Abstract:
Provided is a power converting apparatus which suppresses noise caused by a square wave voltage that is sharply changed according to switching of the power converting apparatus. 
     The invention has a power converting apparatus including a first inverter circuit connected to a DC power supply side; and a second inverter circuit connected to a load side, wherein the first inverter circuit converts DC power from the DC power supply into power having an absolute waveform of an AC waveform, and the second inverter circuit converts the power of the absolute waveform every single cycle thereof into AC power by alternately inverting the power.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a power converting apparatus which connects a DC power supply to a power system or a load. 
       BACKGROUND ART 
       [0002]    A power converting apparatus which receives DC power, converts the DC power into AC power, and outputs the AC power to a load (a rotary electric machine or the like) includes a plurality of switching elements, and converts the supplied DC power into the AC power as the switching elements repeat switching operations. A general single-phase power converting apparatus which converts DC power into AC power is illustrated in  FIG. 2 . As illustrated in  FIG. 2 , the power converting apparatus is configured by connecting a DC power supply E to the input side of a single-phase inverter  3  and connecting a load to the output side of the single-phase inverter  3 , and in order to control AC power supplied to the load, turns on or off switching elements on the basis of a PWM switching signal from a control circuit  7 . The AC power is output to the load on the basis of an output voltage v 6  controlled as above. 
         [0003]    However, in a method of controlling the switching elements on the basis of the above-mentioned PWM switching signal, the output voltage v 6  generates noise base on a square wave-like voltage including a surge voltage or ringing, that is, a sharply changing voltage. Therefore, due to stray capacitances between the single-phase inverter  3  and a housing  18 , leakage current flows and generates noise that affects other devices existing in the periphery of the power converting apparatus. It is known that the noise is a factor of dielectric breakdown of a motor or a reactor or malfunction of electronic devices, and thus there is a demand for a power converting apparatus that suppresses noise. 
         [0004]    An example of a power converting apparatus for the purpose of suppressing noise due to a surge voltage or ringing is disclosed in JP-A-2000-295857 (PTL 1). 
         [0005]    A circuit described in PTL 1 is illustrated in  FIG. 3 . In PTL 1, a power converting apparatus in which an LC filter circuit  8  constituted by an inductor and a capacitor is disposed between a load  6  and a single-phase inverter circuit  3  is disclosed. It is possible for the LC filter circuit  8  to suppress noise in a high frequency band based on a surge voltage output to a motor or ringing. However, in the circuit described in PTL 1, noise in a low frequency band cannot be suppressed. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [PTL 1] JP-A-2000-295857 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    An object of the invention is to provide a power converting apparatus which reduces noise caused by a sharply changing voltage and thus reduces power consumption. 
       Solution to Problem 
       [0008]    In order to solve the problems, the invention provides a power converting apparatus including: a first inverter circuit connected to a DC power supply side; and a second inverter circuit connected to a load side, wherein the first inverter circuit converts DC power from the DC power supply into power having an absolute waveform of an AC waveform, and the second inverter circuit converts the power of the absolute waveform every single cycle thereof into AC power by alternately inverting the power. 
       Advantageous Effects of Invention 
       [0009]    According to the invention, the power converting apparatus which reduces noise caused by a sharply changing voltage and thus reduces power consumption may be provided. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  illustrates a power converting apparatus that represents a first example of the invention. 
           [0011]      FIG. 2  illustrates a general power converting apparatus which converts DC power into AC power. 
           [0012]      FIG. 3  illustrates a general power converting apparatus which suppresses noise. 
           [0013]      FIG. 4  illustrates a control waveform of a capacitor voltage v 1  of a bidirectional buck-boost chopper part. 
           [0014]      FIG. 5  is a schematic diagram of switching signals S 1S  and S 2S  of the bidirectional buck-boost chopper. 
           [0015]      FIG. 6  is a control block diagram of the bidirectional buck-boost chopper. 
           [0016]      FIG. 7  illustrates operation modes of the bidirectional buck-boost chopper. 
           [0017]      FIG. 8  illustrates a waveform of a capacitor voltage v 2  of a low-pass filter part. 
           [0018]      FIG. 9  is a schematic diagram of switching signals S 3S , S 4S , S 5S , and S 6S  of a single-phase inverter and an output voltage v 3 . 
           [0019]      FIG. 10  is a control block diagram of the single-phase inverter. 
