A control system for synchronous rectifiers in a four-switch buck-boost converter

Abstract

The present invention discloses a control system for a synchronous rectifier in a four-switch buck-boost converter, wherein the control system detects the slope of the drain-source voltage of the synchronous rectifier to determine zero voltage turn-on to generate a turn-on enable signal for the synchronous rectifier; and determines that the magnetic reset of the inductor is complete, that is, the current flowing into the source of the synchronous rectifier reaches zero, based on the volt-second product of the voltage difference across the energy transfer inductor, to generate a turn-off enable signal. The control system of the present invention is not affected by the parasitic inductance of the synchronous rectifier package, and can achieve accurate synchronous rectification control, reduce the conduction time of the body diode, and improve the efficiency of the converter. In addition, the control system of the present invention does not need to detect the tiny voltage across the drain-source of the synchronous rectifier, and the circuit is simple to implement. It is integrated with the driver to further improve the power density of the converter.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic conversion, in particular to a control system of a synchronous rectifier tube in a four-switch buck-boost converter. Background Art

[0002] The four-switch buck-boost converter is a non-isolated DC / DC circuit with buck-boost conversion capability. Compared with single-boost or single-buck circuits, it can achieve more efficient power conversion in a wide voltage range and has a simple circuit structure. Therefore, it is widely used in power supply fields such as new energy power generation, communications, and military industry. Figure 1 As shown, the four-switch buck-boost converter consists of the first to fourth switch tubes S1-S4, the energy transfer inductor L f and output filter capacitor C o The third switch tube S3 is also called a synchronous rectifier tube, and the inductor L f One end of the inductor L is connected to the midpoint of the first and second switch tubes S1-S2 to form a switch bridge arm. f The other end is connected to the midpoint of the switch bridge arm formed by the synchronous rectifier tube S3 and the fourth switch tube S4. Figure 1 The control system of the four-switch buck-boost converter shown in the figure includes a main controller and a synchronous rectification driver, wherein the main controller generates control signals v for the first, second and fourth switch tubes (S1, S2 and S4) GS1 、v GS2 and v GS4 , the synchronous rectification drive generates the control signal v of the third switch tube S3 GS3 .

[0003] like Figure 2 As shown in Figure 1, the converter has four main working stages in one switching cycle, including input energy storage stage, direct power transmission stage, freewheeling stage and current clamping stage. In the input energy storage stage (t0-t1), the first and fourth switch tubes (S1 and S4) are turned on, and the input voltage V in Applied to the inductor L f The inductor current i Lf Linear rise, slope is V in / L f At t1, the fourth switch tube S4 is turned off, the input energy storage stage ends, and then the synchronous rectifier tube S3 is turned on, and the converter enters the direct power transmission stage (t1~t2). At this time, the input is connected through the inductor L f To transfer energy to the output, the difference between the input and output voltages is applied to the inductor L f The inductor current i Lf May rise linearly (V in >V o ), remain unchanged (V in =V o) or linearly decrease (V in <V o ), the slope of change is (V in -V o ) / L f At t2, the first switch tube S1 is turned off, and the direct power transmission phase ends. Then the second switch tube S2 is turned on, and the converter enters the freewheeling phase (t2-t3). The output voltage V o Applied to the inductor L f The inductor L f The inductor current i Lf Linear decrease, slope is -V o / L f ; At t3, the inductor current i Lf The current drops to zero, the synchronous rectifier tube S3 is turned off, and the freewheeling stage ends. Then the fourth switch tube S4 is turned on, and the converter enters the current clamping stage (t3-t4). The inductor L f The voltage across the two ends is zero, and the inductor current i Lf Remain unchanged.

