Wireless Power System With in-Band Communications

A wireless power receiving device 24 has a coil 48 that receives wireless power signals from a wireless power transmitting device 12 and has a rectifier 50 that produces direct-current power across rectifier output terminals 76. A load 106 receives a direct-current output voltage from the rectifier output terminals. In-band communications are supported in which an amplitude-shift keying communications scheme or other communications scheme is used by a data transmitter 46T in the wireless power receiving device to transmit in-band data through the coil. In-band data is transmitted by modulating one or more transistors 94, 96 that are coupled to the coil and other wireless power receiving circuitry in series with one or more capacitors 90, 92 and is transmitted by modulating current flow through a ballast transistor or other adjustable load 100 that is coupled across the rectifier output terminals. The wireless power transmitting device may be a wireless charging mat. The wireless power receiving device may be a wristwatch, cellular telephone, tablet computer, laptop computer, or other electronic equipment. The wireless power receiving device may use power from the wireless power transmitting device for powering the device and for charging an internal battery.