Abstract:
A shunt resistor, at least a part of which has a resistive element with pre-set resistivity, is configured to bridge between two electrodes and detect a current value of a current flowing between the electrodes by detecting a voltage drop in the resistive element. The shunt resistor includes two connecting parts affixed to the electrodes via a conductive adhesive, respectively, and the connecting parts electrically connected to the affixed electrodes, a bridging part bridging between the connecting parts by being extended from one of the connecting parts to the other one of the connecting parts, and two bonding wires used to detect a voltage drop in the resistive element. The bonding wires are bonded to the bridging part.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application No. 2014-179482 filed on Sep. 3, 2014, the disclosure of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a shunt resistor to which bonding wires used to detect a current value flowing between electrodes are connected. 
       BACKGROUND ART 
       [0003]    A current value is measured by using a shunt resistor according to a resistance value of a resistive element forming the shunt resistor and a potential difference across the shunt resistor. 
         [0004]    A current detection resistor described in Patent Literature 1 includes a current-passing part through which a current flows and detection parts protruding from the current-passing part. The detection parts are provided integrally with the current-passing part and detect a current value according to a resistance value of the current-passing part and a potential difference between two detection parts. 
         [0005]    A semiconductor module described in Patent Literature 2 includes a connection conductor functioning as a shunt resistor. Bonding wires are bonded to the connection conductor at leg parts which are in contact with switching elements and a lead frame as connection targets. A current value is detected according to a resistance value of the connection conductor and a potential difference between two bonding wires. 
       PRIOR ART LITERATURES 
     Patent Literature 
       [0006]    Patent Literature 1: JP2004-221160A 
         [0007]    Patent Literature 2: JP2013-179744A 
       SUMMARY OF INVENTION 
       [0008]    A current flowing through a resistive element is increasing recently for a shunt resistor employed in an electronic device equipped to, for example, a vehicle. Accordingly, a heating value of the resistive element is also increasing. A need is thus arising from a viewpoint of heat dissipation to directly connect the shunt resistor to a member having a large heat capacity and relatively high heat conductivity, such as a lead frame. 
         [0009]    Nevertheless, according to the technique of Patent Literature 1, machining is not easy because the current-passing part and the detection parts are provided integrally, and also a degree of freedom for a connected-end member is substantially zero because the detection parts are formed in one shape. Hence, the lead frame has to have a space to provide a land pattern of the connected-end member. Such a space may possibly make a required size reduction infeasible. 
         [0010]    Meanwhile, according to the technique described in the Patent Literature 2, the leg parts are connected to the connection targets via a connection member, such as solder. A resistance value between connection points of the two bonding wires is susceptible to conditions of the connection member, such as a material, an amount, and a location. A variance in resistance value gives a direct influence to a measurement error of a current value. In short, the configuration in the related art may not achieve a sufficiently high degree of accuracy in measurement of a current flowing through the connection conductor. 
         [0011]    In view of the foregoing circumstances, the present disclosure has an object to provide a shunt resistor having a higher degree of accuracy in measurement of a current value. 
         [0012]    According to an aspect of the present disclosure, the shunt resistor, at least a part of which has a resistive element with pre-set resistivity, is configured to bridge between two electrodes and detect a current value of a current flowing between the electrodes by detecting a voltage drop in the resistive element. The shunt resistor includes two connecting parts affixed to the electrodes via a conductive adhesive, respectively, and the connecting parts electrically connected to the affixed electrodes, a bridging part bridging between the connecting parts by being extended from one of the connecting parts to the other one of the connecting parts, and two bonding wires used to detect a voltage drop in the resistive element. The bonding wires are bonded to the bridging part. 
         [0013]    In the shunt resistor configured as above, wires used to detect a voltage drop in the resistive element are formed of the bonding wires. Hence, a degree of freedom in shape of a connected-end member can be ensured in contrast to a configuration in which detection portions and a current-passing part are formed integrally as in Patent Literature 1. That is to say, even when the connected-end member is a lead frame or the like, a limitation of a land pattern shape can be eased. Hence, the shunt resistor does not limit a reduction of a physical size of a device in which the shunt resistor is installed. 
