# Patent

## Title
Control circuit for burst switching of power converter and method thereof

## Abstract

This invention provides a control circuit for burst switching of a power converter comprising: an adaptive circuit generating an adaptive threshold in response to a feedback signal correlated to an output load of the power converter; and a switching circuit generating a switching signal to switch a transformer of the power converter in accordance with the adaptive threshold and the feedback signal for regulating an output of the power converter.

## Background
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a power converter, and more particularly, the present invention relates to a burst switching of power converter.
2. Description of Related Art
Switching mode power converter has been widely used to provide regulated voltage and current for computers, home appliances, communication equipments, etc. In recent years, the problem of power saving in switching mode power converter has drawn much attention. A prior art “Switching regulator having low power mode responsive to load power consumptions” U.S. Pat. No. 5,747,977 disclosed the skill using a threshold to achieve lower power consumption. However, the drawback of this prior art is a higher output ripple at the output of the power converter. This higher output ripple is caused by the threshold.
SUMMARY OF THE INVENTION
An object of the present invention is to achieve lower power consumption and generate lower output ripple at the output of the power converter.
To achieve the aforementioned object, the present invention provides a control circuit for burst switching of a power converter, comprising: an adaptive circuit generating an adaptive threshold in response to a feedback signal correlated to an output load of the power converter; and a switching circuit generating a switching signal to switch a transformer of the power converter in accordance with the adaptive threshold and the feedback signal for regulating an output of the power converter.
The aforementioned adaptive circuit generates the adaptive threshold with a first value in response to a value of the feedback signal within a first range, and generates the adaptive threshold with a second value in response to a value of the feedback signal within a second range, wherein the adaptive threshold with the first value is lower than the adaptive threshold with the second value, and the value of the feedback signal within the first range is higher than value of the feedback signal within the second range.
The aforementioned adaptive circuit comprises a power management circuit and a current source, the adaptive threshold generated by the adaptive circuit is formed by the outputs of the power management circuit and the current source.
To achieve the aforementioned object, the present invention further provides a method for controlling a power converter, comprising the steps of: generating an adaptive threshold in response to a feedback signal correlated to a load of the power converter; and generating a switching signal to switch a transformer of the power converter in accordance with the adaptive threshold and the feedback signal for regulating an output of the power converter.
The aforementioned step of generating the adaptive threshold comprising: generating the adaptive threshold with a first value in response to a value of the feedback signal with a first range. The aforementioned step of generating the adaptive threshold further comprising: generating the adaptive threshold with a second value in response to a value of the feedback signal with a second range, wherein the adaptive threshold with the first value is lower than the second value, and the value of the feedback signal within the first range is higher than the value of the feedback signal within the second range.
The aforementioned the adaptive threshold is generated by an adaptive circuit comprising a power management circuit and a current source, the adaptive threshold is sum of the outputs of the power management circuit and the current source, the adaptive threshold with a first value is formed by the output of the current source.
The present invention provides an adaptive threshold for the burst switching of power converter so as to achieve higher efficiency at light load condition and no load condition.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic circuit for a power converter.
FIG. 2 shows a schematic circuit of the control circuit in accordance with a preferred embodiment of the present invention.
FIG. 3 shows a schematic circuit of the power management circuit in accordance with a preferred embodiment of the present invention.
FIG. 4 shows a schematic circuit of the power management circuit in accordance with another preferred embodiment of the present invention.
FIG. 5 shows the burst switching waveforms of the switching signal according to the present invention.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a schematic circuit for a power converter. The power converter comprises a control circuit 100, a transformer 10, a transistor 20, a rectifier 30, a capacitor 35, a resistor 40, a reference device 45, and an opto-coupler 50. The control circuit 100 is coupled to the transistor 20 and controls the transistor 20 to generate a switching signal SW. The transformer 10 comprises a primary winding NP and a secondary winding NS. An input voltage VIN is supplied to one terminal of the primary winding NP. The transistor 20 is connected to the other terminal of the primary winding NP in series. The transistor 20 is used to switch the transformer 10 for transferring power energy from an input of the power converter to an output of the power converter. An output voltage VO is generated across the capacitor 35. The output voltage VO placed at the output of the power converter is regulated through the rectifier 30 and the capacitor 35.
Via the resistor 40, the reference device 45, and the opto-coupler 50, a feedback signal VFB is generated in accordance with the output voltage VO. The reference device 45 can be implemented by a zener diode according to a preferred embodiment of the present invention. The control circuit 100 receives the feedback signal VFB to develop a feedback loop for generating the switching signal SW and regulating the output voltage VO of the power converter.
