Patent Publication Number: US-2011068771-A1

Title: Current sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2009-217745, filed on Sep. 18, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a current sensor for detecting the magnitude of electric current flowing through a conductor. 
     A known current sensor uses a magnetic detection element such as a Hall element or a magnetoresistance effect element. The current detection performed by a current sensor that uses a Hall element will now be described. 
     When current flows through a current path such as a wire, the current forms a magnetic field near the current path. The strength of the magnetic field is proportional to the magnitude of the current. When a Hall element is arranged in the magnetic field formed near the current path, the Hall element generates a Hall voltage that is proportional to the current flowing through the current path. A current sensor that uses the Hall element detects the current flowing through the current path based on the Hall voltage. 
     However, when the strength of the magnetic field acting on the Hall element is low, the proportional relationship of the magnetic field strength and the Hall voltage becomes difficult to maintain. Further, the strength of the magnetic field generated by the current flowing through the current path is low in the first place. To increase the current detection sensitivity of the current sensor, Japanese Laid-Open Patent Publication No. 2002-303642 describes a magnetic core that concentrates the magnetic field generated by the current flowing through a current path and increases the strength of the magnetic field acting on the Hall element. A prior art current sensor including a magnetic core will now be described with reference to  FIG. 8 . 
     The current sensor of  FIG. 8  is coupled to a bus bar  40 . The bus bar  40  is used to supply power to, for example, a vehicle battery. The current sensor includes a magnetic core  31 , a printed circuit board  33 , and a case  34 . The magnetic core  31  concentrates the magnetic field generated by the current flowing through the bus bar  40 . Electronic components including a Hall element  32  are mounted on the printed circuit board  33 . The case  34  accommodates the magnetic core  31  and the printed circuit board  33 . The case  34  includes a sleeve  34   a  through which the bus bar  40  is inserted. The magnetic core  31  is C-shaped and includes a clearance CS (gap). The sleeve  34   a  is inserted into the middle of the space formed in the magnetic core  31  so that the magnetic core  31  surrounds the sleeve  34   a  and the bus bar  40 . The clearance CS (gap) of the magnetic core  31  allows for insertion of the Hall element  32 . The printed circuit board  33  is connected to a male terminal connector  35 , which is arranged on an outer wall of the case  34 . The magnetic core  31  concentrates and increases the magnetic field generated by the current flowing through the bus bar  40 . Leakage flux generated in the clearance CS acts on the Hall element  32 . The magnetic field acting on the Hall element  32  is amplified. This allows for the current sensor to detect the magnitude of a small current flowing through the bus bar  40 . A detection signal corresponding to the Hall voltage of the Hall element  32  is provided to an in-vehicle device (not shown) via a conductor of the printed circuit board  33  and the male terminal connector  35 . 
     SUMMARY OF THE INVENTION 
     The bus bar  40  of the prior art sensor is just inserted into the sleeve  34   a.  Thus, the bus bar  40  may slightly move inside the sleeve  34   a.  Such displacement of the bus bar  40  changes the positional relationship between the magnetic core  31  and the bus bar  40 . This changes the electric field that is concentrated and amplified by the magnetic core  31 . As a result, the current sensor detection may become unstable, and the current detection accuracy may be lowered. 
     When the current sensor is provided with a structure for positioning the bus bar  40  so that the positional relationship between the magnetic core  31  and the bus bar  40  does not change, enlargement of the current sensor is unavoidable. 
     It is an object of the present invention to provide a compact current sensor that detects current with high accuracy. 
