Communicationless Reactance Compensation Using PWM-Controlled Switched Capacitors for Wireless Power Transfer

Self-inductance variations caused by component tolerance and aging decrease the efficiency of wireless power transfer (WPT) systems. This article deals with this issue by compensating for the self-inductance variations using pulsewidth-modulation-controlled switched capacitors. Previous methods relied on wireless communication between the transmitter (Tx) and receiver (Rx) to control the variable capacitors, which is not a feasible approach when considering practical constraints. This article proposes a control scheme in which the switched capacitors are controlled simultaneously by separate closed loops on the Tx and Rx sides, thus eliminating wireless communication. The control scheme is based on a unique condition for perfect resonance derived by focusing on the output power of the WPT circuit. This article also points out the necessity to compensate for the self-inductance variations of both the Tx and Rx coils to achieve maximum ac/ac efficiency in battery charging applications. Furthermore, the proposed control scheme incorporates the soft-switching condition of the inverter to improve the dc/dc efficiency. Experimental results of a 1-kW prototype show that the switched capacitors can flexibly adapt to the self-inductance variations and improve the ac/ac efficiency and dc/dc efficiency by up to 3.04% and 8.63%, respectively.