Feedback compensation circuit for switching power supply

Abstract

The invention provides a feedback compensation circuit for a switching power supply. The feedback compensation circuit comprises a reference voltage source, an error amplifier, a feedback capacitor, a sampling circuit, an optocoupler output circuit and a PWM control chip, output voltage is connected to the inverted input end of the error amplifier through the sampling circuit, the reference voltage source is connected to the in-phase input end of the error amplifier, and the output end of the error amplifier is connected to a feedback pin of the PWM control chip through the optocoupler output circuit. The sampling circuit comprises an upper sampling resistor, a lower sampling resistor, a second resistor and a second capacitor; the feedback capacitor is connected between the inverting input end and the output end of the error amplifier; by selecting feedback capacitors with different capacities, the low-frequency gain of a transfer function from output to control of the feedback compensation circuit is adjusted. According to the invention, the element parameter design process is greatly simplified, the controllability of low-frequency gain is enhanced, and the technical effects of easy parameter design and simple debugging are achieved.

Description

Technical Field

[0001] This invention relates to the field of switching power supply circuit design, and more specifically to a feedback compensation circuit for switching power supplies. Background Technology

[0002] Currently, small and medium power switching power supply circuits still employ hardware feedback compensation circuits composed of resistors, capacitors, a reference voltage source with a built-in operational amplifier, optocouplers, and other components. To achieve good closed-loop control, type II feedback compensation circuits with a single zero and double pole (such as...) are often used. Figure 1 (as shown) or a Type III feedback compensation circuit with dual zeros and multiple poles (such as...) Figure 2 (As shown).

[0003] However, traditional Type II and Type III feedback compensation circuits suffer from low-frequency gain and output voltage in their transfer function from output to control. U 0 sampling resistor R 1 is inversely proportional to the sampling resistor. R When the value is large, the low-frequency gain is too small, and the calculation of the zero and pole frequencies of the transfer function is not intuitive, which makes the design and debugging of the switching power supply feedback compensation circuit difficult and time-consuming, and highly dependent on the experience of engineering technicians. Summary of the Invention

[0004] The purpose of this invention is to address the problems of difficult and time-consuming debugging of feedback compensation circuits in existing switching power supplies, and to provide a feedback compensation circuit for switching power supplies. Through fewer components and simple circuit improvements, it achieves high and controllable low-frequency gain, easy parameter design, and simple debugging.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A feedback compensation circuit for a switching power supply includes a reference voltage source, an error amplifier, a feedback capacitor, a sampling circuit, an optocoupler output circuit, and a PWM control chip. The output voltage is connected to the inverting input of the error amplifier through a sampling circuit, the reference voltage source is connected to the non-inverting input of the error amplifier, and the output of the error amplifier is connected to the feedback pin of the PWM control chip through an optocoupler output circuit. The sampling circuit includes an upper sampling resistor, a lower sampling resistor, a second resistor, and a second capacitor; the output voltage passes through the upper and lower sampling resistors in sequence and is then connected to ground; the second resistor and the second capacitor are connected in series to form a series resistor-capacitor branch, which is then connected in parallel across the upper sampling resistor; the inverting input of the error amplifier is connected to the node between the upper and lower sampling resistors. The feedback capacitor is connected between the inverting input and output of the error amplifier; by selecting feedback capacitors of different capacitances, the low-frequency gain of the transfer function from the output to the control of the feedback compensation circuit is adjusted.

[0006] A feedback compensation circuit for a switching power supply includes a reference voltage source, an error amplifier, a feedback capacitor, a sampling circuit, an optocoupler output circuit, and a PWM control chip. The output voltage is connected to the inverting input of the error amplifier through a sampling circuit, the reference voltage source is connected to the non-inverting input of the error amplifier, and the output of the error amplifier is connected to the feedback pin of the PWM control chip through an optocoupler output circuit. The sampling circuit includes an upper sampling resistor, a lower sampling resistor, a second resistor, and a second capacitor; the output voltage passes through the upper sampling resistor, the second resistor, and the lower sampling resistor in sequence and is then connected to ground; the two ends of the second capacitor are connected in parallel across the two ends of the upper sampling resistor; the inverting input of the error amplifier is connected to the node between the second resistor and the lower sampling resistor. The feedback capacitor is connected between the inverting input and output of the error amplifier; by selecting feedback capacitors of different capacitances, the low-frequency gain of the transfer function from the output to the control of the feedback compensation circuit is adjusted.

