Laminated chip common mode choke coil
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a laminated chip common mode choke coil and, more particularly, to the same which is used for a high frequency circuit. 
     FIG. 6 is an exploded perspective view showing processes of manufacturing a conventional laminated chip common mode choke coil. In this conventional example, the laminated chip common mode choke coil having a winding of 2.5 turns is formed by laminating magnetic substance sheets 31a-31n provided with a conductor pattern and by pressing and thereafter baking them into an integral entity. Ferrite sheets 31b, 31d-31k and 31m are provided with a conductor pattern and a through-holes. The number of turns of this laminated chip common mode choke coil can be increased by repeatedly laminating ferrite sheets 31f-31i for example. 
     On ferrite sheets 31a and 31n, no conductor pattern is formed. On the surface of the ferrite sheet 31b, a conductor pattern 32 is formed which becomes a lead-out electrode at one end of one coil. One end of the conductor pattern 32 is drawn out to the outer edge of the ferrite sheet 31b, and the other end extends to a through-hole 32a. Following the ferrite sheet 31b, a ferrite sheet 31c provided with a through-hole 33a is laminated. 
     A conductor pattern 34 which becomes one end of the other coil is formed, roughly in a U shape, on the ferrite sheet 31d. One end of the conductor pattern 34 is led out to the outer edge of the ferrite sheet 31d to be formed into a conductor pattern 34a that becomes a lead-out electrode, and the other end extends to a through-hole 34b. Furthermore, on the ferrite sheet 31d, a through-hole 34c which connects to the through-hole 33a is formed in a position not contacting with the conductor pattern 34. 
     On the ferrite sheets 31e, 31f, 31g, 31h, 31i and 31k, conductor patterns 35, 36, 37, 38, 39 and 41 are formed respectively, and through-holes 35a, 35b, 36a, 36b, 37a, 37b, 38a, 38b, 39a, 39b and 41a are also formed in the above ferrite sheets. 
     On a ferrite sheet 31j, a conductor pattern 40 which becomes the other end of the other coil is formed roughly in an L shape. On end of the conductor pattern 40 is led out to the outer edge of the ferrite sheet 31j to be formed into a conductor pattern 40a that becomes a lead-out electrode. On the ferrite sheet 31j, a through-hole 40b which connects to the through-hole 39a is formed in a position not contacting with the conductor pattern 40. 
     A conductor pattern 43 is formed which extends to the outer edge of the ferrite sheet 31m and becomes the lead-out electrode of the other end of one coil. 
     In the choke coil, one coil comprises the conductor patterns 35, 37, 39 and 41, and both ends of it are connected to the conductor patterns 32 and 34 respectively. And the other coil comprises the conductor patterns 34, 36, 38 and 40, and both ends Of it are connected to the conductor patterns 34a and 40a respectively. 
     As shown in FIG. 7, in the choke coil, a rectangular coil of 1.0 turn is formed with the ferrite sheets 31f, 31g, 31h and 31i. As shown in FIGS. 8 or 9, a hexagonal coil 45 or a circular coil 46 each having a different pattern from the rectangle may be formed. 
     In a common mode choke coil, normal mode impedance may lead to attenuation of a signal, and thus it is desirable to suppress the impedance to a low level. However, among conventional chip type common mode coils such as the above-mentioned, a good common mode impedance characteristic and a good resistance characteristic could be obtained, while a normal mode impedance characteristic at a high frequency (30-150 MHz) is not good. As shown in FIG. 5, a choke coil having the coil pattern of FIG. 7 causes an impedance of 110Ω at a frequency in the vicinity of 50 MHz. 
     A choke coil having another coil pattern such as shown in FIGS. 8 or 9 similarly has a problem of increase in normal mode impedance although it satisfies other characteristics. It was found that a portion not contributing to coupling between the two coils exists in the lead-out electrode as the cause of increase in normal mode impedance. 
     SUMMARY OF THE INVENTION 
     Therefore, the principal object of the present invention is to provide a laminated chip common mode choke coil which is able to improve the coupling between the two coils and to reduce a peak value of the normal mode impedance. 
     The present invention is a laminated chip common mode choke coil wherein a pair of coils and lead-out electrodes led out from the coils are formed by laminating a plurality of magnetic substance sheets provided with a conductor patterns and through-holes for connecting the conductor patterns, and wherein the permeability around the lead-out electrode is reduced by applying non-magnetic material to the magnetic substance sheets around the conductor patterns that become the lead-out electrodes and by diffusing the non-magnetic material. 
     Because of the low permeability around the lead-out electrode, the magnetic reluctance of that portion becomes high, and thus magnetic flux scarcely generates in the circumference of the lead-out electrode. Therefore, leakage flux in the portion of the lead-out electrode is little. 
