Patent Publication Number: US-2022229093-A1

Title: Current detection device

Description:
CLAIM OF PRIORITY 
     This application is a Continuation of International Application No. PCT/JP2020/037912 filed on Oct. 6, 2020, which claims benefit of priority to Japanese Patent Application No. 2019-185175 filed on Oct. 8, 2019. The entire contents of each application noted above are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to a current detection device capable of measuring a current flowing through a bus bar. 
     2. Description of the Related Art 
     The current sensor described in Japanese Unexamined Patent Application Publication No. 2018-169305 includes a pair of shield plates made of a magnetic material disposed so as to sandwich a bus bar in the thickness direction of the bus bar, and a magnetic detection element disposed between the bus bar and one of the shield plates so as to detect the strength of a magnetic field generated by a current flowing through the bus bar. The shield plates have a length greater than or equal to 20 mm and a width greater than or equal to 24 mm and less than or equal to 38 mm. In this manner, a sufficient shielding performance can be achieved while preventing magnetic saturation in applications that measure large currents. 
     According to the current sensor described in Japanese Unexamined Patent Application Publication No. 2018-169305, the width of the shield plate is set to 24 mm or greater in order to obtain a predetermined shielding effect, and the width of the shield plate is set to 38 mm or less in order to decrease the magnetic saturation ratio. However, when a large current is passed through the bus bar, magnetic saturation is more likely to occur in the shield plate adjacent to the bus bar than in the shield plate adjacent to the magnetic detection element. If magnetic saturation occurs in one of the shield plates, the linearity of the detection result of the magnetic detection element is likely to be lost, and high detection accuracy cannot be maintained, which is problematic. 
     SUMMARY OF THE INVENTION 
     A current detection device includes a plate-shaped bus bar configured to enable a current to be measured to pass therethrough, a magnetic sensor disposed at a position that faces the bus bar in the thickness direction of the bus bar, where the magnetic sensor measures a magnetic field generated when the current to be measured flows through the bus bar, and a first shield and a second shield made of a magnetic material. The first shield and the second shield are disposed so as to sandwich the bus bar and the magnetic sensor in the thickness direction of the bus bar, the first shield is disposed adjacent to the magnetic sensor, and the second shield is disposed adjacent to the bus bar. The first shield and the second shield are configured such that the ratio of the magnetic flux density inside the first shield to the magnetic flux density inside the second shield is in the range about of 1:1 to 1:2 when the current to be measured is flowing through the bus bar. 
     In this manner, by setting the magnetic flux density inside the second shield adjacent to the bus bar to one time to twice the magnetic flux density inside the first shield adjacent to the magnetic sensor, the occurrence of magnetic saturation in one of the shields earlier than in the other can be prevented, thus ensuring the linearity of the detection result. As a result, even a large current can be detected with high accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view illustrating the basic configuration of a current detection device according to an embodiment of the present invention; 
         FIG. 1B  is an exploded perspective view of the current detection device; 
         FIG. 2A  is a cross-sectional view taken along a line IIA-IIA of  FIG. 1A ; 
         FIG. 2B  is a cross-sectional view taken along a line IIB-IIB of  FIG. 1A ; 
         FIG. 3A  is a side view illustrating the position relationship and size relationship among a bus bar, a magnetic sensor, and a pair of upper and lower shields according to a first embodiment; 
         FIG. 3B  is a graph illustrating the relationship between the ratio of thickness of the pair of shields and the ratio of magnetic flux densities in the pair of shields; 
         FIG. 4A  is a side view illustrating the position relationship and size relationship among a bus bar, a magnetic sensor, and a pair of upper and lower shields according to a second embodiment; 
         FIG. 4B  is a graph illustrating the relationship between the ratio of width of the pair of shields and the ratio of magnetic flux densities in the pair of shields; 
         FIG. 5A  is a side view illustrating the position relationship and size relationship among a bus bar, a magnetic sensor, and a pair of upper and lower shields according to a third embodiment; and 
         FIG. 5B  is a graph illustrating the relationship between the ratio of distance of the pair of shields from the bus bar and the ratio of magnetic flux densities in the pair of shields. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Current detection devices according to embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
     The basic configuration of current detection devices  10  according to the embodiments is described first with reference to  FIGS. 1A and 1B  and  FIGS. 2A and 2B . The sizes and relative positions of members of each of the embodiments are described with reference to  FIGS. 3A to 5B .  FIG. 1A  is a perspective view illustrating the basic configuration of the current detection device  10 ,  FIG. 1B  is an exploded perspective view of the current detection device  10 ,  FIG. 2A  is a cross-sectional view taken along a line IIA-IIA of  FIG. 1A , and  FIG. 2B  is a cross-sectional view taken along a line IIB-IIB of  FIG. 1A . 
