Bus voltage-sharing system for three-level Buck Boost converter

Abstract

The invention provides a bus voltage equalizing system for a three-level Buck Boost converter, which comprises a main control unit, a filter capacitor C3, a soft start loop and a main relay, and is characterized in that the main control unit is used for monitoring the working state of a battery and starting the soft start loop when the battery works normally; the soft start loop and the main relay are controlled to be in a closed state, and the voltage between the positive bus and the negative bus is balanced by controlling the duty ratio difference of the switching tube of the first bridge arm and the switching tube of the second bridge arm; when the battery breaks down, the soft start loop and the main relay are disconnected, and the voltage between the positive bus and the negative bus is balanced through the filter capacitor. Therefore, bus voltage sharing under all working conditions can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of hybrid inverters, and more particularly, to a bus voltage equalization system for a three-level Buck-Boost converter. BACKGROUND

[0002] In a three-phase household storage hybrid inverter, the bus voltage is mostly 800V during operation, while the commonly used electrolytic capacitor is 500V, so the electrolytic capacitor needs to be connected in series. In addition, the commonly used three-phase inverter topology is also three-level, with positive and negative half bus lines. In the hybrid inverter shown in the figure, the bus is connected in series with the capacitor. Due to the unequal load of the positive and negative half cycles of the inverter, the difference in hardware parameters, and the difference in the actual drive duty cycle of the three-level Buck-Boost, the positive and negative bus lines will be unbalanced. After the bus imbalance occurs, the inverter output voltage will be "cut off" in one direction, and the capacitor will be damaged due to overvoltage. Therefore, the balance of the positive and negative bus voltages must be ensured for this type of product. Figure 1

[0003] The existing solution 1 (CN201710640288.2) discloses a three-level inverter midpoint balancing circuit control method, which uses an additional balancing circuit to suppress the midpoint potential deviation of the microgrid three-level inverter. The disadvantage of this solution is that it adds many circuits, including transistors and their drivers, inductors, sampling and related control circuits, which increases the cost and increases the size of the product.

[0004] The existing solution 2 (CN202411730034.6) discloses a three-level circuit converter voltage equalization control method and controller, which uses the existing three-level Buck-Boost duty cycle difference to achieve bus voltage equalization. This solution does not need to add new circuits, and is more cost-effective than solution 1. However, this solution relies on the battery to work, and if the battery does not work, the three-level Buck-Boost will not be able to achieve voltage equalization control, so there is a certain limitation in use.

[0005] Therefore, a new solution is needed. SUMMARY

[0006] The purpose of the present application is to provide a bus voltage equalization system for a three-level Buck-Boost converter.

[0007] ​According to an aspect of the present application, a bus voltage equalization system for a three-level Buck-Boost converter is provided, which is connected to the three-level Buck-Boost converter, the three-level Buck-Boost converter comprising a first bridge arm and a second bridge arm, a first end of the first bridge arm being connected to a positive bus, a first end of the second bridge arm being connected to a negative bus, a second end of the first bridge arm and a second end of the second bridge arm being connected to a bus midpoint, the bus voltage equalization system comprising a main control unit, a filter capacitor C3, a soft start circuit and a main relay, the filter capacitor C3 being connected between a third end of the first bridge arm and a third end of the second bridge arm, the third end of the first bridge arm also being connected to a positive electrode of a battery, the soft start circuit and the main relay being connected in parallel between the third end of the second bridge arm and a negative electrode of the battery, wherein the main control unit is configured to monitor an operating state of the battery, when the battery is operating normally, control the soft start circuit and the main relay to be in a closed state and equalize a voltage between the positive bus and the negative bus by controlling a duty cycle difference between a switching tube of the first bridge arm and a switching tube of the second bridge arm, and when the battery fails, disconnect the soft start circuit and the main relay, and equalize the voltage between the positive bus and the negative bus by using the filter capacitor.

