Transformer and dc / dc converter

Abstract

Provided is a transformer comprising: a core having a first base part, a second base part provided facing the first base part, and a first leg part and a second leg part provided between the first base part and the second base part; a primary winding wound around each of the first leg part and the second leg part; a secondary winding wound around each of the first leg part and the second leg part; and a multilayer circuit board having a plurality of layers of coil patterns and a plurality of via conductors for interlayer-connecting the coil patterns. The primary winding is formed of the plurality of layers of coil patterns and the plurality of via conductors. In at least two layers of coil patterns among the plurality of layers of coil patterns, the coil pattern on the first leg part side that is wound around the first leg part and the coil pattern on the second leg part side that is wound around the second leg part are connected in series in the same layer of the multilayer circuit board.

Description

Transformers and DC-DC converters

[0001] The present disclosure relates to a transformer having a primary winding and a secondary winding, and a DC-DC converter including the transformer.

[0002] Transformers having a primary winding and a secondary winding are known. Patent Document 1 discloses a multilayer substrate device having a pattern coil in which two coil patterns are formed spirally extending in the same winding direction and the outer ends of both coil patterns are connected to each other by a connecting pattern.

[0003] Japanese Patent Application Publication No. 9-283335

[0004] The multilayer substrate device described in Patent Document 1 has a problem in that the number of locations where via conductors that connect the coil patterns arranged above and below the sheet layers are formed is large, resulting in a large area for the multilayer substrate device.

[0005] A transformer according to one aspect of the present disclosure includes a core having a first base, a second base opposed to the first base, and first and second legs disposed between the first and second bases, a primary winding wound around each of the first and second legs, a secondary winding wound around each of the first and second legs, and a multilayer circuit board. The multilayer circuit board has multiple layer coil patterns and multiple via conductors connecting the coil patterns between layers. The primary winding is formed by the multiple layer coil patterns and the multiple via conductors, and in at least two layers of the multiple layer coil patterns, the coil pattern wound around the first leg and the coil pattern wound around the second leg are connected in series on the same layer of the multilayer circuit board.

[0006] A DC-DC converter according to one aspect of the present disclosure includes the above-described transformer and a resonant capacitor connected in series to the transformer.

[0007] According to the present disclosure, the area of ​​the transformer can be reduced.

[0008] FIG. 1 is a circuit diagram of an LLC converter. FIG. 2 is a diagram illustrating an example of gain characteristics of an LLC converter. FIG. 3 is a diagram illustrating an example of a combination of cores constituting a transformer. FIG. 4 is a diagram illustrating an example of a core of a transformer. FIG. 5 is a diagram schematically illustrating a cross section of a transformer according to an embodiment. FIG. 6 is a plan view of a coil pattern of a primary winding of a transformer of a comparative example. FIG. 7 is a plan view of a coil pattern of a primary winding of a transformer according to an embodiment. FIG. 8 is a perspective view of a coil pattern of a primary winding of a transformer according to an embodiment. FIG. 9 is a circuit diagram of an LLC converter according to a first example. FIG. 10 is an external view of a transformer according to the first example. FIG. 11 is a schematic view of a core of the transformer according to the first example. FIG. 12 is a diagram schematically illustrating a cross section of a transformer according to the first example. FIG. 13 is a perspective view of a coil pattern of a secondary winding of a transformer according to the first example. FIG. 14 is a perspective view of a coil pattern of a primary winding of a transformer according to the first example. FIG. 15 is a diagram schematically illustrating a cross section of a transformer according to a second embodiment. FIG. 16 is a perspective view of a coil pattern of a primary winding of the transformer according to the second embodiment. FIG. 17 is a diagram schematically illustrating a cross section of a transformer according to a third embodiment. FIG. 18 is a perspective view of a coil pattern of a primary winding of the transformer according to the third embodiment. FIG. 19 is a diagram schematically illustrating a cross section of a transformer according to a fourth embodiment. FIG. 20 is a perspective view of coil patterns of a primary winding and a secondary winding of the transformer according to the fourth embodiment. FIG. 21 is a diagram illustrating an example of a coil pattern of a primary winding of a transformer according to a fifth embodiment. FIG. 22 is a diagram illustrating another example of a coil pattern of a primary winding of a transformer according to the fifth embodiment. FIG. 23 is a diagram illustrating another example of a coil pattern of a primary winding of a transformer according to the fifth embodiment. FIG. 24 is a diagram schematically illustrating a cross section of a transformer according to a sixth embodiment. FIG. 25 is a top view of the transformer according to the sixth embodiment. Fig. 26 is a plan view showing a coil pattern of a first leg, a second leg, and a primary winding of a transformer according to a seventh embodiment. Fig. 27 is a schematic cross-sectional view of a transformer according to an eighth embodiment. Fig. 28 is a diagram showing a portion of a coil pattern of a primary winding of a transformer according to a ninth embodiment. Fig. 29 is a schematic cross-sectional view of a transformer according to a tenth embodiment. Fig. 30 is a perspective view of a coil pattern of a primary winding of a transformer according to an eleventh embodiment.FIG. 31 is a plan view of a coil pattern of a primary winding of a transformer according to a twelfth embodiment. FIG. 32 is a diagram illustrating an example of a cross section of a transformer according to a thirteenth embodiment. FIG. 33 is a diagram illustrating another example of a cross section of a transformer according to the thirteenth embodiment. FIG. 34 is a diagram illustrating a magnetomotive force of a transformer. FIG. 35 is a diagram illustrating a relationship between the number of turns of a winding and winding resistance. FIG. 36 is a circuit diagram illustrating an example of an LLC converter according to a fourteenth embodiment. FIG. 37 is a circuit diagram of a transformer according to the fourteenth embodiment. FIG. 38 is a diagram schematically illustrating a cross section of a transformer according to the fourteenth embodiment. FIG. 39 is a circuit diagram of an LLC converter according to a fifteenth embodiment. FIG. 40 is a circuit diagram of a transformer according to the fifteenth embodiment. FIG. 41 is a diagram schematically illustrating a cross section of a transformer according to the fifteenth embodiment.

[0009] (Background to the Invention of the Disclosure) The background to the invention of the disclosure will be described with reference to FIGS. 1 to 3. FIG.

[0010] FIG. 1 is a circuit diagram of an LLC converter 105.

[0011] 1 shows a transformer, a switch circuit, etc. The figure also shows an example of an input voltage Vin, an output voltage Vout, and a turns ratio N. The circuit also shows the magnetizing inductance Lm, leakage inductance Lr, and capacitance Cr of the resonant capacitor of the transformer, which are shown as an equivalent circuit.

[0012] FIG. 2 is a diagram showing an example of the gain characteristic of the LLC converter 105. In FIG.

[0013] The horizontal axis of the figure represents normalized frequency, and the vertical axis represents a transfer function that is the gain of the LLC converter 105. In the figure, m represents the ratio between the excitation inductance Lm and the leakage inductance Lr, and is calculated by m=Lm / Lr (Equation 1).

[0014] Power supplies for automotive accessories are required to operate over a wide range of input and output voltages. For this reason, for example, a transformer with a large turns ratio N is used to increase the step-down ratio, and the gain (input / output voltage ratio) is adjusted by changing the operating frequency.

[0015] As shown in Figure 2, the smaller the ratio m, the wider the range of gain change relative to frequency change. Based on the above-mentioned (Equation 1), the ratio m can be reduced by increasing the leakage inductance Lr, for example. Therefore, in order to achieve wide-range operation in LLC converter 105, it is considered necessary to increase the leakage inductance Lr of the transformer to a certain extent.

[0016] For example, a large leakage inductance Lr can be obtained by using multiple transformers, but this increases the overall transformer area, which is the mounting area (or footprint) required when mounting the transformer on a mounting board.

[0017] 3 is a diagram showing an example of a combination of cores that make up a transformer. In FIG. 3, the primary winding is indicated by a solid arrow, and the secondary winding is indicated by a dashed arrow. In the transformer of the present disclosure, the primary winding is a winding on the high-voltage side, and the secondary winding is a winding on the low-voltage side.

[0018] Figure 3(a) shows a transformer consisting of two core sets: a concave-convex E-core with three legs and a flat I-core. As shown in the figure, simply using two core sets would increase the overall area of ​​the transformer.

