Abstract:
In some aspects, a direct current power source, formed of a series connection circuit of single power sources, which has three mutually different voltage levels including zero can be provided with first, second, third, and fourth arm pairs, each configured by connecting two arms formed of semiconductor switches in series, an alternating current switch configured by combining semiconductor switches. As such, a plurality of voltage levels can be to be selected from and output by an on and off control of these switch elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/JP2012/63508, filed on May 25, 2012, which is based on and claims priority to Japanese Patent Application No. JP 2011-149269, filed on Jul. 5, 2011. The disclosure of the Japanese priority application and the PCT application in their entirety, including the drawings, claims, and the specification thereof, are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     Embodiments of the invention relate to multilevel converter circuits. 
     2. Related Art 
       FIG. 23  shows one phase&#39;s worth of seven-level converter circuit disclosed in, for example, Japanese Patent Application No. JP-A-11-164567 (also referred to herein as “PTL 1”). 
     A series circuit of semiconductor switches Q 1  to Q 12  is connected between the positive terminal and negative terminal of a direct current combined power source BA 2  wherein direct current single power sources b 11  to b 23  are connected in series, and the connecting point of the semiconductor switches Q 6  and Q 7 , of the semiconductor switches Q 1  to Q 12 , forms an alternating current output point U. Also, the outer side terminals of a diode pair DA 1  formed of diodes D 1  and D 2  are connected between the connecting point of the semiconductor switches Q 1  and Q 2  and the connecting point of Q 7  and Q 8 , and the midpoint terminal of the diode pair DA 1  is connected to the connecting point of the direct current single power sources b 11  and b 12 . 
     In the same way, the outer side terminals of a diode pair DA 2  formed of diodes D 3  and D 4  are connected between the connecting point of the semiconductor switches Q 2  and Q 3  and the connecting point of Q 8  and Q 9 , and the midpoint terminal of the diode pair DA 2  is connected to the connecting point of the direct current single power sources b 12  and b 13 , while the outer side terminals of a diode pair DA 3  formed of diodes D 5  and D 6  are connected between the connecting point of the semiconductor switches Q 3  and Q 4  and the connecting point of Q 9  and Q 10 , and the midpoint terminal of the diode pair DA 3  is connected to the connecting point of the direct current single power sources b 13  and b 21 . In the same way, the outer side terminals of a diode pair DA 4  formed of diodes D 7  and D 8  are connected between the connecting point of the semiconductor switches Q 4  and Q 5  and the connecting point of Q 10  and Q 11 , and the midpoint terminal of the diode pair DA 4  is connected to the connecting point of the direct current single power sources b 21  and b 22 , while the outer side terminals of a diode pair DA 5  formed of diodes D 9  and D 10  are connected between the connecting point of the semiconductor switches Q 5  and Q 6  and the connecting point of Q 11  and Q 12 , and the midpoint terminal of the diode pair DA 5  is connected to the connecting point of the direct current single power sources b 22  and b 23 . 
     In this kind of configuration, when the semiconductor switches Q 1  to Q 6  are turned on, and Q 7  to Q 12  are turned off, a voltage of +3E is output to the alternating current output terminal U, 
     when the semiconductor switches Q 2  to Q 7  are turned on, and Q 8  to Q 12  and Q 1  are turned off, a voltage of +2E is output to the alternating current output terminal U, 
     when the semiconductor switches Q 3  to Q 8  are turned on, and Q 9  to Q 12 , Q 1 , and Q 2  are turned off, a voltage of +1E is output to the alternating current output terminal U, 
     when the semiconductor switches Q 4  to Q 9  are turned on, and Q 1  to Q 3  and Q 10  to Q 12  are turned off, zero voltage is output to the alternating current output terminal U, 
     when the semiconductor switches Q 5  to Q 10  are turned on, and Q 11 , Q 12 , and Q 1  to Q 4  are turned off, a voltage of −1E is output to the alternating current output terminal U, 
     when the semiconductor switches Q 6  to Q 11  are turned on, and Q 12  and Q 1  to Q 5  are turned off, a voltage of −2E is output to the alternating current output terminal U, and 
     when the semiconductor switches Q 7  to Q 12  are turned on, and Q 1  to Q 6  are turned off, a voltage of −3E is output to the alternating current output terminal U. 
     By adjusting on and off of each semiconductor switch Q 1  to Q 12  as above, it is possible to output mutually different seven levels of voltages to the alternating current output terminal U. 
       FIG. 24  shows one phase&#39;s worth of nine-level converter circuit. 
     The converter circuit of  FIG. 24  has nine levels extended from the seven levels of  FIG. 23 , and as the circuit configuration and operation are basically the same as those shown in  FIG. 23 , a description will be omitted. That is, by setting the voltage levels of three terminals of the direct current power source to 4E, 0, and −4E, and adjusting on and off of each semiconductor switch Q 1  to Q 16 , it is possible to output mutually different nine levels of voltages to an alternating current output terminal U. 
     With the circuits of  FIGS. 23 and 24 , the number of semiconductor switches through which output current passes is a maximum of six in series or a maximum of eight in series between the direct current combined power source BA 2  of each circuit and the alternating current output point U. Because of this, steady on loss in the semiconductor switches increases, thus causing a decrease in the efficiency of the whole of the device, and leading to a difficulty in miniaturization and price reduction. 
     Also, in such a general multilevel converter circuit as in  FIGS. 23 and 24 , as powers divided between direct current single power sources b 11  to b 14  and b 21  to b 24  are not the same in principle even when voltage and current output from the alternating current output point U have an alternating current waveform with symmetric positive and negative amplitudes, direct current single power sources independent of one another are necessary. Because of this, as six or eight single power sources which can supply powers independently are necessary for the direct current combined power source BA 2  which is an input, this brings about a big restriction on manufacturing the device. This direct current power source imbalance problem is introduced in, for example, “A multi-level voltage-source converter system with balanced DC voltage” on pp 1144 to 1150 in the conference record of IEEE-PESC &#39;95. 
     Therefore, a problem of the invention is to eliminate an operational restriction by reducing the number of semiconductor switches through which output current passes, thus achieving loss reduction, and by enabling an operation using two single power sources as a direct current input power source. 
     SUMMARY OF INVENTION 
     To address the shortcomings of the related art, in some embodiments of the invention, a multilevel converter circuit which generates a plurality of voltage levels from a direct current power source divided into two, including three terminals, and having three mutually different voltage levels including zero, and selects from and outputs the plurality of voltage levels, can include 
     first, second, third, and fourth arm pairs, each configured by connecting two arms formed of semiconductor switches in series; and 
     a first alternating current switch configured by combining semiconductor switches, the multilevel converter circuit being characterized in that 
     the respective outer side terminals of the first arm pair, second arm pair, and third arm pair are connected in series, in order from a first direct current terminal, between the first direct current terminal wherein the potential of the direct current power source is highest and a third direct current terminal wherein the potential is lowest, 
     the outer side terminals of the fourth arm pair are connected between the midpoint terminal of the first arm pair and the midpoint terminal of the third arm pair, 
     both ends of the first alternating current switch are connected between the midpoint terminal of the fourth arm pair and a second direct current terminal wherein the potential of the direct current power source is intermediate, 
     both ends of each of a first capacitor and second capacitor are connected in parallel to both ends of each of the second arm pair and fourth arm pair respectively, and 
     the midpoint terminal of the second arm pair is formed as an alternating current terminal. 
     In some embodiments, it is possible that both ends of a second alternating current switch are connected between the midpoint terminal of the fourth arm pair and the midpoint terminal of the second arm pair, or it is possible that a fifth arm pair can be connected between the outer side terminals of the second arm pair, and both ends of the second alternating current switch are connected between the midpoint terminal of the fifth arm pair and the midpoint terminal of the fourth arm pair. In some embodiments, it is possible that both ends of a third alternating current switch are connected between the midpoint terminal of the fifth arm pair and the midpoint terminal of the second arm pair. 
