A transformer having auxiliary winding coils for sensing magnetic flux balance is disclosed. The transformer comprises a first winding part having a first channel, a second winding part having a second channel a third winding part having a third channel, a primary winding coil wound around the first winding part, a first secondary winding coil wound around the second winding part, a second secondary winding coil wound around the third winding part, a first auxiliary winding coil for sensing magnetic flux balance wound around the second winding part, a second auxiliary winding coil for sensing magnetic flux balance wound around the third winding part, and a magnetic core assembly having a first extension part disposed in the first channel, a second extension part disposed in the second channel, and a third extension part disposed in the third channel. The first and the second secondary winding coils couple to a plurality of lamps for providing energy to the plurality of lamps, and the first and the second auxiliary winding coils couple to a voltage detecting circuit for detecting if the energy provided to the plurality of lamps is substantially equal.