JPH0822911A - Stressed magnetic core - Google Patents

Abstract

PURPOSE:To provide a stem magnetic core wherein the permeability of a high frequency magnetization is enhanced over a superposed DC magnet field over a wide range in the magnetic core for an inductor such as switching power supply etc. CONSTITUTION:In the magnetic core comprising Fe-Ai alloy foil having Goss orientation, the compresion stress exceeding the elastic deformation range is imposed on the foil. Through these procedures, the magnetic core having the permeability in a specific value upto a high superposed DC magnetic field of 6kA/m can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic core having excellent DC superposition characteristics, which is used for an inductor element or a choke coil of electric equipment such as a switching power supply and an uninterruptible power supply used at high frequencies.

[0002]

2. Description of the Related Art An inductor element is used in a switching power supply as a smoothing choke coil for smoothing current on the secondary side. It is also used as a noise filter to remove high-frequency noise caused by semiconductor switches. The inductor element is formed by winding a magnetic core having a shape such as toroidal or EI.

In the case of the toroidal shape, between the magnetic permeability μ (relative magnetic permeability with respect to the magnetic permeability of vacuum) which is a physical property value of the magnetic core and the inductance value L which is a characteristic value of the inductor element, L = There is a relationship of μ ₀ μAn ² / l. Here, μ ₀ is the magnetic permeability of vacuum, A is the cross-sectional area of the magnetic core,
n is the number of windings and l is the magnetic path length. If the magnetic permeability is high, the number of windings or the cross-sectional area of the magnetic core can be reduced, so that the inductor element can be downsized.

A current obtained by superposing a high frequency current on a direct current is input to the winding of the inductor element. Therefore, the magnetic core receives a high-frequency magnetic field with a small amplitude while being biased with a DC magnetic field. In order to use the inductor element with a higher DC current, it is necessary that the magnetic characteristics of the magnetic core are not saturated with the DC magnetic field generated by the current. That is, the unsaturated characteristic of the magnetic core is required such that the magnetic flux density of the magnetic core material continuously increases with respect to the DC magnetic field.

Means for imparting unsaturated characteristics to the magnetic core include:
Generally, a gap is provided between the cores so that a demagnetizing field acts. However, this method not only has the problem of lengthening the processing steps such as making cut cores and adjusting gaps, but is also not suitable for manufacturing the small cores required for switching power supplies. There is also a problem that core loss increases due to the formation of the gap.

Dust cores and amorphous cores are used to obtain unsaturated characteristics without providing a gap in the magnetic core. The dust core is formed by compacting a metal powder such as Fe-Se-Al alloy, and has an unsaturated characteristic due to the demagnetizing field of the powder particles (see Japanese Patent Publication No. 62-21041). However, there are problems that the obtained value of magnetic permeability is low and that the value of magnetic permeability cannot be controlled. On the other hand, the amorphous core is based on the composition of Fe-Si-B, and the unsaturation of the magnetic core is extracted by partial crystallization (see Japanese Patent Laid-Open No. 5-255820). However, there is a problem in that the crystallization makes the foil brittle, which makes it difficult to manufacture the core.

In order to obtain a high magnetic permeability over a wide range of superimposed DC magnetic fields, it is necessary that the saturation magnetic flux density be high. The saturation magnetic flux density of Fe-Si-B system amorphous is 15 kG, and that of Fe-3% Si alloy is 20 kG.
Therefore, in principle, the Fe—Si magnetic core has better DC superposition characteristics.

The inventors of the present invention have previously conducted Fe-Si in the Goss orientation.
We have shown that by winding an alloy foil, an inductor element with good DC superposition characteristics can be obtained. However, there is no regulation concerning the range of stress, and it was not mentioned that a constant magnetic permeability can be obtained even in a high DC magnetic field by applying a stress exceeding the elastic deformation range.

[0009]

A magnetic core for an inductor element is used for high frequency excitation under superposition of a DC magnetic field. Therefore, it is an object of the present invention to provide a magnetic core for an inductor element used in a switching power supply, especially under superposition of a high DC magnetic field.

