PRINTED COIL TRANSFORMER

Abstract

 A printed coil type transformer in a plane type transformer in which a mid-leg core 33 of EE shaped cores or EI shaped cores is disposed through the center of a spiral of a coil laminate 40 in which a plurality of concentric spiral coils are laminated in a thickness direction by using an insulating resin to obtain a magnetic coupling between the plurality of coils, characterized in that: sectional areas of the core at a feet core 34 and a connecting core 35 are approximately the same; the sectional area (Ae) of the mid-leg core is approximately twice of that of the feet core; the sectional area satisfies the following expression in connection with a core volume (Ve):
   1.4 ≦ Ve^1/3/Ae^1/2 ≦ 1.7; and the following expression is satisfied between a space (w) between the mid-leg core and the feet core of the core and a height (h) of the mid-leg core:
   0.5 ≦ h/w ≦ 2.

Description

Field of the Invention

[0001] The present invention relates to a coil structure suitably used for transformers and choking coils used in electronic equipments and power units and more particularly to a device having an excellent magnetic coupling, a low loss and an excellent high frequency characteristic when used as a transformer.

Description of the Related Art

[0002] A transformer is a magnetic part used in electronic equipments and power units and has a quality of insulating the primary side from the secondary side and of defining a secondary voltage in response to a primary voltage and a turn ratio. For a transformer for a switching power supply, coil transformers constructed by winding a lead wire around a bobbin have spread and the size of the magnetic core has been standardized by the standards of JIS and IEC, etc.

[0003] By the way, as a transformer not using a bobbin, a printed coil type transformer in which a coil is disposed on a single multi-layered printed circuit board has been known as disclosed in Japanese Patent Laid-Open No. 63-173308 for example. In the transformer constructed as described above, a loss due to a leakage inductance is decreased because the winding is disposed in close proximity. When the present inventor studied it in more detail, however, it was found that it would be necessary to decrease the loss further because the losses other than the leakage inductance contribute in a great deal.

[0004] It has been also known to lower the loss of the printed coil type transformer by minimizing a parasitic current of the primary circuit and secondary circuit caused by a switching current as disclosed in Japanese Patent Publication No. Hei. 1-503264. When the present inventor studied it also in more detail, however, it was found that the loss cannot be reduced sufficiently just by considering the lamination order of the multi-layered printed circuit board.

[0005] Accordingly, it is an object of the present invention to solve the aforementioned problems by providing a small printed coil type transformer having a core shape which minimizes the transformer loss.

DISCLOSURE OF THE INVENTION

[0006] According to the present invention for achieving the aforementioned goal, a printed coil type transformer in a plane type transformer in which a mid-leg core 33 of EE shaped cores or EI shaped cores is disposed through the center of a spiral of a coil laminate 40 in which a plurality of concentric spiral coils are laminated in a thickness direction by using an insulating resin to obtain a magnetic coupling between the plurality of coils, is characterized in that: sectional areas of the core at a feet core 34 and a connecting core 35 are approximately the same; an sectional area (Ae) of the mid-leg core is approximately twice of that of the feet core; the sectional area satisfies the following expression in connection with a core volume (Ve):
   1.4 ≦ Ve^1/3/Ae^1/2 ≦ 1.7; and the following expression is satisfied between a space(w) between the mid-leg core and the feet core of the core and a height (h) of the mid-leg core:
   0.5 ≦ h/w ≦ 2.

[0007] According to the present invention constructed as described above, because the sectional area of the core is unified along the magnetic path to keep the magnetic flux density almost constant, no loss will increase locally. Further, while the section of the coil is surrounded by a window section formed by the mid-leg core and the feet cores of the core, the coil resistance can be minimized by optimizing the shape of the core, thus contributing to the miniaturization of the transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] 

FIG. 1 is a structural perspective view of an assembling state, in which a part thereof is cut away, showing one preferred embodiment of the present invention;

FIG. 2 is a section view of a coil laminate 40;

FIGs. 3A through 3C are drawings for explaining the detail of a shape of a core 30;

FIG. 4 is a conceptual graph for explaining a relationship between a transformer loss Ploss and a core sectional area Ae;

FIGs. 5A and 5B are structural perspective views showing a second embodiment of the present invention;

FIGs. 6A and 6B are structural diagrams for explaining a shape of UU shaped cores;

FIGs. 7A and 7B are structural diagrams in which the ratio of dimensions of a core window (h/w) is 1 and which provide the standard in comparing with embodiments in FIGs. 8 and 9;

FIGs. 8A and 8B are structural perspective views showing a third embodiment of the present invention;

FIGs. 9A and 9B are structural perspective views showing a fourth embodiment of the present invention;

FIGs. 10A and 10B are structural perspective views showing a fifth embodiment of the present invention;

FIGs. 11A and 11B are diagrams for explaining a shape of EE shaped cores; and

FIGs. 12A through 12C are structural perspective views showing a sixth embodiment of the present invention.

