Voltage clamping circuit and NPC inverter

Abstract

The utility model relates to the technical field of inverters, and discloses a voltage clamping circuit and an NPC inverter, the first end of an upper bridge arm clamping circuit is respectively connected with the first end of an upper bridge arm and a positive half bus, and the second end of the upper bridge arm clamping circuit is connected with the second end of the upper bridge arm; the first end of the lower bridge arm clamping circuit is connected with the first end of the lower bridge arm, and the second end of the lower bridge arm clamping circuit is connected with the second end of the lower bridge arm and the negative half bus. The second end of the upper bridge arm and the first end of the lower bridge arm are connected to form a common connection point; when the voltage of the common connection point is higher than the positive half bus voltage, the voltage of the common connection point is clamped to the positive half bus voltage through the upper bridge arm clamping circuit; when the voltage of the common connection point is lower than the negative half bus voltage, the voltage of the common connection point is clamped to the negative half bus voltage through the lower bridge arm clamping circuit, so that the phenomenon that the voltage of an inner tube and the voltage of an outer tube are unbalanced is avoided, and the operation reliability of the converter is improved.

Description

Technical Field

[0001] This utility model relates to the field of inverter technology, specifically to a voltage clamping circuit and an NPC inverter. Background Technology

[0002] Due to the distributed parameters of the lines and components in the system, an imbalance in the voltage between the inner and outer tubes can occur during inverter operation. This causes the voltage across the inner tube to exceed half of the bus voltage, threatening its safe operation and reducing system reliability. Existing methods for addressing IGBT stress in NPC three-level inverters involve parallel RC snubber circuits. However, the resistance and capacitance values ​​of the RC snubber are difficult to adjust. The energy absorbed by the capacitor is dissipated through the resistor and during IGBT operation, resulting in significant heat generation in the resistor and increased line losses. Utility Model Content

[0003] In view of this, the present invention provides a voltage clamping circuit and an NPC inverter to solve the problem of voltage imbalance between the internal and external transistors in an NPC three-level inverter.

[0004] In a first aspect, this utility model provides a voltage clamping circuit applied to one arm of a bridge converter. The arm includes an upper arm and a lower arm. The voltage clamping circuit includes an upper arm clamping circuit and a lower arm clamping circuit. The first terminal of the upper arm clamping circuit is connected to the first terminal of the upper arm and the positive half bus, and the second terminal of the upper arm clamping circuit is connected to the second terminal of the upper arm. The first terminal of the lower arm clamping circuit is connected to the first terminal of the lower arm, and the second terminal of the lower arm clamping circuit is connected to the second terminal of the lower arm and the negative half bus. The second terminal of the upper arm and the first terminal of the lower arm are connected to form a common connection point. When the voltage of the common connection point is higher than the voltage of the positive half bus, the voltage of the common connection point is clamped to the voltage of the positive half bus through the upper arm clamping circuit. When the voltage of the common connection point is lower than the voltage of the negative half bus, the voltage of the common connection point is clamped to the voltage of the negative half bus through the lower arm clamping circuit.

[0005] In this invention, when the voltage at the common connection point is higher than the positive half bus voltage, or when the voltage at the common connection point is lower than the negative half bus voltage, the upper bridge arm clamping circuit and the lower bridge arm clamping circuit break down and conduct, clamping the voltage at the common connection point to the positive half bus voltage or the negative half bus voltage, thereby avoiding the imbalance of internal and external tube voltages and improving the operational reliability of the converter.

[0006] In one optional embodiment, the upper arm clamping circuit includes: a first current limiting circuit, a first spike absorption circuit, and a first switching circuit, wherein a first terminal of the first spike absorption circuit is connected to a common connection point, a second terminal of the first spike absorption circuit is connected to a first terminal of the first current limiting circuit, a second terminal of the first current limiting circuit is connected to a first terminal of the first switching circuit, and a second terminal of the first switching circuit is connected to the positive half bus. When the voltage of the common connection point is higher than the voltage of the positive half bus, after the first spike absorption circuit breaks down and conducts, the first switching circuit clamps the voltage of the common connection point to the positive half bus voltage based on its unidirectional conduction characteristic, and the first current limiting circuit limits the loop current.

