Symmetric Unipolar Pulse Width Modulation for Dual Active Bridge Converters

This article proposes a novel Symmetric Unipolar Modulation (SUM) strategy for Dual Active Bridge (DAB) converters, which significantly enhances power density by improving transformer utilization factor and reducing the size of passive components. The proposed SUM technique effectively doubles the transformer frequency (fTX) to 2fsw while maintaining the switching frequency (fsw) of both the primary and secondary side bridge switches at fsw, unlike conventional methods where the semiconductor and transformer frequencies are equal. By pushing the transformer frequency to a higher level, the proposed technique minimizes passive component dimensions without introducing additional stress on the active or passive components. Moreover, the SUM approach reshapes the transformer core flux density from triangular to trapezoidal while reducing its peak and enabling further reduction in magnetic core size. It also reduces the voltage stress across the transformer by half compared to conventional modulation strategies, thereby improving the overall performance of the converter. Experimental validation using a 1-kW prototype confirms the effectiveness of the proposed modulation technique in achieving these benefits.