           [0020]      FIG. 11  illustrates a mechanism for generating a leakage current i leakx  which is a factor of noise. 
           [0021]      FIG. 12  illustrates voltage waveforms of corresponding parts of the general power converting apparatus which converts the DC power into the AC power. 
           [0022]      FIG. 13  illustrates voltage waveforms of corresponding parts of the power converting apparatus that represents the first example of the invention. 
           [0023]      FIG. 14  illustrates a power converting apparatus that represents a second example of the invention. 
           [0024]      FIG. 15  illustrates a power converting apparatus that represents a third example of the invention. 
           [0025]      FIG. 16  is a schematic diagram of a system in which the power converting apparatus that represents the third example of the invention is applied to an EV. 
           [0026]      FIG. 17  illustrates a power converting apparatus that represents a fourth example of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    An embodiment of the invention will be described with reference to the drawings. 
       First Example 
       [0028]    A power converting apparatus  50  according to a first embodiment is illustrated in  FIG. 1 .  FIG. 1  illustrates the power converting apparatus  50  for operating a fan and a pump. The power converting apparatus  50  is constituted by a bidirectional buck-boost chopper  1 , a low-pass filter  2 , a single-phase inverter  3 , a control circuit  4  that controls conduction and shutoff of switching elements S i  and S 2  of the bidirectional buck-boost chopper  1 , and a control circuit  5  that controls conduction and shutoff of switching elements S 3 , S 4 , S 5 , and S 6  of the single-phase inverter  3 . In addition, the control circuit  4  and the control circuit  5  may be separately configured, or may also be configured as a single control circuit  56 . The switching elements S 3  and S 5  of the single-phase inverter  3  constitute the upper arm of an inverter circuit, and the switching elements S 4  and S 6  of the single-phase inverter  3  constitute the lower arm of the inverter circuit. The bidirectional buck-boost chopper  1  is constituted by the switching elements S 1  and S 2 , diodes D 1  and D 2 , an inductor L 1 , and a capacitor C 1 . For example, in a case where NPN-type IGBTs are used as the switching elements S 1  and S 2 , a configuration in which the high potential side of a DC power supply  51  is connected to the collector side of the switching element S 1  and the cathode side of the diode D 1 , the emitter side of the switching element S 1  and the anode side of the diode D 1  are connected to the collector side of the switching element S 2 , the cathode side of the diode D 2 , and one terminal of the inductor L 1 , the emitter side of the switching element S 2  and the anode side of the diode D 2  are connected to the low potential side of the capacitor C 1 , and the high potential side of the capacitor C 1  is connected to the other terminal of the inductor L 1  and the low potential side of the DC power supply E is made. In a case where PNP-type IGBTs are used as the switching elements S 1  and S 2 , all the components are connected to have reverse polarities. 
         [0029]    Conduction and shutoff of the switching elements S 1  and S 2  is controlled by the control circuit  4 , thereby controlling a voltage v 3  of the capacitor C 1 . 
         [0030]    The low-pass filter  2  is constituted by inductors L 2  and L 3  and capacitors C 2 , C 3 , and C 4 . A configuration in which the high potential side of the capacitor C 1  is connected to one terminal of the inductor L 2 , the other terminal of the inductor L 2  is connected to the high potential sides of the capacitors C 2  and C 4 , the low potential side of the capacitor C 4  is connected to the low potential side of the capacitor C 3  and one terminal of the inductor L 3 , the high potential side of the capacitor C 3  is connected to the low potential side of the capacitor C 2  and a ground G, and the other terminal of the inductor L 3  is connected to the low potential side of the capacitor C 1  is made. 
         [0031]    The single-phase inverter  3  is constituted by the switching elements S 3 , S 4 , S 5 , and S 6  and diodes D 3 , D 4 , D 5 , and D 6 . For example, when NPN-type IGBTs are used as the switching elements S 3 , S 4 , S 5 , and S 6 , a configuration in which the high potential side of the capacitor C 4  is connected to the collector sides of the switching elements S 3  and S 5  and the cathode sides of the diodes D 3  and D 5 , the emitter side of the switching element S 3  and the anode side of the diode D 3  are connected to the collector side of the switching element S 4 , the cathode side of the diode D 4 , and one terminal of a load, the emitter side of the switching element S 5  and the anode of the diode D 5  are connected to the collector side of the switching element S 6 , the cathode side of the diode D 6 , and the other terminal of the load, and the emitter side of the switching element S 6  and the anode side of the diode D 6  are connected to the emitter side of the switching element S 4 , the anode side of the diode D 4 , and the low potential side of the capacitor C 4  is made. 