[0007] In one optional embodiment, the lower bridge arm clamping circuit includes: a second current limiting circuit, a second spike absorption circuit, and a second switching circuit, wherein the first terminal of the second spike absorption circuit is connected to the common connection point, the second terminal of the second spike absorption circuit is connected to the first terminal of the second current limiting circuit, the second terminal of the second current limiting circuit is connected to the first terminal of the second switching circuit, and the second terminal of the second switching circuit is connected to the negative half bus. When the voltage of the common connection point is lower than the voltage of the negative half bus, after the second spike absorption circuit breaks down and conducts, the second switching circuit clamps the voltage of the common connection point to the negative half bus voltage based on its unidirectional conduction characteristic, and the second current limiting circuit limits the loop current.

[0008] In one alternative implementation, both the first current limiting circuit and the second current limiting circuit include at least one current limiting resistor.

[0009] In one optional embodiment, both the first peak absorption circuit and the second peak absorption circuit include a Zener diode, wherein, in the upper bridge arm clamping circuit, the cathode of the Zener diode is connected to a common connection point, and the anode of the Zener diode is connected to a first terminal of the first current limiting circuit; in the lower bridge arm clamping circuit, the anode of the Zener diode is connected to a common connection point, and the cathode of the Zener diode is connected to a first terminal of the second current limiting circuit.

[0010] In one optional embodiment, both the first peak absorption circuit and the second peak absorption circuit further include an RC absorption circuit, wherein the first terminal of the RC absorption circuit is connected to a common connection point, and the second terminal of the RC absorption circuit is connected to the first terminal of the second current limiting circuit.

[0011] In one optional embodiment, both the first switching circuit and the second switching circuit include: a first diode, wherein, in the upper bridge arm clamping circuit, the anode of the first diode is connected to the second terminal of the first current limiting circuit, and the cathode of the first diode is connected to the first terminal of the upper bridge arm and the positive half busbar; in the lower bridge arm clamping circuit, the cathode of the first diode is connected to the second terminal of the second current limiting circuit, and the anode of the first diode is connected to the second terminal of the lower bridge arm and the negative half busbar.

[0012] In one alternative implementation, the upper arm clamping circuit includes a first clamping diode, wherein the anode of the first clamping diode is connected to a common connection point, and the cathode of the first clamping diode is connected to the positive half bus.

[0013] In one alternative implementation, the lower arm clamping circuit includes a second clamping diode, wherein the cathode of the second clamping diode is connected to a common connection point, and the anode of the second clamping diode is connected to the negative half bus.

[0014] Secondly, this utility model provides an NPC inverter, comprising: three bridge arms, three voltage clamping circuits according to the first aspect and any optional embodiment thereof, and a support circuit. The three bridge arms are connected in parallel and are respectively connected to the positive half bus and the negative half bus. The two ends of the support circuit are respectively connected to the positive half bus and the negative half bus. The upper and lower bridge arms of each bridge arm are connected to form a common connection point. A voltage clamping circuit is connected to one bridge arm. Specifically, the first end of the upper bridge arm clamping circuit is connected to the first end of the upper bridge arm and the positive half bus. The second end of the circuit is connected to the second end of the upper bridge arm; the first end of the lower bridge arm clamping circuit is connected to the first end of the lower bridge arm, and the second end of the lower bridge arm clamping circuit is connected to the second end of the lower bridge arm and the negative half busbar respectively; the second end of the upper bridge arm is connected to the first end of the lower bridge arm and forms a common connection point; when the voltage of the common connection point is higher than the voltage of the positive half busbar, the voltage of the common connection point is clamped to the voltage of the positive half busbar through the upper bridge arm clamping circuit; when the voltage of the common connection point is lower than the voltage of the negative half busbar, the voltage of the common connection point is clamped to the voltage of the negative half busbar through the lower bridge arm clamping circuit.

[0015] In one optional embodiment, each bridge arm includes: a first power switching device, a second power switching device, a third power switching device, a fourth power switching device, a first freewheeling diode, and a second freewheeling diode, wherein a first terminal of the first power switching device is connected to the positive half busbar, a second terminal of the first power switching device is connected to the cathode of the first freewheeling diode and the first terminal of the second power switching device; a second terminal of the second power switching device is connected to the first terminal of the third power switching device to form a common connection point; a second terminal of the third power switching device is connected to the anode of the second freewheeling diode and the first terminal of the fourth power switching device; a second terminal of the fourth power switching device is connected to the negative half busbar; and the anode of the first freewheeling diode is connected to the cathode of the second freewheeling diode. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of this utility model or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this utility model. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a topology diagram of a three-level NPC inverter based on related technologies;