FIG. 2 is a schematic circuit of the control circuit 100 in accordance with a preferred embodiment of the present invention. One terminal of a pull-high resistor 65 is connected to a supply voltage VCC. The feedback signal VFB is supplied to the other terminal of the pull-high resistor 65. A gate terminal of a level-shift transistor 60 receives the feedback signal VFB. The supply voltage VCC is supplied to a drain terminal of the level-shift transistor 60. A source terminal of the level-shift transistor 60 generates a first feedback signal VA. Therefore, the feedback signal VFB is supplied to the gate terminal of the level-shift transistor 60 for generating the first feedback signal VA. In addition, one terminal of a resistor 61 is coupled to the source terminal of the level-shift transistor 60. The other terminal of the resistor 61 is connected to one terminal of a resistor 62. The other terminal of the resistor 62 is connected to a ground. An attenuator developed by the resistors 61 and 62 generates a second feedback signal VB at a join of the resistors 61 and 62 in accordance with the first feedback signal VA. Therefore, the second feedback signal VB is generated by the first feedback signal VA through the attenuator that acts as a voltage divider. The first feedback signal VA and the second feedback signal VB are correlated to the feedback signal VFB.
A positive input of a comparator 85 is coupled to the join of the resistors 61 and 62 to receive the second feedback signal VB. The second feedback signal VB is supplied to the comparator 85 for generating the switching signal SW. An oscillation circuit (OSC) 80 generates a pulse signal PLS. A clock input CK of a flip-flop 90 receives the pulse signal PLS to turn on the flip-flop 90. The oscillation circuit 80 further generates a ramp signal VRMP. A negative input of the comparator 85 is coupled to the oscillation circuit 80 to receive the ramp signal VRMP for comparing with the second feedback signal VB. An output of the comparator 85 is coupled to a reset terminal R of the flip-flop 90 to turn off an output of the flip-flop 90. An output terminal Q of the flip-flop 90 is connected to a first terminal of an AND gate 92 for generating the switching signal SW. Via an inverter 91, a second terminal of the AND gate 92 receives the pulse signal PLS for limiting the maximum on-time period of the switching signal SW. The supply voltage VCC is supplied to an input terminal D of the flip-flop 90.
An adaptive circuit comprises a current source 74 and a power management circuit (PM) 200. The supply voltage VCC is supplied to one terminal of the current source 74. The other terminal of the current source 74 is coupled to an output of the power management circuit 200. A constant current I1 is generated by the current source 74. An input of power management circuit 200 is coupled to the source terminal of the level-shift transistor 60 and one terminal of the resistor 61 to receive the first feedback signal VA for generating a current I2. That is to say, the current I2 is generated by the output of the power management circuit 200 in accordance with the first feedback signal VA. At a light-load condition, an adaptive threshold can be the constant current I1. At a lighter-load or a no-load condition, the adaptive threshold can be the sum of the constant current I1 and the current I2. Therefore, the adaptive circuit generates the adaptive threshold in the feedback loop of the power converter in accordance with the first feedback signal VA. The first feedback signal VA is correlated to the feedback signal VFB. The adaptive threshold is thus adaptively generated in response to the feedback signal VFB. The feedback signal VFB is correlated to the output load and the output voltage VO of the power converter.
A first terminal of a switch 75 is coupled to an output of the adaptive circuit to receive the adaptive threshold. A second terminal of the switch 75 is coupled to the positive input of the comparator 85 and the join of the resistors 61 and 62. The output of the comparator 85 is further coupled to a control terminal of the switch 75 and controls the switch 75. Via the switch 75, the comparator 85 receives a value of the adaptive threshold for the second feedback signal VB in the feedback loop. The constant current I1, the current I2 and the resistance of the resistors 61, 62 determine the value of the adaptive threshold for the second feedback signal VB.
FIG. 3 is a schematic circuit of the power management circuit 200 in accordance with a preferred embodiment of the present invention. The power management circuit 200 comprises a current source 220, a switch 215 and a comparator 210. One terminal of the current source 220 receives the supply voltage VCC. A first terminal of the switch 215 is coupled to the other terminal of the current source 220. A second terminal of the switch 215 is coupled to the first terminal of the switch 75 and the other terminal of the current source 74 for generating the current I2. The switch 75 and the current source 74 are shown in FIG. 2. A negative input of the comparator 210 is coupled to the join of the source terminal of the level-shift transistor 60 and one terminal of the resistor 61 (shown in FIG. 2) to receive the first feedback signal VA. A threshold VT1 is supplied to a positive input of the comparator 210. An output of the comparator 210 is coupled to a control terminal of the switch 215 and controls the switch 215. That is to say, the comparator 210 is utilized to compare the first feedback signal VA with the threshold VT1. The switch 215 is turned on for generating the current I2 produced by the current source 220 once the value of the first feedback signal VA is lower than the value of the threshold VT1. On the other hand, the switch 215 is turned off to disable the current I2 once the value of the first feedback signal VA is high than the value of the threshold VT1.
Referring to FIG. 2, at the light-load condition, the adaptive threshold with a first value is formed by the constant current I1 once the value of the first feedback signal VA is higher than the value of the threshold VT1. At the lighter-load or the no-load condition, the adaptive threshold with a second value is formed by the sum of the constant current I1 and the current I2 once the value of the first feedback signal VA is lower than the value of the threshold VT1. Besides, the adaptive threshold with the first value (formed by the current source I1) is lower than the adaptive threshold with the second value (formed by the sum of the current source I1 and the current I2). Therefore, the adaptive circuit generates the adaptive threshold with the first value in response to a value of the first feedback signal VA within a first range in the light-load condition. The switch 215 is turned off when the value of the first feedback signal VA is within the first range, and this causes the generation of the adaptive threshold with the first value. The adaptive circuit generates the adaptive threshold with the second value in response to a value of the first feedback signal VA within a second range in the lighter-load or the no-load condition. The switch 215 is turned on when the value of the first feedback signal VA is within the second range, and this causes the generation of the adaptive threshold with the second value. In addition, the value of the first feedback signal VA within the first range is higher than the value of the first feedback signal VA within the second range. Because the first feedback signal VA is correlated to the feedback signal VFB, the adaptive threshold is adaptively generated in response to the feedback signal VFB, and the adaptive threshold is thus adaptively generated in response to the output load and the output voltage VO of the power converter.
