LLC full-bridge synchronous rectification control method

Abstract

The application discloses an LLC full-bridge synchronous rectification control method. In each half cycle of alternating voltage, a rectifier driving control signal is sent to an output full-bridge synchronous rectification network of an LLC resonant converter. The output full-bridge synchronous rectification network comprises two pairs of diagonal conduction synchronous rectifier pairs, and the rectifier driving control signal is used for turning on one synchronous rectifier in the synchronous rectifier pair corresponding to the current half cycle state. In the case that one synchronous rectifier in the synchronous rectifier pair is in the on state, the other synchronous rectifier remains off and completes the freewheeling by using its body diode. The application can solve the current backflow problem from the root, and effectively improve the operation reliability and working efficiency of the converter.

Description

Technical Field

[0001] The embodiments of this application relate to, but are not limited to, the field of switching converter control technology, and particularly to an LLC full-bridge synchronous rectification control method. Background Technology

[0002] LLC resonant converters boast high efficiency and high power density, making them widely used in server power supplies, on-board chargers, and communication power supplies. To reduce secondary-side rectification losses, a full-bridge synchronous rectification circuit composed of four MOSFETs is commonly used to replace diodes. Existing full-bridge synchronous rectification control requires precise detection or prediction of the switching transistor current direction. It turns on the corresponding two diagonal rectifier transistors when the current is in the forward direction and turns them off before the current reverses, preventing the output capacitor current from flowing back into the transformer, thus avoiding efficiency reduction and device damage. Currently, there are two main methods to suppress current backflow: the first is to add an additional current detection circuit, relying on the detection of the current zero-crossing point to determine the rectifier transistor turn-off time; the second is to combine LLC resonant parameters with real-time operating status to calculate the rectifier transistor turn-off time. However, existing solutions have significant drawbacks. The current zero-crossing point detection requires high accuracy and bandwidth from the detection circuit; simultaneously, LLC converters include various operating conditions such as over-resonance, under-resonance, and light / heavy loads, resulting in significant differences in current waveforms, making it impossible to adapt to a uniform turn-off time; furthermore, the overall control logic and drive timing are complex, making it difficult to guarantee the device's operational reliability and anti-interference capabilities. Summary of the Invention

[0003] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0004] This application provides an LLC full-bridge synchronous rectification control method that can solve the current backflow problem at its source and effectively improve the overall reliability and efficiency of the converter.

[0005] This application provides an LLC full-bridge synchronous rectification control method, comprising: sending a rectifier drive control signal to the output full-bridge synchronous rectification network of the LLC resonant converter during each AC voltage half-cycle; wherein the output full-bridge synchronous rectification network includes two pairs of diagonally conducting synchronous rectifier diodes, and the rectifier drive control signal is used to turn on one of the synchronous rectifier diodes in the synchronous rectifier diode pair corresponding to the current half-cycle state; when one of the synchronous rectifier diodes in the synchronous rectifier diode pair is in the conducting state, the other synchronous rectifier diode remains off and completes freewheeling using its body diode.

[0006] In one embodiment of this application, the synchronous rectifier pair includes a first diagonal conducting switch pair and a second diagonal conducting switch pair. The first diagonal conducting switch pair includes a first bridge arm upper tube S1 and a second bridge arm lower tube S4, and the second diagonal conducting switch pair includes a first bridge arm lower tube S2 and a second bridge arm upper tube S3.

[0007] In one embodiment of this application, the LLC resonant converter includes an input full-bridge inverter network, which includes a first primary-side power switch pair and a second primary-side power switch pair. The first primary-side power switch pair includes a first bridge arm upper switch Q1 and a second bridge arm lower switch Q4, and the second primary-side power switch pair includes a first bridge arm lower switch Q2 and a second bridge arm upper switch Q3.

[0008] In one embodiment of this application, sending a rectifier drive control signal to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: if the upper rectifier Q1 of the first bridge arm and the lower rectifier Q4 of the second bridge arm are in a conducting state, sending the rectifier drive control signal to the upper rectifier S1 of the first bridge arm of the output full-bridge synchronous rectifier network to turn on the upper rectifier S1 of the first bridge arm.

[0009] In one embodiment of this application, sending a rectifier drive control signal to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: if the lower rectifier Q2 of the first bridge arm and the upper rectifier Q3 of the second bridge arm are in a conducting state, sending the rectifier drive control signal to the upper rectifier S3 of the second bridge arm of the output full-bridge synchronous rectifier network to turn on the upper rectifier S3 of the second bridge arm.

