A feedback signal stabilized by a capacitor and related to an output voltage of a power converter is used to acquire the output power information of the power converter, and a control circuit uses a second clock not related to the switching frequency of the power converter to count a duration time of the feedback signal being higher than a threshold. When the duration time is higher than a preset time, an abnormal output power of the power converter is distinguished and the power converter will be turned off. The feedback signal will not vary severely even if the output terminal of the power converter is interfered, and the counted duration time will not be influenced when the switching frequency is changing caused by a load changing.