Capacitive wireless power transfer system with double matching transformers for reduced stress and extended ZVS range

Capacitive power transfer uses an electric field to transmit power through physical isolation barrier utilizing capacitances formed by a couple of metal plates. Despite simplicity of the energy link structure, however, the physical dimension and low dielectric constant of the interface medium usually limits the effective link capacitance to be very small and nearly comparable to the main switch output capacitance in the transmitting circuit and thus narrows the soft-switching range. Unlike conventional approach which used very high operating frequency to handle these problems, this paper investigates a half-bridge inverter with double matching transformer networks to obtain required gain with decreased switching frequency and stable zero voltage switching operation for the load range. With mathematical analysis with fundamental harmonic approximation, design guideline is presented to provide a soft-switching scheme even in such a small link capacitances. Simulation and hardware implementation are performed to show that the proposed scheme ensures zero-voltage-switching up to 10% light load condition at switching frequencies that range from 244.5kHz to 345.6kHz for a 5W system equipped with a 256pF link capacitance and a 200pF switch output capacitance.