Current harmonic suppression method, inverter, and storage medium

Abstract

This application relates to the field of microgrid technology, and in particular to a current harmonic suppression method, an inverter, and a storage medium. The method includes: establishing the state equation of the grid-connected inverter based on the grid-connected inverter circuit and Kirchhoff's laws; establishing a transfer function from the disturbance voltage to the harmonic components of the grid-connected current based on the state equation; determining a calculation model for the modulation wave compensation term used to suppress the disturbance voltage based on the transfer function; extracting the harmonic components in the grid-connected current in real time, and obtaining the modulation wave compensation signal based on the calculation model of the harmonic components and the modulation wave compensation term; and superimposing the original modulation wave signal output by the current loop controller and the modulation wave compensation signal to obtain the final modulation wave signal. The method of this application can actively and accurately compensate for periodic voltage disturbances introduced by the switching of the power frequency bridge arm, thereby effectively suppressing the resulting current harmonics and zero-crossing distortion, and significantly reducing the THD of the grid-connected current.

Description

Technical Field

[0001] This application relates to the field of microgrid technology, and in particular to a current harmonic suppression method, an inverter, and a storage medium. Background Technology

[0002] Grid-connected inverters are key interface devices between renewable energy generation systems and the power grid, and their performance directly affects power quality and system efficiency. Common full-bridge grid-connected inverters typically consist of four switching transistors and an output filter (such as an LC or LCL filter). To achieve higher conversion efficiency while ensuring output waveform quality, the industry has proposed a hybrid modulation strategy: one arm of the inverter (usually called the high-frequency arm or left arm) uses high-frequency pulse width modulation (PWM), while the other arm (usually called the power frequency arm or right arm) switches to power frequency near the grid voltage zero-crossing point. This strategy can reduce the switching losses of the power frequency arm to extremely low levels, thereby significantly improving the overall efficiency of the inverter.

[0003] However, while this hybrid modulation strategy brings efficiency advantages, it also introduces a new technical problem: when the power frequency bridge arm switches at the zero-crossing point of the grid-connected current, the voltage at the midpoint of the bridge arm will experience a step jump between the positive and negative DC bus voltages. This voltage step disturbance will be transmitted to the grid side through the output filter, exciting the filter resonance and introducing significant periodic harmonic components into the grid-connected current, especially producing waveform distortion (glitch) near the current zero-crossing point. This not only worsens the total harmonic distortion (THD) of the grid-connected current but may also cause resonance problems with the grid, threatening system stability.

[0004] Traditional solutions primarily rely on increasing the inductance or capacitance parameters of the output filter to passively attenuate harmonics. However, this significantly increases the size, weight, and cost of the equipment, and slows down the system's dynamic response. Abandoning hybrid modulation and adopting full-bridge high-frequency PWM in favor of it would sacrifice its core efficiency advantage of ultra-low switching losses. Therefore, effectively suppressing specific current harmonics introduced by hybrid modulation while maintaining its ultra-low switching losses has become a pressing technical challenge in this field. Summary of the Invention

[0005] The embodiments of this application aim to provide a current harmonic suppression method, inverter and storage medium to solve the problems of high grid current harmonic content and severe zero-crossing distortion caused by hybrid modulation strategies in the prior art, thereby optimizing power quality without sacrificing efficiency.

[0006] To address the aforementioned technical problems, this application provides the following technical solutions:

[0007] According to a first aspect of this application, a current harmonic suppression method is provided, the method comprising:

[0008] Based on the grid-connected inverter circuit and Kirchhoff's laws, establish the state equation of the grid-connected inverter.

[0009] Based on the state equation, a transfer function is established from the disturbance voltage generated at the midpoint of the grid-connected inverter bridge arm to the harmonic components of the grid-connected current.

[0010] Based on the transfer function, a calculation model for the modulation wave compensation term used to suppress the disturbance voltage is determined;

[0011] Harmonic components in the grid-connected current are extracted in real time, and a modulation compensation signal is obtained based on the calculation model of the harmonic components and the modulation compensation term.

