Abstract:
An electronic circuit includes: a bus bar connected to a power source having a positive terminal and a negative terminal; and a plurality of object switching elements as driving objects connected to the bus bar, the object switching elements forming a parallel connected circuit. The object switching elements include minimum on-resistance elements having minimum on-resistance compared to other object switching elements in a corresponding current region among mutually different current regions; and connection points between the minimum on-resistance elements and the bus bar are located at different locations to have mutually different inductance of respective conduction paths between the power source to the connection points located at the different locations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2016-88415 filed Apr. 26, 2016, the description of which is incorporated herein by reference. 
       BACKGROUND 
     Technical Field 
       [0002]    The present disclosure relates to an electronic circuit provided with a plurality of switching elements connected in parallel to each other. 
       Description of Related Art 
       [0003]    The electronic circuit includes, as disclosed by JP-A-2000-197206, a drive circuit that drives a parallel connected circuit in which an IGBT as a Si (silicon) device and a MOSFET as a SiC (silicon carbide) device are connected in parallel. According to this configuration, maximum current value can be increased in a large current region, and large amount of current can flow through a MOSFET having low on-resistance in a small current region so that power loss in the small current region can be reduced. 
         [0004]    Switching elements such as IGBTs or MOSFETs are connected to a power source via a bus bar through which current flows between the switching elements and the power source. Depending on positions of the connection points on the bus bar connected to the switching elements, inductance values of electrical connection are different between respective connection points to the power source. Hence, the inductance variation may influence a driving operation of the switching element. 
       SUMMARY 
       [0005]    The present disclosure provides an electronic circuit capable of avoiding an adverse effect to the switching elements. 
         [0006]    Hereinafter, detailed configuration of the electronic circuit and advantageous effects will be described. 
         [0007]    The present disclosure provides an electronic circuit including: a bus bar connected to a power source having a positive terminal and a negative terminal; and a plurality of object switching elements as driving objects connected to the bus bar, the object switching elements forming a parallel connected circuit. The object switching elements include minimum on-resistance elements having minimum on-resistance compared to other object switching elements in a corresponding current region among mutually different current regions; and connection points between the minimum on-resistance elements and the bus bar are located at different locations to have mutually different inductances of respective conduction paths between the power source to the connection points located at the different locations. 
         [0008]    According to the above-described disclosure, the object switching elements include minimum on-resistance elements having minimum on-resistance compared to other object switching elements in a corresponding current region among mutually different current regions. Hence, in the respective current regions, a large amount of current flows through the minimum on-resistance elements rather than other switching elements. On the other hand, depending on the inductance value of the electrical conduction path from the connection points of the object switching elements to the power source on the bus bar, an amount of surge voltage caused by switching of the object switching elements and an amount of current flowing into the object switching elements are different. 
         [0009]    According to the above-described disclosure, in respective current regions, the connection points of the minimum on-resistance elements on the bus bar are disposed at locations to have different inductances of conduction paths from the power source on the bus bar. Hence, in the object switching elements, the connection of the object switching element having a large amount of current flowing can be designed to have an appropriate inductance value for a large current flow. Thus, adverse effects can be avoided for driving the object switching elements. 
         [0010]    As an embodiment of the above-described disclosure, the following configuration can be employed, that is, the minimum on-resistance elements in the object switching elements, where an on-resistance becomes minimum in a predetermined current region, are located on the bus bar at a location having smaller inductance to the power source than that of other minimum on-resistance elements in the object switching elements, where an on-resistance becomes minimum in a current region smaller than the predetermined current region. 
         [0011]    The larger the current flowing through the parallel connected circuits which are connected to the power source via the bus bar, the larger the current flowing through the minimum on-resistance elements in the large current region. Here, the larger the current flowing through the object switching elements, the larger the surge voltage due to switching operations of the object switching elements. Hence, in the plurality of object switching elements, surge voltage becomes larger at the minimum on-resistance elements in the large current region than other switching elements. 
         [0012]    The smaller the inductance of the electrical conduction path to the power source from the object switching element via the bus bar, the smaller the surge voltage due to the switching of drive state of the switching elements. In other words, the smaller the inductance of the conduction path, the more reduces the surge voltage. In this respect, the following configuration can be used, that is, the minimum on-resistance elements in the object switching elements, where an on-resistance becomes minimum in a predetermined current region, are located on the bus bar at a location having smaller inductance to the power source than that of other minimum on-resistance elements in the object switching elements, where an on-resistance becomes minimum in a current region smaller than the predetermined current region. Thus, in the large current region where large amount of current flows through the above-described parallel connected circuits, the surge voltage can be reduced so that electronic circuits can be prevented from being damaged. 
