Patent ID: 8575944

Claim:
A primary device for inductive power transfer to a secondary device, the primary device comprising: a primary coil, the primary device being configured to (i) operate in a first mode during which the primary coil transfers power through inductive coupling to the secondary device and a load is connected to the secondary device, the secondary device being configured to supply the transferred power to the load, and (ii) operate in a second mode during which the load is disconnected from the secondary device and a foreign object is detected; and a primary controller configured to operate the primary coil (i) using a first frequency during the first mode while the load is connected to the secondary device, and (ii) using a second frequency during the second mode while the load is disconnected from the secondary device; wherein the primary device is configured to measure a primary current in the primary device while the primary coil is being operated at the second frequency during the second mode to detect presence or absence of the foreign object, and wherein the secondary frequency corresponds to an operating frequency at which the primary current does not vary with respect to an air gap between the primary coil and a secondary coil of the secondary device.