FIG. 4 is a schematic circuit of the power management circuit 200 in accordance with another preferred embodiment the present invention. The power management circuit 200 comprises a voltage-to-current circuit and a current mirror circuit formed by transistors 271 and 272. The voltage-to-current circuit comprises an amplifier 260, a buffer amplifier 250, a resistor 265, and a transistor 270. A positive input of the amplifier 260 receives a threshold VT2. An output of the amplifier 260 is coupled to a gate terminal of the transistor 270 to control the transistor 270. A drain terminal of the transistor 270 is coupled to the current mirror circuit. A source terminal of the transistor 270 is coupled to a negative input of the amplifier 260 and one terminal of the resistor 265. The other terminal of the resistor 265 is coupled to a negative input of the buffer amplifier 250 and an output of the buffer amplifier 250. The first feedback signal VA is supplied to a positive input of the buffer amplifier 250. Therefore, the negative input of the buffer amplifier 250 and the output of the buffer amplifier 250 are coupled together. The voltage-to-current circuit is utilized to generate a current I270 once the value of the first feedback signal VA is lower than the value of the threshold VT2. The current I270 is generated by I270=[(VT2−VA)/R265]. Because the threshold VT2 is a constant value, the current I270 is determined by the first feedback signal VA.
The supply voltage VCC is supplied to source terminals of transistors 271 and 272. Gate terminals of transistors 271 and 272 are coupled together. A drain terminal of the transistor 271 is coupled to the drain terminal of the transistor 270 and gate terminals of transistors 271 and 272. A drain terminal of the transistor 272 is coupled to the first terminal of the switch 75 and the other terminal of the current source I1 to generate the current I2. The switch 75 and the current source I1 are shown in FIG. 2. Therefore, the current mirror circuit developed by transistors 271 and 272 receives the current I270 to generate the current I2 at the drain terminal of the transistor 272. That is to say, the current I2 is generated once the value of the first feedback signal VA is lower than the value of the threshold VT2 because the current I270 is correlated to the current I2. In other words, the power management circuit 200 is to enable the current I2 once the value of the first feedback signal VA is lower than the value of the threshold VT2. On the other hands, the power management circuit 200 is to disable the current I2 once the value of the first feedback signal VA is higher than the value of the threshold VT2. As mentioned above, since the threshold VT2 is a constant, the current I2 is produced in response to the value of the first feedback signal VA. In other words, the current I2 is produced in response to the value of the feedback signal VFB. In addition, the value of the first feedback signal VA is correlated to the output load of the power converter; the feedback signal VFB is also correlated to the output load of the power converter because the first feedback signal VA is proportional to the feedback signal VFB.
Referring to FIG. 2, at the light-load condition, the adaptive threshold with a first value is formed by the current source I1 once the value of the first feedback signal VA is higher than the value of the threshold VT2. At the lighter-load or the no-load condition, the adaptive threshold with a second value is formed by the sum of the current source I1 and the current I2 once the value of the first feedback signal VA is lower than the value of the threshold VT2. Besides, the adaptive threshold with the first value is thus lower than the adaptive threshold with the second value. Therefore, the adaptive circuit generates adaptive threshold with the first value in response to a value of the first feedback signal VA within a first range in the light-load condition. The current I2 is disabled when the value of the first feedback signal VA is within the first range, and this causes the generation of the adaptive threshold with the first value. The adaptive circuit generates the adaptive threshold with the second value in response to a value of the first feedback signal VA within a second range in the lighter-load or the no-load condition. The current I2 is enabled when the value of the first feedback signal VA is within the second range, and this causes the generation of the adaptive threshold with the second value. In addition, the value of the first feedback signal VA within the first range is higher than the value of the first feedback signal VA within the second range. Because the first feedback signal VA is correlated to the feedback signal VFB, the adaptive threshold is adaptively generated in response to the feedback signal VFB, and the adaptive threshold is thus adaptively generated in response to the output load and the output voltage VO of the power converter.
FIG. 5 shows the burst switching waveforms of the switching signal according to the present invention. The switching signal SW is generated at a gate terminal of the transistor 20 to control the transistor 20. At a light-load condition, the switching signal SW generates a burst period TBST for prolonging a switching period and shortening a switching frequency, which is shown in FIG. 5. The present invention provides the adaptive threshold for the burst switching of the power converter to achieve higher efficiency at the light load of the power converter. The efficiency will be improved in response to the decrease of the switching and the increase of the switching period. The burst period TBST is determined the value of the adaptive threshold.

