Switching element driving control circuit and switching power supply device

A switching element driving control circuit included in a switching power supply device which includes a transformer having a primary winding and a secondary winding and which converts an input voltage into a desired DC voltage, said switching element driving control circuit controlling a switching operation of a switching element which repeats supplying and stopping a current that flows through the primary winding, said switching element driving control circuit comprising:

• a regulator circuit which generates a power supply voltage having an amplitude;
• a capacitor which smoothes the power supply voltage generated by said regulator circuit to remove a high frequency component;
• a circuit power supply line to which the smoothed power supply voltage is supplied;
• a secondary current on-period detection circuit which detects a first period that is a period from when the switching element turns off until when a secondary current that flows through the secondary winding finishes flowing;
• a secondary current on-duty control circuit which generates a clock signal according to the amplitude of the power supply voltage supplied from said circuit power supply line such that an on-duty ratio of the first period to a third period is maintained, the clock signal turning on the switching element, the third period including the first period and a second period that is a period during which the secondary current does not flow;
• a control circuit which generates a control signal for controlling the switching operation of the switching element, based on the clock signal;
• and
• a driver circuit which supplies the switching element with the control signal.

A switching element driving control circuit included in a switching power supply device which includes a transformer having a primary winding and a secondary winding and which converts an input voltage into a desired DC voltage, said switching element driving control circuit controlling a switching operation of a switching element which repeats supplying and stopping a current that flows through the primary winding, said switching element driving control circuit comprising:

• a regulator circuit which generates a power supply voltage having an amplitude;
• a capacitor which smoothes the power supply voltage generated by said regulator circuit to remove a high frequency component;
• a circuit power supply line to which the smoothed power supply voltage is supplied;
• a secondary current on-period detection circuit which detects a first period that is a period from when the switching element turns off until when a secondary current that flows through the secondary winding finishes flowing;
• a turn-on control circuit which generates an on-signal for turning on the switching element, according to an output of said secondary current on-period detection circuit;
• a control circuit which includes a drain current control circuit that generates an off-signal according to the amplitude of the power supply voltage supplied from said circuit power supply line when a current that flows through the switching element reaches a predetermined current peak level, the off-signal turning off the switching element, said control circuit generating a control signal for controlling the switching operation of the switching element, based on the on-signal and the off-signal;
• and
• a driver circuit which supplies the switching element with the control signal.