Flyback converter control method and flyback converter

Abstract

The invention provides a flyback converter control method and a flyback converter, and the control method comprises the steps: obtaining a sampling signal representing the discharge current of a first capacitor during the conduction period of a second switching tube when the flyback converter operates; when the sampling signal is greater than or equal to the threshold signal, the second switching tube is controlled to be switched off, and the flyback converter is controlled to enter a discharge stage; controlling the first capacitor to intermittently discharge in the discharge stage, and controlling the energy storage energy to be smaller than the discharge energy; the stored energy represents total energy increased by the first capacitor through charging of the input end, and the discharged energy represents total energy reduced by the first capacitor through discharging. In the invention, as long as the resonance current exceeds the safety current, the system can immediately respond to the protection device and enter the discharge stage, so as to control the resonance current within the safety current. The control method provided by the invention can be suitable for any transient working condition, the resonance current can be effectively controlled in any transient working condition, and the safety of the device and the reliability of the system are ensured.

Description

Technical Field

[0001] This invention belongs to the field of switching power supply technology, specifically relating to a control method for a flyback converter and a flyback converter. Background Technology

[0002] Asymmetric half-bridge flyback converters are as follows Figure 1 and Figure 2 In both implementations, under certain transient conditions, there is a large voltage difference between the voltage across the resonant capacitor C1 and the secondary-side refraction voltage N*Vo. Therefore, when the second switch Q2 is turned on, the resonant current i of the resonant capacitor C1... LK Excessive stress can cause the current and voltage stress on system components to exceed specifications, leading to damage and failure of the system components.

[0003] For example, when the power is off, the resonant capacitor C1 has not been fully discharged. When the power is turned on again, there is a large voltage difference between the voltage across the resonant capacitor C1 and the secondary-side refraction voltage N*Vo. Therefore, during the conduction of the second switch Q2, a large resonant current i will be generated when the leakage inductance Lk and the resonant capacitor C1 resonate. LK , specifically Figure 3 As shown. The resonant current i LK This could exceed the current and voltage stress specifications of the second switch Q2 and rectifier D1, causing them to fail. For example, when the system is powered on and has output, and the output capacitor C2 is short-circuited, there is a large voltage difference between the voltage of the resonant capacitor C1 and the secondary-side refraction voltage N*Vo. Therefore, during the conduction of the second switch Q2, the resonant capacitor C1 and the leakage inductance Lk will also generate [a voltage difference]. Figure 3 The larger resonant current i shown LK This can damage system components. Similarly, as... Figure 4 As shown, the active clamp flyback converter can also experience excessive resonant current under the aforementioned transient conditions, potentially damaging system components. For example, during power-off restart or when the output is short-circuited, the clamping capacitor Cclamp and leakage inductance Lk may generate a large resonant current when the second switch Q2 is turned on, thereby damaging system components.

[0004] Taking an asymmetrical half-bridge flyback converter as an example, to address the issue of excessive resonant current during restart due to undischarged resonant capacitor C1 during power-off, which could damage system components, existing technologies propose intermittently turning on the second switch Q2 before restarting to transfer the energy of resonant capacitor C1 to the output and input bus terminals. Furthermore, to address the issue of excessive resonant current damaging system components when the output of the asymmetrical half-bridge flyback converter is short-circuited, existing technologies propose detecting the resonant current; when the resonant current exceeds a set threshold, the second switch Q2 is immediately turned off, thereby preventing excessive resonant current from damaging system components.

[0005] It is obvious that the scheme of intermittently turning on the second switch Q2 before restarting is only applicable to the condition of restarting after shutdown. Similarly, the scheme of controlling the second switch Q2 to turn off based on the magnitude of the resonant current is only applicable to the condition of output short circuit during operation, and not to the condition of restarting after shutdown. This is because when the system is powered on, controlling the second switch Q2 to turn off when the resonant current is too large will cause the resonant capacitor C1 to charge more than it discharges in each switching cycle, making the voltage of the resonant capacitor C1 higher and higher, ultimately leading to system startup failure.

[0006] In summary, existing solutions for different transient conditions are not applicable to all transient conditions. Therefore, there is currently a lack of technical solutions applicable to all transient conditions that cause a large voltage difference between the voltage of the resonant capacitor (clamping capacitor) and the secondary-side refraction voltage, in order to solve the problem of system component damage. Summary of the Invention

[0007] To address the lack of a technical solution applicable to all transient operating conditions causing a large voltage difference between the voltage of the resonant capacitor (clamping capacitor) and the secondary-side refraction voltage, thus preventing system component damage, this invention proposes a control method and a flyback converter. The flyback converter includes a transformer, a first and a second switching transistor located on the primary side of the transformer and connected between the input terminal and a reference ground, and a first capacitor and a first inductor forming a resonant circuit when the second switching transistor is on. Specifically, the control method includes:

[0008] During the operation of the flyback converter, a sampling signal characterizing the magnitude of the discharge current of the first capacitor is acquired during the conduction period of the second switch.

[0009] When the sampled signal is greater than or equal to a threshold signal, the second switch is turned off and the flyback converter is controlled to enter the discharge stage. The threshold signal represents the magnitude of the safe current.

[0010] During the discharge phase, the first capacitor is controlled to discharge intermittently, and the stored energy is controlled to be less than the discharge energy.

[0011] The stored energy represents the total energy gained by the first capacitor through charging at the input terminal, and the discharged energy represents the total energy lost by the first capacitor through discharging.

