Controller with loop impedance modulation for power converter
A control circuit is provided to generate a mode signal at light load of the power converter. The mode signal is coupled to disable the switching signal for saving power. The impedance of an input circuit is increased in response to the mode signal. Furthermore, a soft start circuit is initiated by the mode signal when switching signal is enabled. An external capacitor associates with the impedance of the input circuit determine the off period of the switching signal.
1. A control circuit of a power converter, comprising:
a switching circuit, coupled to a feedback loop of the power converter to generate a switching signal in response to a feedback signal for output regulation, wherein the feedback loop is from a secondary winding of a transformer of the power converter, and the feedback signal is associated with an output voltage of the power converter;
a soft start circuit, coupled to switching circuit and generating a soft start signal to control the switching signal;
a resistive device, coupled to the feedback loop, wherein the resistive device comprises a transistor connected with a first resistor in parallel, the transistor directly connected to a voltage supply and a second resistor connected with the first resistor in series; and
a comparison circuit, coupled to the feedback loop and directly connected to the switching circuit for comparing the feedback signal with a threshold signal and generating a mode signal to the switching circuit,
wherein the mode signal is used to disable the switching signal for saving power, control on/off of the transistor of the resistive device so as to modulate an impedance of the resistive device and reset the soft start circuit for enabling soft start of the switching signal when the switching signal is enabled.
2. The control circuit of the power converter as claimed in claim 1 , further comprising an external capacitor coupled to the resistive device.
3. The control circuit of the power converter as claimed in claim 2 , wherein the external capacitor associated with the resistive device determine an off period of the switching signal of the power converter.
4. The control circuit of the power converter as claimed in claim 1 , wherein the mode signal is enabled once the feedback signal is lower than the threshold signal.
5. The control circuit of the power converter as claimed in claim 1 , wherein the impedance of the resistive device is increased when the mode signal is enabled.
6. A controller of a power converter, comprising:
a switching circuit, coupled to a feedback loop of the power converter to generate a switching signal in response to a feedback signal, wherein the feedback loop is from a secondary winding of a transformer of the power converter, and the feedback signal is associated with an output voltage of the power converter;
a comparison circuit, coupled to the feedback loop and directly connected to the switching circuit to generate a mode signal when the feedback signal of the feedback loop is lower than a threshold signal; and
a resistive device, coupled to the feedback loop, wherein the resistive device comprises a transistor connected with a first resistor in parallel, the transistor directly connected to a voltage supply and a second resistor connected with the first resistor in series;
wherein the mode signal is connected to disable the switching signal for saving power and control on/off of the transistor of the resistive device such that a resistance of the resistive device is increased in response to the mode signal.
7. The controller of the power converter as claimed in claim 6 , further comprising an external capacitor coupled to the resistive device.
8. The controller of the power converter as claimed in claim 7 , wherein the external capacitor associated with the resistive device determine an off period of the switching signal.
9. The controller of the power converter as claimed in claim 6 , further comprising a soft start circuit, wherein the soft start circuit is coupled to the switching circuit and generates a soft start signal to control the switching signal.
10. The controller of the power converter as claimed in claim 9 , wherein the mode signal is coupled to reset the soft start circuit for enabling soft start of the switching signal when the switching signal is enabled.
11. A control circuit of a power converter, comprising:
a switching circuit, coupled to an input circuit of the power converter to generate a switching signal in response to a feedback signal, wherein the feedback loop is from a secondary winding of a transformer of the power converter, and the feedback signal is associated with an output voltage of the power converter;
a resistive device, coupled to the switching circuit, wherein the resistive device comprises a transistor connected with a first resistor in parallel, the transistor directly connected to a voltage supply and a second resistor connected with the first resistor in series; and
a comparison circuit, coupled to the input circuit of the power converter and directly connected to the switching circuit to generate a mode signal when the feedback signal of the input circuit is lower than a threshold signal;
wherein the mode signal is connected to turn off the switching signal for saving power according to the mode signal and control on/off of the transistor of the resistive device such that and an impedance of the input circuit is increased in response to the mode signal.
12. The control circuit of the power converter as claimed in claim 11 , further comprising an external capacitor coupled to the input circuit.
13. The control circuit of the power converter as claimed in claim 12 , wherein the external capacitor coupled to the impedance of the input circuit determine an off period of the switching signal.
14. A controller of a power converter, comprising:
a switching circuit, coupled to an input circuit of the power converter to generate a switching signal in response to a feedback signal, wherein the feedback loop is from a secondary winding of a transformer of the power converter, and the feedback signal is associated with an output voltage of the power converter;
a resistive device, coupled to the switching circuit, wherein the resistive device comprises a transistor connected with a first resistor in parallel, the transistor directly connected to a voltage supply and a second resistor connected with the first resistor in series;
a soft start circuit, coupled to switching circuit and generating a soft start signal to control the switching signal; and
a comparison circuit, coupled to the input circuit of the power converter and directly connected to the switching circuit to generate a mode signal when the feedback signal of the feedback input circuit is lower than a threshold signal;
wherein the mode signal is connected to control on/off of the transistor of the resistive device so as to modulate an impedance of the resistive device, disable the switching signal for saving power and the mode signal is further coupled to reset the soft start circuit for enabling soft start of the switching signal when switching signal is enabled.