Planar transformer

Abstract: Disclosed is a planar transformer comprising a planar primary spiral coil (1) and a planar secondary spiral coil (2), which are mutually insulated and laminated, wherein, (1) when the winding width W₁ of the primary spiral coil and the winding width W₂ of the secondary spiral coil have the relationship of W₁ ≦ W₂, the inner size A i ₁ of the primary spiral coil and the inner size A i ₂ of the secondary spiral coil coincide with each other, and (2) when the winding width W₁ of the primary spiral coil (1) and the winding width W₂ of the secondary spiral coil (2) have the relationship of W₁ > W₂, the central axes of the primary and secondary spiral coils (1), (2) coincide with each other, the outer size A o ₂ of the secondary spiral coil (2) is equal to or smaller than the outer size A o ₁ of the primary spiral coil (1), and the secondary spiral coil (2) is arranged corresponding to a position at which magnetic flux generated in a magnetic layer (3) by a current flowing through the primary spiral coil (1) is largest.

Description: The present invention relates to a planar transformer to be used for various types of circuits such as power supply circuits and inverter circuits for fluorescent tubes. Recently, miniaturization and lightening of all of the electric devices have been strongly required. However, magnetic parts such as inductors and transformers among component parts of electronic circuits have been less miniaturized and lightened than semiconductor elements, resistances and condensers, which becomes a serious reason for preventing the miniaturization and lightening of circuits. Although these magnetic elements are particularly indispensable for various kinds of power inverter circuits, it is difficult to miniaturize and lighten them. In an electronic automatic exchange, for example, many dc-to-dc converters are used in every electronic circuit board in accordance with desired supply voltage, and magnetic elements occupy much of the whole volume of these dc power supplies. The magnetic elements are also used for a back light in the fluorescent tube type utilized for a liquid-crystal display of a word processor or personal computer, and therefore it is indispensable to make magnetic elements thin for a thinner display. In this circumstance, planar inductors or transformers are greatly developed. A planar transformer comprising a planar primary spiral coil and a planar secondary spiral coil which are mutually insulated and laminated is for instance discussed as background art in EP-A-0413348. When a planar transformer is manufactured, it is necessary to sufficiently increase magnetic coupling between primary and secondary coils in order to efficiently transmit a signal or power from a primary coil to a secondary coil. In other words, it is necessary to design a transformer to make coupling coefficient between both coils as close as possible to 100%. The coupling coefficient k of the transformer is indicated by the following formula k = Φ 21 /Φ 1 , where, Φ₁ indicates magnetic flux produced by the primary coil, and Φ₂₁ indicates magnetic flux which interlinks a secondary coil in the magnetic flux produced by the primary coil. Provided that a resistance component is negligible, a primary-to-secondary ratio of voltage is proportional to a product of k and the ratio of winding numbers, and signal transmission is completely performed when k is unity. Provided that various kinds of loss are negligible, efficiency of power transmission from a primary coil to a secondary coil is proportional to square of k. Therefore, slight reduction of coupling coefficient causes remarkable reduction of efficiency of power availability. Thus, increasing of coupling coefficient of a transformer is much important for the performance of the transformer. In general, in order to increase coupling coefficient of a transformer, it is necessary to make a path of magnetic flux produced by a primary coil correspond to that of magnetic flux produced by a secondary coil as much as possible. However, since the distribution of magnetic flux is complicated in the case of a planar transformer, it is difficult to completely realize this condition. Nowadays, there is no unified way of design regarding a method of arranging primary and secondary coils, and this has been decided on the basis of trial and error. When a primary coil remarkably differs from a secondary coil particularly in the winding numbers and sizes, an optimum method of designing coils is not quite obvious from the viewpoint of the performance of the transformer. Accordingly, a planar transformer having a sufficiently high coupling coefficient has not yet been realized for this reason. As described above, it is expected that a planar transformer will contribute to miniaturization and lightening of electronic circuits. However, a designing method for increasing coupling coefficient has not yet been known so that the planar transformer remains far from practical use. The object of the present invention is to provide a planar transformer having high coupling coefficient. In order to achieve this object, the present invention provides for a planar transformer with the features of claim 1 or claim 2. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A-1C are plan views respectively showing the spiral coil used for the planar transformer according to the present invention; Fig. 2 is a schematic showing a structure of the planar transformer according to the present invention; Fig. 3 is a diagram showing the relationship between coupling coefficient k and winding width ratio W₁/W₂ of the spiral coils constituting the planar transformer; Fig. 4A is a schematic showing a change in position of the primary spiral coil while the secondary spiral coil is fixed, and Fig. 4B is a diagram showing the relationship between coupling coefficient k and the position of the primary spiral coil; Fig. 5A is a schematic showing a structure of the planar transformer according to the present invention, and Fig. 5B is a diagram showing the distribution of magnetic flux generated in a magnetic layer by a current flowing through the primary spiral coil; Fig. 6A is a schematic showing a change in position of the secondary spiral coil while the primary spiral coil is fixed, and Fig. 6B is a diagram showing the relationship between coupling coefficient k and the position of the secondary spiral coil; and Fig. 7 is a schematic illustrating parameters used for calculation of the distribution of magnetic flux generated in a magnetic layer by a current flowing through the primary spiral coil. A spiral coil used for the planar transformer of the present invention may be in the round, square, or rectangular shape. Figs. 1A-1C respectively show these shapes. The outer size of each spiral coil is indicated by A o and the inner size is indicated by A i . As shown in Fig. 1C, in the rectangular spiral coil, A o and A i respectively indicate the size parallel to the short side. Winding width w of each spiral coil is indicated by W = (A o - A i )/2 (1) In the present invention, when the winding width W₁ of the primary spiral coil and the winding width W₂ of the secondary spiral coil have the relationship of W₁ ≦ W₂, the inner size A i1 of the primary spiral coil is made to coincide with the inner size A i2 of the secondary spiral coil. Fig. 2 shows a structure of this planar transformer. The primary and secondary spiral coils are mutually insulated and laminated, and an insulating layer (not shown) and magnetic layer 3 are laminated on both sides of this laminated body. The winding width W₁ of the primary spiral coil 1 is smaller than the width W₂ of the secondary spiral coil 2, and the inner size A i1 of the primary spiral coil is identical to the inner size A i2 of the secondary spiral coil. Fig. 3 shows the relationship between coupling coefficient k and the ratio of W₁/W₂ in the planar transformer of the present invention. This figure shows a case (example) where the inner sizes A i of the primary and secondary spiral coils are made to coincide with each other, and a case (comparative example) where the outer sizes A o of the primary and secondary spiral coils are made to coincide with each other. It can be understood from Fig. 3 that the highest coupling coefficient k can be obtained when the ratio of the winding widths of the primary spiral coil to that of secondary spiral coil is unity, and that k reduces as the ratio of W₁/W₂ becomes smaller. When the inner sizes A i of the primary and secondary spiral coils are coincide with each other, reduction of k is more gentle than the case where the outer sizes A o are coincide with each other, and the high coupling coefficient can be sustained even if the ratio of W₁/W₂ is smaller than unity. Fig. 4A shows a change in position of the primary spiral coil while the secondary spiral coil is fixed as to the planar transformer according to the present invention of which ratio of W₁/W₂ is smaller than unity, and Fig. 4B shows the relationship between the position of the primary spiral coil and coupling coefficient k. It can be understood from Fig. 4B that the highest coupling coefficient can be obtained when A i1 is identical to A i2 . (2) In the present invention, when the winding width W₁ of the primary spiral coil and the width W₂ of the secondary spiral coil have the relationship of W₁ > W₂, the central axes of the primary and secondary spiral coils are made to coincide with each other, the outer size A o2 of the secondary spiral coil is made to be equal to or smaller than the outer size A o1 of the primary spiral coil, and the secondary spiral coil is arranged corresponding to a position at which magnetic flux generated by a current flowing through the primary spiral coil is largest. It is more preferable to make the center of the winding width W₂ of secondary spiral coil to coincide with that position. Fig. 5A shows the structure of this planar transformer. The primary spiral coil 1 and secondary spiral coil 