A synchronous rectification control method and flyback converter

Abstract

The application provides a synchronous rectification control method and a flyback converter. The synchronous rectification control method comprises the following steps: when the flyback converter is switched from full load to no load and then operates under no load, the synchronous rectification tube is controlled to be turned off, so that the secondary side flows through the body diode of the synchronous rectification tube. The synchronous rectification control method provided by the application controls the synchronous rectification tube to be turned off and the secondary side to flow through the body diode when the flyback converter is switched from full load to no load and then operates under no load. Since the body diode of the synchronous rectification tube points to the high potential end of the output voltage, when the synchronous rectification tube is turned off, reverse current cannot be formed from the high potential end of the output voltage to the synchronous rectification tube. Therefore, the problem that the reverse current of the resonance capacitor flows back, the positive voltage spike of the switching node is large, and the synchronous rectification tube is damaged is avoided.

Description

Technical Field

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

[0002] like Figure 1 The asymmetric half-bridge flyback converter shown has the following characteristics: if the charge on the resonant capacitor C1 is not completely discharged when it is turned off, a large resonant current i will be generated when the resonant capacitor C1 and the leakage inductance Lk resonate upon restarting. LK Resonant current i LK This could exceed the current and voltage stress specifications of the second switching transistor Q2, causing it to fail. Therefore, a resistor Rr is often connected in parallel across the resonant capacitor C1 to discharge charge during the power-off and restart process.

[0003] For example, in an asymmetric half-bridge flyback converter, the turn-on condition of the secondary-side synchronous rectifier SR is as follows: during the primary-side conduction period, the volt-second product (Vsw-Vo)*Ton and Vsw are detected, and when the volt-second product (Vsw-Vo)*Ton is greater than the set volt-second threshold and Vsw is less than the turn-on voltage, the synchronous rectifier SR is controlled to conduct.

[0004] like Figure 2 As shown, under full load, the asymmetric flyback converter controls the conduction of the synchronous rectifier SR based on the volt-second threshold. After switching from full load to no load, the system will drive the primary and secondary sides again after a period of time T. During this time T, the voltage drop of the resonant capacitor C1 is relatively low, while the output voltage Vo remains almost unchanged under no-load conditions. Therefore, when the system outputs a drive signal again after time T, the synchronous rectifier SR is still controlled to conduct according to the volt-second threshold. Since n*Vo>>Vc1, after the freewheeling of the synchronous rectifier SR ends (the actual freewheeling time is less than the minimum conduction time of the synchronous rectifier SR), the output voltage Vo provides reverse current to the resonant capacitor C1, and a reverse current appears on the secondary side (the current during freewheeling on the secondary side is a forward current). At this time, the voltage generated by the reverse current flowing through the synchronous rectifier SR is Isw*Rdson. At time T2, this voltage reaches the turn-off voltage of the synchronous rectifier SR, and the synchronous rectifier SR turns off. Subsequently, the reverse current flows to the junction capacitance Cds of the synchronous rectifier SR, resulting in a large voltage spike across the drain and source terminals of the synchronous rectifier SR. This voltage spike can easily exceed the withstand voltage of the synchronous rectifier SR, damaging the synchronous rectifier SR.

[0005] In summary, when the system switches from full load to no load and then drives the primary and secondary sides again after a period of time, the drain-source voltage Vsw of the synchronous rectifier tube will exceed its withstand voltage value and be damaged. Summary of the Invention

[0006] To address the technical problem in existing flyback converters where the drain-source voltage Vsw of the synchronous rectifier exceeds its withstand voltage and is damaged when the primary and secondary sides are driven again after a period of time following a switch from full load to no load, this invention proposes a synchronous rectification control method and a flyback converter. The synchronous rectification control method includes:

[0007] When the flyback converter switches from full load to no load and operates under no load, the synchronous rectifier is turned off, allowing the secondary side to freewheel through the body diode of the synchronous rectifier.

[0008] Furthermore, the flyback converter obtains a sampling signal based on the voltage of the first node, and controls the conduction of the synchronous rectifier diode according to the sampling signal and a threshold signal.

[0009] After the flyback converter switches from full load to no load, it pulls up the threshold signal, causing the synchronous rectifier diode to turn off under no load.

[0010] The first node is the connection point between the synchronous rectifier tube and the secondary winding.