         [0013]    As an embodiment of the above-described disclosure, the following configuration can be used, that is, the minimum on-resistance elements in the object switching elements (SUHB, SVHB, SWHB) where an on-resistance becomes minimum in a predetermined current region, are located on the bus bar at a location having smaller inductance to the power source than that of other minimum on-resistance elements in the object switching elements (SUHA, SVHA, SWHA) where an on-resistance becomes minimum in a current region larger than the predetermined current region. 
         [0014]    The larger the current flowing between the above-described parallel connected circuits and the power source via the bus bar, the larger the current flowing through the minimum on-resistance switching elements in the small current region. 
         [0015]    As described, the smaller the inductance of the electrical conduction path between the object switching element and the power source via the bus bar, the larger the current flowing to the switching element or more enhances an effect of reducing the surge voltage which is caused by switching operation of the elements. In this regard, the following configuration is used, that is, the minimum on-resistance elements in the object switching elements where an on-resistance becomes minimum in a predetermined current region, are located on the bus bar at a location having smaller inductance to the power source than that of other minimum on-resistance elements in the object switching elements where an on-resistance becomes minimum in a current region larger than the predetermined current region. Thus, in a current region where a small amount of current flows through the parallel connected circuits, current preferentially flows through the object switching elements having smallest on-resistance (i.e., minimum on-resistance elements) in a small current region. Also, surge voltage due to switching operation of the minimum on-resistance elements can be reduced. As a result, power loss of the electronic circuits can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]    In the accompanying drawings: 
           [0017]      FIG. 1  is a diagram showing an overall configuration of an on-vehicle motor control system according a first embodiment; 
           [0018]      FIG. 2  is a diagram showing voltage-current characteristics for an IGBT and a MOSFET; 
           [0019]      FIG. 3  is a diagram showing a change in inductance depending on a conducting path between a capacitor and an IGBT or a MOSFET; 
           [0020]      FIG. 4  is an overall configuration of an on-vehicle motor control system according to a second embodiment; 
           [0021]      FIG. 5  is a diagram showing a change in inductance depending on a conducting path between a capacitor and an IGBT or a MOSFET; and 
           [0022]      FIG. 6  is a diagram showing an overall configuration of an on-vehicle motor control system according to a third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0023]    Hereinafter, with reference to the drawings, a first embodiment of an on-vehicle inverter of an electronic circuit according to the present disclosure will be described. 
         [0024]    As shown in  FIG. 1 , a control system is provided with a smoothing capacitor  1 , an inverter  20 , a motor generator  30  and a control unit  40 . Also, in  FIG. 1 , bus bars  50   p  (positive side) and  50   n  (negative side) are shown, in which a positional relationship among respective switches (described later) and a positional relationship between bus bars and the switches are illustrated. 
         [0025]    The motor generator  30  serves as an on-vehicle main equipment, capable of transmitting motive power to the driving wheel (not shown). According to the present embodiment, three-phase motor-generator is used as the motor generator  30 . As the motor generator, a permanent-magnet type synchronous motor can be used. 
         [0026]    The inverter  20  has switching portions. Specifically, the inverter  20  is provided with a series-connected body of a U-phase upper arm switch  20 UH and a U-phase lower arm switch  20 UL. The U-phase upper arm switch  20 UH includes a U-phase first upper arm switching element SUHA and a U-phase second upper arm switching element SUHB. The U-phase lower arm switch  20 UL includes a U-phase first lower arm switching element SULA and a U-phase second lower arm switching element SULB. At the output terminals corresponding to the U-phase first upper arm switching element SUHA and the U-phase second upper arm switching element SUHB, respective input terminals corresponding to the U-phase first lower arm switching element SULA and the U-phase second lower arm switching element SULB are connected. 
         [0027]    The inverter  20  is provided with a series-connected body of a V-phase upper arm switch  20 VH and a V-phase lower arm switch  20 VL. The V-phase upper arm switch  20 VH includes a V-phase first upper arm switching element SVHA and a V-phase second upper arm switching element SVHB. The V-phase lower arm switch  20 VL includes a V-phase first lower arm switching element SVLA and a V-phase second lower arm switching element SVLB. 