[0019] Figure 3(b) shows a transformer in which adjacent legs of two sets of cores are joined together to form an integrated unit. In this example, the joined legs are used as the center legs, and the primary winding is wound so that the magnetic flux is canceled at the center legs. As shown in the figure, the area of ​​the transformer is reduced by the integration, but the overall area is still large.

[0020] Figure 3(c) shows an example of the transformer shown in Figure 3(b) with the center leg removed. Because the center leg is the leg where magnetic flux is canceled, removing it does not substantially reduce functionality. By removing the center leg as shown in Figure 3(c), a transformer with a large leakage inductance equivalent to that of the transformer shown in Figure 3(a) can be obtained with a smaller area.

[0021] The present disclosure proposes a structure that further reduces the area of ​​this transformer.

[0022] (Outline of the embodiment) The outline of the embodiment will be described with reference to FIGS.

[0023] FIG. 4 is a diagram illustrating an example of a core of a transformer.

[0024] Fig. 4A shows a core having a first leg 21, a second leg 22, and two outer legs 23, 24 located on either side of the first leg 21 and the second leg 22. Fig. 4B shows the core 10 in a state where the outer legs have been removed from the core shown in Fig. 4A. In these figures, the dashed arrows indicate the magnetic flux φ generated in the core at a predetermined timing (e.g., the positive half cycle of the AC voltage waveform). For example, if the iron loss of the outer legs 23, 24 is small, removing the outer legs 23, 24 does not significantly change the magnetic flux φ. Therefore, the following description will be given using the core 10 in a state where the outer legs are not present, as shown in Fig. 4B.

[0025] Fig. 5 is a diagram schematically illustrating a cross section of the transformer 1 according to the embodiment. In Fig. 5, the core 10 shown in Fig. 4(b) is arranged upside down.

[0026] As shown in Fig. 5, the transformer 1 includes a core 10 and a multilayer circuit board 70 including a plurality of sheet layers 71 to 78. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32. In Fig. 5, the hatching of the multilayer circuit board 70 and the primary winding 31 is omitted.

[0027] The core 10 is composed of a first base 11, a second base 12 arranged opposite the first base 11, and a first leg 21 and a second leg 22 arranged between the first base 11 and the second base 12.

[0028] The first base portion 11 is plate-shaped. The second base portion 12 is plate-shaped and is provided opposite the first base portion 11. When the core 10 is viewed from a first direction x1, which is a direction in which the first base portion 11 and the second base portion 12 face each other, the first base portion 11 and the second base portion 12 are rectangular and at least partially overlap each other.

[0029] Each of the first leg 21 and the second leg 22 has a columnar shape with its axial direction aligned with the first direction x1 and is disposed to extend along the first direction x1. The first leg 21 and the second leg 22 are provided between the first base 11 and the second base 12 and face the second base 12 across a gap g1 in the first direction x1. The first leg 21 and the second leg 22 may be in contact with the second base 12. The first leg 21 and the second leg 22 are disposed at an interval along a second direction x2 that intersects with the first direction x1.

[0030] The primary winding 31 is provided in the area surrounded by the thick dashed line in Fig. 5. The primary winding 31 is wound around each of the first leg 21 and the second leg 22. The secondary winding 32 is also wound around each of the first leg 21 and the second leg 22. The primary winding 31 and the secondary winding 32 are provided inside or on the surface of the multilayer circuit board 70.

[0031] The secondary winding 32 is disposed on both outer sides of the primary winding 31 in the first direction x1, above and below the primary winding 31. The secondary winding 32 is formed by, for example, a 3 / 4 turn coil pattern.

[0032] The primary winding 31 is formed by multiple layers of coil patterns provided on the multilayer circuit board 70 and multiple via conductors that connect the coil patterns between layers. In this example, the multiple layers of coil patterns are configured by four layers, which is an even number of layers. Each coil pattern provided on each layer is a spiral planar coil. The widths of the coil patterns may be the same or different.

[0033] Here, the configuration and problems of the primary winding 131 of the transformer of the comparative example will be described. Note that the transformer of the comparative example also includes a core 10 and a primary winding and a secondary winding provided on a multilayer circuit board 70, as shown in FIG.

[0034] Fig. 6 is a plan view of the coil pattern of the primary winding 131 of the transformer of the comparative example. Fig. 6 also shows the first leg 21 and the second leg 22. In the figure, the coil pattern is indicated by hatched dots, and the via conductors are not indicated by hatching.

[0035] (a) of Figure 6 shows the coil patterns c111 and c112 of the first layer, (b) shows the coil patterns c121 and c122 of the second layer, (c) shows the coil patterns c131 and c132 of the third layer, and (d) shows the coil patterns c141 and c142 of the fourth layer.

[0036] The four-layer coil patterns c111, c121, c131, and c141 wound around the first leg 21 are connected in series vertically via multiple via conductors. The four-layer coil patterns c112, c122, c132, and c142 wound around the second leg 22 are connected in series vertically via multiple via conductors. The fourth-layer coil patterns c141 and c142 are connected in series horizontally to form the primary winding 131.

[0037] More specifically, on the first leg 21 side, the first-layer coil pattern c111 and the second-layer coil pattern c121 are connected via via conductor vz1, the second-layer coil pattern c121 and the third-layer coil pattern c131 are connected via via conductor vz2, and the third-layer coil pattern c131 and the fourth-layer coil pattern c141 are connected via via conductor vz3. In the fourth layer, the coil pattern c141 on the first leg 21 side and the coil pattern c142 on the second leg 22 side are connected in series. On the second leg 22 side, the fourth-layer coil pattern c142 and the third-layer coil pattern c132 are connected via via conductor vz4, the third-layer coil pattern c132 and the second-layer coil pattern c122 are connected via via conductor vz5, and the second-layer coil pattern c122 and the first-layer coil pattern c112 are connected via via conductor vz6.

[0038] In the comparative example, at least six via conductors vz1 to vz6 are required to connect four layers of coil patterns to form the primary winding 131. This increases the number of locations where the via conductors are formed when viewed from the first direction x1, which results in a problem of an increased area of ​​the transformer.

[0039] In the comparative example, the via conductor vz2 is formed at the outer peripheral end of the spiral coil patterns c121 and c131, and the via conductor vz5 is formed at the outer peripheral end of the spiral coil patterns c122 and c132. This requires providing an area for forming the via conductor near the outer periphery of the transformer, which results in a problem of an increased area of ​​the transformer.

[0040] Furthermore, in the comparative example, the coil pattern c112 on the second leg 22 side requires 3.5 turns, and the coil pattern c141 on the first leg 21 side requires 3.5 turns. This results in a problem of a larger transformer area compared to a transformer configured with a three-turn coil pattern. Also, in the comparative example, the coil pattern c121 on the first leg 21 side requires 2.5 turns, and the coil pattern c132 on the second leg 22 side requires 2.5 turns. This results in a problem of ineffective use of the transformer area when a transformer is configured with, for example, a three-turn coil pattern.

[0041] In contrast, the transformer 1 according to the embodiment has the following configuration in order to reduce the area of ​​the transformer.

[0042] Fig. 7 is a plan view of the coil pattern of the primary winding 31 of the transformer 1 according to the embodiment. Fig. 8 is a perspective view of the coil pattern of the primary winding 31 of the transformer 1 according to the embodiment. Note that Figs. 7 and 8 also show the first leg 21 and the second leg 22. In these figures, the coil pattern is hatched, but the via conductors are not. In the perspective view of the coil pattern, the thickness of the coil pattern and the like is not shown.

[0043] (a) of Figure 7 shows the coil patterns c11 and c12 of the first layer, (b) shows the coil patterns c21 and c22 of the second layer, (c) shows the coil patterns c31 and c32 of the third layer, and (d) shows the coil patterns c41 and c42 of the fourth layer.

[0044] Via conductors va1 and va2 are formed on a sheet layer 74 (see FIG. 5) between the first-layer coil patterns c11, c12 and the second-layer coil patterns c21, c22 shown in FIG. 8. Via conductors va1 and va3 are formed on a sheet layer 75 between the second-layer coil patterns c21, c22 and the third-layer coil patterns c31, c32. Via conductors va1 and va4 are formed on a sheet layer 76 between the third-layer coil patterns c31, c32 and the fourth-layer coil patterns c41, c42.