[0010] In one embodiment of this application, sending a rectifier drive control signal to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: if the upper rectifier Q1 of the first bridge arm and the lower rectifier Q4 of the second bridge arm are in a conducting state, sending the rectifier drive control signal to the lower rectifier S4 of the second bridge arm of the output full-bridge synchronous rectifier network to turn on the lower rectifier S4.

[0011] In one embodiment of this application, sending a rectifier drive control signal to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: if the first bridge arm lower transistor Q2 and the second bridge arm upper transistor Q3 are in a conducting state, sending the rectifier drive control signal to the first bridge arm lower transistor S2 of the output full-bridge synchronous rectifier network to turn on the first bridge arm lower transistor S2.

[0012] In one embodiment of this application, when the upper transistor S1 of the first bridge arm is turned on, the body diode of the lower transistor S4 of the second bridge arm is forward-biased; when the upper transistor S3 of the second bridge arm is turned on, the body diode of the lower transistor S2 of the first bridge arm is forward-biased; when the lower transistor S4 of the second bridge arm is turned on, the body diode of the upper transistor S1 of the first bridge arm is forward-biased; when the lower transistor S2 of the first bridge arm is turned on, the body diode of the upper transistor S3 of the second bridge arm is forward-biased.

[0013] This application provides an LLC full-bridge synchronous rectification control method. Within each half-cycle of the AC voltage, a rectifier drive control signal is sent to the output full-bridge synchronous rectification network of the LLC resonant converter. The output full-bridge synchronous rectification network includes two pairs of diagonally conducting synchronous rectifier diodes. The rectifier drive control signal is used to turn on one of the synchronous rectifier diodes in the pair corresponding to the current half-cycle state. When one synchronous rectifier diode in the pair is in the conducting state, the other synchronous rectifier diode remains off and utilizes its body diode for freewheeling. This application eliminates the need to detect the current zero-crossing point and calculate the rectifier diode turn-off time, thus eliminating the need for a high-precision current detection circuit and reducing hardware design requirements. This method can adapt to various operating conditions of the LLC resonant converter, eliminating the need to set corresponding turn-off times for different operating conditions. It also simplifies the control circuit and drive timing, improving the reliability and anti-interference capability of the equipment. This application embodiment can also fundamentally eliminate current backflow. Within each half-cycle of the AC voltage, when reverse current attempts to form a backflow circuit, it will inevitably pass through the always-off synchronous rectifier diode, whose internal body diode is in a reverse-biased state and cannot form a conduction circuit. Even if the working rectifier diode continuously receives the drive signal, no backflow current will be generated, effectively solving the efficiency loss and device damage problems caused by current backflow. Attached Figure Description

[0014] Figure 1 This is a flowchart of the LLC full-bridge synchronous rectification control method provided in the embodiments of this application; Figure 2 This is a schematic diagram of an LLC full-bridge synchronous rectification provided in an embodiment of this application; Figure 3 This is a schematic diagram of the primary and secondary drive signals of an LLC resonant converter provided in one embodiment of this application; Figure 4 This is a schematic diagram of the primary and secondary drive signals of an LLC resonant converter provided in another embodiment of this application. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0016] It should be noted that although the flowchart shows a logical order, in some cases, the steps shown or described may be performed in a different order than that shown in the flowchart. The terms "first," "second," etc., used in the specification, claims, and the aforementioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the structures, proportions, sizes, etc., depicted in the drawings are only used to complement the content disclosed in the specification for those skilled in the art to understand and read, and are not intended to limit the implementation conditions of this application. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to size, without affecting the effects and purposes achieved by this application, should still fall within the scope of the technical content disclosed in this application. Similarly, the terms such as "upper," "lower," "left," "right," "middle," and "one" used in this specification are only for clarity of description and are not used to limit the scope of implementation of this application. Changes or adjustments in their relative relationships, without substantially altering the technical content, should also be considered within the scope of implementation of this application.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing embodiments of this application only and is not intended to limit this application.