[0011] Furthermore, after the flyback converter switches from full load to no load, both the primary-side power transistor and the synchronous rectifier transistor are turned off within the first time period.

[0012] By detecting whether the first duration has elapsed, it can be determined whether the flyback converter has switched from full load to no load.

[0013] Furthermore, during the timing period, the synchronous rectifier tube and / or the primary-side power tube are detected to determine whether the first duration has elapsed.

[0014] Preferably, timing begins after the synchronous rectifier tube is turned on, and during the timing period, it is determined whether the synchronous rectifier tube meets the conduction conditions.

[0015] If not, then determine that the first duration has elapsed.

[0016] Furthermore, if the synchronous rectifier tube meets the conduction condition within the timing period, the current timing ends and the timing restarts, and it is determined that the first duration has not yet arrived.

[0017] Furthermore, the maximum duration of each timing cycle is set to a preset duration, which is longer than the maximum switching cycle of the flyback converter and shorter than the first duration.

[0018] In one embodiment, the voltage change rate is obtained based on the voltage of the first node, and the synchronous rectifier is controlled to turn on when the voltage change rate is greater than a voltage slope threshold.

[0019] After the flyback converter switches from full load to no load, it pulls up the voltage slope threshold, causing the synchronous rectifier to turn off under no load.

[0020] In one embodiment, a first volt-second product is obtained based on the difference between the voltage of the first node and the output voltage, and the synchronous rectifier is controlled to turn on when the first volt-second product is greater than or equal to a volt-second threshold and the voltage of the first node is less than the turn-on voltage.

[0021] After the flyback converter switches from full load to no load, it pulls up the volt-second threshold, causing the synchronous rectifier to turn off under no load.

[0022] In one embodiment, when the flyback converter starts under full load, the synchronous rectifier is turned on according to a first volt-second threshold, and after the synchronous rectifier is turned on for the first time, it is turned on according to a second volt-second threshold.

[0023] After the flyback converter switches from full load to no load, it controls the synchronous rectifier to turn on according to the first volt-second threshold.

[0024] Wherein, the first volt-second threshold is greater than the second volt-second threshold.

[0025] In one embodiment, the synchronous rectifier is controlled to conduct under full load based on a volt-second product threshold.

[0026] After the flyback converter switches from full load to no load, the synchronous rectifier is turned on according to the first volt-second threshold.

[0027] Wherein, the value of the volt-second product threshold is fixed, and the first volt-second threshold is greater than the volt-second product threshold.

[0028] In one embodiment, under full load, the flyback converter controls the synchronous rectifier to turn on according to a second voltage slope threshold.

[0029] After the flyback converter switches from full load to no load, the synchronous rectifier is turned on according to a first voltage slope threshold.

[0030] Wherein, the first voltage slope threshold is greater than the second voltage slope threshold.

[0031] Furthermore, after the flyback converter switches from full load to no load, the conduction time of the original side power transistor decreases when running under no load, and the first volt-second product is less than the volt-second threshold after pull-up when running under no load.

[0032] Furthermore, the negative terminal of the body diode of the synchronous rectifier points to the high potential end of the output voltage.

[0033] A flyback converter that uses the synchronous rectification control method described above to control the synchronous rectifier tube.

[0034] In this invention, after the flyback converter switches from full load to no load and operates under no-load conditions, the synchronous rectifier is turned off, and the secondary side freewheels through the body diode. Since the body diode of the synchronous rectifier points to the high potential end of the output voltage, a reverse current cannot be formed from the high potential end of the output voltage to the synchronous rectifier when it is turned off. Therefore, the problems of reverse current from the resonant capacitor and large forward voltage spikes at the switching node damaging the synchronous rectifier are avoided.

[0035] Furthermore, in this invention, the synchronous rectifier is turned off under no-load conditions by controlling the pull-up of the threshold signal. Only a threshold switching logic needs to be added to the original driving logic of the synchronous rectifier. This solution does not require any modification to the original driving logic of the synchronous rectifier and is easy to implement.

