TWI729617B - Power supply with duty cycle limiting circuit, duty cycle limiting circuit and method of operating the same - Google Patents

Abstract

A power supply with a duty cycle limiting circuit includes a conversion circuit, a drive circuit, a control unit, and a duty cycle limiting circuit. The duty cycle limiting circuit charges and discharges a control signal to generate a control voltage, and determines whether a power switch of a power supply is turned off according to the control voltage and a threshold voltage to limit a maximum duty cycle of the power switch by the threshold voltage.

Description

Power supply with duty-cycle limiting circuit, duty-cycle limiting circuit and operation thereof Working method

The present invention relates to a power supply with a duty-cycle limiting circuit, a duty-cycle limiting circuit and an operation method thereof, and in particular to a power supply and duty-cycle that limit the conduction time of the power switch of the power supply in one cycle Limit the circuit and its method of operation.

In the technical field of power supplies, a switching conversion circuit (such as, but not limited to, conversion circuits such as BUCK, BOOST, and FLYBACK) is usually used in the power supply as the main power conversion circuit of the power supply. The characteristic of the switching conversion circuit is that the power switch of the main loop in the conversion circuit must be controlled by the control unit to switch on so that the switching conversion circuit can convert the input power to the output power. Therefore, if the power switch is damaged, the power supply cannot normally supply the output power to the load.

Due to the above reasons, the protection of the power switch is particularly important when the power supply is abnormal. Specifically, when an abnormality occurs in the power supply, the control unit may detect errors due to misjudgments (for example, but not limited to, damage to current detection components, short circuits, or problems with measuring instruments). Factors such as voltage, input power or output power abnormality) provide a control signal with an incorrect duty cycle. When the duty cycle is too large, it will cause the power switch to be turned on for too long, which will easily cause the power switch to overheat and be damaged, thereby causing the risk of the power supply to burn.

Therefore, how to design a power supply with a duty-cycle limiting circuit, a duty-cycle limiting circuit, and an operating method thereof, and use the duty-cycle limiting circuit to limit the duty cycle of the power switch, so as to prevent the power switch from turning on for a long time. The condition of overheating damage is an important subject for the inventor of this case to study.

In order to solve the above-mentioned problems, the present invention provides a power supply with a duty cycle limiting circuit to overcome the problems of the prior art. Therefore, the power supply with the duty cycle limiting circuit of the present invention includes a conversion circuit including a power switch, and the power switch converts the input power to the output power by switching on. Drive circuit, drive power switch. The control unit is coupled to the drive circuit and provides control signals to the drive circuit. And a duty cycle limiting circuit, coupled to the driving circuit. Among them, the duty cycle limiting circuit charges and discharges the control signal to generate a control voltage, and determines whether to turn off the power switch according to the control voltage and the threshold voltage, so as to limit the maximum duty cycle of the power switch by the threshold voltage.

In order to solve the above-mentioned problems, the present invention provides a duty cycle limiting circuit to overcome the problems of the prior art. Therefore, the duty cycle limiting circuit of the present invention is coupled to the control unit of the power supply and the drive circuit. The drive circuit drives the power switch according to the control signal provided by the control unit. The duty cycle limiting circuit includes a charging and discharging circuit that receives the control signal. The comparison unit is coupled to the charging and discharging circuit. And the switch unit, which is coupled to the comparison unit and the driving circuit. Among them, the charging and discharging circuit charges and discharges the control signal to generate a control voltage, and the comparison unit determines whether the switch is turned on according to the control voltage and the threshold voltage. Unit to limit the maximum duty cycle of the power switch by controlling the driving circuit of the switch unit to turn off the power switch.

In order to solve the above-mentioned problems, the present invention provides an operating method of a duty cycle limiting circuit to overcome the problems of the prior art. Therefore, the operating method of the duty cycle limiting circuit of the present invention includes the following steps: (a) Charge and discharge the control signal of the control unit of the power supply to generate a control voltage. (b) Determine whether to turn off the power switch of the power supply according to the control voltage and the threshold voltage. And (c) when the control voltage is greater than the threshold voltage, the driving circuit of the control power supply turns off the power switch to limit the maximum duty cycle of the power switch through the threshold voltage.

