CN113708638B

CN113708638B - Synchronous rectification control circuit, control method and flyback switching power supply

Info

Publication number: CN113708638B
Application number: CN202110960642.6A
Authority: CN
Inventors: 傅钰泰; 文鹏
Original assignee: Shenzhen Biyi Microelectronics Co ltd
Current assignee: Shenzhen Biyi Microelectronics Co ltd
Priority date: 2021-08-20
Filing date: 2021-08-20
Publication date: 2023-04-18
Anticipated expiration: 2041-08-20
Also published as: CN113708638A

Abstract

The invention discloses a synchronous rectification control circuit, which comprises an LEB self-adaptive control module, wherein the LEB self-adaptive control module comprises a follow current time detection module, an error turn-off detection module and an LEB time adjustment module which are coupled with each other; the follow current time detection module is used for detecting and obtaining secondary side current follow current time and sending the secondary side current follow current time to the LEB time adjustment module and the error turn-off detection module; the error turn-off detection module is used for detecting to obtain a final error turn-off signal and sending the final error turn-off signal to the LEB time adjustment module; and the LEB time adjusting module calculates an LEB final prejudgment value of the next period according to the secondary side current follow current time and the final error turn-off signal. The invention also discloses a synchronous rectification control method and a flyback switching power supply. The invention can self-adaptively adjust the minimum conduction time of the synchronous rectifier tube, thereby adapting to the system requirement to the maximum extent.

Description

Synchronous rectification control circuit, control method and flyback switching power supply

Technical Field

The invention relates to the field of electronic information, in particular to a synchronous rectification control circuit, a control method and a flyback switching power supply.

Background

Synchronous Rectification (SR) is a technology that uses a rectifying MOSFET to replace a rectifying diode to reduce the rectifying loss. At present, the technology is widely applied to the fields of industrial power supplies, consumer electronics and the like. As shown in fig. 1 and 2, fig. 1 is a Flyback architecture using a rectifier diode for secondary side rectification, and fig. 2 is a Flyback architecture using a synchronous rectification technology for secondary side rectification. The synchronous rectifier tube (MOSFET tube) with extremely low on-state resistance is adopted to replace a rectifier diode, so that the rectification loss can be reduced.

In the synchronous rectification technology, voltage control is often adopted for controlling a synchronous rectifier tube, the drain-source voltage VDS of the synchronous rectifier tube is detected in real time, and the synchronous rectifier tube is turned on and off by comparing with a preset turn-on threshold Vth _ on and a turn-off threshold Vth _ off, wherein the basic principle is as shown in fig. 3.

Specifically, as shown in fig. 3, when it is detected that the drain-source voltage VDS is smaller than the turn-on threshold Vth _ on, it is determined that the body diode of the synchronous rectifier tube is turned on, and then the synchronous rectifier tube may be controlled to be turned on. When the drain-source voltage VDS is detected to be larger than the turn-off threshold Vth _ off, the follow current of the secondary side circuit is considered to be finished, and the controller controls the synchronous rectifying tube to be turned off at the moment. However, due to the influence of parasitic parameters such as leakage inductance of the transformer, the freewheeling current ISD of the secondary side circuit at the initial freewheeling stage oscillates, the drain-source voltage VDS of the synchronous rectifier tube also oscillates, and when the drain-source voltage VDS oscillates seriously, the situation that the drain-source voltage VDS passes through the turn-off threshold Vth _ off upwards may occur, which may cause the controller to make a misjudgment to turn off the synchronous rectifier tube, and once the synchronous rectifier tube is turned off by mistake, the loss is increased, and the heat generation is intensified, thereby affecting the system performance and reliability. In order to avoid the early turn-off of the synchronous rectifier, in the prior art, a fixed minimum on-time LEB is usually set after the synchronous rectifier is turned on, so as to shield the forward zero-crossing oscillation of the drain-source voltage VDS in the initial follow-current stage of the secondary side circuit. In the industry, the minimum on-time LEB is generally set to a fixed value, but there is also a method of adjusting the minimum on-time LEB by using an IC Pin and an external resistor, and the minimum on-time LEB is configured in a certain range by the external resistor to adapt to different application systems.

In the prior art, the minimum on-time LEB adopts a fixed threshold, which is simple but has general applicability and high requirements on system design, and may be turned off by mistake once design parameters are changed. The mode that adopts external components and parts configuration is better, can make the application wider, but additionally occupies the Pin foot of IC.

