A FERRITE CORE AND A TRANSFORMER OR INDUCTOR INCLUDING IT

Abstract: A ferrite core for use as a power transformer and/or as a choke coil is assembled from a pair of identical core halves (21) together with a bobbin (31) on which a coil is wound. Each of the core halves (21) has an E-shaped structure with a central portion core (23) around which a coil is wound, a pair of side legs (24) and a boase plate (22) which couples the central portion (23) with the side legs (24). The central portion (23) is oval in cross-section. The core is mounted on a printed circuit board so that the axis of the central portion (23) is parallel to the printed circuit board. This provides a transformer with a low height (H) compared with that of prior transformers which have a circular central portion. Further because of the smooth rounded edges of the central portion (23) the coil is not likely to be damaged, and the coil tightly surrounds the core, resulting the reduction of the undesirable leakage inductance of the transformer.

Description: The present invention relates to the structure of a ferrite core, and in particular, relates to a core for forming the cores of a transformer or a choke coil in a power supply circuit. The particular example of a ferrite core subsequently described is intended to be used in a transformer or a choke coil in a power supply circuit capable of handling up to 1 kW. When used as a power transformer, it may form part of a DC-AC converter and, in this case, a primary power supply is applied to the transformer through a switching circuit to apply an alternate current input to the transformer, and then the required secondary voltage is obtained at the output of the transformer. A ferrite core for such purposes must satisfy the following conditions. a) The core must not magnetically saturate, and preferably, the cross section along the magnetic path is constant along the whole magnetic path in the core. b) The core is preferably closed so that it is shielded and does not interfere with an associated or an external circuit. c) The shape of a core is preferably simple and enables a bobbin containing a winding coil or coils to be mounted on it and enables lead wires of the windings to extend outside the core. d) The core is preferably as small and as light in weight as possible. Also, the power handling capacity to weight ratio should be as large as possible. e) The height of the transformer including the core is as low as possible, so that the transformer is low when mounted on a printed circuit board. The most popular conventional ferrite core half is and E-shape having a constant cross section throughout. Alternatively, a combination of E-shaped and an I-shaped core half is used. However, that core has the disadvantages that it is large in size, its shielding effect is not perfect and further, a bobbin to fit over the core and carry the coil windings must be rectangular in cross-section. Thus the windings are bent sharply at the corners of the bobbin and the insulation on the wire wound on the bobbin is often damaged, further, undesirable leakage inductance increases. We have proposed an improved core in US-A-4,352,080 which intends to improve the above conditions (e). That core is intended to mount on a printed circuit board so that the core axis is perpendicular to the printed circuit plane. However, it has the disadvantage that the height of the transformer using the core is still high, and therefore, the mounting density of the components of a printed circuit board cannot be high. We have proposed another ferrite core in EP-B-68745 which will be described subsequently with respect to Figures 4 and 5. Thus a conventional core half comprises a) a central portion on which, in use, a coil is wound, b) a pair of side legs positioned at both the sides of the central portion, c) a base plate coupling the central portion and the side legs so that the central portion and the side legs together with the base plate form an E-shaped structure, with the free ends of the central portion and legs aligned in a single plane, d) the cross-section of each leg has a flat outer face and curved inner face substantially coaxial with the central portion, e) an empty recess portion is located between ends of the legs, and the side of the base plate, and f) the core half is symmetrical with regard to a first plane including a central axis of the central portion and extending parallel to the flat outer faces of the legs, but is asymmetrical about a second plane including the central axis of central portion and extending perpendicularly to the first plane. Another proposal that has been made for lowering the height of a transformer is the use of a core with a flat centre section which has the enough cross-sectional area for magnetic flux. However, when the centre section is rectangular having sharp corners, it still has the disadvantage that the coil wound on a core does not fit well with the core at the corners, because the coil does not bend at the corners, but curve. So, some space is left between the core and its coil, and that space increases undesirable leakage inductance. Further, since the corners of the core is sharp, the insulation on the coil tends to be injured. According to this invention in such a core half g) the central portion is oval in cross-section and, h) the central portion is positioned on the base plate so a flat face of the central portion formed by the rectangular portion of its cross-section is co-planar with an edge of the base plate. An advantage of the present invention is that it provides a ferrite core which produces a low transformer and/or a low choke coil. It also provides a core which is small in size, and enables a satisfactory winding to be prepared without injury to the wire of the coil. A particular example of a core half, a transformer and an inductor in accordance with this invention will now be described and contrasted with the prior art with reference to the accompanying drawings, in which:- Figure 1(a) is a side elevation of a ferrite core half; Figure 1(b) is an under