In order to have a better understanding of the technology, means and effects adopted by the present invention to achieve the intended purpose, please refer to the following detailed description and drawings of the present invention. I believe that the purpose, features and characteristics of the present invention can be obtained from this in depth and For specific understanding, however, the accompanying drawings are only provided for reference and illustration, and are not intended to limit the present invention.

100, 100’: power supply

1: Conversion circuit

12: Power switch

Rs: current detection resistance

2: drive circuit

Q1: The first switch

Q2: The second switch

3: control unit

4: Duty cycle limiting circuit

42: charge and discharge circuit

422: charging circuit

R1: resistance

C: Energy storage element

424: Discharge Circuit

Qr: Energy release switch

R: impedance element

44: comparison unit

A1: The first input

A2: The second input

Out: output terminal

46: switch unit

X: first end

Y: second end

Z: control end

G: Ground point

200: load

Vin: input power

Vo: output power

Vcc: working voltage

Vc: Control voltage

Vr: threshold voltage

Sc: control signal

Sd: disable signal

Sa: Comparison signal

T1~T4: cycle

t1, t2: time period

(S200)~(S600): steps

Figure 1 is a circuit block diagram of a power supply with a duty cycle limiting circuit of the present invention; Figure 2A is a circuit block diagram of the duty cycle limiting circuit of the present invention and its first coupling method; Figure 2B is a circuit block diagram of the duty cycle limiting circuit of the present invention The circuit block diagram of the ratio limiting circuit and its second coupling method; FIG. 3 is a waveform diagram of the power supply with a duty ratio limiting circuit of the present invention; and FIG. 4 is a flow chart of the operating method of the duty ratio limiting circuit of the present invention Figure.

The technical content and detailed description of the present invention are described as follows in conjunction with the drawings: Please refer to FIG. 1 for a circuit block diagram of a power supply with a duty cycle limiting circuit of the present invention. The power supply 100 includes a conversion circuit 1, a driving circuit 2, a control unit 3, and a duty ratio limiting circuit 4. The conversion circuit 1 includes a power switch 12 that converts an input power Vin into an output power Vo by switching on. The power supply 100 provides the output power Vo to the load 200 to supply power required for the operation of the load 200. The control unit 3 is coupled to the conversion circuit 1 and the driving circuit 2, and provides a control signal Sc to the driving circuit 2 according to the feedback of the output power Vo, so that the driving circuit 2 drives the power switch 12 to switch on according to the control signal Sc. The control unit 3 controls the power supply 100 to stably provide the output power Vo by adjusting the duty ratio of the control signal Sc. The duty cycle limiting circuit 4 is coupled to the driving circuit 2 and the control unit 3, and provides a disable signal Sd according to the control signal Sc to limit the conduction time of the power switch 12 in one cycle, and the maximum conduction time is the maximum duty cycle .

Specifically, the main purpose of the present invention is to prevent the control unit 3 from erroneously providing a control signal Sc with an excessively high duty cycle when an abnormality occurs in the power supply 100 (that is, in a cycle, the ON time is too long. ) To the power switch 12, causing the power switch 12 to be overheated and damaged due to excessive conduction time. Therefore, the present invention uses the duty cycle limiting circuit 4 to limit the maximum on-time (that is, the maximum duty cycle) of the power switch 12, so as to prevent the power switch 12 from being overheated and damaged due to excessive conduction time under any circumstances. . Furthermore, the duty cycle limiting circuit 4 receives the control signal Sc, and charges and discharges the control signal Sc to generate a control voltage. Then, the duty cycle limiting circuit 4 determines whether the drive circuit 2 is controlled by the disable signal Sd to turn off the power switch 12 according to the control voltage and the threshold voltage. The threshold voltage is the voltage corresponding to the maximum duty cycle of the power switch 12. When the control voltage is greater than the threshold voltage, it means that the duty cycle of the control signal Sc is greater than the maximum duty cycle of the power switch 12. Therefore, when the duty ratio limiting circuit 4 determines that the control voltage is greater than the threshold voltage, the duty ratio limiting circuit 4 uses the disable signal Sd to control the driving circuit 2 to turn off the power switch 12. Please refer to the following paragraphs for detailed circuit operation. Since the present invention focuses on the power switch 12 inside the conversion circuit 1 in the power supply 100 Due to the empty ratio limitation method, the embodiments of FIGS. 2A and 2B only present the circuits related to power switch control in the conversion circuit 1 (including the power switch 12, the driving circuit 2, the control unit 3, and the duty ratio limitation circuit 4). The other parts of the circuit in circuit 1 are omitted.

