-Active-Bridge Series-Resonant Converter Designed for Total Loss Minimization

This paper proposes a four-degrees-of-freedom modulation scheme to mitigate the conduction and switching losses in a dual-active-bridge (DAB) series-resonant converter. Under wide-range variations in the voltage gain and output current, the increased reactive power and root-mean-square tank current can contribute significantly to the conduction loss in a DAB converter, while the occurrence of hard switching leads to a switching loss and an increased device stress. The proposed modulation scheme utilizes internal, external phase shifts, and switching frequency as modulation parameters to achieve zero reactive power, minimum-tank-current, and complete soft-switching operation. Analysis of the proposed modulation scheme is given for both buck- and boost-mode operations. The proposed modulation scheme is validated by means of a 1-kW experimental prototype of a DAB series-resonant converter operating at 100 kHz, designed to interface a supercapacitor with a rated output voltage of 48 V to a 250-V dc bus. The effectiveness of the proposed topology for charging/discharging a supercapacitor under wide-range variations in the voltage gain and output current is verified by simulations and experimental results. A maximum efficiency of 97.7% is recorded from the experimental prototype (The experimental setup used for measuring converter's efficiency is shown in the supplementary material).