         [0028]    The inverter  20  is provided with a series-connected body of a W-phase upper arm switch  20 WH and a W-phase lower arm switch  20 WL. The W-phase upper arm switch  20 WH includes a W-phase first upper arm switching element SWHA and a W-phase second upper arm switching element SWHB. The W-phase lower arm switch  20 WL includes a W-phase first lower arm switching element SWLA and a W-phase second lower arm switching element SWLB. 
         [0029]    According to the first embodiment, an IGBT as a Si (silicon) device is used for the first switching elements SUHA, SULA, SVHA, SVLA, SWHA, and SWLA. Hence, the output terminal of each first switching element is an emitter, and the input terminal thereof is a collector. According to the first embodiment, for each of the second switching elements SUHB, SULB, SVHB, SVLB, SWHB and SWLB, an N-channel MOSFET is used as a Sic device. Accordingly, in the second switching elements, each of the output terminals serves as source, and each of the input terminals serves as a drain. 
         [0030]    For the first switching elements SUHA, SULA, SVHA, SVLA, SWHA and SWLA, free-wheel diodes DUHA, DULA, DVHA, DVLA, DWHA, DWLA are connected respectively in reverse direction of the switching elements. These free-wheel diodes are connected in parallel to each other. In the second switching elements SUHB, SULB, SVHB, SVLB, SWHB and SWLB, body diodes DUHB, DULB, DVHB, DVLB, DWHB and DWLB are formed respectively. 
         [0031]    It should be noted that free-wheel diodes may be connected to respective second switching elements in the reverse direction, and the free-wheel diodes are connected in parallel between the second switching elements. 
         [0032]    According to the second embodiment, respective switching parts are configured of a parallel connected circuit including an IGBT and a MOSFET. This is because, current flows, in the small current region, through the MOSFET which has low on-resistance, thereby reducing power loss in the small current region. With reference to  FIG. 2 , the configuration of the first embodiment will be described. In  FIG. 2 , a dashed line represents voltage-current characteristics between the source-drain voltage Vds and the drain current Id of the MOSFET, and a dotted line represents voltage-current characteristics between the collector-emitter voltage Vce and the collector current Ic of the IGBT. Also, a solid line represents the voltage-current characteristics when the IGBT and MOSFET are connected in parallel. 
         [0033]    As shown in  FIG. 2 , in the small current region where an amount of current is less than a predetermined current Ith, the drain-source voltage Vds to the drain current id is lower than the collector-emitter voltage Vce to the collector current Ic. In other words, the on-resistance of the MOSFET is smaller than the on-resistance of the IGBT in the small current region. Hence, in the small current region, a larger amount of current flows at the MOSFET than that flowing through the IGBT when the MOSFET and IGBT are connected in parallel to each other. On the other hand, in the large current region where an amount of current is more than the predetermined current Ith, the collector-emitter voltage Vce to the collector current Ic is lower than the drain-source voltage Vds to the drain current Id. That is, in the large current region, the on-resistance of the IGBT is smaller than the on-resistance of the MOSFET. Hence, in the large current region, larger amount of current flows through the IGBT than through the MOSFET, when the MOSFET and IGBT are connected in parallel to each other. 
         [0034]    According to the first embodiment, the maximum value of the collector current Ic capable of flowing through the first switching elements SUHA, SULA, SVHA, SVLA, SWHA and SWLA is set to be larger than the drain current Id capable of flowing through the second switching elements SUHB, SULB, SVHB, SVLB, SWHB and SWLB. 
         [0035]    Referring back to  FIG. 1 , a first end of the U-phase winding  30 U of the motor generator  30  is connected to the connection point between the U-phase upper arm switch  20 UH and U-phase lower arm switch  20 UL. Similarly, a first end of the V-phase winding  30 V of the motor generator  30  is connected to the connection point between the V-phase upper arm switch  20 VH and V-phase lower arm switch  20 VL. Moreover, a first end of the W-phase winding  30 W of the motor generator  30  is connected to the connection point between the W-phase upper arm switch  20 WH and W-phase lower arm switch  20 WL. The second ends of respective windings  30 U,  30 V and  30 W are mutually connected by the neutral point. 
