Flyback power supply, synchronous rectification control circuit and method
By using a turn-on control module, a turn-off control module, and a drive signal generation module, the turn-off threshold of the synchronous rectifier is automatically adjusted, solving the problems of voltage stress and high device performance requirements in synchronous rectification technology, and achieving efficient and low-cost synchronous rectification control.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CRM ICBG (WUXI) CO LTD
- Filing Date
- 2024-12-02
- Publication Date
- 2026-06-02
Smart Images
Figure CN122137241A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of integrated circuits, and in particular to a flyback power supply, a synchronous rectification control circuit, and a method. Background Technology
[0002] Synchronous rectification circuits are a technology for improving efficiency in switching power supplies. They use one or more power MOSFETs instead of traditional diodes (whose on-resistance is much lower, reducing losses during rectification; avoiding current harmonics and electromagnetic interference generated by traditional diodes during reverse recovery time, thus reducing EMI). Rectification is achieved by controlling the switching time of the MOSFETs, ensuring the rectification process is synchronized with the input signal. When the switch is on, the inductor stores energy; when the switch is off, this energy is released to the load, generating a relatively stable DC output. The development of synchronous rectification technology has brought significant performance improvements to switching power supplies, especially in applications requiring high efficiency and low power consumption. With the continuous advancement of power electronics technology, the application of synchronous rectification technology will become increasingly widespread.
[0003] Current voltage detection technologies need to support continuous mode operation, quasi-resonant mode operation, and discontinuous mode operation. Among them, continuous mode has stringent requirements on the turn-off time of synchronous rectification. The turn-off delay needs to be controlled within a very small time range, almost to the point of being negligible. Otherwise, it will result in a loss of system efficiency or a large voltage stress on the synchronous rectification device, while increasing the system cost.
[0004] Therefore, how to solve the voltage stress problem in synchronous rectification technology, reduce device performance requirements, and reduce system costs has become one of the urgent problems to be solved by those skilled in the art.
[0005] It should be noted that the above description of the technical background is only for the purpose of providing a clear and complete explanation of the technical solutions of the present invention and facilitating understanding by those skilled in the art. It should not be assumed that the above technical solutions are known to those skilled in the art simply because they have been described in the background section of this invention. Summary of the Invention
[0006] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a flyback power supply, a synchronous rectification control circuit and method to solve the problems of voltage stress, high device performance requirements and high system cost in the prior art synchronous rectification circuit.
[0007] To achieve the above and other related objectives, the present invention provides a synchronous rectification control circuit, the synchronous rectification control circuit comprising at least:
[0008] The module includes an on / off control module, an off / return control module, and a drive signal generation module.
[0009] The conduction control module is used to generate the conduction control signal for the synchronous rectifier tube;
[0010] The shutdown control module detects the drain-source voltage of the synchronous rectifier. When the drain-source voltage of the synchronous rectifier rises to the shutdown threshold, it generates a shutdown control signal for the synchronous rectifier. At the same time, it adjusts the shutdown threshold based on the peak value of the drain-source voltage of the synchronous rectifier. The shutdown threshold is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier.
[0011] The drive signal generation module is connected to the output terminals of the turn-on control module and the turn-off control module, and generates the drive signal for the synchronous rectifier tube based on the turn-on control signal and the turn-off control signal.
[0012] The shutdown threshold is a negative value.
[0013] Optionally, the conduction control module includes a comparison unit, which receives the drain-source voltage of the synchronous rectifier and a conduction threshold, and generates the conduction control signal when the drain-source voltage of the synchronous rectifier drops to the conduction threshold.
[0014] The conduction threshold is negative and less than the initial value of the shutdown threshold.
[0015] Alternatively, the shutdown control module includes a first comparator, a peak detection unit, and a shutdown threshold adjustment unit;
[0016] The first comparator receives the drain-source voltage of the synchronous rectifier and the turn-off threshold, and outputs a first comparison result;
[0017] The peak detection unit receives the drain-source voltage of the synchronous rectifier tube, detects the peak value of the drain-source voltage of the synchronous rectifier tube, and obtains a peak detection signal.
[0018] The shutdown threshold adjustment unit is connected to the output of the peak detection unit and adjusts the shutdown threshold based on the peak value of the drain-source voltage of the synchronous rectifier.
[0019] Alternatively, the peak detection unit includes a peak sampling and holding circuit and a voltage divider circuit;
[0020] The peak sampling and holding circuit receives the drain-source voltage of the synchronous rectifier. When the drain-source voltage of the synchronous rectifier is greater than the turn-off threshold, it samples the peak value of the drain-source voltage of the synchronous rectifier and resets when the synchronous rectifier is turned on.
[0021] The voltage divider circuit is connected to the output of the peak sample-and-hold circuit, and the peak detection signal is obtained by dividing the peak value of the drain-source voltage of the synchronous rectifier.
