Switching control circuit and power supply circuit

A switching control circuit for controlling a power supply circuit that includes

• a first capacitor configured to smooth an input voltage of the power supply circuit,
• a transformer including a primary coil provided on a primary side thereof, a secondary coil provided on a secondary side thereof, and an auxiliary coil, the primary coil being configured to have a voltage of the first capacitor applied thereto,
• a second capacitor configured to have a voltage from the auxiliary coil applied thereto, and
• a transistor configured to control an inductor current flowing through the primary coil,
• the switching control circuit being configured to perform switching control of the transistor, the switching control circuit comprising:
• a control circuit configured to output a first control signal when the input voltage is supplied to the power supply circuit, and output a second control signal when supply of the input voltage is stopped,
• a first drive circuit configured to output a first drive signal for performing the switching control of the transistor, in response to the first control signal, and
• a second drive circuit configured to output a second drive signal for controlling on-resistance of the transistor repeatedly to pass the inductor current intermittently to thereby discharge the first capacitor, in response to the second control signal.

A power supply circuit comprising:

• a first capacitor configured to smooth an input voltage of the power supply circuit;
• a transformer including a primary coil provided on a primary side thereof, a secondary coil provided on a secondary side thereof, and an auxiliary coil, the primary coil being configured to have a voltage of the first capacitor applied thereto;
• a second capacitor configured to have a voltage from the auxiliary coil applied thereto;
• a transistor configured to control an inductor current flowing through the primary coil;
• a control circuit configured to
• output a first control signal, when the input voltage is supplied to the power supply circuit, or when supply of the input voltage is stopped yet a first condition that a voltage of the second capacitor reaches a first level is satisfied, and
• output a second control signal, when the supply of the input voltage is stopped, the first condition is satisfied, and thereafter a second condition that the voltage of the second capacitor reaches a second level higher than the first level is satisfied;
• a first drive circuit configured to output a first drive signal for performing switching control of the transistor, in response to the first control signal;
• and
• a second drive circuit configured to output a second drive signal for controlling on-resistance of the transistor to discharge the first capacitor, in response to the second control signal.