An Improved Online Modulation for Resonant DAB DC–AC Converters With Seamless Transition and Enhanced ZVS Performance

The practical deployment of resonant dual-active-bridge (DAB) DC–AC converters is often constrained by the implementation cost associated with complex modulation and modeling, which creates a trade-off between conversion efficiency and real-time implementation. This article proposes a simplified, real-time optimal modulation strategy based on variable-frequency extended phase-shift modulation (VFEPS) that is analytically solvable and readily implementable in real time, while simultaneously lowering conduction and switching losses. Unlike traditional single-phase-shift (SPS) modulation, which suffers from high circulating currents, or RMS-current minimization (RMSM) strategies, which have restricted zero-voltage switching (ZVS) ranges, the proposed strategy dynamically tracks soft-switching boundaries and strategically permits controlled backflow power. This enables the joint optimization of phase-shift angles and switching frequency, thereby ensuring robust soft-switching performance across the entire AC cycle, especially near the AC zero-crossing region. The algorithm is closed-form and non-iterative, making it suitable for low-cost microcontrollers and enabling seamless transitions between different modulation modes through continuous control trajectories. Experimental validation was conducted on a 500-W prototype using monolithic bidirectional GaN (BDS GaN) switches, demonstrating the effectiveness and superior efficiency of the proposed control scheme.