Patent Publication Number: US-9425701-B2

Title: Power conversion device

Description:
TECHNICAL FIELD 
     The present invention relates to a power conversion device or apparatus for converting ac power of utility frequency or commercial power frequency, directly into desired ac power. 
     BACKGROUND ART 
     There is known a matrix converter as a power conversion apparatus for converting ac power to ac power directly and efficiently with a construction requiring a smaller number of component parts and enabling size reduction of the apparatus (Patent Document 1). 
     However, an output line is long in the above-mentioned matrix converter of earlier technology in which a plurality of IGBTs (Insulated Gate Bipolar Transistors) are disposed in an inline arrangement, and the output line is connected collectively from each IGBT. Especially, in a power conversion apparatus in which high frequency ac current flows through the output line, the apparatus becomes susceptible to the influence of L component if the length of a wiring is great. 
     PRIOR ART LITERATURE 
     Patent Document(s) 
     
         
         Patent Document 1: JP2006-333590 A 
       
    
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide power conversion device or apparatus for reducing the length of the output line. 
     According to the present invention, a plurality of switching devices forming a power conversion circuit are arranged so that output terminals of the switching devices are arranged in a row and an output line is drawn out in one direction. 
     According to the present invention, the output line can be drawn out in one direction with an arrangement of the switching devices, so that it is possible to reduce the length of the output line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electric circuit diagram showing a power conversion system to which one embodiment of the present invention is applied. 
         FIG. 2A  is a plan view showing a power conversion apparatus according to the embodiment of the present invention, in an intermediate state under an assembly process. 
         FIG. 2B  is a plan view showing the power conversion apparatus according to the embodiment of the present invention, in an intermediate state under the assembly process. 
         FIG. 2C  is a plan view showing the power conversion apparatus according to the embodiment of the present invention, in an intermediate state under the assembly process. 
         FIG. 2D  is a side view showing the power conversion apparatus according to the embodiment of the present invention, in an intermediate state under the assembly process. 
         FIG. 3  is a view showing a layout of IGBTs and filter condensers of the power conversion apparatus shown in  FIG. 2 , in a plan view and a side view. 
         FIG. 4A  is a plan view showing another layout of the IGBTs and filter condensers shown in  FIG. 3 . 
         FIG. 4B  is a side view of  FIG. 4A . 
         FIG. 5  is a view showing still another layout of the IGBTs and filter condensers shown in  FIG. 3 , in a plan view. 
         FIG. 6  is a view showing still another layout of the IGBTs and filter condensers shown in  FIG. 3 , in a plan view. 
         FIG. 7  is an electric circuit diagram showing a power conversion system to which another embodiment of the present invention is applied. 
         FIG. 8  is a view showing a layout of the IGBTs and filter condensers shown in  FIG. 7 , in a plan view and a side view. 
         FIG. 9  is a view showing another layout of the IGBTs and filter condensers shown in  FIG. 7 , in a plan view and a side view. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     Outline of Power Conversion System  1   
     First,  FIG. 1  is used for illustrating the outline of a power conversion system to which an embodiment of the present invention is applied. A power conversion system  1  of this example is a system to convert three-phase ac power supplied from a three-phase ac power supply or power source  2 , directly to single-phase ac power, with a power conversion apparatus or device  3  according to the embodiment of the present invention, to step up or down the single-phase ac power to an appropriate voltage with a transformer  4 , and thereafter to convert the ac power to dc power with a rectifier  5 , and thereby to charge a secondary battery  6 . There is further provided a smoothing circuit  7 . 
     A filter circuit  8  is provided, in power conversion system  1  of this example, for attenuating higher harmonics for noise suppression for each phase of output lines (R phase, S phase and T phase) to supply the three-phase ac power from three-phase ac power supply or source  2 . Filter circuit  8  of this example includes three filter reactors  81  connected with the three phases R, S and T, respectively, and six filter condensers or capacitors  82 L,  82 R connected among the three phases R, S and T. A layout of filter condensers  82 L,  82 R (shown in  FIGS. 3 ˜ 6 , as filter condensers  821 ˜ 826 ) is explained later. 
