High-dynamic control method and circuit for four-switch buck-boost converter

Abstract

The invention discloses a high-dynamic control method and circuit for a four-switch buck-boost converter. The circuit comprises a four-switch buck-boost converter circuit, an analog-to-digital converter and an FPGA development board. In the four-switch buck-boost converter, the positive electrode of a first switch tube is connected to an input voltage, and the negative electrode is connected to the left end of an inductor; the anode of the second switch tube is connected to the left end of the inductor, and the cathode is grounded; the anode of the third switching tube is connected to the output voltage, and the cathode is connected to the right end of the inductor; the positive electrode of the fourth switch tube is connected to the right end of the inductor; the anode of the output capacitor is connected to the output voltage; in the analog-to-digital converter and the FPGA development board, analog signals are converted into digital signals through the analog-to-digital converter, then the digital signals are input into a digital module, and control signals of four switching tubes are output through a natural switching surface control method and are returned to the four-switch buck-boost converter. According to the invention, high-dynamic control of the four-switch buck-boost converter can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of DC-DC converters of electric energy conversion devices, and in particular to a high dynamic control method and circuit of a four-switch buck-boost converter. Background Art

[0002] With the upgrading of various electronic products, DC power supplies are developing towards technical requirements such as higher efficiency, higher integration and wide input range. In the design of a four-switch buck-boost converter, how to increase the switching rate while ensuring output stability is a very important issue. High dynamic control is of great significance to improving the stable operation capability of switching devices, the conversion efficiency of circuit topology and the safety of the working environment.

[0003] There are two existing control methods for four switch buck-boost (FSBB): the first is a control method based on pulse width modulation (PWM), which simulates the required waveform by changing the duty cycle of the pulse. It has high control accuracy and can achieve good performance in an environment with large noise interference, but the response speed is slow. The second is a control method based on dual input voltage feedforward, which has good voltage stability, but requires the introduction of additional sensors and the dynamic response speed is not fast enough. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a high dynamic control method and circuit for a four-switch buck-boost converter, which can improve the dynamic response speed and complete the high dynamic control of the four-switch buck-boost converter under the premise of ensuring high frequency, high efficiency and wide range voltage regulation.

[0005] Technical solution: A high dynamic control circuit of a four-switch buck-boost converter, including a four-switch buck-boost converter circuit, an analog-to-digital converter, and an FPGA development board carrying a digital control module;

[0006] In the four-switch buck-boost converter, the positive electrode of the first switch tube is connected to the input voltage, and the negative electrode is connected to the left end of the inductor; the positive electrode of the second switch tube is connected to the left end of the inductor, and the negative electrode is grounded; the positive electrode of the third switch tube is connected to the output voltage, and the negative electrode is connected to the right end of the inductor; the positive electrode of the fourth switch tube is connected to the right end of the inductor, and the negative electrode is grounded; the positive electrode of the output capacitor is connected to the output voltage, and the negative electrode is grounded;

[0007] In the analog-to-digital converter and FPGA development board, the analog signal is converted into a digital signal through the analog-to-digital converter, the digital signal is input into the digital module, and then the control signal of the four switch tubes is output through the natural switch surface control method and returned to the four-switch buck-boost converter.

[0008] Furthermore, in the digital module, the digital signal input after conversion by the analog-to-digital converter is read through the SPI interface in a shift register manner.

[0009] Further, in the digital module, the phase change is switched through a three-stage state machine, including a magnetizing inductor linear energy storage stage, a direct transfer stage, and a reset stage, and several intermediate states are included in the soft switching design.

[0010] Furthermore, for the soft switch design, in a steady state, the first state is that the first switch tube and the fourth switch tube are both turned on, and the second switch tube and the third switch tube are both turned off;

[0011] When switching states, the fourth switch tube is first turned off, and when the inductor current charges the junction capacitor at the midpoint of the rear bridge arm to reach the output voltage, the third switch tube is turned on to achieve soft switching and reduce losses;

[0012] When switching from the second state to the third state, the first switch tube is turned off first, and the second switch tube is turned on when the junction capacitance voltage at the midpoint of the front bridge arm drops to 0V, thereby realizing soft switching and reducing losses.

