A planar winding transformer includes a magnetic core set and a multilayer circuit board. The magnetic core set includes two magnetic cores and two magnetic columns. The two magnetic cores are parallel to each other. The multilayer circuit board is disposed between two magnetic cores, and two magnetic columns penetrate through the multilayer circuit board. The multilayer circuit board includes two low voltage winding layers and one high voltage winding layer. Two low voltage winding layers are connected to each other in parallel, and the high voltage winding layer is disposed between two low voltage winding layers. When the high voltage winding layer receives a polarity current, at least one of the low voltage winding layers generates a corresponding induced current. Two magnetic cores and two magnetic columns form a closed path for magnetic flux.