Switching control circuit having off-time modulation to improve efficiency of primary-side controlled power supply

A switching control circuit for a primary-side controlled power supply comprising:

• a switch, for switching a transformer having at least one auxiliary winding and at least one secondary-side winding;
• a switching signal applied to said switch, wherein said switching signal is controlled by a detection voltage in response to an output voltage of said auxiliary winding and a detection current in response to a switching current of said switch;
• a controller, coupled to said transformer, wherein said controller detects a voltage variation of said auxiliary winding and a current variation of said switch, then generates said switching signal with a switching frequency larger than zero for continuously switching said switch to induce continuous energy variation on said auxiliary winding, said controller comprising:
• an oscillator, generating a frequency signal;
• a voltage control loop, developed from said voltage variation of said auxiliary winding, which is in accordance with said switching signal, an error amplifier comparing said voltage variation with a reference voltage, a comparator comparing an output of said error amplifier with said current variation of said switch, and a PWM generator generating said switching signal for said switch in response to an output of said comparator;
• an adder, adding to said frequency signal with said current variation of said switch;
• and
• a duty cycle modulator generating a modulation signal to said PWM generator for adjusting said switching signal and to said oscillator for adjusting said frequency signal in response to said voltage variation of said auxiliary winding and said frequency signal.

A method for switching a primary-side controlled power supply, said method comprising the steps of:

• sensing a current variation of a switching current;
• sensing a voltage variation of an auxiliary winding;
• generating a control signal in response to comparing said voltage variation of said auxiliary winding with a reference voltage;
• generating a modulated current variation signal in response to combining said current variation signal with a frequency signal from an oscillator;
• generating a PWM controlling signal in response to comparing said modulated current variation of said switching current with said control signal;
• generating a PWM switching signal in response to said PWM controlling signal and said frequency signal via a PWM generator by inputting said PWM controlling signal and said frequency signal to said PWM generator;
• and
• generating a modulated duty cycle signal to said oscillator and said PWM generator in response to said control signal and said frequency signal via a duty cycle modulator.