Patent Publication Number: US-2023140022-A1

Title: Vehicle drive unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a vehicle drive unit including a motor and an inverter. 
     2. Description of the Related Art 
     In recent years, cars that travel by using electricity, such as hybrid cars and electric cars, have considerably come into wide use. Cars of such a type include a drive motor and a battery. A drive unit including a motor and a battery drives the motor by converting direct current from the battery to alternating current by an inverter. 
     The inverter has a smoothing capacitor and a power module including a switching element. Since the inverter handles high electric power, a high voltage is applied and a large current flows. Therefore, the heat generation amount at the time of operation is large, and thus cooling is required. A high surge voltage is also generated. Therefore, individual electronic components of the inverter become large, and also tend to be heavy. Consequently, the inverter of the related art is a factor that prevents fuel costs and electricity costs from being reduced. 
     In order to reduce a power transmission distance, the inverter is ordinarily disposed near a motor. However, in the case of a car, there are many devices to be installed and thus a space in which the inverter can be disposed is limited. It is also necessary to consider the balance of a car body. Therefore, it is difficult to adequately dispose a large, heavy inverter in a car. 
     Among structural components of the inverter, in particular, the power module has a high heat generation amount, and thus cooling is required. An inverter disclosed in Japanese Unexamined Patent Application Publication No. 2021-112011 includes a cooling structure of a power module. In the cooling structure, a large portion excluding a lower end portion of the vertically long power module, that is, an upper surface and a side surface are inserted into a cooling liquid path in which a cooling liquid flows and are immersed in the cooling liquid. 
     Since the power module according to Japanese Unexamined Patent Application Publication No. 2021-112011 is vertically long, the area of the upper surface and the area of a lower surface are relatively smaller than the area of the side surface. Therefore, not only the upper surface or the lower surface of the power module, but also the side surface needs to be inserted into the cooling path. Therefore, since the cooling structure of the inverter becomes large, the entire drive unit including a motor and an inverter also becomes large. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made in view of such points, and it is an object of the present disclosure to, while sufficiently cooling a power module, reduce the size of a drive unit including a motor and an inverter. 
     A vehicle drive unit according to the present disclosure includes a motor and an inverter disposed adjacent to each other in an axial direction of the motor, the inverter having a smoothing capacitor and a plurality of power modules. In the vehicle drive unit, the smoothing capacitor and each power module are connected to each other by a busbar. Each power module has a flat shape having a width greater than a thickness thereof, and has a first cooling surface disposed on one surface side and facing the motor, the first cooling surface of each power module is placed side by side on a placement surface orthogonal to the axial direction, the busbar has a plate shape and has a second cooling surface disposed on the one surface side and facing the motor, one end portion of the busbar is connected to the smoothing capacitor, the other end portion of the busbar is connected to each power module, the busbar is formed with a width greater than a thickness thereof so as to extend widthwise along a direction in which each power module is disposed side by side, the second cooling surface is placed on the placement surface, and a cooling portion facing the first cooling surface and the second cooling surface is provided closer to the motor than the placement surface. 
     According to such a structure, since the first cooling surface having a large area of each flat power module having a wide width faces the cooling portion, the cooling area of cooling each power module by the cooling portion can be increased. Therefore, even if only one surface (first cooling surface) of each power module is cooled by the cooling portion, a sufficient cooling ability can be ensured. 
     The busbar can be formed with a wide width by causing the busbar to extend widthwise along a direction in which each power module is disposed side by side. Since the large area of the second cooling surface of the wide, plate-shaped busbar faces the cooling portion, the heat-dissipation area of dissipating heat to the cooling portion from the busbar can be increased. Therefore, even if the cooling ability of cooling each power module by the cooling portion is insufficient, the insufficient cooling ability can be compensated. 
     Since the first cooling surface of each power module is placed side by side on the placement surface orthogonal to the axial direction of the motor, the axial-direction length of the inverter can be reduced. Since the cooling portion needs to be provided only on one surface side of each power module, the inverter can be reduced in size compared with when the cooling portion is provided on two surface sides of each power module. 
     Accordingly, while sufficiently cooling each power module, the size of the drive unit including the motor and the inverter can be reduced. 
     In one embodiment, the cooling portion includes a cooling path in which a cooling medium flows. 
     According to such a structure, the cooling ability of cooling each power module by the cooling portion can be increased by causing a cooling medium to flow in the cooling path. 
     In one embodiment, each power module includes a silicon carbide metal-oxide-semiconductor field-effect transistor (SiC-MOSFET). 
     According to such a structure, since the size of a SiC-MOSFET chip of each power module is small, the size of a heat transfer block (for example, a copper block) that is placed on each SiC-MOSFET chip can also be reduced. If each power module is to be effectively cooled at two surfaces thereof, expensive ceramic substrates (for example, SiN) need to be disposed on two surface sides of the SiC-MOSFET chips. Therefore, cooling one surface of each power module and increasing the effectiveness of cooling the one surface is advantageous in terms of costs than cooling two surfaces of each power module. 
     In one embodiment, the smoothing capacitor has a third cooling surface disposed on the one surface side and facing the motor, the third cooling surface is placed on the placement surface, and the cooling portion also faces the third cooling surface. 
     According to such a structure, the smoothing capacitor, together with the power modules, can be cooled by the cooling portion. 
     In one embodiment, each power module is disposed closer to an outer peripheral side than the smoothing capacitor and the busbar is formed so that the width thereof extends along a peripheral direction of the motor. 
     According to such a structure, the busbar can be easily formed with a wide width. 
