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 method comprises the following steps: estimating the current freewheeling time of the synchronous rectification tube in the current switching period according to the output voltage of the flyback converter and the drain-source voltage of the synchronous rectification tube; and obtaining the minimum conduction time of the synchronous rectification tube in the current switching period according to the estimated current freewheeling time. The synchronous rectification control method provided by the application can adapt to the minimum conduction time requirement 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 secondary freewheeling current oscillates during the initial stage of freewheeling, 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 during 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 under heavy load, the interference signal caused by the switching action lasts for a 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 under light load, the interference signal caused by the switching action lasts for a short time. The minimum conduction time is longer than the time for the secondary side freewheeling to reach zero, causing the synchronous rectification control circuit to turn off too late, resulting in negative current, which can cause abnormal operation 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 use in a flyback converter, characterized in that it includes: estimating the estimated current freewheeling time of the synchronous rectifier in the current switching cycle based on the output voltage of the flyback converter and the drain-source voltage of the synchronous rectifier; and timing the minimum on-time of the synchronous rectifier in the current switching cycle based on the estimated current freewheeling time.

[0008] Optionally, the method further includes: calculating a first volt-second product during the turn-on period of the main power switch based on the drain-source voltage of the synchronous rectifier and the output voltage; triggering a signal to start timing the minimum conduction time based on a comparison result between the first volt-second product and a volt-second product threshold; calculating a second volt-second product during the freewheeling period of the synchronous rectifier based on the output voltage; and terminating timing to obtain the minimum conduction time based on a comparison result between the second volt-second product and the first volt-second product.

[0009] Optionally, the method further includes: detecting the drain-source voltage and the output voltage of the synchronous rectifier; when the drain-source voltage is detected to be greater than the output voltage, performing a volt-second product operation on the portion of the drain-source voltage that is greater than the output voltage until the drain-source voltage is less than the output voltage to obtain a first volt-second product; when the drain-source voltage is detected to be less than the output voltage, integrating the output voltage to obtain a second volt-second product; and terminating the timing when the second volt-second product is greater than the product of a first proportionality coefficient and the first volt-second product to obtain the minimum conduction time.

[0010] Optionally, the method further includes: after the first volt-second product is integrated within the current switching cycle, comparing the value of the first volt-second product with the volt-second product threshold; when the first volt-second product is less than the volt-second product threshold, clearing the first volt-second product to zero; when the first volt-second product is greater than the volt-second product threshold, multiplying the first volt-second product with the first proportional coefficient.

[0011] Optionally, the method further includes: during the current switching cycle, when the timing is terminated to obtain the minimum conduction time, both the first volt-second product and the second volt-second product are cleared to zero.

[0012] Optionally, the first proportionality coefficient is a coefficient in the range (0, 1).

[0013] Optionally, the step of triggering the signal to start timing the minimum conduction time based on the comparison result of the first volt-second product and the volt-second product threshold further includes: when the first volt-second product is greater than the volt-second product threshold, starting to time the minimum conduction time after a first delay.

[0014] According to a second aspect of this application, a synchronous rectification control circuit is provided, applied in a flyback converter, characterized in that it includes: a time estimation module, used to estimate the current freewheeling time of the synchronous rectifier in the current switching cycle based on the output voltage of the flyback converter and the drain-source voltage of the synchronous rectifier, and to send the estimated current freewheeling time; and a timing module, used to time the minimum conduction time of the synchronous rectifier in the current switching cycle based on the current freewheeling time.

[0015] Optionally, the freewheeling time estimation module includes: a voltage detection circuit for detecting the drain-source voltage and output voltage of the synchronous rectifier, and sending a first signal and a second signal according to the detection result; a first volt-second product circuit for receiving the drain-source voltage and the output voltage, and integrating the voltage difference between the drain-source voltage and the output voltage after receiving the first signal to obtain a first volt-second product; a second volt-second product circuit for receiving the output voltage, and integrating the output voltage after receiving the second signal to obtain a second volt-second product; a first comparison circuit for receiving the first volt-second product and a volt-second product threshold, comparing the first volt-second product and the volt-second product threshold, and sending an enable signal; a multiplication circuit for receiving the first volt-second product and the enable signal, and multiplying the first volt-second product by a first proportional coefficient when the enable signal is valid to obtain a third volt-second product; and a second comparison circuit for receiving the second volt-second product and the third volt-second product, and outputting a signal to terminate the timing of the minimum conduction time when the second volt-second product is greater than the third volt-second product.

[0016] Optionally, the timing module receives the enable signal, and when the enable signal is valid, the timing module starts timing the minimum conduction time.

