Patent Publication Number: US-2023136232-A1

Title: Inverter structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a structure of an inverter device mounted on electric vehicles, such as electric automobiles and hybrid automobiles. 
     2. Description of the Related Art 
     Inverters are known which convert direct current to alternating current. The inverters include power modules including switching elements. 
     Since an inverter of this type deals with a large amount of power, a high voltage is applied and large current flows in the inverter. The inverter requires cooling because it generates a large amount of heat when activated. A large surge voltage also occurs. Individual electronic components of the inverter, therefore, tend to be large in size and weight. The presence of such an inverter of the related art has been a hindrance to achieving better fuel economy and lower power consumption. 
     To shorten the distance of power transmission, an inverter is typically disposed near a drive motor. Since an automobile requires many components to be installed, the space for accommodating the inverter is limited. The balance of the vehicle body also needs to be considered. Therefore, it is difficult to properly position a large size, heavy weight inverter in the automobile. 
     As an inverter structure in an alternating current motor combined with an inverter, for example, a structure is known where a positive (+) bus bar and a negative (−) bus bar are molded of resin as an integral part of a doughnut-shaped inverter case, and are connected to a switching element (see, e.g., Japanese Unexamined Patent Application Publication No. 2004-274992). 
     An inverter includes, for example, power modules including switching elements, and a smoothing capacitor. Generally, such electronic components of an inverter that supports a high-voltage power supply are large in size and weight, as described above. 
     Metal strips (bus bars), which are electronic components for connecting the power modules and the smoothing capacitor described above, also allow large current to flow therein and thus are large in size and weight. As the wiring length of bus bars increases, the electric resistance also increases and this results in copper loss when current flows. The bus bars generate a large amount of heat. Moreover, since large current in the inverter is switched on and off at high speed by switching control, significant magnetic changes occur in the bus bars. 
     When the inverter is activated, the magnetic changes cause noise, vibration, and electromagnetic interference in the bus bars. This leads to energy loss and negatively affects the performance of the automobile in various ways. Necessary measures need to be taken to avoid this. If the bus bars have a complex shape, the resulting impact is more significant. 
     In the technique described in Japanese Unexamined Patent Application Publication No. 2004-274992, the switching element and the smoothing capacitor are connected through the positive bus bar and the negative bus bar that are molded of resin as an integral part of the inverter case. With this configuration, due to constraints in wiring length and width, it is not easy to equalize the inductances of the bus bars while reducing them. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the points described above. An object of the present invention is to facilitate equalization of the inductances of wires connected to the smoothing capacitor while reducing the inductances. 
     To achieve the object described above, a first aspect of the present invention provides an inverter structure of an inverter including a smoothing capacitor and a plurality of power modules. The smoothing capacitor includes a plurality of columnar unit capacitors each having electrodes at both ends thereof, a plate-shaped one-end-side bus plate connected to the electrode at one end of each unit capacitor, and a plate-shaped other-end-side bus plate connected to the electrode at the other end of the unit capacitor. The unit capacitors are arranged, with axes thereof parallel to each other, side by side in a direction along a plane perpendicular to the axes. The unit capacitors are arranged at positions equally distant from a predetermined center of the inverter. At the same time, a distance between each power module and at least the unit capacitor closest to the power module is set to be constant. 
     The unit capacitors are arranged, with axes thereof parallel to each other, side by side in a direction along a plane perpendicular to the axes. This can reduce the size of the inverter in the axial direction. The smoothing capacitor includes the plate-shaped one-end-side bus plate and the plate-shaped other-end-side bus plate that are connected to the electrodes at one end and the other end of each unit capacitor. The power modules can thus be easily arranged in such a way as to reduce the distance to the smoothing capacitor. That is, the degree of freedom in the layout of the power modules can be improved. 
     Also, since the distance between each power module and at least the unit capacitor closest to the power module is constant, it is possible to equalize the inductances and offer greater control over the motor. 
     A second aspect of the present invention is characterized in that in the inverter structure according to the first aspect of the present invention, the one-end-side bus plate and the other-end-side bus plate are circular in outer shape. 
     This allows, for example, extension and connection of terminals at any position in the circumferential direction of the smoothing capacitor, and thus can easily increase the degree of freedom in the layout of the power modules. 
     A third aspect of the present invention is characterized in that in the inverter structure according to the first or second aspect of the present invention, the inverter further includes an input bus bar configured to connect one of an outer edge of the one-end-side bus plate and an outer edge of the other-end-side bus plate to the power modules. 
     A fourth aspect of the present invention is characterized in that in the inverter structure according to the first or second aspect of the present invention, an outer edge of at least one of the one-end-side bus plate and the other-end-side bus plate is connected to the power modules. 
     It is thus possible to facilitate connection of the smoothing capacitor to the power modules while reducing and equalizing inductances. 
     A fifth aspect of the present invention is characterized in that in the inverter structure according to any one of the first to fourth aspects of the present invention, an outer surface of at least one of the one-end-side bus plate and the other-end-side bus plate is disposed in the same plane as one of outer surfaces of the power modules. 
     This further facilitates size reduction in the axial direction of the inverter. 
     The present disclosure can facilitate equalization of inductances of wires connected to the smoothing capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram schematically illustrating a configuration of a vehicle system including drive units according to a first embodiment. 
         FIG.  2    is a perspective view of a drive unit including a motor and an inverter. 
         FIG.  3    is a horizontal cross-sectional view of the motor, as viewed from the inverter. 