[0012] The original modulated wave signal output by the current loop controller and the modulated wave compensation signal are superimposed to obtain the final modulated wave signal.

[0013] Optionally, the grid-connected inverter includes an LCL filter composed of a filter inductor, a filter capacitor, and a grid-side inductor, and the state equation of the grid-connected inverter is:

[0014]

[0015] in, The inductance of the filter inductor, The capacitance value of the filter capacitor, The inductance of the grid-side inductor. This is the grid voltage. For the filter inductor current, The voltage at the midpoint of the bridge arm. This is the voltage across the filter capacitor. This is the grid-connected current.

[0016] Optionally, the transfer function is:

[0017]

[0018] in, The transfer function is... The impedance of the LCL filter , For grid-connected current harmonic components, The disturbance voltage is referred to here. s Let be the Laplace complex frequency variable.

[0019] Optionally, the real-time extraction of harmonic components in the grid-connected current includes:

[0020] The fundamental component of the grid-connected current is extracted in real time using a second-order generalized integrator.

[0021] The harmonic component is obtained by subtracting the fundamental component from the grid-connected current.

[0022] Optionally, the calculation model for determining the modulation wave compensation term for suppressing the disturbance voltage based on the transfer function includes:

[0023] Based on the transfer function, a calculation model is determined for the compensation voltage required to suppress the disturbance voltage;

[0024] Based on the calculation model of the compensation voltage, the calculation model of the modulation wave compensation term required to generate the compensation voltage is determined.

[0025] Optionally, the calculation model for the modulation wave compensation term is as follows:

[0026]

[0027] in, For the modulation wave compensation term, The proportionality coefficient of the modulation wave compensation term. This is the PWM modulation gain.

[0028] Optionally, the grid-connected inverter includes a first switch, a second switch, a third switch, and a fourth switch. During the positive half-cycle of the grid-connected current, the fourth switch is normally on, the third switch is normally off, and the first and second switches are high-frequency complementary conductions. During the negative half-cycle of the grid-connected current, the third switch is normally on, the fourth switch is normally off, and the first and second switches are high-frequency complementary conductions.

[0029] According to a second aspect of this application, a grid-connected inverter is provided, the grid-connected inverter including a controller, the controller including: at least one processor and a memory communicatively connected to the at least one processor, the memory storing instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to perform the method described in any of the above.

[0030] Optionally, the grid-connected inverter further includes a first switch, a second switch, a third switch, and a fourth switch. During the positive half-cycle of the grid-connected current, the fourth switch is normally on, the third switch is normally off, and the first and second switches are high-frequency complementary conductions. During the negative half-cycle of the grid-connected current, the third switch is normally on, the fourth switch is normally off, and the first and second switches are high-frequency complementary conductions.

[0031] According to a third aspect of this application, a computer-readable storage medium is provided, the computer-readable storage medium storing a computer program, which, when executed by a processor, performs the steps of any of the methods described above.

[0032] The beneficial effects of this application's embodiments are as follows: Unlike existing technologies, this application provides a current harmonic suppression method. It extracts harmonic components from the grid-connected current in real time and calculates the modulation compensation signal required to offset power frequency switching disturbances using a calculation model based on the transfer function from the disturbance voltage to the grid-connected current harmonic components. This signal is then superimposed onto the original current loop control output. This method can actively and accurately compensate for periodic voltage disturbances introduced by power frequency bridge arm switching, thereby effectively suppressing the resulting current harmonics and zero-crossing distortion, and significantly reducing the grid-connected current THD. While maintaining the advantages of ultra-low switching loss and high efficiency of hybrid modulation, this application effectively improves the quality of the grid-connected current waveform without significantly increasing filter parameters, offering advantages such as low cost, simple implementation, and good dynamic response. Attached Figure Description

[0033] One or more embodiments are illustrated by way of example with reference numerals in the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements with the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the figures in the drawings are not to be limited by scale.