Disclosure of Invention

The invention provides a synchronous rectification control circuit and a control method aiming at the defects in the prior art.

In order to solve the technical problems, the invention is solved by the following technical scheme:

the invention discloses a synchronous rectification control circuit which can adjust the minimum conduction time of a synchronous rectification tube and comprises an LEB self-adaptive control module, wherein the LEB self-adaptive control module comprises

The follow current time detection module is used for detecting and obtaining secondary side current follow current time and respectively sending the secondary side current follow current time to the LEB time adjustment module and the error turn-off detection module;

the error turn-off detection module is used for detecting to obtain a final error turn-off signal and sending the final error turn-off signal to the LEB time adjustment module;

the LEB time adjusting module is used for calculating an LEB final prejudgment value of the next period according to the secondary side current follow current time and the final error turn-off signal;

the output end of the follow current time detection module is respectively coupled with the input end of the error turn-off detection module and the input end of the LEB time adjustment module, and the output end of the error turn-off detection module is coupled with the input end of the LEB time adjustment module.

Optionally, the freewheel time detecting module includes a first timer for calculating the freewheel time of the secondary side current; the starting point of the secondary side current freewheeling time is the rising edge of the driving signal, and the end point of the timing is the moment when the drain-source voltage of the synchronous rectifier tube is greater than the preset voltage.

Optionally, the false turn-off detection module includes a second timer, a first comparator and a second comparator; the output end of the second timer is respectively coupled with the input end of the first comparator and the input end of the second comparator; the second timer is used for calculating the actual conduction time of the synchronous rectifier tube, the timing starting point is the rising edge of the driving signal, and the timing end point is the falling edge of the driving signal; the first comparator compares the actual on-time with the secondary side current freewheeling time; the second comparator compares the actual on-time with the current period value of the LEB of the current period; and the final result of the AND of the output result of the first comparator and the output result of the second comparator is the final error turn-off signal.

Optionally, the LEB time adjusting module includes an accumulator, a third comparator and a fourth comparator; the output end of the accumulator is respectively coupled with the input end of the third comparator and the input end of the fourth comparator; the input of the accumulator is the current LEB period value and the fixed adjusting time, and the output is the LEB preliminary prejudgment value of the next period; the third comparator judges the validity of the LEB preliminary prejudgment value by comparing the LEB preliminary prejudgment value with a first multiple of the secondary side current follow current time; the fourth comparator judges the validity of the LEB preliminary prejudgment value by comparing the LEB preliminary prejudgment value with a maximum LEB time allowed in a system; when both the third comparator and the fourth comparator determine that the LEB preliminary prejudgment value is valid, the LEB final prejudgment value is the LEB preliminary prejudgment value; when at least one of the third comparator and the fourth comparator judges that the LEB preliminary prejudgment value is invalid, the LEB final prejudgment value is the minimum value of the maximum LEB time and the first multiple of the secondary side current freewheeling time.

Optionally, the system further comprises a switching-on detection module, a switching-off detection module, a trigger and a first and gate; two input ends of the trigger are respectively coupled to an output end of the turn-on detection module and an output end of the turn-off detection module, an output end of the trigger and an output end of the LEB adaptive control module are respectively coupled to two input ends of the first AND gate, and an output end of the first AND gate outputs the driving signal.

The invention also discloses a flyback switching power supply which comprises a synchronous rectification control circuit.

The invention also discloses a synchronous rectification control method for adjusting the minimum conduction time of the synchronous rectification tube, which comprises the following steps:

setting an initial value of the minimum on-time to be an LEB initial value, and setting an allowed maximum LEB time in the system;

detecting secondary side current freewheeling time and actual conduction time of the synchronous rectifier tube;

judging whether the synchronous rectifier tube is turned off by mistake;

entering an LEB time adjusting step when the judging result is that the switching-off is mistaken;

when the judgment result is that the LED is not turned off by mistake, the final prejudgment value of the LEB in the next period is equal to the current period value of the LEB;

and obtaining and outputting the final prejudged value of the LEB.

Optionally, the LEB time adjusting step includes the following:

setting an LEB preliminary prejudgment value of a next period as the sum of the LEB current period value and fixed adjustment time;

judging whether the LEB preliminary prejudgment value at the moment is smaller than the maximum LEB time and the secondary side current follow current time of a first multiple;

when the judgment result is yes, the LEB preliminary prejudgment value is valid;

and when the judgment result is negative, the LEB preliminary prejudgment value at the moment is invalid, and the LEB final prejudgment value is made to be the minimum value of the maximum LEB time and the secondary side current follow current time of the first multiple.