         FIG.  4    is a circuit diagram of the inverter. 
         FIG.  5    compares a SiC-MOSFET and an IGBT. 
         FIG.  6    gives a perspective view and a circuit diagram illustrating a detailed structure of a power module. 
         FIG.  7    is a horizontal cross-sectional view of the inverter, as viewed from the side opposite the motor. 
         FIG.  8    is a vertical cross-sectional view of the inverter. 
         FIG.  9    is a perspective view of a bus bar. 
         FIG.  10    gives graphs each illustrating a relation between a bus bar size and inductance sensitivity. 
         FIG.  11    is a horizontal cross-sectional view of a cooling passage in the inverter, as viewed from the motor. 
         FIG.  12    is a horizontal cross-sectional view corresponding to  FIG.  11    and illustrating the cooling passage in the inverter, as viewed from the motor, according to a first modification of the first embodiment. 
         FIG.  13    is a horizontal cross-sectional view corresponding to  FIG.  7    and illustrating the inverter, as viewed from the side opposite the motor, according to a second modification of the first embodiment. 
         FIG.  14    is a horizontal cross-sectional view corresponding to  FIG.  7    and illustrating the inverter, as viewed from the side opposite the motor, according to a second embodiment. 
         FIG.  15    is a vertical cross-sectional view corresponding to  FIG.  8    and illustrating the inverter according to the second embodiment. 
         FIG.  16    is a horizontal cross-sectional view corresponding to  FIG.  7    and illustrating the inverter, as viewed from the side opposite the motor, according to a third embodiment. 
         FIG.  17    is a vertical cross-sectional view corresponding to  FIG.  8    and illustrating the inverter according to the third embodiment. 
         FIG.  18    is a schematic diagram illustrating an exemplary arrangement of power modules and unit capacitors according to another modification. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail on the basis of the drawings. The following description of preferred embodiments is merely illustrative in nature and is in no way intended to limit the present invention, its application, or uses. In the embodiments and modifications described below, components having the same functions as those of the other embodiments or modifications are assigned the same reference numerals and their description will be omitted. 
     First Embodiment 
     (Vehicle Configuration) 
       FIG.  1    illustrates a vehicle  1  including drive units A according to a first embodiment, as viewed from below the vehicle  1 . The vehicle  1  transmits power from at least one of an engine  2  and a drive motor  3  disposed at the front of the vehicle  1  to rear wheels  4  disposed at the rear of the vehicle  1 . That is, the vehicle  1  is a front-engine, rear-wheel drive (FR) hybrid vehicle. 
     As illustrated in  FIG.  1   , the vehicle  1  includes the engine  2 , a transmission  5  coupled to the engine  2 , a drive motor  3  interposed between the engine  2  and the transmission  5 , a propeller shaft  6  coupled to the transmission  5  and configured to transmit power from the engine  2  and the drive motor  3  to the rear wheels  4 , and a differential gear  7  coupled to the propeller shaft  6  and configured to transmit power from the engine  2  and the drive motor  3  to the rear wheels  4  on the right and left sides. 
     The propeller shaft  6  extends in the vehicle front-rear direction on the underside of a floor panel  8 . A tunnel  9  is provided in the center of the floor panel  8  in the vehicle width direction. The propeller shaft  6  is disposed inside the tunnel  9 . 
     The vehicle  1  includes an exhaust pipe  10  that extends from the engine  2  in the vehicle front-rear direction. A catalytic device  11  is disposed on the upstream side of the exhaust pipe  10 . While not shown, a silencer is disposed on the 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 power to be supplied to the motor  3 . The drive motor  3  transmits power to the rear wheels  4 . During deceleration of the vehicle  1 , the drive motor  3  generates regenerative power by being rotationally driven by the propeller shaft  6 , and supplies the generated power to the battery  12 . The battery  12  is composed of a first battery unit  12   a  and a second battery unit  12   b  arranged on both sides 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. The battery units  12   a  and  12   b  each include a plurality of battery cells. The battery cells are, for example, lithium-ion batteries. 
     An in-wheel motor  14  is connected to each of front wheels  13  on the right and left sides. The in-wheel motors  14  function as assist motors that generate power at the start of the vehicle  1  and transmit the power to the front wheels  13 . The in-wheel motors  14  also function as regenerative brakes that generate power during deceleration of the vehicle  1 . Like the drive motor  3 , the in-wheel motors  14  are supplied with power from the battery  12 . 
     As illustrated 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 arranged adjacent to each other in the axial direction of the drive motor  3  (or in the vehicle front-rear direction). An inverter  16  is disposed inside each of the in-wheel motors  14  in the vehicle width direction. The in-wheel motor  14  and the inverter  16  are arranged adjacent to each other in the axial direction of the in-wheel motor  14  (or in the vehicle width direction). The drive motor  3  and the inverter  15  constitute a drive unit A. Similarly, the in-wheel motor  14  and the inverter  16  constitute a drive unit A. 
     The inverters  15  and  16  convert direct-current power stored in the battery  12  to alternating-current power and supply the alternating-current power to the motors  3  and  14 . During deceleration of the vehicle  1 , the inverters  15  and  16  convert alternating-current power generated by the motors  3  and  14  to direct-current power and charge the battery  20 . 
