Coil device

Abstract: A coil device comprises magnetic cores forming a closed magnetic path therein and having a magnetic gap in such path, and a coil wound on the magnetic cores partially. In this structure, the mutually opposed portions of the magnetic cores in the region to form the magnetic gap are so shaped that the cross-sectional area of the fore end becomes smaller than that of the base end, thereby preventing concentration of any leakage magnetic flux on the gap portions while averting abnormal generation of heat in the coil to consequently eliminate harmful influence of noise on peripheral apparatus or components.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements in a coil device for use in a flyback transformer, a switching power transformer, a choke coil or the like. And more particularly, it relates to improvements in a magnetic core with a gap which is filled by magnetic flux and also in a coil device employing such a magnetic core. 2. Description of the Prior Art In any of the conventional transformers, choke coils and so forth known heretofore, it is customary to form a gap in a closed magnetic path so that the magnetic core thereof is not saturated when a desired current is caused to flow. For example, when a ferrite magnetic core usually having a magnetic permeability µ of 5000 or so is used in a transformer, a gap is formed therein to reduce the effective permeability µ within a range of 50 to 300. This signifies that a gap having a great magnetic reluctance needs to be existent in a ferrite magnetic core of which magnetic reluctance is originally small, whereby a great leakage flux is generated in the periphery of the gap. It is generally known that such leakage flux exerts at least two harmful influences as follows. (A) Noise is induced in peripheral apparatus (components) which are prone to be affected by magnetic induction. (B) In case the coil is so wound as to surround the gap, there occurs abnormal generation of heat in the coil around the gap due to the leakage flux. For the purpose of solving the above problems, a variety of improvements have been developed. In an attempt to settle the problem (A), there is contrived an exemplary method of forming a gap merely in the coil alone. However, such method brings about another fault that worsens the problem (B) on the contrary. With regard to the problem (B), some prior examples are known as disclosed in Japanese Patent Laid-open No. 55 (1980)-77115 and Utility Model Laid-open No. 57 (1982)-130402, wherein a gap positioned in a coil is divided magnetically into a plurality of serial portions so as to disperse the concentration of leakage flux. In the prior means developed for solving the problems (A) and (B), there are known examples as disclosed in Japanese Utility Model Publication Nos. 53 (1978)-53850 and 60 (1985)-7448, wherein a gap filler, of which relative permeability is greater than that of air (greater than 1), is used to reduce the magnetic reluctance in the gap porticn so as to diminish the leakage flux. When such gap filler of a material having a greater relative permeability than that of air (greater than 1) is disposed inside of a coil, there exists a possibility that the problems (A) and (B) can be solved to some extent. The use of a coil device to reduce magnetic flux leakage and dispersion is known from US-Patent No. 4 359 706 where the pole peaces are preferably conical or are tapered or cylindrically shaped to provide a high concentration of magnetic flux intensity in the "magnetic gap" between the pole pieces. As an additional arrangement for shaping the magnetic flux and concentrating the high intensity magnetic lines of force in the so-called magnetic gap, coil piece extensions or field concentration members in the form of cylindrical, frusto, conical, single or multiple concentric shields are disposed around the pole pieces. No measurements whatsoever are taken to provide for leakage flux reduction by choosing different coil diameters or by giving the coil pieces a specially logarithmic shape. The coils of course are not comparable with the invention since they are a direct prolongation of each of the coil pieces and are neither isolated from it nor are they supplied with extra current and voltage. It may be pointed out that the US-Patent cited above is a magnetic field generating apparatus and is not adequate for a choke coil or for a transformer or for a power transformer. From the japanese utility model JP-U-57-130402 an extra coil is used as can be seen from Figures 1 and 2 of this application which surrounds the middle pole of the E-shaped coil device and is therefore geometrically comparable with the invention. Nevertheless