2 are mutually insulated and laminated, and an insulating layer (not shown) and magnetic layer 3 are laminated on both sides of this laminated body. The central axes of primary spiral coil 1 and secondary spiral coil 2 coincide with each other, and the outer size A o2 of the secondary spiral coil is smaller than the outer size A o1 of the primary spiral coil. As shown in Fig. 5B, the secondary spiral coil 2 is arranged corresponding to a position at which magnetic flux generated in the magnetic layer 3 by a current flowing through the primary spiral coil 1 becomes greatest. Fig. 6A shows a change in position of the secondary spiral coil while the primary spiral coil is fixed as to the planar transformer according to the present invention of which ratio of W₁/W₂ is greater than unity, and Fig. 6B shows the relationship between the position of the secondary spiral coil and coupling coefficient k. This figure also shows the distribution of magnetic flux generated in the magnetic layer by a current flowing through the primary spiral coil. It can be understood from Fig. 6B that the highest coupling coefficient can be obtained when the secondary spiral coil is arranged corresponding to a position at which magnetic flux in the magnetic layer 3 is largest. The position at which magnetic flux generated in magnetic layer 3 by a current flowing through the primary spiral coil can be calculated by the following formulas (1) to (4). Each parameter in these formulas will be explained with reference to Fig. 7. As regards the magnetic layer, W m indicates a size, t m a thickness, µ s a relative magnetic permeability, and g a gap between the magnetic layers on both sides. The primary spiral coil is interposed between the magnetic layers on both sides. As regards the primary spiral coil, δ indicates the width of a line of the coil conductor, s a distance between the lines of coil conductor, A o the outer size, and A i the inner size. The left end of the magnetic layer is indicated by O, and X-axis extending to the right side along the surface of the magnetic layer indicates the position in the magnetic layer. In the primary spiral coil, there are N numbers of cross sections of lines of the coil conductor at the right and left sides respectively with reference to the central axis. X k indicates the left end of the cross section of the lines of the coil conductor at the left side to the central axis, and X k ' indicates the left end of the cross section of the lines of the coil conductor at the right side. Therefore, coordinate of X k and X k ' are expressed by: X k = (W m - A o )/2 + (k - 1) (δ + s) X k ' = X N + δ + A i + (k - 1) (δ + s). Formulas (1) to (4) indicate magnetic flux generated in the magnetic layer by a current flowing through the primary spiral coil. Formula (1) indicates magnetic flux at the region in the magnetic layer corresponding to between the left end of the magnetic layer and left end X₁ on the cross section of the first line of the coil conductor, formula (2) indicates magnetic flux at the region in the magnetic layer corresponding to the width of the lines of coil conductor, formula (3) indicates magnetic flux at the region in the magnetic layer corresponding to the space between adjacent lines of coil conductor, and formula (4) indicates magnetic flux at the region in the magnetic layer corresponding to between right end X N on the cross section of the Nth line of the coil conductor and the center of the magnetic layer. In each formula, V i (x) indicates a function relating to a magnetic flux component at the region according to the width of each line of coil conductor, W i (x) indicates a function relating to a magnetic flux component at the left side region of each line of coil conductor placed at the left side to the central axis, W i '(x) indicates a function relating to a magnetic flux component at the left side of each line of coil conductor placed at the right side to the central axis, and U i (x) indicates a function relating to a magnetic flux component at the right side of each line of coil conductor placed at the left side to the central axis. where, U i x = cosh[(X i +δ/λ]-cosh(X i /λ) sinh(W m /λ) ·sinh[(W m -x)/λ] V k x =1- cosh(X k /λ)·sinh[(W m -x)/λ]+cosh[(W m -X k -δ)/λ]·sinh x/λ sinh(W m /λ) W i x = cosh[(W m -X i )/λ]-cosh[(W m -X i -δ)/λ] sinh(W m /λ) ·sinh x/λ W i ' x = cosh[(W m -X i ')/λ]-cosh[(W m -X i '-δ)/λ] sinh(W m /λ) ·sinh x/λ λ=(µ s ·g·t m ) 1/2 L(x)= π(W m -2x)·t m .