[0045] 7 and 8, the via conductor va1 is formed to penetrate through the first to fourth layers. Furthermore, the via conductor va1 is formed at a position where it does not overlap with another via conductor va3 and other coil patterns c21 and c31 when viewed from the first direction x1.

[0046] On the first leg 21 side of the first layer, the coil pattern c11 is configured with a 3 / 4 turn winding, and an inner coil pattern c11i is formed inside the coil pattern c11. The inner coil pattern c11i is wound around the first leg 21 in the same layer as the coil pattern c11 and is disposed between the first leg 21 and the coil pattern c11 on the first leg 21 side. The coil pattern c11 and the inner coil pattern c11i are insulated from each other in the first layer. The inner coil pattern c11i is connected to a coil pattern on the first leg 21 side of any layer other than the first layer via a via conductor va1. In this example, the inner coil pattern c11i is connected to a coil pattern c41 on the fourth layer via a via conductor va1.

[0047] In the embodiment, in the first layer, the coil pattern c11 on the first leg 21 side and the coil pattern c12 on the second leg 22 side are connected in series. In the second layer, the coil pattern c21 on the first leg 21 side and the coil pattern c22 on the second leg 22 side are connected in series. In the third layer, the coil pattern c31 on the first leg 21 side and the coil pattern c32 on the second leg 22 side are connected in series. In the fourth layer, the coil pattern c41 on the first leg 21 side and the coil pattern c42 on the second leg 22 side are connected in series. That is, in this example, in all of the coil patterns in the multiple layers, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side wound around the second leg 22 are connected in series on the same layer of the multilayer circuit board 70.

[0048] In the embodiment, the coil pattern c12 on the second leg 22 side of the first layer is connected to the coil pattern c22 on the second leg 22 side of the second layer by a via conductor va2. The coil pattern c21 on the first leg 21 side of the second layer is connected to the coil pattern c31 on the first leg 21 side of the third layer by a via conductor va3. The coil pattern c32 on the second leg 22 side of the third layer is connected to the coil pattern c42 on the second leg 22 side of the fourth layer by a via conductor va4. The coil pattern c41 on the first leg 21 side of the fourth layer is connected to the inner coil pattern c11i of the first layer by a via conductor va1. The multiple via conductors va1 to va4 are each connected to the pattern line pL1 located at the innermost position of the spiral coil pattern.

[0049] As described above, in this embodiment, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side are connected in series on the same layer of the multilayer circuit board 70. With this configuration, the coil patterns on the four layers can be connected using four via conductors va1 to va4 as described above. This allows the number of via conductors to be reduced as viewed from the first direction x1 compared to the comparative example, thereby reducing the area of ​​the transformer 1.

[0050] In the embodiment, the via conductors va1 to va4 are each connected to the pattern line pL1 located at the innermost side of the spiral coil pattern. This configuration eliminates the need to provide a via conductor formation area near the outer periphery of the transformer as in the comparative example, and allows the area of ​​the transformer 1 to be smaller than in the comparative example.

[0051] In the embodiment, the coil patterns of each layer of the coil patterns arranged on the first leg 21 side (e.g., coil patterns c11 and c11i, coil pattern c21, coil pattern c31, and coil pattern c41) have the same number of turns. Also, the coil patterns of each layer of the coil patterns arranged on the second leg 22 side (e.g., coil pattern c12, coil pattern c22, coil pattern c32, and coil pattern c42) have the same number of turns. With this configuration, the number of turns of the coil patterns on the first leg 21 side and the number of turns of the coil patterns on the second leg 22 side can be made equal, thereby making it possible to reduce the area of ​​the transformer 1 compared to the comparative example.

[0052] Furthermore, in the embodiment, in each of the multiple layers, the coil patterns c11 and c11i arranged on the first leg 21 side and the coil pattern c12 arranged on the second leg 22 side have the same number of turns. Similarly, the coil patterns c21 and c22 have the same number of turns, the coil patterns c31 and c32 have the same number of turns, and the coil patterns c41 and c42 have the same number of turns. With this configuration, the number of turns of the coil patterns on the first leg 21 side and the second leg 22 side are equal, allowing the area of ​​the transformer 1 to be smaller than in the comparative example.

[0053] The present disclosure will be described in more detail below with reference to examples of embodiments. Note that each of the examples described below illustrates a specific example of the present disclosure. Therefore, the numerical values, shapes, materials, components, component placement positions, and connection configurations shown in the following examples are merely examples and are not intended to limit the present disclosure. Therefore, among the components in the following examples, components that are not recited in the independent claims will be described as optional components.

[0054] Note that each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, the scales and the like do not necessarily match in each figure. Furthermore, in each figure, the same reference numerals are used for substantially the same configurations, and redundant explanations will be omitted or simplified.

[0055] Each figure also shows x1, x2, and x3 axes, which represent three mutually orthogonal directions, and these axes and the axial directions along these axes are used as necessary for explanation. Note that these axes are added for explanation purposes only and do not limit the direction or posture in which the transformer is used.

[0056] First Embodiment [Configuration of LLC Converter] A description will be given of an LLC converter 5 including a transformer 1A according to a first embodiment. The LLC converter 5 is an example of a DC-DC converter.

[0057] FIG. 9 is a circuit diagram of the LLC converter 5 according to the first embodiment.

[0058] The LLC converter 5 includes a primary circuit 6, a transformer 1A, and a secondary circuit 7. The LLC converter 5 also includes a resonant capacitor Cs. The resonant capacitor Cs is provided on a path connecting the primary circuit 6 and the transformer 1A.

[0059] The primary circuit 6 is composed of a pair of primary ports p11 and p12, a bypass capacitor, and a full bridge circuit having four switch elements s11, s12, s13, and s14.

[0060] One end of switch element s11 is connected to one primary port p11, and the other end is connected to one end of switch element s12. The other end of switch element s12 is connected to the other primary port p12. One end of switch element s13 is connected to one primary port p11, and the other end is connected to one end of switch element s14. The other end of switch element s14 is connected to the other primary port p12.

[0061] The secondary circuit 7 is composed of a pair of secondary ports p21 and p22, four switch elements s21, s22, s23 and s24, and four bypass capacitors.

[0062] The transformer 1A is provided between the primary circuit 6 and the secondary circuit 7. The transformer 1A is a composite transformer including a primary winding 31 and a secondary winding 32. The primary winding 31 is a high-voltage side winding, and the secondary winding 32 is a low-voltage side winding.

[0063] The primary winding 31 is composed of two primary windings 31a and 31b connected in series. One end of the primary winding 31a is connected to a node between the switch elements s11 and s12 via a resonant capacitor Cs. The other end of the primary winding 31a is connected to one end of the other primary winding 31b. The other end of the other primary winding 31b is connected to a node between the switch elements s13 and s14.

[0064] The secondary winding 32 is composed of two secondary windings 32a and 32b connected in parallel. The two secondary windings 32a and 32b are arranged in one-to-one correspondence with the two primary windings 31a and 31b.

[0065] One end of one secondary winding 32a of the two secondary windings is connected to the other secondary port p22 via a switch element s21. The other end of one secondary winding 32a is connected to the other secondary port p22 via a switch element s22. One secondary winding 32a has a center tap, which is connected to one secondary port p21.

[0066] One end of the other secondary winding 32b of the two secondary windings is connected to the other secondary port p22 via a switch element s23. The other end of the other secondary winding 32b is connected to the other secondary port p22 via a switch element s24. The other secondary winding 32b has a center tap, which is connected to one secondary port p21.

[0067] The circuit shown in FIG. 9 also shows the magnetizing inductances Lma and Lmb, leakage inductances Lra and Lrb of the transformer 1A, and the capacitance Cr of the resonant capacitor Cs, which are shown as an equivalent circuit.

[0068] The excitation inductance Lma is the excitation inductance of one primary winding 31 a and one secondary winding 32 a, the leakage inductance Lra is the leakage inductance of one primary winding 31 a and one secondary winding 32 a, the excitation inductance Lmb is the excitation inductance of the other primary winding 31 b and the other secondary winding 32 b, and the leakage inductance Lrb is the leakage inductance of the other primary winding 31 b and the other secondary winding 32 b.

[0069] Although the above describes an LLC converter 5 equipped with a transformer 1A, the LLC converter 5 may be equipped with a transformer 1 according to an embodiment or a transformer according to Example 2 or later, instead of the transformer 1A of Example 1.