[0004] like Figure 1 As shown, the synchronous rectification driver generates a control signal v by detecting the drain-source voltage of the third switch tube S3. GS3When the drain-source voltage is less than the turn-on threshold voltage (ideally zero, but actually -100mV), the synchronous rectifier is turned on; when the drain-source voltage is greater than the turn-off threshold voltage (ideally zero, but actually -1mV to 0mV), the synchronous rectifier is turned off. According to the above introduction, the synchronous rectifier driver needs to detect the tiny voltage (<1mV) of the drain-source of the switch tube to obtain an accurate control signal, and has very high requirements on the accuracy and speed of the internal comparator. The difference in the mV level of the turn-off threshold voltage will cause the current of the synchronous rectifier to reach several amperes at the turn-off moment (the synchronous rectifier has almost no switching loss, and a MOSFET with a small on-resistance will be preferred to reduce the conduction loss), instead of zero, which will reduce the efficiency of the converter. On the other hand, due to the influence of the parasitic inductance of the MOSFET package, the method of generating a control signal by detecting the drain-source voltage still has the problem of premature shutdown, which will further deteriorate the efficiency. The literature "D. Fu, etc. A Novel Driving Scheme for Synchronous Rectifiers in LLC Resonant Converters. IEEE Trans. on Power Electron., vol. 24, no. 5, pp. 1321-1329, May, 2009." proposes a compensation circuit, which requires the addition of components such as resistors, capacitors, switches and non-gates. The circuit structure is complex and will reduce the power density of the converter. The compensation effect is also affected by factors such as parameter tolerance and temperature. At the same time, it is also necessary to adjust the device parameters of the compensation circuit according to parameters such as MOSFET selection, the number of series and parallel connections, and PCB wiring, and the versatility is poor. Summary of the invention

[0005] The object of the present invention is to provide a control system for synchronous rectifier tubes in a four-switch buck-boost converter in view of the problems existing in the prior art.

[0006] The technical solution to achieve the purpose of the present invention is: a control system for a synchronous rectifier tube in a four-switch buck-boost converter, wherein the four-switch buck-boost converter includes first to fourth switch tubes, an energy transfer inductor and an output filter capacitor, wherein the third switch tube is also called a synchronous rectifier tube; the control system includes a synchronous rectifier tube control circuit, which generates a control signal v of the synchronous rectifier tube GS3 , the input source is input voltage V in The power is input to the converter, and the converter realizes power conversion by adjusting the duty ratio and conduction timing of the first to fourth switch control signals, and outputs a voltage V o Supply power to the load.

[0007] Furthermore, the synchronous rectifier control circuit includes three signal input terminals, which are respectively connected to the midpoint voltage v of the switch bridge arm composed of the first and second switch tubes. A , the midpoint voltage v of the switch bridge arm composed of the synchronous rectifier tube and the fourth switch tube B , and the output voltage V o ; Also includes a signal output terminal, output synchronous rectifier control signal v GS3 .

[0008] Furthermore, the synchronous rectifier control circuit includes a zero voltage turn-on detection circuit, a volt-second integration circuit, a volt-second balance control circuit and a drive control circuit;

[0009] The zero voltage turn-on detection circuit determines the zero voltage turn-on by detecting the slope of the drain-source voltage of the synchronous rectifier tube to generate a turn-on enable signal for the synchronous rectifier tube; the circuit has a signal input terminal, which is connected to the output voltage V o connect;

[0010] The volt-second integration circuit and the volt-second balance control circuit determine that the magnetic reset of the inductor is complete, i.e., the current flowing into the source of the synchronous rectifier reaches zero, based on the volt-second product of the voltage difference across the energy transfer inductor, to generate a shutdown enable signal;

[0011] The drive control circuit turns on and off the synchronous rectifier tube under the control of the turn-on and turn-off enable signal.

[0012] Compared with the prior art, the present invention has the following significant advantages:

[0013] 1) The control system of the present invention is not affected by the parasitic inductance of the synchronous rectifier tube package, and can achieve accurate synchronous rectification control, reduce the body diode conduction time, and improve the converter efficiency.

[0014] 2) The control system of the present invention does not need to detect the tiny voltage across the drain and source of the synchronous rectifier tube, the circuit is simple to implement, and the integrated design with the driver can further improve the power density of the converter.

[0015] The present invention is further described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the structure of a four-switch buck-boost converter under conventional control.

[0017] Figure 2 It is the time domain waveform of the four-switch buck-boost converter under conventional control.

[0018] Figure 3 FIG. 1 is a schematic diagram of the control system structure of a synchronous rectifier tube in a four-switch buck-boost converter in an embodiment.

[0019] Figure 4 FIG. 1 is a schematic diagram of a control circuit of a synchronous rectifier tube in an embodiment.

[0020] Figure 5 The following is a waveform diagram of key points of a zero voltage turn-on detection circuit in a synchronous rectifier control circuit in an embodiment.

[0021] Figure 6 (a) is a circuit schematic diagram of a first implementation method of a clamping circuit in a synchronous rectifier control circuit in an embodiment.

[0022] Figure 6 (b) is a circuit schematic diagram of a second implementation method of a clamping circuit in a synchronous rectifier control circuit in an embodiment.