[0036] Furthermore, this invention uses a timing method to detect whether the duration of no drive operation occurs after switching from full load to no load, in order to determine whether the operating condition has changed. This method only requires combining timing with the existing drive logic of the synchronous rectifier tube. This method is easy to implement and can easily distinguish steady-state operating conditions, avoiding errors in judgment results. Attached Figure Description

[0037] Figure 1 A schematic diagram of the current path for the output voltage Vo to cause reverse current to flow into the resonant capacitor C1 and reverse current to appear on the secondary side;

[0038] Figure 2 This is a diagram showing the changes in various signals when the primary and secondary sides are driven again after a period of time following a full-load switch to no-load in the existing technology.

[0039] Figure 3 This is a schematic diagram of the synchronous rectification control method proposed in this invention;

[0040] Figure 4 The volt-second threshold for introducing a hysteresis mechanism;

[0041] Figure 5 This is a schematic diagram of the synchronous rectification control method in the first embodiment;

[0042] Figure 6 This is a schematic diagram of the synchronous rectification control method in the second embodiment.

[0043] Figure 7 This is a schematic diagram of the synchronous rectification control method in the third embodiment;

[0044] Figure 8 This is a diagram showing the changes in various signals when the primary and secondary sides are driven again after a period of time following a full-load switch to no-load in this invention. Detailed Implementation

[0045] 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.

[0046] like Figure 2 As shown, after switching from full load to no load, the system does not drive for a period of time. During this period, the voltage Vc1 of the resonant capacitor C1 drops, while the output voltage Vo remains approximately unchanged. Figure 2 At time T1, the system drives the primary-side power transistor to turn on again. During the primary-side turn-on phase, it acquires the first volt-second product (Vsw-Vo)*Ton. When the first volt-second product is greater than the volt-second threshold and the drain-source voltage Vsw is less than the start-up voltage, it drives the synchronous rectifier transistor SR to turn on. During the conduction of the synchronous rectifier transistor SR, the secondary side freewheels. After the secondary side freewheeling ends, the synchronous rectifier transistor SR remains on (under no-load conditions, due to the minimum conduction time limitation, the actual freewheeling time is less than the minimum conduction time). Since n*Vo >> Vc1, ... Figure 1 As shown, the resonant capacitor C1 experiences reverse current flow, and the synchronous rectifier SR flows with reverse current. At time T2, the synchronous rectifier SR is turned off. After time T2, the reverse current flows to the junction capacitance Cds of the synchronous rectifier SR, causing a positive voltage spike to appear at the first node SW.

[0047] Therefore, to avoid the situation where a large voltage spike occurs at the first node SW after switching from full load to no load, damaging the synchronous rectifier diode during no-load operation, this invention proposes a synchronous rectification control method for flyback converters, including:

[0048] When the flyback converter switches from full load to no load and operates under no load, the synchronous rectifier is turned off, allowing the secondary side to freewheel through the body diode of the synchronous rectifier.

[0049] Furthermore, such as Figure 3 As shown, in a flyback converter, a sampling signal is typically obtained based on the voltage of the first node, and the synchronous rectifier is then controlled to turn on based on the sampling signal and a threshold signal. Therefore, this invention proposes to pull up the threshold signal after switching from full load to no load, so that the synchronous rectifier cannot meet the conduction condition when running under no load, thereby controlling the synchronous rectifier to turn off.

[0050] For example, in one embodiment, during the operation of the flyback converter, a first volt-second product is obtained based on the voltage of the first node during the primary-side conduction phase. Then, the synchronous rectifier is judged to meet the conduction condition based on the first volt-second product, the volt-second threshold, and the voltage of the first node. Therefore, after switching from full load to no load, the volt-second threshold is pulled up, thereby achieving the turn-off of the synchronous rectifier under no-load operation. Since the first volt-second product during the primary-side conduction phase is always less than the pulled-up volt-second threshold under no-load operation, the synchronous rectifier never meets the conduction condition and remains in the off state. The secondary side freewheels through the body diode of the synchronous rectifier.

[0051] For example, in another embodiment, the flyback converter obtains the voltage change rate of the first node after the primary-side power transistor is turned off during operation, and then controls the synchronous rectifier to turn on based on the voltage change rate of the first node and the voltage slope threshold. Therefore, after switching from full load to no load, the voltage slope threshold is pulled up, thereby achieving the turn-off of the synchronous rectifier during no-load operation.