[0018] To reduce secondary-side rectification losses in LLC resonant converters, a full-bridge synchronous rectification circuit consisting of four MOSFETs is commonly used to replace diodes. Existing full-bridge synchronous rectification control requires precise detection or prediction of the switching transistor current direction. It turns on the corresponding two diagonal rectifier transistors when the current is in the forward direction and turns them off before the current reverses, preventing the output capacitor current from flowing back into the transformer, thus avoiding efficiency reduction and device damage. Currently, there are two main methods to suppress current backflow: the first is to add an additional current detection circuit, relying on the detection of the current zero-crossing point to determine the rectifier transistor turn-off time; the second is to calculate the rectifier transistor turn-off time by combining LLC resonant parameters and real-time operating status. However, existing solutions have significant drawbacks. The current zero-crossing point detection requires high accuracy and bandwidth from the detection circuit; simultaneously, LLC converters include various operating conditions such as over-resonance, under-resonance, and light / heavy loads, resulting in significant differences in current waveforms, making it impossible to adapt to a uniform turn-off time; furthermore, the overall control logic and drive timing are complex, making it difficult to guarantee the device's operational reliability and anti-interference capability.

[0019] In view of this, this application provides an LLC full-bridge synchronous rectification control method. Within each half-cycle of the AC voltage, a rectifier drive control signal is sent to the output full-bridge synchronous rectification network of the LLC resonant converter. The output full-bridge synchronous rectification network includes two pairs of diagonally conducting synchronous rectifier diodes. The rectifier drive control signal is used to turn on one of the synchronous rectifier diodes in the pair corresponding to the current half-cycle state. When one synchronous rectifier diode in the pair is in the conducting state, the other synchronous rectifier diode remains off and utilizes its body diode for freewheeling. This application eliminates the need to detect the current zero-crossing point and calculate the rectifier diode turn-off time, thus eliminating the need for a high-precision current detection circuit and reducing hardware design requirements. This method can adapt to various operating conditions of the LLC resonant converter, eliminating the need to set corresponding turn-off times for different operating conditions. It also simplifies the control circuit and drive timing, improving the reliability and anti-interference capability of the equipment. This application embodiment can also fundamentally eliminate current backflow. Within each half-cycle of the AC voltage, when reverse current attempts to form a backflow circuit, it will inevitably pass through the always-off synchronous rectifier diode, whose internal body diode is in a reverse-biased state and cannot form a conduction circuit. Even if the working rectifier diode continuously receives the drive signal, no backflow current will be generated, effectively solving the efficiency loss and device damage problems caused by current backflow.

[0020] The embodiments of this application will be further described below with reference to the accompanying drawings.

[0021] Reference Figure 1 , Figure 1 This is a flowchart of the LLC full-bridge synchronous rectification control method provided in the embodiments of this application. The process may specifically include, but is not limited to, step 110.

[0022] Step 110: During each half-cycle of the AC voltage, a rectifier drive control signal is sent to the output full-bridge synchronous rectifier network of the LLC resonant converter; wherein, the output full-bridge synchronous rectifier network includes two pairs of diagonally conducting synchronous rectifier diodes, and the rectifier drive control signal is used to turn on one of the synchronous rectifier diodes in the synchronous rectifier diode pair corresponding to the current half-cycle state; when one synchronous rectifier diode in the synchronous rectifier diode pair is in the conducting state, the other synchronous rectifier diode remains off and completes freewheeling using its body diode.

[0023] In one feasible embodiment, the primary side of the LLC resonant converter is provided with an input full-bridge inverter network. For example... Figure 2As shown, the input full-bridge inverter network includes a first primary-side power switch pair and a second primary-side power switch pair. The first primary-side power switch pair includes a first bridge arm upper transistor Q1 and a second bridge arm lower transistor Q4, and the second primary-side power switch pair includes a first bridge arm lower transistor Q2 and a second bridge arm upper transistor Q3. In this embodiment, the output full-bridge synchronous rectification network is located on the secondary side of the LLC resonant converter. The synchronous rectification transistor pair of this network includes a first diagonal conducting switch pair and a second diagonal conducting switch pair. The first diagonal conducting switch pair includes a first bridge arm upper transistor S1 and a second bridge arm lower transistor S4, and the second diagonal conducting switch pair includes a first bridge arm lower transistor S2 and a second bridge arm upper transistor S3.

