Analysis and Optimized Design of Compensation Capacitors for a Megahertz WPT System Using Full-Bridge Rectifier

The spatial freedom of wireless power transfer (WPT) systems can be improved using a high operating frequency such as several megahertz (MHz). In the conventional compensations the load of the coupling coils is usually assumed to be pure resistive. However, in MHz WPT systems this assumption is not accurate anymore due to the nonneglectable rectifier input reactance. This paper discusses the impedance characteristics of the full-bridge rectifier at MHz and their influence under the series-series, parallel-series, series-parallel, and parallel-parallel compensation topologies. An undesirable nonzero phase (i.e., none unity power factor) is shown to exist at the primary input port, which leads to decreased power transfer capability. In order to minimize this negative effect, the compensation capacitors are optimally designed, and the series-series topology is found to have the smallest phase under load and coupling variations. Finally, an experimental 6.78 MHz system is built up to verify the optimized design of the compensation capacitors. The results show that the average nonzero phase is effectively reduced together with the improved power factor from 0.916 to 0.982.