[0052] In summary, as Figure 8 As shown in this invention, after the flyback converter switches from full load to no load, the voltage Vc1 of the resonant capacitor C1 drops for a period of time without driving. Therefore, under no-load operation, the situation of n*Vo>>Vc1 still exists. However, at this time, the synchronous rectifier is turned off, and the secondary side freewheels through the body diode. The body diode points to the high potential end of the output voltage, so when the synchronous rectifier is turned off, it is impossible to form a reverse current from the high potential end of the output voltage to the synchronous rectifier. Therefore, the problems of reverse current flowing from the resonant capacitor and large forward voltage spikes at the switching node damaging the synchronous rectifier are avoided.

[0053] Furthermore, since the system does not generate a drive during the first duration after switching from full load to no load, both the primary-side power transistors and synchronous rectifier transistors are turned off during this period. The drive is only generated after the first duration ends. Therefore, in order to accurately pull up the threshold signal, this invention proposes to detect whether the first duration occurs during the operation of the flyback converter. If the arrival of the first duration is detected, it is determined that the flyback converter has switched from full load to light load, and the threshold signal is pulled up.

[0054] Furthermore, the present invention proposes to detect the arrival of the first duration by adding a judgment condition during the timing period, that is, to detect whether the primary side power and / or synchronous rectifier is turned on during the timing period, so as to determine whether the current time is within the first duration.

[0055] Preferably, timing begins after the synchronous rectifier diode is turned on, and the timing period determines whether the synchronous rectifier diode meets the conduction condition. If the synchronous rectifier diode meets the conduction condition within the timing period, it is determined that the first time interval has not been reached, the current timing ends immediately, and timing restarts. If the synchronous rectifier diode does not meet the conduction condition within the timing period, it is determined that the first time interval has been reached, and the threshold signal is pulled up after the timing ends. The maximum duration of each timing interval is greater than the maximum switching cycle of the flyback converter, but less than the first time interval. Because the maximum duration of each timing interval is greater than the maximum switching cycle, the synchronous rectifier diode can be determined to meet the conduction condition within the timing period when the flyback converter is operating normally under heavy or light load conditions. Therefore, the flyback converter will not pull up the threshold signal during normal operation, thus ensuring the normal operation of the system. Simultaneously, since the timing interval is less than the first time interval, after switching from full load to no load, the synchronous rectifier diode cannot be detected to meet the conduction condition within the first time interval of the drive, and the threshold signal can be pulled up immediately after the timing ends.

[0056] In existing technologies, when a flyback converter is driving the primary and secondary sides normally, a fixed volt-second product threshold is often set. Inevitably, when the volt-second product approaches the threshold, due to factors such as sampling errors, noise, or load changes, the volt-second product will fluctuate around the threshold, causing the synchronous rectifier diode SR to be intermittently switched on and off. This results in noise problems in the resonant capacitor C1 due to large output voltage ripple. To overcome this problem, this invention proposes introducing a hysteresis mechanism into the volt-second product threshold setting. The hysteresis mechanism means that the volt-second product threshold is set within a range, rather than a fixed value. Specifically, for example... Figure 4 As shown, two volt-second product thresholds, Vth-H and Vth-L, can be set separately, with Vth-H being greater than Vth-L. During system startup, when (Vsw-Vo)*Ton is greater than Vth-H and Vsw is less than the turn-on voltage, the synchronous rectifier diode SR is turned on. After the synchronous rectifier diode SR is turned on for the first time, the volt-second product threshold is reduced from Vth-H to Vth-L. Therefore, after the synchronous rectifier diode SR is turned on for the first time, it is subsequently turned on when (Vsw-Vo)*Ton is greater than Vth-L and Vsw is less than the turn-on voltage. That is, after the synchronous rectifier diode SR is turned on, the volt-second product threshold is reduced to prevent the volt-second product from fluctuating around Vth-H and causing the synchronous rectifier diode SR to be turned off erroneously. After running for a period of time, the system detects that (Vsw-Vo)*Ton is less than Vth-L, and the synchronous rectifier SR does not meet the turn-on condition. Therefore, the synchronous rectifier SR is turned off, and the secondary side freewheels through the body diode of the synchronous rectifier SR. At the same time, the volt-second product threshold is raised to Vth-H to prevent the volt-second product from floating near Vth-L and causing the synchronous rectifier SR to be mis-turned on.