     According to the present invention, the leakage flux in the portion of the lead-out electrode becomes little, and the coupling between the two coils becomes good. Therefore, the normal mode impedance can be reduced and the attenuation of a signal can be decreased. 
     The above and other objects, features, aspects and advantages of the invention will more fully be apparent from the detailed description of the following embodiments with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing one embodiment of the invention. 
     FIG. 2 is an exploded perspective view showing processes of manufacturing the laminated chip common mode choke coil of FIG. 1. 
     FIG. 3 is a perspective view showing part of the lamination structure of FIG. 2. 
     FIG. 4 is a perspective view showing another part of the lamination structure of FIG. 2. 
     FIG. 5 is a graph showing frequency characteristics of normal mode impedances of the choke coils of the invention and a conventional choke coil. 
     FIG. 6 is an exploded perspective view showing processes of manufacturing a conventional laminated chip common mode choke coil. 
     FIG. 7 is an illustration showing a coil pattern of the conventional laminated chip common mode choke coil. 
     FIG. 8 is an illustration showing another example of a coil pattern of the conventional laminated chip common mode choke coil. 
     FIG. 9 is an illustration showing still another example of a coil pattern of the conventional laminated chip common mode choke coil. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view showing one example of the laminated chip common mode choke coil of the invention, and FIG. 2 is an exploded perspective view showing its manufacturing processes. 
     As shown in FIG. 2, a laminated chip common mode choke coil is formed in such a manner that ferrite sheets 1b, 1d, 1e, 1f, 1g, 1h, 1i, 1j, 1k and 1m each provided with a conductor pattern, and ferrite sheets 1a, 1o, 1p, 1c, 1l, 1q, 1r and 1n each provided with no conductor pattern are laminated and pressed one another and thereafter baked into an integral entity. In the choke coil, as shown in FIG. 8, a hexagonal coil pattern is formed. In addition, a number of turns of the coil can be increased by repeatedly laminating given magnetic substance sheets. 
     On the ferrite sheets 1a and 1n, nothing is formed. On the surface of the ferrite sheet 1b, conductor patterns 3 and 4 for lead-out electrodes at one end of two coils are formed in parallel. First ends of these conductor patterns 3 and 4 are led out to separate positions on the same side of the ferrite sheet 1b. The other ends of the conductor patterns 3 and 4 are extended to through-holes 3a and 4a formed in the ferrite sheet 1b. 
     Between the ferrite sheet 1a and the ferrite sheet 1b, a sheet 1o coated with non-magnetic material is disposed. As shown in FIG. 3, the sheet 1o is a ferrite sheet which the non-magnetic material paste is coated on a portion 2 opposing to the conductor patterns 3 and 4 on the surface of the ferrite sheet 1b. 
     Following the ferrite sheet 1b, a sheet 1p coated with non-magnetic material is laminated. The sheet 1p is a ferrite sheet which the non-magnetic material paste is coated on a portion 5 opposing to the conductor patterns 3 and 4 on the surface of the ferrite sheet 1b. In addition, the sheet 1p coated with non-magnetic material is provided with through-holes 5a and 5b. And the ferrite sheet 1c is provided with through-holes 6a and 6b at the positions corresponding to the through-holes 5a and 5b respectively. 
     Following the ferrite sheet 1c, the ferrite sheet 1d is laminated. On the ferrite sheet 1d, conductor patterns 7 and 8 which become one end of the two coils are formed. First ends of the conductor patterns 7 and 8 are connected to the through-holes 3a and 4a of the conductor patterns 3 and 4 respectively and the other ends are provided with through-holes 7a and 8a. 
     Furthermore, following the ferrite sheet 1d, the ferrite sheet le is laminated. On the ferrite sheet 1e, a conductor pattern 9 is formed which comprises three sides of a hexagon. One end of the conductor pattern 9 is connected to the through-hole 8a of the conductor pattern 8, and the other end is provided with a through-hole 9a. Furthermore, in the ferrite sheet 1e, a through-hole 9b is formed at the position corresponding to the through-hole 7a of the conductor pattern 7. 
     Following the ferrite sheet 1e, the ferrite sheet 1f is laminated. On the ferrite sheet 1f, a conductor pattern 10 is formed which comprises three sides of a hexagon forming the other coil. One end of the conductor pattern 10 is connected to the through-hole 7a of the conductor pattern 7, and the other end is provided with a through-hole 10a. Furthermore, in the ferrite sheet 1f, a through-hole 10b is formed at the position corresponding to the through-hole 9a  of the conductor pattern 9. 