[0012] Preferably, during the discharge phase, the first switch is controlled to be continuously turned off so that the stored energy is zero, and the second switch is controlled to be intermittently turned on so that the first capacitor is intermittently discharged.

[0013] Preferably, during the discharge phase, the first switch is controlled to conduct at least once to make the stored energy greater than zero, and the second switch is controlled to conduct intermittently to make the first capacitor discharge intermittently.

[0014] Furthermore, controlling the intermittent discharge of the first capacitor includes the steps of: acquiring the sampling signal when the first capacitor is discharging, and controlling the first capacitor to stop discharging when the sampling signal is greater than or equal to the threshold signal.

[0015] Furthermore, the method also includes the step of: when the sampled signal is always less than the threshold signal, the flyback converter exits the discharge phase and operates normally.

[0016] In one embodiment, during the discharge phase, a reference signal characterizing the preset on-time of the first switch or the preset peak current on the original side is reduced, and the turn-off time of the first switch is controlled according to the reduced reference signal.

[0017] In one embodiment, during the discharge phase, the feedback signal characterizing the output voltage is reduced, and the turn-off time of the first switch is controlled based on the reduced feedback signal.

[0018] In one embodiment, during the discharge phase, the primary-side sampling signal characterizing the primary-side current is increased, and the turn-off time of the first switch is controlled based on the increased primary-side sampling signal.

[0019] In one embodiment, during the discharge phase, when the first switch is turned on, the increase in voltage of the first capacitor is detected, and the increase in voltage is controlled to be less than the decrease in voltage. The decrease in voltage represents the decrease in voltage of the first capacitor during its last discharge, and the last discharge of the first capacitor occurred immediately before the first switch is turned on.

[0020] A flyback converter includes a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input terminal of the flyback converter and a reference ground, and a first capacitor and a first inductor forming a resonant circuit when the second switch is turned on. Specifically, it further includes:

[0021] The sampling circuit samples a signal characterizing the magnitude of the discharge current of the first capacitor during the operation of the flyback converter while the second switch is on.

[0022] The control circuit controls the second switch to turn off and controls the flyback converter to enter the discharge stage when the sampled signal is greater than or equal to the threshold signal. The threshold signal represents the magnitude of the safe current.

[0023] During the discharge phase, the first capacitor is controlled to discharge intermittently, and the stored energy is controlled to be less than the discharge energy.

[0024] The stored energy represents the total energy gained by the first capacitor through charging at the input terminal, and the discharged energy represents the total energy lost by the first capacitor through discharging.

[0025] Preferably, during the discharge phase, the control circuit controls the first switch to be continuously turned off so that the stored energy is zero, and controls the second switch to be turned on intermittently so that the first capacitor is discharged intermittently.

[0026] Preferably, during the discharge phase, the control circuit controls the first switch to be turned on at least once to make the stored energy greater than zero, and controls the second switch to be turned on intermittently to make the first capacitor discharge intermittently.

[0027] Furthermore, during the discharge phase, when the second switch is turned on, the control circuit controls the second switch to turn off when the sampled signal is greater than or equal to the threshold signal, so that the first capacitor stops discharging.

[0028] Furthermore, during the discharge phase, when the second switch is turned on, the control circuit controls the flyback converter to exit the discharge phase and operate normally when the sampled signal is always less than the threshold signal.

[0029] In one embodiment, during the discharge phase,

[0030] The reference signal characterizing the preset on-time of the first switch or the preset peak current on the original side is reduced, and the control circuit controls the turn-off time of the first switch based on the reduced reference signal.

[0031] Alternatively, the feedback signal characterizing the output voltage is reduced, and the control circuit controls the turn-off time of the first switching transistor based on the reduced feedback signal;

[0032] Alternatively, the primary-side sampling signal characterizing the primary-side current can be increased, and the control circuit can control the turn-off time of the first switching transistor based on the increased primary-side sampling signal.

[0033] In one embodiment, during the discharge phase, when the first switch is turned on, the increase in voltage of the first capacitor is detected, and the increase in voltage is controlled to be less than the decrease in voltage. The decrease in voltage represents the decrease in voltage of the first capacitor during its last discharge, and the last discharge of the first capacitor occurred immediately before the first switch is turned on.

[0034] In one embodiment, the sampling circuit includes a first sampling resistor connected in series in the resonant circuit, and the current flows through the first sampling resistor when the first switch is turned on and when the second switch is turned on. The reference ground interface of the control circuit is connected to the reference ground of the flyback converter, and the sampling interface of the control circuit is connected to the first sampling resistor.

[0035] In one embodiment, the sampling circuit includes a first sampling resistor and a second sampling resistor, which are connected in series between the second switch and the reference ground of the flyback converter. When the second switch is turned on, current flows through the first sampling resistor, and when the first switch is turned on, current flows through the second sampling resistor. The sampling interface of the control circuit is connected to the first sampling resistor, and the reference ground interface of the control circuit is connected to the reference ground of the flyback converter.

[0036] In one embodiment, the sampling circuit includes a first sampling resistor and a second sampling resistor, which are connected in series between the second switch and the reference ground of the flyback converter. When the second switch is turned on, current flows through the first sampling resistor, and when the first switch is turned on, current flows through the second sampling resistor. The reference ground interface of the control circuit is connected to the connection point of the first sampling resistor and the second sampling resistor. The first sampling interface of the control circuit is connected to the other end of the first sampling resistor, and the second sampling interface of the control circuit is connected to the reference ground of the flyback converter.