µ s As described above, the planar transformer with high coupling coefficient can be obtained by determining the positions of the primary and secondary spiral coils. In the planar transformer according to the present invention, a plurality layers of secondary spiral coils may be used. The planar transformer having such a constitution can generate multi-output voltages. A method of producing the planar transformer according to the present invention is not specifically limited. When a thin film process is used, various kinds of thin films such as magnetic material, coil conductor and insulator are formed on an appropriate substrate such as a semiconductor substrate by using methods of spattering, vacuum deposition, CVD, plating, etc. In order to pattern a coil conductor, it is possible to use several kinds of dry etching techniques such as reactive ion etching, ion beam etching and ECR plasma etching, wet etching using an electrolytic solution, and a lift-off method using a photoresist. On the other hand, soft magnetic foils such as amorphous magnetic foils can be used as magnetic layers in order to mechanically sandwich from both sides of the laminated body of the spiral coils via insulating layers. As described above, a producing method can be appropriately selected. In any case, it is preferable to reduce the distance between adjacent lines of coil conductor as much as possible in order to strengthen coupling between them. Example 1 The surface of a silicon substrate was heat-oxidized, a CoZrNb amorphous film having a thickness of 2 µm was formed on this substrate by rf spattering method, and further a SiO₂ film having a thickness of 1 µm was formed. An AℓCu alloy film having a thickness of 10 µm was formed as a coil conductor on the SiO₂ film by dc magnetron spattering method, and further a SiO₂ film having a thickness of 1 µm was formed by rf spattering method. This SiO₂ film was patterned into a square spiral-coil shape, and AℓCu alloy film was patterned into the square spiral-coil shape by using the patterned SiO₂ film as a mask. In the lower spiral coil (secondary coil), the width of lines of coil conductor was 100 µm, the distance between adjacent lines of coil conductor was 5 µm, the inner size was 1 mm, the outer size was 5.5 mm, and the winding number was 20. A polyimide film was formed between lines of coil conductor of the lower spiral coil and on them, and was flattened by using an etch-back method. An AℓCu alloy film having a thickness of 10 µm and a SiO₂ film having a thickness of 1 µm were formed and patterned into a square spiral-coil shape in the same manner as described above. In the upper spiral coil (primary coil), the width of lines of coil conductor is 100 µm, distance between adjacent lines of coil conductor was 5 µm, the inner size was 1 mm, the outer size was 3.3 mm, and the winding number was 10. A polyimide film was formed between lines of coil conductor of the upper spiral coil and on them and was flattened by using an etch-back method. A CoZrNb amorphous film having a thickness of 2 µm was formed on the film to produce a planar transformer. This planar transformer had the outer size of 6 mm, and a thickness of approximately 0.6 mm including that of the substrate. As described above, the winding width W₁ of the upper spiral coil was smaller than W₂ of the lower spiral coil, and the inner sizes of both spiral coils were made to coincide with each other. Electric characteristics were estimated by referring to the upper spiral coil as the primary coil and the lower spiral coil as the secondary coil. The measurement results at a frequency of 5 MHz indicated that primary inductance was approximately 0.9 µH, secondary inductance was approximately 4 µH and mutual inductance was 1.8 µH, and therefore primary-to-secondary coupling coefficient was estimated at approximately 0.95. In this way, a thin-film-type step-up transformer with high coupling coefficient was obtained. Example 2 A thin-film-type step-down transformer was produced in the same manner as described in Example 1. The primary spiral coil had the inner size of 1 mm, the outer size of 5.5 mm, and the winding number of 20, and the secondary spiral coil had the inner size of 2.2 mm, the outer size of 4.5 mm, and the winding number of 10. In this case, the winding width W₁ of the primary spiral coil was greater than W₂ of the secondary spiral coil. The position at which magnetic flux generated in the magnetic layer by a current flowing through the primary spiral coil would be largest was calculated in advance in accordance with formulas (1) to (4), and the inner and outer sizes of the secondary spiral coil were determined so as to arrange the secondary spiral coil in accordance with the calculated position. In this step-down transformer, primary-to-secondary coupling coefficient was approximately 0.9. Example 3 A copper foil having a thickness of 70 µm and a polyimide sheet having a thickness of 10 µm were laminated and wound, and then molded with an insulating resin. The product was sliced at a thickness of 500 µm to form a planar coil (secondary coil) having a round spiral pattern having the outer diameter of 9 mm, the inner diameter of 4 mm, and the winding number of 30. A planar coil (primary coil) having a round spiral pattern having the outer diameter of 6.5 mm, the inner diameter of 4 mm, and the winding number of 15 was formed in the same manner. A polyimide sheet having a thickness of 7 µm was interposed between both coils, and further polyimide sheets and Co amorphous foils respectively