JP2007214425A - Powder magnetic core and inductor using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a powder magnetic core having a high insulation and magnetic permeability with a satisfactory frequency characteristic and a high filling factor of a magnetic material, and to provide an inductor with a high power efficiency using it. <P>SOLUTION: A molding body 1 is press molded with the use of an Fe-Si system soft magnetic alloyed powder including at least either Al or Cr, and having a flat shape of an aspect ratio of not less than two and having an insulating layer on its surface, together with a binder so that a flat direction of the soft magnetic alloyed powder is oriented perpendicular to the pressed direction 2. The powder magnetic core is formed with a plurality of molding bodies 1 laminated and bonded in the direction of the pressed direction 2. The soft magnetic alloyed powder is either one or two kinds of a mixture selected from the soft magnetic alloy having a composition of 82 to 88 wt.% Fe-8 to 10 wt.% Si-4 to 8 wt.% Al (1), and the soft magnetic alloy having a composition of 79 to 93 wt.% Fe-5 to 9 wt.% Si-2 to 12 wt.% Cr (2). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a dust core and an inductor using the same, and more particularly to a dust core suitable for a power supply circuit and an inductor using the same.

  In recent years, the demand for improvement in the power efficiency of elements used in these power supply circuits has increased due to the increase in current accompanying the performance enhancement of CPUs for notebook personal computers and PDAs.

  The inductor core used as a choke coil in these power supply circuits that require a large current requires a high saturation magnetic flux density to prevent magnetic saturation, so that the saturation magnetic flux density is higher than that of a ferrite material. A dust core in which magnetic metal powder and a binder are kneaded is used.

  However, the dust core produced by kneading with the binder has a reduced permeability because the interparticle distance is expanded by the binder, so when used as a magnetic core of a high inductance element, the number of turns can be increased, It was necessary to increase the size of the element itself. As a result, when the number of turns is increased in order to obtain a high inductance, there is a problem that the direct current resistance of the element increases due to an increase in the length of the conductor, resulting in a large copper loss and a problem that miniaturization is difficult. is there.

  On the other hand, Patent Document 1 proposes a method of obtaining high magnetic permeability by orienting flat powder so as to be parallel to the magnetic path direction.

  However, since the specific surface area of the powder surface is increased by flattening the spherical powder, the binder is not uniformly coated on the powder surface when the binder is kneaded, and the insulation is reduced, resulting in an increase in eddy current. However, there is a problem that the frequency characteristics of the initial permeability are deteriorated.

  In general, a thermosetting resin is used as the binder. In this case, the molded product of the flat powder and the resin binder is molded in comparison with the molded product of the normal spherical powder and the resin binder. There was a problem that the subsequent spring back was large, and the filling rate of the magnetic material in the molded body was extremely difficult to increase.

Japanese Patent Laid-Open No. 11-97288

  The object of the present invention is to solve the above problems, and provide a dust core having high insulation and good frequency characteristics, high magnetic material filling rate and high magnetic permeability, and a high power efficiency inductor using the same. There is to do.

  The present invention provides a dust core having a high initial permeability and good frequency characteristics obtained by forming an insulating layer on the surface of a flat powder containing at least one of Al and Cr, and bonding the molded body after split molding. This is a result of studying the production of an inductor in combination with a conductor having a short overall length and a low DC resistance.

  That is, the present invention uses a Fe—Si based soft magnetic alloy powder having a flat shape having an aspect ratio of 2 or more and containing at least one of Al and Cr, and a flat direction of the soft magnetic alloy powder. A powder magnetic core, wherein a plurality of compacts molded together with a binder so as to be oriented perpendicularly to the pressurizing direction are stacked and joined in the pressurizing direction.

  According to the present invention, the soft magnetic alloy powder comprises (1) a soft magnetic alloy having a composition of 82 to 88 wt% Fe −8 to 10 wt% Si −4 to 8 wt% Al, and (2) 79 to 93 wt%. The powder magnetic core according to any one of the above, wherein the powder magnetic core is any one kind or a mixture of two kinds selected from soft magnetic alloys having a composition of Fe-5 to 9 wt% Si-2 to 12 wt% Cr.

