A cascaded H-bridge inverter for heterogeneous power devices and its mixing modulation method

Abstract

The present invention discloses a cascaded H-bridge inverter of heterogeneous power devices and its mixing modulation method, belonging to the field of multilevel converters. The inverter includes three single-phase circuits with the same structure, and each single-phase circuit includes a high-frequency sub-module and n-1 low-frequency sub-modules. An external controller generates a triangular carrier wave and a modulation wave of the high-frequency sub-module and compares them to generate a PWM signal of the switching tubes of the high-frequency sub-module and control the on / off of the switching tubes of the high-frequency sub-module, and the high-frequency sub-module outputs a PWM wave; the controller generates n-1 groups of fundamental-frequency square-wave signals based on the modulation wave and controls the on / off of the switching tubes of the low-frequency sub-modules, and the low-frequency sub-modules output square waves; the PWM wave of one single-phase circuit and n-1 square waves are superimposed to form a phase voltage wave with 2 n +1 level mixing modulation; finally, the inverter outputs three-phase voltages with a phase difference of 120° from each other to an external load. The present invention can achieve performance similar to that of fully wide-bandgap power devices without significantly increasing the device cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of PWM of multilevel converters, and particularly relates to a cascaded H-bridge inverter with heterogeneous power devices and a mixing modulation method thereof. Background Art

[0002] In medium-voltage large-capacity application scenarios such as distributed power generation, electrified transportation, and high-speed drive, the conversion efficiency and power density of power electronic conversion equipment are important indicators to measure its performance. Currently, with the continuous progress of semiconductor technology and the increasing demand of application requirements, the use of wide-bandgap devices with characteristics of high temperature tolerance, high switching frequency, and low switching loss has become an important trend to improve the efficiency and performance of converters.

[0003] As an important form of multilevel converters, the cascaded H-bridge inverter replaces the direct series connection of power devices in a module series connection manner, thus effectively avoiding the occurrence of complex problems such as series voltage equalization of power devices, and also has characteristics such as low voltage stress and low harmonic content. Therefore, in the field of medium-voltage high-power conversion, the cascaded H-bridge shows significant application value. However, at present, the cascaded H-bridge inverter based on all wide-bandgap devices is expensive due to the large number of required power devices and is difficult to be widely applied. Summary of the Invention

[0004] Aiming at the problem that the cascaded H-bridge inverter based on all wide-bandgap devices is expensive due to the large number of power devices and is difficult to be widely applied, the present invention proposes a cascaded H-bridge inverter with heterogeneous power devices and a mixing modulation method thereof, which can achieve performance similar to that of all wide-bandgap power devices without significantly increasing the device cost, and has great engineering promotion value.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] In the first aspect, the present invention discloses a cascaded H-bridge inverter with heterogeneous power devices, which includes three single-phase circuits with the same structure, and each single-phase circuit includes n sub-modules. ; Each sub-module includes four switching tubes and a power supply, and the switching tube includes three ports.

[0007] In the sub-module, the first port of the first switching tube is connected to the second port of the fourth switching tube, the first port of the third switching tube is connected to the second port of the second switching tube, the second port of the first switching tube is connected to the second port of the third switching tube, and the connection node is connected to the positive pole of the power supply; the first port of the fourth switching tube is connected to the first port of the second switching tube, and the connection node is connected to the negative pole of the power supply.

[0008] The first port of the first switching transistor of the first sub-module serves as the first end of the single-phase circuit. The first port of the third switching transistor of the x-th sub-module is connected to the first port of the first switching transistor of the (x + 1)-th sub-module. The first port of the third switching transistor of the last sub-module serves as the second end of the single-phase circuit. The three second ends of the three single-phase circuits are connected together. The three first ends of the three single-phase circuits are connected to the same external load.

[0009] In the single-phase circuit, one of the sub-modules is a sub-module operating at the carrier frequency, and the remaining n - 1 sub-modules are sub-modules operating at the fundamental frequency.