         [0036]    In the smoothing capacitor  10 , a positive side bus bar  50   p  is connected to the positive terminal  10   p  of the smoothing capacitor  10 , and a negative side bus bar  50   n  is connected to the negative terminal  10   n  of the smoothing capacitor  10 . The bus bars  50   p  and  50   n  are conducting members having a long extended shape, specifically a conductive members having a long-extended plate shape. According to the first embodiment, each of the bus bars  50   p  and  50   n  has a linear section formed in a linear shape. In the linear section, cross-sections of the bus bars  50   p  and  50   n  are appropriately the same. 
         [0037]    In the order of proximity from the positive side terminal  10   p , the collector of the U-phase first upper arm switching element SUHA, the drain of the U-phase second upper arm switching element SUHB, the collector of the V-phase first upper arm switching element SVHA, the drain of the V-phase second upper arm switching element SVHB, the collector of the W-phase first upper arm switching element SWHA, and the drain of the W-phase second upper arm switching element SWHB are connected to the positive side bus bar  50   p . According to the first embodiment, collectors and drains are connected to the linear section of the positive side bus bar  50   p.    
         [0038]    In the order of proximity from the negative side terminal  10   n , the emitter of the U-phase first lower arm switching element SULA, the source of the U-phase second lower arm switching element SULB, the emitter of the V-phase first lower arm switching element SVLA, the source of the V-phase second lower arm switching element SVLB, the emitter of the W-phase first lower arm switching element SWLA, and the source of the W-phase second lower arm switching element SWLB are connected in parallel to the negative side bus bar  50   n . Especially, according to the first embodiment, emitters and sources are connected to the linear section of the negative side bus bar. 
         [0039]    The control unit  40  drives the inverter  20  in order to control a controlled variable of the motor generator to be a command value. The controlled variable is, for example, a torque. The control unit  40  outputs drive signals to the drive circuits (not shown) of the respective switches, so as to drive switching elements SUHA, SUHB, SULB, SVHA, SVHB, SVLA, SVLB, SWHA, SWHB, SWLA and SWLB to be ON or OFF. The control unit  40  performs a PWM process based on a comparison between the three-phase command voltages, where respective phases are shifted by 120° of electrical angle, and the carrier signal such as a triangle wave, thereby generating the drive signal corresponding to respective drive circuits. The drive signal includes an ON command that changes the state of the switching element to be ON, and an OFF command that changes the state of the switching element to be OFF. The upper arm drive signal and the lower arm drive signal are complementary signals. Hence, the upper arm switching element and corresponding lower arm switching element are alternately turned ON. According to the first embodiment, in the switches  20 UH to  20 WL, the first switching element and the second switching element are simultaneously turned ON or OFF. 
         [0040]    Subsequently, characteristic configuration of the first embodiment will be described. 
         [0041]    According to the above-described embodiment, In the order of proximity from the positive side terminal  10   p , the collector of the U-phase first upper arm switching element SUHA, the drain of the U-phase second upper arm switching element SUHB, the collector of the V-phase first upper arm switching element SVHA, the drain of the V-phase second upper arm switching element SVHB, the collector of the W-phase first upper arm switching element SWHA, and the drain of the W-phase second upper arm switching element SWHB are connected to the positive side bus bar  50   p.    
         [0042]    Thus, in the respective upper arm switches  20 UH,  20 VH and  20 WH, an inductance of a conduction path from the connection point of the first switching element (IGBT) at the positive side bus bar  50   p  to the positive side terminal  10   p  is smaller than an inductance of a conduction path from the connection point of second switching element (MOSFET) at the positive side bus bar  50   p  to the positive side terminal  10   p.    
         [0043]      FIG. 3  illustrates an example of a change in inductance for the U-phase upper arm switch  20 UH and the V-phase upper arm switch  20 VH. 
         [0044]    According to the above-described configuration, in the large current region, surge voltage caused by switching ON to OFF of the first switching element reduced so that the switching loss occurring at the first switching element can be reduced. Hereinafter, an example of the U-phase upper arm switch  20 UH shown in  FIG. 3  will be described. 
         [0045]    In the large current region, the larger the current supplied to the U-phase upper arm switch  20 UH from the smoothing capacitor via the positive side bus bar  50   p , the larger the collector current flowing through the U-phase first upper arm switching element SUHA. Here, the larger the collector current flowing through the U-phase first upper arm switching element SUHA, the larger the surge voltage caused by turning OFF of the U-phase first upper arm switching element. Accordingly, in the large current region, a surge voltage caused by switching off of the U-phase first upper arm switching element SUHA becomes larger than that of the U-phase second upper arm switching element SUHB. 
