A transformer includes a case, a magnetic device and a thermally conductive layer. The case has a receptacle. The magnetic device is disposed within the receptacle, and includes a winding member and a magnetic core assembly. The thermally conductive layer is arranged between the magnetic device and the case for electrically isolating the magnetic device from the case. The heat generated by the magnetic device is transferred to the case through the thermally conductive layer and dissipated away to ambient air.