  Further, the present invention provides the above compacted powder, wherein the insulating layer is at least one of an Al or Cr passivated film formed by heat treatment in the atmosphere, or an acid-treated film using a solution containing phosphoric acid. It is a magnetic core.

  The present invention is also the above-described dust core, wherein the soft magnetic alloy powder has a powder filling rate of 70% or more, an initial permeability of 40 or more, and a magnetic resonance frequency of 1 MHz or more. Thus, by setting the powder filling rate to 70% or more, the saturation magnetic flux density is increased, and a dust core suitable for an inductor for large current is obtained. Further, by setting the initial permeability to 40 or more by flattening the soft magnetic alloy powder, it is possible to secure necessary inductance when the inductor is configured. Furthermore, by setting the magnetic resonance frequency to 1 MHz or more, the magnetic permeability in the high frequency band can be ensured.

  The present invention is also an inductor characterized in that a conductor is arranged so that a current flows in a direction perpendicular to the flat direction of the soft magnetic alloy powder through the inside of the powder magnetic core.

  Further, the present invention is the inductor described above, wherein the dust core is formed by joining the compact with a plane parallel to the flat surface of the soft magnetic alloy powder.

  The present invention is the above inductor, wherein the dust core has an initial permeability of 40 or more and a DC resistance of the conductor is 1 mΩ or less. Thus, by setting the initial permeability of the dust core to 40 or more and the direct current resistance of the conductor to 1 mΩ or less, a small and highly power-efficient inductor is obtained.

  The present invention is the inductor described above, wherein the number of conductors is one or two, and in the case of two conductors, the conductors are electromagnetically coupled.

  The powder magnetic core of the present invention has a high magnetic permeability because the demagnetizing field is reduced by orienting the flat surface of the flat powder in the magnetic path direction, and the eddy current is suppressed by the insulating layer on the powder surface. It becomes possible to use at.

  In addition, a powder magnetic core having a higher density than that of a conventional powder magnetic core made of flat powder can be obtained by dividing the powder magnetic core and then bonding it. By putting such a high-density powder magnetic core into practical use, a powder magnetic core having a high magnetic permeability and a high saturation magnetic flux density can be put into practical use. As a result, it is possible to improve the performance and size of the inductor.

  In addition, since the direct current resistance of the conductor is as low as 1 mΩ or less in the inductor of the present invention, copper loss due to the direct current resistance during operation can be kept low, and an inductor having extremely high power efficiency can be provided.

  Next, a specific example will be given to describe an embodiment of the present invention.

  FIG. 1 shows a perspective view of a molded body according to the present invention. In FIG. 1, 1 is a molded body, 2 is a pressing direction, and 3 is a cross section of the molded body.

  FIG. 2 shows a scanning electron microscope image of a flat powder of a soft magnetic alloy that is a raw material of the compact. Here, the Fe—Si base alloy having an average particle diameter of 20 μm produced by the water atomization method was flattened by pulverizing in toluene for 2 hours using a bead mill.

  The obtained flat powder of FIG. 2 is heat-treated at 600 ° C. in the air for 1 hour, and the powder having an insulating layer formed on the powder surface is kneaded with 5.0% by weight of a thermosetting resin, dried and crushed. A granulated powder having an average particle size of 200 μm was obtained.

  The obtained granulated powder was filled in a mold, pressure-molded at 700 MPa from the pressing direction 2, and thermoset at 150 ° C. to obtain a molded body 1.

  FIG. 3 shows a stereoscopic microscope image of the cross section 3 (see FIG. 1) of the molded body. It can be seen that the major axis direction of the flat powder is oriented in a direction perpendicular to the pressing direction. Note that a and b correspond to the positions in the cross section indicated in FIG.