         [0032]    Conduction and shutoff of the switching elements S 3 , S 4 , S 5 , and S 6  is controlled by the control circuit  5 , thereby controlling an AC power output to the load. 
         [0033]    Subsequently, using  FIGS. 4 to 7 , a method of controlling a capacitor voltage v 1  will be described.  FIG. 6  is a control block diagram of the control circuit  4 . To the control circuit  4 , a current i 1  that flows to the inductor L 1 , the voltage v 1  applied to the capacitor C 1 , and an output voltage command v 3 * for controlling a voltage output to the load from the single-phase inverter  3  are input. The current i 1  that flows to the inductor L 1  is a current value detected by a resistor (not shown) for current detection connected to the inductor L 1  in series, and the voltage v 1  applied to the capacitor C 1  is a voltage value detected on the basis of the voltage of a resistor (not shown) connected to the capacitor C 1  in parallel. In addition, regarding a current detection method, using a current sensor may be considered. A command generator  9  calculates the absolute value |v 3 *| of the sine wave output voltage command v 3 * to be output to a comparator  100  as a capacitor command voltage v 1 *. The comparator  100  compares the capacitor command voltage v 1 * to the voltage v 1  applied to the capacitor C 1 , and calculates a deviation v 1 ** to be output to a voltage controller  10 . The voltage controller  10  calculates an inductor current command i 1 * to follow the capacitor command voltage v 1 * on the basis of the deviation v 1 ** so as to be output to a comparator  101 . The comparator  101  compares the inductor current command i 1 * to the inductor current command i 1 , and calculates a deviation i 1 ** to be output to a current controller  11 . The current controller  11  calculates a duty command signal D b * for determining conduction widths of the switching elements S 1  and S 2  to cause the current i 1  that flows to the inductor L 1  to follow the inductor current command i 1 * so as to be output to a comparator  12 . The comparator  12  compares the duty command signal D b * to a carrier signal having a predetermined frequency, which is generated by a carrier generator  13 , and outputs a switching signal S 1S  for controlling conduction and shutoff of the switching element to the gate of the switching element S 1  and an inverting circuit  14 . More specifically, in a case where the duty command signal D b * is greater than the carrier signal, a signal for turning on the switching element S 1  of the bidirectional buck-boost chopper  1  and a signal for turning off S 2  are generated, and in a case where the duty command signal D b * is smaller than the carrier signal, a signal for turning on the switching element S 1  of the bidirectional buck-boost chopper and a signal for turning off S 2  are generated. The inverting circuit  14  inverts the switching signal S 1S  so as to be output to the gate of the switching element S 2  as a switching signal S 2S . By performing the above-described control, the AC voltage having the absolute waveform as illustrated in  FIG. 4  is stored in the capacitor C 1 . In addition, a cycle from when the voltage stored in the capacitor C 1  becomes substantially 0 until the voltage becomes substantially 0 subsequent thereto is referred to as 1 cycle of the AC voltage of the absolute value waveform. 
         [0034]    The principle that the voltage of the capacitor C 1  becomes the absolute value waveform of a sine wave-like AC wave will be described using  FIGS. 5 and 7 . 
         [0035]    First, the principle of increasing the voltage of the capacitor C 1  will be described using  FIG. 7 . There is an operation mode of the bidirectional buck-boost chopper controlled on the basis of the switching signals S 1S  and S 2S  of the switching elements S 1  and S 2  of the above-described bidirectional buck-boost chopper  1 . At the time of increasing the capacitor voltage v 1  of the bidirectional buck-boost chopper part, as illustrated in  FIG. 7(   a ), the switching element S 1  is turned on, and the inductor current i 1  is allowed to flow through a loop closed by the DC power supply E, the switching element S 1 , and the inductor L 1  such that magnetic energy is accumulated in the inductor L 1 . Thereafter, as illustrated in  FIG. 7(   b ), by turning off the switching element S 1 , there is a transition to a mode in which the inductor current i 1  flows to the capacitor C 1  from the inductor L 1  via the diode D 2 . Accordingly, the magnetic energy accumulated in the inductor L 1  is absorbed by the capacitor C 1 , thereby increasing the capacitor voltage v 1 . The capacitor voltage v 1  may be set to be greater than the voltage of the DC power supply E by the bidirectional buck-boost chopper. 
