Reactance Decompensation Using PWM-Controlled Switched Capacitors for Maintaining Constant Power and Efficiency in S/S WPT Systems

The flexibility in coil placement and manufacturing tolerance of circuit components cause the self and mutual inductance of the coils to vary in wireless power transfer systems. This article proposes a control method for pulsewidth-modulation-controlled switched capacitors to maintain constant output power and transmission efficiency when such parameter variations occur simultaneously. Instead of compensating for the reactance produced by self-inductance variations, the proposed method deliberately introduces uncompensated reactance into the resonant circuit by adjusting the switched capacitors. The proposed method offers the advantages of constant output power, constant current amplitude, and soft-switching operation of the inverter, regardless of the variations in self and mutual inductance. This article also presents the practical implementation and model-based design of a closed-loop control scheme for the switched capacitors. A 1kW experiment is conducted to compare the performance of the proposed method with the conventional approach of compensating for the self-inductance variations with switched capacitors. The experimental results show that the proposed method maintains a nearly constant dc/dc efficiency between 93.1% and 93.8%, whereas the dc/dc efficiency of the conventional approach varies significantly and drops to 85.2% under the worst-case coupling condition.