         [0046]    Here, the smaller the inductance of the electrical conduction path, on the positive side bus bar  50   p , from the smoothing capacitor  10  to the switching elements, the larger an effect of reduction in the surge voltage due to switching off operation. Hence, the inductance of the electrical conduction path between the collector of the U-phase first upper switching element SUHA and the positive side terminal  10   p  via the positive side bus bar  50   p , where a large amount of current flows, is set to be smaller than the inductance of the electrical conduction path between the drain of the U-phase second upper arm switching element SUHB and the positive side terminal  10   p  via the positive side bus bar  50   p . Thus, an effect of reduction in the surge voltage caused by switching off of the U-phase first upper arm switching element SUHA can be larger, and the switching loss caused by switching off of the U-phase first upper arm switching element SUHA can be reduced. Thus, power loss of the inverter  20  can be reduced. Also, switching loss due to switching-off operations can be reduced for the lower arm switches  20 UL,  20 VL and  20 WL. 
         [0047]    According to the first embodiment, in the large current region, the surge voltages at the upper arm switches  20 UH,  20 VH and  20 WH due to switching-off operations can be reduced. Hereinafter, an example of the U-phase upper arm switch  20 UH will be described. 
         [0048]    When the U-phase first upper arm switching element SUHA turns ON from the OFF state, a recovery current flows through the free-wheel diode DULA connected to the U-phase first lower arm switching element SULA, in the reverse direction. Due to the recovery current, surge voltage occurs at the free-wheel diode DULA and the U-phase first lower arm switching element SULA. According to a configuration in which an inductance of a conduction path from the connection point of the first switching element (IGBT) at the positive side bus bar  50   p  to the positive side terminal  10   p  is smaller than an inductance of a conduction path from the connection point of the second switching element (MOSFET) at the positive side bus bar  50   p  to the positive side terminal  10   p , the surge voltage due to the recovery current can be reduced. As a result, switching loss due to switching-on operations can be reduced so that the power loss of the inverter  20  can be reduced. It should be noted that the switching loss caused by the switching-on operation can be reduced for each of the lower arm switches  20 UL,  20 VL and  20 WL. 
         [0049]    Further, according to the first embodiment, among U, V and W phases, adjacent phases at the connection points onto the positive side bus bar  50   p  are designed such that IGBT switches where a large amount of current flows in the large current region are not adjacent to each other. In other words, IGBT switches and MOSFET switches are alternately placed to be connected to the positive side bus bar  50   p . This configuration reduces the surge voltage. Hereinafter, as shown in  FIG. 3 , the U-phase upper arm switch  20 UH and V-phase upper arm switch  20 VH will be described as examples. 
         [0050]    The surge voltage due to switching of the driving state of the V-phase first upper arm switching element SVHA in the V-phase upper arm switch  20 VH is transmitted to the U-phase upper arm switch  20 UH. Therefore, surge voltage transmitted from the V-phase arm switch  20 VH may be superposed on the surge voltage due to switching operations of the U-phase first upper arm switching element SUHA. In this case, switching loss will be increased. 
         [0051]    According to the first embodiment, on the positive side bus bar  50   p , the connection point of the U-phase second upper arm switching element SUHB is disposed between the connection point of the U-phase first upper arm switching element SUHA and the connection point of the V-phase first upper arm switching element SVHA. Hence, on the positive side bus bar  50   p , the connection point of the V-phase first upper arm switching element SVHA and the U-phase first upper arm switching element SUHA are not adjacent to each other. As a result, the electrical conduction path can be longer between the collector of the V-phase first upper arm switching element SVHA and the collector of the first upper arm switching element SUHA via the positive side bus bar  50   p , so that the inductance value of the electrical conduction path can be increased. Hence, an attenuation effect on surge voltage at the V-phase upper arm switching element SVHA can be enhanced on the above-described electrical conduction path. Therefore, the surge voltage transmitted from the V-phase upper arm switching element SVHA can be prevented from being superposed on the surge voltage due to switching operation of the U-phase first upper arm switching elements SUHA. 