Please refer to FIG. 2A for a circuit block diagram of the duty cycle limiting circuit and its first coupling method of the present invention. For convenience of description, this embodiment takes the power supply 100 with an isolation transformer as an example, but it is not limited to this. In other words, the conversion circuit 1 inside the power supply 100 can be of other types of converter architectures. The primary side of the transformer of the power supply 100 is coupled to the power switch 12 and the current detection resistor Rs, and receives the input power Vin. The driving circuit 2 is coupled between the control unit 3 and the power switch 12, and the duty cycle limiting circuit 4 is coupled to the driving circuit 2. The driving circuit 2 includes a first switch Q1 and a second switch Q2. The first terminal X of the first switch Q1 is coupled to the operating voltage Vcc, and the second terminal Y of the first switch Q1 is coupled to the second terminal Y of the second switch Q2. It is also coupled to the control terminal of the power switch 12. The first terminal X of the second switch Q2 is coupled to the ground point G, and the control terminals Z of the first switch Q1 and the second switch Q2 are coupled to the control unit 3 to receive the control signal Sc. Specifically, the first level of the control signal Sc turns on the first switch Q1 and turns off the second switch Q2, so that the operating voltage Vcc turns on the power switch 12 through the first switch Q1. Conversely, the second level of the control signal Sc will turn off the first switch Q1 and turn on the second switch Q2, so that the control terminal of the power switch 12 is coupled to the ground point G and turned off. Among them, the working voltage Vcc can be provided by the control unit 3 or by an external power supply.

The duty ratio limiting circuit 4 includes a charging and discharging circuit 42, a comparing unit 44 and a switch unit 46, and the comparing unit 44 is coupled to the charging and discharging circuit 42 and the switch unit 46. The charging and discharging circuit 42 is coupled to the control unit 3, and the switch unit 46 is coupled to the second terminal Y of the first switch Q1 and the second switch Q2 (that is, the switch unit 46 is coupled to the control terminal of the power switch 12). The charging and discharging circuit 42 includes a charging circuit 422 and a discharging circuit 424, and the charging circuit 422 is coupled to the control unit 3 and the comparing unit 44, and the discharging circuit 424 is coupled to the charging circuit 422. Specifically, the charging circuit 422 includes a resistor R1 and an energy storage element C, and the discharging circuit 424 includes an energy release switch Qr and an impedance element R. One end of the resistor R1 is coupled to the control unit 3 and receives the control signal Sc. The other end is coupled to one end of the energy storage element C. The energy release switch Qr includes a first terminal X, a second terminal Y, and a control terminal Z. The second terminal Y of the energy release switch Qr is coupled to the other end of the resistor R1 and one end of the energy storage element C, and the control terminal Z is coupled to the resistor One end of R1 is connected to the control unit 3. The first terminal X of the energy release switch Qr is coupled to one end of the impedance element R, and the other end of the impedance element R is coupled to the other end of the energy storage element C and the ground point G. Among them, the energy storage element C can be, for example, but not limited to, an energy storage element such as a capacitor. If cost and design complexity are considered, the capacitor is a better energy storage element C. The impedance element R can be, for example, but not limited to, an element that consumes energy such as a resistor. If cost and design complexity are considered, the resistor is a preferable impedance element R.

