High-Efficiency and Full Range ZVS Single-Stage Semi-DAB AC-DC Converter with Improved Modulation and Variable Frequency Control

In this article, an improved modulation and variable frequency control strategy for a single-stage high-frequency isolated semi-dual-active-bridge-based ac-dc converter is proposed, which can achieve full operating range (the entire ac line cycle and full-load range) zero-voltage switching (ZVS) and high efficiency. Compared with the basic variable frequency control, an improved variable frequency strategy can reduce the envelope of leakage inductor peak current and reduce the dependence and sensitivity to the load power. More accurate ZVS criteria in view of the magnetizing inductor Lm are proposed. Seven operating modes are adopted and the transition among them is seamless. Three control variables are used in every mode. Wide voltage conversion gain and high-quality grid-side ac current can be obtained without mandatory current-tracking loops. In addition, detailed comparisons among different control strategies and different mode operating regions are analyzed. The design of the transformer and inductor is not affected at all despite the switching frequency varies. A 250-W experimental prototype was built to verify the effectiveness of the proposed improved variable frequency control strategy.