  In FIG. 4, expressed by weight ratio, Fe-9.5% Si-2.0% Al, Fe-9.5% Si-4.0% Al, Fe-9.5% Si-5.5% Real part μ ′ of the complex permeability measured in the orientation direction of the flat powder of the compact produced using Al and Fe-9.5% Si-8.0% Al and the flat powder in which the Al addition amount was changed The frequency characteristics of are shown. In a molded body using a powder of Fe-9.5% Si-2.0% Al composition with a small amount of Al added, μ ′ is attenuated at 100 kHz or more. In contrast, Fe-9.5% Si-4.0% Al, Fe-9.5% Si-5.5% Al, and Fe-9.5% Si with an Al addition amount of 4.0% or more. -In the molded body using the powder having the Al composition of 8.0%, μ 'is attenuated at 5000 kHz or more, and the frequency is superior to that of the molded body manufactured using the powder having the Al addition amount of 2.0%. It has characteristics.

  In addition, in a molded body produced using a powder having an Al addition amount of 4.0% or more, a peak due to magnetic resonance is observed immediately before μ ′ attenuation, and it is assumed that μ ′ attenuation is due to magnetic resonance. A molded body produced using powder with Al addition amount of 2.0% does not show a peak of magnetic resonance just before the attenuation of μ ′, and it is considered that the frequency is attenuated by eddy current.

  In FIG. 5, Fe-6.5% Si-1.0% Cr, Fe-6.5% Si-2.0% Cr, Fe-6.5% Si-6.0% expressed by weight ratio. The frequency characteristics of μ ′ measured in the orientation direction of the flat powder of the compact produced using the flat powder in which Cr and Fe-6.5% Si-12.0% Cr and the added amount of Cr were changed are shown. In a compact using a powder of Fe-6.5% Si-1.0% Cr composition with a small amount of added Cr, μ ′ attenuates when the frequency is 10 kHz or more. In contrast, Fe-6.5% Si-2.0% Cr, Fe-6.5% Si-6.0% Cr, and Fe-6.5% Si with a Cr addition amount of 2.0% or more In the molded body using the powder of -12.0% Cr composition, μ ′ is attenuated at 5000 kHz or more, which is superior to the molded body manufactured using the powder having the Cr addition amount of 1.0%. It has a frequency characteristic.

  In addition, in a molded body produced using a powder having a Cr addition amount of 2.0% or more, a peak due to magnetic resonance is observed just before μ ′ attenuation, and it is assumed that μ ′ attenuation is due to magnetic resonance. A compact produced using a powder with 1.0% Cr addition does not show a magnetic resonance peak immediately before the attenuation of μ ′, and it is considered that the frequency is attenuated by an eddy current.

  Next, the relationship between the added amounts of Al and Cr and the specific resistance of the molded body is shown in FIG. It can be seen that the specific resistance of the molded body is remarkably increased when the Al addition amount is 4.0% or more and the Cr addition amount is 2.0% or more. As a result, it can be seen that the improvement of the frequency characteristic of μ ′ due to the addition of Al or Cr is due to the fact that the Al and Cr passivating films formed by the addition of Cr and Al have increased the specific resistance of the compact.

  For comparison, the Fe-9.5% Si-5.5% Al and Fe-6.5% Si-6.0% Cr powders were flattened with a bead mill and expressed in the weight ratio. FIG. 7 shows the frequency characteristics of μ ′ of the molded body made of the unheated powder granulated without heat treatment, together with μ ′ of the molded body made of the heat treated powder. In both 5.5% Al addition and 6.0% Cr addition, the molded body produced using the heat treated powder shows better frequency characteristics than the molded body produced using the unheat treated powder. It was. This is presumably because a passive film made of oxide of Al or Cr in the powder is formed on the surface of the powder by heat treatment in the atmosphere to improve insulation. From the above, compacts made from powders with an insulating layer formed by heat treatment of soft magnetic alloy powder containing Al 4.0% or more or Cr 2.0% or more have excellent frequency characteristics. Is clear. Moreover, you may contain both Al and Cr.

  In FIG. 8, by changing the grinding time by the bead mill, the amount of binder made of thermosetting resin is changed with respect to various Fe—Si—Al flat powders having different aspect ratios of 1 to 50 and flatness. Shows the relationship of the initial permeability μi when the filling rate of the magnetic powder in the molded body is changed. Although the μi of the molded body prepared using the aspect ratio 1 (spherical) powder before flattening is 25 to 30, the molded body manufactured using the flat powder weakly flattened to an aspect ratio of 2 by bead milling. Then, μi is greatly increased to 38-42. Furthermore, μi increases remarkably by further flattening. From the above, it was confirmed that if the aspect ratio of the flat powder is 2 or more, a sufficient effect for improving the magnetic permeability can be exhibited.

