Synchronous rectification control circuit, control method, and flyback converter

Abstract

The application provides a synchronous rectification control circuit, a control method and a flyback converter. The synchronous rectification control circuit is applied to the flyback converter and comprises a voltage detection circuit configured to detect an output voltage of the flyback converter and a time adjustment circuit configured to receive the output voltage and adjust a minimum conduction time of a synchronous rectification tube in a current switching period according to the output voltage. The synchronous rectification control circuit provided by the application can adapt to the demand of the minimum conduction time of the system to the greatest extent, avoids the problems of inflexible application of the fixed minimum conduction time and untimely adjustment of the minimum conduction time, and improves the efficiency of the whole system and the system applicability.

Description

Technical Field

[0001] This invention relates to electronic power technology, and more specifically, to a synchronous rectification control circuit, a control method, and a flyback converter. Background Technology

[0002] Synchronous rectification is a method that uses low-on-state resistance power metal-oxide-semiconductor field-effect transistors (MOSFETs) to replace rectifier diodes to reduce rectification losses. Power MOSFETs are voltage-controlled devices, and their current-voltage characteristics are linear when they are turned on. When using a power MOSFET as a rectifier, the gate voltage must be synchronized with the phase of the voltage being rectified to complete the rectification function. This technology has been widely used in industrial power supplies, consumer electronics, and other fields.

[0003] Figure 1 A structural diagram of a flyback converter in the prior art is shown, such as... Figure 1 As shown, the flyback converter uses a synchronous rectifier (MOSFET) with low on-state resistance as the synchronous rectifier. Existing synchronous rectifier control circuits often use voltage control to control the synchronous rectifier, real-time detecting the drain-source voltage Vds of the synchronous rectifier and comparing it with preset turn-on threshold Vth-on and turn-off threshold Vth-off to turn the synchronous rectifier on or off. However, due to the influence of parasitic parameters such as transformer leakage inductance, the complex current in the initial stage of freewheeling oscillates, and the drain-source voltage Vds of the synchronous rectifier also oscillates. This can easily lead the controller to mistakenly judge that the turn-off threshold Vth-off has been exceeded and turn off the synchronous rectifier. To avoid premature turn-off of the synchronous rectifier, existing technologies usually set a minimum on-time after the synchronous rectifier is turned on to shield the positive zero-crossing oscillation of the drain-source voltage Vds in the initial stage of freewheeling in the secondary circuit. In existing technologies, the minimum on-time is basically set to a fixed value. When the circuit operates at low voltage, the interference signal caused by the switching action lasts for a relatively long time. This causes the waveform of the drain-source voltage Vds of the synchronous rectification to resonate to the turn-off voltage threshold after the minimum conduction time ends, resulting in the synchronous rectification control circuit turning off prematurely. When the circuit operates at high voltage, the interference signal caused by the switching action lasts for a relatively short time. The minimum conduction time is longer than the time it takes for the secondary side freewheeling current to reach zero, causing the synchronous rectification control circuit to turn off too late, resulting in negative current, which can cause malfunctions or circuit damage.

[0004] Therefore, while using a fixed threshold for the minimum conduction time in existing technologies is simple, its applicability is limited, and it places high demands on system design. Errors may occur if the design parameters are changed. Summary of the Invention

[0005] The content of this application is provided to introduce, in a simplified form, some concepts that will be further described in the detailed description below. This content is not intended to identify key or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter.

[0006] The purpose of this application is to provide an improved synchronous rectification control circuit to prevent negative current from flowing through the secondary side, thereby improving efficiency. The foregoing and other objectives are achieved through the features of the independent claims. Further implementations will be apparent from the dependent claims, the specification, and the drawings.

[0007] According to a first aspect of this application, a synchronous rectification control method is provided for a flyback converter, the flyback converter including a synchronous rectifier diode, characterized in that the method includes: detecting the output voltage of the flyback converter; and adjusting the minimum on-time of the synchronous rectifier diode within the current switching cycle according to the output voltage.

[0008] Optionally, adjusting the minimum on-time of the synchronous rectifier tube in the current switching cycle according to the output voltage includes: when the output voltage is detected to be less than a first threshold, determining the minimum on-time in the current switching cycle as a first on-time; when the output voltage is detected to be greater than a second threshold, determining the minimum on-time in the current switching cycle as a second on-time.

[0009] Optionally, the first conduction time is longer than the second conduction time.

[0010] Optionally, the method further includes: when the output voltage is detected to be greater than a first threshold and less than a second threshold, the minimum on-time decreases as the output voltage increases.