Furthermore, when the control signal Sc is at the first level (high level in this embodiment), the control signal Sc at the first level charges the energy storage element C to generate a control voltage Vc, and the resistor R1 The current for charging the energy storage element C is limited (for the energy storage element C as a capacitor, the control signal Sc at the first level charges the energy storage element C to generate the control voltage Vc of the capacitor charging waveform). At this time, since the second terminal Y of the disabling switch Qr receives the control voltage Vc, and the control terminal Z receives the control signal Sc of the first level, the disabling switch Qr is turned off and the discharge circuit 424 is turned off. Therefore, when the control signal Sc is at the first level for a longer time, the peak value of the control voltage Vc generated by the energy storage element C is higher, and vice versa. Therefore, by using this voltage peak value, the length of time that the control signal Sc is at the first level can be known. When the control signal Sc is at the second level (low level in this embodiment), the second terminal Y of the release switch Qr receives the control voltage Vc, and the control terminal Z receives the control signal Sc of the second level. The discharging switch Qr is turned on to cause the discharge circuit 424 to form a discharge path. At this time, the control voltage Vc stored in the energy storage element C is discharged (discharged) by the path of the energy release switch Qr and the impedance element R. Thereby, in each cycle of the control signal Sc, the charging and discharging circuit 42 performs charging and discharging, and generates a control voltage Vc corresponding to the length of time the control signal Sc is at the first level.

The comparison unit 44 includes a first input terminal A1, a second input terminal A2, and an output terminal Out, and the first input terminal A1 is coupled to the resistor R1 and the energy storage element C to receive the control voltage Vc. The second input terminal A2 receives the threshold voltage Vr, and the output terminal Out provides a comparison signal Sa to the control terminal Z of the switch unit 46. open The first terminal X of the switch unit 46 is coupled to the second terminal Y of the first switch Q1 and the second switch Q2, and the second terminal Y of the switch unit 46 is coupled to the ground point G. When the control voltage Vc is less than the threshold voltage Vr, the comparison signal Sa provided by the comparison unit 44 turns off the switch unit 46, and when the control voltage Vc is greater than the threshold voltage Vr, the comparison signal Sa provided by the comparison unit 44 turns on the switch unit 46.

When the switch unit 46 is turned off, the path from the second end Y of the first switch Q1 and the second switch Q2 to the ground point G is open. Therefore, the second terminal Y of the first switch Q1 and the second switch Q2 will not be coupled to the ground point G. Therefore, the operating voltage Vcc when the control signal Sc is at the first level can be provided to the control terminal of the power switch 12 to enable the power switch 12 to be turned on. Therefore, the conduction of the power switch 12 is switched as the level of the control signal Sc changes (that is, the disable signal Sd provided by the switch unit 46 cannot inhibit the drive circuit 2 so that the drive circuit 2 drives according to the control signal Sc Power switch 12). When the switch unit 46 is turned on, the second terminals Y of the first switch Q1 and the second switch Q2 are coupled to the ground point G. Therefore, when the control signal Sc is at the first level, the operating voltage Vcc originally provided to the control terminal of the power switch 12 is pulled to a potential close to ground and cannot turn on the power switch 12, so that the power switch 12 is turned off (that is, the switching unit The disabling signal Sd provided by 46 disables the driving circuit 2 so as to turn off the power switch 12 by disabling the driving circuit 2). Therefore, when the switch unit 46 is turned on, no matter what the level of the control signal Sc is, the power switch 12 cannot be controlled by the driving circuit 2. Thereby, the switch unit 46 is used to turn on the path from the second end Y of the first switch Q1 and the second switch Q2 to the ground point G to limit the maximum duty cycle of the power switch 12.

Please refer to FIG. 2B for a circuit block diagram of the duty cycle limiting circuit and the second coupling method of the present invention, and for the combination thereof, refer to FIGS. 1 to 2A. The difference between the power supply 100' of this embodiment and the power supply 100 of FIG. 2A is that the first terminal X of the switch unit 46 is coupled to the control terminals Z of the first switch Q1 and the second switch Q2. When the switch unit 46 is turned off, the path from the control terminal Z of the first switch Q1 and the second switch Q2 to the ground point G is open. Therefore, the control terminals Z of the first switch Q1 and the second switch Q2 are not coupled to the ground point G. Therefore, the control signal Sc of the first level can turn on the power switch 12 by turning on the first switch Q1. Therefore, whether the power switch 12 is turned on or not is switched as the level of the control signal Sc changes. When open When the shutdown unit 46 is turned on, the control terminals Z of the first switch Q1 and the second switch Q2 are coupled to the ground point G. Therefore, the control signal Sc of the first level is pulled close to the ground potential and cannot turn on the first switch Q1, so that the power switch 12 is turned off. Therefore, when the switch unit 46 is turned on, no matter what the level of the control signal Sc is, the power switch 12 cannot be controlled by the driving circuit 2. Thereby, the switch unit 46 is used to turn on the path from the control terminal Z of the first switch Q1 and the second switch Q2 to the ground point G to limit the maximum duty ratio of the control signal of the power switch 12.

