A kind of boost-buck circuit, device and charge-discharge system and photovoltaic power generation system

Abstract

The utility model discloses a kind of boost-buck circuit, equipment and charge-discharge system and photovoltaic power generation system, belong to power electronics field. Boost-buck circuit includes: first switch tube, second switch tube, third switch tube, first diode, transformer, first capacitor and second capacitor and load resistance. The boost-buck circuit of the application scheme can realize boost and buck, compared with traditional boost-buck circuit, circuit component is less, and volume weight is small. And can realize two-stage boost, so that component stress is small.

Description

Technical Field

[0001] This utility model relates to the field of power electronics technology, and in particular to a step-up / step-down circuit, equipment, charging and discharging system, and photovoltaic power generation system. Background Technology

[0002] Buck-boost DC converters are widely used in photovoltaic power generation systems and in applications where voltage varies over a wide range, such as battery charging and discharging. The output voltage of photovoltaic cells fluctuates significantly with changes in external environmental conditions, requiring the system to adapt to a wide range of input voltage variations.

[0003] Traditional buck-boost DC-DC converters, such as Buck, Boost, and flyback switching converters, can achieve buck-boost conversion, but they have problems such as high device stress and large size and weight, making them unsuitable for some applications. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, this utility model provides a step-up / step-down circuit, equipment, charging and discharging system and photovoltaic power generation system to solve the problems that traditional step-up / step-down DC converters have high stress, large size and weight, and are not suitable for some applications.

[0005] The technical solution adopted by this utility model to solve its technical problem is:

[0006] In a first aspect, a step-up / step-down circuit is provided, comprising: a first switching transistor, a second switching transistor, a third switching transistor, a first diode, a transformer, a first capacitor, a second capacitor, and a load resistor;

[0007] The input terminal of the first switching transistor is connected to the positive terminal of the power supply, and the output terminal of the first switching transistor is connected to the negative terminal of the power supply through a reverse-connected first diode.

[0008] The output terminal of the first switching transistor is also connected to one end of the primary coil of the transformer; the other end of the primary coil of the transformer is connected to the input terminal of the second switching transistor, and the output terminal of the second switching transistor is connected to the negative terminal of the power supply.

[0009] One end of the secondary coil of the transformer is connected to the input terminal of the third switching transistor, and the other end of the secondary coil of the transformer is connected to one end of the first capacitor.

[0010] The other end of the first capacitor is connected to the negative terminal of the power supply, and one end of the first capacitor is also connected to one end of the second capacitor. The other end of the second capacitor is connected to the output terminal of the third switching transistor.

[0011] The other end of the primary coil of the transformer is connected to one end of the first capacitor;

[0012] One end of the load resistor is connected to the other end of the second capacitor, and the other end of the load resistor is connected to the negative terminal of the power supply.

[0013] As an optional implementation of this application, in boost mode, both the first and third switching transistors remain on in each cycle, and the second switching transistor is on for a first preset duration.

[0014] As an optional implementation of this application, in buck mode, in each cycle, the second switch and the third switch remain off, and the first switch is turned on for a second preset duration.

[0015] As an optional implementation of this application, the other end of the primary coil of the transformer is connected to one end of the first capacitor through a positively connected second diode.

[0016] As an optional implementation of this application, a third diode is connected in parallel across the two ends of the second capacitor, with the positive terminal of the third diode connected to one end of the second capacitor and the negative terminal of the third diode connected to the other end of the second capacitor.

[0017] As an optional implementation of this application, one end of the secondary coil of the transformer is connected to the input terminal of the third switching transistor through a positively connected fourth diode.

[0018] As an optional implementation of this application, the negative terminal of the power supply is grounded.

[0019] In a second aspect, a step-up / step-down device is provided, including the step-up / step-down circuit in the technical solution provided in the first aspect.

[0020] Thirdly, a battery charging and discharging system is provided, including the buck-boost circuit in the technical solution provided in the first aspect.

[0021] Fourthly, a photovoltaic power generation system is provided, including the step-up / step-down circuit in the technical solution provided in the first aspect.