[0023] Figure 7 The following is a waveform diagram of key points of a volt-second integration circuit and a volt-second balance control circuit in a synchronous rectifier control circuit in an embodiment.

[0024] Figure 8 (a) is a circuit schematic diagram of a first implementation method of a volt-second integration circuit in a synchronous rectifier control circuit in an embodiment.

[0025] Figure 8 (b) is a circuit schematic diagram of a second implementation method of a volt-second integration circuit in a synchronous rectifier control circuit in an embodiment.

[0026] Fig. 9 Graph showing simulation and experimental results of the present invention in one embodiment.

[0027] Symbol name in the above figure: V in is the input voltage of the converter, V o is the output voltage, C o is the output filter capacitor, S1~S4 are the first to fourth switch tubes, S3 is also called the synchronous rectifier tube, L f is the energy transfer inductor, v GS1 ~v GS4 is the control signal of S1~S4, v A The voltage between the midpoint of the switch bridge arm composed of S1 and S2 and the ground, v B The voltage between the midpoint of the switch bridge arm composed of S3 and S4 and the ground, i Lf is the current flowing through the inductor L f The current, T1 is the duration of the input energy storage phase, T2 is the duration of the direct energy transfer phase, T3 is the duration of the freewheeling phase, T4 is the duration of the current clamping phase, I N is the current flowing through the inductor L during the current clamping stage f The current value, i SD3is the current flowing into the source of S3, t0~t4 is the time, R1~R3 ​​are resistors, C1 and C2 are capacitors, Q1 is a switch tube, D1 is a diode, VCCS is a voltage-controlled current source, CP1~CP3 are comparators, RS1 and RS2 are RS triggers, B1 is a driver, V CC is the voltage source, the reference point is v B , V th1 and V th2 are the first and second threshold voltages, and the reference point is v B , v C1 is the voltage across capacitor C1, v CP3_in+ 、v CP3_in- and v CP3_out are the positive input terminal voltage, negative input terminal voltage and output terminal voltage of comparator CP3 respectively, v AB is the inductance L f Voltage across the board, R 11 and R 12 is the resistance, Q 11 , Q 12 , Q 21 and Q 22 For triode, D 31 and D 32 For diode, D z1 and D z2 For the voltage regulator tube. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0029] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0030] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0031] In one embodiment, in combination Figure 3 , provides a control system for a synchronous rectifier tube in a four-switch buck-boost converter, wherein the four-switch buck-boost converter includes first to fourth switch tubes S1-S4, an energy transfer inductor L f and output filter capacitor C o , wherein the third switch tube S3 is also called a synchronous rectifier tube; the control system includes a synchronous rectifier tube control circuit, and the synchronous rectifier tube control circuit generates a control signal v of the synchronous rectifier tube S3 GS3 , the input source is input voltage V in The converter inputs power, and the converter realizes power conversion by adjusting the duty ratio and conduction timing of the control signals of the first to fourth switch tubes S1 to S4, and outputs a voltage V o Supply power to the load.

[0032] The main controller of the four-switch buck-boost converter includes three signal input terminals and three signal output terminals, wherein the three signal input terminals are respectively connected to the input voltage V in , output voltage V o , and the midpoint voltage v of the switch bridge arm composed of the synchronous rectifier tube S3 and the fourth switch tube S4 B The three signal output terminals respectively output the control signals v of the first, second and fourth switch tubes S1, S2 and S4 GS1 、v GS2 and v GS4 ; Its principle is the same as the conventional control strategy and will not be repeated here.

[0033] The synchronous rectifier control circuit includes three signal input terminals, which are respectively connected to the midpoint voltage v of the switch bridge arm composed of the first and second switch tubes S1-S2. A , the midpoint voltage v of the switch bridge arm composed of the synchronous rectifier tube S3 and the fourth switch tube S4 B , and the output voltage V o Also includes a signal output terminal, output synchronous rectifier tube S3 control signal v GS3 .

[0034] Further, in one embodiment, in combination Figure 4 The synchronous rectifier control circuit includes a zero voltage turn-on detection circuit, a volt-second integration circuit, a volt-second balance control circuit and a drive control circuit; the potential reference point of the synchronous rectifier S3 control circuit is the midpoint of the switch bridge arm composed of the synchronous rectifier S3 and the fourth switch S4, that is, v B .