the Japanese patent application is directed to a non-linear choke coil and has no gap since the opposite cores are in contact with each other. The cores also are tapered but are asymmetric in relation to each other. US-Patent 4 454 557 deals with an alternating currenct transformer containing a yoke with an air gap where said air gap is configured such as to provide a non-linear magnetic response for changes in current. The yoke is claimed to be mostly C-shaped and may be curved spherically in order to reach magnetic satuation as close to the gap as possible. Then the voltage response of the device is as is normal for the BH-curve near the saturation branch and can be said to be logarithmic. This logarithmic response is needed by the inventors to receive a high gain at low currents and a low gain at high currents in a secondary coil, so as to receive a broad dynamic range in a secondary circuit where the signal is used by a sensing meter to regulate or to interrupt the current in case of current overflow or fault-current. The invention does not deal with current transformation but with linear power transformation in the linear range of the BH-curve and does not use different numbers of windings in the primary or secondary circuit respectably. Even if the inventors of the current transformer had intended to increase the non-linearity of the device beyond the performance achieved now, it is not evident that they might have invented a logarithmic shape of poles or an extra coil wound around those coiles. What can be said is that apart from modern computerized numeric tool machine complicated coil geometries like a logarithmic shape can not be fabricated easily or straightforward, since normally the tool for fabricating logarithmic curvatures does not exist, thus confining even excellent experts to normal geometries like cylindres, bowles, linear conuses, hyperbola and similar. However, even in such an improved structure, another problem is still left unsettled that the leakage flux is concentrated on the boundary between the gap and the magnetic core, and in addition a new problem also arises with regard to difficulty in obtaining a satisfactory material which has an adequate permeability as a gap filler and still retains a high saturation flux density and low core loss characteristic equivalent to that of the magnetic core. Consequently, some disadvantages are unavoidable including that the coil wound on the boundary between the gap and the magnetic core is heated to an abnormal extent, and the gap portion is also heated excessively due to the core loss of the gap filler material, and further the B-H curve of the magnetic core with the gap filler inserted therein is rendered nonlinear to eventually cause wave form distortion when the coil device is used in a transformer. Thus, in the current technical stage, completely effective improvements are not available. SUMMARY OF THE INVENTION It is therefore an object of the present invention to solve the above problems and provide an improved coil device which is capable of minimizing the harmful influence of noise to peripheral apparatus (components) and diminishing any leakage flux generated in the periphery of a gap to consequently prevent abnormal generation of heat in the coil around the gap. And another object of the present invention resides in providing an improved coil device which realizes lower production cost and enhanced reliability. For the purpose of achieving the objects mentioned, some alterations have been accomplished relative to the coil device cited above also comprising magnetic cores which form a closed magnetic path therein and have a small gap in such path, and a coil wound on the magnetic cores partially. The first important feature of the invention is the logorithmic shape of the core ends as is described in claim 1, where such geometry adjacent both sides of the gap provides linearity of the magnetic flux in the air gap, resulting in a sufficiently linear B-H-curve and a reduced saturation magnetic flux density. And the second feature of the present invention resides in a structure where the mutually opposed pcrtlons of the magnetic cores are so shaped that the cross-sectional area of the fore end becomes smaller than the cross-sectional area of the base end. Furthermore, with regard to the magnetic core portions in the region to form the gap, the rate of the cross-sectional area of the fore end to that of the base end is defined to be within a range of 1 to 90 percent. In addition, the magnetic cores consist of two E-shaped core elements of which legs butt to each other, and a gap is formed between the opposed faces of center legs, and the coil is so wound as to surround the gap. Due to the constitution mentioned, there occurs no concentration of any leakage flux between the gap and the core end faces, and since no gap filler is used, any core loss is not induced to consequently achieve the above objects. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows an exemplary embodiment of the coil device according to the present invention; Fig. 2 is a schematic diagram illustrating the shape of a gap portion in a magnetic core used in a conventional coil device; Fig. 3 is a schematic diagram illustrating modifications of the gap in the magnetic cores used in the coil device of the present invention; Fig. 4 graphically represents a B-H curve in the conventional coil devices using a magnetic core with the gap shown in Fig. 2; Fig. 5 graphically represent B-H curves in coil devices using magnetic cores with the gaps shown in Fig. 3; Fig. 6 illustrates how temperatures are detected in individual portions of the coil device according to the present invention; and Fig. 7 and 8 are schematic diagrams illustrating modifications of the gap in the magnetic cores used in the coil device of the present invention. Figs. 9 through 11 are perspective views illustrating further modified shapes of the magnetic core used in the coil device of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A coil device (1) shown in Fig. 1 comprises two sectionally E-shaped magnetic cores (2), (3) of which fore ends butt to each other, wherein a gap (5) is formed between opposed faces of center legs (2a), (3a), and a coil (4) is wound thereon. Some examples of such sectionally E-shaped magnetic cores are illustrated in Figs. 9 through 11. In the example of Fig. 9, a rectangular core is shaped into E, and its center leg is shaped to be columnar. The next example of Fig. 10 is called a pot type core with a columnar leg formed at the center of a non-through tubular member. And in the example of Fig. 11, the tubular member of the pot core shown in Fig. 10 is partially cut off. Any of the above exemplary cores has an E-shaped cross section. In the actual coil device, a pair of such cores are combined with each other and a coil is wound on the center legs thereof, although merely a single core is illustrated in each of the above diagrams. And such core is composed of ferrite material. Referring now to the accompanying drawings, the characteristic and the structure of an embodiment of the present invention will be described in comparison with that of a conventional example. Fig. 2 illustrates the shape of gap portions in magnetic cores used in a conventional coil device, wherein the shapes of mutually opposed ends (2b) and (3b) of the magnetic cores and the gap width thereof are so determined that the effective permeability of the magnetic core is rendered uniform in the entirety. The opposed ends (2b) and (3b) of the magnetic cores in the conventional coil device of Fig. 2 are shaped to be columnar in a manner that the sectional areas thereof remain unchanged. And the gap has a width of 3 mm. In the exemplary magnetic cores of the present invention also shown in Fig. 3, opposed ends (2b₂), (3b₂) are so shaped that the sectional areas thereof are reduced by tapered portions (2d), (3d) toward opposed faces (2c), (3c), and the gap (5) is formed to have a width of 2.5 mm respectively so that the effective permeability µ becomes uniform. Fig. 4 graphically represents a B-H curve obtained in a conventional coil device using magnetic cores of the shape shown in Fig. 2. Comparing such curves with one another, the saturation magnetic flux density Bm in the conventional coil device with opposed ends of the known shape shown in Fig. 4 is 5510 Gs. It is also found that the linearity in the latter is not changed, although each density thereof is slightly lower than that in Fig. 4. Table 1 shown below is a list of experimental results obtained by using a tester (6) of Fig. 6 and detecting the temperatures in-coil centers X, coil ends Y, cores Z and peripheries W of coil devices (1) having the opposed ends of the aforementioned shapes (under the testing conditions including a frequency of 100 kHz, a current of 0.8 A, sine wave and ambient temperature of 40°C). (In this table, the shape (a) is shown in Fig. 2 and (d) is shown in Fig. 3. In comparison with the known shape of Fig. 2, the shape in the embodiment of the present invention shown in Fig . 