CN111355392A - Self-adaptive synchronous rectification control system and method of LLC resonant converter - Google Patents

Abstract

The invention relates to a self-adaptive synchronous rectification control system and a method of an LLC resonant converter, wherein the system comprises: the input end of the sampling circuit is connected with the drain electrode of the synchronous rectifying tube and is used for collecting the drain electrode voltage of the synchronous rectifying tube; the input end of the comparator is connected with the output end of the sampling circuit and is used for comparing the input drain voltage with a preset voltage threshold value and outputting a comparison result; and the control unit is connected with the comparator and the grid electrode of the synchronous rectifying tube and is used for controlling the conduction duration of the synchronous rectifying tube in one working period. The invention can directly judge the voltage of the drain terminal of the synchronous rectifier tube so as to obtain whether the synchronous rectifier tube is in the optimal turn-off state, and accordingly, the turn-off control is carried out on the synchronous rectifier tube so as to adjust the synchronous rectifier tube to a proper turn-off point. The method adopts a self-adaptive synchronous rectification algorithm, and can realize self-adaptive control of the conduction time of the synchronous rectification tube according to the change trend of the switching frequency of the primary side switching tube.

Description

Adaptive synchronous rectification control system and method for LLC resonant converter

technical field

The invention relates to synchronous rectification technology, in particular to an adaptive synchronous rectification control system and method of an LLC resonant converter.

Background technique

As an excellent topology, the LLC resonant converter has now become a recognized and widely used power topology. Because the LLC resonant converter can realize zero-voltage turn-on of the primary side power tube and zero-current turn-off of the secondary side rectifier diode in a wide load range, it has the characteristics of higher working efficiency and higher switching frequency than the hard switching topology.

However, when the LLC resonant converter works under heavy load, due to the conduction voltage drop of the secondary side rectifier diode, a large current flows through the rectifier diode, resulting in a large energy loss after passing through the rectifier diode, which reduces the working efficiency of the LLC resonant converter. . In order to solve the above problems, a synchronous rectifier method is proposed, which takes advantage of the low on-resistance of the power tube and replaces the diode with a power tube to greatly improve the working efficiency of the LLC resonant converter when it works under heavy load conditions.

In the LLC resonant converter, the turn-on point of the synchronous rectifier tube is basically the same as the power tube corresponding to the primary side, and the turn-off point of the synchronous rectifier tube will affect the working efficiency and characteristics of the overall system sooner or later. When the synchronous rectifier is turned off earlier than the optimal turn-off point, the current will flow through the body diode of the synchronous rectifier, increasing the loss of the synchronous rectifier; when the synchronous rectifier is turned off later than the optimal turn-off point, the secondary side The current will affect the working state of the primary resonant tank of the LLC resonant converter and cause the distortion of the resonant current. In the prior art, there is a lack of a control method for a synchronous rectifier that can well solve the above problems.

SUMMARY OF THE INVENTION

Based on this, it is necessary to provide a control system and method for a synchronous rectifier tube suitable for an LLC resonant converter.

An adaptive synchronous rectification control system for an LLC resonant converter is used to control the turn-off of a synchronous rectifier in a half-bridge LLC resonant converter with a synchronous rectifier, the system comprising: a sampling circuit, an input end connected to The drain of the synchronous rectifier tube is used to collect the drain voltage of the synchronous rectifier tube; the input end of the comparator is connected to the output end of the sampling circuit, and is used to compare the inputted drain voltage with the preset voltage The threshold value is compared, and the comparison result is output; the control unit, connected with the comparator and the gate of the synchronous rectifier, is used to control the conduction time of the synchronous rectifier in one working cycle, and is also used to control the conduction time of the synchronous rectifier in the When the drain voltage is less than the preset voltage threshold, set the conduction duration of the synchronous rectifier in the next working cycle to be equal to the conduction duration of the current working cycle; the control unit is further configured to obtain the half-bridge LLC resonance The changing trend of the switching frequency of the switching tube of the half-bridge on the primary side of the converter, the synchronous rectification increases when the switching frequency of the switching tube of the half-bridge on the primary side of the half-bridge LLC resonant converter is in a decreasing trend The conduction time of the synchronous rectifier in the next working cycle is reduced when the switching frequency of the switching tube of the half-bridge on the primary side of the half-bridge LLC resonant converter is in an increasing trend to reduce the conduction of the synchronous rectifier in the next working cycle duration.

