WO2019019814A1

WO2019019814A1 - Secondary side feedback control circuit and switching power supply using the same

Info

Publication number: WO2019019814A1
Application number: PCT/CN2018/090364
Authority: WO
Inventors: 肖华; 唐盛斌; 於昌虎
Original assignee: 广州金升阳科技有限公司
Priority date: 2017-07-26
Filing date: 2018-06-08
Publication date: 2019-01-31
Also published as: CN107370384A

Abstract

Disclosed in the present invention is a secondary side feedback control circuit, which is suitable for a isolating switching power supply having a primary circuit formed by a primary winding of a transformer and a secondary circuit formed by a secondary winding of the transformer. The secondary side feedback control circuit comprises: a secondary side degaussing circuit, a detection and decision module, an output voltage coding control module, a feedback switch, an auxiliary winding voltage detection upper resistance, an auxiliary winding voltage detection lower resistance, a feedback switch state detection module, an output voltage decoding feedback module and a duty cycle modulation circuit. Since the feedback switch is connected in series with the secondary side degaussing circuit, the power supply system of the present invention can work stably at a high output voltage. Moreover, charge backflow of the output capacitor can be prevented, the difficulty of controlling the feedback switch can be reduced. At the same time, the feedback switch only needs to use low-voltage devices, reducing the cost of the power supply system.

Description

Secondary side feedback control circuit and switching power supply using the same

Technical field

The invention relates to a switching power supply controller, in particular to a magnetically isolated high output voltage secondary side feedback control circuit and a switching power supply using the same.

Background technique

In order to avoid the interference and damage of the load to the power input, the isolated switching power supply has become an indispensable part of various power supply systems. In order to achieve the isolation between the output voltage and the input voltage and to stabilize the output voltage at the set value, it is necessary to use an isolation device to feed back the magnitude of the output voltage to the input side for adjustment control. Figure 1 shows the commonly used secondary side feedback control technique. The task of isolated feedback is carried out by an isolation amplifier consisting of device TL431, optocoupler and auxiliary device. The basic principle is that the transconductance amplifier composed of TL431 and sampling resistors R1 and R2 amplifies the error voltage signal of the output voltage and the reference voltage into a current signal. When the output voltage is high, the current flowing through the TL431 becomes larger, that is, the current. When the current of the optocoupler becomes larger, the voltage of the controller FB port becomes smaller, so that the controller output duty ratio becomes smaller, so that the transformer transmits less energy to the secondary side output terminal, and the output voltage starts to decrease; otherwise, if the output voltage is biased Low, increase the output voltage by increasing the transmission energy of the transformer by feeding the error signal back to the primary side to control the increase of the duty cycle. Repeatedly adjusting the control so that the output voltage is stabilized at the set value. This kind of feedback technology is called the secondary side feedback because its detection and comparison link is on the secondary side of the switching power supply, that is, the load side. This way of directly detecting the output voltage has the characteristics of high precision, but since the presence of these detection, amplifier, and isolation feedback devices increases the space of the power system board, there is obviously no advantage in cost and size. In particular, optocouplers cannot operate at high temperatures and are prone to aging, making the power supply's high temperature life short and unable to meet some high temperature applications.

In order to solve the above-mentioned technical problems, the invention patent application with the publication number CN 105610306 A and the invention name "secondary feedback control method and its control circuit" proposes a novel secondary side feedback circuit, which is suitable for having An isolated switching power supply of a secondary circuit formed by a primary winding formed by a primary winding of the transformer and a secondary winding of the transformer. 2 is a diagram showing the operation state of the feedback switch of the circuit of FIG. 2 and the voltage waveform diagram of 124, and the secondary side feedback switch switching power supply shown in FIG. 2, which includes: a three-winding transformer, It consists of three windings, a primary winding NP, a secondary winding NS, and an auxiliary winding NA, wherein the winding NP comprises a first port 102 and a second port 103, the winding NS comprising a first port 104 and a second port 105, winding NA A first port 106 and a second port 107 are included. The secondary side modulator is composed of an output voltage coding control module and a detection and determination module. a secondary degaussing circuit comprising two ports of a first port 110 and a second port 111; an output capacitor comprising two ports of a first port 131 and a second port 132; a feedback switch comprising a drain port 133, a source terminal Port 135 and gate port 134 are three ports. An output voltage encoding control module, comprising three ports of a first port 112, a second port 113, and a third port 114; a detection judging module, comprising a first port 115, a second port 116, a third port 117, and a fourth port 118, fifth port 119 five ports; auxiliary winding voltage detection upper resistance, which includes a first port 120 and a second port 121; auxiliary winding voltage detection lower resistance, which includes a first port 122 and a second port 123; a feedback switch a state detecting module comprising two ports of a first port 124 and a second port 125; an output voltage decoding feedback module comprising two ports of a first port 126 and a second port 127; a duty cycle modulation circuit comprising the first The port 128, the second port 129, and the third port 130 have three ports.