     In some embodiments, it is possible that a multilevel converter circuit which generates a plurality of voltage levels from a direct current power source divided into two, including three terminals, and having three mutually different voltage levels including zero, and selects from and outputs the plurality of voltage levels, includes 
     first, second, and third arm pairs, each configured by connecting two arms formed of semiconductor switches in series; and 
     first, second, and third alternating current switches, each configured by combining semiconductor switches, the multilevel converter circuit being characterized in that 
     the respective outer side terminals of the first arm pair and second arm pair are connected in series, in order from a first direct current terminal, between the first direct current terminal wherein the potential of the direct current power source is highest and a third direct current terminal wherein the potential is lowest, 
     the outer side terminals of the third arm pair are connected between the midpoint terminal of the first arm pair and the midpoint terminal of the second arm pair, 
     both ends of the first alternating current switch are connected between the midpoint terminal of the third arm pair and a second direct current terminal wherein the potential of the direct current power source is intermediate, 
     the outer side terminals of a connection circuit wherein two capacitors are connected in series are connected in parallel to the outer side terminals of the third arm pair, 
     both ends of the second alternating current switch is connected between the midpoint terminal of the capacitor series connection circuit and the midpoint terminal of the third arm pair, 
     both ends of the third alternating current switch are connected between the midpoint terminal of the capacitor series connection circuit and the connecting point of the first arm pair and second arm pair, and 
     the connecting point of the first arm pair, second arm pair, and third alternating current switch is formed as an alternating current terminal. 
     In some embodiments, it is possible that a high potential side arm configuring the first arm pair, a low potential side arm configuring the third arm pair, and two arms configuring the fourth arm pair are all configured of diodes, and in some embodiments, it is possible that the high potential side arm configuring the first arm pair, the low potential side arm configuring the third arm pair, the two arms configuring the fourth arm pair, and two arms configuring either the second arm pair or fifth arm pair are all configured of diodes. Also, in some embodiments, it is possible that the high potential side arm configuring the first arm pair, a low potential side arm configuring the second arm pair, and two arms configuring the third arm pair are all configured of diodes. 
     In some embodiments, it is possible that each of the alternating current switches is configured by connecting semiconductor switches having reverse breakdown voltage characteristics in anti-parallel, it is possible that each of the high potential side arm configuring the first arm pair and the low potential side arm configuring the third arm pair is configured of a series connection circuit of a plurality of semiconductor switches having the same function, and semiconductor switches configuring each arm are controlled by their respective individual control signals, and in some embodiments, it is possible that each of the high potential side arm configuring the first arm pair and the low potential side arm configuring the second arm pair is configured of a series connection circuit of a plurality of semiconductor switches having the same function, and semiconductor switches configuring each arm are controlled by their respective individual control signals. 
     In some embodiments, it is possible that the voltage levels of the three terminals of the direct current power source are set to +3E, 0, and −3E, and the levels of the voltages of the first capacitor and second capacitor are maintained at 1E and 2E respectively, and that 
     a total of seven levels of voltages, +3E, +2E, 1E, 0, −1E, −2E, and −3E, are generated using the respective voltages of the direct current power source, first capacitor, and second capacitor, thus enabling an optional selection from and output of the voltage levels. 
     In some embodiments, it is possible that the voltage levels of the three terminals of the direct current power source are set to +4E, 0, and −4E, and the levels of the voltages of the first capacitor and second capacitor are maintained at 1E and 2E respectively, and that 
     a total of nine levels of voltages, +4E, +3E, +2E, 1E, 0, −1E, −2E, −3E, and −4E, are generated using the respective voltages of the direct current power source, first capacitor, and second capacitor, thus enabling an optional selection from and output of the voltage levels, and some embodiments, it is possible that the voltage levels of the three terminals of the direct current power source are set to +3E, 0, and −3E, and the levels of the voltages of the first capacitor and second capacitor are maintained at 1E and 2E respectively, and that 
     a total of seven levels of voltages, +3E, +2E, 1E, 0, −1E, −2E, and −3E, are generated using the respective voltages of the direct current power source, third capacitor, and fourth capacitor, thus enabling an optional selection from and output of the voltage levels. 
     According to some embodiments of the invention, the number of semiconductor switches through which output current passes is a maximum of four between the direct current power source side, which is an input, and the alternating current output, and it is thus possible to realize loss reduction. As a result of this, higher efficiency, price reduction, and miniaturization of a device are possible. Furthermore, as it is possible to form the direct current input power source as a combination of two single power sources, an operational restriction decreases as compared with heretofore known, and it is easy to actually fabricate the device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram showing an embodiment of the invention; 
         FIG. 2  is a waveform diagram showing an output voltage example when the embodiment of  FIG. 1  is caused to operate as a seven-level converter circuit; 
         FIG. 3  is an illustration showing switching operation patterns when the embodiment of  FIG. 1  is caused to operate as the seven-level converter circuit; 
         FIG. 4  is a waveform diagram showing an output voltage example when an embodiment of  FIG. 1  is caused to operate as a nine-level converter circuit; 
         FIG. 5  is an illustration showing switching operation patterns when an embodiment of  FIG. 1  is caused to operate as the nine-level converter circuit; 
         FIG. 6  is a circuit diagram showing a first modification example of an embodiment of  FIG. 1 ; 
         FIG. 7  is an illustration showing a switching operation pattern example of the circuit of  FIG. 6 ; 
         FIG. 8  is a circuit diagram showing a second modification example of an embodiment of  FIG. 1 ; 
         FIG. 9  is an illustration showing a switching operation pattern example of an embodiment of  FIG. 8 ; 
         FIG. 10  is a circuit diagram showing a third modification example of an embodiment of  FIG. 1 ; 
         FIG. 11  is an illustration showing a switching operation pattern example of an embodiment of  FIG. 10 ; 
         FIG. 12  is a circuit diagram showing another embodiment of the invention; 
         FIG. 13  is an illustration showing a switching operation pattern example of an embodiment of  FIG. 12 ; 
         FIG. 14  is a circuit diagram showing a fourth modification example of an embodiment of  FIG. 1 ; 
         FIG. 15  is a circuit diagram showing a fifth modification example of an embodiment of  FIG. 1 ; 
         FIG. 16  is a circuit diagram showing a sixth modification example of an embodiment of  FIG. 1 ; 
         FIG. 17  is a circuit diagram showing a seventh modification example of an embodiment of  FIG. 1 ; 
         FIG. 18  is a circuit diagram showing a first modification example of an embodiment of  FIG. 12 ; 
         FIG. 19  is a circuit diagram showing an eighth modification example of an embodiment of  FIG. 1 ; 
         FIG. 20  is an illustration showing a switching operation pattern example of an embodiment of  FIG. 19 ; 
         FIG. 21  is a circuit diagram showing a second modification example of an embodiment of  FIG. 12 ; 
         FIG. 22  is an illustration showing a switching operation pattern example of an embodiment of  FIG. 21 ; 
         FIG. 23  is a circuit diagram showing a seven-level converter circuit described in PTL 1; and 
         FIG. 24  is a circuit diagram showing a heretofore known example of a nine-level converter circuit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a circuit diagram showing an embodiment of the invention,  FIG. 2  is an example diagram of a seven-level output waveform in  FIG. 1 , and  FIG. 3  is an illustration of switching patterns in  FIG. 1 . 
