High-voltage input flyback converter and switching power supply system

Abstract

Provided in the present application are a high-voltage input flyback converter and a switching power supply system. The flyback converter comprises: a first voltage input control circuit, a second voltage input control circuit, a current sampling circuit and a pulse width modulation controller, wherein the current sampling circuit comprises a first sampling resistor and a second sampling resistor; the first voltage input control circuit is connected to both a power supply and one end of the first sampling resistor, and the other end of the first sampling resistor is connected to one end of the second sampling resistor; the second voltage input control circuit is connected to both the other end of the second sampling resistor and the power supply, and the other end of the second sampling resistor is grounded; and a current sampling end of the pulse width modulation controller is connected to one end of the first sampling resistor. By means of implementation of the present application, a circuit sampling structure in which a resistor is combined with a current transformer is replaced with a structure in which two sampling resistors are combined, so that the design cost can be effectively reduced, the available space on a carrier board can be increased, and the universality of a product can be improved.

Description

A high-voltage input flyback converter and switching power supply system Technical Field

[0001] The present application relates to the field of circuit technology, and in particular to a flyback converter with a high-voltage input and a switching power supply system. Background Art

[0002] In the field of switching power supplies, it is often necessary to sample the current in the primary circuit and then transmit the sampled signal to a pulse-width modulation (PWM) controller to perform pulse-width modulation (PWM) on the semiconductor switches to achieve a stable output voltage. In low-voltage scenarios, such as mains input, current sampling circuits typically use sampling resistors. In high-voltage scenarios, such as three-phase AC input and photovoltaic DC input, related technologies typically use a sampling resistor combined with a current transformer. The sampling signals transmitted by the two sampling devices are combined to produce a final current sampling signal, which the PWM controller uses to modulate the PWM pulse width. However, current transformers are difficult to produce and are large in size, occupying a large board area. Furthermore, using small current transformers, such as those with a core wire diameter of 0.1mm or 0.08mm and 100 or dozens of turns, can easily lead to wire breakage during production and application. Therefore, it is necessary to design a current sampling structure with low cost, high device delivery capability, and stable performance. Technical issues

[0003] The main purpose of this application is to provide a high-voltage input flyback converter and a switching power supply system, aiming to solve the problems of low delivery capability and high cost of devices used in current sampling structures in related technologies. Technical Solutions

[0004] To achieve the above objectives, the present application provides, in a first aspect, a flyback converter with a high-voltage input, comprising: a first voltage input control circuit, a second voltage input control circuit, a current sampling circuit, and a pulse width modulation controller; wherein the current sampling circuit comprises a first sampling resistor and a second sampling resistor, the first voltage input control circuit being electrically connected to one end of the first sampling resistor of a power supply, the other end of the first sampling resistor being electrically connected to one end of the second sampling resistor, the other end of the second sampling resistor being electrically connected to one end of the second voltage input control circuit, and the other end of the second sampling resistor being grounded, the other end of the second voltage input control circuit being electrically connected to the power supply, and the current sampling end of the pulse width modulation controller being electrically connected to one end of the first sampling resistor.

[0005] Furthermore, the first voltage input control circuit includes a first primary winding of a transformer and a first switching tube, and the first sampling resistor includes a first shunt; the first primary winding is coupled to the secondary winding of the transformer, and the secondary winding is used to provide voltage to the corresponding load, one end of the first primary winding is electrically connected to the positive pole of the power supply, the other end of the first primary winding is electrically connected to the first end of the first switching tube, and the second end of the first switching tube is electrically connected to the first shunt; or, one end of the first primary winding is electrically connected to the first end of the first switching tube, the other end of the first primary winding is electrically connected to the first shunt, and the second end of the first switching tube is electrically connected to the positive pole of the power supply.

[0006] Furthermore, the first switch tube includes a MOS tube, the drain of the MOS tube is electrically connected to the first primary winding, the source of the MOS tube is electrically connected to the first shunt, and the gate of the MOS tube is used to be electrically connected to the drive circuit.

[0007] Furthermore, the first voltage input control circuit also includes a first buffer circuit, which includes a first capacitor, a first resistor, a second resistor, a third resistor and a diode; the first capacitor is electrically connected to the first primary winding and one end of the first resistor, respectively, the other end of the first resistor and one end of the second resistor are both electrically connected to the negative pole of the diode, the third resistor is electrically connected to one end of the first primary winding and the other end of the second resistor, and the negative pole of the diode is electrically connected to the other end of the first primary winding.