     In the power conversion system of this example, the three-phase ac power is supplied through filter circuit  8 , to power conversion apparatus  3 , and converted to the signal-phase ac power. Power conversion apparatus  3  of this example includes  6  bidirectional switching devices  31  arranged in a matrix corresponding to the R, S and T phases. Hereinafter, a reference numeral  31  is used, as a generic term, to denote one of the bidirectional switching devices generally, and reference numerals  311 ˜ 316  are used to denote a specific one of the six bidirectional switching devices, as shown in  FIG. 1 . 
     Each of the bidirectional switching devices  31  of this example is an IGBT module including a semiconductor switching element in the form of an IGBT (Insulated Gate Bipolar Transistor), and an anti-parallel freewheel diode or flyback diode combined in an anti-parallel connection. The construction of each bidirectional switching device  31  is not limited to the construction shown in the figure. For example, it is optional to employ a construction including two reverse blocking IGBT elements in the anti-parallel connection. 
     A snubber circuit  32  is provided for each of bidirectional switching devices  31 , to protect the corresponding bidirectional switching device  31  from surge voltage generated with ON/OFF operation of the bidirectional switching device  31 . Snubber circuit  32  is connected with the input side and the output side of the corresponding bidirectional switching device  31  and formed by a combination of one snubber condenser or capacitor and three diodes. Hereinafter, a reference numeral  32  is used, as a generic term, to denote one of the snubber circuits generally, and reference numerals  321 ˜ 326  are used to denote a specific one of the six snubber circuits, as shown in  FIG. 1 . 
     A matrix converter control circuit  9  is provided, in the power conversion system  1  of this example, for ON/OFF control of each of bidirectional switching devices  31  of power conversion apparatus  3 . Matrix converter control circuit  9  receives, as inputs, a value of a voltage supplied from three-phase ac power supply  2 , a value of a dc current currently being outputted, and a value of a target current command, controls the gate signal of each of bidirectional switching devices  31  in accordance with these inputs, adjusts the single-phase ac power outputted to transformer  4 , and thereby obtains the dc power corresponding to a target. 
     Transformer  4  increases or decreases the voltage of single-phase ac power obtained by conversion of power conversion apparatus  3 , to a predetermined value. Rectifier  5  includes four rectifying diodes and convers the single-phase ac power of the adjusted voltage into dc power. Smoothing circuit  7  includes a coil and a condenser or capacitor and smooths pulsation included in the dc current obtained by the rectification, into a condition closer to the dc current. 
     The thus-constructed power conversion system  1  of this example converts the three-phase ac power supplied from three-phase power supply  2 , directly into the single-phase ac power with power conversion apparatus  3 , and convers the single-phase ac power into the dc power after the adjustment to a desired voltage. Thus, secondary battery  6  is charged. The power conversion system  1  is merely one example to which the power conversion apparatus  3  according to the present invention is applied. The present invention is not limited to this example in which the present invention is applied to the power conversion system  1 . The present invention is applicable to other power conversion systems when at least one of the power before conversion and the power after conversion is polyphase ac power. 
     Layout of Parts of Power Conversion Apparatus  3   
       FIGS. 2 ˜ 6  are views for illustrating the spatial layout or arrangement of parts constituting power conversion apparatus  3  shown in  FIG. 1 . In these figures, the same reference numerals are used for identical parts shown in  FIG. 1  to show the correspondence in the figures. 
       FIG. 2  includes  FIGS. 2A ˜ 2 D.  FIG. 2A  is a plan view showing an intermediate state during the assembly process, in which the six bidirectional switching devices  31  (also referred to as the IGBT modules) are mounted on an upper surface of a heat sink  10 .  FIG. 2B  is a plan view showing an intermediate state during the assembly process, in which busbars are further mounted, for connecting terminals of the bidirectional switching devices  31 .  FIG. 2C  is a plan view showing an intermediate state during the assembly process, in which, of the three diodes forming the snubber circuit  32 , and the filter condensers  82  of filter circuit, the left side three filter condensers are mounted.  FIG. 2D  is a side view showing the intermediate state during the assembly process. Since constituent parts of power conversion apparatus  3  of this example are overlapped in the plan view, in the following explanation, main portions are shown in another drawing. 