[0017] 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.

[0018] According to a third aspect of this application, a flyback converter is provided, characterized in that it includes a synchronous rectification control circuit and applies the synchronous rectification control method described above.

[0019] The synchronous rectification control circuit provided in this application utilizes the estimated secondary-side freewheeling time to dynamically control the minimum on-time of the synchronous rectifier 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, solving the problem of excessive oscillation time caused by the switching action of the synchronous rectifier, which exceeds the fixed minimum on-time and causes the synchronous rectifier to turn off erroneously. It also avoids the synchronous rectifier's on-time exceeding the actual secondary-side current freewheeling time, resulting in negative current and thus additional stress and losses. This improves the efficiency of the entire system and has good system applicability. Attached Figure Description

[0020] 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.

[0021] Figure 1 A schematic diagram of a flyback converter in the prior art is shown;

[0022] Figure 2 The diagram shows a linear graph of the minimum on-time and secondary side freewheeling time in an embodiment of this application.

[0023] Figure 3 A voltage waveform diagram of an embodiment of this application is shown;

[0024] Figure 4 A schematic diagram of the module structure of an embodiment of this application is shown;

[0025] Figure 5 A flowchart illustrating the minimum on-time timing according to an embodiment of this application is shown;

[0026] Figure 6 A circuit structure diagram of an embodiment of this application is shown;

[0027] Figure 7A method flowchart of an embodiment of this application is shown.

[0028] In the following text, the same reference numerals denote the same or at least functionally the same features. Detailed Implementation

[0029] 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.

[0030] 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.

[0031] 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.

[0032] This application discloses an embodiment of a synchronous rectification control circuit, which uses the estimated secondary freewheeling time 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 scheduling of minimum on-time, thereby improving the efficiency of the entire system and having good system applicability.

[0033] Figure 2 The following is a linear graph showing the minimum on-time and secondary freewheeling time of an embodiment of this application, as shown below. Figure 2As shown, in order to solve 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, the minimum conduction time MOT can be proportionally related to the secondary side freewheeling time Toff, that is, MOT = k × Toff. At the same time, in order to avoid the synchronous rectifier tube turn-on time exceeding the actual secondary side current freewheeling time and causing negative current, which in turn brings additional voltage stress and loss, the value range of k should be (0, 1). Moreover, an upper limit MOT_max can be set to prevent MOT from being too large and causing the primary and secondary sides to be connected in the transient state.

[0034] In existing technologies, the adjustment of the minimum on-time after detecting the secondary-side freewheeling time typically occurs in the next switching cycle. The synchronous rectification control circuit provided in this application estimates the secondary-side freewheeling time Toff based on the primary-side conduction status, adjusting the system's minimum on-time within the same switching cycle, thus greatly adapting to the system's minimum on-time requirements. The secondary-side freewheeling time can be estimated using the volt-second product. Figure 3 The voltage waveform diagram of an embodiment of this application is shown, such as... Figure 3 As shown, Isw is the secondary-side freewheeling current waveform, Vsw is the drain-source voltage waveform of the secondary-side synchronous rectifier, and Vgate is the gate control voltage waveform of the secondary-side synchronous rectifier. During the conduction period of the primary-side main power switch, the first volt-second product VT1 is the first shaded area in the figure, and its calculation formula is VT1=(Vsw-Vo)×Tonp, where Vo is the output voltage and Tonp is the conduction time of the primary-side main power switch. According to the volt-second balance principle, VT1 should be equal to the volt-second product generated by the secondary-side freewheeling time, i.e., VT1=Vo×Toff. Figure 2 A coefficient k is introduced to represent the relationship between the secondary-side freewheeling time Toff and the minimum on-time MOT. The second volt-second product VT2 generated during the minimum on-time MOT should be the second shaded area in the diagram, i.e., VT2 = Vo × k × Toff = Vo × MOT = k × VT1. Therefore, the minimum on-time MOT is MOT = [(Vsw - Vo) × Tonp × k] / Vo. In practical applications, the turn-on of the main power switch and the synchronous rectifier on the secondary side needs a delay to prevent the primary and secondary sides from being simultaneously on. This delay is denoted as T. delay As shown in the figure, T delay =t2-t1, the minimum conduction time MOT should also be MOT=(k×VT1) / Vo-T delay However, this activation delay is usually very small and can be approximated as negligible in calculations. Within the interval where Vsw > Vo, there are both VT1 and DCM intervals, and their volt-second product VT_DCM is as follows: Figure 3As shown in the third shaded part, in order to distinguish between two intervals, a volt-second product threshold VTref needs to be set. When Vsw > Vo, the circuit starts to integrate and calculates VT1. When Vsw < Vo, if the integrated VT1 < VTref, it means that the obtained VT1 is not in the primary conduction interval, and the integrated quantity VT1 is cleared and the next cycle starts.