         [0036]    On the other hand, at the time of decreasing the capacitor voltage v 1  of the bidirectional buck-boost chopper part, as illustrated in  FIG. 7(   c ), the switching element S 2  is turned on, and an inductor current −i 1  is allowed to flow through a loop closed by the capacitor C 1 , the inductor L 1 , and the switching element S 2  such that magnetic energy is accumulated in the inductor L 1 . Thereafter, as illustrated in  FIG. 7(   d ), by turning off the switching element S 2 , there is a transition to a mode in which the inductor current −i 1  flows to the DC power supply E from the inductor L 1  via the diode D 1 . Accordingly, the energy accumulated in the inductor L 1  is absorbed by the DC power supply E. That is, this means that the energy accumulated in the capacitor C 1  is regenerated in the DC power supply E, thereby decreasing the capacitor voltage v 1 . Here, in order to prevent a short circuit current from flowing, the switching elements S 1  and S 2  are prohibited from being simultaneously turned on.  FIG. 5  illustrates a method of writing conduction timings of the switching elements S 1  and S 2 , and the relationship between the conduction timings of the switching elements S 1  and S 2  and the voltage of the capacitor C 1 . The control circuit  4  calculates the duty command signal D b * that has the absolute waveform of the sine wave-like AC wave on the basis of the sine wave output voltage command v 3 * for controlling the switching elements S 3 , S 4 , S 5 , and S 6  of the single-phase inverter  3 . The switching signals S 1S  and S 2S  of the switching elements S 1  and S 2  are determined by the comparison between the duty command signal D b * and the carrier signal. Specifically, in a case where the duty command signal D b * is greater than the carrier signal, a signal for turning on the switching element S 1  of the bidirectional buck-boost chopper and a signal for turning off S 2  are generated, and in a case where the duty command signal D b * is smaller than the carrier signal, a signal for turning on the switching element S 1  of the bidirectional buck-boost chopper and a signal for turning off S 2  are generated. 
         [0037]      FIG. 5  also illustrates an enlarged diagram of a form at the time of increasing the voltage of the capacitor C 1 . At the time of increasing the voltage, the conduction width of the switching element S 1  is large, and energy stored in the inductor L 1  is increased. When the switching element S 1  is shut off, the energy stored in the inductor L 1  flows to the capacitor C 1 . Here, in a period in which the switching element S 1  is shut off, the switching element S 2  is also conducting. However, since the energy stored in the inductor L 1  is large, current flows in a direction illustrated in  FIG. 7(   b ). Therefore, even though the switching element S 2  is conducting, a current flow as illustrated in  FIG. 7(   c ) does not occur, and the capacitor voltage v 1  is increased. That is, whether to increase or decrease the capacitor voltage v 1  is determined by whether or not the energy stored in the inductor L 1  is large. The energy stored in the inductor L 1  is controlled by the duty command signal D b *. 
         [0038]    The capacitor voltage v 1  of the bidirectional buck-boost chopper  1  controlled on the basis of the above-described operations becomes a sharp waveform including a high frequency component in a switching frequency band of the bidirectional buck-boost chopper  1 . The low-pass filter  2  illustrated in  FIG. 1  may not be included. However, in order to further reduce noise, it is preferable that the bidirectional buck-boost chopper  1  and the single-phase inverter  3  be connected with the low-pass filter  2 . In a case where the low-pass filter  2  is not included, the positive pole side of the capacitor C 1  included in the bidirectional buck-boost chopper  1  is connected to the positive pole side of the single-phase inverter  3 , and the negative pole side of the capacitor C 1  is connected to the negative pole side of the single-phase inverter  3 . 
         [0039]      FIG. 8  illustrates a waveform of the capacitor voltage v 2  of the low-pass filter  2 . The capacitor voltage v 2  of the low-pass filter  2  becomes a waveform obtained by attenuating a high frequency component from the capacitor voltage v 1  of the bidirectional buck-boost chopper  1  by the low-pass filter  2 . A frequency band capable of being attenuated may be determined by Expression (1) depending on a cutoff frequency f c  of the low-pass filter  2 . It is preferable that a low-pass filter  2  capable of deleting a high frequency component to be deleted depending on the application be used. 