     (Drive Unit) 
     The drive unit A of the vehicle  1  will be described by using one that includes the drive motor  3  and the inverter  15  as an example.  FIG.  2    is a perspective view of the drive unit A. As described above, the drive unit A is constituted by the motor  3  and the inverter  15 . The motor  3  and the inverter  15  are coaxially arranged adjacent to each other in the axial direction of the motor  3 . Specifically, a central axis O of the motor  3  coincides with a central axis O of the inverter  15 . The motor  3  (specifically, a casing of the motor  3 ) is cylindrically shaped. The inverter  15  (specifically, a casing of the inverter  15 ) is cylindrically shaped to fit the motor  3 . A rotary shaft  3   a  of the motor  3  penetrates the inverter  15  in the axial direction. The inverter  15  is a thin member that has, for example, a thickness Wiv of 50 mm or less (preferably 30 mm or less). The inverter  15  has an internal cooling passage  61  (described below). An inlet pipe  62  and an outlet pipe  63  for cooling are connected to the upper part of the inverter  15  and communicate with the cooling passage  61 . 
       FIG.  3    is a horizontal cross-sectional view of the motor  3 , as viewed from the inverter  15 . The motor  3  includes coils  17 . Specifically, U-phase, V-phase, and W-phase coils  17   u ,  17   v , and  17   w  are each formed by concentrated winding on the stator of the motor  3 . Two U-phase coils  17   u  are arranged opposite each other in the radial direction of the motor  3 . Similarly, two V-phase coils  17   v  are arranged opposite each other in the radial direction of the motor  3 . Similarly, two W-phase coils  17   w  are arranged opposite each other in the radial direction of the motor  3 . 
     The motor  3  has three motor-side terminal blocks  18  on the outer periphery thereof. The three motor-side terminal blocks  18  correspond to the U-phase, V-phase, and W-phase coils  17   u ,  17   v , and  17   w . A lead wire (not shown) is extended from each of the two U-phase coils  17   u . The two lead wires are tied into a bundle and connected to the corresponding one of the motor-side terminal blocks  18 . The same applies to the V-phase coil  17   v  and the W-phase coil  17   w . An iron core  27  and N-pole and S-pole permanent magnets  28 , which constitute a rotor, are secured to the rotary 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  smooths a voltage applied to the power modules  20 . The plurality of power modules  20  constitute an inverter circuit and convert a direct-current voltage to 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  is connected to the U-phase coil  17   u  of the motor  3 . The V-phase power module  20   v  is connected to the V-phase coil  17   v  of the motor  3 . The W-phase power module  20   w  is connected to the W-phase coil  17   w  of the motor  3 . 
     The power modules  20  are each composed of two arm elements, a lower arm element  21  and an upper arm element  22 , each serving as a switching element. When one of the lower arm element  21  and the upper arm element  22  opens in the power module  20  of each phase, the other of the lower arm element  21  and the upper arm element  22  closes. This allows three-phase alternating current to be supplied to the motor  3 . 
     The power module  20  includes a metal-oxide-semiconductor field-effect transistor (MOSFET) containing silicon carbide (SiC) (hereinafter referred to as SiC-MOSFET).  FIG.  5    compares a SiC-MOSFET and an insulated gate bipolar transistor (IGBT). The SiC-MOSFET constitutes a chip  24  including the lower arm element  21 , the upper arm element  22 , and other control elements. The lower surface of the chip  24  is secured by soldering to a silicon substrate. A copper block  25  serving as a heat transfer block is secured by soldering to the upper surface of the chip  24 . The same applies to the IGBT. 
     As illustrated in  FIG.  5   , the surface area of the chip  24  constituted by the SiC-MOSFET is smaller than the surface area of a chip  24 ′ constituted by the IGBT. Accordingly, the size of the copper block  25  disposed on the upper side of the SiC-MOSFET (chip)  24  is smaller than the size of a copper block  25 ′ disposed on the upper side of the IGBT (chip)  24 ′. The SiC-MOSFET has better heat resistance than the IGBT. 
       FIG.  6    gives a perspective view and a circuit diagram illustrating a detailed structure of the power module  20 . Each power module  20  has a wide flat shape. Specifically, each power module  20  is longer in a width direction W than in a thickness direction t. The power module  20  is substantially in the shape of a rectangular parallelepiped. The width direction W includes a first width direction W 1  and a second width direction W 2  orthogonal to each other. Hereinafter, one side of the power module  20  in the thickness direction t may be referred to as a lower side, and the other side of the power module  20  in the thickness direction t may be referred to as an upper side. 
     The power module  20  has a lower surface  31  to be cooled (first cooled surface), on the lower side thereof (or on one side thereof in the thickness direction t). The power module  20  has an upper surface  32  on the upper side thereof. The power module  20  has a first end face  33  on one side thereof in the first width direction W 1 . The power module  20  has a second end face  34  on the other side thereof in the first width direction W 1 . 
     A negative-side input terminal  35  is connected to the lower side of the first end face  33 , on one side of the first end face  33  in the second width direction W 2 . A positive-side input terminal  36  is connected to the upper side of the first end face  33 , on the other side of the first end face  33  in the second width direction W 2 . The negative-side input terminal  35  and the positive-side input terminal  36  are spaced apart in the up and down direction (thickness direction t). An output terminal  37  is connected to the center of the second end face  34 . 