     A direct current combined power source (also referred to simply as a direct current power source) BA 1  is shown here as one, with direct current single power sources b 1  and b 2  connected in series, having three terminals, a positive terminal P, a negative terminal N, and a zero terminal (a ground terminal) M. An arm pair QA 1  formed of semiconductor switches Q 1  and Q 2 , an arm pair QA 2  formed of semiconductor switches Q 3  and Q 4 , and furthermore, an arm pair QA 3  formed of semiconductor switches Q 5  and Q 6  are connected in series to the direct current power source BA 1 . Also, the outer side terminals of an arm pair QA 4  formed of semiconductor switches Q 7  and Q 8  are connected between the midpoint terminal of the arm pair QA 1  and the midpoint terminal of the arm pair QA 3 , and an alternating current switch SW 1  formed of an inverse parallel connection circuit of semiconductor switches QR 1  and QR 2  having reverse breakdown voltage characteristics is connected between the midpoint terminal of QA 4  and the zero terminal (ground terminal) M of the direct current power source BA 1 . Furthermore, a capacitor C 2  is connected in parallel to the arm pair QA 4 , while a capacitor C 1  is connected in parallel to the arm pair QA 2 , and the midpoint of the arm pair QA 2  forms an alternating current output point U. 
     A description will be given hereafter of a case of causing the kind of configuration of  FIG. 1  to operate in switching patterns P 1  to P 16  shown in  FIG. 3 . 
     Firstly, when the semiconductor switches Q 1 , Q 2 , and Q 3  are turned on, and Q 4 , and QR 1  of SW 1 , are turned off, as in the switching pattern P 1 , voltage Vb 1  of the positive terminal of the direct current power source BA 1  is directly output to the alternating current output point U. Output current, flowing along a path b 1 →Q 1 →Q 2 →Q 3 →U, passes through the three semiconductor switches Q 1 , Q 2 , and Q 3 . Herein, by keeping Q 5 , Q 6 , and Q 7  turned off, Q 4  is clamped by voltage VC 1  of C 1  in a path C 1 →Q 3 , and hereafter, in the same way, Q 5  is clamped by voltage (VC 2 −VC 1 ) in a path C 2 →Q 2 →C 1 , Q 6  is clamped by voltage (−Vb 2 +Vb 1 −VC 2 ) in a path BA 1 →Q 1 →C 2 , Q 7  is clamped by voltage VC 2  in a path Q 8 →C 2 , and QR 1  is clamped by voltage (Vb 1 −VC 2 ) in a path b 1 →Q 1 →C 2 →Q 8 . 
     Herein, when causing the circuit of  FIG. 1  to operate as a seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating output point U is +3E, and the levels of the clamp voltages of SW 1 , Q 4 , Q 5 , Q 6 , and Q 7  are E, 1E, 1E, 4E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as a nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is +4E, and the levels of the clamp voltages of SW 1 , Q 4 , Q 5 , Q 6 , and Q 7  are 2E, 1E, 1E, 6E, and 2E respectively. 
     When the semiconductor switches Q 1 , Q 2 , and Q 4  are turned on, and Q 3 , Q 5 , Q 6 , Q 7 , and QR 1  are turned off, as in the switching pattern P 2 , output current, flowing along a path b 1 →Q 1 →Q 2 →C 1 →Q 4 →U, passes through the three semiconductor switches Q 1 , Q 2 , and Q 4 , and voltage (Vb 1 −VC 1 ) is output to the alternating current output point U. At this time, Q 3  is clamped by VC 1 , Q 5  is clamped by voltage (VC 2 −VC 1 ), Q 6  is clamped by voltage (−Vb 2 +Vb 1 −VC 2 ), Q 7  is clamped by voltage VC 2 , and QR 1  is clamped by voltage (Vb 1 −VC 2 ). 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating current output point U is +2E, and the clamp voltages of SW 1 , Q 3 , Q 5 , Q 6 , and Q 7  are 1E, 1E, 1E, 4E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is +3E, and the clamp voltages of SW 1 , Q 3 , Q 5 , Q 6 , and Q 7  are 2E, 1E, 1E, 6E, and 2E respectively. 
     When the semiconductor switches Q 1 , Q 3 , and Q 5  are turned on, and Q 2 , Q 4 , Q 6 , Q 7 , and QR 1  are turned off, as in the switching pattern P 3 , output current, flowing along a path b 1 →Q 1 →C 2 →Q 5 →C 1 →Q 3 →U, passes through the three semiconductor switches Q 1 , Q 3 , and Q 5 , and voltage (Vb 1 −VC 2 +VC 1 ) is output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating current output point U is +2E, and the clamp voltages of SW 1 , Q 2 , Q 4 , Q 6 , and Q 7  are 1E, 1E, 1E, 4E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is +3E, and the clamp voltages of SW 1 , Q 2 , Q 4 , Q 6 , and Q 7  are 2E, 1E, 1E, 6E, and 2E respectively. 
     When SW 1 , Q 2 , Q 3 , and Q 8  are turned on, and Q 1 , Q 4 , Q 5 , Q 6 , and Q 7  are turned off, as in the switching pattern P 4 , output current, flowing along a path SW 1 →Q 8 →C 2 →Q 2 →Q 3 →U, passes through the four semiconductor switches SW 1 , Q 8 , Q 2 , and Q 3 , and voltage (VC 2 ) is output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating current output point U is +2E, and the clamp voltages of Q 1 , Q 4 , Q 5 , Q 6 , and Q 7  are 1E, 1E, 1E, 3E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is +2E, and the clamp voltages of Q 1 , Q 4 , Q 5 ,  6 , and Q 7  are 2E, 1E, 1E, 4E, and 2E respectively. 
     When Q 1 , Q 4 , and Q 5  are tuned on, and Q 2 , Q 3 , Q 6 , Q 7 , and QR 1  are turned off, as in the switching pattern P 5 , output current, flowing along a path b 1 →Q 1 →C 2 →Q 5 →Q 4 →U, passes through the three semiconductor switches Q 1 , Q 5 , and Q 4 , and voltage (Vb 1 −VC 2 ) is output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating current output point U is +1E, and the clamp voltages of SW 1 , Q 2 , Q 3 , Q 6 , and Q 7  are 1E, 1E, 1E, 4E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is +2E, and the clamp voltages of SW 1 , Q 2 , Q 3 , Q 6 , and Q 7  are 2E, 1E, 1E, 4E, and 2E respectively. 
     When SW 1 , Q 2 , Q 4 , and Q 8  are turned on, and Q 1 , Q 3 , Q 5 , Q 6 , and Q 7  are turned off, as in the switching pattern P 6 , output current, flowing along a path SW 1 →Q 8 →C 2 →Q 2 →C 1 →Q 4 →U, passes through the four semiconductor switches SW 1 , Q 8 , Q 2 , and Q 4 , and voltage (VC 2 −VC 1 ) is output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating current output point U is +1E, and the clamp voltages of Q 1 , Q 3 , Q 5 , Q 6 , and Q 7  are 1E, 1E, 1E, 3E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is +1E, and the clamp voltages of Q 1 , Q 3 , Q 5 , Q 6 , and Q 7  are 2E, 1E, 1E, 4E, and 2E respectively. 
     When SW 1 , Q 3 , Q 5 , and Q 8  are turned on, and Q 1 , Q 2 , Q 4 , Q 6 , and Q 7  are turned off, as in the switching pattern P 7 , output current, flowing along a path SW 1 →Q 8 →Q 5 →C 1 →Q 3 →U, passes through the four semiconductor switches SW 1 , Q 8 , Q 5 , and Q 3 , and voltage (VC 1 ) is output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating current output point U is +1E, and the clamp voltages of Q 1 , Q 2 , Q 4 , Q 6 , and Q 7  are 1E, 1E, 1E, 3E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is +1E, and the clamp voltages of Q 1 , Q 2 , Q 4 , Q 6 , and Q 7  are 2E, 1E, 1E, 4E, and 2E respectively. 
