KR102032869B1 - Power Supply Apparatus including DC-AC Inverter and Method of Controlling the same - Google Patents

Abstract

In accordance with an aspect of the present invention, there is provided a power control method including a DC-AC inverter, the method comprising: inputting DC power to input terminals of a plurality of DC-AC inverters in a switching mode connected in series with each other; And a plurality of controls independent of each other from the control unit to the plurality of DC-AC inverters so as to independently control outputs of the plurality of DC-AC inverters based on load sizes connected to output terminals of the plurality of DC-AC inverters. Outputting the signals sequentially.

Description

Power Supply Apparatus including DC-AC Inverter and Method of Controlling the same}

The present invention relates to a power supply device, and more particularly, to a power supply device and a power control method including a DC-AC inverter.

In order to improve the power conversion efficiency of the high frequency DC-AC inverter and reduce the size of the output filter design, a plurality of series-connected DC-AC inverters are widely used. Flexibility in system design is possible.

However, as shown in FIG. 5, in the case of performing pulse width modulation (PWM) control to have the same output in the inverter connected in series, all DC-AC inverters even in a low load region in which all inverters do not need to be operated. Operation could cause unnecessary switching losses in the power semiconductor.

The present invention has been made to meet the above-described needs, and according to the present invention, an object of the present invention is to independently control a plurality of DC-AC inverter outputs to reduce switching losses. However, these problems are exemplary, and the scope of the present invention is not limited thereby.

In accordance with another aspect of the present invention, there is provided a method of controlling a power supply of a DC-AC inverter, the method comprising: inputting DC power to input terminals of a plurality of DC-AC inverters of a switching mode connected in series with each other; And a plurality of controls independent of each other from the control unit to the plurality of DC-AC inverters so as to independently control outputs of the plurality of DC-AC inverters based on load sizes connected to output terminals of the plurality of DC-AC inverters. Outputting the signals sequentially.

In this case, the control signal may include a pulse width modulation (PWM) signal generated by phase shifting a basic waveform signal.

The sequentially outputting of the plurality of control signals may include outputting a control signal having a phase shift different from that of the other DC-AC inverters to at least one DC-AC inverter of the plurality of DC-AC inverters. It may include.

The sequentially outputting of the plurality of control signals may include outputting a control signal for outputting at least one DC-AC inverter of the plurality of DC-AC inverters to zero.

Further, in the step of sequentially outputting the plurality of control signals, the control unit may output the control signal so that the voltage output from the plurality of DC-AC inverter is transformed from a pulse waveform to a sinusoidal wave like waveform.

In addition, the sinusoidal wave like waveform may have a symmetrical peak, the lowest point and at least one intermediate point between the highest point and the lowest point.

In addition, each of the plurality of DC-AC inverters comprises a plurality of switching elements; And a transformer connected to the plurality of switching elements and receiving a voltage according to the switching of the plurality of switching elements, wherein the plurality of switching elements may be switched based on the control signal.

On the other hand, a power supply device including a DC-AC inverter according to an embodiment of the present invention includes a plurality of DC-AC inverters of the switching mode connected in series with each other to receive DC power from the power source and output the AC power to the output terminal; And a plurality of control signals independent of each other by the plurality of DC-AC inverters to independently control the outputs of the plurality of DC-AC inverters based on a load size connected to the output terminals of the plurality of DC-AC inverters. It may include a control unit for outputting sequentially.

In this case, the control signal may include a pulse width modulation (PWM) signal.

The control unit may output the control signal such that voltages output from the plurality of DC-AC inverters are transformed from a pulse waveform into a sinusoidal wave like waveform.

In addition, the sinusoidal wave like waveform may have a symmetrical peak, the lowest point and at least one intermediate point between the highest point and the lowest point.

In addition, each of the plurality of DC-AC inverters comprises a plurality of switching elements; And a transformer connected to the plurality of switching elements and receiving a voltage according to the switching of the plurality of switching elements, wherein the plurality of switching elements may be switched based on the control signal.

In addition, output terminals of the plurality of DC-AC inverters may be connected to a chamber of a remote plasma generator (RPG), and the controller may sequentially output the plurality of control signals to generate plasma to the chamber.