[0008] In the bus voltage equalization system for the three-level Buck-Boost converter provided by the present application, the first bridge arm comprises a first capacitor C1, a first switching tube Q1, a second switching tube Q2 and a first inductor L1, a first end of the first capacitor C1 and a first end of the first switching tube Q1 being connected to the positive bus, a second end of the first switching tube Q1 being connected to a first end of the second switching tube Q2 and a first end of the first inductor L1, a second end of the second switching tube Q2 and a second end of the first capacitor C1 being connected to the bus midpoint, a second end of the first inductor L1 being connected to a first end of the filter capacitor C3:

[0009] The second bridge arm comprises a second capacitor C2, a third switching tube Q3, a fourth switching tube Q4 and a second inductor L2, a first end of the second capacitor C2 and a first end of the third switching tube Q3 being connected to the bus midpoint, a second end of the third switching tube Q3 being connected to a first end of the fourth switching tube Q4 and a first end of the second inductor L2, a second end of the fourth switching tube Q4 and a second end of the second capacitor C2 being connected to the negative bus, a second end of the second inductor L2 being connected to a second end of the filter capacitor C3;

[0010] A control end of the first switching tube Q1, a control end of the second switching tube Q2, a control end of the third switching tube Q3 and a control end of the fourth switching tube Q4 are connected to the main control unit.

[0011] In the bus voltage equalization system for the three-level Buck-Boost converter, when the battery is in normal operation and the voltage of the first capacitor C1 is not equal to the voltage of the second capacitor C2, the main control unit is configured to control the soft start circuit and the main relay to be in a closed state, and is further configured to:

[0012] control the first switch Q1 and the fourth switch Q4 to be turned on at the same time;

[0013] when the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, control the fourth switch Q4 to be turned off after a first energy storage time, and then control the first switch Q1 to be turned off after a first discharge time; when the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, control the first switch Q1 to be turned off after a first energy storage time, and then control the fourth switch Q4 to be turned off after a first discharge time;

[0014] control the first switch Q1 and the fourth switch Q4 to be turned on at the same time after a first freewheeling time, and repeat the process until the voltage of the first capacitor C1 is equal to the voltage of the second capacitor C2.

[0015] In the bus voltage equalization system for the three-level Buck-Boost converter, the first energy storage time is related to the duty cycle of the fourth switch Q4 or the first switch Q1, the first discharge time is related to the difference between the voltage of the first capacitor C1 and the voltage of the second capacitor C2, and the first freewheeling time is equal to the difference between the first energy storage time and the first discharge time.

[0016] In the bus voltage equalization system for the three-level Buck-Boost converter, when the battery is in a fault state and the voltage of the first capacitor C1 is not equal to the voltage of the second capacitor C2, the main control unit is configured to disconnect the soft start circuit and the main relay, and is further configured to:

[0017] when the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, control the first switch Q1, the second switch Q2, the third switch Q3 and the fourth switch Q4 to be turned off, so that the charge of the filter capacitor C3 is discharged to the second capacitor C2 to make the voltage of the filter capacitor C3 lower than a first preset value, and then the charge of the first capacitor C1 is discharged to the filter capacitor C3 to make the voltage of the filter capacitor C3 higher than a second preset value;

[0018] When the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, the main control unit is configured to:

[0019] When the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, the main control unit is configured to:

[0020] control the second switch Q2 and the third switch Q3 to be opened at the same time, control the third switch Q3 to be closed and the fourth switch Q4 to be opened after a second energy storage time, control the third switch Q3 to be opened and the fourth switch Q4 to be closed after a first charging time, and repeat the above operations until the voltage of the filter capacitor C3 is less than the first preset value;

[0021] control the first switch Q1 and the third switch Q3 to be opened at the same time, control the first switch Q1 to be closed and the second switch Q2 to be opened after a third energy storage time, control the first switch Q1 to be opened and the second switch Q2 to be closed after a second charging time, and repeat the above operations until the voltage of the filter capacitor C3 is greater than the second preset value.

[0022] In the bus voltage equalization system for the three-level Buck-Boost converter provided by the application, the second energy storage time is related to the duty cycle of the third switch Q3 and the working cycle, and the sum of the first charging time and the second energy storage time is equal to the working cycle; the third energy storage time is related to the duty cycle of the first switch Q1 and the working cycle, and the sum of the second charging time and the third energy storage time is equal to the working cycle.

