PWM controller for compensating a maximum output power of a power converter
A PWM controller having an oscillator, a control circuit and a two-level limiter is provided. The oscillator generates a pulse signal. The control circuit couples to the oscillator for generating a PWM signal in response to the pulse signal, wherein the PWM signal controls a power switch. The two-level limiter couples to the control circuit for generating a two-level limit signal in response to an on-time of the PWM signal, wherein the two-level limit signal is formed by a first-level signal and a second-level signal during a switching period of the PWM signal, and the first-level signal is used to limit the maximum output power of the power converter under a high-line input voltage with a heavy-load condition, and the second-level signal is used to limit the maximum output power of the power converter under a low-line input voltage with the heavy-load condition.
1. A PWM controller for compensating a maximum output power of a power converter, comprising:
an oscillator, for generating a pulse signal;
a control circuit, coupled to the oscillator for generating a PWM signal in response to the pulse signal, wherein the PWM signal controls a power switch; and
a two-level limiter, coupled to the control circuit for generating a two-level limit signal in response to an on-time of the PWM signal, wherein the two-level limit signal is formed by a first-level signal and a second-level signal during a switching period of the PWM signal, and the first-level signal is used to limit the maximum output power of the power converter under a high-line input voltage with a heavy-load condition, and the second-level signal is used to limit the maximum output power of the power converter under a low-line input voltage with the heavy-load condition.
2. The PWM controller of claim 1 , wherein the two-level limiter comprises:
a comparing circuit, coupled to the control circuit for generating a switching signal in response to the PWM signal; and
a two-level compensation circuit, coupled to the comparing circuit for generating the two-level limit signal in response to the switching signal,
wherein the first-level signal is generated in response to the switching signal is logic-low, the second level signal is generated in response to the switching signal is logic-high.
3. The PWM controller of claim 2 , wherein the comparing circuit comprises:
a saw-tooth circuit, coupled to the control circuit for generating a saw-tooth signal in response to an on/off status of the PWM signal; and
a comparator, coupled to the saw-tooth circuit for comparing the saw-tooth signal with a reference signal to generate the switching signal.
4. A PWM controller for compensating a maximum output power of a power converter, comprising:
an oscillator, for generating a pulse signal;
a control circuit, coupled to the oscillator for generating a PWM signal in response to the pulse signal, wherein the PWM signal controls a power switch; and
a two-level limiter, coupled to the control circuit for generating a two-level limit signal in response to an on-time of the PWM signal, wherein the two-level limit signal is formed by a first-level signal and a second-level signal during a switching period of the PWM signal, and the first-level signal is used to limit the maximum output power of the power converter under a high-line input voltage with a heavy-load condition, and the second-level signal is used to limit the maximum output power of the power converter under a low-line input voltage with the heavy-load condition;
wherein the two-level limiter comprises:
a comparing circuit, coupled to the control circuit for generating a switching signal in response to the PWM signal; and
a two-level compensation circuit, coupled to the comparing circuit for generating the two-level limit signal in response to the switching signal; wherein the first-level signal is generated in response to the switching signal is logic-low, the second level signal is generated in response to the switching signal is logic-high;
wherein the comparing circuit comprises:
a saw-tooth circuit, coupled to the control circuit for generating a saw-tooth signal in response to an on/off status of the PWM signal; and
a comparator, coupled to the saw-tooth circuit for comparing the saw-tooth signal with a reference signal to generate the switching signal;
wherein the reference signal determines a holding period of the first-level signal, and the holding period is to solve an over-compensation problem under the high-line input voltage with the heavy-load condition.
5. A method for compensating a maximum output power of a power converter, comprising the steps of:
generating a pulse signal;
generating a PWM signal in accordance with the pulse signal, wherein the PWM signal controls a power switch; and
generating a two-level limit signal in accordance with an on-time of the PWM signal, wherein the two-level limit signal is formed by a first-level signal and a second-level signal during a switching period of the PWM signal.
6. The method of claim 5 , wherein the first-level signal is used to limit the maximum output power of the power converter under a high-line input voltage with a heavy-load condition.
7. The method of claim 5 , wherein the second-level signal is used to limit the maximum output power of the power converter under a low-line input voltage with a heavy-load condition.