     In one embodiment, an inductance of the busbar is a function of a length extending from the one end portion to the other end portion of the busbar, the function has a minimum value so that the inductance becomes the same at a first length and a second length that differ from each other, the smoothing capacitor and each power module are connected to each other by two of the busbars, the two busbars being a negative-electrode-side busbar and a positive-electrode-side busbar, a length of one of the negative-electrode-side busbar and the positive-electrode-side busbar is the first length, a length of the other of the negative-electrode-side busbar and the positive-electrode-side busbar is the second length, and at least one of the negative-electrode-side busbar and the positive-electrode-side busbar has the second cooling surface. 
     According to such a structure, even though the length of the negative-electrode-side busbar and the length of the positive-electrode-side busbar differ from each other, the inductance of the negative-electrode-side busbar and the length of the positive-electrode-side busbar can be equalized. 
     According to the present disclosure, while sufficiently cooling each power module, the drive unit including a motor and an inverter can be reduced in size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic structural view of a vehicle system including a drive unit according to a first embodiment. 
         FIG.  2    is a perspective view of the drive unit including a motor and an inverter. 
         FIG.  3    is a cross sectional view of the motor when seen from the inverter. 
         FIG.  4    is a circuit diagram of the inverter. 
         FIG.  5    shows a comparison between a SiC-MOSFET and an IGBT. 
         FIG.  6    is a perspective view and a circuit diagram of a detailed structure of a power module. 
         FIG.  7    is a cross sectional view of the inverter when seen from a side opposite to the motor. 
         FIG.  8    is a vertical sectional view of the inverter. 
         FIG.  9    is a perspective view of a busbar. 
         FIG.  10    shows graphs showing the relationship between size and inductance sensitivity of busbar. 
         FIG.  11    is cross sectional view of a cooling path of the inverter when seen from the motor. 
         FIG.  12    is a view, corresponding to  FIG.  11   , of Modification 1 of the first embodiment, and is a cross sectional view of a cooling path of an inverter when seen from a motor. 
         FIG.  13    is a view, corresponding to  FIG.  7   , of Modification 2 of the first embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  14    is a view, corresponding to  FIG.  7   , of a second embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  15    is a view, corresponding to  FIG.  8   , of the second embodiment, and is a vertical sectional view of the inverter. 
         FIG.  16    is a view, corresponding to  FIG.  7   , of a third embodiment, and is a cross sectional view of an inverter when seen from a side opposite to a motor. 
         FIG.  17    is a view, corresponding to  FIG.  8   , of the third embodiment, and is a vertical sectional view of the inverter. 
         FIG.  18    is a view, corresponding to  FIG.  8   , of Modification 1 of the third embodiment, and is a vertical sectional view of an inverter. 
         FIG.  19    is a perspective view of power modules and busbars according to Modification 2 of the third embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present disclosure are described in detail below based on the drawings. The description of preferable embodiments below merely substantially gives exemplifications, and is not intended at all to limit the present disclosure, applicable matters thereof, or use thereof. 
     First Embodiment 
     Vehicle Structure 
       FIG.  1    shows a vehicle  1  including drive units A according to a first embodiment when seen from below the vehicle. In the vehicle  1 , power from at least one of an engine  2  and a drive motor  3  disposed at the front of the vehicle is transmitted to rear wheels  4  disposed at the rear of the vehicle. That is, the vehicle  1  is a hybrid vehicle of a front engine/rear drive (FR) type. 
     As shown in  FIG.  1   , the vehicle  1  includes the engine  2 , a transmission  5  that is connected to the engine  2 , the drive motor  3  that is disposed between the engine  2  and the transmission  5 , a propeller shaft  6  that is connected to the transmission  5  and that transmits power from the engine  2  and the drive motor  3  to the rear wheels, and a differential  7  that is connected to the propeller shaft  6  and that transmits power from the engine  2  and the drive motor  3  to the left and right rear wheels  4 . 
     The propeller shaft  6  extends below a floor panel  8  in a vehicle front-rear direction. A tunnel portion  9  is provided on a central side in a vehicle width direction of the floor panel  8 . The propeller shaft  6  is disposed on an inner side of the tunnel portion  9 . 
     The vehicle  1  includes an exhaust pipe  10  extending in the vehicle front-rear direction from the engine  2 . A catalytic device  11  is disposed on an upstream side of the exhaust pipe  10 . Although not shown, a silencer is disposed on a downstream side of the exhaust pipe  10 . 
     The vehicle  1  includes a fuel tank (not shown) that stores fuel to be supplied to the engine  2 , and a battery  12  that stores electric power to be supplied to the motor  3 . The drive motor  3  transmits power to the rear wheels  4 , and is rotationally driven by the propeller shaft  6  and generates regenerative power when the speed of the vehicle is reduced to supply the generated electric power to the battery  12 . The battery  12  includes a first battery unit  12   a  and a second battery unit  12   b , which are disposed one on each side in the vehicle width direction. The second battery unit  12   b  is longer than the first battery unit  12   a  in the vehicle front-rear direction. Each of the battery units  12   a  and  12   b  includes a plurality of battery cells. The battery cells are, for example, lithium-ion batteries. 
     An in-wheel motor  14  is connected to each of left and right front wheels  13 . Each in-wheel motor  14  functions as an assist motor that generates power and transmits the power to the front wheels  13  when the vehicle  1  is started. Each in-wheel motor  14  functions as a regenerative brake that generates electric power when the speed of the vehicle is reduced. Similarly to the drive motor  3 , each in-wheel motor  14  has electric power supplied thereto from the battery  12 . 