[0008] Furthermore, the second voltage input control circuit includes a second primary winding of the transformer, a second switching tube and a second buffer circuit, and the second sampling resistor includes a second shunt; two ends of the second buffer circuit are electrically connected to two ends of the second primary winding, the second primary winding is coupled to the secondary winding, one end of the second primary winding is electrically connected to the second shunt, the other end of the second primary winding is electrically connected to the first end of the second switching tube, and the second end of the second switching tube is electrically connected to the negative pole of the power supply; or, one end of the second primary winding is electrically connected to the first end of the second switching tube, the other end of the second primary winding is electrically connected to the negative pole of the power supply, and the second end of the second switching tube is electrically connected to the second shunt.

[0009] Furthermore, it includes a second capacitor and a third capacitor, the second capacitor is electrically connected to the positive electrode of the power supply and the other end of the first sampling resistor respectively, and the third capacitor is electrically connected to the negative electrode of the power supply and one end of the second sampling resistor respectively.

[0010] Furthermore, a filter circuit is included, one end of the filter circuit is electrically connected to one end of the first sampling resistor, and the other end of the filter circuit is electrically connected to the pulse width modulation controller.

[0011] Furthermore, the resistance of the first sampling resistor is equal to that of the second sampling resistor.

[0012] A second aspect of the present application provides a switching power supply system, comprising a load and the flyback converter as described in the first aspect of the present application, wherein the load is electrically connected to the voltage output terminal of the flyback converter. Beneficial effects

[0013] From the above description, it can be seen that this application is aimed at the scenario of high-voltage input, and adopts a current sampling structure combining two sampling resistors, and realizes the current sampling function through the internal conversion function of the pulse width modulation controller. Compared with the related technology that uses a sampling resistor combined with a current transformer structure, it has lower design cost, smaller occupied volume and wider universality. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without paying any creative work.

[0015] FIG1 is a schematic structural diagram of a flyback converter with a high-voltage input according to an embodiment of the present application;

[0016] FIG2 is a circuit diagram of a first high-voltage input flyback converter according to an embodiment of the present application;

[0017] FIG3 is a circuit diagram of a second high-voltage input flyback converter according to an embodiment of the present application;

[0018] FIG4 is a circuit diagram of a third high-voltage input flyback converter according to an embodiment of the present application;

[0019] FIG5 is a schematic structural diagram of another high-voltage input flyback converter according to an embodiment of the present application. Modes for Carrying Out the Invention

[0020] In order to make the purpose, features, and advantages of the invention of this application more obvious and easy to understand, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are only part of the embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without making creative efforts shall fall within the scope of protection of this application.

[0021] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the embodiments of this application, the meaning of "plurality" is two or more, unless otherwise clearly specified.

[0022] In the related art, due to the problems of low delivery capability and high cost of devices used in the current sampling structure, an embodiment of the present application provides a flyback converter with a high-voltage input.

[0023] FIG1 is a schematic diagram of a high-voltage input flyback converter according to an embodiment of the present application. The high-voltage input flyback converter includes a first voltage input control circuit 100, a second voltage input control circuit 200, a current sampling circuit 300, and a pulse width modulation controller 400. The current sampling circuit 300 includes a first sampling resistor 310 and a second sampling resistor 320. The first voltage input control circuit 100 is electrically connected to a power supply and one end of the first sampling resistor 310, respectively. The other end of the first sampling resistor 310 is electrically connected to one end of the second sampling resistor 320, and the other end of the second sampling resistor 320 is electrically connected to one end of the second voltage input control circuit 200. The other end of the second sampling resistor 320 is grounded, and the other end of the second voltage input control circuit 200 is electrically connected to the power supply. A current sampling terminal of the pulse width modulation controller 400 is electrically connected to one end of the first sampling resistor 310. Furthermore, the resistance values ​​of the first sampling resistor 310 and the second sampling resistor 320 are equal.

