Wireless inductive power transfer

A wireless power transfer system comprising:

• a power receiver and a power transmitter,
• wherein the power transmitter comprises:
• 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 a power receiver;
• a driver circuit, wherein the driver circuit is configured to generate a drive signal for the resonance circuit,
• a message receiver, wherein the message receiver is configured to receive at least one message from the power receiver,
• a power loop control circuit,
• wherein the power loop control circuit is configured to change a power of the drive signal in response to the at least one message,
• wherein the changing is subject to at least one of a current of the resonance circuit being below maximum limit, a voltage of the resonance circuit being below a maximum limit, and a power of the drive signal being below a maximum limit;
• and
• an adapter circuit,
• wherein the adaptor circuit is configured to change the maximum limit in response to a load indication,
• wherein the load indication is determined based on the change in power of the drive signal,
• the power receiver comprising:
• a data generator,
• wherein the data generator is configured to generate load data,
• wherein the load data reflects a loading of the power receiver;
• and
• a communication transmitter,
• wherein the communication transmitter is configured to transmit at least one message to the power transmitter,
• wherein the at least one message comprises the load data.

A method of operating a power transmitter, wherein the power transmitter comprises a resonance circuit, wherein the resonance circuit has a capacitive impedance and an inductive impedance, wherein the inductive impedance comprises a transmitter coil, wherein the transmitter coils is configured to generate a power transfer signal, wherein the power transfer signal is configured to transferring power to a power receiver, the method comprising:

• generating a drive signal for the resonance circuit;
• receiving at least one message from the power receiver;
• adapting a power of the drive signal in response to at least one message received from the power receiver, wherein the adapting is subject to a constraint of a current of the resonance circuit;
• determining a load indication, wherein the load indication is determined based on the change in power of the drive signal due to the impact of a load on the resonant circuit;
• and
• changing the maximum limit in response to the load indication.