     As shown in  FIG.  1   , an inverter  15  is interposed between the drive motor  3  and the transmission  5 . The drive motor  3  and the inverter  15  are disposed adjacent to each other in an axial direction of the drive motor  3  (vehicle front-rear direction). Inverters  16  are disposed on inner sides of the respective in-wheel motors  14  in the vehicle width direction. The in-wheel motors  14  and the respective inverters  16  are disposed adjacent to each other in an axial direction of each in-wheel motor  14  (vehicle width direction) . The drive motor  3  and the inverter  15  constitute the drive unit A. Similarly, the in-wheel motors  14  and the respective inverters  16  constitute the drive units A. 
     The inverter  15  and the inverters  16  each convert direct-current electric power stored in the battery  12  into alternating-current electric power and supply the converted electric power to a corresponding one of the motor  3  and the motors  14 , and each convert the alternating-current electric power generated at the corresponding one of the motor  3  and the motors  14  into direct-current electric power to charge the battery when the speed of the vehicle is reduced. 
     Drive Units 
     The drive units A of the vehicle  1  are described by taking as an example, the drive unit A including the drive motor  3  and the inverter  15 .  FIG.  2    is a perspective view of the drive unit A. As described above, the drive unit A includes the motor  3  and the inverter  15 . The motor  3  and the inverter  15  are disposed adjacent and coaxially to each other in the axial direction of the motor  3 . Specifically, a center axis 0 of the motor  3  and a center axis  0  of the inverter  15  coincide with each other. The motor  3  (specifically, a casing of the motor  3 ) is formed with a cylindrical shape. The inverter  15  (specifically, a casing of the inverter  15 ) is formed with a cylindrical shape corresponding to that of the motor  3 . A rotation shaft  3   a  of the motor  3  extends through the inverter  15  in the axial direction. A thickness Wiv of the inverter  15  is small, and is, for example, 50 mm or less (preferably, 30 mm or less). A cooling path  61  (described later) is provided inside the inverter  15 . A cooling inlet pipe  62  and a cooling outlet pipe  63  that communicate with the cooling path  61  are connected to an upper portion of the inverter. 
       FIG.  3    is a cross sectional view of the motor  3  when seen from the inverter  15 . The motor  3  has coils  17 . Specifically, U-phase coils  17   u , V-phase coils  17   v , and W-phase coils  17   w  are wound in a concentrated manner on a stator of the motor  3 . The U-phase coils  17   u  are disposed at two locations so as to oppose each other in a radial direction of the motor  3 . Similarly, the V-phase coils  17   v  are disposed at two locations so as to oppose each other in the radial direction of the motor  3 . Similarly, the W-phase coils  17   w  are disposed at two locations so as to oppose each other in the radial direction of the motor  3 . 
     Three motor-side terminal blocks  18  are provided at an outer peripheral portion of the motor  3 . The three motor-side terminal blocks  18  correspond to the U-phase coils  17   u , the V-phase coils  17   v , and the W-phase coils  17   w . Lead wires (not shown) extend from the U-phase coils  17   u  disposed at the two locations. The two lead wires are bundled into one lead wire and the bundle is then connected to the motor-side terminal block  18 . This also applies to the V-phase coils  17   v  and the W-phase coils  17   w . As a rotor, an iron core  27  and a permanent magnet  28  having a N pole and a S pole are fixed to the rotation shaft  3   a . 
       FIG.  4    is a circuit diagram of the inverter  15 . The inverter  15  includes a smoothing capacitor  19  and a plurality of power modules  20 . The smoothing capacitor  19  smoothens a voltage that is applied to the power modules  20 . The plurality of power modules  20  constitute an inverter circuit, and convert direct-current voltage into an alternating-current voltage. 
     The plurality of power modules  20  include a U-phase power module  20   u , a V-phase power module  20   v , and a W-phase power module  20   w . The U-phase power module  20   u  corresponds to the U-phase coils  17   u  of the motor  3 . The V-phase power module  20   v  corresponds to the V-phase coils  17   v  of the motor  3 . The W-phase power module  20   w  corresponds to the W-phase coils  17   w  of the motor  3 . 
     Each power module  20  includes two arm elements, that is, a lower arm element  21  and an upper arm element  22 , serving as switching elements. In each phase power module  20 , when one of the lower arm element  21  and the upper arm element  22  is opened, the other of the lower arm element  21  and the upper arm element  22   is closed. Therefore, a three-phase alternating current is supplied to the motor  3 . 
     Here, each power module  20  includes a SiC-MOSFET.  FIG.  5    shows a comparison between a SiC-MOSFET and an IGBT. The SiC-MOSFET is a MOSFET (metal-oxide-semiconductor field-effect transistor) including silicon carbide (SiC), and constitutes a chip  24  including the lower arm element  21  and the upper arm element  22  and, for example, other control elements. A lower surface of the chip  24  is soldered and fixed to a silicon substrate. A copper block  25 , serving as a heat transfer block, is soldered and fixed to an upper surface of the chip  24 . This also applies to the IGBT (insulated gate bipolar transistor). 
     As shown in  FIG.  5   , a surface area of the chip  24  constituted by the SiC-MOSFET is smaller than a surface area of a chip  24 ′ constituted by the IGBT. Therefore, the size of the copper block  25  disposed on an upper side of the SiC-MOSFET (chip)  24  is smaller than the size of a copper block  25 ′ disposed on an upper side of the IGBT (chip)  24 ′. The SiC-MOSFET has better heat resistance than the IGBT. 