[0018] Figure 2 This is a composition diagram of the voltage clamping circuit according to an embodiment of the present utility model;

[0019] Figure 3 This is a schematic diagram of another voltage clamping circuit according to an embodiment of the present invention;

[0020] Figure 4 This is a topology diagram of a T-type NPC inverter based on related technologies;

[0021] Figure 5 This is a detailed circuit structure diagram of the voltage clamping circuit according to an embodiment of the present utility model;

[0022] Figure 6 This is a detailed circuit structure diagram of another voltage clamping circuit according to an embodiment of the present utility model;

[0023] Figure 7 This is a detailed circuit structure diagram of another voltage clamping circuit according to an embodiment of the present utility model;

[0024] Figure 8 This is a partial topology diagram of a three-level NPC inverter according to an embodiment of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of this utility model clearer, the technical solutions of the embodiments of this utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this utility model, not all embodiments. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this utility model.

[0026] In the description of this utility model, it should be noted that the terms "first", "second" and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0027] In the description of this utility model, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can also refer to the internal connection of two components; and they can refer to a wireless connection or a wired connection. Those skilled in the art can understand the specific meaning of the above terms in this utility model based on the specific circumstances.

[0028] Furthermore, the technical features involved in the different embodiments of this utility model described below can be combined with each other as long as they do not conflict with each other.

[0029] This embodiment provides a voltage clamping circuit applied to one arm of a bridge converter, which includes an upper arm and a lower arm. The bridge converter is primarily an NPC bridge circuit, for example... Figure 1 The three-level NPC inverter shown is... Figure 1 In a power switching device, a MOSFET and a diode connected in parallel form a power switching device. For example, S1 and D1 connected in parallel form a power switching device. The NPC bridge circuit is a three-phase full-bridge circuit. Each bridge arm includes an upper bridge arm and a lower bridge arm. Both the upper and lower bridge arms are composed of two power switching devices connected in series. Figure 1 The structure shown is as follows: Figure 2 As shown, the voltage clamping circuit includes: an upper bridge arm clamping circuit and a lower bridge arm clamping circuit, wherein, Figure 2 by Figure 1 Take the bridge arm consisting of S1, S2, S3, S4, D1, D2, D3, D4, D13, and D14 as an example.

[0030] like Figure 2 As shown, the voltage clamping circuit includes an upper bridge arm clamping circuit and a lower bridge arm clamping circuit. The first terminal of the upper bridge arm clamping circuit is connected to the first terminal of the upper bridge arm and the positive half bus (i.e., BUS+), and the second terminal of the upper bridge arm clamping circuit is connected to the second terminal of the upper bridge arm. The first terminal of the lower bridge arm clamping circuit is connected to the first terminal of the lower bridge arm, and the second terminal of the lower bridge arm clamping circuit is connected to the second terminal of the lower bridge arm and the negative half bus (i.e., BUS-). The second terminal of the upper bridge arm and the first terminal of the lower bridge arm are connected to form a common connection point, i.e., point A.

[0031] When the voltage at the point of common coupling is higher than the positive half-bus voltage, the voltage at the point of common coupling is clamped to the positive half-bus voltage through the upper bridge arm clamping circuit; when the voltage at the point of common coupling is lower than the negative half-bus voltage, the voltage at the point of common coupling is clamped to the negative half-bus voltage through the lower bridge arm clamping circuit.

[0032] Specifically, Figure 2 S2 and S3 are internal transistors, and S1 and S4 are external transistors. Due to the voltage imbalance between the internal and external transistors, the voltage across S2 and S3 will be higher than half of the DC bus voltage. Therefore, an upper bridge arm clamping circuit and a lower bridge arm clamping circuit are set up. When the voltage at point A is higher than the positive half bus voltage or lower than the negative half bus voltage, it indicates that the voltage across S2 and S3 is higher than half of the DC bus voltage. At this time, the upper bridge arm clamping circuit and the lower bridge arm clamping circuit clamp the voltage at point A to the positive half bus voltage or the negative half bus voltage, thereby reducing the voltage at point A.

[0033] Specifically, the first end of the upper bridge arm clamping circuit and the second end of the lower bridge arm clamping circuit should be connected as close as possible to the BUS capacitor in order to clamp the voltage at point A.

