Abstract:
In a power conversion device including: a converter which steps up or down a voltage of a direct current power supply; and an inverter which converts the direct current voltage obtained by the converter into an alternating voltage to drive an electric motor, a plurality of magnetic parts are arranged above a switching element assembly unit with a water-cooled type second heat sink interposed between the switching element assembly unit and the plurality of magnetic parts, the switching element assembly unit configured by mounting all the switching element modules on upper and lower surfaces of a water-cooled type first heat sink. Accordingly, it is possible to provide a power conversion device capable of housing many switching element modules in a compact space and cooling them effectively, while preventing the influence of a noise due to the magnetic parts from acting on the switching element modules as much as possible.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application Nos. 2009-145642, 2009-145643 and 2009-145644 filed on Jun. 18, 2009 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power conversion device comprising: a converter which includes a plurality of switching element modules and a plurality of magnetic parts, and steps up or down a voltage of a direct current power supply; and an inverter which includes a plurality of switching element modules other than the switching element modules of the converter and converts the direct current voltage obtained by the converter into an alternating voltage to drive an electric motor. 
     2. Description of the Related Art 
     Japanese Patent Application Laid-open No. 2005-73374 discloses a power conversion device in which a main circuit is configured by interposing multiple switching element modules between a pair of coolant tubes in a cooling system, and an inverter is configured by interposing the main circuit between a power wiring section including magnetic parts and a control circuit board having a control circuit to control the switching element modules. 
     Incidentally, the power conversion device disclosed by the above-mentioned Laid-open Publication is an inverter. In a power conversion device including a converter and an inverter, the converter and the inverter each include multiple switching element modules, and those many switching element modules need to be cooled. However, it is desirable to avoid an increase in the size of the structure for cooling many switching element modules. Also, the influence of a noise due to the magnetic parts included in the converter needs to be prevented from acting on the switching element modules as much as possible. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of such background, and an object of the invention is to provide a power conversion device capable of housing many switching element modules in a compact space and cooling them effectively, while preventing the influence of a noise due to the magnetic parts from acting on the switching element modules as much as possible. 
     In order to achieve the above-mentioned object, according to a first feature of the present invention, there is provided a power conversion device comprising: a converter which includes a plurality of switching element modules and a plurality of magnetic parts, and steps up or down a voltage of a direct current power supply; and an inverter which includes a plurality of switching element modules other than the switching element modules of the converter and converts the direct current voltage obtained by the converter into an alternating voltage to drive an electric motor, wherein the plurality of magnetic parts are arranged above a switching element assembly unit with a water-cooled type second heat sink interposed between the switching element assembly unit and the plurality of magnetic parts, the switching element assembly unit configured by mounting all the switching element modules on upper and lower surfaces of a water-cooled type first heat sink. 
     According to the configuration of the first feature, the switching element assembly unit is configured by mounting the multiple switching element modules constituting part of the converters and the inverter on the upper and lower surfaces of the water-cooled type first heat sink, and the multiple magnetic parts are arranged above the switching element assembly unit with the water-cooled type second heat sink interposed between the switching element assembly unit and the multiple magnetic parts. This allows the power conversion device to be compact, while allowing the multiple switching element modules and magnetic parts to be cooled efficiently. In addition, an influence of a noise of the magnetic parts can be prevented from acting on the switching element modules by the second heat sink. 
     According to a second feature of the present invention, in addition to the first feature, a plurality of stud bolts each having threaded shank portions at opposite ends thereof are implanted in the first heat sink in such a manner that the threaded shank portions project from the upper and lower surfaces of the first heat sink, and the switching element modules are mounted on the upper and lower surfaces of the first heat sink in such a manner that the switching element modules are fastened to the threaded shank portions of selected stud bolts out of the stud bolts. 
     According to the above configuration, the switching element modules can be easily mounted on the upper and lower surfaces of the first heat sink by using few components. 
     According to a third feature of the present invention, in addition to the first or second feature, a cooling water supply source is connected in parallel to the first and second heat sinks in order to distribute and supply cooling water to the first and second heat sinks. 
     According to the above configuration, optimal cooling performance can be obtained for cooling the switching element modules and the magnetic parts. 
     According to a fourth feature of the present invention, in addition to the first feature, an even number of the switching element modules are mounted on the upper and lower surfaces of the first heat sink in a substantially symmetrical arrangement with respect to the first heat sink. 
     According to the above configuration, the cooling performance for each switching element module can be optimized. 
     According to a fifth feature of the present invention, in addition to the first feature, the plurality of switching element modules that constitute part of the converter and mounted on the upper and lower surfaces of the first heat sink, and magnetic parts that are directly coupled to the switching element modules constituting part of the converter out of all the magnetic parts, are connected to each other by bus bars. 
     According to the above configuration, the switching element assembly unit is configured by mounting the switching element modules of the converter on the first heat sink, multiple magnetic parts in the converter are arranged above the switching element assembly unit with the second heat sink interposed between the switching element assembly unit and the multiple magnetic parts; and the switching element modules and the magnetic parts directly coupled to each other are connected by the bus bars. Thus, the lengths of the bus bars can be minimized, and the assembly can be made easier. 
     According to a sixth feature of the present invention, in addition to the first feature, the plurality of magnetic parts and a capacitor unit are disposed above the switching element assembly unit with the second heat sink interposed between the switching element assembly unit and the plurality of magnetic parts and between the switching element assembly unit and the capacitor unit, the capacitor unit formed by integrally including input capacitors respectively included in first and second converters which step up or down voltages of a pair of the direct current power supplies. 
     According to the above configuration, the capacitor unit is formed by integrally including the input capacitors respectively included in the pair of converters. Compact arrangement of the both input capacitors allows the power conversion device to be compact, while heat transfer from the switching element modules side to the both input capacitors can be suppressed by the second heat sink. 
     According to a seventh feature of the present invention, in addition to the sixth feature, the capacitor unit includes a single grounding terminal common to the both input capacitors. 
     According to the above configuration, since the capacitor unit has the single grounding terminal common to the both input capacitors, not only the capacitor unit can be made more compact, but also its wiring inductance can be reduced. 
     According to an eighth feature of the present invention, in addition to the first feature, a DC link capacitor unit having a smoothing capacitor is provided between the converter and the inverter, positive side connection terminals of the plurality of switching element modules mounted on the upper and lower surfaces of the first heat sink are connected to positive side connection terminals provided on the DC link capacitor unit and are also connected in common to a positive side external bus bar, negative side connection terminals of the plurality of switching element modules mounted on the first heat sink are connected to negative side connection terminals provided on the DC link capacitor unit and are also connected in common to a negative side external bus bar, and the positive side and negative side external bus bars arranged outside the DC link capacitor unit are stacked with an insulating member interposed between the positive side and negative side external bus bars to be integrated into an external bus bar unit. 
