Primary side feedback flyback circuit based on LM5021

Abstract

The invention relates to an LM5021-based primary side feedback flyback circuit, belongs to the technical field of flyback circuit topology, and solves the problem of complex design of a primary side feedback circuit in the prior art. The flyback circuit comprises a transformer, an LM5021, a voltage sampling circuit, a pull-down resistor R170 and a loop control circuit; a synonym terminal of a primary side output winding of the transformer is connected to a VIN port of the LM5021; the voltage sampling circuit is connected between the dotted terminal and the synonym terminal of the primary side output winding and is used for sampling the output voltage of the primary side output winding; the output end of the voltage sampling circuit is connected with the input end of the loop control circuit, the output end of the loop control circuit is connected with one end of the pull-down resistor R170, and the other end of the pull-down resistor R170 is connected to a COM port of the LM5021; the loop control circuit enables the voltage of the COM port of the LM5021 to be stabilized in an effective range by comparing the sampling voltage with a preset reference voltage. And the number of components of the flyback circuit when primary side feedback is utilized is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of flyback circuit topology, and in particular to a primary-side feedback flyback circuit based on LM5021. Background Art

[0002] The flyback transformer switching power supply has a relatively simple circuit and uses less of a large energy storage filter inductor and a freewheeling diode. Therefore, the volume of the flyback transformer switching power supply is smaller than that of the forward transformer switching power supply, and the cost is also lower. It is often used in small-power DC-DC conversion situations and as an auxiliary power supply for other products.

[0003] In the prior art, the flyback transformer switching power supply is topologically controlled by the LM5021 controller, but the LM5021 controller is only applicable to the occasion of output voltage isolation feedback, and an isolation circuit needs to be added. The isolation circuit adopts the form of an optocoupler and consists of three parts: a secondary voltage sampling circuit, a PID closed-loop circuit, and an optocoupler. However, the introduction of these three circuits requires the addition of additional circuit parts when the space for the flyback circuit is limited, and the use of optocouplers will inevitably make the insulation layout between the primary and secondary sides difficult, increasing the area of ​​the control circuit. Summary of the invention

[0004] In view of the above analysis, an embodiment of the present invention aims to provide a primary-side feedback flyback circuit based on LM5021, so as to solve the problem of complex design of the primary-side feedback circuit in the prior art.

[0005] The embodiment of the present invention provides a primary feedback flyback circuit based on LM5021, wherein the flyback circuit includes a transformer, LM5021, a voltage sampling circuit, a pull-down resistor R170 and a loop control circuit;

[0006] The transformer's primary output winding opposite end is connected to the VIN port of LM5021;

[0007] The voltage sampling circuit is connected between the same-name terminal and the opposite-name terminal of the primary output winding, and is used to sample the output voltage of the primary output winding;

[0008] The output end of the voltage sampling circuit is connected to the input end of the loop control circuit, the output end of the loop control circuit is connected to one end of the pull-down resistor R170, and the other end of the pull-down resistor R170 is connected to the COM port of LM5021; the loop control circuit compares the sampled voltage with a preset reference voltage to stabilize the COM port voltage of LM5021 within a valid range.

[0009] Based on the further improvement of the above circuit, the voltage sampling circuit includes a first voltage dividing resistor R316 and a second voltage dividing resistor R317;

[0010] The opposite-name end of the primary output winding is connected to one end of the first voltage-dividing resistor R316;

[0011] The other end of the first voltage-dividing resistor R316 is connected to one end of the second voltage-dividing resistor R317, and the other end of the second voltage-dividing resistor R317 is simultaneously connected to the same-name end of the primary output winding and the ground; the connection point between the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317 serves as the output end of the voltage sampling circuit.

