Patent ID: 8000119

Claim:
A switching mode power supply, comprising: a rectifier for generating an input DC voltage from an input AC voltage; a switching transistor coupled to a primary coil of a transformer for converting the input DC voltage and supplying power to a secondary side of the transformer according to an operation of the switching transistor; and a switching controller that receives a feedback voltage corresponding to an output voltage of the switching mode power supply, a sensing signal corresponding to a current flowing through the switching transistor, and a first signal corresponding to a voltage difference between a first electrode and a second electrode of the switching transistor, the switching controller for controlling the turning on and off of the switching transistor; wherein the switching controller sets a threshold period whenever the first signal has a value greater than a reference value, thereby setting a plurality of threshold periods during operation of the switching mode power supply; wherein for each threshold period the switching controller turns on the switching transistor at a point after a variable delay time from a previous point at which the switching transistor was turned on; wherein the switching controller turns on the switching transistor when a voltage between the first electrode and the second electrode of the switching transistor is equal to the lowest possible value if the input DC voltage is less than or equal to a modulation reference value.