Optionally, the step of determining whether the synchronous rectifying tube is turned off by mistake includes: judging whether the actual on-time is simultaneously less than or equal to a second multiple of the current period value of the LEB and a third multiple of the secondary side current freewheeling time or not;

if so, the synchronous rectifier tube is turned off by mistake;

and if the judgment result is negative, the synchronous rectifier tube is not turned off by mistake.

Optionally, the first multiple is a coefficient in the range of [0,1 ]; the second multiple is a coefficient not less than 1; the third multiple is a coefficient in the range of [0,1 ].

1. The invention judges whether the current period is mistakenly turned off or not by judging the actual conduction condition of the synchronous rectifier tube, further judges whether the minimum conduction time of the subsequent period needs to be increased or not, and simultaneously determines the upper limit of the minimum conduction time of the next period according to the maximum LEB time and the secondary side current freewheeling time of the current period, thereby being capable of adapting to the requirement of the system on the minimum conduction time to the maximum extent.

2. The invention prevents the problems of additional stress and loss caused by the fact that the opening time of the synchronous rectifier tube exceeds the actual secondary side current follow current time to cause negative current due to too much increase of the LEB preliminary prejudged value by adding the limitation of the maximum LEB time to the LEB final prejudged value of the next period and adding comparison with the secondary side current follow current time of the previous period.

3. The synchronous rectification control circuit and the control method provided by the invention can ensure that the minimum conduction time of the synchronous rectification tube in an actual system can be adapted to the system requirement to the greatest extent, not only avoids the problem of inflexible application of fixed minimum conduction time, but also avoids the adoption of additional Pin Pin external regulation, and has good system applicability while greatly improving the efficiency.

Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention and do not constitute a limitation of the invention. In the drawings:

FIG. 1 shows a Flyback architecture with a diode for secondary side rectification;

FIG. 2 shows a Flyback architecture with synchronous rectification for secondary side rectification;

FIG. 3 is a graph illustrating the operating principle of a synchronous rectifier;

FIG. 4 is a schematic diagram of an embodiment of a synchronous rectification control circuit;

FIG. 5 is a diagram showing the structure of an LEB adaptive control module of the synchronous rectification control circuit according to the embodiment;

FIG. 6 shows a flywheel time detection module schematic of an embodiment synchronous rectification control circuit;

FIG. 7 is a schematic diagram of a false turn-off detection module of the synchronous rectification control circuit of the embodiment;

FIG. 8 shows a schematic diagram of an LEB time adjustment block of an embodiment synchronous rectification control circuit;

FIG. 9 is a flow chart diagram illustrating an embodiment synchronous rectification control method;

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the drawings of the embodiments of the present invention. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the described embodiments of the invention without inventive step, are within the scope of protection of the invention.

Unless defined otherwise, technical or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The use of "first," "second," and similar terms in the description and claims of the present application do not denote any order, quantity, or importance, but rather the terms are used to distinguish one element from another.

The invention discloses an embodiment of a synchronous rectification control circuit, which is coupled with a synchronous rectification tube and used for automatically controlling the minimum on-time (LEB) of the synchronous rectification tube. According to the embodiment, the minimum on-time of the synchronous rectifier tube can be adjusted in a self-adaptive manner under the condition that the synchronous rectifier tube is turned off by mistake because the actual on-time t2 (n) is smaller than the secondary side current freewheeling time ton (n), so that the problem of continuous wrong turn-off of the synchronous rectifier tube is avoided, and the system performance and the reliability can be improved. In this embodiment, whether the minimum on-time of the next cycle needs to be adjusted and the specific adjustment amount are determined according to the on-state condition of the synchronous rectifier in the current cycle, so as to provide the minimum on-time pre-determined value of the next cycle. Since the system has the preset allowable maximum LEB time t3 (max), the embodiment can determine the final minimum on-time prejudged value of the next period according to the given minimum on-time prejudged value and the maximum LEB time t3 (max), and the actual on-time t2 (n) of the current period, and execute the LEB logic according to the final calculated final minimum on-time in the next period.

