Inductively coupled systems used in applications such as RFID and wireless power often require high Q factor resonant transmitters to maximize the magnetic field and achieve high overall efficiency. However, these are sensitive to environmental detuning as well as component tolerances. Existing methods for accurate tuning require search algorithms, usually requiring the suspension of normal operation in order to calibrate the resonant inductor-capacitor circuit, thus reducing power throughput and increasing system complexity. We describe here how zero-voltage switched fractional capacitance techniques may be used to achieve continuous and real-time adaptive tuning of large-signal resonant inductor-capacitor circuits. Minimal additional circuitry is required and tuning is maintained without disrupting normal operation. Many variants are possible for the implementation of the system, and some tradeoffs relating to the available tuning range and operating voltages are analyzed for two alternative topologies. Experimental results are presented for a 125-kHz demonstration system.
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