Symmetrical and Asymmetrical Modulation in Dual Active Bridge Converters: Analysis of Degree-of-Freedom Control and Light-Load Soft-Switching in 800 V Electric Vehicles

Symmetrical and asymmetrical modulation create different control spaces in an 800 V dual-active-bridge (DAB) converter. The distinction matters most away from unity conversion ratio and at partial load, where zero-voltage switching (ZVS) is governed by the current available at each commutation rather than by semiconductor capability alone. This review examines switching symmetry as a constraint on transformer flux balance, leakage-inductor boundary conditions, and reactive-power circulation. Symmetrical SPS and symmetric EPS/DPS/TPS preserve half-cycle waveform invariance, limiting how independently current slopes can be shaped and potentially narrowing the ZVS region as power decreases. Asymmetrical schemes relax these constraints through unequal phase shifts, duty-ratio shaping, and additional control variables. The resulting degrees of freedom can separate active-power control from reactive-current injection, extend soft-switching operation, reduce backflow and RMS current, and redistribute device stress. The review links modulation symmetry, degree-of-freedom expansion, current-stress minimization, and ZVS-boundary behaviour to provide a practical basis for selecting partial-load modulation strategies for SiC DAB converters used across the wide voltage range of 800 V electric-vehicle systems.