[0035] The zero voltage turn-on detection circuit determines the zero voltage turn-on by detecting the slope of the drain-source voltage of the synchronous rectifier S3 to generate a turn-on enable signal for the synchronous rectifier S3; the circuit has a signal input terminal, which is connected to the output voltage V o connect;

[0036] The volt-second integration circuit and the volt-second balance control circuit are configured according to the energy transfer inductance L f The volt-second product of the voltage difference between the two ends determines that the inductor magnetic reset is complete, that is, the current flowing into the source of the synchronous rectifier reaches zero, to generate a shutdown enable signal;

[0037] The drive control circuit turns on and off the synchronous rectifier tube S3 under the control of the turn-on and turn-off enable signal.

[0038] Further, in one embodiment, in combination Figure 4 , the zero voltage turn-on detection circuit includes a second capacitor C2, a second resistor R2, a first diode D1, a third resistor R3, a third comparator CP3, a second monostable trigger, and a clamping circuit;

[0039] One end of the second capacitor C2 is connected to the output voltage V o The other end is connected to the positive input terminal of the third comparator CP3, and is connected to the power supply V through the second resistor R2. CC , the clamp circuit is connected in parallel with the second resistor R2; the first diode D1 is connected in series with the third resistor R3, and the positive electrode of the first diode D1 is connected to the power supply V CC The other end of the third resistor R3 is connected to the midpoint voltage v B The common end of the first diode D1 and the third resistor R3 is connected to the negative input end of the third comparator CP3, and the output end of the third comparator CP3 is connected to the drive control circuit through the second monostable trigger. The key working waveform is as follows: Figure 5 shown.

[0040] According to Kirchhoff's voltage law, the drain-source voltage v of the synchronous rectifier S3 is DS3 =V o -v B The voltage across the second capacitor C2 is v C2 and the voltage across the second resistor R2 is v R2The following relationship exists:

[0041] v DS3 =v C2 +v R2 +V CC (1)

[0042] Derivative both sides of the above equation:

[0043]

[0044] The comparator CP3 meets the "virtual off" characteristic, and the voltage across the second resistor R2 is v R2 and the positive voltage clamping value V of the clamping circuit in the zero voltage turn-on detection circuit Z+ And negative voltage clamp value V Z- The following relationship exists:

[0045]

[0046] Substituting formula (3) into formula (2), we can get:

[0047]

[0048] Ignoring the quadratic differential term, we can obtain:

[0049]

[0050] That is, the voltage across the second capacitor C2 is v C2 The rate of change and the drain-source voltage v of the synchronous rectifier S3 DS3 When the converter is working in the input energy storage stage, the first and fourth switch tubes S1 and S4 are turned on, and the drain-source voltage v of the synchronous rectifier tube S3 is DS3 =V o , the rate of change is zero, at this time the positive input voltage v of the third comparator CP3 CP3_in+ =V CC , higher than the negative input voltage (v CP3_in- =V CC -V FD1 , V FD1 is the conduction voltage drop of the first diode D1), the comparator CP3 outputs a high level.

[0051] At time t1, the fourth switch tube S4 is turned off, the input energy storage phase ends, and then the energy transfer inductor current i Lf The junction capacitance of the synchronous rectifier tube S3 is discharged, and the junction capacitance of the fourth switch tube S4 is charged. The voltage at the positive input terminal of the third comparator CP3 is:

[0052]

[0053] And dvDS3 / dt<0, when the drain-source voltage of synchronous rectifier S3 v DS3 When the downward slope of is large, there is:

[0054]

[0055] That is, the voltage at the positive input terminal of the third comparator CP3 is lower than the voltage at the negative input terminal, and the third comparator CP3 outputs a low level. When the junction capacitance of the synchronous rectifier tube S3 is discharged to zero, its body diode is turned on, and the drain-source voltage v DS3 ≈V o The change rate is zero, and the comparator CP3 outputs a high level again. The output signal of the third comparator CP3 generates a high level pulse signal through the second monostable trigger (input rising edge trigger), enabling the drive control circuit to output a high level and turn on the synchronous rectifier S3.