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         [0040]    A control method of controlling the single-phase inverter  3  will be described using  FIGS. 9 and 10 .  FIG. 10  is a control block diagram of the control circuit  5  that controls the switching elements S 3 , S 4 , S 5 , and S 6  included in the single-phase inverter  3 . To the control circuit  5 , a high-order command signal (for example, a voltage command required for a load or a frequency command) output from a high-order controller (not shown) and the voltage v 2  of the capacitor C 2  included in the low-pass filter  2  are input. In a case where the low-pass filter  2  is not included, the voltage v 1  of the capacitor C 1  included in the bidirectional buck-boost chopper  1  is input to the control circuit  5 . The high-order command signal is input to a command generator  15 , and the sine wave output voltage command v 3 * is calculated on the basis of the input command signal and is output to the comparator  12 . The sine wave output voltage command v 3 * and 0 (zero) are input to the comparator  12 , and by comparing the sine wave output voltage command v 3 * to 0 (zero), a duty command D INV * of the switching elements S 3 , S 4 , S 5 , and S G  of the single-phase inverter is generated. The duty command D INV * is output to a D flip-flop  17 . On the other hand, the voltage v 1  or the voltage v 2  input to the control circuit  5  is input to a zero voltage detector  16 , and a clock signal Z CLK  for switching a conduction or shutoff timing of the switching elements S 3  S 4 , S 5 , and S 6  in a case where the voltage v 1  or the voltage v 2  becomes substantially zero is output to the D flip-flop  17 . The D flip-flop  17  updates the duty command D INV * of the switching elements S 3 , S 4 , S 5 , and S 6  of the single-phase inverter at the time of rising of the clock signal Z CLK , and generates switching signals S 3s  and S 6S  of the switching elements S 3  and S 6  of the single-phase inverter to be output to the gates of the switching elements S 3  and S 6  and a NOT circuit  14 . The NOT circuit  14  generates switching signals S 4S  and S 5S  of the switching elements S 4  and S 5  by reversing the input switching signals S 3S  and S 66  so as to be output to the gates of the switching elements S 4  and S 5 . 
         [0041]    When the switching elements S 3  and S 6  of the single-phase inverter are turned on and the switching elements S 4  and S 5  are turned off, a sine wave voltage v 3  on the positive pole side may be output. On the other hand, when the switching elements S 4  and S 5  of the single-phase inverter are turned on and the switching elements S 3  and S 6  are turned off, a sine wave voltage v 3  on the negative pole side may be output. Here, in order to prevent a short circuit current from flowing, the switching elements S 3  and S 4  and the switching elements S 5  and S 6  of the single-phase inverter are prohibited from being simultaneously turned on. 
         [0042]    Since the switching elements S 3 , S 4 , S 5 , and S 6  of the single-phase inverter are switched at the time when the capacitor voltage v 2  of the low-pass filter  2  is a zero voltage, a surge voltage or ringing may be suppressed, and switching loss may be reduced. Moreover, since switching of the switching elements S 3 , S 4 , S 5 , and S 6  of the single-phase inverter is performed every single cycle of the capacitor voltage v 2  of the low-pass filter part, the number of switching operations may be reduced, and as a result, switching loss may be reduced. 
         [0043]      FIG. 11  illustrates a mechanism for generating a leakage current i leak  which is a factor of noise. As stray capacitances, there are C S1  that is generated between connection lines from the single-phase inverter  3  to the load  6 , C S2  that is generated between the single-phase inverter  3  and a housing  18  that stores the single-phase inverter  3 , the load  6 , and the like, C S3  that is generated between the high potential side connection line connected from the single-phase inverter  3  to the load  6  and the housing  18 , C S4  that is generated between the low potential side connection line connected from the single-phase inverter  3  to the load  6  and the housing  18 , and C S5  that is generated between the load  6  and the housing  18 . The leakage current leak in each part is generated due to a voltage change dv/dt between the stray capacitances C S  that are present in the respective parts and may be expressed by Expression (2). 
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         [0044]    From Expression (2), it can be seen that the leakage current i leak  which is a factor of noise may be reduced by suppressing the stray capacitances C S  in the respective parts and the voltage change dv/dt. 