In one of the embodiments, a gate driver is further included, the input terminal of the gate driver is connected to the output terminal of the control unit, the output terminal of the gate driver is connected to the gate of the synchronous rectifier, and the control unit is The on-time duration of the synchronous rectifier in one duty cycle is controlled by the gate driver.

In one embodiment, a PWM timer is further included, the input end of the PWM timer is connected to the output end of the control unit, the output end of the PWM timer is connected to the gate of the synchronous rectifier, the The control unit controls the conduction time of the synchronous rectifier in one working cycle by adjusting the duty ratio of the PWM signal output by the PWM timer.

In one of the embodiments, the system includes an MCU integrated with the comparator, the control unit and the PWM timer.

In one embodiment, the inverting input terminal of the comparator is connected to the output terminal of the sampling circuit, the non-inverting input terminal of the comparator is connected to the voltage setting terminal of the control unit, and the control unit also uses for setting the preset voltage threshold.

In one of the embodiments, the sampling circuit has an absolute value taking function, and sets the drain voltage output to the comparator to a positive potential.

In one of the embodiments, the absolute values of the increased on-time and the decreased on-time are equal.

An adaptive synchronous rectification control method for an LLC resonant converter, which is used for turning off a synchronous rectifier tube in a half-bridge LLC resonant converter with a synchronous rectifier tube, the method comprising: step A, in the synchronous rectifier tube Obtain the drain voltage of the synchronous rectifier tube when the tube is turned off; step B, if the drain voltage is less than a preset voltage threshold, set the on-time duration of the synchronous rectifier tube in the next working cycle to be the same as the current working cycle The turn-on durations of the cycles are equal; in step C, if the drain voltage is greater than or equal to the preset voltage threshold, obtain the changing trend of the switching frequency of the switching tubes of the half-bridge on the primary side of the half-bridge LLC resonant converter ; Step D, increase the on-time length of the synchronous rectifier tube in the next working cycle when the switching frequency is in a decreasing trend, and return to step A in the next working cycle; when the switching frequency is in an increasing trend The on-time duration of the synchronous rectifier in the next work cycle is reduced, and the process returns to step A in the next work cycle.

In one embodiment, if the switching frequency remains stable, after step B, wait for a preset time period before returning to step A, and the preset time period is greater than the duty cycle.

In one of the embodiments, the absolute values of the increased on-time duration and the decreased on-time duration of the step D are equal each time.

The above-mentioned adaptive synchronous rectification control system and method of LLC resonant converter can directly judge the drain voltage of the synchronous rectifier tube, so as to obtain whether the synchronous rectifier tube is in the best off state, and accordingly, the synchronous rectifier tube is in the best off state. The rectifier tube is turned off and adjusted to an appropriate turn-off point. The method adopts an adaptive synchronous rectification algorithm, which can realize the adaptive control of the conduction time of the synchronous rectifier according to the change trend of the switching frequency of the primary side switch. And each cycle only needs to make at most one judgment, which simplifies the realization difficulty of the adaptive synchronous rectification system.

Description of drawings

In order to better describe and illustrate embodiments and/or examples of those inventions disclosed herein, reference may be made to one or more of the accompanying drawings. The additional details or examples used to describe the drawings should not be construed as limiting the scope of any of the disclosed inventions, the presently described embodiments and/or examples, and the best mode presently understood of these inventions.

Figure 1 is a circuit topology diagram of a half-bridge LLC resonant converter with a synchronous rectifier;

Fig. 2 is the principle block diagram of the adaptive synchronous rectification control system of LLC resonant converter in one embodiment;

Fig. 3 is the flow chart of the adaptive synchronous rectification control method of LLC resonant converter in one embodiment;

FIG. 4 is a waveform diagram of controlling the circuit shown in FIG. 1 by using the method shown in FIG. 3 when the switching frequency of the primary side switch tube is reduced;

FIG. 5 is a waveform diagram of using the method shown in FIG. 3 to control the circuit shown in FIG. 1 when the switching frequency of the primary side switch tube increases.