[0007] Furthermore, the optocoupler output circuit includes an optocoupler, an optocoupler current-limiting resistor, a bias resistor, and an auxiliary power supply; The anode of the input stage LED of the optocoupler is connected to the auxiliary power supply through the optocoupler current-limiting resistor, and the cathode of the input stage LED of the optocoupler is connected to the output terminal of the error amplifier. The bias resistor is connected in parallel between the cathode and anode of the input stage LED of the optocoupler to reduce the resistance value of the optocoupler current-limiting resistor and improve the low-frequency gain. The collector of the output stage phototransistor of the optocoupler is connected to the feedback pin of the PWM control chip.

[0008] Furthermore, the optocoupler output circuit also includes a compensation capacitor, which is connected in parallel between the collector and emitter of the phototransistor in the output stage of the optocoupler.

[0009] Furthermore, the optocoupler output circuit also includes a third capacitor, which is connected in parallel across the optocoupler current-limiting resistor.

[0010] Furthermore, the optocoupler output circuit also includes a fifth resistor and a third capacitor. The fifth resistor and the third capacitor are connected in series to form a series RC branch, which is connected in parallel across the optocoupler current-limiting resistor.

[0011] The application provides a feedback compensation circuit for a switching power supply, which is improved by fewer components and a simple circuit, greatly simplifies the component parameter design process and enhances the controllability of low-frequency gain, and only needs to select feedback capacitors with different capacities to conveniently adjust the low-frequency gain, so that higher and controllable low-frequency gain can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is a structural schematic diagram of a conventional type II feedback compensation circuit in the prior art.

[0013] Figure 2 is a structural schematic diagram of a conventional type III feedback compensation circuit in the prior art.

[0014] Figure 3 is a structural schematic diagram of a single-zero-point double-pole-point feedback compensation circuit for a switching power supply provided by an embodiment of the application.

[0015] Figure 4 is a structural schematic diagram of a single-zero-point double-pole-point feedback compensation circuit for a switching power supply provided by an embodiment of the application.

[0016] Figure 5 is a structural schematic diagram of a double-zero-point multi-pole-point feedback compensation circuit for a switching power supply provided by an embodiment of the application.

[0017] Figure 6 is a structural schematic diagram of a double-zero-point double-pole-point feedback compensation circuit for a switching power supply provided by an embodiment of the application. DETAILED DESCRIPTION

[0018] The technical solutions of the application will be described in detail below with reference to the drawings and specific embodiments. It should be noted that the drawings are only used for illustrative description and cannot be understood as a limitation of the patent; in order to better illustrate the embodiments, some components in the drawings may be omitted, enlarged or reduced, and do not represent the actual product circuit; for those skilled in the art, it is understandable that some known components and their descriptions in the drawings may be omitted.

[0019] Embodiment one

[0020] As shown in Figure 3 , the embodiment of the application provides a single-zero-point double-pole-point feedback compensation circuit for a switching power supply, which comprises a reference voltage source U 1 , an error amplifier EA , a feedback capacitor C 1, a sampling circuit, an optical coupling output circuit and a PWM control chip.

[0021] wherein the output voltage U 0 is connected to the inverting input end of the error amplifier EA through the sampling circuit, the reference voltage source U 1 is connected to the non-inverting input end of the error amplifier EA , and the output end of the error amplifier EA is connected to the feedback pin of the PWM control chip through the optical coupling output circuit.

[0022] The sampling circuit comprises an upper sampling resistor R 1 , a lower sampling resistor R d , a second resistor R 2 and a second capacitor C 2 ; the output voltage U 0 passes through the upper sampling resistor R 1 and the lower sampling resistor R d in sequence and is then connected to the ground; the second resistor R 2 and the second capacitor C 2 are connected in series to form a resistor-capacitor series branch, and the resistor-capacitor series branch is connected in parallel to both ends of the upper sampling resistor R 1 ; the inverting input end of the error amplifier EA is connected to the node between the upper sampling resistor R 1 and the lower sampling resistor R d ; and the feedback capacitor C 1 is connected between the inverting input end and the output end of the error amplifier EA .

[0023] In order to simplify the circuit structure, a controllable precision voltage source with a model number of TL431 is selected in the embodiment, and the TL431 integrates the reference voltage source U 1 and the error amplifier EA , thereby improving the overall integration of the circuit.

