Abstract:
A magnetic core is obtained by hardening or curing a mixture of magnetic powder and resin. The magnetic core shows a superior DC bias characteristic which does not become drastically saturated but is gently saturated even beyond 1000*10 3 /4π [A/m]. Therefore, the magnetic core has sufficient relative permeability more than ten.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a magnetic core and a coil component using the same. In particular, his invention relates to the magnetic core for the coil component which is used as a reactor in a high-power system such as an energy control of a battery mounted on an electrically-powered car or a hybrid car including an electromotor and an internal-combustion engine.  
         [0002]     A known coil component is disclosed in JP-A2001-185421. The disclosed coil component is used for a low-power system. The disclosed coil component comprises a coil and first and second magnetic core members. The first magnetic core member includes magnetic metal powder of 50-70%, by volume, and thermosettable resin of 50-30%, by volume. The second magnetic core member is a dust core made of sintered ferrite body or magnetic metal powder. The first and the second magnetic core members are magnetically connected in series. The coil is embedded in the first magnetic core member.  
         [0003]     One of the purposes of JP-A 2001-185421 is to provide a magnetic component such as an inductor, a choke coil and a transformer, which is suitable for use in a large-current electronic component.  
         [0004]     However, note here that the term “large current” is a relative term. The actual target of an electric current range of JP-A 2001-185421 is from several amperes to several tens of amperes as disclosed in paragraph [0002] of JP-A 2001-185421. In addition, because a coil component is normally designed to have a better DC bias characteristic in its target electric-current range, i.e. the range from several amperes to several tens of amperes in JP-A2001-185421. Furthermore, according to conventional techniques, beyond the target electric-current range, its DC bias characteristic becomes drastically saturated and its relative permeability becomes lowered.  
         [0005]     On the other hand, in a high-power system such as an energy control of a battery mounted on an electrically-powered car or a hybrid car, a coil component is used in an electric current of two hundreds amperes or more. It is therefore conceivable that the coil component of JP-A 2001-185421 is not suitable for the high-power system.  
       SUMMARY OF THE INVENTION  
       [0006]     It is therefore an object of the present invention to provide a magnetic core, which is suitable for use in a high-power coil component, and to provide a coil component using the magnetic core.  
         [0007]     According to one aspect of the present invention, a magnetic core is a core obtainable by hardening a mixture of magnetic powder and resin. The magnetic core shows a superior DC bias characteristic which does not become drastically saturated but is gently saturated even beyond 1000*10 3 /4π [A/m]. Therefore, the magnetic core has sufficient relative permeability more than ten at a magnetic field of 1000*10 3 /4π [A/m].  
         [0008]     According to another aspect of the present invention, a coil component comprises the aforementioned magnetic core and a coil wound around the magnetic core.  
         [0009]     According to another aspect of the present invention, another coil component comprises the magnetic core made of the mixture and a coil, wherein the magnetic core is arranged in the vicinity of the coil to constitute at least one part of a magnetic path in relation to the coil.  
         [0010]     According to another aspect of the present invention, another coil component comprises the magnetic core made of the mixture and a coil, wherein at least one part of the coil is embedded in the magnetic core.  
         [0011]     An appreciation of the objectives of the present invention and a more complete understanding of its structure and a fabrication method thereof may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS:  
       [0012]      FIG. 1  is a graph showing a DC bias characteristic of a magnetic core according to an embodiment of the present invention, wherein the magnetic core is made of a mixture of resin and magnetic powder;  
         [0013]      FIG. 2  is a perspective view showing a coil component using the magnetic core made of the mixture;  
         [0014]      FIG. 3  is a perspective view showing another coil component using the magnetic core made of the mixture;  
         [0015]      FIG. 4  is a perspective view showing another coil component using the magnetic core made of the mixture, wherein another magnetic core is inserted in the magnetic core made of the mixture;  
         [0016]      FIG. 5  is a perspective view showing another coil component using the magnetic core made of the mixture, wherein a high magnetic reluctance member is inserted in the magnetic core made of the mixture;  
         [0017]      FIG. 6  is a cross-sectional view showing a structure of a coil insulated;  
         [0018]      FIG. 7  is a perspective view showing another coil component using the magnetic core made of the mixture, wherein the coil component is enclosed in a rectangular parallelepiped case; and  
         [0019]      FIG. 8  is a partially-sectional, perspective view showing another coil component using the magnetic core made of the mixture, wherein the coil component is enclosed in a spherical case. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]     According to an embodiment of the present invention, a magnetic core is made of a mixture of magnetic powder and resin. In detail, the magnetic core of the embodiment is a casting, which is obtainable by casting the mixture into a predetermined shaped container for molding. In consideration of the size of the high-power coil component, it is preferable that the mixture is composed of the materials which are capable of casting without any solvents.  
