A power converter includes a transformer, a switch element, a control circuit, an over-current detecting circuit, and a compensation signal generating circuit. The transformer includes a primary winding and at least an auxiliary winding. The control circuit is coupled to the switch element, and controls on/off status of the switch element to control a primary current flowing through the primary winding. The over-current detecting circuit compares the primary current with a current limit, and then controls the control circuit to turn off the switch element if the primary current is greater than the current limit. The compensation signal generating circuit is coupled between the auxiliary winding and over-current detecting circuit, and provides an adjustment value to calibrate the current limit according to an output of the auxiliary winding; wherein the adjustment value varies with time when the switch element is turned on.