Apparatus and method for enabling a two-stage inverter to switch between modes

Abstract

The present disclosure relates to an apparatus and method for enabling a two-stage inverter to switch between modes. An apparatus for enabling a two-stage inverter to switch between modes, comprising: a first inverter unit; a second inverter unit; a load connected between the first inverter unit and the second inverter unit; a mode switching unit connected between the load and the second inverter unit; and a control unit configured to drive the load in a single-stage inverter mode or a two-stage inverter mode by performing control to turn on or turn off the mode switching unit.

Description

Technical Field

[0001] This disclosure relates to apparatus and methods for enabling two-stage inverters to switch between modes, and more specifically, to apparatus and methods for enabling two-stage inverters to switch between modes based on the inverter's temperature and operating characteristics to achieve high output and high efficiency. Background Technology

[0002] Typically, unlike ordinary three-phase motors that use Y-connections or Δ-connections to configure the voltage source internally, open-end winding machines have a structure in which the three wires for the voltage source can be connected to the outside of the open-end winding machine, rather than to the inside of the open-end winding machine.

[0003] Typically, open-end winding machines are driven by two inverters. In this case, the voltage applied to the open-end winding machine can be increased because the voltage can be output from both sides. Furthermore, by increasing the voltage applied to the open-end winding machine, the same effect as in a maximum 4-level inverter can be achieved.

[0004] In the use of dual inverters (two-stage inverters) to drive an open-end winding machine, the power supply is typically connected to each of the two inverters, providing power to the machine. However, in another scenario, the power supply is connected to only one inverter, while the other is disconnected. Therefore, regardless of the power supply, the DC link voltage to the disconnected inverter can be increased as needed and used. In this case, the disconnected inverter cannot continuously output or receive effective power. For this reason, the disconnected inverter can function as an ineffective power source.

[0005] However, this structure cannot be used to increase output when the motor is running at high speed, because the motor requires a lot of unused power when running at high speed.

[0006] Typically, in existing dual inverters (two-stage inverters), space vector PWM (SVPWM) is used to control the two inverters on both sides.

[0007] However, in the case of next-generation electric vehicles, the current situation requires a control method for controlling a single-stage inverter and a two-stage inverter with a topology to ensure high output and high efficiency in batteries with limited capacity, thereby improving engine control performance by achieving high output and high efficiency.

[0008] The background technology disclosed herein is disclosed in Korean Patent No. 10-1423054 (registered on July 18, 2014, entitled "Apparatus and Method for Controlling a Dual Inverter System"). Summary of the Invention

[0009] The present invention aims to solve the above-mentioned problems and its purpose is to provide an apparatus and method for enabling two-stage inverters to switch between modes, which enables the two-stage inverters to switch between modes based on the inverter's temperature and operating characteristics to achieve high output and high efficiency.

[0010] According to one aspect of this disclosure, an apparatus is provided for enabling a two-stage inverter to switch between modes, the apparatus comprising: a first inverter unit; a second inverter unit; a load connected between the first inverter unit and the second inverter unit; a mode switching unit connected between the load and the second inverter unit; and a control unit configured to drive the load in a single-stage inverter mode or a two-stage inverter mode by performing control of turning the mode switching unit on or off.

[0011] In this device, the control unit can drive the load in two-stage inverter mode through the shutdown mode switching unit, and can drive the load in one-stage inverter mode through the turn-on mode switching unit.

[0012] In this device, the mode switching unit may include three switches S7, S8 and S9 respectively connected for phases U, V and W, wherein the input terminals of the three switches S7, S8 and S9 may be connected to the lines for phases U, V and W respectively, and their output terminals are connected to each other in a shared manner.

[0013] In this device, the switching element used in the first inverter unit may include a SiC MOSFET, and the switching element used in the second inverter unit may include a Si IGBT.

[0014] In this device, when the current inverter drive mode is the first-level inverter mode, the current temperature Temp of the switching element (SiC MOSFET) is lower than the first reference temperature T_limit_1, and the modulation index MI is lower than the minimum value Hys.Min of the specified hysteresis loop, the control unit can drive the inverter in the first-level inverter mode, but using space vector pulse width modulation (SVPWM) technology.