         [0021]     In this embodiment, the casting process is basically carried out without pressure or with reduction of pressure. Once the casting process is finished, the casting may be subjected to some pressure for the purpose of increasing the density of the magnetic core according to the present embodiment. There is no limitation on the mold shape, and the magnetic core of the mixture can be formed in any shapes.  
         [0022]     The magnetic powder is soft magnetic metal powder, especially, Fe base powder in this embodiment. Specifically, the Fe base powder is powder selected from the group comprising Fe—Si system powder, Fe—Si—Al system powder, Fe—Ni system powder and Fe system amorphous powder. In case of Fe—Si system powder, an average content of Si is preferably in a range of from 0.0 percent, by weight, to 11.0 percents, by weight, both inclusive. In case of Fe—Si—Al system powder, an average content of Si is preferably in a range of from 0.0 percent, by weight, to 11.0 percents, by weight, both inclusive; while another average content of Al is preferably in a range of from 0.0 percent, by weight, to 7.0 percents, by weight, both inclusive. In case of Fe—Ni system powder, an average content of Ni is in a range of from 30.0 percents, by weight, to 85.0 percents, by weight, both inclusive.  
         [0023]     In this embodiment, the magnetic powder is substantially spherical powder, which can be obtained by, e.g., gas atomization. The spherical or the almost spherical powder is suitable for increasing its filling factor or filling ratio in the mixture of the magnetic powder and the resin. In this embodiment, it is recommended that the spherical or the almost spherical powder has an average diameter of 500 μm or less as the most normal diameter in its particle size distribution. The magnetic powder may be non-spherical powder such as powder obtained by another intentional gas atomization or indefinitely-shaped powder obtained by water atomization, when its anisotropy is used. If the magnetic powder of non-spherical powder or indefinitely-shaped powder is used, the mixture of the magnetic powder and the resin is subjected to an anisotropic alignment under the predetermined magnetic field before the mixture becomes completely hardened.  
         [0024]     In this embodiment, the resin is epoxy resin. In this embodiment, the epoxy resin is required to be liquid which has a small coefficient of viscosity. Therefore, the mutual solubility of resin and additives, hardenings or catalysts and the lifetime of the resin, in particular, are important items to be considered in deciding the actual epoxy resin. Based on the considerations, it is preferable that the base compound is selected from the group of bisphenol A epoxy resin, bisphenol F epoxy resin, polyfunctional epoxy resin and so on, while the hardener or curing agent is selected from the group of aromatic polyamine system, carboxylic anhydride system, initiative hardener system and so on. In this embodiment, bisphenol A epoxy resin is selected as a base compound of resin, and low-viscosity solventless aromatic amine liquid is selected as a hardener.  
         [0025]     The resin may be another thermosettable resin such as silicone resin. Also, the resin may be another curable or hardenable resin such as light-curable or photo-settable resin, ultraviolet curable resin, chemical-reaction curable resin, or the like.  
         [0026]     In consideration of fluidity of the mixture of the resin and the magnetic powder, the mixing ratio of the resin in the mixture is in a range of from 20 percents, by volume, to 90 percents, by volume, both inclusive. Preferably, the mixing ratio is in a range of from 40 percents, by volume, 5 to 70 percents, by volume, both inclusive.  