     As shown in  FIG. 2  and  FIG. 3 , each bidirectional switching device  31  of this example includes an input terminal, an output terminal and an intermediate or midpoint terminal between the two IGBTs arranged in a pair, and the inpute terminal, output terminal and intermediate terminal are provided on the upper side of the module package. Among the six bidirectional switching devices  311 ˜ 316  shown in  FIG. 3 , the left side terminals of the three left side bidirectional switching devices  311 ,  313  and  315  are input terminals, the right side terminals of the three left side bidirectional switching devices  311 ,  313  and  315  are output terminals, and the central terminals of the three left side bidirectional switching devices  311 ,  313  and  315  are intermediate terminals. Among the six bidirectional switching devices  311 ˜ 316  shown in  FIG. 3 , the right side terminals of the three right side bidirectional switching devices  312 ,  314  and  316  are input terminals, the left side terminals of the three right side bidirectional switching devices  312 ,  314  and  316  are output terminals, and the central terminals of the three right side bidirectional switching devices  312 ,  314  and  316  are intermediate terminals. Gate terminals of bidirectional switching devices  31  are provided in another part of the module package and omitted in the figure. 
     As shown in  FIG. 2  and  FIG. 3 , the six bidirectional switching devices  311 ˜ 316  are fixed on the upper surface of heat sink  10 , by fastening means such as bolts. As shown in these figures, the six bidirectional switching devices  311 ˜ 316  are arranged in three pairs: a first pair of bidirectional switching devices  311  and  312  disposed, respectively, on the left and right sides of a center line CL, a second pair of bidirectional switching devices  313  and  314  disposed, respectively, on the left and right sides of the center line CL, and a third pair of bidirectional switching devices  315  and  316  disposed, respectively, on the left and right sides of the center line CL. In other words, the bidirectional switching devices  311  and  312  are disposed side by side, on the left and right side of center line CL, respectively, along the extending direction in which the three terminals (input terminal, output terminal and intermediate terminals) of each bidirectional switching device  31  are extended or arranged; the bidirectional switching devices  313  and  314  are disposed side by side, on the left and right side of center line CL, respectively, along the extending direction; and the bidirectional switching devices  315  and  316  are disposed side by side, on the left and right side of center line CL, respectively, along the extending direction. Hereinafter, this arrangement is also expressed as “juxtaposition, or parallel arrangement, with respect to center line CL or output lines P, N connecting the output terminals”. This arrangement is different from the arrangement shown in  FIG. 5 . The paired bidirectional switching devices are two bidirectional switching devices connected with the same one of the R, S, T phases of the input line. 
     With this arrangement or juxtaposition including the bidirectional switching devices  311  and  312 ;  313  and  314 ; or  315  and  316  of each pair disposed on the left and right sides of center line CL, it is possible to employ a layout to draw out the output lines P and N (busbars  331  and  332 ) in one direction at a minimum distance. Since influence of L component is increased by an increase of wiring outputting high frequency ac power, the arrangement of this example can restrain the influence of the L component. This effect of the arrangement of this example is more advantageous as compared to the example shown in  FIG. 5 . Thus, the output lines P and N are almost straight up to transformer  4 . 
     As mentioned before, the right end terminals of left side bidirectional switching devices  311 ,  313  and  315  on the left side of center line CL are all output terminals, and the left end terminals of left side bidirectional switching devices  311 ,  313  and  315  are all input terminals. The left end terminals of right side bidirectional switching devices  312 ,  314  and  316  on the right side of center line CL are all output terminals, and the right end terminals of right side bidirectional switching devices  312 ,  314  and  316  are all input terminals. 
     To the input terminals at the left ends of bidirectional switching devices  311 ,  313  and  315  on the left side of center line CL, the input lines R, S and T of one branch branching off from the input lines of three-phase ac power supply  2  are connected in an inward direction toward the center line CL. To the input terminals at the right end of bidirectional switching devices  312 ,  314  and  316  on the right side of center line CL, the input lines R, S and T of the other branch branching off from the input lines of three-phase ac power supply  2  are connected in an inward direction toward the center line CL. The R phase is connected to the input terminals of bidirectional switching devices  311  and  312 ; the S phase is connected to the input terminals of bidirectional switching devices  313  and  314 ; and the T phase is connected to the input terminals of bidirectional switching devices  315  and  316 . The input lines R, S and T on the left side are extended and connected in the inward direction toward center line CL, and the input lines R, S and T on the right side are also extended and connected in the inward direction toward center line CL. With this connecting arrangement of the input lines, it is possible to decreases the distance in the left and right direction, of heat sink  10  as compared to the arrangement in the other example shown in  FIG. 6 . 
