Switching power supply device and synchronous rectifier circuit

A switching power supply device comprising:

• a transformer including a primary winding and a secondary winding;
• a first transistor coupled to one end of the primary winding;
• a first control circuit that outputs a first control voltage used for controlling a switching operation of the first transistor;
• a delay circuit that delays the first control voltage and supplies the delayed first control voltage to the first transistor;
• a second transistor that has a first terminal coupled to one end of the secondary winding, a second terminal at a reference potential, and a third terminal to which a second control voltage is applied and that performs a switching operation in accordance with the second control voltage;
• a diode having a cathode coupled to the one end of the secondary winding and an anode at the reference potential;
• a second control circuit that outputs a third control voltage used for controlling a switching operation of the second transistor in accordance with a first voltage of the first terminal;
• a control voltage generation circuit that generates the second control voltage in accordance with the first control voltage and the third control voltage;
• and
• a delay time control circuit that determines an ON period in which the diode is switched on in accordance with the first voltage and controls a delay time so that the delay time by which the delay circuit delays the first control voltage is shorter for a longer ON period.