[0037] In one embodiment, the sampling circuit includes a second capacitor and a sampling resistor. The second capacitor and the sampling resistor are connected in series between the positive terminal of the first capacitor and a reference ground. The positive terminal of the second capacitor is connected to the positive terminal of the first capacitor. The sampling interface of the control circuit is connected to the connection point of the second capacitor and the sampling resistor. The reference ground interface of the control circuit is connected to the reference ground of the flyback converter.

[0038] The control scheme proposed in this invention detects the resonant current in real time when the second switch is turned on and resonance occurs. When the resonant current exceeds the safe current, the second switch is turned off to prevent damage to the device due to excessive resonant current. Simultaneously, the flyback converter is controlled to enter a discharge phase. During the discharge phase, the first switch is continuously turned off or turned on at least once, the second switch is intermittently turned on, and the increase in charge of the first capacitor is controlled to be less than the decrease in charge. Therefore, the charge of the first capacitor can be discharged during the discharge phase until the maximum value of the resonant current is within the safe current range. Thus, during operation, the resonant current can be controlled to be below the safe current in a timely manner, making the voltage of the first capacitor close to the secondary-side refraction voltage, ensuring the normal operation of the flyback converter and preventing damage to system components. Clearly, in this invention, as long as the resonant current exceeds the safe current during flyback converter operation, the system can immediately respond to protect the device and enter the discharge phase, controlling the resonant current below the safe current to protect the system components from damage. Therefore, the control method proposed in this invention is applicable to any transient operating condition, effectively controlling the magnitude of the resonant current under any transient condition, ensuring device safety and system reliability. Attached Figure Description

[0039] Figure 1 and Figure 2 This is a structural diagram of an asymmetric half-bridge flyback converter.

[0040] Figure 3 The waveform diagram shows the operation of the asymmetric half-bridge flyback converter.

[0041] Figure 4 This is a structural diagram of an active clamp flyback converter;

[0042] Figure 5 This is a flowchart of the control method proposed in this invention;

[0043] Figure 6 This is a flowchart of the control method in the first embodiment;

[0044] Figure 7 The waveform diagram is shown in the first embodiment.

[0045] Figure 8 and Figure 9 These are two waveform diagrams corresponding to the control method in the second embodiment;

[0046] Figures 10-13 This allows for various configuration options for the sampling circuit. Detailed Implementation

[0047] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in various forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0048] As described in the background section, under certain transient conditions, asymmetric half-bridge flyback converters (AHB) and active clamp flyback converters (ACF) may experience a situation where the resonant capacitor voltage or clamping capacitor voltage exceeds the secondary-side refraction voltage. This leads to excessive resonant current during primary-side resonance, damaging system components and reducing system reliability. However, existing solutions have certain limitations and can only address specific transient conditions, not all. To address these limitations, this invention proposes a control method for flyback converters, wherein…

[0049] The flyback converter includes a transformer, a first switch Q1 and a second switch Q2 located on the primary side of the transformer and connected between the input terminal Vin and the reference ground Gnd, and a first capacitor C1 and a first inductor Lk forming a resonant circuit when the second switch Q2 is turned on. Obviously, this flyback converter includes... Figure 1 , Figure 2 and Figure 4 The asymmetric half-bridge flyback converter and the active clamp flyback converter are shown.

[0050] Furthermore, such as Figure 5 As shown, the control method includes the following steps:

[0051] When the flyback converter is powered on, a sampling signal representing the magnitude of the discharge current of the first capacitor C1 is acquired when the second switch Q2 is turned on.

[0052] When the sampled signal is greater than or equal to the threshold signal, the second switch Q2 is turned off and the flyback converter enters the discharge stage. The threshold signal represents the magnitude of the safe current. When the current flowing through the second switch Q2 exceeds the safe current, the second switch is damaged.

[0053] During the discharge phase, the first capacitor C1 is controlled to discharge intermittently, and the stored energy is controlled to be less than the discharge energy. The stored energy represents the energy gained by the first capacitor through charging at the input terminal, and the discharge energy represents the energy lost by the first capacitor through discharging.

[0054] In the first embodiment, during the discharge phase, the first switch Q1 is continuously turned off to reduce the stored energy to zero, and the second switch Q2 is intermittently turned on to intermittently discharge the first capacitor C1.

[0055] In the second embodiment, during the discharge phase, the first switch Q1 is controlled to conduct at least once to ensure that the stored energy is not zero, and the second switch Q2 is controlled to conduct intermittently to cause the first capacitor C1 to discharge intermittently.

[0056] Therefore, the control scheme proposed in this invention detects the resonant current in real time when the second switch is turned on and resonates. When the resonant current exceeds the safe current, the second switch is turned off, thus preventing excessive resonant current from damaging the device. Simultaneously, the flyback converter enters the discharge phase. During this phase, the increase in charge of the first capacitor is controlled to be less than the decrease in charge, allowing the first capacitor to discharge until the maximum value of the resonant current is within the safe current range. Therefore, during operation, the resonant current can be controlled to be below the safe current in a timely manner, ensuring that the voltage of the first capacitor is close to the secondary-side refraction voltage, guaranteeing the normal operation of the flyback converter and preventing damage to system components. Clearly, in this invention, whenever the resonant current exceeds the safe current during flyback converter operation, the system can immediately respond with protective devices and enter the discharge phase, controlling the resonant current below the safe current to protect system components from damage. Therefore, the control method proposed in this invention is applicable to any transient operating condition, effectively controlling the magnitude of the resonant current under any transient condition, ensuring device safety and system reliability.