     In the configuration of  FIG. 1 , the input lines R, S and T extending from three-phase ac power supply  2  to power conversion apparatus  3  branch off at the position between the filter reactors  81  and the filter condensers  82 L and  82 R. However, it is possible to employ a configuration in which the input lines R, S and T are divided into two branches on the upstream side of filter reactors  81 , and the filter reactors  81  are provided for each of the branches of the input lines R, S and T. 
     A busbar  331  forming an output line P of power conversion apparatus  3  is connected with the right end output terminals of bidirectional switching devices  311 ,  313  and  315  on the left side of center line CL. A busbar  332  forming an output line N of power conversion apparatus  3  is connected with the left end output terminals of bidirectional switching devices  312 ,  314  and  316  on the right side of center line CL. The forward ends of the busbars  331  and  332  are connected with transformer  4 . Busbars including these busbars  331  and  332  and busbars mentioned herein below are made of conductor such as copper, superior in the electrical conductivity. 
     A busbar  333  connects the input terminals of bidirectional switching devices  311  and  312  paired with each other and disposed on the left and right sides of center line CL. A busbar  334  connects the input terminals of bidirectional switching devices  313  and  314  paired with each other and disposed on the left and right sides of center line CL. A busbar  335  connects the input terminals of bidirectional switching devices  315  and  316  paired with each other and disposed on the left and right sides of center line CL. In the equivalent circuit shown in  FIG. 1 , the wirings corresponding to the busbars are shown with the same reference numerals, respectively. These busbars  333 ˜ 335  are not essential for the function of power conversion apparatus  3 , and therefore, it is optional to omit these busbars. 
     These busbars  333 ˜ 335  are arranged to intersect the busbars  331  and  332  forming the output lines P and N as viewed in a plan view. However, as shown in the side view of  FIG. 3 , the busbars  333 ˜ 335  connecting the input terminals are formed at a position higher than the busbars  331  and  332 , and thereby arranged to avoid interference therebetween with a multilevel crossing structure of overpass or underpass. 
     The filter condensers  82 L and  82 R provided between two of the phases can be used in common by employing the arrangement in which the bidirectional switching devices  311  and  312  disposed on the left and right sides of center line in the first pair are connected, the bidirectional switching devices  313  and  314  in the second pair are connected, and the bidirectional switching devices  315  and  316  in the third pair are connected. Specifically, filter condenser  821  is provided between the R and S phases on the left side in  FIG. 3 , and filter condenser  824  is provided between the R and S phases on the right side in  FIG. 3 . The busbar  333  connects the input terminals of bidirectional switching devices  311  and  312  to which the R phase is inputted. Therefore, noises in the R phase of three-phase ac power supply  2  are removed by cooperative filtering operation of the two filter condensers  821  and  824 . Consequently, it is possible to reduce the capacity of one filter condenser and hence to reduce the sizes of the filter condensers. The same is applied to the S phase and the T phase. 
     The filter circuit in this example includes the six filter condensers  821 ˜ 826  so arranged that three of the six filter condensers are connected among the input lines on the left side of center line CL and the remaining three are connected among the input lines on the right side of center line CL, as shown in  FIG. 3 . The left side filter condenser  821  is provided between the S phase and the R phase which corresponds to the input terminal of bidirectional switching device  311 . Similarly, the left side filter condenser  822  is provided between the T phase and the S phase which corresponds to the input terminal of bidirectional switching device  313 . The left side filter condenser  823  is provided between the R phase and the T phase which corresponds to the input terminal of bidirectional switching device  315 . Similarly, the right side filter condenser  824  is provided between the S phase and the R phase corresponding to the input terminal of bidirectional switching device  313 . The right side filter condenser  825  is provided between the T phase and the S phase corresponding to the input terminal of bidirectional switching device  314 . The right side filter condenser  826  is provided between the R phase and the T phase corresponding to the input terminal of bidirectional switching device  316 . 
     With the arrangement in which the six filter condensers  821 ˜ 826  are arranged so that three are on the left side of center line CL and the other three filter condensers are on the right side, for the six bidirectional switching devices  311 ˜ 316  arranged so that three are on the left side of center line CL and the other three switching devices are on the right side, it is possible to reduce the distance or length of connection wire routing for each of filter condensers  821 ˜ 826  and bidirectional switching devices  311 ˜ 316 . 