     According to the above configuration, the current flowing through the internal wiring of the DC link capacitor unit is decreased. Thus, the internal wiring of the DC link capacitor unit is prevented from generating heat which causes an adverse thermal effect on the smoothing capacitor. 
     According to a ninth feature of the present invention, in addition to the eighth feature, the external bus bar unit is in direct contact with the first heat sink. 
     According to the above configuration, the heat generated in the external bus bar unit is directly transferred to the first heat sink side so that a temperature rise near the DC link capacitor unit can be suppressed. 
     The above and other objects, characteristics and advantages of the present invention will be clear from detailed descriptions which will be provided below for the preferred embodiment while referring to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall schematic circuit diagram of a power conversion device. 
         FIG. 2  is a side view of a switching element assembly unit. 
         FIG. 3  is a diagram viewed from an arrow  3  in  FIG. 2  with a control circuit omitted. 
         FIG. 4  is a cross-sectional view taken along a line  4 - 4  in  FIG. 3 . 
         FIG. 5  is a cutaway perspective view of an essential portion of the switching element assembly unit. 
         FIG. 6  is a cross-sectional view taken along a line  6 - 6  in  FIG. 2 . 
         FIGS. 7A and 7B  are diagrams showing a longitudinal sectional view of the switching element assembly unit and a change of a thermal conductivity in a direction of cooling water circulation in a first heat sink in comparison. 
         FIG. 8  is a perspective view of the power conversion device. 
         FIG. 9  is a plan view viewed from an arrow  9  in  FIG. 8 . 
         FIG. 10  is a diagram viewed from an arrow  10  in  FIG. 9 . 
         FIG. 11  is a cross-sectional view taken along a line  11 - 11  in  FIG. 9 . 
         FIG. 12  is a cross-sectional view of a second heat sink taken along a line  12 - 12  in  FIG. 10 . 
         FIG. 13  is a diagram showing a circulation circuit of cooling water. 
         FIG. 14  is a cross-sectional view taken along a line  14 - 14  in  FIG. 9 . 
         FIG. 15  is an exploded perspective view of a capacitor unit. 
         FIG. 16  is an exploded perspective view of the switching element assembly unit, a DC link capacitor unit, and an external bus bar unit. 
         FIG. 17  is an exploded perspective view of the external bus bar unit. 
         FIGS. 18A and 18B  are simplified diagrams showing current paths between a converter and an inverter in two cases for comparison,  FIG. 18A  being a case without the external bus bar unit, and  FIG. 18B  being a case with the external bus bar unit. 
         FIG. 19  is a simplified diagram showing an example of a path of commutation current in the DC link capacitor unit in the case with the external bus bar unit. 
         FIG. 20  is a simplified diagram showing another example of a path of commutation current in the DC link capacitor unit in the case with the external bus bar unit. 
         FIG. 21  is a simplified diagram showing yet another example of a path of commutation current in the DC link capacitor unit in the case with the external bus bar unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following, an embodiment of the present invention is described referring to  FIGS. 1 to 21 . At first, referring to  FIG. 1 , a power conversion device is a device to be installed in a vehicle to convert direct current power obtained from a fuel cell  15 , which is a first direct current power supply, and direct current power obtained from a storage battery  16 , which is a second direct current power supply, into three-phase alternating current power to be supplied to an electric motor  17  generating power to drive a driving wheel. The power conversion device includes a first converter  18  which steps up or down the direct current voltage obtained from the fuel cell  15 , a second converter  19  which steps up or down the direct current voltage obtained from the storage battery  16 , an inverter  20  which converts the direct current voltage from the first and second converters  18  and  19  into an alternating voltage to drive the electric motor  17 , and a DC link capacitor unit  21  provided between the both converters  18 ,  19  and the inverter  20 . 
     The first converter  18  includes a first input capacitor  22 , a first inductor  24 , a three-phase transformer  26 , a first, a second, and a third switching element modules  27 A,  27 B, and  27 C. The three-phase transformer  26  has a primary winding  26 A, a secondary winding  26 B, and a tertiary winding  26 C. 
     The negative terminal of the fuel cell  15  is connected to a ground line  28  common to the first converter  18 , the second converter  19 , and the inverter  20 . The first input capacitor  22  is provided between the ground line  28  and a first input-side positive line  29 , which is connected to the positive terminal of the fuel cell  15 . One end of the first inductor  24  is connected to the first input-side positive line  29 . Respective one ends of the primary winding  26 A, the secondary winding  26 B, and the tertiary winding  26 C in the three-phase transformer  26  are connected in parallel to the other end of the first inductor  24 . 
     The first switching element module  27 A includes a first positive side switching element  31 A and a first negative side switching element  32 A. The first positive side switching element  31 A is disposed between a common positive line  30  and the primary winding  26 A of the three-phase transformer  26 , the common positive line  30  being common to the first converter  18 , the second converter  19 , and the inverter  20 . The first negative side switching element  32 A is disposed between the primary winding  26 A and the ground line  28 . The second switching element module  27 B includes a second positive side switching element  31 B and a second negative side switching element  32 B. The second positive side switching element  31 B is disposed between the common positive line  30  and the secondary winding  26 B of the three-phase transformer  26 . The second negative side switching element  32 B is disposed between the secondary winding  26 B and the ground line  28 . The third switching element module  27 C includes a third positive side switching element  31 C and a third negative side switching element  32 C. The third positive side switching element  31 C is disposed between the common positive line  30  and the tertiary winding  26 C of the three-phase transformer  26 . The third negative side switching element  32 C is disposed between the tertiary winding  26 C and the ground line  28 . 
     The second converter  19  includes a second input capacitor  23 , a second inductor  25 , a two-phase transformer  33  having a primary winding  33 A and a secondary winding  33 B, and a fourth and a fifth switching element modules  27 D and  27 E. 
     The second input capacitor  23  is provided between a second input-side positive line  34  connected to the positive side terminal of the storage battery  16  and the ground line  28  connected to the negative side terminal of the storage battery  16 . One end of the second inductor  25  is connected to the second input-side positive line  34 . Respective one ends of the primary winding  33 A and the secondary winding  33 B in the two-phase transformer  33  are connected in parallel to the other end of the second inductor  25 . 