[0022] The application employs the above technical solution and has at least the following beneficial effects:

[0023] This application provides a step-up / step-down circuit, device, charging / discharging system, and photovoltaic power generation system. The step-up / step-down circuit includes: a first switching transistor, a second switching transistor, a third switching transistor, a first diode, a transformer, a first capacitor, a second capacitor, and a load resistor; the input terminal of the first switching transistor is connected to the positive terminal of the power supply, and the output terminal of the first switching transistor is connected to the negative terminal of the power supply through the reverse-connected first diode; the output terminal of the first switching transistor is also connected to one end of the primary coil of the transformer; the other end of the primary coil of the transformer is connected to the input terminal of the second switching transistor, and the output terminal of the second switching transistor is connected to the negative terminal of the power supply; one end of the secondary coil of the transformer is connected to the input terminal of the third switching transistor, and the other end of the secondary coil of the transformer is connected to one end of the first capacitor; the other end of the first capacitor is connected to the negative terminal of the power supply, and one end of the first capacitor is also connected to one end of the second capacitor, the other end of the second capacitor being connected to the output terminal of the third switching transistor; the other end of the primary coil of the transformer is connected to one end of the first capacitor; one end of the load resistor is connected to the other end of the second capacitor, and the other end of the load resistor is connected to the negative terminal of the power supply. The buck-boost circuit proposed in this application can achieve both voltage boost and buck conversion. Compared to traditional buck-boost circuits, it has fewer components and is smaller in size and weight. Furthermore, it can achieve two-stage voltage boost, thus reducing stress on the components. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a circuit diagram of a step-up / step-down circuit provided in an embodiment of the present invention;

[0026] Figure 2 This is a specific step-up / step-down circuit schematic diagram provided by an embodiment of the present invention;

[0027] Figure 3 This is one of the embodiments provided by this utility model. Figure 2 The equivalent circuit diagram of the step-up / step-down circuit shown is shown.

[0028] Figure 4 This is one of the embodiments provided by this utility model. Figure 2 The equivalent circuit diagram of the buck converter circuit shown is shown.

[0029] Figure 5 This is a schematic diagram of the control logic of a step-up / step-down circuit provided in an embodiment of this utility model.

[0030] Figure 6This is a schematic diagram of the control logic of a fuzzy PID controller provided in an embodiment of this utility model.

[0031] Explanation of reference numerals in the attached figures:

[0032] VIN - Power supply, S1 - First switching transistor, S2 - Second switching transistor, S3 - Third switching transistor, D1 - First diode, D2 - Second diode, D3 - Third diode, D4 ​​- Fourth diode, T1 - Transformer; NP - Primary coil, NS - Secondary coil, C1 - First capacitor, C2 - Second capacitor, RL - Load resistor. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this utility model will be described in detail below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0034] Traditional step-up / step-down circuits, such as Cuk circuits, have the advantage of continuous current, but their adjustable voltage range is limited. When adjusting voltage over a wider range, the switching frequency is high, the stress is high, and they may not even be applicable to a wide range of voltage adjustments.

[0035] To solve the above problems, refer to Figure 1 This utility model embodiment provides a step-up / step-down circuit, including: a first switch S1, a second switch S2, a third switch S3, a first diode D1, a transformer T1, a first capacitor C1, a second capacitor C2, and a load resistor RL;

[0036] The input terminal of the first switching transistor S1 is connected to the positive terminal of the power supply VIN, and the output terminal of the first switching transistor S1 is connected to the negative terminal of the power supply VIN through the reverse-connected first diode D1.

[0037] The output terminal of the first switch S1 is also connected to one end of the primary coil NP of the transformer T1; the other end of the primary coil NP of the transformer T1 is connected to the input terminal of the second switch S2, and the output terminal of the second switch S2 is connected to the negative terminal of the power supply VIN.

[0038] One end of the secondary coil NS of the transformer T1 is connected to the input terminal of the third switch S3, and the other end of the secondary coil NS of the transformer T1 is connected to one end of the first capacitor C1.