Detailed ways

In order to facilitate understanding of the present invention, the present invention will be described more fully hereinafter with reference to the related drawings. Preferred embodiments of the invention are shown in the accompanying drawings. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

Unless otherwise defined, all technical and 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 terms used herein in the description of the present invention are for the purpose of describing specific embodiments only, and are not intended to limit the present invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Fig. 1 is a circuit topology diagram of a half-bridge LLC resonant converter with a synchronous rectifier tube. The circuit topology of the synchronous rectifier LLC resonant converter includes: transformer primary winding _NP , transformer secondary first winding N _S1 , transformer secondary winding side second winding N _S1 ; power tubes M1, M2 on the primary side of the transformer; resonant tank, including resonant capacitor Cr, resonant inductance Lr and excitation inductance Lm; synchronous rectifier tubes M3, M4 on the secondary side of the transformer, output capacitor CO, etc. . Among them, the diodes D1 and D2 on the primary side of the transformer are the parasitic diodes of the power transistors M1 and M2 respectively, the capacitors C1 and C2 are the parasitic capacitors of M1 and M2 respectively; the D3 and D4 on the secondary side of the transformer are the synchronous rectifier transistors M3 and M2 respectively. The parasitic diodes of M4, C3 and C4 are the parasitic capacitances of the synchronous rectifiers M3 and M4, respectively. The power supply V _IN provides the input power for the primary side of the transformer, the output voltage of the output terminal of the LLC resonant converter is V _O , the output current is I _O , and the equivalent resistance of the load is R.

The adaptive synchronous rectification control method of the LLC resonant converter of the present application is used to control the turn-off of the synchronous rectifier in the half-bridge LLC resonant converter with the synchronous rectifier. For the circuit shown in FIG. The rectifiers M3 and M4 are turned off, and the control methods of the synchronous rectifiers M3 and M4 are the same. The following describes the adaptive synchronous rectification control method of the LLC resonant converter in one embodiment with reference to FIG. 3 .

When the input/output parameters of the LLC resonant converter change, the optimal turn-off point of the synchronous rectifier on the secondary side of the transformer will also change. The inventive idea of the present invention is that when the optimal turn-off point of the synchronous rectifier changes, the turn-off time of the synchronous rectifier is also adjusted accordingly. Referring to FIG. 3 , the adaptive synchronous rectification control method of the LLC resonant converter in one embodiment includes the following steps:

S310: Obtain the drain voltage of the synchronous rectifier when the synchronous rectifier is turned off.

In one embodiment, a sampling circuit may be set to sample the drain voltage of the synchronous rectifier. Since the on-time of the current duty cycle of the synchronous rectifier is known (obtained by the adaptive synchronous rectification control method of the LLC resonant converter of the present invention in the previous duty cycle), the synchronous rectifier can be judged according to the on-time of the current duty cycle The moment when the rectifier is turned off.

S320, if the drain voltage is less than the preset voltage threshold, set the on-time duration of the synchronous rectifier in the next working cycle to be equal to the on-time duration of the current working cycle.

If the obtained drain voltage V _SR of the synchronous rectifier is less than the preset voltage threshold V _H , it means that the synchronous rectifier in this working cycle is turned off at a low level when it is turned off, and the turn-off time of the synchronous rectifier in the current working cycle is Appropriate turn-off time, so the on-time duration of the synchronous rectifier remains unchanged in the next duty cycle.

If the obtained drain voltage V _SR of the synchronous rectifier is greater than or equal to the preset voltage threshold V _H , it means that the synchronous rectifier in this working cycle is turned off at a high level when it is turned off, and it is necessary to turn off the synchronous rectifier. Adjustment. In this embodiment, it is determined whether the synchronous rectifier is turned off earlier or later than the optimal turn-off point by obtaining the switching frequency change trend of the half-bridge switching tube on the primary side of the half-bridge LLC resonant converter. When the switching frequency of the switch tube is on an increasing trend, go to step S330; and when the switching frequency is on a decreasing trend, go to step S340.

S330, reducing the conduction time of the synchronous rectifier in the next working cycle.

In this embodiment, when the switching frequency of the switch tube is increasing, the on-time of the synchronous rectifier tube in the next working cycle is reduced by Δt, and the process returns to step S310, and the above actions are performed once in the next working cycle. That is, if the measured V _SR >V _H in each subsequent cycle, the on-time of the next duty cycle of the synchronous rectifier continues to decrease by Δt...until the measured V _SR < after one or more cycles of adjustment V _H , adjust the turn-off time of the synchronous rectifier to an appropriate value.

