Wireless inductive power transfer

A wireless power transfer system comprising a power transmitter and a power receiver, the power transmitter being arrange to wirelessly provide power to the power receiver via an inductive signal;

• the power transmitter comprising:
• a variable resonance circuit for generating the inductive signal in response to a drive signal, the variable resonance circuit comprising an inductive impedance and a capacitive impedance, the resonance circuit having a variable resonance frequency and the inductive impedance comprising a transmitter coil arranged to generate the inductive signal;
• a driver for generating the drive signal for the variable resonance circuit;
• a modulator for amplitude modulating the inductive signal for transmitting data values to the power receiver by varying the variable resonance frequency in response to the data values:
• a receiver coil for receiving the inductive signal;
• a demodulator for demodulating amplitude modulation of the inductive signal;
• and
• a first power extractor for extracting power from the inductive signal and for powering at least part of the power receiver;
• wherein the variable resonance circuit comprises a resonance modification circuit for controlling 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 each of at least some cycles of the drive signal, and the modulator is arranged to adapt the duration of the fractional time interval in response to the data values.

A method of operation for a power transmitter wirelessly providing power to a power receiver via an inductive signal, the power transmitter comprising an variable resonance circuit comprising an inductive impedance and a capacitive impedance, the resonance circuit having a variable resonance frequency and the inductive impedance comprising a transmitter coil;

• the method comprising:
• the variable resonance circuit generating the inductive signal in response to a drive signal;
• generating the drive signal for the variable resonance circuit;
• amplitude modulating the inductive signal for transmitting data values to the power receiver by varying the variable resonance frequency in response to the data values;
• wherein the variable resonance circuit comprises a resonance modification circuit controlling the variable resonance frequency by slowing a state change for at least one of the capacitive impedance and the inductive impedance for each of a fractional time interval of at least some cycles of the drive signal, and the amplitude modulating comprises adapting the duration of the fractional time interval in response to the data values.

A method of operation for a wireless power transfer system comprising a power transmitter and a power receiver, the power transmitter being arranged to wirelessly provide power to the power receiver via an inductive signal and comprising an variable resonance circuit comprising an inductive impedance and a capacitive impedance, the resonance circuit having a variable resonance frequency and the inductive impedance comprising a transmitter coil;

• the method comprising the power transmitter performing the steps of:
• the variable resonance circuit generating the inductive signal in response to a drive signal;
• generating the drive signal for the variable resonance circuit;
• amplitude modulating the inductive signal for transmitting data values to the power receiver by varying the variable resonance frequency in response to the data values;
• and the power receiver performing the steps of:
• demodulating amplitude modulation of the inductive signal;
• and
• extracting power from the inductive signal and for powering at least part of the power receiver;
• wherein the variable resonance circuit comprises a resonance modification circuit controlling 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 each of at least some cycles of the drive signal, and the amplitude modulating comprises adapting the duration of the fractional time interval in response to the data values.