     One aspect of the present invention is a current sensor for outputting a detection signal corresponding to a current flowing through a bus bar. The current sensor includes a magnetic core that concentrates and amplifies a magnetic field generated by the current near a detection portion of the bus bar. A magnetic detection element detects the magnetic field concentrated by the magnetic core and outputs an electrical signal corresponding to the detected magnetic field. The detection portion of the bus bar and the magnetic core are molded integrally with each other. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a perspective view showing a current sensor according to one embodiment of the present invention; 
         FIG. 2  is a perspective exploded view showing the current sensor of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view showing the current sensor of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view taken along line  4 - 4  in  FIG. 3 ; 
         FIG. 5  is a perspective view showing a first modification of the current sensor; 
         FIG. 6  is a perspective view showing a second modification of the current sensor; 
         FIG. 7  is a plan view showing a third modification of the current sensor; and 
         FIG. 8  is an exploded perspective view showing a current sensor of the prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A current sensor according to one embodiment of the present invention will now be discussed with reference to  FIGS. 1 to 4 . First, the structure of the current sensor will be described with reference to  FIG. 1 . 
     As shown in  FIG. 1 , a case  1  covers electronic components of the current sensor. The case  1  protects the electronic components from the ambient environment. A connector  21  is arranged on the front of the case  1 . The connector  21  is connected to a harness or the like (not shown) and may be used to supply the current sensor with power and to output a detection signal of the current sensor to an external device. A bus bar  11  having an elongated planar shape is attached to the case  1  in a state extending vertically through the case  1  as viewed in the drawing. The bus bar  11  is a power supply conductor and connects, for example, an in-vehicle inverter device and an in-vehicle motor. The bus bar  11  includes end portions that define coupling portions for coupling the current sensor to external devices. In the illustrated example, the two end portions of the bus bar  11  respectively have insertion holes  11   a  and  11   b  through which bolts  2   a  and  2   b  are inserted. The in-vehicle inverter device and the in-vehicle motor, which serve as the external devices, respectively include threaded holes  3   a  and  3   b  that correspond to the bolts  2   a  and  2   b.  The bolts  2   a  and  2   b  fasten the bus bar  11  to the in-vehicle inverter device and in-vehicle motor. 
     As shown in  FIG. 2 , the case  1  includes an upper case  10  and a lower case  20 . The bus bar  11  is attached to the upper case  10 . A tab  13  having a through hole  13   a  extends from the bottom of each of two opposing side walls of the upper case  10 . The connector  21  is arranged on the lower case  20 . A hook  22  is arranged on each of two opposing side walls of the lower case  20  to engage with the corresponding tab  13  of the upper case  10 . The upper case  10  may be referred to as a first member, and the lower case  20  may be referred to as a second member. The case  1  is separable into the first and second members  10  and  20 . This increases the design freedom for the case  1  and convenience for assembling the current sensor. In the illustrated example, the cases  10  and  20  are resin members formed from a resin material. 
     The engagement of the tabs  13  of the upper case  10  with the hooks  22  of the lower case  20  integrally couples the upper case  10  and the lower case  20  and forms the case  1 . The tabs  13  and hooks  22  may be referred to as a fastening structure. The tabs  13  may be formed on the second member  20 , and the hooks  22  may be formed on the first member  10 . 
     A planar substrate mount  23  projects from an upper surface of the lower case  20 . The substrate mount  23  includes catches  23   a.  A printed circuit board  24  is fastened to the substrate mount  23  by the catches  23   a.  In the illustrated example, the printed circuit board  24  is T-shaped and includes a laterally extending plate  24   a,  which is held by the catches  23   a,  and a vertically extending plate  24   b,  which extends upward from the laterally extending plate  24   a.  A Hall IC  25  is mounted on the vertically extending plate  24   b.  A Hall element serving as a magnetic detection element (magnetoelectric conversion element) and its peripheral circuits are integrated in the Hall IC  25 . Although not shown in the drawings, a processing circuit for processing output signals of the Hall IC  25  is also mounted on the printed circuit board  24 . Basal portions of metal pins T 1  to T 3  are soldered to the laterally extending plate  24   a  of the printed circuit board  24 . The metal pins T 1  to T 3  have distal portions extending into the connector  21  and functioning as a power supply terminal, an output terminal, and a ground (GND) terminal. Referring to  FIG. 3 , when molding the lower case  20  from resin, the metal pins T 1  to T 3  are integrally insert-molded, or embedded, in the lower case  20 . An insertion hole  20   a  extends through the lower case  20  at the rear of the substrate mount  23 . The bus bar  11  is inserted through the insertion hole  20   a.    