  However, as flattening progresses, the effect of springback increases, and the compact filling ratio decreases. Therefore, the compact using a flat powder with an aspect ratio that satisfies the desired characteristics in consideration of the magnetic permeability and saturation magnetic flux. It is necessary to produce.

  Therefore, FIG. 9 shows the magnetic powder filling rate after molding and curing of a molded body molded at a height different from 3 mm, 6 mm, and 12 mm using various aspect ratio powders and cured at 150 ° C. As the aspect ratio increases, the magnetic powder filling ratio in the molded body greatly decreases due to expansion by springback during curing. However, by reducing the height of the molded body, the spring back after molding is greatly reduced. The reason will be described with reference to the drawings. FIG. 10 shows the distribution of the magnetic powder filling rate inside the compacts having different heights, FIG. 10 (a) shows the case where the molding height is large, and FIG. 10 (b) shows the case where the molding height is small. . This is because, as shown in the figure, when the molded body height is smaller, the molding pressure is transmitted to the inside of the molded body, and a dense molded body having a high filling rate can be obtained.

  FIG. 11 shows the initial permeability of these compacts measured in the orientation direction of the flat powder. Since the influence of springback is small when the aspect ratio is low, the initial permeability of the molded body after molding and after curing is not much different, but in the molded body made of powder with an aspect ratio of 50, each molded body is thin. The smaller the decrease in permeability. This is because a compact with a high magnetic material filling rate and a small demagnetizing field is realized by the effect of reducing the spring back described above.

  Next, the above-described molded body 1 produced at various heights was bonded with an epoxy resin, so that dust cores joined as shown in a perspective view in FIG. 12 were produced. The dust core after bonding was kept constant at a height of 12 mm. Table 1 shows the height of each molded body, the number of bonded bodies, the density of the dust core, and the initial permeability.

  Since the magnetic powder filling rate is increased by reducing the height of the molded body as described above, the density of the dust core after bonding is also increased, and the initial permeability is accordingly increased. As described above, according to the present invention, it is possible to provide a powder magnetic core superior to a powder magnetic core using a conventional flat powder.

  Next, a linear structure type inductor shown in a perspective view in FIG. In the figure, 1 is a compact, 4 is a dust core, 5 is a flat copper wire, and 6 is an inductor. The inserted rectangular copper wire 5 is a straight rectangular copper wire having a thickness of 0.5 mm and a width of 2 mm and coated on the surface with a polyamideimide, and was bonded to the dust core with an epoxy resin.

  As a comparative example, a wound structure type inductor shown in a perspective view in FIG. 14 was fabricated using a spherical Fe—Si—Al powder and a rectangular conductor having a wound structure. In the figure, 7 is a wound flat rectangular copper wire, 8 is a dust core, and 9 is an inductor. The wound flat rectangular copper wire was manufactured using the same rectangular copper wire as described above with the number of turns of 3.5 turns.

  Table 2 shows the values of the inductance L (μH), the DC specific resistance Rdc (mΩ), and the copper loss Pcu (mW) at various current values.

  Both inductors have the same L value of 0.56 μH, but the Rdc value of the linear structure type of the present invention is as low as 0.42 mΩ. Since the value of the copper loss Pcu is also proportional to Rdc, it can be said that an extremely excellent inductor has been put into practical use.

  Next, a step-down DC-DC converter in which these inductors are mounted as choke coils was manufactured, and the power efficiency thereof was measured and shown in FIG. Although the power efficiency between the two is almost the same on the low current side, the linear structure type inductor of the present invention shows superior power efficiency compared to the conventional wound structure type inductor in a large current region of 10 A or more. ing. This is because the Rdc of the inductor of the present invention is extremely low, and thus it is possible to significantly reduce the copper loss in the large current region. The present invention as described above makes it possible to provide an inductor that greatly improves power efficiency.