[0023] When the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, the main control unit is configured to:

[0024] control the second switch Q2 and the third switch Q3 to be opened at the same time, control the second switch Q2 to be closed and the first switch Q1 to be opened after a fourth energy storage time, control the second switch Q2 to be opened and the first switch Q1 to be closed after a third charging time, and repeat the above operations until the voltage of the filter capacitor C3 is less than the first preset value;

[0025] The fourth switch Q4 and the second switch Q2 are controlled to be opened at the same time, after the fifth energy storage time, the fourth switch Q4 is controlled to be closed and the third switch Q3 is controlled to be opened, after the fourth charging time, the fourth switch Q4 is controlled to be opened and the third switch Q3 is controlled to be closed, and the process is repeated until the voltage of the filter capacitor C3 is higher than the second preset value.

[0026] In the bus voltage equalization system for the three-level Buck-Boost converter provided by the application, the fourth energy storage time is related to the duty cycle of the second switch Q2 and the working cycle, and the sum of the third charging time and the fourth energy storage time is equal to the working cycle; the fifth energy storage time is related to the duty cycle of the fourth switch Q4 and the working cycle, and the sum of the fourth charging time and the fifth energy storage time is equal to the working cycle.

[0027] The bus voltage equalization system for the three-level Buck-Boost converter provided by the application has the following beneficial effects: the main control unit of the application can monitor the state of the battery in real time and determine the running state of the main circuit, when the battery is working normally, the normally closed relay of the soft start circuit keeps closed; when the battery cannot be charged or discharged due to failure, too high or too low SOC, that is, cannot participate in the circuit work, the normally closed relay of the soft start circuit and the main relay are opened to disconnect the battery from the positive and negative buses, and only the filter capacitor is used for voltage equalization; thus, bus voltage equalization in all working conditions can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only constitute the embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor based on the provided drawings:

[0029] Figure 1 The figure shows a schematic diagram of a bus voltage equalization system for a three-level Buck-Boost converter provided by the application;

[0030] Figures 2-7 The figure shows a current flow direction schematic diagram of a bus voltage equalization system for a three-level Buck-Boost converter provided by the application in a certain state. DETAILED DESCRIPTION

[0031] For the purpose of promoting an understanding of the application, the application will now be described in greater detail with reference to the figures. The embodiments shown in the figures are illustrative of the application. However, the application can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete. It is therefore desirable that the present application be summed in the broadest terms.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for describing particular embodiments only and is not intended to be limiting of the application.

[0033] Figure 1 A schematic diagram of a bus voltage equalization system for a three-level Buck-Boost converter is shown. As shown in the figure, the bus voltage equalization system for a three-level Buck-Boost converter is connected to the three-level Buck-Boost converter, which includes a first bridge arm composed of a first capacitor C1, a first switch Q1, a second switch Q2 and a first inductor L1, and a second bridge arm composed of a second capacitor C2, a third switch Q3, a fourth switch Q4 and a second inductor L2. The bus voltage equalization system includes a main control unit (not shown in the figure), a filter capacitor C3, a soft start circuit and a main relay K1. The soft start circuit includes a normally closed relay K2, a resistor and a diode connected in series. Figure 1 Specifically, in an embodiment of the application, a first end of the first capacitor C1 and a first end of the first switch Q1 are connected to a positive bus, a second end of the first switch Q1 is connected to a first end of the second switch Q2 and a first end of the first inductor L1, a second end of the second switch Q2 and a second end of the first capacitor C1 are connected to a bus midpoint, a second end of the first inductor L1 is connected to a first end of the filter capacitor C3; a first end of the second capacitor C2 and a first end of the third switch Q3 are connected to the bus midpoint, a second end of the third switch Q3 is connected to a first end of the fourth switch Q4 and a first end of the second inductor L2, a second end of the fourth switch Q4 and a second end of the second capacitor C2 are connected to a negative bus, a second end of the second inductor L2 is connected to a second end of the filter capacitor C3; control ends of the first switch Q1, the second switch Q2, the third switch Q3 and the fourth switch Q4 are connected to the main control unit; the soft start circuit and the main relay are connected in parallel between the second end of the filter capacitor C3 and the negative electrode of the battery.