[0010] In a second aspect, the present invention discloses a mixing modulation method for the heterogeneous power device cascaded H-bridge inverter as described above, including the following steps:

[0011] The sub-module operating at the carrier frequency is a high-frequency sub-module, and the sub-module operating at the fundamental frequency is a low-frequency sub-module. An external controller generates a triangular carrier wave and a modulation wave for the high-frequency sub-module, compares the triangular carrier wave with the modulation wave, generates PWM signals for the switching transistors of the high-frequency sub-module, and controls the on-off of the four switching transistors of the high-frequency sub-module based on the PWM signals. The output voltage of the high-frequency sub-module is the PWM wave of v H The controller generates n - 1 groups of fundamental frequency square wave signals based on the modulation wave of the high-frequency sub-module. One group of fundamental frequency square wave signals controls the on-off of the four switching transistors of one low-frequency sub-module. The n - 1 low-frequency sub-modules output n - 1 square waves with different phases. The PWM wave of one single-phase circuit and the n - 1 square waves are superimposed to form a phase voltage wave with 2n + 1 level mixing modulation. The phase voltage waves of the three single-phase circuits are 120 degrees out of phase with each other. Finally, the heterogeneous power device cascaded H-bridge inverter outputs three-phase voltages to the external load.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] The method of the present invention introduces a wide bandgap power device H-bridge sub-module into the traditional cascaded H-bridge inverter. Through the technical means of mixing modulation, only the high-frequency sub-module using the wide bandgap power device operates at the carrier frequency, and the power devices of the remaining low-frequency sub-modules are switched at the fundamental frequency. Therefore, the switching loss of the inverter is significantly reduced. This technology improves the system efficiency at a very low cost and still can ensure the output harmonic performance, and has great potential application value in the fields of renewable energy power generation and energy storage. Description of the Drawings

[0014] Figure 1 is a schematic diagram of the three-phase inverter topology connection of the heterogeneous power device cascaded H-bridge provided by the present invention;

[0015] Figure 2It is a schematic diagram of a single-phase inverter circuit based on a cascaded H-bridge of heterogeneous power devices provided by the present invention;

[0016] Figure 3 It is a mixing modulation method for a cascaded H-bridge inverter provided by the present invention;

[0017] Figure 4 It is the modulation wave of the high-frequency sub-module of the single-phase cascaded seven-level inverter and the gate signals of each switching tube in an embodiment of the present invention;

[0018] Figure 5 It is the gate signals of some switching tubes and the output voltage waveforms of the first and second low-frequency sub-modules of the single-phase cascaded seven-level inverter in an embodiment of the present invention;

[0019] Figure 6 It is the output phase voltage, phase current waveforms and phase voltage harmonic spectrum diagram after applying the proposed mixing modulation in a specific embodiment of the present invention. Specific Embodiments

[0020] The present invention will be further elaborated and described below in conjunction with specific embodiments. The embodiments are only demonstrations of the present disclosure content and do not delimit the scope of limitation. Without conflict, the technical features of each embodiment of the present invention can be combined accordingly.

[0021] Benefiting from the modular characteristics of the cascaded H-bridge, the present invention provides a cascaded H-bridge inverter topology structure of a combined heterogeneous power device and its mixing modulation method. In the single-phase circuit of the present invention, only one sub-module uses a silicon carbide metal oxide semiconductor field effect transistor SiC MOSFET with low switching loss and operates at a high carrier frequency, and the remaining sub-modules use silicon-based insulated gate bipolar transistors Si IGBTs and switch at the fundamental frequency. Therefore, the switching loss is reduced. Through this modulation method, similar performance to that of all wide-bandgap power devices can be achieved using a small number of wide-bandgap devices, thereby improving the conversion efficiency and power density of the inverter.

