Patent ID: 7158394

Claim:
A switching power supply circuit comprising: a main switching element arranged to control an output voltage output from the switching power supply circuit with a switch ON/OFF operation; a soft-start circuit arranged to output a soft-start voltage whose voltage value increases over time during a start-up time from when the switching power supply circuit starts driving to when the output voltage of the switching power supply circuit reaches a predetermined normal-operation output voltage; and a control circuit arranged to start an output of a switching-control signal of the main switching element based on the soft-start voltage after the soft-start voltage reaches a predetermined signal-output starting voltage to control the switch ON/OFF operation of the main switching element and to cause the output voltage of the switching power supply circuit to gently rise towards the normal-operation output voltage; wherein the soft-start circuit includes a time-constant circuit having a soft-start capacitor which is charged during the start-up time; the soft-start circuit outputs a charge voltage of the soft-start capacitor as the soft-start voltage; the soft-start circuit includes a time-constant switching control circuit arranged to control the switching of a time constant to charge the soft-start capacitor during the start-up time; and the time-constant switching control circuit sets a time constant that causes the charge voltage of the soft-start capacitor to sharply increase at least until the charge voltage of the soft-start capacitor reaches the signal-output starting voltage after the switching power supply circuit starts driving and switches to a time constant that causes a rising trend of the charge voltage of the soft-start capacitor to become gentle with a predetermined time-constant switching timing after the charge voltage of the soft-start capacitor reaches the signal-output starting voltage.