[0034]

[0035] ​Specifically, in an embodiment of the present application, the main control unit is configured to monitor the working state of the battery, when the battery is working normally, control the soft start circuit and the main relay to be in a closed state and balance the voltage between the positive bus and the negative bus by controlling the duty cycle difference between the switch tube of the first bridge arm and the switch tube of the second bridge arm; when the battery fails, the soft start circuit and the main relay are disconnected, and the filter capacitor is used to balance the voltage between the positive bus and the negative bus. During the operation of the product, the main control unit will monitor the state of the battery in real time and then determine the operation state of the main circuit. When the battery is working normally, the normally closed relay of the soft start circuit remains in a closed state. When the battery cannot be charged or discharged due to failure, too high or too low SOC, i.e., cannot participate in the circuit operation, the normally closed relay of the soft start circuit and the main relay are opened to disconnect the connection between the battery and the positive and negative buses, and only the filter capacitor is used for voltage balancing. Thus, the bus voltage balancing in all working conditions can be realized.

[0036] Further, in an embodiment of the present application, when the battery is working normally and the voltage of the first capacitor C1 is not equal to the voltage of the second capacitor C2, i.e., the positive and negative bus voltages are not balanced, the main control unit is configured to control the soft start circuit and the main relay to be in a closed state. Then, at t0, the first switch tube Q1 and the fourth switch tube Q4 are controlled to be opened at the same time, as shown in FIG. 2, the first inductor L1 and the second inductor L2 store energy, and the first capacitor C1 and the second capacitor C2 discharge together to the battery end. Figure 2

[0037] Further, in an embodiment of the present application, when the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, the fourth switch tube Q4 is turned off at t1 because the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2. The first switch tube Q1 and the fourth switch tube Q4 are turned off at the same time when there is no voltage balancing requirement, but now the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, so the fourth switch tube Q4 is turned off first. The current path in the circuit is as shown in FIG. 3. Figure 3 ​As shown, the first capacitor C1 will continue to discharge to the battery end, while the discharge of the second capacitor C2 is stopped, i.e. the energy consumption of the first capacitor C1 is greater than that of the second capacitor C2. Then after a delay of a certain time t' (such as 1 μs), the first switch Q1 is turned off, and t' is dynamically adjusted according to the voltage difference between the first capacitor C1 and the second capacitor C2, if the voltage difference is large, t' can be appropriately increased, and vice versa. Then in the remaining period, the circuit enters the normal freewheeling state, and the inductor current is freewheeled through the diodes of the second switch Q2 and the third switch Q3. Until t1 (t1 = t0 + x μs, x is the duty cycle), return to state 1, i.e. control the first switch Q1 and the fourth switch Q4 to be turned on at the same time. Repeat this process, which can reduce the voltage of the first capacitor C1, and thus achieve voltage balance between the first capacitor C1 and the second capacitor C2. That is, when the voltage of the first capacitor C1 is greater than that of the second capacitor C2, after a first energy storage time, the fourth switch Q4 is controlled to be turned off, and after a first discharge time, the first switch Q1 is controlled to be turned off; after a first freewheeling time, the first switch Q1 and the fourth switch Q4 are controlled to be turned on at the same time, and the process is repeated until the voltage of the first capacitor C1 is equal to that of the second capacitor C2. Among them, the first energy storage time is related to the working period and the duty cycle of the fourth switch Q4, for example, the working period is 40 us, and the duty cycle of the fourth switch Q4 is 50%, then the first energy storage time is 20 us; the first discharge time is related to the difference between the voltage of the first capacitor C1 and the voltage of the second capacitor C2, and the first freewheeling time is equal to the difference between the first energy storage time and the first discharge time.

[0038] Further, in an embodiment of the present application, when the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, the first switch Q1 is turned off at t1 because the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2. The second capacitor C2 continues to discharge to the battery, while the discharge of the first capacitor C1 is stopped, i.e., the energy consumption of the second capacitor C2 is greater than that of the first capacitor C1. Then, after a delay of a certain time t' (e.g., 1 μs), the fourth switch Q4 is turned off. The time t' is dynamically adjusted according to the voltage difference between the first capacitor C1 and the second capacitor C2. If the voltage difference is large, the time t' can be appropriately increased, and vice versa. Then, in the remaining period, the circuit enters a normal freewheeling state, and the inductor current freewheels through the diode of the second switch Q2 and the diode of the third switch Q3. Until t1 (t1 = t0 + x μs, x is the duty cycle), the first switch Q1 and the fourth switch Q4 are turned on again. This is repeated to reduce the voltage of the second capacitor C2, thereby balancing the voltages of the first capacitor C1 and the second capacitor C2. That is, when the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, after a first energy storage time, the first switch Q1 is turned off, and after a first discharge time, the fourth switch Q4 is turned off. After a first freewheeling time, the first switch Q1 and the fourth switch Q4 are turned on, and the process is repeated until the voltage of the first capacitor C1 is equal to the voltage of the second capacitor C2. The first energy storage time is related to the duty cycle and the duty cycle of the first switch Q1. For example, when the duty cycle is 40 μs and the duty cycle of the first switch Q1 is 50%, the first energy storage time is 20 μs. The first discharge time is related to the difference between the voltage of the first capacitor C1 and the voltage of the second capacitor C2. The first freewheeling time is equal to the difference between the first energy storage time and the first discharge time.