[0034] Figure 1 This is a schematic diagram of the grid-connected inverter provided in the embodiments of this application;

[0035] Figure 2 This is a schematic diagram of the controller provided in an embodiment of this application;

[0036] Figure 3 This is a schematic diagram of the hybrid modulation mode driving method provided in the embodiments of this application;

[0037] Figure 4 This is a schematic diagram of a conventional grid-connected current control strategy provided in an embodiment of this application;

[0038] Figure 5 This is a flowchart of a current harmonic suppression method provided in an embodiment of this application;

[0039] Figure 6 This is a schematic diagram of a control strategy using a current harmonic suppression method provided in an embodiment of this application;

[0040] Figure 7 This is a comparison diagram of the control effects before and after inhibition provided in the embodiments of this application. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0042] Furthermore, the technical features involved in the various embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0043] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0044] Please see Figure 1 , Figure 1 This is a schematic diagram of the grid-connected inverter provided in an embodiment of this application. For example... Figure 1 The grid-connected inverter shown includes an inverter circuit 10 and a controller 20. The inverter circuit 10 includes a DC voltage input source. First switching transistor Second switching transistor Third switching transistor Fourth switching transistor Filter inductor Filter capacitor and grid-side inductor In addition, the grid-connected inverter is connected to the power grid. The inverter bridge arm midpoint voltage is expressed as .

[0045] The controller 20 is connected to the switching transistors in the inverter circuit 10. ~ The connection is based on the built-in control program that controls the switching transistor. ~ The controller 20 can be turned on and off. In some embodiments, the controller 20 may be a microcontroller unit (MCU) or a digital signal processing (DSP) controller, etc.

[0046] Please refer to Figure 2 , Figure 2 An exemplary structure of controller 20 is shown. For example... Figure 2As shown, the controller 20 includes at least one processor 21 and a memory 22. The memory 22 can be built into the controller 20 or external to the controller 20. The memory 22 can also be a remotely configured memory connected to the controller 20 via a network.

[0047] Memory 22, as a non-volatile computer-readable storage medium, can be used to store non-volatile software programs, non-volatile computer-executable programs, and modules. Memory 22 may include a program storage area and a data storage area, wherein the program storage area may store the operating system and application programs required for at least one function; the data storage area may store data created based on the use of the terminal, etc. Furthermore, memory 22 may include high-speed random access memory and may also include non-volatile memory, such as at least one disk storage device, flash memory device, or other non-volatile solid-state storage device. In some embodiments, memory 22 may optionally include memory remotely located relative to processor 21, and these remote memories can be connected to the terminal via a network. Examples of such networks include, but are not limited to, the Internet, corporate intranets, local area networks, mobile communication networks, and combinations thereof.

[0048] The processor 21 performs various functions of the terminal and processes data by running or executing software programs and / or modules stored in the memory 22 and calling data stored in the memory 22, thereby performing overall monitoring of the terminal, such as implementing the current harmonic suppression method described in any embodiment of this application.

[0049] Processor 21 can be one or more. Figure 2 The example provided is a processor 21. Processor 21 and memory 22 can be connected via a bus or other means. Processor 21 may include a central processing unit (CPU), a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a controller, a field-programmable gate array (FPGA) device, etc. Processor 21 can also be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors combined with a DSP core, or any other such configuration.

[0050] Please refer to Figure 3 , Figure 3 This is a schematic diagram of the hybrid modulation mode driving method provided in an embodiment of this application. For example... Figure 3 As shown, during the positive half-cycle of the grid-connected current, the fourth switching transistor... Normally on, third switching transistor Normally off, first switching transistor Second switching transistor High-frequency complementary conduction; during the negative half-cycle of the grid-connected current, the third switching transistor... Normally on, fourth switching transistor Normally off, first switching transistor Second switching transistor High-frequency complementary conduction.