It is worth mentioning that the first terminal X of the switch unit 46 in this embodiment is coupled to the control terminals Z of the first switch Q1 and the second switch Q2. Therefore, when the switch unit 46 is turned on, the first switch Q1 and the second switch Q2 cannot be driven by the control signal Sc, which happens to be opposite to the first switch Q1 and the second switch Q2 in the embodiment of FIG. 2A. Therefore, compared with the coupling method of the embodiment in FIG. 2A, the coupling method of this embodiment can achieve the effect of reducing the power consumption of the power supply 100'.

Please refer to FIG. 3 for a waveform diagram of a power supply with a duty cycle limiting circuit according to the present invention, and refer to FIGS. 1 to 2B for complex cooperation. The duty of the control signal Sc during the period T1~T2 is relatively small, and the duty during the period T3~T4 is relatively large. When the control signal Sc is at the first level (meaning high level), the control signal Sc at the first level charges the energy storage element C (capacitor) to generate a control voltage Vc, and the second level (meaning The low level) control signal Sc turns on the release switch Qr to release the control voltage Vc. Then, the comparison unit 44 compares the control voltage Vc with the threshold voltage Vr. Since the duty ratio of the control signal Sc in the period T3~T4 is relatively large, the charging time of the energy storage element C is relatively long, so that the voltage peak value is relatively high. This further causes the control voltage Vc in the periods t1 and t2 to be greater than the threshold voltage Vr in the periods T3 to T4, so that the comparison unit 44 provides a high-level comparison signal Sa during the periods t1 and t2 to turn on the switch unit 46, so that the switch unit 46 provides The disable signal Sd disables the drive circuit 2. Therefore, the driving circuit 2 is disabled during the time periods t1 and t2, so that the power switch 12 is turned off during the time periods t1 and t2. Therefore, comparing the control signal Sc of Fig. 3 with the signal received by the control terminal of the power switch 12, it is obvious that the duty cycle of the control terminal signal of the power switch 12 is limited to the maximum duty cycle. Below the duty ratio, the duty ratio limiting circuit 4 has the effect of shielding the control signal Sc that is larger than the maximum duty ratio.

Please refer to FIG. 4 for a flow chart of the operation method of the duty cycle limiting circuit of the present invention, and refer to FIGS. 1 to 3 for compound cooperation. The method first includes: the duty cycle limiting circuit charges and discharges the control signal of the control unit to generate a control voltage (S200). When the control signal Sc is at the first level, the control signal Sc at the first level charges the energy storage element C of the charging circuit 422 to generate a control voltage Vc, and when the control signal Sc is at the second level, the discharge circuit The energy release switch Qr of 424 is turned on so that the control voltage Vc stored in the energy storage element C is quickly released from the path of the energy release switch Qr and the impedance element R. Then, the duty ratio limiting circuit determines whether to turn off the power switch according to the control voltage and the threshold voltage (S400). The comparison unit 44 of the duty cycle limiting circuit 4 receives the control voltage Vc and the threshold voltage Vr, and provides a comparison signal Sa to the switch unit 46 according to the control voltage Vc and the threshold voltage Vr. When the control voltage Vc is less than the threshold voltage Vr, the comparison signal Sa provided by the comparison unit 44 turns off the switch unit 46, and when the control voltage Vc is greater than the threshold voltage Vr, the comparison signal Sa provided by the comparison unit 44 turns on the switch unit 46.