[0012] Based on the further improvement of the above circuit, the loop control circuit includes a first capacitor C5, a second capacitor C321, a first resistor R332 and TLV431;

[0013] One end of the first capacitor C5, one end of the first resistor R332 and the REF port of the TLV431 are connected, and the connection point serves as the input end of the loop control circuit;

[0014] The other end of the first resistor R332 is connected to one end of the second capacitor C321;

[0015] The other end of the first capacitor C5, the other end of the second capacitor C321 and the CATHODE port of the TLV431 are connected, and the connection point serves as the output end of the loop control circuit. The ANODE port of the TLV431 is grounded.

[0016] Based on the further improvement of the above circuit, the flyback circuit further includes a rectifier diode D8, a third capacitor C125 and a fourth capacitor C318;

[0017] The opposite-name end of the primary output winding is connected to the anode of the rectifier diode D8; the cathode of the rectifier diode D8 is simultaneously connected to one end of the first voltage-dividing resistor R316, one end of the third capacitor C125 and one end of the fourth capacitor C318;

[0018] The other end of the third capacitor C125 and the other end of the fourth capacitor C318 are both grounded.

[0019] Based on the further improvement of the above circuit, the flyback circuit further includes a first diode D10 and a second diode D31;

[0020] The opposite-name end of the primary output winding is connected to the anode of the first diode D10 and the anode of the second diode D31 at the same time;

[0021] The cathode of the first diode D10 and the cathode of the second diode D31 are connected to the VIN port of LM5021 at the same time.

[0022] Based on the further improvement of the above circuit, the flyback circuit also includes a high-voltage startup resistor circuit and an input capacitor circuit;

[0023] The positive electrode of the starting voltage is connected to the input end of the high-voltage starting resistor circuit, and the output end of the high-voltage starting resistor circuit is connected to the VIN port of LM5021;

[0024] The input capacitor circuit is connected between the positive electrode of the starting voltage and the negative electrode of the starting voltage, and the negative electrode of the starting voltage is grounded.

[0025] Based on the further improvement of the above circuit, the flyback circuit also includes a high-voltage startup charging capacitor circuit;

[0026] The input end of the high-voltage startup charging capacitor circuit is connected to the VIN port of LM5021 and the output end of the high-voltage startup resistor circuit at the same time; the output end of the high-voltage startup charging capacitor circuit is grounded;

[0027] When the starting voltage meets the conditions, the positive electrode of the starting voltage charges the high-voltage starting charging capacitor circuit through the high-voltage starting resistor circuit.

[0028] Based on the further improvement of the above circuit, the flyback circuit further includes a MOS tube Q300 and an RCD absorption circuit;

[0029] The OUT port of LM5021 is connected to the gate of MOS tube Q300 through resistor R33, and is connected to the source of MOS tube Q300 through protection resistor R325;

[0030] The drain of MOS tube Q300 is connected to the input end of the RCD absorption circuit and the opposite end of the primary input winding of the transformer;

[0031] The output end of the RCD absorption circuit is connected to the same end of the primary input winding of the transformer.

[0032] Based on the further improvement of the above circuit, the RCD absorption circuit includes a third diode D304, a second resistor R41, a third resistor R10, a fifth capacitor C305 and a sixth capacitor C98;

[0033] The anode of the third diode D304 serves as the input end of the RCD absorption circuit, and the cathode of the third diode D304 is connected to one end of the second resistor R41, one end of the third resistor R10, one end of the fifth capacitor C305, and one end of the sixth capacitor C98;

[0034] The other end of the second resistor R41, the other end of the third resistor R10, the other end of the fifth capacitor C305, and the other end of the sixth capacitor C98 are connected, and the connection point serves as the output end of the RCD absorption circuit. The output end of the RCD absorption circuit is connected to the same-name end of the primary input winding of the transformer.

[0035] Based on the further improvement of the above circuit, the SS port of LM5021 is grounded through the seventh capacitor C13; the VCC port of LM5021 is grounded through the eighth capacitor C34; the GND port of LM5021 is grounded;

[0036] The RT port of LM5021 is grounded through the fourth resistor R318;

[0037] The CS port of LM5021 is connected to one end of the ninth capacitor C320 and one end of the fourth resistor R330 at the same time, and the other end of the ninth capacitor C320 is grounded; the other end of the fourth resistor R330 is connected to the source of the MOS tube Q300 and one end of the fifth resistor R326 at the same time; the other end of the fifth resistor R326 is grounded.

