Synchronous rectifier tube control circuit and switching power supply

Abstract

The invention provides a synchronous rectifier tube control circuit and a switching power supply, relates to the technical field of switching power supplies, and provides three embodiments including a first module and / or a third module and a second module, the problem that the synchronous rectifier tube is easy to be mistakenly conducted is solved through the first module and / or the third module, and the detection efficiency is ensured; and the second module is high in detection stability and accuracy, so that when the first module and / or the third module are / is failed in detection, the second module can solve the problem that the synchronous rectification tube is easy to be mistakenly conducted, the detection reliability is ensured, and the purposes of accurately controlling the conduction of the synchronous rectification switch tube and improving the power supply efficiency are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of switching power supplies, and in particular to a synchronous rectifier control circuit and a switching power supply. Background Art

[0002] In the secondary side synchronous rectification scheme of the switching power supply, the on and off of the synchronous rectifier is usually controlled according to the drain-source voltage (Vds) of the synchronous rectifier (specifically MOSFET tube). However, in this control method, the synchronous rectifier is often mis-turned on due to voltage ringing.

[0003] In the prior art, many detection schemes have been proposed to solve the problem of synchronous rectifier mis-conduction. However, the existing detection schemes are still not ideal, and there may be detection failures, resulting in the synchronous rectifier being unable to conduct normally, causing power supply efficiency loss. Summary of the invention

[0004] The embodiments of the present invention provide a synchronous rectifier control circuit and a switching power supply to overcome the above technical problems, achieve accurate control of the conduction of the synchronous rectifier switch tube, and ensure the power supply efficiency.

[0005] In order to achieve the above-mentioned object of the invention, the first embodiment of the present invention discloses a synchronous rectifier control circuit, including:

[0006] The first module detects the falling rate of the drain-source voltage of the synchronous rectifier tube based on the falling rate threshold value to output a first detection signal;

[0007] The second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage for a first time period, and output a second detection signal;

[0008] A logic module, based on the first detection signal or the second detection signal, outputs a conduction control signal to control the conduction of the synchronous rectifier;

[0009] Among them, the detection time of the second module is longer than the detection time of the first module.

[0010] In order to achieve the above-mentioned object of the invention, a second embodiment of the present invention provides a synchronous rectifier control circuit, comprising:

[0011] The first module detects the falling rate of the drain-source voltage of the synchronous rectifier tube based on the falling rate threshold value to output a first detection signal;

[0012] The second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage for a first time period, and output a second detection signal;

[0013] A third module is used to output a third detection signal of a valid level if it is detected that the sampling voltage VDS decreases below a fifth threshold voltage after the sampling voltage VDS continues to be greater than the fourth threshold voltage for a third period of time;

[0014] A logic module, based on the first detection signal, the second detection signal or the third detection signal, outputs a conduction control signal to control the conduction of the synchronous rectifier tube;

[0015] The detection duration of the second module is greater than the detection duration of the first module and / or the detection duration of the third module.

[0016] Optionally, the falling rate is characterized by the falling time length of the sampling voltage VDS of the drain-source voltage from a third threshold voltage to a second threshold voltage, and the falling rate threshold is characterized by the second time length; when the first module detects that the falling time length of the sampling voltage VDS from the third threshold voltage to the second threshold voltage is less than or equal to the second time length, the first detection signal with a valid level is output; wherein, the third threshold voltage>the first threshold voltage.

[0017] Optionally, the first threshold voltage and the second threshold voltage are both negative voltages.

[0018] The first threshold voltage is lower than the second threshold voltage.

[0019] Optionally, the first duration is greater than the second duration.

[0020] Optionally, the drop rate is characterized by a drop amplitude of a sampled voltage VDS of the drain-source voltage within a fixed time period, and the drop rate threshold is characterized by a drop amplitude threshold;

[0021] The first module outputs a first detection signal of a valid level when detecting that the drop amplitude of the sampling voltage VDS within a fixed time period is greater than a drop amplitude threshold.

[0022] In order to achieve the above-mentioned object of the invention, a third embodiment of the present invention provides a synchronous rectifier control circuit, comprising:

[0023] The second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage for a first time period, and output a second detection signal;

[0024] A third module is used to output a third detection signal of a valid level if it is detected that the sampling voltage VDS decreases below a fifth threshold voltage after the sampling voltage VDS continues to be greater than the fourth threshold voltage for a third period of time;

[0025] The logic module outputs a conduction control signal based on the second detection signal or the third detection signal to control the conduction of the synchronous rectifier tube.