[0010]

Means for Solving the Problems In the present invention, Fe- in which the plate plane orientation of the crystal grains is (110) and the rolling direction is [001].
A stress-applied magnetic core characterized in that a compressive stress exceeding an elastic deformation range is applied in a rolling direction in a magnetic core made of a Si alloy foil. The present invention uses a method of bending a foil as one of means for applying a stress, and by applying a compressive stress exceeding the elastic range, a magnetic core having a constant magnetic permeability up to a high DC superimposed magnetic field can be obtained.

The present invention will be described in detail below. In order to obtain a certain degree of magnetic permeability when a DC magnetic field is superimposed, the magnetic core must be provided with unsaturated characteristics. As a means for this, the present inventors have utilized magnetostriction, which is one of the important properties of magnetic materials. Regarding the Goss orientation of the grain-oriented silicon steel sheet, the rolling surface is (110) and the rolling direction is [001]. The [001] direction of the rolling direction is the easy axis of magnetization, and the magnetic flux density in that direction is extremely sensitive to stress. [001]
When a stress σ is applied in the direction, the magnetostriction constant in the [001] direction is set to λ ₁₀₀ , and E _m = −3 / 2λ by the magnetoelastic effect
An energy change of ₁₀₀ σ occurs. Silicon steel [0
Since the magnetostriction constant λ ₁₀₀ is positive in the [01] direction, E _m > 0 by applying compressive stress (σ <0), and the in-plane direction perpendicular to the rolling direction becomes the easy magnetization direction. That is, the rolling direction is the direction in which the magnetization is difficult, which makes it possible to obtain the unsaturated characteristics of the magnetic core.

In the present invention, bending stress is applied to the foil by winding the foil in a toroidal shape. Compressive stress acts inside the center line of the cross section of the bent foil, and tensile stress of the same magnitude acts outside. The inventors of the present invention have stated that in the portion of the foil where pressure stress is applied, the easy magnetization direction is perpendicular to the rolling direction, and the magnetic flux density increases with a constant inclination with respect to the application of the magnetic field in the rolling direction. I have knowledge.
On the other hand, in the portion outside the center line where the tensile stress is applied, the rolling direction is the easy axis of magnetization, and the magnetic flux density is saturated immediately when a small magnetic field is applied. As a result of these two regions coexisting in a bent foil, the high-frequency excitation permeability when a DC magnetic field is superposed shows a very large value in a small magnetic field near zero, and is constant in a certain range of a DC magnetic field higher than that. The magnetic permeability of

In the present invention, in order to evaluate the direct current superposition characteristic of the magnetic core, the average magnetic permeability of the portion where the magnetic permeability is constant with respect to the direct current magnetic field coming from the compressive stress portion is defined as μ _p, and the portion where the magnetic permeability is constant is set. The magnetic field (anisotropic magnetic field) at which is ended is defined as H _a . H
_The value a was a value at which the magnetic permeability was 80% of μ _p . The high magnetic permeability near the zero magnetic field from the tensile stress part is a useful characteristic as a smooth choke coil for a self-excited switching power supply.

As a result of evaluation by various wound cores, the present inventors have found that when the compressive stress on the surface of the foil exceeds the range of elastic deformation of the material, the magnetic permeability μ _p and the anisotropic magnetic field H _a are the compressive stress σ. It has been found that (or strain ε) becomes less dependent. This property is practically important when making a ring-shaped core having a large wall thickness. This is because, when stress is applied by the winding operation, the stress applied to the side closer to the inner diameter of the core and the stress applied to the side closer to the outer shape may differ greatly. In the range of elastic deformation, the DC superimposition characteristic of magnetic permeability greatly changes depending on the stress.Therefore, when making a thick core as described above, the specifications differ greatly between the inner diameter side and the outer diameter side. Become. Such problems can be avoided by making the core within the range of plastic deformation.

The present inventors have found that by applying a compressive stress exceeding the elastic range, it is possible to manufacture a magnetic core having a high magnetic permeability of around 100 up to a high superimposed DC magnetic field of up to 6 kA / m. It came to completion.

The details of the present invention will be described below with reference to specific examples. The Si content of the Fe-Si alloy foil used in the present invention is
The weight percentage is preferably 1 to 6%. Because most of iron loss at high frequency consists of overcurrent loss,
This is because the iron loss decreases as the Si content increases and the electrical resistance increases. On the other hand, if the Si content is less than 1%, the electrical resistance is low and the iron loss is large, so that it cannot be put to practical use. Further, in the present invention, the effect of magnetostriction is utilized in order to bring out the direct current superposition characteristic, and the Si content is preferably 6% or less at which a positive magnetostriction constant can be obtained in the Fe-Si alloy foil.