[0038] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0039] 1. The present invention breaks through the limitation of the prior art that LM5021 can only realize secondary side feedback;

[0040] 2. During primary-side feedback, voltage feedback can be directly performed through the flyback circuit power supply winding, which reduces the number of closed-loop components. In situations where layout space is limited, the number of circuit components can be greatly reduced, and there is no need to introduce additional operational amplifiers, voltage references for PID control loops, optocouplers and other components, which can effectively reduce costs while increasing power density.

[0041] 3. By introducing the TLV431 chip, the feedback circuit is set on the primary side of the transformer to avoid the increased layout difficulty caused by the introduction of the isolation circuit and the increase in the number of components caused by the isolation feedback.

[0042] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0044] Figure 1 A schematic diagram of the structure of a primary-side feedback flyback circuit based on LM5021 provided in an embodiment of the present invention;

[0045] Figure 2 Schematic diagram of an equivalent circuit of the TLV431 provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0047] According to the official application and technical support of LM5021, the LM5021 controller is only suitable for occasions where the output voltage is isolated and fed back, and an isolation circuit needs to be added. The isolation circuit adopts the form of an optocoupler and consists of three parts: a secondary voltage sampling circuit, a PID closed-loop circuit and an optocoupler.

[0048] However, the introduction of these three circuit parts requires the addition of additional circuit parts when the space for the flyback circuit is limited. The use of optocouplers will inevitably make the insulation layout between the primary and secondary sides difficult, increasing the area of ​​the control circuit.

[0049] Based on this, the official LM5021 proposed the LM5020 chip. However, the LM5020 chip increased the number of chip pins due to the introduction of pins related to the EA function, and the design of separating the UVLO from the VIN pin made the undervoltage circuit design of the chip more complicated when the input voltage exceeded the VIN maximum input voltage of 100V.

[0050] At the same time, the LM5021 includes an ultra-low (25μA) startup current feature to minimize power consumption in the high-voltage startup network, while the skip mode reduces power consumption under light load conditions, thereby enabling energy-saving applications. Additional features include undervoltage lockout, cycle-by-cycle current limit, hiccup mode overload protection, slope compensation, soft start, and oscillator synchronization functions.

[0051] The LM5021 is a high-performance 8-pin IC chip with a total propagation delay of less than 100ns, and can set a 1MHz oscillator through a single resistor. The use of the LM5021 can effectively reduce the size of the control circuit and power components while improving the efficiency of the flyback circuit.

[0052] The present invention proposes a primary-side feedback flyback circuit based on LM5021 by studying the above LM5020, LM5021 and flyback circuit.

[0053] A specific embodiment of the present invention discloses a primary feedback flyback circuit based on LM5021, such as Figure 1 As shown, the flyback circuit includes a transformer, LM5021, a voltage sampling circuit, a pull-down resistor R170 and a loop control circuit;

[0054] The transformer's primary output winding opposite end is connected to the VIN port of LM5021;

[0055] The voltage sampling circuit is connected between the same-name terminal and the opposite-name terminal of the primary output winding, and is used to sample the output voltage of the primary output winding;

[0056] The output end of the voltage sampling circuit is connected to the input end of the loop control circuit, the output end of the loop control circuit is connected to one end of the pull-down resistor R170, and the other end of the pull-down resistor R170 is connected to the COM port of LM5021; the loop control circuit compares the sampled voltage with a preset reference voltage to stabilize the COM port voltage of LM5021 within a valid range.

[0057] Specifically, Figure 1 As shown, the flyback circuit includes a transformer, LM5021, a voltage sampling circuit, a pull-down resistor R170 and a loop control circuit.