         [0027]     The magnetic core has an elastic modulus of 3000 MPa or more. The resin is selected such that, in case of the magnetic core has the foregoing elastic modulus under a specific condition, the resin has an elastic modulus of 100 MPa or more if only the resin is hardened in accordance with the specific condition. The value of the elastic modulus of the magnetic core or the hardened resin is measured in accordance with a standard of measurement called JIS K6911 (Testing methods for thermosetting plastics).  
         [0028]     In this embodiment, the magnetic core has the elastic modulus of 15000 MPa. The resin is selected such that the hardened resin has 1500 MPa if only the resin is hardened under the same condition where the mixture is hardened to have the elastic modulus of 15000 MPa. When the magnetic core has the elastic modulus of 15000 MPa or more, its thermal conductivity drastically becomes better. Specifically the thermal conductivity becomes 2[WK −1 m −1 ]. Therefore, it is preferable that the magnetic core has the elastic modulus of 15000 MPa or more.  
         [0029]      FIG. 1  shows a DC bias characteristic of the magnetic core made of the mixture of Fe—Si system powder and epoxy resin. The mixing ratio of the epoxy resin in the mixture is 50 percents, by volume. Namely, the Fe—Si system powder has mixing ratio of 50 percents, by volume. From  FIG. 1 , it is clearly seen that the DC bias characteristic of the mixture of the embodiment does not drastically saturated and has high relative permeability μ e  over fifteen even at a magnetic field of 1000*10 3 /4π [A/m].  
         [0030]     The above-mentioned magnetic core can be modified as far as the magnetic core has relative permeability of 10 or more at a magnetic field of 1000*10 3 /4π [A/m]. For example, each of particles of the magnetic powder may be provided with a high permeability thin layer, such as a Fe—Ni base thin layer. The high permeability thin layer is formed on a surface of each particle of the magnetic powder. Also, each of particles of the magnetic powder may be coated with at least one insulator layer in advance of the mixing of the powder and the resin. In case of the magnetic powder particle with the high permeability thin layer, the insulator layer is formed on the high permeability thin layer. The mixture of the resin and the magnetic powder may further include non-magnetic filler such as filler selected from the group comprising glass fiber, granular resin, and inorganic material base powder, which includes silica powder, alumina powder, titanium oxide powder, silica glass powder, zirconium powder, calcium carbonate powder and aluminum hydroxide powder. Also, the mixture of the resin and the magnetic powder may include a small amount of permanent magnetic powder.  
         [0031]     Next explanation will be directed to a coil component using the above-mentioned magnetic core with reference to FIGS.  2  to  8 .  
         [0032]     A first coil component  100  shown in  FIG. 2  is a toroidal magnetic core  10  made of the above-mentioned mixture and a coil  20  wound around the magnetic core  10 .  
         [0033]     A second coil component  110  shown in  FIG. 3  is one of modifications of toroidal coil component. The coil  20  is completely embedded in the magnetic core  10  made of the mixture, except for end portions  21 ,  22  of the coil  20 . The coil  20  may be partially exposed out of the magnetic core  10 .  
         [0034]     A third coil component  120  shown in  FIG. 4  is another modification of toroidal coil component, which comprises a specific magnetic core member  30  in addition to the magnetic core  10  made of the aforementioned mixture and the coil  20 . The coil  20  is completely embedded in the magnetic core  10  made of the mixture, except for end portions  21 ,  22  of the coil  20 . The coil  20  is wound around the specific magnetic core  30  which is also completed embedded in the magnetic core  10 . As far as the specific magnetic core  30  constitutes one part of the magnetic path in relation to the coil  20 , the specific magnetic core  30  can be disposed anywhere. For example, the specific magnetic core member  30  can be disposed around the coil  20  and/or within a hollow portion or inner portion of the coil  20 . The hollow portion or inner portion of the coil  20  is also referred to as a magnetomotive force portion.  
