JP2844337B2 - Power transformer core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a magnetic core such as a power transformer.

<Prior Art> Mn-Zn ferrite is used for a magnetic core of a power transformer used for a switching power supply or the like.

Meanwhile, in recent years, power transformers used for these devices have been required to be miniaturized, and thus the operating frequency has been increasing to a high frequency region of 500 kHz or more.

However, in such a frequency domain, the conventional Mn
-Zn ferrite results in a large power loss.

Then, the present inventors considered that TiO ₂ , Ta ₂ O ₅ , S
An ultra low loss ferrite containing iO ₂ and CaO has been proposed (Japanese Patent Application No. 61-205223).

<Problems to be Solved by the Invention> However, even with the above-described ferrite containing these subcomponents, the reduction of power loss is not sufficient, and a ferrite with lower loss is demanded.

An object of the present invention is to provide a transformer core for power supply with low loss.

<Means for Solving the Problems> Such an object is achieved by the present invention described below.

That is, the present invention is formed from a ferrite sintered body containing MnO, ZnO and Fe ₂ O ₃ as main components, has an average crystal grain size of 5 μm or less, and has a crystal grain size of 10 μm.
The number of crystal grains exceeding 3% is less than 3% of the total crystal grains, and the loss at 80 ° C, 1000 kHz and 50 mT is 410 mW / cm ³ or less. This is a transformer core for power supply used at an operating frequency of 500 kHz or more.

The ferrite sintered body is made of TiO ₂ , Ta ₂ O ₅ , SiO ₂
And at least one selected from CaO as an accessory component.

 Hereinafter, a specific configuration of the present invention will be described in detail.

Ferrite sintered body of the present invention MnO, a Mn-Zn ferrite as a main component of ZnO and Fe ₂ O _3, no particular limitation on the composition ratio, average grain size and particle size distribution as described above , The effect of the present invention is realized. However, in order to obtain a ferrite sintered body with lower loss, it is preferable that at least one, and preferably all, of the above-mentioned subcomponents are contained.

These subcomponents are preferably contained in the following range.

500 ppm ≦ TiO ₂ ≦ 6000 ppm 100 ppm ≦ Ta ₂ O ₅ ≦ 2000 ppm 150 ppm ≦ SiO ₂ ≦ 270 ppm 500 ppm ≦ CaO ≦ 2000 ppm The reasons for limiting the content of the subcomponents are as follows.

When the TiO ₂ falls below the above range, the loss increases. Further, when the ferrite sintered body is used as a power supply ferrite, heat is generated, but when the TiO ₂ is less than the above range, the temperature characteristic curve of the loss is shifted too much to the high temperature side, and low loss in a practical temperature range. I can't get anything.

If TiO ₂ exceeds the above range, the loss decreases, but the curve of the temperature characteristic of the loss shifts to a lower temperature side, which is not preferable as a ferrite for power supply accompanied by heat generation.

When Ta ₂ O ₅ is out of the above range, the loss increases.

When SiO ₂ is less than the above range, the specific resistance decreases and Q deteriorates.

When SiO ₂ exceeds the above range, the loss increases and the magnetic permeability decreases.

In some cases, SiO ₂ may be contained as an impurity in the raw material of the main component at about 100 ppm.

In this case, it is preferable that the amount of SiO ₂ added after the calcination is reduced by that amount so that the above range of SiO ₂ is contained after sintering.

If CaO is less than the above range, the crystal grain boundaries will be thin,
Eddy current loss increases.

 When CaO exceeds the above range, the loss increases.

When CaO is contained in the ferrite sintered body of the present invention, Ca in the crystal particles is preferably present in the following distribution.

That is, when performing Auger analysis while performing ion milling, or performing Auger analysis after each ion milling intermittently performing ion milling and measuring the Ca content profile, the Ca content gradually increases toward the peak or from the peak. At this time, the distance from the peak when the Ca content is halved with respect to the peak is 5
It is preferably about 10 °. Note that the peak of the Ca content is considered to coincide with the crystal grain boundary. like this
By having a Ca distribution, the losses are even lower.

The contents of the main components MnO, ZnO and Fe ₂ O ₃ are as follows: 31.1 mol% ≦ MnO ≦ 43.8 mol% 4.0 mol% ≦ ZnO ≦ 13.5 mol% 52.2 mol% ≦ Fe ₂ O ₃ ≦ 55.4 mol% Preferably, there is.