[0057] It should be noted that the resonant circuit resonates during the period when the first switch is turned off and the second switch is turned on, generating a resonant current in the resonant circuit. The discharge current of the first capacitor is equivalent to the resonant current.

[0058] Preferably, in the first embodiment, as Figure 6 As shown, to avoid excessive resonant current damaging components in asymmetric half-bridge flyback converters and active clamp flyback converters during operation, this invention proposes the following control method:

[0059] During the operation of the flyback converter, the discharge current of the first capacitor is detected when the second switch Q2 is turned on, thereby obtaining a sampling signal characterizing the magnitude of the discharge current, and comparing the sampling signal with the threshold signal;

[0060] If the discharge current exceeds the safe current (i.e., the sampled signal is greater than or equal to the threshold signal), the second switch Q2 is immediately turned off and the flyback converter enters the discharge stage. If the discharge current is within the safe current (i.e., the sampled signal is less than the threshold signal), the flyback converter is controlled to operate normally (the first and second switches are turned on alternately), transferring the energy from the input terminal to the output terminal to power the load, and repeating the above operation.

[0061] Specifically, during the discharge phase, the first switch Q1 remains off, while the second switch Q2 is intermittently turned on. During the intermittent conduction of the second switch Q2, the discharge current is detected to obtain a sampling signal. This sampling signal is compared to a threshold signal. When the discharge current exceeds the safe current (i.e., the sampling signal is greater than or equal to the threshold signal), the second switch Q2 is turned off to stop the first capacitor C1 from discharging. After a set time, the second switch Q2 is turned on again to allow the first capacitor C1 to discharge again. This process is repeated until the maximum or minimum discharge current is within the safe current range (i.e., the maximum value of the sampling signal is less than the threshold signal). At this point, the discharge phase ends, and the flyback converter resumes normal operation and repeats the above steps.

[0062] Specifically, such as Figure 7As shown, assuming the flyback converter starts powering on at time t0 (or, time t0 is a certain moment in the flyback converter's operation), the PWM drive signal first turns on the first switch Q1 and turns off the second switch Q2, storing energy in the primary winding. Then, at time t1, the PWM drive signal turns off the first switch Q1 and turns on the second switch Q2, transferring energy from the primary winding to the secondary winding. Simultaneously, when the first switch Q1 is off and the second switch Q2 is on, the discharge current iLK of the first capacitor C1 is collected in real time. At time t2, iLK exceeds the safe current iref, causing the second switch Q2 to immediately turn off, and the flyback converter enters the discharge phase. During the discharge phase, the first switch Q1 is continuously off, and the second switch Q2 is intermittently on. Specifically, at time t1, the first switch Q1 is turned off and the second switch Q2 is turned on, and the discharge current iLK of the first capacitor C1 is collected in real time when the second switch Q2 is on. At time t2, iLK is greater than the safe current iref, so the second switch Q2 is immediately turned off. After a certain period, at time t3, the first switch Q1 is turned off and the second switch Q2 is turned on again. While the second switch Q2 is on, the discharge current iLK of the first capacitor C1 is collected in real time. At time t4, iLK is greater than the safe current iref, so the second switch Q2 is immediately turned off. After a certain period, at time t5, the first switch Q1 is turned off and the second switch Q2 is turned on again. At time t6, the maximum value of iLK is less than the safe current iref, so at time t7, after the second switch Q2 finishes conducting, the discharge phase ends, and the flyback converter begins normal operation and repeats the above operations. During the discharge phase, as the voltage of the first capacitor C1 decreases, the conduction time of the second switch Q2 gradually increases. Specifically, at time t7, the first switch Q1 is turned on and the second switch Q2 is turned off, storing energy in the primary winding. After that, the first switch Q1 is turned off and the second switch Q2 is turned on, and the primary winding transfers energy to the secondary winding. When the second switch Q2 is turned on, iLK is sampled. At this time, iLK is less than iref. This operation is repeated until iLK is greater than iref. Then the above operation is repeated again to enter the discharge stage.

[0063] It should be noted that in this field, the resonant current is usually obtained by sampling the voltage of the sampling resistor. Depending on the setting of the sampling resistor, the voltage of the sampling resistor may be positive or negative. Positive and negative indicate the direction of the current, not the magnitude of the current. Figure 7 In the waveform diagram shown, the discharge current iLK of the first capacitor is negative. When comparing the magnitudes of the discharge current and the safe current in the above description, the absolute value of this negative value is compared. Figure 7 In the process, the current iLK flowing through the second switch during its conduction is the resonant current iLK. When the resonant current iLK is positive, it indicates that the leakage inductance Lk is discharging; when the resonant current iLK is negative, it indicates that the first capacitor C1 is discharging.

[0064] In summary, once the flyback converter is running, the resonant current is monitored in real time during the conduction of the second switch Q2. As long as the resonant current exceeds the safe current, the first switch Q1 is immediately turned off and the second switch Q2 is turned on intermittently, so that the amplitude of the resonant current is attenuated to within the safe current, avoiding damage to the system components and ensuring the normal operation of the flyback converter thereafter.

