JP4033852B2 - Common mode filter - Google Patents

Description

  The present invention relates to a common mode filter, and more particularly to a common mode filter capable of adjusting a characteristic impedance and allowing a high-frequency signal to pass therethrough.

  The common mode filter is a coil component used for filtering radiation noise or the like caused by various electronic devices, and has a structure in which a plurality of windings are tightly wound around a core.

  FIG. 5A is a diagram showing a conventional common mode filter 101 in which two windings 104 and 105 are closely attached to a magnetic core 102. The magnetic core 102 has a cylindrical core 121 and substantially prismatic flanges 122 and 123 formed at both ends thereof, and the flange 122 is connected to an external terminal (not shown). The collar 123 includes electrodes 126 and 127 connected to another external terminal (not shown). The windings 104 and 105 are each covered with an insulator, and are wound bifilarly around the core 121 in a spiral manner in a state of being in close contact with each other. Both ends of the winding 104 are connected to the electrodes 124 and 126, and both ends of the winding 105 are connected to the electrodes 125 and 127. The two windings 104 and 105 each function as a differential signal line.

  FIG. 5B is a diagram showing a conventional common mode filter 201 in which three windings 204, 205, and 206 are closely attached to a magnetic core 202. The magnetic core 202 has a cylindrical core portion 221 and substantially prismatic flange portions 222 and 223 formed at both ends thereof, and the flange portion 222 is connected to an external terminal (not shown). The flange 223 includes electrodes 227, 228, and 229 that are connected to other external terminals (not shown).

  The windings 204, 205, and 206 are each covered with an insulator, and are spirally wound around the core portion 221 in a trifilar manner in a state of being in close contact with each other. Both ends of the winding 204 are connected to electrodes 224 and 227, both ends of the winding 205 are connected to electrodes 225 and 228, and both ends of the winding 206 are connected to electrodes 226 and 229. The windings 204 and 205 each function as a differential signal line, and the winding 206 functions as a ground line (see, for example, Patent Document 1).

  The operation of the common mode filter will be described using the common mode filter 101 shown in FIG. 5A as an example. The signals are input to the two windings 104 and 105 as signals in opposite directions. Thus, the magnetic fluxes generated on the magnetic core 102 by the differentially input signals are opposite to each other and cancel each other. Therefore, no magnetic field is generated and the impedance (inductance) is not increased, so that the signal is output without being attenuated.

  On the other hand, since common mode noise is generated in the same direction of each of the windings 104 and 105, magnetic fluxes generated on the magnetic core 102 become magnetic fluxes in the same direction and are superimposed and strengthened. Therefore, a magnetic field is generated and impedance (inductance) is increased, so that common mode noise can be attenuated. In this way, the common mode filter 101 can attenuate only noise without attenuating the signal.

The common mode filter 201 shown in FIG. 5B has a winding 206 that functions as a ground line in addition to the differential signal lines 204 and 205. The common mode filter 101 shown in FIG. However, common mode noise can be similarly attenuated according to the above-described operation principle.
Japanese Patent Laid-Open No. 2003-77730

  By the way, in recent years, high-speed differential transmission using a signal transmission method such as DVI or IEEE has appeared, and a common mode filter is also used to remove noise in such high-speed differential transmission. However, since the common mode filter does not attenuate the differential signal at all, when transmitting a signal having a high frequency component to the signal, depending on the characteristics of the common mode filter, not only the noise but also the signal of the high frequency component may be included. It may be attenuated.

  For example, in the DVI transmission method used when connecting a liquid crystal display (LCD) and a personal computer (PC), the signal transmission frequency is 560 MHz when the resolution of the LCD is SXGA. Considering even the third and fifth harmonics, a common mode filter having characteristics that do not attenuate at least 2.5 GHz signals is required, but it is very difficult to manufacture a common mode filter having such characteristics. It is.