[0034] In some alternative implementations, the NPC inverter can also provide... Figure 3 The T-shaped structure shown. Figure 3 In this circuit, S1 and S4 are the main power switching transistors, and S2 and S3 are freewheeling transistors. Both the upper and lower bridge arm clamping circuits include a diode, D0. When the voltage spike at point A is higher than the BUS+ voltage, D0 clamps the voltage at point A to BUS+. The cathode of D7 is connected to point A. When the voltage spike at point A is lower than the BUS- voltage, D1 clamps the voltage at point A to BUS-.

[0035] In some alternative implementations, such as Figure 4 As shown, the upper bridge arm clamping circuit includes: a first current limiting circuit, a first spike absorption circuit, and a first switching circuit.

[0036] Specifically, such as Figure 4 As shown, the first terminal of the first spike absorption circuit is connected to the common connection point (i.e., point A), the second terminal of the first spike absorption circuit is connected to the first terminal of the first current limiting circuit, the second terminal of the first current limiting circuit is connected to the first terminal of the first switching circuit, and the second terminal of the first switching circuit is connected to the positive half bus (i.e., BUS+).

[0037] Specifically, when the voltage at the point of common coupling is higher than the voltage of the positive half bus, after the first spike absorption circuit breaks down and conducts, the first switching circuit clamps the voltage at the point of common coupling to the voltage of the positive half bus based on the unidirectional conduction characteristic, and the first current limiting circuit limits the loop current.

[0038] Optionally, when the voltage at point A is higher than the voltage of the positive half-bus, a clamping channel needs to be constructed so that the voltage at point A is the BUS+ voltage when the positive half-axis is in the positive half-axis. Therefore, a first spike absorption circuit is set up. When the voltage at point A is higher than the voltage of the positive half-bus, the first spike absorption circuit breaks down and conducts.

[0039] In some alternative implementations, such as Figure 4As shown, the lower bridge arm clamping circuit includes: a second current limiting circuit, a second spike absorption circuit, and a second switching circuit.

[0040] Specifically, such as Figure 4 As shown, the first terminal of the second spike absorption circuit is connected to the common connection point, the second terminal of the second spike absorption circuit is connected to the first terminal of the second current limiting circuit, the second terminal of the second current limiting circuit is connected to the first terminal of the second switching circuit, and the second terminal of the second switching circuit is connected to the negative half bus.

[0041] Specifically, when the voltage at the point of common coupling is lower than the negative half-bus voltage, after the second spike absorption circuit breaks down and conducts, the second switching circuit clamps the voltage at the point of common coupling to the negative half-bus voltage based on its unidirectional conduction characteristic, and the second current limiting circuit limits the loop current.

[0042] Optionally, when the voltage at point A is lower than the negative half-bus voltage, a clamping channel needs to be constructed so that the voltage at point A is the BUS- voltage during the negative half-axis. Therefore, a second spike absorption circuit is set up so that when the voltage at point A is lower than the negative half-bus voltage, the second spike absorption circuit breaks down and conducts.

[0043] Optionally, in order to prevent the BUS+ and BUS- currents from flowing in reverse to point A, a first switching circuit and a second switching circuit are provided. The first switching circuit and the second switching circuit have unidirectional conduction characteristics and serve as anti-reverse circuits.

[0044] In some alternative implementations, such as Figure 5 As shown, both the first current limiting circuit and the second current limiting circuit include at least one current limiting resistor R0.

[0045] In some alternative implementations, such as Figure 5 As shown, both the first and second peak absorption circuits include a Zener diode ZD0. In the upper bridge arm clamping circuit, the cathode of the Zener diode ZD0 is connected to the common connection point (i.e., point A), and the anode of the Zener diode ZD0 is connected to the first terminal (i.e., R0) of the first current limiting circuit. In the lower bridge arm clamping circuit, the anode of the Zener diode ZD0 is connected to the common connection point, and the cathode of the Zener diode ZD0 is connected to the first terminal of the second current limiting circuit.

[0046] In one optional embodiment, both the first peak absorption circuit and the second peak absorption circuit further include an RC absorption circuit, wherein the first terminal of the RC absorption circuit is connected to a common connection point, and the second terminal of the RC absorption circuit is connected to the first terminal of the second current limiting circuit.

