Coils' Current Distortion Due to Variable Series Compensation Capacitance in EV Wireless Charging for a Constant Optimum Load

When considering EV wireless charging that uses inductive power transfer with magnetic resonance, the coils' current distortion must be minimized to guarantee compliance with the electromagnetic compatibility limits on the radiated magnetic field set by the relevant industrial standards. This paper analyzes the current distortion caused by switch-controlled capacitors (SCCs) used as series compensation to achieve constant optimum load (COL) matching at different coils' alignments. First, the proposed COL charging method is explained where the SCCs have either the half-wave or the full-wave modulation. Their impact on the measured coils' current distortion has been analyzed up to 30MHz by computing the fast Fourier transform (FFT). Additionally, the currents' FFT from the half-wave modulation has been compared to those resulting from the conventional series-series compensation with fixed capacitance. The SCCs using the half-wave modulation result in the highest total-lumped distortion. However, the individual amplitudes corresponding to the critical frequencies of the radiated magnetic field's limit from SAE J2954 are comparable or lower than those resulting from the other implementations. Finally, the radiated magnetic field resulting from each strategy has been evaluated using the finite element method. All results are well within the SAE J2954 recommended limits at 10 m. Moreover, a minimum distance of 25 cm from the outer sides of the coupled coils ensures a safe exposure to both the general public and implanted medical devices according to the ICNIRP reference levels.