       FIG.  6    is a perspective view and a circuit diagram of a detailed structure of a power module  20 . Each power module  20  has a flat shape having a wide width. Specifically, each power module  20  is longer in a width direction W than in a thickness direction t. Each power module  20  has a substantially parallelepiped shape. The width direction W includes a first width direction W1 and a second width direction W2 orthogonal to each other. Hereunder, one side of the power module  20  in the thickness direction may be the lower side and an opposite side of the power module  20  in the thickness direction may be the upper side. 
     The power module  20  has a lower surface  31 , serving as a first cooling surface, on the lower side (one surface side, the one side in the thickness direction). The power module  20  has an upper surface  32  on the upper side. The power module  20  has a first end surface  33  on one side in the first width direction W1. The power module  20  has a second end surface  34  on an opposite side in the first width direction W1. 
     A negative-electrode-side input terminal  35  is connected to a lower side of the first end surface  33  and on one side in the second width direction W2. A positive-electrode-side input terminal  36  is connected to an upper side of the first end surface  33  and on an opposite side in the second width direction W2. The negative-electrode-side input terminal  35  and the positive-electrode-side input terminal  36  are disposed apart from each other in an up-down direction (thickness direction). An output terminal  37  is connected to a central portion of the second end surface  34 . 
     The lower arm element  21  and the upper arm element  22  are accommodated inside a package (box body) of the power module  20 . The negative-electrode-side input terminal  35  is connected to the lower arm element  21 . The positive-electrode-side input terminal  36  is connected to the upper arm element  22 . The output terminal  37  is connected between the lower arm element  21  and the upper arm element  22 . 
       FIG.  7    is a cross sectional view of the inverter  15  when seen from a side opposite to the motor  3 .  FIG.  8    is a vertical sectional view of the inverter  15  along line VIII-VIII. As shown in  FIG.  7   , an axial through hole  40  through which the rotation shaft  3   a  of the motor  3  passes is provided in the center of the inverter  15 . A cylindrical boss portion  41  is formed around the axial through hole  40 . The smoothing capacitor  19  is disposed along the boss portion  41 . 
     Each power module  20  (the U-phase power module  20   u , the V-phase power module  20   v , and the W-phase power module  20   w ) are disposed closer to an outer peripheral side than the smoothing capacitor  19 . On the outer peripheral side of the smoothing capacitor  19 , each power module  20  is disposed side by side in a peripheral direction of the motor  3 . The input terminals  35  and  36  (the first end surface  33 ) and the output terminal  37  (the second end surface  34 ) of each power module  20  face the peripheral direction of the motor  3  (the inverter  15 ). Each power module  20  is disposed so that the thickness direction t coincides with the axial direction of the motor  3 . The smoothing capacitor  19  and each power module  20  are disposed in a space defined by an outer peripheral wall portion  42  and the boss portion  41  of the inverter  15 . 
     As shown in  FIGS.  7  and  8   , a heat sink  60  is provided on a motor  3  side of the inverter  15 . The heat sink  60  is primarily used for cooling each power module  20 . The heat sink  60  is disposed between the outer peripheral wall portion  42  and the boss portion  41  of the inverter  15 . The heat sink  60  has an upper wall portion  60   a , an outer peripheral wall portion  60   b , a lower wall portion  60   c , and an inner peripheral wall portion  60   d . 
     An upper surface  65  of the upper wall portion  60   a  of the heat sink  60  constitutes a placement surface orthogonal to the axial direction of the motor  3  (may hereunder be referred to as “placement surface  65 ”). The lower surface (first cooling surface)  31  of each power module  20  (the U-phase power module  20   u , the V-phase power module  20   v , and the W-phase power module  20   w ) faces the motor  3 . Specifically, the lower surface (first cooling surface)  31  of each power module  20  is placed side by side on the same placement surface  65 . 
     As shown in  FIGS.  7  and  8   , the smoothing capacitor  19  is one in which an assembly of a plurality of columnar capacitors  45  is covered by a flat plate from both sides in the thickness direction. The smoothing capacitor  19  has a lower surface  19   a , serving as a third cooling surface, on a lower side (one surface side, the one side in the thickness direction). The smoothing capacitor  19  has an upper surface  19   b  on a side opposite to the lower surface  19   a . The lower surface  19   a  and the upper surface  19   b  of the smoothing capacitor  19  are constituted by flat plates. The lower surface (third cooling surface)  19   a  of the smoothing capacitor  19  faces the motor  3 . Specifically, the lower surface (third cooling surface)  19   a  of the smoothing capacitor  19  is placed on the placement surface  65 . 
     As shown in  FIGS.  7  and  8   , the smoothing capacitor  19  and each power module  20  are connected to each other by a negative-electrode-side busbar  51  and a positive-electrode-side busbar  52 , serving as input busbars  50  (may hereunder be simply referred to as “busbars 50”). The negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  each have a plate shape. Specifically, the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  are longer in the width direction W and a length direction L than in the thickness direction t. 
     As shown in  FIG.  7   , the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  are formed with wide widths so as to be extend along the peripheral direction of the motor  3  (the inverter  15 ). In other words, the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  are formed with wide widths so as to extend along a direction in which each power module  20  is disposed side by side. The width direction W of the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  is the peripheral direction of the motor  3  (an arc shape is formed). The negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  each have a fan shape. The length direction L of the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  is the radial direction of the motor  3 . 
