Capacitive Power Transfer

The simplicity and low cost of capacitive interfaces makes them very attractive for wireless charging stations and galvanically isolated power supplies. Major benefits include low electromagnetic radiation and the amenability of combined power and data transfer over the same interface. We present a capacitive power transfer circuit using series resonance that enables efficient high frequency, moderate voltage operation through soft-switching. An included analysis predicts fundamental limitations on the maximum achievable efficiency for a given amount of coupling capacitance and is used to find the optimum circuit component values and operating point. A prototype capacitive charger achieves near 80 efficiency at 3.7W with only 63 pF of coupling capacitance. An automatic tuning loop adjusts the frequency from 4.2MHz down to 4MHz to allow for 25 variation in the nominal coupling capacitance. The duty cycle is also automatically adjusted to maintain over 70 efficiency for light loads down to 0.3 W. Simulation results from a galvanically isolated LED driver work in progress indicate that efficiencies over 90 at 12.6W output power are possible using only 500 pF of capacitance. Regulation of LED current is accomplished by tuning the frequency of the series resonant circuit, eliminating the need for secondary-side current sense and regulation electronics.