     The fourth switching element module  27 D includes a fourth positive side switching element  31 D and a fourth negative side switching element  32 D. The fourth positive side switching element  31 D is disposed between the common positive line  30  and the primary winding  33 A of the two-phase transformer  33 . The fourth negative side switching element  32 D is disposed between the primary winding  33 A and the ground line  28 . The fifth switching element module  27 E includes a fifth positive side switching element  31 E and a fifth negative side switching element  32 E. The fifth positive side switching element  31 E is disposed between the common positive line  30  and the secondary windings  33 B of the two-phase transformer  33 . The fifth negative side switching element  32 E is disposed between the secondary winding  33 B and the ground line  28 . 
     The inverter  20  includes a sixth, a seventh, and an eighth switching element modules  27 F,  27 G, and  27 H. 
     The sixth switching element module  27 F includes a sixth positive side switching element  31 F and a sixth negative side switching element  32 F. The sixth positive side switching element  31 F is disposed between the common positive line  30  and a U-phase power supply line  35 U connected to the electric motor  17  being a three-phase AC motor. The sixth negative side switching element  32 F is disposed between the U-phase power supply line  35 U and the ground line  28 . The seventh switching element module  27 G includes a seventh positive side switching element  31 G and a seventh negative side switching element  32 G. The seventh positive side switching element  31 G is disposed between the common positive line  30  and a V-phase power supply line  35 V connected to the electric motor  17 . The seventh negative side switching element  32 G is disposed between the V-phase power supply line  35 V and the ground line  28 . The eighth switching element module  27 H includes an eighth positive side switching element  31 H and an eighth negative side switching element  32 H. The eighth positive side switching element  31 H is disposed between the common positive line  30  and a W-phase power supply line  35 W connected to the electric motor  17 . The eighth negative side switching element  32 H is disposed between the W-phase power supply line  35 W and the ground line  28 . 
     Now, in this embodiment, the first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H in the first to eighth switching element modules  27 A to  27 H each includes an Insulated Gate Bipolar Transistor (IGBT), and a diode connected in parallel to each IGBT with a forward direction being a direction from an emitter to a collector. 
     The DC link capacitor unit  21  includes smoothing capacitors  36  provided between the common positive line  30  and the ground line  28 . Although only one smoothing capacitor  36  is shown for the sake of simplicity in  FIG. 1 , the DC link capacitor unit  21  includes the smoothing capacitors  36  corresponding to respective phases of U-phase, V-phase, and W-phase of the three-phase AC electric motor  17 , which are provided between the common positive line  30  and the ground line  28 . 
     A series circuit including a pair of discharge resistances  37  is connected between the common positive line  30  and the ground line  28 . 
     Referring to both  FIGS. 2 and 3 , the first, second, and third switching element modules  27 A,  27 B,  27 C included in the first converter  18 , the fourth and fifth switching element modules  27 D,  27 E included in the second converter  19 , and the sixth, seventh, and eighth switching element modules  27 F,  27 G,  27 H included in the inverter  20  are disposed on the upper and lower surfaces of a water-cooled type first heat sink  40 . 
     Further referring to  FIGS. 4 to 6  together, the first heat sink  40  includes a heat sink body  41  formed extending longer in one direction, and a lid  42  coupled to the heat sink body  41  from above to form a first cooling water passage  43  between the heat sink body  41  and the lid  42 . Thus, the first cooling water passage  43  is formed in such a manner that an outward passage portion  43   a  extending in the longitudinal direction of the heat sink  40  from one end of the first heat sink  40  to the other end thereof and a return passage portion  43   b  extending in parallel to the outward passage portion  43   a  from the other end of the first heat sink  40  to the one end thereof communicate with each other at the other end side of the first heat sink  40 . One end of the first heat sink  40  includes a cooling water inlet pipe  44  leading to the outward passage portion  43   a  in the first cooling water passage  43  and a cooling water outlet pipe  45  leading to the return passage portion  43   b  in the first cooling water passage  43 . The cooling water introduced from the cooling water inlet pipe  44  into the first cooling water passage  43  flows to the other end side of the first heat sink  40  through the outward passage portion  43   a  along the circulation direction shown by an arrow  46  in  FIG. 6 . The cooling water further flows around to the return passage portion  43   b  side at the other end side of the first heat sink  40  and is drawn from the cooling water outlet pipe  45  at the one end side of the first heat sink  40 . 
     Incidentally, the first converter  18  includes three switching element modules, i.e., the first, second, and third switching element modules  27 A,  27 B,  27 C, the second converter  19  includes two switching element modules, i.e., the fourth and fifth switching element modules  27 D,  27 E, and the inverter  20  includes three switching element modules, i.e., the sixth, seventh, and eighth switching element modules  27 F,  27 G,  27 H. Thus, an even number of switching element modules are disposed on the upper and the lower surfaces of the first heat sink  40 , specifically, eight switching element modules  27 A to  27 H in this embodiment. These switching element modules  27 A to  27 H are mounted on the upper and lower surfaces of the first heat sink  40  in a substantially symmetrical arrangement with respect to the first heat sink  40 . 
     When the heat generation amount of the first to third switching element modules  27 A to  27 C is e.g. 700 W, the heat generation amount of the fourth and the fifth switching element modules  27 D,  27 E is e.g. 500 W, and the heat generation amount of the sixth to eighth switching element modules  27 F to  27 H is e.g. 1100 W. Thus, a switching element module with a higher heat generation amount is disposed on the first heat sink&#39;s  40  lower surface where cooling is easier, while the first to eighth switching element modules  27 A to  27 H are mounted on the upper and lower surfaces of the first heat sink  40  in an substantially symmetrical arrangement with respect to the first heat sink  40 . In the present embodiment, the second and third switching element modules  27 B,  27 C, and the fourth and fifth switching element modules  27 D,  27 E are disposed on the upper surface of the first heat sink  40 , while the first switching element module  27 A, and the sixth, seventh, and eighth switching element modules  27 F,  27 G,  27 H are disposed on the lower surface of the first heat sink  40 . 
     Directing attention to  FIG. 4 , the first to eighth switching element modules  27 A to  27 H are on the upper and lower surfaces of the first heat sink  40  in such a manner that the first to eighth positive side switching elements  31 A to  31 H are arranged at one side of the first heat sink  40  in its width direction, i.e., the outward passage portion  43   a  side of the first cooling water passage  43  in the present embodiment, while the first to eighth negative side switching elements  32 A to  32 H are arranged at the other side of the first heat sink  40  in its width direction, i.e., the return passage portion  43   b  side of the first cooling water passage  43  in the present embodiment. 