[0026] Among them, the detection time of the second module is longer than the detection time of the third module.

[0027] Optionally, the fifth threshold voltage and the second threshold voltage are both negative voltages.

[0028] The fifth threshold voltage is equal to or lower than the second threshold voltage.

[0029] Furthermore, the second module includes:

[0030] A second comparison submodule, used for comparing the sampling voltage VDS with the first threshold voltage, and outputting a second comparison signal;

[0031] The second timer is connected to the output end of the second comparison submodule, and is used to start timing when the level of the second comparison signal represents that the sampling voltage VDS is less than the first threshold voltage, and output a second detection signal with a valid level after timing for a first time period.

[0032] The embodiment of the present invention further discloses a switching power supply, comprising:

[0033] A primary winding, a primary switch tube connected to the primary winding, a secondary winding, a synchronous rectifier tube connected to the secondary winding, and a synchronous rectifier tube control circuit as described in an embodiment of the present invention.

[0034] The embodiments of the present invention include the following advantages:

[0035] The synchronous rectifier control circuit provided by the present invention provides three embodiments including a first module and / or a third module and a second module, wherein the first module and / or the third module is used to solve the problem that the synchronous rectifier is easily mis-conducted, thereby ensuring detection efficiency; and the second module has high detection stability and accuracy. When the first module and / or the third module fails to detect, the second module can solve the problem that the synchronous rectifier is easily mis-conducted, thereby ensuring detection reliability, achieving the purpose of accurately controlling the conduction of the synchronous rectifier switch tube and ensuring power supply efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention.

[0037] Figure 1 is a schematic diagram of the structure of a switching power supply provided by an embodiment of the present invention;

[0038] Figure 2 is a schematic diagram of the principle of the first embodiment of the present invention;

[0039] Figure 3is a schematic structural diagram of a first embodiment of the present invention;

[0040] Figure 4 is a schematic diagram of the principle of the second embodiment of the present invention;

[0041] Figure 5 is a schematic structural diagram of a second embodiment of the present invention;

[0042] Figure 6 is a schematic diagram of the principle of the third embodiment of the present invention;

[0043] Figure 7 4 is a schematic structural diagram of a fourth embodiment of the present invention. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0045] In the following, the terms "second", "first", etc. are used only for convenience of description and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "second", "first", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0046] In the embodiments of the present invention, unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium. In addition, the term "coupled" can be a direct electrical connection or an indirect electrical connection through an intermediate medium. The term "contact" can be a direct contact or an indirect contact through an intermediate medium.

[0047] In the embodiment of the present invention, "and / or" describes the association relationship of the associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist at the same time, and B exists alone, where A and B may be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0048] Figure 1 The schematic diagram is a structural diagram of a switching power supply, which includes a primary winding, a primary switch tube M1 connected to the primary winding, a secondary winding, a synchronous rectifier tube M2 connected to the secondary winding, and a synchronous rectifier tube control circuit.

[0049] by Figure 1Taking the switching power supply structure shown in FIG. 1 as an example, in the secondary synchronous rectification scheme, the on and off of the synchronous rectifier M2 is usually controlled according to the drain-source voltage (Vds) of the synchronous rectifier M2 (specifically, a MOSFET tube). However, this control method often causes the problem of mis-conduction of the synchronous rectifier M2 due to the voltage ringing generated after the synchronous rectifier M2 is turned off. If the primary switch tube M1 and the synchronous rectifier M2 are turned on at the same time, additional chip loss may be generated at best, and the chip may be burned at worst.

[0050] In the prior art, many detection schemes are proposed to solve the problem of mis-conduction of the synchronous rectifier tube M2. However, the existing detection schemes are still not ideal, and there may be detection failures, resulting in the synchronous rectifier tube being unable to conduct normally, causing power supply efficiency loss.

[0051] To solve the above problems, the first embodiment of the present invention provides a synchronous rectifier control circuit, such as Figure 2 As shown, the control circuit includes a first module, a second module and a logic module.

[0052] The first module detects the falling rate of the drain-source voltage of the synchronous rectifier M2 based on the falling rate threshold to output a first detection signal ctrl1; the second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage VTH1 for a first time ("reach" can be understood as greater than or equal to) a first time length, and outputs a second detection signal ctrl2; the logic module outputs a conduction control signal TON based on the first detection signal ctrl1 or the second detection signal ctrl2 to control the conduction of the synchronous rectifier M2; wherein, the detection time length T2 of the second module is greater than the detection time length T1 of the first module.