[0015] In this device, when the current inverter drive mode is the first-stage inverter mode, the current temperature Temp of the switching element (SiC MOSFET) of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit can drive the inverter in a two-stage inverter mode, but using space vector pulse width modulation (SVPWM) technology.

[0016] In this device, when the current inverter drive mode is the first-level inverter mode, the current temperature Temp of the switching element (SiC MOSFET) of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit may not change the current inverter drive mode.

[0017] In this device, when the current inverter drive mode is not the first-stage inverter mode and the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is not lower than the second reference temperature T_limit_2, the control unit can drive the inverter in a two-stage inverter mode, but using DPWM technology.

[0018] In this device, when the current inverter drive mode is not the first-stage inverter mode, the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is lower than the second reference temperature T_limit_2, and the modulation index MI is not lower than the maximum value Hys.Max of the specified hysteresis loop. The control unit can drive the inverter in two-stage inverter mode, but using SVPWM technology.

[0019] In this device, when the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit can drive the inverter in the first-level inverter mode, but using SVPWM technology.

[0020] In this device, when the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit may not change the current inverter drive mode.

[0021] According to another aspect of this disclosure, a method is provided to enable a two-stage inverter to switch between modes, the method comprising: a controller of the two-stage inverter checking whether the current inverter drive mode is a first-stage inverter mode; the controller comparing the current temperature Temp of the switching element (SiC MOSFET) of the first inverter unit with a first reference temperature T_limit_1; and when the current temperature Temp of the switching element (SiC MOSFET) of the first inverter unit is not lower than the first reference temperature T_limit_1, the controller drives the inverter in the first-stage inverter mode, but using DPWM technology.

[0022] The method may also include driving the inverter in first-stage inverter mode by the control unit when the current inverter drive mode is first-stage inverter mode, the current temperature Temp of the switching element (SiC MOSFET) of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is lower than the minimum value Hys.Min of the specified hysteresis loop, but using SVPWM technology.

[0023] The method may also include driving the inverter in a two-stage inverter mode by the control unit when the current inverter drive mode is a single-stage inverter mode, the current temperature Temp of the switching element (SiC MOSFET) of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop. However, SVPWM technology is used.

[0024] The method may further include the following: when the current inverter drive mode is a first-level inverter mode, the current temperature Temp of the switching element (SiC MOSFET) of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit does not change the current inverter drive mode.

[0025] The method may also include driving the inverter in a two-stage inverter mode by the control unit when the current inverter drive mode is not a single-stage inverter mode and the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is not lower than the second reference temperature T_limit_2, but using DPWM technology.

[0026] The method may also include driving the inverter in a two-stage inverter mode by the control unit when the current inverter drive mode is not a single-stage inverter mode, the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is lower than the second reference temperature T_limit_2, and the modulation index MI is not lower than the maximum value Hys.Max of the specified hysteresis loop, but using SVPWM technology.

[0027] The method may also include driving the inverter in first-level inverter mode by the control unit when the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop, but using SVPWM technology.

[0028] The method may further include the following: when the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element (Si IGBT) of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit does not change the current inverter drive mode.

[0029] According to one aspect of this disclosure, the disclosure relates to a method for enabling a two-stage inverter to switch between modes, applicable to both single-stage and two-stage inverters. Using this method, a two-stage inverter can switch between modes based on temperature and the inverter's operating characteristics to achieve high output and high efficiency.

[0030] Furthermore, when the inverter needs to switch between modes, the switching does not occur immediately, and the impossible parts of motor control are minimized through hysteresis loop control. Therefore, it is possible to achieve effects that may improve the motor control performance. Attached Figure Description

[0031] Figure 1 This is a view illustrating a schematic configuration of a device for enabling a two-stage inverter to switch between modes according to a first embodiment of the present disclosure.

[0032] Figure 2 It shows the basis Figure 1 The table shows the temperature distribution that varies depending on the inverter's drive mode.

[0033] Figure 3 It is shown Figure 1 A table showing the rated operating temperatures of the switching elements in the first and second inverter units.

[0034] Figure 4 This is a flowchart describing a method for enabling a two-stage inverter to switch between modes according to a second embodiment of the present disclosure.

[0035] Figure 5 It is used to describe Figure 4 The curve of the hysteresis loop part in the graph. Detailed Implementation

[0036] The apparatus and method for enabling a two-stage inverter to switch between modes according to the first and second embodiments of the present disclosure will now be described with reference to the accompanying drawings.

