A DC-Voltage-Controlled Variable Capacitor for Stabilizing the ZVS Frequency of a Resonant Converter for Wireless Power Transfer

Varactors are often used in phase-locked-loop (PLL) circuits for dynamic frequency control. However, the voltage and current ratings of varactors are too low to be used in most power electronic circuits. This paper proposes a transistor-controlled variable capacitor (TCVC), which functions similar to varactors in that its equivalent capacitance can be controlled by a dc voltage. However, TCVC can handle high voltages and currents so that it can be used in dc-ac power converters of wireless power transfer (WPT) systems such as an autonomous push-pull resonant converter to adjust its zero voltage switching (ZVS) frequency so that the operating frequency of the system can be stabilized to simplify the circuit and EMI filter design particularly for WPT systems with multiple power pickups while maintaining soft-switching operation of the converter against magnetic coupling and load variations. The relationship between the equivalent capacitance of the TCVC and the dc control voltage is developed by theoretical analysis and verified by experimental results. A prototype circuit is built with a PLL controller to demonstrate that the soft-switching condition of the converter is maintained when the operating frequency is locked in at 1.65 MHz under load and magnetic coupling variations.