The composition of the ferrite sintered body is determined by chemical analysis or fluorescence X
It can be measured by line analysis or the like.

The ferrite sintered body of the present invention has an average crystal grain size of 5 μm.
Or less, and the number of crystal grains having a crystal grain size exceeding 10 μm is 3% or less of the entire crystal grains.

 When the average crystal grain size exceeds 5 μm, the loss increases.

Even if the number of crystal grains having a crystal grain size exceeding 10 μm exceeds 3% of the entire crystal grains, the loss increases.

 The average crystal grain size in this case is defined as follows.

First, the average of the cross-sectional areas of the crystal grains appearing in the cross section of the ferrite sintered body, that is, the cross-sectional area per crystal grain is determined. Next, the diameter of a sphere giving a great circle having the same area as this cross-sectional area is obtained. In the present invention, this value is defined as the average crystal grain size.

Further, the crystal particles having a crystal grain size exceeding 10 μm mean that the major axis of the cross section of the crystal particles appearing in the cross section of the ferrite sintered body exceeds 10 μm.

Such a measurement, for example, after a mirror-polished ferrite sintered body, etching with hydrochloric acid or the like, this is 500-1000
The measurement may be performed by using a photograph taken by a scanning electron microscope at a magnification of about 2 times and measuring at least an area of 2500 μm ² or more.

In order to obtain a ferrite sintered body with a lower loss, it is preferable to have the following crystal grain size distribution in addition to the above average grain size and crystal grain size distribution.

That is, when the above-mentioned measurement method is used, the area of crystal grains having a crystal grain size of 10 μm or more is 15% of the total area.
Or less, and the maximum crystal grain size is 15μ
Preferably, it does not exceed m. The crystal grain size in this case also
Similar to the above, the cross-sectional major axis of the crystal grain.

Such a ferrite sintered body can be obtained by a known ferrite sintering method, but in order to easily obtain the average crystal grain size and the crystal grain size distribution as described above, the ferrite sintered body must be manufactured by the following method. Is preferred.

First, ZnO is added to a mixed powder of Fe ₂ O ₃ and Mn ₂ O ₃ , which is preferably produced by spray roasting described in JP-B-47-11550 or the like, and mixed and pulverized by a ball mill or the like.
The average primary particle diameter after pulverization is preferably about 0.8 to 1.0 μm.

In addition, when adding said subcomponent, it is preferable to perform addition before mixing and grinding.

 The dried powder is then molded.

This compact is sintered. The sintering temperature depends on the atmosphere during sintering, but is preferably 1150 to 1250 ° C. The atmosphere during sintering is preferably a nitrogen atmosphere containing 1.0 to 3.0% of oxygen. The sintering time is preferably 3 to 4 hours. The heating rate up to the sintering temperature is 10
0 to 200 ° C / hour, temperature falling rate from sintering temperature is 150 to 200 ° C
/ Hour. When the temperature is lowered, it is preferable to control the atmosphere in accordance with the temperature drop so as to maintain the spinel phase equilibrium.

Although the ferrite sintered body of the present invention described above is suitably used for a transformer core for a switching power supply, etc., in particular, an operating frequency of about 500 to 1000 kHz and a magnetic flux density of 20 to 100 m
When used in a magnetic core for a power transformer used at T and a temperature of about 60 to 120 ° C., low loss characteristics are effectively realized.

<Example> Hereinafter, the present invention will be described in more detail with reference to specific examples of the present invention.

Example 1 A mixed powder of Fe ₂ O ₃ and Mn ₂ O ₃ was produced by spray roasting,
In addition, ZnO and TiO ₂ , Ta ₂ O ₅ , SiO ₂ and CaO
Was mixed and pulverized by a ball mill. The average primary particle size after pulverization was 0.85 μm.

 The powder after pulverization was dried and then molded.

This compact was sintered at a sintering temperature of 1200 ° C. in a nitrogen atmosphere containing 2.0% of oxygen. The sintering time was 3 hours. The rate of temperature rise to the sintering temperature was 100 ° C./hour in air, and the rate of temperature decrease from the sintering temperature was 150 ° C./hour.

The composition of the ferrite sintered body thus obtained was analyzed by X-ray fluorescence analysis. As a result, MnO 37 mol%, ZnO 9 mol%, Fe ₂ O ₃ 54 mol%, TiO ₂ 3000 ppm, Ta ₂ O ₅ 600 ppm, SiO _2
2 200 ppm and CaO 1200 ppm. This is sample N
o.1.

