TW201543795A - Flyback converter with over current protection and control circuit thereof - Google Patents

Abstract

The present invention discloses a flyback converter with over current protection and control circuit thereof. The flyback converter includes a primary winding, a switching element coupling to the primary winding, a secondary winding and an auxiliary winding. The auxiliary winding is used to detect the output voltage for outputting a first voltage. A control circuit outputs a control signal for controlling a switch according to the first voltage. The control circuit includes an overcurrent determining unit and an output voltage detection unit. The overcurrent determining unit coupled to the switch outputs a control signal according to a current sensing signal and a setting voltage. The current sensing signal is corresponding to the current of the primary winding. The output voltage detection unit is coupled to the auxiliary winding and the overcurrent determining unit, and adjusts the setting voltage according to the first voltage. Thus, the control signal is adaptively adjusted according to the first voltage.

Description

Flyback converter with improved overcurrent protection and its control circuit

The present invention provides a flyback converter and its control circuit, and more particularly to a flyback converter with improved overcurrent protection and its control circuit.

Since the flyback converter has advantages of high efficiency, low loss, small size, and light weight, it has been widely used as a power conversion device for various electronic products. In order for the flyback converter to operate normally and stably, the protection mechanism of the flyback converter is a very important part, such as overcurrent protection.

The traditional flyback converter only uses the detection of the primary peak current as a protection mechanism for the output overcurrent. Traditionally, the overcurrent protection point of the primary current is fixed, so a constant voltage protection mechanism with lower voltage and larger current can be realized at the output end. However, when the output voltage of the flyback converter is caused by an overcurrent, the excessive protection current not only increases the input power dissipation but also causes unnecessary power consumption. And the need to use higher current rated power components, and greatly increase the burden of cost.

The present invention provides a control circuit for a flyback converter with improved overcurrent protection. The flyback converter includes a primary side winding, a secondary side winding, and an auxiliary winding. The auxiliary winding detects the output voltage and correspondingly outputs the first voltage. The control circuit outputs a control signal correspondingly according to the first voltage to control the switching element of the flyback converter. The control circuit includes an overcurrent determination unit and an output voltage detection unit. Over electricity The flow determining unit is coupled to the switching element, and outputs a control signal according to the current sensing signal and the set voltage. The current sensing signal corresponds to the current of the primary side winding. The output voltage detecting unit is coupled between the auxiliary winding and the overcurrent determining unit. The output voltage detecting unit adjusts the set voltage correspondingly according to the first voltage, so that the control signal is adaptively adjusted according to the first voltage.

The present invention provides a flyback converter with improved overcurrent protection including a transformer, a switching element, a current sensing unit, and a control circuit. The transformer has a primary winding, a secondary winding and an auxiliary winding, wherein the auxiliary winding is configured to detect an output voltage of the flyback converter and correspondingly output the first voltage. The switching element is coupled to the primary side winding. The current sensing side circuit is coupled between the switching element and the ground end to convert the current of the primary side winding into a current sensing signal. The control circuit outputs a control signal according to the first voltage and correspondingly to control the switching element, wherein the control circuit includes an overcurrent determining unit and an output voltage detecting unit. The overcurrent determination unit is coupled to the switching component, and outputs a control signal according to the current sensing signal and the set voltage. The output voltage detecting unit is coupled between the auxiliary winding and the overcurrent determining unit. The output voltage detecting unit adjusts the set voltage correspondingly according to the first voltage, so that the control signal adaptively adjusts the current sensing signal according to the first voltage.

In summary, the flyback converter with improved overcurrent protection and the control circuit thereof according to the embodiments of the present invention can automatically adjust the set voltage according to the manner in which the auxiliary winding detects the output voltage, so as to achieve more effective protection. Flyback converter. In other words, when the output voltage is low due to the overcurrent, the output voltage detecting unit adjusts the set voltage correspondingly according to the first voltage generated by the output voltage detected by the auxiliary winding, so that the overcurrent judging unit can act early To limit the transfer of more energy to the output. Therefore, not only the overall power consumption of the flyback converter can be reduced, but also the power components with higher current ratings are not required, and the burden of cost is not increased.