[0047] Figure 6 In the circuit, the RC snubber circuit is composed of a capacitor C0 and a resistor R1 connected in parallel.

[0048] In some alternative implementations, such as Figure 5 As shown, both the first and second switching circuits include a first diode D0. In the upper bridge arm clamping circuit, the anode of the first diode D0 is connected to the second terminal (i.e., R0) of the first current limiting circuit, and the cathode of the first diode D0 is connected to the first terminal of the upper bridge arm and the positive half bus (i.e., BUS+). In the lower bridge arm clamping circuit, the cathode of the first diode D0 is connected to the second terminal of the second current limiting circuit, and the anode of the first diode D0 is connected to the second terminal of the lower bridge arm and the negative half bus (i.e., BUS-).

[0049] Specifically, refer to Figure 5 S1, S2, S3, and S4 can each withstand half of the BUS voltage in application, which is beneficial for cost advantage. Due to the existence of distributed parameters of the lines and components in the system, during the operation of the inverter, there will be peak values ​​of voltage at point A that are higher than the BUS+ voltage or lower than the BUS- voltage. This will cause excessive voltage stress on the inner tubes of the upper tubes S1, S2, or S3, S4 when equalizing the voltage, threatening their safe operation. The current optimization proposes to add a clamping circuit at the midpoint of the inner tube of each phase inverter line IGBT. When the voltage at point A is higher than the BUS+ voltage, the Zener diode ZD0 quickly breaks down to absorb part of the peak value, and the diode D0 conducts to clamp the voltage at point A to the BUS+ voltage. The current-limiting resistor R0 is used for loop current limiting to protect the Zener diode and diode from overcurrent breakdown. The negative half-cycle operates on the same principle.

[0050] In some alternative implementations, such as Figure 7 As shown, the upper bridge arm clamping circuit includes: a first clamping diode D01, wherein the anode of the first clamping diode D01 is connected to the common connection point (i.e., point A), and the cathode of the first clamping diode D01 is connected to the positive half bus (i.e., BUS+).

[0051] In some alternative implementations, such as Figure 7 As shown, the lower bridge arm clamping circuit includes: a second clamping diode D02, wherein the cathode of the second clamping diode D02 is connected to the common connection point (i.e., point A), and the anode of the second clamping diode D02 is connected to the negative half bus (i.e., BUS-).

[0052] Specifically, Figure 7 The functions of the first clamping diode D01 and the second clamping diode D02 are equivalent to Figure 5 The specific function of the Zener diode ZD0 in this circuit will not be elaborated here.

[0053] This embodiment provides an NPC inverter, including: three bridge arms, three or more voltage clamping circuits according to embodiments and any optional embodiments, and a support circuit. The three bridge arms are connected in parallel and are respectively connected to the positive half bus and the negative half bus. The two ends of the support circuit are respectively connected to the positive half bus and the negative half bus. The upper and lower bridge arms of each bridge arm are connected to form a common connection point. A voltage clamping circuit is connected to one bridge arm. Specifically, the first end of the upper bridge arm clamping circuit is connected to the first end of the upper bridge arm and the positive half bus, and the second end of the upper bridge arm clamping circuit is connected to the second end of the upper bridge arm. The first end of the lower bridge arm clamping circuit is connected to the first end of the lower bridge arm, and the second end of the lower bridge arm clamping circuit is connected to the second end of the lower bridge arm and the negative half bus. The second end of the upper bridge arm is connected to the first end of the lower bridge arm to form a common connection point.

[0054] Specifically, when the voltage at the point of common coupling is higher than the positive half-bus voltage, the voltage at the point of common coupling is clamped to the positive half-bus voltage through the upper bridge arm clamping circuit; when the voltage at the point of common coupling is lower than the negative half-bus voltage, the voltage at the point of common coupling is clamped to the negative half-bus voltage through the lower bridge arm clamping circuit.

[0055] Specifically, refer to Figure 8 S1~S4, D1~D4, D13, and D14 form the R-phase bridge arm; S5~S8, D5~D8, D15, and D16 form the S-phase bridge arm; and S9~S12, D9~D12, D17, and D18 form the T-phase bridge arm. Each upper bridge arm is connected in parallel with one... Figures 2-5 The upper bridge arm clamping circuit shown has one connected in parallel for each lower bridge arm. Figures 2-4 The lower bridge arm clamping circuit shown, or one in parallel for each upper bridge arm. Figure 5 The upper bridge arm clamping circuit shown has one connected in parallel for each lower bridge arm. Figure 5 The lower bridge arm clamping circuit is shown.

