A transformer includes a covering member, a bobbin, a primary winding coil, plural secondary winding coils, and a magnetic core assembly. The covering member includes plural pins. The bobbin is combined with the covering member, and includes a bobbin body and a channel. A first winding section and plural single-trough second winding sections are defined on the bobbin body. The single-trough second winding sections are arranged at bilateral sides of the first winding section. The channel runs through the bobbin body. The primary winding coil is wound around the first winding section of the bobbin, and connected with the pins. The secondary winding coils are wound around respective single-trough second winding sections of the bobbin. The magnetic core assembly is partially embedded into the channel of the bobbin.