In the present embodiment, the minimum on-time has an allowable maximum value in the system, i.e., the maximum LEB time t3 (max); the minimum on-time pre-determined value is abbreviated as LEB preliminary pre-determined value t1 (n + 1), and the final minimum on-time pre-determined value is abbreviated as LEB final pre-determined value tLEB (n + 1).

As shown in fig. 4, the synchronous rectification control circuit disclosed in this embodiment includes a synchronous rectifier, an LEB adaptive control module 300, an on detection module 100, an off detection module 200, a flip-flop 400, and a first and gate 500, two input terminals of the flip-flop 400 are respectively coupled to the on detection module 100 and the off detection module 200, an output terminal of the flip-flop 400 and an output terminal of the LEB adaptive control module 300 are electrically connected to two input terminals of the first and gate 500 together, an output terminal of the first and gate 500 outputs a driving signal SR PWM, and the driving signal SR PWM is fed back to the synchronous rectification control circuit of this embodiment. In this embodiment, the flip-flop 400 is an RS flip-flop, so in this embodiment, the output terminal of the turn-on detection module 100 is electrically connected to the S input terminal of the flip-flop 400, the turn-off detection module 200 is electrically connected to the R input terminal of the flip-flop 400, the output signal of the flip-flop 400 is an on signal or an off signal of the synchronous rectifier, that is, the flip-flop 400 determines the final output by internal logic according to the output of the turn-on detection module 100 and the output of the turn-off detection module 200, the output on signal indicates that the synchronous rectifier is in an on state, and the output off signal indicates that the synchronous rectifier is in an off state.

As shown in fig. 5, in one embodiment, the LEB adaptive control module 300 includes a freewheel time detection module 310, a false turn-off detection module 320, and an LEB time adjustment module 330, which are electrically connected to each other. The output terminal of the freewheel time detection module 310 is coupled to the input terminal of the false turn-off detection module 320 and the input terminal of the LEB time adjustment module 330, respectively, and the output terminal of the false turn-off detection module 320 is coupled to the input terminal of the LEB time adjustment module 330. The input of the LEB adaptive control module is a drain-source voltage Vds of the synchronous rectifier tube and a driving signal SR PWM, the output of the output end is a LEB final pre-judgment value tLEB (n + 1) of the next period of the synchronous rectifier tube, the LEB final pre-judgment value tLEB (n + 1) output by the LEB adaptive control module 300 and an output signal of the trigger 400 are jointly connected to two input ends of the first and gate 500, the output end of the first and gate 500 outputs the driving signal SR PWM after the phase and operation, and the driving signal SR PWM is fed back to the LEB adaptive control module 300.

As shown in fig. 6, in one embodiment, the freewheel time detection module 310 is configured to obtain the secondary side current freewheel time ton (n) through calculation, and send the secondary side current freewheel time ton (n) to the LEB time adjustment module 330 and the false turn-off detection module 320 for processing. The freewheel time detecting module 310 includes a first timer 311, and the first timer 311 is configured to calculate a secondary-side current freewheel time ton (n). The starting point of the first timer 311 is the high-level rising edge of the driving signal SR PWM, and the ending point of the timing is the time when the drain-source voltage Vds of the synchronous rectifier tube is greater than the preset voltage, and the time period between the starting point and the ending point of the timing by the first timer 311 is the time value of the secondary-side current freewheeling time ton (n). In other embodiments, the value of the preset voltage can be set according to actual requirements.

As shown in fig. 7, in one embodiment, the false shutdown detection module 320 is configured to calculate a final false shutdown signal and send the final false shutdown signal to the LEB time adjustment module 330 for processing. The false shutdown detection module 320 includes a second timer 323, a first comparator 321, and a second comparator 322. An output terminal of the second timer 323 is coupled to an input terminal of the first comparator 321 and an input terminal of the second comparator 322, respectively

The second timer 323 is configured to calculate an actual on-time t2 (n) of the synchronous rectifier, where a timing start point is a high-level rising edge of the driving signal SR PWM, a timing end point is a falling edge of the driving signal SR PWM, and a time period between the timing start point and the timing end point of the second timer 323 is a time value of the actual on-time t2 (n). The output of the second timer 323 is the actual on-time t2 (n).

