A planar transformer includes a primary winding including a first primary winding layer, a secondary winding including a first secondary winding layer, where an Ns1th turn of coil to an (Ns1+B1−1)th turn of coil in the secondary winding are disposed at the first secondary winding layer, and a charge balance winding including a first charge balance winding layer, where the first charge balance winding layer is disposed between the first primary winding layer and the first secondary winding layer, and is adjacent to the first primary winding layer and the first secondary winding layer. N1 turns of coils are disposed at the first charge balance winding layer, where N1 is greater than or equal to Nb1.