[0065] Preferably, in the second embodiment, such as Figure 8 and Figure 9 As shown, the present invention proposes the following control method:

[0066] During the operation of the flyback converter, the discharge current of the first capacitor is detected when the second switch Q2 is turned on, thereby obtaining a sampling signal characterizing the magnitude of the discharge current, and comparing the sampling signal with the threshold signal;

[0067] If the sampled signal is greater than or equal to the threshold signal, the second switch Q2 is immediately turned off and the flyback converter enters the discharge stage; if the sampled signal is less than the threshold signal, the flyback converter is controlled to operate normally (the first and second switches are turned on alternately), transferring the input energy to the output to power the load, and repeating the above operation.

[0068] Specifically, during the discharge phase, the first switch Q1 is turned on at least once, while the second switch Q2 is turned on intermittently. Simultaneously, during the intermittent conduction of the second switch Q2, the discharge current is detected each time it is turned on. If the discharge current exceeds the safe current, the second switch Q2 is turned off, stopping the first capacitor C1 from discharging. After a certain period, the second switch Q2 is turned on again, or the second switch Q2 is turned on again after the first switch Q1 is turned off, until the maximum or minimum discharge current is within the safe current range. At this point, the discharge phase ends, and the flyback converter operates normally and repeats the above process. During the discharge phase, the number of times the first switch Q1 is turned on is not limited, as long as the energy gained by the first capacitor through charging at the input terminal is less than the energy lost by discharging the first capacitor.

[0069] For example, such as Figure 8As shown, assuming the flyback converter starts powering on at time t0 (or, time t0 is a certain moment in the flyback converter's operation), the PWM drive signal first turns on the first switch Q1 and turns off the second switch Q2, storing energy in the primary winding. Then, the PWM drive signal turns off the first switch Q1 and turns on the second switch Q2, transferring energy from the primary winding to the secondary winding. Simultaneously, when the first switch Q1 is off and the second switch Q2 is on, the discharge current iLK of the first capacitor C1 is collected in real time. At time t1, if iLK is greater than the safe current iref, the second switch Q2 is immediately turned off, and the flyback converter enters the discharge phase. During the discharge phase, at time t2, the first switch Q1 is turned on to charge the first capacitor C1 at the input. At time t3, the first switch Q1 is turned off and the second switch Q2 is turned on. At time t4, if the discharge current iLK is detected to be greater than the safe current iref, the second switch Q2 is turned off. At time t5, the first switch Q1 is turned on. At time t6, the first switch Q1 is turned off and the second switch Q2 is turned on. During the conduction of the second switch Q2, the discharge current iLK is always less than the safe current iref. Therefore, the flyback converter operates normally after the conduction time of the second switch Q2 ends. At time t8, the first switch Q1 is turned off and the second switch Q2 is turned on. Energy is transferred from the primary winding to the secondary winding. iLK is sampled when the second switch Q2 is turned on. At this time, iLK is less than iref. This operation is repeated until iLK is greater than iref, at which point the above operation is repeated again to enter the discharge phase.

[0070] Furthermore, if the primary side of the flyback converter is based on peak current control, then the preset peak current during normal operation of the flyback converter is ipaek. Figure 8During the discharge phase, the preset peak current is reduced to set the preset peak current to k*ipeak (0 < k < 1), thus ensuring that the energy increase of the first capacitor is less than the energy decrease during the discharge phase. Alternatively, in other embodiments, if the primary side of the flyback converter is controlled based on a constant on-time, the on-time of the first switch Q1 during normal operation is set to T1, and the on-time of the first switch Q1 is reduced to k*T1 (0 < k < 1) during the discharge phase. Alternatively, the output feedback voltage Vfb is received during normal operation of the flyback converter, and the received output feedback voltage Vfb is processed during the discharge phase. The on-time of the first switch Q1 is controlled according to k*Vfb (0 < k < 1) during the discharge phase, thereby ensuring that the stored energy is less than the discharged energy during the discharge phase. Alternatively, the primary side sampling signal Vcs of the primary current is sampled during normal operation of the flyback converter, and the sampled primary side sampling signal Vcs is processed during the discharge phase. The on-time of the first switch Q1 is controlled according to m*Vcs (m > 1) during the discharge phase. The above-mentioned methods for controlling the conduction time of the first switch Q1 during the discharge phase to achieve a storage energy less than the discharge energy are open-loop control. Therefore, the values ​​of k and m need to be calculated in advance to obtain appropriate values.

[0071] Furthermore, during the discharge phase, closed-loop control can be used to ensure that the stored energy is less than the discharged energy. During the discharge phase, the voltage increase ΔV+ of the first capacitor is detected during the conduction of the first switch, and the voltage increase ΔV+ is controlled to be less than the voltage decrease ΔV-. Here, the voltage decrease ΔV- represents the voltage decrease during the most recent discharge of the first capacitor before the current conduction of the first switch. Figure 9 As shown, if this closed-loop feedback method is used in this process, if the discharge current iLK is detected to be greater than the safe current iref at time t1, the second switch Q2 is immediately turned off, and the flyback converter enters the discharge stage. During the entire discharge stage, the first switch Q1 is turned on only at time t2, and the voltage increase of the first capacitor during the turn-on period of the first switch Q1 is ΔV1. The most recent discharge of the first capacitor C1 before time t2 occurred between t0 and t2. During the discharge period of the first capacitor C1 between t0 and t2, the voltage decrease was detected to be ΔV0. Therefore, the on-time of the first switch Q1 during the period t2-t3 needs to satisfy ΔV1 < ΔV0. Similarly, during the discharge stage, the first switch Q1 can also be continuously turned off during t2-t3, and turned on once during t3-t4. In this case, the voltage increase of the first capacitor during t3-t4 needs to be less than the voltage decrease of the first capacitor during t2-t3.