     The lower arm element  21  and the upper arm element  22  are housed in a package (housing) of the power module  20 . The negative-side input terminal  35  is connected to the lower arm element  21 , and the positive-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 horizontal cross-sectional view of the inverter  15 , as viewed from the side opposite the motor  3 .  FIG.  8    is a vertical cross-sectional view taken along line VIII-VIII in the inverter  15  illustrated in  FIG.  7   . As illustrated in  FIG.  7   , the inverter  15  has, in the center thereof, a shaft through-hole  40  that allows the rotary shaft  3   a  of the motor  3  to pass therethrough. The shaft through-hole  40  is defined by a cylindrical boss  41  formed therearound. The smoothing capacitor  19  is disposed along the boss  41 . 
     The power modules  20  (U-phase power module  20   u , V-phase power module  20   v , and W-phase power module  20   w ) are arranged on the outer side of the smoothing capacitor  19 . The power modules  20  are arranged side by side in the circumferential direction of the motor  3 , on the outer side of the smoothing capacitor  19 . That is, for example, the power modules  20  are arranged at positions (on a circular arc) equally distant from the center of the rotary shaft  3   a  of the motor  3 . At the same time, the power modules  20  are arranged at positions equally distant from the smoothing capacitor  19  (i.e., positions in respective radial directions of the smoothing capacitor  19 ). More specifically, for example, when x is the distance between the negative-side input terminal  35  (or positive-side input terminal  36 ) of the U-phase power module  20   u  and the outer edge of the one-end-side bus plate  19   c  (or the other-end-side bus plate  19   d ) of the smoothing capacitor  19 , the same distance x is set for the V-phase power module  20   v  and the W-phase power module  20   w . The distance between only one of the negative-side input terminal  35  and the positive-side input terminal  36  and the smoothing capacitor  19 , described above, may be equal for all the power modules  20 . 
     The input terminals  35  and  36  (on the first end face  33 ) and the output terminal  37  (on the second end face  34 ) of each power module  20  are configured to face in the circumferential direction of the motor  3  (inverter  15 ). The power modules  20  are arranged on respective lines radially extending from the center O of the inverter  15  (motor  3 ). The power modules  20  are each disposed in such a way that the thickness direction t coincides with the axial direction of the motor  3 . The smoothing capacitor  19  and the power modules  20  are arranged in a space defined by an outer peripheral wall  42  and the boss  41  in the inverter  15 . 
     As illustrated in  FIGS.  7  and  8   , a heat sink  60  is disposed on a side of the inverter  15  adjacent to the motor  3 . The heat sink  60  is mainly used for cooling the power modules  20 . The heat sink  60  is disposed between the outer peripheral wall  42  and the boss  41  in the inverter  15 . The heat sink  60  has an upper wall  60   a , an outer peripheral wall  60   b , a lower wall  60   c , and an inner peripheral wall  60   d.    
     An upper surface  65  of the upper wall  60   a  of the heat sink  60  constitutes a mounting surface (which may hereinafter be referred to as “mounting surface  65 ”) orthogonal to the axial direction of the motor  3 . The lower surface (first cooled surface)  31  of each of the power modules  20  (U-phase power module  20   u , V-phase power module  20   v , and W-phase power module  20   w ) faces toward the motor  3 . Specifically, the lower surfaces (first cooled surfaces)  31  of the power modules  20  are arranged side by side on the same mounting surface  65 . 
     As illustrated in  FIGS.  7  and  8   , the smoothing capacitor  19  is formed by a group of columnar unit capacitors  45  each having electrodes T at both ends (one end  19   a  and the other end  19   b ) thereof. The columnar unit capacitors  45  are arranged, with axes thereof parallel to each other (i.e., parallel to the rotary shaft  3   a  of the motor  3 ), side by side in a direction along a plane perpendicular to the axes, and connected together by being held from both sides in the axial direction by the one-end-side bus plate  19   c  and the other-end-side bus plate  19   d  that are circular in outer shape. The one-end-side bus plate  19   c  on the lower side of the smoothing capacitor  19  serves as a surface to be cooled (third cooled surface) and faces toward the motor  3 . Specifically, the one-end-side bus plate  19   c  of the smoothing capacitor  19  is mounted on the mounting surface  65 . 
     The configuration of the electrodes T at the one end  19   a  and the other end  19   b  of each unit capacitor  45  is not particularly limited, and electrodes of various types may be used. For example, the electrodes T may be lead wire electrodes, band electrodes, or plate-shaped electrodes. Also, the way of connecting the electrodes T to the one-end-side bus plate  19   c  and the other-end-side bus plate  19   d  is not particularly limited, and various techniques, such as welding, soldering, or mechanical pressure bonding, may be used. 
     The plurality of unit capacitors  45  are arranged in such a way that some of them are equally distant, for example, from the center of the rotary shaft  3   a  of the motor  3 . Also, a pattern of arrangement of each of the power modules  20  and at least some unit capacitors  45  close to the power module  20  is set to be constant. Additionally or alternatively, the distance between each power module  20  and at least the unit capacitor  45  closest to the power module  20  is set to be constant. More specifically, for example, when y is the distance between the negative-side input terminal  35  (or positive-side input terminal  36 ) of the U-phase power module  20   u  and the electrode T of a unit capacitor  451  closest thereto, the same distance y is set for the V-phase power module  20   v  and the W-phase power module  20   w . The distance between only one of the negative-side input terminal  35  and the positive-side input terminal  36  and the unit capacitor  45  closest thereto may be equal for all the power modules  20 . 
     With the configuration described above, the inductances of connecting wires between the smoothing capacitor  19  and the power modules  20  can be easily reduced, and/or the inductances described above can be easily equalized by making the power modules  20  equally distant from the smoothing capacitor  19 . 