  Further, by inserting the common mode filter into the signal transmission path, the characteristic impedance of the transmission path is changed by the inductance component and stray capacitance. When the characteristic impedance changes, the signal is reflected at the characteristic impedance mismatch point in the transmission path, and the normal band of the signal is narrowed. Therefore, the signal is attenuated or distorted. Therefore, the characteristic impedance is usually matched, but the characteristic impedance cannot be adjusted with the conventional common mode filter.

  In the case of the common mode filter 101 shown in FIG. 5A, when the common mode filter 101 is disposed on the printed circuit board, stray capacitance exists between the windings 104 and 105 and the ground layer of the printed circuit board. . Due to the capacitance component of the stray capacitance, the characteristic impedance of the common mode filter changes. One of the factors that cause the characteristic impedance to change is that the value of the stray capacitance varies depending on the distance between the windings 104 and 105 and the ground layer. For this reason, the characteristic impedance changes depending on the thickness of the substrate to be used. Further, since the common mode filter 1 does not have a ground line, there may be a case where a sufficient noise attenuation effect cannot be obtained when noise is superimposed on the ground line.

  In the case of the common mode filter 201 shown in FIG. 5B, if the winding 206 located in the middle of the trifilar winding is a ground line, the ground line is sandwiched between the winding 204 and the winding 205 which are signal lines. As a result, the distance between the windings 204-205 is increased. When the distance between the signal lines is increased, the magnetic coupling is weakened, so that the magnetic flux cancellation is reduced. Therefore, the inductance due to the magnetic field with respect to the differential signal increases, and the high-frequency signal is attenuated. In addition, since the characteristic impedance increases due to the increase in inductance, mismatching of the characteristic impedance occurs.

  An object of the present invention is to provide a common mode filter that can remove noise superimposed on a ground line, pass a high-frequency signal, and adjust the characteristic impedance.

In order to achieve the above object, the invention according to claim 1 is wound around a magnetic core having a core part, a plurality of electrodes respectively provided at both ends of the core part, and the core part. And a common mode filter having at least two windings each connected to an electrode at both ends, wherein the windings are formed on a plate-like conductor spirally wound around a winding core, and on the plate-like conductor. Similarly, two linear conductors wound in a spiral are provided, and the plate-like conductor is used as a ground line .

According to such a configuration, since the plate conductor is used as a ground line, the stray capacitance between the linear conductor and the plate conductor becomes dominant, and the stray capacitance between the linear conductor and the ground layer of the printer board can be ignored. Become.

  The invention described in claim 2 is characterized in that, in the invention described in claim 1, the magnetic core has flange portions at both ends of the core portion, and the electrodes are formed at the flange portions. Yes.

  According to a third aspect of the present invention, in the first aspect of the present invention, the plate-like conductor is formed directly on the winding core.

The invention described in claim 4 is characterized in that, in the invention described in claim 1, a plurality of linear conductors are provided and insulated from each other.

The invention described in claim 5 is characterized in that, in the invention described in claim 1, at least one of the plate-like conductor and the linear conductor is coated with an insulating film.

According to such a configuration, the plate conductor and the linear conductor are insulated.

The invention described in claim 6 is characterized in that, in the invention described in claim 1, an insulating sheet is provided between the plate-like conductor and the linear conductor .

According to such a configuration, the plate conductor and the linear conductor are insulated.

  The invention described in claim 7 is characterized in that, in the invention described in claim 1, a plate core is provided which sandwiches the winding in cooperation with the core portion.

  According to such a configuration, a closed magnetic path is formed by the plate core, and the magnetic permeability of the magnetic flux increases.

According to an eighth aspect of the present invention, in the first aspect of the present invention, the winding includes two linear conductors that are similarly spirally wound on the plate-like conductor, The width is characterized by being wider than the total value of the radii of the linear conductors.

The invention described in claim 9 is characterized in that the two linear conductors are similarly spirally wound on the plate-like conductor .

