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Analytical Expressions of Differential-mode Harmonics in Loosely-coupled Series-resonant Wireless Power Transfer System

The fundamental frequency approximation (FFA) for power transfer is commonly used to linearize a wireless power transfer (WPT) system which is equipped with non-linear switching converters such as a Class D inverter and rectifier. However, the FFA for power transfer disables us to analyze harmonics in a WPT system. The system characteristics only at the fundamental frequency can be analyzed with the FFA.In this paper, a new FFA for induced voltages is proposed so that analytical expressions of differential-mode voltage and current harmonics in a loosely-coupled series-resonant WPT system can be derived. Fourier series representations of the coil currents, inverter-output voltage, and rectifier-input voltage are derived and verified experimentally.