A power supply circuit for receiving an input voltage and outputting a driving voltage to at least one LED string. The power supply circuit includes a rear-stage converting circuit and a control circuit. The rear-stage converting circuit is used for receiving the input voltage and converting the input voltage into a compensating voltage. The control circuit is connected to the rear-stage converting circuit and the LED string for detecting the magnitude of a current passing through the LED string, thereby controlling the current passing through the LED string to be identical. The driving voltage is outputted from the power supply circuit. The driving voltage is a summation of the input voltage and the compensating voltage.