[0024] In one feasible embodiment, the primary side of the LLC resonant converter further includes an input power supply, a resonant inductor Lr, a resonant capacitor Cr, and the primary winding of a transformer. The input full-bridge inverter circuit consists of switching transistors Q1, Q2, Q3, and Q4. The secondary side also includes the secondary winding of a transformer and an output load. The output full-bridge synchronous rectifier circuit consists of synchronous rectifier diodes S1, S2, S3, and S4, and the output load... It is connected between the positive and negative output terminals.

[0025] In a feasible embodiment, the AC voltage half-cycle refers to half a working cycle of the AC voltage output by the secondary winding of the transformer, which can be determined based on the operating state of the LLC resonant converter, such as by combining the phase of the AC voltage on the secondary side of the transformer, the switching sequence of the primary full-bridge inverter circuit, or the change of the resonant current.

[0026] In one feasible embodiment, during each half-cycle of the AC voltage, while sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter, if the upper transistor Q1 of the first bridge arm and the lower transistor Q4 of the second bridge arm are in a conducting state, a rectifier drive control signal is sent to the upper transistor S1 of the first bridge arm of the output full-bridge synchronous rectifier network to turn on the upper transistor S1 of the first bridge arm. The primary-side input full-bridge inverter circuit of the LLC resonant converter consists of two bridge arms: the first bridge arm includes upper transistor Q1 and lower transistor Q2, and the second bridge arm includes upper transistor Q3 and lower transistor Q4; the output full-bridge synchronous rectifier network also consists of two bridge arms: the first bridge arm includes upper transistor S1 and lower transistor S2, and the second bridge arm includes upper transistor S3 and lower transistor S4. Within each half-cycle of the AC voltage, the primary side forms a fixed-direction current path through the diagonally conducting switching transistors. When the upper transistor Q1 of the first bridge arm and the lower transistor Q4 of the second bridge arm are in the on state, the primary side current flows out from the positive terminal of the input power supply, passes through Q1, the resonant inductor Lr, the primary winding of the transformer, and the resonant capacitor Cr in sequence, and then flows back to the negative terminal of the input power supply through Q4. At this time, the current direction of the primary winding of the transformer is fixed. According to the coupling relationship of the winding terminals, the secondary winding will induce an AC voltage of a certain polarity, corresponding to one half-cycle of the AC voltage on the secondary side. During this half-cycle, the potential on the terminal side of the secondary winding is positive. At this time, a rectifier drive control signal is sent to the upper transistor S1 of the first bridge arm of the output full-bridge synchronous rectifier network, causing S1 to conduct.

[0027] In a feasible embodiment, when the upper transistor S1 of the first bridge arm is turned on, the body diode of the lower transistor S4 of the second bridge arm is forward-biased, meaning that S4 remains off and utilizes its body diode to complete the freewheeling current, thereby forming a complete rectified current path within this half-cycle. Since S4 is always off, when the current reverses, its body diode will enter a reverse bias state and cannot conduct to form a reverse-current loop. This eliminates the need to additionally turn off S1 at the current zero-crossing point, thus eliminating the need for the current zero-crossing detection stage and simplifying the control timing and circuit design.

[0028] In one feasible embodiment, during each half-cycle of the AC voltage, while sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter, if the lower transistor Q2 of the first bridge arm and the upper transistor Q3 of the second bridge arm are in a conducting state, a rectifier drive control signal is sent to the upper transistor S3 of the second bridge arm of the output full-bridge synchronous rectifier network to turn on the upper transistor S3. Similarly, during each half-cycle of the AC voltage, the primary side forms a current path in a fixed direction through the diagonally conducting switching transistors. When the lower transistor Q2 of the first bridge arm and the upper transistor Q3 of the second bridge arm are in a conducting state, the primary side current flows out from the positive terminal of the input power supply, passes through Q3, the resonant capacitor Cr, the primary winding of the transformer, the resonant inductor Lr, and then flows back to the negative terminal of the input power supply through Q2. At this time, the current direction of the primary winding of the transformer is opposite to that before. According to the coupling relationship of the winding terminals, the secondary winding will induce an AC voltage with the opposite polarity to that before, corresponding to the other half-cycle of the AC voltage on the secondary side. During this half-cycle, the potential on the non-identical end of the secondary winding is positive. At this time, a rectifier drive control signal is sent to the upper tube S3 of the second bridge arm of the output full-bridge synchronous rectifier network to turn on S3.