[0024] Further, the optical coupling output circuit comprises an optical coupler, an optical coupler current limiting resistor R 3 and a bias resistor R 4Auxiliary power supply U’ 0 and compensation capacitor C FB The anode of the LED in the input stage of the optocoupler is connected to the optocoupler's current-limiting resistor. R 3 Connect to auxiliary power supply U’ 0 The cathode of the input stage LED of the optocoupler is connected to the error amplifier. EA The output terminal of the bias resistor R 4 It is connected in parallel between the cathode and anode of the LED in the input stage of the optocoupler to reduce the current-limiting resistance of the optocoupler. R 3 The resistance value is adjusted to improve low-frequency gain; the collector of the phototransistor in the output stage of the optocoupler is connected to the feedback pin of the PWM control chip. The compensation capacitor... C FB It is connected in parallel between the collector and emitter of the phototransistor in the output stage of the optocoupler.

[0025] According to the circuit structure of the present invention, it is not difficult to prove that, when the following conditions are met... R 1 >> R 2 In this case, Figure 3 The transfer function from output to control of the feedback compensation circuit shown is: ; in, This is the AC component of the optocoupler output voltage; This represents the AC component of the output voltage. This represents the current transfer ratio of the optocoupler.

[0026] Furthermore, in this embodiment of the invention, the upper sampling resistor... R 1 Second capacitor C 2 The zero point that constitutes the feedback compensation circuit, the zero point frequency Second resistor R 2 Second capacitor C 2 The first pole of the right half-plane of the feedback compensation circuit, the frequency of the first pole. Compensation capacitor C FB The pull-up resistor built into the feedback pin of the PWM control chip R FB The second pole forms the left half-plane of the feedback compensation circuit, and the frequency of the second pole is... .

[0027] In summary, the low-frequency gain of this embodiment of the invention... , and connected to the output voltage U 0 sampling resistor on R 1 It's irrelevant. Therefore, it's simply a matter of selecting feedback capacitors of different capacitance values. C 1 This allows for convenient adjustment of the low-frequency gain of the transfer function from the output to the control of the feedback compensation circuit, thereby improving the controllability of the low-frequency gain.

[0028] In contrast, Figure 1 In the traditional Type II feedback compensation circuit shown, C 1 >> C 2 In this case, the transfer function from output to control is: ; Its low-frequency gain It is not difficult to see that the sampling resistor is located on the top. R 1 In cases where the gain is large, the low-frequency gain is relatively small.

[0029] The parameter design process of the single-zero-point double-pole feedback compensation circuit of this invention will be described in detail below with specific data.

[0030] Assuming output voltage U 0 If the voltage is 48V, then according to the conventional design of existing technology, the upper sampling resistor... R 1 Approximately 120kΩ, auxiliary power supply U’ 0 12V corresponds to the optocoupler current-limiting resistor. R 3 A 3.6kΩ resistor can be used; since the operating voltage of the built-in LED in the optocoupler is 1.0~1.2V, the bias resistor... R 4 A pull-up resistor of 1.0–1.2 kΩ is acceptable; the feedback pin of the PWM control chip has a built-in pull-up resistor. R FB The typical value is 18 kΩ. The parameters of all the above components are known; the parameter of the undetermined component in the circuit design is the second resistor. R 2 Feedback capacitor C 1 Second capacitor C 2 Compensation capacitor C FB .

[0031] In the process of calculating the parameters of the to-be-determined elements, the zero point and pole frequency of the transfer function need to satisfy At the same time, in order to improve the intermediate frequency gain , , generally 3-5 times of are taken.After the zero point and pole frequency of the transfer function are determined, since the up-sampling resistance R 1 is known, the second capacitance 2 can be obtained from C , the second resistance 2 can be obtained from R , and the compensation capacitance C FB can be obtained from the preset low frequency gain . C 1 .

[0032] Specifically, if the crossover frequency of the closed-loop transfer function of the feedback compensation circuit is taken as 1.0 kHz (typical value), then the zero point frequency , the first pole frequency , and the second pole frequency .

[0033] If the low frequency gain of the feedback compensation circuit is to be realized , the parameters of the to-be-determined elements can be obtained through the following calculations: ; ; ; .

