The present invention relates to a switch controller, a switch control method, and a power supply including the switch controller. An exemplary embodiment of the present invention detects an on-time of a power switch of the power supply and decreases a frequency of a clock signal according to a period during which the detected on-time is shorter than or equal to the minimum on-time. According to the exemplary embodiment, switching of the power switch is controlled according to a clock signal, and the minimum on-time is an on period of the power switch that cannot be shortened.