[0070] [Configuration of Transformer] The configuration of a transformer 1A according to the first embodiment will be described with reference to Fig. 10 to Fig. 14. In the first embodiment, an example in which the number of layers of the coil pattern is two will be described.

[0071] FIG. 10 is an external view of a transformer 1A according to the first embodiment.

[0072] FIG. 10A shows a perspective view of the transformer 1A, and FIG. 10B shows a view of the transformer 1A shown in FIG. 10A with the first base portion 11 temporarily removed.

[0073] 10 , transformer 1A includes core 10 and multilayer circuit board 70. Multilayer circuit board 70 is provided with primary winding 31 and secondary winding 32. In Example 1, core 10 does not have an outer leg, and has first leg 21 and second leg 22.

[0074] FIG. 11 is a schematic diagram of the core 10 of the transformer 1A.

[0075] 11 shows the core 10 disassembled into two parts. As shown in FIG. 11 , the core 10 has a first core 10a and a second core 10b. The first core 10a is composed of a first base 11, a first leg 21, and a second leg 22. The first core 10a is formed by integrally molding the first base 11, the first leg 21, and the second leg 22. The second core 10b is composed of a second base 12.

[0076] That is, the core 10 has a first base portion 11, a second base portion 12, a first leg portion 21, and a second leg portion 22. The first leg portion 21 and the second leg portion 22 are connected to the first base portion 11. Each of the first base portion 11, the second base portion 12, the first leg portion 21, and the second leg portion 22 is formed of a magnetic material such as Mn—Zn ferrite.

[0077] The first base portion 11 and the second base portion 12 are each plate-shaped, and are arranged to face each other in the first direction x1.

[0078] Each of the first leg 21 and the second leg 22 has a columnar shape with its axis aligned in the first direction x1 and is arranged to extend along the first direction x1. The first leg 21 and the second leg 22 are arranged at an interval along a second direction x2 that intersects with the first direction x1. In this embodiment, the first direction x1 and the second direction x2 are perpendicular to each other.

[0079] Hereinafter, the direction in which the first base 11 and the second base 12 face each other will be referred to as the first direction x1, and the direction in which the first leg 21 and the second leg 22 are arranged side by side will be referred to as the second direction x2. In addition, the direction perpendicular to both the first direction x1 and the second direction x2 will sometimes be referred to as the third direction x3.

[0080] The first leg 21 and the second leg 22 have an elliptical cross section perpendicular to the first direction x1. The first leg 21 and the second leg 22 are arranged so that the width of the elliptical shape is parallel to an axis along the second direction x2. The cross sections of the first leg 21 and the second leg 22 may be circular, elliptical, polygonal, or oval.

[0081] FIG. 12 is a diagram schematically illustrating a cross section of a transformer 1A according to the first embodiment.

[0082] The primary winding 31 and secondary winding 32 shown in FIG. 12 are provided, for example, inside or on the surface of a multilayer circuit board 70 (see FIG. 10 ). The multilayer circuit board 70 has two through holes. The first leg 21 and the second leg 22 of the core 10 are inserted into the two through holes of the multilayer circuit board 70, respectively. By inserting the first leg 21 and the second leg 22 into the two through holes, the primary winding 31 and the secondary winding 32 are wound around the respective legs. Note that in this example, the primary winding 31 and the secondary winding 32 are not wound in contact with the respective legs, but are wound with a material forming a sheet layer sandwiched between the legs. The actual multilayer circuit board 70 is larger than the size shown in FIG. 10 , and the above-mentioned primary circuit 6, secondary circuit 7, etc. are mounted on it.

[0083] The primary winding 31 includes two primary coils. In this example, the primary winding 31 is composed of two primary coils 41 and 42. The primary coil 41 is wound around the first leg 21, and the primary coil 42 is wound around the second leg 22. In this example, the primary coil 41 corresponds to the primary winding 31a shown in FIG. 9, and the primary coil 42 corresponds to the primary winding 31b.

[0084] FIG. 12 shows the current flow direction in the coil pattern of the primary winding 31 at a predetermined timing (e.g., a positive half cycle of the AC voltage waveform). For example, in the coil pattern on the first leg 21 side of the primary coil 41, the current flow direction in the pattern line arranged on the left side of the first leg 21 is the positive direction of the third direction x3, and the current flow direction in the pattern line arranged on the right side of the first leg 21 is the negative direction of the third direction x3. Also, in the coil pattern on the second leg 22 side of the primary coil 42, the current flow direction in the pattern line arranged on the left side of the second leg 22 is the negative direction of the third direction x3, and the current flow direction in the pattern line arranged on the right side of the second leg 22 is the positive direction of the third direction x3. The current flow as described above generates magnetic flux in the core 10. FIGS. 13 and 14 (described later) show the directions of the magnetic flux generated in the first leg 21 and the second leg 22. In these figures, the magnetic flux in the first leg 21 is generated in the negative direction of the first direction x1, and the magnetic flux in the second leg 22 is generated in the positive direction of the first direction x1.

[0085] FIG. 13 is a perspective view of the coil pattern of the secondary winding 32 of the transformer 1A according to the first embodiment.

[0086] The secondary winding 32 includes two or more secondary coils. In this example, the secondary winding 32 is made up of four secondary coils 51, 52, 53, and 54. Each of the secondary coils 51 to 54 is formed, for example, by a U-shaped planar coil.

[0087] The secondary winding 32 is composed of two secondary windings 32a and 32b (see FIG. 9). Each of the two secondary windings 32a and 32b is composed of two secondary coils connected in parallel. Specifically, one secondary winding 32a is composed of two secondary coils 51 and 52 connected in parallel, and the other secondary winding 32b is composed of two secondary coils 53 and 54 connected in parallel. Since the secondary windings 32a and 32b are connected in parallel, the four secondary coils 51 to 54 are connected in parallel with each other. As shown in FIG. 12, the secondary coils 51 and 52 are wound around the first leg 21, and the secondary coils 53 and 54 are wound around the second leg 22.

[0088] FIG. 14 is a perspective view of the coil pattern of the primary winding 31 of the transformer 1A according to the first embodiment.

[0089] 14 shows the coil patterns c11 and c12 of the first layer and the coil patterns c21 and c22 of the second layer. Via conductors vb1 and vb2 that penetrate the sheet layer 74 are formed in the sheet layer 74 between the coil patterns c11 and c12 of the first layer and the coil patterns c21 and c22 of the second layer.

[0090] On the first leg 21 side of the first layer, the coil pattern c11 is configured with a 3 / 4 turn winding, and an inner coil pattern c11i is formed inside the coil pattern c11. The inner coil pattern c11i is wound around the first leg 21 in the same layer as the coil pattern c11 and is disposed between the first leg 21 and the coil pattern c11 on the first leg 21 side. The coil pattern c11 and the inner coil pattern c11i are insulated from each other in the first layer. The inner coil pattern c11i is connected to the coil pattern c21 of the second layer via a via conductor vb1. An input / output line ioL for inputting current to the primary winding 31 or outputting current from the primary winding 31 is provided at the outermost end of each of the coil pattern c11 and the inner coil pattern c11i.

[0091] In Figure 14, arrows indicate the direction of current flowing through each coil pattern at a predetermined timing (e.g., a positive half cycle of the AC voltage waveform). At this predetermined timing, current is input from the input / output line ioL of the coil pattern c11, and current is output from the input / output line ioL of the inner coil pattern c11i. When the transformer 1A is viewed from the opposite direction of the arrow in the first direction x1 in Figure 14, current flows clockwise through the coil patterns c11, c11i, and c21 wound around the first leg 21, and current flows counterclockwise through the coil patterns c12 and c22 wound around the second leg 22. In other words, the current flows in opposite directions between the coil patterns c11 and c11i and the coil pattern c12 located on the same layer, and the current flows in opposite directions between the coil patterns c21 and c22 located on the same layer.

[0092] In Example 1, in the first layer, the coil pattern c11 on the first leg 21 side and the coil pattern c12 on the second leg 22 side are connected in series. In the second layer, the coil pattern c21 on the first leg 21 side and the coil pattern c22 on the second leg 22 side are connected in series. In this example, in the coil patterns of all layers among the multiple layers of coil patterns, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side wound around the second leg 22 are connected in series on the same layer of the multilayer circuit board 70.

