Optimization of the Compensation Capacitors for Megahertz Wireless Power Transfer Systems

A wireless power transfer system can increase its system frequency to several Megahertz for large spatial freedom. A MHz system usually uses the same compensation as those kHz systems. Traditionally, the rectifier is modeled as a pure resistive load for the coupling coils when designing the compensation capacitors. However, the rectifier input reactance cannot be ignored for MHz systems. This reactance can affect the system resonance and is usually not considered when designing the compensation capacitors. This paper analyzes the rectifier input impedance and discusses its undesirable effects. A non-zero phase is shown to exist seeing into the coupling coils for MHz WPT system. In order to limit the phase, the optimal compensation capacitors are discussed and found. Finally, a 6.78 MHz system is used to verify the proposed optimal compensation method. It shows the phase variation can be limited from the original range [15 degrees - 35 degrees] to the optimal range [-5 degrees - 5 degrees].