In order to further understand the technology, method and effect of the present invention in order to achieve the intended purpose, reference should be made to the detailed description and drawings of the present invention. Understand, of course The drawings and the annexed drawings are only for the purpose of illustration and description, and are not intended to limit the invention.

1, 2, 3‧‧‧ flyback converter

10‧‧‧Transformers

20‧‧‧Switching elements

30‧‧‧ Current sensing unit

40‧‧‧Control circuit

50‧‧‧secondary circuit

60, 80‧‧‧ voltage divider circuit

410, 710‧‧‧ Output voltage detection unit

420, 720‧‧‧Overcurrent judgment unit

510‧‧‧Rectifier filter circuit

520‧‧‧Return circuit

521a‧‧‧Lighting diode

521b‧‧‧Photoelectric crystal

U1‧‧‧Optocoupler

U2‧‧‧Three-terminal shunt regulator

D1‧‧‧First Diode

D2‧‧‧ second diode

D3‧‧‧Rected Diode

R1‧‧‧first resistance

R2‧‧‧second resistance

C1‧‧‧first capacitor

R3, R4, R5, R6, R7, R8‧‧‧ resistors

C2, C3‧‧‧ capacitor

V1‧‧‧ first voltage

V2‧‧‧Set voltage

VG‧‧‧ control signal

VB‧‧‧ DC bias

IN‧‧‧Input voltage

OUT, VOUT‧‧‧ output voltage

IOUT‧‧‧Output current

GND‧‧‧ ground terminal

NP‧‧‧ primary winding

NA‧‧‧Auxiliary winding

NS‧‧‧ secondary winding

VCS‧‧‧ current sensing signal

VFB‧‧‧Responsive signal voltage

RCS‧‧‧ sense resistor

OCP_Vref‧‧‧reference voltage

COMP1‧‧‧First Comparator

FIG. 1 is a schematic diagram of a control circuit for a flyback converter with improved overcurrent protection according to an embodiment of the present invention.

FIG. 2 is a specific circuit diagram of a flyback converter with improved overcurrent protection according to another embodiment of the present invention.

3 is a graph showing the relationship between the output voltage and the output current of the flyback converter with improved overcurrent protection of FIG. 2.

4 is a specific circuit diagram of a flyback converter with improved overcurrent protection according to another embodiment of the present invention.

Various illustrative embodiments are described more fully hereinafter with reference to the accompanying drawings. However, the inventive concept may be embodied in many different forms and should not be construed as being limited to the illustrative embodiments set forth herein. Rather, these exemplary embodiments are provided so that this invention will be in the In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Similar numbers always indicate similar components.

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, such elements are not limited by the terms. These terms are used to distinguish one element from another. Thus, a first element discussed below could be termed a second element without departing from the teachings of the inventive concept. As used herein, the term "or" may include all combinations of any one or more of the associated listed items.

[Embodiment of a flyback converter with improved overcurrent protection]

First, please refer to FIG. 1 , which is an improvement provided by an embodiment of the present invention. Schematic diagram of a flyback converter with overcurrent protection. As shown in FIG. 1, the flyback converter 1 includes a transformer 10, a switching element 20, a current sensing unit 30, a control circuit 40, and a secondary side circuit 50. The transformer 10 has a primary side winding NP, a secondary side winding NS, and an auxiliary winding NA. The switching element 20 is coupled to the primary side winding NP. The current sensing unit 30 is coupled between the switching element 20 and the ground GND. The control circuit 40 includes an output voltage detecting unit 410 and an overcurrent determining unit 420. The secondary side circuit 50 is coupled to the secondary side winding NS to form an output voltage VOUT and supply power to a load (not shown).

In the present embodiment, the sensing resistor RCS can be used as the current sensing unit 30 and coupled between the switching element 20 and the ground GND. When the switching element 20 is turned on, the sensing resistor RCS can be used to sense the primary side current flowing through the primary side winding NP and generate a current sensing signal VCS representing the primary side current. However, in other embodiments, the circuit architecture of the current sensing unit 30 may have different circuit architectures, and those skilled in the art may design according to actual usage, and the present invention is not limited thereto.