         [0014]    In the shunt resistor configured as above, the connecting parts are connected to the electrodes as connection targets. The connecting parts and the electrodes are connected via the conductive adhesive, such as solder. The bonding wires in the shunt resistor configured as above are bonded to the bridging part bridging between the two connecting parts. Hence, a resistance value between connection points of the two bonding wires is unsusceptible to conditions of the conductive adhesive, for example, condition of solder, such as a material, an amount, and a location. Consequently, a variance in resistance value caused by the conductive adhesive can be restricted and hence accuracy in measurement of a current flowing through the resistive element can be increased. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
           [0016]      FIG. 1  is a perspective view showing a schematic configuration of a shunt resistor according to a first embodiment; 
           [0017]      FIG. 2  is a top view showing a relation between a connection configuration of bonding wires and a loop area of a sense current in a shunt resistor according to a configuration in the related art; 
           [0018]      FIG. 3  is a top view showing a relation between a connection configuration of bonding wires and a loop area of a sense current in the shunt resistor according to the first embodiment; 
           [0019]      FIG. 4  is a top view showing a schematic configuration of the shunt resistor; 
           [0020]      FIG. 5  is a top view showing a schematic configuration of a shunt resistor according to a second embodiment; 
           [0021]      FIG. 6  is a perspective view showing a schematic configuration of the shunt resistor; 
           [0022]      FIG. 7  is a perspective view showing a schematic configuration of a shunt resistor according to another embodiment; and 
           [0023]      FIG. 8  is a perspective view showing a schematic configuration of a shunt resistor according to still another embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Hereinafter, embodiments of the present disclosure will be described according to the drawings. In respective drawings referred to in the following, same or equivalent parts are labeled with same reference numerals. Directions referred to in the following are an x direction, a y direction orthogonal to the x direction, and a z direction orthogonal to an x-y plane defined by the x direction and the y direction. That is to say, the x direction, the y direction, and the z direction are linearly independent to one another. 
       First Embodiment 
       [0025]    Firstly, a schematic configuration of a shunt resistor of the present embodiment will be described with reference to  FIG. 1 . 
         [0026]    As is shown in  FIG. 1 , a shunt resistor  100  has a surface conforming to the x-y plane and electrically connects two electrodes  200  aligned side by side in the x direction to each other. The shunt resistor  100  described herein connects a first electrode  200   a  and a second electrode  200   b.  The electrodes  200  may be, for example, lands provided on a board or a lead frame. In short, a configuration of the electrodes  200  is not particularly limited. 
         [0027]    The shunt resistor  100  includes two connecting parts  10  connected to the electrodes  200  via solders  300  as a conductive adhesive and a bridging part  20  bridging between the two connecting parts  10 . The bridging part  20  has a main part  21 , an intermediate part  22 , and a resistive element  23 . The shunt resistor  100  also includes bonding wires  30  used to detect a current value of a current flowing through the resistive element  23 . 
         [0028]    As is shown in  FIG. 1 , the connecting parts  10  have a first terminal  10   a  connected to the first electrode  200   a  and a second terminal  10   b  connected to the second electrode  200   b.  The connecting parts  10  are shaped like a plane conforming to the x-y plane. Surfaces of the connecting parts  10  opposing the electrodes  200  are connected to the electrodes  200  via the solders  300 . 
         [0029]    The main part  21  of the bridging part  20  includes a first main part  21   a  and a second main part  21   b,  each of which is a plate-like member conforming to the x-y plane. Likewise, the resistive element  23  is provided so as to conform to the x-y plane and sandwiched between the first main part  21   a  and the second main part  21   b.  As is shown in  FIG. 1 , the first main part  21   a,  the resistive element  23 , and the second main part  21   b  are aligned in the x direction in order of description and bonded to each other to form an integrated conductor as a whole. The conductor integrally formed of the first main part  21   a,  the resistive element  23 , and the second main part  21   b  is extended in the x direction to electrically connect the first terminal  10   a  and the second terminal  10   b.  The main part  21  together with the resistive element  23  is provided at a higher position than the connecting parts  10  in the z direction. 