     The multiple chips included in each of the first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H, i.e., six chips  47 ,  48 ,  49 ,  50 ,  51 ,  52  in the present embodiment, are arranged in such a manner that every two chips are aligned side-by-side along the circulation direction  46  of the cooling water in the first cooling water passage  43  as shown in  FIG. 3 . As shown in  FIG. 4 , the chips are disposed on a copper electrode  53  with a solder layer  54  interposed therebetween. A copper electrode  56  interposing an insulating substrate  55  between itself and the copper electrodes  53 , is fixed to a copper base plate  58  with a solder layer  57  interposed between the copper electrode  56  and the copper base plate  58 . 
     Synthetic resin cases  60  formed in a rectangular frame shape are arranged on the copper base plate  58 . Each case  60  has an opening  59  to dispose corresponding one of pairs of the first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H for the first to eighth switching element modules  27 A to  27 H, respectively. The copper base plates  58  and the cases  60  are fixed to the first heat sink  40 . 
     On the first heat sink  40 , as clearly shown in  FIG. 5 , multiple stud bolts  63  each having threaded shank portions  63   a  at opposite ends thereof are implanted at the positions corresponding to four corners of each case  60  in such a manner that the threaded shank portions  63   a  project from the upper and lower surfaces of the first heat sink  40 . The cases  60  and the copper base plates  58  of the first to eighth switching element modules  27 A to  27 H are secured by fastening the nuts  64  engaging with the threaded shank portions  63   a  of selected ones of stud bolts  63  out of all the stud bolts  63 , and thereby the first to eighth switching element modules  27 A to  27 H are mounted on the upper and lower surfaces of the first heat sink  40 . 
     Also a coated layer  65  made of a synthetic resin is formed inside each case  60  so as to embed therein the chips  47  to  52 , the copper electrode  53 , the solder layer  54 , the insulating substrate  55 , the copper electrode  56 , and the solder layer  57 . The first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H of the first to the eighth switching element modules  27 A to  27 H are sealed inside the coated layer  65 . Also, each case  60  is mounted with a control circuit  66  to control switching on/off of the corresponding one of first to eighth positive side switching elements  31 A to  31 H and the corresponding one of first to eighth negative side switching elements  32 A to  32 H of the first to eighth switching element modules  27 A to  27 H, in a manner that each control circuits  66  cover the corresponding one of first to eighth switching element modules  27 A to  27 H. 
     The first heat sink  40 , the first to eighth switching element modules  27 A to  27 H mounted on the upper and lower surfaces of the first heat sink  40 , and the control circuits  66  form a switching element assembly unit  67 . 
     Now, the first to eighth switching element modules  27 A to  27 H are mounted to the first heat sink  40  in such a manner that terminals to be connected to the three-phase transformer  26 , the two-phase transformer  33 , and the electric motor  17  are arranged at one side of the first heat sink  40  in its width direction. Terminal members  68 A,  68 B,  68 C of the first to third switching element modules  27 A to  27 C, which are coupled to the terminals to be connected to the three-phase transformer  26 , terminal members  68 D,  68 E of the fourth and fifth switching element modules  27 D,  27 E, which are coupled to the terminals to be connected to the two-phase transformer  33 , and terminal members  68 F,  68 G,  68 H of the sixth to eighth switching element modules  27 F to  27 H, which are coupled to the terminal to be connected to the electric motor  17  are mounted on the cases  60  so as to be arranged on one side of the first heat sink  40  in its width direction. 
     A positive side connection terminal  69  to be connected to the common positive line  30  and a negative side connection terminals  70  to be connected to the ground line  28  in each of the switching element modules  27 A to  27 H are mounted on the case  60  in a side-by-side arrangement for each of the switching element modules  27 A to  27 H at the other side of the first heat sink  40  in its width direction. 
     The first cooling water passage  43  included in the first heat sink  40  has multiple cooling fins  71 . The cooling fins  71  are configured by arranging in parallel multiple plate members formed of a light metal such as aluminum alloy with V-shaped cross sections in a direction perpendicular to the circulation direction  46 . The cooling fins  71  are disposed along the circulation direction  46  while dividing the inside of the first cooling water passage  43  into multiple portions in the width direction. 
     On the other hand, the multiple chips  47  to  52  included in each of the first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H of the first to eighth switching element modules  27 A to  27 H are arranged by twos side-by-side along the circulation direction  46  of the cooling water in the first cooling water passage  43 , i.e., the chips  47 ,  48 , the chips  49 ,  50 , and the chips  51 ,  52  are arranged in such a manner that each pair is aligned side-by-side along the circulation direction  46 . As shown in  FIG. 7A , a set of the cooling fins  71  is separately arranged for each of sets of chips aligned along the circulation direction  46 , i.e., the sets of chips  47 ,  49 ,  51  and the sets of chips  48 ,  50 ,  52 . In addition, as clearly shown in  FIG. 6 , the sets of cooling fins  71  for the set of chips  47 ,  49 ,  51  are arranged to be offset from the sets of cooling fins  71  for the set of chips  48 ,  50 ,  52  in a direction perpendicular to the circulation direction  46 . 
     By the way, it is known that when cooling water flows through each of the cooling fins  71 , a thermal entrance region exists near the inlet of each of the cooling fins  71  due to a change in the flow velocity distribution. In the entrance region, the thermal conductivity decreases in a downstream direction, and approaches to a certain thermal conductivity in a hydrodynamically developed region where the flow velocity becomes constant after the cooling water passes through the entrance region. Though relatively high thermal conductivity is achieved in the entrance region, in the hydrodynamically developed region, a boundary layer with the surface of the cooling fin  71  becomes thick and the heat transfer efficiency is reduced. Accordingly, when long, continuous fins are disposed along the circulation direction  46  in the first cooling water passage  43 , it is difficult to obtain a high cooling efficiency across the entire first cooling water passage  43 . However, as described above, the set of cooling fins  71  is separately arranged for each of the sets of chips of  47 ,  49 ,  51  and the sets of chips  48 ,  50 ,  52  aligned along the circulation direction  46 , and also the sets of cooling fins  71  for the set of chips  47 ,  49 ,  51  are arranged to be offset from the sets of cooling fins  71  for the set of chips  48 ,  50 ,  52  in a direction perpendicular to the circulation direction  46 . Accordingly, a thermal entrance region as shown in  FIG. 7B  can be formed at an inlet of each set of the cooling fins  71  separately arranged for the corresponding one of sets of chips  47 ,  49 ,  51  and sets of chips  48 ,  50 ,  52  aligned along the circulation direction  46 . Thus, it becomes possible to obtain a high cooling efficiency across the entire first cooling water passage  43 . 