[0022] As Figure 1 shown, the heterogeneous power device cascaded H-bridge inverter of the present invention is a three-phase inverter topology composed of three single-phase circuits with the same structure and a phase difference of 120° from each other. Each single-phase circuit includes a high-frequency sub-module and n - 1 low-frequency sub-modules, and n is an integer, where n is the total number of sub-modules in the single-phase inverter circuit.

[0023] In a specific embodiment of the present invention, as Figure 2 shown in (a) of, the first module in the single-phase circuit is a high-frequency sub-module, and the 2nd to the nth modules in the single-phase circuit are low-frequency sub-modules. The low-frequency sub-module includes switching tube S 1i and switching tube S 2i, switching transistor S 3i , switching transistor S 4i and a DC-side input voltage source, and i is an integer, switching transistor S 1i , switching transistor S 2i , switching transistor S 3i and switching transistor S 4i are all silicon-based insulated gate bipolar transistors Si IGBTs; and in the present invention, the amplitudes of all DC-side input voltage sources are equal.

[0024] The high-frequency sub-module includes switching transistor S 11 , switching transistor S 21 , switching transistor S 31 , switching transistor S 41 and a DC-side input voltage source, switching transistor S 11 , switching transistor S 21 , switching transistor S 31 and switching transistor S 41 are all silicon carbide-based metal oxide semiconductor field effect transistors SiC MOSFETs or gallium nitride high electron mobility transistors GaN HEMTs.

[0025] In the high-frequency sub-module, the source of switching transistor S 11 is connected to the drain of switching transistor S 41 , the source of switching transistor S 31 is connected to the drain of switching transistor S 21 , the drain of switching transistor S 11 is connected to the drain of switching transistor S 31 , and after connection, it is connected to the positive pole of the DC-side input voltage source; the source of switching transistor S 41 is connected to the source of switching transistor S 21 , and after connection, it is connected to the negative pole of the DC-side input voltage source. In a single-phase circuit, the source of switching transistor S 11 serves as the first end of the single-phase circuit, and the three first ends of the three single-phase circuits are connected to the same external load. Specifically, the three first ends of the three single-phase circuits are respectively connected to the A, B, and C phase terminals of the external load; the source of switching transistor S 31 is connected to the emitter of switching transistor S 12 of the first low-frequency sub-module.

[0026] In the low-frequency sub-module, the emitter of switching transistor S 1i is connected to the collector of switching transistor S 4i , the emitter of switching transistor S 3i is connected to the collector of switching transistor S 2i , the collector of switching transistor S 1i is connected to the collector of switching transistor S 3iis connected to the collector, and after connection, it is connected to the positive pole of the DC-side input voltage source; the emitter of the switching transistor S 4i is connected to the emitter of the switching transistor S 2i , and after connection, it is connected to the negative pole of the DC-side input voltage source. In a single-phase circuit, the emitter of the switching transistor S 3c of the c-th low-frequency sub-module is connected to the emitter of the switching transistor S 1（c+1） of the (c + 1)-th low-frequency sub-module, where and c is an integer; the emitter of the switching transistor S 3n of the last low-frequency sub-module serves as the second terminal of the single-phase circuit, and the three second terminals of the three single-phase circuits are connected to serve as the neutral point N of the three-phase inverter.

[0027] The gates of all the switching transistors are connected to an external controller for controlling the turning on and off of the switching transistors.

[0028] In the present invention, the high-frequency sub-module can be located at any position in the single-phase circuit, that is, the high-frequency sub-module can not only be used as the first sub-module in the single-phase circuit, but also be used as the last sub-module in the single-phase circuit.