Finally, when the control voltage is greater than the threshold voltage, the duty cycle limiting circuit controls the driving circuit to turn off the power switch (S600). When the switch unit 46 is turned off, the path from the second end Y of the first switch Q1 and the second switch Q2 to the ground point G is open. Therefore, the conduction of the power switch 12 is switched as the level of the control signal Sc changes (that is, the disable signal Sd provided by the switch unit 46 cannot inhibit the drive circuit 2 so that the drive circuit 2 drives according to the control signal Sc Power switch 12). When the switch unit 46 is turned on, the path from the second end Y of the first switch Q1 and the second switch Q2 to the ground point G is turned on. Therefore, when the control signal Sc is at the first level, the signal at the control terminal of the power switch 12 (ie, the second terminal Y of the first switch Q1 and the second switch Q2) is pulled down to a level that is insufficient to turn on the power switch 12, so that the power The switch 12 is turned off (that is, the disabling signal Sd provided by the switch unit 46 disables the driving circuit 2 to turn off the power switch 12 by disabling the driving circuit 2). In this way, the switch unit 46 is used to turn on the path from the second terminal Y of the first switch Q1 and the second switch Q2 to the ground point G to limit the maximum duty cycle of the signal at the control terminal of the power switch 12. Among them, can pass The effect of disabling the driving circuit 2 is achieved by coupling the control signal Sc of the first level to the ground point G or coupling the driving end of the driving circuit 2 (ie, the control end of the power switch 12) to the ground point G.

In summary, the main advantages and effects of the embodiments of the present invention are that the duty cycle limiting circuit of the present invention is coupled to the control unit and the driving circuit of the power supply, which can be avoided by the operation of the duty cycle limiting circuit. When the power supply is abnormal, the control unit erroneously provides a control signal with an excessively high duty cycle to the power switch of the power supply, causing the power switch to be overheated and damaged due to the excessive conduction time.

However, the above are only detailed descriptions and drawings of the preferred embodiments of the present invention. However, the characteristics of the present invention are not limited to these, and are not intended to limit the present invention. The full scope of the present invention should be referred to the following application The scope of the patent shall prevail. All embodiments that conform to the spirit of the scope of the patent application of the present invention and similar variations should be included in the scope of the present invention. Anyone familiar with the art in the field of the present invention can easily think of it. Changes or modifications can be covered in the following patent scope of this case. In addition, the features mentioned in the scope of the patent application and the specification can be implemented individually or in any combination.

100: power supply

1: Conversion circuit

12: Power switch

2: drive circuit

3: control unit

4: Duty cycle limiting circuit

200: load

Vin: input power

Vo: output power

Sc: control signal

Sd: disable signal

Claims (19)

