Techniques for detecting the presence of a foreign object on the surface of a wireless charging transmitter by measuring quality factor during a charging session

An inductive charging method, comprising:

• providing a wireless charging pad comprising a transmitter coil configured to, when active, inductively transfer power to a receiver coil of a wireless device, wherein the wireless device comprises rectifier diodes between the receiver coil and a receiver load;
• detecting, by a controller of the wireless charging pad, the wireless device upon a charging surface proximate to the transmitter coil;
• in response to detecting the wireless device, determining, by the controller, a first quality factor of the transmitter coil and initiate a charging session during which the transmitter coil is activated to inductively couple with the receiver coil and inductively transfer electrical energy thereto;
• and
• during the charging session, periodically performing, by the controller, a foreign object detection (FOD) technique comprising:
• adjusting an operating point of the transmitter coil according to a specific sequence of operating points that causes the rectifier diodes to be reverse biased to disconnect the receiver load from the receiver coil;
• in response to adjusting the operating point of the transmitter coil according to the specific sequence, determining a second quality factor of the transmitter coil;
• and
• detecting whether a foreign object is present on the charging surface based on a comparison between the first and second quality factors.

An induction charging system, comprising:

• a transmitter coil means for, when active, inductively transferring power to a receiver coil of a wireless device, wherein the wireless device comprises rectifier diodes between the receiver coil and a receiver load;
• and
• a controller means for:
• detecting the wireless device upon a charging surface proximate to the transmitter coil;
• in response to detecting the wireless device, determining a first quality factor of the transmitter coil and initiating a charging session during which the transmitter coil is activated to inductively couple with the receiver coil and inductively transfer electrical energy thereto;
• and
• during the charging session, periodically performing a foreign object detection (FOD) technique comprising:
• adjusting an operating point of the transmitter coil according to a specific sequence of operating points that causes the rectifier diodes to be reverse biased to disconnect the receiver load from the receiver coil;
• in response to adjusting the operating point of the transmitter coil according to the specific sequence, determining a second quality factor of the transmitter coil;
• and
• detecting whether a foreign object is present on the charging surface based on a comparison between the first and second quality factors.