Low-profile transformer and method for producing same

Abstract

Provided are: a low-profile transformer which is equipped with a plurality of cylindrical secondary coils and is capable of being more compact and having a lower product height in comparison to the prior art; and a method for producing the same.　Thus, a transformer equipped with a primary coil 34 which is wound around the outer circumference of the center legs 4c, 6c of first and second cores 4, 6 which constitute a magnetic core, and also equipped on the outer circumference thereof with cylindrical secondary coils 22, 24 which are concentrically arranged relative to one another and also relative to the primary coil 34, wherein: the cylindrical secondary coils 22, 24 are provided with cylindrical secondary coil main body sections 22d, 24d which encircle and sandwich slits 22c, 24c therebetween, and terminal units 22a, b, 24a, b which each extend toward the outside from a region near the end section of the cylindrical secondary coil main body sections, which face one another with the slits 22c, 24c sandwiched therebetween; and the cylindrical secondary coils 22, 24 are positioned in a manner such that there is overlap between one of the terminal units 22a, 24a in each of said coils, and there is no contact between the other of the terminal units 22b, 24b in each of said coils.

Description

Low-profile transformer and its manufacturing method

[0001] The present invention relates to a low-profile transformer and a manufacturing method thereof, and more particularly to a low-profile transformer for power conversion in which a primary coil and a secondary coil are concentrically arranged and which is used in step-down DC-DC converters of electric vehicles and hybrid vehicles (EV / HEV vehicles), and a manufacturing method thereof.

[0002] In recent years, power conversion transformers for switching power supplies have been required to have specifications of an output voltage of approximately 12 V and an output current of approximately 100 A due to the demand for step-down DC-DC converters for EV / HEV vehicles. Furthermore, there is a demand for smaller and lighter on-board converter units, and there is an urgent need to reduce the size and weight of the main transformer, which is a large component. A known prior art for achieving the miniaturization of high-current transformers is a transformer 120 disclosed in Patent Document 1 (particularly FIG. 1 ), which is illustrated in FIG. 7 of this application and which houses a primary coil 134 and a secondary coil 136 made of a cylindrical copper plate between two PQ cores 146, 148.

[0003] Patent Publication No. 2012-209446

[0004] In such a power conversion transformer, it is necessary to use a plurality of secondary coils to obtain a larger current, but in the above-mentioned patent document 1, a plurality of secondary coils 136, 136 are stacked in the winding axis direction around the primary coil 134, concentrically with the primary coil 134, and an output is obtained from each of the terminal portions 136a, 136a of the stacked secondary coils 136, 136. However, in the patent document 1, the product height increases according to the number of stacked secondary coils 136, 136, making it difficult to promote a reduction in the height and size of the product.

[0005] The present invention has been made in view of the above circumstances, and aims to provide a low-profile transformer and a manufacturing method thereof that can facilitate a reduction in the height and size of the product when multiple secondary coils are arranged outside the primary coil.

[0006] The low-profile transformer of the present invention comprises: a magnetic core; a primary coil wound around the magnetic core of the magnetic core; and at least two cylindrical secondary coils arranged concentrically with the primary coil and concentrically with each other around the primary coil, wherein each of the at least two cylindrical secondary coils has a cylindrical secondary coil body having a circumferential cylindrical shape with a slit sandwiched therebetween, and terminal portions extending outward from respective end-neighbor regions of the cylindrical secondary coil body facing each other across the slit at one longitudinal end side of the slit, wherein one of the terminal portions of each of the at least two cylindrical secondary coils overlaps and is electrically connected to each other, and the other of the terminal portions is arranged so as to be out of contact with each other. Here, the "ends of the cylindrical secondary coil body" refers to the regions facing each other across the slit of the cylindrical secondary coil body.