[0039] The other end of the first capacitor C1 is connected to the negative terminal of the power supply VIN. One end of the first capacitor C1 is also connected to one end of the second capacitor C2. The other end of the second capacitor C2 is connected to the output terminal of the third switch S3.

[0040] The other end of the primary coil NP of the transformer T1 is connected to one end of the first capacitor C1;

[0041] One end of the load resistor RL is connected to the other end of the second capacitor C2, and the other end of the load resistor RL is connected to the negative terminal of the power supply VIN.

[0042] The buck-boost circuit in this embodiment can both boost and buck voltage.

[0043] During voltage boosting, both the first switch S1 and the third switch S3 remain on, while the second switch S2 is on for a first preset duration (i.e., duty cycle, determined according to actual requirements). When both the first switch S1 and the third switch S3 remain on, the voltage at the first capacitor C1 is related to the duty cycle and the input voltage VIN, while the voltage at the second capacitor C2 is related to the transformer T1. In contrast, the voltage in a traditional buck-boost circuit is only related to the input voltage and the duty cycle. (The traditional relationship between input and output voltage is: Output voltage = Input voltage / (1 - Duty cycle). To achieve a larger voltage boost, a larger duty cycle is required, meaning a longer switch conduction time and greater stress. If a larger boost ratio is required, the switch conduction time will also be longer. If the buck-boost circuit operates for a long time, it will cause excessive heat generation in the switches, affecting their lifespan. Therefore, a long switch operation time is not allowed in the design; that is, traditional buck-boost circuits cannot achieve a large boost ratio.)

[0044] During the step-down process, the second switch S2 and the third switch S3 remain off, while the first switch S1 is turned on for a second preset duration (i.e., duty cycle, set according to actual needs). After the second switch S2 and the third switch S3 remain off, it can be equivalent to a traditional Buck circuit.

[0045] As an optional implementation of this application, the first switch S1, the second switch S2, and the third switch S3 can be any of the following: transistors, MOSFETs, and IGBTs (although the figures in this application all show MOSFETs, transistors and IGBTs can also function as switch transistors). The input and output terminals of the switch transistors correspond to different pins in different types of switch transistors, but all must achieve the following effect: when the switch transistor is turned on, current flows from the input terminal to the output terminal; when the switch transistor is turned off, current cannot flow from the input terminal to the output terminal.

[0046] The buck-boost circuit provided in this embodiment includes: a first switch S1, a second switch S2, a third switch S3, a first diode D1, a transformer T1, a first capacitor C1, a second capacitor C2, and a load resistor RL; the input terminal of the first switch S1 is connected to the positive terminal of the power supply VIN, and the output terminal of the first switch S1 is connected to the negative terminal of the power supply VIN through the reverse-connected first diode D1; the output terminal of the first switch S1 is also connected to one end of the primary coil NP of the transformer T1; the other end of the primary coil NP of the transformer T1 is connected to the input terminal of the second switch S2, and the output terminal of the second switch S2 is connected to the input terminal of the transformer T1. The negative terminal of the power supply VIN is connected; one end of the secondary coil NS of transformer T1 is connected to the input terminal of the third switch S3, and the other end of the secondary coil NS of transformer T1 is connected to one end of the first capacitor C1; the other end of the first capacitor C1 is connected to the negative terminal of the power supply VIN, and one end of the first capacitor C1 is also connected to one end of the second capacitor C2, and the other end of the second capacitor C2 is connected to the output terminal of the third switch S3; the other end of the primary coil NP of transformer T1 is connected to one end of the first capacitor C1; one end of the load resistor RL is connected to the other end of the second capacitor C2, and the other end of the load resistor RL is connected to the negative terminal of the power supply VIN. The buck-boost circuit of this application can achieve both boost and buck voltage. Compared with traditional buck-boost circuits, it has fewer components and is smaller in size and weight. Furthermore, it can achieve two-stage boost voltage, thus reducing component stress.

