A Mode-Switching Four-Degree-of-Freedom Variable-Frequency Modulation Strategy for Dual-Active-Bridge Microinverters

This paper addresses the challenge of maintaining low current stress and high efficiency in dual-active-bridge (DAB) microinverters, where the voltage conversion ratio and instantaneous transferred power vary continuously over the line-frequency cycle. A mode-switching, four-degree-of-freedom, variable-frequency modulation strategy with analytical parameter calculation is proposed. Four representative low-current-stress operating modes are selected for the G<1 and G>1 regions. Under unity-power-factor and sinusoidal grid-current-tracking constraints, analytical switching-frequency expressions are derived for each mode, enabling coordinated control of the primary-side duty ratio, secondary-side duty ratio, external phase-shift ratio, and switching frequency without iterative online optimization. Duty-ratio limits, switching-frequency bounds, ZVS commutation-current requirements, and an SPS-based hysteresis transition near G=1 are incorporated. A 500 W experimental prototype was built for validation. The proposed strategy achieved a peak efficiency of 97.2% at 300 W and an efficiency of 96.5% with a grid-current THD of 2.8% at 500 W. Compared with a fixed-frequency minimum-current-stress TPS strategy, the measured peak leakage-inductor current at 500 W was reduced from 8.23 A to 7.42 A. The results validate the proposed modulation method under the reported experimental conditions.