Zero-crossing slotted foreign object detection

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

providing a charging current to a resonant circuit when a receiving device is present on a surface of the wireless charging device;

providing a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or to transitions of a current in the resonant circuit through a zero ampere level;

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

sampling the voltage or current in the resonant circuit after a delay following each of a plurality of edges in the zero-crossing signal to obtain samples of voltage or current; and

determining whether an object other than the receiving device is present on a surface of the wireless charging device based on measurements of the samples of voltage or current captured based on timing provided by the zero-crossing signal, wherein the samples of voltage or current are captured during the measurement slot.

2. The method of claim 1 , wherein the zero volt level corresponds to a current amplitude midway between maximum and minimum amplitudes of an alternating current (AC) measured in the resonant circuit.

3. The method of claim 1 , wherein the zero volt level corresponds to a voltage level midway between maximum and minimum amplitudes of an AC voltage measured across the resonant circuit.

4. The method of claim 1 , further comprising:

determining that the object other than the receiving device is present on the surface of the wireless charging device based on a rate of decrease in voltage or current measured using the samples of voltage or current.

5. The method of claim 1 , further comprising:

determining that the object other than the receiving device is present on the surface of the wireless charging device based on a rate of decrease of energy stored in the resonant circuit, wherein the energy stored in the resonant circuit is indicated by the samples of voltage or current.

6. The method of claim 1 , further comprising:

determining that the object other than the receiving device is present on the surface of the wireless charging device based on a Q factor of the resonant circuit, wherein the Q factor of the resonant circuit is indicated by a plurality of the samples of voltage or current.

7. The method of claim 1 , wherein the delay is calculated to cause sampling of the voltage or current when the voltage or current has a maximum amplitude.

8. A charging device, comprising:

a resonant circuit comprising a transmitting coil;

a driver circuit configured to provide a charging current to the resonant circuit;

a zero-crossing detector configured to provide a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or corresponding to transitions of a current in the resonant circuit through a zero ampere level;

a sample and hold circuit configured to sample voltage or current in the resonant circuit after a delay following each of a plurality of edges in the zero-crossing signal to obtain samples of voltage or current and

a controller configured to:

cause the driver circuit to provide the charging current to the resonant circuit when a receiving device is present on a surface of the charging device;

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

determine whether an object other than the receiving device is present on a surface of the charging device based on measurements of the samples of voltage or current captured based on timing provided by the zero-crossing signal, wherein the samples of voltage or current are captured during the measurement slot.

9. The charging device of claim 8 , wherein the zero volt level corresponds to a current amplitude midway between maximum and minimum amplitudes of an alternating current (AC) measured in the resonant circuit.

10. The charging device of claim 8 , wherein the zero volt level corresponds to a voltage level midway between maximum and minimum amplitudes of an AC voltage measured across the resonant circuit.

11. The charging device of claim 8 , wherein the controller is further configured to:

determine that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease in voltage or current measured using the samples of voltage or current.

12. The charging device of claim 8 , wherein the controller is further configured to:

determine that the object other than the receiving device is present on the surface of the charging device based on a rate of decrease of energy stored in the resonant circuit, wherein the energy stored in the resonant circuit is indicated by the samples of voltage or current.

13. The charging device of claim 8 , wherein the controller is further configured to:

determine that the object other than the receiving device is present on the surface of the charging device based on a Q factor of the resonant circuit, wherein the Q factor of the resonant circuit is indicated by a plurality of the samples of voltage or current.

14. The charging device of claim 8 , wherein the delay is calculated to cause sampling of the voltage or current when the voltage or current has a maximum amplitude.

15. A non-transitory processor-readable storage medium having instructions stored thereon which, when executed by at least one processor of a processing circuit, cause the processing circuit to:

provide a charging current to a resonant circuit when a receiving device is present on a surface of a wireless charging device;

provide a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or to transitions of a current in the resonant circuit through a zero ampere level;

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

sample the voltage or current in the resonant circuit after a delay following each of a plurality of edges in the zero-crossing signal to obtain the samples of voltage or current; and

determine whether an object other than the receiving device is present on a surface of the wireless charging device based on measurements of samples of voltage or current captured based on timing provided by the zero-crossing signal, wherein the samples of voltage or current are captured during the measurement slot.

16. The non-transitory processor-readable storage medium of claim 15 , wherein the instructions further cause the processing circuit to:

determine that the object other than the receiving device is present on the surface of the wireless charging device based on a rate of decrease in energy, voltage or current measured using the samples of voltage or current.

17. The non-transitory processor-readable storage medium of claim 15 , wherein the instructions further cause the processing circuit to:

determine that the object other than the receiving device is present on the surface of the wireless charging device based on a Q factor of the resonant circuit, wherein the Q factor of the resonant circuit is indicated by a plurality of the samples of voltage or current.