[0057] It should be noted that the technical solution proposed above in this invention, which involves "controlling the synchronous rectifier to turn off by pulling up the volt-second threshold when the flyback converter switches from full load to no load, so that the secondary side can freewheel through the body diode of the synchronous rectifier", is applicable to both the prior art where the synchronous rectifier is controlled to turn on based on a fixed volt-second threshold and the prior art where the synchronous rectifier is controlled to turn on based on a hysteresis volt-second threshold.

[0058] Specifically, such as Figure 5 As shown, in the first embodiment, the flyback converter starts up under full load at time T1. The controller drives the primary-side main power transistor to turn on. During the primary-side turn-on phase, the controller acquires the first volt-second product (Vsw-Vo)*Ton and the drain-source voltage Vsw of the synchronous rectifier (Vsw is the voltage of the first node SW). When the first volt-second product (Vsw-Vo)*Ton is greater than the first volt-second threshold and the drain-source voltage Vsw is less than the turn-on voltage, the controller controls the synchronous rectifier to turn on. After the synchronous rectifier turns on for the first time, during the primary-side turn-on phase, the controller acquires the first volt-second product (Vsw-Vo)*Ton and the drain-source voltage Vsw of the synchronous rectifier. When the first volt-second product (Vsw-Vo)*Ton is greater than the second volt-second threshold and the drain-source voltage Vsw is less than the turn-on voltage, the controller controls the synchronous rectifier to turn on. At time T2, the flyback converter switches from full load to no load. During this period Tx after the switch from full load to no load, the controller does not generate a drive signal to drive the primary-side power transistor and the synchronous rectifier to turn on.

[0059] In the above process, timing begins when the synchronous rectifier is turned on, and the maximum duration of each timing is set to a set Tset. During the timing period, the first volt-second product (Vsw-Vo)*Ton and the drain-source voltage Vsw are acquired, and it is used to determine whether the synchronous rectifier meets the conduction condition. If the synchronous rectifier meets the conduction condition during the timing period, the first duration has not yet expired, the current volt-second threshold—the second volt-second threshold—is maintained, and the timing is reset to zero and restarted. If the synchronous rectifier does not meet the conduction condition during the timing period, the current volt-second threshold is pulled up and switched to the first volt-second threshold after the timing ends. Specifically, by Figure 5 It can be seen that under full load, timing begins when the synchronous rectifier is turned on. Since Tset is greater than the maximum switching period, timing ends prematurely and restarts each time the synchronous rectifier turns on (Ta). This continues until the load switches to no-load. Because no drive is output for a period Tx, Tset is less than Tx. Therefore, timing reaches the maximum duration Tset, and the synchronous rectifier does not meet the turn-on condition within Tset. Thus, after the timing Tset ends, the pull-up volt-second threshold is the first volt-second threshold. After a period Tx, the drive outputs again. Since the synchronous rectifier is then turned off, the timing start condition cannot be met, and timing stops.

[0060] Specifically, in the second embodiment, such as Figure 6 As shown, under full load, the flyback converter starts up at time T1. The controller drives the primary-side main power transistor to turn on. During the primary-side turn-on phase, it acquires the first volt-second product (Vsw-Vo)*Ton and the drain-source voltage Vsw of the synchronous rectifier. When the first volt-second product (Vsw-Vo)*Ton is greater than a fixed volt-second product threshold and the drain-source voltage Vsw is less than the turn-on voltage, it controls the synchronous rectifier to turn on. After the synchronous rectifier turns on for the first time, during the primary-side turn-on phase, it acquires the first volt-second product (Vsw-Vo)*Ton and the drain-source voltage Vsw of the synchronous rectifier. When the first volt-second product (Vsw-Vo)*Ton is greater than the volt-second product threshold and the drain-source voltage Vsw is less than the turn-on voltage, the controller controls the synchronous rectifier to turn on. At time T2, the flyback converter switches from full load to no load. During this period, the controller does not generate a drive signal to drive the primary-side power transistor and the synchronous rectifier to turn on for a period of time after the switch from full load to no load.