     In this example, the left three and the right three of filter condensers  821 ˜ 826  are disposed on the outer sides of the region in which the six bidirectional switching devices  311 ˜ 316  are formed, with respect to center line CL. Concretely, as shown in  FIG. 2D , the left three and the right three of filter condensers  821 ˜ 826  are fixed on upper part of the busbars. With the arrangement in which filter condensers  821 ˜ 826  are so arranged that the bidirectional switching devices  311 ˜ 316  are located between the left three filter condensers and the right three filter condensers, it is possible to minimize the spacing between the left and right bidirectional switching devices  31 L and  31 R in the left and right direction. Therefore, it is possible to set the distance or length in the left and right direction, of heat sink  10  at a minimum value. As a result, it is possible to reduce the size of heat sink  10  as compared to the arrangement in another example shown in  FIG. 4A . 
     The left three and the right three of filer condensers  821 ˜ 826  are mounted on the left and right sides of center line CL as shown in  FIG. 2  showing the plan view and side view of an actual apparatus. 
     Beforehand, the explanation is directed to the connection structure of the busbars. As shown in  FIG. 2B , busbar  331  forms the output line P connecting the output terminals of bidirectional switching devices  311 ,  313  and  315  and leading to transformer  4 . Busbar  332  forms the output line N connecting the output terminals of bidirectional switching devices  312 ,  314  and  316  and leading to transformer  4 . Busbar  333  is a busbar connecting the input terminals of bidirectional switching devices  311  and  312 , and including a first end portion which extends outwards in a leftward direction beyond the input terminal of bidirectional switching device  311  and which is connected with a busbar  336  to connect the filter condenser  823 , and a second end portion which extends outwards in a rightward direction beyond the input terminal of bidirectional switching device  312  and which is connected with a busbar  337  to connect the filter condenser  826  (cf.  FIG. 2C  and  FIG. 3  for the connection state of filter condensers  823  and  826 ). Busbars  336  and  337  connected with both ends of busbar  333  are inclined with respect to a line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 , that is a line extending in an up and down direction as viewed in  FIG. 2C . 
     Busbar  334  is a busbar connecting the input terminals of bidirectional switching devices  313  and  314 , and including a first end portion which extends outwards in the leftward direction beyond the input terminal of bidirectional switching device  313  and which is connected with a busbar  338  to connect the filter condensers  821  and  822 , and a second end portion which extends outwards in the rightward direction beyond the input terminal of bidirectional switching device  314  and which is connected with a busbar  339  to connect the filter condensers  824  and  825  (cf.  FIG. 2C  and  FIG. 3  for the connection state of filter condensers  821 ,  822 ,  824  and  825 ). Busbars  338  and  339  connected with both ends of busbar  334  extend along the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 , that is the line extending in the up and down direction as viewed in  FIG. 2C . 
     Busbar  335  is a busbar connecting the input terminals of bidirectional switching devices  315  and  316 , and including a first end portion which extends outwards in the leftward direction beyond the input terminal of bidirectional switching device  315  and which is connected with a busbar  340  to connect the filter condenser  823 , and a second end portion which extends outwards in the rightward direction beyond the input terminal of bidirectional switching device  316  and which is connected with a busbar  341  to connect the filter condenser  826  (cf.  FIG. 2C  and  FIG. 3  for the connection state of filter condensers  823  and  826 ). Busbars  340  and  341  connected with both ends of busbar  335  are inclined with respect to the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 , that is the line extending in the up and down direction as viewed in  FIG. 2C . 
     As shown in  FIG. 2D , these busbars  333 ,  334  and  335  are connected with the input terminals of bidirectional switching devices  311 ˜ 316  through a plurality of busbars  345  and  346 , and disposed at a position or level above the busbars  331  and  332  forming the output lines P and N. With this arrangement, the busbars  333 ˜ 335  and the busbars  331  and  332  are separated in the height or vertical direction with a predetermined clearance without interference in the manner of grade separation or multilevel crossing. 