[0056] In some optional embodiments, each bridge arm includes: a first power switching device, a second power switching device, a third power switching device, a fourth power switching device, a first freewheeling diode, and a second freewheeling diode, wherein a first terminal of the first power switching device is connected to the positive half busbar, a second terminal of the first power switching device is connected to the cathode of the first freewheeling diode and the first terminal of the second power switching device; a second terminal of the second power switching device is connected to the first terminal of the third power switching device and forms a common connection point; a second terminal of the third power switching device is connected to the anode of the second freewheeling diode and the first terminal of the fourth power switching device; a second terminal of the fourth power switching device is connected to the negative half busbar; and the anode of the first freewheeling diode is connected to the cathode of the second freewheeling diode.

[0057] Specifically, refer to Figure 8The parallel connections of S1 and D1, S5 and D5, and S9 and D9 together form the first power switching device. The parallel connections of S2 and D2, S6 and D6, and S10 and D10 together form the second power switching device. The parallel connections of S3 and D3, S7 and D7, and S11 and D11 together form the third power switching device. The parallel connections of S4 and D4, S8 and D8, and S12 and D12 together form the fourth power switching device. D13, D15, and D17 are the first freewheeling diodes, and D14, D16, and D18 are the second freewheeling diodes.

[0058] Optionally, Figure 8 The freewheeling diode in the circuit can also be a MOS or an IGBT.

[0059] Specifically, refer to Figure 8 Capacitors C1 and C2 are both supporting capacitors. Figure 8 The filter circuit in the circuit is an LC filter circuit, in which inductor L1 and capacitor C3 form an R-phase filter circuit, inductor L2 and capacitor C4 form an R-phase filter circuit, and inductor L3 and capacitor C5 form an R-phase filter circuit.

[0060] Optionally, the topology of the NPC inverter can also be configured to... Figure 4 The structure shown is merely an example and is not intended to limit the understanding of the subject.

[0061] Although embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A voltage clamping circuit, characterized by, The voltage clamping circuit is applied to a bridge arm of a bridge converter, the bridge arm includes an upper bridge arm and a lower bridge arm, and the voltage clamping circuit includes an upper bridge arm clamping circuit and a lower bridge arm clamping circuit, wherein a first end of the upper bridge arm clamping circuit is connected with a first end of the upper bridge arm and a positive half bus respectively, and a second end of the upper bridge arm clamping circuit is connected with a second end of the upper bridge arm; a first end of the lower bridge arm clamping circuit is connected with a first end of the lower bridge arm, and a second end of the lower bridge arm clamping circuit is connected with a second end of the lower bridge arm and a negative half bus respectively; a second end of the upper bridge arm is connected with a first end of the lower bridge arm and forms a common connection point; when a voltage of the common connection point is higher than a voltage of the positive half bus, the voltage of the common connection point is clamped to the voltage of the positive half bus through the upper bridge arm clamping circuit; when the voltage of the common connection point is lower than the voltage of the negative half bus, the voltage of the common connection point is clamped to the voltage of the negative half bus through the lower bridge arm clamping circuit.

2. The voltage clamping circuit of claim 1, wherein, The upper bridge arm clamping circuit includes a first current limiting circuit, a first peak absorption circuit and a first switch circuit, wherein a first end of the first peak absorption circuit is connected with the common connection point, and a second end of the first peak absorption circuit is connected with a first end of the first current limiting circuit; a second end of the first current limiting circuit is connected with a first end of the first switch circuit; a second end of the first switch circuit is connected with the positive half bus; when the voltage of the common connection point is higher than the voltage of the positive half bus, after the first peak absorption circuit is broken down and turned on, the first switch circuit clamps the voltage of the common connection point to the voltage of the positive half bus based on a unidirectional conduction characteristic, and the first current limiting circuit limits a loop current.