plan; Figure 1(c) is a cross-section taken along the line A-A shown in Figure 1(b); Figure 2(a) is a side elevation of a bobbin which fits on the core half; Figure 2(b) is an under plan of the bobbin; Figure 3 is an exploded perspective view of a transformer using the core halves omitting a coil, Figure 4(a) is a side elevation of a prior ferrite core half as described in EP-B-68745; Figure 4(b) is an under plan of the prior core half; Figure 4(c) is a cross section taken along the line B-B shown in Figure 4(b); Figure 5(a) is a side elevation of a bobbin for the use with the prior art core; and, Figure 5(b) is an underplan of the bobbin. The ferrite core half 1 described in EP-B-68745 has a substantially rectangular base plate 2 which has a recess 2A, a ciruclar centre core 3 mounted on the centre of the base plate 2, and a pair of side legs 4 mounted on both sides of the base plate 2. The centre core 3 is positioned so that it contacts both the recess 2A and one side of the base plate 2. The cross section of each side leg 4 has linear lines 4A, 4B, 4C, a curve 4D which defines the recess 2A, an arc 4E which is coplanar with the centre core 3, and the linear line 4F between the arc 4E and the line 4C. The bobbin 11 which fits to the core has hollow cylindrical body 12 with a pair of flanges 12A at both the ends of the same, and the terminal plate 13 having a plurality of terminal pins 13A. The terminal plate 13 is composed integrally with the body 12 by using plastics. A transformer or a choke coil is obtained by winding a coil on a bobbin to which a pair of cores are inserted. The transformer is mounted on a printed circuit board so that the pins 13A pass through the printed circuit board. The structure of the core of Figures 4 and 5 has the advantage that the height of the trnasformer on a printed circuit board is lower than that of earlier formers described in US-A-4,352,080 because the core is mounted on a printed circuit board so that the axis of the circular centre core 3 is parallel to the plane of the printed circuit board. However, it still has the disadvantage that the height on a printed circuit board is not low enough for a miniaturized electronic component. When the spacing between printed circuit boards is restricted to 25.4 mm, the height of a transformer or the board must be less than 16 mm, considering the spacing necessary for insulation and soldering. Figure 1(a) is a vertical view of the present core half, Figure 1(b) is a bottom view of the present core half, and Fig.1C is a cross sectional view at the line A-A of Fig.1B. Figs.2A and 2B show a bobbin, and Fig.3 shows a perspective view of a transformer using the cores of Figs.1A through 1C and the bobbin of Figs.2A and 2B. In the figures, the ferrite core half 21 according to the present invention has an essentially rectangular base plate 22 made of ferrite material, a center core 23 mounted on the center of the base plate 22, and a pair of side legs 24 mounted at the ends of the base plate 22. Those members 22, 23 and 24 are integrally molded by using ferrite material. The center core 23 is positioned on the base plate 22 so that the center core 23 inscribes with the side of the base plate 22. The cross section of the center core 23 is not rectangular, nor circular, but is oval. Thus it has a cross-section which is a combination of a pair of arcs 23A with radius (a), and a rectangle with the first side of the length (2a) and the second side of the length (b-2a). The area of the cross-section of the center core 23 is 2a(b-2a)+¶a², which must be enough for the path of the magnetic flux. In other words, when the core is designed, the cross sectional area is first determined so that the magnetic flux in the core does not saturate in operation, and it should be noted that the diameter (2a) of the center core 23 is smaller than that of a circular center core of Fig.4A. That flat center core is the important feature of the present invention. Further, it should be appreciated that the center core 23 of the present invention has no sharp edges, but it has a pair of arcs 23A. Because of no sharp edges, a wire of a coil wound on the center core is no injured. Further, the shape of a coil wound on the center core fits well with the shape of the center core. This fact provides the further small size of a transformer, and reduction of undesirable leakage inductance. If the cross section of the center core is rectangular, the coil would not fit with the shape of the core, but some spacing would be left between the center core and the coil. The cross section of each of side legs 24 has the linear line 24A which inscribes with the short side of the base plate 22, a pair of linear lines 24B which is parallel to the long side of the base plate 22, and arc 24C which is coplanar with the arc 23A of the center core 23, a linear line 24D between the end of the arc 23C and the line 24B, and another linear line 24E between the other end of the arc 23C and the line 24B. The side legs 24 are positioned so that the linear line 24B inscribes with the base plate 22, or that linear line 24B is on the extension of the linear line of the center core 23. The length of the line 24A is longer than the short side of the base plate 22, so that some recess area 22A is defined by a pair of side legs 24 and the base plate 22. The depth of the recess is preferably the same as the thickness of a flange of a bobbin. The cross section of the base plate 22 has a step 22B, and the end of the base plate 22 has a slanted slope 22C, so that no sharp edge of the base plate is provided. That slanted slope 22C is advantageous in the manufacturing process of the core in taking out the core from a die in the molding process. In order to provide enough reduction of height of a transformer, it is preferable that the length (b) is considerably longer than the radius (a). In a preferable embodiment, the ratio of (b) which is the lateral length of the center core, to (2a) which is the height of the center core, is larger than 1.2 and still preferably that ratio is larger than 2.0. A pair