     As illustrated in  FIGS.  7  and  8   , with respect to the smoothing capacitor  19 , the power modules  20  are arranged side by side in a direction along a plane perpendicular to the axial direction. The smoothing capacitor  19  is connected to the power modules  20  by a negative-side bus bar  51  and a positive-side bus bar  52  serving as input bus bars  50  (which may hereinafter be simply referred to as “bus bars  50 ”). The negative-side bus bar  51  and the positive-side bus bar  52  are plate-shaped. Specifically, the negative-side bus bar  51  and the positive-side bus bar  52  are longer in the width direction W and the length direction L than in the thickness direction t. 
     As illustrated in  FIG.  7   , the negative-side bus bar  51  and the positive-side bus bar  52  are wide members that extend along the circumferential direction of the motor  3  (inverter  15 ). In other words, the negative-side bus bar  51  and the positive-side bus bar  52  are wide members that extend along the direction in which the power modules  20  are arranged side by side. The width direction W of the negative-side bus bar  51  and the positive-side bus bar  52  is along the circumferential direction of the motor  3  (or along an arc). The negative-side bus bar  51  and the positive-side bus bar  52  are fan-shaped. The length direction L of the negative-side bus bar  51  and the positive-side bus bar  52  is along the radial direction of the motor  3 . 
     The negative-side bus bar  51  is connected at one end portion  51   i  thereof to the outer edge of the one-end-side bus plate  19   c  on the lower side of the smoothing capacitor  19 . The negative-side bus bar  51  is also connected at the other end portion  510  thereof to the negative-side input terminal  35  of each power module  20 . The positive-side bus bar  52  is connected at one end portion  52   i  thereof to the outer edge of the other-end-side bus plate  19   d  on the upper side of the smoothing capacitor  19 . The positive-side bus bar  52  is also connected at the other end portion  52   o  thereof to the positive-side input terminal  36  of each power module  20 . The outer edge of at least one of the one-end-side bus plate  19   c  and the other-end-side bus plate  19   d  may be directly connected to the negative-side input terminal  35  or the positive-side input terminal  36  of the power module  20 . 
     The negative-side bus bar  51  may have a lower surface  51   a  to be cooled (second cooled surface), on the lower side thereof (or on one side thereof in the thickness direction t). The lower surface (second cooled surface)  51   a  of the negative-side bus bar  51  faces toward the motor  3 . Specifically, the lower surface (second cooled surface)  51   a  of the negative-side bus bar  51  may be mounted on the mounting surface  65 . 
     As illustrated in  FIG.  7   , an output bus bar  54  is connected to the output terminal  37  of each power module  20 . There are a total of three output bus bars  54  corresponding to the U-phase, the V-phase, and the W-phase. The output bus bars  54  are each interposed between the power module  20  and a corresponding one of the coils  17 . The output bus bar  54  is plate-shaped. In addition to the output bus bar  54 , a wire harness may be interposed between the power module  20  and a corresponding one of the coils  17 . 
     The inverter  15  has three inverter-side terminal blocks  46  on the outer periphery thereof. The inverter-side terminal blocks  46  correspond to the respective power modules  20 . The output bus bars  54  each extend to a corresponding one of the inverter-side terminal blocks  46 . Electrically conducting members (such as a bus bar and a wire harness) are interposed between each inverter-side terminal block  46  and a corresponding one of the motor-side terminal blocks  18 . 
     (Inductance Sensitivity of Bus Bar) 
       FIG.  9    is a perspective view of the bus bar  50 .  FIG.  10    gives graphs each illustrating a relation between a size of the bus bar  50  and inductance sensitivity. As a result of dedicated studies, the inventors of the present application made the following discoveries about the relation between the size of the bus bar  50  and inductance sensitivity. 
     As illustrated in  FIGS.  9  and  10   , the inductance sensitivity (nH) of the bus bar  50  decreases as the width dimension W (mm) of the bus bar  50  increases. 
     Basically, the inductance sensitivity (nH) of the bus bar  50  increases as the length dimension L (mm) of the bus bar  50  increases. However, as shown by the graph in the middle of  FIG.  10   , the relation between the length dimension L (mm) and the inductance sensitivity (nH) of the bus bar  50  has a local minimum M. This means that different length dimensions L correspond to the same inductance sensitivity (nH). Specifically, the inductance sensitivity (nH) of the bus bar  50  ( 51  or  52 ) is a function of the length dimension L (mm) from the one end portion  51   i  or  52   i  (i.e., the one-end-side bus plate  19   c  or other-end-side bus plate  19   d  of the smoothing capacitor  19 ) to the other end portion  510  or  52   o  (i.e., the input terminal  35  or  36  of each power module  20 ) of the bus bar  50  ( 51  or  52 ). The function has the local minimum M such that different lengths, the first length L 1  (mm) and the second length L 2  (mm), correspond to the same inductance sensitivity K (nH). The second length L 2  (mm) is longer than the first length L 1  (mm). 
     The inductance sensitivity (nH) of the bus bar  50  shows little change with the change in the thickness dimension t (mm) of the bus bar  50 . 
     As illustrated in  FIG.  7   , the width dimension of the negative-side bus bar  51  and the width dimension of the positive-side bus bar  52  are substantially the same. As illustrated in  FIG.  8   , the length dimension L− of the negative-side bus bar  51  and the length dimension L+ of the positive-side bus bar  52  differ from each other. The length dimension L− of the negative-side bus bar  51  corresponds to the first length L 1 . The length dimension L+ of the positive-side bus bar  52  corresponds to the second length L 2 . The length dimension L+ of the positive-side bus bar  52  (second length L 2 ) is longer than the length dimension L− of the negative-side bus bar  51  (first length L 1 ). Because of the presence of the local minimum M, however, the inductance of the negative-side bus bar  51  and the inductance of the positive-side bus bar  52  are equal. 