         [0030]    As is shown in  FIG. 1 , the intermediate part  22  of the bridging part  20  connects the connecting parts  10  and the main part  21 . The main part  21  and the connecting parts  10  are provided integrally via the intermediate part  22 . More specifically, the first main part  21   a  and the first terminal  10   a  are connected via a first intermediate part  22   a  and the second main part  21   b  and the second terminal  10   b  are connected via a second intermediate part  22   b . When the shunt resistor  100  is viewed from a front in the y direction, the bridging part  20  corresponds to an upper base and leg parts of substantially a trapezoidal shape. More specifically, the bridging part  20  forms a trapezoidal shape having a plate-like member integrally formed of the main part  21  and the resistive element  23  as an upper base and the intermediate part  22  as leg parts. 
         [0031]    The main part  21  and the intermediate part  22  of the bridging part  20  are conductive parts made of metal, for example, copper and have smaller resistivity than the resistive element  23 . The resistive element  23  is chiefly made of, for example, CnMnSn or CuMnNi. 
         [0032]    The bonding wires  30  are made of a generally known material, for example, aluminum. The bonding wires  30  are connected to sensing electrodes  400  used to detect potential across the bonding wires  30 . The bonding wires  30  are a first wire  30   a  and the second wire  30   b.  As is shown in  FIG. 1 , the first wire  30   a  is bonded to the first main part  21   a  at a first end and connected to a first sensing electrode  400   a  as one of the sensing electrodes  400  at a second end. The second wire  30   b  is bonded to the second main part  21   b  at a first end and connected to a second sensing electrode  400   b  as the other one of the sensing electrodes  400  at a second end. That is to say, one end of each bonding wire  30  of the present embodiment is bonded to the main part  21  of the bridging part  20  corresponding to the upper base of substantially a trapezoidal shape. 
         [0033]    An operational-effect of the shunt resistor  100  of the present embodiment will now be described with reference to  FIG. 2  to  FIG. 4 . 
         [0034]    In the configuration as above, when a potential difference is generated between the first electrode  200   a  and the second electrode  200   b,  a current flows through the resistive element  23  by way of the connecting parts  10 , the intermediate part  22 , and the main part  21 . A potential difference observed between the first wire  30   a  and the second wire  30   b  depends on bonding positions of the bonding wires  30 . Such dependency is attributed in part to a distance between connection positions of the bonding wires  30  in the bridging part  20  or the connecting parts  10 . A resistance value between the connection positions increases as the distance becomes longer and hence a potential difference observed between the first wire  30   a  and the second wire  30   b  increases, too. 
         [0035]    Besides the distance, the dependency is also attributed to conditions of the solders  300 , such as an ingredient, an amount, a location, and a shape in a fixed state. In a connection conductor described in Patent Literature 2 (JP2013-179744A), bonding wires are bonded to portions corresponding to the connecting parts  10 . The solders  300  are disposed beneath the connecting parts  10 . Hence, when a resistance value or a TCR (Temperature Coefficient of Resistance) between the connection positions of the bonding wires  30  fluctuates with the condition of the solders  300 , such as an ingredient, an amount, a location, and a shape in a fixed state, an influence appears in a potential difference observed between the first wire  30   a  and the second wire  30   b.    
         [0036]    In contrast, in the shunt resistor  100  of the present embodiment, the bonding wires  30  are bonded to the bridging part  20 , to be more specific, the main part  21 . Because the solders  300  are interposed between the connecting parts  10  and the electrodes  200  as described above, the solders  300  are not in contact with the main part  21 . Hence, presence of the solders  300  does not give an influence to a potential difference observed between the first wire  30   a  and the second wire  30   b.  That is to say, a variance in potential difference caused by the solders  300  can be restricted and hence a current value of a current flowing through the resistive element  23  can be detected at a higher degree of accuracy. 
         [0037]    In the present embodiment, the bonding wires  30  are bonded to the bridging part  20  of substantially a trapezoidal shape at the main part  21  corresponding to the upper base. Because the bridging part  20  has a trapezoidal arch structure, deflection of the bridging part  20  can be restricted against a force acting on the main part  21  from an upper base side to a lower base side. That is to say, because the bonding wires  30  can be bonded in a stable manner, connection reliability can be enhanced. 