[0011] Optionally, the method further includes: receiving the output voltage, generating a first current proportional to the output voltage; and using the first current to charge a first capacitor to a third threshold to obtain the minimum conduction time.

[0012] Optionally, the method further includes: the first threshold is equal to the second threshold, wherein when the second threshold is equal to the first threshold, after the output voltage reaches the first threshold, the synchronous rectifier completes the switching between the first conduction time and the second conduction time within the current switching cycle.

[0013] According to a second aspect of this application, a synchronous rectification control circuit is provided for use in a flyback converter, characterized in that it includes: a voltage detection circuit for detecting the output voltage of the flyback converter; and a time adjustment circuit for receiving the output voltage and adjusting the minimum on-time of the synchronous rectifier tube in the current switching cycle according to the output voltage.

[0014] Optionally, the time adjustment circuit includes: a first current source for outputting a bias current, the bias current being used to determine the maximum value of the minimum on-time; a voltage-controlled current source for receiving the output voltage and generating a first current proportional to the output voltage; a first capacitor for receiving the bias current and the first current, generating a capacitor voltage, and resetting the capacitor voltage at the beginning of each switching cycle; a voltage comparator for receiving the capacitor voltage and a third threshold, and outputting a termination signal when the capacitor voltage is greater than the third threshold; and a trigger configured such that a first input terminal receives an on signal, a second input terminal receives the termination signal, and an output terminal outputs a first off signal, wherein the time from the start of the on signal to the output terminal outputting the first off signal is the minimum on-time.

[0015] Optionally, the synchronous rectification control circuit further includes: a first logic circuit configured to receive the first turn-off signal at a first input terminal, receive the second turn-off signal at a second input terminal, and output a turn-off signal at an output terminal; and a first flip-flop configured to receive the turn-on signal at a first input terminal, receive the turn-off signal at a second input terminal, and output a control signal for the synchronous rectifier tube at an output terminal.

[0016] According to a third aspect of this application, a flyback converter is provided, characterized in that it includes the synchronous rectification control circuit described above.

[0017] The synchronous rectification control circuit provided in this application has the following advantages: 1. It uses the detection of output voltage to dynamically control the minimum conduction time of the synchronous rectifier tube on the secondary side of the flyback converter in the current cycle, so as to meet the system's requirements for minimum conduction time to the greatest extent.

[0018] 2. This solves the problem of excessive oscillation time caused by the switching action of the synchronous rectifier tube, which exceeds the fixed minimum conduction time and causes the synchronous rectifier tube to turn off erroneously.

[0019] 3. It avoids the negative current caused by the synchronous rectifier tube turning on for longer than the actual secondary current freewheeling time, which would lead to additional voltage stress and losses.

[0020] 4. The circuit structure is simple and easy to implement. The output voltage detection can share the detection pin with other detections in the control circuit, which improves the efficiency of the entire system and has good system applicability, while the cost is also easy to control. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 A schematic diagram of a flyback converter in the prior art is shown;

[0023] Figure 2 The diagram shows a linear graph of the minimum on-time and output voltage according to an embodiment of this application;

[0024] Figure 3 A schematic diagram of the module structure of an embodiment of this application is shown;

[0025] Figure 4 A schematic diagram of the structure of a time adjustment circuit according to an embodiment of this application is shown;

[0026] Figure 5 A method flowchart of an embodiment of this application is shown.

[0027] In the following text, the same reference numerals denote the same or at least functionally the same features. Detailed Implementation

[0028] The technical solutions of the embodiments of this application will now be described with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0029] For example, it should be understood that the disclosure of the described method applies to the corresponding apparatus or system for performing the method, and vice versa. For example, if specific method steps are described, the corresponding apparatus may include units that perform the described method steps, even if such units are not described or shown in detail in the accompanying drawings. On the other hand, for example, if a particular device is described based on functional units, the corresponding method may include steps that perform the described functions, even if such steps are not explicitly described or illustrated in the accompanying drawings. Furthermore, it should be understood that features of the various exemplary aspects described herein can be combined with each other unless otherwise specifically stated.

[0030] It should be understood that the connection / coupling of A and B in the embodiments of this application means that A and B can be connected in series or in parallel, or A and B can be connected through other devices. The embodiments of this application do not limit this.