[0037] For clarity and convenience, the thickness of the lines and the dimensions of the components may be shown in the accompanying drawings at a non-precise scale. Furthermore, terms will be used below that are defined by consideration of the function of the components according to this disclosure, and these terms are assigned to the intended user or administrator or practice in the art. Therefore, the terms should be defined in the context of this specification.

[0038] Figure 1 This is a view illustrating a schematic configuration of a device for enabling a two-stage inverter to switch between modes according to a first embodiment of the present disclosure.

[0039] like Figure 1 As shown, the apparatus for enabling two-stage inverters to switch between modes according to a first embodiment of the present disclosure includes a first inverter unit 10, a second inverter unit 20, a mode switching unit 110 formed between the first inverter unit 10 and the second inverter unit 20, and a control unit 120.

[0040] A load (e.g., a motor) is connected between the first inverter unit 10 and the mode switching unit 110. Power supply V dc Connected to the first inverter unit 10.

[0041] Under the control of the control unit 120, the first inverter unit 10 can apply a step voltage (e.g., a six-step voltage) to the load. Under the control of the control unit 120, the mode switching unit 110 can be turned on and off. Therefore, the load can be driven in either a single-stage inverter mode or a two-stage inverter mode.

[0042] For example, under the control of the control unit 120, when the mode switching unit 110 is turned off, the load is driven in a two-stage inverter mode (i.e., in high output mode), and when the mode switching unit 110 is turned on, the load is driven in a single-stage inverter mode (i.e., in high efficiency mode).

[0043] The mode switching unit 110 includes three switches S7, S8 and S9 respectively connected for phases U, V and W, and the input terminals of the three switches S7, S8 and S9 are respectively connected to the lines for phases U, V and W, and their output terminals are connected to each other in a shared manner.

[0044] When the control unit 120 drives the load in first-level inverter mode, the control unit 120 can select any one of a number of inverter drive technologies (space vector pulse width modulation (SVPWM) and discontinuous pulse width modulation (DPWM) technologies) according to pre-specified conditions (e.g., temperature or modulation index MI) and can drive the load.

[0045] Furthermore, when the control unit 120 drives the load in a two-stage inverter mode, the control unit 120 can select any one of multiple inverter drive technologies (SVPWM and DPWM technologies) according to pre-specified conditions (e.g., temperature or modulation index MI) and drive the load.

[0046] For reference, in SVPWM technology, the six-phase inverter switches operate at the PWM carrier frequency, and the four-phase inverter switches operate at the PWM carrier frequency. Therefore, at the same carrier frequency, the number of switching phases in DPWM technology is less than that in SVPWM technology. Consequently, the switching losses are correspondingly lower. The lower the switching losses, the lower the heat loss of the switching elements. Therefore, the switching elements are subjected to less heat.

[0047] When high engine output is not required, the control unit 120 can improve efficiency by controlling the first-level inverter mode (i.e., high-efficiency drive). That is, high-efficiency drive can be performed by controlling only the first inverter unit 10.

[0048] Furthermore, when high engine output is required, the control unit 120 can increase the output by controlling the two-stage inverter mode (i.e., high output drive). That is, high output drive can be executed by controlling both the first inverter unit 10 and the second inverter unit 20.

[0049] At this time, controlling the mode switching unit 110 to shut down via control unit 120 to drive the load in two-stage inverter mode means that control unit 120 performs high-output drive using open-end winding technology. Furthermore, controlling the mode switching unit 110 to turn on via control unit 120 to drive the load in single-stage inverter mode means that control unit 120 performs high-efficiency drive using three-phase technology (see reference). Figure 2 ). Figure 2 It shows the basis Figure 1 The table shows the temperature distribution that varies depending on the inverter's drive mode.

[0050] The control unit 120 can be the vehicle's ECU (electronic control unit) or MCU (microcontroller unit).

[0051] Figure 3 It is shown Figure 1 The table below shows the rated operating temperatures of the switching elements in the first and second inverter units. The rated operating temperature range of the switching element (SiC MOSFET) used in the first inverter unit 10 (which may be described hereinafter as the first switching element) is -55 to 175 degrees Celsius (°C). The rated operating temperature range of the switching element (Si IGBT) used in the second inverter unit 20 (which may be described hereinafter as the second switching element) is -40 to 175 degrees Celsius (°C).