[0056] Furthermore, in one embodiment, Figure 6 Two specific embodiments of the clamping circuit in the S3 control circuit are given. Figure 6 (a), the clamping circuit includes a second diode D connected in parallel 31 , the third diode D 32 , the second diode D 31 The cathode of the third diode D 32 The positive terminal of the power supply V CC , the positive and negative voltage clamping values ​​are equal to the conduction voltage drop of the diode;

[0057] Combination Figure 6 (b), or comprising a first voltage zener diode D in series z1 , the second voltage zener diode D z2 , the cathodes of the two Zener diodes are connected, the first Zener diode D z1 The positive terminal of the power supply V CC The positive and negative voltage clamping values ​​are equal to the voltage regulation value of the Zener diode plus the forward conduction voltage drop.

[0058] Further, in one embodiment, the drive control circuit includes a second RS trigger RS2 and a driver; the S terminal of the second RS trigger RS2 is connected to the output of the zero voltage turn-on detection circuit, the R terminal is connected to the output of the volt-second balance control circuit, and the Q terminal outputs the control signal v of the synchronous rectifier S3 through the driver. GS3 .

[0059] Furthermore, in one embodiment, Figure 8Two specific embodiments of the volt-second integration circuit in the synchronous controller control circuit are given. The volt-second integration circuit includes a first resistor R1 and a first capacitor C1. The first resistor R1 is connected in series with the first capacitor C1. The other end of the first resistor R1 is connected to the midpoint voltage v A The other end of the first capacitor C1 is connected to the midpoint voltage v B , the common end of the first resistor R1 and the first capacitor C1 is output to the volt-second balance control circuit;

[0060] Or including a first resistor R1, an eleventh resistor R 11 , the twelfth resistor R 12 、The first NPN transistor Q 11 、The second NPN transistor Q 12 、The first PNP transistor Q 21 , the second PNP transistor Q 22 , a first capacitor C1; one end of the first resistor R1 is connected to the midpoint voltage v A , the other end is connected to the emitters of four transistors; the eleventh resistor R 11 One end is connected to the midpoint voltage v B , the other end is connected to the first NPN transistor Q 11 The base and collector of the second NPN transistor Q 12 The base of the twelfth resistor R 12 One end is connected to the midpoint voltage v B The other end is connected to the first PNP transistor Q 21 The base and collector of the second PNP transistor Q 22 The base of the second NPN transistor Q 12 The collector of the second PNP transistor Q 22 The collector of is connected to the midpoint voltage v through the first capacitor C1 B , and output to the volt-second balance control circuit.

[0061] The volt-second balance control circuit comprises a switch tube Q1, a first comparator CP1, a second comparator CP2, a first RS trigger RS1 and a first monostable trigger; the positive input terminal of the first comparator CP1 is connected to the midpoint voltage v A , the negative input terminal inputs the first threshold voltage V th1 The output end is connected to the R end of the first RS trigger RS1; the negative input end of the second comparator CP2 is connected to the midpoint voltage v through the switch tube Q1 B , the positive input terminal inputs the second threshold voltage V th2 The output end is connected to the S end of the first RS trigger RS1 through the first monostable trigger, and the output is connected to the drive control circuit; the output Q end of the first RS trigger RS1 controls the opening and closing of the switch tube Q1.

[0062] The key working waveforms of the volt-second integration circuit and the volt-second balance control circuit are as follows: Figure 7 As shown in the figure. From the above analysis, it can be seen that the four-switch buck-boost converter has four working stages in one switching cycle, namely the input energy storage stage, the direct power transmission stage, the freewheeling stage and the current clamping stage. In the current clamping stage, the second and fourth switch tubes (S2 and S4) are turned on, and the inductor L f The voltage across the two ends is zero, that is, v A =v B , the positive input voltage v of the first comparator CP1 CP1_in+ =v A -v B =v AB =0, which is lower than the negative input voltage, i.e. the first threshold voltage V th1 , the first comparator CP1 outputs a low level, and the first RS flip-flop RS1 will not be reset.

[0063] The converter enters the input energy storage stage from the current clamping stage, the first and fourth switch tubes (S1 and S4) are turned on, and the positive input voltage v of the first comparator CP1 is CP1_in+ =v AB =V in , higher than the first threshold voltage V th1 , the first comparator CP1 outputs a high level, the first RS flip-flop RS1 is reset and outputs a low level, the control switch Q1 is turned off, and the volt-second integration circuit is enabled. Figure 8 Taking (a) as an example, the time constant of resistor R1 and capacitor C1 is designed to be much larger than the switching period of the converter, then:

[0064]

[0065] Energy transfer inductor current i Lf The rate of change and the voltage difference v AB The following relationship exists:

[0066]

[0067] Substituting formula (9) into formula (8), we can obtain:

[0068]

[0069] That is, the voltage across the first capacitor C1 is v C1 The rate of change and energy transfer inductance L f The current i Lf There is a certain proportional relationship between the change rate of the first capacitor C1 and the voltage v C1 The energy transfer inductor current i can be obtained LfThe information, and the energy transfer inductor magnetic reset complete moment and the voltage v across the first capacitor C1 C1 The moment of returning to the initial value is the same.