[0022] Alternatively, the shutdown threshold adjustment unit includes a second comparator, a third comparator, a logic circuit, a negative voltage generation circuit, and a switch;
[0023] The second comparator compares the peak detection signal with the maximum set voltage and outputs a second comparison result;
[0024] The third comparator compares the turn-off threshold with the minimum turn-off voltage and outputs a third comparison result;
[0025] The logic circuit is connected to the output terminals of the second comparator and the third comparator; when the peak detection signal is greater than the maximum set voltage and the turn-off threshold is greater than or equal to the minimum turn-off voltage, a turn-on signal is generated; when the peak detection signal is less than or equal to the maximum set voltage or the turn-off threshold is less than the minimum turn-off voltage, a turn-off signal is generated.
[0026] The negative voltage generating circuit is controlled by the peak detection signal to generate an adjustment amount for the turn-off threshold, and the adjustment amount is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier tube, and the adjustment amount is negative.
[0027] One end of the switch is connected to the output of the negative voltage generating circuit, and the other end is connected to the shutdown threshold. It is controlled by the output signal of the logic circuit and is used to adjust the shutdown threshold.
[0028] Optionally, the drive signal generation module includes a trigger and a drive unit;
[0029] The trigger is connected to the output terminals of the turn-on control module and the turn-off control module, and generates a switch control signal based on the turn-on control signal and the turn-off control signal;
[0030] The driving unit is connected to the output of the trigger and drives the synchronous rectifier tube based on the switch control signal.
[0031] To achieve the above and other related objectives, the present invention also provides a flyback power supply, which includes at least: a flyback topology circuit and the above-mentioned synchronous rectification control circuit;
[0032] The synchronous rectification control circuit provides a drive signal for the synchronous rectifier diodes in the flyback topology circuit.
[0033] To achieve the above and other related objectives, the present invention also provides a synchronous rectification control method, the synchronous rectification control method comprising:
[0034] When the drain-source voltage of the synchronous rectifier drops to the conduction threshold, the synchronous rectifier is turned on.
[0035] When the drain-source voltage of the synchronous rectifier rises to the turn-off threshold, the synchronous rectifier is turned off; and the turn-off threshold is adjusted based on the peak value of the drain-source voltage of the synchronous rectifier, wherein the turn-off threshold is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier.
[0036] Wherein, both the turn-on threshold and the turn-off threshold are negative values, and the turn-on threshold is less than the turn-off threshold.
[0037] Optionally, when the drain-source voltage of the synchronous rectifier rises to the turn-off threshold, the specific steps include: detecting the drain-source voltage of the synchronous rectifier; if the drain-source voltage of the synchronous rectifier is greater than the turn-off threshold, turning off the synchronous rectifier; and further detecting the peak value of the drain-source voltage of the synchronous rectifier.
[0038] If the peak detection signal is greater than the maximum set voltage, an adjustment amount is added to the shutdown threshold to update the shutdown threshold, which is limited to not less than the minimum shutdown voltage; if the peak detection signal is less than or equal to the maximum set voltage, the shutdown threshold is maintained.
[0039] The adjustment amount is a negative value.
[0040] Optionally, the adjustment amount is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier.
[0041] As described above, the flyback power supply, synchronous rectification control circuit, and method of the present invention have the following beneficial effects:
[0042] 1. The flyback power supply, synchronous rectification control circuit and method of the present invention automatically adjust the turn-off threshold according to the drain-source voltage of the synchronous rectifier tube, solves the voltage stress problem caused by common operation in continuous mode, and enables the system to work safely and efficiently in continuous mode; at the same time, it obtains the maximum conduction time in discontinuous mode, resulting in high system efficiency.
[0043] 2. The flyback power supply, synchronous rectification control circuit and method of the present invention reduce the delay requirements of the comparator and do not require specific processing of the primary side control drive, making them simple and easy to implement. Attached Figure Description
[0044] Figure 1 The diagram shows a schematic of a synchronous rectification control principle.
[0045] Figure 2 The diagram shown is a schematic representation of the synchronous rectification control circuit of the present invention.
[0046] Figure 3 The diagram shown is a flowchart of the synchronous rectification control method of the present invention.
[0047] Figure 4 The diagram shown is a structural schematic of the flyback power supply of the present invention.
[0048] Figure 5 The diagram shows a waveform representation of the synchronous rectification control circuit and method of the present invention.