[0053] The detection principle of the first module: when the primary switch tube M1 is turned off, the rate of decrease of the drain-source voltage of the synchronous rectifier tube M2 is very large, while the rate of decrease of the drain-source voltage of the synchronous rectifier tube M2 caused by ringing is relatively small. The present invention utilizes such a feature to determine whether the decrease of the synchronous rectifier tube M2 is caused by the turn-off of the primary switch tube or by the ringing, and then, when the drain-source voltage of the synchronous rectifier tube M2 decreases based on the turn-off of the primary switch tube M1, the first detection signal ctrl1 with an effective level is output, so as to solve the problem of mis-conduction of the synchronous rectifier tube M2.

[0054] The detection principle of the second module: if the sampling voltage VDS of the drain-source voltage of the synchronous rectifier tube M2 drops to less than the first threshold voltage VTH1 (in some of the following descriptions, the first threshold voltage VTH1 is represented by VTH1), the synchronous rectifier tube M2 can be turned on. Then the duration of the sampling voltage VDS dropping to VTH1 due to ringing will not be that long; if it is caused by the shutdown of the primary switch tube M1, the duration of the sampling voltage VDS dropping to VTH1 will be longer. The embodiment of the present invention uses the first duration as the detection threshold, and by detecting whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold for the first duration, it can effectively distinguish between the shutdown and ringing of the primary switch tube M1, and then when the drain-source voltage of the synchronous rectifier tube M2 drops based on the shutdown of the primary switch tube M1, the second detection signal ctrl2 of the effective level is output to control the conduction of the synchronous rectifier tube M2.

[0055] In an embodiment of the present invention, two solutions are provided to solve the misleading turn-on of the synchronous rectifier tube M2. The logic module can output the turn-on control signal TON based on the first detection signal ctrl1 or the second detection signal ctrl2 to control the synchronous rectifier tube M2 to turn on. Among them, the detection time length T2 of the second module is greater than the detection time length T1 of the first module. Compared with the drop rate detection scheme using only the first module, the misleading turn-on solution provided by the second module can be effectively applied when the detection of the first module fails (such as in light load or light-heavy load mode, the drop rate of the source-drain voltage of the synchronous rectifier tube M2 may be very slow, and the first module may not be able to effectively distinguish whether the drop of the drain-source voltage of the synchronous rectifier tube M2 is caused by the shutdown of the primary switch tube M1 or the ringing), that is, when the first module is interfered with and there is a leakage detection, so as to control the synchronous rectifier tube M2 to be normally and effectively turned on when the primary switch tube M1 is turned off, thereby improving the reliability of the synchronous rectification control and ensuring the power efficiency of the switching power supply.

[0056] In the embodiment of the present invention, the detection time T2 of the second module is greater than the detection time T1 of the first module, which can be understood as the detection speed of the first module is greater than the detection speed of the second module. Or it can be understood that in the case where both the first module and the second module can effectively detect (such as non-light load or light-heavy load mode), compared with the second module, the first module can give priority to outputting the detection result of "whether the drop in the drain-source voltage of the synchronous rectifier tube M2 is caused by the shutdown of the primary switch tube M1 or the ringing", and then control or not control the conduction of the synchronous rectifier tube M2. Or it can be understood more simply that the solution provided by the second module to solve the problem of mis-conduction of the synchronous rectifier tube M2 is actually a patch solution for the first module: when the first module can effectively detect, the conduction of the synchronous rectifier tube M2 is actually determined by the first detection signal ctrl1 output by the first module to ensure the detection efficiency; and when the first module cannot effectively detect (i.e., there is a leakage detection problem), the conduction of the synchronous rectifier tube M2 is actually determined by the second detection signal ctrl2 output by the second module. Since the detection of the second module is longer, it is not easily interfered with, and its detection stability and accuracy are high, so that it can prevent the synchronous rectifier tube M2 from mis-conduction due to ringing when the synchronous rectifier tube M2 is turned off, and can control the synchronous rectifier tube M2 to be normally and effectively turned on when the first module fails, so as to ensure the power supply efficiency. It can be considered that the synchronous rectifier tube control circuit provided by the present invention has both detection efficiency and detection reliability in solving the problem that the synchronous rectifier tube M2 is easily mis-conducted.