Their connection relationship is: port 102 is connected to the positive terminal 101 of the input power source, port 103 is connected to the port 130; port 104, port 131, port 119 are connected together, the connection point forms the positive port 108 of the switching power supply output voltage; port 105, The port 110, the drain port 133 of the feedback switch, and the port 115 are connected together; the port 111, the port 118, the source port 135 of the feedback switch, and the port 132 are connected together, and the connection point forms the negative port 109 of the switching power supply output voltage; the feedback switch The gate port 134 is connected to the port 112; the port 113 is connected to the port 116; the port 114 is connected to the port 117; the port 120 is connected to the port 106; the port 121, the port 122, and the port 124 are connected together; the port 125 is connected to the port 126; Port 127 is connected to port 128; port 129, port 123, and port 107 are connected together, and the connection point forms the negative terminal of the input power. The excitation process of the flyback power switching power supply is the same as that of the conventional flyback switching power supply. The difference is how to feed back the change of the output voltage from the secondary side to the primary side during the degaussing phase. The specific working principle is detailed in paragraphs 0086-0090 of the relevant manual.

Switching Power Supply Switching Power Supply The feedback control simplification process of the above scheme is as follows: secondary side sampling output voltage → coding → control feedback switch resistance state change → primary side detection resistance state change → decoding → generating voltage modulation duty ratio. This solution requires neither optocoupler devices nor other additional isolated transfer devices, thereby avoiding some inherent defects caused by these devices themselves, and there are no devices added to assist the operation of these devices, which can reduce the volume. And cost, to optimize the size, cost, performance, and a wider range of applications. At the same time, there is no problem that the output voltage of the primary feedback technique is low and the output voltage cannot be changed by the secondary side.

However, the above scheme needs to ensure that ΔVref < ΔV ₁₂₄ , that is, the detection module can determine such a sudden change in voltage. If the power system output voltage is V _OUT =12V, V ₁₂₄ =3V, V _BE =3V, ΔV _ref =0.1V, and the feedback switch turns on voltage drop V _sdon =0.06V.

By the formula:

Can be derived:

Resulting in ΔV _ref >ΔV ₁₂₄ . Therefore, at a high output voltage, the feedback switch state detecting module is difficult to detect whether the conduction state of the feedback switch changes, and the decoding and the duty cycle modulation module cannot complete the decoding of the feedback switch conduction state and the duty ratio adjustment, and further The power system is not working properly. In addition, the feedback switch cannot be turned on after the excitation phase and degaussing of the transformer. Otherwise, the charge of the output capacitor may be reversed, which may cause a power system failure. The timing diagram of the feedback switch operation is shown in FIG. 3, where H represents the output voltage encoding control module 112. The port outputs a high level, the drive feedback switch is turned on, and the feedback switch is in a low resistance region; L indicates that the output voltage code control module 112 port outputs a low level, the drive feedback switch is turned off, and the source port 135 of the feedback switch is connected to the switching power supply. The negative terminal 109 of the output voltage is in the high resistance region when the feedback switch is turned off. The excitation phase of the transformer is 0~t1, t4~t5. If the feedback switch needs to be turned on during the degaussing phase of the transformer t1~t3, t5~t6, the detection and judgment module needs to detect the voltage at 133 through port 115, and the voltage at port 133. When the threshold value is reached, the feedback switch is turned off, that is, the feedback switch is turned off at t2, so that the feedback switch of the output capacitor is not backed up in the time period from t3 to t4 and t6 to t7 after the degaussing of the transformer is completed. Feedback switch control is relatively complicated. Since the feedback switch is connected in parallel with the degaussing circuit, the feedback switch needs to use a high voltage resistant device to increase the power system cost.

Summary of the invention

The technical problem to be solved by the present invention is to propose a secondary side feedback control circuit suitable for a high output voltage, a switching power supply.

