Inductive power supply

An inductive power transfer system for transferring power to at least one secondary device comprising:

• a primary unit including a power transfer surface and a plurality of field generators arranged in an array, each of said field generators operable to generate an electromagnetic field for inductive power transfer, said primary unit including switching circuitry to selectively activate at least one of said plurality of field generators by selectively providing power from a power source to said at least one field generator, 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, wherein said field generators are positioned to provide power to said at least one secondary device at any location in proximity to said power transfer surface;
• said at least one secondary device separable from said primary unit, said at least one secondary device including a power receiver operable to inductively couple with said field generators for inductive power transfer;
• wherein said system of said primary unit and said at least one secondary device has a resonant frequency;
• 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 said resonant frequency of said inductive power transfer 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;
• 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 switch network are selectively controlled to select an impedance of said variable impedance circuitry.

A method for use in an inductive power transfer system, the system comprising a primary unit and a secondary device separable from the primary unit, the primary unit including a power transfer surface, drive circuitry, and a plurality of field generators arranged in an array and each being operable to generate an electromagnetic field to transfer power in proximity to the power transfer surface, said method comprising:

• selectively activating a number of the field generators to transfer power inductively to the secondary device to provide power to the secondary device at any location in proximity to the power transfer surface, wherein each of the field generators has an axis substantially perpendicular to the power transfer surface and is located at a different position relative to said power transfer surface such that the axes of the field generators are spaced apart with respect to each other;
• controlling the drive circuitry to supply power to the number of active field generators in order to form an inductive coupling between the active field generators and the secondary device, wherein the inductive coupling has a resonant frequency;
• adjusting an impedance of the drive circuitry to maintain inductive power transfer at or near the resonant frequency based on which of said field generators are selectively activated to transfer power, wherein the impedance circuitry is adjusted in operation to a predetermined impedance based on which of the plurality of field generators are selectively activated;
• wherein the variable impedance circuitry is operably coupled between a power source and the plurality of field generators such that the variable impedance circuitry is shared by the plurality of field generators, wherein the variable impedance circuitry includes a plurality of capacitors arranged in a switched network shared by the plurality of field generators;
• and
• selectively controlling one or more switches of the switch network to select an impedance of said variable impedance circuitry.

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 and disposed adjacent said power transfer surface, 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;
• determining circuitry to determine a position and orientation of the secondary device relative to said power transfer surface;
• driver circuitry operable to provide power from a power source to said field generators as a function of said determined position and said determined orientation, 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 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, wherein said variable impedance circuitry is adjusted in operation to a predetermined impedance based on which of said plurality of field generators are selectively powered;
• 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.