[0070] The working principles of the volt-second integration circuit and the volt-second balance control circuit are further explained below in conjunction with the four working stages of the four-switch buck-boost converter.

[0071] The converter enters the input energy storage stage from the current clamping stage. The first and fourth switch tubes (S1 and S4) are turned on, the volt-second integration circuit is enabled, and the voltage difference across the energy transfer inductor is v AB =V in , the voltage across the first capacitor C1 is v C1 Linear rise, slope is V in / (R1C1). At t1, the fourth switch tube S4 is turned off, and then the zero voltage turn-on detection circuit controls the synchronous rectifier tube S3 to turn on, and the converter enters the direct power transmission stage. The voltage difference between the two ends of the energy transfer inductor is v AB =V in -V o , the voltage across the first capacitor C1 is v C1 May rise linearly (V in >V o ), remain unchanged (V in =V o ) or linearly decrease (V in <V o ), the slope of change is (V in -V o ) / (R1C1). At t2, the first switch tube S1 is turned off, and then the second switch tube S2 is turned on. The converter enters the freewheeling stage, and the voltage difference across the energy transfer inductor is v AB =-V o , the voltage across the first capacitor C1 is v C1 Linear decrease, slope is -V o / (R1C1).

[0072] The voltage across the first capacitor C1 is v C1 The initial value (the value at the beginning of the energy storage phase) is equal to zero. C1 When it decreases to zero, the energy transfer inductor is completely reset. After the synchronous rectifier tube S3 is turned off, the junction capacitance energy of the fourth switch tube S4 will affect the energy transfer inductor L f Reverse excitation generates a negative current I N , which is the initial value of the next switching cycle. When the energy transfer inductor is completely reset in the next switching cycle and the synchronous rectifier tube S3 is turned off, the junction capacitance energy of the fourth switch tube S4 will further increase the negative current I N , forming positive feedback, which will significantly increase the inductor current iLf The peak value and effective value of the current will reduce the converter efficiency. Therefore, in order to achieve accurate synchronous rectification control and improve system stability, the turn-off time of the synchronous rectifier tube S3 needs to be advanced by a certain amount compared to the time when the energy transfer inductor is completely reset. Figure 4 and Figure 7 As shown, when v C1 Below the second threshold voltage V th2 When the second comparator CP2 outputs a high level, the driving control circuit outputs a low level, and the synchronous rectifier S3 is turned off, realizing early shutdown for a certain period of time. The output signal of the second comparator CP2 generates a high level pulse signal through the first monostable trigger (input rising edge trigger), controls the first RS trigger RS1 to output a high level, the switch tube Q1 is turned on, and the voltage across the first capacitor C1 immediately drops to zero. After that, the fourth switch tube S4 is turned on, and the converter enters the current clamping stage. The voltage difference v across the energy transfer inductor is AB =0, the switch tube Q1 remains on, and the voltage across the first capacitor C1 is v C1 =0.

[0073] Figure 8 (b) shows a second specific embodiment of the volt-second integration circuit. When the voltage difference v across the energy transfer inductor is AB >0, transistor Q 11 and Q 12 Cut-off, resistors R1, R 12 And transistor Q 21 ~Q 22 The current mirror circuit composed of charges the capacitor C1; when the voltage difference v across the energy transfer inductor AB <0, transistor Q 21 and Q 22 Cut-off, resistors R1, R 11 And transistor Q 11 ~Q 12 The current mirror circuit composed of discharges the capacitor C1,

[0074]

[0075] By setting the resistor R 11 The resistance is slightly smaller than R 12 , to realize the discharge current of capacitor C1 with respect to the voltage v across the energy transfer inductor AB The proportionality coefficient is slightly larger than the charging current with respect to v AB The proportional coefficient of the synchronous rectifier S3 is used to control the shutdown time in advance. Figure 8 The circuit shown in (a) is similar and will not be described in detail.