The technical solution of the present invention to solve the above technical problems is as follows:

A secondary side feedback control circuit is applicable to an isolated switching power supply having a secondary side circuit formed by a primary side winding formed by a primary winding of a transformer and a secondary winding formed by a secondary winding of the transformer, comprising: a secondary side degaussing circuit, a detection judging module, and an output Voltage coding control module, feedback switch, auxiliary winding voltage detection upper resistance, auxiliary winding voltage detection lower resistance, feedback switch state detection module, output voltage decoding feedback module and duty cycle modulation circuit;

The secondary degaussing circuit is in a conducting state during the degaussing stage of the transformer, providing a path for the storage of the transformer to charge the output capacitor; in the non-degaussing stage of the transformer, the high resistance state is prevented, and the charge of the output capacitor is prevented from being reversed;

The detection judging module detects the output voltage of the isolating switching power supply, compares the voltage with the internal reference voltage, outputs the voltage change information, and sends the voltage to the output voltage encoding control module;

The output voltage coding control module performs coding according to the received communication change protocol according to the agreed communication protocol, and controls the working state of the feedback switch according to the coding;

The feedback switch state detecting module samples the voltage of the auxiliary winding by the auxiliary winding voltage detecting upper resistor and the auxiliary winding voltage detecting the partial pressure of the resistor at the appointed time of the degaussing phase of each switching cycle, and detects the current detected voltage with the previous detected voltage. The voltage is compared, the magnitude of the change of the voltage and the direction of the change of the voltage are obtained, and the change information of the working state of the feedback switch is output and sent to the output voltage decoding feedback module;

The output voltage decoding feedback module receives the feedback state change state information of the feedback switch, decodes according to the agreed communication protocol, determines whether the output voltage is high or low, outputs the modulation voltage, and sends it to the duty cycle to adjust to the circuit;

The duty cycle modulation circuit receives the modulation voltage, and modulates the duty ratio of the excitation of the main side winding of the transformer according to the magnitude of the voltage, and increases the duty ratio when the modulation voltage increases, and vice versa;

The utility model is characterized in that: the control end of the feedback switch is connected to the output voltage coding control module, the conduction current outflow end of the feedback switch is connected to the secondary side degaussing circuit, and the conduction current inflow end of the feedback switch is connected to the negative port of the output voltage of the switching power supply.

Preferably, if the feedback switch operating state change information is that the increased amplitude exceeds the set value, it is considered that the working state of the feedback switch jumps from the linear region to the saturation region; otherwise, the feedback switch operating state change information is the reduced amplitude exceeds the set value. The value is considered to be the transition from the saturation region to the linear region.

Preferably, the output voltage decoding feedback module outputs the result of the period is “output voltage is high”, and then gradually reduces the modulation voltage until “output voltage is low”; otherwise, if the result of the current decoding is “output voltage” If it is low, the modulation voltage is gradually increased until the "switching power supply output voltage is high".

Preferably, the feedback switch is a MOS tube.

A specific implementation of the isolated switching power supply applying the technical solution includes: a three-winding transformer, which is composed of three windings of a primary winding NP, a secondary winding NS, and an auxiliary winding NA, which respectively include a first port and a first a two-port; a secondary degaussing circuit comprising two ports of a first port and a second port; an output capacitor comprising two ports of a first port and a second port; and a feedback switch comprising a drain port, a source port, and Three ports of the gate port; an output voltage code control module, which includes a first port and a second port; a detection and determination module, which includes a first port and a second port; and an auxiliary winding voltage detecting upper resistor, which includes the first port and a second port; an auxiliary winding voltage detecting resistor, comprising a first port and a second port; a feedback switch state detecting module comprising a first port and a second port; an output voltage decoding feedback module comprising the first port and the Two ports and two ports; a duty cycle modulation circuit, which includes three ports of a first port, a second port, and a third port;