     One end portion  51   i  of the negative-electrode-side busbar  51  is connected to a negative-electrode terminal  19   c  provided on the lower surface  19   a  side of the smoothing capacitor  19 . The other end portion  51   o  of the negative-electrode-side busbar  51  is connected to a negative-electrode-side input terminal  35  of each power module  20 . One end portion  52   i  of the positive-electrode-side busbar  52  is connected to a positive-electrode terminal  19   d  provided on the upper surface  19   b  side of the smoothing capacitor  19 . An other end portion  52   o  of the positive-electrode-side busbar  52  is connected to a positive-electrode-side input terminal  36  of each power module  20 . 
     The negative-electrode-side busbar  51  has a lower surface  51   a , serving as a second cooling surface, on the lower side (one surface side, the one side in the thickness direction). The lower surface (second cooling surface)  51   a  of the negative-electrode-side busbar  51  faces the motor  3 . Specifically, the lower surface (second cooling surface)  51   a  of the negative-electrode-side busbar  51  is placed on the placement surface  65 . 
     As shown in  FIG.  7   , output busbars  54  are each connected to the output terminal  37  of a corresponding one of the power modules  20 . There are a total of three output busbars  54  corresponding to the U phase, the V phase, and the W phase. The output busbars  54  are each interposed between the corresponding one of the power modules  20  and a corresponding one of the coils  17 . The output busbars  54  each have a plate shape. In addition to the output busbars  54 , a wire harness or the like may be interposed between the corresponding one of the power modules  20  and the corresponding one of the coils  17 . 
     Three inverter-side terminal blocks  46  are provided at an outer peripheral portion of the inverter  15 . The inverter-side terminal blocks  46  corresponding to the respective power modules  20 . The output busbars  54  extend up to the respective inverter-side terminal blocks  46 . Electrically conductive members (busbars, wire harnesses, or the like) are each interposed between a corresponding one of the inverter-side terminal blocks  46  and a corresponding one of the motor-side terminal blocks  18 . 
     Busbar Inductance Sensitivity 
       FIG.  9    is a perspective view of a busbar  50 .  FIG.  10    shows graphs showing the relationship between size and inductance sensitivity of busbar  50 . As a result of assiduous studies, the inventor of the present application and others have found the following with regard to the relationship between the size and inductance sensitivity of busbar  50 . 
     As shown in  FIGS.  9  and  10   , the larger a width W (mm) of busbar  50 , the smaller an inductance sensitivity (nH) of busbar  50 . 
     Basically, the larger a length L (mm) of busbar  50 , the larger the inductance sensitivity (nH) of busbar  50 . However, as shown in the middle graph in  FIG.  10   , a minimum value (inflection point) M exists in the relationship between the length L (mm) and the inductance sensitivity (nH) of busbar  50 . Therefore, even if lengths L differ from each other, the inductance sensitivity (nH) may become the same. Specifically, the inductance sensitivity (nH) of busbar  50  ( 51 ,  52 ) is a function of the length L (mm) extending from the one end portion  51   i  or  52   i  of busbar  50  ( 51 ,  52 ) (the terminal  19   c  or  19   d  of the smoothing capacitor  19 ) to the other end portion  51   o  or  52   o  of busbar  50  ( 51 ,  52 ) (the input terminal  35  or  36  of each power module  20 ). The function has the minimum value M so that the inductance sensitivity K (nH) becomes the same at a first length L1 (mm) and a second length L2 (mm) that differ from each other. The second length L2 (mm) is larger than the first length L1 (mm). 
     Even if a thickness t (mm) of busbar  50  changes, the inductance sensitivity (nH) of busbar  50  does not change. 
     As shown in  FIG.  7   , the width of the negative-electrode-side busbar  51  and the width of the positive-electrode-side busbar  52  are substantially the same. As shown in  FIG.  8   , a length L- of the negative-electrode-side busbar  51  and a length L+ of the positive-electrode-side busbar  52  differ from each other. The length L- of the negative-electrode-side busbar  51  corresponds to the first length L1. The length L+ of the positive-electrode-side busbar  52  corresponds to the second length L2. The length L+ of the positive-electrode-side busbar  52  (the second length L2) is larger than the length L- of the negative-electrode-side busbar  51  (the first length L1). However, due to the existence of the minimum value M above, the inductance of the negative-electrode-side busbar  51  and the inductance of the positive-electrode-side busbar  52  are equal to each other. 
     Cooling Path 
       FIG.  11    is cross sectional view of a cooling path  61  of the inverter  15  when seen from the motor  3 . As shown in  FIGS.  8  and  11   , the cooling path (cooling jacket)  61 , serving as a cooling portion, is provided inside the heat sink  60 . The cooling path  61  is defined by the upper wall portion  60   a , the outer peripheral wall portion  60   b , the lower wall portion  60   c , and the inner peripheral wall portion  60   d . The cooling path  61  is formed with a doughnut shape (ring shape, cylindrical shape) over the entire periphery when seen in the axial direction of the motor  3  (the inverter  15 ). The rotation shaft  3   a  of the motor  3  passes through an inner side of the inner peripheral wall portion  60   d . As described above, the upper surface of the upper wall portion  60   a  of the heat sink  60  is the placement surface  65 . 
     The cooling path  61  is provided closer to the motor  3  than the placement surface  65 . A cooling medium H flows in the cooling path  61 . The cooling medium H is, for example, cooling water or cooling oil. 
     A plurality of fins  64 , serving as cooling portions, are provided inside the heat sink  60  (the cooling path  61 ). The fins  64  extend downward inside the cooling path  61  from the upper wall portion  60   a . That is, the fins  64  are provided closer to the motor  3  than the placement surface  65 . 
