Switching power supply device with a switching control circuit

A switching power supply device comprising:

• a power transformer including a primary winding, a secondary winding, and an auxiliary winding;
• a switching element which is connected to said primary winding and performs a switching operation to supply and stop supplying a first direct-current voltage to said primary winding;
• an output voltage generation circuit which converts, into a second direct-current voltage, an alternating-current voltage induced in said secondary winding through the switching operation of said switching element, and supplies the second direct-current voltage to a load;
• a transformer reset detection circuit which monitors a voltage signal of said auxiliary winding and generates a transformer reset signal according to a decrease in the voltage signal;
• a secondary-side on-time signal generation circuit which generates a secondary-side on-time signal indicating a secondary-side on-time that is a time period from when said switching element is turned off to when the transformer reset signal is generated;
• a feedback control circuit which generates a feedback signal corresponding to a voltage level of the second direct-current voltage;
• a switching control circuit which controls the switching operation of said switching element according to the feedback signal so that the second direct-current voltage falls within a normal operation specified range;
• and
• an overvoltage detection circuit which generates an overvoltage detection signal when the secondary-side on-time indicated by the secondary-side on-time signal becomes smaller than a set value,
• wherein the set value is smaller than the secondary-side on-time resulting when the second direct-current voltage is controlled to be within the normal operation specified range, and
• said switching control circuit controls the switching operation of said switching element so that the second direct-current voltage becomes equal to or below an overvoltage specified value when said overvoltage detection circuit generates the overvoltage detection signal.