Improved Control Methods of Bidirectional Single-Phase AC-DC DAB Converter for Light EV Charging Applications

Abstract

Dual Active Bridge (DAB) based unfolder is a promising converter for Light Electric Vehicle (LEV) battery charging applications in single phase ac systems owing to several advantages such as high efficiency, high power density and galvanic isolation. For single phase applications, however, a unique challenge appears in the DAB operation, wherein the RMS value of the DAB inductor current increases much above the RMS grid current, often 1.45 times or more. This increases the conduction losses and negatively impacts the system efficiency. Additionally, distortions near zero crossing are often encountered in the converter’s ac side voltage profile, adversely affecting the grid current THD. A novel current minimization technique is proposed in this paper where the DAB inductor RMS current is reduced significantly, to only 1.15 times the output grid current. The closed loop control of the DAB is modified to accurately control the output capacitor voltage using a closed-form solution, resulting in a smooth zero transition. Further, a grid distortion mitigation technique is incorporated to improve the output grid current THD. The proposed work is verified using MATLAB Simulink software and experimentally validated using a lab prototype with a 230V, 50Hz single phase ac grid.