     As illustrated in  FIGS. 1A and 1B , the current detection device  10  includes a substantially rectangular parallelepiped housing  11  constituted by fixing a cover member  11   a  located on the upper side (a Z 1  side in  FIGS. 1A and 1B ) and a case member  11   b  located on the lower side (a Z 2  side in  FIGS. 1A and 1B ) to each other. Three bus bars  21 ,  22 , and  23  pass through the case member  11   b  in the width direction of the housing  11  (a Y 1 -Y 2  direction in  FIGS. 1A and 1B ). 
     The three bus bars  21 ,  22 , and  23  are conductive plates having the same shape. The bus bars  21 ,  22 , and  23  are disposed such that two opposing plate surfaces correspond to the top and bottom of the housing  11 , respectively, in the width direction of the housing  11 . The bus bars  21 ,  22 , and  23  extend in a strip shape in the width direction of the housing  11  and are disposed at equal intervals in the longitudinal direction of the housing  11  (an X 1 -X 2  direction in  FIGS. 1A and 1B ). 
     As illustrated in  FIGS. 1B and 2B , a circuit board  30  is disposed inside the housing  11  so as to extend in the longitudinal direction (the X 1 -X 2  direction). Magnetic sensors  31 ,  32 , and  33  are disposed on the circuit board  30  at positions corresponding to the bus bars  21 ,  22 , and  23 , respectively, in an X-Y plane (a plane including the X 1 -X 2  direction and a Y 1 -Y 2  direction). At least part of a main portion of each of the magnetic sensors  31 ,  32 , and  33  faces the corresponding bus bar in the vertical direction (a Z 1 -Z 2  direction). Note that the magnetic sensors  31 ,  32 , and  33  may be provided on either the upper surface or the lower surface of the circuit board  30 . 
     To take the magnetic sensor  32  as an example, as illustrated in  FIG. 2A , the magnetic sensor  32  is disposed at a position corresponding to the center in the width direction (the Y 1 -Y 2  direction) of the housing  11 . The bus bar  22  and the magnetic sensor  32  face each other in the vertical direction. As illustrated in  FIG. 2B , the magnetic sensor  32  is disposed so as to face the bus bar  22  at the same position as the bus bar  22  in the width direction (the X 1 -X 2  direction) of the bus bar  22  on the X-Y plane. Since the magnetic sensor  32  is disposed so as to correspond to the bus bar  22  in this way, the magnetic sensor  32  can measure the current value of a current to be measured by detecting the magnetic field induced by the current (the current to be measured) flowing through the bus bar  22 . The magnetic sensor  32  is configured by using, for example, a magnetoresistive element, such as a GMR element (giant magnetoresistive element). 
     The magnetic sensor  32  is sandwiched from above and below in the thickness direction of the bus bar  22  by a pair of shields (a first shield  41   a  disposed in the cover member  11   a  and a second shield  41   b  disposed in the case member  11   b ). It is desirable that the first shield  41   a  and the second shield  41   b  be made of a ferromagnetic material as magnetic shields made of the same magnetic material. The first shield  41   a  and the second shield  41   b  are disposed so as to face each other in parallel in the vertical direction. Each of the first shield  41   a  and the second shield  41   b  has a configuration in which a plurality of metal plates having the same rectangular shape and the same size in plan view are stacked in the vertical direction. By arranging the first shield  41   a  and the second shield  41   b  so as to sandwich the magnetic sensor  32  in this way, the magnetic sensor  32  blocks a foreign magnetic field (an external magnetic field), such as an induced magnetic field due to the currents flowing through the adjacent bus bars  21  and  23 . Thus, the magnetic sensor  32  decreases the influence of the external magnetic field. 
