I. Introduction

Wireless power transfer systems utilizing magnetically coupled coils has gained more and more research interest due to its wide range of applications such as electric vehicles [1], wireless sensor networks [2], and implantable biomedical devices [3], [4]. Although the WPT concept has been explored at the beginning of the century by Nikola Tesla [5], the recent work by a group at MIT [6] has led to a massive increase in research and commercial activity. In particular, the MIT group proposed an improved inductive power transfer system based on magnetic resonance coupling using a four-coil system which allows a more efficient operation over a farther distance range than similar previous methods [6]. This long distance operation has opened up many applications for charging electronic devices without annoying wires.