The power supply and control method of the DC-AC inverter according to the embodiment of the present invention made as described above independently controls the output of the plurality of DC-AC inverter, reducing the switching loss in the light load region, The conversion efficiency can be improved. In addition, the size of the filter design can be reduced because the output voltage of the DC-AC inverter can be transformed into a sine wave-like waveform so that the component can be made relatively larger than the existing waveform. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart of a power control method including a DC-AC inverter according to an embodiment of the present invention.
2 is a circuit diagram of a power supply device including a DC-AC inverter according to an embodiment of the present invention.
3 is an illustration of a control signal waveform for a power supply including a DC-AC inverter according to an embodiment of the present invention.
4 is a circuit diagram illustrating a current conduction path for a portion of an output waveform of a power supply including a DC-AC inverter according to an embodiment of the present invention.
5 is a signal waveform of a transformer primary side voltage and a secondary side voltage of a power supply including a DC-AC inverter.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in various forms, and the following embodiments are intended to complete the disclosure of the present invention, the scope of the invention to those skilled in the art It is provided to inform you completely. In addition, for convenience of description, the components may be exaggerated or reduced in size.

In addition, the following embodiments are provided to those skilled in the art to fully understand the present invention can be modified in various forms, the scope of the present invention is limited to the embodiments described below It doesn't happen.

In addition, the terms including or having in the following embodiments means that there is a feature or component described in the specification, and does not preclude the possibility of adding one or more other features or components. .

Hereinafter, a power supply device 100 including a DC-AC inverter according to an embodiment of the present invention will be described in detail with reference to FIG. 2. 2 is a circuit diagram of a power supply device 100 including a DC-AC inverter according to an embodiment of the present invention.

Referring to FIG. 2, the power supply device 100 may include a plurality of DC-AC inverters 110a, 110b, and 110c and a controller 150. The plurality of DC-AC inverters 110a, 110b, and 110c may receive DC power and output AC power to an output terminal.

For example, the plurality of DC-AC inverters 110a, 110b, and 110c may operate in a switching mode. The plurality of DC-AC inverters 110a, 110b and 110c may be connected in series with each other, and the output terminals of the plurality of DC-AC inverters 110a, 110b and 110c may be connected to the load 140. Each of the DC-AC inverters 110a, 110b, and 110c may include a plurality of switching elements 120a, 120b, 120c, and 120d and a transformer 130. The transformer 130 may be connected to the plurality of switching elements 120a, 120b, 120c, and 120d to receive a voltage according to the switching of the plurality of switching elements 120a, 120b, 120c, and 120d.

The plurality of switching elements 120a, 120b, 120c, and 120d in each of the DC-AC inverters 110a, 110b, and 110c may be switched based on a control signal input from the controller 150, for example, illustrated in FIG. 2. As described above, the power transistor may include four power transistors (TR).

For example, the first DC-AC inverter 110a may include the first switching elements 120a and S _{1 and M1} , the second switching elements 120b and S _{2 and M1} , and the third switching elements 120c and S _{3 and M1} . And fourth switching elements 120d, S _{4, and M1} . In this case, a series connection structure of the first switching elements 120a, S _{1, M1} and the fourth switching elements 120d, S _{4, M1} , the second switching elements 120b, S _{2, M1} , and the third switching element The series connection structure of 120c, S _{3, M1} may be connected in parallel with each other. The transformer 130 in the first DC-AC inverter 110a is connected to intermediate nodes of two series connection structures such that the first to fourth switching elements S _{1, M1} , S _{2, M1} , S _{3, M1,.} The output voltage V _INV1 may be input from S _{4, M1} ).

Similarly, the n-th DC-AC inverter 110c may include the first switching elements 120a, S _{1, MN} , the second switching elements 120b, S _{2, MN} , and the third switching elements 120c, S _{3, MN} . And a fourth switching element 120d, S _{4, MN} . In this case, a series connection structure of the first switching elements 120a, S _{1, MN} and the fourth switching elements 120d, S _{4, MN} , the second switching elements 120b, S _{2, MN} , and the third switching element The series connection structure of 120c, S _{3, MN} may be connected in parallel with each other. The transformer 130 may be connected to intermediate nodes of two series connection structures to receive the output voltage V _INVN from the first to fourth switching elements 120a, 120b, 120c, and 120d. The transformer 130 in the n-th DC-AC inverter 110c is connected to intermediate nodes of two series connection structures such that the first to fourth switching elements S _{1, MN} , S _{2, MN} , S _{3, MN,.} S _{4, MN} ) can receive the output voltage (V _INVN ).