[0072] It should be noted that there is no limit to the number of times the first switching transistor is turned on during the entire discharge phase, such as... Figure 8As shown, the number of times the first switch is turned on can be controlled to be the same as the number of times the second switch is intermittently turned on, or it can be as follows: Figure 9 The first switch is controlled to turn on only once, and the time when the first switch is turned on can be during any of the second switch's turn-off periods within the discharge phase.

[0073] Correspondingly, the present invention also proposes a flyback converter, comprising: a transformer, a first switch Q1 and a second switch Q2 located on the primary side of the transformer and connected between the input terminal Vin and the reference ground GND, a first capacitor C1 and a first inductor Lk forming a resonant circuit when the second switch Q2 is turned on, and further comprising:

[0074] The sampling circuit samples a signal characterizing the discharge current of the first capacitor C1 during the operation of the flyback converter when the second switch Q2 is turned on.

[0075] The control circuit controls the second switch Q2 to turn off and controls the flyback converter to enter the discharge stage when the sampled signal is greater than or equal to the threshold signal. The threshold signal represents the magnitude of the safe current.

[0076] During the discharge phase, the first capacitor C1 discharges intermittently, and the stored energy is less than the discharge energy. The stored energy represents the total energy gained by the first capacitor through charging at the input terminal, while the discharge energy represents the total energy lost by the first capacitor through discharging.

[0077] Specifically, during the discharge phase, the control circuit continuously turns off the first switch to reduce the stored energy to zero, and intermittently turns on the second switch to cause the first capacitor to discharge intermittently. Alternatively, during the discharge phase, the control circuit turns on the first switch at least once to increase the stored energy to greater than zero, and intermittently turns on the second switch to cause the first capacitor to discharge intermittently. The control circuit can use the control method mentioned above to control the conduction duration of the first switch during the discharge phase.

[0078] Furthermore, taking an asymmetric half-bridge flyback converter as an example, such as... Figure 10As shown, the sampling circuit includes a first sampling resistor Rcs1. When the first switch Q1 is turned on, current flows through the first sampling resistor Rcs1, which is connected in series in the resonant circuit. When the second switch Q2 is turned on, current flows through the first sampling resistor Rcs1. The first sampling resistor Rcs1 is used to sample the primary current when the first switch Q1 is turned on and the second switch Q2 is turned off to output a sampling voltage Vcs1. Simultaneously, the first sampling resistor Rcs1 is used to sample the resonant current (including the discharge current of the first capacitor C1) to output a sampling voltage Vcs2. The control circuit has a sampling interface CS, which is connected to the other end of the first sampling resistor Rcs1. The reference ground GND of the control circuit is connected to the reference ground of the flyback converter. In this embodiment, the sampling voltage Vcs2 is a negative voltage sample when sampling the discharge current, and the corresponding threshold signal Vref can also be set to a negative voltage. The control circuit receives the sampling voltage Vcs2. When Vcs2 is less than Vref, it controls the second switch Q2 to turn off immediately and controls the flyback converter to enter the discharge stage. During the discharge phase, the control circuit keeps the first switch Q1 off and the second switch Q2 intermittently on. During the intermittent conduction of the second switch Q2, when Vcs2 is less than Vref, the control circuit turns off the second switch Q2 to stop the first capacitor C1 from discharging. After a set time, the control circuit turns the second switch Q2 on again to discharge the first capacitor C1. This intermittent conduction of the second switch Q2 is repeated until the minimum value of Vcs2 is greater than Vref, at which point the discharge phase ends, and the control circuit outputs a drive signal to control the flyback converter to operate normally. The above steps are repeated when the flyback converter is operating normally. In this embodiment, both Vcs2 and Vref are set to negative voltages. However, in practice, Vcs2 can be made positive by applying an absolute value, in which case Vref should be set to a positive voltage. This embodiment does not represent a limitation of the solution.

[0079] Furthermore, such as Figure 11As shown, the sampling circuit includes a first sampling resistor Rcs1 and a second sampling resistor Rcs2. Rcs1 and Rcs2 are connected in series between the second switch Q2 and the reference ground of the flyback converter. Rcs1 is connected in series in the resonant circuit. When the first switch Q1 is turned on, current flows through the second sampling resistor Rcs2. The second sampling resistor Rcs2 is used to sample the primary-side current when the first switch Q1 is turned on and the second switch Q2 is turned off to output a sampling voltage Vcs2. The first sampling resistor Rcs1 is used to sample the resonant current (including the discharge current of the first capacitor C1) to output a sampling voltage Vcs1. The controller circuit has a sampling interface CS, which is connected to the first sampling resistor Rcs1. The first and second sampling resistors Rcs1 share this sampling interface. The reference ground GND of the controller circuit is connected to the reference ground of the flyback converter. In this embodiment, the sampling voltage Vcs2 is a positive voltage sample when sampling the discharge current. The control circuit receives the sampling voltage Vcs2. When Vcs2 is greater than Vref, it controls the second switch Q2 to turn off immediately and controls the flyback converter to enter the discharge stage. During the discharge stage, the control circuit controls the first switch Q1 to remain off, while the second switch Q2 is intermittently turned on. During the intermittent conduction of the second switch Q2, when Vcs2 is greater than Vref while the second switch Q2 is on, the control circuit controls the second switch Q2 to turn off, stopping the first capacitor C1 from discharging. After a set time, the control circuit controls the second switch Q2 to turn on again, allowing the first capacitor C1 to discharge again. The above steps of intermittently turning on the second switch Q2 are repeated until the maximum value of Vcs2 is less than Vref, at which point the discharge stage ends, the control circuit outputs a drive signal to control the flyback converter to operate normally, and the above steps are repeated when the flyback converter is operating normally.

