Improving Light-Load Performance in Bidirectional AC–DC DAB Using Asymmetric Semivariable Frequency Modulation

Abstract

This article proposes a triangular modulation (TRM) based on asymmetrical semivariable frequency (ASVF) for a bidirectional quasi single-stage ac–dc dual-active-bridge (DAB) converter, aimed at improving light-load performance. In the conventional TRM, the phase-shift ratio serves as the controlling variable responsible for regulating output power. However, at lower phase-shift ratios under light-load conditions measured at grid frequency, the rms current through the transformer significantly adds to device conduction loss relative to the processed output power. Along with retaining the benefits of conventional TRM, the proposed modulation technique leverages asymmetrical soft-switching capability by identifying the optimal phase-shift ratio and frequency through optimization. These optimal control variables lead to the reduced transformer rms current, improved total harmonics distortion (THD) and decreased current stress compared to conventional TRM for the same output power, leading to improved light-load efficiency. To validate the proposed ASVF TRM, a 500 W prototype was developed using the TMS320F28335 microcontroller for battery charging applications. Experimental results show that under light-load conditions, efficiency improves by 6.11% compared to the conventional TRM. In addition, THD improves from 5.8% to 4.2%. © 1986-2012 IEEE.