The thickness of the foil is preferably 10 to 200 μm. This is because it is difficult to make a foil having a thickness of less than 10 μm by rolling, and a foil having a thickness of more than 200 μm has a large iron loss and cannot be put to practical use.

When a grain-oriented electrical steel sheet (thickness: 0.15 to 0.4 mm) of a Fe-Si alloy having a Goss orientation is rerolled and annealed under appropriate conditions, fine crystals are formed by primary recrystallization. It is known to return to a Goss orientation composed of grains (Wada et al., The Japan Institute of Metals, 40, 1158 (1976)).
By utilizing this phenomenon, an Fe-Si alloy foil having a desired Goss orientation can be obtained.

In order to obtain a foil having an optimum Goss orientation, the rolling rate is preferably 50% or more, and the heat treatment temperature for recrystallization is preferably 700 ° C. or more and 1200 ° C. or less. Especially when it is necessary to reduce the iron loss, the heat treatment temperature is set to 800 to 900 ° C. and the crystal grain size is set to 20 to
It is effective to grow to 100 μm. The heat treatment atmosphere may be a condition that does not participate, and argon, nitrogen, hydrogen, or a mixed atmosphere of two or more gases thereof can be used.

Next, the obtained Goth-oriented foil is annularly wound to form a toroidal core. When a foil is used as a wound core, insulation is necessary to prevent electrical conduction between layers. For the insulating treatment, an oxide such as Al ₂ O ₃ may be applied to the foil surface before the recrystallization heat treatment, or may be applied after the heat treatment.
More simply, a silicate-based resin may be coated as an insulating film after heat treatment.

The method for controlling the stress of the foil will be described below. A compressive stress can be easily generated by winding the foil. The undistorted foil before winding may be straight, but if the foil has a curvature, the range of stress control is expanded. Specifically, recrystallization heat treatment is performed in the state of a wound body having an average diameter d _i to prepare a foil without distortion and having a curvature d _i / 2. The average diameter d of the foil after the recrystallization heat treatment is
_It may be wound around the diameter of _i and heat treated again for strain relief. When this is rewound on a toroidal core having an average diameter D _f , a strain of ε = t (l / d _f ± 1 / d _i ) can be given to the surface of the foil.

Here, the reference numerals in parentheses are negative when rewinding is performed in the same direction, and are positive when rewinding is performed in the opposite direction. Assuming elastic deformation, a compressive stress of σ = Eε is applied to the inner surface of the foil (E is Young's modulus), and a tensile stress of the same magnitude is applied to the outer surface of the foil. In the plastic deformation range, the stress does not follow the above formula, but even in this case, the strain becomes a criterion for judging whether the strain is within the elastic deformation range or outside the range.

The yield stress of Fe-3% Si alloy is 37 kg.
/ Mm ² , and the Young's modulus in the [001] direction is 1.27.
It is x10 ⁴ kg / mm ² . Therefore, Fe-Si
The elastic limit stress of the alloy can be set to about 40 kg / mm ^2, and the corresponding strain ε is 0.31%.

[0024]

EXAMPLES The present invention will be further described below based on examples. Example 1 A 280 [mu] m thick grain-oriented electrical steel sheet having a Goss orientation of Fe-3% Si was cold-rolled to obtain a 50 [mu] m thick foil. Next, the foil was cut so that the rolling direction was the longitudinal direction, and a ribbon-shaped foil having a width of 5 mm was prepared. 85 in nitrogen
Heat treatment at 0 ℃ for 10 minutes, average crystal grain size 50μm
A Goss-oriented Fe-Si alloy foil composed of the above fine crystal grains was obtained. The heat treatment was performed in a state of being wound with an average diameter of 50 mm and 32 mm, and a foil having no stress of that diameter was prepared.