The connection relationship is: the first port of the primary winding NP is connected to the positive pole of the input power supply, the second port is connected to the third port of the duty cycle modulation circuit; the first port of the secondary winding NS, the first port of the output capacitor, The first ports of the detection and determination module are connected together, the connection point forms a positive port of the switching power supply output voltage; the second port of the secondary winding NS and the first port of the secondary degaussing circuit are connected together; the second port of the secondary degaussing circuit, The source port of the feedback switch is connected; the second port of the output voltage code control module is connected to the second port of the detection and judgment module; the drain port of the feedback switch and the second port of the output capacitor are connected together, and the connection point forms a switching power supply output voltage a negative port; the gate port of the feedback switch is connected to the first port of the output voltage encoding control module; the first port of the auxiliary winding voltage detecting upper resistor is connected to the first port of the auxiliary winding NA; the auxiliary winding voltage is detected by the upper resistor a second port, a first port of the auxiliary winding voltage detecting resistor, and a first end of the feedback switch state detecting module Connected together; the second port of the feedback switch state detection module is connected with the first port of the output voltage decoding feedback module; the second port of the output voltage decoding feedback module is connected with the first port of the duty cycle modulation circuit; The second port of the modulation circuit, the second port of the auxiliary winding voltage detecting lower resistor, and the second port of the auxiliary winding NA are connected together, and the connection point forms a negative terminal of the input power source.

Preferably, the feedback switch is a MOS tube.

The above related terms are explained as follows:

The agreed communication protocol means that the output voltage of the sampled switching power supply is numbered according to a predetermined rule, and the encoding rule is defaulted in the decoding process of the primary side, and the purpose is to determine whether the output voltage is high or low. The specific encoding and decoding process can be understood by the detailed explanation of the embodiments.

The appointment time of the degaussing phase means that the secondary side feedback switch is scheduled to operate at a certain time or time in the degaussing phase, and the detection module of the primary side senses the action to be valid during the predetermined time period.

The control terminal of the feedback switch: the port that controls the conduction switch to be turned on and off. For example, for the MOS transistor, it refers to the gate of the MOS transistor; for the triode, it refers to the base of the triode.

On-in current of the feedback switch: When the feedback switch is turned on, the current flowing into the port, such as the MOS transistor, refers to the drain of the MOS transistor, regardless of N-channel, P-channel, enhanced or depleted MOS When the tube is turned on, the current flows from the drain with a high voltage to the source with a low voltage. For the triode, it refers to the collector of the triode. When turned on, the current flows from the collector with a high voltage to a low voltage. The emitter.

The conduction current flowing out of the feedback switch: after the feedback switch is turned on, the port through which the current flows, such as the MOS tube, refers to the source of the MOS tube; for the triode, it refers to the emitter of the triode.

Feedback switch operating state: refers to the appropriate driving voltage to the feedback switch to operate in the saturation zone or linear zone, the saturation zone and the linear zone are relative rather than set absolute limits, they differ only in the feedback switch Whether the control voltage can be detected on the primary side and correctly judge the change of its working state.

In particular, in order to make it easier to understand the working principle in the present invention, the logic relationship in the encoding process in the embodiment is expressed in a specific form, for example, the message "the output voltage is high" is saturated with the "feedback switch". "Representation", in the actual product implementation, the "output voltage is too high" information can also be carried by "feedback switch in the linear region", only to better illustrate the invention, and is not intended to limit the invention.

Compared with the prior art, the present invention has the following beneficial effects:

(1) No optocoupler devices are required and no additional isolation transfer devices are required, thereby avoiding not only some of the inherent defects inherent in these devices, but also devices added to assist in the operation of these devices, reducing the size and The cost is optimized for volume, cost and performance, and the scope of application is wider.

(2) At the same time, there is no problem that the output voltage of the primary feedback technique is low and the output voltage cannot be changed by the secondary side.

(3) At high output voltage, ΔV _ref <ΔV ₁₂₄ can be ensured to make the power supply system work stably. The relevant principle analysis is shown in the embodiment part with specific switching power supply design and calculation.

(4) In addition, in the invention, the feedback switch is connected in series with the degaussing path, and the degaussing circuit will automatically cut off after the excitation phase and the degaussing to prevent the charge of the output capacitor from being reversed. The timing of the working state diagram of the feedback switch is as shown in FIG. 5, wherein H indicates that the output voltage encoding control module 112 port outputs a high level, the driving feedback switch is turned on, the feedback switch operates in a linear region; L indicates that the output voltage encoding control module 112 port outputs a low level, and the drain of the feedback switch in the present invention 135 is connected to the negative port 109 of the switching power supply output voltage. When the port 112 is low, the feedback switch operates in the saturation region. The transformer excitation phase is 0 to t1, t3 to t4, and the degaussing end phase is t2 to t3 and t5 to t6.