     As shown in  FIGS.  8  and  11   , when seen in the axial direction of the motor  3  (the inverter  15 ), the cooling path (cooling portion)  61  faces all of the lower surface (first cooling surface)  31  of each power module  20 , the lower surface (second cooling surface)  51   a  of the negative-electrode-side busbar  51 , and the lower surface (third cooling surface)  19   a  of the smoothing capacitor  19 . 
     Similarly, when seen in the axial direction of the motor  3 , the fins (cooling portions)  64  face all of the lower surface (first cooling surface)  31  of each power module  20 , the lower surface (second cooling surface)  51   a  of the negative-electrode-side busbar  51 , and the lower surface (third cooling surface)  19   a  of the smoothing capacitor  19 . 
     As shown in  FIG.  11   , the inlet pipe  62  and the outlet pipe  63  are connected to an upper portion of the outer peripheral wall portion  42  of the inverter  15 . The inlet pipe  62  and the outlet pipe  63  communicate with the cooling path  61 . The cooling medium H introduced into the cooling path  61  via the inlet pipe  62  is discharged to the outside via the outlet pipe  63  after being guided to the outer peripheral wall portion  60   b  and the inner peripheral wall portion  60   d  and having flowed inside the cooling path  61  in the peripheral direction. Note that a guide plate  66  may be provided on a downstream side of the inlet pipe  62 . 
     Operational Effects of First Embodiment 
     According to the present embodiment, since the lower surface (first cooling surface)  31  having a large area of each flat power module  20  having a wide width faces the cooling path  61   and the fins  64 , serving as cooling portions, the cooling area of cooling each power module  20  by the cooling portions  61  and  64  can be increased. Therefore, even if only one surface (lower surface, first cooling surface)  31  of each power module  20  is cooled by the cooling portions  61  and  64 , a sufficient cooling ability can be ensured. 
     The negative-electrode-side busbar  51  can be formed with a wide width by causing the negative-electrode-side busbar  51  to extend along a direction in which each power module  20  is disposed side by side (the peripheral direction of the motor  3 ). Since the lower surface (the second cooling surface)  51   a  having a large area of the plate-shaped negative-electrode-side busbar  51  having a wide width faces the cooling portions  61  and  64 , the heat-dissipation area of dissipating heat to the cooling portions  61  and  64  from the negative-electrode-side busbar  51  can be increased. Therefore, even if the cooling ability of cooling each power module  20  by the cooling portions  61  and  64  is insufficient, the insufficient cooling ability can be compensated. 
     Since the lower surface (the first cooling surface)  31  of each power module  20  (the U-phase power module  20   u , the V-phase power module  20   v , and the W-phase power module  20   w ) is placed side by side on the same placement surface  65  orthogonal to the axial direction of the motor  3 , the axial-direction length of the inverter  15  can be reduced. Since the cooling portions  61  and  64  need to be provided only on one surface (lower surface, first cooling surface)  31  side of each power module  20 , the inverter  15  can be reduced in size compared with when the cooling portions  61  and  64  are provided on two surface sides of each power module  20 . 
     Accordingly, while sufficiently cooling each power module  20 , the size of the drive unit A including the motor  3  and the inverter  15  can be reduced. 
     By causing the cooling medium H to flow in the cooling path  61  serving as a cooling portion, the cooling ability of cooling each power module  20  by the cooling portion can be increased. 
     Since the size of the SiC-MOSFET chip  24  of each power module  20  is small, the size of the copper block  25 , serving as a heat transfer block, that is placed on each SiC-MOSFET chip  24  can also be reduced (see  FIG.  5   ). If each power module  20  is to be effectively cooled at two surfaces thereof, expensive ceramic substrates (for example, SiN) need to be disposed on two surface sides of the SiC-MOSFET chips  24 . Therefore, cooling one surface of each power module  20  and increasing the effectiveness of cooling the one surface is advantageous in terms of costs than cooling two surfaces of each power module  20 . 
     The smoothing capacitor  19 , together with the power modules  20 , can be cooled by the cooling portions  61  and  64 . 
     The negative-electrode-side busbar  51  can be easily formed with a wide width by disposing each power module  20  side by side in the peripheral direction of the motor  3  on the outer peripheral side of the smoothing capacitor  19  and by causing the width direction W of the negative-electrode-side busbar  51  to be the peripheral direction of the motor  3 . Therefore, the heat-dissipation area of dissipating heat to the cooling portions  61  and  64  from the negative-electrode-side busbar  51  can be easily increased. 
     Since the cooling portions  61  and  64  are provided on the side of the motor  3 , this is advantageous in terms of also cooling wires (for example, the output busbars  54 ) that connect the coils  17  of the motor  3  and the respective power modules  20  to each other. 
     As shown by an alternate long and short dash lines in  FIG.  1   , in the related art, an inverter  15 ′ is often disposed near a second battery unit  12   b ′ of a battery  12 ′. According to the present embodiment, since the inverter  15  can be disposed adjacent to the motor  3  in the axial direction, the inverter  15  no longer needs to be disposed near the second battery unit  12   b . Therefore, the layout of the second battery unit  12   b  can be provided with greater freedom, and the second battery unit  12   b  can be increased in size. 
     Reduction and Equalization of Inductance 
     The inductance of the busbar  51  and the inductance of the busbar  52  can be reduced by increasing the width of the busbar  51  and the width of the busbar  52 . 