     As shown by broken lines in  FIG. 2C , filter condensers  821 ,  822  and  823  are disposed on the outer side with respect to center line CL, and arranged so that the centers of filter condensers  821 ,  822  and  823  are located, respectively, at the apexes of a triangle (preferably an isosceles triangle or an equilateral or regular triangle) which is oriented so that one of the apexes is directed in the outward direction. With the arrangement of the three filter condensers  821 ,  822  and  823  located at the apexes of the triangle, it is possible to set the wiring lengths among the condensers at minimum distances, to reduce the size of power conversion apparatus  3 , and to attain tuning among the condensers properly. Furthermore, with the arrangement in which the triangle is so oriented that one of the apexes of the triangle is directed in the outward direction, it is possible to improve the balance of wiring connecting to the condensers and to decrease the distance to each of the busbars  333 ,  334  and  335 , as compared to the arrangement in which one of the apexes of the triangle is directed in the inward direction, 
     Filter condenser  821  connected between the R phase and S phase is mounted on the upper surface of a busbar  342 . Filter condenser  822  connected between the S phase and T phase is mounted on the upper surface of a busbar  343 . These two busbars  342  and  343  are inclined with respect to a line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 , that is, a line extending in the up and down direction in  FIG. 2C . Moreover, these two busbars  342  and  343  are extended across the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 , that is, the line extending in the up and down direction in  FIG. 2C , and connected with busbars  333 ,  342  and  335 . Filter condensers  824  and  825  on the right side of center line CL are arranged symmetrically with respect to center line CL. 
     With the arrangement in which busbars  342  and  343  are inclined with respect to the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 , it is possible to make the wiring distance equal to the wiring distance of the filter condenser  823  connected between the R phase and T phase, as much as possible. Therefore, it is possible to attain tuning among filter condensers  821 ,  822  and  823 . Moreover, with the arrangement in which busbars  342  and  343  are provided across the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 , it is possible to reduce the connection distances of filter condensers  821  and  822  with busbars  333 ,  334  and  335 , and hence it is possible to reduce the size of power conversion apparatus  3 . With the arrangement in which each of the filter condensers  821 ˜ 826  is disposed on the upper surface of the busbars, namely the arrangement in which the bidirectional switching devices  311 ˜ 316  are disposed on one side of the busbars, and the filter condensers  821 ˜ 826  are on the opposite side of the busbars, the design freedom or flexibility of layout of filter condensers  821 ˜ 826  is increased. 
     Filter condenser  823  connected between the R phase and T phase is mounted on the upper surface of a busbar  344  connected between busbars  336  and  340 . This busbar  344  is disposed so that busbar  344  is parallel to the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 . 
     Following is explanation on an implementing example of three diodes and one snubber condenser or capacitor forming one of the snubber circuits  32  shown in  FIG. 1 . In the case of the snubber circuit  321  of bidirectional switching device  311 , for example, as shown in  FIG. 1 , a first terminal of snubber circuit  321  is connected with the input terminal of bidirectional switching device  311 , a second terminal of snubber circuit  321  is connected with the intermediate terminal of bidirectional switching device  311 , and a third terminal is connected with the output terminal of bidirectional switching device  311 . Therefore, as shown in  FIGS. 2C and 2D , the three diodes are fixed and connected, respectively, with brackets  351 ˜ 356  which are made of conductor and connected with the intermediate terminals of bidirectional switching devices  31 L and  31 R.  FIG. 2D  shows only the bracket  355 . 
     In this example, the conversion system uses a relatively large sized electrolytic condenser for the snubber condensers, and employs a snubber condenser  327  common to the six snubber circuits  321 ˜ 326  (cf.  FIG. 3 ). Busbars  347  and  348  for connecting this snubber condenser  327  and the three diodes are formed to extend, between the busbars  331  and  332  forming the output lines P and N, in the same direction as the output lines. 
     As shown in  FIG. 2D  and  FIG. 3 , the two busbars  347  and  348  connected with snubber condenser  327  are fixed at a level higher than the busbars  331  and  332  forming the output lines P and N, and lower than the busbars  333 ,  334  and  335 . These two busbars  347  and  348  are supported by heat sink  10  or a base (not shown) other than heat sink  10 . It is optional to provide insulating coating on the surfaces of busbars  347  and  348  to prevent short-circuit with busbars  333 ,  334  and  335 . 
     As to the layout of busbar  311  and  312  forming output lines P and N and busbars  347  and  348  leading to snubber condenser  327 , the disposition of busbars  347  and  348  between busbars  311  and  312  makes it possible to reduces the wiring distances of output lines P and N and the wiring distances to snubber condenser  327 . Moreover, the setting of busbars  347  and  348  at the position higher than busbar  311  and  312  makes it possible to reduce the distances from the diodes of snubber circuits  321 ˜ 326 . 