S340, increasing the on-time duration of the synchronous rectifier in the next working cycle.

In this embodiment, when the switching frequency of the switch tube is in a decreasing trend, the on-time of the synchronous rectifier tube in the next working cycle is extended by Δt, and the process returns to step S310 to perform the above actions once in the next working cycle. That is, if the measured V _SR >V _H in each subsequent cycle, the on-time of the next duty cycle of the synchronous rectifier will continue to extend Δt...until the measured V _SR < after one or more cycles of adjustment V _H , adjust the turn-off time of the synchronous rectifier to a suitable value.

It can be understood that, in one embodiment, the Δt reduced each time in step S330 and the Δt extended each time in step S340 are equal. The specific value of Δt can be set by those skilled in the art according to experience.

The above-mentioned adaptive synchronous rectification control method of LLC resonant converter can directly judge the drain voltage of the synchronous rectifier, so as to obtain whether the synchronous rectifier is in the best off state, and accordingly, the synchronous rectifier can be adjusted accordingly. Perform shutdown control and adjust to an appropriate shutdown point. The method adopts an adaptive synchronous rectification algorithm, which can realize the adaptive control of the conduction time of the synchronous rectifier according to the change trend of the switching frequency of the primary side switch. And only one judgment is needed in each cycle, which simplifies the realization difficulty of the adaptive synchronous rectification system.

After step S320, the off-time of the synchronous rectifier is appropriate, and at this time, if the system is in a stable state, the current off-time should be maintained. Therefore, in one embodiment, if the switching frequency of the switching transistors of the half-bridge on the primary side of the LLC resonant converter remains stable, after step S320, it waits for a preset time period and then returns to step S310 to synchronize the rectifier transistors within the preset time period. The on-time does not need to be adjusted to save system resources. It can be understood that the preset duration can be significantly longer than the duty cycle of the synchronous rectifier.

The invention also provides an adaptive synchronous rectification control system of the LLC resonant converter, which is used to control the turn-off of the synchronous rectifier in the half-bridge LLC resonant converter with the synchronous rectifier. FIG. 2 is a schematic block diagram of an adaptive synchronous rectification control system of an LLC resonant converter in an embodiment. An adaptive synchronous rectification control system 100 of the LLC resonant converter (hereinafter referred to as the control system 100 ) includes a sampling circuit 10 and a comparator 20 and the control unit 30.

The input end of the sampling circuit 10 is connected to the drain of the synchronous rectifier M4 for collecting the drain voltage V _SR of the synchronous rectifier M4 . It can be understood that the synchronous rectifier M3 is also connected to an identical control system 100 for control (not shown in FIG. 2 ). The following takes the control system 100 connected to the synchronous rectifier M4 as an example for specific description.

The input end of the comparator 20 is connected to the output end of the sampling circuit 30 for comparing the drain voltage input by the sampling circuit 30 with a preset voltage threshold and outputting the comparison result.

The control unit 30 is connected to the comparator 20 and the gate of the synchronous rectifier M4, and is used to control the conduction time of the synchronous rectifier M4 in one working cycle, and is also used to control the ON time of the synchronous rectifier M4 when the drain voltage V _SR is less than the preset voltage threshold V _H. , set the conduction duration of the synchronous rectifier in the next working cycle to be equal to the conduction duration of the current working cycle. The control unit 30 is further configured to obtain the change trend of the switching frequency of the switching transistors of the half-bridge on the primary side of the half-bridge LLC resonant converter (for example, the power transistor M1 and the power transistor M2 in FIG. 1 ). When the trend is small, the on-time of the synchronous rectifier M4 in the next working cycle is increased, and when the switching frequency is on an increasing trend, the on-time of the synchronous rectifier M4 in the next working cycle is decreased.

The adaptive synchronous rectification control system of the above LLC resonant converter can directly judge the drain voltage of the synchronous rectifier tube, so as to obtain whether the synchronous rectifier tube is in the best off state, and based on this, the synchronous rectifier tube can be judged directly. Perform shutdown control and adjust to an appropriate shutdown point. The method adopts an adaptive synchronous rectification algorithm, which can realize the adaptive control of the conduction time of the synchronous rectifier according to the change trend of the switching frequency of the primary side switch. And only one judgment is needed in each cycle, which simplifies the realization difficulty of the adaptive synchronous rectification system.

