Patent ID: 7265999

Claim:
A power supply regulation circuit, comprising: a transformer including a first winding fed with input voltage and a second winding for generating output voltage; a switching element including a first terminal, a second terminal and a control terminal, and oscillating so as to electrically couple or decouple the first and second terminals in response to a control signal received on the control terminal, so that on/off of current passing through the first winding is controlled; and a control circuit for generating the control signal, the power supply regulation circuit controlling an energy supply to the first and second windings by controlling the oscillation of the switching element, and supplying a constant output current to a load connected to the second winding, the control circuit having: an element current detection function of detecting an element current passing through the switching element; a current limit varying function of changing, between a first level and a second level higher than the first level, a current limit dictating a maximum value of the element current in a period during which the switching element couples the first and second terminals; a function of outputting a signal for turning off the switching element when the element current reaches the current limit changing between the first and second levels; a function of detecting an on duty of secondary current passing through the second winding, and outputting a signal for turning on the switching element so as to keep the on duty of the secondary current at a fixed value; and a function of generating the control signal based on the signal for turning on the switching element and the signal for turning off the switching element.