I. Introduction
Wireless power transmission using magnetic fields has recently received considerable attention as one of the attractive methods to recharge mobile consumer electronics, such as smartphones and laptops, by simply placing them over flat charging surfaces without power cords [1]. As a result, an industrial consortium has been formed to standardize such devices, and several companies are now offering new products that operate on the basis of inductive power transmission [2]–[6]. To prevent overheating and interference with other nearby devices, in compliance with the Federal Communications Commission (FCC) rules, the power transmission efficiency (PTE) should be maintained at a high level [7].