In one embodiment, the control system 100 also includes a gate driver 50 . The input terminal of the gate driver 50 is connected to the output terminal of the control unit 30, and the output terminal of the gate driver 50 is connected to the gate of the synchronous rectifier transistor M4. The control unit 30 controls the conduction time of the synchronous rectifier M4 in one working cycle through the gate driver 50 .

In one embodiment, the control system 100 may be specifically implemented by using an MCU (Microcontroller Unit, microcontroller). In one embodiment, the MCU may be integrated with the control unit 30 and the comparator 20 . Using MCU to realize digital control, the circuit structure is simple, no need for external complex circuits, and the parasitic effect caused by the parasitic parameters of the loop is reduced.

In the embodiment shown in FIG. 2 , the adaptive synchronous rectification control system 100 of the LLC resonant converter further includes a PWM timer 40 . The input end of the PWM timer 40 is connected to the output end of the control unit 30, the output end of the PWM timer 40 is connected to the gate driver 50, and the control unit 30 controls the synchronous rectifier tube M4 by adjusting the duty cycle of the PWM signal output by the PWM timer. On-time in one duty cycle. In other embodiments, control system 100 may also include PWM timer 40 without providing gate driver 50 .

In one embodiment, the MCU of the control system 100 may also be integrated with the PWM timer 40 .

In the embodiment shown in FIG. 2 , the inverting input terminal of the comparator 20 is connected to the output terminal of the sampling circuit 10 , and the non-inverting input terminal of the comparator 20 is connected to the voltage setting terminal of the control unit 30 . The control unit 30 is also used for setting the preset voltage threshold V _H provided to the non-inverting input terminal of the comparator 20 .

In one embodiment, when the output current _IO of the LLC converter changes, the adjustment direction of the conduction time of the secondary-side synchronous rectifier M4 is determined according to the change trend of the frequency f _SW of the primary-side switch. When the primary side switching frequency f _SW increases, the conduction time of the secondary side synchronous rectifier M4 should decrease accordingly; when the primary side switching frequency f _SW decreases, the conduction time of the secondary side synchronous rectifier M4 should increase accordingly.

The following describes the working principle of the control system 100 through an embodiment, please refer to FIG. 3 together:

First, it is judged whether the synchronous rectifier M4 is turned off, and if it is established, the sampling circuit 10 starts to work; otherwise, it returns to judgment. When the synchronous rectifier M4 is turned off, the sampling circuit 10 collects the voltage signal V _SR when the rectifier is turned off from the drain end of the synchronous rectifier M4 on the secondary side of the LLC converter, and transmits it to the MCU through the comparator inside the MCU 20. Compare V _SR with the set high level V _H of the MCU comparator and make a logical judgment. If V _SR < V _H is established, the synchronous rectifier M4 is turned off at a low level, and the control process ends; if V _SR < V _H is not established, the synchronous rectifier M4 is turned off at a high level. The change of the frequency f _SW is used to adjust the conduction time of the synchronous rectifier M4 in the next cycle. If the increase of f _SW is established, the conduction time of the synchronous rectifier M4 in the next cycle is reduced by Δt. If it is not established, that is, f _SW is reduced, the conduction time of the synchronous rectifier M4 in the next cycle is increased by Δt, and returns to the working steps of the sampling circuit 10 , repeat the above steps.

In one embodiment, the control process of the control system 100 is as follows: in the detection time period of the off state of the synchronous rectifier M4 in each working cycle, from the detected change trend of the switching frequency f _SW of the primary switch, Detect and logically judge the state of the LLC converter after the secondary side synchronous rectifier M4 is turned off in the current cycle. During the off period of the LLC converter's secondary synchronous rectifier M4, the sampling circuit 10 starts to work, and the sampling circuit 10 will The collected drain voltage V _SR of the synchronous rectifier tube M4 is transmitted to the inverting input terminal of the comparator 20 for comparison with the threshold voltage V _H set by the control unit 30 to perform logical judgment and judgment, and according to the detection result, the synchronization in the next working cycle is performed. The conduction time of the rectifier tube M4 is controlled, and the specific process is as follows:

(1) When the primary side switching frequency f _SW decreases, the on-time of the secondary side synchronous rectifier M4 should increase accordingly, see Figure 4 .