       Second Embodiment 
       [0052]    Hereinafter, with reference to the drawings, differences from the above-described first embodiment will mainly be described. According to the second embodiment, in the respective switches  20 UH to  20 WL, connection points of the first switching element and the second switching element on the respective bus bars  50   p  and  50   n  are changed. In  FIG. 4 , for convenience reasons, the same reference symbols are added to the same configuration as that shown in  FIG. 1 . 
         [0053]    As shown in  FIG. 4 , in the order of proximity from the positive side terminal  10   p , the drain of the U-phase second upper arm switching element SUHB, the collector of the U-phase first upper arm switching element SUHA, the drain of the V-phase second upper arm switching element SVHB, the collector of the V-phase first upper arm switching element SVHA, the drain of the W-phase second upper arm switching element SWHB, and the collector of the W-phase first upper arm switching element SWHA are connected to the positive side bus bar  50   p . According to the second embodiment, collectors and drains are connected to the linear section of the positive side bus bar  50   p.    
         [0054]    In the order of proximity from the negative side terminal  10   n , the source of the U-phase second lower arm switching element SULB, the emitter of the U-phase first lower arm switching element SULA, the source of the V-phase second lower arm switching element SVLB, the emitter of the V-phase first lower arm switching element SVLA, the source of the W-phase second lower arm switching element SWLB and the emitter of the W-phase first lower arm switching element SWLA are connected in parallel to the negative side bus bar  50   n . Especially, according to the second embodiment, emitters and sources are connected to the linear section of the negative side bus bar. 
         [0055]    Thus, in the respective upper arm switches  20 UH,  20 VH and  20 WH, an inductance of a conduction path from the connection point of the second switching element (MOSFET) at the positive side bus bar  50   p  to the positive side terminal  10   p  is smaller than an inductance of a conduction path from the connection point of the first switching element (IGBT) at the positive side bus bar  50   p  to the positive side terminal  10   p .  FIG. 5  illustrates an example of a change in inductance for the U-phase upper arm switch  20 UH and the V-phase upper arm switch  20 VH. 
         [0056]    According to the above-described configuration, the power loss of the inverter  20  in a small current region can be reduced. Hereinafter, an example of the U-phase upper arm switch  20 UH shown in  FIG. 5  will be described. 
         [0057]    In the small current region, the larger the current supplied to the U-phase upper arm switch  20 UH from the smoothing capacitor  10  via the positive side bus bar  50   p , the larger the current flowing through the U-phase second upper arm switching element SUHB. Further, the larger the drain current flowing through the U-phase second upper arm switching element SUHB, the larger the surge voltage due to the switching-off operation of the U-phase second upper arm switching element SUHB. Accordingly, in the small current region, the surge voltage due to the switching-off operation in the U-phase second upper arm switching element SUHB becomes larger than that of the U-phase first upper arm switching SUHA. 
         [0058]    According to the second embodiment, the inductance of the electrical conduction path from the drain of the U-phase second upper arm switching element SUHB at which large amount of current flows in the small current region, to the positive side terminal  10   p  via the positive side bus bar  50   p  is set to be smaller than the inductance of the electrical conduction path from the collector of the U-phase first upper arm switching element SUHA to the positive side terminal  10   p  via the positive side bus bar. Thus, an effect of reduction in the surge voltage caused by switching off of the U-phase first upper arm switching element SUHA can be larger. Also, by setting the above-described inductances, the current going to flow through the U-phase first upper arm switching element SUHA can preferentially flow through the U-phase upper arm switching element SUHB. Therefore, since the current flows through the MOSFET rather than the IGBT in the small current region, power loss can be significantly reduced. Therefore, the power loss of the inverter  20  can be reduced. 
         [0059]    According to the second embodiment, similarly to the first embodiment, the surge voltage can be reduced, the surge voltage being produced when the second switching elements of the upper arm switches  20 UH to  20 WH switch the state from off to on in the small current region. 
         [0060]    According to the second embodiment, according to the first embodiment, similarly to the first embodiment, among U, V and W phases, adjacent phases at the connection points onto the positive side bus bar  50   p  are designed such that surge voltage transmitted from other phases can be reduced. 
       Third Embodiment 
       [0061]    Hereinafter, with reference to the drawings, differences from the above-described first and second embodiments will mainly be described. According to the third embodiment, as shown in  FIG. 6 , as a control system, two pairs of motor generator and inverter are provided in the motor systems. In  FIG. 6 , for reasons of convenience, the same reference symbols are added to the same configuration as that shown in  FIG. 1 . 