[0007] In each of the at least two cylindrical secondary coils, the terminals of the cylindrical secondary coil are preferably formed by the conductive plate material constituting the cylindrical secondary coil body, and a notch is preferably provided in a corner formed by a side surface of each terminal in the extension direction that is located on the opposite side of the terminal from the slit and an edge surface of the cylindrical secondary coil body. In each of the at least two cylindrical secondary coils, it is also preferable that the one terminal and the other terminal are substantially parallel to each other and are formed to extend outward while bending.

[0008] In addition, it is preferable that the cross section of the cylindrical secondary coil main body of each of the at least two cylindrical secondary coils is rectangular, the long side surfaces of the rectangle form the circumferential surface of the cylinder, and the short side surfaces of the rectangle form the radial surface of the cylinder. It is also preferable that the primary coil is an air-core coil in which a conducting wire is wound multiple times.

[0009] The method for manufacturing a low-profile transformer of the present invention includes: a first step of arranging a primary coil in a wound state around the outer periphery of a center leg portion of one of a first core and a second core that constitute a magnetic core; a second step of forming a cylindrical secondary coil body portion of at least two cylindrical secondary coils, the cylindrical secondary coil body portion being made of a predetermined metal material and surrounding a slit therebetween, and extending one terminal portion in a predetermined direction from each of end vicinity regions of the cylindrical secondary coil body portion facing each other across the slit on one longitudinal end side of the slit; a third step of arranging the at least two cylindrical secondary coils around the outer periphery of the primary coil concentrically with the primary coil and concentrically with each other; and a fourth step of overlapping and electrically connecting one terminal portion of each of the two terminal portions of each of the at least two cylindrical secondary coils and arranging the other terminal portions so that they are not in contact with each other. a fifth step of combining the first core and the second core to form the magnetic core by butting one middle leg portion against the other middle leg portion located at the center of the concentrically arranged primary coil and at least two cylindrical secondary coils, and storing the primary coil and at least two cylindrical secondary coils within this magnetic core; and

[0010] Furthermore, the second step includes a manufacturing step of forming the cylindrical secondary coil body by sheet metal processing a plate made of the specified metal material, and extending terminal portions one by one in a specified direction from each of the end vicinity regions of the cylindrical secondary coil body, and when extending the terminal portions in the specified direction, it is preferable that a notch is provided in a corner formed by the side surface of each terminal portion in the extending direction that is located on the opposite side from the slit and the edge of the cylindrical secondary coil body, so that the extending direction of the terminal portions can be adjusted.

[0011] According to the low-profile transformer and its manufacturing method of the present invention, by arranging multiple cylindrical secondary coils concentrically on the same plane, the height can be significantly reduced compared to conventional transformers in which multiple secondary coils are stacked in the winding axis direction, thereby facilitating the reduction in the height and size of the product.

[0012] 1A and 1B are perspective views showing the structure of a low-profile transformer according to an embodiment of the present invention, as viewed obliquely from above the front, and as viewed obliquely from above the back.

[0024] FIG. 1 is an exploded perspective view showing the structure of a low-profile transformer according to an embodiment of the present invention.

[0025] FIG. 2 is an enlarged perspective view showing a cylindrical secondary coil of a low-profile transformer according to an embodiment of the present invention.

[0026] FIG. 3 is a schematic diagram showing two cylindrical secondary coils according to an embodiment of the present invention ((a) shows the overlapping state of terminal portions of two cylindrical secondary coils, (b) shows the shape of a first cylindrical secondary coil, and (c) shows the shape of a second cylindrical secondary coil).

[0027] FIG. 4 is a diagram for explaining a cylindrical secondary coil manufactured using a sheet metal processing technique, where (A) is a perspective view viewed obliquely from below the bottom, and (B) is a developed view showing the shape of a plate-shaped conductive material before sheet metal processing.

[0028] FIG. 4 is a schematic diagram for explaining the difference in height dimension when a comparative example (A) and an example (B) are assembled ((a) is a front view of the transformer, and (b) is a perspective view showing the shape of a cylindrical secondary coil).

[0029] FIG. 5 is a perspective view of a transformer according to a conventional technology, as viewed obliquely from above the front.