3 is so improved that, as listed in Table 1, the temperature in the coil center X is lower by 5 to 20°C; the temperature in the coil end Y is lower by 3 to 12°C; the temperature in the core Z is lower by 1.5 to 10°C; and the temperature in the periphery W is lower by 2.5 to 5.5°C. Judging from the above results in combination with the machining facility and the production cost, it is obvious that the embodiments of the present invention are superior to the known one. Thus, according to the exemplary embodiments described in detail hereinabove, it will be understood that the present invention is structurally simple and ensures satisfactory linearity in the B-H curve with another advantage of preventing abnormal generation of heat that may be caused in the coil around the gap by some leakage magnetic flux. The B-H curve of the coil device shown in Fig. 3 is quite identically to the B-H curve of Fig. 5. The present invention is not limited to the above embodiments alone, and a variety of modifications may be contrived as well. For example, a gap filler of a suitable material free from exerting any harmful influence on the magnetic permeability µ may be inserted in the gap, and the gap may be formed between some other legs than the center legs. As for the shape of the opposed ends, similar effects can be achieved in modified ones as well as in the exemplary shapes of the aforementioned embodiments on condition that the sectional area is reduced toward the opposed faces. Although embodiments can be concerned with a device having two closed magnetic paths, the present invention is applicable also to any device with one, three or more closed magnetic paths. It is a matter of course that the invention can be carried into effect in any other coil device. In each embodiments equivalent effects are attainable if, with regard to the mutually opposed core portions in the region to form a gap, the rate of the cross-sectional area of the fore end to that of the base end is within a range of 1 to 90 percent. In addition, if the fore ends of the magnetic cores (10a), (10b) are so curved as defined by logarithmic functions, as illustrated in Fig. 7, then the characteristics. can further be enhanced when such magnetic cores are employed in the coil device. The curves of such fore end shape are expressed by the following logarithmic functions: γ s - γ = x g ℓ n x s x When the fore ends of magnetic cores (2b₂), (3b₂) are furnished with the planar members (2d), (3d) as illustrated in Fig. 3, remarkable convenience is achieved since the areas of the fore end faces remain unchanged in adjusting the gap therebetween by partially grinding the planar faces of such members in parallel with each other. In another example where projections (14), (14) are formed on the faces of fore ends of magnetic cores (13a), (13b) as illustrated in Fig. 9, there is attainable an advantage of rendering the flux density uniform in the gap and reducing the leakage flux that interlinks with the coil.

Claims: A coil device (1) having two E-shaped magnetic cores (2 and 3) wherein each of the center legs (2a and 3a) of the E-shaped magnetic cores is shorter than other legs, the legs of the two E-shaped magnetic cores are positioned in opposition to each other, a gap (5) is formed between the opposed faces of the center legs of the magnetic cores, and a coil (4) is wound for completely surrounding said gap (5), characterized in that each of the center legs of the magnetic cores that are opposed to form the gap (5) is tapered from the basis end toward the center of the fore end and the taper provides a LOG curve represented by the following equation: γ s - γ = χ g l n x s x wherein χ g : distance from the center of the gap (5) to the fore end surface of the core (2a), χ s : distance from the center of the gap (5) to the base end of the core (2b), χ : distance from the origin of coordinates along the χ axis, γ s : radius of the core (2a or 2b), and γ : radius of the core (2a or 2b) in the coordinate corresponding to the position. A coil device (1) according to claim 1 characterized in that the magnetic core portion (2a and 2b) in the region of said gap (5) are so shaped that the rate of the cross-sectional area of the fore end (2a) to that of the base end (2b) is within a range of 1 to 90%. A coil device (1) according to claim 1 characterized in that the members (2a and 2b) each have a planar surface which is in parallel to the other and is opposite to each other. A coil device (1) according to claim 3, characterized in that the members (2a and 2b) have the center axis in common. A coil device (1) according to claim 3, characterized in that projections are formed on the face of said fore end (2a).

Inventors: ITO SHINICHIRO TDK CORPORATION, KINOSHITA YUKIHARU TDK CORPORA, ITO, SHINICHIRO, TDK CORPORATION, KINOSHITA, YUKIHARU, TDK CORPORATION
Applicants: TDK CORP, TDK CORPORATION
Application year: 1990
Main classification: H01F3
IPC codes: H01F38, H01F3, H01F27