The present invention relates to a magnetic element such as an conductor, a choke coil, a transformer, or the like in electronic equipment, particularly a miniature magnetic element used under a large current and to a method of manufacturing the same.
With the reduction in size and thickness of electronic equipment, the reduction in size and thickness of components and devices used therein also has been demanded strongly. On the other hand, LSIs such as a CPU and the like have come to be made up of an increasing number of circuit components and a current of several amperes to several tens of amperes may be supplied to a power circuit provided in the LSIs. Therefore, similarly an inductor such as a choke coil used therein has been required to reduce its size, to lower the resistance, although being contrary to the size reduction, by enlarging the cross-sectional area of a coil conductor, and not to lower the inductance greatly with DC bias. The operation frequency has come to be higher and therefore it has been required that the loss in a high frequency area is low. Furthermore, in order to reduce the cost, it has been necessary that component elements with simple shapes can be assembled in easy processes. In other words, it has been demanded that a miniaturized thin inductor that can be used under a large current and at a high frequency is provided at a low cost.
In the case where an inductor is formed by providing a winding around a toroidal core, the inductance of the inductor is expressed by the following formula:
Lxcx9cxcexcxc3x97S=N2/r
wherein L indicates inductance, xcexc magnetic permeability, S the cross-sectional area of a magnetic path, N the number of turns, and r the length of the magnetic path. From this formula, it is understood that a large value of L is obtained when the magnetic permeability xcexc, the cross-sectional area S of a magnetic path, and the number of turns N are increased and the length r of the magnetic path is reduced. However, when the magnetic permeability is increased, the magnetic flux density is saturated even at a small current value. The magnetic permeability is decreased at higher current values, thus deteriorating the DC bias characteristics (the inductance value (L) characteristics dependent on a direct current). The enlargement of the cross-sectional area of the magnetic path is contrary to the size reduction and results in a long lead wire in the case of the same number of turns, thus causing a high resistance. The use of a lead wire with a large cross-sectional area to prevent this further goes against the size reduction. The increase in number of turns is contrary to the size reduction and also causes a high resistance. To shorten the magnetic path leads to the size reduction but the number of turns cannot be increased in that case. Therefore, generally it has been difficult to obtain a miniature inductor that has a high inductance, excellent DC bias characteristics, and a low resistance in a winding and that can be used not only at low frequencies but also at high frequencies.
An inductor that has been used practically will be described as follows.
In an EE-type or EI-type ferrite core and a coil that have been used most commonly, because a ferrite material has a relatively high magnetic permeability and a lower saturation magnetic flux density compared to that of a metallic magnetic material, the inductance is decreased greatly due to the magnetic saturation when the ferrite material is used without being modified, resulting in poor DC bias characteristics. Therefore, in order to improve the DC bias characteristics, usually such a ferrite core and a coil have been used by providing a gap in any position in a magnetic path of the core to decrease the apparent magnetic permeability.
In an inductor in which a Fexe2x80x94Sixe2x80x94Al based alloy, a Fexe2x80x94Ni based alloy, or the like that has a higher saturation magnetic flux density than that of ferrite is used as a core material, because such a metallic material has a low electrical resistance, the increase in high operation frequency to several hundreds of kHz to MHz as in the recent situation results in the increase in eddy current loss and thus the inductor cannot be used without being modified. Therefore, a so-called dust core has been used, which is obtained by superposing members, which have been formed to have thin bodies, via an insulating layer or which is formed using a pulverized material that is insulated.
It also has been proposed to combine and use a plurality of magnetic bodies. One obtained by winding a coil around a ferrite core with rib and then dipping them into a mixed solution of magnetic powder and a resin material (JP-A-61-136213) and one obtained by preparing two members formed through the superposition of a plurality of thin magnetic metal bodies, providing a planar coil between the two members, and fixing magnetic powder with a dispersed adhesive (JP-A-9-270334) have been described as being effective for reducing the size of an inductor. In addition, one obtained by providing a planar coil between two ferrite sheets and fixing ferrite powder with a dispersed adhesive in order to reduce the leakage flux has been proposed (JP-A-6-342725), although it is not described as achieving the size reduction.