     In terms of the relationship between the sizes of the first shield  41   a  and the second shield  41   b  and the distance between the magnetic sensor  32  and each of the first shield  41   a  and the second shield  41   b  in the vertical direction (the Z 1 -Z 2  direction), the relationship illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B  is schematic. The particular relationships are described in each of the embodiments described below. In each of the embodiments, when a current to be measured flows in the bus bars  21 ,  22 , or  23 , the ratio of the magnetic flux density inside the first shield  41   a  to the magnetic flux density inside the first shield  41   b  is in the range of about 1:1 to 1:2. The specific configuration is described below in the description of each of the embodiments. Note that the term “range of about 1:1 to 1:2” includes the ratio of about 1:1 and the ratio of 1:2. Similarly, in the description below, the upper limit and the lower limit are included in a described range. 
     Note that the location of the magnetic sensor  32  relative to the bus bar  22 , the locations of the two shields  41   a  and  41   b  relative to the magnetic sensor  32 , and the operations and effects of the locations similarly apply to the magnetic sensors  31  and  33  that are located at either side of the magnetic sensor  32 . 
     First Embodiment 
       FIG. 3A  is a side view illustrating the relationship between the location and size among a bus bar  120 , a magnetic sensor  130 , and a pair of upper and lower shields  141   a  and  141   b  according to the first embodiment. In  FIG. 3A , each of members is illustrated in a simplified manner.  FIG. 3B  is a graph illustrating the relationship between the ratio of thickness of the pair of shields  141   a  and  141   b  and the ratio of magnetic flux densities in the pair of shields  141   a  and  141   b.    
     According to the first embodiment, as illustrated in  FIG. 3A , the bus bar  120 , the magnetic sensor  130 , and the pair of upper and lower shields  141   a  and  141   b  are disposed in the current detection device  10  illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B  and have the size and position relationship described below. The other configurations are the same as those of the current detection device  10  illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B . The plurality of bus bars  120  made of the same material as the bus bars  21 ,  22 , and  23  pass through the housing  11 . A plurality of magnetic sensors  130  are disposed on the circuit board  30  in the housing  11  so as to correspond to the plurality of bus bars  120 , and each of the plurality of magnetic sensors  130  is sandwiched by the two shields  141   a  and  141   b  facing each other in the vertical direction. 
     The bus bar  120  illustrated in  FIG. 3A  is a plate member extending in a strip shape in the width direction (the Y 1 -Y 2  direction) of the housing  11  (refer to  FIGS. 1A and 1B ). The bus bar  120  has a thickness of D 10  in the vertical direction (the Z 1 -Z 2  direction) and a width of W 10  in the right-left direction (the X 1 -X 2  direction). 
     As illustrated in  FIG. 3A , the magnetic sensor  130  is disposed so that the center in the width direction (the X 1 -X 2  direction) coincides with a center AX in the width direction of the bus bar  120 . In addition, the magnetic sensor  130  is disposed so as to be separated from the bus bar  120  by a distance C 10  in the vertical direction (the Z 1 -Z 2  direction). 
     The first shield  141   a  and the second shield  141   b  are disposed so that the center in the width direction (the X 1 -X 2  direction) coincides with the center AX in the width direction of the bus bar  120 . The thickness of the first shield  141   a  is T 11 , and the distance of the first shield  141   a  from the bus bar  120  in the vertical direction (the Z 1 -Z 2  direction) is set to D 11 . The thickness of the second shield  141   b  is set to T 12 , which is greater than that of the first shield  141   a,  and the distance of the second shield  141   b  from the bus bar  120  in the vertical direction is set to D 12 , which is less than the above-described D 11 . The widths (in the X 1 -X 2  directions) of the two shields  141   a  and  141   b  are the same and are set to be greater than the width W 10  of the bus bar  120 . In addition, the lengths of the first shield  141   a  and the second shield  141   b  in an extending direction (the Y 1 -Y 2  direction) are the same, and the planar shapes thereof are also the same. 
       FIG. 3B  illustrates the ratio of the magnetic flux densities in the two shields  141   a  and  141   b  when the thicknesses T 11  and T 12  of the two shields  141   a  and  141   b  are changed while keeping the distances D 11  and D 12  of the two shields  141   a  and  141   b  from the bus bar  120  constant. As can be seen from the result, the ratio of the magnetic flux density increases with increasing thickness of the second shield  141   b  adjacent to the bus bar  120 , as compared with the case where the thicknesses T 11  and T 12  of the two shields  141   a  and  141   b  are the same. That is, the magnetic flux density in the first shield  141   a  adjacent to the magnetic sensor  130  relatively increases with increasing thickness of the second shield  141   b.  As a result, by adjusting the ratio of the thicknesses of the pair of the upper and lower shields  141   a  and  141   b,  the ratio of the magnetic flux densities in the two shields can be set to a desired value. In this manner, even when as illustrated in  FIG. 3A , the second shield  141   b  is disposed closer to the bus bar  120  than the first shield  141   a,  it can be prevented that magnetic saturation occurs in the second shield  141   b  adjacent to the bus bar  120  earlier than in the first shield  141   a.  Consequently, the linearity of the detection result of the magnetic sensor  130  can be ensured and, thus, high accuracy measurement can be made even when a large current is passed through the bus bar  120 . 