Next, a silicate resin is coated on the surface of the foil for insulation and is rewound to have an outer diameter of 18 m.
m core (thickness 1.5 mm) and outer diameter 28 mm core (thickness 1 mm) were produced. Here, when the heat-treated foil having a diameter of 32 mm was rewound on the actual core, the winding directions were opposite to each other. That is, the foil surface, which was the back side at the time of heat treatment, was made to come to the surface of the actual core. With this operation, 4
A stress in the plastic deformation range of 0 kg / cm ² or more is applied. The strains applied to the core at this time are ε = 0.34% and 0.46% corresponding to the size of the core. on the other hand,
When the heat-treated foil having a diameter of 50 mm was rewound on the actual core, the winding direction was the same direction. That is, the foil surface, which was on the front side during the heat treatment, was made to be on the front side even with the actual core. Under such conditions, stress within the elastic deformation range is applied. The strains applied to the core at this time are ε = 0.09% and 0.20% corresponding to the size of the measurement core.

Conductor winding is applied to the produced core, and LC
High frequency excitation (50k) when superimposing DC current by R meter
Hz) and the magnetic permeability μ was calculated from the measured inductance. The results of plotting the permeability μ against the applied DC magnetic field (DC superposition characteristics) are shown in FIGS. 1 and 2. FIG. 1 shows the measurement result of the core in the plastic deformation range, and FIG. 2 shows the measurement result of the core in the elastic deformation range.

As can be seen from FIG. 1, in the plastic deformation range, the DC superposition characteristics do not depend so much on the deformation amount, and the anisotropic magnetic field H
_{As a} , a value as high as 6 kA / m can be obtained. on the other hand,
As can be seen from FIG. 2, in the elastic deformation range, the DC superimposition characteristic greatly changes depending on the deformation amount, and a high value cannot be obtained even for H _a .

Example 2 As in Example 1, the thickness of the Fe-3% Si alloy was 50 μm.
The foil was heat treated as a wound body having an average diameter of 38 mm and 54 mm. The width of the foil was 11 mm. Next, these foils were rewound to produce a thick core having an inner diameter of 15 mm and an outer diameter of 27 mm. Here, when the core having a diameter of 38 mm was rewound, the core was wound in the opposite direction and a stress in the plastic deformation range was applied. Further, when the core having a diameter of 54 mm was rewound, the core was rewound in the same direction, and a stress within the elastic deformation range was applied. The DC superimposition specification of the rewound core was measured under the same conditions as in Example 1, and the results are shown in FIG.

As can be seen from FIG. 3, the core in the plastic deformation range exhibits a constant magnetic permeability over a wide range of DC magnetic field. The magnetic permeability of the core in the elastic deformation range changes greatly with respect to the DC magnetic field.

[0030]

In the conventional Fe-Si alloy foil, a gap is provided in the core in order to obtain a DC superposition characteristic.
The present invention derives the DC superposition characteristic by applying stress without a gap. Since there is no gap, the manufacturing process can be simplified and the magnetic core can be provided at low cost. Since the magnetic permeability does not depend on the stress in the plastic deformation range of the present invention, a constant magnetic permeability that does not depend on the part is obtained in a thick core in which the additional stress is significantly different between the inner diameter side and the outer diameter side of the core. be able to.

The stress-applied magnetic core of the present invention is 6 kA / m.
Since it exhibits high magnetic permeability up to a high magnetic field, it is possible to realize an inductor (or choke coil) that requires high inductance under high current conditions.

[Brief description of drawings]

FIG. 1 is a diagram showing a DC superimposition characteristic of magnetic permeability of a core in a plastic deformation range.

FIG. 2 is a diagram showing a DC superimposition characteristic of magnetic permeability of a core in an elastic deformation range.

FIG. 3 is a diagram showing DC superimposition characteristics of magnetic permeability of a core in a plastic deformation range and a core in an elastic deformation range.

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Claims (2)

[Claims] 1. A magnetic core composed of a Fe—Si alloy foil having a grain surface orientation of (110) and a rolling direction of [001], wherein a compressive stress exceeding an elastic deformation range is applied to the rolling direction. A stress-applied magnetic core characterized by being provided. 2. The compressive stress on the surface of the alloy foil is 4
The stress-applied magnetic core according to claim 1, wherein the stress-applied magnetic core is 0 kg / mm ² or more, and the compressive stress is generated by bending the foil.