BEST MODES FOR IMPLEMENTING THE PRESENT INVENTION

[0009] The present invention will be explained below by using the drawings. FIG. 1 is a structural perspective view of an assembling state, in which a part of a coil laminate is cut away, showing one preferred embodiment of the present invention. In the figure, a coil laminate 40 is what a conventional bobbin and a lead wire are put together in one body and a concrete detailed structure thereof has been disclosed in Japanese Patent Laid-Open No. Hei. 6-310345 for example which the present applicant has proposed. A core hole 41 is created at the center of the coil laminate 40 and a mid-leg core of an upper core 31 and a lower core 32 are inserted thereto. Terminals 42 are embedded in two sides crossing with other sides where feet cores 34 of the upper core 31 and the lower core 32 are attached.

[0010] FIG. 2 is a section view of the coil laminate 40, showing a section along a direction of 2-2 in FIG. 1. In the figure, two layers of secondary coils 45 exist in the middle while being sandwiched by respective upper and lower layers of primary coils 44. An internal interconnecting terminal 43a for interconnecting the respective upper and lower layers of the primary coil 44 and an internal interconnecting terminal 43b for interconnecting all of the primary coils 44 and the secondary coils 45 are provided near the core hole 41. Terminals 42 are provided at the both ends of the coil laminate 40. One terminal is a primary terminal 42a for interconnecting the respective upper and lower layers of the primary coils 44 and the other terminal is a secondary terminal 42b for interconnecting the two layers of the secondary coils 45. As for a number of turns of the coils, each layer of the primary coil 44 is would by three turns of coil and each layer of the secondary coil 45 is wound by two turns of coil. Because the gaps between each layer of the coil are filled with an insulating resin and an isolating distance required in obtaining various safety standards is so thin as 0.6 mm, the transformer can be miniaturized further.

[0011] FIGs. 3A through 3C are drawings for explaining the detail of the shape of a core 30, wherein FIG. 3A is a front view when the upper core 31 and the lower core 32 are assembled, FIG. 3B is a plan view of the core, and FIG. 3C is a perspective view for explaining sectional areas of the core. A diameter of the mid-leg core 33 is represented by D, which is the same value with a width C of the feet core 34 (D = C). A length of the connecting core 35 is represented by A, a space between the inner sides of the feet core 34 is represented by E and a thickness of the feet core 34 is represented by b. A space between the inner side of the feet core 34 and a peripheral face of the mid-leg core 33 facing thereto is represented by w. Then, the following relation holds:

   A height of the connecting core 35 when the upper core 31 and the lower core 32 has been assembled is represented by H and a space between the inner faces of the connecting core 35 facing to each other is represented by h. A thickness of the connecting core 35 is represented by t. Then, the following relation holds:

   Then, a sectional area Ae 35 of the connecting core 35, a sectional area Ae 34 of the feet core 34 and a sectional area Ae 33 of the mid-leg core 33 in the magnetic flux direction are defined to a value almost equal with respect to a sectional area per one line of magnetic flux passing through the connecting core 35 → the feet core 34 → the mid-leg core 33 to prevent the flux density from locally increasing and thus increasing the loss. Because two lines of magnetic flux pass through the mid-leg core 33, the sectional area Ae 33 is two times of the sectional area of other cores Ae 35 and Ae 34. When the core sectional area Ae is defined to be the sectional area Ae 33 of the mid-leg core 33, it may be expressed as follows:

Here, the thicknesses t and b fulfill the following relation:

   Further, a core volume Ve is expressed as a total of a volume Ve35 of the connecting core 35, a volume Ve34 of the feet core 34 and a volume Ve33 of the mid-leg core 33 in the direction of the magnetic flux, so that the following expression holds:

   Here, a ratio between the core volume Ve and the core sectional area Ae is represented by a dimensionless coefficient k which is defined by the following expression:

   Next, a relation between an iron loss PFe and a copper loss PCu of the coil will be explained. Here, the iron loss refers to an electric power consumed in the magnetic iron core due to a time-varying magnetizing force and includes a hysteresis loss and an eddy current loss. The iron loss PFe is expressed by the following expression:

Where, C1 is a constant defined by the shape and material of the coil, B is a flux density and fsw is a switching frequency. When the flux density B and the switching frequency fsw are constant, the iron loss PFe is proportional to the core volume Ve.