3. The voltage clamping circuit of claim 2, wherein, The lower bridge arm clamping circuit includes a second current limiting circuit, a second peak absorption circuit and a second switch circuit, wherein a first end of the second peak absorption circuit is connected with the common connection point, and a second end of the second peak absorption circuit is connected with a first end of the second current limiting circuit; a second end of the second current limiting circuit is connected with a first end of the second switch circuit; a second end of the second switch circuit is connected with the negative half bus; when the voltage of the common connection point is lower than the voltage of the negative half bus, after the second peak absorption circuit is broken down and turned on, the second switch circuit clamps the voltage of the common connection point to the voltage of the negative half bus based on a unidirectional conduction characteristic, and the second current limiting circuit limits a loop current.

4. The voltage clamping circuit of claim 3, wherein, The first current limiting circuit and the second current limiting circuit each include at least one current limiting resistor.

5. The voltage clamping circuit of claim 3, wherein, The first peak absorption circuit and the second peak absorption circuit each include a voltage stabilizing tube, wherein in the upper bridge arm clamping circuit, a cathode of the voltage stabilizing tube is connected with the common connection point, and an anode of the voltage stabilizing tube is connected with the first end of the first current limiting circuit; in the lower bridge arm clamping circuit, the anode of the voltage stabilizing tube is connected with the common connection point, and the cathode of the voltage stabilizing tube is connected with the first end of the second current limiting circuit.

6. The voltage clamping circuit of claim 3, wherein, The first peak absorption circuit and the second peak absorption circuit further each comprise an RC absorption circuit, wherein a first end of the RC absorption circuit is connected to the common connection point, and a second end of the RC absorption circuit is connected to a first end of the second current limiting circuit.

7. The voltage clamping circuit of claim 3, wherein, The first switch circuit and the second switch circuit each comprise a first diode, wherein in the upper bridge arm clamping circuit, an anode of the first diode is connected to a second end of the first current limiting circuit, and a cathode of the first diode is connected to a first end of the upper bridge arm and a positive half bus; in the lower bridge arm clamping circuit, a cathode of the first diode is connected to a second end of the second current limiting circuit, and an anode of the first diode is connected to a second end of the lower bridge arm and a negative half bus.

8. The voltage clamping circuit of claim 1, wherein, The upper bridge arm clamping circuit comprises a first clamping diode, wherein an anode of the first clamping diode is connected to the common connection point, and a cathode of the first clamping diode is connected to the positive half bus.

9. The voltage clamping circuit of claim 1, wherein, The lower bridge arm clamping circuit comprises a second clamping diode, wherein a cathode of the second clamping diode is connected to the common connection point, and an anode of the second clamping diode is connected to the negative half bus.

10. An NPC inverter, characterized by, Comprise: three bridge arms, three voltage clamping circuits according to any one of claims 1-8, a support circuit, wherein the three bridge arms are connected in parallel and then connected to a positive half bus and a negative half bus respectively, two ends of the support circuit are connected to the positive half bus and the negative half bus respectively, and upper and lower bridge arms of each bridge arm are connected and form a common connection point; one voltage clamping circuit is connected to one bridge arm, and the specific connection manner is: a first end of the upper bridge arm clamping circuit is connected to a first end of the upper bridge arm and a positive half bus respectively, and a second end of the upper bridge arm clamping circuit is connected to a second end of the upper bridge arm; a first end of the lower bridge arm clamping circuit is connected to a first end of the lower bridge arm, and a second end of the lower bridge arm clamping circuit is connected to a second end of the lower bridge arm and a negative half bus respectively; the second end of the upper bridge arm is connected to the first end of the lower bridge arm and forms a common connection point; when the voltage of the common connection point is higher than the voltage of the positive half bus, the voltage of the common connection point is clamped to the voltage of the positive half bus through the upper bridge arm clamping circuit; when the voltage of the common connection point is lower than the voltage of the negative half bus, the voltage of the common connection point is clamped to the voltage of the negative half bus through the lower bridge arm clamping circuit.

11. The NPC inverter of claim 10, wherein, Each bridge arm comprises a first power switching device, a second power switching device, a third power switching device, a fourth power switching device, a first freewheeling diode, and a second freewheeling diode, wherein a first end of the first power switching device is connected to the positive half bus, and a second end of the first power switching device is connected to a cathode of the first freewheeling diode and a first end of the second power switching device; a second end of the second power switching device is connected to a first end of the third power switching device and forms the common connection point; a second end of the third power switching device is connected to an anode of the second freewheeling diode and a first end of the fourth power switching device respectively; The second end of the fourth power switching device is connected with the negative half bus bar; The anode of the first freewheeling diode is connected with the cathode of the second freewheeling diode.

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