With respect to the configurations of inductors, many conventional inductors have been formed of an EE or EI type core and a coil. However, in order to obtain a thin inductor, JP-A-9-92540 describes using one formed by winding the coil spirally in a plane. Further, JP-A-9-205023 describes that the terminal on the internal circumference side (hereinafter referred to as an xe2x80x9cinner terminalxe2x80x9d) of a spirally wound coil is lead out by providing a cutout in a core, so that the thickness corresponding to that of the lead wire is reduced.
However, when a ferrite material is used and a gap is provided anywhere in a magnetic path to decrease the apparent magnetic permeability, there has been a problem that a core vibrates in this gap portion when being operated with an alternating current, thus generating noise.
When thin metallic magnetic bodies with a high saturation magnetic flux density are superposed via insulating layers, the thin bodies that can be used at high frequencies should be formed to be sufficiently thin. Therefore, the cost increases and no complicated shape can be formed, which have been problems. Further, in order to obtain a dust core with characteristics good enough, it is necessary to make the dust core dense by the application of a very high pressure of about 10t/cm2 in a molding process. Therefore, there have been problems that a special high-strength mold is required and complicated shapes are formed with difficulty.
In the types disclosed in JP-A-61-136213 and JP-A-6-342725 that are included in the types in which a plurality of magnetic bodies are combined and used, a member obtained by dispersing ferrite in a resin is used. However, since there is a limitation in the filling rate of the ferrite, there has been a problem that the saturation magnetic flux density of this member is low and therefore the DC bias characteristics are poor. Furthermore, in the type disclosed in JP-A-9-270334, the kind of the magnetic body to be mixed with resin is not described, but it is necessary to prepare a member formed by superposing a plurality of thin magnetic metal bodies in all cases, resulting in a high cost. In addition, since the upper and lower surfaces of an element are formed of metallic magnetic bodies, the electrical resistance is low and therefore insulation is required, and complicated shapes cannot be formed, which also have been problems.
The present invention seeks to provide a magnetic element, such as an inductor, a choke coil, a transformer, or the like, that is suitable for the use under a large current in various types of electronic equipment.
A magnetic element of the present invention includes: a composite magnetic member A containing a metallic magnetic powder in an amount of 50-70 vol. % and a thermosetting resin in an amount of 50-30 vol. %; a magnetic member B that is a ferrite sintered body or a pressed-powder magnetic body of the metallic magnetic powder; and a coil. A magnetic path determined by the arrangement of the coil passes the magnetic member A and the magnetic member B in series. The coil is embedded in the magnetic member A.
In the magnetic element of the present invention, it is preferable that the gaps in the coil are filled with the magnetic member A. Further, it is preferable that the coil is wound around the magnetic member B.
It is preferable that the magnetic member B is positioned outside the magnetic member A in which the coil is embedded. In this case, it is further preferable that a plurality of plate-like magnetic members Bare included and are spaced from one another at 500-xcexcm intervals or less, particularly at 300-xcexcm intervals or less, the magnetic member A in which the coil is embedded is arranged in the intervals, and the coil is formed of a conductor wound in a planar shape.
Furthermore, it is preferable that an oxide insulating layer is formed on the surface of the metallic magnetic powder contained in the magnetic member A. In this case, it is further preferable that the metallic magnetic powder contained in the magnetic member A contains Fe as the main component and Al, and the oxide insulating layer on the surface of the metallic magnetic powder is an insulating layer containing aluminum oxide as the main component, which is formed by a heat treatment in the presence of oxygen.
In this specification, the main component denotes a constituent accounting for at least 50 wt. %.
The present invention also provides a method of manufacturing the above-mentioned magnetic element. A first method of manufacturing the magnetic element according to the present invention includes: preparing a paste containing magnetic powder and thermosetting resin; filling gaps around the coil with the paste; and forming the magnetic member A from the paste by curing the thermosetting resin through a treatment with heat.
A second method of manufacturing a magnetic element according to the present invention includes: preparing a slurry containing magnetic powder and thermosetting resin; forming an uncured composite sheet from the slurry; and forming the magnetic member A from the uncured composite sheet by curing the thermosetting resin through a treatment with heat and pressure.