     Referring to  FIGS. 8 to 11  together, at one side of the switching element assembly unit  67 , a substrate  72  to dispose a current sensor and the like is arranged in a manner that the terminal members  68 A to  68 H mounted to the cases  60  arranged at the one side of the first heat sink  40  in its width direction penetrates the substrate  72 . The substrate  72  is fixed to the first heat sink  40 . Above the switching element assembly unit  67 , a second heat sink  73  is arranged. The first inductor  24  and the three-phase transformer  26  in the first converter  18 , the second inductor  25  and the two-phase transformer  33  in the second converter  19 , a capacitor unit  38  configured by integrating the first input capacitor  22  of the first converter  18  and the second input capacitor  23  of the second converter  19  are arranged above the second heat sink  73  with the second heat sink  73  interposed between the switching element assembly units  67  and the components. 
     The second heat sink  73  includes a heat sink body  74  formed extending longer in the same direction as the longitudinal direction of the first heat sink  40 , and a lid  75  coupled to the heat sink body  74  from below to form a second cooling water passage  76  between the heat sink body  74  and the lid  75 . The second heat sink  73  is formed to be thinner than the first heat sink  40 . 
     In  FIG. 12 , the second cooling water passage  76  is formed between the heat sink body  74  and the lid  75  in such a manner that an outward passage portion  76   a  extending in the longitudinal direction of the heat sink  73  from one end of the second heat sink  73  toward the other end thereof and a return passage portion  76   b  extending in parallel to the outward passage portion  76   a  from the other end of the second heat sink  73  toward the one end thereof communicate with each other at the other end side of the second heat sink  73 . One end of the second heat sink  73  includes a cooling water inlet pipe  78  leading to the outward passage portion  76   a  and a cooling water outlet pipe  79  leading to the return passage portion  76   b . The cooling water introduced from the cooling water inlet pipe  78  into the second cooling water passage  76  flows to the other end side of the second heat sink  73  through the outward passage portion  76   a  of the second cooling water passage  76  along the circulation direction shown by an arrow  77 . The cooling water further flows around to the return passage portion  76   b  side at the other end side of the second heat sink  73  and is drawn from the cooling water outlet pipe  79  at the one end side of the second heat sink  73 . The heat sink body  74  in the second heat sink  73  is integrally provided with a straightening vane portion  74   a , which is arranged in the outward passage portion  76   a  of the second cooling water passage  76  in a manner extending along the circulation direction  77 . 
     In  FIG. 13 , the first cooling water passage  43  in the first heat sink  40 , and the second cooling water passage  76  in the second heat sink  73  are connected in parallel to an exhaust port of a cooling water pump  80  as a cooling water supply source. Cooling water drawn from the first and second cooling water passages  43  and  76  cools the electric motor  17 , and is then cooled by a radiator  81  and returns to the intake port of the cooling water pump  80 . 
     The three-phase transformer  26  of the first converter  18  is fixed on the second heat sink  73  so as to be arranged at a position at one side of the first and second heat sinks  40  and  73  in their width direction, i.e., the side at which substrate  72  is arranged, and at one end side of the first and second heat sinks  40  and  73 , i.e., the side at which the cooling water inlet pipes  44 ,  78  and the cooling water outlet pipes  45 ,  79  are provided. A first cover  84  covering the three-phase transformer  26  is fixed to the second heat sink  73 . 
     The two-phase transformer  33  of the second converter  19  is arranged at the one side of the first and second heat sinks  40 ,  73  in their width direction so as to be aligned with the three-phase transformer  26 . The second inductor  25  of the second converter  19  is arranged at the one side of the first and second heat sinks  40 ,  73  in their width direction with the two-phase transformer  33  interposed between the second inductor  25  and the three-phase transformer  26 . The two-phase transformer  33  and the second inductor  25  fixed on the second heat sink  73  are both covered with the second cover  85  fixed to the second heat sink  73 . Also, a first terminal block  86  located above the substrate  72  is fixed to the first cover  84 , and a second terminal block  87  located above the substrate  72  is fixed to the second cover  85 . 
     The first terminal block  86  includes a common terminal  88  to connect the winding wires  26 A,  26 B,  26 C of the three-phase transformer  26  to the first inductor  24 , and individual terminals  89 ,  90 ,  91  to individually connect the winding wires  26 A,  26 B,  26 C to the first to third switching element modules  27 A to  27 C, respectively. The individual terminals  89  to  91  are connected to the terminal members  68 A,  68 B,  68 C, which penetrate through the substrate  72  while coupling to the terminals to be connected to the three-phase transformer  26  of the first to third switching element modules  27 A to  27 C, via bus bars  92 ,  93 ,  94  located outside the substrate  72 , respectively. 
     The second terminal block  87  includes a common terminal  95  to connect the winding wires  33 A,  33 B of the two-phase transformer  26  to the second inductor  25 , and individual terminals  96 ,  97  to connect the winding wires  33 A,  33 B to the fourth and fifth switching element module  27 D,  27 E, respectively. Here, the individual terminal  96  is arranged below the common terminal  95 , and the individual terminal  97  is arranged above the common terminal  95 . The second terminal block  87  also includes terminals  98 ,  99  which couple to opposite ends of the second inductor  25 , respectively. The individual terminals  96 ,  97  are connected to terminal members  68 D,  68 E, which penetrate through the substrate  72  while coupling to the terminals to be connected to the two-phase transformer  33  of the fourth and fifth switching element modules  27 D,  27 E, via bus bars  100 ,  101  located outside the substrate  72 , respectively. Also, the common terminal  95  and the terminal  98  are connected via a bus bar  102 . 
     The first inductor  24  of the first converter  18  is fixed on the second heat sink  73  so as to be aligned with the three-phase transformer  26  at the other side of the first and second heat sinks  40 ,  73  in their width direction, i.e., the opposite side from where the substrate  72  is arranged, and at the one end side of the first and second heat sinks  40 ,  73 , i.e., the side at which the cooling water inlet pipes  44 ,  78  and the cooling water outlet pipes  45 ,  79  are provided. A third cover  105  covering the first inductor  24  is fixed to the second heat sink  73 . Also, a third terminal block  106  is fixed to the third cover  105  so as to be arranged at one end side of the first and second heat sinks  40 ,  73 . 