[0029] In a specific embodiment of the present invention, as shown in (b) of Figure 2 , when the high-frequency sub-module serves as the last sub-module in the single-phase circuit, both the high-frequency sub-module and the low-frequency sub-module include the switching transistors S 1a , the switching transistor S 2a , the switching transistor S 3a , the switching transistor S 4a and a power source (DC-side input voltage source), and a is an integer, and the switching transistor includes three ports;

[0030] The first port of the switching transistor S 1a is connected to the second port of the switching transistor S 4a , the first port of the switching transistor S 3a is connected to the second port of the switching transistor S 2a , the second port of the switching transistor S 1a is connected to the second port of the switching transistor S 3a , and after connection, it is connected to the positive pole of the power source; the first port of the switching transistor S 4a is connected to the first port of the switching transistor S 2a , and after connection, it is connected to the negative pole of the power source;

[0031] The first port of the switching transistor S 11 serves as the first terminal of the single-phase circuit, the first port of the switching transistor S 3x is connected to the first port of the switching transistor S 1(x+1) , and x is an integer; the switching transistor S3n The first port of 3n serves as the second end of a single-phase circuit, and the three second ends of the three single-phase circuits are connected; the three first ends of the three single-phase circuits are connected to the same external load.

[0032] Among them, the switching transistors S of the low-frequency sub-module 1j , switching transistor S 2j , switching transistor S 3j and switching transistor S 4j are all silicon-based insulated gate bipolar transistors Si IGBTs, and j is an integer; the first port of switching transistor S bj is the emitter, the second port of switching transistor S bj is the collector, the third port of switching transistor S bj is the gate, where b is 1, 2, 3, or 4.

[0033] The four switching transistors of the high-frequency sub-module are silicon carbide metal oxide semiconductor field effect transistors SiC MOSFETs or gallium nitride high electron mobility transistors GaN HEMTs; the first port of switching transistor S bn is the source, the second port of switching transistor S bn is the drain, and the third port of switching transistor S bn is the gate.

[0034] In this embodiment, a mixing modulation method for the heterogeneous power device cascaded H-bridge inverter described above is also provided, including the following steps:

[0035] An external controller generates a triangular carrier wave and a modulation wave of the high-frequency sub-module, compares the triangular carrier wave with the modulation wave, generates a PWM signal for the switching transistors of the high-frequency sub-module, controls the on and off of the four switching transistors of the high-frequency sub-module based on the PWM signal, and then the output voltage of the high-frequency sub-module is a PWM wave of v H ; the controller generates n - 1 groups of fundamental frequency square wave signals based on the modulation wave of the high-frequency sub-module, and a group of fundamental frequency square wave signals controls the on and off of the four switching transistors of a low-frequency sub-module (that is, the fundamental frequency square wave signal corresponds to a low-frequency sub-module one by one), and n - 1 low-frequency sub-modules generate n - 1 square waves with mutually staggered phases; the PWM wave of a single-phase circuit and n - 1 square waves are superimposed to form a phase voltage wave with 2n + 1 level mixing modulation, and the phases of the three phase voltage waves are mutually different by 120°; finally, the heterogeneous power device cascaded H-bridge inverter outputs three-phase voltages of A, B, and C to an external load.

[0036] In this embodiment, at this time, the high-frequency sub-module is the last sub-module in the single-phase circuit, and the modulation wave of the high-frequency sub-module within a fundamental frequency period (0, 2π] is a composite waveform with quarter symmetry:

[0037] The expression of the modulation wave in the interval (0, π] is:

[0038] ;

[0039] The expression of the modulation wave in the interval (π, 2π] is:

[0040] ;

[0041] where, v m1 is the modulation wave of the high-frequency sub-module; n is the sum of the numbers of the high-frequency sub-module and the low-frequency sub-module; m is the modulation degree, 0 < m < 1; , and j is an integer; ; θ is the angle of the modulation wave;

[0042] When 0 < θ ≤ π, the switch tube S 2n of the high-frequency sub-module is always on, and the switch tube S 3n is always off. If the modulation wave is greater than or equal to the triangular carrier wave at this time, the switch tube S 1n turns on and the switch tube S 4n turns off. If the modulation wave is less than the triangular carrier wave at this time, the switch tube S 4n turns on and the switch tube S 1n turns off; when π < θ ≤ 2π, the switch tube S 4n is always on, and the switch tube S 1n is always off. If the modulation wave is greater than or equal to the triangular carrier wave at this time, the switch tube S 3n turns on and the switch tube S 2n turns off. If the modulation wave is less than the triangular carrier wave at this time, the switch tube S 2n turns on and the switch tube S 3n turns off.

