The most common WPT technology now is the electromagnetic
induction method, which is a very efficient nonradiative
WPT. However, it generally has a small air gap at a few
centimeters and recently improved to approximately 10 cm by
increasing its frequency to 20–40 kHz. Furthermore, its efficiency
drops severely when there is a misalignment between its
transmitting and receiving coils, even when the misalignment
is only several centimeters. While this technology is sufficient
to charge most electric appliances, it might not be convenient
enough to realize a charge-on-the-go system. Recently, a highly
efficient midrange WPT technology using magnetic resonance
coupling (MRC) was rediscovered and proposed [10]–[12] and
has received much attention due to its high efficiency and practical
midrange [13]–[15]. It has efficiencies of approximately
90% within 1 m and 45% at 2 m [10], [11].
[4] and medical implants [5]–[7] and bigger high-power devices
such as EVs [8], [9]. To make wireless charging more practical,