     When SW 1 , Q 4 , Q 5 , and Q 8  are turned on, and Q 1 , Q 2 , Q 3 , Q 6 , and Q 7  are turned off, as in the switching pattern P 8 , output current, flowing along a path SW 1 →Q 8 →Q 5 →Q 4 →U, passes through the four semiconductor switches SW 1 , Q 8 , Q 5 , and Q 4 , and the potential of the zero terminal M is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the voltage of the alternating current output point U is 0, and the clamp voltages of Q 1 , Q 2 , Q 3 , Q 6 , and Q 7  are 1E, 1E, 1E, 3E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is 0, and the clamp voltages of Q 1 , Q 2 , Q 3 , Q 6 , and Q 7  are 2E, 1E, 1E, 4E, and 2E respectively. 
     When SW 1 , Q 2 , Q 3 , and Q 7  are turned on, and Q 1 , Q 4 , Q 5 , Q 6 , and Q 8  are turned off, as in the switching pattern P 9 , output current, flowing along a path SW 1 →Q 7 →Q 2 →Q 3 →U, passes through the four semiconductor switches SW 1 , Q 7 , Q 2 , and Q 3 , and the potential of the zero terminal is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is 0, and the clamp voltages of Q 1 , Q 4 , Q 5 , Q 6 , and Q 8  are 3E, 1E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is 0, and the clamp voltages of Q 1 , Q 4 , Q 5 , Q 6 , and Q 8  are 4E, 1E, 1E, 2E, and 2E respectively. 
     When SW 1 , Q 2 , Q 4 , and Q 7  are turned on, and Q 1 , Q 3 , Q 5 , Q 6 , and Q 8  are turned off, as in the switching pattern P 10 , output current, flowing along a path SW 1 →Q 7 →Q 2 →C 1 →Q 4 →U, passes through the four semiconductor switches SW 1 , Q 7 , Q 2 , and Q 4 , and voltage (VC 1 ) is output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is −1E, and the clamp voltages of Q 1 , Q 3 , Q 5 , Q 6 , and Q 8  are 3E, 1E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is −1E, and the clamp voltages of Q 1 , Q 3 , Q 5 , Q 6 , and Q 8  are 4E, 1E, 1E, 2E, and 2E respectively. 
     When SW 1 , Q 3 , Q 5 , and Q 7  are turned on, and Q 1 , Q 2 , Q 4 , Q 6 , and Q 8  are turned off, as in the switching pattern P 11 , output current, flowing along a path SW 1 →Q 7 →C 2 →Q 5 →C 1 →Q 3 →U, passes through the four semiconductor switches SW 1 , Q 7 , Q 5 , and Q 3 , and voltage (−VC 2 +VC 1 ) is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is −1E, and the clamp voltages of Q 1 , Q 2 , Q 4 , Q 6 , and Q 8  are 3E, 1E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is −1E, and the clamp voltages of Q 1 , Q 2 , Q 4 , Q 6 , and Q 8  are 4E, 1E, 1E, 2E, and 2E respectively. 
     When Q 2 , Q 3 , and Q 6  are turned on, and Q 1 , Q 4 , Q 5 , Q 8 , and QR 2  are turned off, as in the switching pattern P 12 , output current, flowing along a path b 2 →Q 6 →C 2 →Q 2 →Q 3 →U, passes through the three semiconductor switches Q 6 , Q 2 , and Q 3 , and voltage (Vb 2 +VC 2 ) is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is −1E, and the clamp voltages of SW 1 , Q 1 , Q 4 , Q 5 , and Q 8  are 1E, 4E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is −2E, and the clamp voltages of SW 1 , Q 1 , Q 4 , Q 5 , and Q 8  are 2E, 6E, 1E, 1E, and 2E respectively. 
     When SW 1 , Q 4 , Q 5 , and Q 7  are turned on, and Q 1 , Q 2 , Q 3 , Q 6 , and Q 8  are turned off, as in the switching pattern P 13 , output current, flowing along a path SW 1 →Q 7 →C 2 →Q 5 →Q 4 →U, passes through the four semiconductor switches SW 1 , Q 7 , Q 5 , and Q 4 , and voltage (−VC 2 ) is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is −2E, and the clamp voltages of Q 1 , Q 2 , Q 3 , Q 6 , and Q 8  are 3E, 1E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is −2E, and the clamp voltages of SW 1 , Q 1 , Q 2 , Q 3 , Q 6 , and Q 8  are 4E, 1E, 1E, 2E, and 2E respectively. 
     When Q 2 , Q 4 , and Q 6  are turned on, and Q 1 , Q 3 , Q 5 , Q 8 , and QR 2  are turned off, as in the switching pattern P 14 , output current, flowing along a path b 2 →Q 6 →C 2 →Q 2 →C 1 →Q 4 →U, passes through the three semiconductor switches Q 6 , Q 2 , and Q 4 , and voltage (Vb 2 +VC 2 −VC 1 ) is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is −2E, and the clamp voltages of SW 1 , Q 1 , Q 3 , Q 5 , and Q 8  are 1E, 4E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is −3E, and the clamp voltages of SW 1 , Q 1 , Q 3 , Q 5 , and Q 8  are 2E, 6E, 1E, 1E, and 2E respectively. 
     When Q 3 , Q 5 , and Q 6  are turned on, and Q 1 , Q 2 , Q 4 , Q 8 , and QR 2  are turned off, as in the switching pattern P 15 , output current, flowing along a path b 2 →Q 6 →Q 5 →C 1 →Q 3 →U, passes through the three semiconductor switches Q 6 , Q 5 , and Q 3 , and voltage (Vb 2 +VC 1 ) is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is −2E, and the clamp voltages of SW 1 , Q 1 , Q 2 , Q 4 , and Q 8  are 1E, 4E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is −3E, and the clamp voltages of SW 1 , Q 1 , Q 2 , Q 4 , and Q 8  are 2E, 6E, 1E, 1E, and 2E respectively. 
     When Q 4 , Q 5 , and Q 6  are turned on, and Q 1 , Q 2 , Q 3 , Q 8 , and QR 2  are turned off, as in the switching pattern P 16 , output current, flowing along a path b 2 →Q 6 →Q 5 →Q 4 →U, passes through the three semiconductor switches Q 6 , Q 5 , and Q 4 , and voltage (Vb 2 ) is directly output to the alternating current output point U. 
     Herein, when causing the circuit of  FIG. 1  to operate as the seven-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +3E and −3E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (⅓) and (⅔) of Vb 1 , the potential of the alternating current output point U is 3E, and the clamp voltages of SW 1 , Q 1 , Q 2 , Q 3 , and Q 8  are 1E, 4E, 1E, 1E, and 2E respectively. 
     Meanwhile, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, in the event that the levels of Vb 1  and Vb 2  are set to +4E and −4E, and VC 1  and VC 2  are maintained at levels 1E and 2E corresponding to (¼) and ( 2/4) of Vb 1 , the voltage of the alternating current output point U is −4E, and the clamp voltages of SW 1 , Q 1 , Q 2 , Q 3 , and Q 8  are 2E, 6E, 1E, 1E, and 2E respectively. 
     When both Vb 1  and Vb 2  in  FIG. 1  are +3E as the seven-level converter circuit, seven levels of voltages, +3E, +2E, +1E, 0, −1E, −2E, and −3E, can be output to the alternating current output point U in accordance with the switching patterns P 1  to P 16 , and in the same way, when both Vb 1  and Vb 2  in  FIG. 1  are +4E as the seven-level converter circuit, nine levels of voltages, +4E, +3E, +2E, +1E, 0, −1E, −2E, −3E, and −4E, can be output to the alternating current output point U in accordance with the switching patterns P 1  to P 16 . 