[0051] Please refer to Figure 4 , Figure 4 This is a schematic diagram of a conventional grid-connected current control strategy provided in an embodiment of this application. For example... Figure 4 As shown, the grid-connected current reference value Inverter grid-connected current sampling value The difference is then fed into the current loop controller. , to obtain the modulated wave Modulated wave The first switching transistor is generated after comparison with the triangular carrier wave. Or the second switching transistor The high-frequency drive signal is used to obtain the midpoint voltage of the inverter bridge arm. Inverter bridge arm midpoint voltage With the voltage of the filter capacitor The voltage of the filter inductor is obtained by subtraction. Voltage of the filter inductor With filter inductor reactance Divide the two to obtain the current of the filter inductor. The current of the filter inductor With the current of the filter capacitor The difference is used to obtain the grid-connected current. .

[0052] Please refer to Figure 5 , Figure 5 This is a flowchart illustrating a current harmonic suppression method provided in an embodiment of this application. The method is applied to a grid-connected inverter. The grid-connected inverter may include an inverter circuit and a controller, etc. In some embodiments, the grid-connected inverter can... Figures 1-2 The implementation of the structure is described in detail in the above embodiments and will not be repeated here.

[0053] like Figure 5 As shown, the current harmonic suppression method includes:

[0054] Step S501: Based on the grid-connected inverter circuit and Kirchhoff's laws, establish the state equations of the grid-connected inverter.

[0055] by Figure 1 Taking the grid-connected inverter shown as an example, according to Kirchhoff's voltage and current laws, the state equation of this grid-connected inverter is:

[0056] (1)

[0057] in, For the inductance of the filter inductor, The capacitance value of the filter capacitor, The inductance of the grid-side inductor. This is the grid voltage. For the filter inductor current, The voltage at the midpoint of the bridge arm. This is the voltage across the filter capacitor. This is the grid-connected current.

[0058] Step S502: Based on the state equation, establish the transfer function from the disturbance voltage generated at the midpoint of the grid-connected inverter bridge arm to the harmonic component of the grid-connected current.

[0059] like Figure 3 As shown, when the grid-connected inverter adopts hybrid modulation mode, the right bridge arm switches to power frequency near the zero-crossing point of the grid-connected current, causing the midpoint voltage of the bridge arm to change. exist The abrupt changes between these phases introduce discrete, periodic harmonic interference at the input port of the LCL filter, causing the grid-connected current to... Distortion spikes appear near the zero crossing point, and harmonic components increase.

[0060] Therefore, the grid-connected current can be Rewritten in the form of a superposition of fundamental and harmonic components:

[0061] (2)

[0062] in, For grid-connected current The fundamental component of For grid-connected current Harmonic components, s Let be the Laplace complex frequency variable.

[0063] Currently, commonly used current harmonic suppression control methods employ improved control strategies based on dual-loop control, such as PI + repetitive control and PI + resonant controllers. These common improved control strategies present a trade-off between response speed and steady-state accuracy: repetitive control has high latency and slow dynamic response; multi-resonant controllers are sensitive to parameters, experience gain degradation at high frequencies, and have limited compensation effects. Furthermore, to effectively suppress all major harmonics, multiple resonant controllers need to be connected in parallel, placing high demands on the processor's computational power.

[0064] From a principle perspective, when the right arm of the inverter switches at the zero-crossing point of the grid current, the ideal output voltage of the inverter arm is:

[0065] (3)

[0066] The actual output voltage of the bridge arm exist The step transition occurs between these two points. Fourier series decomposition and spectral analysis are then performed on the step signal.

[0067] (4)

[0068] in, h The harmonic order is (the above formula only includes odd harmonics). This refers to the disturbance voltage introduced by the switching, which corresponds to the sum of all harmonic components. It is a wide-spectrum periodic disturbance source that will excite harmonic currents of the corresponding frequency through the LCL filter.

[0069] Since only harmonic components are of concern, the fundamental frequency circuit can be ignored when analyzing the harmonic propagation path, and the ideal output voltage can be used as a reference. and grid voltage All are considered to be 0, meaning the voltage applied to the input of the LCL filter is only the disturbance voltage. .

[0070] Assumption After performing a Laplace transform on the state equation of equation (1), the transfer function from the disturbance voltage to the harmonic components of the grid-connected current can be obtained. :

[0071] (5)

[0072] in, The impedance of the LCL filter , For grid-connected current harmonic components, This is the disturbance voltage.