     Furthermore, from the viewpoint of the linearity of the detection result of the magnetic sensor  130 , the ratio of the magnetic flux densities in the pair of shields  141   a  and  141   b  is most preferably 1. In consideration of  FIG. 3B , the ratio of thickness of the first shield  141   a  adjacent to the magnetic sensor  130  and the second shield  141   b  adjacent to the bus bar  120  (T 11 : 112 ) is 1:2.5. Furthermore, in practical uses, it is desirable that the ratio of thickness of the first shield  141   a  and the second shield  141   b  be 1:1 or greater, and therefore, when combined with the above-mentioned most preferable thickness ratio, it is desirable that the ratio be in the range of about 1:1 to 1:2.5. As can be seen from  FIG. 3B , according to this range, the ratio of the magnetic flux density in the first shield  141   a  to that in the second shield  141   b  is in the range of about 1:1 to 1:2. 
     Note that according to the first embodiment, the magnetic sensor  130  is disposed so that the center in the width direction (the X 1 -X 2  direction) coincides with the center AX in the width direction of the bus bar  120 . However, the magnetic sensor  130  and the bus bar  120  may be disposed such that the center in the width direction of the magnetic sensor  130  is shifted from that of the bus bar  120  within a region in which the first shield  141   a  and the second shield  141   b  face each other. For example, if a signal terminal and a power supply terminal of the magnetic sensor  130  are moved away from the bus bar  120  by shifting in this way, the influence on the detection result can be reduced even when the bus bar  120  generates noise at the time of switching on and off of a voltage for controlling the current to be measured flowing in the bus bar  120 . 
     Second Embodiment 
       FIG. 4A  is a side view illustrating the position relationship and size relationship among a bus bar  220 , a magnetic sensor  230 , and a pair of upper and lower shields  241   a  and  241   b  according to the second embodiment. In  FIG. 4A , the members are illustrated in a simplified manner.  FIG. 4B  is a graph illustrating the relationship between the ratio of width of the pair of shields  241   a  and  241   b  and the ratio of magnetic flux densities in the pair of shields  241   a  and  241   b.    
     According to the second embodiment, as illustrated in  FIG. 4A , the bus bar  220 , the magnetic sensor  230 , and the pair of upper and lower shields  241   a  and  241   b  are disposed in the current detection device  10  illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B  and have the size and position relationship described below. The other configurations are the same as those of the current detection device  10  illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B . The plurality of bus bars  220  made of the same material as the bus bars  21 ,  22 , and  23  pass through the housing  11 . A plurality of magnetic sensors  230  are disposed on the circuit board  30  in the housing  11  so as to correspond to the plurality of bus bars  220 , and each of the plurality of magnetic sensors  230  is sandwiched by the two shields  241   a  and  241   b  facing each other in the vertical direction. 
     The bus bar  220  illustrated in  FIG. 4A  is a plate member extending in a strip shape in the width direction (the Y 1 -Y 2  direction) of the housing  11 . The bus bar  220  has a thickness of D 20  in the vertical direction (the Z 1 -Z 2  direction) and a width of W 20  in the right-left direction (the X 1 -X 2  direction). 
     As illustrated in  FIG. 4A , the magnetic sensor  230  is disposed so that the center in the width direction (the X 1 -X 2  direction) coincides with a center AX in the width direction of the bus bar  220 . In addition, the magnetic sensor  230  is disposed so as to be separated from the bus bar  220  by a distance C 20  in the vertical direction (the Z 1 -Z 2  direction). The thickness D 20  and the width W 20  of the bus bar  220  and the distance C 20  between the bus bar  220  and the magnetic sensor  230  are the same as the thickness D 10  and the width W 10  of the bus bar  120  and the distance C 10  between the bus bar  120  and the magnetic sensor  130  according to the first embodiment, respectively. 
