Single-Degree-of-Freedom Hybrid Modulation Strategy and Light-Load Efficiency Optimization for Dual-Active-Bridge Converter

Dual-active-bridge (DAB) converters either adopt single-degree-of-freedom (SDOF) modulation or multiple-degree-of-freedom modulation. The SDOF single-phase-shift (SPS) modulation has low power transmission efficiency under light load, and the multiple-degree-of-freedom modulation has high requirements for model accuracy and controller design. Focusing on these limitations, this article proposes an SDOF hybrid modulation method to optimize transmission efficiency, switching device current stress, and zero-voltage-switching (ZVS) implementation under the light-load condition. To this end, first, the SDOF SPS, primary-side internal phase shift (PS-IPS), and secondary-side IPS (SS-IPS) modulation methods are comprehensively analyzed in terms of current stress, ZVS feature, and transmission power. Second, by comparing the current stress, ZVS, and loss features of SDOF modulation methods under the light-load condition, the case-dependent optimal modulation method is determined. On this basis, an SDOF hybrid modulation strategy within the light-load range is developed by resorting to a proper switching rule, which judges the voltage ratio between the two ends of the inductor and the size of the transmission power. Finally, a 600-W experimental prototype is built, and the results show that the proposed strategy effectively improves the transmission efficiency of the converter under the light-load condition.