Switching power supply device with synchronous rectification

A switching power supply device with synchronous rectification, comprising:

• a transformer that receives an input voltage on a primary side;
• a synchronous rectifier element that conducts/breaks a current of a secondary-side coil of the transformer;
• and
• a synchronous rectifier control circuit that drives the synchronous rectifier element to be on/off,
• wherein the synchronous rectifier control circuit includes:
• an ON-timing detection circuit that detects a turn-on timing to turn on the synchronous rectifier element based on a terminal voltage of the synchronous rectifier element;
• an OFF-timing detection circuit that detects a turn-off timing to turn off the synchronous rectifier element by comparing the terminal voltage of the synchronous rectifier element with a predetermined turn-off threshold voltage;
• an ON/OFF control circuit that generates an ON/OFF control signal for the synchronous rectifier element based on an output signal of the ON-timing detection circuit and an output signal of the OFF-timing detection circuit;
• a discharge circuit that performs discharge of releasing a charge of a gate terminal of the synchronous rectifier element before the turn-off timing of the synchronous rectifier element to reduce a gate voltage of the gate terminal;
• a voltage adjustment circuit that, based on a change in the terminal voltage of the synchronous rectifier element, corrects the predetermined turn-off threshold voltage such that the turn-off timing of the synchronous rectifier element gets closer to a point at which a current flowing in the synchronous rectifier element becomes zero, the predetermined turn-off threshold voltage being used by the OFF-timing detection circuit to detect the turn-off timing of the synchronous rectifier element;
• a first detection circuit that detects a state in which the terminal voltage of the synchronous rectifier element is equal to or less than a predetermined potential;
• and
• a second detection circuit that detects a decrease in the terminal voltage of the synchronous rectifier element after the discharge circuit starts the discharge, wherein the voltage adjustment circuit includes:
• a first latch circuit that takes in an output signal of the second detection circuit in response to a change in an output signal of the first detection circuit;
• a pulse generation circuit that generates a pulse signal having a predetermined pulse width in response to a change in the output signal of the ON-timing detection circuit;
• a first logic circuit that obtains a logical product of an output signal of the first latch circuit and the pulse signal generated by the pulse generation circuit;
• and
• a second logic circuit that obtains a logical product of an inverted output signal of the first latch circuit and the pulse signal generated by the pulse generation circuit, and
• wherein the voltage adjustment circuit changes the predetermined turn-off threshold voltage to be higher by a first predetermined amount based on an output signal of the first logic circuit, and changes the predetermined turn-off threshold voltage to be lower by a second predetermined amount based on an output signal of the second logic circuit, the second predetermined amount being identical with or different from the first predetermined amount.