[0058] Among them, port 1 of the transformer is the same-name terminal of the primary input winding, port 3 of the transformer is the opposite-name terminal of the primary input winding, port 5 of the transformer is the opposite-name terminal of the primary output winding, and port 4 of the transformer is the same-name terminal of the primary output winding.

[0059] Among them, LM5021 includes 8 ports, namely COM port, VIN port, CS port, RT port, SS port, VCC port, OUT port and GND port. COM port is PWM control input terminal, VIN port is internal bias circuit input terminal, CS port is current monitoring terminal, RT port is clock signal input terminal, SS port is soft start or "hiccup" working mode timing input, VCC port is internal bias circuit output terminal, OUT port is PWM control output terminal, GND port is common ground.

[0060] Specifically, the opposite-name end of the primary output winding of the transformer is connected to the VIN port of the LM5021 to provide an input voltage for an internal power supply circuit of the LM5021. When the input voltage of the LM5021 reaches a threshold, the internal regulator is started.

[0061] The voltage sampling circuit is connected between the same-name terminal and the opposite-name terminal of the primary output winding, and is used to sample the output voltage of the primary output winding. The sampled voltage is connected to the input end of the loop control circuit. The output end of the loop control circuit is connected to one end of the pull-down resistor R170, and the other end of the pull-down resistor R170 is connected to the COM port of LM5021.

[0062] Specifically, the loop control circuit compares the sampled voltage with a preset reference voltage so that the COM port voltage of LM5021 is stabilized within an effective range, thereby achieving control over the flyback circuit.

[0063] Preferably, if Figure 1As shown, the voltage sampling circuit includes a first voltage dividing resistor R316 and a second voltage dividing resistor R317;

[0064] The opposite-name end of the primary output winding is connected to one end of the first voltage-dividing resistor R316;

[0065] The other end of the first voltage-dividing resistor R316 is connected to one end of the second voltage-dividing resistor R317, and the other end of the second voltage-dividing resistor R317 is simultaneously connected to the same-name end of the primary output winding and the ground; the connection point between the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317 serves as the output end of the voltage sampling circuit.

[0066] like Figure 1 As shown, the output point VCCP12 of the opposite-name end of the primary output winding is input through the first voltage-dividing resistor R316, and the same-name end of the primary output winding is connected to the second voltage-dividing resistor R317. The connection point of the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317 serves as the output end of the voltage sampling circuit. The voltage at the output end of the voltage sampling circuit can be adjusted by adjusting the first voltage-dividing resistor and the second voltage-dividing resistor. It can be understood that the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317 are connected in series to divide the voltage that needs to be actually output, and the resistance value is adjusted according to the voltage-dividing ratio.

[0067] Preferably, the flyback circuit further includes a rectifier diode D8, a third capacitor C125 and a fourth capacitor C318;

[0068] The opposite-name end of the primary output winding is connected to the anode of the rectifier diode D8; the cathode of the rectifier diode D8 is simultaneously connected to one end of the first voltage-dividing resistor R316, one end of the third capacitor C125 and one end of the fourth capacitor C318;

[0069] The other end of the third capacitor C125 and the other end of the fourth capacitor C318 are both grounded.

[0070] Specifically, Figure 1 As shown, after the opposite-name end of the primary output winding is rectified by the rectifier diode D8, it is used as the output of the opposite-name end of the primary output winding. At the same time, the same-name end of the primary output winding is grounded, and the third capacitor C125 and the fourth capacitor C318 are connected in parallel between the opposite-name end of the primary output winding and the same-name end of the primary output winding.

[0071] Preferably, the loop control circuit includes a first capacitor C5, a second capacitor C321, a first resistor R332 and a TLV431;

[0072] One end of the first capacitor C5, one end of the first resistor R332 and the REF port of the TLV431 are connected, and the connection point serves as the input end of the loop control circuit;

[0073] The other end of the first resistor R332 is connected to one end of the second capacitor C321;

[0074] The other end of the first capacitor C5, the other end of the second capacitor C321 and the CATHODE port of the TLV431 are connected, and the connection point serves as the output end of the loop control circuit. The ANODE port of the TLV431 is grounded.