The first comparator 321 determines whether the synchronous rectifier is turned off by mistake by comparing the actual on-time t2 (n) of the synchronous rectifier with a third multiple of the secondary-side current freewheeling time ton (n), where the third multiple is 0.5 times in the present embodiment, specifically, when the actual on-time t2 (n) is less than or equal to 0.5 times of the secondary-side current freewheeling time ton (n), that is:

and when t2 (n) is less than or equal to 0.5 × ton (n), judging that the synchronous rectifier tube is turned off by mistake.

The second comparator 322 determines whether the synchronous rectifier is turned off by mistake by comparing the actual on-time t2 (n) of the synchronous rectifier with the second multiple of the current period value tLEB (n), in this embodiment, the second multiple is 1.1 times. Specifically, when the actual on-time t2 (n) is less than or equal to 1.1 times the LEB current period value tLEB (n), i.e.:

and when t2 (n) is less than or equal to 1.1 × tLEB (n), judging that the synchronous rectifier tube is turned off by mistake.

The final result of the two output judgment results of the first comparator 321 and the second comparator 322 after the and-taking process is used as the final error shutdown signal. I.e. when the actual on-times t2 (n) are simultaneously satisfied

And when t2 (n) is less than or equal to 0.5 × ton (n) and t2 (n) is less than or equal to 1.1 × tleb (n), the final error turn-off signal is judged as error turn-off. The false shutdown detection module 320 sends the final false shutdown signal to the LEB time adjustment module 330 for processing. In one embodiment, the output terminal of the first comparator 321 and the output terminal of the second comparator 322 are respectively connected to two input terminals of a second and gate 324, and the output terminal of the second and gate 324 outputs the final false shutdown signal.

As shown in fig. 8, in one embodiment, the LEB time adjustment module 330 obtains the LEB final predetermined value tLEB (n + 1) of the next period by computationally analyzing the secondary side current freewheel time ton (n) and the final false turn-off signal. Specifically, the LEB time adjustment module 330 includes an accumulator 331, a third comparator 333, and a fourth comparator 334. The output of the accumulator 331 is coupled to the input of the third comparator 333 and the input of the fourth comparator 334, respectively.

The LEB time adjustment module 330 operates the accumulator 331 according to the final miscut signal received. The input of the accumulator 331 is the minimum on-time of the current period, i.e. the current period value tLEB (n), of the LEB, and the input of the fixed adjusting time T, and the output of the output is the preliminary prejudged value T1 (n + 1) of the LEB of the next period. The accumulator 331 functions to add the LEB current period value tLEB (n) and the fixed adjustment time T, so the output of the accumulator 331 is T1 (n + 1) = [ tLEB (n) + T ]. In the present embodiment, the fixed adjustment time T is 50ns, and in other embodiments, the fixed adjustment time T may be set to other time values according to system requirements.

The third comparator 333 compares the LEB preliminary prediction value t1 (n + 1) output from the accumulator 331 with the first multiple of the secondary-side current freewheel time ton (n), thereby determining whether the LEB preliminary prediction value t1 (n + 1) may exceed the secondary-side current freewheel time ton (n). In this embodiment, the first multiple is 2/3 times, so the specific judgment method for comparing the LEB preliminary predetermined value t1 (n + 1) with the secondary side current freewheel time ton (n) of 2/3 times is as follows:

when t1 (n + 1) < 2/3 × ton (n), the LEB preliminary prejudgment value t1 (n + 1) output by the accumulator 331 is determined to be valid, and the LEB final prejudgment value tLEB (n + 1) of the next cycle is equal to the LEB preliminary prejudgment value t1 (n + 1) determined to be valid at this time, that is, tLEB (n + 1) = t1 (n + 1);

when t1 (n + 1) — is not less than 2/3 × ton (n), the preliminary LEB prediction value t1 (n + 1) output from the accumulator 331 is determined to be invalid, and the final LEB prediction value tLEB (n + 1) of the next cycle is equal to 2/3 times the secondary side current freewheel time ton (n), that is, tLEB (n + 1) =2/3 × ton (n).

The fourth comparator 334 is used to determine whether the LEB preliminary predetermined value t1 (n + 1) output by the accumulator 331 exceeds the maximum LEB time t3 (max) allowed in the circuitry. The LEB preliminary prejudice value t1 (n + 1) output by the accumulator 331 is compared with the maximum LEB time t3 (max), i.e.:

when t1 (n + 1) < t3 (max), the LEB preliminary pre-determined value t1 (n + 1) output by the accumulator 331 is determined to be valid, and the LEB final pre-determined value tLEB (n + 1) of the next cycle is equal to the LEB preliminary pre-determined value t1 (n + 1) determined to be valid at this time, that is, tLEB (n + 1) = t1 (n + 1);

when t1 (n + 1) has no negative or positive speed with respect to t3 (max), the LEB preliminary prediction value t1 (n + 1) output from the accumulator 331 is determined to be invalid, and the LEB final prediction value tLEB (n + 1) of the next cycle is equal to the maximum LEB time t3 (max), i.e., tLEB (n + 1) = t3 (max).

