In this article, a dual active bridge (DAB) based single-stage isolated ac-dc converter is studied. By analyzing the root mean square value of the leakage inductor current, the high-frequency transformer turns ratio is optimized for higher conversion efficiency. Three operating modes of the DAB converter are utilized in the control strategy. The current and the voltage waveforms of these operating modes are analyzed. The control variables, including the phase-shift ratio, the primary side duty cycle d_1, and the secondary side duty cycle d_2 are combined based on instantaneous calculation according to achieve boundary zero voltage switching (ZVS). The mode transition is seamless while the switching frequency is constant. Furthermore, the magnetizing inductor of the transformer is designed to help the secondary side switches to achieve ZVS. ZVS conditions and parameter design are given in this article. Besides, a 1 kW prototype was fabricated to verify the effectiveness of the design and the control strategy.
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