[0047] Based on the same inventive concept, this application provides a specific buck-boost circuit, such as... Figure 2 As shown: It includes: a first switch S1, a second switch S2, a third switch S3, a first diode D1, a transformer T1, a first capacitor C1, a second capacitor C2, and a load resistor RL;

[0048] The input terminal of the first switching transistor S1 is connected to the positive terminal of the power supply VIN, and the output terminal of the first switching transistor S1 is connected to the negative terminal of the power supply VIN through the reverse-connected first diode D1.

[0049] The output terminal of the first switch S1 is also connected to one end of the primary coil NP of the transformer T1; the other end of the primary coil NP of the transformer T1 is connected to the input terminal of the second switch S2, and the output terminal of the second switch S2 is connected to the negative terminal of the power supply VIN.

[0050] One end of the secondary coil NS of the transformer T1 is connected to the input terminal of the third switch S3 through the positively connected fourth diode D4, and the other end of the secondary coil NS of the transformer T1 is connected to one end of the first capacitor C1; wherein, the fourth diode D4 can stabilize the voltage across the second capacitor C2.

[0051] The other end of the first capacitor C1 is connected to the negative terminal of the power supply VIN. One end of the first capacitor C1 is also connected to one end of the second capacitor C2. The other end of the second capacitor C2 is connected to the output terminal of the third switch S3.

[0052] The other end of the primary coil NP of the transformer T1 is connected to one end of the first capacitor C1 through a positively connected second diode D2;

[0053] One end of the load resistor RL is connected to the other end of the second capacitor C2, and the other end of the load resistor RL is connected to the negative terminal of the power supply VIN.

[0054] A third diode D3 is connected in parallel across the second capacitor C2. The anode of the third diode D3 is connected to one end of the second capacitor C2, and the cathode of the third diode D3 is connected to the other end of the second capacitor C2. When the third diode D3 is conducting, it short-circuits the second capacitor C2. In buck mode, the circuit is equivalent to a conventional Buck circuit.

[0055] In addition, as an optional implementation of this application, the negative terminal of the power supply VIN is grounded. By grounding the negative terminal of the power supply VIN, the stability of equipment operation and personnel safety can be ensured.

[0056] In boost mode, both the first switch S1 and the third switch S3 remain on in each cycle, while the second switch S2 is on for a first preset duration. The second diode D2 ensures that the capacitor charging voltage waveform does not fluctuate significantly, while the first diode D1, having no conductive path, is considered an open circuit, and the third diode D3, lacking a bias voltage, is also considered an open circuit. Because the transformer T1's terminals are configured as follows... Figure 2 As shown, at this time, transformer T1, fourth diode D4, third switch S3, and second capacitor C2 form a forward topology. When the second switch S2 is turned on, the input voltage is mapped to the secondary coil NS through the turns ratio of transformer T1, serving as the voltage source of the forward topology. Combining the above boosting process, it can be seen that the output voltage is the sum of the voltage across the first capacitor C1 and the voltage across the second capacitor C2, achieving the boost. Its equivalent circuit diagram is shown below. Figure 3 As shown.

[0057] The duty cycle D of the second switch S2 is defined by the following formula:

[0058]

[0059] t on The time t is the time during which the second switch S2 remains on. off The time during which the second switch S2 remains off.

[0060] Assuming the inductor operates in steady state, then we have

[0061] V in ×t on =V off ×t off Formula 2

[0062] Because V off =V o -V in

[0063] V off V is the voltage across the inductor when the second switch S2 is turned off. o V is the output voltage. in This is the input voltage.

[0064] Combined equations 1 and 2

[0065] achievable

[0066] The turns ratio of transformer T1 is n;

[0067] Therefore, the voltage across the second capacitor C2 is V at this time. C2 for:

[0068] V C2 =V in ×n

[0069] It can be known that the voltage across the load resistor RL is:

[0070]

[0071] In buck mode, during each cycle, the second switch S2 and the third switch S3 remain off, while the first switch S1 is on for a second preset duration. When the first switch S1 is off, the first diode D1 provides a freewheeling path for the primary coil NP of the transformer T1. The equivalent circuit diagram at this time is as follows: Figure 4 As shown.