     Similarly, on the ferrite sheets 1g, 1h, 1i and 1j, conductor patterns 11, 12, 13 and 14 are formed and through-holes 11a, 11b, 12a, 12b, 13a, 13b, 14a and 14b are formed in the above ferrite sheets respectively. 
     On the ferrite sheet 1k, conductor patterns 15 and 16 which become the other ends of the two coils are so formed that they become sides of the respective hexagons. One end of the conductor pattern 15 is connected to the through-hole 14a of the conductor pattern 14, and one end of the conductor pattern 16 is connected to the through-hole 13a of the conductor pattern 13. Furthermore, the other end of the conductor pattern 15 is provided with a through-hole 15a, and the other end of the conductor pattern 16 is provided with a through-hole 16a. 
     In the ferrite sheet 11, through-holes 17a and 17b are formed at the positions corresponding to the through-holes 15a and 16a. Furthermore, on the ferrite sheet 1m, conductor patterns 19 and 20 which become lead-out electrodes are formed in parallel. Between the ferrite sheets 1l and 1m, a sheet 1q coated with non-magnetic material is laminated. As shown in FIG. 4, the sheet 1q coated with non-magnetic material is a ferrite sheet which the non-magnetic material paste is coated on the portion 18 opposing to the conductor patterns 19 and 20 on the surface of the ferrite sheet 1m. 
     Following the ferrite sheet 1m, the ferrite sheet 1r is laminated. The sheet 1r is a ferrite sheet which the nonmagnetic material paste is coated on the portion 21 opposing to the conductor patterns 19 and 20 on the surface of the ferrite sheet 1m. Following this ferrite sheet 1r, the ferrite sheet 1n is laminated. 
     The laminated chip common mode choke coil is formed by laminating and integrally baking the ferrite sheets 1a-1n and by making external electrodes 22a, 22b, 22c and 22d. In this choke coil, one coil comprises the conductor patterns 7, 10, 12, 14 and 15, while the other coil comprises the conductor patterns 8, 9, 11, 13 and 16. Both ends of one coil are connected to the lead-out electrodes 3 and 19 respectively, and both ends of the other coil are connected to the lead-out electrodes 4 and 20 respectively. In this choke coil, the two coils having a hexagonal shape and a 1.0 turn are formed with the ferrite sheets 1g1j. 
     Furthermore, the external electrodes 22a, 22b, 22c and 22d are respectively connected to the lead-out electrodes formed with the conductor patterns 3, 4, 19 and 20. 
     In order to obtain connections between the electrodes, it is necessary that the through-holes formed in the ferrite sheets are coated with Ag paste or Ag - Pd paste. This is the same as to the ferrite sheets 1p and 1q, however no through-hole is necessary for the ferrite sheets 1o and 1r. 
     As for the material of the ferrite sheets, Ni-Cu-Zn ferrite or the like is used for example. Furthermore, as to the material of the non-magnetic material paste, anything which diffuses into the ferrite sheet and reduces its permeability may be used. However, some of the non-magnetic materials may cause cracks of a coating surface of the non-magnetic material paste, and thus it is necessary to check a particle size of the material powder and an amount of varnish before preparing the paste. In this experiment, the choke coils were formed using three kinds of non-magnetic material paste, that is, (1) Co-ferrite, (2) Co 3  O 4 , and (3) Borosilicate glass containing alkaline-earth metal, and the characteristics of the choke coils were measured, and the data is shown in FIG. 5. The three kinds of the paste had a 20 μm in thickness before sintering. The Ni-Cu-Zn ferrite having a permeability of 600 was used. 
     In a choke coil of the invention, the non-magnetic material diffuses and the permeability around the lead-out electrode becomes low by baking the ferrite lamination. Therefore, the magnetic reluctance of that portion becomes high, thus making it hard to generate magnetic flux. Thereby, leakage flux generated in the portion of the lead-out electrode becomes little, and thus coupling between the two coils becomes good. Thus, as shown in FIG. 5, the peak value of a normal mode impedance could be reduced to less than 20Ω. 
     In addition, if a number of the ferrite sheets coated with the non-magnetic material paste is increased, an effect of reducing the normal mode impedance can be further enhanced. The non-magnetic material paste may be coated on both sides of the ferrite sheet. Furthermore, a choke coil having a desired number of turns can be obtained by adjusting a number of the ferrite sheets provided with a conductor pattern. The coil pattern to be formed can be made in the shape of FIGS. 7 or 9 other than a hexagon. 
     It will be apparent from the foregoing that, while the present invention has been described in detail and illustrated, these are only particular illustrations and examples and the invention is not limited to these. The spirit and scope Of the invention is limited only by appended claims.