The plurality of transformers 130 connected to the plurality of DC-AC inverters 110a, 110b, and 110c are connected in series to each other, and the voltage signal V _{TR_S to which the} secondary voltage signals are _summed after boosting the input primary voltage signal. ) May be output to the load 140.

The controller 150 independently controls the outputs of the plurality of DC-AC inverters 110a, 110b and 110c based on the size of the load 140 connected to the output terminals of the plurality of DC-AC inverters 110a, 110b and 110c. can do.

In more detail, the controller 150 may perform control so that control signals of the plurality of DC-AC inverters 110a, 110b, and 110c may sequentially transmit energy to the transformer secondary side according to the load 140. have. The controller 150 may sequentially output a plurality of control signals independent of each other to the plurality of DC-AC inverters 110a, 110b, and 110c. The control signal may include a pulse width modulation (PWM) signal, and preferably, may include a pulse width modulation (PWM) signal generated by phase shifting a basic waveform signal.

For example, the controller 150 may output a control signal having a phase shift different from that of the other DC-AC inverters to at least one DC-AC inverter of the plurality of DC-AC inverters 110a, 110b, and 110c. . In more detail, the controller 150 may output control signals having different phase shifts to at least one DC-AC inverter instead of all the same control signals to the plurality of DC-AC inverters.

As another example, the controller 150 may output a control signal for the output of at least one DC-AC inverter of the plurality of DC-AC inverters 110a, 110b, and 110c to be zero.

In addition, the controller 150 may output a control signal such that voltages output from the plurality of DC-AC inverters 110a, 110b, and 110c are transformed from a pulse waveform into a sinusoidal wave like waveform. The voltages output from the plurality of DC-AC inverters 110a, 110b, and 110c may refer to transformer secondary side voltages.

For example, the sine wave-like waveform may refer to a waveform that is not the same as the sine wave but is modulated to be more similar to the sine wave in the basic pulse waveform. More specifically, the sine wave-like waveform may refer to a waveform capable of symmetrically having at least one midpoint between the highest point, the lowest point, and the highest point and the lowest point. In this case, the waveform similar to the sine wave can bring the components larger than the existing waveform, thereby reducing the size of the output filter design.

The power supply device 100 described above may be applied to various electronic devices. For example, the power supply device 100 may be used as a power supply device of a remote plasma generator (RPG) among semiconductor devices. In this case, output terminals of the plurality of DC-AC inverters 110a, 110b, and 110c may be connected to a chamber of the remote plasma generator RPG, and the controller may sequentially output a plurality of control signals to generate plasma to the chamber. .

3A and 3B are examples of control signal waveforms for a power supply including a DC-AC inverter according to an embodiment of the present invention. 3A and 3B, V _{GS1 and MN} denote control signals input to the first switching element 120a in the n-th DC-AC inverter 120c, and V _{GS2 and MN} denote the second switching element 120b. Means a control signal input to V _{GS3, MN} means a control signal input to the third switching element (120c), V _{GS4, MN} means a control signal input to the fourth switching element (120d). . In each inverter, the control signals (V _{GS1, MN} , V _{GS2, MN} , V _{GS3, MN} , V _{GS4, MN} ) are shown with + pulse waveforms without shades and-pulse waveforms with shades. In addition, the sum output signal V _TRS output to the load is illustrated as a simple sum of the primary voltages of the transformer 130 without considering the voltage boost of the transformer 130 for convenience.

3A and 3B, the controller 150 differently outputs control signals to the plurality of switching elements 120a, 120b, 120c, and 120d of the plurality of DC-AC inverters 110a, 110b, and 110c, respectively. The plurality of switching elements 120a, 120b, 120c, and 120d may be switched. 3A shows an example of outputting a low voltage, and FIG. 3B shows an example of outputting a high voltage.

Although a low voltage or a high voltage has been described as an example in FIGS. 3A and 3B, this is only an example, and those skilled in the art may use various embodiments.

Meanwhile, the first switching element 120a, the fourth switching element 120d, the second switching element 120b, and the third switching element 120c connected in series in the plurality of switching elements 120a, 120b, 120c, and 120d, respectively. ) Is switched based on a control signal, and may be sequentially supplied with a positive voltage or a negative voltage according to a predetermined time interval.