     The third terminal block  106  includes terminals  107 ,  108  which couple to opposite ends of the first inductor  24 , respectively. The one terminal  107  is connected to a bus bar  109  which extends to the other end side of the second heat sink  73  in its longitudinal direction between the switching element assembly unit  67  and the second heat sink  73 . The other terminal  108  is connected to the common terminal  88  provided on the first terminal block  86  via a bus bar  110 . 
     The capacitor unit  38  is fixed to the second heat sink  73  so as to be aligned with the first inductor  24  at the other side of the first and second heat sinks  40 ,  73  in their width direction, i.e., the opposite side from where the substrate  72  is arranged. The capacitor unit  38  is covered with a fourth cover  111  fixed to the second heat sink  73 . 
     In  FIGS. 14 and 15 , the first input capacitor  22 , which is a partial component of the capacitor unit  38 , is configured by connecting multiple first capacitor devices  112  arranged in parallel. The second input capacitor  23 , which is the rest of the components of the capacitor unit  38 , is configured by connecting multiple second capacitor devices  113  aligned parallel to the arrangement direction of the first capacitor devices  112 . The first capacitor devices  112  and the second capacitor devices  113 , which are included in the same number, are arranged in such a manner that their negative sides face each other. A common bus bar  114 , which has multiple negative side connection pieces  114   a  to be connected to the negative sides of the first and second capacitor devices  112 ,  113  by solder, is arranged so as to cover the first and second capacitor devices  112 ,  113  from the above. A first individual bus bar  115 , which has multiple positive side connection pieces  115   a  to be connected to the positive sides of the first capacitor devices  112  by solder, and a second individual bus bar  116 , which has multiple positive side connection pieces  116   a  to be connected to the positive sides of the second capacitor devices  113  by solider, are arranged on the common bus bar  114  with an insulating paper  117  interposed between the common bus bar  114  and the first individual bus bar  115 , as well as between the common bus bar  114  and the second individual bus bar  116 . The capacitor unit  38  configured by connecting the first and second capacitor devices  112 ,  113  with the common bus bar  114  and the first and second bus bars  115 ,  116  is housed in a case  119  so as to be embedded in a coated layer  118  made of a synthetic resin, and the case  119  is covered with the fourth cover  111 . 
     Also, the common bus bar  114  is integrally provided with a grounding terminal  120 , which is common to the first and second input capacitors  22 ,  23 , and projects from the coated layer  118 . The first and second individual bus bars  115 ,  116  are integrally provided with projecting positive terminals  121 ,  122  corresponding to the first and second input capacitors  22 ,  23 , respectively, in a manner that the positive terminals  121 ,  122  interpose the grounding terminal  120  therebetween. Both positive terminals  121  and  122  also project from the coated layer  118 . 
     A fourth terminal block  123  is fixed on the other end of the second heat sink  73 , at such position where the fourth terminal block  123  and the first inductor  24  interpose the capacitor unit  38 . The fourth terminal block  123  includes positive side and negative side terminals  124 ,  125  for fuel cell to be connected to the fuel cell  15 , and positive side and negative side terminals  126 ,  127  for storage battery to be connected to the storage battery  16 . 
     The positive terminal  121  of the capacitor unit  38 , and the bus bar  109  connected to the terminal  107  provided on the third terminal block  106  coupling to the first inductor  24  are connected to the fourth terminal block  123  so as to couple to the positive side terminal  124  for fuel cell. The positive terminal  122  of the capacitor unit  38 , and a bus bar  128  connected to the terminal  99  provided on the second terminal block  87  so as to couple to the second inductor  25  are connected to the fourth terminal block  123  so as to couple to the positive side terminal  126  for storage battery. 
     A fifth terminal block  130  to be connected to the U-phase, V-phase, and W-phase power supply lines  35 U,  36 V, and  35 W coupled to the electric motor  17  is fixed to the other end of the first heat sink  40 . Bus bars  131 ,  132 ,  133 , whose respective one ends are connected to the terminal members  68 F to  68 H of the sixth to eighth switching element modules  27 F to  27 H which penetrate through the substrate  72  while coupling to the terminals to be connected to the electric motor  17 , are extended to the fifth terminal block  130 . 
     Now, on the opposite side of the substrate  72 , the DC link capacitor unit  21  is arranged, which is supported on the first heat sink  40  by stays  135 . An external bus bar unit  136  is arranged between the first heat sink  40  and the DC link capacitor unit  21 . 
     In  FIG. 16 , from the lateral face of the DC link capacitor unit  21  which faces the first heat sink  40 , positive side connection terminals  139  and negative side connection terminals  140  project. The positive and negative side connection terminals  139 ,  140  are to be connected to the positive side connection terminals  69  and the negative side connection terminals  70  provided on the first to eighth switching element modules  27 A to  27 H mounted on the upper and lower surface of the first heat sink  40 . 
     In  FIG. 17 , the external bus bar unit  136  includes: a positive side external bus bar  141  having multiple positive side connection pieces  141   a  projecting from its both sides, the positive side connection pieces  141   a  connected to the positive side connection terminals  69  of the first to eighth switching element modules  27 A to  27 H and the positive side connection terminals  139  of the DC link capacitor unit  21 ; and a negative side external bus bar  142  having multiple negative side connection pieces  142   a  projecting from its both sides, the negative side connection pieces  142   a  connected to the negative side connection terminals  70  of the first to eighth switching element modules  27 A to  27 H, and the negative side connection terminals  140  of the DC link capacitor unit  21 . The external bus bar unit  136 , which is arranged outside the DC link capacitor unit  21 , is configured by stacking the positive side and negative side external bus bars  141 ,  142 , a plate-shaped insulating member  143  interposed between the positive side external bus bar  141  and the negative side external bus bar  142 , and plate-shaped insulating members  144 ,  145  which interpose the positive side and negative side external bus bars  141 ,  142  between themselves and the plate-shaped insulating member  143 , respectively. 
     One end of the negative side external bus bar  142  in the external bus bar unit  136  is integrally provided with a mounting plate portion  142   b  which projects more than one end of the positive side external bus bar  141 . The mounting plate portion  142   b  is fixed to the first heat sink  40  with a bolt  146 , and the external bus bar unit  136  is in direct contact with the first heat sink  40 . 