In addition, in the embodiment, the switching element 20 is a metal oxide semiconductor field effect transistor (MOSFET), the gate G of the switching element 20 is coupled to the overcurrent determining unit 420, and the drain D of the switching element 20 is coupled once. The side winding NP and the source S of the switching element 20 are coupled to the current sensing unit 30. However, in other embodiments, the switching element 20 may be a bipolar junction transistor (BJT), other types of transistors, or other types of switches, and thus the invention is not limited.

In the above, after the flyback converter 1 receives the input voltage VIN, the switching of the switching element 20 is controlled by the control circuit 40, thereby transmitting the input voltage VIN to the primary winding NP of the transformer 10, so that the transformer 10 will input the voltage. VIN is converted to an output voltage VOUT to supply power to a load (not shown). Further, when the flyback converter 1 is turned on at the switching element 20, the energy will be stored in the transformer 10, and the output corresponding to the change in the magnitude of the primary current flowing through the switching element 20 The current sensing signal VCS is to the overcurrent determining unit 420. this In addition, once the switching element 20 is turned off, the energy stored by the transformer 10 will be discharged to the output terminal OUT of the flyback converter 1 via the secondary winding NS, while the auxiliary winding NA of the transformer 10 will be returned according to the return. The output voltage VOUT outputted by the converter 1 generates a first voltage V1 correspondingly, and inputs the first voltage V1 to the output voltage detecting unit 410.

The overcurrent determining unit 420 is coupled to the switching element 20 for comparing the current sensing signal VCS with a set voltage V2 and generating a control signal VG according to the comparison result to control switching of the switching element 20. On the other hand, the output voltage detecting unit 410 is coupled to the auxiliary winding NA and the overcurrent determining unit 420 for receiving the first voltage V1, and correspondingly adjusting the set voltage V2 according to the first voltage V1, so that the control signal VG can be The adaptive adjustment is made based on the first voltage V1 to further control the switching of the switching element 20.

For example, since the auxiliary winding NA is used to detect the output voltage VOUT and correspondingly output the first voltage V1, the output terminal OUT of the flyback converter 1 with improved overcurrent protection is output due to overcurrent. When the voltage VOUT falls, the first voltage V1 generated by the auxiliary winding NA also decreases. Accordingly, the output voltage detecting unit 410 adjusts the set voltage V2 correspondingly according to the reduced first voltage V1, that is, lowers the set voltage V2. In other words, due to the decrease of the set voltage V2, the overcurrent determination unit 420 can operate early to control the switching of the switching element 20 to limit the transfer of greater energy to the output terminal OUT of the flyback converter 1 to achieve protection tripping. The effect of the converter 1.

[Embodiment of a flyback converter circuit with improved overcurrent protection]

Please refer to FIG. 2. FIG. 2 is a circuit diagram of a flyback converter with improved overcurrent protection according to an embodiment of the present invention. The flyback converter 2 of FIG. 2 is similar in structure to the outer flyback converter 1 of FIG. 1, and the same elements that are included in the following will be denoted by the same reference numerals, and further, the output voltage is further illustrated. The circuit structure of the detection unit 410 and the overcurrent determination unit 420 is detailed.

In this embodiment, the output voltage detecting unit 410 includes a first diode D1. The first resistor R1, the first capacitor C1, the second resistor R2, and the second diode D2. The anode end of the first diode D1 is coupled to the auxiliary winding NA. One end of the first resistor R1 is coupled to the cathode end of the first diode D1. The first capacitor C1 is coupled between the other end of the first resistor R1 and the ground GND. The second resistor R2 is coupled between the other end of the first resistor R1 and the ground GND. The anode end of the second diode D2 is coupled to one end of the second resistor R2, and the cathode end of the second diode D2 is coupled to the overcurrent determining unit 420.

The overcurrent determination unit 420 includes a first comparator COMP1, wherein the first comparator COMP1 has a first input, a second input, and a first output. The first input end is coupled to the cathode end of the second diode D2 for receiving the first voltage V1. The second input terminal is coupled to the source terminal S of the switching element 20 to receive the current sensing signal VCS. The first output terminal controls the switching of the switching element 20 according to the comparison result of the current sensing signal VCS and the set voltage V2, wherein the set voltage V2 is a reference voltage OCP_Vref.

