Mn-Zn ferrite and coil component using same

Details Mn-Zn ferrite and coil component using same Mn-Zn ferrite and coil component using same

2002-08-02

2004-07-27

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

C252S062590, C252S062600, C252S062640, C252S062580

Reexamination Certificate

active

06767478

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to Mn—Zn ferrite and a coil component using Mn—Zn ferrite as a magnetic core, and in particular to Mn—Zn ferrite and a coil component suitable for switching power supplies, noise filters, choke coils and so forth.
2. Description of the Related Art
Magnetic core materials for use in noise filters such as line filters for EMI countermeasures are desired to have high impedance in a frequency band where noises should be removed. Impedance in a high frequency band can be increased through the phenomenon of resonance between the stray capacitance (C) of a coil and the inductance (L) of a core. It is known that impedance is maximum at the resonance frequency (1/LC). It is also known that impedance at a frequency lower than the resonance frequency is proportional to the initial permeability of a core, and impedance at a frequency higher than the resonance frequency is inversely proportional to the stray capacitance of a coil.
Mn—Zn ferrite that has high permeability in a wide frequency band has been studied intensively and disclosed in, for example, Japanese Patent Laid-open Nos. 2000-353613 and Hei 9-180925. The initial permeability of Mn—Zn ferrite is closely related to a real part &egr;′ of complex relative permittivity, and when &egr;′ has a large value, the initial permeability is relaxed in a high frequency band and cannot be maintained through up to a high frequency band. As a result, it is difficult to obtain excellent impedance properties in a high frequency band.
While Japanese Patent Laid-open No. 2000-353613 mentioned above does not describe dielectric properties or impedance properties, Japanese Patent Laid-open No. Hei 9-180925 refers to permittivity (relative permittivity) &egr;, and describes the relative permittivity &egr; as reading 50,000 to 1,000,000 at 1 KHz, which is very high. Usually, complex relative permittivity &egr; is expressed as (&egr;′-j&egr;″), and although the above-mentioned relative permittivity &egr; is not separated into a complex component and a real component, generally, in case of polycrystalline Mn—Zn ferrite, &egr;″/&egr;′=0.5 to 1.5. Accordingly, &egr;′ may be estimated to range from 30,000 to 60,000 at the lowest, which is a very high value. The real part &egr;′ of complex relative permittivity with such a high value at 1 kHz not only can never be 50 or less at 1 MHz, but also causes relaxation of initial permeability in the vicinity of 500 kHz making it impossible to maintain the initial permeability through up to a high frequency band of 1 MHz or more.
The present inventors have found that a coil component comprising a magnetic core of conventional Mn—Zn ferrite having a high real part &egr;′ of complex relative permittivity scarcely reduces stray capacitance despite whatever optimizations of its winding conditions. The stray capacitance of a coil component is roughly classified into capacitance between coils and capacitance between a coil and a core. The former is determined based on the winding conditions of a coil and the latter is determined based on the material properties of a magnetic core, both increasing as the real part &egr;′ of complex relative permittivity increases. Therefore, in the coil component with a magnetic core of conventional Mn—Zn ferrite having a high real part &egr;′ of complex relative permittivity, the capacitance between a coil and a core is extremely high as compared with the capacitance between coils. As a result, the optimization of winding conditions has been hardly effective in reducing the stray capacitance of the component.
When designing a magnetic core material and a coil component to be used in a high frequency band, two requirements must be met to reduce stray capacitance: one is to use a magnetic core material that maintains high permeability through up to a high frequency band and the other is to optimize its winding conditions. The coil component using conventional Mn—Zn ferrite as its magnetic core satisfies neither of the two requirements, making it difficult to obtain high impedance in a wide frequency band.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above-mentioned conventional problems, and an object of the present invention is therefore to provide Mn—Zn ferrite that maintains initial permeability over a wide frequency band, has small stray capacitance when provided with a coil, and has excellent impedance properties over a wide frequency band, and also a coil component made thereof.
In Mn—Zn ferrite according to the present invention, basic components include 44.0 to 50.0 mol % (50.0 mol % excluded) Fe
2
O
3
, 4.0 to 26.5 mol % ZnO and the remainder MnO, and its real part &egr;′ of complex relative permittivity is 20,000 or less at 1 kHz and 50 or less at 1 MHz, respectively.
The Mn—Zn ferrite of the present invention may further contain as additive at least one of 0.01 to 4.0 mass % SnO
2
and 0.01 to 3.0 mass % TiO
2
(provided that an upper limit is 4% in total when both thereof are contained). The Mn—Zn ferrite of the present invention may also contain as additive at least one of 0.01 to 2.0 mass % CuO, 0.01 to 2.0 mass % NiO, 0.01 to 2.0 mass % CoO, 0.01 to 2.0 mass % MgO, 0.01 to 2.0 mass % Al
2
O
3
and 0.01 to 2.0 mass % Cr
2
O
3
(provided that an upper limit in total is 2 mass % when two or more thereof are contained).
A coil component according to the present invention uses the above-described Mn—Zn ferrite as a magnetic core.
The Mn—Zn ferrite and the coil component according to the present invention can provide good impedance properties over a wide frequency band and effectively convert noises at a practically critical frequency band of 10 to 100 MHz into thermal energy and absorb them. Thus, they prove to be very useful.
BRIEF DESCRIPTION OF THE ATTACHED DRAWING
FIG. 1
is a graph illustrating frequency properties of impedance of the present invention samples and comparative samples.


REFERENCES:
patent: 6210598 (2001-04-01), Kobayashi et al.
patent: 6296791 (2001-10-01), Kobayashi et al.
patent: 6403017 (2002-06-01), Kobayashi et al.
patent: 6440323 (2002-08-01), Kobayashi et al.
patent: 6461531 (2002-10-01), Kobayashi et al.
patent: 2003/0059365 (2003-03-01), Ito et al.
patent: 1 101 736 (2001-05-01), None
patent: Hei 9-180925 (1997-07-01), None
patent: 2000-353613 (2000-12-01), None
patent: WO 98/32140 (1998-07-01), None

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