A coupled inductor has two pillars that are aligned in a vertical direction, wherein a first coil and a second coil are respectively wound around one of the two pillars, respectively, wherein the bottom surface of winding turns of the first coil and the bottom surface of winding turns of the second coil are separated by a gap, wherein a magnetic material is disposed in the gap and a straight line that is enclosed by each of the first coil and the second coil passes through the two pillars.