Comparison of two high frequency converters for capacitive power transfer

Capacitive power transfer (CPT) method has been proposed recently as an alternative to inductive power transfer (IPT) for wireless/contactless power transfer. Unlike IPT, which uses magnetic fields as medium for power transfer, CPT is based on electric field coupling. Power transfer using CPT offers significant advantages such as low standing power loss, low electromagnetic interference (EMI), and the ability to transfer power through metal objects, etc. However, due to the very low permittivity of the air, the effective coupling capacitance between the primary and the secondary pick-up is very small (only at picofarads to nanofarads level), which limits the maximum power capability that a CPT system can achieve. To overcome such a limitation, high frequency operation is desirable to increase the power transfer capability with a given coupling condition. With fast development of power electronics over the past decades it is possible to increase the operating frequency from kHz to MHz level without compromising much on power efficiency of the system. This paper presents a comparison between two converters which are suitable for high frequency operation of CPT systems. One is an autonomous current-fed push-pull converter, and the other is a class E converter. Both converters can achieve MHz level frequency operation, but their operating principles are different. The key features of these two converters are summarized in this research, and the performance of CPT systems based on both converters are analyzed and evaluated.