[0076] To further illustrate the effectiveness of the system described in the present invention, Fig. 9The simulation and experimental results of the specific embodiment are given. It can be seen from the figure that the drain-source voltage v of the synchronous rectifier tube S3 DS3 When it drops rapidly, the third comparator CP3 outputs a low level, v DS3 After decreasing to zero, an S3 turn-on enable signal is generated. The S3 control circuit proposed in the present invention realizes the zero voltage turn-on of S3. The voltage across the first capacitor C1 is v C1 Waveform and energy transfer inductor current i Lf The shapes of the waveforms are consistent, which shows that the S3 control circuit proposed in the present invention can accurately obtain the energy transfer inductor current i Lf The information is used to generate a shutdown enable signal by judging that the energy transfer inductor is magnetically reset completely, that is, the current flowing into the source of the synchronous rectifier reaches zero, thereby achieving accurate synchronous rectification control.

[0077] The control system proposed in the present invention is not affected by the parasitic inductance of the synchronous rectifier tube package, can realize accurate synchronous rectification control, reduce the body diode conduction time, and improve the converter efficiency. There is no need to detect the tiny voltage across the drain and source of the synchronous rectifier tube. The circuit is simple to implement and can be integrated with the driver to further improve the power density of the converter.

[0078] The above is only a preferred embodiment of the present invention. It should be pointed out that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A control system for a synchronous rectifier in a four-switch buck-boost converter, the four-switch buck-boost converter comprising first to fourth switch tubes (S1-S4), an energy transfer inductor (L f ) and output filter capacitor (C o ), wherein the third switch tube (S3) is also called a synchronous rectifier tube; characterized in that, The control system includes a synchronous rectifier control circuit, which generates a control signal v of the synchronous rectifier (S3). GS3 , the input source is input voltage V in The converter inputs power, and the converter realizes power conversion by adjusting the duty ratio and conduction timing of the control signal of the first to fourth switch tubes (S1-S4), and outputs a voltage V o Supply power to the load; The synchronous rectifier control circuit includes a zero voltage turn-on detection circuit, a volt-second integration circuit, a volt-second balance control circuit and a drive control circuit; The zero voltage turn-on detection circuit determines the zero voltage turn-on by detecting the slope of the drain-source voltage of the synchronous rectifier (S3) to generate a turn-on enable signal for the synchronous rectifier (S3); the circuit has a signal input terminal connected to the output voltage V o connect; The volt-second integration circuit and the volt-second balance control circuit are based on the energy transfer inductance (L f ) The volt-second product of the voltage difference across the two ends is used to determine that the inductor magnetic reset is complete, that is, the current flowing into the source of the synchronous rectifier tube reaches zero, to generate a shutdown enable signal; The drive control circuit turns on and off the synchronous rectifier tube (S3) under the control of the turn-on and turn-off enable signal; The zero voltage turn-on detection circuit comprises a second capacitor (C2), a second resistor (R2), a first diode (D1), a third resistor (R3), a third comparator (CP3), a second monostable trigger, and a clamping circuit; One end of the second capacitor (C2) is connected to the output voltage V o The other end is connected to the positive input of the third comparator (CP3), and is connected to the power supply V through the second resistor (R2). CC The clamp circuit is connected in parallel with the second resistor (R2); the first diode (D1) is connected in series with the third resistor (R3), and the positive electrode of the first diode (D1) is connected to the power supply V CC The other end of the third resistor (R3) is connected to the midpoint voltage v B , a common end of the first diode (D1) and the third resistor (R3) is connected to a negative input end of a third comparator (CP3), and an output end of the third comparator (CP3) is connected to a drive control circuit via a second monostable trigger; According to Kirchhoff's voltage law, the drain-source voltage v of the synchronous rectifier (S3) DS3 =V o -v B The voltage across the second capacitor (C2) is v C2 and the voltage across the second resistor (R2) v R2 The following relationship exists: v DS3 =v C2 +v R2 +V CC (1) Take the derivative of both sides of the above equation: The third comparator (CP3) satisfies the "virtual off" characteristic, and the voltage across the second resistor (R2) is v R2 and the positive voltage clamping value V of the clamping circuit in the zero voltage turn-on detection circuit Z+ And negative voltage clamp value V Z- The following relationship exists: Substituting formula (3) into formula (2), we can get: Ignoring the quadratic differential term, we can obtain: That is, the voltage across the second capacitor (C2) is v C2 The rate of change and the synchronous rectifier (S3) drain-source voltage v DS3 The rate of change is the same.

