1. Field of the Invention
The present invention relates to a common mode choke coil and a line filter used in various electronic circuits, and more particularly to a common mode choke coil and a line filter having edgewise windings of a rectangular insulated wire.
2. Description of the Related Art
In recent years, since the miniaturization and enhanced performance of an electronic apparatus have been strongly demanded, a common mode choke coil used in a line filter is required to be downsized and improved in performance. Meanwhile, the characteristic of a common mode choke coil, for example, conductive noise level is controlled by regulation in a frequency band between 150 kHz and 30 MHz. Conventionally, a common mode choke coil uses a round insulated wire (refer to FIG. 1 of Japanese Patent Publication No. 2000-150243). Such a conventional common mode choke coil, as shown in FIG. 4, comprises: a bobbin 413 shaped cylindrical with a cylindrical hollow, and having first and second windings 411 and 412 wound therearound; a closed magnetic path core 414 (hereinafter referred to simply as “magnetic core” as appropriate) of square type with its center core leg inserted in the hollow of the bobbin 413; a terminal stand 415 having the bobbin 413 and the magnetic core 414 mounted thereon; and four terminal pins 416 (only two are shown in the figure) having their one ends embedded in the terminal stand 415 and also connected to respective lead wires of the first and second windings 411 and 412.
The bobbin 413 is composed of two parts each shaped semi-cylindrical and put together with each other so as to enclose the center core leg of the magnetic core 414. The bobbin 413 has a partition 417, by which the first and second windings 411 and 412 wound around the bobbin 413 are separated from each other. The terminal stand 415 includes a frame section 418 having an opening and a base section 419 having the terminal pins 416 embedded therein. The frame section 418 is arranged to stand at an end portion of the base section 419 thereby forming a substantially L-letter in its side view. The magnetic core 414 is brought into contact with an outer peripheral part of the frame section 418 of the terminal stand 415 and a portion of the bobbin 413 is inserted in the opening of the frame section 418 so that the magnetic core 414 and the bobbin 413 are positioned securely and correctly. Also, insulation plates 420 are provided entirely at upper and lower ends of the frame section 418, respectively, so as to go through respective gaps between the both outer core legs of the magnetic core 414 and the bobbin 413 thereby insulating the magnetic core 414 from the first and second windings 411 and 412 wound on the bobbin 413.
On the other hand, a common mode choke coil using edgewise windings of a rectangular insulated wire is increasingly used because the edgewise winding has the following advantages over the winding of a round insulated wire. Firstly, the edgewise winding can better achieve higher performance, higher efficiency, miniaturization and lower-profile due to its larger conductor occupation ratio. Secondly, the edgewise winding has a smaller stray capacity and therefore can realize better frequency characteristics. And thirdly, the edgewise winding does not require a process of winding a wire on a bobbin, and is easier to assemble, resulting in an easier automation of the manufacturing process.
FIGS. 5A and 5B show a conventional common mode choke coil using edgewise windings, perspectively viewed from two opposing directions, respectively (refer to FIG. 1 of Japanese Patent Publication No. H09-134827). The common mode choke coil comprises two edgewise windings 59 each formed of a rectangular insulated wire, each provided around a core leg 54a of a magnetic core 54 consisting of two core pieces, and having the same number of winding turns, with one winding 59 of the two being stacked on the other winding 510 concentrically. The one winding 59 has two terminations 59a and 59b, which lead out without crossing each other at one side of a bobbin 57, have their insulation peeled off, and which are hooked respectively around termination tying sections 57e and 57f of the bobbin 57 thereby constituting terminals. And, the other winding 510 has two terminations 510a and 510b, which lead out crossing each other at a side of the bobbin 57 opposite to the aforementioned one side, have their insulation peeled off, and which are hooked respectively around termination tying sections 57g and 57h of the bobbin 57 thereby constituting terminals.
FIG. 6 shows another conventional common mode choke coil using edgewise windings (refer to FIG. 2 of Japanese Patent Publication No. H11-273975). The common mode choke coil comprises: a magnetic core 617 consisting of two core pieces, shaped square and forming a closed magnetic path; and four edgewise windings, i.e., first to fourth windings 611, 612, 613 and 614 provided around the magnetic core 617. The first and second windings 611 and 612 are provided respectively around two core legs 617a butting each other, and the third and fourth windings are provided respectively around the other two core legs 617a butting each other. The first and third windings are connected in series to each other, and the second and fourth windings are connected in series to each other. Magnetic fluxes generated respectively by the first and second windings cancel out each other with a line current, which is the case also with magnetic fluxes of the second and third windings, the third and fourth windings, and the fourth and first windings. On the other hand, magnetic fluxes generated respectively by the first and third windings are aggregated with a line current, and also magnetic fluxes generated respectively by the second and fourth windings are aggregated by a line current. And, the first and fourth windings are arranged side by side with their winding directions set opposite to each other, and the second and third windings are arranged side by side with their winding directions set opposite to each other so that magnetic fluxes generated by the first to fourth windings are aggregated with currents flowing in the same direction (noise current).
