Battery-powered devices (e.g., consumer electronic devices, electric and hybrid automobiles, etc.) are charged from a power source (e.g., AC power outlet) through a charging device. The charging device couples the battery to the power source through an adaptor. The cord extending between the power source and the battery-powered device can take up space. In situations where multiple devices require charging, each with their own charger and cord, the charging area can become cramped and inconvenient.
Approaches are being developed that use over-the-air or wireless power transmission between a transmitter and a receiver coupled to the electronic device. Wireless power transmission using inductive coils is one method considered as an un-tethered method for transferring power wirelessly through a coupled wireless power signal. In wireless power transmission, power is transferred by transmitting a wireless power signal through a transmit coil. On the receiver side, a receive coil may couple with the transmit coil through the wireless power signal, thus, receiving the transmitted power wirelessly. The distance between the transmitter coil and receive coil, at which efficient power transfer can take place, is a function of the transmitted energy and the required efficiency. The coupling coefficient (k) is a function of the distance between the coils, the coil sizes, and materials. The power conversion efficiency (e.g., coupling factor, coupling quality) may be significantly improved if the coils are sized and operated at such a frequency that they are physically within the so-called “near-field zone” of each other.
Communication in conventional wireless power charging arrangements typically cause an impedance change (e.g., capacitive or resistive) on the receive coil that is used to send information from the wireless power receiver to the wireless power transmitter. For example, capacitors may be coupled with a plurality of switches to function as a capacitive modulation circuit that may modulate (e.g., add and subtract) capacitance to the LC network (resonant tank). Conventional wireless power receivers may control the switches to modulate the amount of power coupled to the LC network, which is detected by the wireless power transmitter. As a result, communication from the wireless power receiver to the wireless power transmitter may be enabled through modulation of the parallel capacitance of the receive coil within the LC network of the wireless power receiver. This impedance change may result in an increased system losses and lower system efficiency. Large load changes on the output of the wireless power receiver may also cause false communication signals, which may be typically resolved by increasing the amount of output capacitance and passively changing the receiver control loop compensation to have low sensitivity to these events.