2. The control system of the synchronous rectifier tube in the four-switch buck-boost converter according to claim 1, characterized in that: The synchronous rectifier control circuit comprises three signal input terminals, which are respectively connected to the midpoint voltage v of the switch bridge arm composed of the first and second switch tubes (S1-S2). A , the midpoint voltage v of the switch bridge arm composed of the synchronous rectifier tube (S3) and the fourth switch tube (S4) B , and the output voltage V o ; Also includes a signal output terminal, output synchronous rectifier (S3) control signal v GS3 .

3. The control system of the synchronous rectifier tube in the four-switch buck-boost converter according to claim 1, characterized in that: The clamping circuit includes a second diode (D 31 )、The third diode (D 32 ), the second diode (D 31 ) and the cathode of the third diode (D 32 ) is connected to the positive terminal of the power supply V CC ; or comprising a first voltage regulator diode (D z1 )、The second Zener diode (D z2 ), the cathodes of the two Zener diodes are connected, the first Zener diode (D z1 ) is connected to the positive terminal of the power supply V CC .

4. The control system of the synchronous rectifier tube in the four-switch buck-boost converter according to claim 1, characterized in that: The volt-second integration circuit comprises a first resistor (R1) and a first capacitor (C1), wherein the first resistor (R1) and the first capacitor (C1) are connected in series, and the other end of the first resistor (R1) is connected to a midpoint voltage v A The other end of the first capacitor (C1) is connected to the midpoint voltage v B , a common end of the first resistor (R1) and the first capacitor (C1) is output to a volt-second balance control circuit; or includes a first resistor (R1), an eleventh resistor (R 11 )、The twelfth resistor (R 12 ), the first NPN transistor (Q 11 ), the second NPN transistor (Q 12 ), the first PNP transistor (Q 21 ), the second PNP transistor (Q 22 ), a first capacitor (C1); one end of the first resistor (R1) is connected to a midpoint voltage v A , the other end is connected to the emitters of the four transistors; the eleventh resistor (R 11 ) is connected to the midpoint voltage v B , the other end is connected to the first NPN transistor (Q 11 ) and the base and collector of the second NPN transistor (Q 12 ) base; the twelfth resistor (R 12 ) is connected to the midpoint voltage v B The other end is connected to the first PNP transistor (Q 21 ) and the base and collector of the second PNP transistor (Q 22 ) base; the second NPN transistor (Q 12 ) and the collector of the second PNP transistor (Q 22 ) is connected to the midpoint voltage v through the first capacitor (C1). B , and output to the volt-second balance control circuit.

5. The control system of the synchronous rectifier tube in the four-switch buck-boost converter according to claim 4, characterized in that: The time constant of the first resistor (R1) and the first capacitor (C1) is much larger than the switching period of the converter, so: or Energy transfer inductor current i Lf The rate of change and the voltage difference v AB The following relationship exists: Substituting formula (8) into (6), we can obtain: Or substitute formula (8) into (7), we can get: That is, the voltage across the first capacitor (C1) is v C1 The rate of change and energy transfer inductance (L f ) of the current i Lf There is a proportional relationship between the rate of change of .

6. The control system of the synchronous rectifier tube in the four-switch buck-boost converter according to claim 1, characterized in that: The volt-second balance control circuit comprises a switch tube (Q1), a first comparator (CP1), a second comparator (CP2), a first RS trigger (RS1) and a first monostable trigger; The positive input terminal of the first comparator (CP1) is connected to the midpoint voltage v A , the negative input terminal inputs the first threshold voltage V th1 The output end is connected to the R end of the first RS trigger (RS1); the negative input end of the second comparator (CP2) is connected to the midpoint voltage v through the switch tube (Q1) B , the positive input terminal inputs the second threshold voltage V th2 , the output end is connected to the S end of the first RS trigger (RS1) through the first monostable trigger, and the output is connected to the drive control circuit; The output Q end of the first RS trigger (RS1) controls the opening and closing of the switch tube (Q1).

7. The control system of the synchronous rectifier tube in the four-switch buck-boost converter according to claim 1 or 6, characterized in that: The drive control circuit comprises a second RS trigger (RS2) and a driver; the S terminal of the second RS trigger (RS2) is connected to the output of the zero voltage turn-on detection circuit, the R terminal is connected to the output of the volt-second balance control circuit, and the Q terminal outputs a control signal v of the synchronous rectifier (S3) through the driver. GS3 .

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