     As shown in  FIG. 3 , the upper case  10  is capable of accommodating the substrate mount  23 , the printed circuit board  24 , and the Hall IC  25 . The upper case  10  is divided into a large accommodation compartment  10   a  and a small accommodation compartment  10   b  respectively corresponding to the laterally extending plate  24   a  and vertically extending plate  24   b  of the printed circuit board  24 . A detection portion of the bus bar  11  and the magnetic core  12  are integrally insert-molded, or embedded, in a wall of the small accommodation compartment  10   b.  The detection portion of the bus bar  11  and the magnetic core  12  are embedded integrally in the upper case  10 , for example, when molding the upper case  10 . 
     With reference to  FIG. 4 , the structure of the magnetic core  12  will now be described in detail. 
     The magnetic core  12  is a magnetic body. As shown in  FIG. 4 , the magnetic core  12  is a C-shaped member that surrounds the detection portion of the bus bar  11 . The C-shaped member includes a clearance CT corresponding to the small accommodation compartment  10   b.  The magnetic core  12  has two opposing ends that define the clearance CT in between. The opposing ends of the magnetic core  12  are thicker than the other parts of the magnetic core  12 . Each opposing end includes a stepped surface. The stepped surface is formed so that the clearance CT narrows from the inner side of the magnetic core  12  toward the outer side of the magnetic core  12 . The Hall IC  25  accommodated in the small accommodation compartment  10   b  is located in the central part of the clearance CT. 
     Due to such a structure, the magnetic core  12  concentrates and amplifies the magnetic field generated by the current flowing through the bus bar  11  in the current sensor. The leakage flux in the clearance CT acts on the Hall IC  25  in the small accommodation compartment  10   b.  The Hall IC  25  outputs an electrical signal in correspondence with the current flowing through the bus bar  11 . 
     In the magnetic core  31  of the prior art current sensor, the clearance CS has a constant width. The magnetic flux generated in the clearance CS of the constant width becomes smaller as the outer side of the magnetic core becomes closer. Further, the magnetic flux generated in the clearance CS becomes larger as the width of the clearance CS becomes smaller. In the present embodiment, the clearance CT narrows from the inner side of the magnetic core  12  toward the outer side of the magnetic core  12 . Thus, magnetic flux is evenly generated in the clearance CT. This obtains the advantages described below. 
     In the prior art sensor, the magnetic field acting on the Hall element  32  slightly changes in accordance with the position of the Hall element  32  in the clearance CS of the magnetic core  31 . Thus, when the Hall element  32  is displaced in the clearance CS, the current sensor may not be able to detect current with high accuracy. To detect current with high accuracy, the Hall element  32  and the magnetic core  31  must be accurately positioned. However, accurate positioning of the Hall element  32  and the magnetic core  31  would increase manufacturing processes for the current sensor. This would raise the manufacturing cost for the current sensor. In this respect, the magnetic core  12  of the present embodiment evenly generates magnetic flux in the clearance CT. Thus, the magnetic field acting on the Hall IC  25  subtly changes even when, for example, assembling tolerances of the cases  10  and  20  displace the Hall IC  25  in the clearance CT. Consequently, the current sensor of the present embodiment detects current with high accuracy and reduces manufacturing costs without requiring the Hall IC  25  to be positioned with high accuracy. 
     In the present embodiment, the detection portion of the bus bar  11  and the magnetic core  12  are molded integrally with each other. This prevents relative displacement of the detection portion of the bus bar  11  and the magnetic core  12 . Thus, the current sensor stably detects current with high accuracy. Further, since the detection portion of the bus bar  11  and the magnetic core  12  are molded integrally with each other, there is no need for a structure that positions the bus bar  11 . This allows for the current sensor to be compact and detect current with high accuracy. 
