JP2004153079A - Common mode filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress common mode noises in high-speed differential signal transmissions including USB 2.0, IEEE 1394 or DVI without adversely affecting the high-speed transmission signals. <P>SOLUTION: A coaxial wire W having a center conductor 1 and an outer conductor 2 with an insulating layer 3 for surrounding the conductor 1 being interposed therebetween, is wound around the center core 11 of a magnetic or non-magnetic drum core 10. A pair of electrodes 21, 22 are arranged at each of rectangular flanges 12 of the drum core 10. Then, the conductor 1 is connected to one of the electrodes 21, 22, and the conductor 2 is connected to the other electrode 22. The wire W is wound around the core 11 of the drum core 10 with a required interval therebetween. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a common mode filter for suppressing a common mode noise such as a signal line, and particularly to a high-speed differential such as USB 2.0, IEEE1394, DVI (used for connection of a liquid crystal display or the like) in a personal computer or the like. The present invention relates to a surface mount type common mode filter used for attenuating common mode noise passing through two signal lines in signal transmission.
[0002]
[Prior art]
Conventionally, as a common mode filter of this type, as shown in FIG. 6, an electrode 25 is formed on a drum core 10 made of a magnetic material such as ferrite by plating or the like, and a pair of wires 15 is attached to a core portion 11 of the drum core 10. , 16 are wound in close contact with each other, and winding ends (each wire end) are connected to electrodes 25 provided on the square flange 12 of the core 10 by soldering or the like. Patent Document 1 listed below is an example of this type of common mode filter.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-93756
[Problems to be solved by the invention]
In recent years, high-speed differential transmission such as DVI (TMDS) and IEEE 1394 (S800) has appeared in signal transmission between a personal computer and peripheral devices, and depending on the transmission characteristics of a common mode noise filter, a high frequency component of a transmission signal may be reduced. Since it has an adverse effect such as attenuation, it is difficult to use it for high-speed signals. Considering that the degree of influence on the transmission signal depends on (1) the signal cutoff frequency and (2) the characteristic impedance matching, it is impossible to increase the signal cutoff frequency with the conventional wound-type common mode noise filter, and the characteristic impedance depends on the application. Cannot be adjusted according to the condition, and the signal quality may be affected. For example, in high-speed differential transmission such as DVI (TMDS) and IEEE 1394 (S800), it is required that a cutoff frequency reaches a GHz band. Also, it has been difficult to adjust the characteristic impedance according to each application with the conventional wire-wound common mode noise filter.
[0004]
In view of the above, the present invention provides a common mode filter capable of suppressing common mode noise without adversely affecting a high speed transmission signal in high speed differential signal transmission such as USB 2.0, IEEE 1394, DVI, or the like. Aim.
[0005]
Other objects and novel features of the present invention will be clarified in embodiments described later.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, a common mode filter according to the present invention is configured such that a coaxial wire provided with an outer conductor with an insulating layer interposed around a center conductor is wound around a core portion of a magnetic or non-magnetic drum core. A pair of electrodes are provided on each of the flange portions of the drum core, and the center conductor is connected to one of the pair of electrodes, and the external conductor is connected to the other of the pair of electrodes.
[0007]
The common mode filter is characterized in that the coaxial wire is wound with a wire interval.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of a common mode filter according to the present invention will be described with reference to the drawings.
[0009]
FIG. 1 is an exploded perspective view of a common mode filter according to an embodiment of the present invention, FIG. 2 is an enlarged perspective view showing a terminal processing of a coaxial wire in the embodiment, and FIG. 3 is a sectional view of a coaxial wire used.
[0010]
In these figures, reference numeral 10 denotes a magnetic core (e.g., ferrite) or a non-magnetic (e.g., ceramic) drum core, which has a square flange 12 on both sides of a core 11. A pair of electrodes 21 and 22 are formed on each of the rectangular flange portions 12 of the drum core 10 by baking a metal paste, plating a metal, or the like. The electrodes 21 and 22 are continuously formed over the upper surface, the outer end surface, and the lower surface (mounting surface) of both ends of the rectangular flange portion 12 to constitute a surface-mounted terminal. A concave portion 13 is formed at the center of the upper surface of the rectangular flange portion 12, and extended portions 21 a and 22 a of the electrodes 21 and 22 are formed to extend from the bottom surface of the concave portion 13.