[0013] When switching from the third state to the first state, first turn off the third switch tube, wait for the midpoint voltage of the rear bridge arm to drop to 0V, turn on the fourth switch and turn off the second switch tube at the same time, wait for the reverse inductor current to charge the midpoint junction capacitor of the front bridge arm to the input voltage, turn on the first switch tube, and realize soft switching to reduce losses.

[0014] Furthermore, there is an anti-overshoot design in the digital module. When the inductor current is greater than the set value, the state will be switched regardless of whether the switching condition is met, ensuring that the system will not be damaged due to excessive instantaneous current at startup.

[0015] The control method of any one of the above-mentioned high dynamic control circuits of the four-switch buck-boost converter is implemented by the following steps:

[0016] The input voltage, inductor current, output voltage, and bridge arm midpoint voltage of the four-switch buck-boost converter are collected and converted into digital signals through an analog-to-digital converter;

[0017] The digital signal is collected via SPI interface to unify the sampling rate;

[0018] The digital signal is processed by a natural switch surface control method, and nonlinear control is performed by a three-stage state machine Verilog program. The control signals of the four switch tubes are output and returned to the four-switch buck-boost converter.

[0019] Furthermore, in the control method of the natural switch surface, a four-switch buck-boost converter in the boundary current mode state has three working states in one cycle: the linear energy storage stage of the excitation inductance, the direct transmission stage and the reset stage; the switch state of each stage is different, and the circuit state trajectory is normalized and derived according to the inductance equation and the capacitance equation. Under the premise that the trajectories of the linear energy storage stage and the reset stage of the excitation inductance are determined, the trajectory of the direct transmission stage is determined based on the principle of minimum conduction loss, ensuring that there is an intersection between the three trajectories, and adding soft switching and anti-overshoot design;

[0020] In steady state, the first state is that the first switch tube and the fourth switch tube are both turned on, and the second switch tube and the third switch tube are both turned off. When switching states, the fourth switch tube is turned off first, and the inductor current is waited for to charge the junction capacitor at the midpoint of the rear bridge arm to reach the output voltage, and then the third switch tube is turned on to achieve soft switching to reduce losses; when switching from the second state to the third state, the first switch tube is turned off first, and the junction capacitor voltage at the midpoint of the front bridge arm is waited to drop to 0V, and then the second switch tube is turned on to achieve soft switching to reduce losses; when switching from the third state to the first state, the third switch tube is turned off first, and the voltage at the midpoint of the rear bridge arm is waited to drop to 0V, and then the fourth switch is turned off while the second switch tube is turned off, and the reverse inductor current is waited for to charge the junction capacitor at the midpoint of the front bridge arm to the input voltage, and then the first switch tube is turned on to achieve soft switching to reduce losses;

[0021] Finally, four switch tube signals are output to the four-switch buck-boost converter to complete the control.

[0022] Compared with the prior art, the present invention has the following significant effects:

[0023] The present invention converts the input voltage, inductor current, output voltage, output current and rear bridge arm current of the sampling circuit into digital signals through an analog-to-digital converter and enters the digital module for nonlinear control, and outputs four switch signals to return to the circuit; compared with the existing pulse width modulation control method or the control method based on dual input voltage feedforward, the dynamic response speed is greatly improved; at the same time, under the premise of ensuring high frequency, high efficiency and wide range of voltage regulation, the high dynamic control of the four-switch buck-boost converter is completed. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a circuit diagram of a four-switch buck-boost converter;

[0025] FIG. 2( a ) is a module diagram of the high dynamic control method of the present invention, and FIG. 2( b ) is a specific circuit diagram of the high dynamic control method of the present invention;

[0026] Figure 3The three states of the working cycle of the four-switch buck-boost converter in the boundary current mode. The solid line represents the switch on, and the dotted line represents the switch off. Among them, (a) is the state of the linear energy storage stage of the excitation inductor, (b) is the state of direct transmission, and (c) is the state of the reset stage;

[0027] FIG4(a) is a diagram showing the working trajectory of a four-switch buck-boost converter in a buck state, FIG4(b) is a diagram showing the working trajectory of a four-switch buck-boost converter in a boost state, and FIG4(c) is a diagram showing the working trajectory of a four-switch buck-boost converter when the input and output voltages are similar;