[0057] Among them, reference Figure 3 , the first module can be implemented in the following ways:

[0058] In one embodiment, the falling rate is characterized by the falling time of the sampling voltage VDS of the drain-source voltage of the synchronous rectifier M2 from the third threshold voltage VTH3 to the second threshold voltage VTH2, and the falling rate threshold is characterized by the second time. When the first module detects that the falling time of the sampling voltage VDS from the third threshold voltage VTH3 to the second threshold voltage VTH2 is less than or equal to the second time, the first detection signal ctrl1 of the effective level is output. This embodiment adopts the principle of fixing the falling amplitude of the sampling voltage VDS (from the third threshold voltage VTH3 to the second threshold voltage VTH2) to detect the falling rate of the drain-source voltage of the synchronous rectifier M2 by detecting the falling time corresponding to the fixed falling amplitude. Generally, the falling time caused by ringing is several times the falling time caused by the turning off of the primary switch tube M1, that is, in the ringing stage, the falling time of VDS from the third threshold voltage VTH3 to the second threshold voltage VTH2 is longer, and when the primary switch tube M1 is turned off, the falling time of VDS from the third threshold voltage VTH3 to the second threshold voltage VTH2 is shorter. By setting a second time as the falling speed threshold and comparing the second time with the falling time, it can be determined whether the falling of the synchronous rectifier tube M2 is caused by the turning off of the primary switch tube or by the ringing, and then when the drain-source voltage of the synchronous rectifier tube M2 drops based on the turning off of the primary switch tube M1, the first detection signal ctrl1 with a valid level is output to control the normal conduction of the synchronous rectifier tube M2.

[0059] In this embodiment, the third threshold voltage VTH3>the first threshold voltage VTH1.

[0060] In this embodiment, the first threshold voltage VTH1 and the second threshold voltage VTH2 are both negative voltages. By setting the first threshold voltage VTH1 lower than the second threshold voltage VTH2, the second module can play a patch role. For example, the first threshold voltage VTH1 is -200mv, and the second threshold voltage VTH2 is -75mv. The present invention can design the first threshold voltage VTH1 to be lower than or much lower than the second threshold voltage VTH2. The advantage is that it increases the difficulty of the second module as a patch solution to drive the synchronous rectifier tube M2 to turn on (also called turn on), that is, it makes it more difficult for the patch solution to meet the turn-on conditions, and meets the characteristics of the synchronous rectification solution that allows the synchronous rectifier tube M2 to be turned off, but cannot mis-turn on the synchronous rectifier tube M2.

[0061] In this embodiment, the first duration may be set greater than the second duration to extend the detection duration T2 of the second module so that the second module can function as a patch. When the first duration is set greater than the second duration, the first threshold voltage VTH1 may also be equal to the second threshold voltage VTH2.

[0062] Of course, in this embodiment, the first threshold voltage VTH1 can be set to be lower than the second threshold voltage VTH2, and the first time length can be set to be greater than the second time length, so as to fully reflect the patching function of the second module.

[0063] In actual applications, the present invention does not limit the specific method used to achieve the purpose of making the detection time T2 of the second module longer than the detection time T1 of the first module.

[0064] In another embodiment (not shown), the rate of decrease is characterized by the decrease amplitude of the sampled voltage VDS of the drain-source voltage of the synchronous rectifier M2 within a fixed time period, and the rate of decrease threshold is characterized by the decrease amplitude threshold. Among them, when the first module detects that the decrease amplitude of the sampled voltage VDS within a fixed time period is greater than the decrease amplitude threshold, it outputs a first detection signal ctrl1 of a valid level. This embodiment adopts the principle of detecting the decrease amplitude of the sampled voltage VDS within a fixed time period to detect the rate of decrease of the drain-source voltage of the synchronous rectifier M2. In general, the decrease amplitude caused by the turning off of the primary switch tube M1 is several times the decrease amplitude caused by the ringing, that is, the decrease amplitude of VDS within the fixed time period is small during the ringing stage, and the decrease amplitude of VDS within the fixed time period is large when the primary switch tube M1 is turned off. By setting a drop amplitude threshold as a drop speed threshold, and comparing the drop amplitude of VDS within a fixed time with the drop amplitude threshold, it is possible to determine whether the drop of the synchronous rectifier tube M2 is caused by the shutdown of the primary switch tube M1 or by ringing, and then, when the drain-source voltage of the synchronous rectifier tube M2 drops based on the shutdown of the primary switch tube M1, the first detection signal ctrl1 of the effective level is output to normally turn on the synchronous rectifier tube M2. In this embodiment, the patching effect of the second module can be reflected by setting the first time length to be greater than the fixed time length.