[0013] A low-profile transformer according to an embodiment of the present invention and a method for manufacturing the same will be described below with reference to the drawings. The low-profile transformer according to this embodiment is applied, for example, to an in-vehicle power conversion transformer. Fig. 1(A) is a perspective view of the low-profile transformer according to the embodiment of the present invention, seen obliquely from above on the front side, and Fig. 1(B) is a perspective view of the low-profile transformer according to the embodiment of the present invention, seen obliquely from above on the rear side. Fig. 2 is an exploded perspective view of the low-profile transformer according to the embodiment of the present invention.

[0014] 1A, 1B, and 2, the low-profile transformer 10 of this embodiment includes, as its main components, a first core 4 and a second core 6 made of a PQ core that constitute a magnetic core, a primary coil 34 (preferably an air-core coil) formed by winding a conductor wire, and first and second cylindrical secondary coils 22, 24 formed by cylindrical metal plates. The first core 4 and the second core 6 are made of, for example, a ferrite core, and each have a cylindrical center leg 4c, 6c, outer legs 4a, 4b, 6a, 6b arranged on both sides of the center leg 4c, 6c, and a back surface 4d, 6d connecting the center leg 4c, 6c and the outer legs 4a, 4b, 6a, 6b. The first core 4 and the second core 6 are symmetrical, and the two cores 4, 6 are assembled so that corresponding components face each other.

[0015] The outer legs 4a, 4b, 6a, and 6b have a plate shape with an arc-shaped inner surface and a flat outer surface. The three legs 4a, 4b, and 4c constituting the first core 4 and the three legs 6a, 6b, and 6c constituting the second core 6 are preferably arranged to face each other with a small gap (not shown) between them.

[0016] The transformer preferably has a closed magnetic circuit structure, and an EE core, EI core, or ER core can be used instead of a PQ core. Of course, an open magnetic circuit structure using an I core or the like is also possible. Furthermore, an air-core primary coil 34 formed by winding a conductor around the center legs (magnetic cores) 4c and 6c of the first core 4 and second core 6 constituting the PQ core described above is disposed. Instead of forming the primary coil 34 as an air-core coil, the primary coil 34 may be formed by placing an insulating resin bobbin around the center legs (magnetic cores) 4c and 6c and winding a conductor around the resin bobbin. Of course, even when using the air-core primary coil 34 described above, a resin bobbin may be disposed around the center legs (magnetic cores) 4c and 6c.

[0017] The inner diameter of the second cylindrical secondary coil 24 is larger than the outer diameter of the first cylindrical secondary coil 22, and is sized so that the first cylindrical secondary coil 22 fits within the second cylindrical secondary coil 24 with a slight gap between them. That is, the primary coil 34, first cylindrical secondary coil 22, and second cylindrical secondary coil 24 are concentrically arranged in this order around the central axes of the center legs (magnetic cores) 4c, 6c of the first core 4 and second core 6 that constitute the PQ core. Although not shown, in practice, bobbins made of an insulating material such as phenolic resin, separators made of an insulating material molded to correspond to the shapes of the cylindrical secondary coils 22, 24, etc. are interposed between the coils 34, 22, 24.

[0018] The first cylindrical secondary coil 22 and the second cylindrical secondary coil 24 are formed by processing a plate-shaped conductive material such as copper or aluminum into a cylindrical shape. Each of the cylindrical secondary coils 22 and 24 has a cylindrical secondary coil body 22d, 24d formed by wrapping the plate-shaped conductive material around a slit 22c, 24c. The cylindrical secondary coils 22 and 24 also have terminals 22a, 22b, 24a, and 24b extending from the respective end vicinity regions of the cylindrical secondary coil body 22d, 24d at the longitudinal lower ends of both circumferential ends of the coil that face each other across the slit 22c, 24c. As shown in FIGS. 1A and 1B , the terminals 34a and 34b of the primary coil 34 are arranged to extend upward from the primary coil 34 in the drawing, but the extending direction of the terminals 34a and 34b is not limited thereto.