[0049] Component designation explanation
[0050] 1. Synchronous Rectification Control Circuit
[0051] 11. Conductor control module
[0052] 12 Shutdown Control Module
[0053] 121 First Comparator
[0054] 122 peak detection units
[0055] 123 Shutdown Threshold Adjustment Unit
[0056] 12a Peak Sample and Hold Circuit
[0057] 12b voltage divider circuit
[0058] 12c Second Comparator
[0059] 12d Third Comparator
[0060] 12e logic circuits
[0061] 12f Negative Voltage Generating Circuit
[0062] 12g switch
[0063] 13 Drive signal generation module
[0064] 131 trigger
[0065] 132 drive units
[0066] 2. Flyback Topology Circuit
[0067] 21 Drive control circuit Detailed Implementation
[0068] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.
[0069] Please see Figures 1-5 It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.
[0070] The synchronous rectification circuit is located on the secondary side of the transformer and consists of a control circuit and a synchronous rectification MOSFET. It serves as an auxiliary circuit to the primary side, and the switching on and off of the synchronous rectification MOSFET is a passive process. Taking a flyback power supply using synchronous rectification technology as an example, the voltage detection synchronous rectification control technology aims to detect the drain-source voltage of the synchronous rectifier diode. Utilizing the characteristic that the forward voltage drop across the body diode of the synchronous rectifier diode is much greater than the forward voltage drop of the synchronous rectifier diode, during the turn-off process of the primary side power switch (when the drive signal DRV_Q1 of the power switch diode is low), the drain-source voltage of the primary side power switch diode rises, corresponding to a decrease in the drain-source voltage of the synchronous rectifier diode. Current Isd (i.e., -Ids) flows through the body diode of the synchronous rectifier diode, generating a forward voltage drop (which is negative). When the forward voltage drop is less than the conduction threshold... When Vth_on, the synchronous rectifier's turn-on signal DRV_Q2 is generated (high level). After the synchronous rectifier is turned on, its drain-source voltage drop equals the product of the on-current and the on-resistance, i.e., Vds_Q2 = -Ids*Rds(on). During the synchronous rectifier's conduction period, the current Isd continuously decreases, and the synchronous rectifier's drain-source voltage Vds_Q2 approaches 0V. During this period, an appropriate turn-off threshold Vth_off is set. When the synchronous rectifier's drain-source voltage Vds_Q2 approaches the fixed turn-off threshold Vth_off, a drive signal to turn off the synchronous rectifier is issued. The drive control circuit inevitably has logic delays and propagation delays, which can cause the synchronous rectifier's drive signal to lag. In continuous mode systems, this can lead to the simultaneous conduction of the power switch on the primary side and the synchronous rectifier on the secondary side. The turn-off comparator requires a high-performance comparator with extremely short propagation and turn-off delays. It is not well-suited for high-speed continuous-mode systems, such as those using mainstream third-generation semiconductors like GaN or superjunction silicon MOSFETs. Furthermore, the turn-off threshold is typically a negative voltage (for example, a few millivolts negative). In continuous mode, this leads to a short period of simultaneous conduction of the power switch and synchronous rectifier. The degree of this simultaneous conduction is highly dependent on the comparator speed, delay, and driver capability. It also necessitates the use of soft-drive circuitry on the primary-side controller or slowing down the turn-on speed of the primary-side power transistor. Even then, it still cannot adequately support relatively high-frequency continuous-mode systems. Reducing the turn-off threshold can avoid the problems caused by the simultaneous conduction of the power switch and synchronous rectifier, but this shortens the conduction time of the synchronous rectifier. In discontinuous or quasi-resonant operating modes, although the body diode does not need to conduct when the synchronous rectifier is turned off, premature turn-off still occurs, resulting in a loss of system efficiency. Figure 1 As shown.
[0071] To address the above problems, this invention proposes a synchronous rectification control circuit 1, which includes:
[0072] The module includes a conduction control module 11, a shutdown control module 12, and a drive signal generation module 13.
[0073] like Figure 2 As shown, the conduction control module 11 is used to generate the conduction control signal for the synchronous rectifier tube.
[0074] Specifically, in this embodiment, the turn-on control module 11 detects the drain-source voltage Vds of the synchronous rectifier. When the drain-source voltage Vds of the synchronous rectifier drops to the turn-on threshold Von_th, a turn-on control signal for the synchronous rectifier is generated. Since current flows through the body diode of the synchronous rectifier, the turn-on threshold Von_th is the opposite (negative) of the turn-on threshold of the synchronous rectifier. As an example, the turn-on control module 11 includes a comparison unit. The comparison unit receives the drain-source voltage Vds and the turn-on threshold Von_th of the synchronous rectifier, compares the drain-source voltage Vds and the turn-on threshold Von_th, and outputs the comparison result. In actual use, the turn-on timing of the synchronous rectifier can be determined according to the specific control strategy, which will not be elaborated here.