[0065] Figure 3This is an example of an implementation of the first module. The first module can be implemented by a first comparison submodule and a first judgment submodule. The first comparison submodule is used to compare the voltages of the first input terminal and the second input terminal, and output a first comparison signal CMP1, wherein the first input terminal receives the sampling voltage VDS, and the second input terminal receives the third threshold voltage VTH3 and the second threshold voltage VTH2 in time-sharing; the first judgment submodule is used to determine whether the duration of the sampling voltage VDS falling from the third threshold voltage VTH3 to the second threshold voltage VTH2 is less than or equal to the second duration according to the first comparison signal CMP1, and output a first detection signal ctrl1. In this example, by switching the voltage value received by the second input terminal of the first comparison submodule to VTH3 or VTH2 in time-sharing, the first comparison submodule can be reused in the VDS falling slope detection, thereby reducing the chip area and reducing power consumption. The time-sharing switching means can refer to the relevant prior art, which will not be elaborated here. Further, the first judgment submodule may include: a first judgment unit (not shown in the figure) and a second judgment unit (not shown in the figure); the first judgment unit judges whether VDS is less than the third threshold voltage VTH3 based on the first comparison signal CMP1, and outputs a first judgment signal; the second judgment unit judges whether the sampling voltage VDS drops to the second threshold voltage VTH2 within the second time period based on the first judgment signal and the first comparison signal CMP1, and outputs a first detection signal ctrl1. The first judgment unit is implemented by a trigger, and the second judgment unit can be implemented by a trigger and a first timer.

[0066] It should be noted that, based on the different implementations of the first module given in the embodiments of the present invention, the implementation logic corresponding to the corresponding implementation can be implemented by a related logic circuit. A person skilled in the art can naturally assemble a corresponding logic circuit based on a comparator, a trigger, a timer, etc. when providing the implementation logic of the first module as described above. Such assembly means belongs to conventional technical means and is not limited here.

[0067] Correspondingly, the implementation logic based on the second module can also be implemented through relevant logic circuits. Figure 3 The second module can be implemented by a second comparison submodule and a second timer. The second comparison submodule is used to compare the sampling voltage VDS with the first threshold voltage VTH1 and output a second comparison signal CMP2; the second timer is connected to the output end of the second comparison submodule and is used to start timing when the level of the second comparison signal CMP2 indicates that the sampling voltage VDS is less than the first threshold voltage VTH1, and after timing for a first time, output a second detection signal ctrl2 of a valid level.

[0068] Continue to refer Figure 3The logic module of the embodiment of the present invention can be implemented by an OR gate, which is used to perform an OR operation on the first detection signal ctrl1 and the second detection signal ctrl2, and output a conduction control signal TON; wherein, when the level of the first detection signal ctrl1 is valid or the level of the second detection signal ctrl2 is valid, the level of the conduction control signal TON is valid, and the synchronous rectifier tube M2 is controlled to be turned on.

[0069] To solve the above problems, the second embodiment of the present invention provides a synchronous rectifier control circuit, such as Figure 4 As shown, the control circuit includes a first module, a second module, a third module and a logic module.

[0070] The first module detects the falling rate of the drain-source voltage of the synchronous rectifier M2 based on the falling rate threshold to output a first detection signal ctrl1; the second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage VTH1 for a first time period, and outputs a second detection signal ctrl2; the third module is used to output a third detection signal ctrl3 of a valid level if it is detected that the sampling voltage VDS drops below the fifth threshold voltage VTH5 after the sampling voltage VDS is continuously greater than the fourth threshold voltage VTH4 for a third time period ("reach" can be understood as greater than or equal to) the third time period; the logic module outputs a conduction control signal TON based on the first detection signal ctrl1 or the second detection signal ctrl2 or the third detection signal ctrl3 to control the conduction of the synchronous rectifier M2; wherein, the detection time T2 of the second module is greater than the detection time T1 of the first module and / or the detection time T3 of the third module.

[0071] Regarding the detection principles of the first module and the second module and related implementable means, reference may be made to the related description in the aforementioned first embodiment, and the embodiment of the present invention will not be elaborated herein.