[0011]
In the present embodiment, the coaxial wire W of FIG. 3 is used for the winding. The coaxial wire W has a cylindrical outer conductor 2 provided around a cylindrical center conductor 1 with an insulating layer 3 interposed therebetween. The center conductor 1 and the outer conductor 2 are concentric and The outer diameter a, the outer diameter b of the outer conductor 2, the thickness c of the insulating layer 3, and the dielectric constant ε. It is preferable to use a low dielectric constant material having a relative dielectric constant of 5 or less for the insulating layer 3. If the relative dielectric constant exceeds 5, the line capacitance is undesirably increased.
[0012]
This coaxial wire W is wound around the core portion 11 of the drum core 10 a plurality of times with a required winding interval, and the ends of the coaxial wire W after the winding are extended to the concave portions 13 of the square flange portion 12, respectively. Are connected (connected) to the extending portions 21a and 22a of the electrodes 21 and 22 that are being used. That is, as shown in an enlarged manner in FIG. 2, the center conductor 1 is electrically connected to the extension 21a of the electrode 21 and the outer conductor 2 is electrically connected to the extension 22a of the electrode 22 by thermocompression bonding, soldering, or the like.
[0013]
After the winding of the coaxial wire W, a magnetic or non-magnetic flat core 20 is fixed between the upper surfaces of the two rectangular flange portions 12 of the drum core 10 with an adhesive or the like, thereby forming a surface-mounted common mode. The filter is completed. The flat core 20 is provided so that the upper surface of the product is made flat and can be sucked by the suction nozzle of the automatic mounting machine. If both the drum core 11 and the flat core 20 are made of a magnetic material, the leakage magnetic flux can be reduced. Also, it can contribute to improvement of the common mode noise suppression characteristics.
[0014]
To increase the signal cut-off frequency of the common mode filter, differential mode inductance (hereinafter, Ld hereinafter) and interline capacitance (hereinafter, C ₁₂₎ it is necessary to reduce the. _Ld, the cutoff frequency _{C 12} is large low, Ld _Conversely, the cutoff frequency _{C 12} is small tends to be higher. In the present embodiment, winding is performed using a coaxial wire W provided with a cylindrical outer conductor 2 with a low dielectric constant insulating layer 3 (preferably having a relative dielectric constant of 5 or less) interposed around a cylindrical central conductor 1. Thus, one winding portion is formed by the center conductor 1 and the other winding portion is coupled to the outer conductor 2 by the outer conductor 2, thereby reducing leakage magnetic flux, increasing inter-line coupling, and suppressing Ld. . As a result, the signal cutoff frequency can be extended to a high frequency (for example, GHz band).
[0015]
FIG. 4 shows Sdd21 characteristics (attenuation characteristics for a differential mode) in the case of the present embodiment in comparison with the case of a conventional product. However, in the conventional product, two wires of 40 μm in diameter are wound 7 turns, and in the present embodiment, a coaxial wire having a diameter a of the central conductor 1 of 10 μm and an outer diameter b of the outer conductor 2 of 100 μm in FIG. The core was wound and the drum core 10 was the same (however, the cross section of the core was elliptical and the cross-sectional area was 0.7 mm ² ). It can be seen that the embodiment has less attenuation in a high frequency region and a higher cutoff frequency.
[0016]
In addition, the characteristic impedance can be controlled to be adjusted to a specific value by changing the thickness of the insulating layer 3 or the insulating material, so that impedance matching according to the application can be achieved.
[0017]
In the case of a coaxial wire, the inductance L and the capacitance C per unit length for a high frequency can be approximated as follows.
L ≒ (μ / 2π) ln (b / a)
C ≒ 2πε / ln (b / a)
Here, μ is the magnetic permeability of the insulating layer, and ε is the dielectric constant of the insulating layer.
The characteristic impedance Zid is given by the following approximate expression.