If V _SR1 <V _H , and the comparator output is 1, the turn-off time of the synchronous rectifier M4 in the current cycle is the appropriate turn-off time; wherein, V _SR1 represents the drain voltage of the synchronous rectifier M4 in the first cycle.

If V _SR1 >V _H , the output of the comparator is 0, the control unit 30 will increase a high level time Δt in the next cycle, prolong the conduction time of the synchronous rectifier M4 in the next cycle, and the sampling circuit 10 will collect the second cycle The drain terminal voltage V _SR2 of the synchronous rectifier M4 is transmitted to the inverting input terminal of the comparator 20 for comparison with the threshold voltage V _H set by the control unit 30 ;

If the measured V _SR >V _H in each next cycle, the output of the comparator 20 is 0, then the next cycle continues to increase a high level time Δt;

Until the measured V _SR <V _H in the current cycle, the output of the comparator 20 is 1, and the synchronous rectifier M4 of the cycle is turned off at a low level when it is turned off.

(2) When the switching frequency f _SW of the primary side increases, the on-time of the secondary side synchronous rectifier M4 should decrease, see Fig. 5 .

If V _SR1 < V _H , the comparator output is 1, then the SR of the cycle is turned off at a low level;

If V _SR1 >V _H , the output of the comparator is 0, then a high level time Δt is reduced in the next cycle to shorten the conduction time of the synchronous rectifier M4 in the next cycle, and the synchronous rectification of the next cycle collected by the sampling circuit 10 The drain terminal voltage V _SR2 of the tube M4 is transmitted to the inverting input terminal of the comparator 20 for comparison with the threshold voltage V _H set by the control unit 30;

If the measured V _SR >V _H in each next cycle, the output of the comparator 20 is 0, then the next cycle continues to reduce a high level time Δt;

Until V _SR <V _H measured in the current cycle, the output of the comparator 20 is 1, then the synchronous rectifier M4 of the cycle is turned off at a low level when it is turned off.

According to the change trend of the switching frequency f _SW of the primary side switch tube of the LLC converter to guide the adjustment trend of the conduction time of the secondary side synchronous rectifier tube M4, when sampling the drain-source voltage V _SR value of the synchronous rectifier tube M4 of the LLC resonant converter, Use the comparator 20 inside the MCU to output 0 or 1. In each switching cycle, the on-time length of the secondary-side synchronous rectifier M4 is changed by the control unit 30, so that the secondary-side synchronous rectifier M4 can be turned off at a low voltage level. In this way, the secondary side synchronous rectifier tube M4 of the LLC converter can be turned off at a low voltage point through the adjustment of several working cycles, so as to realize the function of self-adaptive synchronous rectification. Compared with the method of judging the state at the time of turn-off by collecting the current passing through the synchronous rectifier tube, the embodiment shown in FIG. 2 does not need to perform current sampling through the detection resistor, which has advantages in terms of circuit loss; The drain-source voltage VDS of the synchronous rectifier is determined by comparing the voltage and the turn-on threshold of the synchronous rectifier to determine whether the body diode of the synchronous rectifier is turned on, thereby controlling the turn-on and turn-off time of the synchronous rectifier. The body diode will switch in the on or off state, and the on-state body diode will increase the loss, so the embodiment shown in Figure 2 also has advantages in circuit loss; The length of the conduction time of its body diode determines the way to adjust the conduction time of the synchronous rectifier in the next cycle. When the system works at high frequency, the influence of the added parasitic parameters of the circuit cannot be ignored. The circuit oscillation of the MCU will affect the sampling results of the MCU, so the design of the anti-noise capability of the circuit must be considered. The circuit structure of the embodiment shown in Figure 2 is simple, and no external complex circuits are required. Parasitic effects caused by parasitic parameters.

