Inductor devices – Core forms casing
Reexamination Certificate
1999-11-18
2001-06-12
Mai, Anh (Department: 2832)
Inductor devices
Core forms casing
C336S192000, C336S175000, C336S225000
Reexamination Certificate
active
06246310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to noise suppressing apparatuses, and more particularly, to a noise suppressing apparatus for suppressing high-frequency noise radiating through an interface cable or a power cable from a circuit board.
2. Description of the Related Art
Conventionally, an inductance element having a coil provided either on the surface of ferrite or inside of ferrite has been utilized for suppressing noise leaking through a signal cable or a power cable.
A ceramic ferrite, which is commonly used for this type of inductance element, complies with Snoeks' limit. According to Snoeks' limit, the permeability of a high-permeability ceramic ferrite tends to start decreasing in a comparatively low frequency band. For example, when ferrite having a relative permeability of 500 is used, there is a tendency for the permeability of the ferrite to start decreasing in the frequency band above several megahertz and to further decrease at higher frequency bands.
In the above-described inductance element including a magnetic body such as the ceramic ferrite, a noise suppressing effect on the order of tens to hundreds of megahertz can be obtained in the low frequency band. Conversely, in a high frequency band, as the permeability of the magnetic body decreases, the noise cannot be suppressed adequately because of a decrease in the noise suppressing effect.
When a ceramic ferrite having a low permeability is used, a constant permeability is maintained up to a relatively high frequency band. However, there is a problem in that a desired impedance of the magnetic body in the high frequency band is obtained, which causes the impedance thereof to decrease in the low frequency band. When a ceramic ferrite having a relative permeability of approximately 15 is used, the permeability starts decreasing in the frequency band above approximately 100 MHz, thereby complying with Snoeks' limit. There is a problem in that the noise suppressing effect can be obtained in the frequency band below 1 GHz, whereas it cannot be sufficiently obtained in the frequency band above 1 GHz.
When a ceramic ferrite having high impedance is desired, the number of turns of the coil must be increased. However, the increase in the number of turns of the coil causes stray capacitance to increase as well. In the frequency band above a particular frequency, the stray capacitance allows noise to pass because it functions as a capacitor. Thus, the noise suppressing effect is not achieved sufficiently and thus, cannot effectively suppress noise.
T-type filters and &pgr;-type filters, obtained by combining an inductor and a capacitor, are known for suppressing noise. These filters show a remarkable noise suppressing effect, due to the combination of characteristics of the inductance and of the capacitance, up to a particular frequency. However, it is noted that the noise suppressing effect cannot be obtained sufficiently in the frequency band above the particular frequency because influences caused by residual inductance and stray capacitance prevent the noise suppressing effect from functioning properly in the high frequency band.
For example, in Japanese Unexamined Patent Publication No. 8-204486, there is disclosed a signal transmission element which defines a known noise suppressing apparatus.
FIG. 8
shows a perspective view of the signal transmission element defining the noise suppressing apparatus. In
FIG. 8
, a signal transmission wire
52
defines a coil by being wrapped around an insulated magnetic body
51
in the form of a ferrite rod which has a rod-like shape, and is embedded in a magnetic resin
53
obtained by combining a magnetic metal powder and a resin. A pair of external electrodes
54
a
and
54
b
are provided on the magnetic resin
53
allowing an electric current to flow through the signal transmission wire
52
, and an electrode for ground connection or a ground electrode
55
is formed so as to substantially cover the entire surface of the magnetic resin
53
between the pair of external electrodes
54
a
and
54
b.
The signal transmission element is constructed to maximize use of a magnetic loss generated by a ferromagnetic metal powder by effectively applying a high frequency magnetic field inside of the ferromagnetic metal particles in spite of the skin effect of the ferromagnetic metal. This construction enables this element to positively absorb the high-frequency signal components occurring in the high frequency domain.
However, in this noise suppressing apparatus defining the signal transmission element, although the noise suppressing effect is obtained sufficiently in the high frequency band above a gigahertz waveband, while it is not obtained in the low frequency band below a gigahertz waveband. The cutoff frequency of this noise suppressing apparatus is defined as the frequency at which the attenuation value of the transmission characteristic is about −3 dB. Since a noise suppressing effect with the attenuation value of only −10 dB or lower is obtained in the low frequency band below a gigahertz waveband, it is difficult to obtain the desired noise suppressing effect substantially in the low frequency band below a gigahertz waveband.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a noise suppressing apparatus that is constructed to produce a noise suppressing effect in a high frequency band above a gigahertz waveband as well as a noise suppressing effect in a low frequency band below a gigahertz waveband.
To this end, a preferred embodiment of the present invention provides a noise suppressing apparatus including a magnetic body including a ferrite, a coiled conductor provided in the magnetic body, a pair of external electrodes provided on the surface of the magnetic body and electrically connected to both ends of the coiled conductor, and a ground electrode disposed between the pair of external electrodes and covering at least a main portion of the surface of the magnetic body.
The coiled conductor is disposed in the magnetic body including ferrite material so that the magnetic body functions as a magnetic shield. Because the ground electrode is further disposed between the pair of external electrodes arranged along the magnetic body so as to cover at least a main portion of the surface of the magnetic body, an electromagnetic wave absorption function is achieved due to the combined actions of the magnetic body and the ground electrode. Therefore, the emission of noise is efficiently suppressed due to the magnetic shield function of the magnetic body, and moreover, some of the noise, which is not suppressed by only the magnetic shield function of the magnetic body, is absorbed by the combined actions of the magnetic body and the ground electrode, which can sufficiently suppress the emission of noise.
The longer the length of the wire conductor, the greater the noise suppressing effect which can be obtained. Since the conductor is arranged in a coil configuration, the required wire length can be secured without causing an increase in size of a product including the conductor, which enables a sufficient noise suppressing effect to be obtained. In the noise suppressing apparatus of this invention, the noise suppressing effect is achieved from around approximately 300 MHz, and it can be sufficiently obtained up to several gigahertz.
In a noise suppressing apparatus according to another preferred embodiment of the present invention, the magnetic body including the ferrite may be a magnetic resin obtained by dispersing a ferrite powder in a resin.
By using as a magnetic body, the magnetic resin obtained by dispersing the ferrite powder in the resin, a magnetic body having ferrite sufficiently dispersed therein is obtained. In addition, this magnetic body can be molded into a desired shape, which is capable of enhancing characteristics of the magnetic body as well as the degree of freedom of the design thereof. When the magnetic resin is u
Sugitani Masami
Uchida Katsuyuki
Keating & Bennett LLP
Mai Anh
Murata Manufacturing Co. Ltd
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