The present invention discloses a transformer used in a charger circuit, in which the input side of the transformer is connected to an AC input and PWM control circuit of the charger circuit, the output side of the transformer is connected to a constant current and/or constant voltage control circuit of the charger circuit. The AC input, PWM control circuit, the constant current and/or constant voltage control circuit are coupled through an optical coupling element, wherein the transformer includes a main output winding and an auxiliary output winding, and the auxiliary output winding is used to supply power for the optical coupling element and the constant current and/or constant voltage control circuit. The present invention further provides a high performance charger circuit adopting the transformer described above for improving electromagnetic compatibility, conversion efficiency and short circuit characteristic.