According to the common mode filter of the first aspect of the present invention, since the plate conductor is used as the ground line, the stray capacitance between the line conductor and the plate conductor becomes dominant, and the linear conductor-printer board The stray capacitance between the ground layers can be ignored. Therefore, by changing the width of the plate-shaped conductor, the characteristic impedance of the common mode filter is adjusted in advance and the crosstalk between windings is reduced without considering the stray capacitance between the linear conductor and the ground layer of the printer board. Can be made.

  According to the common mode filter of the second aspect of the present invention, since the flange portions are provided at both ends of the core portion, it can be easily mounted when mounted on a substrate or the like.

  According to the common mode filter of the third aspect of the present invention, since the plate-like conductor is formed directly on the core portion, the work of winding the plate-like conductor around the core portion is not necessary. The size of the mode filter itself can also be reduced.

  According to the common mode filter of the present invention, differential signals can be handled.

  According to the common mode filter of the fifth aspect of the present invention, the plate conductor and the winding can be insulated with a simple configuration in which either the plate conductor or the winding is covered.

  According to the common mode filter of the sixth aspect of the present invention, the plate-like conductor and the winding can be insulated with a simple configuration in which an insulating sheet is placed between the plate-like conductor and the winding.

  According to the common mode filter of claim 7 of the present invention, since the magnetic permeability of the common mode filter is increased by providing the plate core, the common mode impedance is increased and the common mode noise can be further reduced. .

According to the common mode filter of claim 8 of the present invention, the width of the plate-like conductor is made wider than the total value of the radii of the line-like conductors, so that the two line conductors can be reliably placed on the plate-like conductor. Therefore, the stray capacitance generated between the linear conductor and the plate conductor can be surely dominant.

According to the common mode filter of the ninth aspect of the present invention, since the two linear conductors are similarly spirally wound on the plate-like conductor, the crosstalk between the windings can be further reduced. Can do.

  A common mode filter according to a first embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 is a diagram showing a configuration of a common mode filter 1 according to the first embodiment of the present invention. The common mode filter 1 includes a magnetic core 2, a plate-like conductor 3, and windings 4 and 5. 1 is drawn with the windings 4 and 5 separated from the plate-like conductor 3 for the sake of convenience, the windings 4 and 5 are actually in close contact with the plate-like conductor 3 and firmly attached to the magnetic core 2. It is wound.

  FIG. 2A is a perspective view of the magnetic core 2, and FIG. 2B is a bottom view of the magnetic core 2. The magnetic core 2 has a cylindrical core portion 21 and substantially prismatic flange portions 22 and 23 formed at both ends thereof, and the flange portion 22 is connected to an external terminal (not shown). The flange 23 includes electrodes 26 and 27 connected to other external terminals (not shown). The plate-like conductor 3 is a strip-like thin film conductor made of copper or silver, and is directly formed in a spiral shape on the core portion 21. Both end portions of the plate-like conductor 3 extend to the flange portions 22 and 23 and also serve as electrodes. Trimming technique is used to form the plate-like conductor 3, and the entire magnetic core 2 is plated, and plating other than the portions necessary for the plate-like conductor 3 is removed.

  The windings 4 and 5 are respectively covered with an insulator and are bifilar wound on the plate-like conductor 3 in a state of being in close contact with each other. Both ends of the winding 4 are connected to the electrodes 24 and 26, and both ends of the winding 5 are connected to the electrodes 25 and 27. The windings 4 and 5 each serve as a differential signal line, and the plate conductor 3 serves as a ground line.

  As will be described later, stray capacitance exists between the plate-like conductors 3 and the windings 4 and 5, and in this embodiment, this stray capacitance is increased. In order to increase the stray capacitance between the plate-like conductor 3 and the windings 4 and 5, the width of the plate-like conductor 3 is increased and the windings 4 and 5 are arranged on the plate-like conductor 3. Specifically, the width of the plate-like conductor 3 is made wider than at least the total value of the radii of the winding 4 and the winding 5. On the other hand, if the width of the plate conductor 3 is too wide, it intersects with the plate conductor 3 of the adjacent turn. In this case, the entire core portion 21 becomes a thin film conductor. However, due to the current flowing through the windings 4 and 5, a current also flows through the winding core portion 21 that has become a thin film conductor due to mutual induction. The magnetic field which interrupts the electric current which flows into 5 will be produced. Therefore, the width of the plate-like conductor 3 must be a width that does not intersect with the plate-like conductor 3 of the adjacent turn.