[0072] The duty cycle of the first switching transistor S1 in buck mode is D;

[0073] Then the output voltage V o With input voltage V in relation:

[0074] V o =V in ×D

[0075] Let the ripple rate of the buck converter be r and the switching frequency be f. sw Load current I O

[0076] Then we can obtain:

[0077] Where L1 is the inductance of the primary coil NP of transformer T1 in the step-up / step-down circuit at this time.

[0078] In both boost and buck modes, the duty cycle of the first switch S1 and the duty cycle of the second switch S2 are controlled by a PID controller.

[0079] This application uses a control scheme based on existing technology for the PID controller without making any improvements. A control scheme based on existing technology is provided below.

[0080] First, PID controllers are widely used, and their general form is as follows:

[0081]

[0082] Where e(t) is the system error; Kp, Ki, and Kd are the weights of the system error signal and its integral and differential components, respectively. The controller can calculate the control signal u(t) through such weighting to drive the switching transistor.

[0083] The functions of each component in a PID controller are as follows:

[0084] The proportional element reflects the deviation signal e(t) of the control system in real time and proportionally. Once a deviation occurs, the controller immediately takes control action to reduce the deviation.

[0085] The integral term is mainly used to eliminate steady-state error and improve the accuracy of the system. The strength of the integral action depends on the integral time constant Ki; the larger Ki is, the weaker the integral action, and vice versa.

[0086] The differential element reflects the changing trend of the deviation signal and can introduce an effective early correction signal into the system before the deviation signal becomes too large, thereby accelerating the system's response speed and shortening the adjustment time.

[0087] The performance of a PID controller depends on the rationality of three parameters: Kp, Ki, and Kd. Therefore, optimizing the PID controller parameters is of great significance. Thus, the embodiments of this application are based on... Figure 5 The control is implemented using a dual closed-loop control method. The outer loop is for voltage control, and the inner loop is for current control. The voltage loop employs variable universe of discourse fuzzy PI control; the current loop employs deadbeat control.

[0088] Vfdb1: DC-DC output voltage, Vset: output voltage setting, error1: deviation 1, Vout1: voltage loop controller output, Vin: input voltage, Iset: current setting, Ifdb1: inductor current, error2: deviation 2, Iout1: current loop controller output.

[0089] like Figure 6 As shown, the fuzzy controller used in this embodiment is a two-dimensional fuzzy controller. The inputs are the error *e* and the error rate of change *ec*, and the outputs are the correction values ​​of the three parameters Kp, Ki, and Kd of the PID controller. In the fuzzy PID control, the magnitudes of *e* and *ec* are continuously acquired and then multiplied by a quantization factor as the input variables of the fuzzy control system. The adjustment parameters of the control system are obtained according to fuzzy rules formulated based on expert experience. Then, defuzzification is performed, and the values ​​are multiplied by a scaling factor to obtain the adjustment values ​​of Kp, Ki, and Kd. These adjustment values ​​then interact with the initial values ​​of Kp, Ki, and Kd to obtain the final control parameters. In this way, the controller can meet the requirement of automatic adjustment of control parameters when the error and the error rate of change change, improving the dynamic response characteristics of the system and enhancing its anti-interference capability.

[0090] The deadbeat control principle in the current loop is as follows.

[0091] (1) The controller samples the current at the beginning of each cycle, obtaining I(k,1) and I(k,2), which represent the sampled currents of the first and second cycles of the k-th clock cycle, respectively. Since the duty cycle of the two cycles within each clock cycle is the same, the input and output voltages can also be considered constant, so the change in current is also equal. Based on this, linear interpolation can be performed to obtain the inductor current value one cycle in advance:

[0092] I SET (k)=2I(k,2)-I(k,1) (1)

[0093] (2) The deviation value Δv between the measured value of the output voltage Vfdb of the DC-DC converter and the set value of the reference voltage Vref is obtained by calculating the deviation value. The reference current Iref is obtained through the voltage loop controller.