In one embodiment, after the LEB preliminary predetermined value t1 (n + 1) output by the accumulator 331 passes through the determination processes of the third comparator 333 and the fourth comparator 334 respectively:

when both the two judgments judge that the preliminary LEB prejudgment value t1 (n + 1) is valid, the final LEB prejudgment value tLEB (n + 1) of the next period which is finally output is the preliminary LEB prejudgment value t1 (n + 1) at the moment, namely: tLEB (n + 1) = t1 (n + 1).

When at least one judgment t1 (n + 1) is invalid, the final prejudgment value tLEB (n + 1) of the last output next cycle is the minimum value of the secondary side current free-wheeling time ton (n) and the maximum LEB time t3 (max) which are 2/3 times, namely: tLEB (n + 1) = Min [2/3 × ton (n), t3 (max) ].

Furthermore, it should be understood that the specific values of the first multiple, the second multiple and the third multiple are not limited to the specific values in the present embodiment. The first multiple of the present embodiment is 2/3 times, but in other embodiments, the first multiple may be a coefficient within a range of [0,1] according to a preset requirement. In other embodiments, the second multiple may be set to a coefficient not less than 1 according to a predetermined requirement. In other embodiments, the third multiple may be a coefficient within a range of [0,1] according to a preset requirement.

The invention also discloses an embodiment of the flyback switching power supply, which comprises the embodiment of the synchronous rectification control circuit disclosed in the foregoing.

The present invention further discloses an embodiment of a synchronous rectification control method, as shown in fig. 9, which is suitable for the embodiment of the synchronous rectification control circuit disclosed above, and is mainly used for the LEB adaptive control module 300 to adjust the control method of the LEB final predetermined value tLEB (n + 1) of the next period, that is, to adjust the minimum on-time of the synchronous rectification tube in the system. The method specifically comprises the following steps:

step S10: the initial value of the minimum conduction time of the synchronous rectifier tube is set as an LEB initial value, and the allowed maximum LEB time t3 (max) in the system is set. It should be understood that the system itself is set with an allowable maximum LEB time t3 (max), and the control method of the present embodiment starts with the initial LEB value first by adjusting the LEB preliminary prejudice value t1 (n + 1) after starting the start-up.

Furthermore, in one embodiment, the LEB is initialized to N times the maximum LEB time t3 (max), i.e.:

LEB initial value = N × t3 (max), N being a coefficient within a value [0,1 ].

Step S20: the secondary-side current freewheeling time ton (n) is detected, and the actual on-time t2 (n) of the synchronous rectifier is detected. In this step, the secondary-side current freewheel time ton (n) is calculated by the first timer 311 in the freewheel time detection module 310, where the start time of the first timer 311 is the time when the high-level rising edge of the driving signal SR PWM is located, and the end time of the timing is the time when the drain-source voltage Vds is greater than 3V (preset voltage).

Step S30: and judging whether the synchronous rectifier tube is turned off by mistake or not, and obtaining a final error turn-off signal. In this step, the false turn-off detection module 320 determines the relationship between the actual on-time t2 (n) and the secondary-side current freewheeling time ton (n) through the first comparator 321, and determines the relationship between the actual on-time t2 (n) and the current period value tLEB (n) of the LEB through the second comparator 322. When the actual on-time t2 (n) is less than or equal to 0.5 times of the secondary current freewheeling time ton (n) (i.e. the third time of the secondary current freewheeling time ton (n)), and when the actual on-time t2 (n) is less than or equal to 1.1 times of the current period value tLEB (n) (i.e. the second time of the current period value tLEB (n)), the synchronous rectifier tube is determined to be turned off by mistake. That is, when t2 (n) is equal to or less than 0.5 × ton (n) and t2 (n) is equal to or less than 1.1 × tleb (n) are simultaneously satisfied, it is determined that the synchronous rectifier tube is turned off by mistake, and the final false turn-off signal output by the false turn-off detection module 320 is turned off by mistake; otherwise, when at least one of the terms is not satisfied, it is determined that the synchronous rectifier tube is not turned off by mistake, and the final false turn-off signal output by the false turn-off detection module 320 is that the false turn-off is not generated.