[0029] In a feasible embodiment, when the upper transistor S3 of the second bridge arm is turned on, the body diode of the lower transistor S2 of the first bridge arm is forward-biased, meaning S2 remains off and utilizes its body diode to complete the freewheeling current, thus forming a complete rectified current path within this half-cycle. Since S2 is always off, when the current reverses, its body diode will enter a reverse-biased state and cannot conduct to form a reverse-current loop. Therefore, there is no need to additionally turn off S3 at the current zero-crossing point, eliminating the need for a current zero-crossing detection step and simplifying the control timing and circuit design.

[0030] In a feasible embodiment, during each half-cycle of the AC voltage, while sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter, if the upper transistor Q1 of the first bridge arm and the lower transistor Q4 of the second bridge arm are in a conducting state, a rectifier drive control signal can also be sent to the lower transistor S4 of the second bridge arm of the output full-bridge synchronous rectifier network to turn on the lower transistor S4. When the upper transistor Q1 of the first bridge arm and the lower transistor Q4 of the second bridge arm on the primary side are simultaneously turned on, a current path with a fixed direction is formed on the primary side. The current in the primary winding of the transformer flows in from the same-name terminal. According to the coupling relationship of the same-name terminals of the windings, the secondary winding induces an AC voltage with the same-name terminal being positive and the non-same-name terminal being negative, corresponding to one half-cycle of the AC voltage on the secondary side. In the secondary-side output full-bridge synchronous rectifier network, S1 and S4 are a diagonal pair of transistors, adapted to the rectification requirements of the half-cycle. During this half-cycle, in addition to sending a drive signal to the upper tube S1 of the first bridge arm to turn it on, a rectifier tube drive control signal can also be sent to the lower tube S4 of the second bridge arm to turn it on. The rectified current path differs between the two conduction modes: when S1 is on, the current flows from the same-name terminal of the secondary winding, through S1 to the positive output terminal, and then through the load. The current flows back to the negative output terminal, and finally flows back to the non-same-name terminal of the secondary winding through the body diode of S4; when S4 is turned on, the current flows out from the same-name terminal of the secondary winding, flows through the body diode of S1 to the negative output terminal, and then through the load. The current flows back to the positive output terminal and finally flows to the non-same-name terminal of the secondary winding through S4. Both methods can form an effective rectifier circuit. Since the other switching transistor in the diagonal pair is always turned off, it relies solely on its body diode to complete the freewheeling. When the current shows a reverse trend, the body diode of the turn-off transistor will be in a reverse bias state, and a reverse-flow circuit cannot be formed. There is no need to turn off the conducting transistor, and the current zero-crossing detection step is also eliminated.

[0031] In one feasible embodiment, when the lower transistor S4 of the second bridge arm is turned on, the body diode of the upper transistor S1 of the first bridge arm is forward turned on, that is, S1 remains off and uses its body diode to complete the freewheeling current, thereby forming a complete rectified current path within this half cycle.

[0032] In a feasible embodiment, during each half-cycle of the AC voltage, the process of sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: if the lower transistor Q2 of the first bridge arm and the upper transistor Q3 of the second bridge arm are in a conducting state, a rectifier drive control signal can also be sent to the lower transistor S2 of the first bridge arm of the output full-bridge synchronous rectifier network to turn on the lower transistor S2. Similarly, in the secondary-side output full-bridge synchronous rectifier network, S2 and S3 form a diagonal pair to meet the rectification requirements of the half-cycle. During this half-cycle, in addition to sending a drive signal to S3 to turn it on, a rectifier drive control signal can also be sent to S2 to turn it on. The rectified current path is different under the two conduction modes: when S3 is on, the current flows out from the non-same-name terminal of the secondary winding, flows through S3 to the positive terminal of the output, and then through the load. The current flows back to the negative output terminal, and finally flows back to the same-name terminal of the secondary winding through the body diode of S2; when S2 is turned on, the current flows out from the non-same-name terminal of the secondary winding, flows to the positive output terminal through the body diode of S3, and then through the load. The current flows back to the negative output terminal and finally flows to the corresponding terminal of the secondary winding through S2. Both methods can form an effective rectified circuit. Since the other switch in the diagonal pair is always turned off, it relies solely on its body diode to complete the freewheeling. When the current shows a reverse trend, the body diode of S3 will be in a reverse bias state, and a reverse-flow circuit cannot be formed. There is no need to turn off the conducting tube, and the current zero-crossing detection step is also eliminated.