     The first shield  241   a  and the second shield  241   b  are disposed so that the center in the width direction (the X 1 -X 2  direction) coincides with the center AX in the width direction of the bus bar  220 . The thickness of the first shield  241   a  is set to T 20 , the width of the first shield  241   a  is set to W 21 , and the distance from the bus bar  220  in the vertical direction (the Z 1 -Z 2  direction) is set to D 21 . The thickness of the second shield  241   b  is set to T 20 , which is the same as the thickness of the first shield  241   a,  the width is set to W 22 , which is less than the width of the first shield  241   a,  and the distance from the bus bar  220  in the vertical direction is set to D 22 , which is less than the above-described D 21 . Furthermore, the lengths of the first shield  241   a  and the second shield  241   b  in the extending direction (the Y 1 -Y 2  direction) are the same. 
       FIG. 4B  illustrates the ratio of the magnetic flux densities in the two shields  241   a  and  241   b  when the widths W 21  and W 22  of the two shields  241   a  and  241   b  are changed while keeping the distances D 21  and D 22  of the two shields  241   a  and  241   b  from the bus bar  220  constant. As can be seen from the result, the ratio of the magnetic flux density increases with decreasing width of the second shield  241   b  adjacent to the bus bar  220 , as compared with the case where the widths W 21  and W 22  of the two shields  241   a  and  241   b  are the same. That is, the magnetic flux density in the first shield  241   a  adjacent to the magnetic sensor  230  relatively increases with decreasing width of the second shield  241   b.  As a result, by adjusting the ratio of width of the pair of the upper and lower shields  241   a  and  241   b,  the ratio of the magnetic flux densities in the two shields can be set to a desired value. In this manner, even when as illustrated in  FIG. 4A , the second shield  241   b  is disposed closer to the bus bar  220  than the first shield  241   a,  it can be prevented that magnetic saturation occurs in the second shield  241   b  adjacent to the bus bar  220  earlier than in the first shield  241   a.  Consequently, the linearity of the detection result of the magnetic sensor  230  can be ensured and, thus, high accuracy measurement can be made even when a large current is passed through the bus bar  220 . 
     Furthermore, from the viewpoint of the linearity of the detection result of the magnetic sensor  230 , the ratio of the magnetic flux densities in the pair of shields  241   a  and  241   b  is most preferably 1. In consideration of  FIG. 4B , the ratio of width of the first shield  241   a  adjacent to the magnetic sensor  230  and the second shield  241   b  adjacent to the bus bar  220  is 1:0.3. Furthermore, in practical uses, it is desirable that the ratio of width of the first shield  241   a  and the second shield  241   b  (W 21 :W 22 ) be 1:1 or greater (W 21 ≥W 22 ), and therefore, when combined with the above-mentioned most preferable width ratio, it is desirable that the ratio be in the range of about 1:1 to 1:0.3. As can be seen from  FIG. 4B , according to this range, the ratio of the magnetic flux densities in the first shield  241   a  and the second shield  241   b  is in the range of about 1:1 to 1:2. Note that the other operations, effects, and modifications are the same as those of the first embodiment. 
     Third Embodiment 
       FIG. 5A  is a side view illustrating the position relationship and size relationship among a bus bar  320 , a magnetic sensor  330 , and a pair of upper and lower shields  341   a  and  341   b  according to the third embodiment. In  FIG. 5A , the members are illustrated in a simplified manner.  FIG. 5B  is a graph illustrating the relationship between the ratio of distance of the pair of shields  341   a  and  341   b  from the bus bar  320  and the ratio of magnetic flux densities in the pair of shields  341   a  and  341   b.    
     According to the third embodiment, as illustrated in  FIG. 5A , the bus bar  320 , the magnetic sensor  330 , and the pair of upper and lower shields  341   a  and  341   b  are disposed in the current detection device  10  illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B  and have the size and position relationship described below. The other configurations are the same as those of the current detection device  10  illustrated in  FIGS. 1A and 1B  and  FIGS. 2A and 2B . The plurality of bus bars  320  made of the same material as the bus bars  21 ,  22 , and  23  pass through the housing  11 . A plurality of magnetic sensors  330  are disposed on the circuit board  30  in the housing  11  so as to correspond to the plurality of bus bars  320 , and each of the plurality of magnetic sensors  330  is sandwiched by the two shields  341   a  and  341   b  facing each other in the vertical direction. 