[0075] like Figure 2 As shown, the shutdown control module 12 detects the drain-source voltage Vds of the synchronous rectifier. When the drain-source voltage Vds of the synchronous rectifier rises to the shutdown threshold Voff_th, a shutdown control signal for the synchronous rectifier is generated. At the same time, the shutdown threshold Voff_th is adjusted based on the peak value Vds_max of the drain-source voltage of the synchronous rectifier. The shutdown threshold Voff_th is negatively correlated with the peak value Vds_max of the drain-source voltage of the synchronous rectifier.
[0076] Specifically, in this embodiment, the shutdown control module 12 includes a first comparator 121, a peak detection unit 122, and a shutdown threshold adjustment unit 123.
[0077] The first comparator 121 receives the drain-source voltage Vds of the synchronous rectifier and the turn-off threshold Voff_th, and outputs the first comparison result. In this example, the inverting input of the first comparator 121 is connected to the turn-off threshold Voff_th, and the non-inverting input is connected to the drain-source voltage Vds of the synchronous rectifier. When the drain-source voltage Vds of the synchronous rectifier is greater than the turn-off threshold Voff_th, it outputs a high level; when the drain-source voltage Vds of the synchronous rectifier is less than or equal to the turn-off threshold Voff_th, it outputs a low level. In actual use, the correspondence between the input signal and the input polarity can be adjusted as needed.
[0078] The peak detection unit 122 receives the drain-source voltage Vds of the synchronous rectifier and detects the peak value Vds_max of the drain-source voltage of the synchronous rectifier to obtain the peak detection signal Vds_max_det. As an example, the peak detection unit 122 includes a peak sample-and-hold circuit 12a and a voltage divider circuit 12b. The peak sample-and-hold circuit 12a receives the drain-source voltage Vds of the synchronous rectifier. When the drain-source voltage Vds of the synchronous rectifier is greater than or equal to the turn-off threshold Voff_th, it samples the peak value Vds_max of the drain-source voltage of the synchronous rectifier and resets when the synchronous rectifier is turned on. In this example, the enable terminal EN of the peak sample-and-hold circuit 12a is connected to the output terminal of the first comparator 121, and the reset terminal RST is connected to the drive signal generation module 13 (which obtains information on whether the synchronous rectifier is turned on from the drive signal generation module 13). In actual use, any structure that can realize the peak detection of the drain-source voltage of the synchronous rectifier is applicable to the present invention and is not limited to this embodiment. The voltage divider circuit 12b is connected to the output of the peak sampling and holding circuit 12a. It divides the peak value Vds_max of the drain-source voltage of the synchronous rectifier to obtain the peak detection signal Vds_max_det. In this example, the voltage divider circuit 12b is composed of a first resistor R1 and a second resistor R2 connected in series. Any circuit structure that can realize the voltage divider function is applicable to this invention, and will not be described in detail here.
[0079] The turn-off threshold adjustment unit 123 is connected to the output of the peak detection unit 122 and adjusts the turn-off threshold Voff_th based on the peak value Vds_max of the drain-source voltage of the synchronous rectifier. As an example, the turn-off threshold adjustment unit 123 includes a second comparator 12c, a third comparator 12d, a logic circuit 12e, a negative voltage generation circuit 12f, and a switch 12g. The second comparator 12c compares the peak detection signal Vds_max_det with the maximum set voltage Vds_max_set and outputs a second comparison result. In this example, the inverting input of the second comparator 12c is connected to the peak detection signal Vds_max_det, and the non-inverting input is connected to the maximum set voltage Vds_max_set. When the peak detection signal Vds_max_det is greater than the maximum set voltage Vds_max_set, it outputs a low level; when the peak detection signal Vds_max_det is less than or equal to the maximum set voltage Vds_max_set, it outputs a high level. The third comparator 12d compares the turn-off threshold Voff_th with the minimum turn-off voltage Voff_min and outputs the third comparison result. In this example, the inverting input of the third comparator 12d is connected to the minimum turn-off voltage Voff_min, and the non-inverting input is connected to the turn-off threshold Voff_th. It outputs a low level when the minimum turn-off voltage Voff_min is greater than the turn-off threshold Voff_th, and a high level when the minimum turn-off voltage Voff_min is less than or equal to the turn-off threshold Voff_th. In practical applications, the correspondence between the input signal and input polarity can be adjusted as needed to achieve the same function; this embodiment is not the only option. Logic circuit 12e is connected to the outputs of the second comparator 12c and the third comparator 12d. When the peak detection signal Vds_max_det is greater than the maximum set voltage Vds_max_set and the turn-off threshold Voff_th is greater than or equal to the minimum turn-off voltage Voff_min, a turn-on signal is generated. When the peak detection signal Vds_max_det is less than or equal to the maximum set voltage Vds_max_set, or the turn-off threshold Voff_th is less than the minimum turn-off voltage Voff_min, a turn-off signal is generated. In this example, logic circuit 12e includes an inverter NOT and an AND gate. The input of the inverter NOT is connected to the output of the second comparator 12c, and the input of the AND gate is connected to the output of the inverter NOT and the output of the third comparator 12d, respectively. The negative voltage generation circuit 12f is controlled by the peak detection signal Vds_max_det and is used to generate the adjustment amount V1 (negative value) of the turn-off threshold. The adjustment amount V1 is negatively correlated with the peak value Vds_max of the drain-source voltage of the synchronous rectifier (linear or non-linear change). That is, the larger the peak value Vds_max of the drain-source voltage of the synchronous rectifier, the smaller the adjustment amount V1 (the larger the absolute value).One end of switch 12g is connected to the output of negative voltage generation circuit 12f, and the other end is connected to the turn-off threshold Voff_th. Controlled by the output signal of logic circuit 12e, it adjusts the turn-off threshold Voff_th. Specifically, when switch 12g is on, the adjustment amount V1 is added to the original turn-off threshold Voff_th, causing Voff_th to decrease. When switch 12g is on, the turn-off threshold Voff_th retains its original value. The turn-off threshold Voff_th is negative, and its initial value is greater than the on-th threshold Von_th.