[0039] Further, in an embodiment of the present application, when the battery fails and the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, i.e., the positive and negative bus voltages are unbalanced, the main control unit disconnects the soft start circuit and the main relay, and turns off the first switch Q1, the second switch Q2, the third switch Q3, and the fourth switch Q4, so that the charge of the filter capacitor C3 is discharged to the second capacitor C2 to make the voltage of the filter capacitor C3 lower than a first preset value, and then the charge of the first capacitor C1 is discharged to the filter capacitor C3 to make the voltage of the filter capacitor C3 higher than a second preset value.

[0040] Specifically, as shown in FIG. 4, the soft start circuit includes a first capacitor C1, a second capacitor C2, a third capacitor C3, a first switch Q1, a second switch Q2, a third switch Q3, a fourth switch Q4, and a main control unit. Figure 4As shown, at time t0, the second switch Q2 and the third switch Q3 are opened. The energy of the filter capacitor C3 is released through the path L1-Q2-Q3-L2 and stored in the first inductor L1 and the second inductor L2, and the voltage of the filter capacitor C3 decreases. Next, as shown in Figure 5 As shown, at time t1, the third switch Q3 is closed and the fourth switch Q4 is opened, i.e., the third switch Q3 and the fourth switch Q4 are complementary on in this stage, and the inductor current continues to flow through the path L1-Q2-C2-Q4-L2. At this time, the energy of the filter capacitor C3 continues to be released, and the second capacitor C2 is charged, and the voltage of the second capacitor C2 increases. Then, the above two processes are repeated, so that the energy of the filter capacitor C3 is released to the second capacitor C2, and the voltage of the second capacitor C2 increases, until the filter capacitor C3 is discharged to a certain voltage value. Subsequently, as shown in Figure 6 As shown, at time t2, the first switch Q1 and the third switch Q3 are opened. The energy of the first capacitor C1 is released through the path Q1-L1-C3-L2-Q3, and the voltage of the first capacitor C1 decreases, and the first inductor L1 and the second inductor L2 are stored. At the same time, the filter capacitor C3 is charged and the voltage increases. Next, as shown in Figure 7 As shown, at time t3, the first switch Q1 is closed and the second switch Q2 is opened, i.e., the first switch Q1 and the second switch Q2 are complementary on in this stage, and the inductor current continues to flow through the path L1-C3-L2-Q3-Q2. At this time, the filter capacitor C3 continues to be charged, and the voltage of C3 increases. Then, the above two processes are repeated, so that the energy of the first capacitor C1 is released to the filter capacitor C3, and the voltage of the first capacitor C1 decreases, until the filter capacitor C3 is charged to a certain voltage value. Thus, by transferring the energy of the first capacitor C1 to the equalization C3, and then transferring the energy of the filter capacitor C3 to the second capacitor C2, the voltage of the first capacitor C1 and the second capacitor C2 is equalized. Figure 6 and Figure 7 As shown, at time t2, the first switch Q1 and the third switch Q3 are opened. The energy of the first capacitor C1 is released through the path Q1-L1-C3-L2-Q3, and the voltage of the first capacitor C1 decreases, and the first inductor L1 and the second inductor L2 are stored. At the same time, the filter capacitor C3 is charged and the voltage increases. Next, as shown in

