Switching power supply and planar transformer thereof

The invention provides a planar transformer, comprising: a PCB winding board comprising at most four layers of PCBs arranged in a laminated manner, each layer of PCB winding board being provided with a through hole, the PCB winding board being provided with an opening penetrating through the PCB winding board, the opening being used for accommodating a center pillar of a magnetic core; the winding unit is arranged on the PCB winding board, and the winding unit comprises a primary winding and a secondary winding; the silk-screen layers are arranged on the top surface and the bottom surface of the PCB winding board, and the winding units are covered with the silk-screen layers; wherein a first winding and a power supply winding which are connected in series are arranged at the center tap of the primary winding, and a second winding and a third winding which are connected in series are arranged at the center tap of the secondary winding. The planar transformer can meet the balance requirements of different commo