[0061] Similarly, timing begins when the synchronous rectifier is turned on, and the maximum duration of each timing is set to a predetermined Tset. During the timing period, the first volt-second product (Vsw-Vo)*Ton and the drain-source voltage Vsw are acquired, and this is used to determine whether the synchronous rectifier meets the turn-on condition. If the synchronous rectifier meets the turn-on condition within the timing period, the current volt-second threshold—the volt-second product threshold—is maintained, and the timing is reset to zero and restarted. If the synchronous rectifier does not meet the turn-on condition within the timing period, the current volt-second threshold is pulled up, switching to the first volt-second threshold. Specifically, by... Figure 6 It is known that under full load, timing begins when the synchronous rectifier is turned on. Since Tset is greater than the maximum switching period, timing ends prematurely each time it reaches Ta (when the synchronous rectifier is turned on). This continues until the load switches to no-load. Because no drive is output for a period Tx, Tset is less than Tx, so timing ends at Tset. Furthermore, the synchronous rectifier does not meet the turn-on condition within Tset. Therefore, after timing Tset ends, the pull-up volt-second threshold is the first volt-second threshold. After a period Tx, the drive outputs again. Since the synchronous rectifier is then turned off, the timing start condition cannot be met, and timing stops.

[0062] Specifically, in the third embodiment, such as Figure 7As shown, under full load, the flyback converter starts up at time T1. The controller drives the primary-side main power transistor to turn on. When the primary-side power transistor turns off, it detects the rate of change dv / dt of the drain-source voltage Vsw. When the rate of change dv / dt of Vsw is greater than the second voltage slope threshold, it controls the synchronous rectifier to turn on. After the synchronous rectifier turns on for the first time, it detects the rate of change dv / dt of Vsw when the primary-side power transistor turns off, and controls the synchronous rectifier to turn on when the rate of change dv / dt is greater than the second voltage slope threshold. At time T2, the flyback converter switches from full load to no load. During this period, the controller does not generate a drive signal to drive the primary-side power transistor and the synchronous rectifier to turn on for a period of time after the switch from full load to no load.

[0063] Similarly, timing begins when the synchronous rectifier is turned on, and the maximum duration of each timing is set to a predetermined Tset. During the timing period, the rate of change dv / dt is detected to determine whether the synchronous rectifier meets the conduction condition. If the synchronous rectifier meets the conduction condition within the timing period, the current voltage slope threshold—the second voltage slope threshold—is maintained, and the timing is reset to zero and restarted. If the synchronous rectifier does not meet the conduction condition within the timing period, the second voltage slope threshold is raised, switching to the first voltage slope threshold. Specifically, by... Figure 7 It can be seen that under full load, timing begins when the synchronous rectifier is turned on, and ends prematurely each time the timing reaches Ta (when the synchronous rectifier is turned on). This continues until the load switches to no-load. Because no drive is output for a period Tx, Tset is less than Tx, so timing continues until Tset. Furthermore, the synchronous rectifier does not meet the turn-on condition within Tset. Therefore, the pull-up voltage slope threshold after timing Tset ends is the first voltage slope threshold. After a period Tx, a drive is output again. Since the synchronous rectifier is then turned off, the timing start condition cannot be met, and timing stops.

[0064] Furthermore, it should be noted that in the above embodiments, the detection of the first duration is based on whether the synchronous rectifier meets the conduction condition within the timing period, and the timing start point is the conduction moment of the synchronous rectifier. However, in other embodiments, the timing start point can be set with other conditions, such as the turn-off moment of the synchronous rectifier, the pull-down moment of the synchronous rectifier's drive voltage, the moment when the synchronous rectifier's current begins to rise or falls to zero, etc., all of which can be used as the timing start point. Alternatively, parameters such as the drive voltage and primary current of the primary-side power transistor can be sampled to set the timing start point, thereby detecting whether the primary-side power transistor is conducting within the timing period. However, in the primary-secondary driving of a flyback converter, the information transmission between the primary and secondary sides often requires additional design, and the original control logic of the synchronous rectifier itself needs to determine whether the synchronous rectifier meets the conduction condition. Therefore, it is preferable to use the secondary side information to determine whether the synchronous rectifier is conducting to set the additional condition for the timing period. This allows direct reuse of the original control logic of the synchronous rectifier, eliminating the need for primary-secondary information transmission, resulting in a simple implementation and accurate detection results. Similarly, the overshoot amplitude of the output voltage can be used to determine whether the flyback converter has switched from full load to no load. However, compared with the timing judgment method with additional judgment conditions mentioned above, this detection method needs to distinguish the output under steady-state conditions.