[0035] Some examples of the method for estimating the secondary freewheeling time in the embodiments of the present application are described above. However, the embodiments of the present application are not limited thereto, and there may be other ways of extension and variation.

[0036] For example, the secondary freewheeling time of this cycle can be estimated based on the secondary freewheeling time of the previous switching cycle, and thus the minimum on-time MOT of this switching cycle can be determined according to the proportionality coefficient.

[0037] At the same time, those of ordinary skill in the art can realize that, in combination with the structures and methods of the various examples described in the embodiments disclosed herein, different configuration methods or adjustment methods can be used to implement the described functions for each structure or reasonable deformation of the structure, but such implementation should not be considered to exceed the scope of the present application. And it should be understood that the connection relationships between the various components of the amplifier in the foregoing figures in the embodiments of the present application are illustrative examples and do not impose any limitations on the embodiments of the present application.

[0038] Any range or device value given herein can be extended or changed without losing the desired effect. In addition, any embodiment can be combined with another embodiment that is not explicitly prohibited.

[0039] Although the subject matter has been described in language specific to structural features and / or acts, it should be understood that the subject matter defined in the appended claims need not be limited to the above specific features or acts. On the contrary, the above specific features and acts are disclosed as examples for implementing the claims, and other equivalent features and acts are intended to fall within the scope of the claims.

[0040] It should be understood that the above benefits and advantages may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to the embodiments that solve any or all of the described problems, or have any or all of the described benefits and advantages. It should also be understood that the reference to "one" item may refer to one or more of those items.

[0041] Figure 4 A schematic diagram of the module structure of an embodiment of the present application is shown, as Figure 4As shown, the synchronous rectification control circuit includes a time estimation module 101 and a timing module 102. The time estimation module 101 is used to estimate the estimated current freewheeling time of the synchronous rectification circuit in the current switching cycle and send the estimated current freewheeling time. The timing module 102 is used to adjust the minimum on-time (MOT) of the synchronous rectifier tube in the current switching cycle according to the estimated current freewheeling time. The first turn-off signal associated with the minimum on-time (MOT) and the second turn-off signal controlling the turn-off of the synchronous rectifier tube 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 tube. Here, the second turn-off signal is the turn-off signal obtained through the system feedback loop.

[0042] Figure 5 A flowchart illustrating the minimum conduction time of an embodiment of this application is shown, as follows: Figure 5 As shown, at the beginning of the process, the drain-source voltage Vsw and the output voltage Vo are detected first. When Vsw is greater than Vo, the integration of the (Vsw-Vo) portion begins and is recorded as the first volt-second product VT1. Integration stops when Vo is detected to be greater than Vsw. The first volt-second product VT1 obtained after integration is compared with the volt-second product threshold VTref. When VT1 is detected to be less than VTref, the first volt-second product VT1 is cleared to zero, and the detection of Vsw and Vo starts again. When VT1 is detected to be greater than VTref, the minimum on-time timing starts, and the Vo portion begins to be integrated and recorded as the second volt-second product VT2. When the second volt-second product VT2 is detected to be greater than k×VT1, the minimum on-time MOT ends, and the integration of VT1 and VT2 is cleared to zero, and the next loop is restarted.

[0043] As an example, Figure 6 A circuit structure diagram of an embodiment of this application is shown, as follows: Figure 6As shown in the figure, the time estimation module 101 includes a voltage detection circuit 1011, a first volt-second integration circuit 1012, a second volt-second integration circuit 1013, a first comparison circuit 1014, a second comparison circuit 1015, and a multiplication circuit 1016. The timing module includes a MOT timing circuit 1021. Among them, the voltage detection circuit 1011 receives the input voltage Vo and the drain-source voltage Vsw. When it detects that Vsw > Vo, it outputs a first signal S1 to the first volt-second integration circuit 1012. The first volt-second integration circuit 1012 receives Vsw and Vo, and starts to integrate the part of (Vsw - Vo) after receiving the first signal S1, denoted as the first volt-second integration VT1. When the voltage detection circuit 1011 detects that Vsw < Vo, it cuts off the first signal S1 and starts to output a second signal S2. After cutting off the first signal S1, the first volt-second integration circuit 1012 ends the integration and sends VT1 to the first comparison circuit 1014. The first comparison circuit 1014 receives a volt-second integration threshold VTref. When the first comparison circuit detects that VT1 < VTref, it sends a Reset signal to the first volt-second integration circuit 1012 to clear the VT1 integration. When it detects that VT1 > VTref, it sends an enable signal to the multiplication circuit 1016 and the MOT timing circuit 1021. When the enable signal is valid, the MOT timing circuit 1021 starts timing. At the same time, the multiplication circuit 1016 calculates k × VT1 as the third volt-second integration and sends it to the second comparison circuit 1015. The second volt-second integration circuit 1013 receives the output voltage Vo. When the second volt-second integration circuit 1013 receives the second signal S2, it starts to integrate Vo, denoted as the second volt-second integration VT2 and sends it to the second comparison circuit 1015. When the second comparison circuit 1015 detects that the second volt-second integration VT2 is greater than k × VT1, the second comparison circuit 1015 sends a timing end signal to the MOT timing circuit 1021, and at the same time sends a Reset signal to the first volt-second integration circuit 1012 and the second volt-second integration circuit 1013 to clear the first volt-second integration VT1 and the second volt-second integration VT2.

