This paper presents a wireless power transfer system for mobile devices. To achieve high efficiency in the receiver circuit, an optimal switching frequency is required. Through experimental analysis, we suggest the optimal efficiency of a receiver circuit. The switching frequency of this receiver circuit can vary from 500 kHz to 13.56 MHz. The received power has a range of 0 to 5 W, and it has a maximum efficiency of 81%. This circuit was fabricated with a 0.18 µm BCD process, and it occupies an area of 2.5 mm × 2.5 mm.