Transmitter Coil Resonant Frequency Selection for Wireless Power Transfer

This paper proposes the optimum resonance frequency of a transmitter (Tx) coil driven by a series-resonant inverter in wireless power transfer. While typical wireless power systems include a dc-dc buck converter at the receiver (Rx) side to output a constant voltage, it is revealed that the Tx design affects the operation of an Rx buck converter and Tx-to-Rx transfer efficiency. Specifically, it is found that if the resonant frequency of the Tx coil is too close to inverter switching frequency, the Rx buck converter is not allowed to operate at high duty cycle, which degrades the Tx-to-Rx transfer efficiency. Therefore, the Tx resonance frequency should be sufficiently lower than the inverter switching frequency in order to allow high-duty operation of the Rx buck converter. This is to guarantee high Q-factor of the Rx coil and thus the Tx-to-Rx transfer efficiency. The required Tx resonant frequency is proposed as a function of magnetic coupling, Rx coil property, Rx buck duty cycle, and Rx load condition. Simulation and experimental results with a constant-output-voltage system support the discussions.