Patent ID: 8270185

Claim:
A switching mode power supply, comprising: a rectifier; a transformer having a primary winding coupled to the rectifier and a secondary winding; a switching circuit coupled to the primary winding of the transformer to control current flow through the primary winding of the transformer based on a switching control signal; and a switching control circuit to generate the switching control signal as a function of a timing signal having a varying frequency, wherein the varying frequency of the timing signal causes a switching frequency of the switching circuit to vary over a period of time, wherein the switching control circuit comprises: a flip-flop that outputs the switching control signal as a function of a set input and a reset input; a feedback circuit that generates the reset input to the flip-flop, the feedback circuit being configured to monitor the current flow through the primary winding of the transformer and to produce a state change on the reset input when the current flow through the primary winding reaches a threshold; and a timing circuit that generates the timing signal as the set input to the flip-flop, the timing circuit including a spectrum shaping circuit that causes the varying frequency of the timing signal.