     (Cooling Passage) 
       FIG.  11    is a horizontal cross-sectional view of the cooling passage  61  in the inverter  15 , as viewed from the motor  3 . As illustrated in  FIGS.  8  and  11   , the heat sink  60  has therein the cooling passage (cooling jacket)  61  constituting a cooling zone. The cooling passage  61  is defined by the upper wall  60   a , the outer peripheral wall  60   b , the lower wall  60   c , and the inner peripheral wall  60   d . As viewed in the axial direction of the motor  3  (inverter  15 ), the cooling passage  61  is a doughnut-shaped (annular or cylindrical) passage that extends throughout the perimeter. The rotary shaft  3   a  of the motor  3  penetrates inside the inner peripheral wall  60   d . As described above, the upper surface of the upper wall  60   a  of the heat sink  60  is the mounting surface  65 . 
     The cooling passage  61  is disposed closer to the motor  3  than the mounting surface  65  is. A cooling medium H flows in the cooling passage  61 . For example, the cooling medium H is cooling water or cooling oil. 
     A plurality of fins  64  constituting the cooling zone are provided in the interior (cooling passage  61 ) of the heat sink  60 . In the cooling passage  61 , the fins  64  extend downward from the upper wall  60   a . That is, the fins  64  are disposed closer to the motor  3  than the mounting surface  65  is. 
     As illustrated in  FIGS.  8  and  11   , as viewed in the axial direction of the motor  3  (inverter  15 ), the cooling passage (cooling zone)  61  faces toward the entire area of both the lower surface (first cooled surface)  31  of each power module  20  and the one-end-side bus plate (third cooled surface)  19   c  on the lower side of the smoothing capacitor  19 . 
     Similarly, as viewed in the axial direction of the motor  3 , the fins (cooling zone)  64  face toward the entire area of both the lower surface (first cooled surface)  31  of each power module  20  and the one-end-side bus plate (third cooled surface)  19   c  on the lower side of the smoothing capacitor  19 . 
     As illustrated in  FIG.  11   , the inlet pipe  62  and the outlet pipe  63  are connected to the upper part of the outer peripheral wall  42  of the inverter  15 . The inlet pipe  62  and the outlet pipe  63  communicate with the cooling passage  61 . The cooling medium H introduced through the inlet pipe  62  into the cooling passage  61  is guided by the outer peripheral wall  42  and the boss  41  to circumferentially flow in the cooling passage  61 , and is then discharged through the outlet pipe  63  to the outside. The inlet pipe  62  may be provided with a guide plate  66  on the downstream side. 
     Operation and Effect of First Embodiment: Reduced and Equalized Inductances 
     In the present embodiment, increasing the width dimensions of the bus bars  51  and  52  can reduce the inductances of the bus bars  51  and  52 . 
     The smoothing capacitor  19  and the power modules  20   u ,  20   v , and  20   w  are mounted on the same mounting surface  65 . Since this reduces the lengths of the bus bars  51  and  52  that connect the smoothing capacitor  19  to the power modules  20   u ,  20   v , and  20   w , the inductances of the bus bars  51  and  52  can be reduced. 
     The motor  3  and the inverter  15  are arranged adjacent to each other in the axial direction. This reduces the length of an electric path between each of the power modules  20   u ,  20   v , and  20   w  and a corresponding one of the coils  17   u ,  17   v , and  17   w . It is thus possible to reduce the inductance of the electric path (including the output bus bar  54 ) that connects each of the power modules  20   u ,  20   v , and  20   w  to a corresponding one of the coils  17   u ,  17   v , and  17   w.    
     The power modules  20   u ,  20   v , and  20   w  are arranged side by side in the circumferential direction of the motor  3 , on the outer side of the smoothing capacitor  19 . This can equalize the distances between the smoothing capacitor  19  and each of the power modules  20   u ,  20   v , and  20   w . With the bus bars  51  and  52  that are wide members extending along the circumferential direction of the motor  3 , the inductances of the electric paths between the smoothing capacitor  19  and each of the power modules  20   u ,  20   v , and  20   w  can be equalized. 
     With the local minimum M (see  FIG.  10   ), even though the length dimension L− of the negative-side bus bar  51  (first length L 1 ) and the length dimension L+ of the positive-side bus bar  52  (second length L 2 ) differ from each other, the inductance of the negative-side bus bar  51  and the inductance of the positive-side bus bar  52  can be equalized. 
     As described above, a pattern of arrangement of each power module  20  and some unit capacitors  45  close to the power module  20  is set to be constant. Also, the distance between each power module  20  and at least the unit capacitor  45  closest to the power module  20  (i.e., the distance of wiring connection between the electrode and the terminal) is set to be constant. Thus, the inductances of the connecting wires between the smoothing capacitor  19  and the power modules  20  can be more easily reduced, and/or the inductances described above can be easily equalized by making the power modules  20  equally distant from the smoothing capacitor  19 . 