[0031] This application discloses an embodiment of a synchronous rectification control circuit, which utilizes the output voltage to dynamically control the minimum on-time of the synchronous rectifier tube on the secondary side of the flyback converter in the current cycle. This maximizes the adaptation to the system's requirements for the minimum on-time, avoiding both the inflexibility of fixed minimum on-time applications and the untimely adjustment of the minimum on-time, thereby improving the efficiency of the entire system and having good system applicability.

[0032] Figure 2 The following is a linear graph of the minimum on-time and output voltage of an embodiment of this application, as shown: Figure 2 As shown, to address the issue of excessively long oscillation time caused by the switching action of the synchronous rectifier, exceeding the fixed minimum on-time and leading to false turn-off, it is necessary to introduce a relationship between the output voltage and the minimum on-time (MOT). Since the switching oscillation time is longer under low-voltage conditions, the required minimum on-time (MOT) under low-voltage conditions should be longer. Therefore, the minimum on-time (MOT) decreases as the output voltage (Vo) increases. When the output voltage (Vo) is between the first threshold (V1) and the second threshold (V2), the minimum on-time should be between the first on-time (MOT1) and the second on-time (MOT2). Between these two values, the first conduction time MOT1 is greater than the second conduction time MOT2. At the same time, in order to avoid the synchronous rectifier tube's turn-on time exceeding the actual secondary current freewheeling time, resulting in negative current and thus causing additional voltage stress and losses, the system's output voltage should be set with an upper limit. When the output voltage Vo is less than the first threshold V1, the synchronous rectifier control circuit selects the first conduction time MOT1 as the system's minimum conduction time. When the output voltage Vo is greater than the second threshold V2, the synchronous rectifier control circuit selects the second conduction time MOT2 as the system's minimum conduction time.

[0033] This application adjusts the minimum on-time within the current cycle by adjusting the output voltage. First, it does not require additional pins, as the output voltage can share the voltage feedback pin in the control circuit or be obtained by sampling the drain-source voltage of the synchronous rectifier and then filtering. Second, this application's solution can adjust the minimum on-time within the current cycle in real time, allowing for timely and rapid adjustment.

[0034] The foregoing describes an example of the output voltage and minimum on-time of an embodiment of this application. However, the embodiments of this application are not limited thereto, and there may be other extensions and variations.

[0035] For example, it should be understood that, such as Figure 2As shown, the minimum on-time and output voltage are not limited to a linear relationship; it is only necessary to ensure that the minimum on-time MOT decreases as the output voltage Vo increases.

[0036] For example, it should be understood that the first threshold and the second threshold can be equal. The first threshold can be set as the dividing point for the minimum conduction time. When the output voltage is greater than the first threshold, the system selects a smaller minimum conduction time; when the output voltage is less than the first threshold, the system selects a larger minimum conduction time. This control scheme is convenient to operate and control, relatively easy to implement in circuits, and can balance cost and efficiency in applications where high precision is not required.

[0037] As an example, Figure 3 A schematic diagram of the module structure of an embodiment of this application is shown, as follows: Figure 3 As shown, the synchronous rectification control circuit 10 includes a voltage detection circuit 101 and a time adjustment circuit 102. The voltage detection circuit 101 is used to detect the output voltage of the flyback converter. The time adjustment circuit 102 is used to adjust the minimum on-time of the synchronous rectifier in the current switching cycle according to the output voltage. After receiving the input signal Vo, it outputs the minimum on-time MOT. The first turn-off signal associated with the minimum on-time MOT and the second turn-off signal controlling the synchronous rectifier are logically ANDed by the logic circuit 103, and the final turn-off signal is output to the reset terminal R of the first flip-flop 104. The set terminal S of the first flip-flop 104 receives the turn-on signal, and the output terminal Q outputs the switch control signal Gate of the synchronous rectifier. Here, the second turn-off signal is the turn-off signal obtained through the system feedback loop.

[0038] As an example Figure 4 A schematic diagram of the time adjustment circuit according to an embodiment of this application is shown, as follows: Figure 4As shown, the time adjustment circuit includes a first current source I0, a voltage-controlled current source I1, a voltage comparator U1, a first capacitor C1, a first switching transistor Q1, and a trigger T1. The first current source I0 is configured to be connected to the first capacitor C1 to output a bias current, which determines the maximum value of the minimum on-time. The voltage-controlled current source I1 is configured to receive the output voltage and generate a first current proportional to the output voltage. The first capacitor C1 is configured to receive the bias current and the first current to generate a capacitor voltage. The voltage comparator U1 is configured to receive the capacitor voltage at its first input terminal and receive a third threshold voltage Vr at its second input terminal. When the capacitor voltage is greater than the third threshold Vref, the output terminal outputs the first turn-off signal. When the output voltage Vo is larger, the first current is larger, and the current received by the positive terminal of the first capacitor C1 is larger. For the same third threshold Vref, the charging time of the first capacitor C1 is shorter. In the current switching cycle, the time elapsed from the turn-on signal timing to the output of the first turn-off signal is the minimum conduction time, that is, the charging time of the first capacitor. The control terminal of the first switching transistor Q1 synchronously receives the inverted signal of the turn-on signal of the synchronous rectifier to release the voltage of the first capacitor C1 before the start of each switching cycle and start timing.

