A wireless system consists of a charging pad (transmitter, TX or primary) and the secondary-side equipment (receiver, RX or secondary). There is a coil in the charging pad and in the secondary equipment which are magnetically coupled to each other when the secondary is placed on the primary. Power is then transferred from the transmitter to the receiver through coupled inductors (effectively an air-core transformer). Controlling the amount of power transferred is achieved by sending feedback (error signal) communication to the primary (to increase or decrease power).

The receiver communicates with the transmitter by changing the load seen by the transmitter. This load variation results in a change in the transmitter coil current, which is measured and interpreted by a processor in the charging pad. The communication is digital; packets are transferred from the receiver to the transmitter. Differential bi-phase encoding is used for the packets. The bit rate is 2-kbps.

Various types of communication packets have been defined. These include identification and authentication packets, error packets, control packets, end power packets, and power usage packets.

The transmitter coil stays powered off most of the time. It occasionally wakes up to see if a receiver is present. When a receiver authenticates itself to the transmitter, the transmitter will remain powered on. The receiver maintains full control over the power transfer using communication packets.

Figure 9-1 WPC Wireless Power System Indicating the Functional Integration of the BQ51013B-Q1