     Since the smoothing capacitor  19  and each of the power modules  20   u ,  20   v , and  20   w  are placed on the same placement surface  65 , the length of the busbars  51  and  52 , each connecting the smoothing capacitor  19  to each of the power modules  20   u ,  20   v , and  20   w , can be reduced. Therefore, the inductance of the busbar  51  and the inductance of the busbar  52  can be reduced. 
     Since the motor  3  and the inverter  15  are disposed adjacent to each other in the axial direction, lengths of electrical paths between the power modules  20   u ,  20   v , and  20   w  and the respective coils  17   u ,  17   v , and  17   w  are reduced. Therefore, inductances of the electrical paths (including the output busbars  54 ) connecting the power modules  20   u ,  20   v , and  20   w  and the respective coils  17   u ,  17   v , and  17   w  can be reduced. 
     Since the power modules  20   u ,  20   v , and  20   w  are disposed side by side in the peripheral direction of the motor  3  on the outer peripheral side of the smoothing capacitor  19 , the distances between the smoothing capacitor  19  and the power modules  20   u ,  20   v , and  20   w  can be made equal to each other. Further, at each of the busbars  51  and  52 , inductances of electrical paths between the smoothing capacitor  19  and the power modules  20   u ,  20   v , and  20   w  can be equalized by forming the busbars  51  and  52  with wide widths so as to extend along the peripheral direction of the motor  3 . 
     By using the minimum value (the inflection point) M (see  FIG.  10   ), even though the length L- of the negative-electrode-side busbar  51  (the first length L1) and the length L+ of the positive-electrode-side busbar  52  (the second length L2) differ from each other, the inductance of the negative-electrode-side busbar  51  and the inductance of the positive-electrode-side busbar  52  can be equalized with respect to each other. 
     Modification 1 of First Embodiment 
       FIG.  12    is a view, corresponding to  FIG.  11   , of Modification 1 of the first embodiment. According to the present modification, the cooling portions (the cooling path  61  and the fins  64 ) do not face the lower surface (the third cooling surface)  19   a  of the smoothing capacitor  19  at all. The cooling portions  61  and  64  do not face a part of the lower surface (the second cooling surface)  51   a  of the negative-electrode-side busbar  51 . 
     Since the heat generation amount of the smoothing capacitor  19  is smaller than the heat generation amount of each power module  20 , even if the smoothing capacitor  19  is not cooled by the cooling portions  61  and  64 , this may not be a problem. 
     As long as the cooling portions  61  and  64  face a part of the lower surface (the second cooling surface)  51   a  of the negative-electrode-side busbar  51 , the heat-dissipation effect of dissipating heat to the cooling portions  61  and  64  from the negative-electrode-side busbar  51  can be somewhat provided. 
     Modification 2 of First Embodiment 
       FIG.  13    is a view, corresponding to  FIG.  7   , of Modification 2 of the first embodiment. The inverters  16  according to the present modification are disposed adjacent to the respective in-wheel motors  14  in the axial direction (vehicle width direction) of the in-wheel motors  14  (see  FIG.  1   ). The inverters  16  do not have an axial through hole  40  and a boss portion  41 . The smoothing capacitor  19  has a columnar shape, and is disposed at the center of the inverter  16 . The heat sink  60  does not have the inner peripheral wall portion  60   d . The cooling path  61  has a circular shape without a hole when seen in the axial direction of the motor  3 . 
     Other Modifications of First Embodiment 
     The cooling medium H that flows in the cooling path  61  may be, for example, air. Further, the cooling portions may be constituted by, for example, only the fins  64  without providing the cooling path  61 . The cooling portions may be solid cooling members. 
     The cooling portions need not be provided along the entire periphery, and may be provided only at portions in the peripheral direction facing the power modules  20 . 
     Instead of the negative-electrode-side busbar  51 , the positive-electrode-side busbar  52  may have a second cooling surface that faces the motor  3  and that is placed on the placement surface  65 . 
     Although not shown, each output busbar  54  may be formed with a wide width so as to extend along the peripheral direction of the motor  3 . In other words, the width direction of each output busbar  54  may be the peripheral direction (an arc shape is formed) . Each output busbar  54  may have a fan shape. Therefore, since each output busbar  54  is easily formed with a wide width, the inductance of each output busbar  54  is easily reduced (see  FIG.  10   ). 
     The placement surface  65  may include a plurality of surfaces that are positioned in the same plane orthogonal to the axial direction of the motor  3 . 
     Second Embodiment 
       FIG.  14    is a view, corresponding to  FIG.  7   , of a second embodiment, and is a cross sectional view of an inverter 215 when seen from a side opposite to the motor  3 .  FIG.  15    is a view, corresponding to  FIG.  8   , of the second embodiment, and is a vertical sectional view of the inverter 215. A detailed description of structures that are the same as those of the embodiment above may not be given. 
     In the present embodiment, each power module  20  (the U-phase power module  20   u , the V-phase power module  20   v , and the W-phase power module  20   w ) is disposed closer to an outer peripheral side of a smoothing capacitor  19 . On the outer peripheral side of the smoothing capacitor  19 , the power modules  20  are disposed side by side in a peripheral direction of the motor  3 . 
     The input terminals  35  and  36  (first end surface  33 ) and the output terminal  37  (second end surface  34 ) of each power module  20  face a radial direction of the motor  3  (the inverter 215). Specifically, the input terminals  35  and  36  (the first end surface  33 ) of each power module  20  face an inner peripheral side. The output terminal  37  (the second end surface  34 ) of each power module  20  faces the outer peripheral side. Each power module  20  is disposed radially from the center  0 , which is the origin, of the inverter 215 (the motor  3 ). 