[0093] In Example 1, the coil pattern c12 on the second leg 22 side of the first layer is connected to the coil pattern c22 on the second leg 22 side of the second layer by a via conductor vb2. The coil pattern c21 on the first leg 21 side of the second layer is connected to the inner coil pattern c11i of the first layer by a via conductor vb1. The multiple via conductors vb1 and vb2 are each connected to the pattern line pL1 located at the innermost position of the spiral coil pattern.

[0094] As described above, in Example 1, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side are connected in series on the same layer of the multilayer circuit board 70. With this configuration, the coil patterns on the two layers can be connected using two via conductors vb1 and vb2 as described above. This reduces the number of locations where the via conductors are formed when viewed from the first direction x1, thereby reducing the area of ​​the transformer 1A.

[0095] In addition, in Example 1, the multiple via conductors vb1 and vb2 are each connected to the pattern line pL1 located at the innermost side of the spiral coil pattern. This configuration eliminates the need to provide a via conductor formation area near the outer periphery of the transformer, for example, and allows the area of ​​the transformer 1A to be reduced.

[0096] In addition, in Example 1, the coil patterns of each layer of the coil patterns arranged on the first leg 21 side (e.g., coil patterns c11 and c11i and coil pattern c21) have the same number of turns. Also, the coil patterns of each layer of the coil patterns arranged on the second leg 22 side (e.g., coil pattern c12 and coil pattern c22) have the same number of turns. With this configuration, the number of turns of the coil patterns on the first leg 21 side and the number of turns of the coil patterns on the second leg 22 side can be made equal, thereby reducing the area of ​​the transformer 1A.

[0097] Furthermore, in Example 1, in each of the multiple layers, the coil patterns c11 and c11i arranged on the first leg 21 side and the coil pattern c12 arranged on the second leg 22 side have the same number of turns. Similarly, the coil patterns c21 and c22 have the same number of turns. With this configuration, the number of turns of the coil patterns on the first leg 21 side and the second leg 22 side are equal, allowing the area of ​​the transformer 1A to be reduced.

[0098] Example 2 A transformer 1B according to Example 2 will be described with reference to Fig. 15 and Fig. 16. Example 2 describes an example in which two coil patterns in the first layer are not connected in series, and two coil patterns in the other layers are connected in series.

[0099] FIG. 15 is a diagram schematically illustrating a cross section of a transformer 1B according to the second embodiment.

[0100] 15 , the transformer 1B includes a core 10 and a multilayer circuit board 70. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32. The configurations of the core 10 and the secondary winding 32 of the transformer 1B are the same as those in the first embodiment.

[0101] FIG. 16 is a perspective view of a coil pattern of a primary winding 31 of a transformer 1B according to the second embodiment.

[0102] 16 shows coil patterns c11n and c12n in the first layer, coil patterns c21 and c22 in the second layer, coil patterns c31 and c32 in the third layer, and coil patterns c41 and c42 in the fourth layer. An input / output line ioL for inputting a current to the primary winding 31 or outputting a current from the primary winding 31 is provided at the outermost end of each of the coil patterns c11n and c12n.

[0103] Via conductors vc1 and vc4 are formed in a sheet layer 74 between the first-layer coil patterns c11n and c12n and the second-layer coil patterns c21 and c22. Via conductors vc2 and vc4 are formed in a sheet layer 75 between the second-layer coil patterns c21 and c22 and the third-layer coil patterns c31 and c32. Via conductors vc3 and vc4 are formed in a sheet layer 76 between the third-layer coil patterns c31 and c32 and the fourth-layer coil patterns c41 and c42. The via conductor vc4 is formed to penetrate from the first layer to the fourth layer. The via conductor vc4 is formed in a position that does not overlap with the other via conductors vc2 and the other coil patterns c22 and c32 when viewed from the first direction x1.

[0104] In Fig. 16, arrows indicate the direction of current flowing through each coil pattern at a predetermined timing (e.g., a positive half cycle of the AC voltage waveform). At this predetermined timing, current is input from the input / output line ioL of coil pattern c11n, and current is output from the input / output line ioL of coil pattern c12n. When transformer 1B is viewed from the opposite direction of the arrow in the first direction x1 in Fig. 16, current flows clockwise through coil patterns c11n, c21, c31, and c41 wound around first leg 21, and current flows counterclockwise through coil patterns c12n, c22, c32, and c42 wound around second leg 22. In other words, the coil patterns c11n and c12n located on the same layer have opposite current flow directions, the coil patterns c21 and c22 located on the same layer have opposite current flow directions, the coil patterns c31 and c32 located on the same layer have opposite current flow directions, and the coil patterns c41 and c42 located on the same layer have opposite current flow directions.

[0105] In Example 2, in the first layer, the coil pattern c11n on the first leg 21 side and the coil pattern c12n on the second leg 22 side are not connected to each other on the same layer of the multilayer circuit board 70 and are insulated from each other. However, in the second layer, the coil pattern c21 on the first leg 21 side and the coil pattern c22 on the second leg 22 side are connected in series, in the third layer, the coil pattern c31 on the first leg 21 side and the coil pattern c32 on the second leg 22 side are connected in series, and in the fourth layer, the coil pattern c41 on the first leg 21 side and the coil pattern c42 on the second leg 22 side are connected in series. That is, in this example, in all remaining coil patterns different from the coil pattern on the first layer among the multiple layers, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side wound around the second leg 22 are connected in series on the same layer of the multilayer circuit board 70.

[0106] In Example 2, the coil pattern c11n on the first leg 21 side of the first layer and the coil pattern c21 on the first leg 21 side of the second layer are connected by a via conductor vc1. The coil pattern c22 on the second leg 22 side of the second layer and the coil pattern c32 on the second leg 22 side of the third layer are connected by a via conductor vc2. The coil pattern c31 on the first leg 21 side of the third layer and the coil pattern c41 on the first leg 21 side of the fourth layer are connected by a via conductor vc3. The coil pattern c42 on the second leg 22 side of the fourth layer and the coil pattern c12n on the second leg 22 side of the first layer are connected by a via conductor vc4. The multiple via conductors vc1 to vc4 are each connected to the pattern line pL1 located at the innermost position of the spiral coil pattern.

[0107] As described above, in Example 2, in at least two of the coil patterns in multiple layers, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side are connected in series on the same layer of the multilayer circuit board 70. With this configuration, the coil patterns in the four layers can be connected using four via conductors vc1 to vc4 as described above. This reduces the number of via conductor locations when viewed from the first direction x1, thereby reducing the area of ​​the transformer 1B.

[0108] In Example 2, the via conductors vc1 to vc4 are each connected to the pattern line pL1 located at the innermost side of the spiral coil pattern. This configuration eliminates the need to provide a via conductor formation area near the outer periphery of the transformer, thereby reducing the area of ​​the transformer 1B.

[0109] In Example 2, the coil patterns of each layer of the coil patterns arranged on the first leg 21 side (e.g., coil pattern c11n, coil pattern c21, coil pattern c31, and coil pattern c41) have the same number of turns. The coil patterns of each layer of the coil patterns arranged on the second leg 22 side (e.g., coil pattern c12n, coil pattern c22, coil pattern c32, and coil pattern c42) have the same number of turns. This configuration allows the number of turns of the coil patterns on the first leg 21 side and the second leg 22 side to be equal, thereby reducing the area of ​​the transformer 1B compared to the comparative example.

[0110] Furthermore, in Example 2, in each of the multiple layers, the coil pattern c11n arranged on the first leg 21 side and the coil pattern c12n arranged on the second leg 22 side have the same number of turns. Similarly, the coil patterns c21 and c22 have the same number of turns, the coil patterns c31 and c32 have the same number of turns, and the coil patterns c41 and c42 have the same number of turns. With this configuration, the number of turns of the coil patterns on the first leg 21 side and the second leg 22 side are equal, allowing the area of ​​the transformer 1B to be reduced.

[0111] Third Embodiment A transformer 1C according to a third embodiment will be described. In the third embodiment, the number of turns of the coil pattern in one layer is smaller than that in the first embodiment.

[0112] FIG. 17 is a diagram schematically illustrating a cross section of a transformer 1C according to a third embodiment.

[0113] 17 , the transformer 1C includes a core 10 and a multilayer circuit board 70. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32. The configurations of the core 10 and the secondary winding 32 are the same as those in the first embodiment.