[0080] It should be noted that the implementation of the turn-off threshold adjustment unit 123 is not limited to this embodiment, and any circuit structure that can adjust the turn-off threshold Voff_th based on the peak detection signal Vds_max_det is applicable.
[0081] like Figure 2 As shown, the drive signal generation module 13 is connected to the output terminals of the turn-on control module 11 and the turn-off control module 12, and generates the drive signal DRV_SR of the synchronous rectifier based on the turn-on control signal and the turn-off control signal.
[0082] Specifically, when the turn-on control signal is valid, the drive signal DRV_SR of the synchronous rectifier is set to the control level corresponding to the synchronous rectifier being turned on; when the turn-off control signal is valid, the drive signal DRV_SR of the synchronous rectifier is set to the control level corresponding to the synchronous rectifier being turned off. As an example, the drive signal generation module 13 includes a trigger 131 and a drive unit 132. The trigger 131 is connected to the output terminals of the turn-on control module 11 and the turn-off control module 12, generating a switch control signal based on the turn-on and turn-off control signals. As an example, the trigger 131 is implemented using an RS trigger, with the set terminal S connected to the output terminal of the turn-on control module 11, the reset terminal R connected to the output terminal of the turn-off control module 12, and the output terminal Q outputting the switch control signal. The drive unit 132 is connected to the output terminal of the trigger 131, driving the synchronous rectifier based on the switch control signal.
[0083] It should be noted that the implementation of the drive signal generation module 13 is not limited, as long as it can generate the drive signal DRV_SR of the synchronous rectifier based on the turn-on control signal and the turn-off control signal.
[0084] like Figure 3 As shown, the present invention also provides a synchronous rectification control method. In this embodiment, the method can be implemented based on the synchronous rectification control circuit 1 of the present invention; in practical use, any hardware circuit or software module that can implement this method is applicable. The method includes:
[0085] When the drain-source voltage of the synchronous rectifier drops to the turn-on threshold, the synchronous rectifier is turned on.
[0086] Specifically, in this embodiment, when the synchronous rectifier is turned off, when an inverting current flows through the body diode of the synchronous rectifier, the drain-source voltage of the synchronous rectifier drops to a negative value. When the drain-source voltage of the synchronous rectifier reaches the conduction threshold, the synchronous rectifier is controlled to conduct.
[0087] When the drain-source voltage of the synchronous rectifier rises to the turn-off threshold, the synchronous rectifier is turned off; and the turn-off threshold is adjusted based on the peak value of the drain-source voltage of the synchronous rectifier, and the turn-off threshold is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier; wherein, the turn-off threshold is negative and the turn-on threshold is less than the turn-off threshold.
[0088] Specifically, as the reverse current flowing through the synchronous rectifier tube decreases, the drain-source voltage of the synchronous rectifier tube continuously increases (to a negative value). When the turn-off threshold is reached, the synchronous rectifier tube is turned off.
[0089] Specifically, such as Figure 3 As shown, after the synchronous rectifier is turned off, the peak value of the drain-source voltage of the synchronous rectifier is further detected. If the peak detection signal Vds_max_det is less than or equal to the maximum set voltage Vds_max_set, the turn-off threshold is maintained. If the peak detection signal Vds_max_det is greater than the maximum set voltage Vds_max_set, an adjustment amount is added to the turn-off threshold to update the turn-off threshold, i.e., Voff_th(n) = Voff_th(n-1) + V1, where n is a positive integer greater than or equal to 1, and V1 is the adjustment amount (negative value). Simultaneously, this invention limits the turn-off threshold Voff_th to not less than the minimum turn-off voltage Voff_min. That is, when the updated turn-off threshold Voff_th(n) is greater than or equal to the minimum turn-off voltage Voff_min, the updated turn-off threshold is used as the turn-off threshold for the next cycle; when the updated turn-off threshold Voff_th(n) is less than the minimum turn-off voltage Voff_min, the minimum turn-off voltage Voff_min is used as the turn-off threshold for the next cycle.