         [0035]     Preferably, the specific magnetic core member  30  is fixed to the coil  20  by means of the magnetic core  10  made of the mixture. Also, it is preferable that the specific magnetic core member  30  is a dust core made of powder selected from the group comprising Fe system amorphous powder, Fe—Si system powder, Fe—Si—Al system powder and Fe—Ni system powder, or a laminated core made of Fe base thin sheets.  
         [0036]     A fourth coil component  130  shown in  FIG. 3  is another modification of toroidal coil component, which comprises a high magnetic reluctance member  40 . The high magnetic reluctance member  40  has a magnetic reluctance higher than the mixture, i.e. the material of the magnetic core  10 . The high magnetic reluctance member  40  is inserted into the magnetic path formed by the coil  20  so that the magnetic fluxes due to the coil  20  penetrate the high magnetic reluctance member  40 . In other words, the illustrated high magnetic reluctance member  40  is placed within the hollow portion of the coil  20 . The illustrated high magnetic reluctance member  40  is embedded in the magnetic core  10  made of the mixture. For example, the high magnetic reluctance member  40  is made of a material which comprises the same resin as the resin of the mixture. In addition, the high magnetic reluctance member  40  may be made of another material comprising the same resin as the resin of the mixture and magnetic powder as far as the high magnetic reluctance member  40  has the magnetic reluctance higher than the magnetic core  10 .  
         [0037]     The high magnetic reluctance member  40  constitutes a region which has relative permeability of  20  or less within the magnetic core  10  made of the mixture.  
         [0038]     As shown in  FIG. 6 , the coil  20  may be enclosed by an insulator  50  to ensure insulation between turns of the coil  20 . The illustrated insulator  50  comprises a bobbin  60  and a cylindrical cover  70 . The bobbin  60  has on its peripheral part thereof a spiral groove  61 . Neighboring spiral turns of the groove  61  constitute the separations  62  of the turns of the coil  20 . The coil  20  is accommodated in a space defined by the spiral groove  61  and the cylindrical cover  70 . Therefore, if there are two or more coils  20 , they can be insulated from each other.  
         [0039]     Preferably, the material of the insulator  50  is the same resin as that of the mixture. The insulator  50  may be molded by using the same material. In addition, the illustrated coil  20  is an edgewise coil but may be another type coil such as a toroidal coil.  
         [0040]     A fifth coil component  140  shown in  FIG. 7  further comprises a case  80 , which has a rectangular parallelepiped shape, although its upper surface is omitted in  FIG. 7  for the sake of better understanding. The coil  20  of the fifth coil component  140  is an edgewise coil. The coil  20  is arranged within the case  80  The magnetic core  10  made of the mixture is filled between the coil  20  and the case  80  and encapsulates the coil  20  therein. For example, the case  80  is made of metal such as aluminum alloy or Fe—Ni alloy. It is preferable that, on the inner surface of the metal case  80 , an insulation layer is formed The case  80  may be a ceramic case such as an alumina mold.  
         [0041]     A six coil component  150  shown in  FIG. 8  also has a case  84  but the shape of the case  84  is spherical. In detail, the case comprises a metal container  82  and an insulator layer  84  formed on the inner surface of the metal container  82 . The metal container  82  is made of aluminum alloy or Fe—Ni alloy.  
         [0042]     In every coil component  100 ,  110 ,  120 ,  130 ,  140 ,  150 , the magnetic core  10  made of the mixture constitutes a loop of a magnetic path passing a center of the coil  30 . In every coil component  100 ,  110 ,  120 ,  130 ,  140 ,  150 , the magnetic core  10  constitutes at least one part of a magnetic path in relation to the coil  20 .  
         [0043]     The preferred embodiments of the present invention will be better understood by those skilled in the art by reference to the above description and figures. The description and preferred embodiments of this invention illustrated in the figures are not to intend to be exhaustive or to limit the invention to the precise form disclosed. They are chosen to describe or to best explain the principles of the invention and its applicable and practical use to thereby enable others skilled in the art to best utilize the invention.  
         [0044]     While there has been described what is believed to be the preferred embodiment of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the sprit of the invention, and it is intended to claim all such embodiments that fall within the true scope of the invention.