An inductive power
transfer system comprises a
transmitter coil TX and a
receiver coil RX spaced from the
transmitter coil. A
transmitter circuit comprises the
transmitter coil and is in the form of a
Class E amplifier with a first
inductor Uchoke and a
transistor in series between the terminals of a power supply, a first transmitter
capacitor Cpar in parallel with the
transistor between the first
inductor and a power supply terminal, a primary tank circuit in parallel with the first transmitter
capacitor, the primary tank circuit comprising the
transmitter coil and a second transmitter
capacitor Cres arranged in parallel with the
transmitter coil, and a third transmitter capacitor Cser in series with the first
inductor between the first transmitter capacitor and the primary tank circuit. The
transistor is arranged to switch at a first frequency ωd and the
capacitance of the second transmitter capacitor is selected such that the resonant frequency ωOTX of the primary tank circuit is greater than the first frequency. The
receiver circuit comprises a Class E
rectifier having a first
receiver capacitor CL arranged in parallel with a load RL and a secondary tank circuit in parallel with the first receiver capacitor. The secondary tank circuit comprises the
receiver coil and a second receiver capacitor Cres arranged in parallel or series with the
receiver coil. A first
diode Dr2 is provided between the secondary tank circuit and the first receiver capacitor. The
capacitance of the second receiver capacitor is selected such that the resonant frequency ωoRX of the secondary tank circuit differs from the first frequency, so that the secondary tank circuit operates in semi-
resonance and maintains some reactive impedance. The transmitter circuit is configured to vary the first frequency, in order to achieve a desired impedance of the primary tank circuit.