The above-mentioned embodiments only represent several embodiments of the present invention, and the descriptions thereof are specific and detailed, but should not be construed as a limitation on the scope of the invention patent. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can also be made, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims (10)

1. a self-adaptive synchronous rectification control system of LLC resonant converter, for the synchronous rectifier tube in the half-bridge LLC resonant converter with synchronous rectifier tube is turned off and controlled, it is characterized in that, described system comprises: a sampling circuit, the input end is connected to the drain of the synchronous rectifier tube, and is used for collecting the drain voltage of the synchronous rectifier tube; a comparator, whose input end is connected to the output end of the sampling circuit, for comparing the inputted drain voltage with a preset voltage threshold, and outputting a comparison result; a control unit, connected to the comparator and the gate of the synchronous rectifier, for controlling the conduction time of the synchronous rectifier in one working cycle, and for when the drain voltage is less than a preset voltage threshold , set the conduction duration of the synchronous rectifier in the next working cycle to be equal to the conduction duration of the current working cycle; the control unit is also used to obtain the half-bridge on the primary side of the half-bridge LLC resonant converter. The changing trend of the switching frequency of the switching tube, when the switching frequency of the switching tube of the half-bridge on the primary side of the half-bridge LLC resonant converter is in a decreasing trend, increase the conduction time of the synchronous rectifier tube in the next working cycle , when the switching frequency of the switching tube of the half-bridge on the primary side of the half-bridge LLC resonant converter is in an increasing trend, the on-time duration of the synchronous rectifier tube in the next working cycle is reduced. 2 . The adaptive synchronous rectification control system of LLC resonant converter according to claim 1 , further comprising a gate driver, the input end of the gate driver is connected to the output end of the control unit, and the gate driver The output end of the synchronous rectifier is connected to the gate of the synchronous rectifier, and the control unit controls the conduction time of the synchronous rectifier in one working cycle through the gate driver. 3. the adaptive synchronous rectification control system of LLC resonant converter according to claim 1, is characterized in that, also comprises PWM timer, the input end of described PWM timer is connected the output end of described control unit, described The output end of the PWM timer is connected to the gate of the synchronous rectifier tube, and the control unit controls the conduction of the synchronous rectifier tube in one working cycle by adjusting the duty ratio of the PWM signal output by the PWM timer duration. 4 . The adaptive synchronous rectification control system for LLC resonant converter according to claim 3 , characterized in that it comprises an MCU integrated with the comparator, a control unit and a PWM timer. 5 . 5. The adaptive synchronous rectification control system of an LLC resonant converter according to claim 1, wherein the inverting input terminal of the comparator is connected to the output terminal of the sampling circuit, and the non-inverting input terminal of the comparator is connected to the output terminal of the sampling circuit. The terminal is connected to the voltage setting terminal of the control unit, and the control unit is further used for setting the preset voltage threshold. 6. The adaptive synchronous rectification control system of LLC resonant converter according to claim 1, wherein the sampling circuit has an absolute value taking function, and the drain voltage output to the comparator is set to a positive potential . 7 . The adaptive synchronous rectification control system of an LLC resonant converter according to claim 1 , wherein the absolute values of the increased on-time duration and the decreased on-time duration are equal. 8 . 8. an adaptive synchronous rectification control method of LLC resonant converter, for the synchronous rectifier tube in the half-bridge LLC resonant converter with synchronous rectifier tube to be turned off and controlled, it is characterized in that, described method comprises: Step A, obtaining the drain voltage of the synchronous rectifier when the synchronous rectifier is turned off; Step B, if the drain voltage is less than the preset voltage threshold, set the on-time duration of the synchronous rectifier in the next working cycle to be equal to the on-time duration of the current working cycle; Step C, if the drain voltage is greater than or equal to the preset voltage threshold, acquiring the switching frequency variation trend of the switching transistors of the half-bridge on the primary side of the half-bridge LLC resonant converter; Step D, when the switching frequency is in a decreasing trend, increase the conduction time of the synchronous rectifier in the next working cycle, and return to step A in the next working cycle; decrease when the switching frequency is in an increasing trend. The on-time duration of the synchronous rectifier tube in the next working cycle is reduced, and the process returns to step A in the next working cycle. 9 . The adaptive synchronous rectification control method of LLC resonant converter according to claim 8 , wherein if the switching frequency remains stable, then after step B, wait for a preset time period and then return to step A, and the preset The duration is greater than the duty cycle. 10 . The adaptive synchronous rectification control method for an LLC resonant converter according to claim 8 , wherein the absolute value of the on-time duration increased and the on-time duration decreased each time in the step D are equal. 11 .

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