     Also, when the circuit of  FIG. 1  operates as the seven-level converter circuit, the same +2E is output in the switching patterns P 2  to P 4 , but when the direction of alternating current output current i from the alternating current output point U is positive, the capacitor C 1  is charged in the switching pattern P 2 . As opposed to this, the capacitor C 1  is discharged, and the capacitor C 2  is charged, in the switching pattern P 3 , while the capacitor C 2  is discharged in the switching pattern P 4 . With regard to the switching patterns P 5  to P 7  when the output voltage of the alternating current output point U is +1E too, C 2  is charged in the switching pattern P 5 , C 1  is charged, while C 2  is discharged, in the switching pattern P 6 , and C 1  is discharged in the switching pattern P 7 , in accordance with the direction of the alternating current output current i. 
     That is, by appropriately selecting from the switching patterns P 2  to P 4  in order to output the voltage of +2E to the alternating current output point U, and the switching patterns P 5  to P 7  in order to output the voltage of +1E to the alternating current output point U, it is possible to adjust VC 1  and VC 2  independently of each other, and it is possible to always control each of VC 1  and VC 2  to +1E and +2E. 
     From the symmetry of the circuit, the same relationship is established in the switching patterns P 10  and P 15  too. Also, when in the switching patterns P 1 , P 8 , P 9 , and P 16 , as no current flows through the capacitor C 1  or C 2 , no change occurs in VC 1  or VC 2 .  FIGS. 2 and 3  show an output waveform diagram when the circuit of  FIG. 1  is caused to operate as the seven-level converter circuit, and an example of the switching patterns P 1  to P 16 , respectively. 
     Next, when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, the voltage of +3E is output to the alternating current output point U in both the switching patterns P 2  and P 3 , but +2E is output in the switching patterns P 4  and P 5 , and +1E is output in the switching patterns P 6  and P 7 . 
     Herein, considering a case in which the direction of the alternating current output current i from the alternating current output point U is positive, the capacitor C 1  is charged by the alternating current output current i in the switching pattern P 2  when outputting +3E, while the capacitor C 1  is discharged, and the capacitor C 2  is charged, in the switching pattern P 3 . 
     Also, C 2  is discharged in the switching pattern P 4  when outputting +2E, and C 2  is charged in the switching pattern P 5 . Also, C 1  is charged, and C 2  is discharged, in the switching pattern P 6  when outputting +1E, while C 1  is discharged in the switching pattern P 7 . 
     That is, by appropriately selecting the switching patter P 2  or P 3  when outputting the voltage of +3E to the alternating current output point U, the switching patter P 4  or P 5  when outputting the voltage of +2E, and the switching pattern P 6  or P 7  when outputting the voltage of +1E, it is possible to adjust VC 1  and VC 2  independently of each other, and it is possible to always control each of VC 1  and VC 2  to +1E and +2E. 
     From the symmetry of the circuit, the same relationship is established in the switching patterns P 10  and P 15  too. Also, when in the switching patterns P 1 , P 8 , P 9 , and P 16 , as no current flows through the capacitor C 1  or C 2 , no change occurs in VC 1  or VC 2 .  FIGS. 4 and 5  show an output waveform diagram when causing the circuit of  FIG. 1  to operate as the nine-level converter circuit, and an example of the switching patterns P 1  to P 16 , respectively. 
       FIG. 6  is a circuit diagram showing a first modification example of  FIG. 1 . The difference from  FIG. 1  is that an alternating current switch SW 2  is added between the neutral terminal of the arm pair QA 4  and the neutral terminal of the arm pair QA 2 . 
     When causing this circuit as the seven-level converter circuit, in all the modes other than the switching patterns P 8  and P 9 , the operations of the semiconductor switches Q 1  to Q 8  and SW 1  are quite the same as in the case of  FIG. 1 . The operations of QR 3  and QR 4  of the added alternating current switch SW 2  are such that QR 3  is turned off, and QR 4  is turned on, in the switching patterns P 1  to P 7 , and conversely, QR 3  is turned on, and QR 4  is turned off, in the switching patterns P 10  to P 16 . 
     In any of the heretofore described switching patterns, as no current is caused to flow through the alternating switch SW 2 , and the alternating switch SW 2  is clamped by the voltage VC 1  or VC 2 , and maintains a standby state, there is no effect on the operation of the whole of the circuit. Only when outputting 0 in the switching patterns P 8  and P 9 , QR 3  and QR 4  are both turned on, thus allowing output current to flow through SW 2 . That is, as the current, flowing from the zero terminal M of BA 1  along a path SW 1 →SW 2 →U, passes through only the two semiconductor switches SW 1  and SW 2 , it is possible to reduce semiconductor conduction loss as compared with the circuit of  FIG. 1 , and thus possible to hope for higher efficiency. 
       FIG. 7  shows an example of switching patterns when causing the circuit of  FIG. 6  to operate as the seven-level converter circuit. 
       FIG. 8  is a circuit diagram showing a second modification example of  FIG. 1 . The difference from  FIG. 1  is that an arm pair QA 5  is additionally connected in parallel to QA 2 , and an alternating current switch SW 2  is additionally connected between the midpoint terminal of the arm pair QA 4  and the midpoint terminal of QA 5 . When causing the circuit of  FIG. 8  to operate as the seven-level converter circuit, in all the modes other than the switching patterns P 7  to P 10 , the operations of the semiconductor switches Q 1  to Q 8  and SW 1  are quite the same as in the case of  FIG. 1 . The operations of QR 3  and QR 4  of the added SW 2  and of Q 9  and Q 10  of the added QA 5  are such that QR 3  is turned off, QR 4  is turned on, Q 9  is turned off, and Q 10  is turned on, in the switching patterns P 1  to P 6 . Also, QR 3  is turned on, QR 4  is turned off, Q 9  is turned on, and Q 10  is turned off, in the switching patterns P 11  to P 16 . In any of the switching patterns, as no current is caused to flow through the added SW 2  and QA 5 , and SW 2  and QA 5  are clamped by VC 1  and voltage (VC 2 −VC 1 ) respectively, and maintained in a standby state, there is no effect on the operation of the whole of the circuit. 
     In the circuit of  FIG. 8 , when in the switching pattern P 7 , SW 1 , SW 2 , Q 3 , and Q 10  are turned on, output current, flowing along a path SW 1 →SW 2 →Q 10 →C 1 →Q 3 , passes through the four semiconductor switches, and VC 1  is directly output to the alternating current output point U. In the switching pattern P 8 , SW 1 , SW 2 , Q 4 , and Q 10  are turned on, output current, flowing along a path SW 1 →SW 2 →Q 10 →Q 4 , passes through the four semiconductor switches, and the potential 0 of the zero terminal of BA 1  is directly output to the alternating current output point U. In the switching pattern P 9 , SW 1 , SW 2 , Q 3 , and Q 9  are turned on, output current, flowing along a path SW 1 →SW 2 →Q 9 →Q 3 , passes through the four semiconductor switches, and the potential 0 of the zero terminal of BA 1  is directly output to the alternating current output point U. Also, in the switching pattern P 10 , SW 1 , SW 2 , Q 4 , and Q 9  are turned on, output current, flowing along a path SW 1 →SW 2 →Q 9 →C 1 →Q 4 , passes through the four semiconductor switches, and −VC 1  is directly output to the alternating current output point U. 
     As above, in any of the switching patterns P 7  to P 10 , the output current passes through four semiconductor switches, and in terms of number, the circuit of  FIG. 8  is equivalent to the circuit of  FIG. 1 . In the circuit of  FIG. 1 , it is always necessary for the output current to pass through the arm pair QA 4  (Q 7  and Q 8 ), and breakdown voltage characteristics which can withstand the voltage of 2E are required of this QA 4  when in operation. As opposed to this, in  FIG. 8 , semiconductor switches through which the output current passes are SW 1 , SW 2 , QA 5 , and QA 2 , and breakdown voltage required of all these semiconductor switches is 1E. Because of this, with the circuit of  FIG. 8 , it is possible to hope for higher efficiency as compared with the circuit of  FIG. 1 .  FIG. 9  shows an operation pattern example of  FIG. 8 . 