Referring to FIG. 3A, the controller 150 applies a control signal V _{GS1, M1} , V _{GS2, M1} , V _{GS3, M1} , V _{GS4, M1} without a phase shift to the first DC-AC inverter 110a. The control signal V _{GS1, M2} , V _{GS2, M2} , V _{GS3, M2} , V _{GS4, M2} , which performs the phase shift of the first interval, is applied to the second DC-AC inverter 110b and the third DC is applied. The control signals V _{GS1, M3} , V _{GS2, M3} , V _{GS3, M3} , V _{GS4, and M3} performing the phase shift of the second interval may be applied to the AC inverter 110c.

More specifically, the transformer primary side voltage pulse waveform of the first DC-AC inverter 110a is generated with a first pulse width 200, and the transformer primary side voltage pulse waveform of the second DC-AC inverter 110b is formed with a first pulse width 200. Two pulse widths 210 are generated, and the transformer primary voltage pulse waveform of the third DC-AC inverter 110c may not generate pulse widths.

The control signals applied to the plurality of switching elements 120a, 120b, 120c, and 120d of the third DC-AC inverter 110c add up the voltages of the plurality of switching elements 120a, 120b, 120c, and 120d due to the phase shift. The transformer primary voltage may be zero, resulting in no pulse width.

The summation output signal V _TRS output to the load may appear in the form of the voltage pulse waveform of the transformer of the second DC-AC inverter 110b being summed on the voltage pulse waveform of the transformer of the first DC-AC inverter 110a. Accordingly, the output signal VTRS may have a sinusoidal wave like waveform having a midpoint in addition to the highest point and the lowest peak.

As such, when the output voltage of the power supply device 100 is low, only a part of the DC-AC inverter applies the control signal to the plurality of DC-AC inverters 110a, 110b, and 110c without applying the same control signal. It can work. Meanwhile, in this example, only three DC-AC inverters are operated and controlled, and a person skilled in the art may dynamically change the number of operations of the DC-AC inverter.

Referring to FIG. 3B, the controller 150 applies the control signals V _{GS1, M1} , V _{GS2, M1} , V _{GS3, M1} , V _{GS4, M1} to the first DC-AC inverter 110a without phase shift. The control signal V _{GS1, M2} , V _{GS2, M2} , V _{GS3, M2} , V _{GS4, M2} is also applied to the second DC-AC inverter 110b without the phase shift, and the n-th DC-AC inverter 110c is applied. ), The control signals V _{GS1, M3} , V _{GS2, M3} , V _{GS3, M3} , V _{GS4, and M3} may be applied. Although not shown in the figure, a control signal may be applied to the third to n-1 DC-AC inverters without a phase shift like the first and second DC-AC inverters 110a and 110b.

In more detail, the transformer primary voltage pulse waveforms of the first DC-AC inverter 110a and the second DC-AC inverter 110b are each generated with a third pulse width 220 and an n-th DC-AC inverter ( The transformer primary voltage pulse waveform of 110c may generate a fourth pulse width 230. The first pulse width 200 and the third pulse width 220 may be the same.

The sum output signal V _TRS output to the load may be equal to the sum of the pulse waveforms of the transformer primary side voltage of the n-th DC-AC inverter 110c to the pulse waveforms of the first to n-1 inverters having the same waveform. Can be. Accordingly, the peak waveform may be generated through the voltage pulse waveform of the transformer of the n-th DC-AC inverter 110c, thereby generating a pulse waveform similar to the sine wave.

FIG. 4 is a circuit diagram illustrating a current conduction path for a portion of an output waveform of a power supply apparatus according to an exemplary embodiment of the present invention. Referring to FIG. 4, a plurality of circuits each generated in a section from t0 to t5 of FIG. The switching elements 120a, 120b, 120c, and 120d will be described.

For example, looking at the interval between to and t1, in the first DC-AC inverter 110a, a positive voltage is applied to the first switching element 120a and the second switching element 120b so that the first switching element 120a is applied. ) And only the second switching element 120b may be turned on, and the third switching element 120c and the fourth switching element 120d may be turned off.