[0080] Furthermore, such as Figure 12As shown, the sampling circuit includes a first sampling resistor Rcs1 and a second sampling resistor Rcs2. The first and second sampling resistors Rcs1 and Rcs2 are connected in series between the second switch Q2 and the reference ground of the flyback converter. The first sampling resistor Rcs1 is connected in series in the resonant circuit. When the first switch Q1 is turned on, current flows through the second sampling resistor Rcs2. The second sampling resistor Rcs2 is used to sample the primary-side current sampling voltage Vcs2 when the first switch Q1 is turned on and the second switch Q2 is turned off. The first sampling resistor Rcs1 is used to sample the resonant current (including the discharge current of the first capacitor C1) to output the sampling voltage Vcs1. The control circuit (Controller) has two sampling interfaces: the first sampling interface CS1 is connected to the first sampling resistor Rcs1, and the second sampling interface CS2 is connected to the reference ground of the flyback converter. The reference ground of the control circuit is connected to the first sampling resistor Rcs1. Similarly, in this embodiment, when sampling the discharge current, the sampling voltage Vcs2 is a positive voltage sample. See the specific reference... Figure 12 The embodiments shown are not described in detail here.

[0081] Furthermore, such as Figure 13 As shown, the sampling circuit includes a second capacitor C2 and a sampling resistor Rcs. The second capacitor C2 and the sampling resistor Rcs are connected in series between the positive terminal of the first capacitor C1 and the reference ground. The sampling interface CS of the control circuit is connected to the connection point of the second capacitor C2 and the sampling resistor Rcs. When the voltage of the first capacitor C1 changes, the second capacitor C2 changes synchronously. Therefore, the discharge current characterizing the first capacitor C1 can be obtained by collecting the voltage change of the second capacitor C2.

[0082] In summary, when the flyback converter proposed in this invention is running, it will detect the resonant current in real time during the conduction of the second switch Q2. As long as the resonant current exceeds the safe current, it will immediately control the first capacitor C1 to discharge intermittently, so that the amplitude of the resonant current is reduced to within the safe current, thereby avoiding damage to the system devices and ensuring the normal operation of the flyback converter thereafter.

[0083] It should be noted that the above sampling circuit is based on an asymmetric half-bridge flyback circuit, but those skilled in the art can also apply the above sampling circuit to an active clamp flyback circuit.

[0084] It should be noted that the specific implementations and corresponding illustrations provided are merely one way of describing the implementation method of the present invention, and are not intended to limit the specific structure of the implementation scheme of the present invention. Various changes or modifications can be made to these implementation schemes without departing from the principles and essence of the present invention, but all such changes and modifications fall within the protection scope of the present invention.

[0085] Although the embodiments are described and illustrated separately above, some common technologies are involved. Those skilled in the art can replace and integrate them between the embodiments. If there is any content not explicitly described in one embodiment, then another embodiment that is described can be referred to.

[0086] The embodiments described above do not constitute a limitation on the scope of protection of this technical solution. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the above embodiments should be included within the scope of protection of this technical solution.

Claims

1. A control method for a flyback converter, the flyback converter comprising a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input terminal of the flyback converter and a reference ground, and a first capacitor and a first inductor forming a resonant circuit when the second switch is in the on state, characterized in that, The control method includes: During the operation of the flyback converter, a sampling signal characterizing the magnitude of the discharge current of the first capacitor is acquired during the conduction period of the second switch. When the sampled signal is greater than or equal to the threshold signal, the second switch is turned off and the flyback converter is controlled to enter the discharge stage. The threshold signal represents the magnitude of the safe current. During the discharge phase, the first capacitor is controlled to discharge intermittently, and the stored energy is controlled to be less than the discharge energy. The stored energy represents the total energy gained by the first capacitor through charging at the input terminal, and the discharged energy represents the total energy lost by the first capacitor through discharging.

2. The control method as described in claim 1, characterized in that, During the discharge phase, the first switch is controlled to be continuously turned off so that the stored energy is zero, and the second switch is controlled to be intermittently turned on so that the first capacitor is intermittently discharged.

3. The control method as described in claim 1, characterized in that, During the discharge phase, the first switch is controlled to conduct at least once to make the stored energy greater than zero, and the second switch is controlled to conduct intermittently to make the first capacitor discharge intermittently.

4. The control method as described in claim 1, characterized in that, Controlling the intermittent discharge of the first capacitor includes the steps of: acquiring the sampling signal when the first capacitor is discharging, and controlling the first capacitor to stop discharging when the sampling signal is greater than or equal to the threshold signal.

5. The control method as described in claim 4, characterized in that, It also includes the step of: when the sampled signal is always less than the threshold signal, the flyback converter exits the discharge phase and operates normally.