  Further, in the present invention, as shown in a perspective view in FIG. 16, two conductors such as flat copper wires 11a and 11b are installed, and each conductor is electromagnetically coupled through a dust core with a coupling coefficient of 1 or less. It is also possible to adopt a four-terminal inductor structure that operates in the same manner.

  In the above embodiment, an example in which an insulating layer is formed on the surface of the soft magnetic powder particles by heat treatment in the atmosphere has been shown. It was separately confirmed that the same effect can be obtained even if formed.

The perspective view of the molded object which concerns on this invention. The scanning electron microscope image of the flat powder of the soft-magnetic alloy used as the raw material of the compact | molding | casting which concerns on this invention. Stereomicroscopic image of the cross section of the compact. The figure which shows the frequency dependence of (micro | micron | mu) 'of the molded object produced with Al addition flat powder. The figure which shows the frequency dependence of (micro | micron | mu) 'of the molded object produced with Cr addition flat powder. The figure which shows the specific resistance of the molded object with respect to Al and Cr addition amount. The figure which shows the frequency dependence of (micro | micron | mu) 'of the molded object produced with the heat processing powder and the non-heat processing powder. The figure which shows the relationship between (micro | micron | mu) i for every aspect ratio, and a magnetic powder filling rate. The figure which shows the magnetic powder filling rate of the powder magnetic core produced with various molded object height. FIG. 10A shows the distribution of the magnetic powder filling rate in the compacts having different heights, FIG. 10A shows a case where the molding height is large, and FIG. 10B shows a case where the molding height is small. The figure which shows the initial magnetic permeability of the powder magnetic core produced with various molded object height. The perspective view of the joined powder magnetic core which concerns on this invention. The perspective view of the linear structure type inductor concerning the present invention. The perspective view of the coil structure type inductor of a comparative example. The figure which shows the power efficiency of the DC-DC converter using the inductor of this invention. The perspective view of the 4-terminal inductor which has two conductors which concern on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Forming body 2 Pressing direction 3 Forming body cross section 4, 8, 10 Powder magnetic core 5, 11a, 11b Flat copper wire 6, 9 Inductor 7 Winding flat copper wire 12 4-terminal inductor

Claims (8)

  Fe-Si based soft magnetic alloy powder having a flat shape with an aspect ratio of 2 or more and containing at least one of Al and Cr and having an insulating layer on the surface is used, and the flat direction of the soft magnetic alloy powder is perpendicular to the pressing direction. A powder magnetic core comprising a plurality of compacts formed by pressure molding together with a binder so as to be oriented in a stacked manner, and stacked and joined in the pressure direction.   The soft magnetic alloy powder includes (1) a soft magnetic alloy having a composition of 82 to 88 wt% F −8 to 10 wt% Si − 4 to 8 wt% Al, and (2) 79 to 93 wt% Fe − 5 to 9. 2. The powder magnetic core according to claim 1, wherein the powder magnetic core is any one selected from soft magnetic alloys having a composition of wt% Si −2 to 12 wt% Cr, or a mixture of two kinds.   3. The insulating layer according to claim 1, wherein the insulating layer is at least one of an Al or Cr passivated film formed by heat treatment in the atmosphere, or an acid-treated film using a solution containing phosphoric acid. Powder magnetic core.   The powder filling rate of the soft magnetic alloy powder is 70% or more, the initial permeability is 40 or more, and the magnetic resonance frequency is 1 MHz or more, The magnetic resonance frequency according to any one of claims 1 to 3, Powder magnetic core.   A conductor is disposed so that a current flows in a direction perpendicular to the flat direction of the soft magnetic alloy powder through the inside of the dust core according to any one of claims 1 to 4. Inductor.   6. The inductor according to claim 5, wherein the dust core is formed by joining the compact with a plane parallel to the flat surface of the soft magnetic alloy powder.   The inductor according to claim 5 or 6, wherein an initial permeability of the dust core is 40 or more and a direct current resistance of the conductor is 1 mΩ or less.   The inductor according to any one of claims 5 to 7, wherein the number of the conductors is one or two, and in the case of two, the conductors are electromagnetically coupled.