Switching power supply unit and semiconductor device for switching power supply

A switching power supply unit which does output through the conversion of an inputted first direct-current voltage to a second direct-current voltage via a transformer, the switching power supply unit comprising:

• a switching device connected to the primary side of the transformer to which the first direct-current voltage is applied;
• an output voltage control circuit which stabilizes the second direct-current voltage;
• a control signal transmission circuit which transmits a signal from the output voltage control circuit to the primary side of the transformer;
• and
• a control circuit which controls the operation of the switching device,
• the control circuit having:
• a switching device current detection circuit which detects a current flowing through the switching device and which outputs the current as a device current detection signal;
• a feedback signal control circuit which receives the signal from the control signal transmission circuit as a feedback signal, outputs a feedback output signal varying in response to the feedback signal, and outputs a charging start signal for overload protection when the feedback signal has reached a specified feedback signal level;
• a comparator which compares the output signal from the switching device current detection circuit and the feedback output signal and which outputs the comparison signal thus produced;
• a switching signal control circuit which controls the amount of the current and the output of the switching device based on the comparison signal;
• a clamp circuit which fixed the maximum value of the feedback output signal to limit the maximum value of the current of the switching device;
• an overload protection delay time control circuit which receives the charging start signal for the overload protection as an input to supply a charging current;
• a capacitor for adjusting overload protecting operation delay time which is charged with the charging current of the overload protection delay time control circuit;
• and
• an overload signal output circuit which detects a voltage at the capacitor for adjusting the overload protecting operation delay time and which outputs a switching stop signal when the voltage has exceeded a specified voltage,
• the switching signal control circuit controlling the peak value of the current of the switching device according to the feedback signal to the feedback signal control circuit and stopping the operation of the switching device based on the switching stop signal of the overload signal output circuit.