[0075] Specifically, Figure 1 As shown, one end of the first capacitor C5, one end of the first resistor R332 and the REF port of TLV431 are connected, and the connection point serves as the input end of the loop control circuit; the other end of the first resistor R332 is connected to one end of the second capacitor C321; the other end of the first capacitor C5, the other end of the second capacitor C321 and the CATHODE port of TLV431 are connected, and the connection point serves as the output end of the loop control circuit, and the ANODE port of TLV431 is grounded VIN-.

[0076] like Figure 2 As shown, TLV431 is a voltage reference with a rated voltage of 1.25V. The effective voltage range of the COM port of LM5021 is 1.25V-2.75V. Using 1.25V TLV431 can make the COM port of LM5021 within the effective voltage range.

[0077] It is worth noting that the primary feedback flyback circuit based on LM5021 provided in the embodiment of the present invention samples the output voltage of the primary output winding of the transformer through the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317, and then compares it with the internal reference voltage of TLV431. At the same time, the COM port voltage of LM5021 is adjusted through the transistor inside TLV431 in the loop formed by the first capacitor C5, the second capacitor C321 and the first resistor R332, thereby forming a complete voltage closed loop.

[0078] Preferably, the flyback circuit further includes a first diode D10 and a second diode D31;

[0079] The opposite-name end of the primary output winding is connected to the anode of the first diode D10 and the anode of the second diode D31 at the same time;

[0080] The cathode of the first diode D10 and the cathode of the second diode D31 are connected to the VIN port of LM5021 at the same time.

[0081] Specifically, Figure 1 As shown, the output point VCCP12 of the opposite-name end of the primary output winding is also connected to the VIN port VCC-IN of LM5021 through two parallel first diodes D10 and second diodes D31 to further protect the transformer.

[0082] Preferably, the flyback circuit further includes a high-voltage startup resistor circuit and an input capacitor circuit;

[0083] The positive electrode of the starting voltage is connected to the input end of the high-voltage starting resistor circuit, and the output end of the high-voltage starting resistor circuit is connected to the VIN port of LM5021;

[0084] The input capacitor circuit is connected between the positive electrode of the starting voltage and the negative electrode of the starting voltage, and the negative electrode of the starting voltage is grounded.

[0085] Specifically, Figure 1 As shown, the positive electrode VIN+ of the startup voltage is connected to the input end of the high-voltage startup resistor circuit, and the output end of the high-voltage startup resistor circuit is connected to the VIN port VCC-IN of the LM5021. Figure 1 As shown, the high-voltage starting resistance circuit includes resistor R148, resistor R149, resistor R168, resistor R169, resistor R146, resistor R11, resistor R155 and resistor R156; any two of the above resistors are connected in series to form four series branches, and then the four series branches are connected in parallel, and finally the circuit after parallel connection is used as the high-voltage starting resistance circuit.

[0086] It is understandable that in an environment with a complex starting voltage, if the starting voltage is too high, the high-voltage starting resistor circuit can buffer and protect the flyback circuit.

[0087] Specifically, Figure 1 As shown, the input end of the input capacitor circuit is connected to the positive electrode VIN+ of the startup voltage, and the output end of the input capacitor circuit is grounded VIN-, located between the positive electrode of the startup voltage and the negative electrode of the startup voltage, and is used as an input filter function of the flyback power supply.

[0088] Preferably, the flyback circuit further includes a high-voltage startup charging capacitor circuit;

[0089] The input end of the high-voltage startup charging capacitor circuit is connected to the VIN port of LM5021 and the output end of the high-voltage startup resistor circuit at the same time; the output end of the high-voltage startup charging capacitor circuit is grounded;

[0090] When the starting voltage meets the conditions, the positive electrode of the starting voltage charges the high-voltage starting charging capacitor circuit through the high-voltage starting resistor circuit.