In addition, the resistor R3 and the resistor R4 form a voltage dividing circuit 60 for dividing the DC bias voltage VB to generate a reference voltage OCP_Vref, and inputting the reference voltage OCP_Vref to the first input terminal of the comparator COMP1.

Next, the working principle of the flyback converter 2 will be further explained. Specifically, in operation, after the flyback converter 1 receives the input voltage VIN, the first comparator COMP1 in the overcurrent determination unit 420 transmits a comparison result of the sensing signal VCS and the set voltage V2 to generate a control signal. VG, and switching the switching element 20 according to the control signal VG, so that the transformer 10 converts the input voltage VIN into an output voltage VOUT to supply power to a load (not shown).

However, when the output terminal of the flyback converter 1 is caused by an overcurrent, the output voltage VOUT is decreased. At this time, the auxiliary winding NA correspondingly generates the first voltage V1 according to the detected output voltage VOUT, and passes the first a diode D1 and a voltage dividing circuit formed by the resistor R1 and the resistor R2 adjust the first voltage V1 to form a lower first voltage V1, wherein the first capacitor C1 is used to filter the first voltage V1. Noise.

Then, the lower first voltage V1 is injected into the second via the second diode D2. A first input of the comparator COMP1, thereby reducing the set voltage V2 (ie, the reference voltage OCP_Vref) at the first input of the first comparator COMP1. In other words, the set voltage V2 at the first input terminal of the first comparator COMP1 can be appropriately adjusted via the output voltage detecting unit 410. Therefore, when the first comparator COMP1 compares the current sensing signal VCS with the set voltage V2 (ie, the reference voltage OCP_Vref) due to the decrease of the set voltage V2 (ie, the reference voltage OCP_Vref), the current sensing signal VCS is higher than the set voltage V2 ( That is, the reference voltage OCP_Vref) will occur early, that is, the first comparator COMP1 will perform an early output operation to output a control signal VG to control the switching of the switching element 20. Accordingly, the magnitude of the primary side current is reduced, and further energy transfer to the output terminal is further limited to achieve the effect of protecting the flyback converter 2.

In other words, referring to FIG. 3, FIG. 3 is a diagram showing the relationship between the output voltage and the output current of the flyback converter with improved overcurrent protection of FIG. 2. In FIG. 3, the vertical axis represents the output voltage VOUT, and the horizontal axis represents the output current IOUT. Curve C1 represents the relationship between the output voltage VOUT of the conventional flyback converter and the output current IOUT. A curve C2 shows a relationship between the output voltage VOUT of the flyback converter 1 of the present embodiment and the output current IOUT.

It can be seen from FIG. 3 that when the conventional flyback converter and the flyback converter 1 of the present embodiment simultaneously generate an overcurrent at the output end, the flyback converter 1 of the present embodiment has The current determination unit 420 performs an earlier action than the conventional flyback converter to limit greater energy transfer to the output. In addition, the curves C1 and C2 can be known that the output voltage VOUT outputted by the flyback converter 1 of the present embodiment is lower than that of the conventional flyback converter when the overcurrent occurs at the output terminal, and The output current IOUT is also low. Therefore, by detecting the output voltage VOUT by the output voltage detecting unit 410 to correspondingly adjust the set voltage V2 of the first output end of the first comparator COMP1, the power consumption of the input power source can be reduced, and a higher current is not required. Rated power components.

[Another embodiment of a flyback converter circuit with improved overcurrent protection]

Please refer to FIG. 4. FIG. 4 is a circuit diagram of a flyback converter with improved overcurrent protection according to another embodiment of the present invention. The flyback converter 3 of FIG. 4 is similar in structure to the flyback converters 1, 2 of FIGS. 1, 2, and the same elements that are included in the following will be denoted by the same reference numerals. The difference between the flyback converters 2 and 4 is that the coupling manners of the second diodes D2 in the output voltage detecting units 410 and 710 are not the same. Further, the set voltage V2 received by the first output terminal of the first comparator COMP1 in the overcurrent determination units 420, 720 is also different. In addition, in this embodiment, the circuit structure of the detail of the secondary side circuit 50 is further explained.

