Asymmetric Half-Frequency Modulation in DAB to Optimize the Conduction and Switching Losses in EV Charging Applications

Dual active bridge (DAB) dc-dc converters are used widely in electric vehicle (EV) charging systems with the advantage of high-power density, bidirectional power flow, and possibility of soft switching. The maximum converter efficiency of DAB-based EV chargers happens at unity voltage gain, while it degrades at the start of the charging cycle where the gain is lower. This article presents a novel gate signal modulation method called asymmetric half-frequency modulation (AHFM) that modifies the duty ratio and switching frequency of the source bridge switches to achieve the higher converter efficiency at low-gain operation. Generalized harmonic approximation (GHA) method is utilized to analyze the circuit operation with applied AHFM, and evaluation of zero-voltage-switching (ZVS) requirements is explained. Then, a loss optimization problem is developed with an objective to select the optimal set of phase shift control variables depending on the modulation method to minimize the sum of conduction and switching losses. Five modulation methods based on conventional modulation and proposed AHFM are compared in terms of resultant switching network efficiency at a wide gain/power range that gives an understanding of optimal method at a specific operating point. The proposed AHFM and loss optimization are applied to the EV battery charging profile to improve the converter efficiency during the whole charging cycle. A 1.1-kW-rated proof-of-concept DAB dc-dc converter has been developed to validate the performance of the proposed method.