     Operation and Effect of First Embodiment: Improved Cooling Performance 
     In each power module  20  having a wide and flat shape, the lower surface (first cooled surface)  31  having a large area faces toward the cooling passage  61  and the fins  64  constituting a cooling zone. This increases the area of the power modules  20  cooled by the cooling passage  61  and fins  64  (cooling zone). Thus, even when only one side (lower surface, first cooled surface)  31  of the power module  20  is cooled by the cooling passage  61  and fins  64  (cooling zone), it is possible to ensure sufficient cooling performance. 
     The lower surfaces (first cooled surfaces)  31  of the power modules  20  (U-phase power module  20   u , V-phase power module  20   v , and W-phase power module  20   w ) are arranged side by side on the same mounting surface  65  orthogonal to the axial direction of the motor  3 . This can reduce the axial length of the inverter  15 . Also, since the cooling passage  61  and fins  64  (cooling zone) simply need to be provided on one side (lower surfaces, first cooled surfaces)  31  of the power modules  20 , the inverter  15  can be made smaller than when the cooling passage  61  and fins  64  (cooling zone) are provided on both sides of the power modules  20 . 
     With the configuration described above, it is possible to reduce the size of the drive unit A composed of the motor  3  and the inverter  15  while sufficiently cooling the power modules  20 . 
     By allowing the cooling medium H to flow in the cooling passage  61  constituting a cooling zone, the power modules  20  can be more effectively cooled by the cooling zone. 
     Since the SiC-MOSFET chip  24  included in each power module  20  is small in size, the copper block  25  (heat transfer block) disposed on the SiC-MOSFET chip  24  is also small in size (see  FIG.  5   ). To enable the power module  20  to be effectively cooled on both sides, it is necessary to provide expensive ceramic substrates (e.g., SiN substrates) on both sides of the SiC-MOSFET chip  24 . Cooling one side of the power module  20  and improving the effectiveness of cooling the one side is more advantageous costwise than cooling both sides of the power module  20 . 
     Together with the power modules  20 , the smoothing capacitor  19  can also be cooled by the cooling passage  61  and fins  64  (cooling zone). 
     The power modules  20  are arranged side by side in the circumferential direction of the motor  3 , on the outer side of the smoothing capacitor  19 . At the same time, by extending the negative-side bus bar  51  in the circumferential direction of the motor  3 , the negative-side bus bar  51  can be easily widened in the width direction W. This can easily increase the area of heat dissipation from the negative-side bus bar  51  toward the cooling passage  61  and fins  64  (cooling zone). 
     The cooling passage  61  and fins  64  (cooling zone) are disposed adjacent to the motor  3 . This is also advantageous for cooling the wires (e.g., output bus bars  54 ) that connect the motor  3  to the power modules  20 . 
     In the related art, as indicated by a two-dot chain line in  FIG.  1   , an inverter  15 ′ has often been disposed near a second battery unit  12   b ′ of a battery  12 ′. In the present embodiment, where the inverter  15  can be disposed adjacent to the motor  3  in the axial direction, the inverter  15  does not need to be disposed near the second battery unit  12   b . This can increase the degree of freedom in the layout of the second battery unit  12   b  and can increase the size of the second battery unit  12   b.    
     First Modification of First Embodiment 
       FIG.  12    corresponds to  FIG.  11    and illustrates a first modification of the first embodiment. In the present modification, the cooling passage  61  and fins  64  (cooling zone) do not at all face toward the one-end-side bus plate (third cooled surface)  19   c  on the lower side of the smoothing capacitor  19 . 
     There may be no problem even when the smoothing capacitor  19 , which generates less heat than the power modules  20 , is not cooled by the cooling passage  61  and fins  64  (cooling zone). 
     Second Modification of First Embodiment 
       FIG.  13    corresponds to  FIG.  7    and illustrates a second modification of the first embodiment. The inverter  16  according to the present modification is disposed adjacent to the in-wheel motor  14  in the axial direction of the in-wheel motor  14  (or in the vehicle width direction) (see  FIG.  1   ). The inverter  16  does not have the shaft through-hole  40  and the boss  41 . In the smoothing capacitor  19 , the unit capacitors  45  are also provided in and around the center. The heat sink  60  does not have the inner peripheral wall  60   d . The cooling passage  61  is in the shape of a circle without a hole, as viewed in the axial direction of the motor  3 . 
     Other Modifications of First Embodiment 
     The cooling medium H flowing in the cooling passage  61  may be, for example, air. The cooling zone may not include the cooling passage  61 , and may be constituted by the fins  64  alone. The cooling zone may be constituted by a solid cooling member. 
     The cooling zone does not necessarily need to be provided throughout the perimeter, and may be provided only in an area facing toward the power modules  20  and extending in the circumferential direction. 
     Instead of the negative-side bus bar  51 , the positive-side bus bar  52  may have the second cooled surface on one side thereof (or on one side in the thickness direction t) facing toward the motor  3  and mounted on the mounting surface  65 . 
     While not shown, the output bus bars  54  may be wide members that extend along the circumferential direction of the motor  3 . In other words, the width direction of the output bus bars  54  may be along the circumferential direction (or along an arc). The output bus bars  54  may be fan-shaped. This makes it easier to widen the output bus bars  54 , and thus easier to reduce the inductances of the output bus bars  54  (see  FIG.  10   ). 
     The mounting surface  65  may be constituted by a plurality of surfaces in the same plane orthogonal to the axial direction of the motor  3 . 
     Second Embodiment 
       FIG.  14    is a horizontal cross-sectional view corresponding to  FIG.  7    and illustrating the inverter  15 , as viewed from the side opposite the motor  3 , according to a second embodiment.  FIG.  15    is a vertical cross-sectional view corresponding to  FIG.  8    and illustrating the inverter  15  according to the second embodiment. Note that the same components as those of the aforementioned embodiment may not be described in detail. 
