Bidirectional ZVS Operation of All Switches for a DAB Converter Over a Full Range of Loads With Optimized Current Stress

All-switches zero voltage switching (ZVS) and lower current stress are the most important challenges for improving the efficiency of dual-active-bridge (DAB) converters operating in the bidirectional full-load range and wide voltage range applications. Triple-phase-shift (TPS) has three control degrees of freedom and it is a more flexible way to extend the ZVS range in all kinds of operating conditions. In order to improve the efficiency of DAB converters, a new optimal current stress in TPS (OCS-TPS) control is proposed to minimize the current stress as well as achieve all-switches ZVS within the bidirectional full-load range and wide voltage range applications. Firstly, seven specific switching modes satisfying the ZVS conditions under step-up and step-down are selected from 12 TPS switching modes. Secondly, the issue of current stress minimization is turned into an extreme optimization problem with equality constraints of desired transmission power and inequality constraints of ZVS conditions in selected switching modes. The Karush-Kuhn-Tucker (KKT) algorithm is employed to solve this optimization issue. Then two switching modes are chosen to form the high-load range ZVS and low-load range partial-switches ZCS/ZVS in the case of step-down and step-up operations respectively. Therefore, the phase-shift ratios of selected switching modes in the low-load range are further optimized by adding an offset value to achieve all-switches ZVS. Finally, the effectiveness of the proposed strategy is verified by the experimental results.