[0114] FIG. 18 is a perspective view of the coil pattern of the primary winding 31 of the transformer 1C according to the third embodiment.

[0115] 18 shows first-layer coil patterns c11 and c12 and second-layer coil patterns c21 and c22. Via conductors vb1 and vb2 that penetrate the sheet layer are formed in the sheet layer between the first-layer coil patterns c11 and c12 and the second-layer coil patterns c21 and c22. The coil pattern of Example 3 has one less turn of the coil pattern than Example 1, but the other configurations are the same as Example 1. Example 3 can also achieve the same effects as Example 1.

[0116] Fourth Embodiment A transformer 1D according to a fourth embodiment will be described. In the fourth embodiment, an example will be described in which the coil pattern of the primary winding 31 and the coil pattern of the secondary winding 32 are alternately arranged.

[0117] Fig. 19 is a diagram schematically illustrating a cross section of a transformer 1D according to Example 4. Fig. 20 is a perspective view of the coil patterns of a primary winding 31 and a secondary winding 32 of the transformer 1D according to Example 4.

[0118] 19 includes a core 10 and a multilayer circuit board 70. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32. The configuration of the core 10 is the same as that of the embodiment.

[0119] Each of the primary winding 31 and the secondary winding 32 is formed by a plurality of layers of coil patterns provided on the multilayer circuit board 70 and a plurality of via conductors that connect the coil patterns between layers.

[0120] 19 and 20 , the primary winding 31 is configured with a four-layer coil pattern, and the secondary winding 32 is configured with a four-layer coil pattern. The coil patterns of the primary winding 31 and the secondary winding 32 are alternately arranged in the first direction x1. In this example, the coil patterns of the secondary winding 32 are arranged in the first, third, fifth, and seventh layers of the overall coil pattern, and the coil patterns of the primary winding 31 are arranged in the second, fourth, sixth, and eighth layers. In Example 4, the coil patterns of the primary winding 31 and the coil patterns of the secondary winding 32 are alternately arranged, thereby realizing a transformer 1D with an interleaved winding structure.

[0121] The transformer 1D according to the fourth embodiment has the same effects as the transformer 1 according to the embodiment, and further has the following effects. By alternately arranging the coil patterns of the primary winding 31 and the secondary winding 32 as in the transformer 1D according to the fourth embodiment, the magnetomotive force generated in the transformer 1D can be reduced. This allows the winding resistance of the primary winding 31 and the secondary winding 32 to be reduced.

[0122] (Example 5) A description will be given of the coil pattern of the primary winding 31 of the transformer 1E according to Example 5. In Example 5, an example will be described in which the via conductors are arranged inside the center line mL1 in the width direction of the coil pattern.

[0123] A transformer 1E according to the fifth embodiment includes a core 10 and a multilayer circuit board 70 (see FIG. 12). The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32.

[0124] Fig. 21 is a diagram illustrating an example of a coil pattern of the primary winding 31 of the transformer 1E according to Example 5. Fig. 22 is a diagram illustrating another example of the coil pattern of the primary winding 31 of the transformer 1E according to Example 5. Fig. 23 is a diagram illustrating another example of the coil pattern of the primary winding 31 of the transformer 1E according to Example 5.

[0125] 21 to 23 show a portion of the coil pattern of the first leg 21 and the primary winding 31 shown in Fig. 14. These figures also show perspective views of the multiple via conductors and the coil pattern from the first direction x1. As shown in Fig. 21 to 23, the multiple via conductors va1 and va3 are located between the first leg 21 and a center line mL1 that passes through the center of the primary winding 31 in the width direction, and are arranged more inward than the center line mL1.

[0126] As shown in Fig. 21 , the via conductors va1 and va3 are arranged along the outer periphery of the first leg 21, thereby reducing the area of ​​the wiring pattern. In Fig. 21 , the via conductors va1 and va3 are arranged in order along the outer periphery of the first leg 21, but as shown in Fig. 22 , the via conductors va1 and va3 may be arranged alternately along the outer periphery of the first leg 21. Furthermore, as shown in Fig. 23 , the positions of the coil patterns of the primary winding 31 may be slightly shifted so that the via conductor va1 and the coil pattern of the primary winding 31 do not overlap. The fifth embodiment can also achieve the same effects as the first embodiment.

[0127] Sixth Embodiment A transformer 1F according to a sixth embodiment will be described. In the sixth embodiment, two secondary windings 32 are provided on both outer sides of the primary winding 31, and a bus bar 60 is connected to the secondary windings 32.

[0128] Fig. 24 is a diagram schematically illustrating a cross section of a transformer 1F according to Example 6. Fig. 25 is a diagram illustrating the transformer 1F according to Example 6 as viewed from above.

[0129] 24 and 25 includes a core 10 and a multilayer circuit board 70. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32. The configurations of the core 10, the primary winding 31, and the secondary winding 32 are the same as those in the embodiment.

[0130] The secondary coils 51 and 52 of the transformer 1F are arranged on both outer sides of the primary coil 41 in the first direction x1. The secondary coils 53 and 54 are arranged on both outer sides of the primary coil 42 in the first direction x1. Note that "on both outer sides of the primary coil 41" means both the first base 11 side and the second base 12 side as viewed from the primary coil 41, and "on both outer sides of the primary coil 42" means both the first base 11 side and the second base 12 side as viewed from the primary coil 42.

[0131] The transformer 1F also has four bus bars 60 connected to the secondary coils 51 to 54. The bus bars 60 are formed of copper plates and have a U-shape when viewed from the first direction x1, as shown in FIG. 25 . The thickness of the bus bars 60 is, for example, 5 to 15 times the thickness of each of the secondary coils 51 to 54. Two of the four bus bars 60 are arranged on both outer sides of the secondary coils 51 and 52 in the first direction x1. The other two bus bars 60 are arranged on both outer sides of the secondary coils 53 and 54 in the first direction x1.

[0132] The transformer 1F according to Example 6 has the same effects as the transformer 1 according to the embodiment, and also has the following effects. In the transformer 1F according to Example 6, two secondary coils are arranged on both outsides of the primary coil in the first direction x1. Therefore, the secondary coil, through which a large current flows and which is likely to generate heat, is arranged on the outside, thereby improving the heat dissipation performance of the transformer 1F. Furthermore, in the transformer 1F according to Example 6, a bus bar 60 is connected to each of the secondary coils 51 to 54. This makes it possible to reduce the wiring resistance of the wiring connected to the secondary coils 51 to 54.

[0133] Example 7 A coil pattern of the primary winding 31 of the transformer 1G according to Example 7 will be described. In Example 7, an example will be described in which a plurality of via conductors are arranged in the first recessed portion 21 a of the first leg portion 21 and the second recessed portion 22 a of the second leg portion 22.

[0134] The transformer 1G according to the seventh embodiment includes a core 10 and a multilayer circuit board 70 (see FIG. 5). The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32.

[0135] FIG. 26 is a plan view of the coil patterns of the first leg 21, the second leg 22, and the primary winding 31 of the transformer 1G according to the seventh embodiment.

[0136] 26 , the first leg 21 has a first recess 21a recessed from a portion of the outer periphery of the first leg 21 toward the central axis. The second leg 22 has a second recess 22a recessed from a portion of the outer periphery of the second leg 22 toward the central axis. A plurality of via conductors are arranged in the first recess 21a and the second recess 22a. Specifically, the via conductors va1 and va3 are arranged in the first recess 21a, and the via conductors va2 and va4 are arranged in the second recess 22a.

[0137] The transformer 1G according to the seventh example has the same effects as the transformer 1 according to the embodiment, and further has the following effects: As in the transformer 1G according to the seventh example, by providing recesses in the first leg 21 and the second leg 22 and arranging via conductors in the recesses, the area of ​​the transformer 1G can be reduced.

[0138] Eighth Embodiment A transformer 1H according to an eighth embodiment will be described. In the eighth embodiment, an example will be described in which the secondary winding 32 is located away from the end E of the leg portion and on the first base portion 11 side.

[0139] FIG. 27 is a diagram schematically illustrating a cross section of a transformer 1H according to an eighth embodiment.

[0140] The transformer 1H according to the eighth embodiment includes a core 10 and a multilayer circuit board 70. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32.

