Flyback power converter and control circuit and control method thereof

A control circuit of a flyback power converter, wherein the flyback power converter includes a power stage circuit, a voltage sense circuit, a current sense circuit, and the control circuit, wherein the power stage circuit operates a power switch therein to convert an input voltage to an output voltage according to a pulse width modulation (PWM) signal, and the voltage sense circuit is coupled to the power stage circuit, for generating a voltage sense signal according to the output voltage, and the current sense circuit is coupled to the power stage circuit, for generating a current sense signal according to a switch current flowing through the power switch, and the control circuit is coupled to the power stage circuit, the voltage sense circuit, and the current sense circuit, for generating the PWM signal according to the voltage sense signal and the current sense signal, wherein when the flyback power converter operates in a quasi-resonant (QR) mode, the control circuit determines a time point of turning ON the power switch according to a zero voltage crossing event (ZVC) of the output voltage which is indicated by the voltage sense signal that the output voltage changes from a non-zero voltage to zero voltage, and determines a time point of turning OFF the power switch according to the current sense signal, a PWM frequency of the PWM signal, and a predetermined threshold, wherein the control circuit adaptively adjusts an ON period of the power switch according to the PWM frequency to compensate an output voltage ripple, the control circuit comprising:

• a QR control circuit, which is coupled to the voltage sense circuit, for generating a set signal when a ZVC of the output voltage occurs, and generating a QR mode indication signal when the flyback power converter operates in the QR mode;
• a frequency sense circuit, for generating a period length signal according to the PWM frequency;
• a gain control circuit, which is coupled to the QR control circuit and the frequency sense circuit, for generating an adjusted threshold by adaptively adjusting the predetermined threshold according to the QR mode indication signal and the period length signal;
• a comparison circuit, which is coupled to the gain control circuit and the current sense circuit, for comparing the current sense signal with the adjusted threshold, to generate a reset signal;
• and
• a PWM signal generation circuit, which is coupled to the comparison circuit and the QR control circuit, for generating the PWM signal according to the set signal and the reset signal, such that when the voltage sense signal indicates that a ZVC of the output voltage occurs, the power switch is turned ON, and when the current sense signal reaches the adjusted threshold, the power switch is turned OFF.

A control circuit of a flyback power converter, wherein the flyback power converter includes a power stage circuit, a voltage sense circuit, a current sense circuit, and the control circuit, wherein the power stage circuit operates a power switch therein to convert an input voltage to an output voltage according to a pulse width modulation (PWM) signal, and the voltage sense circuit is coupled to the power stage circuit, for generating a voltage sense signal according to the output voltage, and the current sense circuit is coupled to the power stage circuit, for generating a current sense signal according to a switch current flowing through the power switch, and the control circuit is coupled to the power stage circuit, the voltage sense circuit, and the current sense circuit, for generating the PWM signal according to the voltage sense signal and the current sense signal, wherein when the flyback power converter operates in a quasi-resonant (QR) mode, the control circuit determines a time point of turning ON the power switch according to a zero voltage crossing event (ZVC) of the output voltage indicated by the voltage sense signal, and determines a time point of turning OFF the power switch according to the current sense signal, a PWM frequency of the PWM signal, and a predetermined threshold, wherein the control circuit adaptively adjusts an ON period of the power switch according to the PWM frequency to compensate an output voltage ripple, the control circuit comprising:

• a QR control circuit, which is coupled to the voltage sense circuit, for generating a set signal when a ZVC of the output voltage occurs, and generating a QR mode indication signal when the flyback power converter operates in the QR mode;
• a frequency sense circuit, for generating a period length signal according to the PWM frequency;
• a gain control circuit, which is coupled to the QR control circuit, the frequency sense circuit, and the current sense circuit, for adaptively processing the current sense signal to generate a current indication signal according to the QR mode indication signal and the period length signal;
• a comparison circuit, which is coupled to the gain control circuit, for comparing the current indication signal with the predetermined threshold, to generate a reset signal;
• and
• a PWM signal generation circuit, which is coupled to the comparison circuit and the QR control circuit, for generating the PWM signal according to the set signal and the reset signal, such that when the voltage sense signal indicates that a ZVC of the output voltage occurs, the power switch is turned ON, and when the current indication signal reaches the adjusted threshold, the power switch is turned OFF.

