Patent ID: 8587154

Claim:
A primary unit for use in an inductive power transfer system, the system including said primary unit and a secondary device separable from said primary unit, the system of said primary unit and the secondary device having a resonant frequency and being operable to transfer power inductively to the secondary device, said primary unit comprising: a power transfer surface capable of enabling inductive coupling with said secondary device in a plurality of different working positions; a plurality of field generators arranged in an array, each of said field generators operable to generate an electromagnetic field for transferring power inductively to the secondary device, each of said field generators having an axis substantially perpendicular to said power transfer surface and being located at a different position relative to said power transfer surface such that said axes of said field generators are spaced apart with respect to each other; and driver circuitry operable to provide power from a power source to said field generators, said driver circuitry including: switching circuitry to selectively activate at least one of said plurality of field generators by selectively powering said at least one field generator in order to generate said electromagnetic field, wherein said driver circuitry selectively activates said at least one field generator to transfer power inductively to the secondary device, wherein said driver circuitry is operable to drive one or more of said field generators to provide power to the secondary device in at least one of said plurality of different working positions; variable impedance circuitry operably coupled between said power source and said plurality of field generators such that said variable impedance circuitry is shared by said plurality of field generators, said variable impedance circuitry to be adjusted to affect the resonant frequency of the system based on which of said plurality of field generators are selectively activated by said switching circuitry to generate said electromagnetic field, wherein said variable impedance circuitry is adjusted in operation to a predetermined impedance based on which of said plurality of field generators are selectively activated; and wherein said variable impedance circuitry includes a plurality of capacitors arranged in a switched network shared by the plurality of field generators, wherein one or more switches of said switched network are selectively controlled to select an impedance of said variable impedance circuitry.