[0043] The voltage of the square wave generated by the jth low-frequency sub-module is:

[0044] ;

[0045] where, E is the voltage amplitude of the power supply;

[0046] When θ j ≤ θ ≤ π - θ n-j , the switch tubes S 1j , S 2j of the jth low-frequency sub-module are turned on, and the switch tubes S 4j , S 3j are turned off; when π + θ j ≤ θ ≤ 2π - θ n-j , the switch tubes S 3j , S 4j are turned on, and S 2j , S1j Turn off; when θ is in other cases, switch S 1j 、S 3j Turn off, switch S 4j 、S 2j Turn on.

[0047] In a specific embodiment of the present invention, the number of single-phase cascade units n = 3. Analyzing the heterogeneous power device cascaded H-bridge inverter and its mixing modulation method proposed by the present invention can take into account both the high efficiency and low THD (Total Harmonic Distortion) value of the inverter. At this time, the single-phase circuit of the inverter includes a sub-module using a silicon carbide metal oxide semiconductor field effect transistor SiC MOSFET as a switch, that is, a high-frequency sub-module, and two sub-modules using a silicon-based insulated gate bipolar transistor Si IGBT integrated with a reverse diode as switches, that is, the first and second low-frequency sub-modules.

[0048] The embodiment of the present invention also provides a mixing modulation method for a heterogeneous power device cascaded H-bridge inverter when n = 3. The method includes:

[0049] As Figure 3 shown, an external controller generates a triangular carrier wave and a modulation wave of the first H-bridge sub-module, and compares the triangular carrier wave with the modulation wave to generate high-frequency PWM signals of the four switches of the high-frequency sub-module. The output voltage of the high-frequency module is a PWM wave of v H ; the controller generates 2 groups of fundamental frequency square wave signals based on the modulation wave of the high-frequency sub-module. Each group of fundamental frequency square wave signals controls the on and off of the four switches of a low-frequency sub-module. The two low-frequency sub-modules generate 2 square waves with different phases, which are superimposed to form a five-level stepped wave. In this way, the PWM wave output by the high-frequency sub-module and the stepped wave output by the low-frequency sub-module are superimposed, and the average output waveform of the entire single-phase circuit follows a seven-level fundamental frequency sine wave. The phases of the three-phase voltage waves are 120° out of phase with each other. Finally, the heterogeneous power device cascaded H-bridge inverter outputs three-phase voltages of A, B, and C to an external load.

[0050] In a preferred embodiment of the present invention, the modulation principle of a single-phase heterogeneous power device cascaded seven-level inverter composed of n = 3 H-bridge sub-modules is as Figure 4 and Figure 5 shown. To describe the modulation wave signal of the high-frequency sub-module within a fundamental frequency period, the second half of the modulation wave in the interval (0, π] is segmented and defined as Equation (1).

[0051] ;

[0052] The second half of the modulation wave in the interval (π, 2π] is segmented and defined as Equation (2).

[0053] ;

[0054] Among them, 0 < m < 1 is the modulation degree. At this time , . When 0 < θ ≤ π, the switch S of the high-frequency sub-module 23 is always on, and the switch S 33 is always off. When the modulation wave is greater than or equal to the triangular carrier wave v m1 ≥ v cr , the switch S 13 turns on, while the switch S 43 turns on; when π < θ < 2π, the switch S 43 is always on, the switch S 13 is always off, and the modulation wave is greater than the carrier wave v m1 > v cr , the switch S 33 turns on, while the switch S 23 turns on. The upper and lower two switches of the same bridge arm are a pair of complementary switch pairs.