         [0038]    Further, according to the shunt resistor  100  of the present embodiment, an influence of a magnetic flux induced by a current (main current of  FIG. 2  and  FIG. 3 ) flowing between the two electrodes  200  given to a potential difference observed between the first wire  30   a  and the second wire  30   b  can be reduced, which will be described in detail in the following. 
         [0039]      FIG. 2  is a top view showing a configuration when the bonding wires  30  are connected to the connecting parts  10  in a manner in the related art. A magnetic flux induced by the main current passes through a region enclosed by a current path of a sense current flowing through the bonding wires  30  (a shaded region of  FIG. 2 ). When the magnetic flux varies with a variance in main current with time, an induced electromotive force is generated in the current path of the sense current, and the induced electromotive force is undesirably superimposed on a potential difference observed between the first wire  30   a  and the second wire  30   b  as a noise. The induced electromotive force becomes larger as an area of the region enclosed by the current path of the sense current (hereinafter, referred to as a loop area) becomes larger. 
         [0040]      FIG. 3  is a top view showing a configuration of the shunt resistor  100  of the present embodiment. In the shunt resistor  100 , the bonding wires  30  are connected to the main part  21  of the bridging part  20 . Hence, a loop area S 2  of the present embodiment can be smaller than a loop area S 1  in the configuration in the related art. Consequently, an induced electromotive force generated in the current path of the sense electrode can be smaller than an induced electromotive force generated in the configuration in the related art, which can in turn reduce an influence of the magnetic flux to a potential difference observed between the first wire  30   a  and the second wire  30   b.    
         [0041]    It is preferable to set bonding positions on the main part  21  in close proximity to boundaries between the main part  21  and the resistive element  23  as is shown in  FIG. 4 , in which case a distance between bonding positions of the first wire  30   a  and the second wire  30   b  becomes substantially a minimum in an extending direction of the bridging part  20  (the x direction in  FIG. 4 ). 
         [0042]    When configured as in  FIG. 4 , an influence of a resistance value and a TCR of the conductive parts of the bridging part  20  except for the resistive element  23  given to a potential difference observed between the first wire  30   a  and the second wire  30   b  can be substantially a minimum. In addition, because the loop area of the sense current can be reduced, a noise superimposed on a potential difference observed between the first wire  30   a  and the second wire  30   b  can be reduced by restricting an induced electromotive force generated due to the main current. In short, a current value of the main current flowing through the resistive element  23  can be detected at a higher degree of accuracy. 
       Second Embodiment 
       [0043]    The first embodiment above has described the bonding wires  30  as to the bonding positions in detail. In the present embodiment, attention is paid to routing of bonding wires  30 . 
         [0044]    As is shown in  FIG. 5 , two bonding wires  30 , namely, the first wire  30   a  and the second wire  30   b,  in the shunt resistor  100  of the present embodiment are extracted substantially parallel to an extending direction of the bridging part  20  (an x direction in  FIG. 5 ) to substantially a same direction. Herein, “to a same direction” means that both of the first wire  30   a  and the second wire  30   b  are extracted toward a left side on a sheet surface of  FIG. 5 . That is to say, the first wire  30   a  and the second wire  30   b  are extended in the x direction and extracted side by side in a y direction. The present embodiment is of a same configuration as the first embodiment above except for routing of the bonding wires  30 . 
         [0045]    According to the configuration as above, a distance between the first wire  30   a  and the second wire  30   b  can be shorter than in a configuration in which the bonding wires  30  are extracted in a direction (the y direction) substantially orthogonal to the extending direction of the bridging part  20  as in the related art shown in  FIG. 2  and in the first embodiment above shown in  FIG. 3 . Hence, a loop area of a sense current can be reduced further than in the first embodiment above. Consequently, an induced electromotive force generated due to a main current can be restricted, which can in turn reduce a noise superimposed on a potential difference observed between the first wire  30   a  and the second wire  30   b.  In short, a current value of the main current flowing through the resistive element  23  can be detected at a higher degree of accuracy. 