(5) The detection and judgment module does not need to detect whether the transformer is in the degaussing phase t1~t2, t4~t5 by sampling the voltage at the first port of the degaussing circuit of the secondary side, and only needs to detect the output voltage of the switching power supply through the positive port of the output voltage of the switching power supply. And compare the voltage with the internal reference voltage. The result of the comparison determines the working state of the feedback switch and reduces the difficulty of controlling the feedback switch. This is its outstanding advantage.

(6) The feedback switch of the present invention only needs to use a low voltage device to reduce the power system cost.

DRAWINGS

Figure 1 is a typical circuit diagram of a switching power supply using a conventional secondary side feedback controller;

2 is a typical circuit diagram of a switching power supply of a novel application secondary side feedback control circuit;

Figure 3 is a diagram showing the working state of the feedback switch of the circuit of Figure 2 and the voltage waveform at 124;

4 is a block diagram showing the principle of a switching power supply to which a secondary side feedback control circuit according to a first embodiment of the present invention is applied;

FIG. 5 is a diagram showing the working state of the feedback switch and the voltage waveform at 124 in the present invention.

Detailed ways

In order to make the present invention more apparent, the present invention will be further described in detail below with reference to the accompanying drawings. It is understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Embodiment 1

4 is a block diagram showing a principle of a switching power supply to which a secondary side feedback control circuit according to a first embodiment of the present invention is applied. The switching power supply includes a three-winding transformer including three windings of a primary winding NP, a secondary winding NS, and an auxiliary winding NA. Composition, wherein the winding NP comprises a first port 102 and a second port 103, the winding NS comprises a first port 104 and a second port 105, the winding NA comprises a first port 106 and a second port 107; a secondary side degaussing circuit, which comprises Two ports of one port 110 and second port 111; an output capacitor comprising two ports of a first port 131 and a second port 132; a feedback switch comprising a drain port 135, a source port 133 and a gate port 134 a port; an output voltage encoding control module, comprising a first port 112, a second port 114; a detection judging module comprising a first port 119, a second port 117; an auxiliary winding voltage detecting upper resistor R _FA1 , which includes the first port 120 and second port 121; the auxiliary winding voltage detection resistor R _FA2, which comprises a first port 122 and second port 123; feedback switch state detection module, comprising a first and second port 124 Port 125 two ports; an output voltage decoding feedback module, which includes two ports, a first port 126 and a second port 127; a duty cycle modulation circuit, which includes a first port 128, a second port 129, and a third port 130 Ports.

Their connection relationship is: port 102 is connected to the positive terminal 101 of the input power source, port 103 is connected to the port 130; port 104, port 131, port 119 are connected together, the connection point forms the positive port 108 of the switching power supply output voltage; port 105, The ports 110 are connected together; the port 111 and the source port 133 of the feedback switch are connected together; the drain port 135 and the port 132 of the feedback switch are connected together, the connection point forms the negative port 109 of the switching power supply output voltage; and the gate port 134 of the feedback switch Ports 112 are connected together; port 114 is connected to port 117; port 120 is connected to port 106; port 121, port 122, port 124 are connected together; port 125 is connected with port 126; port 127 is connected with port 128; port 129, Port 123 and port 107 are connected together, and the connection point forms the negative terminal of the input power.

The specific working principle is:

The function of the detection and judgment module is: detecting the output voltage of the switching power supply through the port 119, and comparing the voltage with the internal reference voltage, the result of the comparison can determine whether the feedback switch changes the working state, and passes the information through its port port. 117 is passed to the output voltage encoding control module.

The output voltage coding control module includes two functions: one is the coding function, and the coding rule is set. When the output voltage changes from a high state to a low state, the feedback switch operation state needs to change from a saturation region to a linear region, and vice versa. When the voltage changes from a low state to a high state, the feedback switch operating state needs to change from a linear region to a saturated region. Then, when the output voltage is high, the feedback switch should be in the saturation region, and the feedback switch drive level should be low; when the output voltage is low, the feedback switch should be in the linear region, and the feedback switch drive level is high. Second, the control function controls the feedback switch gate to be at the corresponding encoding level, that is, the port 112 outputs a low level when the output voltage is high, and the port 112 outputs a high level when the output voltage is low.