[0072] The detection principle of the third module is: after the primary switch tube M1 is turned on, the drain-source voltage of the synchronous rectifier tube M2 will suddenly increase, and its sampling voltage VDS will also increase. When the primary switch tube M1 is turned off, VDS will drop sharply again. Therefore, if VDS continues to be greater than the fourth threshold voltage VTH4 for more than the third time period, it indicates that the primary switch tube M1 is normally turned on. After that, if VDS drops below the fifth threshold voltage VTH5, it can be considered that the primary switch tube M1 is turned off again after being normally turned on. In this way, it can be clearly detected that the drain-source voltage of the synchronous rectifier tube M2 decreases based on the shutdown of the primary switch tube M1, so as to control the normal conduction of the synchronous rectifier tube M2.

[0073] In the embodiment of the present invention, three solutions are provided to solve the problem of mis-conduction of the synchronous rectifier tube M2. The logic module can output a conduction control signal TON based on the first detection signal ctrl1, the second detection signal ctrl2, or the third detection signal ctrl3 to control the conduction of the synchronous rectifier tube M2.

[0074] Compared with the falling rate detection scheme using only the first module, the third module can not only solve the problem of misleading the synchronous rectifier tube M2, but also ensure that the synchronous rectifier tube M2 can still be turned on (conducted) normally when it is lightly loaded or lightly loaded, thereby ensuring the power conversion efficiency. However, considering that in actual applications, the detection conditions of the third module may still not be met and the detection failure may occur (such as after the primary switch tube M1 is turned on, the duration of VDS continuously greater than the fourth threshold voltage VTH4 may not reach the third duration), the misleading turn-on solution provided by the second module in the embodiment of the present invention can be effectively applied when the first module and / or the third module fail to detect. That is, when the first module and / or the third module fail to detect, whether the synchronous rectifier tube M2 is turned on or not is actually determined by the second detection signal ctrl2 output by the second module. Since the detection of the second module is relatively long, it is not easily interfered with, and its detection stability and accuracy are high, so that it can prevent the synchronous rectifier tube M2 from misleading when the synchronous rectifier tube M2 is turned off due to ringing, and can ensure the normal and effective conduction of the synchronous rectifier tube M2, thereby ensuring the power efficiency and improving the reliability of the synchronous rectification control. It can be considered that the synchronous rectifier control circuit provided by the present invention has both detection efficiency and detection reliability in solving the problem that the synchronous rectifier M2 is easily mis-conducted.

[0075] The synchronous rectifier control circuit provided in the embodiment of the present invention includes the three solutions to the mis-conduction of the synchronous rectifier M2. In specific applications, the three solutions may have different configurations in timing to achieve both detection efficiency and detection reliability.

[0076] In one embodiment, the second module can be used only as a patch solution for the first module, that is, the detection time T2 of the second module is greater than the detection time T1 of the first module. In this embodiment, the relationship between the detection time T1 of the first module and the detection time T3 of the third module, and the relationship between the detection time T2 of the second module and the detection time T3 of the third module are not limited by the present invention.

[0077] In one embodiment, the second module can be used only as a patch solution for the third module, that is, the detection time T2 of the second module is greater than the detection time T3 of the third module. In this embodiment, the relationship between the detection time T1 of the first module and the detection time T3 of the third module, and the relationship between the detection time T2 of the second module and the detection time T1 of the first module are not limited by the present invention.

[0078] In one embodiment, if Figure 4 As shown, the second module can be used as a patch solution for the first module and the third module, the detection time T2 of the second module is greater than the detection time T3 of the third module, and the detection time T2 of the second module is greater than the detection time T1 of the first module. In this implementation, the detection time T1 of the first module and the detection time T3 of the third module are not limited by the present invention.

[0079] In the embodiment of the present invention, the magnitudes of the fifth threshold voltage VTH5 and the second threshold voltage VTH2 can be designed differently based on different applications. Generally speaking, the fifth threshold voltage VTH5 and the second threshold voltage VTH2 are both negative voltages, and the fifth threshold voltage VTH5 is equal to or lower than the second threshold voltage VTH2. Figure 3 When the implementation method of the fixed drop amplitude shown in the figure is implemented, the VDS drop time is detected (the first module outputs a first detection signal ctrl1 of a valid level when detecting that the drop time of the sampling voltage VDS from the third threshold voltage VTH3 to the second threshold voltage VTH2 is less than or equal to the second time). Figure 5 As shown, the fifth threshold voltage VTH5 may be equal to the second threshold voltage VTH2, so that part of the circuit, such as the comparator circuit, can be shared to the greatest extent possible, thereby reducing circuit complexity and chip area.