[0090] More specifically, as an example, the adjustment amount V1 is a fixed value. As another example, the adjustment amount V1 is a voltage negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier, that is, the larger the peak value of the drain-source voltage of the synchronous rectifier, the more negative the adjustment amount V1 is, and the larger its absolute value is; the adjustment amount V1 is linearly or non-linearly correlated with the peak value of the drain-source voltage of the synchronous rectifier.
[0091] like Figure 4 As shown, the present invention also provides a flyback power supply, which includes: a flyback topology circuit 2 and the synchronous rectification control circuit 1 of the present invention.
[0092] like Figure 4As shown, in this embodiment, the flyback topology circuit 2 includes a transformer, a power switch PRI, a sampling resistor Rcs, a drive control circuit 21, a synchronous rectifier SR, and an output capacitor C. One end of the primary winding of the transformer is connected to a DC voltage VDC, and the other end is grounded via the power switch PRI and the sampling resistor Rcs. The power switch PRI is controlled by the drive control circuit 21; as an example, the drive control circuit 21 is a PWM controller 21, which generates a control signal DRV_PRI to turn the power switch PRI on or off based on the sampled signal on the sampling resistor Rcs. One end of the secondary winding of the transformer is connected to the non-inverting output terminal V+, and the other end is connected to the inverting output terminal V- via the synchronous rectifier SR (the source of the synchronous rectifier SR is connected to the inverting output terminal, and the drain is connected to the secondary winding of the transformer). The output capacitor C is connected between the non-inverting output terminal V+ and the inverting output terminal V-.
[0093] like Figure 4 As shown, the synchronous rectification control circuit 1 provides the drive signal DRV_SR to the synchronous rectifier SR in the flyback topology circuit 2. By detecting the drain-source voltage on the synchronous rectifier, the turn-off threshold is adjusted. The higher the drain-source voltage of the synchronous rectifier, the earlier it turns off, thus avoiding the problem of the power switch PRI and the synchronous rectifier SR being on simultaneously. At the same time, it achieves the maximum conduction time in discontinuous mode, reducing the speed requirements of the comparator and the turn-off capability requirements of the drive in the control circuit. The structure of the synchronous rectification control circuit 1 is described above and will not be repeated here.
[0094] It should be noted that the synchronous rectification circuit of the present invention can be applied to fields including but not limited to power adapters, LED drivers, ballasts, PC power supplies, etc., especially in applications requiring high efficiency and low voltage and high current; it is not limited to this embodiment.
[0095] The working principle and synchronous rectification control method of the synchronous rectification control circuit of the present invention are explained based on the flyback power supply, such as... Figure 5 As shown, the drive control circuit 21 generates a turn-on signal for the power switch PRI (DRV_PRI is high), turning on the power switch PRI. The voltage at the same terminal (point P) of the primary winding of the transformer is approximately zero. At this time, point P generates a negative voltage relative to the DC voltage VDC, and the same terminal (point S) of the secondary winding of the transformer is also negative. The body diode of the synchronous rectifier SR cannot conduct, and energy is stored in the primary winding of the transformer. When the drive control circuit 21 generates a turn-off signal for the power switch PRI (DRV_PRI becomes low), the voltage at point P begins to rise, and a positive voltage is generated at point P relative to the DC voltage VDC. That is, the voltage at the same terminal (point S) of the secondary side becomes positive, the body diode of the synchronous rectifier SR conducts, and the energy stored in the transformer during the conduction of the power switch PRI begins to be released to the secondary side. This process is called flyback.
[0096] When the body diode of the secondary-side synchronous rectifier SR is turned on, due to the direction of current flow, the voltage at the drain (cathode of the body diode) of the synchronous rectifier SR is lower than that at the source (anode of the body diode). The synchronous rectification control circuit 1 detects the drain-source voltage of the synchronous rectifier SR. When the drain-source voltage Vds of the synchronous rectifier decreases to the turn-on threshold Von_th, a turn-on signal for the synchronous rectifier SR is generated. During the turn-on process, the current flowing through the synchronous rectifier SR gradually decreases, and the drain-source voltage of the synchronous rectifier SR gradually increases. When the drain-source voltage Vds of the synchronous rectifier increases to the turn-off threshold Von_off, a turn-off signal for the synchronous rectifier SR is generated.