In detail, as shown in FIG. 4, the output voltage detecting unit 710 includes a first diode D1, a first resistor R1, a first capacitor C1, a second resistor R2, and a second diode D2. The anode end of the first diode D1 is coupled to the auxiliary winding NA. One end of the first resistor R1 is coupled to the cathode end of the first diode D1. The first capacitor C1 is coupled between the other end of the first resistor R1 and the ground GND. The second resistor R2 is coupled between the other end of the first resistor R1 and the ground GND. The cathode end of the second diode D2 is coupled to one end of the second resistor R2, and the anode end of the second diode D2 is coupled to the overcurrent determining unit 720.

The overcurrent determination unit 720 includes a first comparator COMP1, wherein the first comparator COMP1 has a first input, a second input, and a first output. The first input end is coupled to the anode end of the second diode D2. The second input terminal is coupled to the source terminal S of the switching element 20 to receive the current sensing signal VCS. The first output end of the first comparator COMP1 controls the switching of the switching element 20 according to the comparison result of the current sensing signal VCS and the set voltage V2, wherein, in this embodiment, the set voltage V2 is one. The voltage VFB of the feedback signal.

On the other hand, the secondary side circuit 50 is coupled to the secondary side winding NS to form an output voltage VOUT, and supplies power to a load (not shown). The secondary side circuit 50 includes a rectification filter circuit 510 and a feedback circuit 520. The rectifying and filtering circuit 510 includes a rectifying diode D3 and a capacitor C2. The anode of the rectifier diode D2 is coupled to the secondary winding NS, the cathode of the rectifier diode D2 is coupled to one end of the capacitor C2, and the capacitor C2 is coupled. Between the diode diode D2 and the ground GND. The rectifying and filtering circuit 510 is configured to rectify and filter the voltage outputted by the secondary winding NS.

The feedback circuit 520 is configured to sample the output voltage VOUT to generate a voltage VFB of the feedback signal corresponding to the load, wherein the feedback circuit 520 includes the resistors R5 R R7 , the capacitor C3 , the light emitting diode 521 a of the photocoupler U1 , and the third The three-terminal shunt regulator U2 has an anode terminal A, a cathode terminal K and a reference terminal R. It is worth mentioning that the photonic crystal 521b of the photocoupler U1 is disposed in the primary side circuit and is connected in series with the resistor R8 to form a voltage dividing circuit 80 for forming a voltage VFB of the feedback signal and inputting it to the first A first input of a comparator COMP1.

Further, the feedback circuit 520 samples the output voltage VOUT through the resistors R6 and R7, and then the sampled output voltage is input to the reference terminal R of the three-terminal shunt regulator U2, and the three-terminal shunt regulator U2 passes through the inside. The circuit in turn generates a corresponding current If flowing through resistor R5. It can be seen from this that the magnitude of the current If corresponds to the magnitude of the load. Further, when the current If passes through the light-emitting diode 521a of the photocoupler U1, a corresponding current is also generated in the photo-crystal 521b of the photocoupler U1 in the primary-side circuit. Therefore, the magnitude of the current generated in the photo-crystal 521b of the photocoupler U1 also corresponds to the amount of load, and can be used as the voltage VFB of the feedback signal.