Transformer transmission process: Because there is the storage capacity of the output capacitor C _OUT , the output voltage of the switching power supply will not change greatly in one or even several cycles, so the capacitor C _OUT voltage V _OUT can be ignored in a short time. The change. According to the present invention, it is now necessary to superimpose a control voltage on V _OUT . For this purpose, we use a rectifying diode as the secondary side degaussing path, as shown in Figure 4, which is the simplest and most common way. Because of the junction voltage drop V _BE of the diode, when the feedback switch operates in the linear region during the degaussing phase, the minimum voltage difference between the winding NS port 104 and the port 105 is (V _OUT +V _BE +V _dson ), where V _Dson is the voltage difference between the drain and the source when the feedback switch operates in the linear region; when the feedback switch operates in the saturation region during the degaussing phase, the voltage difference between the winding NS port 104 and the port 105 is (V _OUT +V _BE + V _DS ), where V _DS is the voltage difference between the drain and the source when the feedback switch operates in the saturation region. It can be seen that the feedback MOS switch in the degaussing stage operates in the linear region or the saturation region, so that the voltage difference across the winding has undergone a large abrupt change. By conversion, the voltage variation at the port 124 of the feedback switch state detecting module is:

Where n _A / n _S is the ratio of the winding NA to the number of turns of the winding NS.

The detection and determination process of the feedback switch state detecting module: In order to make the transmission process more intuitive, a power switching power supply is specifically designed to be explained below. Select V _OUT =12V, V ₁₂₄ =3V, ΔV _ref =0.1V, V _BE =0.4V, the feedback switch operates in the saturation region with a turn-on voltage drop of V _DS =3V, and the feedback switch operates in the linear region. The pressure drop V _dson = 0.06V.

By the formula:

Can be derived:

ΔV _ref <ΔV ₁₂₄ , it can be judged that the working state of the feedback switch changes from the linear region to the saturation region; conversely, it can be judged that the working state of the feedback switch changes from the saturation region to the linear region.

Decoding and duty cycle modulation process: According to the coding rules set in the encoding process, the decoding rule should be set to decode the corresponding result when receiving the judgment result of “the feedback switch operating state changes from the saturation region to the linear region”. The output voltage changes from a high state to a low state. On the contrary, when the judgment result of the "feedback switch operating state changes from the linear region to the saturation region" is received, the output voltage is correspondingly decoded from the low state to the high state. status". If the decoding result is "the output voltage changes from the high state to the low state", it indicates that the output voltage is too high before the change, and the current output voltage is low until the state change in the opposite direction is received again; otherwise, it can be obtained Know that the current output voltage is too high. It can be seen that as long as the communication protocol formed by the two settings is controlled and transmitted, the output voltage of the secondary side of the isolation transformer can be fed back to its primary side. When the output voltage is high, the modulation voltage _Vcrl is gradually decreased, which controls the duty ratio to gradually decrease, thereby causing the output voltage to decrease; conversely, when the output voltage is low, the duty ratio is gradually increased to rise again. Repeat this way to stabilize the output voltage at the set value.

The detection switch module and the output voltage code control module enable the feedback switch tube to operate in the linear region or the saturation region, thereby increasing the amplitude of the voltage change at the port 124, so that the feedback switch state detection module can be reliable when the power system is at a high output voltage. The ground switch detects the change of the working state of the feedback switch, and then adjusts the duty ratio after decoding to stabilize the output voltage at the set value.

The above is only a preferred embodiment of the present invention, and it should be noted that the above-described preferred embodiments are not to be construed as limiting the scope of the invention, and the scope of the invention should be determined by the scope defined by the claims. It will be apparent to those skilled in the art that various modifications and improvements can be made without departing from the spirit and scope of the invention.

Claims

A secondary side feedback control circuit is applicable to an isolated switching power supply having a secondary side circuit formed by a primary side winding formed by a primary winding of a transformer and a secondary winding formed by a secondary winding of the transformer, comprising: a secondary side degaussing circuit, a detection judging module, and an output Voltage coding control module, feedback switch, auxiliary winding voltage detection upper resistance, auxiliary winding voltage detection lower resistance, feedback switch state detection module, output voltage decoding feedback module and duty cycle modulation circuit;

The secondary degaussing circuit is in a conducting state during the degaussing stage of the transformer, providing a path for the storage of the transformer to charge the output capacitor; in the non-degaussing stage of the transformer, the high resistance state is prevented, and the charge of the output capacitor is prevented from being reversed;

The detection judging module detects the output voltage of the isolating switching power supply, compares the voltage with the internal reference voltage, outputs the voltage change information, and sends the voltage to the output voltage encoding control module;