[0141] The core 10 includes a first core 10a and a second core 10b. The first core 10a is composed of a first base 11, a first leg 21, and a second leg 22. The first leg 21 and the second leg 22 are arranged side by side along the second direction x2. The first leg 21 and the second leg 22 are provided between the first base 11 and the second base 12 and face the second base 12 in the first direction x1 across a predetermined gap g1. For example, by providing the gap g1 between the first leg 21 and the second leg 22 and the second base 12, the saturation magnetic flux density in the core 10 can be reduced.

[0142] Furthermore, the secondary winding 32 is located closer to the first base 11 in the first direction x1 than the predetermined distance from the end E of the first leg 21 facing the second base 12 and the end E of the second leg 22 facing the second base 12. By moving the secondary winding 32 away from the end E in this manner, it is possible to reduce eddy current loss in the secondary winding 32 caused by leakage flux from the core 10.

[0143] While the above description has been given with respect to the secondary winding 32, the same applies to the busbar 60. For example, the busbar 60 may be located on the first base portion 11 side in the first direction x1, away from the end E of the first leg 21 facing the second base portion 12 and the end E of the second leg 22 facing the second base portion 12 by a distance L1 that is greater than the above-mentioned predetermined distance. By moving the busbar 60 away from the end E in this way, it is possible to reduce eddy current loss in the busbar 60 caused by leakage flux from the core 10.

[0144] (Example 9) A coil pattern of a transformer 1i according to Example 9 will be described. In Example 9, an example will be described in which the width of the outermost pattern line is wider than the width of the innermost pattern line.

[0145] The transformer 1i according to the ninth embodiment includes a core 10 and a multilayer circuit board 70 (see FIG. 12). The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32.

[0146] Fig. 28 is a diagram showing a part of the coil pattern of the primary winding 31 of the transformer 1i according to Example 9. Fig. 28 shows the coil pattern of the first layer of the primary winding 31 shown in Fig. 14 .

[0147] When the coil pattern is close to a concentric circle, the winding resistance Rdc of the innermost first turn １ , the winding resistance Rdc of the second winding line in the middle ２ , and the winding resistance Rdc of the outermost third winding line ３ are expressed by the following (Equation 1), (Equation 2), and (Equation 3), respectively. ρ is the electrical resistivity of the coil pattern, and h is the wiring thickness of the line of the coil pattern. clr is the clearance between adjacent coil patterns in the radial direction Dr centered on the center pc of the coil pattern. clr ｃｏｒｅ is the clearance between the first innermost turn of the coil pattern and the leg 21 in the radial direction Dr. The center pc is also the center of the leg 21, and r is the radius of the leg 21 centered on the center pc. The width C of the first innermost turn of the coil pattern １ and the width C of the second winding ２ and the width C of the outermost third turn of the track ３ is Rdc １ = Rdc ２ = Rdc ３ The width of each line in this example is determined based on the relationship １ ≦C ２ and C ２ ≦C ３ The following relationship exists.

[0148] ...(Formula 1)

[0149] ...(Formula 2)

[0150] ...(Formula 3)

[0151] By determining the width of each line as described above, it is possible to make the wiring resistance uniform according to the number of turns, regardless of how many turns the coil pattern has, and to reduce copper loss.

[0152] Tenth Embodiment A transformer 1J according to a tenth embodiment will be described. In the tenth embodiment, an example in which the core 10 includes an outer leg will be described.

[0153] FIG. 29 is a diagram schematically illustrating a cross section of a transformer 1J according to a tenth embodiment.

[0154] 29 , the first core 10a of the transformer 1J has a first leg 21, a second leg 22, and two outer legs 23, 24 located on both outer sides of the first leg 21 and the second leg 22. The transformer 1J according to the tenth example also achieves the same effects as the transformer 1 according to the embodiment.

[0155] (Example 11) A description will be given of a coil pattern of a transformer 1K according to Example 11. In Example 11, an example will be described in which the number of turns of the coil pattern of the primary winding 31 differs between the first layer and the second layer.

[0156] The transformer 1K according to the eleventh embodiment includes a core 10 and a multilayer circuit board 70 (see FIG. 12). The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32.

[0157] FIG. 30 is a perspective view of the coil pattern of the primary winding 31 of the transformer 1K according to the eleventh embodiment.

[0158] 30 , the primary winding 31 of the transformer 1K according to the eleventh embodiment has a coil pattern of three turns in the first layer and a coil pattern of two turns in the second layer. The other configurations of the transformer 1K according to the eleventh embodiment are substantially the same as those of the transformers 1 and 1C according to the first and third embodiments. Therefore, the transformer 1K according to the eleventh embodiment has the same effects as the transformers 1 and 1C according to the first and third embodiments.

[0159] (Example 12) A coil pattern of a transformer 1L according to Example 12 will be described. In Example 12, an example will be described in which the coil pattern is not square wound but circular wound.

[0160] The transformer 1L according to the twelfth embodiment includes a core 10 and a multilayer circuit board 70 (see FIG. 5). The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32.

[0161] FIG. 31 is a plan view of the coil pattern of the primary winding 31 of the transformer 1L according to the twelfth embodiment.

[0162] 31 , the coil pattern of the primary winding 31 of the transformer 1L according to Example 12 is circular. The other configurations of the transformer 1L according to Example 12 are the same as those of the embodiment. Therefore, the transformer 1L according to Example 12 also achieves the same effects as the transformer 1 according to the embodiment.

[0163] (Example 13) A transformer 1M according to Example 13 will be described. In Example 13, a configuration capable of suppressing the influence of the close arrangement of the primary windings 31 will be described.

[0164] FIG. 32 is a diagram illustrating an example of a cross section of a transformer 1M according to a thirteenth embodiment.

[0165] 32 includes a core 10 and a multilayer circuit board 70. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32. The configuration of the core 10 is the same as that of the embodiment.

[0166] Each of the primary winding 31 and the secondary winding 32 is formed by a plurality of layers of coil patterns provided on the multilayer circuit board 70 and a plurality of via conductors that connect the coil patterns between layers.

[0167] 32, the primary winding 31 is composed of four layers of coil patterns, and the secondary winding 32 is composed of four layers of coil patterns. Here, if the coil pattern of the primary winding 31 is P and the coil pattern of the secondary winding 32 is S, the coil patterns of the primary winding 31 and the secondary winding 32 are arranged in the order S-P-P-S and S-P-P-S in the first direction x1. In this example, the coil pattern P of the primary winding 31 and the coil pattern S of the secondary winding 32 are arranged in layers to realize a transformer 1M with an interleaved winding structure.

[0168] FIG. 33 is a diagram illustrating another example of a cross section of a transformer 1M according to the thirteenth embodiment.

[0169] 33 includes a core 10 and a multilayer circuit board 70. The multilayer circuit board 70 is provided with a primary winding 31 and a secondary winding 32. The configuration of the core 10 is the same as that of the embodiment.

[0170] 33, the primary winding 31 is composed of four layers of coil patterns, and the secondary winding 32 is composed of eight layers of coil patterns. Here, if the coil pattern of the primary winding 31 is P and the coil pattern of the secondary winding 32 is S, the coil patterns of the primary winding 31 and the secondary winding 32 are arranged in the order S-P-S, S-P-S, S-P-S, and S-P-S in the first direction x1. In this example, the coil pattern P of the primary winding 31 and the coil pattern S of the secondary winding 32 are arranged in layers to realize a transformer 1M with an interleaved winding structure.

[0171] The transformer 1M according to Example 13 has the same effects as the transformer 1 according to the embodiment, and further has the following effects. By stacking the coil pattern of the primary winding 31 and the coil pattern of the secondary winding 32 as in the transformer 1M according to Example 13, the magnetomotive force generated in the transformer 1M can be reduced. This allows the winding resistance of the primary winding 31 and the secondary winding 32 to be reduced.

[0172] Fig. 34 is a diagram showing the magnetomotive force of a transformer, and Fig. 35 is a diagram showing the relationship between the number of turns of a winding and the winding resistance.

[0173] 34(a) shows a cross section of a transformer in which secondary windings 32 are arranged on both outer sides of four-layer primary windings 31 in the first direction x1, and FIG. 34(b) shows the magnetomotive force corresponding to the coordinates in the first direction x1 of FIG. 34(a). Also, FIG. 34(c) shows a cross section of a transformer in which primary windings 31 and secondary windings 32 are arranged alternately in the first direction x1, and FIG. 34(d) shows the magnetomotive force corresponding to the coordinates in the first direction x1 of FIG. 34(c). The magnetomotive force is a value obtained by multiplying the number of turns of the winding by the current value.