Next, the working principle of the flyback converter 3 will be further explained. In detail, after the flyback converter 1 receives the input voltage VIN, the first comparator COMP1 in the overcurrent determination unit 720 transmits a comparison between the sense signal VCS and the set voltage V2 (ie, the voltage VFB of the feedback signal). As a result, the control signal VG is generated, and the switching element 20 is switched in accordance with the control signal VG to cause the transformer 10 to convert the input voltage VIN into the output voltage VOUT to supply power to a load (not shown). Therefore, in normal operation, the auxiliary winding NA correspondingly generates the first voltage V1 according to the output voltage VOUT, and then the first voltage V1 passes through the first diode D1 and the voltage dividing circuit formed by the resistor R1 and the resistor R2. At this time, the voltage level of the first voltage V1 is higher than the voltage VFB of the feedback signal, that is, the second diode D2 at this time. The voltage level at the cathode end is higher than the voltage level at the anode end of the second diode D2, and due to the characteristics of the diode, the current can only flow in a single direction. That is, current can only flow from the anode to the cathode, but not from the cathode to the anode. Therefore, when the flyback converter 3 is in normal operation, the output voltage detecting unit 710 cannot output the first voltage V1 to the first input terminal of the first comparator COMP1.

However, when the output terminal of the flyback converter 3 is caused by an overcurrent, the output voltage VOUT is decreased. At this time, the first voltage V1 correspondingly output by the auxiliary winding NA according to the detected output voltage VOUT is also lowered, so that The voltage level at the cathode end of the second diode D2 is lower than the voltage level at the anode terminal of the second diode D2. At this time, the voltage VFB of the feedback signal is discharged through the second diode D2, thereby reducing the voltage. The set voltage V2 at the first input of the first comparator COMP1 (ie, the voltage VFB of the feedback signal). In other words, the set voltage V2 (ie, the voltage VFB of the feedback signal) located at the first input end of the first comparator COMP1 is appropriately adjusted by turning on the second diode D2.

Accordingly, the current sense signal is compared when the first comparator COMP1 compares the current sense signal VCS with the set voltage V2 (ie, the voltage VFB of the feedback signal) due to the decrease of the set voltage V2 (ie, the voltage VFB of the feedback signal). The VCS is higher than the set voltage V2 (ie, the voltage VFB of the feedback signal), that is, the first comparator COMP1 operates early to output the control signal VG to control the switching of the switching element 20, and further, due to the switching element 20 The switching can be switched early, thus reducing the magnitude of the primary side current and further limiting the transfer of more energy to the output to achieve the effect of protecting the flyback converter 3.

[The possible effects of the invention]

In summary, the flyback converter with improved overcurrent protection and the control circuit thereof according to the embodiments of the present invention can detect the output voltage according to the auxiliary winding and adjust the set voltage correspondingly, so that the switching component 20 can be early The operation to protect the flyback converter more effectively.

In other words, when the output voltage is low due to overcurrent, the output is low. The voltage detecting unit can adjust the set voltage correspondingly according to the output voltage detected by the auxiliary winding, so that the overcurrent determining unit can act early according to the adjusted set voltage to output a control signal to control the switching element to limit greater energy transfer. To the output. Therefore, not only the overall power consumption of the flyback converter can be reduced, but also the power components with higher current ratings are not required, and the burden of cost is not increased.

In summary, the flyback converter with improved overcurrent protection and the control circuit thereof provided by the embodiments of the present invention can reduce the input power dissipation by using fewer components and minimum line changes. .

The above description is only the preferred embodiment of the present invention, but the features of the present invention are not limited thereto, and any one skilled in the art can easily change or modify it in the field of the present invention. Covered in the following patent scope of this case.

1‧‧‧return converter

10‧‧‧Transformers

20‧‧‧Switching elements

30‧‧‧ Current sensing unit

40‧‧‧Control circuit

50‧‧‧secondary circuit

410‧‧‧Output voltage detection unit

420‧‧‧Overcurrent judgment unit

V1‧‧‧ first voltage

VG‧‧‧ control signal

IN‧‧‧Input voltage

OUT‧‧‧ output voltage

GND‧‧‧ ground terminal

NP‧‧‧ primary winding

NA‧‧‧Auxiliary winding

NS‧‧‧ secondary winding

VCS‧‧‧ current sensing signal

RCS‧‧‧ sense resistor

Claims (10)