  Next, the operation of the common mode filter 1 shown in FIG. 1 will be described. The signals are input to the two windings 4 and 5 as signals in opposite directions. Thus, the magnetic fluxes generated on the magnetic core 2 by the differentially input signals are opposite to each other and cancel each other. Therefore, no magnetic field is generated and the impedance (inductance) is not increased, so that the signal is output without being attenuated.

  On the other hand, since common mode noise is generated in the same direction of the windings 3, 4, and 5, magnetic fluxes generated on the magnetic core 2 become magnetic fluxes in the same direction, and are superimposed and strengthened. Therefore, a magnetic field is generated and impedance (inductance) is increased, so that common mode noise can be attenuated. In this way, the common mode filter 1 can attenuate only noise without attenuating the signal.

  FIG. 3 is a longitudinal sectional view showing a state in which the common mode filter 1 is arranged on the printed circuit board 6. The printed circuit board 6 has a laminated structure, and has a ground layer 61 as an intermediate layer. The ground layer 61 is electrically connected to a plate-like conductor 3 as an electrode on the flange portions 22 and 23 on the common mode filter 1 (not shown).

  The common mode filter 1 floats between a pair of windings 4 and 5 (hereinafter referred to as a pair line 7) and a plate-like conductor 3, between a pair line 7 and a ground layer 61, and between adjacent pair lines 7. Capacitances C1, C2, and C3 exist. The characteristic impedance of the transmission line changes depending on the total value of the stray capacitances C1, C2, and C3.

Here, the capacity C is
C = ε · S / d (ε is a constant)
It is represented by This means that the capacitance C is proportional to the cross-sectional area S of the substance that generates the capacitance component and inversely proportional to the distance d.

  In the common mode filter 1 according to the present embodiment, since the area S of the plate conductor 3 that is a ground line is large and the distance d between the pair line 7 and the plate conductor 3 is short, the stray capacitance C1 becomes large. On the other hand, since the distance between the pair line 7 and the ground layer 61 is much longer than the distance between the pair line 7 and the plate-like conductor 3, the stray capacitance C2 is much smaller than the stray capacitance C1. Therefore, the stray capacitance C1 becomes dominant and the stray capacitance C2 can be ignored.

  In addition, the stray capacitance C3 exists between the adjacent pair lines 7. In this case, the stray capacitance C3 is much smaller than the stray capacitance C1 for the same reason. Therefore, the stray capacitance C1 is dominant. Become.

  In this way, the stray capacitance generated in the common mode filter 1 is substantially the same as the stray capacitance C1 between the pair line 7 and the plate-like conductor 3 regardless of the printed circuit board 6 on which the common mode filter 1 is mounted. Since it can be assumed that there is, the characteristic impedance is also constant regardless of the printed circuit board 6. On the other hand, the stray capacitance generated in the common mode filter 1 can be changed by changing the area of the plate-like conductor 3, that is, changing the width of the plate-like conductor 3. Therefore, the characteristic impedance when the printed circuit board 6 is arranged can be adjusted in advance simply by changing the width of the plate-like conductor 3.

  Further, the stray capacitance C3 causes crosstalk between the pair lines 7. However, also in this case, since the stray capacitance C3 is much smaller than the stray capacitance C1, the stray capacitance C1 is dominant. That is, since the combination of the pair line 7 and the plate-like conductor 3 is stronger than the connection between the pair lines 7, electric power is less likely to leak between adjacent pair lines. In this way, crosstalk between each pair line 7 can be reduced.