[0041] That is, when the battery fails and the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, the second switch Q2 and the third switch Q3 are controlled to be opened at the same time, after a second energy storage time, the third switch Q3 is controlled to be closed and the fourth switch Q4 is controlled to be opened, after a first charging time, the third switch Q3 is controlled to be opened and the fourth switch Q4 is controlled to be closed, and the process is repeated until the voltage of the filter capacitor C3 is lower than a first preset value; the first switch Q1 and the third switch Q3 are controlled to be opened at the same time, after a third energy storage time, the first switch Q1 is controlled to be closed and the second switch Q2 is controlled to be opened, after a second charging time, the first switch Q1 is controlled to be opened and the second switch Q2 is controlled to be closed, and the process is repeated until the voltage of the filter capacitor C3 is higher than a second preset value. The second energy storage time is related to the duty cycle and the duty cycle of the third switch Q3, and the sum of the first charging time and the second energy storage time is equal to the duty cycle; the third energy storage time is related to the duty cycle and the duty cycle of the first switch Q1, and the sum of the second charging time and the third energy storage time is equal to the duty cycle.

[0042] Similarly, when the battery fails and the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, that is, the positive and negative bus voltages are unbalanced, at this time, the main control unit disconnects the soft start circuit and the main relay, and controls the conduction of the first switch tube Q1, the second switch tube Q2, the third switch tube Q3 and the fourth switch tube Q4 to be closed, so that the electric quantity of the filter capacitor C3 is released to the first capacitor C1 to make the voltage of the filter capacitor C3 lower than the first preset value, and then the electric quantity of the second capacitor C2 is released to the filter capacitor C3 to make the voltage of the filter capacitor C3 higher than the second preset value. Specifically, when the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, the main control unit is configured to: control the second switch tube Q2 and the third switch tube Q3 to be opened at the same time, control the second switch tube Q2 to be closed after a fourth energy storage time, and control the first switch tube Q1 to be opened at the same time, control the second switch tube Q2 to be opened and the first switch tube Q1 to be closed after a third charging time, and repeat the above process until the voltage of the filter capacitor C3 is lower than the first preset value; control the fourth switch tube Q4 and the second switch tube Q2 to be opened at the same time, control the fourth switch tube Q4 to be closed after a fifth energy storage time, and control the third switch tube Q3 to be opened at the same time, control the fourth switch tube Q4 to be opened and the third switch tube Q3 to be closed after a fourth charging time, and repeat the above process until the voltage of the filter capacitor C3 is higher than the second preset value. Wherein, the fourth energy storage time is related to the duty cycle of the second switch tube Q2 and the working cycle, and the sum of the third charging time and the fourth energy storage time is equal to the working cycle; the fifth energy storage time is related to the duty cycle of the fourth switch tube Q4 and the working cycle, and the sum of the fourth charging time and the fifth energy storage time is equal to the working cycle.

[0043] The bus voltage equalization system for three-level Buck-Boost converter provided by the application, through the main control unit, the working state of the battery is monitored in real time, when the battery is in normal working state, the main control unit will continue to keep the connection of the battery, and through controlling the soft start circuit and the main relay in closed state and controlling the duty cycle difference of the switching tube of the first bridge arm and the switching tube of the second bridge arm, voltage equalization is carried out. When the battery fails, such as abnormal battery voltage, too high or too low SOC, etc., the main control unit will judge that the battery cannot participate in normal work, and then disconnect the connection of the battery and the bus. When the battery is in normal working state, the normally closed relay of the soft start circuit keeps closed state, so that the battery can supply power normally, and at the same time, the main control unit can balance the voltage between the positive bus and the negative bus by adjusting the duty cycle difference of the switching tube. When the battery fails or cannot work, the main control unit will disconnect the connection of the soft start circuit and the main relay, at this time, the battery cannot provide power. At this time, the system will rely on the filter capacitor to maintain the voltage balance between the positive bus and the negative bus. Through the above control strategy, the system can ensure that the voltage of the bus is always balanced in different working states (including normal working state of the battery, failure state, etc.). Whether the battery is in normal working state or not, the system can realize bus voltage equalization in all working conditions through the soft start circuit, the capacitor or the duty cycle adjustment.

[0044] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the application can be practiced without these specific details. In some instances, well-known methods, structures and techniques have not been described in detail in order to not obscure the understanding of this description.