[0034] Since the feedback capacitance C 1 only affects the low frequency gain and does not affect the compensation frequency, during the debugging process, the feedback capacitance C 1 may also take other values such as 2.2 nF, 2.7 nF, 3.9 nF, or 4.7 nF, so as to realize the adjustment of the low frequency gain without changing the compensation frequency. For the convenience of adjustment, two capacitors in series can also be used on the PCB to obtain the required feedback capacitance C 1 .

[0035] ​It is evident from the above process that the calculation of the component parameters in this embodiment of the invention is highly intuitive and can be directly obtained from the designed zero-point and pole-point frequencies. Compared with the traditional Type II feedback compensation circuit, the number of circuit components in this embodiment of the invention has not increased; only the positions and parameters of the components have changed. By improving the connection positions of the components, this invention significantly simplifies the component parameter design process and enhances the controllability of low-frequency gain, requiring only the selection of feedback capacitors of different capacitances. C 1 allows for convenient adjustment of low-frequency gain.

[0036] Example 2

[0037] like Figure 4 As shown, the single-zero-point double-pole feedback compensation circuit for switching power supplies provided in this embodiment is a variant design based on Embodiment 1.

[0038] Compared with Embodiment 1, the main difference in this embodiment lies in the second capacitor. C 2 Second resistor R 2 The connection location. Specifically, the output voltage in this embodiment of the invention. U 0 Connect the sampling resistors in sequence R 1 Second resistor R 2 and the sampling resistor R d Then, connect to ground; second capacitor C 2 The two ends are connected in parallel to the sampling resistor. R 1 Both ends; error amplifier EA The inverting input terminal is connected to the second resistor. R 2 With the sampling resistor R d On the nodes between.

[0039] Apart from the above, the other technical features of this embodiment are the same as those of Embodiment 1, and will not be repeated here.

[0040] Example 3

[0041] like Figure 5 As shown, this embodiment of the invention provides a dual-zero-point multi-pole feedback compensation circuit for switching power supplies, including a reference voltage source. U 1 Error amplifier EA Feedback capacitor C 1 Sampling circuit, optocoupler output circuit and PWM control chip.

[0042] wherein the output voltage U 0 is connected to the inverting input of the error amplifier EA , the reference voltage source U 1 is connected to the non-inverting input of the error amplifier EA , the output of the error amplifier EA is connected to the feedback pin of the PWM control chip through the opto-coupler output circuit.

[0043] The sampling circuit comprises an upper sampling resistor R 1 , a lower sampling resistor R d , a second resistor R 2 and a second capacitor C 2 ; the output voltage U 0 passes through the upper sampling resistor R 1 and the lower sampling resistor R d in sequence and is connected to the ground; the second resistor R 2 and the second capacitor C 2 are connected in series to form a resistor-capacitor series branch, and the whole is connected in parallel across the upper sampling resistor R 1 ; the inverting input of the error amplifier EA is connected to the node between the upper sampling resistor R 1 and the lower sampling resistor R d ; and the feedback capacitor C 1 is connected between the inverting input and the output of the error amplifier EA .

[0044] Further, the opto-coupler output circuit comprises an opto-coupler, an opto-coupler current-limiting resistor R 3 , a bias resistor R 4 , an auxiliary power supply U’ 0 , a third capacitor C 3 , a fifth resistor R 5 and a compensation capacitor C FB . The anode of the input-stage light-emitting diode of the opto-coupler is connected to the auxiliary power supply through the opto-coupler current-limiting resistorR 3 Connect to auxiliary power supply U’ 0 The fifth resistor R 5 and the third capacitor C 3 After being connected in series, it is then connected in parallel to the optocoupler current-limiting resistor. R 3 The two ends form a series resistor-capacitor branch. The cathode of the LED in the input stage of the optocoupler is connected to the error amplifier. EA The output terminal of the bias resistor R 4 It is connected in parallel between the cathode and anode of the LED in the input stage of the optocoupler to reduce the current-limiting resistance of the optocoupler. R 3 The resistance value is adjusted to improve low-frequency gain; the collector of the phototransistor in the output stage of the optocoupler is connected to the feedback pin of the PWM control chip. The compensation capacitor... C FB It is connected in parallel between the collector and emitter of the phototransistor in the output stage of the optocoupler.

