Power receiving unit reflected reactance and tuning methods

1. An apparatus for providing a desired output from a power receiving unit in a wireless power transfer system, comprising:

a resonant network including a variable reactance element, wherein the resonant network is configured to resonate when the variable reactance element is at a resonant reactance value;

a control circuit operably coupled to the variable reactance element and configured to:

determine a first reactance value and a second reactance value, wherein an output of the power receiving unit is the desired output when the variable reactance element is either the first reactance value or the second reactance value, and the first reactance value is below the resonant reactance value and the second reactance value is above the resonant reactance value;

determine an indication of a source reactance associated with a wireless power source; and

adjust the variable reactance element to either the first reactance value or the second reactance value based on the indication of the source reactance.

2. The apparatus of claim 1 wherein the control circuit is configured to receive an information element from the wireless power source and to determine the indication of the source reactance based on the information element.

3. The apparatus of claim 2 wherein the information element is a single bit.

4. The apparatus of claim 1 wherein the control circuit includes a look-up-table to determine the indication of the source reactance.

5. The apparatus of claim 1 further comprising a reactance measuring circuit configured to determine the indication of the source reactance.

6. The apparatus of claim 1 wherein the variable reactance element includes at least one variable capacitor.

7. The apparatus of claim 1 wherein the variable reactance element includes a plurality of capacitors and at least one switch operably coupled to the control circuit.

8. The apparatus of claim 1 wherein the control circuit is configured to receive a communication signal and adjust the variable reactance element to either the first reactance value or the second reactance value based on the communication signal.

9. The apparatus of claim 1 wherein the power receiving unit comprises at least one coil configured to inductively couple power via a magnetic field.

10. The apparatus of claim 1 wherein the resonant network is capacitive when the variable reactance element is at the first reactance value.

11. The apparatus of claim 1 wherein the resonant network is inductive when the variable reactance element is at the second reactance value.

12. The apparatus of claim 1 wherein the indication of the source reactance indicates a reflected reactance measured in the wireless power source is inductive and the control circuit is configured to adjust the variable reactance element to the second reactance value.

13. The apparatus of claim 1 wherein the indication of the source reactance indicates a reflected reactance measured in the wireless power source is capacitive and the control circuit is configured to adjust the variable reactance element to the first reactance value.

14. A method of reducing reflected reactance in a wireless power transfer system, comprising:

regulating an output of a power receiving unit, wherein the power receiving unit includes a resonant circuit and the output is regulated to a desired output based on a variable reactance element in the resonant circuit;

determining a first reactance value and a second reactance value, wherein the output of the power receiving unit is the desired output when the variable reactance element is either the first reactance value or the second reactance value, and the first reactance value is below a resonant reactance value and the second reactance value is above the resonant reactance value;

determining an indication of a source reactance associated with a wireless power source; and

adjusting the variable reactance element to either the first reactance value or the second reactance value based on the indication of the source reactance.

15. The method of claim 14 further comprising receiving an information element from the wireless power source and determining the indication of the source reactance based on the information element.

16. The method of claim 15 wherein the information element is a single bit.

17. The method of claim 14 wherein determining the indication of the source reactance includes querying a look-up-table.

18. The method of claim 14 wherein determining the indication of the source reactance includes measuring a reactance of the resonant circuit with a reactance measuring circuit.

19. The method of claim 14 wherein adjusting the variable reactance element includes providing a control signal to at least one variable capacitor.

20. The method of claim 14 wherein adjusting the variable reactance element includes providing a control signal to at least one switch operably coupled to a plurality of capacitors in the resonant circuit.

21. The method of claim 14 further comprising reducing the output of the power receiving unit to a nominal value prior to adjusting the variable reactance element.

22. The method of claim 14 further comprising receiving a communication signal and adjusting the variable reactance element to either the first reactance value or the second reactance value based on the communication signal.

23. The method of claim 14 wherein the resonant circuit is capacitive when the variable reactance element is at the first reactance value.

24. The method of claim 14 wherein the resonant circuit is inductive when the variable reactance element is at the second reactance value.

25. The method of claim 14 further comprising:

determining that the indication of the source reactance indicates a reflected reactance measured in the wireless power source is inductive; and

adjusting the variable reactance element to the second reactance value.

26. The method of claim 14 further comprising:

determining that the indication of the source reactance indicates a reflected reactance measured in the wireless power source is capacitive; and

adjusting the variable reactance element to the first reactance value.

27. An apparatus for reducing reflected reactance in a wireless power transfer system, comprising:

means for varying a reactance of a power receiving unit,

means for determining a first reactance value and a second reactance value, wherein an output of the power receiving unit is a desired output when the reactance of the power receiving unit is either the first reactance value or the second reactance value, and the first reactance value is below a resonant reactance value and the second reactance value is above the resonant reactance value;

means for determining an indication of a source reactance associated with a wireless power source; and

means for adjusting the reactance of the power receiving unit to either the first reactance value or the second reactance value based on the indication of the source reactance.

28. A non-transitory processor-readable storage medium comprising processor-readable instructions configured to cause at least one processor to reduce reflected reactance in a wireless power transfer system, comprising:

code for regulating an output of a power receiving unit, wherein the power receiving unit includes a resonant circuit and the output is regulated to a desired output based on a variable reactance element in the resonant circuit;

code for determining a first reactance value and a second reactance value, wherein the output of the power receiving unit is the desired output when the variable reactance element is either the first reactance value or the second reactance value, and the first reactance value is below a resonant reactance value and the second reactance value is above the resonant reactance value;

code for determining an indication of a source reactance associated with a wireless power source; and

code for adjusting the variable reactance element to either the first reactance value or the second reactance value based on the indication of the source reactance.

29. The storage medium of claim 28 further comprising code for receiving an information element from a power transmitting unit and code for determining the indication of the source reactance based on the information element.

30. The storage medium of claim 28 wherein the code for adjusting the variable reactance element includes code for providing a control signal to at least one variable capacitor.