of ferrite core halves of Figs. 1A through 1C are coupled with a bobbin of Figs. 2A and 2B, after a coil is wound on the bobbin. The bobbin 31 has a hollow cylindrical body 32 and a pair of flanges 32A at both the ends of the body 32, and a terminal plate 33 having a plurality of terminal pins 33A. The internal cross section of the cylindrical body 32 is the same as the cross section of the center core 23 of a core half. Preferably, the terminal plate 33 has some stoppers 33B in the direction of the pins 33A. The bobbin having the cylindrical body 32, the flanges 31, and the terminal plate 33 having stoppers 33B and pins 33A is integrally composed of dielectric plastics. It should be noted that the external shape of the cylindrical body 32 of the bobbin has no sharp edges because of the curved structure of the center core, therefore, the coil wound on the bobbin is not injured, and no spacing is left between the bobbin (or the core) and the coil, since a coil does not bend but fits well to the profile of the bobbin. When the coil is wound on the bobbin, a pair of ferrite core halves are inserted in the bobbin so that the center cores 23 of two core halves touches with each other in the hollow cylindrical body 32 of the bobbin, then, it should be appreciate of course that the side legs of two core halves touches with each other, since the height of the center core is the same as that of the side legs. The assembled transformer or choke coil is mounted on a printed circuit board, so that the pins 33A passes through the printed circuit board. In this case, the stopper 33B defines the level of the transformer, by abutting the stopper with a printed circuit board PL. Therefore, it should be noted that the total height H of the transformer on the printed circuit board is the length between the end of the stopper 33B and the top of the flanges 32A. That height H may be less than 16 mm when the capacity of the transformer is up to 100 watts, and so, the printed circuit board may be mounted with the interval of 25.4 mm. Preferably, the cross sectional area along magnetic path in the core is uniform, so that no magnetic saturation occurs. In that regard, it is preferable that cross sectional area of the center core 23 is the same as the cross sectional area of the base plate 22, and it is also the same as the sum of the cross sectional area of the side legs 24. However, when the core half is not large, the size of the side leg may be larger than that defined by magnetic flux saturation condition mentioned above, because the side legs designed by the above condition would be too small to keep the necessary mechanical strength. It should be noted that the height S of the side leg 24 is longer than (2a) which is the height of the center core 23. That relation allows to reduce the leakage inductance of the transformer, because the flux from the center core 23 is well received by the large side legs through the base plate 22. As described above in detail, the present invention provides a core half for a transformer or a choke coil which is low when mounted on a printed circuit board, by mounting the cores so that the axis of the center cores are positioned parallel to the printed circuit board. As the cross section of the center core is flat, but not circular, the height is further reduced. Further, as the corner of the center core is not sharp, but smooth arc, the coil wound on the cores is not injured, and fits well with the cores. This reduces not only the size of the transformer, but also the undesirable leakage inductance of the transformer.

Claims: 1. A ferrite core half (21) comprising a) a central portion (23) on which, in use, a coil is wound, b) a pair of side legs (24) positioned at both the sides of the central portion (23), c) a base plate (22) coupling the central portion (23) and the side legs (24) so that the central portion (23) and the side legs (24) together with the base plate (22) form an E-shaped structure, with the free ends of the central portion and legs aligned in a single plane, d) the cross-section of each leg (24) has a flat outer face and curved inner face substantially coaxial with the central portion (23), e) an empty recess portion (22A) is located between ends of the legs (24), and the side of the base plate (22), and f) the core half (21) is symmetrical with regard to a first plane including a central axis of the central portion and extending parallel to the flat outer faces of the legs, but is asymmetrical about a second plane including the central axis of the central portion and extending perpendicularly to the first plane, characterised in that: g) the central portion (23) is oval in cross-section and, h) the central portion (23) is positioned on the base plate (22) so a flat face of the central portion formed by the rectangular portion of its cross-section is co-planar with an edge of the base plate (22). 2. A ferrite core half according to claim 1, wherein the cross-section of the leg has a linear face (24E) facing the empty recess (22), and another linear face (24D) close to the end of the side leg. 3. A ferrite core half according to claim 1 or 2, wherein a step (22B) is provided on the base plate (22) between the central portion (23) and the empty recess (22). 4. A ferrite core half according to any one of the preceding claims, wherein the base plate (22) has a bevelled edge (22C). 5. A ferrite core half according to any one of the preceding claims, wherein the ratio of the length of the straight portions to the width of the oval-shaped cross-section of the central portion is greater than 1.2. 6. A ferrite core comprising a pair of core halves in accordance with any one of the preceeding claims placed with the free ends of their legs and central portions together. 7. A transformer or inductor comprising a ferrite core in accordance with claim 6 with a bobbin (31) carrying a coil mounted on the central portions (23) of the core halves.

Inventors: MITSUI, TADASHI
Applicants: TDK CORPORATION
Application year: 1987
Main classification: H01F27
IPC codes: H01F27, H01F30, H01F17