A pulse density modulation method for ZVS full-bridge converters in wireless power transfer systems

This paper aims to combine two advanced techniques, namely, pulse density modulation (PDM) and zero-voltage-switching (ZVS) class-D converters, for wireless power transfer (WPT) systems. PDM enables voltage regulation and efficiency maximization without additional dc/dc stages. ZVS class-D converters ensure soft switching for various coupling and load conditions. However, these two techniques cannot be directly combined because the ZVS relies on the symmetric and continuous switching operation, which would be disrupted by PDM. This paper modifies the topology of the ZVS class-D converter and proposes new operating principles and modulation logics for the modified converter, which is called PDM ZVS full-bridge converter, so that the two techniques are successfully combined. The effectiveness of the proposed method was verified in experiment. The overall efficiency of the experimental WPT system is 94 ~ 72 % when the power transfer distance was 0.1 ~ 0.4 m, while the diameter of the coils was 0.3 m.