[0097] It should be noted that the control logic of the power switch PRI differs slightly for different operating modes, which will not be detailed here. As an example, in continuous operation mode, when the switching cycle time of the internal oscillator of the drive control circuit 21 reaches a preset duration, a high-level signal (DRV_PRI is high) is generated to turn on the power switch PRI; when the voltage generated on the sampling resistor Rcs by the current flowing through the power switch PRI reaches a certain set value (controlled by the feedback from the drive control circuit 21 and the system), the drive control circuit 21 generates a turn-off signal for the power switch PRI (DRV_PRI becomes low); this cycle repeats. In intermittent operation mode, the drive control circuit 21 turns on the power switch PRI based on the switching cycle of the internal oscillator (same as in continuous operation mode); the timing of the drive control circuit 21 generating the turn-off signal for the power switch PRI depends on the turn-off management mechanism of the drive control circuit 21, which will not be specified here.
[0098] Furthermore, for the synchronous rectification section, during the first turn-on cycle n0, the synchronous rectifier SR is turned off using the initially set turn-off threshold Voff_th(1). Subsequently, during the turn-off period of the synchronous rectifier SR, if the peak value Vds_max of the drain-source voltage of the synchronous rectifier exceeds the maximum set value (at this time, Vds_max_det is greater than Vds_max_set), an adjustment amount V1 is superimposed on the turn-off threshold Voff_th(1), making the updated turn-off threshold Voff_th(2) less than the turn-off threshold Voff_th(1) of the previous cycle. During the second turn-on cycle n1, the synchronous rectifier SR is turned off based on the updated turn-off threshold Voff_th(2). The turn-off threshold is advanced, generating a freewheeling time t1 for the turn-off body diode. The duration of t1 is proportional to the drain-source voltage Vds of the synchronous rectifier. During the turn-off period of the synchronous rectifier SR after the second turn-on cycle n1, the drain-source voltage Vds of the synchronous rectifier is still higher than the maximum set value. In the third turn-on cycle n2, the turn-off threshold is adjusted again by an additional amount V1 based on the previous turn-on cycle. The corresponding body diode freewheeling time is t2, so the body diode freewheeling time in the third turn-on cycle n2 is t1 + t2. Since the voltage at t2 is lower than the voltage at t1 (i.e., the drain-source voltage Vds of the synchronous rectifier decreases, and the adjustment amount V1 increases), t2 is less than t1. During the turn-off period of the next synchronous rectifier SR, if the drain-source voltage Vds of the synchronous rectifier is detected to be lower than the maximum set value, then the turn-off threshold in the fourth turn-on cycle n4 remains the same as the previous turn-on cycle, achieving closed-loop adjustment of the turn-off threshold Voff_th. If, after multiple cycles of adjustment, the turn-off threshold has reached the minimum turn-off voltage Voff_min, then the turn-off threshold will be set to the minimum turn-off voltage Voff_min.
[0099] In summary, this invention provides a flyback power supply, a synchronous rectification control circuit, and a method, comprising: a turn-on control module, a turn-off control module, and a drive signal generation module; the turn-on control module generates a turn-on control signal for the synchronous rectifier; the turn-off control module detects the drain-source voltage of the synchronous rectifier, and generates a turn-off control signal for the synchronous rectifier when the drain-source voltage rises to a turn-off threshold; simultaneously, the turn-off threshold is adjusted based on the peak value of the drain-source voltage of the synchronous rectifier, and the turn-off threshold is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier; the drive signal generation module is connected to the output terminals of the turn-on control module and the turn-off control module, and generates a drive signal for the synchronous rectifier based on the turn-on control signal and the turn-off control signal; wherein, the turn-off threshold is a negative value. The flyback power supply, synchronous rectification control circuit, and method of this invention automatically adjust the turn-off threshold based on the drain-source voltage of the synchronous rectifier diode, solving the voltage stress problem caused by common current in continuous mode, enabling the system to operate safely and efficiently in continuous mode; simultaneously, it achieves maximum conduction time in discontinuous mode, resulting in high system efficiency; and it reduces the comparator delay requirements and eliminates the need for specific processing of the primary-side control drive, making it simple and easy to implement. Therefore, this invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.
[0100] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.
Claims
1. A synchronous rectification control circuit, characterized in that, The synchronous rectification control circuit includes at least: The module includes an on / off control module, an off / return control module, and a drive signal generation module. The conduction control module is used to generate the conduction control signal for the synchronous rectifier tube; The shutdown control module detects the drain-source voltage of the synchronous rectifier. When the drain-source voltage of the synchronous rectifier rises to the shutdown threshold, it generates a shutdown control signal for the synchronous rectifier. At the same time, it adjusts the shutdown threshold based on the peak value of the drain-source voltage of the synchronous rectifier. The shutdown threshold is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier. The drive signal generation module is connected to the output terminals of the turn-on control module and the turn-off control module, and generates the drive signal for the synchronous rectifier tube based on the turn-on control signal and the turn-off control signal. The shutdown threshold is a negative value.