[0091] like Figure 1 As shown, the high-voltage startup charging capacitor circuit can be a circuit in which capacitors C11 and C12 are connected in parallel. The input end of the high-voltage startup charging capacitor circuit is simultaneously connected to the VIN port VCC-IN of LM5021 and the output end of the high-voltage startup resistor circuit; and the output end of the high-voltage startup charging capacitor circuit is grounded VIN-.

[0092] Preferably, the flyback circuit further includes a MOS tube Q300, an RCD absorption circuit, a resistor R33 and a protection resistor R325;

[0093] The OUT port of LM5021 is connected to the gate of MOS tube Q300 through resistor R33, and is connected to the source of MOS tube Q300 through protection resistor R325;

[0094] The drain of MOS tube Q300 is connected to the input end of the RCD absorption circuit and the opposite end of the primary input winding of the transformer;

[0095] The output end of the RCD absorption circuit is connected to the same end of the primary input winding of the transformer.

[0096] Preferably, the RCD absorption circuit includes a third diode D304, a second resistor R41, a third resistor R10, a fifth capacitor C305 and a sixth capacitor C98;

[0097] The anode of the third diode D304 serves as the input end of the RCD absorption circuit, and the cathode of the third diode D304 is connected to one end of the second resistor R41, one end of the third resistor R10, one end of the fifth capacitor C305, and one end of the sixth capacitor C98;

[0098] The other end of the second resistor R41, the other end of the third resistor R10, the other end of the fifth capacitor C305, and the other end of the sixth capacitor C98 are connected, and the connection point serves as the output end of the RCD absorption circuit. The output end of the RCD absorption circuit is connected to the same-name end of the primary input winding of the transformer.

[0099] Specifically, Figure 1 As shown, the gate of the MOS tube Q300 can be connected to the OUT port of LM5021 through the resistor R33; the source of the MOS tube Q300 can be connected to the OUT port of LM5021 through the protection resistor R325; the drain of the MOS tube Q300 is simultaneously connected to the input end of the RCD absorption circuit and the opposite end of the primary input winding of the transformer.

[0100] Specifically, Figure 1 As shown, the RCD absorption circuit includes a third diode D304, a second resistor R41, a third resistor R10, a fifth capacitor C305 and a sixth capacitor C98. The second resistor R41, the third resistor R10, the fifth capacitor C305 and the sixth capacitor C98 are connected in parallel, one end of the parallel connection is connected to the cathode of the third diode D304, and the other end is connected to the same end of the primary input winding of the transformer.

[0101] Preferably, the SS port of LM5021 is grounded through the seventh capacitor C13; the VCC port of LM5021 is grounded through the eighth capacitor C34; the GND port of LM5021 is grounded;

[0102] The RT port of LM5021 is grounded through the fourth resistor R318;

[0103] The CS port of LM5021 is connected to one end of the ninth capacitor C320 and one end of the fourth resistor R330 at the same time, and the other end of the ninth capacitor C320 is grounded; the other end of the fourth resistor R330 is connected to the source of the MOS tube Q300 and one end of the fifth resistor R326 at the same time; the other end of the fifth resistor R326 is grounded.

[0104] Specifically, Figure 1 As shown, the SS port of LM5021 is grounded to VIN- through the seventh capacitor C13; the VCC port of LM5021 is grounded to VIN through the eighth capacitor C34; the GND port of LM5021 is grounded to VIN-; the RT port of LM5021 is grounded to VIN- through the fourth resistor R318; the CS port of LM5021 is grounded to VIN- through the ninth capacitor C320, and is connected to one end of the fourth resistor R330, the other end of the fourth resistor R330, and the source of the MOS tube Q300 and one end of the fifth resistor R326, and the other end of the fifth resistor R326 is grounded to VIN-.