[0028] Figure 5 It is the inductor current waveform and switch timing diagram of the four-switch buck-boost converter in the buck state;

[0029] Figure 6 It is the inductor current waveform and switch timing diagram of the four-switch buck-boost converter in the boost state;

[0030] Figure 7 It is the simulation waveform of the four-switch buck-boost converter in the buck state;

[0031] Figure 8 It is the simulation waveform diagram of the four-switch buck-boost converter in the boost state;

[0032] In the figure, the reference numerals S1-S4 are control signals of the first to fourth switches, V in is the input voltage, C in is the input resistance, L is the inductance, V a is the midpoint voltage of the front bridge arm, V b is the midpoint voltage of the rear bridge arm, i L is the inductor current, C0 is the output capacitance, I0 is the output current, and V0 is the output voltage. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0034] The present invention provides a high dynamic control method and circuit for a four-switch buck-boost converter. The input voltage, inductor current, output voltage, output current and rear bridge arm midpoint voltage of the four-switch buck-boost converter are sampled and converted into digital signals by an analog to digital converter (ADC). The sampling rate is unified through a serial peripheral interface (SPI). The control signals of the four switches of the circuit are output in a digital module according to a high dynamic control method of a natural switch surface. The dynamic switching speed is improved under the premise of ensuring high frequency, high efficiency and wide range voltage regulation, thereby solving the technical problem of slow dynamic response speed of the existing four-switch buck-boost converter.

[0035] The circuit of the four-tube buck-boost converter is as follows Figure 1 As shown, it is composed of a first switch tube Q1, a second switch tube Q2, a third switch tube Q3, a fourth switch tube Q4, and an inductor L. The first switch tube Q1 and the second switch tube Q2 are connected in series to form a front bridge arm, and an input capacitor C is connected between the two terminals of the front bridge arm. in The third switch tube Q3 and the fourth switch tube Q4 are connected in series to form a rear bridge arm. The output capacitor C0 is connected between the two terminals of the rear bridge arm. The inductor L is connected between the midpoint of the front bridge arm and the midpoint of the rear bridge arm. The input voltage source V in The third switch tube Q3 and the secondary ground constitute the converter output port. The output voltage V0 of the converter output port is equivalent to the current source I0 connected to the output port.

[0036] like Figure 2(a) , 2(b) As shown, the present invention needs to first collect the input voltage V of the four-switch buck-boost converter in , inductor current i L , output voltage V0, output current I0 and rear bridge arm midpoint voltage V b , and then the collected V in 、i L , V0, I0 and V b All are converted into digital signals and then input into the digital module for high dynamic control. The digital module outputs control signals S1 to S4 of the first to fourth switch tubes to complete the closed-loop high dynamic control of the circuit.

[0037] In the digital module, the buck-boost converter has three states per working cycle, such as Figure 3(a), (b), and (c) in the figure are respectively the linear energy storage stage T1 of the excitation inductance, the direct transmission stage T2, and the reset stage T3. The solid line represents the switch opening, and the dotted line represents the switch opening. S1 is opened as u1=1, and S3 is opened as u2=1. S1 opening means S2 is closed, and S3 opening means S4 is closed. The inductance equation and the capacitance equation can be written as:

[0038]

[0039] set up

[0040]

[0041] Among them, Z0 and f0 are characteristic impedance and characteristic frequency respectively.

[0042] The output reference voltage is set as V r , remember V n 、i n ,t n Normalized values ​​of V, i, and t respectively. Normalization operation: Divide the variables such as voltage, current, and time by the reference value to obtain the normalized variable.

[0043]

[0044] Among them, Z r 、f r are the reference impedance and reference frequency respectively.

[0045] In the T1 phase, the first switch tube Q1 and the fourth switch tube Q4 are turned on, and the second switch tube Q2 and the third switch tube Q3 are turned off. In formula (1), let u1 = 1 and u2 = 0, and we get formula (6).

[0046]

[0047] The time, voltage and current in formula (6) are normalized and substituted by those in formula (5) to obtain formula (7).

[0048]

[0049] Among them, V inn ,I on 、v on 、i Ln V in ,I o 、V o 、i L The normalized variable.

[0050] Substituting formula (3) and (4) into formula (7) and simplifying it, we get formula (8).

[0051]

[0052] Dividing the two equations in equation (8) yields equation (9).