A line filter using one of the above-described various common mode choke coils is shown in FIG. 7, which comprises a common mode choke coil 70 and by-pass capacitors 71 such that one termination of the common mode choke coil 70 serves as an input terminal and the other termination thereof is connected to the by-pass capacitors 71 thereby serving as an output terminal, and a load 72 is connected to both ends of the output terminal, whereby conductive noise in the frequency band mentioned above is removed. In the line filter structured as described above, noise is cut mostly such that noise current in a low frequency is cut by impedance due to an inductance factor of the common mode choke coil 70, deterioration of inductance performance in a high frequency band is made up for by the by-pass capacitors 71, and that a high frequency noise current is caused to flow toward the ground.
Another line filter using two of the above-described various common mode choke coils is shown in FIG. 8, which comprises two common mode choke coils 70 and 80 and by-pass capacitors 71 such that the common mode choke coils 70 and 80 are connected in cascade to each other, the by-pass capacitors 71 are connected to the connections between the common mode choke coils 70 and 80, one termination of the common mode choke coil 70 serves as an input terminal, and that one termination of the common mode choke coil 80 serves as an output terminal, whereby the performance of the filter is improved. Thus, the line filter comprises one or more common mode choke coils and by-pass capacitors in order to remove conductive noise in the above-described frequency band.
The performance of the above-described common mode choke coils will hereinafter be explained with reference to an equivalent circuit shown in FIG. 9, in which L represents inductance, Cs represents stray capacity, and R represents wire resistance.
The following equation (1) is valid:Z=R+jωL/(1−ω2LCs)  (1)where Z is impedance between terminals c and d of the coil, f is frequency, and ω is resonant frequency and equal to 2πf.
Here, resonant frequency ω can be expressed by the following equation (2):ω=(LCs)−1/2  (2)
The above equations can hold true only when L is constant relative to the frequency, but in practice, the initial permeability μ of the core varies according to the frequency as shown in FIG. 10, indicating a sharp decline in the high frequency band.
Referring to FIG. 10, S1 represents characteristics of a conventional Mn—Zn ferrite core, and S2 represents frequency characteristics of a new Mn—Zn ferrite core (refer to Japanese Patent Publication Nos. 2001-220221 and 2001-220222) used in the present invention and described later in detail. In the conventional Mn—Zn ferrite core, the initial permeability μ is as large as 5,000 in a low frequency band but declines sharply, for example, to approximately ⅓ at 1 MHz, as indicated by symbol A. On the other hand, in the new Mn—Zn ferrite core, the initial permeability μ is 4,000, somewhat smaller than that of the conventional Mn—Zn ferrite, in a low-frequency band but declines less sharply, as indicated by symbol B, only to approximately {fraction (1/2.5)} at 1 MHz, and keeps declining less sharply from 1 MHz upward than in the conventional Mn—Zn ferrite core.
For the ease of understanding problems associated with the conventional Mn—Zn ferrite core, changes of characteristics of Mn—Zn ferrite cores depending on difference in their initial permeability will hereinafter be explained with reference to FIGS. 10 and 11. FIG. 11 shows characteristics of three toroidal cores each having an outer diameter of 25 mm, an inner diameter of 15 mm and a thickness of 13 mm, and having about 20 winding turns of a round insulated wire therearound thus limiting the number of winding turns in order to prevent stray capacity Cs of winding from having influence thereby allowing the characteristics of the cores to distinctly show up. In FIG. 11, symbols Z1 and Z2 show frequency characteristics of impedance of common mode choke coils using the conventional Mn—Zn ferrite core (whose characteristics are shown by S1 in FIG. 10 ) and the new Mn—Zn ferrite core (whose characteristics are shown by S2 in FIG. 10), respectively. Specifically, the common mode choke coil having the characteristics shown by Z1 uses a core whose initial permeability μ declines sharply from 1 MHz upward, while the common mode choke coil having the characteristics shown by Z2 uses a core whose initial permeability μ measures 100 or more even at 10 MHz.
