A wireless power transmitter configured to transfer power to a wireless power receiver, comprising: 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 driving each of coils included in the coil assembly individually, wherein 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, and wherein a distance from the center of the first and second bottom coils to the center of the top coil is set to 23 mm, wherein the first and second bottom coils have a height of 49 mm and a width of 44 mm, and the through hole in the first and second bottom coils has a height of 26 mm and a width of 22 mm, wherein the top coil has a height of 46 mm and a width of 49.5 mm, and the through hole in the top coil has a height of 21 mm and a width of 25.5 mm, and wherein the first and second bottom coils and the top coil have a thickness of 0.9 mm to 1.3 mm, wherein a level of power transferred to the wireless power receiver by the coil assembly is controlled based on a level of input voltage applied to the full-bridge inverter, wherein the level of voltage applied to the full-bridge inverter is adjusted within a range of 1 V to 18 V, wherein an operating frequency of power signals transmitted from the wireless power transmitter is fixed within a range of 140 kHz to 150 kHz, wherein the first and second bottom coils and the top coil have a same inductance value within a range of 10.6 μH to 12.0 μH. 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 driving each of coils included in the coil assembly individually, wherein 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, and wherein a distance from the center of the first and second bottom coils to the center of the top coil is set to 23 mm, wherein the first and second bottom coils have a height of 49 mm and a width of 44 mm, and the through hole in the first and second bottom coils has a height of 26 mm and a width of 22 mm, wherein the top coil has a height of 46 mm and a width of 49.5 mm, and the through hole in the top coil has a height of 21 mm and a width of 25.5 mm, and wherein the first and second bottom coils and the top coil have a thickness of 0.9 mm to 1.3 mm, wherein a level of power transferred to the wireless power receiver by the coil assembly is controlled based on a level of input voltage applied to the full-bridge inverter, wherein the level of voltage applied to the full-bridge inverter is adjusted within a range of 1 V to 18 V, wherein an operating frequency of power signals transmitted from the wireless power transmitter is fixed within a range of 140 kHz to 150 kHz, wherein the first and second bottom coils and the top coil have a same inductance value within a range of 10.6 μH to 12.0 μH.