[0073] Step S503: Based on the transfer function, determine the calculation model for the modulation wave compensation term used to suppress disturbance voltage.

[0074] In one embodiment, a modulation compensation signal is injected into the original modulation signal output by the grid-connected inverter current loop controller to suppress disturbance voltage. This modulation compensation signal, after being modulated by a PWM modulation stage, yields the compensation voltage for the inverter arm. :

[0075] (6)

[0076] in, For modulation wave compensation term, For PWM modulation gain, This represents the amplitude of the PWM triangular carrier wave.

[0077] Under the action of the compensation voltage, the total disturbance voltage of the inverter bridge arm It becomes:

[0078] (7)

[0079] To minimize the impact of the total disturbance voltage on the harmonic current, the total disturbance voltage must be zero. Therefore:

[0080] (8)

[0081] Disturbance voltage introduced by switch switching It cannot be measured directly, but it can be deduced by back-calculation using equation (5):

[0082] (9)

[0083] Combining equation (8), the calculation model for the compensation voltage is obtained:

[0084] (10)

[0085] In an LCL filter, the filter inductor is typically in the microhenry level, and the filter capacitor is typically in the microfarad level. Therefore, the impedance of an LCL filter... It can be simplified to:

[0086] (11)

[0087] Combining equations (6), (10), and (11), the calculation model for the modulation wave compensation term can be obtained:

[0088] (12)

[0089] in, This is the proportional coefficient of the modulation wave compensation term, used to limit the modulation wave compensation range and avoid overmodulation of the high-frequency bridge arm.

[0090] Step S504: Extract the harmonic components in the grid-connected current in real time, and obtain the modulation compensation signal based on the calculation model of the harmonic components and the modulation compensation term.

[0091] The harmonic components in the grid-connected current contain the sum of multiple harmonic components, making direct extraction difficult. In one embodiment, the fundamental component of the grid-connected current is first extracted in real time using a second-order generalized integrator (SOGI).

[0092] (13)

[0093] (14)

[0094] in, For SOGI's transfer function, The angular frequency of the power frequency component of the grid-connected current. This is the SOGI gain coefficient.

[0095] The harmonic components in the grid-connected current for:

[0096] (15)

[0097] After obtaining the harmonic components, the modulated wave compensation signal can be calculated based on the calculation model of the modulation wave compensation term.

[0098] In step S505, the original modulation wave signal and the modulation wave compensation signal output by the current loop controller are superimposed to obtain the final modulation wave signal.

[0099] After introducing the modulation compensation term into the control loop, the final modulation signal for:

[0100] (16)

[0101] in, The original modulated wave signal output by the grid-connected current closed-loop control. This is the modulated wave compensation signal.

[0102] Please refer to Figure 6 , Figure 6 This is a schematic diagram of a control strategy employing a current harmonic suppression method provided in an embodiment of this application. Based on Figure 5 Methods for obtaining modulated wave compensation signals Then, the modulated wave compensation signal With current loop controller The original modulated wave signal output The sum is compared with a triangular carrier wave to generate the first switching transistor. Or the second switching transistor The high-frequency drive signal is used to compensate for the current harmonics caused by the power frequency switching of the power frequency bridge arm while maintaining the ultra-low switching loss of the right bridge arm, thereby optimizing the grid-connected current waveform quality.

[0103] Please refer to Figure 7 , Figure 7 This is a comparison diagram of the control effects before and after inhibition provided in the embodiments of this application. Figure 7 It can be seen that after adopting the suppression strategy provided in this application, the harmonic components in the grid-connected current are significantly reduced compared with the case without the suppression strategy, thus optimizing the waveform quality of the grid-connected current.