       FIG. 10  is a third modification example of  FIG. 1 , wherein one portion of  FIG. 8  is changed. The difference from  FIG. 8  is that an alternating current switch SW 3  is added between the neutral terminal of the arm pair QA 5  and the neutral terminal of QA 2 . 
     With regard to operations in  FIG. 10 , in all the modes other than the switching patterns P 8  and P 9 , the operations of the semiconductor switches Q 1  to Q 10 , SW 1 , and SW 2  are quite the same as in the case of  FIG. 8 . The operations of QR 5  and QR 6  of the added SW 3  are such that QR 5  is turned off, and QR 6  is turned on, in the switching patterns P 1  to P 7 , while QR 5  is turned on, and QR 6  is turned off, in the switching patterns P 10  to P 16 , thus setting the operations to be in the reverse relation between in the switching patterns P 1  to P 7  and P 10  to P 16 . 
     In any of the heretofore described modes, as no current is caused to flow through the added SW 3 , and SW 3  is clamped by VC 1  or voltage 0, and maintained in a standby state, there is no effect on the operation of the whole of the circuit. Only in the switching patterns P 8  and P 9  in which voltage 0 is output, QR 5  and QR 6  are both turned on, and current flows through SW 3 . At this time, output current, flowing from the zero terminal M of BA 1  along a path SW 1 →SW 2 →SW 3 →U, passes through only the three semiconductor switches SW 1 , SW 2 , and SW 3 , meaning that semiconductor conduction loss decreases as compared with the circuits of  FIGS. 1 and 8 , and it is possible to hope for higher efficiency.  FIG. 11  shows operation patterns in  FIG. 10 . 
       FIG. 12  shows another embodiment of the invention. 
     Herein, a direct current combined power source BA 1 , with direct current power sources b 1  and b 2 , each having the voltage of 3E, connected in series, which has three terminals, a positive terminal, a zero terminal, and a negative terminal, is connected as a direct current side power source. In this multilevel converter circuit, a series circuit of an arm pair QA 1  formed of semiconductor switches Q 1  and Q 2 , and an arm pair QA 2  formed of semiconductor switches Q 3  and Q 4 , is connected between the positive terminal and negative terminal of the direct current combined power source BA 1 . Also, the outer side terminals of an arm pair QA 3  formed of semiconductor switches Q 5  and Q 6  are connected between the midpoint terminal of the arm pair QA 1  and the midpoint terminal of the arm pair QA 2 , and an alternating current switch SW 1  formed of an inverse parallel circuit of semiconductor switches QR 1  and QR 2  having reverse breakdown voltage characteristics is connected between the midpoint terminal of the arm pair QA 3  and the zero terminal of the direct current combined power source BA 1 . 
     Also, a capacitor series circuit CA 1 , wherein capacitors C 3  and C 4  are connected in series, is connected in parallel to the arm pair QA 3 , an alternating current switch SW 2  formed of an inverse parallel circuit of semiconductor switches QR 3  and QR 4  having reverse breakdown voltage characteristics is connected between the midpoint terminal of the capacitor series circuit CA 1  and the midpoint terminal of the arm pair QA 3 , an alternating current switch SW 3  formed of an inverse parallel circuit of semiconductor switches QR 5  and QR 6  having reverse breakdown voltage characteristics is connected between the midpoint terminal of the capacitor series circuit CA 1  and the connection point of the arm pair QA 1  and arm pair QA 2 , and the connecting point of QA 1 , QA 2 , and SW 3  forms an alternating current output point U. 
     The circuit of  FIG. 12  operates in the following way in accordance with kinds of switching patterns P 1 ′ to P 13 ′ in  FIG. 13 . 
     Firstly, when the semiconductor switches Q 1 , Q 2 , and Q 6  are turned on, and Q 3 , Q 4 , Q 5 , QR 1 , QR 3 , and QR 5  are turned off, as in the switching pattern P 1 ′, voltage Vb 1  of the positive terminal of the direct current combined power source BA 1  is directly output to the alternating current output point U. Output current, flowing along a path b 1 →Q 1 →Q 2 →U, passes through the two semiconductor switches Q 1  and Q 2 . At this time, SW 3  is clamped by voltage VC 3  of C 3  in the path of C 3  and Q 2 , SW 2  is clamped by voltage VC 4  of C 4  in the path of Q 6  and C 4 , and SW 1  is clamped by voltage (Vb 1 −VC 3 −VC 4 ) in the path of b 1 , Q 1 , C 3 , C 4 , and Q 6 . 
     In the same way, Q 3  is clamped by voltage (VC 3 +VC 4 ) in the path of C 4 , C 3 , and Q 2 , Q 4  is clamped by voltage (−Vb 2 +Vb 1 −VC 3 −VC 4 ) in the path of b 2 , b 1 , C 3 , and C 4 , and Q 5  is clamped by voltage (VC 3 +VC 4 ) in the path of Q 6 , C 4 , and C 3 . 
     Herein, in the event that VC 3  and VC 4  are both controlled to a level 1E corresponding to (⅓) of Vb 1  by using the direct current power source BA 1  wherein the levels of Vb 1  and Vb 2  are +3E and −3E respectively, the levels of the clamp voltages of Q 3 , Q 4 , Q 5 , SW 1 , SW 2 , and SW 3  are 2E, 4E, 2E, 1E, 1E, and 1E respectively. 
     When the semiconductor switches Q 1 , Q 6 , and SW 3  are turned on, and Q 2 , Q 3 , Q 4 , Q 5 , QR 1 , and QR 3  are turned off, as in the switching pattern P 2 ′, output current, flowing along a path b 1 →Q 1 →C 3 →SW 3 →U, passes through the two semiconductor switches Q 1  and SW 3 , and voltage (Vb 1 −VC 3 ) is output to the alternating current output point U. At this time, Q 2  is clamped by VC 3 , Q 3  is clamped by VC 4 , Q 4  is clamped by (Vb 1 −VC 3 −VC 4 ), Q 5  is clamped by (VC 3 +VC 4 ), SW 1  is clamped by (Vb 1 −VC 3 −VC 4 ), and SW 2  is clamped by the voltage VC 3  of VC 4 . 
     Also, in the event that the levels of Vb 1  and Vb 2  are +3E and −3E respectively, and VC 3  and VC 4  are both controlled to a level 1E corresponding to (⅓) of Vb 1 , the potential of the alternating current output point U is +2E, and the clamp voltages of SW 1 , SW 2 , Q 2 , Q 3 , Q 4 , and Q 5  are 1E, 1E, 1E, 1E, 4E, and 2E respectively. 
     When the semiconductor switches Q 2 , Q 6 , QR 1 , and QR 2  are turned on, and Q 1 , Q 3 , Q 4 , Q 5 , QR 3 , and QR 5  are turned off, as in the switching pattern P 3 ′, output current, flowing along a path SW 1 →Q 6 →C 4 →C 3 →Q 2 →U, passes through the three semiconductor switches SW 1 , Q 6 , and Q 2 , and voltage (VC 3 +VC 4 ) is output to the alternating current output point U. 
     At this time, in the event that the levels of Vb 1  and Vb 2  are +3E and −3E respectively, and VC 1  and VC 2  are both controlled to a level 1E corresponding to (⅓) of Vb 1 , the potential of the alternating current output point U is +2E, and the clamp voltages of SW 1 , SW 2 , Q 1 , Q 3 , Q 4 , and Q 5  are 1E, 1E, 1E, 2E, 3E, and 2E respectively. 
     When the semiconductor switches Q 1 , Q 3 , and Q 6  are turned on, and Q 2 , Q 4 , Q 5 , QR 1 , QR 3 , and QR 6  are turned off, as in the switching pattern P 4 ′, output current, flowing along a path b 1 →Q 1 →C 3 →C 4 →Q 3 →U, passes through the two semiconductor switches Q 1  and Q 3 , and voltage (Vb 1 −VC 3 −VC 4 ) is output to the alternating current output point U. 