[0045] Similarly, it is to be understood that the embodiments of the present application can be readily combined with one another, and / or with other embodiments or implementations not expressly described in order to produce additional embodiments of the application that are within the scope of the present application. Thus, the foregoing description of the embodiments of the application has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the application to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the application be limited not with this detailed description, but rather by the claims appended hereto.

[0046] Furthermore, to one skilled in the art, it will be apparent that the embodiments described herein are applicable to other than the particular embodiments described, and that the moieties and the methods described herein have general applicability. Changes in form and substitution of equivalents are contemplated as circumstances can suggest or render expedient. As such, although specific embodiments of, and examples for, the application are described herein, the present application is not limited to these described embodiments or examples. Instead, it is to be understood that the scope of the application encompasses all modifications, combinations and permutations of the described embodiments and examples.

[0047] It should be noted that the above-mentioned embodiments illustrate rather than limit the application, and that one skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'comprising' does not exclude the presence of elements or steps other than those listed in a claim. The word 'a' or 'an' preceding an element does not exclude the presence of a plurality of such elements. The application can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a unitary claim, several devices, apparatuses or means can be listed, comprising means for carrying out a certain function. The functions of the separate means can be carried out by one specific means. The use of the terms first, second and third, etc. does not imply any ordering but rather are used for naming the elements and should be interpreted as 'first','second' and 'third' etc.

Claims

1. A bus equalization system for a three-level BuckBoost converter, connected to the three-level BuckBoost converter, the three-level BuckBoost converter including a first bridge arm and a second bridge arm, a first end of the first bridge arm connected to a positive bus, a first end of the second bridge arm connected to a negative bus, and second ends of the first bridge arm and the second bridge arm connected to the midpoint of the bus, characterized in that, The bus voltage equalization system includes a main control unit, a filter capacitor C3, a soft-start circuit, and a main relay. The filter capacitor C3 is connected between the third end of the first bridge arm and the third end of the second bridge arm. The third end of the first bridge arm is also connected to the positive terminal of the battery. The soft-start circuit and the main relay are connected in parallel between the third end of the second bridge arm and the negative terminal of the battery. The main control unit is used to monitor the battery's operating status. When the battery is working normally, it controls the soft-start circuit and the main relay to be in a closed state and equalizes the voltage between the positive bus and the negative bus by controlling the difference in the duty cycle of the switching transistors of the first bridge arm and the second bridge arm. When the battery fails, it disconnects the soft-start circuit and the main relay and equalizes the voltage between the positive bus and the negative bus by using the filter capacitor.

2. The bus voltage equalization system for a three-level BuckBoost converter according to claim 1, characterized in that, The first bridge arm includes a first capacitor C1, a first switch Q1, a second switch Q2, and a first inductor L1. The first terminal of the first capacitor C1 and the first terminal of the first switch Q1 are connected to the positive bus. The second terminal of the first switch Q1 is connected to the first terminal of the second switch Q2 and the first terminal of the first inductor L1. The second terminal of the second switch Q2 and the second terminal of the first capacitor C1 are connected to the midpoint of the bus. The second terminal of the first inductor L1 is connected to the first terminal of the filter capacitor C3. The second bridge arm includes a second capacitor C2, a third switch Q3, a fourth switch Q4, and a second inductor L2. The first end of the second capacitor C2 and the first end of the third switch Q3 are connected to the midpoint of the bus. The second end of the third switch Q3 is connected to the first end of the fourth switch Q4 and the first end of the second inductor L2. The second end of the fourth switch Q4 and the second end of the second capacitor C2 are connected to the negative bus. The second end of the second inductor L2 is connected to the second end of the filter capacitor C3. The control terminals of the first switch Q1, the second switch Q2, the third switch Q3, and the fourth switch Q4 are connected to the main control unit.

3. The bus voltage equalization system for a three-level BuckBoost converter according to claim 2, characterized in that, When the battery is operating normally and the voltage of the first capacitor C1 is not equal to the voltage of the second capacitor C2, the main control unit is used to control the soft-start circuit and the main relay to be in a closed state, and is also used to: The first switch Q1 and the fourth switch Q4 are simultaneously turned on; When the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, the fourth switch Q4 is turned off after the first energy storage time, and the first switch Q1 is turned off after the first discharge time. When the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, the first switch Q1 is turned off after the first energy storage time, and the fourth switch Q4 is turned off after the first discharge time. After the first freewheeling time, the first switch Q1 and the fourth switch Q4 are turned on simultaneously, and this process is repeated until the voltage of the first capacitor C1 is equal to the voltage of the second capacitor C2.