In the second DC-AC inverter 110b and the third DC-AC inverter 110c, a positive voltage is applied to the second switching element 120b and a negative voltage is applied to the fourth switching element 120d, respectively. Only the second switching element 120b and the fourth switching element 120d may be turned on, and the first switching element 120a and the third switching element 120c may be turned off.

The sections between t1 and t2 are also driven in the same operation as the sections between t0 and t1, and when looking at the sections between t2 and t3, the first DC-AC inverter 110a and the second DC-AC inverter 110b ), The positive voltage is applied to the first switching element 120a and the second switching element 120b, respectively, so that only the first switching element 120a and the second switching element 120b are turned on and the third switching element 120c is And the fourth switching device 120d may be turned off.

In addition, in the third DC-AC inverter 110c, a negative voltage is applied to the fourth switching device 120d and a positive voltage is applied to the second switching device 120b, so that the second switching device 120b and the fourth switching device 120b are applied. Only the switching element 120d may be turned on, and the first switching element 120a and the third switching element 120c may be turned off.

Referring to the section between t3 and t4, a positive voltage is applied to the first switching element 120a and the second switching element 120b in the first DC-AC inverter 110a and the second DC-AC inverter 110b, respectively. In addition, only the first switching element 120a and the second switching element 120b may be turned on, and the third switching element 120c and the fourth switching element 120d may be turned off.

In the third DC-AC inverter 110c, a positive voltage is applied to the first switching element 120a and a negative voltage is applied to the third switching element 120c, so that the first switching element 120a and the third switching element are applied. Only 120c may be turned on, and the second switching element 120b and the fourth switching element 120d may be turned off.

Looking at the interval between t4 ~ t5, in the first DC-AC inverter 110a and the second DC-AC inverter 110b, a positive voltage is applied to the first switching element 120a and the second switching element 120b. In addition, only the first switching element 120a and the second switching element 120b may be turned on, and the third switching element 120c and the fourth switching element 120d may be turned off.

In the second DC-AC inverter 110b and the third DC-AC inverter 110c, a positive voltage is applied to the first switching element 120a and a negative voltage is applied to the third switching element 120c, so that the first switching is performed. Only the device 120a and the third switching device 120c may be turned on, and the second switching device 120b and the fourth switching device 120d may be turned off.

1 is a flowchart illustrating a power control method including a DC-AC inverter.

Referring to FIG. 1, DC power may be input to input terminals of a plurality of DC-AC inverters 110a, 110b, and 110c in a switching mode connected in series with each other (step S100).

In more detail, when the plurality of switching elements 120a, 120b, 120c, and 120d of the plurality of DC-AC inverters 110a, 110b, and 110c are switched modes, the plurality of DC-AC inverters 110a, 110b, 110c) may receive DC power through an input terminal.

The plurality of DC-AC inverters 110a, 110b, and 110c receive DC power and sequentially control the plurality of control signals independent of each other to the plurality of switching elements 120a, 120b, 120c, and 120d through the controller 150. You can print (Step S110)

More specifically, the controller 150 outputs the plurality of DC-AC inverters 110a, 110b and 110c based on the size of the load 140 connected to the output terminals of the plurality of DC-AC inverters 110a, 110b and 110c. Can be controlled independently.

The control signal may include a pulse width modulation (PWM) signal generated by phase shifting the basic waveform signal. The controller 150 may output a control signal having a phase shift different from that of the remaining DC-AC inverters to at least one DC-AC inverter of the plurality of DC-AC inverters 110a, 110b, and 110c. For example, it is also possible to output a control signal such that the output of at least one DC-AC inverter of the plurality of DC-AC inverters 110a, 110b, 110c becomes zero.

The controller 150 may output a control signal such that voltages output from the plurality of DC-AC inverters 110a, 110b, and 110c are transformed from a pulse waveform into a sinusoidal waveform.

Subsequently, the plurality of switching elements 120a, 120b, 120c, and 120d included in each of the plurality of DC-AC inverters 110a, 110b, and 110c may be switched based on the control signal (step S120).

The plurality of switching elements 120a, 120b, 120c, and 120d may receive a positive voltage or a negative voltage according to a switching of which the switch is turned on or off based on a control signal. The DC-AC inverter may generate the transformer primary side voltage by adding voltages of the plurality of switching elements 120a, 120b, 120c, and 120d, respectively.

Subsequently, AC power may be output to the output terminals of the plurality of DC-AC inverters 110a, 110b, and 110c (step S130).