The output voltage coding control module performs coding according to the received communication change protocol according to the agreed communication protocol, and controls the working state of the feedback switch according to the coding;

The feedback switch state detecting module samples the voltage of the auxiliary winding through the auxiliary winding voltage detecting upper resistor and the auxiliary winding voltage detecting the partial pressure of the resistor at the appointed time of the degaussing phase of each switching cycle, and the current detected voltage is compared with the previously detected voltage. The voltage is compared, the magnitude of the change of the voltage and the direction of the change of the voltage are obtained, and the change information of the working state of the feedback switch is output and sent to the output voltage decoding feedback module;

The output voltage decoding feedback module receives the feedback state change state information of the feedback switch, decodes according to the agreed communication protocol, determines whether the output voltage is high or low, outputs the modulation voltage, and sends it to the duty cycle to adjust to the circuit;

The duty cycle modulation circuit receives the modulation voltage, and modulates the duty ratio of the excitation of the main side winding of the transformer according to the magnitude of the voltage, and increases the duty ratio when the modulation voltage increases, and vice versa;

The utility model is characterized in that: the control end of the feedback switch is connected to the output voltage coding control module, the conduction current outflow end of the feedback switch is connected to the secondary side degaussing circuit, and the conduction current inflow end of the feedback switch is connected to the negative port of the output voltage of the switching power supply.
The secondary side feedback control circuit according to claim 1, wherein the feedback switch operating state change information is that the increased amplitude exceeds the set value, and then the operating state of the feedback switch is changed from a linear region to a saturated region; If the feedback switch operating state change information is that the reduced amplitude exceeds the set value, it is considered that the working state of the feedback switch jumps from the saturation region to the linear region.
The secondary side feedback control circuit according to claim 1, wherein the output voltage decoding feedback module outputs "the output voltage is too high", and the modulation voltage is gradually decreased until the "output voltage is low" occurs. On the other hand, if the result of the decoding of this period is "the output voltage is low", the modulation voltage is gradually increased until the "switching power supply output voltage is high" appears.
The secondary side feedback control circuit according to claim 1, wherein the feedback switch is a MOS transistor.
A switching power supply applying the secondary side feedback control circuit according to claim 1, comprising: a three-winding transformer comprising three windings of a primary winding NP, a secondary winding NS, and an auxiliary winding NA, respectively a port and a second port; a secondary degaussing circuit comprising two ports of a first port and a second port; an output capacitor comprising two ports of a first port and a second port; a feedback switch comprising a drain port, Three ports of source port and gate port; output voltage coding control module, which includes a first port and a second port; a detection and determination module, which includes a first port and a second port; and an auxiliary winding voltage detection upper resistor, which includes a first port and a second port; an auxiliary winding voltage detecting lower resistor, comprising a first port and a second port; a feedback switch state detecting module comprising a first port and a second port; and an output voltage decoding feedback module comprising Two ports of one port and two ports; a duty cycle modulation circuit, which includes three ports of a first port, a second port, and a third port.

The connection relationship is: the first port of the primary winding NP is connected to the positive pole of the input power supply, the second port is connected to the third port of the duty cycle modulation circuit; the first port of the secondary winding NS, the first port of the output capacitor, The first ports of the detection and determination module are connected together, the connection point forms a positive port of the switching power supply output voltage; the second port of the secondary winding NS and the first port of the secondary degaussing circuit are connected together; the second port of the secondary degaussing circuit, The source port of the feedback switch is connected; the second port of the output voltage code control module is connected to the second port of the detection and judgment module; the drain port of the feedback switch and the second port of the output capacitor are connected together, and the connection point forms a switching power supply output voltage a negative port; a gate port of the feedback switch is connected to the first port of the output voltage encoding control module; a first port of the auxiliary winding voltage detecting upper resistor is connected to the first port of the auxiliary winding NA; and an auxiliary winding voltage detecting upper resistor The first port of the two-port, auxiliary winding voltage detection resistor, and the first port of the feedback switch state detection module Connected; the second port of the feedback switch state detection module is connected with the first port of the output voltage decoding feedback module; the second port of the output voltage decoding feedback module is connected with the first port of the duty cycle modulation circuit; The second port of the modulation circuit, the second port of the auxiliary winding voltage detecting lower resistor, and the second port of the auxiliary winding NA are connected together, and the connection point forms a negative terminal of the input power source.
A switching power supply according to claim 5, wherein the feedback switch is a MOS transistor.