Patent ID: 7768802

Claim:
A switching mode power supply (SMPS) 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 for receiving a feedback voltage corresponding to a voltage of the secondary side of the transformer, a sense signal corresponding to current flowing at the switching transistor, and a first signal corresponding to a voltage difference between first and second electrodes of the switching transistor obtained by using a voltage of a second coil of the transformer, the switching controller for controlling the turning on and off of the switching transistor, wherein after the switching transistor is turned off, the switching controller detects and counts a number of times that a voltage level of the first signal and a reference voltage are equal, the switching controller causing the switching transistor to turn-on when the count of the number of times reaches a variable reference number changed at random.