[0033] In one feasible embodiment, when the lower transistor S2 of the first bridge arm is turned on, the body diode of the upper transistor S3 of the second bridge arm is forward turned on, that is, S3 remains off and uses its body diode to complete the freewheeling current, thereby forming a complete rectified current path within the half-cycle.

[0034] In a feasible embodiment, the rectifier drive control signals S1 and S3 can be directly generated corresponding to the conduction states of the primary-side Q1 and Q3, without requiring additional calculations based on the resonant parameters or operating state of the LLC resonant converter. In other words, the conduction state of the primary-side switches directly corresponds to the required rectification stage on the secondary side. As long as the primary-side Q1 is detected to be in the conduction state, the rectifier drive control signal corresponding to S1 can be generated and output synchronously; similarly, when the primary-side Q3 is detected to be in the conduction state, the rectifier drive control signal corresponding to S3 can be generated and output synchronously. The entire process requires no additional acquisition of circuit parameters, determination of current state, or complex calculations; the generation of the secondary-side drive signal is entirely based on the operating state of the primary-side switches.

[0035] The LLC full-bridge synchronous rectification control method of this application will be described below with some specific embodiments.

[0036] Example 1: When the LLC resonant converter is operating normally, the drive control signals for the primary-side power switches Q1, Q2, Q3, Q4 and the secondary-side synchronous rectifiers S1, S2, S3, S4 are as follows: Figure 3 As shown.

[0037] When the upper transistor Q1 of the first bridge arm and the lower transistor Q4 of the second bridge arm are in the conducting state, the voltage of the primary winding of the transformer is positive at the same-name terminal and negative at the opposite-name terminal (first polarity). At this time, the secondary winding of the transformer also induces a voltage with positive at the same-name terminal and negative at the opposite-name terminal, sending a rectifier drive control signal to the upper transistor S1 of the first bridge arm of the output full-bridge synchronous rectifier network, causing S1 to conduct, while S2, S3, and S4 remain in the off state. At this time, the forward current flows out from the same-name terminal of the secondary winding of the transformer, flowing sequentially through the drain and source of S1, the positive terminal of the output, and the output load. The current flows from the negative output terminal, through the body diode of the lower transistor S4 in the second bridge arm, to the opposite-named terminal of the transformer secondary winding. The body diode of S4 is forward-biased, completing the freewheeling. When the current decreases to zero and attempts to reverse, if the reverse current is to form a loop, it needs to return from the positive output terminal through the still potentially conducting S1 to the transformer secondary winding, and then flow into S4 from the opposite-named terminal of the secondary winding. However, S4 is always in the off state, and the direction of its body diode is from the negative output terminal (anode) to the opposite-named terminal of the transformer secondary winding (cathode). When the reverse current attempts to flow into S4 from the opposite-named terminal of the transformer secondary winding, the body diode is in a reverse biased state and cannot conduct to form a loop, thus completely blocking the reverse current flow.

[0038] Similarly, when the lower transistor Q2 of the first bridge arm and the upper transistor Q3 of the second bridge arm are in the conducting state, the voltage of the primary winding of the transformer is negative at the same-name terminal and positive at the opposite-name terminal (second polarity). At this time, the secondary winding of the transformer also induces a voltage with negative at the same-name terminal and positive at the opposite-name terminal, sending a rectifier drive control signal to the upper transistor S3 of the second bridge arm of the output full-bridge synchronous rectifier network, causing S3 to conduct, while S1, S2, and S4 remain in the off state. At this time, the forward current flows out from the opposite-name terminal of the secondary winding of the transformer, flowing sequentially through the drain and source of S3, the positive output terminal, and the output load. The current flows from the negative output terminal to the body diode of the lower transistor S2 in the first bridge arm, and finally to the corresponding terminal of the transformer secondary winding. The body diode of S2 is forward biased, completing the freewheeling. When the current attempts to reverse, it will also be unable to form a reverse flow circuit due to the reverse bias of the body diode of S2.

[0039] In this embodiment, the rectifier drive control signals of S1 and S3 can be directly generated corresponding to the conduction states of Q1 and Q3 on the primary side, without the need for additional calculations based on the resonance parameters or operating state of the LLC resonant converter.