[0105] like Figure 1As shown, it is worth noting that when the starting voltage VIN+ reaches the starting voltage, the high-voltage starting resistor circuit and the high-voltage starting charging capacitor circuit form a charging circuit. At this time, the OUT port of LM5021 sends out a PWM wave to drive the MOS tube Q300 to work. As the PWM wave is sent out, the same-name terminal 4 and the opposite-name terminal 5 of the primary output winding of the transformer begin to generate alternating voltage and current. After rectification by the rectifier diode D8, a voltage VCCP12 is generated in the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317. VCCP12 obtains a sampling voltage after the voltage is divided by the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317, and is input into the loop control circuit. In the loop control circuit, the sampling voltage is compared with the reference voltage 1.25V inside the TLV431. If it is lower than 1.25V, the transistor between the ANODE port and the CATHODE port of the TLV431 controls the current flowing through the pull-down resistor R170 to decrease, thereby increasing the voltage of the COM port of the LM5021, thereby increasing the duty cycle of the OUT port output of the LM5021, thereby increasing the voltage at VCCP12; if it is higher than 1.25V, the transistor between the ANODE port and the CATHODE port of the TLV431 controls the current flowing through the pull-down resistor R170 to increase, thereby decreasing the voltage of the COM port of the LM5021, thereby reducing the duty cycle of the OUT port output of the LM5021, thereby reducing the voltage at VCCP12; ultimately, the voltage at CVVP12 is maintained at a steady-state voltage.

[0106] Compared with the prior art, the primary-side feedback flyback circuit based on LM5021 provided in this embodiment breaks through the limitation of the prior art that only secondary-side feedback can be achieved by using LM5021; at the same time, during primary-side feedback, voltage feedback can be directly performed through the flyback circuit power supply winding, reducing the number of closed-loop components. In situations where layout space is limited, the number of circuit components can be greatly reduced, and there is no need to introduce additional operational amplifiers, voltage references for PID control loops, optocouplers and other components, thereby effectively reducing costs while improving power density; and by introducing the TLV431 chip, the feedback circuit is set on the primary side of the transformer, thereby avoiding the increased layout difficulty caused by the introduction of an isolation circuit and the increased number of components caused by isolated feedback.

[0107] Those skilled in the art will appreciate that all or part of the processes of the above-mentioned embodiments can be implemented by instructing related hardware through a computer program, and the program can be stored in a computer-readable storage medium, wherein the computer-readable storage medium is a disk, an optical disk, a read-only storage memory, or a random access memory, etc.

[0108] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A primary feedback flyback circuit based on LM5021, characterized in that: The flyback circuit includes a transformer, LM5021, a voltage sampling circuit, a pull-down resistor R170 and a loop control circuit; The transformer's primary output winding opposite end is connected to the VIN port of LM5021; The voltage sampling circuit is connected between the same-name terminal and the opposite-name terminal of the primary output winding, and is used to sample the output voltage of the primary output winding; The output end of the voltage sampling circuit is connected to the input end of the loop control circuit, the output end of the loop control circuit is connected to one end of the pull-down resistor R170, and the other end of the pull-down resistor R170 is connected to the COM port of LM5021; the loop control circuit compares the sampled voltage with a preset reference voltage to stabilize the COM port voltage of LM5021 within a valid range.

2. The flyback circuit according to claim 1, characterized in that: The voltage sampling circuit includes a first voltage dividing resistor R316 and a second voltage dividing resistor R317; The opposite-name end of the primary output winding is connected to one end of the first voltage-dividing resistor R316; The other end of the first voltage-dividing resistor R316 is connected to one end of the second voltage-dividing resistor R317, and the other end of the second voltage-dividing resistor R317 is simultaneously connected to the same-name end of the primary output winding and the ground; the connection point between the first voltage-dividing resistor R316 and the second voltage-dividing resistor R317 serves as the output end of the voltage sampling circuit.

3. The flyback circuit according to claim 2, characterized in that: The loop control circuit includes a first capacitor C5, a second capacitor C321, a first resistor R332 and a TLV431; One end of the first capacitor C5, one end of the first resistor R332 and the REF port of the TLV431 are connected, and the connection point serves as the input end of the loop control circuit; The other end of the first resistor R332 is connected to one end of the second capacitor C321; The other end of the first capacitor C5, the other end of the second capacitor C321 and the CATHODE port of the TLV431 are connected, and the connection point serves as the output end of the loop control circuit. The ANODE port of the TLV431 is grounded.