[0052] In this case, the temperature of each switching element can be measured using its own built-in temperature sensor (not shown). The temperature of a switching element that does not have its own built-in temperature sensor can be measured using a separate external temperature sensor (not shown) connected to it.

[0053] For reference, SiC MOSFETs are suitable for low power ratings and therefore cannot be used for high output. In contrast, SiIGBTs are suitable for high output but cannot achieve high efficiency. Therefore, each switching element of the first inverter unit 10 can be implemented as a SiC MOSFET, and each switching element of the second inverter unit 20 can be implemented as a Si IGBT.

[0054] Furthermore, in single-stage inverter mode, the battery voltage cannot be fully utilized, and therefore, it is typically only used up to the linear modulation region (modulation index MI of 1.15). In two-stage inverter mode, the maximum amplitude of the output phase voltage is the power supply voltage V. dc The voltage amplitude can be controlled, and the battery voltage can be fully utilized. Therefore, a two-stage inverter mode, instead of a single-stage inverter mode, can be used for high output. However, compared to the single-stage inverter mode, the two-stage inverter mode uses more switches. Therefore, the two-stage inverter mode is characterized by reduced power conversion efficiency due to increased switching losses and conduction losses.

[0055] The following will also refer to Figure 4 The flowchart in the document describes the operation of the control unit 120 in more detail.

[0056] Figure 4 This is a flowchart describing a method for enabling a two-stage inverter to switch between modes according to a second embodiment of the present disclosure.

[0057] Reference Figure 4 The control unit 120 calculates (or measures) the temperature and modulation index MI (S101) of each of the switching elements (e.g., SiC MOSFETs and Si IGBTs) in the first inverter unit 10 and the second inverter unit 20.

[0058] Additionally, if the current inverter drive mode is the first-level inverter mode (as in S102), the control unit 120 checks whether the current temperature Temp of the first switching element (SiC MOSFET) is lower than the first reference temperature T_limit_1 (S103).

[0059] At this point, the first reference temperature T_limit_1 is set to approximately 70% of the maximum operating temperature of the first switching element (SiC MOSFET).

[0060] When the result of the check (S103) is that the current temperature Temp of the first switching element (SiC MOSFET) is not lower than the first reference temperature T_limit_1 (No in S103), the control unit 120 drives the inverter in first-level inverter mode, but uses DPWM technology (S105).

[0061] Additionally, when the result of the check (S103) is that the current temperature Temp of the first switching element (SiC MOSFET) is lower than the first reference temperature T_limit_1 (Yes in S103), the control unit 120 checks whether the modulation index MI is lower than the minimum value Hys.Min of the specified hysteresis loop (S104) (reference). Figure 5 ).

[0062] When the result of the check (S104) is that the modulation index MI is lower than the minimum value Hys.Min of the specified hysteresis loop (Yes in S104), the control unit 120 drives the inverter in first-level inverter mode, but uses SVPWM technology (S106).

[0063] In addition, the control unit 120 checks whether the result of the check (S104) is that the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop (S107).

[0064] When the result of the check (S107) is that the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop (No in S107), the controller 120 drives the inverter in two-stage inverter mode, but uses SVPWM technology (S108).

[0065] Additionally, when the result of the check (S107) is that the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop (Yes in S107), the control unit 120 does not change the current inverter drive mode (S112).

[0066] If the result of the check (S102) is that the current inverter drive mode is not the first-level inverter mode (No in S102), the control unit 120 checks whether the current temperature of the second switching element (Si IGBT) is lower than the second reference temperature T_limit_2 (S109).

[0067] In this case, the second reference temperature T_limit_2 is set to approximately 70% of the maximum operating temperature of the second switching element (Si IGBT).

[0068] When the result of the check (S109) is that the current temperature Temp of the second switching element (Si IGBT) is not lower than the second reference temperature T_limit_2 (No in S109), the control unit 120 drives the inverter in a two-stage inverter mode, but using DPWM technology (S113).