4. The bus voltage equalization system for a three-level BuckBoost converter according to claim 3, characterized in that, The first energy storage time is related to the working cycle and the duty cycle of the fourth switch Q4 or the first switch Q1. The first discharge time is related to the difference between the voltage of the first capacitor C1 and the voltage of the second capacitor C2. The first freewheeling time is equal to the difference between the first energy storage time and the first discharge time.

5. The bus voltage equalization system for a three-level BuckBoost converter according to claim 2, characterized in that, When the battery malfunctions and the voltage of the first capacitor C1 is not equal to the voltage of the second capacitor C2, the main control unit is used to disconnect the soft-start circuit and the main relay, and is also used to: When the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, by controlling the conduction and shutdown of the first switch Q1, the second switch Q2, the third switch Q3 and the fourth switch Q4, the charge of the filter capacitor C3 is released to the second capacitor C2 so that the voltage of the filter capacitor C3 is lower than the first preset value, and then the charge of the first capacitor C1 is released to the filter capacitor C3 so that the voltage of the filter capacitor C3 is higher than the second preset value. When the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, by controlling the conduction and shutdown of the first switch Q1, the second switch Q2, the third switch Q3 and the fourth switch Q4, the charge of the filter capacitor C3 is released to the first capacitor C1, so that the voltage of the filter capacitor C3 is lower than a first preset value, and then the charge of the second capacitor C2 is released to the filter capacitor C3, so that the voltage of the filter capacitor C3 is higher than a second preset value.

6. The bus voltage equalization system for a three-level BuckBoost converter according to claim 5, characterized in that, When the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, the main control unit is used to: The second switch Q2 and the third switch Q3 are turned on simultaneously. After the second energy storage time, the third switch Q3 is turned off while the fourth switch Q4 is turned on. After the first charging time, the third switch Q3 is turned on while the fourth switch Q4 is turned off. This process is repeated until the voltage of the filter capacitor C3 is lower than the first preset value. The first switch Q1 and the third switch Q3 are turned on simultaneously. After a third energy storage time, the first switch Q1 is turned off while the second switch Q2 is turned on. After a second charging time, the first switch Q1 is turned on while the second switch Q2 is turned off. This process is repeated until the voltage of the filter capacitor C3 is higher than a second preset value.

7. The bus voltage equalization system for a three-level BuckBoost converter according to claim 6, characterized in that, The second energy storage time is related to the working cycle and the duty cycle of the third switch Q3, and the sum of the first charging time and the second energy storage time is equal to the working cycle; the third energy storage time is related to the working cycle and the duty cycle of the first switch Q1, and the sum of the second charging time and the third energy storage time is equal to the working cycle.

8. The bus voltage equalization system for a three-level BuckBoost converter according to claim 5, characterized in that, When the voltage of the first capacitor C1 is less than the voltage of the second capacitor C2, the main control unit is used to: The second switch Q2 and the third switch Q3 are turned on simultaneously. After a fourth energy storage time, the second switch Q2 is turned off while the first switch Q1 is turned on. After a third charging time, the second switch Q2 is turned on while the first switch Q1 is turned off. This process is repeated until the voltage of the filter capacitor C3 is lower than a first preset value. The fourth switch Q4 and the second switch Q2 are turned on simultaneously. After a fifth energy storage time, the fourth switch Q4 is turned off while the third switch Q3 is turned on. After a fourth charging time, the fourth switch Q4 is turned on while the third switch Q3 is turned off. This process is repeated until the voltage of the filter capacitor C3 is higher than the second preset value.

9. The bus equalization system for a three-level BuckBoost converter according to claim 8, characterized in that, The fourth energy storage time is related to the working cycle and the duty cycle of the second switch Q2, and the sum of the third charging time and the fourth energy storage time is equal to the working cycle; the fifth energy storage time is related to the working cycle and the duty cycle of the fourth switch Q4, and the sum of the fourth charging time and the fifth energy storage time is equal to the working cycle.

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