[0053]

[0054] Convert the differential equation into a difference equation and we get formula (10).

[0055]

[0056] Among them, i Ln(t0) 、V on(t0) They represent the normalized inductor current and normalized output voltage at the beginning of T1 phase respectively.

[0057] Multiply both sides by the product of the denominators and transpose the terms to obtain the equation for the first trajectory.

[0058] λ1:I on ·(i Ln -i Ln(t0) )+V inn ·(v on -v on(t0) )=0 (11)

[0059] It can be obtained that the trajectory of the first segment of the linear energy storage stage of the excitation inductance is a line segment.

[0060] In the T2 phase, the first switch tube Q1 and the third switch tube Q3 are turned on, the second switch tube Q2 and the fourth switch tube Q4 are turned off, and the input directly transfers energy to the output. In formula (1), let u1 = 1, u2 = 1, and we get formula (12).

[0061]

[0062] The time, voltage and current in formula (12) are normalized and substituted by those in formula (5) to obtain formula (13).

[0063]

[0064] Substituting formula (3) and (4) into formula (13) and simplifying them, we get formula (14).

[0065]

[0066] Dividing the two equations in equation (14) yields equation (15).

[0067]

[0068] Multiply both sides of the equation by the product of the denominators and integrate both sides to obtain formula (16).

[0069]

[0070] The integral calculation of formula (16) and the shifted term yield formula (17).

[0071] λ2:(i Ln -I on ) 2 +(v on -V inn ) 2 -[(i Ln(t1) -I on ) 2 +(v on(t1) -V inn ) 2 ]=0 (17)

[0072] It can be obtained that the trajectory of the second direct transmission stage is an arc, where i Ln(t1) 、v on(t1) Represents the normalized inductor current and normalized output voltage at the end of T1 and the beginning of T2.

[0073] In the reset stage T3, the second switch tube Q2 and the third switch tube Q3 are turned on, the first switch tube Q1 and the fourth switch tube Q4 are turned off, and the inductor current begins to decrease. In formula (1), let u1 = 0, u2 = 1, and we get formula (18).

[0074]

[0075] The time, voltage and current in formula (18) are normalized and substituted by those in formula (5) to obtain formula (19).

[0076]

[0077] Substituting formula (3) and (4) into formula (19) and simplifying it, we get formula (20).

[0078]

[0079] Dividing the two equations in equation (20) yields equation (21).

[0080]

[0081] Multiply both sides of the equation by the product of the denominators and integrate both sides to obtain formula (22).

[0082]

[0083] By integrating formula (22) and shifting the term, we can obtain formula (23).

[0084] λ3:(i Ln -I on ) 2 +V on 2 -[(i Ln(t3) -I on ) 2 +v on(t3) 2 ]=0 (23)

[0085] Here Ln(t3) 、v on(t3) Represents the normalized inductor current and normalized output voltage at the end of T3 and the beginning of T1.

[0086] Assuming steady state, v on(to) =v on(t3) =V rn =1,i Ln(to) =i Ln(t3) = 0. Then the trajectories of λ1 and λ3 are determined, and now we need to determine the trajectory of λ2. The center of λ2 is known to be (V inn ,I on ), now we need to determine its radius. From the perspective of reducing conduction loss, the smaller the radius of λ2, the smaller the peak current and the smaller the conduction loss. The center of the circle (V inn ,I on ) to λ1 is Center(V inn ,I on ) to λ3’s tangent point (V2,I2) is On the basis of ensuring that the three segments of the trajectory intersect, the conduction loss is minimized and the radius of λ2 should be the larger value of the distance between the two points.

[0087] V on is the horizontal axis, i Ln Plotted as the vertical axis Figure 4(a) , 4(b) , 4(c) three pictures.

[0088] Figure 4(a) shows the three-segment trajectory under the voltage reduction condition. Under the control method requirements of the natural switch surface, i Ln It should start from (1, 0), rise in the T1 stage (blue), rise in the T2 stage (green), fall in the T3 stage (red), and finally return to (1, 0). At this time, (1, 0), blue point 1, and purple point 3 are the switching points. The switching conditions are when the inductor current at the operating point is greater than or equal to the inductor current value at the blue point 1, when the operating point is outside the red track, and when the inductor current at the operating point is less than or equal to 0.

