Variable reactive element in a resonant circuit

A current fed push-pull resonant converter including an input for connection to a substantially direct current electrical supply, a resonant circuit including a primary conductive path of an ICPT system, first switching apparatus to selectively provide current from the input to the resonant circuit, second switching apparatus associated with a reactive element of the resonant circuit, a switch controller operable to control the second switching apparatus to vary the effective reactance of the reactive element, and a phase sensor to sense the phase of a voltage or current in the resonant circuit whereby the controller may actuate the second switching apparatus to allow the reactive element to be electrically connected to or disconnected from the resonant circuit in each cycle of the voltage or current dependant on the sensed phase to vary the effective reactance of the reactive element and thereby vary the resonant frequency of the resonant circuit toward a selected frequency.

An ICPT system including:

• a. A power supply comprising a resonant converter having a resonant circuit including a primary conductive path of an ICPT system, first switching apparatus to selectively provide current from an input to the resonant circuit, second switching apparatus associated with a reactive element of the resonant circuit, and a switch controller operable to control the second switching apparatus to vary the effective reactance of the reactive element, and a phase sensor to sense the phase of a voltage or current in the resonant circuit whereby the controller may actuate the second switching apparatus to allow the reactive element to be electrically connected to or disconnected from the resonant circuit in each cycle of the voltage or current dependant on the sensed phase to vary the effective reactance of the reactive element and thereby vary the resonant frequency of the resonant circuit toward a selected frequency.
• b. One or more secondary pick-ups, each pick-up having a pick resonant circuit comprising a pick-up coil and a tuning capacitor whereby power from the primary conductive path may be transferred to the pick-up by mutual induction between the primary conductive path and the coil.

A method of frequency stabilisation for a resonant converter having a resonant circuit comprising an inductive reactive element and a capacitive reactive element, the method including the steps of sensing the phase of a voltage or current in the resonant circuit and selectively switching one of the reactive elements into or out of the resonant circuit in each cycle of the voltage or current dependant on the sensed phase to alter the effective inductance or capacitance of the reactive element to thereby control the resonant frequency of the resonant circuit by varying the resonant frequency toward a selected frequency.