In more detail, AC power may be output to the load 140 connected to the output terminals of the plurality of DC-AC inverters 110a, 110b, and 110c.

According to the power supply apparatus and the control method according to the present invention described above, by controlling the output of the plurality of DC-AC inverter independently, it is possible to reduce the switching loss in the light load region, and improve the power conversion efficiency. In addition, since the output voltage of the DC-AC inverter can be transformed into a sinusoidal wave-like waveform, the component can be made relatively larger than the existing waveform, thereby reducing the filter design size.

In the detailed description and the accompanying drawings of the present invention, specific embodiments have been described, but the present invention is not limited to the disclosed embodiments and does not depart from the technical spirit of the present invention for those skilled in the art. Various substitutions, modifications and variations are possible within the scope. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be construed as including not only the claims below but also equivalents thereof.

110a, 110b, 110c: multiple DC-AC inverters
120a, 120b, 120c, 120d: plurality of switching elements
130: transformer
140: load
150: control unit

Claims (13)

Inputting a DC power source into input terminals of a plurality of DC-AC inverters of a switching mode connected in series with each other; And
A plurality of control signals independent of each other from the control unit to the plurality of DC-AC inverters so as to independently control the outputs of the plurality of DC-AC inverters based on the load magnitudes connected to the output terminals of the plurality of DC-AC inverters Outputting sequentially;
Output terminals of the plurality of DC-AC inverters are connected to a chamber of a remote plasma generator (RPG),
The control unit sequentially outputs the plurality of control signals to generate plasma in the chamber,
The control signal includes a pulse width modulation (PWM) signal generated by phase shifting a basic waveform signal,
The outputting of the plurality of control signals sequentially includes outputting a control signal having a phase shift different from that of the other DC-AC inverters to at least one DC-AC inverter of the plurality of DC-AC inverters. And outputting a control signal for outputting at least one DC-AC inverter of the plurality of DC-AC inverters to zero when the output voltage required by the load is lower than a predetermined magnitude.
Power control method of DC-AC inverter. delete delete delete The method of claim 1,
In the step of sequentially outputting the plurality of control signals, the control unit outputs the control signal so that the voltage output from the plurality of DC-AC inverter is transformed from a pulse waveform to a sinusoidal wave like waveform, the power supply of the DC-AC inverter Control method. The method of claim 5,
And the sinusoidal wave like waveform has symmetrical peaks, troughs and at least one midpoint between the peaks and the troughs. The method of claim 1,
Each of the plurality of DC-AC inverters
A plurality of switching elements; And
And a transformer connected to the plurality of switching elements and receiving a voltage according to the switching of the plurality of switching elements.
And the plurality of switching elements are switched based on the control signal. A plurality of DC-AC inverters connected in series with each other so as to receive DC power from a power source and output AC power to an output terminal; And
The plurality of DC-AC inverters sequentially control a plurality of control signals independent of each other to independently control the outputs of the plurality of DC-AC inverters based on a load size connected to the output terminals of the plurality of DC-AC inverters. And a control unit for outputting the
Output terminals of the plurality of DC-AC inverters are connected to a chamber of a remote plasma generator (RPG),
The control unit sequentially outputs the plurality of control signals to generate plasma in the chamber,
The control signal includes a pulse width modulation (PWM) signal generated by phase shifting a basic waveform signal,
The control unit may output a control signal having a phase shift different from that of the other DC-AC inverters to at least one DC-AC inverter of the plurality of DC-AC inverters, and the output voltage required by the load may be lower than a predetermined magnitude. In the case of outputting a control signal such that the output of at least one DC-AC inverter of the plurality of DC-AC inverter is 0,
Power supply comprising a DC-AC inverter. delete The method of claim 8,
The controller includes a DC-AC inverter for outputting the control signal so that the voltage output from the plurality of DC-AC inverter is transformed from a pulse waveform to a sinusoidal wave like waveform. The method of claim 10,
And the sinusoidal wave like waveform has a highest point, a lowest point, and at least one midpoint between the highest point and the lowest point symmetrically. The method of claim 8,
Each of the plurality of DC-AC inverters
A plurality of switching elements; And
And a transformer connected to the plurality of switching elements and receiving a voltage according to the switching of the plurality of switching elements.
And the plurality of switching elements are switched based on the control signal. delete

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