A power supply with a duty cycle limiting circuit includes: a conversion circuit, including a power switch, the power switch is used to convert an input power source into an output power source by switching on; a driving circuit is used to drive the Power switch; a control unit coupled to the drive circuit and used to provide a control signal to the drive circuit; and a duty cycle limiting circuit coupled to the drive circuit; wherein the duty cycle limiting circuit controls the The signal is charged and discharged to generate a control voltage, and the duty cycle limiting circuit determines whether to turn off the power switch according to the control voltage and a threshold voltage, so as to limit a maximum duty cycle of the power switch by the threshold voltage. For the power supply described in claim 1, wherein when the duty ratio limiting circuit determines that the control voltage is greater than the threshold voltage, the duty ratio limiting circuit controls the driving circuit to turn off the power switch. For the power supply described in claim 1, wherein the duty ratio limiting circuit includes: a charging and discharging circuit, which receives the control signal; a comparison unit, which is coupled to the charging and discharging circuit; and a switch unit, which is coupled The comparison unit is connected to the drive circuit; wherein the charge and discharge circuit generates the control voltage according to the control signal to charge and discharge, and the comparison unit determines whether the switch unit is turned on according to the control voltage and the threshold voltage to pass The switch unit controls the drive circuit to turn off the power switch. For the power supply described in item 3 of the scope of patent application, when the control voltage is greater than the threshold voltage, the comparison unit turns on the switch unit to cause the drive circuit to turn off the power switch. For the power supply described in claim 3, the charging and discharging circuit includes: a charging circuit coupled to the control unit and the comparing unit; and a discharging circuit coupled to the charging circuit; wherein the control A first level of the signal causes the charging circuit to charge to generate the control voltage, and a second level of the control signal causes the discharge circuit to discharge the control voltage. The power supply according to item 5 of the scope of patent application, wherein the discharge circuit includes: a discharging switch coupled to the charging circuit; and an impedance element coupled to the discharging switch; wherein, the second level The control signal turns on the release switch, so that the control voltage is released through the impedance element. The power supply according to claim 1, wherein the driving circuit includes: a first switch coupled to a working voltage and the control unit; and a second switch coupled to the first switch and the control unit Unit and a ground point; wherein, a first level of the control signal turns on the first switch, and turns off the second switch, so that the operating voltage turns on the power switch; a second level of the control signal turns off The first switch is turned off, and the second switch is turned on, so that a control terminal of the power switch is coupled to the ground point and turned off. For the power supply described in claim 7, wherein the duty cycle limiting circuit is coupled to the control unit, and it is determined whether the control signal of the first level is coupled to the ground point according to the control voltage. For the power supply described in item 7 of the scope of patent application, wherein the duty cycle limiting circuit is coupled to the control terminal of the power switch, and it is determined whether the control terminal of the power switch is coupled to the connection according to the control voltage. location. A duty cycle limiting circuit is coupled to a control unit of a power supply and a drive circuit. The drive circuit drives a power switch according to a control signal provided by the control unit. The duty cycle limiting circuit includes: a charge and discharge A circuit for receiving the control signal; a comparison unit coupled to the charging and discharging circuit and receiving a threshold voltage; and a switch unit coupled to the comparison unit and the driving circuit; wherein the charging and discharging circuit controls the The signal is charged and discharged to generate a control voltage, and the comparison unit determines whether to turn on the switch unit according to the control voltage and the threshold voltage, so as to limit a maximum duty of the power switch by turning off the power switch by the switch unit ratio. For the duty cycle limiting circuit described in item 10 of the scope of patent application, when the control voltage is greater than the threshold voltage, the comparison unit turns on the switch unit to cause the drive circuit to turn off the power switch. According to the duty cycle limiting circuit described in claim 10, the charging and discharging circuit includes: a charging circuit coupled to the control unit and the comparing unit; and a discharging circuit coupled to the charging circuit; wherein, A first level of the control signal causes the charging circuit to charge to generate the control voltage, and a second level of the control signal causes the discharge circuit to discharge the control voltage. The duty cycle limiting circuit according to item 12 of the scope of patent application, wherein the discharging circuit includes: a discharging switch coupled to the charging circuit; and an impedance element coupled to the discharging switch; wherein, the second The control signal at the level turns on the discharging switch, so that the control voltage is discharged through the impedance element. In the duty cycle limiting circuit described in claim 10, the switch unit is coupled to the control unit, and it is determined whether the control signal is coupled to a ground point according to the control voltage. In the duty cycle limiting circuit described in claim 10, the switch unit is coupled to the power switch, and determines whether a control terminal of the power switch is coupled to a ground point according to the control voltage. An operating method of a duty cycle limiting circuit is suitable for limiting the duty cycle of a power switch of a power supply. The operating method includes the following steps: (a) A control unit of the power supply controls the power switch A control signal is charged and discharged to generate a control voltage; (b) judge whether to turn off the power switch according to the control voltage and a threshold voltage; and (c) when the control voltage is greater than the threshold voltage, control the power supply A driving circuit of the power switch turns off the power switch to limit a maximum duty cycle of the power switch through the threshold voltage. According to the operating method of the duty cycle limiting circuit described in the scope of patent application, step (a) further includes: (a1) charging according to a first level of the control signal to generate the control voltage, and according to A second level of the control signal discharges the control voltage. The operating method of the duty cycle limiting circuit as described in item 17 of the scope of patent application, wherein step (a1) further includes: According to the control voltage, it is determined whether the control signal with the first level is coupled to a ground point so that the driving circuit turns off the power switch. According to the operating method of the duty ratio limiting circuit described in item 17 of the scope of patent application, step (a1) further includes: judging whether a control terminal of the power switch is coupled to a ground point to make the driving circuit according to the control voltage Turn off the power switch.

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