[0019] The terminals 22a, 22b of the first cylindrical secondary coil 22 and the terminals 24a, 24b of the second cylindrical secondary coil 24 are all configured to extend substantially parallel to one another. The terminals 22a, 22b, 24a, 24b are formed to extend outward while bending. That is, the terminals 22a, 22b, 24a, 24b extend forward from the cylindrical secondary coil main bodies 22d, 24d, then bend downward and further bend forward. The bottom surfaces of the forward-extending terminals 22a, 22b, 24a, 24b are configured to be electrically connected, for example, by soldering, to a conductive pattern on a circuit board.

[0020] One terminal 24a of the second cylindrical secondary coil 24 is placed on one terminal 22a of the first cylindrical secondary coil 22, and these two terminals 22a, 24a are electrically connected (set at a common potential) (see FIGS. 4(a), (b), and (c)). On the other hand, the other terminal 24b of the second cylindrical secondary coil 24 is arranged so as not to overlap the other terminal 22b of the first cylindrical secondary coil 22, and the two terminals 22b, 24b are configured to be spaced apart from each other (see FIGS. 4(a), (b), and (c)). For this reason, the tip portions of both terminal portions 22a, b of the first cylindrical secondary coil 22 are fabricated to be placed on the same plane (substrate surface), whereas the tip portion of terminal portion 24a, b of the second cylindrical secondary coil 24 is configured to be positioned higher than the tip portion of terminal portion 24b by the thickness of terminal portion 22a. This is because the tip portion of terminal portion 24a is superimposed on the tip portion of terminal portion 22a of the first cylindrical secondary coil 22, whereas the tip portion of terminal portion 24b needs to be configured to be placed on the substrate surface in the same way as terminal portions 22a, b of the first cylindrical secondary coil 22.

[0021] If there is a demand to reduce the width of the slits 22c, 24c of each cylindrical secondary coil 22, 24, the overlapping terminal portions 22a, 24a will be disposed between the other terminal portions 22b, 24b (see FIG. 4(a)), but if there is no limit to the width of the slits 22c, 24c, the overlapping terminal portions 22a, 24a do not need to be disposed between the other terminal portions 22b, 24b. In particular, if three or more cylindrical secondary coils are concentrically stacked, it is necessary to appropriately change the position of one of the overlapping terminal portions or the width of the slit so that the other terminal portions can be disposed at predetermined positions spaced apart from each other.

[0022] However, all or part of the terminals 22a, 22b, 24a, and 24b do not necessarily have to be configured to extend outward while bending as in the above embodiment, and may be configured to extend linearly. Furthermore, for example, when connecting to a conductive pattern on the back surface by passing through a through hole in a substrate, it is important to configure the downward extending portions of the terminals 22a, 22b, 24a, and 24b to be longer.

[0023] Furthermore, the cross sections of the cylindrical secondary coil bodies 22d, 24d of the two cylindrical secondary coils 22, 24 are rectangular, with the long sides of the rectangle forming the circumferential surface of the cylinder and the short sides of the rectangle forming the radial surface of the cylinder. This allows for a thin radial thickness, making it easier to arrange the two cylindrical secondary coils 22, 24 concentrically. Since the two cylindrical secondary coils 22, 24 have different coil diameters, their overall coil lengths are different, and typically, their output current values ​​are different. If the output current values ​​between the two coils need to be equal (or at some other predetermined ratio), it is essential to adjust the cross-sectional area of ​​each coil 22, 24 according to the coil diameter.

[0024] The cylindrical secondary coils 22, 24 of the low-profile transformer 10 according to the above embodiment can be manufactured by various methods, such as die molding and sheet metal processing. However, sheet metal processing is preferred because it is simple. When using this sheet metal processing method, for example, as shown in FIG. 5A , which shows the cylindrical secondary coil 24 as viewed from diagonally below the bottom, notches 24 e, f are provided in the edge portions (corners) of the cylindrical secondary coil body 24 d where the terminals 24 a, 24 b branch off. By providing these notches 24 e, f, when the cylindrical secondary coil body 24 d is formed into a cylindrical shape, the two terminals 24 a, 24 b can extend forward in a parallel shape rather than flaring out in a V-shaped manner.

