Wireless inductive power transfer

1. A power transmitter comprising:

a variable resonance circuit, wherein the variable resonance circuit is configured to generate a power transfer signal in response to a drive signal, wherein the variable resonance circuit comprises a capacitive impedance and an inductive impedance, wherein the inductive impedance comprises a transmitter coil, wherein the transmitter coil is arranged to generate the power transfer signal, wherein the variable resonance circuit has a variable resonance frequency, wherein the variable resonance frequency is arranged by varying at least one of the capacitive impedance and the inductive impedance in response to a control signal;

a driver circuit, wherein the driver circuit is configured to generate the drive signal for the variable resonance circuit, wherein the drive signal has a variable drive frequency;

a frequency modulator, wherein the frequency modulator is configured to apply a frequency modulation to the drive signal by varying the variable drive frequency in response to data values, wherein the data values are transmitted to a power receiver; and

an adapter circuit, wherein the adapter circuit is configured to generate the control signal in response to the data values such that the variable resonance frequency follows variations in the variable drive frequency.

2. The power transmitter of claim 1 , wherein the frequency modulator is arranged to select the variable drive frequency from a set of frequencies in response to the data values, wherein the adapter circuit is arranged to generate the control signal to adapt at least one of the capacitive impedance and the inductive impedance between a set of impedances, wherein the set of impedances corresponds to variable resonance frequencies, wherein the variable resonance frequencies correspond to the set of frequencies.

3. The power transmitter of claim 1 , wherein the adapter circuit is arranged to generate the control signal to have transitions, wherein the transitions are synchronized to transitions of the variable drive frequency between at least two of a set of frequencies.

4. The power transmitter of claim 1 , wherein the adapter circuit is arranged to generate the control signal to have transitions, wherein the transitions are synchronized to at least one data symbol time(s) of the frequency modulation.

5. The power transmitter of claim 1 , wherein the variable resonance circuit comprises a switch, wherein the switch is arranged to limit a current flow to a reactive component of at least one of the capacitive impedance and the inductive impedance in response to the control signal.

6. The power transmitter of claim 5 , wherein the control signal is a binary control signal, wherein the switch is arranged in an open state in response to a first value of the binary control signal and in a closed state in response to a second value of the binary control signal.

7. The power transmitter of claim 5 , wherein the adapter circuit is arranged to change the control signal only when the frequency modulation causes the variable drive frequency to change.

8. The power transmitter of claim 1 , wherein the capacitive impedance comprises a switch, wherein the switch is arranged to short circuit and/or disconnect a portion of the capacitive impedance in response to the control signal.

9. The power transmitter of claim 1 , wherein the inductive impedance comprises a switch, wherein the switch is arranged to short circuit and/or disconnect a portion of the inductive impedance in response to the control signal.

10. The power transmitter of claim 1 , wherein the variable resonance circuit comprises a resonance modification circuit, wherein the resonance modification circuit is configured to control the variable resonance frequency by slowing a state change for at least one of the capacitive impedance and the inductive impedance for a fractional time interval of at least a portion of cycles of the drive signal, wherein the resonance modification circuit is arranged to adapt the duration of the fractional time interval in response to the control signal.

11. The power transmitter of claim 10 , wherein the resonance modification circuit is arranged to determine one of a start time and an end time of the fractional time interval in response to a signal of the variable resonance circuit, wherein the resonance modification circuit is arranged to determine the other of the start time and the end time in response to the control signal.

12. The power transmitter of claim 1 , wherein the frequency modulator is arranged to cause frequency transitions in the variable drive frequency, wherein the adapter circuit is arranged to generate the control signal to cause transitions in the variable resonance frequency, wherein the transitions are synchronized to the frequency transitions of the variable drive frequency.

13. The power transmitter of claim 1 , wherein the frequency modulator is arranged to cause frequency transitions during a data symbol, wherein the circuit adapter is arranged to generate the control signal to cause transitions in the variable resonance frequency during the data symbol.

14. A wireless power transfer system comprising a power transmitter and a power receiver, the power transmitter comprising:

a variable resonance circuit, wherein the variable resonance circuit is configured to generate a power transfer signal in response to a drive signal, wherein the variable resonance circuit comprises a capacitive impedance and an inductive impedance, wherein the inductive impedance comprises a transmitter coil, wherein the transmitter coil is arranged to generate the power transfer signal, wherein the variable resonance circuit has a variable resonance frequency, wherein the variable resonance frequency is arranged by varying at least one of the capacitive impedance and the inductive impedance in response to a control signal;

a driver circuit, wherein the driver circuit is configured to generate the drive signal for the variable resonance circuit, wherein the drive signal has a variable drive frequency;

a frequency modulator, wherein the frequency modulator is configured to apply a frequency modulation to the drive signal by varying the variable drive frequency in response to data values, wherein the data values are transmitted to the power receiver; and

an adapter circuit, wherein the adapter circuit is configured to generate the control signal in response to the data values such that the variable resonance frequency follows variations in the variable drive frequency.

15. A method of operation of a power transmitter the power transmitter comprising a variable resonance circuit, wherein the variable resonance circuit is configured to generate a power transfer signal in response to a drive signal, wherein the variable resonance circuit comprises a capacitive impedance and an inductive impedance, wherein the inductive impedance comprises a transmitter coil, wherein the transmitter coil is arranged to generate the power transfer signal, wherein the variable resonance circuit has a variable resonance frequency, wherein the variable resonance frequency is arranged by varying at least one of the capacitive impedance and the inductive impedance in response to a control signal; the method comprising:

generating the drive signal for the variable resonance circuit, wherein the drive signal has a variable drive frequency;

applying a frequency modulation to the drive signal by varying the variable drive frequency in response to data values, wherein the data values are transmitted to a power receiver; and

generating the control signal in response to the data values such that the variable resonance frequency follows variations in the variable drive frequency resulting from the frequency modulation of the drive signal.

16. The method of claim 15 , further comprising:

selecting the variable drive frequency from a set of frequencies in response to the data values; and

generating the control signal to adapt at least one of the capacitive impedance and the inductive impedance between a set of impedances, wherein the set of impedances corresponds to at least one variable resonance frequency, wherein the variable resonance frequencies correspond to the set of frequencies.

17. The method of claim 15 , further comprising:

generating the control signal to have transitions, wherein the transitions are synchronized to transitions of the variable drive frequency between at least two frequencies.

18. The method of claim 15 , further comprising:

generating the control signal to have transitions, wherein the transitions are synchronized to at least one data symbol time(s) of the frequency modulation.

19. The method of claim 15 , further comprising:

limiting a current flow to a reactive component of at least one of the capacitive impedance and the inductive impedance in response to the control signal.

20. A computer program stored on a non-transitory medium, wherein the computer program when executed on a processor performs the method as claimed in claim 15 .