     Similarly to the embodiment above, the lower surface (first cooling surface)  31  of each power module  20 , the lower surface (second cooling surface)  51   a  of the negative-electrode-side busbar  51 , and the lower surface (third cooling surface)  19   a  of the smoothing capacitor  19  are placed on the placement surface  65 . 
     As shown in  FIG.  14   , the width of the negative-electrode-side busbar  51  and the width of the positive-electrode-side busbar  52  are the same. As shown in  FIG.  15   , a length L- of the negative-electrode-side busbar  51  and a length L+ of the positive-electrode-side busbar  52  are the same. Therefore, the inductance of the negative-electrode-side busbar  51  and the inductance of the positive-electrode-side busbar  52  are equal to each other. 
     The other structures are the same as those of the first embodiment. 
     Third Embodiment 
       FIG.  16    is a view, corresponding to  FIG.  7   , of a third embodiment, and is a cross sectional view of an inverter 315 when seen from a side opposite to the motor  3 .  FIG.  17    is a view, corresponding to  FIG.  8   , of the third embodiment, and is a vertical sectional view of the inverter 315. A detailed description of structures that are the same as those of the embodiments above may not be given. 
     As shown in  FIGS.  16  and  17   , the negative-electrode-side input terminals  35  are each connected to a lower side of the first end surface  33  of a corresponding one of power modules  20 . The positive-electrode-side input terminals  36  are each connected to a lower side of the second end surface  34  of the corresponding one of the power modules  20 . The output terminals  37  are each connected to the center of the upper surface  32  of the corresponding one of the power modules  20 . 
     The negative-electrode-side busbar  51  connects the negative-electrode terminal  19   c  of the smoothing capacitor  19  and the negative-electrode-side input terminal  35  of each power module to each other. The positive-electrode-side busbar  52  connects the positive-electrode terminal  19   d  of the smoothing capacitor  19  and the positive-electrode-side input terminal  36  of each power module  20  to each other. 
     The positive-electrode-side busbar  52  starts out from the positive-electrode terminal  19   d  of the smoothing capacitor  19  (one end portion  52   i ), and extends along the upper surface  32  to the second end surface  34  from the first end surface  33  of each power module  20 . Then, the positive-electrode-side busbar  52  bends downward and extends along the second end surface  34  up to the positive-electrode-side input terminal  36  (other end portion  52   o ) on the lower side of each power module  20 . In other words, the positive-electrode-side busbar  52  extends so as to be wound around each power module  20  from the upper surface  32 . Each output busbar  54  starts out from the output terminal  37  at the upper surface  32  of each power module  20 , and extends upward. The positive-electrode-side busbar  52  has three opening portions for passing three upwardly extending output busbars  54  therethrough. 
     Similarly to the embodiments above, the lower surface (first cooling surface)  31  of each power module  20 , the lower surface (second cooling surface)  51   a  of the negative-electrode-side busbar  51 , and the lower surface (third cooling surface)  19   a  of the smoothing capacitor  19  are placed on the placement surface  65 . 
     As shown in  FIG.  16   , the width of the negative-electrode-side busbar  51  and the width of the positive-electrode-side busbar  52  are the same. As shown in  FIG.  17   , the length (L-) of the negative-electrode-side busbar  51  and the length (sum of La+ and Lb+) of the positive-electrode-side busbar  52  differ from each other. The length (L-) of the negative-electrode-side busbar  51  corresponds to the first length L1. The length (sum of La+ and Lb+) of the positive-electrode-side busbar  52  corresponds to the second length L2. The length (sum of La+ and Lb+, the second length L2) of the positive-electrode-side busbar  52  is larger than the length (L-, the first length L1) of the negative-electrode-side busbar  51 . However, due to the existence of the minimum value M (see  FIG.  10   ), the inductance of the negative-electrode-side busbar  51  and the inductance of the positive-electrode-side busbar  52  are equal to each other. 
     Note that the conditions (for example, materials) of the negative-electrode-side busbar  51  and the positive-electrode-side busbar  52  according to the present embodiment differ from those of the embodiments above. Therefore, the mode of the minimum value (M above) (see  FIG.  10   ) also differs. Specifically, the difference between the first length L1 and the second length L2 is larger than that in the embodiments above. 
     The other structures are the same as those of the second embodiment. 
     Modification 1 of Third Embodiment 
       FIG.  18    is a view, corresponding to  FIG.  8   , of Modification 1 of the third embodiment, and is a vertical sectional view of the inverter 315. Although in the embodiments above, each power module  20  accommodates both the lower arm element  21  and the upper arm element  22  in one package, the present modification differs. In the present modification, each power module  20  is separated into a first package  20   a  that accommodates the lower arm element  21  and a second package  20   b  that accommodates the upper arm element  22 . 
     Modification 2 of Third Embodiment 
       FIG.  19    is a perspective view of the power modules  20  and the busbars  50  according to Modification 2 of the third embodiment. In the present modification, each power module  20  (the U-phase power module  20   u , the V-phase power module  20   v , and the W-phase power module  20   w ) is linearly disposed side by side. The width direction of each of the busbars  51  and  52  is a linear direction (direction in which each power module  20  is disposed side by side). Other Embodiments 
     Although the present disclosure has been described by way of preferred embodiments above, such descriptions are not limiting matters, and, naturally, various modifications are possible. 
     The present disclosure is applicable to a vehicle drive unit, and thus is very effective and has high industrial applicability.