[0024] It is understandable that in the field of switching power supplies, when the input source is low voltage, such as mains input, since the 220V AC input voltage is generally lower than 400V after passing through the rectifier bus, the voltage input control circuit can use conventional devices, and voltage conversion can be achieved using a single voltage input control circuit; when the input source is high voltage, such as using three-phase AC as the input source (bus voltage exceeds 780V) or using photovoltaic DC input exceeding 700V, conventional devices will no longer be applicable, and there are fewer and more expensive optional high-voltage control devices. Therefore, the circuit topology can be changed to adapt to high-voltage scenarios. The optional topology structures include dual or triple flyback structures. The current sampling structure designed in this example is applied to the dual overlapping flyback structure.

[0025] The double-overlap flyback converter in this embodiment includes two voltage control circuits, which have similar structures. The current sampling circuit adopts a topological structure of two current sampling resistors. The two sampling resistors are respectively connected to the two voltage control circuits. The input current flows through the first voltage control circuit, the first sampling resistor, the second sampling resistor, and the second voltage control circuit in sequence. Then, a reference ground GND is set at the common connection terminal of the second sampling resistor and the second voltage control circuit to achieve the same function as the sampling resistor + current transformer solution. The two sampling resistors can be made of the same material, and the manufacturing process of the sampling resistor is mature and the price is relatively low, so the design cost and the universality of the product can be effectively improved. In addition, according to the sampling resistor and the current sampling voltage V CS The corresponding relationship: V CS = R CS1 *I pri + R CS2 *I pri ; Among them, I pri is the input current value, R CS1 、R CS2 The resistance values ​​of the first sampling resistor and the second sampling resistor, respectively, can be set to be equal to obtain a relatively ideal current sampling signal. However, when using a sampling resistor + current transformer, it is necessary to select a suitable transformer primary-to-secondary turns ratio to adjust the resistance of the current transformer so that the resistance of the current transformer is equal to that of the sampling resistor. Therefore, the current sampling structure in this embodiment is simpler and more applicable than the structure of the related art.

[0026] As shown in FIG2 and FIG3 , respectively, are circuit schematic diagrams of a first flyback converter and a second flyback converter provided in this embodiment. Referring to FIG2 and FIG3 , the first voltage input control circuit 100 includes a first primary winding N1 of a transformer T1 and a first switch tube Q1, and the first sampling resistor 310 includes a first shunt RCS1. The first primary winding N1 is coupled to a secondary winding N3 of the transformer T1, and the secondary winding N3 is used to provide a voltage to a corresponding load. One end of the first primary winding N1 is electrically connected to the positive electrode of the power supply, the other end of the first primary winding N1 is electrically connected to the first end of the first switch tube Q1, and the second end of the first switch tube Q1 is electrically connected to the first shunt RCS1. Alternatively, one end of the first primary winding N1 is electrically connected to the first end of the first switch tube Q1, the other end of the first primary winding N1 is electrically connected to the first shunt RCS1, and the second end of the first switch tube Q1 is electrically connected to the positive electrode of the power supply.

[0027] Specifically, the first voltage control circuit in this embodiment mainly includes a first primary winding of a transformer and a first switching tube, the first switching tube is connected in series to the first primary winding, and the secondary winding of the transformer is used to output voltage to an external power load. A rectifier circuit structure can also be added between the secondary winding and the load. The rectifier circuit can rectify the pulsating voltage, such as a diode rectifier, which includes a diode and a capacitor. The positive electrode of the diode is connected to one end of the secondary winding N3, the negative electrode of the diode is connected to the load, and the two ends of the capacitor are connected to the two ends of the secondary winding N3. Other topologies such as synchronous rectification can also be selected according to actual topology and actual needs; one end of the first primary winding of the transformer is used to connect to AC power, and accordingly, an electromagnetic interference filter (EMI) can also be set on the power input side. Filter) and a full-bridge rectifier circuit to obtain a smooth and stable input power supply. The other end of the first primary winding is connected to the first end of the first switching tube, and the second end of the first switching tube is connected to the signal drive terminal of the PWM controller. The PWM controller obtains the input current sampling signal and adjusts the output voltage based on the current sampling signal. Specifically, the PWM controller calculates the duty cycle based on the current sampling voltage of the primary circuit and the secondary output voltage feedback. Then, the bias of the switching tube, such as the base of the transistor or the gate of the MOS tube, is modulated to achieve a change in the conduction time of the transistor or MOS tube, thereby achieving a change in the output of the switching regulated power supply. The third end of the first switching tube is connected to the first sampling resistor to transmit the input current to the first sampling resistor. In this embodiment, the first sampling resistor can be a shunt. A shunt is a resistor that can pass a very large current and can be used to detect large current values.