4. The flyback circuit according to claim 2, characterized in that: The flyback circuit also includes a rectifier diode D8, a third capacitor C125 and a fourth capacitor C318; The opposite-name end of the primary output winding is connected to the anode of the rectifier diode D8; the cathode of the rectifier diode D8 is simultaneously connected to one end of the first voltage-dividing resistor R316, one end of the third capacitor C125 and one end of the fourth capacitor C318; The other end of the third capacitor C125 and the other end of the fourth capacitor C318 are both grounded.

5. The flyback circuit according to claim 1, characterized in that: The flyback circuit also includes a first diode D10 and a second diode D31; The opposite-name end of the primary output winding is connected to the anode of the first diode D10 and the anode of the second diode D31 at the same time; The cathode of the first diode D10 and the cathode of the second diode D31 are connected to the VIN port of LM5021 at the same time.

6. The flyback circuit according to claim 1, characterized in that: The flyback circuit also includes a high-voltage startup resistor circuit and an input capacitor circuit; The positive electrode of the starting voltage is connected to the input end of the high-voltage starting resistor circuit, and the output end of the high-voltage starting resistor circuit is connected to the VIN port of LM5021; The input capacitor circuit is connected between the positive electrode of the starting voltage and the negative electrode of the starting voltage, and the negative electrode of the starting voltage is grounded.

7. The flyback circuit according to claim 6, characterized in that: The flyback circuit also includes a high-voltage startup charging capacitor circuit; The input end of the high-voltage startup charging capacitor circuit is connected to the VIN port of LM5021 and the output end of the high-voltage startup resistor circuit at the same time; the output end of the high-voltage startup charging capacitor circuit is grounded; When the starting voltage meets the conditions, the positive electrode of the starting voltage charges the high-voltage starting charging capacitor circuit through the high-voltage starting resistor circuit.

8. The flyback circuit according to claim 1, characterized in that: The flyback circuit also includes a MOS tube Q300 and an RCD absorption circuit; The OUT port of LM5021 is connected to the gate of MOS tube Q300 through resistor R33, and is connected to the source of MOS tube Q300 through protection resistor R325; The drain of MOS tube Q300 is connected to the input end of the RCD absorption circuit and the opposite end of the primary input winding of the transformer; The output end of the RCD absorption circuit is connected to the same end of the primary input winding of the transformer.

9. The flyback circuit according to claim 8, characterized in that: The RCD absorption circuit includes a third diode D304, a second resistor R41, a third resistor R10, a fifth capacitor C305 and a sixth capacitor C98; The anode of the third diode D304 serves as the input end of the RCD absorption circuit, and the cathode of the third diode D304 is connected to one end of the second resistor R41, one end of the third resistor R10, one end of the fifth capacitor C305, and one end of the sixth capacitor C98; The other end of the second resistor R41, the other end of the third resistor R10, the other end of the fifth capacitor C305, and the other end of the sixth capacitor C98 are connected, and the connection point serves as the output end of the RCD absorption circuit. The output end of the RCD absorption circuit is connected to the same-name end of the primary input winding of the transformer.

10. The flyback circuit according to claim 1, characterized in that: The SS port of LM5021 is grounded through the seventh capacitor C13; the VCC port of LM5021 is grounded through the eighth capacitor C34; the GND port of LM5021 is grounded; The RT port of LM5021 is grounded through the fourth resistor R318; The CS port of LM5021 is connected to one end of the ninth capacitor C320 and one end of the fourth resistor R330 at the same time, and the other end of the ninth capacitor C320 is grounded; the other end of the fourth resistor R330 is connected to the source of the MOS tube Q300 and one end of the fifth resistor R326 at the same time; the other end of the fifth resistor R326 is grounded.

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