[0044] Some examples of the circuit structure of the embodiments of the present application are described above. However, the embodiments of the present application are not limited thereto, and there may be other ways of expansion and deformation.

[0045] For example, when it detects that VT1 > VTref, an enable signal can be sent to the second comparison circuit 1015 to enable the comparison between the third volt-second integration k × VT1 and the second volt-second integration VT2. The other previous steps and subsequent steps are the same as the principles described above and will not be elaborated here.

[0046] Another example is that in practical applications, there needs to be a delay when the main power switch tube and the synchronous rectifier tube on the secondary side are turned on to prevent the primary and secondary sides from being connected in common. This delay is denoted as T delay , as shown in the figure, T delay=t2-t1, the minimum conduction time MOT should also be MOT=(k×VT1) / Vo-T delay As an example, the synchronous rectifier circuit 10 may also include a delay circuit (not shown in the figure). The delay circuit may be configured such that its input is connected to the output of the first comparator circuit 1014. After the first comparator circuit 1014 sends an enable signal, it sends an enable signal to the MOT timing circuit 1021 after a first delay so that the MOT timing circuit 1021 starts timing the minimum conduction time.

[0047] In this embodiment, the control circuit adjusts the minimum on-time in the current cycle in real time according to the estimated secondary side freewheeling time, resulting in better precision control and improved dynamic response and system efficiency.

[0048] This application also provides a synchronous rectification control method, which is applied to the synchronous rectification control circuit described above. The method includes: estimating the estimated current freewheeling time of the synchronous rectifier in the current switching cycle based on the output voltage of the flyback converter and the drain-source voltage of the synchronous rectifier; and obtaining the minimum on-time of the synchronous rectifier in the current switching cycle based on the estimated current freewheeling time.

[0049] Figure 7 A method flowchart of an embodiment of this application is shown, as follows: Figure 7 As shown, the synchronous rectification control method includes steps S01-S04. This synchronous rectification control method is applied to the synchronous rectification control circuit in a flyback converter and can be applied to applications such as... Figure 4 , Figure 6 In the structure shown.

[0050] In step S01, the first volt-second product during the turn-on period of the main power switch is calculated based on the drain-source voltage and output voltage of the synchronous rectifier.

[0051] In step S02, based on the comparison result between the first volt-second product and the volt-second product threshold, a signal to start timing the minimum conduction time is triggered;

[0052] In step S03, the second volt-second product during the freewheeling period of the synchronous rectifier tube current is calculated based on the output voltage;

[0053] In step S04, the timing is terminated based on the comparison result between the second volt-second product and the first volt-second product to obtain the minimum conduction time.

[0054] Optionally, the method further includes: detecting the drain-source voltage and the output voltage of the synchronous rectifier; when the drain-source voltage is detected to be greater than the output voltage, performing a volt-second product operation on the portion of the drain-source voltage that is greater than the output voltage until the drain-source voltage is less than the output voltage to obtain a first volt-second product; when the drain-source voltage is less than the output voltage, integrating the output voltage to obtain a second volt-second product; and when the second volt-second product is greater than the product of a first proportionality coefficient and the first volt-second product, terminating the timing to obtain the minimum conduction time.

[0055] To distinguish between the two intervals, a volt-second product threshold needs to be set. Optionally, the method further includes: after the first volt-second product is integrated within the current switching cycle, comparing the value of the first volt-second product with the volt-second product threshold; when the first volt-second product is less than the volt-second product threshold, clearing the first volt-second product to zero; when the first volt-second product is greater than the volt-second product threshold, multiplying the first volt-second product with the first proportional coefficient.