[0028] 2 , the first switch Q1 includes a MOS tube, a drain of the MOS tube is electrically connected to the first primary winding N1 , a source of the MOS tube is electrically connected to the first shunt RCS1 , and a gate of the MOS tube is electrically connected to the drive circuit.

[0029] Specifically, in this embodiment, the first switching transistor is a MOS transistor. The gate of the MOS transistor is connected to the drive circuit and is used to receive a switching control signal. The switching control signal can be used to control the conduction of the switching transistor to sample the input current and control the output voltage. For example, when the MOS transistor is on, the input source charges the transformer T1. When the MOS transistor is off, the energy stored in the transformer T1 is released on the secondary side, and the secondary-side rectifier circuit rectifies the pulsating energy into a command value. Depending on the input and output voltages and power levels, the voltage input control circuit can operate in continuous mode or discontinuous mode.

[0030] Further, referring to Figure 2, the first voltage input control circuit 100 also includes a first buffer circuit, which includes a first capacitor C1, a first resistor R1, a second resistor R2, a third resistor R3 and a diode D1; the first capacitor C1 is electrically connected to the first primary winding N1 and one end of the first resistor R1, respectively, the other end of the first resistor R1 and one end of the second resistor R2 are both electrically connected to the cathode of the diode D1, the third resistor R3 is electrically connected to one end of the first primary winding N1 and the other end of the second resistor R2, respectively, and the cathode of the diode D1 is electrically connected to the other end of the first primary winding N1.

[0031] Specifically, the first voltage control circuit in this embodiment also includes a snubber circuit, which can reduce transient spikes caused by the rapid rise and fall of the switching device. The snubber circuit can be a passive snubber circuit or an active snubber circuit. Active snubber circuits primarily use switching transistors. This embodiment uses a passive snubber circuit, which primarily includes a capacitor, a resistor, and a diode. This circuit is used to control oscillations generated by reactive components in the circuit, thereby improving circuit reliability and efficiency, reducing electromagnetic interference (EMI), and achieving a higher operating frequency. Furthermore, the diode in the snubber circuit in this embodiment can be configured as shown in FIG4 , where the anode of diode D1 is connected to capacitor C1 and resistor R3, respectively, and the cathode is connected to one end of the first primary winding N1.

[0032] Further, referring to Figures 2, 3, and 4, the second voltage input control circuit 200 includes a second primary winding N2 of the transformer T1, a second switch tube Q2, and a second buffer circuit. The second sampling resistor 210 includes a second shunt RCS2. Two ends of the second buffer circuit are electrically connected to two ends of the second primary winding N2. The second primary winding N2 is coupled to the secondary winding N3. One end of the second primary winding N2 is electrically connected to the second shunt RCS2, the other end of the second primary winding N2 is electrically connected to the first end of the second switch tube Q2, and the second end of the second switch tube Q2 is electrically connected to the negative electrode of the power supply. Alternatively, one end of the second primary winding N2 is electrically connected to the first end of the second switch tube Q2, the other end of the second primary winding N2 is electrically connected to the negative electrode of the power supply, and the second end of the second switch tube Q2 is electrically connected to the second shunt RCS2.

[0033] Specifically, the flyback converter in this embodiment employs a dual-switch flyback topology. It is understood that, compared to a single-switch circuit, the input voltage of the dual-switch flyback converter can be doubled. While maintaining the same output power as a single-switch flyback converter, the current capacity of the switch used in the dual-switch flyback converter, operating at twice the input voltage of the single-switch flyback converter, can be reduced to half that of a single-ended converter. This effectively addresses the voltage withstand issue of the switch. The first voltage input control circuit in this embodiment has a similar structure to the second voltage input control circuit, also including a primary winding, a switch, and a snubber circuit. The second sampling resistor connected thereto can also be a shunt. The snubber circuit also employs the same passive snubber circuit, including resistors R4, R5, and R6, capacitor C2, and diode D2. The second switch Q2 can also be a MOS transistor, and its gate is also electrically connected to the drive circuit to receive a drive signal. Similar to D1, diode D2 in the snubber circuit can be located at either the upper or lower end of the capacitor and resistor unit.