2. The synchronous rectification control circuit according to claim 1, characterized in that: The conduction control module includes a comparison unit, which receives the drain-source voltage of the synchronous rectifier and a conduction threshold. When the drain-source voltage of the synchronous rectifier drops to the conduction threshold, the conduction control signal is generated. The conduction threshold is negative and less than the initial value of the shutdown threshold.
3. The synchronous rectification control circuit according to claim 1 or 2, characterized in that: The shutdown control module includes a first comparator, a peak detection unit, and a shutdown threshold adjustment unit. The first comparator receives the drain-source voltage of the synchronous rectifier and the turn-off threshold, and outputs a first comparison result; The peak detection unit receives the drain-source voltage of the synchronous rectifier tube, detects the peak value of the drain-source voltage of the synchronous rectifier tube, and obtains a peak detection signal. The shutdown threshold adjustment unit is connected to the output of the peak detection unit and adjusts the shutdown threshold based on the peak value of the drain-source voltage of the synchronous rectifier.
4. The synchronous rectification control circuit according to claim 3, characterized in that: The peak detection unit includes a peak sampling and holding circuit and a voltage divider circuit; The peak sampling and holding circuit receives the drain-source voltage of the synchronous rectifier. When the drain-source voltage of the synchronous rectifier is greater than the turn-off threshold, it samples the peak value of the drain-source voltage of the synchronous rectifier and resets when the synchronous rectifier is turned on. The voltage divider circuit is connected to the output of the peak sample-and-hold circuit, and the peak detection signal is obtained by dividing the peak value of the drain-source voltage of the synchronous rectifier.
5. The synchronous rectification control circuit according to claim 3, characterized in that: The shutdown threshold adjustment unit includes a second comparator, a third comparator, a logic circuit, a negative voltage generation circuit, and a switch. The second comparator compares the peak detection signal with the maximum set voltage and outputs a second comparison result; The third comparator compares the turn-off threshold with the minimum turn-off voltage and outputs a third comparison result; The logic circuit is connected to the output terminals of the second comparator and the third comparator; when the peak detection signal is greater than the maximum set voltage and the turn-off threshold is greater than or equal to the minimum turn-off voltage, a turn-on signal is generated; when the peak detection signal is less than or equal to the maximum set voltage or the turn-off threshold is less than the minimum turn-off voltage, a turn-off signal is generated. The negative voltage generating circuit is controlled by the peak detection signal to generate an adjustment amount for the turn-off threshold, and the adjustment amount is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier tube, and the adjustment amount is negative. One end of the switch is connected to the output of the negative voltage generating circuit, and the other end is connected to the shutdown threshold. It is controlled by the output signal of the logic circuit and is used to adjust the shutdown threshold.
6. The synchronous rectification control circuit according to claim 1, characterized in that: The drive signal generation module includes a trigger and a drive unit; The trigger is connected to the output terminals of the turn-on control module and the turn-off control module, and generates a switch control signal based on the turn-on control signal and the turn-off control signal; The driving unit is connected to the output of the trigger and drives the synchronous rectifier tube based on the switch control signal.
7. A flyback power supply, characterized in that, The flyback power supply includes at least: a flyback topology circuit and a synchronous rectification control circuit as described in any one of claims 1-6; The synchronous rectification control circuit provides a drive signal for the synchronous rectifier diodes in the flyback topology circuit.
8. A synchronous rectification control method, characterized in that, The synchronous rectification control method includes: When the drain-source voltage of the synchronous rectifier drops to the conduction threshold, the synchronous rectifier is turned on. When the drain-source voltage of the synchronous rectifier rises to the turn-off threshold, the synchronous rectifier is turned off; and the turn-off threshold is adjusted based on the peak value of the drain-source voltage of the synchronous rectifier, wherein the turn-off threshold is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier. Wherein, both the turn-on threshold and the turn-off threshold are negative values, and the turn-on threshold is less than the turn-off threshold.
9. The synchronous rectification control method according to claim 8, characterized in that: When the drain-source voltage of the synchronous rectifier rises to the turn-off threshold, the specific steps include: detecting the drain-source voltage of the synchronous rectifier; if the drain-source voltage of the synchronous rectifier is greater than the turn-off threshold, turning off the synchronous rectifier; and further detecting the peak value of the drain-source voltage of the synchronous rectifier. If the peak detection signal is greater than the maximum set voltage, an adjustment amount is added to the shutdown threshold to update the shutdown threshold, which is limited to not less than the minimum shutdown voltage; if the peak detection signal is less than or equal to the maximum set voltage, the shutdown threshold is maintained. The adjustment amount is a negative value.
10. The synchronous rectification control method according to claim 8, characterized in that: The adjustment amount is negatively correlated with the peak value of the drain-source voltage of the synchronous rectifier.