High-Efficiency Variable Turns-Ratio Semi-Dual Active Bridge Converter for a DC Fast Charging Station With Energy Storage

In electric vehicle fast charging applications, the isolated dc/dc converter charging a battery electric vehicle from a battery energy storage system should provide high efficiency over a wide voltage gain. The semi-Dual Active Bridge (semi-DAB) converter is an excellent choice for this unidirectional application. However, achieving the high efficiency of these converters is challenging when the input and output voltages vary over a wide range. Thus, a novel two-winding semi-DAB converter is proposed in this article to improve the overall efficiency with a variable turns-ratio, which minimizes the converter conversion effort, so that the required effective voltage gain is within a reasonable range even as input and output voltages vary widely. Furthermore, a control law is also proposed to determine the operational structure of the converter, i.e., the turns-ratio and the bridge configuration, which reduces the converter peak and rms currents. A 550-V, 10-kW semi-DAB converter is built on a four-layer PCB to test the proposed converter and control. The proposed converter with control provides efficiency improvement of up to 3.5% in comparison to the standard structure and control, and has the most improvement during the common fast charging scenario with high input and low output voltages. The peak efficiency achieved is 98.5%.