A control circuit for a flyback converter, the flyback converter comprising a primary side winding, a switching element, a secondary side winding and an auxiliary winding, the switching element being coupled to the primary side winding, The auxiliary winding is configured to detect an output voltage of the flyback converter and correspondingly output a first voltage, and the control circuit outputs a control signal according to the first voltage to control switching of the switching element, the control circuit The method includes: an overcurrent determining unit coupled to the switching component, the overcurrent determining unit comparing a current sensing signal and a set voltage to generate the control signal, wherein the current sensing signal corresponds to the primary winding And an output voltage detecting unit coupled to the auxiliary winding and the overcurrent determining unit, the output voltage detecting unit adjusting the set voltage according to the first voltage, so that the control signal is according to the first voltage Make adaptive adjustments. The control circuit of claim 1, wherein the output voltage detecting unit comprises: a first diode, an anode end of the first diode is coupled to the auxiliary winding; and a first resistor, the first resistor One end of the first resistor is coupled to the cathode end of the first diode; a first capacitor is coupled between the other end of the first resistor and a ground; a second resistor, the first a second resistor is coupled between the other end of the first resistor and the ground; and a second diode, one end of the second diode is coupled to one end of the second resistor, the second diode The other end of the circuit is coupled to the overcurrent judging unit. The control circuit of claim 1, wherein the overcurrent determining unit comprises: a first comparator having a first input terminal, a second input terminal and a first output terminal, the first input terminal Receiving the set voltage, the second input receiving The current sensing signal outputs a control signal according to the current sensing signal and the set voltage comparison result to control switching of the switching element, wherein the set voltage is a reference voltage. The control circuit of claim 1, wherein the overcurrent determining unit comprises: a first comparator having a first input terminal, a second input terminal and a first output terminal, the first input terminal Receiving the set voltage, the second input terminal receives the current sensing signal, and the first output terminal outputs the control signal according to the comparison result of the current sensing signal and the set voltage to control switching of the switching element, wherein the setting The voltage is the voltage of a feedback signal. The control circuit of claim 4, wherein the voltage of the feedback signal is generated by an optical coupler according to the output voltage of the flyback converter. A flyback converter with improved overcurrent protection includes: a transformer having a primary side winding, a secondary side winding and an auxiliary winding, wherein the auxiliary winding is configured to detect one of the flyback converters Outputting a voltage and correspondingly outputting a first voltage; a switching element coupled to the primary side winding; a current sensing unit coupled between the switching element and a ground, converting the current of the primary winding to a current sensing signal; and a control circuit, according to the first voltage and correspondingly outputting a control signal to control switching of the switching element, wherein the control circuit comprises: an overcurrent determining unit coupled to the switching element, And the overcurrent determining unit compares the current sensing signal with a set voltage to generate the control signal; and an output voltage detecting unit coupled to the auxiliary winding and the overcurrent determining unit, the output voltage detecting unit The set voltage is correspondingly adjusted according to the first voltage, so that the control signal is adaptively adjusted according to the first voltage. A flyback converter with improved overcurrent protection as described in claim 6 The output voltage detecting unit includes: a first diode, an anode end of the first diode is coupled to the auxiliary winding; and a first resistor, one end of the first resistor is coupled to the first diode a cathode of the body; a first capacitor coupled between the other end of the first resistor and a ground; a second resistor coupled to the other end of the first resistor And a second diode, one end of the second diode is coupled to one end of the second resistor, and the other end of the second diode is coupled to the overcurrent determining unit. The flyback converter with improved overcurrent protection according to claim 6, wherein the overcurrent determination unit comprises: a first comparator having a first input terminal, a second input terminal, and a first input terminal An output terminal receives the set voltage, the second input terminal receives the current sensing signal, and the first output terminal outputs the control signal according to the current sensing signal and the set voltage comparison result to control Switching of the switching element, wherein the set voltage is a reference voltage. The flyback converter with improved overcurrent protection according to claim 6, wherein the overcurrent determination unit comprises: a first comparator having a first input terminal, a second input terminal, and a first input terminal An output terminal receives the set voltage, the second input terminal receives the current sensing signal, and the first output terminal outputs the control signal according to the current sensing signal and the set voltage comparison result to control Switching of the switching element, wherein the set voltage is a voltage of a feedback signal. The flyback converter with improved overcurrent protection according to claim 9, wherein the voltage of the feedback signal is generated by an optical coupler according to the output voltage of the flyback converter.