[0080] Figure 5 This is an example of an implementation of the third module. The third module includes a third comparison submodule, a third timer, and a third judgment submodule. The third comparison submodule is used to compare the sampling voltage VDS and the fourth threshold voltage VTH4 and output a third comparison signal CMP3. The third timer is connected to the third comparison submodule, and is used to judge whether the sampling voltage VDS continues to be greater than the fourth threshold voltage VTH4 for a third time period according to the third comparison signal CMP3, and output an indication signal INT; the third judgment submodule detects whether the sampling voltage VDS is reduced to below the fifth threshold voltage VTH5 in response to the indication signal INT, and outputs a third detection signal ctrl3. Among them, the third judgment submodule can have a variety of implementation means, such as through a trigger, or through a trigger and an AND gate. Similarly, the above example does not constitute a structural limitation on the third module of the embodiment of the present invention. Based on the implementation logic of the third module given in the embodiment of the present invention, those skilled in the art can also implement it through other related logic circuits, and the present invention is not limited here.

[0081] Continue to refer Figure 5The logic module of the embodiment of the present invention can be implemented by OR gate 1 and OR gate 2. OR gate 1 is used to perform OR operation on the first detection signal ctrl1 and the third detection signal ctrl3, and output a first logic signal; OR gate 2 is connected to the output end of OR gate 1, and is used to perform OR operation on the first logic signal and the second detection signal ctrl2, and output a conduction control signal TON; wherein, when the level of any one of the first detection signal ctrl1, the second detection signal ctrl2 and the third detection signal ctrl3 is valid, the level of the conduction control signal TON is valid, and the synchronous rectifier tube M2 is controlled to be turned on.

[0082] To solve the above problems, the third embodiment of the present invention provides a synchronous rectifier control circuit, such as Figure 6 As shown, the control circuit includes a second module, a third module and a logic module.

[0083] The second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage VTH1 for a first time period, and output a second detection signal ctrl2; the third module is used to output a third detection signal ctrl3 of a valid level if it is detected that the sampling voltage VDS decreases below the fifth threshold voltage VTH5 after the sampling voltage VDS is continuously greater than the fourth threshold voltage VTH4 for a third time period ("reach" can be understood as greater than or equal to) the third time period; the logic module outputs a conduction control signal TON based on the second detection signal ctrl2 or the third detection signal ctrl3 to control the conduction of the synchronous rectifier M2; wherein, the detection time T2 of the second module is greater than the detection time T3 of the third module.

[0084] Regarding the detection principles of the second module and the third module and related implementable means, reference may be made to the related descriptions in the aforementioned first embodiment and second embodiment, and the embodiments of the present invention will not be elaborated herein.

[0085] In the embodiment of the present invention, two solutions are provided to solve the mis-conduction of the synchronous rectifier M2. The logic module can output a conduction control signal TON based on the second detection signal ctrl2 or the third detection signal ctrl3 to control the conduction of the synchronous rectifier M2.

[0086] Compared with the falling rate detection scheme that only uses the third module, the mis-conduction solution provided by the second module can be effectively applied when the detection of the third module fails (for example, after the primary switch tube M1 is turned on, the time that VDS continues to be greater than the fourth threshold voltage VTH4 may not reach the third time), which not only ensures that the mis-conduction problem of the synchronous rectifier tube M2 is effectively solved, but also ensures that the synchronous rectifier tube M2 is effectively turned on when the detection of the third module fails, thereby improving the reliability of the synchronous rectification control and ensuring the power efficiency of the switching power supply.

[0087] In the embodiment of the present invention, the detection time T2 of the second module is greater than the detection time T3 of the third module. The detection efficiency in solving the problem of mis-conduction of the synchronous rectifier tube M2 is achieved through the third module, and the second module is used as a backup to accurately control the conduction of the synchronous rectifier switch tube and ensure the power supply efficiency. It can be considered that the synchronous rectifier tube control circuit of the embodiment of the present invention has both detection efficiency and detection reliability.

[0088] Figure 7 This is an example of an implementation of the synchronous rectifier control circuit of the embodiment of the present invention. The implementation structure description of the second module and the third module can refer to Figure 5 , which will not be elaborated here. The logic module of the embodiment of the present invention can be implemented by an OR gate, which is used to perform an OR operation on the third detection signal ctrl3 and the second detection signal ctrl2, and output a conduction control signal TON; wherein, when the level of the third detection signal ctrl3 is valid or the level of the second detection signal ctrl2 is valid, the level of the conduction control signal TON is valid, and the synchronous rectifier tube M2 is controlled to be turned on.