[0065] The present invention also proposes a flyback converter that uses the synchronous rectification control method proposed above to control the synchronous rectifier tube.

[0066] 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.

[0067] 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.

[0068] 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 synchronous rectification control method applied to a flyback converter, characterized in that, When the flyback converter switches from full load to no load and operates under no load, the synchronous rectifier is turned off, allowing the secondary side to freewheel through the body diode of the synchronous rectifier.

2. The synchronous rectification control method as described in claim 1, characterized in that, The flyback converter obtains a sampled signal based on the voltage of the first node, and controls the conduction of the synchronous rectifier diode according to the sampled signal and a threshold signal. After the flyback converter switches from full load to no load, it pulls up the threshold signal, causing the synchronous rectifier diode to turn off during no-load operation. The first node is the connection point between the synchronous rectifier tube and the secondary winding.

3. The synchronous rectification control method as described in claim 1, characterized in that, After the flyback converter switches from full load to no load, both the primary-side power transistors and the synchronous rectifier transistors are turned off during the first time period. By detecting whether the first duration has elapsed, it is determined that the flyback converter has switched from full load to no load.

4. The synchronous rectification control method as described in claim 3, characterized in that, During the timing period, the synchronous rectifier tube and / or the primary-side power tube are checked to determine whether the first duration has elapsed.

5. The synchronous rectification control method as described in claim 4, characterized in that, Timing begins after the synchronous rectifier tube is turned on, and it is determined within the timing period whether the synchronous rectifier tube meets the conduction conditions. If not, then determine that the first duration has elapsed.

6. The synchronous rectification control method as described in claim 5, characterized in that, If the synchronous rectifier tube meets the conduction condition within the timing period, the current timing ends and the timing restarts, and it is determined that the first duration has not yet arrived.

7. The synchronous rectification control method as described in claim 5, characterized in that, The maximum duration of each timing cycle is set to a preset duration, which is greater than the maximum switching cycle of the flyback converter and less than the first duration.

8. The synchronous rectification control method as described in claim 2, characterized in that, The voltage change rate is obtained based on the voltage of the first node, and the synchronous rectifier is controlled to turn on when the voltage change rate is greater than the voltage slope threshold. After the flyback converter switches from full load to no load, it pulls up the voltage slope threshold so that the synchronous rectifier is turned off when running under no load.

9. The synchronous rectification control method as described in claim 2, characterized in that, The first volt-second product is obtained based on the difference between the voltage of the first node and the output voltage. When the first volt-second product is greater than or equal to the volt-second threshold and the voltage of the first node is less than the turn-on voltage, the synchronous rectifier is controlled to turn on. After the flyback converter switches from full load to no load, it pulls up the volt-second threshold, causing the synchronous rectifier to turn off when running under no load.

10. The synchronous rectification control method as described in claim 9, characterized in that, When the flyback converter starts under full load, the synchronous rectifier is turned on according to a first volt-second threshold, and after the synchronous rectifier is turned on for the first time, it is turned on according to a second volt-second threshold. After the flyback converter switches from full load to no load, it controls the synchronous rectifier to turn on according to the first volt-second threshold. Wherein, the first volt-second threshold is greater than the second volt-second threshold.

11. The synchronous rectification control method as described in claim 9, characterized in that, Under full load, the synchronous rectifier tube is controlled to conduct based on the volt-second product threshold. After the flyback converter switches from full load to no load, the synchronous rectifier is turned on according to the first volt-second threshold. The value of the volt-second product threshold is fixed, and the first volt-second threshold is greater than the volt-second product threshold.

12. The synchronous rectification control method as described in any one of claims 9-11, characterized in that, After the flyback converter switches from full load to no load, the conduction time of the original side power transistor decreases when running under no load, and the first volt-second product is less than the volt-second threshold after pull-up when running under no load.

13. The synchronous rectification control method as described in claim 1, characterized in that, The negative terminal of the body diode of the synchronous rectifier tube points to the high potential end of the output voltage.

14. A flyback converter, characterized in that, The synchronous rectifier tube is controlled by the synchronous rectification control method according to any one of claims 1-13.

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