     Thus, as shown in  FIG. 11 , the positive side connection terminals  139  of the DC link capacitor unit  21 , positive side connection pieces  141   a  of the external bus bar unit  136 , and the positive side connection terminals  69  of the first to eighth switching element modules  27 A to  27 H are connected to each other by a bolt  147  with the positive side connection pieces  141   a  interposed between the positive side connection terminals  139  and the positive side connection terminals  69 . Also, the negative side connection terminals  140  of the DC link capacitor unit  21 , the negative side connection pieces  142   a  of the external bus bar unit  136 , and the negative side connection terminals  70  of the first to eighth switching element modules  27 A to  27 H are connected to each other by a bolt  148  with the negative side connection pieces  142   a  interposed between the negative side connection terminals  140  and the negative side connection terminals  70 . 
     Now, DC power obtained by the first converter  18  or the second converter  19  is converted by the inverter  20  into AC power, which is supplied to the electric motor  17 . The power from the first converter  18  or the second converter  19  is once stored in the smoothing capacitor  36  of the DC link capacitor unit  21 , and then the stored power is drawn by the inverter  20 . Such flow of power can be represented by a flow of current. When the external bus bar unit  136  is not provided, as shown by narrow line arrows in  FIG. 18A , a current i 1  supplied from the smoothing capacitor  36 , and a current i 2  supplied from the first converter  18  or the second converter  19  without passing through the smoothing capacitor  36  flow through the internal wiring of the DC link capacitor unit  21  to flow into the inverter  20 . If the internal wiring of the DC link capacitor unit  21  is designed to have the load of all currents as such, the internal wiring will generate heat or become larger in size. Thus, the heat generated at the wiring causes an adverse thermal effect on the smoothing capacitor  36 . 
     On the contrary, if the external bus bar unit  136  is interposed between the first converter  18  and the DC link capacitor unit  21  as well as the second converter  19  and the DC link capacitor unit  21 , as shown by narrow line arrows in  FIG. 18B , a portion i 2 ′ of the current i 2  supplied from the first converter  18  or the second converter  19  directly flow into the inverter  20  side through the positive side external bus bar  141  in the external bus bar unit  136 . Thus, the current flowing through the internal wiring of the DC link capacitor unit  21  can be suppressed to be low, and the thermal effect on the smoothing capacitor  36  can be suppressed to a lower level. 
     Also, commutation current flows into the DC link capacitor unit  21  by switching on/off of the sixth to eighth positive side switching elements  31 F to  31 H and the sixth to eighth negative side switching elements  32 F to  32 H of the sixth to eighth switching element modules  27 F to  27 H of the inverter  20 . Referring to  FIGS. 19 to 21 , the path of the commutation current flowing inside the DC link capacitor unit  21  and the external bus bar unit  136  when the sixth and seventh positive side switching elements  31 F,  31 G, and the sixth and seventh negative side switching elements  32 F,  32 G are switched on/off is described. Here, for the sake of simplicity, a portion corresponding to W-phase of the inverter  20  is omitted. 
     First, when the sixth positive side switching element  31 F and the seventh negative side switching element  32 G are switched on, and the sixth negative side switching element  32 F and the seventh positive side switching element  31 G are switched off, as shown by narrow line arrows in  FIG. 19 , currents i 3 ′, i 4 ′ corresponding to currents i 3 , i 4  flowing through the internal wiring of the DC link capacitor unit  21  flow through the positive side and negative side bus bars  141 ,  142  of the external bus bar unit  136 . Thus, the current flowing through the internal wiring of the DC link capacitor unit  21  can be suppressed to be low. 
     Also when the sixth and seventh negative side switching elements  32 F,  32 G are switched on, and the sixth and seventh positive side switching elements  31 F,  31 G are switched off, as shown by narrow line arrows in  FIG. 20 , a current i 5 ′ corresponding to a current i 5  flowing through the internal wiring of the DC link capacitor unit  21  flows through the negative side bus bar  142  of the external bus bar unit  136 . Thus, the current flowing through the internal wiring of the DC link capacitor unit  21  can be suppressed to be low. 
     Further, when the sixth and seventh positive side switching elements  31 F,  31 G are switched on, and the sixth and seventh negative side switching elements  32 F,  32 G are switched off, as shown by narrow line arrows in  FIG. 21 , a current i 6 ′ corresponding to a current i 6  flowing through the internal wiring of the DC link capacitor unit  21  flows through the positive side bus bar  141  of the external bus bar unit  136 . Thus, the current flowing through the internal wiring of the DC link capacitor unit  21  can be suppressed to be low. 
     In other words, common connection of the positive side connection terminals  69  of the first to eighth switching element modules  27 A to  27 H to each other outside the DC link capacitor unit  21  by the external bus bar unit  136 , and common connection of the negative side connection terminals  70  of the first to eighth switching element modules  27 A to  27 H to each other outside the DC link capacitor unit  21  by the external bus bar unit  136  allow a portion of the current supplied from the first converter  18  or the second converter  19  to the inverter  20  to flow outside the DC link capacitor unit  21 , and allow a portion of the commutation current generated by switching on/off the sixth to eighth positive side switching elements  31 F to  31 H and the sixth to eighth negative side switching elements  32 F to  32 H in the inverter  20  to flow outside the DC link capacitor unit  21 . 
     Next, an operation of the embodiment is described. The switching element assembly unit  67  is configured by including part of the first converter  18 , the second converter  19  and the inverter  20  on the upper and lower surfaces of the water-cooled type first heat sink  40  with the first to eighth switching element modules  27 A to  27 H mounted on the upper and lower surfaces. Above the switching element assembly unit  67 , the first inductor  24  and the three-phase transformer  26  in the first converter  18 , and the second inductor  25  and the two-phase transformer  33  in the second converter  19  are arranged with the water-cooled type second heat sink  73  interposed between the switching element assembly unit  67  and the above components. Thus, the power conversion device can be made small in size while efficiently cooling the first to eighth switching element modules  27 A to  27 H, the first inductor  24 , the three-phase transformer  26 , the second inductor  25 , and the two-phase transformer  33 . In addition, the second heat sink  73  allows the first to eighth switching element modules  27 A to  27 H to be less effected by a noise due to the first inductor  24 , the three-phase transformer  26 , the second inductor  25 , and the two-phase transformer  33 . 
     Also, the capacitor unit  38  formed by integrating the first input capacitor  22  included in the first converter  18  and the second input capacitor  23  included in the second converter  19  is arranged above the switching element assembly unit  67  with the second heat sink  73  interposed between the switching element assembly unit  67  and the capacitor unit  38 . Thus, compact arrangement of the first and second input capacitors  22 ,  23  allows the power conversion device to be compact, while heat transfer from the first to eighth switching element modules  27 A to  27 H to the first and second input capacitors  22 ,  23  is suppressed by the second heat sink  73 . 