         [0046]    When viewed from a front of a bonding surface on which the bonding wires  30  are bonded, that is, when viewed from a front in a z direction shown in  FIG. 6 , it is preferable to configure in such a manner that bonding positions of the first wire  30   a  and the second wire  30   b  on the bonding surface fall on a virtual line L along the extending direction (the x direction). According to the configuration as above, the first wire  30   a  and the second wire  30   b  are extended in the x direction and extracted side by side in the z direction. 
         [0047]    According to the configuration as above, y coordinates of the first wire  30   a  and the second wire  30   b  on the main part  21  coincide with each other, and when viewed in a plane in the z direction, the first wire  30   a  and the second wire  30   b  lie one on the other. Hence, the loop area of the sense current can be smaller than in the configuration as shown in  FIG. 5  in which the y coordinates are at positions different from each other. Consequently, an induced electromotive force due to the main current can be restricted, which can in turn reduce a noise superimposed on a potential difference observed between the first wire  30   a  and the second wire  30   b.    
       Other Embodiments 
       [0048]    The present disclosure is not limited to the embodiments mentioned above, and can be changed and modified to various embodiments which are also within the spirit and scope of the present disclosure. 
         [0049]    The respective embodiments above have described the configuration in which the bridging part  20  is of substantially a trapezoidal shape when viewed from a front in the y direction by way of example. However, the present disclosure is not limited to the configuration as above. For example, the intermediate part  22  may be of a rectangular shape orthogonal to connecting parts  10  or the intermediate part  22  connecting the connecting parts  10  and the main part  21  may be bent. Further, the present disclosure can be also applied to a configuration as is shown in  FIG. 7  in which the bridging part  20  does not have an intermediate part and connecting parts  10 , the main part  21 , and the resistive element  23  together form a flat plate as a whole. Herein, bonding wires  30  are bonded to the main part  21  not in contact with solders  300 . 
         [0050]    Accordingly, presence of the solders  300  does not give an influence to a potential difference observed between the first wire  30   a  and the second wire  30   b.  That is to say, a variance in potential difference caused by the solders  300  can be restricted and hence a current value of a current flowing through the resistive element  23  can be detected at a higher degree of accuracy. 
         [0051]    The respective embodiments above have described a case where the resistive element  23  as a part of the bridging part  20  is sandwiched between the first main part  21   a  and the second main part  21   b.  However, the present disclosure is not limited to the described case. The present disclosure can be also applied to a configuration in which connecting parts  10 , the main part  21 , and the intermediate part  22  are formed integrally using a same material as the resistive element  23 . Owing to the configuration to bond bonding wires  30  to portions corresponding to the main part  21 , a potential difference observed between the first wire  30   a  and the second wire  30   b  becomes unsusceptible to solders  300  and a variance in potential difference caused by the solders  300  can be restricted. According to the configuration as above, a resistance value used to calculate a current value flowing through the resistive element  23  is calculated using resistivity of the resistive element  23 , a sectional area of the bridging part  20 , and a distance between bonding positions of the bonding wires  30 . 
         [0052]    The second embodiment above has described a case where the first wire  30   a  and the second wire  30   b  are extracted substantially parallel to the extending direction of the bridging part  20  to substantially a same direction. Herein, “substantially parallel” and “substantially a same” do not necessarily mean that the bonding wires  30  have to be extracted perfectly parallel to the extending direction to exactly a same direction. That is to say, the operational-effect described above can be achieved when the first wire  30   a  and the second wire  30   b  are extracted in directions substantially parallel to the extending direction of the bridging part  20  and the extracted directions are substantially same. 
         [0053]    It is preferable to fix bonding wires  30  at positions as close as possible to boundaries between the resistive element  23  and the main part  21 . Further, it should be understood that a configuration as is shown in  FIG. 8  in which bonding wires  30  are fixed directly above the boundaries and a configuration in which bonding wires  30  are fixed at positions closer to the resistive element  23  than to the boundaries or on the resistive element  23  are also within the scope of the present disclosure. 
         [0054]    While the present disclosure has been described with reference to embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. The present disclosure is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the present disclosure.