[0094] (3) The controller performs current estimation according to equation (1), replacing I(k+1,1) with Iset(k), thus obtaining current information one cycle in advance. After performing current estimation, the controller calculates the duty cycle d of the next cycle based on the error between Iest(k) and the reference current Iref.

[0095]

[0096] In the formula: T is the switching period; d(k+1) is the duty cycle of the (k+1)th period.

[0097] (4) Compare the measured value of the output voltage Vfdb with the set value of the reference voltage Vref, and continuously adjust the duty cycle of the switching device so that the switching control quantity accurately follows the control reference value within one switching cycle, thereby achieving stable output voltage.

[0098] Of course, this application can also use a conventional PID controller for control, that is, once the parameters Kp, Ki, and Kd are tuned, they will not be changed. Since the specific control is not within the scope of protection of this application, it is only described in detail.

[0099] It is understood that the same or similar parts in the above embodiments can be referred to each other, and the contents not described in detail in some embodiments can be referred to the same or similar contents in other embodiments.

[0100] It should be noted that in the description of this application, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, in the description of this application, unless otherwise stated, "a plurality of" means at least two.

[0101] Any process or method described in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of the preferred embodiments of this application includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the function involved, as will be understood by those skilled in the art to which embodiments of this application pertain.

[0102] It should be understood that various parts of this application can be implemented using hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented using software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0103] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0104] Furthermore, the functional units in the various embodiments of this application can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into a module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

[0105] The storage media mentioned above can be read-only memory, disk, or optical disk, etc.

[0106] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0107] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A step-up / step-down circuit, characterized in that, include: The components include a first switching transistor, a second switching transistor, a third switching transistor, a first diode, a transformer, a first capacitor, a second capacitor, and a load resistor. The input terminal of the first switching transistor is connected to the positive terminal of the power supply, and the output terminal of the first switching transistor is connected to the negative terminal of the power supply through a reverse-connected first diode. The output terminal of the first switching transistor is also connected to one end of the primary coil of the transformer; the other end of the primary coil of the transformer is connected to the input terminal of the second switching transistor, and the output terminal of the second switching transistor is connected to the negative terminal of the power supply. One end of the secondary coil of the transformer is connected to the input terminal of the third switching transistor, and the other end of the secondary coil of the transformer is connected to one end of the first capacitor. The other end of the first capacitor is connected to the negative terminal of the power supply, and one end of the first capacitor is also connected to one end of the second capacitor. The other end of the second capacitor is connected to the output terminal of the third switching transistor. The other end of the primary coil of the transformer is connected to one end of the first capacitor; One end of the load resistor is connected to the other end of the second capacitor, and the other end of the load resistor is connected to the negative terminal of the power supply.

2. The step-up / step-down circuit according to claim 1, characterized in that: In boost mode, both the first and third switching transistors remain on during each cycle, and the second switching transistor remains on for a first preset duration.

3. The step-up / step-down circuit according to claim 1, characterized in that: In buck mode, during each cycle, the second and third switching transistors remain off, while the first switching transistor is turned on for a second preset duration.

4. The step-up / step-down circuit according to claim 1, characterized in that: The other end of the primary coil of the transformer is connected to one end of the first capacitor via a positively connected second diode.

5. The step-up / step-down circuit according to claim 1, characterized in that: A third diode is connected in parallel across the two ends of the second capacitor. The positive terminal of the third diode is connected to one end of the second capacitor, and the negative terminal of the third diode is connected to the other end of the second capacitor.

6. The step-up / step-down circuit according to claim 1, characterized in that: One end of the secondary coil of the transformer is connected to the input terminal of the third switching transistor via a positively connected fourth diode.

7. The step-up / step-down circuit according to claim 1, characterized in that: The negative terminal of the power supply is grounded.

8. A pressure boosting device, characterized in that, Includes the buck-boost circuit as described in any one of claims 1-7.

9. A charging and discharging system, characterized in that: Includes the buck-boost circuit as described in any one of claims 1-7.

10. A photovoltaic power generation system, characterized in that: Includes the buck-boost circuit as described in any one of claims 1-7.

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