         [0045]    The DC power output from the DC power supply  51  is stored in the capacitor C 1  of the bidirectional buck-boost chopper  1  to become a waveform of the absolute value of the AC power, and when the power stored to become the waveform of the absolute value of the AC power is at the zero voltage, conduction and shutoff of the switching elements S 3 , S 4 , S 5 , and S 6  of the single-phase inverter  3  is controlled, thereby performing switching operations in a state where a voltage change is small. Therefore, a surge voltage or ringing may be suppressed without a sharp change in the voltage output to the load  6 . Moreover, since a voltage change between the parts illustrated in  FIG. 11  is reduced, leakage current may be suppressed, and it is possible to reduce noise. 
         [0046]      FIG. 12  illustrates voltage waveforms v 6 , v 7 , and v 8  of corresponding parts of a general power converting apparatus illustrated in  FIG. 2 . The voltage waveforms v 6 , v 7 , and v 8  of the corresponding parts are square wave-like voltage waveforms, and sharp voltage changes are repeated. That is, as can be seen from Expression (2), high leakage current flows, resulting in increase in noise. 
         [0047]      FIG. 13  illustrates voltage waveforms v 3 , v 4 , and v 5  of corresponding parts of the power converting apparatus that represents the first example of the invention. The voltage waveforms v 3 , v 4 , and v 5  of the corresponding parts are sine wave-like voltage waveforms, and compared to the voltage waveforms v 6 , v 7 , and v 8  of the corresponding parts of the general power converting apparatus that converts DC power into AC power, which is illustrated in  FIG. 2 , a sharp voltage change may be suppressed. Therefore, leakage current may be reduced, and a harmonic component of the output current i 2  may be reduced. Therefore, the power converting apparatus that represents the first example of the invention can reduce noise, motor loss, and motor noise. 
         [0048]    As described above, since timing of conduction or shutoff is switched in the case where the voltage v 1  or the voltage v 2  becomes substantially zero, the number of switching operations in the single-phase inverter  3  may be reduced. Moreover, conduction or shutoff is switched in a state where a voltage is rarely applied to the switching elements, and thus reduction in switching loss may be achieved. 
         [0049]    In addition, when sine wave-like AC power is to be made only by the single-phase inverter, switching loss is generated due to the number of conduction and shutoff operations of the four switching elements. On the other hand, as in this example, a sine wave-like voltage is generated by the bidirectional buck-boost chopper  1  having the two switching elements. Therefore, switching loss is basically generated due to the number of conduction and shutoff operations of the two switching elements. Therefore, reduction in the number of switching operations and reduction in switching loss in the entire power converting apparatus may be achieved, and thus reduction in power consumption in the entire power converting apparatus may be achieved. 
         [0050]    Moreover, the inductor L 2  and L 3  of the low-pass filter  2  of the power converting apparatus that represents the first example of the invention function as high impedances for a high frequency voltage. That is, the inductor L 2  and L 3  of the low-pass filter part act as high impedances for a voltage change that occurs at the time of switching of the bidirectional buck-boost chopper, and thus leakage current that flows to the ground G may be suppressed, thereby reducing noise. In addition, a MOSFET or the like other than an IGBT may be applied to each of the switching elements S 1  to S 6 , and a bidirectional buck chopper or the like other than the bidirectional buck-boost chopper  1  may be applied to a DCDC converter. 
       Second Example 
       [0051]      FIG. 14  illustrates a power converting apparatus that represents a second example of the invention, and the power converting apparatus is for operating a plurality of fans and pumps. The power converter that represents the second example of the invention is constituted by connecting a single DC power supply E and a plurality of bidirectional buck-boost choppers, single-phase inverters, and loads in parallel, and like elements that have the same functions as those of the first example illustrated in  FIG. 1  are denoted by like reference numerals. Switching of each of the bidirectional buck-boost choppers and each of the single-phase inverter is controlled by each of control circuits  1  and  2  on the basis of each of output voltage commands and v 15 *, v 16 *, and v 17 * and the above-described control order. As a result, output voltages v 15 , v 16 , v 17  of the respective single-phase inverters may be generated to be sine wave-like, and thus the plurality of fans and pumps may be operated while reducing noise. 
