Switching control circuit and power supply circuit

A switching control circuit for a power supply circuit that generates an output voltage of a target level from an input voltage thereof, the power supply circuit including

• a transformer including a primary coil, a secondary coil, and an auxiliary coil,
• a first transistor and a second transistor configured to control a current flowing through the primary coil, and
• a first capacitor that forms a resonant circuit with the primary coil,
• the switching control circuit being configured to control switching of the first transistor and the second transistor, the switching control circuit comprising:
• a load detection circuit configured to detect a state of a load of the power supply circuit, based on a resonant current flowing through the resonant circuit;
• and
• a drive signal output circuit configured to output a drive signal, which
• causes the power supply circuit to operate in a burst mode when the load is in a light-load state, and
• causes the power supply circuit to operate in a normal mode when the load is in a heavy-load state, wherein
• the drive signal output circuit is configured to operate in:
• a first state, in which the drive signal is outputted based on a predetermined condition,
• a second state, in which the drive signal is outputted to gradually reduce an ON period of the first transistor and the second transistor,
• a third state, in which an output of the drive signal is stopped, and
• a fourth state, in which the drive signal is outputted to gradually increase the ON period of the first transistor and the second transistor, and
• the drive signal output circuit is further configured to
• repeat an operation of entering the first state, the second state, the third state, and the fourth state, and
• reduce an amount of change in the ON period of the first transistor and the second transistor, in response to the input voltage rising, in each of the second state and the fourth state.

A power supply circuit configured to generate an output voltage of a target level from an input voltage thereof, the power supply circuit comprising:

• a transformer including a primary coil, a secondary coil, and an auxiliary coil;
• a first transistor and a second transistor configured to control a current flowing through the primary coil;
• a first capacitor that forms a resonant circuit with the primary coil;
• and
• a switching control circuit configured to control switching of the first transistor and the second transistor, the switching control circuit including:
• a load detection circuit configured to detect a state of a load of the power supply circuit, based on a resonant current flowing through the resonant circuit;
• and
• a drive signal output circuit configured to output a drive signal, which
• causes the power supply circuit to operate in a burst mode when the load is in a light-load state, and
• causes the power supply circuit to operate in a normal mode when the load is in a heavy-load state, wherein
• the drive signal output circuit is configured to operate in:
• a first state, in which the drive signal is outputted based on a predetermined condition,
• a second state, in which the drive signal is outputted to gradually reduce an ON period of the first transistor and the second transistor, and
• a third state, in which an output of the drive signal is stopped;
• and
• the drive signal output circuit is further configured to enter the third state from the first state, without going through the second state, immediately after the power supply circuit starts operating in the burst mode.

A switching control circuit for a power supply circuit that generates an output voltage of a target level from an input voltage thereof, the power supply circuit including

• a transformer including a primary coil, a secondary coil, and an auxiliary coil,
• a first transistor and a second transistor configured to control a current flowing through the primary coil, and
• a first capacitor that forms a resonant circuit with the primary coil,
• the switching control circuit being configured to control switching of the first transistor and the second transistor, the switching control circuit comprising:
• a drive signal output circuit configured to output a plurality of types of drive signals to drive the first transistor and the second transistor, the plurality of types of drive signals including:
• a first drive signal outputted based on a resonant current of the resonant circuit and a feedback voltage corresponding to the output voltage,
• a second drive signal outputted based on a dead time and a timing at which a voltage at the auxiliary coil reaches any of a plurality of predetermined levels,
• a third drive signal outputted so as to gradually reduce an ON period of the first transistor and the second transistor, and
• a fourth signal outputted so as to stop driving of the first transistor and the second transistor, wherein
• the drive signal output circuit is configured to
• operate in a first pattern of outputting the first drive signal, then the second drive signal, and then the fourth drive signal, without outputting the third drive signal, and thereafter
• operate in a second pattern of outputting in an order of the second drive signal, the third drive signal, and the fourth drive signal.