Detection of device removal from a surface of a multi-coil wireless charging device

1. A method for operating a charging device, comprising:

determining that a receiving device has been placed on a surface of the charging device;

providing a charging current to a resonant circuit after determining that the receiving device has been placed on the surface of the charging device;

while transferring power to the receiving device, determining that a change or rate of change in voltage or current level associated with the resonant circuit indicates a potential removal of the receiving device from the surface;

providing a measurement slot by decreasing or terminating the charging current for a period of time; and

during the measurement slot:

detecting a change or rate of change in voltage or current associated with the resonant circuit; and

determining that the receiving device has been removed from the surface of the charging device when a step change in the voltage or current associated with the resonant circuit exceeds a threshold value.

2. The method of claim 1 , wherein the resonant circuit includes a transmitting coil.

3. The method of claim 2 , further comprising:

determining that the receiving device has been removed from the surface of the charging device when a voltage measured at a terminal of the transmitting coil exceeds a threshold voltage level.

4. The method of claim 3 , wherein the threshold voltage level is maintained by a lookup table and determined when the transmitting coil is electromagnetically uncoupled.

5. The method of claim 3 , wherein the threshold voltage level is determined when the receiving device is first placed on the surface of the charging device.

6. The method of claim 1 , further comprising:

determining that the receiving device has been removed from the surface of the charging device when a current measured in the resonant circuit has an amplitude that is less than a threshold current amplitude.

7. The method of claim 6 , wherein the threshold current amplitude is maintained by a lookup table and determined when no object is electromagnetically coupled with a coil in the resonant circuit.

8. The method of claim 6 , wherein the threshold current amplitude is determined when the receiving device is first placed on the surface of the charging device.

9. The method of claim 1 , further comprising:

using a low-pass filter to filter short-duration or low-magnitude changes in a signal representing voltage or current in the resonant circuit.

10. The method of claim 1 , further comprising:

receiving measurements from one or more sensors in the charging device; and

providing the measurement slot when one of the measurements indicates physical removal of the receiving device.

11. The method of claim 10 , wherein the one or more sensors include a strain measuring sensor.

12. The method of claim 10 , wherein the one or more sensors include an accelerometer.

13. The method of claim 10 , wherein the one or more sensors comprise an infrared or ultrasonic sensing element.

14. The method of claim 10 , wherein the one or more sensors comprise a hall-effect device.

15. A charging device, comprising:

a charging circuit; and

a controller configured to:

determine that a receiving device has been placed on a surface of the charging device;

cause the charging circuit to provide a charging current to a resonant circuit after determining that the receiving device has been placed on the surface of the charging device;

determine that a change or rate of change in voltage or current level associated with the resonant circuit indicates a potential removal of the receiving device from the surface while the charging circuit is transferring power to the receiving device;

provide a measurement slot by causing the charging circuit to decrease or terminate the charging current for a period of time; and

during the measurement slot:

detect a change or rate of change in a voltage or current associated with the resonant circuit; and

determine during the measurement slot that the receiving device has been removed from the surface of the charging device when a step change in the the voltage or current associated with the resonant circuit exceeds a threshold value.

16. The charging device of claim 15 , wherein the resonant circuit includes a transmitting coil.

17. The charging device of claim 16 , wherein the controller is further configured to:

determine that the receiving device has been removed from the surface of the charging device when a voltage measured at a terminal of the transmitting coil exceeds a threshold voltage level.

18. The charging device of claim 17 , wherein the threshold voltage level is maintained by a lookup table and determined when the transmitting coil is electromagnetically uncoupled.

19. The charging device of claim 17 , wherein the threshold voltage level is determined when the receiving device is first placed on the surface of the charging device.

20. The charging device of claim 15 , wherein the controller is further configured to:

determine that the receiving device has been removed from the surface of the charging device when a current measured in the resonant circuit has an amplitude that is less than a threshold current amplitude.

21. The charging device of claim 20 , wherein the threshold current amplitude is maintained by a lookup table and determined when no object is electromagnetically coupled with a coil in the resonant circuit.

22. The charging device of claim 20 , wherein the threshold current amplitude is determined when the receiving device is first placed on the surface of the charging device.

23. The charging device of claim 15 , further comprising:

a low-pass filter configured to filter short-duration or low-magnitude changes in a signal representing voltage or current in the resonant circuit, wherein the change or rate of change in the voltage or current level is detected at an output of the low-pass filter.

24. The charging device of claim 15 , wherein the controller is further configured to:

receive measurements from one or more sensors in the charging device; and

provide the measurement slot when one of the measurements indicates physical removal of the receiving device.

25. The charging device of claim 24 , wherein the one or more sensors include a strain measuring sensor.

26. The charging device of claim 24 , wherein the one or more sensors include an accelerometer.

27. The charging device of claim 24 , wherein the one or more sensors comprise an infrared or ultrasonic sensing element.

28. The charging device of claim 24 , wherein the one or more sensors comprise a hall-effect device.