[0025] Figure 5(B) is a development view showing the shape of the plate-shaped conductive material before sheet metal processing of the cylindrical secondary coil 24. As shown in this development view, notches 24e and f, which are cut out so as to taper inward, are provided in the edge portions (corners) of the cylindrical secondary coil main body 24d where the terminal portions 24a and 24b branch off before sheet metal processing. Note that the edges of the notches 24e and f on the cylindrical secondary coil main body 24d side have an R (=2.0). An example of other dimensions of the cylindrical secondary coil 24 is also shown in Figure 5(B).

[0026] (Method of Manufacturing Low-Profile Transformer) A method of manufacturing a low-profile transformer according to this embodiment will now be briefly described using an example. First, a first step is performed in which the primary coil 34, which is an air-core coil formed by winding a conductor around the center leg (magnetic core) 4c of the first core 4, which is made of a PQ core that constitutes a magnetic core. As described above, the first step may also be performed by placing an insulating resin bobbin around the center leg (magnetic core) 4c and winding a conductor around this resin bobbin to form the primary coil 34.

[0027] Next, the second step is performed to create the two cylindrical secondary coils 22, 24. In the second step, cylindrical secondary coil bodies 22d, 24d are formed from a metal material such as copper or aluminum, and the bodies are wound around the slits 22c, 24c. The bent terminals 22a, 22b and 24a, 24b are protruded from the cylindrical secondary coil bodies 22d, 24d, respectively, on one longitudinal end side of the slits 22c, 24c, in regions that face each other across the slits 22c, 24c. Note that the second step can also be performed before the first step.

[0028] Next, a third step is performed in which the two cylindrical secondary coils 22, 32 formed as described above are arranged concentrically around the outer periphery of the primary coil 34. That is, from the inside to the outside, the primary coil 34, the first cylindrical secondary coil 22, and the second cylindrical secondary coil 24 are arranged concentrically with the middle legs (magnetic cores) 4c, 6c as the centers.

[0029] Next, a fourth step is carried out in which one of the two terminal portions 22a, 24a of each of the two cylindrical secondary coils 22, 24 is overlapped with one another to electrically connect them, and the other terminal portions 22b, 24b are arranged so that they are not in contact with one another.

[0030] Finally, in the fifth step, the first core 4 and the second core 6 are combined so that the center leg 6c and the outer legs 6a, 6b of the second core 6, which is made of a PQ core constituting the magnetic core, face the center leg 6c and the outer legs 6a, 6b of the first core 4 across a gap. This allows the primary coil 34 and the two cylindrical secondary coils 22, 24 to be housed concentrically between the first core 4 and the second core 6. Note that the third and fourth steps are performed after the second step, but the first step can also be performed between the second and third steps or after the third step.

[0031] EXAMPLES Hereinafter, by comparing an example with a comparative example, it will be specifically shown that the low-profile transformer according to this embodiment can promote a low profile (see FIG. 6).

[0032] (A) Configuration of the Comparative Example As shown in Figure 6(A)(a), the transformer according to the comparative example has a primary coil 234 sandwiched between two secondary coils 222, 224 shown in (A)(b), which are arranged so as to overlap in the axial direction. The secondary coils 222, 224 of this comparative example are formed by winding a copper rectangular wire, and have an annular portion with two bent terminal portions extending from this annular portion, but the cross section of the annular portion is short in the axial direction. Specifically, this rectangular wire has, for example, a long side of 6 mm and a short side of 3.3 mm (cross-sectional area of ​​19.8 mm). 2 ) and the annular portion is arranged so that the long sides are radial and the short sides are axial. The transformer of the comparative example arranged in this way is formed to have a width of 50 mm and a height of 36 mm, as shown in the front view (A)(a).

