Inductor devices – With closed core interrupted by an air gap
Reexamination Certificate
2001-11-28
2003-09-16
Mai, Anh (Department: 2832)
Inductor devices
With closed core interrupted by an air gap
C336S083000, C148S300000, C148S301000
Reexamination Certificate
active
06621398
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a magnetic core (which will hereinunder be often referred to as “core” simply) which is used in an inductance element such as a choke coil and a transformer for use in a switching power supply or the like and, in particular, to a magnetic core comprising a permanent magnet for magnetically biasing.
In a choke coke and a transformer used in, for example, a switching power supply or the like, a voltage is usually applied thereto with an AC component superposed to a DC component. Therefore, a magnetic core used in those choke coil and transformer is required to have a magnetic characteristic of a good magnetic permeability so that the core is not magnetically saturated by the superposition of the DC component. This magnetic characteristic will be referred to as “DC superposition characteristic” or simply “superposition characteristic” in the art.
As magnetic cores in application fields within high frequency bands, there have been used a ferrite magnetic core and a dust magnetic core. These magnetic cores have individual features due to physical properties of their materials. That is, the ferrite magnetic core has a high intrinsic magnetic permeability and a low saturated magnetic flux density while the dust magnetic core has a low intrinsic magnetic permeability and a high saturated magnetic flux density. Accordingly, the dust magnetic core is often used as one having a toroidal shape. On the other hand, the ferrite magnetic core has an E-shape core part having a central leg formed with a magnetic gap so as to prevent magnetic saturation from being caused by the superposition of the DC component.
Recently, since electronic parts are required to be small-sized as electronic devices are more compact-sized, the magnetic core with the magnetic gap is small-sized too. So, there is a strong demand for magnetic cores having an increased magnetic permeability against superposition of DC component.
Generally, it is necessary for the demand to select a magnetic core having a high saturation magnetization, that is, to select a magnetic core that is not magnetically saturated by a high magnetic field applied. However, the saturation magnetization is inevitably determined by materials and cannot be made as high as desired.
As a solution, it has been conventionally proposed to dispose a permanent magnet in a magnetic gap formed in a magnetic path of a magnetic core, that is, to magnetically bias the magnetic core, to thereby cancel a DC magnetic flux caused by the superposition of DC component.
The magnetic bias by use of the permanent magnet is a good solution to improve the DC superposition characteristic. However, this method have hardly been brought into a practical use for reasons as follows. More specifically, use of a sintered metallic magnet resulted in considerable increase of a core loss of the magnetic core. In addition, use of a ferrite magnet led in unstable superposition characteristic.
Means to resolve the problems is disclosed, for example, in Japanese Unexamined Patent Publication No. S50-133453 or JP 50-133453 A. This Publication uses, as a magnetically biasing magnet, a bond magnet comprising rare-earth magnetic powder with a high magnetic coercive force and binder which are mixed together with each other and compacted into a shape. Thereby, the DC superposition characteristic and temperature elevation of the core are improved.
Recently, a power supply has been more and more strongly required to improve its power transformation efficiency. Accordingly, this requirement has been became to a high level that it is difficult to determine good and bad of magnetic cores for choke coils and transformers by core temperatures measured. It is therefore inevitable to determine it from core loss data measured by use of a core-loss measuring device. According to the study by the present inventors, it was confirmed that the core loss has a degraded value in cores having the resistance value disclosed in JP 50-133453 A.
SUMMARY OF THE INVENTION
It is therefore a first object of this invention to provide, in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, easily and cheaply the magnetic core having an excellent DC superposition characteristic and an excellent core-loss characteristic in consideration of the above description.
In addition, there have recently been demands for coil parts of a surface-mounted type. Those coil parts are subjected to reflow soldering process so as to be surface-mounted on a circuit board. It is desired that a magnetic core of the coil part be not degraded in its magnetic properties under conditions of the reflow soldering process. Further, an oxidation-resistant rare-earth magnet is indispensable.
It is a second object of this invention to provide, in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, easily and cheaply the magnetic core which has an excellent DC superposition characteristic, an excellent core-loss characteristic, and oxidation resistance without affecting the characteristics under conditions of the reflow soldering process in consideration of the above description.
Furthermore, it is desired that not only magnetic powder has an improved oxidation resistance but also a rare-earth magnet has a high specific resistance.
It is a third object of this invention to provide, in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, easily and cheaply the magnetic core which has an excellent DC superposition characteristic, an excellent core-loss characteristic, oxidation resistance, and a high specific resistance in consideration of the above description.
According to a first aspect of this invention, in order to achieve the above-mentioned first object in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, there is provided the magnetic core comprising the magnetically biasing magnet, wherein the magnetically biasing magnet comprises a bond magnet comprising rare-earth magnetic powder and a binder resin, the rare-earth magnetic powder has an intrinsic coercive force of 5 kOe or more, a Curie temperature of 300° C. or more, and an average particle size of 2.0-50 &mgr;m, and the rare-earth magnetic powder consists of an aggregation of magnetic particles surfaced with a coating of a metallic layer containing an oxidation-resistant metal.
Preferably, the oxidation-resistant metal may be, for example, at least one metal or alloy thereof selected from a group of zinc, aluminum, bismuth, gallium, indium, magnesium, lead, antimony, tin.
Preferably, the bond magnet may comprise the binder resin content thereof which is 20% or more on the base of a volumetric percentage and the bond magnet may have a specific resistance of 1 &OHgr;·cm or more. The binder resin may be polyamideimide resin.
In addition, the magnetic powder preferably may comprise the oxidation-resistant metal content thereof which is 0.1-10% on the base of a volumetric percentage.
Furthermore, it is possible to obtain an inductance part by winding at least one winding by one or more turns on the above-mentioned magnetic core comprising the magnetically biasing magnet.
In addition, the inductance part includes a coil, a choke coil, a transformer, and other parts each of which generally essentially comprises a core and winding or windings.
Accord
Ambo Tamiko
Fujiwara Teruhiko
Hoshi Haruki
Ishii Masayoshi
Isogai Keita
Frishauf Holtz Goodman & Chick P.C.
Mai Anh
NEC Tokin Corporation
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