         [0062]    As shown in  FIG. 6 , the control system is provided with a first inverter  20   a , a first motor generator  30   a , a second inverter  20   b , and a second motor generator  30   b . According to the third embodiment, as the motor generators  30   a  and  30   b , the same permanent magnet synchronous motor as the motor generator  30  of the first embodiment is used. 
         [0063]    The first motor generator  30   a  serves as an on-vehicle main equipment similar to the motor generator  30  of the first embodiment. In  FIG. 6 , windings included in the first motor generator  30   a  are shown as windings  30 Ua,  30 Va and  30 Wa. 
         [0064]    The second motor generator  30   b  serves as a generator and a starter that applies initial rotation to the output shaft of the engine as an on-vehicle main equipment. In  FIG. 6 , windings included in the second motor generator  30   b  are shown as windings  30 Ub,  30 Vb and  30 Wb. 
         [0065]    According to the third embodiment, the first inverter  20   a  is configured similar to the inverter  20  of the first embodiment shown in  FIG. 1 . Hence, according to the third embodiment, the same symbols as the switching elements of the inverter  20  shown in  FIG. 1  are applied to the switching elements of the first inverter  20   a . The second inverter  20   b  has similar configuration to the inverter  20  of the second embodiment shown in  FIG. 4 . Hence, according to the third embodiment, the same symbols as used in the switching elements included in the inverter  20  shown in  FIG. 4  are applied to the switching elements included in the second inverter  20   b.    
         [0066]    A positive side bus bar  51   p  is connected to the positive side terminal  10   p  of the smoothing capacitor  10 . A negative side bus bar  51   n  is connected to the negative terminal  10   n  of the smoothing capacitor  10 . The bus bars  50   p  and  50   n  are conducting member having a long extended shape, specifically a conductive member having a long-extended plate shape. According to the third embodiment, each of the bus bars  51   p  and  51   n  has a linear section formed in linear shape. In the linear section, cross-sections of the bus bars  51   p  and  51   n  are appropriately the same. 
         [0067]    In the order of proximity from the positive side terminal  10   p , input terminals of the upper arm switching elements included in the first inverter  20   a  are connected to the positive side bus bar  51   p . Specifically, in the order of proximity from the positive side terminal  10   p , the collector of the U-phase first upper arm switching element SUHA, the drain of the U-phase second upper arm switching element SUHB, the collector of the V-phase first upper arm switching element SVHA, the drain of the V-phase second upper arm switching element SVHB, the collector of the W-phase first upper arm switching element SWHA and the drain of the W-phase second upper arm switching element SWHB are connected to the positive side bus bar  51   p.    
         [0068]    In addition to the upper arm switching elements included in the first inverter  20   a , input terminals of the upper arm switching elements included in the second inverter  20   b  are connected to the positive side bus bar  51   p . Specifically, in the order of proximity from the drain side of the W-phase second upper arm switching element SWHB, the drain of the U-phase second upper arm switching element SUHB, the collector of the U-phase first upper arm switching element SUHA, the drain of the V-phase second upper arm switching element SVHB, the collector of the V-phase first upper arm switching element SVHA, the drain of the W-phase second upper arm switching element SWHB, the collector of the W-phase first upper arm switching element SWHA are connected to the positive side bus bar  51   p . According to the third embodiment, each collector and the drain are connected to the linear section of the positive side bus bar  51   p.    
         [0069]    In the order of proximity from the negative side terminal  10   n , input terminals of the lower arm switching elements included in the first inverter  20   a  are connected to the negative side bus bar  51   n . Specifically, in the order of proximity from the negative side terminal  10   n , the emitter of the U-phase first lower arm switching element SULA, the source of the U-phase second lower arm switching element SULB, the emitter of the V-phase first lower arm switching element SVLA, the source of the V-phase second lower arm switching element SVLB, the emitter of the W-phase first lower arm switching element SWLA and the source of the W-phase second lower arm switching element SWLB are connected to the positive side bus bar  50   n.    