Zid ≒ (L / C) ^1/2 (1)
[0018]
FIG. 5 shows the relationship between b / a and the characteristic impedance Zid when the dielectric constant ε of the insulating layer 3 is used as a parameter. From the characteristic impedance expression (1) and FIG. 5, it can be seen that the characteristic impedance can be set to a desired value by changing b / a and the dielectric constant ε.
[0019]
According to this embodiment, the following effects can be obtained.
[0020]
(1) A coaxial wire W provided with an outer conductor 2 with an insulating layer 3 interposed around a center conductor 1 is wound around a core 11 of a magnetic or non-magnetic drum core 10, and a square flange 12 of the drum core 10 is wound. Are provided with a pair of electrodes 21 and 22, respectively, and the center conductor 1 is connected to one electrode 15 and the external conductor 2 is connected to the other electrode 16, so that the line-to-line coupling can be sufficiently increased and Ld can be reduced. For this reason, it is possible to increase the signal cutoff frequency of the common mode filter.
[0021]
(2) By winding the coaxial wire W with an interval between the wires, the stray capacitance between the outer conductors 2 can be reduced, and the transmission characteristics can be improved.
[0022]
(3) When the diameter of the center conductor 1 is a and the outer diameter of the outer conductor 2 is b, the characteristic impedance is set to be suitable for the application by changing the value of b / a and the dielectric constant ε of the insulating layer 3. it can.
[0023]
(4) By fixing the magnetic or non-magnetic flat core 20 between the upper surfaces of both the rectangular flanges 12 of the magnetic or non-magnetic drum core 10, the upper surface of the product is made a flat surface, and is suctioned by the suction nozzle of the automatic mounting machine. When the drum core 10 and the plate-shaped core 20 are both made of a magnetic material, the leakage magnetic flux can be reduced and the common mode noise suppression characteristics can be improved.
[0024]
In addition, the electrodes 21 and 22 in each embodiment may be configured by attaching a terminal fitting to the rectangular flange 12 in addition to baking of a metal paste, metal plating, or the like.
[0025]
Although the embodiments of the present invention have been described above, it will be obvious to those skilled in the art that the present invention is not limited to the embodiments and various modifications and changes can be made within the scope of the claims.
[0026]
【The invention's effect】
As described above, according to the common mode filter of the present invention, a coaxial wire provided with an outer conductor with an insulating layer interposed around a center conductor is wound around a core portion of a magnetic or non-magnetic drum core. By providing a pair of electrodes on the flange portion of the drum core and connecting the center conductor to one of the pair of electrodes and the external conductor to the other of the pair of electrodes, the signal cutoff frequency is increased, and the USB2 In high-speed differential signal transmission such as 2.0, IEEE1394, DVI, etc., common mode noise can be suppressed without adversely affecting high-speed transmission signals.
[Brief description of the drawings]
FIG. 1 is an exploded perspective view showing an embodiment of a common mode filter according to the present invention.
FIG. 2 is a partially enlarged perspective view of the embodiment. FIG. 3 is a cross-sectional view of a coaxial wire used in the embodiment.
FIG. 4 is a graph showing transmission characteristics (Sdd21 attenuation characteristics) of the embodiment in comparison with a conventional product.
FIG. 5: When the diameter of the center conductor of the coaxial wire is a, the outer diameter of the outer conductor is b, and the dielectric constant of the insulating layer is ε, b / a and characteristic impedance Zid when the dielectric constant ε is used as a parameter. 6 is a graph showing the relationship of.
FIG. 6 is a perspective view showing an example of a conventional common mode filter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Center conductor 2 Outer conductor 3 Insulating layer 10 Drum core 11 Core 12 Square flange 15, 16 Wire 20 Flat core 21, 22, 25 Electrode W Coaxial wire

Claims (2)

A coaxial wire provided with an outer conductor with an insulating layer interposed around a central conductor is wound around a core portion of a magnetic or non-magnetic drum core, and a pair of electrodes are provided on a flange portion of the drum core, respectively. A common mode filter, wherein the central conductor is connected to one of the electrodes, and the external conductor is connected to the other. The common mode filter according to claim 1, wherein the coaxial wire is wound with a wire interval.

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