[0034] Furthermore, referring to FIG2 , a second capacitor C3 and a third capacitor C4 are also included. The second capacitor C3 is electrically connected to the positive electrode of the power supply and the other end of the first sampling resistor RCS1, respectively. The third capacitor C4 is electrically connected to the negative electrode of the power supply and one end of the second sampling resistor RCS2, respectively.

[0035] Specifically, in this embodiment, capacitors C3 and C4 are used to establish equal operating voltages for the upper and lower transistors at the moment of power-up. Even after the power supply is operational, they continue to balance the voltages and filter out midpoint deviations caused by transient disturbances. It should be noted that the dual-overlap flyback converter, which simply connects the switches in series, can result in voltage balancing between the two switches. However, by using symmetrical primary windings N1 and N2 and identical capacitors C3 and C4, the circuit automatically corrects any deviation in the midpoint voltages of the two switches (i.e., the midpoints of the two capacitor connection lines), thereby achieving automatic voltage balancing.

[0036] FIG5 is a schematic diagram of the structure of another flyback converter with a high-voltage input according to this embodiment. Referring to FIG5 , the flyback converter further includes a filter circuit 500 . One end of the filter circuit 500 is electrically connected to one end of the first sampling resistor 310 , and the other end of the filter circuit 500 is electrically connected to the pulse width modulation controller 400 .

[0037] Specifically, the filter circuit 500 includes a fourth resistor R510 and a fourth capacitor C520. One end of the fourth resistor R510 is electrically connected to one end of the first sampling resistor RCS1, the other end of the fourth resistor R510 is electrically connected to one end of the fourth capacitor C520, and the other end of the fourth capacitor C520 is grounded. The common connection terminal of the fourth resistor R510 and the fourth capacitor C520 is electrically connected to the pulse width modulation controller 400. The filter circuit in this embodiment is a first-order low-pass filter. The connection terminal between the resistor R510 and the fourth capacitor C520 serves as the output terminal of the low-pass filter, which is used to output the filtered current sampling voltage VCS to the current sampling terminal CS of the pulse width modulation controller. In addition, the PWM controller in this embodiment can generate a corresponding control signal based on the result of the sampling signal to drive the switch tube to control the on-time and switching frequency of the switch tube, thereby controlling the constant voltage output of the switching power supply. When the switch tube is on, current flows through the switch tube, and the PWM controller can obtain the voltage across the sampling resistor immediately before the switch tube is turned off. In actual circuits, due to the potential turn-off delay of the switching tube, the sampling resistor's sampling action may be delayed relative to the actual turn-off moment of the switching tube, resulting in a certain sampling error. Therefore, a compensation resistor can be provided to compensate for the sampling error to accurately obtain the current sampling voltage Vcs. The compensation resistor can be provided between the first sampling resistor RCS1 and the PWM controller, with one end of the compensation resistor connected to one end of the first sampling resistor RCS1 and the other end connected to the current sampling CS terminal of the PWM controller. In this way, the current in the switching power supply increases as the AC input voltage rises, and this can be further compensated by the compensation resistor. If the turn-off delay of the switching tube is long and more compensation is required, the resistance of the compensation resistor can be further increased.

[0038] The flyback converter with high-voltage input provided in the embodiment of the present application adopts a current sampling structure combining two sampling resistors, and realizes the current sampling function through the internal conversion function of the pulse width modulation controller. Compared with the structure of combining a sampling resistor with a current transformer in the related art, it has lower design cost, smaller occupied volume and wider universality.

[0039] An embodiment of the present application further provides a switching power supply system, which includes a load and the above-mentioned flyback converter, wherein the load is electrically connected to the voltage output terminal of the flyback converter.

[0040] It should be noted that the various embodiments in the present application are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same and similar parts between the various embodiments can be referenced to each other.

[0041] It should also be noted that, in the present application, relational terms such as first and second, etc. are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article, or device. In the absence of further restrictions, an element defined by the statement "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or device comprising the element.