         [0070]    In addition to the lower arm switching elements included in the first inverter  20   a , output terminals of the lower arm switching elements included in the second inverter  20   b  are connected to the negative side bus bar  51   n . Specifically, in the order of proximity from the source side of the W-phase second lower arm switching element SWLB, the source of the U-phase second lower arm switching element SULB, the emitter of the U-phase first lower arm switching element SULA, the source of the V-phase second lower arm switching element SVLB, the emitter of the V-phase first lower arm switching element SVLA, the source of the W-phase second lower arm switching element SWLB, and the emitter of the W-phase first lower arm switching element SWLA are connected to the negative side bus bar  51   n . According to the third embodiment, each emitter and the source are connected to the linear section of the positive side bus bar  51   p.    
         [0071]    According to the third embodiment described above, in the positive side bus bar  51   p , the connection points corresponding to the W-phase second upper arm switching element SWHB included in the first inverter  20   a , and the U-phase second upper arm switching element SUHB included in the second inverter  20   b  are disposed between the connection point of the W-phase first upper arm switching element SWHA included in the first inverter  20   a  and the connection point of the U-phase first upper arm switching element SUHA included in the second inverter  20   b . Hence, a distance between the first upper arm switching elements included in the first inverter  20   a  and the first upper arm switching elements included in the second inverter  20   b  through the positive side bus bar  51   b  can be longer, so that inductance can be increased. Thus, for example, surge voltage produced when switching the U-phase first upper arm switching element SUHA included in the second inverter  20   b  can be effectively attenuated in the transmission path of the bus bar  51   p  when the surge voltage is transmitted to the W-phase first upper arm switching element SWHA included in the first inverter  20   a . As a result, surge voltage transmitted between the first inverter  20   a  and the second inverter  20   b  can be reduced. 
         [0072]    According to the third embodiment, in the first inverter  20   a  and the second inverter  20   b , switching elements included in either the first inverter  20   a  or the second inverter  20   b  correspond to object switching elements and the switching elements included in the other inverter correspond auxiliary switching elements. 
       Other Embodiment 
       [0073]    The above-described embodiments may be modified as follows. 
         [0074]    According to the first and second embodiments, by changing the distances from the positive terminal  10   p  of the smoothing capacitor  10  to the connection points of the input terminals of the upper arm switching elements, the inductance value is changed. However, it is not limited thereto. For example, the width of the bus bar may be changed so as to change the inductance. Here, the wider the bus bar, the lower the inductance value. The inductance of the negative bus bar side  50   n  is similar to that of the positive side bus bar  50   p.    
         [0075]    According to the above-described first embodiment, the switching part is configured of two switching elements having minimum on-resistance in the respective two current regions. However, it is not limited thereto. For example, three or more switching elements may be used in which switching elements have minimum on-resistance in the respective current regions. When using three switching elements having minimum on-resistance in three current regions such as small, intermediate, large regions, to configure a switching part, the connection point of the input terminal of the switching element corresponding to the large current region may be disposed, in the positive side bus bar, between the connection point of the input terminal corresponding to the small current region and the connection point of the input terminal corresponding to the intermediate current region. 
         [0076]    In the third embodiment, a system provided with three or more sets of motor generator and inverter may be used. For example, in the case where three sets of motor generator and inverter are used in a system, among three inverters connected via the positive side bus bar  51   p , adjacent two inverters correspond to an inverter provided with an object switching elements, and an inverter provided with other switching elements. 
         [0077]    As a switching element which configures each of the switch parts, the switching element is not limited to a combination of the IGBT and MOSFET, but a combination of other devices may be used. 
         [0078]    As the positive and negative bus bars, it is not limited to a bus bar having a linear section. For example, a bus bar having a part of curved long-extended plate shape may be used. The positive side and the negative side bus bars are not limited to having a plate shape, but a rod like shape may be used. 
         [0079]    Inverters are not limited to three-phase inverters. However, inverters having one phase, two phases or four or more phases may be used. As an electronic circuit configured of a plurality of switching elements connected in parallel to each other, it is not limited to inverters. 
         [0080]    As a power source connected to the bus bar, not only the smoothing capacitors but also secondary batteries may be used. 
         [0081]    The motor generator is not limited to a permanent-magnet type synchronous motor, but a field-winding synchronous motor may be used. Further, as a motor generator, it is not limited to the synchronous motor, but an induction motor may be used. Also, the motor generator is not limited to on-vehicle main equipment. However, the motor generator may be used for other purposes such as motors used for an electric power steering system or an air-conditioning compressor.