A wireless power transmitter configured to transfer power to a wireless power receiver, including a coil assembly comprising first and second bottom coils placed adjacent to each other in a line and each consisting of a single layer of 11 turns and a top coil stacked on the first and second bottom coils and consisting of a single layer of 12 turns; and a full-bridge inverter. The first and second bottom coils and the top coil have a substantially rectangular frame structure with a through hole in the center, wherein the top coil lies on a plane surface in the middle between the first and second bottom coils, a distance from the center of the first and second bottom coils to the center of the top coil is set to a range of 21 mm to 25 mm, the first and second bottom coils have a height of 48 mm to 50 mm and a width of 43 mm to 45 mm, and the through hole in the first and second bottom coils has a height of 25 mm to 27 mm and a width of 21 mm to 23 mm, the top coil has a height of 45 mm to 47 mm and a width of 48.5 mm to 50.5 mm, and the through hole in the top coil has a height of 20 mm to 22 mm and a width of 24.5 mm to 26.5 mm, the first and second bottom coils and the top coil have a thickness of 0.9 mm to 1.3 mm, the wireless power transmitter uses an input voltage of the full-bridge inverter to control an amount of power which is transferred, the input voltage has a range of 1 V to 18 V, wherein an operating frequency to control the amount of the power is within a range of 140 kHz to 150 kHz, and the first and second bottom coils and the top coil have a inductance value within a range of 10.6 μH to 12.0 μH.