[0039] In this embodiment, the control circuit adjusts the minimum conduction time within the current cycle in real time according to the output voltage, resulting in better precision control and improved dynamic response and system efficiency.

[0040] It should be understood that the above description is an example of a time adjustment circuit according to an embodiment of this application. However, the embodiments of this application are not limited thereto, and there may be other extensions and modifications.

[0041] For example, when the first threshold is equal to the second threshold, a voltage comparator can be used to detect the output voltage and compare it with a set third threshold. When the output voltage is greater than the first threshold, the system selects a smaller minimum on-time. When the output voltage is less than the first threshold, the system selects a larger minimum on-time.

[0042] For example, it should be understood that in practical applications, the turn-on of the main power switch and the synchronous rectifier on the secondary side needs to be delayed to prevent the primary and secondary sides from being simultaneously turned on. This delay is denoted as T. delay However, this turn-on delay is usually very small and can be approximated as negligible in calculations. As an example, the time adjustment circuit may also include a delay circuit (not shown in the figure), configured with its input connected to the turn-on signal and its output connected to the set terminal of the first flip-flop, for receiving the turn-on signal and, after a first delay, sending the turn-on signal to the first flip-flop.

[0043] Furthermore, those skilled in the art will recognize that the structures and methods described in conjunction with the embodiments disclosed herein can be used with different configuration or adjustment methods to achieve the described functions for each structure or reasonable variations thereof, but such implementations should not be considered beyond the scope of this application. Moreover, it should be understood that the connection relationships between the various components of the amplifier in the foregoing figures in the embodiments of this application are illustrative examples and do not impose any limitations on the embodiments of this application.

[0044] Any ranges or device values ​​given herein may be extended or modified without loss of the desired effect. Furthermore, any embodiment may be combined with another embodiment that is not expressly prohibited.

[0045] Although the subject matter has been described in language specific to structural features and / or actions, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or actions described above. Rather, the specific features and actions described above are disclosed as examples of implementing the claims, and other equivalent features and actions are intended to fall within the scope of the claims.

[0046] It should be understood that the above benefits and advantages may relate to one embodiment or several embodiments. The embodiments are not limited to embodiments that solve any or all of the described problems, or embodiments that have any or all of the described benefits and advantages. It should also be understood that a reference to "one" item may refer to one or more of those items.

[0047] Figure 5 A method flowchart of an embodiment of this application is shown, as follows: Figure 5 As shown, this synchronous rectification control method is applied to a flyback converter, which includes the synchronous rectification control circuit described above. The synchronous rectification control method includes steps S01-S02 and can be applied to applications such as... Figure 3 , Figure 4 In the structure shown.

[0048] In step S01, the output voltage of the flyback converter is detected;

[0049] In step S02, the minimum on-time of the synchronous rectifier tube in the current switching cycle is adjusted according to the output voltage.

[0050] Optionally, adjusting the minimum on-time of the synchronous rectifier tube in the current switching cycle according to the output voltage includes: when the output voltage is detected to be less than a first threshold, determining the minimum on-time in the current switching cycle as a first on-time; when the output voltage is detected to be greater than a second threshold, determining the minimum on-time in the current switching cycle as a second on-time.

[0051] Optionally, the first conduction time is longer than the second conduction time.

[0052] Optionally, the method further includes: when the output voltage is detected to be greater than a first threshold and less than a second threshold, the minimum on-time decreases as the output voltage increases.

[0053] Optionally, the method further includes: receiving the output voltage, generating a first current proportional to the output voltage; and using the first current to charge a first capacitor to a third threshold to obtain the minimum conduction time.

[0054] Optionally, the method further includes: the first threshold is equal to the second threshold, wherein when the second threshold is equal to the first threshold, the synchronous rectifier completes the switching between the first conduction time and the second conduction time within the current switching cycle.