     According to this embodiment, it is possible to provide following advantages. 
     1) To the six bidirectional switching devices  311 ˜ 316 , three devices being on one of the left and right sides of center line CL, and the other three devices being on the other side, the six filter condensers  821 ˜ 826  are disposed so that three of the six filter condensers are disposed on the left side of center line CL to the three devices on the left side, and the remaining three filter condensers are disposed on the right side of center line CL to the right three bidirectional switching devices. Therefore, it is possible to reduce the routing or wiring distances of filter condensers  821 ˜ 826  and bidirectional switching devices  311 ˜ 316 . 
     2) In this example, the pair of bidirectional switching devices  311  and  312 , the pair of bidirectional switching devices  313  and  314 , and the pair of bidirectional switching devices  315  and  316  are arranged so that the two devices of each pair are arranged side by side on the left and right sides of center line CL, respectively. This layout makes it possible to draw out the output lines P and N (busbars  331  and  332 ) in one direction shortly. Therefore, the layout of this example can restrain the influence of the L component or inductance though a longer wire for outputting high frequency ac power would be susceptible to the influence of the L component. 
     3) In this example, the three of filter condensers  821 ˜ 826  on the left side and the other three filter condensers on the right side are disposed on the outer sides of the region in which the six bidirectional switching devices  311 ˜ 316  are provided, with respect to center line CL so that the region of the bidirectional switching devices is located between the three filter condensers on the left side and the other three filter condensers on the right side. Therefore, it is possible to minimize the spacing, in the left and right direction, between the left side bidirectional switching devices  31 L and the right side bidirectional switching devices  31 R. Consequently, it is possible to set the distance or dimension of heat sink  10  in the left and right direction at a minimum distance, and hence to reduce the size of heat sink  10 . 
     4) In this example, busbars  333 ,  334  and  335  connect the input terminals of bidirectional switching devices  311  and  312  arranged on the left and right side of center line CL in a pair, the input terminals of bidirectional switching devices  313  and  314  arranged on the left and right side of center line CL in a pair, and the input terminals of bidirectional switching devices  315  and  316  arranged on the left and right side of center line CL in a pair, respectively. Therefore, filter condensers  82 L and  82 R provided between the phases can be utilized for common use. Consequently, it is possible to reduce a capacity of each filter condenser and hence to reduce the size of the filter condensers. 
     5) In this example, to the input terminals of bidirectional switching devices  31 L, the left side input lines R, S and T are extended in the inward direction toward center line CL. Similarly, to the input terminals of bidirectional switching devices  31 R, the right side input lines R, S and T are extended in the inward direction toward center line CL. Therefore, it is possible to reduce the distance or dimension of heat sink  10  in the left and right direction. 
     6) In this example, filter condensers  821 ˜ 826  are disposed on the upper side of the busbars. In other words, the bidirectional switching devices  311 ˜ 316  are disposed on one side of the busbars, and the filter condensers  821 ˜ 826  are disposed on the other side of the busbars. Therefore, the freedom of layout design of filter condensers  821 ˜ 826  is increased. 
     7) In this example, as to the arrangement of busbars  331  and  332  forming output lines P and N and busbars  347  and  348  to snubber condenser  327 , the busbars  347  and  348  are disposed between busbars  331  and  332 . Therefore, it is possible to reduce the distances including the distances of output lines P and N and the wiring distance to snubber condensers  327 . 
     8) In this example, the busbars  347  and  348  are disposed at the position higher than busbars  331  and  332 . Therefore, it is possible to reduce the distances from the diodes of snubber circuits  321 ˜ 326 . 
     9) In this example, the three filter condensers  821 ,  822  and  823  are positioned at the apexes of a triangle, respectively. Therefore, it is possible to minimize the wiring distances among the condensers, to reduce the size of power conversion apparatus  3 , and to achieve tuning among the condensers. 
     10) In this example, the three condensers positioned so as to form a triangle are arranged so that one apex of the triangle is directed in the outward direction. Therefore, it is possible to improve the balance of wiring connected with the condensers as compared to the arrangement in which one apex is directed in the inward direction, and to reduce the distance to each of busbars  333 ,  334  and  335 . 
