In 2006, the MIT (Massachusetts Institute of Technology) research team reports their experiment on lighting up a 60-W bulb with two 60-cm diameter coils which are separated by a distance of 2 m [1]. The wireless power transfer technology has, since then, received a surge of interests from all over the world and has now been applied to many areas such as household appliances, RFID (radio frequency identification devices), medical implanted devices, electric vehicles and maglev trains [2]-[4].