[0040] like Figure 3As shown, the rectifier drive control signal of S1 is consistent with the conduction state of Q1 on the primary side, and the rectifier drive control signal of S3 is consistent with the conduction state of Q3 on the primary side. The remaining synchronous rectifiers are not connected to effective drive signals and are always in the off state. Even if the LLC resonant converter operates under light load, over-resonance, or other conditions, and the resonant current drops to zero early and shows a reverse trend, the body diode of the off rectifier will also block it, the reverse waveform of the resonant current is cut off, and no negative current is generated. Therefore, there will be no current backflow problem, and the risk of synchronous rectification running out of control is completely eliminated.

[0041] Example 2: This embodiment is an adjustment based on Embodiment 1, focusing only on the drive control signal for the lower rectifier output of the secondary-side synchronous rectifier network. When the transformer primary winding voltage is in the first polarity (positive at the same terminal and negative at the opposite terminal), only the lower rectifier S4 of the second bridge arm is driven to conduct, while the upper rectifier S1 of the first bridge arm remains off, relying on its own diode for freewheeling. When the transformer primary winding voltage is in the second polarity (negative at the same terminal and positive at the opposite terminal), only the lower rectifier S2 of the first bridge arm is driven to conduct, while the upper rectifier S3 of the second bridge arm remains off, relying on its own diode for freewheeling. The working principle of this embodiment is symmetrical to that of Embodiment 1, and it can also effectively block reverse current, reliably avoiding the problem of current backflow.

[0042] Example 3: When the driving logic of Example 1 is continuously operating, the primary voltage of the transformer is in the first polarity stage, and only S1 is controlled to conduct. The S4, which is turned off, relies on its own diode to complete the freewheeling current. After long-term operation, the temperature of S4, which is continuously generating heat from the freewheeling current, will be significantly higher than that of S1, which is conducting. Similarly, when the primary voltage of the transformer is in the second polarity stage, only S3 is controlled to conduct, and the S2, which is turned off, relies on its own diode to complete the freewheeling current. After long-term operation, the device temperature of S2 will also be significantly higher than that of S3, ultimately resulting in an excessively large temperature difference among the four synchronous rectifier diodes and an uneven distribution of heat generation in the devices.

[0043] To balance the operating temperature of the four synchronous rectifier diodes S1, S2, S3, and S4, this embodiment uses the main control chip to alternately run the rectifier diode drive control logic of Embodiment 1 and Embodiment 2. The specific drive timing is as follows: Figure 4As shown. In the first polarity cycle, the driving method of Embodiment 1 is used, controlling S1 to be on and S4 to remain off, relying on the body diode for freewheeling. In the first second polarity cycle, the driving method of Embodiment 1 is still used, controlling S3 to be on and S2 to remain off, relying on the body diode for freewheeling. In the second first polarity cycle, the driving method of Embodiment 2 is switched, controlling S4 to be on and S1 to remain off, relying on the body diode for freewheeling. In the second second polarity cycle, the driving logic is switched synchronously, controlling S2 to be on and S3 to remain off, relying on the body diode for freewheeling. The circuit cycles through the two driving modes according to this pattern, allowing the four synchronous rectifier diodes to take turns undertaking the main on-state operation and the body diode freewheeling operation, thereby achieving temperature balance of all rectifier diodes and improving the problem of long-term overheating of a single diode.

[0044] The core logic of this application's embodiment is as follows: In the LLC full-bridge synchronous rectifier circuit, within each half-cycle of the AC voltage, only one rectifier drive control signal is sent to one of the diagonal synchronous rectifier diodes in a pair to turn it on, while the other synchronous rectifier diode in the same pair remains off, relying on its own body diode for freewheeling. The rectifier diode in the diagonal pair, which is always off, has a body diode with unidirectional conductivity. When the load current drops to zero and a reverse trend appears, the body diode will be reverse-biased and cut off, directly cutting off the reverse current path. This method eliminates the risk of current backflow without relying on precise timing to turn off the rectifier diode. Furthermore, this application's embodiment can adapt to various operating conditions of the LLC resonant converter. Whether it's over-resonance, under-resonance, or light-load and full-load conditions, it can operate stably without switching control strategies or adjusting operating parameters, solving the problem that traditional synchronous rectification control methods are easily affected by resonance states and load changes.

