Wireless power transfer (WPT) is continuing to gain popularity as a convenient way to power devices that require electrical energy. Wireless power transfer can be used as a direct source of power, but is frequently used to charge internal batteries within a device. Examples of devices that are currently incorporating WPT technology include electric automobiles, radio-frequency identification (RFID) tags, electric toothbrushes, mobile phones, watches, other consumer electronics, and medical devices such as pacemakers.
A standard WPT system consists of four elements, typically a source loop, a load loop, and two additional loops that act as resonators. As seen in FIGS. 1 and 2, a conventional transmitter TX includes a planar resonator and source while the receiver RX includes a planar load and resonator. However, one issue with this WPT system is that it is only efficient when the transmitter and receiver elements are resonating at the same frequency and are properly aligned (i.e., they must have the same Q factor). In products incorporating WPT, it is often impractical or inconvenient to ensure proper alignment, which results in dramatic decreases in energy transfer efficiency. What is therefore needed is a WPT system that is insensitive to misalignment.