Step S40: when the judgment result in the step S30 is that the switch-off is mistaken, entering an LEB time adjusting step;

when the error shutdown does not occur in step S30, the last output final prejudged value tLEB (n + 1) of the next period is equal to the current period value tLEB (n) of the LEB, that is, tLEB (n + 1) = tLEB (n).

Step S50: and obtaining and outputting a final prejudgment value tLEB (n + 1) of the LEB of the next period.

And the system repeats the second step to the fifth step under the action of the driving signal SR PWM.

In one embodiment, the LEB time adjustment step comprises the sub-steps of:

substep S41: let the preliminary LEB prediction value T1 (n + 1) of the next period be the sum of the current period value tLEB (n) of the LEB and the fixed adjustment time T, wherein the fixed adjustment time T is 50ns in the present embodiment. Namely: t1 (n + 1) = tLEB (n) +50ns. In this step, when the final false turn-off signal is received as the false turn-off, the accumulator 331 in the LEB time adjustment module 330 calculates to obtain the LEB preliminary predetermined value t1 (n + 1).

Substep S42: it is determined whether the LEB preliminary prediction value t1 (n + 1) output by the accumulator 331 at this time is smaller than both the maximum LEB time t3 (max) and 2/3 times the secondary-side current freewheel time ton (n) (the first multiple of the secondary-side current freewheel time ton (n)). Here, this step compares the LEB preliminary prey value t1 (n + 1) with the secondary-side current freewheel time ton (n) by the third comparator 333, and compares the LEB preliminary prey value t1 (n + 1) with the maximum LEB time t3 (max) by the fourth comparator 334.

If the determination result is yes, that is, if it is determined that t1 (n + 1) < 2/3 × ton (n) and t1 (n + 1) < t3 (max), it is determined that the LEB preliminary pre-determination value t1 (n + 1) at this time is valid, and then let the last output LEB final pre-determination value tLEB (n + 1) of the next cycle be the LEB preliminary pre-determination value t1 (n + 1) at this time, that is, tLEB (n + 1) = t1 (n + 1):

if the judgment result is negative, that is, at least one of the maximum LEB time t3 (max) and the secondary side current flywheel time ton (n) which is 2/3 times of the LEB preliminary pre-judgment value t1 (n + 1) is not greater than the LEB preliminary pre-judgment value t1 (n), the minimum value of the secondary side current flywheel time ton (n) which is 2/3 times of the LEB final pre-judgment value tLEB (n + 1) in the next period is set, that is, the minimum value is: tLEB (n + 1) = Min [2/3 × ton (n), t3 (max) ].

In summary, the LEB adaptive control module 300 can adaptively adjust the minimum on-time by the synchronous rectification control method. In the embodiment, whether the current cycle is turned off by mistake is judged by judging the actual on-state condition of the synchronous rectifier tube, and then whether the minimum on-state time of the following cycle needs to be increased is judged, and meanwhile, the upper limit of the minimum on-state time of the next cycle is determined according to the maximum LEB time t3 (max) and the secondary side current freewheeling time ton (n) of the current cycle, so that the requirement of the system on the minimum on-state time can be met to the greatest extent.

In this embodiment, once the synchronous rectifier is turned off by mistake, the accumulator 331 increases the current period value tLEB (n) by 50ns to obtain the preliminary predetermined value t1 (n + 1) of LEB. Meanwhile, the LEB adaptive control module 300 determines whether the increased LEB preliminary predetermined value t1 (n + 1) exceeds the maximum LEB time t3 (max) and determines whether 2/3 times of the secondary side current freewheel time ton (n) is exceeded. The above embodiment adds the limit of the maximum LEB time t3 (max) to the LEB final predetermined value tLEB (n + 1) of the next period and adds the comparison with the secondary side current freewheeling time ton (n) of the previous period, so as to prevent the problem of additional stress and loss caused by the negative current caused by the fact that the on-time of the synchronous rectifier exceeds the actual secondary side current freewheeling time ton (n) due to the fact that the LEB initial predetermined value t1 (n + 1) is increased too much.

