Inductive power receiver

1. An inductive power receiver comprising:

a power receiving coil;

a switch mode regulator that varies at least one of a switching duty cycle or a switching frequency to regulate at least one of an output voltage or an output current;

a linear regulator that regulates at least one of an output voltage or an output current; and

a modulator configured to communicate to an inductive power transmitter via the power receiving coil during one or more communicating periods and not communicate during one or more non-communicating periods,

wherein:

the switch mode regulator operates to control power flow from the power receiving coil during the non-communicating period and does not operate to control power flow from the power receiving coil during the communicating period; and

the linear regulator operates to control power flow from the power receiving coil during the communicating period and does not operate to control power flow from the power receiving coil during the non-communicating period.

2. The receiver of claim 1 further comprising a bypass switch configured to bypass the switch mode regulator.

3. The receiver of claim 1 wherein the switch mode regulator operates with a substantially fixed control effort during the communicating period.

4. The receiver of claim 3 wherein the substantially fixed control effort is determined based on a duty cycle of the switch mode regulator in the non-communicating period.

5. The receiver of claim 1 wherein the switch mode regulator operates with a substantially 100% or substantially 0% duty cycle during the communicating period.

6. The receiver of claim 1 wherein the switch mode regulator operates with a maximum rate of change of a duty cycle or a range of duty cycle values during the communicating period.

7. The receiver of claim 6 wherein the maximum rate is 1% per millisecond.

8. The receiver of claim 1 wherein the linear regulator is a low dropout regulator.

9. The receiver of claim 1 wherein the linear regulator is configured to operate substantially as a closed switch during the non-communicating period.

10. The receiver of claim 1 further comprising a load disconnect switch.

11. The receiver of claim 1 wherein the switch mode regulator is a coupled coil regulator.

12. The receiver of claim 1 wherein the switch mode regulator is configured to provide an AC voltage in series with the power receiving coil to regulate AC power from the receiving coil.

13. The receiver of claim 1 wherein the modulator is configured to use amplitude modulation.

14. The receiver of claim 1 wherein components in the modulator are shared with components in the switch mode regulator.

15. The receiver of claim 1 wherein a control effort of the switch mode regulator is adjusted during the communicating period.

16. The receiver of claim 15 , wherein the control effort is responsive to a regulated output of the receiver processed by a feedback path, and wherein during the communicating period the feedback path is modified.

17. The receiver of claim 15 , wherein an error signal is determined by a comparison of an output voltage of the switch mode regulator and a set-point voltage, the error signal being used to control a duty cycle of the switch mode regulator, and wherein the error signal is increased or decreased during the communicating period.

18. The receiver of claim 15 , further comprising a rectifier having a input arranged to receive a regulated output dependent on the switch mode regulator and an output to provide a rectified voltage, wherein an error signal is determined by a comparison of the rectified voltage and a set-point voltage, the error signal being used to control the switch mode regulator, and wherein the error signal is increased or decreased during the communicating period.

19. The inductive power receiver of claim 1 , wherein the switch mode regulator is selected from the group consisting of: a buck regulator, a boost regulator, a buck-boost regulator, and a coupled coil regulator.