     The bus bar  320  illustrated in  FIG. 5A  is a plate member extending in a strip shape in the width direction (the Y 1 -Y 2  direction) of the housing  11 . The bus bar  320  has a thickness of D 30  in the vertical direction (the Z 1 -Z 2  direction) and a width of W 30  in the right-left direction (the X 1 -X 2  direction). 
     As illustrated in  FIG. 5A , the magnetic sensor  330  is disposed so that the center in the width direction (the X 1 -X 2  direction) coincides with a center AX in the width direction of the bus bar  320 . In addition, the magnetic sensor  330  is disposed so as to be separated from the bus bar  320  by a distance C 30  in the vertical direction (the Z 1 -Z 2  direction). The thickness D 30  and the width W 30  of the bus bar  320  and the distance C 30  between the bus bar  320  and the magnetic sensor  330  are the same as the thickness D 10  and the width W 10  of the bus bar  120  and the distance  010  between the bus bar  120  and the magnetic sensor  130  according to the first embodiment, respectively. 
     The first shield  341   a  and the second shield  341   b  are disposed so that the center in the width direction (the X 1 -X 2  direction) coincides with the center AX in the width direction of the bus bar  320 . The first shield  341   a  has a thickness of T 30 , and the distance from the bus bar  320  in the vertical direction (the Z 1 -Z 2  direction) is set to D 31 . The thickness of the second shield  341   b  is set to T 30 , which is the same as the thickness of the first shield  341   a,  and the width of the second shield  341   b  is also the same as that of the first shield  341   a.  The distance from the bus bar  320  in the vertical direction is set to D 32 , which is less than the above-described D 31 . Furthermore, the lengths of the first shield  341   a  and the second shield  341   b  in the extending direction (the Y 1 -Y 2  direction) are the same, and the planar shapes are also the same. 
       FIG. 5B  illustrates the ratio of the magnetic flux densities in the two shields  341   a  and  341   b  when the distances of the two shields  341   a  and  341   b  from the bus bar  320  are changed. As can be seen from the result, the ratio of the magnetic flux densities is less than 1 in the range in which the distance between the second shield  341   b  and the bus bar  320  is less than the distance between the first shield  341   a  and the bus bar  320 . In addition, the ratio of magnetic flux densities is 1 when the distances between the bus bar  320  and each of the shields  341   b  and the bus bar  320  are the same. 
     That is, the magnetic flux density in the first shield  341   a  adjacent to the magnetic sensor  330  relatively increases with increasing distance D 32  between the second shield  341   b  and the bus bar  320 . As a result, by adjusting the ratio of the distance of the pair of upper and lower shields  341   a  and  341   b  from the bus bar  320 , the ratio of the magnetic flux densities in the two shields can be set to a desired value. In this manner, it can be prevented that magnetic saturation occurs in the second shield  341   b  adjacent to the bus bar  320  earlier than in the first shield  341   a,  and the linearity of the detection result of the magnetic sensor  330  can be ensured. Thus, high accuracy measurement can be made even when a large current is passed through the bus bar  320 . 
     Furthermore, from the viewpoint of the linearity of the detection result of the magnetic sensor  330 , the ratio of the magnetic flux densities in the pair of shields  341   a  and  341   b  is most preferably 1. In consideration of  FIG. 5B , the ratio of distance of the first shield  341   a  adjacent to the magnetic sensor  330  and the second shield  341   b  adjacent to the bus bar  320  from the bus bar  320  (D 31 :D 32 ) is 1:1. Furthermore, in practical uses, it is desirable that the ratio of distance of the first shield  341   a  and the second shield  341   b  from the bus bar  320  be 1:0.2 or greater, and therefore, when combined with the above-mentioned most preferable thickness ratio, it is desirable that the ratio be in the range of about 1:0.2 to 1:1. As can be seen from  FIG. 5B , according to this range, the ratio of the magnetic flux densities in the first shield  341   a  and the second shield  341   b  is in the range of about 1:1 to 1:2. Note that the other operations, effects, and modifications are the same as those of the first embodiment or the second embodiment. 
     While the present invention has been described with reference to the above embodiments, the present invention is not limited to the above embodiments, and a variety of improvements and modifications can be made within the purpose of the improvement or the scope and spirit of the present invention. 
     As described above, the current detection device according to the present invention can prevent loss of the linearity of the detection result caused by the occurrence of magnetic saturation in one of the pair of shields when a large current is passed through the bus bar. The current detection device according to the present invention is useful in that a large current can be measured with high accuracy.