Switching power supply unit

A switching power supply apparatus comprising:

• a main transformer (T1) comprising a primary winding (n1) and a secondary winding (n2);
• a main switching element (Q1);
• a main converter (10) that turns "on" and "off" a DC current applied to said primary winding of said main transformer using said main switching element and outputs a main output voltage by rectifying and smoothing current induced in said secondary winding of said main transformer;
• an error amplifier (20) that generates an error voltage between said main output voltage and a first reference voltage (Vref1);
• a PWM control circuit (30) that outputs an on-off control signal for said main switching element so that said error voltage outputted from said error amplifier is minimized;
• a photo-coupler (PC) that receives said error voltage from said error amplifier and outputs said error voltage to said PWM control circuit;
• an over-voltage protection circuit (50) that generates an over-voltage protection signal when said main output voltage exceeds a second reference voltage (Vref2);
• an on-off circuit (60) that outputs either a high level potential or a common level potential depending on whether the power is to be turned "on" or "off";
• a shut down latch circuit (70) that outputs a shut down signal from an output terminal thereof when the over-voltage protection signal from said over-voltage protection circuit represents an over voltage state;
• and
• a shut down implementation circuit (80) that holds an input voltage of said photo-coupler at a value which suppresses input to said photo-coupler when said potential level from said on-off circuit specifies a power off state or when the shut down signal from said shut down latch circuit is provided.