  Furthermore, since the magnetic coupling between the windings 4 and 5 is strengthened by shortening the distance between the windings 4 and 5, the magnetic fluxes cancel each other more efficiently. Accordingly, there is no increase in inductance due to the magnetic field, and a high-frequency signal can be passed.

  A common mode filter according to a second embodiment of the present invention will be described with reference to FIG. FIG. 4 is a diagram showing a state in which the plate core 8 is attached to the common mode filter 1 according to the first embodiment. The plate core 8 is a plate-like core such as ferrite or dielectric, and is attached to the upper surface of the common mode filter. By attaching the plate core 8 to the common mode filter 1, the magnetic core 2 and the plate core 8 can form a closed magnetic circuit. Since the magnetic permeability of the magnetic flux is increased by this closed magnetic path, only the impedance with respect to the common mode noise can be increased, and the common mode noise can be further reduced.

  The common mode filter 1 according to the present invention is not limited to the above-described embodiment, and various modifications and improvements can be made within the scope described in the claims. For example, instead of covering the windings 4 and 5, the plate-like conductor 3 may be coated, or both the windings 4 and 5 and the plate-like conductor 3 may be covered. Further, an insulating sheet may be placed between the windings 4 and 5 and the plate-like conductor 3. The plate-like conductor 3 may be a flat wire or a metal film formed by a plating method, a sputtering method, a vapor deposition method, or the like. Further, the plate-like conductor 3 may not be formed directly on the core part 21. Further, the number of windings may be one, or two or more. The plate-like conductor 3 may not be connected to the ground.

1 is a perspective view showing a configuration of a common mode filter 1 according to a first embodiment of the present invention. (A) is a perspective view of the magnetic core 2, and (b) is a bottom view of the magnetic core 2. FIG. 3 is a longitudinal sectional view showing a state in which a common mode filter 1 is arranged on a printed circuit board 6. These are figures which showed a mode that the plate core 8 was attached to the common mode filter 1. FIG. (A) is a perspective view showing a common mode filter in which two windings are closely attached to a ferrite or dielectric drum core, and (b) is a three-dimensional view of a ferrite or dielectric drum core. It is the perspective view which showed the common mode filter which wound the coil | winding closely.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Common mode filter 2 Magnetic body core 3 Plate-shaped conductor 4, 5 Winding | winding 7 Pair line 8 Plate core 21 Core part 22 and 23 Kaki part 24 and 25 Electrode

Claims (9)

A magnetic core having a winding core, and
A plurality of electrodes respectively provided at both ends of the core part;
At least two windings wound around the core and both ends connected to the electrodes;
A common mode filter comprising:
The winding includes a plate-like conductor wound spirally around the winding core portion, and two linear conductors similarly wound on the plate-like conductor, the plate-like conductor comprising: A common mode filter characterized by being used as a ground line .   2. The common mode filter according to claim 1, wherein the magnetic core has flange portions at both ends of the core portion, and the electrodes are respectively provided at the flange portions of the both ends.   The common mode filter according to claim 1, wherein the plate-like conductor is formed directly on the core portion.   The common mode filter according to claim 1, wherein a plurality of the linear conductors are insulated from each other.   5. The common mode filter according to claim 1, wherein at least one of the plate-like conductor and the linear conductor is coated with an insulating film.   The common mode filter according to claim 1, wherein an insulating sheet is placed between the plate-like conductor and the linear conductor.   The common mode filter according to any one of claims 1 to 6, further comprising a plate core that sandwiches the winding in cooperation with the winding core portion.   The winding includes two linear conductors that are similarly spirally bifurcated on the plate-shaped conductor, and the width of the plate-shaped conductor is wider than the total value of the radii of the line-shaped conductors. The common mode filter according to any one of claims 1 to 7, wherein: The common mode filter according to any one of claims 1 to 8, wherein the two linear conductors are similarly wound in a spiral manner on the plate-like conductor .

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