[0045] In this embodiment of the invention, the upper sampling resistor R 1 Second capacitor C 2 The first zero point constituting the feedback compensation circuit, the frequency of the first zero point Optocoupler current limiting resistor R 3 and the third capacitor C 3 The second zero point constitutes the feedback compensation circuit; the frequency of the second zero point. Second resistor R 2 Second capacitor C 2 The first pole of the right half-plane of the feedback compensation circuit, the frequency of the first pole. Fifth resistor R 5 and the third capacitor C 3 The second pole forms the middle part of the feedback compensation circuit, and the frequency of the second pole is... Compensation capacitor C FB The pull-up resistor built into the feedback pin of the PWM control chip R FB The third pole of the left half-plane of the feedback compensation circuit has a frequency of [missing information]. .

[0046] According to the circuit structure of the present invention, it is not difficult to prove that, when the following conditions are met... R1 >> R 2 , R 3 >> R 5 In this case, Figure 5 The transfer function from output to control of the feedback compensation circuit shown is: ; in, This is the AC component of the optocoupler output voltage; This represents the AC component of the output voltage. This represents the current transfer ratio of the optocoupler.

[0047] In summary, the low-frequency gain of this embodiment of the invention... , and connected to the output voltage U O sampling resistor on R 1 It's irrelevant. Therefore, it's simply a matter of selecting feedback capacitors of different capacitance values. C 1 This allows for convenient adjustment of the low-frequency gain of the transfer function from the output to the control of the feedback compensation circuit, thereby improving the controllability of the low-frequency gain.

[0048] In contrast, Figure 2 In the traditional Type III feedback compensation circuit shown, C 1 >> C 2 , R 1 >> R 2 In this case, the transfer function from output to control is: ; Its low-frequency gain Sampling resistor R 1 In cases where the gain is large, the low-frequency gain is relatively small.

[0049] The parameter design process of the dual-zero-point multi-pole feedback compensation circuit of this invention will be described in detail below with specific data.

[0050] Assuming output voltage U 0 If the voltage is 36V, then according to the conventional design of existing technology, the upper sampling resistor... R 1 Approximately 82kΩ; Auxiliary power supply voltage U’ 0 12V corresponds to the optocoupler current-limiting resistor. R3.9kΩ; since the operating voltage of the built-in LED of the optocoupler is 1.0-1.2V, the bias resistor R 4 1.0-1.2kΩ; the pull-up resistor built in the feedback pin of the PWM control chip R FB is typically 18 kΩ. The parameters of the above elements are known, and the to-be-determined element parameters in the circuit design are the second resistor R 2 , the fifth resistor R 5 , the feedback capacitor C 1 , the second capacitor C 2 , the third capacitor C 1 , and the compensation capacitor C FB .

[0051] In the process of designing the to-be-determined element parameters, the zero point and pole frequency of the transfer function need to satisfy , and in order to improve the mid-frequency gain , , the mid-frequency gain is generally taken as 3-5 times . After the zero point and pole frequency of the transfer function are determined, since the upper sampling resistor R 1 is known, the second capacitor 2 can be obtained from C , the second resistor 2 can be obtained from R , the third capacitor 3 can be obtained from C , the fifth resistor 5 can be obtained from R , and the compensation capacitor can be obtained from C FB . C 1 .

[0052] Specifically, if the loop crossover frequency of the feedback compensation circuit f C is 8 kHz, then the first zero point frequency f ZC1 = 200 Hz, the second zero point frequency f ZC2 = 400 Hz, the first pole frequency f PC1 = 2.0 kHz, and the second pole frequency f ​PC2 = 4.0 kHz, third pole frequency f PC3 = 10 kHz.

[0053] In order to realize the low frequency gain of the feedback circuit H CV ( f ZC1 )> 15 dB, each undetermined element parameter can be obtained by the following calculation: ; ; ; ; ; .

[0054] It can be seen from the above process that the calculation of each element parameter in the embodiment of the present application is intuitive and can be directly obtained according to the designed zero point and pole frequency. Compared with the traditional type III feedback compensation circuit, the number of circuit elements in the embodiment of the present application is not increased, only the position and parameter of the element are changed. By improving the connection position of the element, the present application greatly simplifies the element parameter design process and enhances the controllability of the low frequency gain. Only by selecting a feedback capacitor C 1 with different capacity, the low frequency gain can be conveniently adjusted.

[0055] It should be noted that in the third embodiment, the compensation capacitor C FB between the collector and the emitter of the light-sensitive triode in parallel with the output stage of the optocoupler is an optional element. If the compensation capacitor C FB is removed on the basis of the third embodiment, the third pole of the feedback compensation circuit will disappear, so that the circuit degenerates into a double-zero double-pole feedback compensation circuit.

