A Low-Subharmonic, Full-Range, and Rapid Pulse Density Modulation Strategy for ZVS Full-Bridge Converters

The pulse-density-modulation (PDM) zero-voltage-switching (ZVS) full-bridge converter is a promising power converter for wireless power transfer (WPT) systems. The converter has the advantages of both direct conversion ratio control and load-independent soft switching. These advantages reduce the overall system complexity and power loss. However, the converter suffers from the limitations of large low-frequency subharmonics, a narrowed modulation range, and a large modulation delay. These limitations are caused by the existing PDM strategy, which was designed to generate a symmetric ZVS current to ensure the ideal ZVS for minimizing the switching loss. This paper finds that even with an asymmetric ZVS current, the ideal ZVS can still be ensured by the negative feedback effect of the dead-time voltage. Based on this finding, a PDM strategy that allows asymmetric ZVS currents is proposed to overcome the aforementioned limitations. In experiments, a ZVS full-bridge converter was modulated by the existing and the proposed PDM strategies, respectively. The converter performances and responses were compared and the results showed that the proposed PDM strategy can overcome the limitations while achieving ZVS. The proposed PDM strategy was also tested in a WPT system for verification.