[0033] (B) Configuration of the Example On the other hand, the low-profile transformer according to the example has two cylindrical secondary coils 22, 24 as shown in (B)(b) concentrically arranged around the outer periphery of a primary coil (not shown) as shown in Fig. 6(B)(a). The cylindrical secondary coils 22, 24 of this comparative example are formed by winding a copper rectangular wire around it, and have a cylindrical portion and two bent terminal portions extending from this cylindrical portion, similar to the above comparative example, but the cross section of the cylindrical portion is made shorter in the radial direction. Specifically, this rectangular wire has, for example, a long side of 10 mm and a short side of 2.0 mm (cross-sectional area of ​​20.0 mm). 2 ) and the cylindrical portion is arranged so that the long sides are axial and the short sides are radial. As shown in the front view (B)(a), the low-profile transformer of this embodiment arranged in this manner has a width of 50 mm, the same as the comparative example, but a height of 28.4 mm. As such, the low-profile transformer of this embodiment has a significant height reduction of 7.6 mm, a reduction rate of 21%, compared to the conventional transformer of the comparative example.

[0034] (Modifications) The low-profile transformer and the method for manufacturing the low-profile transformer of the present invention are not limited to those of the above-described embodiment, and various other modifications are also applicable. For example, in the above-described embodiment, two cylindrical secondary coils are arranged concentrically, but three or more cylindrical secondary coils may be arranged concentrically.

[0035] In the above embodiment, the terminals of the cylindrical secondary coil are formed so as to bend and extend outward, but the terminals may be configured to extend linearly (for example, downward) without being bent. Furthermore, as mentioned above, the core is not limited to a type in which two PQ cores are combined to form a closed magnetic circuit, but may be a type in which a closed magnetic circuit is formed by combining two E cores, ER cores, or other cores. Furthermore, an open magnetic circuit formed by an I core or the like may also be used.

[0036] In the above embodiment, the two terminals of the cylindrical secondary coil are arranged substantially parallel to each other, allowing the terminals to be arranged compactly, but they do not necessarily have to be parallel to each other if there is sufficient space to arrange the terminals. In the above embodiment, a notch is provided in the corner formed by the side surface of the terminal opposite to the slit among the side surfaces in the extension direction and the circumferential surface of the cylindrical secondary coil body, but the notch does not have to be provided if the cylindrical secondary coil is formed by a method such as die molding instead of sheet metal processing, or if the terminals are connected to the cylindrical secondary coil body by welding or the like.

[0037] Furthermore, according to the manufacturing method of the low-profile transformer of this embodiment, the first to fifth steps are performed in this order, but as long as the manufacturing order is such that at least the third and fourth steps come after the second step, and the fifth step comes after the first, second, third, and fourth steps, the other steps can be performed in an appropriate order depending on the circumstances. For example, the second step may be performed before the first step, or the second and fourth steps may be performed in this order, and then the first and third steps may be performed in this order.

[0038] 4 First core 4a, 4b, 6a, 6b Outer leg portion 4c, 6c Center leg portion 4d, 6d Back portion 6 Second core 10 Low-profile transformer 22 First cylindrical secondary coil 22a, 22b, 24a, 24b, 34a, 34b, 136a Terminal portion 22c, 24c Slit 22d, 24d Cylindrical secondary coil main body portion 24 Second cylindrical secondary coil 24e, 24f Notch 120 Transformer 146, 148 PQ core 34, 134, 234 Primary coil 222, 136, 224 Secondary coil

Claims

1. A magnetic core; a primary coil wound around the outer periphery of the magnetic core; At least two cylindrical secondary coils are disposed around the primary coil, concentrically with the primary coil and overlapping each other in the radial direction, Each of the at least two cylindrical secondary coils is provided with a cylindrical secondary coil body made of a predetermined conductive material having a cylindrical shape wound around a slit, and terminal portions extending outward from respective end vicinity regions of the cylindrical secondary coil body facing each other across the slit at one end side of the longitudinal direction of the slit in the cylindrical secondary coil body, the at least two cylindrical secondary coils have their one terminal portions overlapping and electrically connected to each other, and the other terminal portions are arranged so as to be in a non-contact state with each other, A low-profile transformer, characterized in that the cross-sectional areas of the at least two cylindrical secondary coils are adjusted according to the coil diameters of the respective cylindrical secondary coils so that the output current values ​​are equivalent.