[0089] Figure 4(b) shows the three-segment trajectory under boost condition. Under the control method requirements of the natural switch surface, i Ln It should start from (1, 0), rise in T1 stage (blue), fall in T2 stage (green), fall in T3 stage (red), and finally return to (1, 0). At this time, (1, 0), 4 purple points, and 2 red points are switching points. The switching conditions are when the operating point is outside the green track, when the inductor current at the operating point is less than or equal to the inductor current value at the 2 red points, and when the inductor current at the operating point is less than or equal to 0.

[0090] Figure 4(c) shows the three-segment trajectory when the input and output voltages are close. Under the control method requirements of the natural switching surface, in the steady state, i Ln It should start from (1, 0), rise in the T1 stage (blue), rise or fall in the T2 stage (green), fall in the T3 stage (red), and finally return to (1, 0). At this time, (1, 0), blue point 1, and purple point 3 are the switching points. The switching conditions are when the inductor current at the operating point is greater than or equal to the inductor current value at the blue point 1, when the operating point is outside the green track, and when the inductor current at the operating point is less than or equal to 0.

[0091] Combining the switching conditions of the three cases, the condition for switching from T1 to T2 is: the inductor current value is greater than or equal to the inductor current value at the blue point 1 and is outside the green track. The condition for switching from T2 to T3 is: the inductor current value is less than or equal to the inductor current value at the red point 2 or is outside the red track. The condition for switching from T3 to T1 is: the inductor current is less than or equal to 0.

[0092] When performing switching rate control, anti-overshoot and soft switching designs also need to be added.

[0093] The anti-overshoot design is that when the inductor current is greater than the set value, the state will be switched regardless of whether the switching conditions are met, ensuring that the system will not be damaged due to excessive instantaneous current at startup.

[0094] The soft switch design is that when state switching is required, the junction capacitance is first charged or discharged through the inductor current, so that the voltages on both sides of the switch tube are equal before turning it on, reducing switching losses and improving overall efficiency. When switching from T1 to T2, the fourth switch tube Q4 is first turned off, and the voltages on both sides of the third switch tube Q3 are equal when the inductor current charges the junction capacitance of the rear bridge arm to V0, and the third switch tube Q3 is turned on to complete the state switching. When switching from T2 to T3, the first switch tube Q1 is first turned off, and the voltage of the junction capacitance of the front bridge arm is reduced to 0V. At this time, the voltages on both ends of the second switch tube Q2 are equal, and the second switch tube Q2 is turned on to complete the switching from T2 to T3. When switching from T3 to T1, first turn off the third switch tube Q3, at which time the inductor current reverses, wait for the voltage at the midpoint of the rear bridge arm to drop to 0V, at which time the voltages on both sides of the fourth switch tube Q4 are equal, turn on the fourth switch tube Q4 and turn off the second switch tube Q2 at the same time, wait for the reverse inductor current to charge the voltage at the midpoint of the front bridge arm to Vin, at which time the voltages at both ends of the first switch tube Q1 are equal, turn on the first switch tube Q1, and complete the switch from T3 to T1. At this point, the full cycle of soft switching is completed, and the inductor current waveforms under buck and boost conditions and the timing diagrams of the four switches are shown in Figure 1. Figure 5 , Figure 6 As shown, the simulation waveform is as follows Figure 7 , Figure 8 shown.

[0095] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A high dynamic control circuit for a four-switch buck-boost converter, characterized in that: Includes a four-switch buck-boost converter circuit, an analog-to-digital converter, and an FPGA development board that hosts a digital control module; In the four-switch buck-boost converter, the positive electrode of the first switch tube is connected to the input voltage, and the negative electrode is connected to the left end of the inductor; the positive electrode of the second switch tube is connected to the left end of the inductor, and the negative electrode is grounded; the positive electrode of the third switch tube is connected to the output voltage, and the negative electrode is connected to the right end of the inductor; the positive electrode of the fourth switch tube is connected to the right end of the inductor, and the negative electrode is grounded; the positive electrode of the output capacitor is connected to the output voltage, and the negative electrode is grounded; In the analog-to-digital converter and FPGA development board, the analog signal is converted into a digital signal through the analog-to-digital converter, the digital signal is input into the digital module, and then the control signal of the four switch tubes is output through the natural switch surface control method and returned to the four-switch buck-boost converter.