The shape of the sample is 20mm in outer diameter, 10mm in inner diameter, height
5 mm toroidal shape.

Next, by changing the sintering conditions, ferrite sintered bodies having the same composition as Sample No. 1 and having various average crystal grain sizes and crystal grain size distributions were produced (Sample Nos. 2 to 7). Table 1 shows the sintering conditions for each sample.

Sample No. 1 and Sample No. 5 ferrite sintered bodies were mirror-polished and then etched with hydrochloric acid to make the polished surface 750
Photos taken with a scanning electron microscope at × 2 magnification are shown in FIGS. 1 and 2, respectively. The average grain size (d) of these samples and other samples, the ratio of the number of crystal grains having a grain size exceeding 10 μm to the total number of crystal grains (N ₁₀ ), the crystal grains having a grain size exceeding 10 μm Area ratio (S ₁₀ ) to the total area of
Table 1 shows the case where crystal grains exceeding m are present, and the case where they do not exist is represented by x).

The calculation of the average crystal grain size and the crystal grain size distribution was performed as follows.

First, 50 μm × 50 on the photograph obtained as described above
A square section of μm was taken, and the number of crystal grains present in this section was calculated.

However, the number of crystal grains existing at the boundaries of the sections was counted as 1/2. With this number as n, the average crystal grain size d was calculated by the following equation.

For calculating N ₁₀ , S ₁₀ and the maximum crystal grain size, the major axis of the crystal grains was used.

S ₁₀ calculates the average value of the crystal grains having a crystal grain size exceeding 10 μm and the largest one having the largest crystal grain size, and calculating the area of a circle having this value as the diameter.
The ratio of the area obtained by multiplying the total number of crystal grains having a crystal grain size of more than 10 μm to the total area was determined.

For these samples, power loss was measured at the frequency (f) and the magnetic flux density (B) shown in Table 1 below. The temperature of the sample at the time of measurement was 80 ° C.

 Table 1 shows the results.

[Example 2] composition, MnO37 mol%, ZnO10 mol%, Fe ₂ O ₃ 53 mol%
A ferrite sintered body sample having the same average crystal grain size and crystal grain size distribution as those of the above sample Nos. 1 to 7 was prepared according to Example 1.

When the same measurement as in Example 1 was performed on these samples, power loss characteristics depending on the average crystal grain size and the crystal grain size distribution were observed. The power loss characteristics depended on the particle size distribution.

[Example 3] The Ca content profiles of the ferrite sintered bodies of Sample Nos. 1 and 5 obtained in Example 1 were measured by Auger analysis while performing ion milling.

 The results are shown in FIG.

In FIG. 3, the vertical axis indicates the Ca content, and the horizontal axis indicates the distance from the position where the peak of the Ca content is shown.

<Effect of the Invention> The ferrite sintered body of the transformer core for a power supply of the present invention is:
The loss is low because it has a predetermined average crystal grain size and crystal grain size distribution.

Further, when the ferrite sintered body of the transformer core for a power supply of the present invention contains a predetermined subcomponent, the loss is further reduced.

[Brief description of the drawings]

FIG. 1 and FIG. 2 are photographs substituted for drawings showing crystal grains of a ferrite sintered body. FIG. 3 shows a Ca content profile of the ferrite sintered body.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Saito 1-13-1 Nihonbashi, Chuo-ku, Tokyo Inside TDK Corporation (56) References JP-A-58-84177 (JP, A) JP-A-62 −219903 (JP, A)

Claims (3)

(57) [Claims] The present invention relates to a ferrite sintered body mainly composed of MnO, ZnO and Fe ₂ O ₃ , wherein the number of crystal grains having an average crystal grain size of 5 μm or less and having a crystal grain size exceeding 10 μm is determined. A transformer core for power supply used at an operating frequency of 500 kHz or more, which is less than 3% of the total and the loss at 80 ° C, 1000 kHz, and 50 mT is 410 mW / cm ³ or less. 2. The area of crystal grains exceeding 10 μm is 15
%. 3. The transformer core for a power supply according to claim 1, wherein the transformer core contains at least one selected from TiO ₂ , Ta ₂ O ₅ , SiO ₂ and CaO as an auxiliary component.