having a thickness of 7 µm were provided on both sides of the laminated coils to produce a planar transformer. This planar transformer had the outer size of 10 mm and a thickness of approximately 1 mm. As described above, the winding width W₁ of the primary spiral coil was smaller than W₂ of the secondary spiral coil, and the inner sizes of both spiral coils were made to coincide with each other. An estimation of electric characteristics of this planar transformer indicated that primary inductance was approximately 30 µH, secondary inductance was approximately 7 µH, mutual inductance was 13.5 µH, and therefore primary-to-secondary coupling coefficient was approximately 0.93. In this way, a planar step-up transformer having high primary-to-secondary coupling coefficient was obtained. Example 4 A step-down transformer was produced in the same manner as described in Example 3. The primary spiral coil had the outer diameter of 9 mm, the inner diameter of 4 mm, and the winding number of 30, and the secondary spiral coil had the outer diameter of 8 mm, the inner diameter of 5.5 mm, and the winding number of 15. In this case, the winding width W₁ of the primary spiral coil was greater than W₂ of the secondary spiral coil. The position at which magnetic flux generated in the magnetic layer by a current flowing through the primary spiral coil would be largest was calculated in accordance with formulas (1) to (4) in advance, and the inner and outer diameters of the secondary spiral coil were determined so that the secondary spiral coil would be arranged in accordance with this position. The primary-to-secondary coupling coefficient of this step-down transformer was 0.92. As described above in detail, it is possible to increase coupling coefficient in both cases of the step-up and step-down transformers of the present invention by optimizing the relative position of the primary and secondary spiral coils, and therefore excellent effect for improving performance can be obtained.

Claims: A planar transformer comprising a planar primary spiral coil (1) and a planar secondary spiral coil (2) which are mutually insulated and laminated, characterized by a magnetic layer (3) laminated on at least one of said spiral coils with an insulating layer between them, in that the winding width W₁ of the primary spiral coil (1) and the winding width W₂ of the secondary spiral coil (2) have the relationship of W₁ ≤ W₂, and in that the inner size A i1 of the primary spiral coil and the inner size A i2 of the secondary spiral coil coincide with each other. A planar transformer comprising a planar primary spiral coil (1) and a planar secondary spiral coil (2) which are mutually insulated and laminated, characterized by a magnetic layer (3) laminated on at least one of said spiral coils with an insulating layer between them, in that the winding width W₁ of the primary spiral coil (1) and the winding width W₂ of the secondary spiral coil (2) have the relationship of W₁ > W₂, in that the central axes of the primary and secondary spiral coils (1,2) coincide with each other, in that the outer size A o2 of the secondary spiral coil (2) is equal to or smaller than the outer size A o1 of the primary spiral coil (1), and in that the inner size A i2 of the secondary spiral coil (2) is so dimensioned, that the secondary spiral coil (2) is in a position at which magnetic flux generated in the magnetic layer (3) by a current flowing through the primary spiral coil (1) is largest. A planar transformer according to claim 2, characterized in that the center of the winding width W₂ of secondary spiral coil (2) coincides with the position at which magnetic flux generated in the magnetic layer (3) by a current flowing through the primary spiral coil (1) is largest. A planar transformer according to claim 1 or claim 2, characterized by a plurality of layers of planar secondary spiral coils (2). A planar transformer according to claim 1 or claim 2, characterized in that a magnetic layer (3) is laminated on each side of the laminated body of the spiral coils (1, 2) with an insulating layer between the magnetic layer (3) and the laminated body. A planar transformer according to claim 5, characterized in that it is laminated on a semiconductor substrate. A planar transformer according to claim 6, characterized in that polyimide is used as the insulating layer. A planar transformer according to claim 6, characterized in that silicon oxide is used as the insulating layer. A planar transformer according to claim 5, characterized in that each magnetic layer (3) is a soft magnetic foil. A planar transformer according to claim 1 or claim 2, characterized in that said planar transformer is designed for use in a dc-to-dc converter.

Inventors: MIZOGUCHI TETSUHIKO, SATO TOSHIRO, MIZOGUCHI, TETSUHIKO, SATO, TOSHIRO, MIZOGUCHI, TETSUHIKO, INTELLECTUAL PROPERTY DIV., Sato, Toshiro, c/o Intellectual Property Division
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: H01F27
IPC codes: H01F27, H01F19