[0056] Optionally, the method further includes: during the current switching cycle, when the timing is terminated to obtain the minimum conduction time, both the first volt-second product and the second volt-second product are cleared to zero.

[0057] To avoid the synchronous rectifier's turn-on time exceeding the actual secondary current freewheeling time, which would cause negative current and thus additional voltage stress and losses, the minimum on-time MOT should be less than the secondary freewheeling time Toff. As an example, the first proportional coefficient is a coefficient in the range (0, 1).

[0058] In practical applications, a delay is required for the turn-on of the main power switch and the synchronous rectifier on the secondary side to prevent the primary and secondary sides from being simultaneously powered on. This delay is denoted as T. delay As shown in the figure, T delay =t2-t1, the minimum conduction time MOT should also be MOT=(k×VT1) / Vo-T delay As an example, the above method also includes: when the first volt-second product is detected to be greater than the volt-second product threshold, the minimum conduction time is started after a first delay.

[0059] 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.

[0060] 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.

[0061] 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 for use in a flyback converter, characterized in that, include: Detect the drain-source voltage and output voltage of the synchronous rectifier diode; When the drain-source voltage is detected to be greater than the output voltage, the portion of the drain-source voltage that is greater than the output voltage is subjected to a volt-second product operation until the drain-source voltage is less than the output voltage, so as to obtain the first volt-second product; Based on the comparison result between the first volt-second product and the volt-second product threshold, a signal is triggered to start timing the minimum conduction time; When the drain-source voltage is detected to be less than the output voltage, the output voltage is integrated to obtain the second volt-second product; When the second volt-second product is greater than the product of the first proportional coefficient and the first volt-second product, the timing is terminated to obtain the minimum conduction time.

2. The synchronous rectification control method according to claim 1, characterized in that, The method further includes: During the current switching cycle, after the first volt-second product is integrated, the value of the first volt-second product is compared with the volt-second product threshold. When the first volt-second product is less than the volt-second product threshold, the first volt-second product is cleared to zero; When the first volt-second product is greater than the volt-second product threshold, the first volt-second product is multiplied by the first proportional coefficient.

3. The synchronous rectification control method according to claim 1, characterized in that, The method further includes: During the current switching cycle, when the timing is terminated to obtain the minimum conduction time, both the first volt-second product and the second volt-second product are cleared to zero.

4. The synchronous rectification control method according to claim 1, characterized in that, The first proportionality coefficient is a coefficient in the range (0, 1).

5. The synchronous rectification control method according to claim 1, characterized in that, The step of triggering the signal to start timing the minimum conduction time based on the comparison result of the first volt-second product and the volt-second product threshold further includes: When the first volt-second product is greater than the volt-second product threshold, the minimum conduction time is started after the first delay.

6. A synchronous rectification control circuit, applied in a flyback converter, characterized in that, include: The voltage detection circuit is used to detect the drain-source voltage and output voltage of the synchronous rectifier diode, and send a first signal and a second signal based on the detection results. The first volt-second product circuit is used to receive the drain-source voltage and the output voltage. After receiving the first signal, it integrates the voltage difference between the drain-source voltage and the output voltage to obtain the first volt-second product. The second volt-second product circuit is used to receive the output voltage and, after receiving the second signal, integrate the output voltage to obtain the second volt-second product; A first comparison circuit is used to receive the first volt-second product and the volt-second product threshold, and to send an enable signal when the first volt-second product is greater than the volt-second product threshold. The multiplication circuit is used to receive the first volt-second product and the enable signal. When the enable signal is valid, the first volt-second product is multiplied by the first proportional coefficient to obtain the third volt-second product. The second comparison circuit is used to receive the second volt-second product and the third volt-second product, and send a timing end signal when the second volt-second product is greater than the third volt-second product; A timing module is configured to receive the enable signal, and when the enable signal is valid, start timing for the minimum conduction time; and receive the timing end signal, and when the timing end signal is valid, terminate timing to obtain the minimum conduction time.

7. The synchronous rectification control circuit according to claim 6, characterized in that, Also includes: The first logic circuit is configured to receive a first turn-off signal associated with the minimum on-time at its first input terminal, receive a second turn-off signal controlling the turn-off of the synchronous rectifier at its second input terminal, and output a turn-off signal at its output terminal. The first trigger is configured to receive an on signal at its first input terminal, receive the off signal at its second input terminal, and output the control signal of the synchronous rectifier at its output terminal.

8. A flyback converter, characterized in that, It includes a synchronous rectification control circuit and applies the synchronous rectification control method according to any one of claims 1-5.

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