A power supply system includes an electromagnetic interference (EMI) filter circuit, a rectifier and filter circuit, a switch circuit, a transformer, a feedback circuit and a controller. The feedback circuit includes a voltage divider circuit, a filter circuit, and a voltage and temperature compensation circuit. The voltage divider circuit generates a voltage dividing signal, and includes a first lossless element and a second lossless element connected in series between an output of the transformer and the ground, and the voltage dividing signal is generated at a node of the first and second lossless elements. The filter circuit filters the voltage dividing signal into a direct current (DC) signal. The voltage and temperature compensation circuit does voltage compensation and temperature compensation to the DC signal to generate a feedback signal, and sends the feedback signal to the controller.