[0012] The copper loss is a load loss and includes an I²R loss caused by the eddy current and load in the coil, a stray loss caused by a leakage current and a loss caused by circulating currents in parallel coils. The copper loss PCu is expressed by the following expression:

where, C2 is a constant and N is a number of turns. Meanwhile, the following relation holds between the flux density B and the number of turns N:

where, C3 is a constant. Substituting the expression (9) into the number of turns N of the expression (8), the following expression is obtained:

That is, the copper loss PCu is inversely proportional to the square of the core sectional area Ae.

[0013] FIG. 4 is a conceptual graph for explaining a relationship between a transformer loss Ploss and a core sectional area Ae. The transformer loss is defined by a sum of the iron loss PFe and the copper loss PCu of the coil. As represented by the expression (7), the iron loss PFe is apt to increase along the increase of the core sectional area Ae. On the other hand, the copper loss PCu is apt to decrease along the increase of the core sectional area Ae. Accordingly, there exists an optimum core sectional area Ae which minimizes the loss in the transformer loss Ploss represented by the sum of the both.

[0014] It is now questioned how to decide the shape of the core which minimizes the transformer loss Ploss with reference to FIG. 3. When the shape of the core (w and h) related to the copper loss PCu is constant, the iron loss PFe takes the minimum value when h = w from the expression (5). When the core window width E is a given condition, a coil passing sectional width w is expressed from the expression (1), as follows:

Assuming h = w and substituting it into the expression (10), the copper loss PCu is expressed by the following expression:

The transformer loss may be minimized when the expression (12) is minimized. The value of the expression (12) is minimized when the following condition holds:

   Next, when the core width A is a given condition, the coil passing sectional width w is expressed from the expressions (1) and (4), as follows:

Assuming h = w and substituting it into the expression (10), the copper loss PCu may be expressed by the following expression:

The transformer loss may be minimized when the expression (15) is minimized. The value of the expression (15) is minimized when the following condition holds:

   Then, the shape of the core which minimizes the transformer loss within the range of the core window width E and the core width A described above is within the following range from the expressions (13) and (16):

At this time, the coefficient k is within the following range from the expression (6):

   FIGs. 5A and 5B are structural perspective views showing a second embodiment of the present invention, wherein FIG. 5A shows a state in which UU shaped cores are mounted to a coil laminate and FIG. 5B shows a simplex of the coil laminate. The U shaped core has a connecting core 37 and two feet cores 36 provided on the both sides thereof. The two-holed coil laminate 50 has two core holes 51a and 51b and the detailed structure thereof has been disclosed in Japanese Patent Laid-Open No. Hei. 6-333759 for example which the present applicant has proposed. Rows of terminals 52 are provided respectively at the edge of the both sides along the direction in which the core of the two-holed coil laminate 50 is mounted. The UU shaped cores or UI shaped cores are mounted to the two-holed coil laminate 50 to create a closed magnetic path.

[0015] FIGs. 6A and 6B are structural diagrams for explaining the shape of the UU shaped cores, wherein FIG. 6A is a front view in a state when the UU shaped cores have been assembled and FIG. 6B is a plan view of the U shaped core. Although the same reference numerals with those in FIG. 3 are used in FIG. 6 for the convenience of the explanation, they have values intrinsic in FIG. 6. A diameter of the feet core 36 is represented by D. A length of the connecting core 37 is represented by A, a thickness thereof t, a width thereof C and a space between inner peripheral faces of the feet cores 36 as 2w. Then, the following expression holds with respect to a core volume Ve and a core sectional area Ae:

   Accordingly, the coefficient k is expressed as follows:

   It is now questioned how to decide the shape of the core which minimizes the transformer loss Ploss with reference to FIG. 6. When the shape of the core (w and h) related to the copper loss PCu is constant, the iron loss PFe takes the minimum value when h = w from the expression (19). When the core window width E is a given condition, the coil passing sectional width w is expressed from FIG. 6B, as follows:

Assuming h = w and substituting it into the expression (10), the copper loss PCu is expressed by the same expression with the expression (12) described above, so that the value which minimizes the value of the expression (12) under the condition of the expression (13) also minimizes the transformer loss Ploss:

   Next, when the core width A is a given condition, a coil passing sectional width w is expressed from FIG. 6B, as follows:

Assuming h = w and substituting it into the expression (10), the copper loss PCu may be expressed by the following expression:

The transformer loss may be minimized when the expression (24) is minimized. The value of the expression (24) is minimized when the following condition holds:

   Then, the shape of the core which minimizes the transformer loss within the range of the core window width E and the core width A described above is within the following range from the expressions (13) and (25):

At this time, the coefficient k is within the following range from the expression (6):

   Now a case when h ≠ w while fulfilling the condition that hw = constant will be considered. FIGs. 7A and 7B are structural diagrams in which the ratio of dimensions of the core window (h/w) is 1 and which provide the standard in comparing with embodiments in FIGs. 8 and 9, wherein FIG. 7A is a plan view in a state when EE shaped cores are mounted to a coil laminate and FIG. 7B is a section view along B-B line in FIG. 7A and substantially shows a state in which the device shown in FIG. 1 has been assembled. As described in connection with the expression (11), the iron loss PFe takes a minimum value when h = w.

[0016] FIGs. 8A and 8B are structural views showing a third embodiment of the present invention in which the ratio of dimensions of the core window (h/w) is 1/2, wherein FIG. 8A is a plan view when EE cores are mounted to a coil laminate and FIG. 8B is a section view along B-B line in FIG. 8A. A coefficient which minimizes the transformer loss when

is within the following range when it is calculated with the same procedures in the expressions (11) through (16) described above:

   The iron loss PFe increases by about 5 % as compared to the case when

. However, it is considered to cause no trouble practically as a transformer.

[0017] Meanwhile, when the dimension of the core window is flat as described above, the section of the coil laminate 40 can be flat and the conductor as well. Then, because the copper loss decreases from the quality that the larger the conductor surface area, the smaller an AC resistance related to the copper loss can be from the skin effect, the increase of the transformer loss is actually less than 5 %.

[0018] FIGs. 9A and 9B are structural views showing a fourth embodiment of the present invention in which the ratio of dimensions of the core window (h/w) is 2, wherein FIG. 9A is a plan view when EE cores are mounted to a coil laminate and FIG. 9B is a section view along B-B line in FIG. 9A. A coefficient which minimizes the transformer loss when

is within the following range when it is calculated with the same procedures in the expressions (11) through (18) described above:

   The iron loss PFe increases by about 5 % as compared to the case when

. However, it is considered to cause no trouble practically as a transformer.

[0019] Meanwhile, when the dimension of the core window is vertically long as described above, the section of the coil laminate 40 can be made vertically long. As a result, the plan dimension of the coil laminate 40 can be reduced and the transformer mounting area can be reduced. When FIG. 9 is compared with FIG. 7 for example, the transformer mounting area of the case in FIG. 9 in which the core window is vertically long can be 1/2 of the case in FIG. 7 in which the lengths are equal. Then, it is suitable for the use in which a small transformer mounting area is a merit like the field in which a high density mounting is required.

[0020] As described in the first through fourth embodiments, the shape of the transformer which minimizes the transformer loss when the ratio of the dimensions of the core window (h/w) is within the range of 1/2 through 2 may be realized when the coefficient k is within the following range as described in connection with the expressions (18),(27), (28) and (29):

   Next, a comparison of a product of the present invention which is equivalent to the first embodiment with a conventional transformer using JIS FEER 25.5 will be explained. Table 1 compares characteristics of the conventional transformer with those of the present invention (coefficient k = 1.61), wherein six upper items relate to the shape of the core and two lower items relate to the transformer loss.