[0069] Additionally, when the result of the check (S109) is that the current temperature Temp of the second switching element (Si IGBT) is lower than the second reference temperature T_limit_2 (Yes in S109), the control unit 120 checks whether the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop (S110) (reference). Figure 5 ).

[0070] When the result of the check (S110) is that the modulation index MI is not lower than the maximum value Hys.Max of the specified hysteresis loop (No in S110), the control unit 120 drives the inverter in two-stage inverter mode, but uses SVPWM technology (S108).

[0071] Additionally, when the result of the check (S110) is that the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop (Yes in S110), the control unit 120 checks whether the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop (S111).

[0072] When the result of the check (S111) is that the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop (No in S111), the control unit 120 drives the inverter in first-level inverter mode, but uses SVPWM technology (S106).

[0073] Additionally, when the result of the check (S111) is that the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop (Yes in S111), the control unit 120 does not change the current inverter drive mode (S112).

[0074] Figure 5 It is used to describe Figure 4 The graph shows the hysteresis loop portion of the inverter. When immediately entering inverter mode with reference MI, engine control performance degrades. Therefore, the hysteresis loop portion is set at reference MI, which serves as a benchmark for switching inverter modes. When the modulation index MI is lower than the minimum value Hys.Min of the specified hysteresis loop, the inverter operates in single-stage inverter mode. When the modulation index MI is higher than the maximum value Hys.Max of the specified hysteresis loop, the inverter operates in two-stage inverter mode. Therefore, when performing mode switching, the impossible parts of engine control are minimized.

[0075] As described above, the mode switching method according to the second embodiment of this disclosure is applicable to both single-stage and two-stage inverters. The two-stage inverter can switch between single-stage and two-stage inverter modes based on the inverter's temperature and operating characteristics. Therefore, high output and high efficiency can be achieved. Furthermore, when the inverter needs to switch between modes, the switching does not occur immediately, and the impossibility of engine control is minimized through hysteresis loop control. Therefore, effects that may improve engine control performance can be achieved.

[0076] The embodiments of this disclosure are described by way of example only with reference to the accompanying drawings. It will be understood by those skilled in the art to which this disclosure pertains that various other modifications and equivalents are possible based on this disclosure. Therefore, the technical scope of this disclosure should be defined by the following claims. Furthermore, for example, the features described in this specification may be implemented in the form of a method, process, apparatus, software program, data stream, or signal. Although these features are described in a single form context (e.g., described only as a method), these features may also be implemented in different forms (e.g., as an apparatus or program). The apparatus may be implemented in suitable hardware, software, firmware, etc. For example, the method may be implemented in a computer, microprocessor, or device such as a processor, which generally refers to a processing device such as an integrated circuit or programmable logic device.

Claims

1. An apparatus for enabling a two-stage inverter to switch between modes, the apparatus comprising: First inverter unit; Second inverter unit; The load connected between the first inverter unit and the second inverter unit; A mode switching unit connected between the load and the second inverter unit; as well as The control unit is configured to drive the load in either a single-stage inverter mode or a two-stage inverter mode by controlling the mode switching unit to turn on or off. The control unit is further configured to determine the drive mode of the two-stage inverter based on the current inverter drive mode and the current temperature of the switching elements of the first inverter unit or the second inverter unit.

2. The apparatus according to claim 1, wherein, The control unit drives the load in the two-stage inverter mode by turning off the mode switching unit, and drives the load in the single-stage inverter mode by turning on the mode switching unit.

3. The apparatus according to claim 1, wherein, The mode switching unit includes three switches S7, S8 and S9 respectively connected for phases U, V and W, wherein the input terminals of the three switches S7, S8 and S9 are respectively connected to the lines for phases U, V and W, and their output terminals are connected to each other in a shared manner.

4. The apparatus according to claim 1, wherein, The switching elements used in the first inverter unit include SiC MOSFETs, and The switching element used in the second inverter unit includes Si IGBT.

5. The apparatus according to claim 1, wherein, When the current inverter drive mode is the first-level inverter mode, the current temperature Temp of the switching element SiC MOSFET is lower than the first reference temperature T_limit_1, and the modulation index MI is lower than the minimum value Hys.Min of the specified hysteresis loop, the control unit drives the inverter in the first-level inverter mode, but uses space vector pulse width modulation (SVPWM) technology.