     In the present embodiment, the power modules  20  (U-phase power module  20   u , V-phase power module  20   v , and W-phase power module  20   w ) are arranged on the outer side of the smoothing capacitor  19 . The power modules  20  are arranged side by side in the circumferential direction of the motor  3 , on the outer side of the smoothing capacitor  19 . 
     The input terminals  35  and  36  (on the first end face  33 ) and the output terminal  37  (on the second end face  34 ) of each power module  20  are configured to face in the radial direction of the motor  3  (inverter  15 ). Specifically, the input terminals  35  and  36  (on the first end face  33 ) of each power module  20  face inward, and the output terminal  37  (on the second end face  34 ) of each power module  20  faces outward. The power modules  20  are arranged on respective lines radially extending from the center O of the inverter  15  (motor  3 ). 
     As in the embodiments described above, the lower surface (first cooled surface)  31  of each power module  20  and the one-end-side bus plate (third cooled surface)  19   c  on the lower side of the smoothing capacitor  19  are mounted on the mounting surface  65 . 
     As illustrated in  FIG.  14   , the width dimension of the negative-side bus bar  51  and the width dimension of the positive-side bus bar  52  are the same. As illustrated in  FIG.  15   , the length dimension L− of the negative-side bus bar  51  and the length dimension L+ of the positive-side bus bar  52  are the same. Accordingly, the inductance of the negative-side bus bar  51  and the inductance of the positive-side bus bar  52  are equal. 
     The other configurations are the same as those of the first embodiment. 
     Third Embodiment 
       FIG.  16    is a horizontal cross-sectional view corresponding to  FIG.  7    and illustrating the inverter  15 , as viewed from the side opposite the motor  3 , according to a third embodiment.  FIG.  17    is a vertical cross-sectional view corresponding to  FIG.  8    and illustrating the inverter  15  according to the third embodiment. Note that the same components as those of the aforementioned embodiments may not be described in detail. 
     As illustrated in  FIGS.  16  and  17   , the negative-side input terminal  35  is connected to the lower side of the first end face  33  of each power module  20 . The positive-side input terminal  36  is connected to the lower side of the second end face  34  of each power module  20 . The output terminal  37  is connected to the center of the upper surface  32  of each power module  20 . 
     The negative-side bus bar  51  connects the one-end-side bus plate  19   c  of the smoothing capacitor  19  to the negative-side input terminal  35  of each power module  20 . The positive-side bus bar  52  connects the other-end-side bus plate  19   d  of the smoothing capacitor  19  to the positive-side input terminal  36  of each power module  20 . 
     The positive-side bus bar  52  leaves the other-end-side bus plate  19   d  of the smoothing capacitor  19  (one end portion  52   i ), and extends along the upper surface  32  from the first end face  33  toward the second end face  34  of each power module  20 . The positive-side bus bar  52  then bends downward and extends along the second end face  34  to reach the positive-side input terminal  36  (the other end portion  52   o ) on the lower side. In other words, the positive-side bus bar  52  extends along the upper surface  32  to wrap the power module  20 . Each output bus bar  54  leaves the output terminal  37  on the upper surface  32  of the power module  20  and extends upward. The positive-side bus bar  52  has three openings that allow passage of the respective output bus bars  54  that extend upward. 
     As in the embodiments described above, the lower surface (first cooled surface)  31  of each power module  20  and the one-end-side bus plate (third cooled surface)  19   c  on the lower side of the smoothing capacitor  19  are mounted on the mounting surface  65 . 
     As illustrated in  FIG.  16   , the width dimension of the negative-side bus bar  51  and the width dimension of the positive-side bus bar  52  are the same. As illustrated in  FIG.  17   , the length dimension (L−) of the negative-side bus bar  51  and the length dimension (sum of La+ and Lb+) of the positive-side bus bar  52  differ from each other. The length dimension (L−) of the negative-side bus bar  51  corresponds to the first length L 1 . The length dimension (sum of La+ and Lb+) of the positive-side bus bar  52  corresponds to the second length L 2 . The length dimension (sum of La+ and Lb+, second length L 2 ) of the positive-side bus bar  52  is longer than the length dimension (L−, first length L 1 ) of the negative-side bus bar  51 . Because of the presence of the local minimum M (see  FIG.  10   ), however, the inductance of the negative-side bus bar  51  and the inductance of the positive-side bus bar  52  are equal. 
     The conditions (e.g., materials) of the negative-side bus bar  51  and the positive-side bus bar  52  according to the present embodiment differ from those in the embodiments described above. This means that the mode of the local minimum M (see  FIG.  10   ) also differs from that in the embodiments described above. Specifically, the difference between the first length L 1  and the second length L 2  is greater than in the embodiments described above. 
     The other configurations are the same as those of the second embodiment. 
     Other Embodiments 
     The embodiments described above show an example where three power modules  20  are closely arranged side by side on the circumference, but the arrangement of the power modules  20  is not limited to this. For example, as illustrated in  FIG.  18   , the power modules  20  may be arranged at intervals of 120° about the central axis, and the unit capacitors  45  may accordingly be arranged in a rotationally symmetrical pattern. This arrangement can further facilitate equalization of inductances. 
     Although the present disclosure has been described with reference to the preferred embodiments, the description above is not intended to be limiting and various modifications can be made.