[0055] For the switching devices of the first low-frequency sub-module, when θ 1 ≤ θ ≤ π - θ 2 , the switches S 11 and S 21 of the first low-frequency sub-module conduct, and the switches S 41 and S 31 turn off; when π + θ 1 ≤ θ ≤ 2π - θ 2 , the switches S 31 and S 41 conduct, and S 21 and S 11 turn off; at other times, the switches S 11 and S 31 turn off, and the switches S 41 and S 21 conduct.

[0056] Then the output voltage of the first low-frequency sub-module is

[0057] ;

[0058] For the switching devices of the second low-frequency sub-module, when θ 2 ≤ θ ≤ π - θ 1 , the S 12 and S 22 of the second low-frequency sub-module conduct, and S 42 and S 32 turn off; when π + θ 2 ≤ θ ≤ 2π - θ 1 , the switch S 32, S 42 conducts, and the switching transistor S 22 , S 12 turns off; during the remaining time, the switching transistor S 11 , S 31 turns off, and the switching transistor S 41 , S 21 conducts.

[0059] Then the output voltage of the second low-frequency sub-module is

[0060] ;

[0061] The modulation principle of the high-frequency sub-module and the relationship between the gate signals of each switching transistor are shown in Figure 4 , and the gate signals and output voltages of some switching transistors of the low-frequency sub-module are shown in Figure 5 . It can be seen that only the high-frequency sub-module using low-switching-loss power devices operates at the carrier frequency in the present invention, and the remaining low-frequency sub-modules all switch at the fundamental frequency. Therefore, the switching loss of the inverter is reduced and the efficiency is effectively improved.

[0062] In a specific embodiment, the DC output voltage E of the sub-module of the single-phase heterogeneous power device cascaded seven-level inverter of the present invention is 100V, operating at a modulation index m = 1, fundamental frequency 50Hz, high-frequency sub-module switching frequency f cr = 20kHz, under the condition of a load power factor of 0.9, the simulation waveforms and THD analysis of the output phase voltage and current on the AC side are shown in Figure 6 . It can be seen that the single-phase output voltage of the inverter has seven levels of ±3E, ±2E, ±E, and 0 within a fundamental frequency period of 0.02s and follows the sine transformation law. The total harmonic distortion rate is only 18.20%, and the harmonic frequencies are also only concentrated near the carrier frequency (carrier ratio 400) of the high-frequency sub-module and its integer multiples, realizing the steady-state output of sinusoidal current.

