A transformer structure includes a bobbin, a conductive base, a first winding coil, plural second winding coils, and a magnetic core assembly. The bobbin includes a main body and a channel. The main body has a first winding section and plural first pins. The plural first pins are located at bilateral sides of the main body. The channel runs through the main body. The conductive base is disposed on a bottom side of the bobbin, and includes at least one connecting part. Through the connecting part of the conductive base, at least a portion of the plural first pins are electrically connected with each other. The first winding coil is wound around the first winding section. The second winding coils are connected with corresponding first pins. The magnetic core assembly is partially embedded into the channel of the bobbin.