[0042] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure is not limited to the embodiments shown herein but is intended to be applied in the broadest manner consistent with the principles and novel features disclosed herein.

Claims

1. A flyback converter with high voltage input, characterized in that: include: A first voltage input control circuit, a second voltage input control circuit, a current sampling circuit and a pulse width modulation controller; The current sampling circuit includes a first sampling resistor and a second sampling resistor, the first voltage input control circuit is electrically connected to a power supply and one end of the first sampling resistor, respectively, the other end of the first sampling resistor is electrically connected to one end of the second sampling resistor, the other end of the second sampling resistor is electrically connected to one end of the second voltage input control circuit, and the other end of the second sampling resistor is grounded. The other end of the second voltage input control circuit is electrically connected to the power supply, and the current sampling end of the pulse width modulation controller is electrically connected to one end of the first sampling resistor.

2. The flyback converter according to claim 1, wherein: The first voltage input control circuit includes a first primary winding of a transformer and a first switching tube, and the first sampling resistor includes a first shunt; the first primary winding is coupled to a secondary winding of the transformer, and the secondary winding is used to provide voltage to a corresponding load, one end of the first primary winding is electrically connected to the positive electrode of the power supply, the other end of the first primary winding is electrically connected to the first end of the first switching tube, and the second end of the first switching tube is electrically connected to the first shunt; Alternatively, one end of the first primary winding is electrically connected to the first end of the first switching tube, the other end of the first primary winding is electrically connected to the first shunt, and the second end of the first switching tube is electrically connected to the positive pole of the power supply.

3. The flyback converter according to claim 2, wherein: The first switch tube includes a MOS tube, a drain of the MOS tube is electrically connected to the first primary winding, a source of the MOS tube is electrically connected to the first shunt, and a gate of the MOS tube is electrically connected to a drive circuit.

4. The flyback converter according to claim 2, wherein: The first voltage input control circuit also includes a first buffer circuit, which includes a first capacitor, a first resistor, a second resistor, a third resistor and a diode; the first capacitor is electrically connected to the first primary winding and one end of the first resistor, respectively, the other end of the first resistor and one end of the second resistor are both electrically connected to the negative pole of the diode, the third resistor is electrically connected to one end of the first primary winding and the other end of the second resistor, and the negative pole of the diode is electrically connected to the other end of the first primary winding.

5. The flyback converter according to claim 2, wherein: The second voltage input control circuit includes a second primary winding of the transformer, a second switching tube, and a second buffer circuit; the second sampling resistor includes a second shunt; two ends of the second buffer circuit are electrically connected to two ends of the second primary winding, the second primary winding is coupled to the secondary winding, one end of the second primary winding is electrically connected to the second shunt, the other end of the second primary winding is electrically connected to the first end of the second switching tube, and the second end of the second switching tube is electrically connected to the negative electrode of the power supply; Alternatively, one end of the second primary winding is electrically connected to the first end of the second switch tube, the other end of the second primary winding is electrically connected to the negative pole of the power supply, and the second end of the second switch tube is electrically connected to the second shunt.

6. The flyback converter according to claim 1, wherein: The device further includes a second capacitor and a third capacitor, wherein the second capacitor is electrically connected to the positive electrode of the power supply and the other end of the first sampling resistor, respectively, and the third capacitor is electrically connected to the negative electrode of the power supply and one end of the second sampling resistor, respectively.

7. The flyback converter according to claim 1, wherein: It also includes a filter circuit, one end of which is electrically connected to one end of the first sampling resistor, and the other end of which is electrically connected to the pulse width modulation controller.

8. The flyback converter according to claim 7, wherein: The filtering circuit includes a fourth resistor and a fourth capacitor, one end of the fourth resistor is electrically connected to one end of the first sampling resistor, the other end of the fourth resistor is electrically connected to one end of the fourth capacitor, the other end of the fourth capacitor is grounded, and a common connection end of the fourth resistor and the fourth capacitor is electrically connected to the pulse width modulation controller.

9. The flyback converter according to any one of claims 1 to 8, characterized in that: The resistance values ​​of the first sampling resistor and the second sampling resistor are equal.

10. A switching power supply system, characterized in that: The invention comprises a load and the flyback converter according to any one of claims 1 to 9, wherein the load is electrically connected to the voltage output terminal of the flyback converter.

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