Switching-mode power supplies capable of operating at valley switching, and relevant control methods

A power controller suitable for use in a power supply with an inductive device and a power switch, wherein when the power switch is OFF the inductive device is capable of providing an oscillation signal with at least one signal valley, the power controller comprising:

• a valley detector coupled to the inductive device, for detecting the signal valley to provide a valley signal;
• and
• a valley selector receiving the valley signal, for counting an occurrence number of the signal valley, comparing the occurrence number with a lock number, and when the occurrence number and the lock number fit a predetermined condition, providing a pass signal that allows the valley signal to make the power switch turned ON so a cycle time starts at a start moment;
• wherein the valley selector checks if the start moment falls within an expectation window, and changes the lock number if the start moment falls outside of the expectation window.