A control circuit of a flyback power converter, wherein the flyback power converter includes a power stage circuit, a voltage sense circuit, a current sense circuit, and the control circuit, wherein the power stage circuit operates a power switch therein to convert an input voltage to an output voltage according to a pulse width modulation (PWM) signal, and the voltage sense circuit is coupled to the power stage circuit, for generating a voltage sense signal according to the output voltage, and the current sense circuit is coupled to the power stage circuit, for generating a current sense signal according to a switch current flowing through the power switch, and the control circuit is coupled to the power stage circuit, the voltage sense circuit, and the current sense circuit, for generating the PWM signal according to the voltage sense signal and the current sense signal, wherein when the flyback power converter operates in a quasi-resonant (QR) mode, the control circuit determines a time point of turning ON the power switch according to a zero voltage crossing event (ZVC) of the output voltage which is indicated by the voltage sense signal that the output voltage changes from a non-zero voltage to zero voltage, and determines a time point of turning OFF the power switch according to the current sense signal, a PWM frequency of the PWM signal, and a predetermined threshold, the control circuit comprising:

• a QR control circuit, which is coupled to the voltage sense circuit, for generating a QR mode indication signal when the flyback power converter operates in the QR mode;
• a frequency sense circuit, for generating a period length signal according to the PWM frequency;
• an error amplifier circuit, including:
• a bandwidth control circuit, which is coupled to the QR control circuit and the frequency sense circuit, for generating an adjustment signal according to the QR mode indication signal and the period length signal;
• an amplification circuit, which is coupled to the voltage sense circuit, for generating an amplification signal according to a reference voltage and the voltage sense signal;
• a resistance compensation circuit, which is coupled to the bandwidth control circuit and the amplification circuit;
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• a capacitance compensation circuit, which is coupled to the bandwidth control circuit and the amplification circuit;
• and
• a PWM signal generation circuit, which is coupled to the error amplifier circuit and the current sense circuit, for generating the PWM signal according to the amplification signal and the current sense signal;
• wherein the error amplifier circuit adaptively adjusts the operation bandwidth of the error amplifier circuit to compensate the output voltage ripple by adjusting a compensation resistance of the resistance compensation circuit or a compensation capacitance of the capacitance compensation circuit or both according to the adjustment signal.

A control circuit of a flyback power converter, wherein the flyback power converter includes a power stage circuit, a voltage sense circuit, a current sense circuit, and the control circuit, wherein the power stage circuit operates a power switch therein to convert an input voltage to an output voltage according to a pulse width modulation (PWM) signal, and the voltage sense circuit is coupled to the power stage circuit, for generating a voltage sense signal according to the output voltage, and the current sense circuit is coupled to the power stage circuit, for generating a current sense signal according to a switch current flowing through the power switch, and the control circuit is coupled to the power stage circuit, the voltage sense circuit, and the current sense circuit, for generating the PWM signal according to the voltage sense signal and the current sense signal, wherein when the flyback power converter operates in a quasi-resonant (QR) mode, the control circuit determines a time point of turning ON the power switch according to a zero voltage crossing event (ZVC) of the output voltage which is indicated by the voltage sense signal that the output voltage changes from a non-zero voltage to zero voltage, and determines a time point of turning OFF the power switch according to the current sense signal, a PWM frequency of the PWM signal, and a predetermined threshold, wherein the control circuit adaptively an amplifier gain of an error amplifier therein according to the PWM frequency, the control circuit comprising:

• a QR control circuit, which is coupled to the voltage sense circuit, for generating a QR mode indication signal when the flyback power converter operates in the QR mode;
• a frequency sense circuit, for generating a period length signal according to the PWM frequency;
• the error amplifier circuit, including:
• an amplifier gain control circuit, which is coupled to the QR control circuit and the frequency sense circuit, for generating an amplifier gain adjustment signal according to the QR mode indication signal and the period length signal;
• an amplification circuit, which is coupled to the voltage sense circuit, for adjusting the amplifier gain according to the amplifier gain adjustment signal, and generating an amplification signal according to a reference voltage, the voltage sense signal, and the amplifier gain;
• a resistance compensation circuit, which is coupled to the amplification circuit;
• and
• a capacitance compensation circuit, which is coupled to the amplification circuit;
• and
• a PWM signal generation circuit, which is coupled to the error amplifier circuit and the current sense circuit, for generating the PWM signal according to the amplification signal and the current sense signal.

A control method of a flyback power converter, comprising:

• operating a power switch of a power stage circuit to convert an input voltage to an output voltage according to a pulse width modulation (PWM) signal;
• generating a voltage sense signal according to the output voltage;
• generating a current sense signal according to a switch current flowing through the power switch;
• generating the PWM signal according to the voltage sense signal and the current sense signal, whereby a feedback control loop is formed;
• when the flyback power converter operates in a quasi-resonant (QR) mode, determining a time point of turning ON the power switch according to a zero voltage crossing event (ZVC) of the output voltage which is indicated by the voltage sense signal that the output voltage changes from a non-zero voltage to zero voltage, and determining a time point of turning OFF the power switch according to the current sense signal, a PWM frequency of the PWM signal, and a predetermined threshold;
• and
• adaptively adjusting an ON period of the power switch or an operation bandwidth or an amplifier gain of an error amplifier circuit in the feedback control loop according to the PWM frequency, to compensate an output voltage ripple.