A power-efficient wireless capacitor charging system through an inductive link

A power-efficient wireless capacitor charging system for inductively powered applications has been presented. A bank of capacitors can be directly charged from an ac source by generating a current through a series charge injection capacitor and a capacitor charger circuit. The fixed charging current reduces energy loss in switches, while maximizing the charging efficiency. An adaptive capacitor tuner compensates for the resonant capacitance variations during charging to keep the amplitude of the ac input voltage at its peak. We have fabricated the capacitor charging system prototype in a 0.35-\mu\hbox{m} 4-metal 2-poly standard CMOS process in 2.1 mm² of chip area. It can charge four pairs of capacitors sequentially. While receiving 2.7-V peak ac input through a 2-MHz inductive link, the capacitor charging system can charge each pair of 1 \mu \hbox{F} capacitors up to \pm2 V in 420 \mu\hbox{s}, achieving a high measured charging efficiency of 82%.