Wireless inductive power transfer

A wireless power transfer system comprising a power transmitter for inductively transferring power to a power receiver, the power transmitter comprising:

• a resonance circuit comprising a capacitive impedance and an inductive impedance,
• wherein the inductive impedance comprises a transmitter coil,
• wherein the transmitter coil is configured to generate a power transfer signal,
• wherein the power transfer signal is configures to wirelessly transfer transferring power to the power receiver;
• a sampler, wherein the sampler is configured to generate samples by sampling at sample times at least one of a current through the transmitter coil and a voltage over the transmitter coil;
• a message receiver,
• wherein the message receiver is configured to receive messages load modulated onto the power transfer signal by the power receiver,
• wherein the message receiver is configured to detect the modulation in response to the samples;
• a driver, wherein the driver is configured to generate a drive signal for the resonance circuit;
• a resonance modification circuit,
• wherein the resonance modification circuit is configured to reduce the resonance frequency of the resonance circuit by slowing a state change for at least one of the capacitive impedance and the inductive impedance in a fractional time interval of each of at least a plurality of cycles of the drive signal,
• wherein the state change is a change of at least one of a voltage of the capacitive impedance and a current of the inductive impedance,
• wherein the resonance modification circuit is configured to begin slowing the state change at a start-time of the fractional time interval and to stop slowing the state change at an end-time of the fractional time interval;
• and
• a sample time controller, wherein the sample time controller is configured to control the sample times in response to at least one of the start-times and the end-times of the fractional time intervals.

A method of operation for a power transmitter inductively transferring power to a power receiver, the power transmitter comprising a resonance circuit, wherein the resonance circuit comprises a capacitive impedance and an inductive impedance, wherein the inductive impedance comprises a transmitter coil, wherein the transmitter coil is configured to generate a power transfer signal, wherein the power transfer signal is configured to wirelessly transfer power to the power receiver;

• the method comprising:
• generating samples by sampling at least one of a current through the transmitter coil and a voltage over the transmitter coil at sample times;
• receiving messages load modulated onto the power transfer signal by the power receiver in response to the samples;
• generating a drive signal for the resonance circuit;
• reducing the resonance frequency of the resonance circuit by slowing a state change for at least one of the capacitive impedance and the inductive impedance in a fractional time interval of each of at least a plurality of cycles of the drive signal,
• wherein the state change is a change of at least one of a voltage of the capacitive impedance and a current of the inductive impedance,
• wherein the slowing of the state change is begun at a start-time of the fractional time interval and stopped at an end-time of the fractional time interval;
• and
• controlling the sample times in response to at least one of the start-times and the end-times of the fractional time intervals.