Resonance voltage attenuation detection circuit, semiconductor device for switching power, and switching power supply

A semiconductor device for a switching power which generates a drive signal that turns on and off a switching element connected in series with a primary winding of a transformer for voltage conversion which includes an auxiliary winding, the semiconductor device comprising:

• a resonance voltage attenuation detection circuit to which a voltage induced in the auxiliary winding is applied and which detects attenuation of a resonance voltage;
• a turn-on control circuit that generates a timing signal that turns on the switching element based on an attenuation detection signal output by the resonance voltage attenuation detection circuit;
• and
• a turn-off control circuit that generates a timing signal that turns off the switching element,
• wherein the semiconductor device is able to be operated in a quasi-resonance mode.

A semiconductor device for a switching power which generates a drive signal that turns on and off a switching element connected in series with a primary winding of a transformer for voltage conversion which includes an auxiliary winding, the semiconductor device comprising:

• a first external terminal to which a feedback voltage corresponding to a voltage output from a secondary side of the transformer is applied;
• a second external terminal to which a voltage induced in the auxiliary winding is applied;
• a resonance detection circuit that detects resonance of the voltage of the auxiliary winding based on a voltage of the second external terminal;
• a resonance voltage attenuation detection circuit that detects attenuation of a resonance voltage of the auxiliary winding based on the voltage of the second external terminal;
• a turn-on control circuit that generates a timing signal that turns on the switching element based on a detection signal of the resonance detection circuit and on a detection signal of the resonance voltage attenuation detection circuit;
• a turn-off control circuit that generates a timing signal that turns off the switching element based on a voltage of the first external terminal and a voltage proportional to a current flowing through the switching element;
• and
• a drive pulse generation circuit that generates a pulse signal which is a source of the drive signal based on the signal output from the turn-on control circuit and on the signal output from the turn-off control circuit,
• wherein the turn-on control circuit generates a timing signal that turns on the switching element when resonance of the voltage of the auxiliary winding is around a bottom so that the semiconductor device is able to be operated in a quasi-resonance mode.