2. The high dynamic control circuit of the four-switch buck-boost converter according to claim 1, characterized in that: In the digital module, the digital signal input after conversion by the analog-to-digital converter is read through the SPI interface in the form of shift register.

3. The high dynamic control circuit of the four-switch buck-boost converter according to claim 1, characterized in that: In the digital module, the phase change is switched by a three-stage state machine, including the linear energy storage stage of the excitation inductor, the direct transfer stage and the reset stage, and several intermediate states are included in the soft switching design.

4. The high dynamic control circuit of the four-switch buck-boost converter according to claim 3, characterized in that: For the soft switch design, in steady state, the first state is that the first switch tube and the fourth switch tube are both turned on, and the second switch tube and the third switch tube are both turned off; When switching states, the fourth switch tube is first turned off, and when the inductor current charges the junction capacitor at the midpoint of the rear bridge arm to reach the output voltage, the third switch tube is turned on to achieve soft switching and reduce losses; When switching from the second state to the third state, the first switch tube is turned off first, and the second switch tube is turned on when the junction capacitance voltage at the midpoint of the front bridge arm drops to 0V, thereby realizing soft switching and reducing losses. When switching from the third state to the first state, first turn off the third switch tube, wait for the midpoint voltage of the rear bridge arm to drop to 0V, turn on the fourth switch and turn off the second switch tube at the same time, wait for the reverse inductor current to charge the midpoint junction capacitor of the front bridge arm to the input voltage, turn on the first switch tube, and realize soft switching to reduce losses.

5. The high dynamic control circuit of the four-switch buck-boost converter according to any one of claims 1 to 3, characterized in that: There is an anti-overshoot design in the digital module. When the inductor current is greater than the set value, the state will be switched regardless of whether the switching conditions are met, ensuring that the system will not be damaged due to excessive instantaneous current at startup.

6. The control method of the high dynamic control circuit of the four-switch buck-boost converter according to any one of claims 1 to 3, characterized in that: The implementation steps are as follows: The input voltage, inductor current, output voltage, and bridge arm midpoint voltage of the four-switch buck-boost converter are collected and converted into digital signals through an analog-to-digital converter; The digital signal is collected via SPI interface to unify the sampling rate; The digital signal is processed by a natural switch surface control method, and nonlinear control is performed by a three-stage state machine Verilog program. The control signals of the four switch tubes are output and returned to the four-switch buck-boost converter.

7. The control method of the high dynamic control circuit of the four-switch buck-boost converter according to claim 6, characterized in that: In the control method of the natural switching surface, a four-switch buck-boost converter in the boundary current mode state has three working states in one cycle: the linear energy storage stage of the excitation inductance, the direct transmission stage and the reset stage. The switch state of each stage is different. The circuit state trajectory is normalized and derived according to the inductance equation and the capacitance equation. Under the premise that the trajectories of the linear energy storage stage and the reset stage of the excitation inductance are determined, the trajectory of the direct transmission stage is determined based on the principle of minimum conduction loss, ensuring that there is an intersection between the three trajectories, and adding soft switching and anti-overshoot design. In steady state, the first state is that the first switch tube and the fourth switch tube are both turned on, and the second switch tube and the third switch tube are both turned off. When switching states, the fourth switch tube is turned off first, and the inductor current is waited for to charge the junction capacitor at the midpoint of the rear bridge arm to reach the output voltage, and then the third switch tube is turned on to achieve soft switching to reduce losses; when switching from the second state to the third state, the first switch tube is turned off first, and the junction capacitor voltage at the midpoint of the front bridge arm is waited to drop to 0V, and then the second switch tube is turned on to achieve soft switching to reduce losses; when switching from the third state to the first state, the third switch tube is turned off first, and the voltage at the midpoint of the rear bridge arm is waited to drop to 0V, and then the fourth switch is turned off while the second switch tube is turned off, and the reverse inductor current is waited for to charge the junction capacitor at the midpoint of the front bridge arm to the input voltage, and then the first switch tube is turned on to achieve soft switching to reduce losses; Finally, four switch tube signals are output to the four-switch buck-boost converter to complete the control.

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