[0104] The current harmonic suppression method provided in this application extracts harmonic components from the grid-connected current in real time. It then calculates the modulation compensation signal required to offset power frequency switching disturbances using a calculation model based on the transfer function from the disturbance voltage to the grid-connected current harmonic components. This signal is then superimposed onto the original current loop control output. This method actively and accurately compensates for periodic voltage disturbances introduced by power frequency bridge arm switching, effectively suppressing the resulting current harmonics and zero-crossing distortion, and significantly reducing the THD of the grid-connected current. While maintaining the advantages of ultra-low switching loss and high efficiency of hybrid modulation, this application effectively improves the waveform quality of the grid-connected current without significantly increasing filter parameters, offering advantages such as low cost, simple implementation, and good dynamic response.

[0105] This application also provides a non-volatile computer-readable storage medium storing computer-executable instructions that are executed by one or more processors, for example, to perform the method steps described above.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them; under the concept of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of this application as described above, which are not provided in detail for the sake of brevity; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method of current harmonic suppression, characterized by, The method includes: Based on the grid-connected inverter circuit and Kirchhoff's laws, establish the state equation of the grid-connected inverter. Based on the state equation, a transfer function is established from the disturbance voltage generated at the midpoint of the grid-connected inverter arm due to the use of a hybrid modulation strategy to the harmonic components of the grid-connected current. Based on the transfer function, a calculation model for the modulation wave compensation term used to suppress the disturbance voltage is determined; Harmonic components in the grid-connected current are extracted in real time, and a modulation compensation signal is obtained based on the calculation model of the harmonic components and the modulation compensation term. The original modulated wave signal output by the current loop controller and the modulated wave compensation signal are superimposed to obtain the final modulated wave signal. The grid-connected inverter includes an LCL filter composed of a filter inductor, a filter capacitor, and a grid-side inductor, and the transfer function is: in, The transfer function is... The impedance of the LCL filter , For grid-connected current harmonic components, The disturbance voltage is referred to here. s Let Laplace be the complex frequency variable. The inductance of the filter inductor, The capacitance value of the filter capacitor, The inductance of the grid-side inductor; The calculation model for the modulation wave compensation term is as follows: wherein is the modulation wave compensation term, is a proportional factor of the modulation wave compensation term, is a PWM modulation gain.

2. The method of claim 1, wherein, The state equation of the grid-connected inverter is: wherein, is the grid voltage, is the filter inductor current, is the bridge leg midpoint voltage, is the filter capacitor voltage, is the grid current.

3. The method of claim 1, wherein, The real-time extraction of harmonic components from the grid-connected current includes: The fundamental component of the grid-connected current is extracted in real time using a second-order generalized integrator. The harmonic component is obtained by subtracting the fundamental component from the grid-connected current.

4. The method of claim 1, wherein, The calculation model for determining the modulation wave compensation term for suppressing the disturbance voltage based on the transfer function includes: Based on the transfer function, a calculation model is determined for the compensation voltage required to suppress the disturbance voltage; Based on the calculation model of the compensation voltage, the calculation model of the modulation wave compensation term required to generate the compensation voltage is determined.

5. The method according to any one of claims 1 to 4, characterized in that, The grid-connected inverter includes a first switch, a second switch, a third switch, and a fourth switch. During the positive half-cycle of the grid-connected current, the fourth switch is normally on, the third switch is normally off, and the first and second switches conduct in a complementary manner at high frequency. During the negative half-cycle of the grid-connected current, the third switch is normally on, the fourth switch is normally off, and the first and second switches conduct in a complementary manner at high frequency.

6. A grid-connected inverter, characterized by The grid-connected inverter includes a controller, the controller comprising: at least one processor and a memory communicatively connected to the at least one processor, the memory storing instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to perform the method according to any one of claims 1 to 4.

7. The grid-tied inverter of claim 6, wherein, The grid-connected inverter also includes a first switch, a second switch, a third switch, and a fourth switch. During the positive half-cycle of the grid-connected current, the fourth switch is normally on, the third switch is normally off, and the first and second switches conduct in a high-frequency complementary manner. During the negative half-cycle of the grid-connected current, the third switch is normally on, the fourth switch is normally off, and the first and second switches conduct in a high-frequency complementary manner.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, performs the steps of the method as described in any one of claims 1-5.

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