     At this time, in the event that the levels of Vb 1  and Vb 2  are +3E and −3E respectively, and VC 1  and VC 2  are both controlled to a level 1E corresponding to (⅓) of Vb 1 , the potential of the alternating current output point U is +1E, and the clamp voltages of SW 1 , SW 2 , SW 3 , Q 2 , Q 4 , and Q 5  are 1E, 1E, 1E, 2E, 4E, and 2E respectively. 
     When SW 1 , SW 2  and Q 2  are turned on, and Q 1 , Q 3 , Q 4 , Q 5 , Q 6 , and QR 5  are turned off, as in the switching pattern P 5 ′, output current, flowing along a path SW 1 →SW 2 →C 3 →Q 2 →U, passes through the three semiconductor switches SW 1 , SW 2 , and Q 2 , and voltage (VC 3 ) is output to the alternating current output point U. 
     At this time, in the event that the levels of Vb 1  and Vb 2  are +3E and −3E respectively, and VC 1  and VC 2  are both controlled to a level 1E corresponding to (⅓) of Vb 1 , the potential of the alternating current output point U is +1E, and the clamp voltages of SW 3 , Q 1 , Q 3 , Q 4 , Q 5 , and Q 6  are 1E, 2E, 2E, 2E, 1E, and 1E respectively. 
     When SW 1 , SW 3 , and Q 6  are turned on, and Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and QR 3  are turned off, as in the switching pattern P 6 ′, output current, flowing along a path SW 1 →Q 6 →C 4 →SW 3 →U, passes through the three semiconductor switches SW 1 , Q 6  and SW 3  and voltage (VC 4 ) is output to the alternating current output point U. 
     At this time, in the event that the levels of Vb 1  and Vb 2  are +3E and −3E respectively, and VC 1  and VC 2  are both controlled to a level 1E corresponding to (⅓) of Vb 1 , the potential of the alternating current output point U is +1E, and the clamp voltages of SW 2 , Q 1 , Q 2 , Q 3 , Q 4 , and Q 5  are 1E, 1E, 1E, 1E, 3E, and 2E respectively. 
     When SW 1 , SW 2  and SW 3  are turned on, and Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6  are turned off, as in the switching pattern P 8 ′, output current, flowing along a path SW 1 →SW 2 →SW 3 →U, passes through the three semiconductor switches SW 1 , SW 2 , and SW 3 , and the potential 0 of the zero terminal M is output to the alternating current output point U. 
     At this time, in the event that the levels of Vb 1  and Vb 2  are +3E and −3E respectively, and VC 1  and VC 2  are both controlled to a level 1E corresponding to (⅓) of Vb 1 , the potential of the alternating current output point U is +1E, and the clamp voltages of Q 1 , Q 2 , Q 3 , Q 4 , Q 5  and Q 6  are 2E, 1E, 1E, 2E, 1E, and 1E respectively. 
     As the respective operations in the switching pattern P 7 ′ to the switching pattern P 13 ′ are the same as those in the switching pattern P 1 ′ to the switching pattern P 6 ′ from the symmetry of the circuit, a description will be omitted. 
     In accordance with the switching pattern P 1 ′ to the switching pattern P 13 ′, it is possible to output seven levels of voltages, 3E, 2E, 1E, 0, −1E, −2E, and −3E, to the alternating current output point U. Herein, the same +2E is output in the switching patterns P 2 ′ and P 3 ′, but considering an active power output time at which the direction of alternating current output current i from the alternating current output point U is the same, the capacitor C 3  is charged at the current i in P 2 ′, and the capacitors C 3  and C 4  are both discharged at the current i in P 2 . 
     Also, with regard to P 4 ′ to P 6 ′ when +1E is output to the alternating current output point U, the capacitors C 3  and C 4  are both discharged in P 4 ′, the capacitor C 3  is discharged in P 5 ′, and the capacitor C 4  is discharged in P 6 ′, at the alternating current output current i. 
     In the same way, with regard to P 8 ′ to P 10 ′ when −1E is output to the alternating current output point U, the capacitor C 3  is discharged in P 8 ′, the capacitor C 4  is discharged in P 9 ′, and the capacitors C 3  and C 4  are both charged in P 10 ′, at the alternating current output current i. 
     With regard to P 11 ′ and P 12 ′ when −1E is output to the alternating current output point U, the capacitors C 3  and C 4  are both discharged in P 11 ′, and the capacitor C 4  is charged in P 12 ′. 
     From the above, by appropriately selecting the switching patterns P 2 ′ and P 3 ′ when outputting the voltage of +2E to the alternating current output point U, P 4 ′ to P 6 ′ when outputting the voltage of +1E, P 8 ′ to P 10 ′ when outputting the voltage of −1E, and P 11 ′ and P 12 ′ when outputting the voltage of −2E, it is possible to adjust VC 3  and VC 4 , and it is possible to always maintain VC 3  and VC 4  at 1E. Also, in the switching patterns P 1 ′, P 7 ′, and P 13 ′, as no current flows through the capacitor C 3  or C 4 , no change occurs in VC 3  or VC 4 .  FIG. 13  shows operation patterns of the circuit of  FIG. 12 . 
       FIG. 14  shows a fourth modification example of  FIG. 1 . The difference from  FIG. 1  is that the semiconductor switches Q 1 , Q 6 , Q 7 , and Q 8  are changed to diodes D 1 , D 6 , D 7 , and D 8  respectively. 
     In the circuit of  FIG. 1 , both an inverter operation which converts direct current to alternating current and a rectifier operation which converts alternating current to direct current are possible, but if limited to only the rectifier operation, it is possible to apply the circuit of  FIG. 14 . In the circuit of  FIG. 14 , as inexpensive diodes are used in place of controllable semiconductor switches such as Q 1 , Q 6 , Q 7 , and Q 8 , price reduction is possible. 
     A description will hereafter be given of a case of causing  FIG. 14  to operate as a seven-level converter. 
     “In the case of the switching pattern P 1 ” 
     In this case, the alternating current voltage is +3E, and the path of current is U→Q 3 →Q 2 →D 1 →b 1 . 
     “In the case of the switching pattern P 2 ” 
     In this case, the alternating current voltage is +2E, and the path of current is U→Q 4 →C 1 →Q 2 →D 1 →b 1 . 
     “In the case of the switching pattern P 3 ” 
     In this case, the alternating current voltage is +2E, and the path of current is U→Q 3 →C 1 →Q 5 →C 2 →D 1 →b 1 . 
     “In the case of the switching pattern P 4 ” 
     In this case, the alternating current voltage is +2E, and the path of current is U→Q 3 →Q 2 →C 2 →D 8 →SW 1 . 
     “In the case of the switching pattern P 5 ” 
     In this case, the alternating current voltage is +1E, and the path of current is U→Q 4 →Q 5 →C 2 →D 1 →b 1 . 
     “In the case of the switching pattern P 6 ” 
     In this case, the alternating current voltage is +1E, and the path of current is U→Q 4 →C 1 →Q 2 →C 2 →D 8 →SW 1 . 
     “In the case of the switching pattern P 7 ” 
     In this case, the alternating current voltage is +1E, and the path of current is U→Q 3 →C 1 →Q 5 →D 8 →SW 1 . 
     “In the case of the switching pattern P 8 ” 
     In this case, the alternating current voltage is 0, and the path of current is U→Q 4 →Q 5 →D 8 →SW 1 . 
     “In the case of the switching pattern P 9 ” 
     In this case, the alternating current voltage is 0, and the path of current is SW 1 →D 7 →Q 2 →Q 3 →U. 
     Hereafter, as the operations are the same as heretofore described in the switching patterns P 10  to P 16  too from the symmetry of the circuit, a description will be omitted. 