[0045] The beneficial effects of the embodiments of this application include at least the following: (1) It can fundamentally eliminate current backflow. During each half-cycle of AC voltage, when the current attempts to flow in the reverse direction, it will inevitably pass through the synchronous rectifier tube that is kept off. The body diode of this device is in a reverse biased state and cannot form a conduction loop. Even if the working rectifier tube continues to receive the drive signal, no backflow current will be generated in the circuit.

[0046] (2) The overall control logic is simple. The drive signal of the secondary rectifier tube can be synchronized with the state of the primary switch tube, eliminating the need for complex processing steps such as phase detection, current zero-crossing detection, and adaptive dead-time adjustment.

[0047] (3) The operating state is not affected by the resonance mode and the load size. The converter can work normally under different operating conditions such as over-resonance, under-resonance, no-load, and full-load, without the need to change the control scheme according to the operating conditions.

[0048] (4) There is no risk of bridge arm shoot-through in the circuit. At any given time, only one synchronous rectifier tube receives the drive signal and conducts, which avoids the bridge arm shoot-through problem from a structural point of view.

[0049] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for controlling a full-bridge synchronous rectifier in LLC, characterized in that, include: Within each half-cycle of the AC voltage, a rectifier drive control signal is sent to the output full-bridge synchronous rectifier network of the LLC resonant converter. The output full-bridge synchronous rectifier network includes two pairs of diagonally conducting synchronous rectifier diodes. Each pair includes a first diagonally conducting switch pair and a second diagonally conducting switch pair. The first diagonally conducting switch pair includes a first bridge arm upper diode S1 and a second bridge arm lower diode S4, and the second diagonally conducting switch pair includes a first bridge arm lower diode S2 and a second bridge arm upper diode S3. The rectifier drive control signal is used to turn on one of the synchronous rectifier diodes in the pair corresponding to the current half-cycle state. When one synchronous rectifier diode in the pair is in the conducting state, the other synchronous rectifier diode remains off and utilizes its body diode for freewheeling. Specifically, when the upper transistor S1 of the first bridge arm is turned on, the body diode of the lower transistor S4 of the second bridge arm is forward-biased; when the upper transistor S3 of the second bridge arm is turned on, the body diode of the lower transistor S2 of the first bridge arm is forward-biased; when the lower transistor S4 of the second bridge arm is turned on, the body diode of the upper transistor S1 of the first bridge arm is forward-biased; and when the lower transistor S2 of the first bridge arm is turned on, the body diode of the upper transistor S3 of the second bridge arm is forward-biased.

2. The LLC full-bridge synchronous rectification control method according to claim 1, characterized in that, The LLC resonant converter includes an input full-bridge inverter network, which includes a first primary-side power switch pair and a second primary-side power switch pair. The first primary-side power switch pair includes a first bridge arm upper switch Q1 and a second bridge arm lower switch Q4, and the second primary-side power switch pair includes a first bridge arm lower switch Q2 and a second bridge arm upper switch Q3.

3. The LLC full-bridge synchronous rectification control method according to claim 2, characterized in that, Sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: If the first bridge arm upper tube Q1 and the second bridge arm lower tube Q4 are in a conducting state, the rectifier tube drive control signal is sent to the first bridge arm upper tube S1 of the output full-bridge synchronous rectifier network to turn on the first bridge arm upper tube S1.

4. The LLC full-bridge synchronous rectification control method according to claim 2, characterized in that, Sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: If the first bridge arm lower tube Q2 and the second bridge arm upper tube Q3 are in a conducting state, the rectifier tube drive control signal is sent to the second bridge arm upper tube S3 of the output full-bridge synchronous rectifier network to turn on the second bridge arm upper tube S3.

5. The LLC full-bridge synchronous rectification control method according to claim 2, characterized in that, Sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: If the first bridge arm upper tube Q1 and the second bridge arm lower tube Q4 are in a conducting state, the rectifier tube drive control signal is sent to the second bridge arm lower tube S4 of the output full-bridge synchronous rectifier network to turn on the second bridge arm lower tube S4.

6. The LLC full-bridge synchronous rectification control method according to claim 2, characterized in that, Sending rectifier drive control signals to the output full-bridge synchronous rectifier network of the LLC resonant converter includes: If the first lower bridge arm tube Q2 and the second upper bridge arm tube Q3 are in a conducting state, the rectifier tube drive control signal is sent to the first lower bridge arm tube S2 of the output full-bridge synchronous rectifier network to turn on the first lower bridge arm tube S2.

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