## Claims

1. A control circuit for burst switching of a power converter, comprising:
an adaptive circuit generating an adaptive threshold in response to a feedback signal correlated to an output load of the power converter, wherein an output of the adaptive circuit is coupled to a first terminal of a switch; and
a switching circuit generating a switching signal to switch a transformer of the power converter in accordance with the adaptive threshold and the feedback signal for regulating an output of the power converter, wherein an input terminal of a comparing circuit of the switching circuit is coupled to a second terminal of the switch, and an output of the comparing circuit of the switching circuit is coupled to a control terminal of the switch;
wherein the feedback signal is applied in the generation of the switching signal and the adaptive threshold;
wherein the adaptive threshold increases as the output load decreases from a light load to no load.

2. The control circuit as claimed in claim 1, wherein the adaptive circuit generates the adaptive threshold with a first value in response to a value of the feedback signal within a first range, and generates the adaptive threshold with a second value in response to a value of the feedback signal within a second range.

3. The control circuit as claimed in claim 2, wherein the adaptive threshold with the first value is lower than the adaptive threshold with the second value, and the feedback signal within the first range is higher than the feedback signal within the second range.

4. The control circuit as claimed in claim 1, wherein the adaptive circuit comprises a power management circuit and a current source.

5. The control circuit as claimed in claim 4, wherein the adaptive threshold generated by the adaptive circuit is sum of the outputs of the power management circuit and the current source.

6. A method for controlling a power converter, comprising the steps of:
generating an adaptive threshold, by an adaptive circuit, in response to a feedback signal correlated to a load of the power converter, wherein an output of the adaptive circuit is coupled to a first terminal of a switch; and
generating a switching signal, by a switching circuit, to switch a transformer of the power converter in accordance with the adaptive threshold and the feedback signal for regulating an output of the power converter;
wherein an input terminal of a comparing circuit of the switching circuit is coupled to a second terminal of the switch, and an output of the comparing circuit of the switching circuit is coupled to a control terminal of the switch;
wherein an output of the comparing circuit of the switching circuit is arranged to control the switch for adjusting the feedback signal according to the adaptive threshold;
wherein the feedback signal is applied in the generation of the switching signal and the adaptive threshold;
wherein the adaptive threshold increases as the output load decreases from a light load to no load.

7. The method as claimed in claim 6, wherein the step of generating the adaptive threshold comprising:
generating the adaptive threshold with a first value in response to a value of the feedback signal with a first range.

8. The method as claimed in claim 7, wherein the step of generating the adaptive threshold further comprising:
generating the adaptive threshold with a second value in response to a value of the feedback signal with a second range.

9. The method as claimed in claim 8, wherein the adaptive threshold with the first value is lower than the adaptive threshold with the second value, and the value of the feedback signal within the first range is higher than the value of the feedback signal within the second range.

10. The method as claimed in claim 6, wherein the adaptive threshold is generated by the adaptive circuit comprising a power management circuit and a current source.

11. The method as claimed in claim 6, wherein the adaptive threshold is generated by sum of the outputs of a power management circuit and a current source.