A transformer includes a magnetic core, a first coil unit and a second coil unit. The first coil unit is disposed within the magnetic core and includes a laminated board having layers laminated therein and conductive patterns. Respective ones of the conductive patterns are disposed on the laminated layers. The second coil unit includes a conductive wire spaced apart from the conductive patterns of the laminated board by an insulating distance. The conductive wire includes a triple-insulated wire surrounded by three sheets of insulating paper to maintain the insulating distance from the conductive patterns.