6. The control method as described in claim 3, characterized in that, During the discharge phase, the reference signal representing the preset on-time of the first switch or the preset peak current on the original side is reduced, and the turn-off time of the first switch is controlled according to the reduced reference signal.

7. The control method as described in claim 3, characterized in that, During the discharge phase, the feedback signal characterizing the output voltage is reduced, and the turn-off time of the first switching transistor is controlled based on the reduced feedback signal.

8. The control method as described in claim 3, characterized in that, During the discharge phase, the primary-side sampling signal, which characterizes the primary-side current, is increased, and the turn-off time of the first switch is controlled based on the increased primary-side sampling signal.

9. The control method as described in claim 3, characterized in that, During the discharge phase, when the first switch is turned on, the increase in voltage of the first capacitor is detected, and the increase in voltage is controlled to be less than the decrease in voltage. The decrease in voltage represents the decrease in voltage of the first capacitor during the last discharge, and the last discharge of the first capacitor was immediately before the first switch was turned on.

10. A flyback converter, comprising a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input terminal of the flyback converter and a reference ground, and a first capacitor and a first inductor forming a resonant circuit when the second switch is in the on state, characterized in that, Also includes: The sampling circuit samples a signal characterizing the magnitude of the discharge current of the first capacitor during the operation of the flyback converter while the second switch is on. The control circuit controls the second switch to turn off and controls the flyback converter to enter the discharge stage when the sampled signal is greater than or equal to the threshold signal. The threshold signal represents the magnitude of the safe current. During the discharge phase, the first capacitor is controlled to discharge intermittently, and the stored energy is controlled to be less than the discharge energy. The stored energy represents the total energy gained by the first capacitor through charging at the input terminal, and the discharged energy represents the total energy lost by the first capacitor through discharging.

11. The flyback converter as claimed in claim 10, characterized in that, During the discharge phase, the control circuit controls the first switch to be continuously turned off so that the stored energy is zero, and controls the second switch to be turned on intermittently so that the first capacitor is discharged intermittently.

12. The flyback converter as described in claim 10, characterized in that, During the discharge phase, the control circuit controls the first switch to be turned on at least once to make the stored energy greater than zero, and controls the second switch to be turned on intermittently to make the first capacitor discharge intermittently.

13. The flyback converter as described in claim 11 or 12, characterized in that, During the discharge phase, when the second switch is turned on, the control circuit controls the second switch to turn off so that the first capacitor stops discharging when the sampled signal is greater than or equal to the threshold signal.

14. The flyback converter as described in claim 13, characterized in that, During the discharge phase, when the second switch is turned on, the control circuit controls the flyback converter to exit the discharge phase and operate normally when the sampled signal is always less than the threshold signal.

15. The flyback converter as described in claim 12, characterized in that, During the discharge phase, The reference signal characterizing the preset on-time of the first switch or the preset peak current on the original side is reduced, and the control circuit controls the turn-off time of the first switch based on the reduced reference signal. Alternatively, the feedback signal characterizing the output voltage is reduced, and the control circuit controls the turn-off time of the first switching transistor based on the reduced feedback signal; Alternatively, the primary-side sampling signal characterizing the primary-side current can be increased, and the control circuit can control the turn-off time of the first switching transistor based on the increased primary-side sampling signal.

16. The flyback converter as described in claim 12, characterized in that, During the discharge phase, when the first switch is turned on, the increase in voltage of the first capacitor is detected, and the increase in voltage is controlled to be less than the decrease in voltage. The decrease in voltage represents the decrease in voltage of the first capacitor during the last discharge, and the last discharge of the first capacitor is immediately before the first switch is turned on.

17. The flyback converter as claimed in claim 10, characterized in that, The sampling circuit includes a first sampling resistor connected in series in the resonant circuit. When the first switch is turned on and when the second switch is turned on, the current flows through the first sampling resistor. The reference ground interface of the control circuit is connected to the reference ground of the flyback converter, and the sampling interface of the control circuit is connected to the first sampling resistor.

18. The flyback converter as claimed in claim 10, characterized in that, The sampling circuit includes a first sampling resistor and a second sampling resistor. The first sampling resistor and the second sampling resistor are connected in series between the second switch and the reference ground of the flyback converter. When the second switch is turned on, the current flows through the first sampling resistor, and when the first switch is turned on, the current flows through the second sampling resistor. The sampling interface of the control circuit is connected to the first sampling resistor, and the reference ground interface of the control circuit is connected to the reference ground of the flyback converter.

19. The flyback converter as described in claim 10, characterized in that, The sampling circuit includes a first sampling resistor and a second sampling resistor, which are connected in series between the second switch and the reference ground of the flyback converter. When the second switch is turned on, current flows through the first sampling resistor, and when the first switch is turned on, current flows through the second sampling resistor. The reference ground interface of the control circuit is connected to the connection point of the first sampling resistor and the second sampling resistor. The first sampling interface of the control circuit is connected to the other end of the first sampling resistor, and the second sampling interface of the control circuit is connected to the reference ground of the flyback converter.

20. The flyback converter as claimed in claim 10, characterized in that, The sampling circuit includes a second capacitor and a sampling resistor. The second capacitor and the sampling resistor are connected in series between the positive terminal of the first capacitor and the reference ground. The positive terminal of the second capacitor is connected to the positive terminal of the first capacitor. The sampling interface of the control circuit is connected to the connection point of the second capacitor and the sampling resistor. The reference ground interface of the control circuit is connected to the reference ground of the flyback converter.

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