2. A low-profile transformer as described in claim 1, characterized in that all of the at least two cylindrical secondary coils are made by processing plate-shaped conductive material made of the same material into a cylindrical shape.

3. A low-profile transformer as described in claim 2, characterized in that the same material is copper or aluminum.

4. 2. The low-profile transformer according to claim 1, characterized in that in each of the at least two cylindrical secondary coils, the terminal portion of the cylindrical secondary coil is formed by a plate material made of the specified conductive material that constitutes the cylindrical secondary coil body, and a notch is provided in a corner formed by a side surface of the cylindrical secondary coil body that is located on the opposite side of the terminal portion from the slit among side surfaces in the extension direction of each terminal portion, and an edge surface of the cylindrical secondary coil body.

5. 2. The low-profile transformer according to claim 1, wherein in each of the at least two cylindrical secondary coils, the one terminal portion and the other terminal portion are substantially parallel to each other and are formed so as to extend outward while being bent.

6. 2. The low-profile transformer according to claim 1, characterized in that in each of the at least two cylindrical secondary coils, the cross section of the cylindrical secondary coil body is rectangular, the long side surfaces of the rectangle form the circumferential surface of the cylinder, and the short side surfaces of the rectangle form the radial surfaces of the cylinder.

7. 2. The low-profile transformer according to claim 1, wherein the primary coil is an air-core coil formed by winding a conductor wire a plurality of times.

8. A first step of disposing a primary coil in a wound state on an outer periphery of a center leg portion of one of a first core and a second core that constitute a magnetic core; a second step of forming a cylindrical secondary coil body portion of a predetermined metal material for each of at least two cylindrical secondary coils, the cylindrical secondary coil body portion being wound around a slit, and extending terminal portions in a predetermined direction from each of the end vicinity regions of the cylindrical secondary coil body portions facing each other across the slit on one longitudinal end side of the slit, and further adjusting the cross-sectional area of ​​each cylindrical secondary coil body portion in forming the cylindrical secondary coil body portion so that the output current values ​​are equivalent in accordance with the coil diameter of each cylindrical secondary coil; a third step of arranging the at least two cylindrical secondary coils around an outer periphery of the primary coil, concentrically with the primary coil and overlapping each other in a radial direction; a fourth step of electrically connecting one of the terminal portions of each of the at least two cylindrical secondary coils to each other by overlapping the other terminal portions, and disposing the other terminal portions so as to be in a non-contact state with each other; a fifth step of combining the first core and the second core to form the magnetic core by butting one center leg portion against the other center leg portion located at the center of the primary coil and the at least two cylindrical secondary coils arranged concentrically, and housing the primary coil and the at least two cylindrical secondary coils in the magnetic core; A method for manufacturing a low-profile transformer, characterized in that the manufacturing is performed in an order such that at least the third and fourth steps are performed after the second step, and the fifth step is performed after the first step, the second step, the third step, and the fourth step.

9. the second step includes a manufacturing step of forming the cylindrical secondary coil body by sheet metal processing a plate material made of the predetermined metal material, and extending terminal portions one by one in a predetermined direction from each of the end vicinity regions of the cylindrical secondary coil body; 9. The method for manufacturing a low-profile transformer according to claim 8, characterized in that when the terminal portions are extended in the predetermined direction, a notch is provided in a corner formed by a side surface of each of the terminal portions in the extending direction that is located on the opposite side to the slit and an edge of the cylindrical secondary coil body, so that the extending direction of the terminal portions can be adjusted when the terminal portions are extended in the predetermined direction.