       Third Embodiment 
       [0052]      FIG. 15  illustrates a power converting apparatus that represents a third example of the invention, and the power converting apparatus drives a three-phase motor mounted in a hybrid vehicle (hereinafter, “HEV”), a plug-in hybrid vehicle (hereinafter, “PHEV”), or an electric vehicle (hereinafter, “EV”). The power converting apparatus that represents the third example of the invention is constituted by connecting a single DC power supply E, three bidirectional buck-boost choppers, and three single-phase inverters in parallel and respectively connecting the single-phase inverters corresponding UVW phases and windings L u , L v , and L w , of the three-phase motor, and like elements that have the same functions as those of the first example illustrated in FIG.  1  are denoted by like reference numerals. Switching of each of the bidirectional buck-boost choppers and the single-phase inverters for the respective phases is controlled by each of control circuits  1  and  2  on the basis of output voltage commands v u *, v v *, and v w * having phases shifted by 120 degrees from each other and the above-described control order. As a result, output voltages v u , v v , and v w  of the respective single-phase inverters for the WW phases are generated to have sine wave waveforms with phases shifted by 120 degrees from each other, thereby driving the three-phase motor. In addition, it can be easily understood that even though a plurality of DC power supplies E are provided, the three-phase motor may be driven. In addition,  FIG. 16  illustrates an example of a system schematic diagram in which the power converting apparatus as the third example of the invention is applied to the EV. 
       Fourth Example 
       [0053]      FIG. 17  is a power converting apparatus that represents a fourth example of the invention, and the power converting apparatus connects the battery of a PHEV or EV to a power system. The power converting apparatus that represents the fourth example of the invention is a system in which the load part of the power converting apparatus described in the first example is replaced with an inductor L 6  for connection and the power system, and like elements that have the same functions are denoted by like reference numerals. As described above, an output voltage v 3  of a single-phase inverter may be generated to be sine wave-like, and thus harmonic current that flows into the power system may be suppressed when the battery and the power system are connected. Moreover, transmitting and receiving of power between the battery and the power system may be performed with high efficiency when battery voltage control and power factor control of the output current i 3  to are applied to switching control of a bidirectional buck-boost chopper and the single-phase inverter. 
         [0054]    By providing the configuration of this example, even when the battery mounted in the PHEV or EV and the power system are connected, inflow of harmonic current due to noise to the power system may be reduced. 
         [0055]    According to the invention, the output voltage v 3  of the single-phase inverter is generated to be sine wave-like, and switching of the switching elements S 3  to S 6  of the single-phase inverter is performed every single cycle of the capacitor voltage v 1  of the bidirectional buck-boost chopper  1  or the capacitor voltage v 2  of the low-pass filter  2  at the time of the zero voltage. Therefore, a surge voltage or ringing that occurs due to switching may be suppressed, and switching loss of the inverter may be reduced. Moreover, by using the low-pass filter  2 , each of the inductor L 2  and L 3  of the low-pass filter part functions as a high impedance for a sharp voltage change that is generated due to switching of the bidirectional buck-boost chopper. Therefore, leakage current that flows to the stray capacitances may further be reduced, noise may be reduced. Furthermore, in a case where the load is a motor, a harmonic component of the load current i 2  may be suppressed, and thus motor loss and motor noise may be reduced. Moreover, since leakage current may be reduced by reduction in noise, malfunction of other electronic devices due to leakage current may be prevented, and thus it is possible to provide a power converting apparatus having high reliability. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  bidirectional buck-boost chopper 
           2  low-pass filter 
           3  single-phase inverter 
           4 ,  5  control circuit 
           6  load 
           8  LC filter 
           50  power converting apparatus 
           52 ,  53  output terminal 
           60 ,  70  series circuit part 
           80 ,  90  phase of single-phase inverter 
         E DC power supply 
         S 1 , S 2 , S 3 , S 4 , S 5 , S 6  switching element 
         D 1 , D 2 , D 3 , D 4 , D 5 , D 6  diode 
         L 1 , L 2 , L 3 , L 4 , L 5 , L 6  inductor 
         C 1 , C 2 , C 3 , C 4  capacitor 
         i 1  inductor current of bidirectional buck-boost chopper 
         i 2 , i 3  output current of single-phase inverter 
         v 1  capacitor voltage of bidirectional buck-boost chopper 
         v 2  capacitor voltage of low-pass filter part 
         v 3  output voltage 
         v 4 , v 5  voltage of corresponding part 
         v 1 * capacitor voltage command 
         v 3 * output voltage command of single-phase inverter 
         θ phase angle of AC voltage