       FIG. 15  shows a fifth modification example of  FIG. 1 . As this example is such that an alternating current switch SW 2  is added to  FIG. 14 , a description will be omitted. 
       FIG. 16  is a sixth modification example of  FIG. 1 , and a modification example of  FIG. 8  too. The difference from  FIG. 8  is that the semiconductor switches Q 1 , Q 6 , Q 7 , Q 8 , Q 9 , and Q 10  are changed to diodes D 1 , D 6 , D 7 , D 8 , D 9 , and D 10  respectively. 
     In the circuit of  FIG. 8 , an inverter operation which converts direct current to alternating current and a rectifier operation which converts alternating current to direct current are both possible, but if limited to only the rectifier operation, it is possible to apply the circuit of  FIG. 16 . In the circuit of  FIG. 16 , as inexpensive diodes are used in place of controllable semiconductor switches such as Q 1 , Q 6 , Q 7 , and Q 8 , price reduction is possible. 
     As the operations of  FIG. 16  is the same as those of  FIG. 14  in the switching patterns P 1  to P 6 , a description will be omitted. 
     In the switching pattern P 7 , the alternating current voltage is +1E, and the path of current is U→Q 3 →C 1 →D 10 →SW 2 →SW 1 . 
     In the switching pattern P 8 , the alternating current voltage is 0, and the path of current is U→Q 4 →D 10 →SW 2 →SW 1 . 
     In the switching pattern P 9 , the alternating current voltage is 0, and the path of current is SW 1 →SW 2 →D 9 →Q 3 →U. 
     Hereafter, as the operations are the same as heretofore described in the switching patterns P 10  to P 16  too from the symmetry of the circuit, a description will be omitted. 
       FIG. 17  is a seventh modification example of  FIG. 1 , and a modification example of  FIG. 16  too. That is, as this example is such that an alternating current switch SW 3  is added to  FIG. 16 , a description will be omitted. The switching patterns P 1  to P 16  used in  FIGS. 14, 15, 16, and 17  are the same as their respective switching patterns P 1  to P 16  shown in  FIGS. 3, 7, 9, and 11 . 
       FIG. 18  is a first modification example of  FIG. 12 . The difference from  FIG. 12  is that the semiconductor switches Q 1 , Q 4 , Q 5 , and Q 6  are changed to diodes D 1 , D 4 , D 5 , and D 6  respectively. 
     In the circuit of  FIG. 12 , an inverter operation which converts direct current to alternating current and a rectifier operation which converts alternating current to direct current are both possible, but if limited to only the rectifier operation, it is possible to apply the circuit of  FIG. 18 . In the circuit of  FIG. 18 , as inexpensive diodes are used in place of controllable semiconductor switches such as Q 1 , Q 4 , Q 5 , and Q 6 , price reduction is possible. 
     Herein, a description will be given of the operations. Herein, it is taken to use the same switching patterns P 1 ′ to P 13 ′ as those of  FIG. 13 . 
     In the switching pattern P 1 ′, the alternating current voltage is +3E, and the path of current is U→Q 2 →D 1 →b 1 . 
     In the switching pattern P 2 ′, the alternating current voltage is +2E, and the path of current is U→SW 3 →C 3 →D 1 →b 1 . 
     In the switching pattern P 3 ′, the alternating current voltage is +2E, and the path of current is U→Q 2 →C 3 →C 4 →D 6 →SW 1 . 
     In the switching pattern P 4 ′, the alternating current voltage is +1E, and the path of current is U→Q 3 →C 4 →C 3 →D 1 →b 1 . 
     In the switching pattern P 5 ′, the alternating current voltage is +1E, and the path of current is U→Q 2 →C 3 →SW 2 →SW 1 . 
     In the switching pattern P 6 ′, the alternating current voltage is +1E, and the path of current is U→SW 3 →C 4 →D 6 →SW 1 . 
     In the switching pattern P 7 ′, the alternating current voltage is 0, and the path of current is U→SW 3 →SW 2 →SW 1 . 
     Hereafter, as the operations are the same as heretofore described in the switching patterns P 8 ′ to P 13 ′ too from the symmetry of the circuit, a description will be omitted. 
       FIG. 19  is an eighth modification example of  FIG. 1 . The difference from  FIG. 1  is that the semiconductor switch Q 1  is substituted by a series circuit of Q 1   a  and Q 1   b , and the semiconductor switch Q 6  is substituted by a series circuit of Q 6   a  and Q 6   b.    
     Herein, the substitute semiconductor switches are such that when in switching patterns P 1  to P 3  and P 5  of a seven-level operation, Q 1   a  and Q 1   b  are both turned on, and Q 6   a  and Q 6   b  are both turned off. Also, when in switching patterns P 4  and P 6  to P 8 , only one of Q 1   a  or Q 1   b  is turned on, and Q 6   a  and Q 6   b  are both turned off. When in switching patterns P 9  to P 11  and P 13 , Q 1   a  and Q 1   b  are both turned off, and only one of Q 6   a  or Q 6   b  is turned on. Further, when in switching patterns P 12  and P 14  to P 16 , Q 1   a  and Q 1   b  are both turned off, and Q 6   a  and Q 6   b  are both turned on. 
     On and off patterns in  FIG. 19  are shown in  FIG. 20 . 
     Each of the pair of Q 1   a  and Q 1   b  and the pair of Q 6   a  and Q 6   b , by being caused to operate in this way, can be replaced with a semiconductor switch series connection circuit with in the order of half the principled minimum breakdown voltage 4E required of Q 1  and Q 6  in  FIG. 1 . In this case, as they are individually switched by their respective low-voltage semiconductor elements when making a transition from one mode to another, it is possible to reduce switching loss, thus enabling loss reduction. For example, when using high-voltage IGBTs with several kV or more as switching elements, when they are IGBTs with a certain breakdown voltage or higher, there is a case in which the switching characteristics or steady loss characteristics deteriorate extremely, but by taking the heretofore described kinds of measures in this kind of case, it is possible to apply low-voltage elements superior in the characteristics, thus possible to contribute to loss reduction. 
       FIG. 21  shows a second modification example of  FIG. 12 . The difference from  FIG. 12  is that the semiconductor switch Q 1  is substituted by a series circuit of Q 1   a  and Q 1   b , and likewise, the semiconductor switch Q 4  is substituted by a series circuit of Q 4   a  and Q 4   b , in the same way as in  FIG. 19 . 
     Herein, the substitute semiconductor switches are such that when in switching patterns P 1 ′, P 2 ′, and P 4 ′ of a seven-level operation, Q 1   a  and Q 1   b  are both turned on, and Q 4   a  and Q 4   b  are both turned off. Also, when in switching patterns P 5 ′ to P 9 ′, only one of Q 1   a  or Q 1   b  and only one of Q 4   a  or Q 4   b  are turned on. When in switching patterns P 10 ′, P 12 ′, and P 13 ′, Q 1   a  and Q 1   b  are both turned off, and Q 4   a  and Q 4   b  are both turned on. Further, when in switching patterns P 3 ′ and P 11 ′, Q 1   a , Q 1   b , Q 4   a , and Q 4   b  are all turned off. 
     On and off patterns in  FIG. 21  are shown in  FIG. 22 . I to VII in  FIG. 22  represent operation modes, wherein I represents a case of causing the semiconductor switches to operate in the switching pattern P 1 ′, IIa in the switching pattern P 2 ′, IIb in the switching pattern P 3 ′, . . . VII in the switching pattern P 13 ′. 
     Each of the pair of Q 1   a  and Q 1   b  and the pair of Q 4   a  and Q 4   b , by being caused to operate in this way, can be replaced with a semiconductor switch series connection circuit with in the order of half the principled minimum breakdown voltage 4E required of Q 1  and Q 4  in  FIG. 12 , and by applying low-voltage elements superior in the characteristics, it is possible to contribute to loss reduction.