The synchronous rectification control circuit and the control method provided by the embodiment can ensure that the minimum on-time of the synchronous rectification tube in an actual system can be adapted to the system requirement to the greatest extent, not only can the problem of inflexible application of fixed minimum on-time be avoided, but also the adoption of additional Pin Pin external regulation is avoided, and the efficiency can be greatly improved, and meanwhile, the synchronous rectification control circuit and the control method have good system applicability.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

In summary, the above-mentioned embodiments are only preferred embodiments of the present invention, and all equivalent changes and modifications made in the claims of the present invention should be covered by the claims of the present invention.

Claims

1. The synchronous rectification control circuit is characterized by comprising an LEB self-adaptive control module, wherein the LEB self-adaptive control module comprises

2. The synchronous rectification control circuit of claim 1, wherein the freewheel time detection module comprises a first timer for calculating the freewheel time of the secondary side current; the starting point of the secondary side current freewheeling time is the rising edge of the driving signal, and the end point of the timing is the moment when the drain-source voltage of the synchronous rectifier tube is greater than the preset voltage.

3. The synchronous rectification control circuit of claim 1, wherein the false turn-off detection module comprises a second timer, a first comparator and a second comparator; the output end of the second timer is respectively coupled with the input end of the first comparator and the input end of the second comparator;

the second timer is used for calculating the actual conduction time of the synchronous rectifier tube, the timing starting point is the rising edge of the driving signal, and the timing end point is the falling edge of the driving signal;

the first comparator compares the actual on-time with the secondary side current freewheeling time; the second comparator compares the actual on-time with the current period value of the LEB of the current period; and the final result of the AND of the output result of the first comparator and the output result of the second comparator is the final error turn-off signal.

4. The synchronous rectification control circuit of claim 1, wherein the LEB time adjustment module comprises an accumulator, a third comparator and a fourth comparator; the output end of the accumulator is respectively coupled with the input end of the third comparator and the input end of the fourth comparator;

the input of the accumulator is the current LEB period value and the fixed adjusting time, and the output is the LEB preliminary prejudgment value of the next period;

the third comparator is used for judging the effectiveness of the LEB preliminary prejudgment value by comparing the LEB preliminary prejudgment value with the secondary side current follow current time of a first multiple;

the fourth comparator judges the validity of the LEB preliminary prejudgment value by comparing the LEB preliminary prejudgment value with a maximum LEB time allowed in a system;

when the third comparator and the fourth comparator both judge that the LEB preliminary prejudgment value is valid, the LEB final prejudgment value is the LEB preliminary prejudgment value;

when at least one of the third comparator and the fourth comparator determines that the LEB preliminary prejudgment value is invalid, the LEB final prejudgment value is a minimum value of the maximum LEB time and a first multiple of the secondary side current freewheel time.

5. The synchronous rectification control circuit of any one of claims 1 to 4, further comprising a turn-on detection module, a turn-off detection module, a flip-flop and a first AND gate; two input ends of the trigger are respectively coupled to an output end of the turn-on detection module and an output end of the turn-off detection module, an output end of the trigger and an output end of the LEB adaptive control module are respectively coupled to two input ends of the first AND gate, and an output end of the first AND gate outputs a driving signal.

6. A flyback switching power supply comprising the synchronous rectification control circuit of claim 5.

7. A synchronous rectification control method is used for adjusting the minimum conduction time of a synchronous rectification tube, and is characterized by comprising the following steps:

setting the initial value of the minimum on-time as an LEB initial value, and setting the allowed maximum LEB time in the system;

judging whether the synchronous rectifier tube is turned off by mistake;

and obtaining and outputting the final prejudged value of the LEB.

8. The synchronous rectification control method of claim 7, wherein the LEB time adjustment step includes the following:

judging whether the LEB preliminary prejudgment value at the moment is smaller than the maximum LEB time and the secondary side current follow current time of a first multiple at the same time;

9. The synchronous rectification control method of claim 8, wherein the step of determining whether the synchronous rectification tube is turned off by mistake comprises: judging whether the actual on-time is simultaneously less than or equal to a second multiple of the current period value of the LEB and a third multiple of the secondary side current freewheeling time or not;

if so, the synchronous rectifier tube is turned off by mistake;

10. The synchronous rectification control method of claim 9, wherein the first multiple is a coefficient in a range of [0,1 ]; the second multiple is a coefficient not less than 1; the third multiple is a coefficient in the range of [0,1 ].