[0089] It should be noted that the various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0090] The technical solution provided by the present invention is described in detail above. The principle and implementation mode of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the present invention, and the content of this specification should not be understood as limiting the present invention. At the same time, for those skilled in the art, according to the present invention, there will be different forms of changes in the specific implementation mode and application scope. It is not necessary and impossible to list all the implementation modes here, and the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A synchronous rectifier control circuit, characterized in that: include: A first module detects a decreasing rate of the drain-source voltage of the synchronous rectifier tube based on a decreasing rate threshold value to output a first detection signal; A second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage for a first time period, and output a second detection signal; A logic module, based on the first detection signal or the second detection signal, outputs a conduction control signal to control the conduction of the synchronous rectifier; The detection duration of the second module is longer than the detection duration of the first module.

2. A synchronous rectifier control circuit, characterized in that: include: A first module detects a decreasing rate of the drain-source voltage of the synchronous rectifier tube based on a decreasing rate threshold value to output a first detection signal; A second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage for a first time period, and output a second detection signal; A third module is used for outputting a third detection signal of a valid level if it is detected that the sampling voltage VDS decreases below a fifth threshold voltage after the sampling voltage VDS continues to be greater than the fourth threshold voltage for a third period of time; a logic module, which outputs a conduction control signal based on the first detection signal, the second detection signal, or the third detection signal, so as to control the conduction of the synchronous rectifier; The detection duration of the second module is greater than the detection duration of the first module and / or the detection duration of the third module.

3. The synchronous rectifier control circuit according to claim 1 or 2, characterized in that: The falling rate is characterized by the falling time length of the sampling voltage VDS of the drain-source voltage falling from the third threshold voltage to the second threshold voltage, and the falling rate threshold is characterized by the second time length; The first module outputs the first detection signal of a valid level when detecting that the duration of the sampling voltage VDS decreasing from the third threshold voltage to the second threshold voltage is less than or equal to the second duration; Wherein, the third threshold voltage>the first threshold voltage.

4. The synchronous rectifier control circuit according to claim 3, characterized in that: The first threshold voltage and the second threshold voltage are both negative voltages, The first threshold voltage is lower than the second threshold voltage.

5. The synchronous rectifier control circuit according to claim 3, characterized in that: The first duration is greater than the second duration.

6. The synchronous rectifier control circuit according to claim 1 or 2, characterized in that: The drop rate is characterized by the drop amplitude of the sampled voltage VDS of the drain-source voltage within a fixed time period, and the drop rate threshold is characterized by the drop amplitude threshold; When the first module detects that the drop amplitude of the sampling voltage VDS within the fixed time period is greater than the drop amplitude threshold, the first module outputs the first detection signal of a valid level.

7. A synchronous rectifier control circuit, characterized in that: include: A second module is used to detect whether the sampling voltage VDS of the drain-source voltage is continuously less than the first threshold voltage for a first time period, and output a second detection signal; A third module is used for outputting a third detection signal of a valid level if it is detected that the sampling voltage VDS decreases below a fifth threshold voltage after the sampling voltage VDS continues to be greater than the fourth threshold voltage for a third period of time; The logic module outputs a conduction control signal based on the second detection signal or the third detection signal to control the conduction of the synchronous rectifier; The detection duration of the second module is greater than the detection duration of the third module.

8. The synchronous rectifier control circuit according to claim 2 or 7, characterized in that: The fifth threshold voltage and the second threshold voltage are both negative voltages, The fifth threshold voltage is equal to or lower than the second threshold voltage.

9. The synchronous rectifier control circuit according to any one of claims 1, 2 and 7, characterized in that: The second module includes: A second comparison submodule, used for comparing the sampled voltage VDS with the first threshold voltage, and outputting a second comparison signal; A second timer is connected to the output end of the second comparison submodule, and is used to start timing when the level of the second comparison signal indicates that the sampling voltage VDS is less than the first threshold voltage, and output a second detection signal with a valid level after timing for a first time period.

10. A switching power supply, characterized in that: include: A primary winding, a primary switch tube connected to the primary winding, a secondary winding, a synchronous rectifier tube connected to the secondary winding, and a synchronous rectifier tube control circuit as described in any one of claims 1 to 9.

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