〈Table 1〉

	JIS FEER25.5	Embodiment(K=1.61)
Core Shape A[mm]	25.5	23.2
H[mm]	18.6	12.8
h[mm]	3.4	6.2
W[mm]	6.2	4.0
Ve[mm³]	2160	1782
Ae[mm²]	44.8	58.1
Loss Copper Loss	1	0.6 ∝ Ae⁻²
Iron Loss	1	0.82 ∝ Ve

[0021] As compared to the conventional transformer, a height H of the EE shaped core is lower and a length A is also shorter by about 10 % in the present invention. Then, a core volume Ve thereof is smaller by about 20 % and a core sectional area Ae is larger by about 30 % in contrary. Because an amount of magnetic material used may be small if the core volume Ve is small, a light weight transformer can be manufactured at low cost. While the ratio of dimensions of the core window is flat as

in the conventional transformer, it is vertically long as

in the present invention. Then, it can be seen that the performance of the present invention as a transformer is better because the copper loss is reduced by 40 % and the iron loss is reduced by 18 % in terms of the transformer loss.

[0022] FIG. 10A and 10B are structural perspective views showing a fifth embodiment of the present invention, wherein FIG. 10A shows a state in which EE shaped cores are mounted to a coil laminate and FIG. 10B shows a simplex of the coil laminate. FIGs. 11A and 11B are diagrams for explaining the shape of the EE shaped cores, wherein FIG. 11A is a front view when the EE shaped cores are assembled and FIG. 11B is a plan view of the E shaped core. This fifth embodiment is a variation of the embodiment shown in FIG. 3. While the diameter D of the mid-leg core 33 and the width C of the connecting core 35 have been equal in the case of the embodiment shown in FIG. 3, it is selected to be D < C in the present embodiment. By constructing as such, a thickness b of the feet core can be thinned and dimensions of the core window h and w can be large, so that a core window area hw can be increased. As a result, it brings about an effect that the transformer can be miniaturized and thinned further.

[0023] FIGs. 12A through 12C are structural perspective views showing a sixth embodiment of the present invention, wherein FIG. 12A shows a state in which EI shaped cores are mounted to the coil laminate, FIG. 12B shows the simplex of the coil laminate and FIG. 12C is a drawing for explaining a state in which the EI shaped cores are assembled. This sixth embodiment is a variation of the embodiment shown in FIG. 3, in which the EE shaped cores of the core 30 are replaced with the EI shaped cores. The same effect with the first embodiment can be obtained even by adopting such shape by having substantially the same core window dimensions h and w.

[0024] As described above, according to the present invention, the ratio of the dimensions of the core window (h/w) is selected to be within 0.5 through 2 and the coefficient which represents the shape of the core (= Ve^1/3Ae^1/2) to be within 1.4 through 1.7, so that it brings about an effect that the transformer loss which is defined by the copper loss and iron loss is minimized. Further, it brings about an effect that the shape of the transformer can be miniaturized as compared to the conventional transformer stipulated in the JIS FEER 25.5 and the like.

[0025] In this case, there are the one-holed coil laminate 40 and the two-holed coil laminate 50 and the cores mounted thereto are selected respectively from the EE shaped and UU shaped cores, so that the ratio of dimensions of the core window and the coefficient k are defined in accordance to the expression (30).

Claims

1. A printed coil type transformer in a plane type transformer in which a mid-leg core 33 of EE shaped cores or EI shaped cores is disposed through the center of a spiral of a coil laminate 40 in which a plurality of concentric spiral coils are laminated in a thickness direction by using an insulating resin to obtain a magnetic coupling between said plurality of coils, characterized in that:
   sectional areas of said core at a feet core 34 and a connecting core 35 are approximately the same;
   the sectional area (Ae) of said mid-leg core is approximately twice of that of said feet core;
   the sectional area satisfies the following expression in connection with a core volume (Ve):
   1.4 ≦ Ve^1/3/Ae^1/2 ≦ 1.7; and
   the following expression is satisfied between a space (w) between said mid-leg core and said feet core of said core and a height (h) of said mid-leg core:
   0.5 ≦ h/w ≦ 2.

2. A printed coil type transformer in a plane type transformer in which a plurality of concentric spiral coils are laminated in a thickness direction by using an insulating resin and feet cores 36 of UU shaped cores or UI shaped cores are disposed through a two-holed coil laminate 50 having a plurality of center of said spiral to obtain a magnetic coupling between said plurality of coils, characterized in that:
   a sectional area of said feet core (Ae) is approximately the same with a sectional area of a connecting core 37;
   the sectional area satisfies the following expression in connection with a core volume (Ve):
   1.4 ≦ Ve^1/3/Ae^1/2 ≦ 1 7; and
   the following expression is satisfied between a space (2w) between inner peripheral faces of said middle feet cores and a height (h) of said feet core:
0.5 ≦ h/w ≦ 2.

Drawing