[0063] The above-described embodiments merely represent several implementation manners of the present invention, and their descriptions are relatively specific and detailed, but should not be construed as limiting the scope of the present invention. For those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A frequency mixing modulation method for a cascaded H-bridge inverter of heterogeneous power devices, characterized in that: The heterogeneous power device cascade H-bridge inverter includes three single-phase circuits with the same structure, each single-phase circuit includes n submodules, ; Each submodule includes four switch tubes and a power supply, and the switch tube includes three ports; In the submodule, the first port of the first switch tube is connected to the second port of the fourth switch tube, the first port of the third switch tube is connected to the second port of the second switch tube, the second port of the first switch tube is connected to the second port of the third switch tube, and the connection is connected to the positive electrode of the power supply; the first port of the fourth switch tube is connected to the first port of the second switch tube, and the connection is connected to the negative electrode of the power supply; The first port of the first switch tube of the first submodule serves as the first end of the single-phase circuit, the first port of the third switch tube of the xth submodule is connected to the first port of the first switch tube of the x+1th submodule, The first port of the third switch tube of the last submodule is used as the second end of the single-phase circuit, and the three second ends of the three single-phase circuits are connected; the three first ends of the three single-phase circuits are connected to the same external load; In a single-phase circuit, one of the submodules is a submodule operating at a carrier frequency, and the remaining n-1 submodules are submodules operating at a fundamental frequency; The mixing modulation method comprises the following steps: The submodule running at the carrier frequency is the high-frequency submodule, and the submodule running at the fundamental frequency is the low-frequency submodule; the external controller generates a triangular carrier and a modulation wave of the high-frequency submodule, and compares the triangular carrier with the modulation wave to generate a PWM signal for the switch tube of the high-frequency submodule, and controls the on and off of the four switch tubes of the high-frequency submodule based on the PWM signal. The output voltage of the high-frequency submodule is v H PWM wave; the controller generates n-1 groups of base frequency square wave signals based on the modulation wave of the high frequency submodule, and one group of base frequency square wave signals controls the on and off of four switch tubes of a low frequency submodule, and n-1 low frequency submodules output n-1 square waves with different phases; the PWM wave of a single-phase circuit and n-1 square waves are superimposed to form a 2n+1 level mixing modulated phase voltage wave, and the phase voltage waves of the three single-phase circuits are 120 degrees out of phase with each other; finally, the heterogeneous power device cascade H-bridge inverter outputs a three-phase voltage to an external load; The modulation wave of the high frequency submodule within a base frequency period (0, 2π] is a composite waveform; The expression of the modulated wave in the interval (0, π] is: ； The expression of the modulated wave in the interval (π, 2π] is: ； Among them, v m1 is the modulation wave of the high-frequency submodule; m is the modulation index, 0 <m<1; , and j is an integer; ;θ is the angle of the modulation wave; When 0<θ≤π, the second switch tube of the high-frequency submodule is always on and the third switch tube is always off. If the modulation wave is greater than or equal to the triangular carrier at this time, the first switch tube is turned on and the fourth switch tube is turned off. If the modulation wave is less than the triangular carrier at this time, the fourth switch tube is turned on and the first switch tube is turned off. When π<θ≤2π, the fourth switch tube is always on and the first switch tube is always off. If the modulation wave is greater than or equal to the triangular carrier at this time, the third switch tube is turned on and the second switch tube is turned off. If the modulation wave is less than the triangular carrier at this time, the second switch tube is turned on and the third switch tube is turned off. The voltage of the square wave generated by the jth low-frequency submodule for: ； Where, E is the voltage amplitude of the power supply; When θ j ≤θ≤π-θ n-j , the first switch tube and the second switch tube of the jth low-frequency submodule are turned on, and the third switch tube and the fourth switch tube are turned off; when π+θ j ≤θ≤2π-θ n-j When θ is the other conditions, the first switch tube and the third switch tube are turned off, and the second switch tube and the fourth switch tube are turned on.

2. The frequency mixing modulation method of the heterogeneous power device cascade H-bridge inverter according to claim 1, characterized in that: The four switch tubes of the submodule running at the fundamental frequency are all silicon-based insulated gate bipolar transistors Si IGBT; the first port of the switch tube of the submodule running at the fundamental frequency is the emitter, the second port is the collector, and the third port is the gate.

3. The frequency mixing modulation method of the heterogeneous power device cascade H-bridge inverter according to claim 2, characterized in that: The four switch tubes of the submodule running at the carrier frequency are all silicon carbide-based metal oxide semiconductor field effect transistors SiCMOSFET or gallium nitride high electron mobility transistors GaN HEMT; the first port of the switch tube of the submodule running at the carrier frequency is the source, the second port is the drain, and the third port is the gate.

4. The frequency mixing modulation method of the heterogeneous power device cascade H-bridge inverter according to claim 2 or 3, characterized in that: The gates of all the switch tubes are connected to an external controller to control the on and off of the switch tubes.

5. The frequency mixing modulation method of the heterogeneous power device cascade H-bridge inverter according to claim 1, characterized in that: The power supply is a DC side input voltage source, and the amplitudes of all DC side input voltage sources are equal.

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