6. The apparatus according to claim 1, wherein, When the current inverter drive mode is the first-stage inverter mode, the current temperature Temp of the switching element SiC MOSFET of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit drives the inverter in the two-stage inverter mode, but uses space vector pulse width modulation (SVPWM) technology.

7. The apparatus according to claim 1, wherein, When the current inverter drive mode is the first-level inverter mode, the current temperature Temp of the switching element SiC MOSFET of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit does not change the current inverter drive mode.

8. The apparatus according to claim 1, wherein, When the current inverter drive mode is not the first-stage inverter mode, and the current temperature Temp of the switching element Si IGBT of the second inverter unit is not lower than the second reference temperature T_limit_2, the control unit drives the inverter in the two-stage inverter mode, but uses discontinuous pulse width modulation (DPWM) technology.

9. The apparatus according to claim 1, wherein, When the current inverter drive mode is not the first-stage inverter mode, the current temperature Temp of the switching element Si IGBT of the second inverter unit is lower than the second reference temperature T_limit_2, and the modulation index MI is not lower than the maximum value Hys.Max of the specified hysteresis loop. The control unit drives the inverter in the two-stage inverter mode, but uses SVPWM technology.

10. The apparatus according to claim 1, wherein, When the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element Si IGBT of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit drives the inverter in the first-level inverter mode, but uses SVPWM technology.

11. The apparatus according to claim 1, wherein, When the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element Si IGBT of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit does not change the current inverter drive mode.

12. A method for enabling a two-stage inverter to switch between modes, wherein, The two-stage inverter includes a first inverter unit and a second inverter unit, a load is connected between the first inverter unit and the second inverter unit, and a mode switching unit is connected between the load and the second inverter unit. The method includes: The control unit of the two-stage inverter checks whether the current inverter drive mode is the single-stage inverter mode. The control unit compares the current temperature Temp of the SiC MOSFET switching element of the first inverter unit with the first reference temperature T_limit_1; and When the current temperature Temp of the switching element SiC MOSFET of the first inverter unit is not lower than the first reference temperature T_limit_1, the control unit drives the inverter in the first-stage inverter mode, but uses discontinuous pulse width modulation (DPWM) technology.

13. The method according to claim 12, wherein the method further comprises: When the current inverter drive mode is the first-level inverter mode, the current temperature Temp of the switching element SiC MOSFET of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is lower than the minimum value Hys.Min of the specified hysteresis loop, the control unit drives the inverter in the first-level inverter mode, but uses space vector pulse width modulation (SVPWM) technology.

14. The method according to claim 12, wherein the method further comprises: When the current inverter drive mode is the first-stage inverter mode, the current temperature Temp of the switching element SiC MOSFET of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop, the inverter is driven by the control unit in the two-stage inverter mode, but using SVPWM technology.

15. The method according to claim 12, further comprising: When the current inverter drive mode is the first-level inverter mode, the current temperature Temp of the SiC MOSFET of the first inverter unit is lower than the first reference temperature T_limit_1, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit does not change the current inverter drive mode.

16. The method of claim 12, further comprising: When the current inverter drive mode is not the first-level inverter mode, and the current temperature Temp of the switching element Si IGBT of the second inverter unit is not lower than the second reference temperature T_limit_2, the control unit drives the inverter in a two-level inverter mode, but uses the DPWM technology.

17. The method of claim 12, further comprising: When the current inverter drive mode is not the first-stage inverter mode, the current temperature Temp of the switching element SiIGBT of the second inverter unit is lower than the second reference temperature T_limit_2, and the modulation index MI is not lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit drives the inverter in a two-stage inverter mode, but using SVPWM technology.

18. The method of claim 12, further comprising: When the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element SiIGBT of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and not lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit drives the inverter in the first-level inverter mode, but uses SVPWM technology.

19. The method of claim 12, further comprising: When the current inverter drive mode is not the first-level inverter mode, the current temperature Temp of the switching element SiIGBT of the second inverter unit is lower than the second reference temperature T_limit_2, the modulation index MI is lower than the maximum value Hys.Max of the specified hysteresis loop, and the modulation index MI is higher than the minimum value Hys.Min of the specified hysteresis loop and lower than the maximum value Hys.Max of the specified hysteresis loop, the control unit does not change the current inverter drive mode.

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