A transformer and a method of manufacturing the same are disclosed. The transformer comprises a magnetic core, a winding coil with a primary winding coil and a secondary winding coil, a bobbin with a primary input port and a bobbin connecting member, and an insulating slipcase. The bobbin is mounted by the winding coil. The insulating slipcase includes a first opening to receive the magnetic core, the winding coil and the bobbin, and includes a first side wall, a second side wall with a slipcase connecting member which is engaged with the bobbin connecting member, and a secondary output port for dealing with the output of the transformer. The first side wall is opposite to the first opening and has a second opening for the output of the secondary winding coil.