Robust foreign objects detection

1. A foreign object detection method for a wireless power transmitter having a matching network and transmit coil, the method comprising:

(A) energizing the matching network and transmit coil and exciting resonance between the matching network and transmit coil;

(B) allowing the resonance to decay;

(C) measuring a temporal characteristic of the decay; and

(D) analyzing the temporal characteristic to determine whether a foreign object is coupled to an electromagnetic field generated by the transmit coil,

wherein (A) comprises switching an inverter of the wireless power transmitter, and

wherein (A) transfers sufficient energy to a wireless power receiver to charge a rectifier filter capacitor of the wireless power receiver to a level sufficient to reverse bias rectifier diodes of the wireless power receiver prior to (B).

2. The foreign object detection method of claim 1 , wherein (B) is performed without energy input to the matching network or transmit coil.

3. The foreign object detection method of claim 2 , wherein (A) comprises switching an inverter of the wireless power transmitter and (B) comprises stopping the switching of the inverter and holding its output in a low impedance state.

4. The foreign object detection method of claim 1 , wherein (C) is performed using continuous time measurements or discrete time measurements.

5. The foreign object detection method of claim 1 , wherein (D) comprises determining a quality factor.

6. The foreign object detection method of claim 1 , further comprising:

(E) determining whether to allow or inhibit wireless power transfer based on (D).

7. A foreign object detection method for a wireless power transmitter having a matching network and transmit coil, the method comprising:

(A) energizing the matching network and transmit coil and exciting resonance between the matching network and transmit coil;

(B) allowing the resonance to decay;

(C) measuring a temporal characteristic of the decay; and

(D) analyzing the temporal characteristic to determine whether a foreign object is coupled to an electromagnetic field generated by the transmit coil,

wherein (A) comprises switching an inverter of the wireless power transmitter, and

wherein (A) comprises switching the inverter for a first period of time and a second period of time after the first period of time, wherein the inverter transfers a lower energy level in the second period of time than in the first period of time.

8. The foreign object detection method of claim 7 , wherein a power supply voltage of the inverter is lower in the second period of time than in the first period of time.

9. The foreign object detection method of claim 7 , wherein the inverter has a first switching frequency during the first period of time and a second switching frequency during the second period of time, the first and second switching frequencies being different.

10. The foreign object detection method of claim 9 , wherein the resonance has a resonant frequency, and the first switching frequency is closer than the second switching frequency to the resonant frequency.

11. The foreign object detection method of claim 10 , wherein (A) transfers sufficient energy to a wireless power receiver to charge a rectifier filter capacitor of the wireless power receiver to a level sufficient to reverse bias rectifier diodes of the wireless power receiver prior to (B).

12. A foreign object detection method for a wireless power transmitter having a matching network and transmit coil, the method comprising:

(A) energizing the matching network and transmit coil and exciting resonance between the matching network and transmit coil;

(B) allowing the resonance to decay;

(C) measuring a temporal characteristic of the decay; and

(D) analyzing the temporal characteristic to determine whether a foreign object is coupled to an electromagnetic field generated by the transmit coil,

wherein (D) comprises determining a quality factor, and

wherein (D) comprises comparing the quality factor to an acceptable quality factor for wireless power transmission.

13. The foreign object detection method of claim 12 , wherein the acceptable quality factor is at least in part derived from a quality factor value provided to the wireless power transmitter by a wireless power receiver via in-band or out-of-band communication.

14. An apparatus for driving a wireless power transmitter and performing foreign object detection, the apparatus comprising:

a drive circuit configured to energize a matching network and transmit coil of the wireless power transmitter, excite resonance between the matching network and transmit coil, and allow the resonance to decay; and

a controller configured to control the drive circuit, measure a temporal characteristic of the decay and analyze the temporal characteristic to determine whether a foreign object is coupled to an electromagnetic field generated by the transmit coil,

wherein the drive circuit comprises an inverter configured to energize and excite the resonance by switching in response to a control signal produced using the controller, and

wherein the inverter is switched for a first period of time and a second period of time after the first period of time to energize and excite resonance of the matching network and transmit coil, wherein the inverter transfers a higher energy level in the first period of time than in the second period of time.

15. The apparatus of claim 14 , wherein the controller reduces a power supply voltage of the inverter in the second period of time to a lower value than in the first period of time.

16. The apparatus of claim 14 , wherein the inverter has a first switching frequency during the first period of time and a second switching frequency during the second period of time, the first and second switching frequencies being different.

17. The apparatus of claim 16 , wherein the resonance has a resonant frequency, and the first switching frequency is closer than the second switching frequency to the resonant frequency.

18. The apparatus of claim 14 , wherein the controller is configured to determine a quality factor.

19. An apparatus for driving a wireless power transmitter and performing foreign object detection, the apparatus comprising:

a drive circuit configured to energize a matching network and transmit coil of the wireless power transmitter, excite resonance between the matching network and transmit coil, and allow the resonance to decay; and

a controller configured to control the drive circuit, measure a temporal characteristic of the decay and analyze the temporal characteristic to determine whether a foreign object is coupled to an electromagnetic field generated by the transmit coil,

wherein the controller is configured to determine a quality factor, and

wherein the controller is configured to compare the quality factor to an acceptable quality factor for wireless power transmission.

20. The apparatus of claim 19 , wherein the controller is configured to compare the quality factor to a quality factor provided by a wireless power receiver via in-band or out-of-band communication.