     The current sensor of the present embodiment has the advantages described below. 
     (1) The detection portion of the bus bar  11  and the magnetic core  12  are molded integrally with each other. This prevents relative displacement of the detection portion of the bus bar  11  and the magnetic core  12  without a structure for positioning the bus bar  11 , and allows for the current sensor to be compact and detect current with high accuracy. 
     (2) The case  1  includes the upper case  10 , which accommodates the detection portion of the bus bar  11  and the magnetic core  12 , and the lower case  20 , which accommodates the printed circuit board  24  on which the Hall IC  25  is mounted. The current sensor is assembled just by coupling the case  10  and  20  to each other. In other words, the current sensor is formed by the separable cases  10  and  20 . This facilitates the assembling of the current sensor. 
     (3) The opposing ends of the magnetic core  12  defining the clearance CT each includes a stepped surface that is formed so that leakage flux is evenly generated in the clearance CT. Thus, displacement of the Hall IC in the clearance CT would only subtly change the magnetic field acting on the Hall IC  25 . This eliminates the need for positioning the Hall IC  25  with high accuracy and thereby reduces manufacturing cost for the current sensor. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
     As shown in  FIG. 5 , the bus bar  11  may include undulated portions  11   c  and  11   d  between the case  1  and the insertion holes  11   a  and  11   b.  The undulated portions  11   c  and  11   d  absorb the stress applied to the bus bar  11  when fastening the bolts  2   a  and  2   b  and decrease or eliminate the stress applied to the electronic components of the current sensor. This prevents the electronic components of the current sensor from being damaged. Instead of the undulated portions  11   c  and  11   d,  the bus bar  11  may include through holes  11   e  (refer to  FIG. 6 ) or recesses  11   f  and  11   g  (refer to  FIG. 7 ). The recesses  11   f  and  11   g  of  FIG. 7  are formed in the front and rear surfaces of the bus bar  11  and provide the bus bar  11  with partially thin portions. These structures also absorb the stress applied to the bus bar  11  and thereby have the advantages described above. 
     In the above-discussed embodiment, to form the clearance CT in the magnetic core  12  that narrows from the inner side toward the outer side of the magnetic core  12 , stepped surfaces are formed in the opposing ends defining the clearance CT. The stepped surfaces may each be a smooth sloped surface. Depending on the shape of the magnetic core  12  and the shape of the detection portion of the bus bar  11 , the clearance CT may be formed so that it widens from the inner side toward the outer side of the magnetic core  12 . It is only required that the clearance CT be adjusted so that leakage flux is evenly generated in the clearance CT when concentrating and amplifying the magnetic field generated near the detection portion of the bus bar  11 . Adjustment of the clearance CT includes, for example, widening or narrowing the clearance CT continuously or in a stepped manner. 
     In the above-discussed embodiment, the clearance CT of the magnetic core  12  is formed so as to narrow from the inner side toward the outer side of the magnetic core  12 . Instead, the clearance CT of the magnetic core  12  may have a constant width when relative displacement of the Hall IC  25  relative to the magnetic core  12  is ignorable such as when the magnetic core  12  is sufficiently larger than the Hall IC  25 . Such a structure would also have advantages that are the same or similar to advantages (1) and (2), which are described above. 
     In the above-discussed embodiment, the Hall IC  25  is used to detect leakage flux generated in the clearance CT. A magnetoresistance effect element may be used in lieu of the Hall IC  25 . The magnetoresistance effect element has a resistance that is changed by a magnetoresistance effect in accordance with the magnetic field. 
     In the above-discussed embodiment, the bus bar  11  is a conductor that connects the in-vehicle inverter device and in-vehicle motor. The bus bar  11  may also be a power supply conductor connected to a vehicle battery. 
     The in-vehicle inverter device and in-vehicle motor may be arranged in a so-called hybrid vehicle. 
     The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.