[0056] Embodiment Four

[0057] As shown in Figure 6 , the double-zero double-pole feedback compensation circuit for a switching power supply provided by the present embodiment is a variant design made on the basis of the third embodiment.

[0058] Compared with the third embodiment, the difference of the present embodiment mainly lies in that the fifth resistor R 5 is cancelled. In the case of cancelling the fifth resistor R 5Afterwards, the second pole of the third embodiment will disappear, so that the circuit degenerates into the double-zero double-pole feedback compensation circuit of the fourth embodiment.

[0059] In addition, other technical features of the present embodiment are the same as those of the third embodiment, and will not be described herein.

[0060] The above-described embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the present application. It should be noted that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims.

Claims

1. A feedback compensation circuit for a switching power supply, characterized in that, It includes a reference voltage source, an error amplifier, a feedback capacitor, a sampling circuit, an optocoupler output circuit, and a PWM control chip; The output voltage is connected to the inverting input of the error amplifier through a sampling circuit, the reference voltage source is connected to the non-inverting input of the error amplifier, and the output of the error amplifier is connected to the feedback pin of the PWM control chip through an optocoupler output circuit. The sampling circuit includes an upper sampling resistor, a lower sampling resistor, a second resistor, and a second capacitor; the output voltage passes through the upper and lower sampling resistors in sequence and is then connected to ground; the second resistor and the second capacitor are connected in series to form a series resistor-capacitor branch, which is then connected in parallel across the upper sampling resistor; the inverting input of the error amplifier is connected to the node between the upper and lower sampling resistors. The feedback capacitor is connected between the inverting input and output of the error amplifier; by selecting feedback capacitors of different capacitances, the low-frequency gain of the transfer function from the output to the control of the feedback compensation circuit is adjusted.

2. A feedback compensation circuit for a switching power supply, characterized in that, It includes a reference voltage source, an error amplifier, a feedback capacitor, a sampling circuit, an optocoupler output circuit, and a PWM control chip; The output voltage is connected to the inverting input of the error amplifier through a sampling circuit, the reference voltage source is connected to the non-inverting input of the error amplifier, and the output of the error amplifier is connected to the feedback pin of the PWM control chip through an optocoupler output circuit. The sampling circuit includes an upper sampling resistor, a lower sampling resistor, a second resistor, and a second capacitor; the output voltage passes through the upper sampling resistor, the second resistor, and the lower sampling resistor in sequence and is then connected to ground; the two ends of the second capacitor are connected in parallel across the two ends of the upper sampling resistor; the inverting input of the error amplifier is connected to the node between the second resistor and the lower sampling resistor. The feedback capacitor is connected between the inverting input and output of the error amplifier; by selecting feedback capacitors of different capacitances, the low-frequency gain of the transfer function from the output to the control of the feedback compensation circuit is adjusted.

3. The feedback compensation circuit for a switching power supply according to claim 1 or 2, characterized in that, The optocoupler output circuit includes an optocoupler, an optocoupler current-limiting resistor, a bias resistor, and an auxiliary power supply; The anode of the input stage LED of the optocoupler is connected to the auxiliary power supply through the optocoupler current-limiting resistor, and the cathode of the input stage LED of the optocoupler is connected to the output terminal of the error amplifier. The bias resistor is connected in parallel between the cathode and anode of the input stage LED of the optocoupler to reduce the resistance value of the optocoupler current-limiting resistor and improve the low-frequency gain. The collector of the output stage phototransistor of the optocoupler is connected to the feedback pin of the PWM control chip.

4. The feedback compensation circuit for a switching power supply according to claim 3, characterized in that, The optocoupler output circuit also includes a compensation capacitor, which is connected in parallel between the collector and emitter of the phototransistor in the output stage of the optocoupler.

5. The feedback compensation circuit for a switching power supply according to claim 3, characterized in that, The optocoupler output circuit also includes a third capacitor, which is connected in parallel across the optocoupler current-limiting resistor.

6. The feedback compensation circuit for a switching power supply according to claim 3, characterized in that, The optocoupler output circuit also includes a fifth resistor and a third capacitor. The fifth resistor and the third capacitor are connected in series to form a series RC branch, which is connected in parallel across the optocoupler current-limiting resistor.

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