As apparent in FIG. 11, the impedance of the common mode choke coil (whose characteristics are shown by Z1), which uses the conventional Mn—Zn ferrite core whose initial permeability μ starts declining already at 1 MHz, comes down to as low as 1 kΩ at 35 MHz. On the other hand, the impedance of the common mode choke coil (whose characteristics are shown by Z2), which uses the new Mn—Zn ferrite core whose initial permeability μ measures at 100 or more even at 10 MHz, starts declining at 5 MHz or higher and maintains as high as 3 kΩ at 35 MHz. Thus, a core which has a high initial permeability in the high frequency band has a large impedance in the high frequency band. However, as the number of winding turns increases, the stray capacity Cs of winding becomes increased causing a resonance at a frequency gained by the aforementioned equation (2) before the initial permeability μ starts changing, and then the impedance Z decreases according to the characteristics defined by an interaction between the stray capacity Cs of winding and the inductance L. As a result, even in the core having a high initial permeability in the high frequency band, its impedance starts declining at a lower frequency due to the conventional round insulated wire having a large stray capacity Cs, and consequently its impedance curve tends to shift entirely to a lower frequency, thereby producing characteristics shown by symbol Z3 in FIG. 11.
The conventional common mode choke coil usually uses a Mn—Zn ferrite core whose initial permeability varies according to the frequency, measuring high in a low frequency band but declining sharply in a high frequency band as shown by S1 in FIG. 10. The conventional common mode choke coil may alternatively use a Ni—Zn ferrite core. In the Ni—Zn ferrite, the initial permeability does not decline sharply in a high frequency band but is low in a low frequency band.
In order to efficiently remove the conductive noise factor of the common mode choke coil, it is essential to make the impedance Z of the common mode choke coil as large as possible. The following three methods are available to increase the impedance Z when a wire is set to have a constant diameter in view of current capacity based on a temperature rise of the coil.                1. The initial permeability μ of the core is maximized;        2. The number of winding turns is increased; and        3. The core constant (=effective magnetic path length/effective section area) is reduced.        
The above three methods, however, have respective problems described below.
1. The maximization of the initial permeability μ results in an increased impedance. However, the initial permeability μ has frequency characteristics as discussed above. In the conventional Mn—Zn ferrite core, as shown by S1 in FIG. 10, the initial permeability ranges around 5,000 in the low frequency band, specifically only up to 300 kHz or 500 kHz, and then declines sharply as indicated by symbol A to approximately ⅓ at 1 MHz. There is a ferrite core which has an initial permeability of 10,000 or higher, but the initial permeability starts declining at a still lower frequency, for example 100 kHz, and measures a still lower value in a high frequency band. On the other hand, the Ni—Zn ferrite core has a large impedance between 10 MHz and 30 MHz but has a low initial permeability in the low frequency band. Therefore, the number of winding turns must be increased to increase impedance, which limits its usage to special application.
In any case, the frequency band between 150 kHz and 30 MHz, in which conductive noise is regulated, cannot be successfully covered.
2. Inductance is proportional to a square of the number of winding turns. However, the increase in the number of turns causes stray capacity of winding to increase. Consequently, not only the resonant frequency becomes lower, but also the impedance in a high frequency band decreases. As a result, the impedance starts declining at a lower frequency due to the effect of the stray capacity Cs, which prohibits an excellent core material from fully demonstrating its excellence. And, the increased number of winding turns deteriorates heat radiation effect thus causing a temperature rise, which results in requirement of an increased diameter of the wire.
3. The shape of the core is determined by a space factor of a board and therefore cannot be freely determined.
Thus, the following problem exists in acquiring a common mode choke coil that works duly in a wide frequency band (between 10 kHz and 30 MHz). Since the conventional Mn—Zn ferrite core has a high initial permeability, its impedance can be increased without increasing the number of winding turns, thus preventing the increase of the stray capacity. However, the sharp decline of the initial permeability in the high frequency band causes the impedance to decrease. On the other hand, since a core having a high initial permeability in the high frequency band has a low initial permeability in the low frequency band, the number of winding turns must be increased in order to obtain a large impedance in the low frequency band. However, the increased number of winding turns causes the stray capacity to increase, thus resulting in a reduced impedance in the high frequency band.
Also, a line filter using common mode choke coils inherently incurs the above problem. Therefore, by-pass capacitors for removing noises in the high frequency band are required in the line filter. However, the provision of the by-pass capacitors encounters space and cost problems.