[0055] The method provided in this application utilizes the dynamic control of the minimum on-time of the synchronous rectifier on the secondary side of the flyback converter based on the output voltage. This maximizes the adaptation to the system's requirements for the minimum on-time, avoiding both the inflexibility of fixed minimum on-time applications and the untimely adjustment of the minimum on-time. The circuit implementation is simple in structure, improving the efficiency of the entire system and providing good system applicability while saving costs.

[0056] The steps of the methods described herein can be performed in any suitable order, or simultaneously where appropriate. Furthermore, individual blocks can be removed from any method without departing from the spirit and scope of the subject matter described herein. Aspects of any of the embodiments described above can be combined with aspects of any other described embodiments to form further embodiments without loss of the desired effects.

[0057] The terms “comprising,” “including,” or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase “comprising one…” does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0058] It should be understood that the above description is given by way of example only, and various modifications can be made by those skilled in the art. The above description, examples, and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments with a certain degree of specificity have been described above, or reference has been made to one or more individual embodiments, those skilled in the art can make various changes to the disclosed embodiments without departing from the spirit or scope of this specification.

Claims

1. A synchronous rectification control method applied to a flyback converter, the flyback converter including synchronous rectifier diodes, characterized in that, The method includes: Detect the output voltage of the flyback converter; Adjusting the minimum on-time of the synchronous rectifier tube within the current switching cycle according to the output voltage, wherein adjusting the minimum on-time of the synchronous rectifier tube within the current switching cycle according to the output voltage includes: When the output voltage is detected to be less than a first threshold, the minimum on-time within the current switching cycle is determined as the first on-time. When the output voltage is detected to be greater than the second threshold, the minimum on-time within the current switching cycle is determined as the second on-time, wherein the second threshold is greater than or equal to the first threshold.

2. The synchronous rectification control method according to claim 1, characterized in that, The first conduction time is greater than the second conduction time.

3. The synchronous rectification control method according to claim 1, characterized in that, The step of adjusting the minimum on-time of the synchronous rectifier tube within the current switching cycle according to the output voltage includes: When the output voltage is detected to be greater than a first threshold and less than a second threshold, the minimum conduction time decreases as the output voltage increases.

4. The synchronous rectification control method according to claim 3, characterized in that, The step of adjusting the minimum on-time of the synchronous rectifier tube within the current switching cycle according to the output voltage includes: Receive the output voltage and generate a first current that is directly proportional to the output voltage; The first capacitor is charged to the third threshold using the first current to obtain the minimum conduction time.

5. The synchronous rectification control method according to claim 1, further comprising: The first threshold is equal to the second threshold. Specifically, when the second threshold is equal to the first threshold, after the output voltage reaches the first threshold, the synchronous rectifier completes the switching between the first conduction time and the second conduction time within the current switching cycle.

6. A synchronous rectification control circuit, applied to a flyback converter, characterized in that, include: A voltage detection circuit is used to detect the output voltage of the flyback converter; The timing adjustment circuit receives the output voltage and adjusts the minimum on-time of the synchronous rectifier diode within the current switching cycle based on the output voltage. When the output voltage is detected to be less than a first threshold, the minimum on-time within the current switching cycle is determined as the first on-time. When the output voltage is detected to be greater than the second threshold, the minimum on-time within the current switching cycle is determined as the second on-time, wherein the second threshold is greater than or equal to the first threshold.

7. The synchronous rectification control circuit according to claim 6, characterized in that, The time adjustment circuit includes: The first current source is used to output a bias current, which is used to determine the maximum value of the minimum on-time; A voltage-controlled current source is used to receive the output voltage and generate a first current that is proportional to the output voltage. A first capacitor is used to receive the bias current and the first current, generate a capacitor voltage, and reset the capacitor voltage according to the turn-on signal at the beginning of each switching cycle. A voltage comparator is used to receive the capacitor voltage and a third threshold. When the capacitor voltage is greater than the third threshold, a first shutdown signal is output. The time from the start of the turn-on signal to the output of the first turn-off signal is the minimum turn-on time.

8. The synchronous rectification control circuit according to claim 7, characterized in that, Also includes: A first logic circuit is configured to receive the first shutdown signal at its first input terminal, receive the second shutdown signal at its second input terminal, and output a shutdown signal at its output terminal. The first trigger is configured to receive the turn-on signal at the first input terminal, receive the turn-off signal at the second input terminal, and output the control signal of the synchronous rectifier at the output terminal.

9. A flyback converter, characterized in that, Includes the synchronous rectification control circuit as described in any one of claims 6-8.

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