Abstract: A transformer circuit for a power source having a first voltage level and a second voltage level. The first coil is coupled to the first voltage level at a terminal of the first coil to generate a first induced voltage by a first current. The demagnetization loop switch is coupled between another terminal of the first coil and the second voltage level to switch the first current according to a switching signal provided by an outside circuit. The demagnetization circuit is coupled between another terminal of the first coil and the first voltage level to consume the energy stored in the first coil when the demagnetization loop switch is turned off. The second coil is coupled to the second voltage level at a terminal of the second coil to generate a signal having a second induced voltage according to the first induced voltage.