     11) In this example, the busbars  342  and  343  are inclined with respect to the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 . Therefore, it is possible to make the wiring distance equal to the wiring distance of the filter condenser  823  connected between the R phase and T phase, as much as possible. Therefore, it is possible to attain tuning among filter condensers  821 ,  822  and  823 . 
     12) In this example, the busbars  342  and  343  are provided across the line connecting the input terminals of bidirectional switching devices  311 ,  313  and  315 . Therefore, it is possible to reduce the connection distances of filter condensers  821  and  822  with busbars  333 ,  334  and  335 , and hence it is possible to reduce the size of power conversion apparatus  3 . 
     Other Embodiments 
     According to the present invention, variations and modifications are possible, besides the preceding embodiment. Following is explanation on variation examples according to the present invention. However, there is no intention of limiting the present invention to the above-mentioned embodiment, and following embodiments. Members used in the above-mentioned embodiment are given the same reference numerals and explanation is omitted appropriately. 
     In the above-mentioned embodiment, as shown in  FIG. 3 , the left side three filter condensers  82 L and the right side three filter condensers  82 R are disposed, respectively, on the outer sides of bidirectional switching devices  311 ,  313  and  315 , and on the outer side of bidirectional switching devices  312 ,  314  and  316  with respect to center line CL as the center. However, as shown in  FIGS. 4A and 4B , it is possible to place the left side three filter condensers  82 L and the right side three filter condensers  82 R, between the bidirectional switching devices  311 ,  313  and  315  on the left side of center line CL and the bidirectional switching devices  312 ,  314  and  316  on the right side of center line CL. 
     Moreover, in the above-mentioned embodiment, as shown in  FIG. 3 , the bidirectional switching devices  311 ,  313  and  315  are disposed on the right side of center line CL, and the bidirectional switching devices  312 ,  314  and  316  are disposed on the right side of center line CL. However, it is possible to employ an arrangement in which, as shown in  FIG. 5 , the bidirectional switching devices  311 ,  313  and  315  and the bidirectional switching devices  312 ,  314  and  316  are disposed along the center line CL. 
     In the above-mentioned embodiment, as shown in  FIG. 3 , the three bidirectional switching devices  311 ,  313  and  315  are disposed on the left side of center line CL, the three bidirectional switching devices  312 ,  314  and  316  are disposed on the right side of center line, and the input terminals and the output terminals of these six bidirectional switching devices  311 ˜ 316  are arranged symmetrically with respect to center line CL in a manner of line symmetry or reflection symmetry. However, as shown in  FIG. 6 , it is possible to employ an arrangement in which the three bidirectional switching devices  311 ,  313  and  315  are disposed on the left side of center line CL, the three bidirectional switching devices  312 ,  314  and  316  are disposed on the right side of center line, and the input and output terminals of the left side three bidirectional switching devices  311 ,  313  and  315  and the input and output terminals of the right side bidirectional switching devices  312 ,  314  and  316  are arranged in the same manner. In this case, the two set of the input lines R, S and T are extended in the same direction (in the rightward direction in the illustrated example) and connected with the input terminals of the respective bidirectional switching devices. 
     Moreover, in the above-mentioned embodiment, as shown in  FIG. 3 , filter condensers  821 ˜ 826  are provided between two phases so that each of the six bidirectional switching devices  311 ˜ 316  corresponds uniquely to one of the six filter condensers. However, as shown in  FIG. 7 , it is possible to employ an arrangement in which filter condensers  821 ˜ 826  are provided between two phases so that each of the six bidirectional switching devices  311 ˜ 316  corresponds uniquely to a plurality of filter condensers (two of filter condensers in the illustrated example). 
     In this case, the filter condensers may be disposed in a center region of the power conversion apparatus  3  as shown in  FIG. 8 , or may be disposed on the outer sides of power conversion apparatus  3 , as shown in  FIG. 9 . In the case of the arrangement in which the filter condensers are disposed in the center region of the power conversion apparatus  3  as shown in  FIG. 8 , it is possible to utilize free space and hence to restrain or reduce the size of power conversion apparatus  3  as much as possible. 
     The bidirectional switching devices  311 ,  313  and  315  correspond to a first switching device or element in the claims of the present invention, and the bidirectional switching devices  312 ,  314  and  316  correspond to a second switching device or element in the claims of the present invention. The power conversion apparatus  3  corresponds to a conversion circuit in the claims of the present invention. The busbars  331  and  332  corresponds to an output line in the claims of the present invention.