     Moreover, since the capacitor unit  38  has the single grounding terminal  120  common to the first and second input capacitors  22 ,  23 , not only the capacitor unit  38  can be made more compact, but also its wiring inductance can be reduced. 
     The first to eighth switching element modules  27 A to  27 H, the number of which is even, are mounted on the upper and lower surfaces of the first heat sink  40  in a substantially symmetrical arrangement with respect to the first heat sink  40 . Thus, cooling performance of the first to eighth switching element modules  27 A to  27 H can be optimized. 
     Also, the first to eighth switching element modules  27 A to  27 H are mounted on the first heat sink  40  with their connection terminals arranged in the same direction. The first to the fifth switching element modules  27 A to  27 E, which are part of the first and second converters  18 ,  19  and are mounted on the first heat sink  40 , and the three-phase transformer  26  and the two-phase transformer  33 , which are part of the first and second converters  18 ,  19  and are disposed on the second heat sink  73  while being directly coupled to the first to the fifth switching element modules  27 A to  27 E, are connected to each other by the bus bars  92 ,  93 ,  94 ,  100 , and  101 . Thus, the lengths of the bus bars  92  to  94 ,  100 ,  101  can be minimized, and also the assembly of the unit can be made easier. 
     Incidentally, the multiple stud bolts  63  each having threaded shank portions  63   a  at opposite ends thereof are implanted in the first heat sink  40  in such a manner that the threaded shank portions  63   a  project from the upper and lower surfaces of the first heat sink  40 . The cases  60  and the copper base plates  58  of the first to eighth switching element modules  27 A to  27 H are secured by fastening the nuts  64  engaged with the threaded shank portions  63   a  of selected ones of stud bolts  63  out of all the stud bolts  63 , and thereby the first to eighth switching element modules  27 A to  27 H are mounted on the upper and lower surfaces of the first heat sink  40 . The first to eighth switching element modules  27 A to  27 H can be easily mounted on the upper and lower surfaces of the first heat sink  40  with fewer components. 
     The cooling water pump  80  which supplies cooling water to the first cooling water passage  43  in the first heat sink  40  and the second cooling water passage  76  in the second heat sink  73  is connected in parallel to the first and second cooling water passages  43 ,  76  to distribute and supply the cooling water from the cooling water pump  80  to the first and second heat sinks  40 ,  73 . Thus, optimal cooling performance can be obtained for cooling the first to eighth switching element modules  27 A to  27 H, the first inductor  24 , the three-phase transformer  26 , the second inductor  25 , and the two-phase transformer  33 . 
     The first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H included in the first to eighth switching element modules  27 A to  27 H each have multiple chips  47 ,  48 ,  49 ,  50 ,  51 , and  52 , which are arranged on the first heat sink  40  by twos side-by-side along the circulation direction  46  of the cooling water in the first cooling water passage  43 . The set of the cooling fins  71  is separately arranged for each set of chips of  47 ,  49 ,  51  and each set of chips  48 ,  50 ,  52  aligned along the circulation direction  46  in the first cooling water passage  43  included in the first heat sink  40 . Also, as clearly shown in  FIG. 6 , the sets of cooling fins  71  for the sets of chips of  47 ,  49 ,  51  are arranged to be offset from the sets of cooling fins  71  for the sets of chips  48 ,  50 ,  52  in a direction perpendicular to the circulation direction  46 . 
     Thereby, thermal entrance regions as shown in  FIG. 7B  can be formed at the inlet of each set of the cooling fins  71  separately arranged for corresponding one of sets of chips  47 ,  49 ,  51  and sets of chips  48 ,  50 ,  52  aligned along the circulation direction  46 . Accordingly, a high cooling efficiency across the entire first cooling water passage  43  can be obtained. 
     The first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H included in the first to eighth switching element modules  27 A to  27 H are arranged at positions spaced apart along the circulation direction  46  in the first cooling water passage  43 . In the present embodiment, the first to eighth positive side switching elements  31 A to  31 H are arranged at positions corresponding to the outward passage portion  43   a  in the first cooling water passage  43 , whereas the first to eighth negative side switching elements  32 A to  32 H are arranged at positions corresponding to the return passage portion  43   b  in the first cooling water passage  43 . Thus, a thermal entrance region is formed for each of the first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H, and the cooling efficiency can be increased. 
     Also, the first to eighth switching element modules  27 A to  27 H are mounted on the first heat sink  40  in such a manner that the first to eighth positive side switching elements  31 A to  31 H and the first to eighth negative side switching elements  32 A to  32 H included in the first to eighth switching element modules  27 A to  27 H are sealed inside the coated layers  65  made of a synthetic resin. Thus, the first to eighth switching element modules  27 A to  27 H can be effectively cooled by the heat dissipation from the coated layers  65  and the cooling from the first heat sink  40 . 
     Now, the DC link capacitor unit  21  having the smoothing capacitor  36  is provided between the first converter  18  and the inverter  20  as well as between the second converter  19  and the inverter  20 , and the positive side connection terminals  69  and the negative side connection terminals  70  of the first to eighth switching element modules  27 A to  27 H included in the first converter  18 , the second converter  19 , and the inverter  20  are connected to the positive side connection terminals  139  and the negative side connection terminals  140  provided on the DC link capacitor unit  21 . The positive side connection terminals  69 ,  139  are connected in common to the positive side external bus bar  141 , and the negative side connection terminals  70 ,  140  are connected in common to the negative side external bus bar  142 . Since the external bus bar unit  136  is integrally formed by stacking the positive side and negative side external bus bars  141 ,  142  arranged outside the DC link capacitor unit  21  with the insulating member  143  interposed therebetween, the current flowing through the internal wiring of the DC link capacitor unit  21  is decreased. Thus, the internal wiring of the DC link capacitor unit  21  can be prevented from generating heat which causes an adverse thermal effect on the smoothing capacitor  36 . 
     Furthermore, since the external bus bar unit  136  is in direct contact with the first heat sink  40 , the heat generated in the external bus bar unit  136  is directly transferred to the first heat sink  40  side. Thus, a temperature rise near the DC link capacitor unit  21  can be suppressed. 
     An embodiment of the present invention is explained above, but the present invention is not limited to the above-mentioned embodiment and may be modified in a variety of ways as long as the modifications do not depart from the gist of the present invention.