[0174] As shown in Figures 34(a) and (b), when adjacent windings have currents flowing in the same direction, the range of change in magnetomotive force becomes large. On the other hand, as shown in Figures 34(c) and (d), when the primary winding 31 and the secondary winding 32 are wound alternately, the range of change in magnetomotive force becomes small. In the case of Figure 34(c), it is possible to make the magnetic field distribution more uniform than in (a).

[0175] Furthermore, in the case of the structure shown in FIG. 34(a), the number of turns (mt) of adjacent windings in the first direction x1 increases, resulting in a higher winding resistance (see FIG. 35). This increases copper loss and may result in a decrease in conversion efficiency. On the other hand, in the case of the structure shown in FIG. 34(c), the number of turns (mt) of adjacent windings in the first direction x1 decreases, resulting in a decrease in winding resistance. This prevents an increase in copper loss and a decrease in conversion efficiency.

[0176] Fourteenth Embodiment An LLC converter 5A and a transformer 1N according to a fourteenth embodiment will be described. The LLC converter 5A is an example of a DC-DC converter.

[0177] FIG. 36 is a circuit diagram showing an example of an LLC converter 5A according to a fourteenth embodiment.

[0178] As shown in Fig. 36, the LLC converter 5A includes a primary circuit 6, a transformer 1N, and a secondary circuit 7. The LLC converter 5A also includes two resonant capacitors Cs. The resonant capacitors Cs are provided on two paths connecting the primary circuit 6 and the transformer 1N. In Fig. 36, the H path and L path relative to the center tap on the secondary side are indicated by arrows.

[0179] FIG. 37 is a circuit diagram of a transformer 1N according to a fourteenth embodiment.

[0180] As shown in Fig. 37, the winding ratio of the transformer 1N of Example 14 is primary:secondary = 24: 1. Fig. 37 shows the inductance Lo when the secondary side is open and the inductance Ls when the secondary side is shorted.

[0181] FIG. 38 is a diagram schematically illustrating a cross section of a transformer 1N of an LLC converter 5A according to a fourteenth embodiment.

[0182] The primary winding 31 of the transformer 1N is formed by a four-layer coil pattern. The secondary winding 32 is formed by a two-layer coil pattern and is arranged on both outer sides of the four-layer coil pattern in the first direction x1. Bus bars 60 are arranged on both outer sides of the secondary winding 32 in the first direction x1. In this example, the coil pattern of the secondary winding 32 and the bus bar 60 located above the primary winding 31 form an H path to the center tap on the secondary side, and the coil pattern of the secondary winding 32 and the bus bar 60 located below the primary winding 31 form an L path to the center tap on the secondary side.

[0183] According to the transformer 1N of the fourteenth embodiment, it is possible to increase the turn ratio and generate a large leakage inductance.

[0184] Fifteenth Embodiment An LLC converter 5B and a transformer 1o according to a fifteenth embodiment will be described. The LLC converter 5B is an example of a DC-DC converter.

[0185] FIG. 39 is a circuit diagram showing an example of an LLC converter 5B according to a fifteenth embodiment.

[0186] As shown in Fig. 39, the LLC converter 5B includes a primary circuit 6, a transformer 1o, and a secondary circuit 7. The LLC converter 5B also includes two resonant capacitors Cs. The resonant capacitors Cs are provided on two paths connecting the primary circuit 6 and the transformer 1o. The secondary side of the LLC converter 5B is formed by a full-bridge circuit.

[0187] FIG. 40 is a circuit diagram of a transformer 1o according to a fifteenth embodiment.

[0188] As shown in Fig. 40, the winding ratio of the transformer 1o in Example 15 is primary:secondary = 24: 1. Fig. 40 shows the inductance Lo when the secondary side is open and the inductance Ls when the secondary side is shorted.

[0189] FIG. 41 is a diagram schematically illustrating a cross section of a transformer 1o of an LLC converter 5B according to a fifteenth embodiment.

[0190] The primary winding 31 of the transformer 1o is formed by a four-layer coil pattern. The secondary winding 32 is formed by a two-layer coil pattern and is arranged on both outer sides of the four-layer coil pattern in the first direction x1. The bus bars 60 are arranged further on both outer sides of the secondary winding 32 in the first direction x1.

[0191] The transformer 1o according to the fifteenth embodiment can increase the turn ratio and generate a large leakage inductance.

[0192] (Summary) Transformers (transformers 1 to 1o) according to one aspect of the present disclosure will be illustrated.

[0193] The transformer of aspect 1 includes a core 10 having a first base 11, a second base 12 provided opposite the first base 11, and a first leg 21 and a second leg 22 provided between the first base 11 and the second base 12, a primary winding 31 wound around each of the first leg 21 and the second leg 22, a secondary winding 32 wound around each of the first leg 21 and the second leg 22, and a multilayer circuit board 70. The multilayer circuit board 70 has multiple layer coil patterns and multiple via conductors connecting the coil patterns between layers. The primary winding 31 is formed by the multiple layer coil patterns and multiple via conductors. In at least two layers of the multiple layer coil patterns, the coil pattern wound around the first leg 21 on the first leg 21 side and the coil pattern wound around the second leg 22 on the second leg 22 side are connected in series on the same layer of the multilayer circuit board 70.

[0194] This configuration allows the number of via conductors to be reduced compared to conventional configurations, allowing multiple layers of coil patterns to be connected, thereby reducing the area of ​​the transformer.

[0195] The transformer of aspect 2 is the transformer described in aspect 1, and in the coil patterns of all layers among the multiple layers of coil patterns, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side may be connected in series on the same layer of the multilayer circuit board 70.

[0196] This configuration allows the coil patterns of all layers to be connected with a minimum number of via conductors, thereby reducing the area of ​​the transformer.

[0197] The transformer of aspect 3 is the transformer described in aspect 2, and further has an inner coil pattern wound around the first leg 21 in one of the multiple layers and arranged between the first leg 21 and the coil pattern on the first leg 21 side, and the inner coil pattern may be connected to the coil pattern on the first leg 21 side of any other layer different from the one layer via a via conductor.

[0198] This configuration allows the coil patterns on all layers to be efficiently connected using via conductors, thereby reducing the area of ​​the transformer.

[0199] The transformer of aspect 4 is the transformer described in aspect 1, wherein the multi-layer coil patterns are three or more layer coil patterns, and in one of the multi-layer coil patterns, the coil pattern on the first leg side and the coil pattern on the second leg side are insulated from each other on the same layer of the multilayer circuit board, and in all remaining coil patterns different from the coil pattern on the one layer of the multilayer coil patterns, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side may be connected in series on the same layer of the multilayer circuit board 70.

[0200] This configuration allows the number of via conductors to be reduced compared to conventional configurations, allowing multiple layers of coil patterns to be connected, thereby reducing the area of ​​the transformer.

[0201] The transformer of aspect 5 is a transformer according to any one of aspects 1 to 4, wherein the coil pattern is spiral, and the multiple via conductors may each be connected to a pattern line pL1 located at the innermost side of the coil pattern.

[0202] This configuration eliminates the need to provide an area for forming via conductors near the outer periphery of the transformer, thereby reducing the area of ​​the transformer.

[0203] The transformer of aspect 6 is a transformer according to any one of aspects 1 to 5, wherein the coil pattern is spiral, and the coil patterns of each layer of the coil pattern arranged on the first leg 21 side may have the same number of turns, and the coil patterns of each layer of the coil pattern arranged on the second leg 22 side may have the same number of turns.

[0204] In this way, by making the number of turns of the coil pattern on the first leg 21 side equal to that of the coil pattern on the second leg 22 side equal to that of the coil pattern on the first leg 21 side, the area of ​​the transformer can be reduced.

[0205] The transformer of aspect 7 is a transformer according to any one of aspects 1 to 6, wherein the coil pattern is spiral, and in each of the multiple layers, the coil pattern arranged on the first leg 21 side and the coil pattern arranged on the second leg 22 side may have the same number of turns.

[0206] In this way, by making the number of turns of the coil patterns on the first leg 21 side and the second leg 22 side equal, the area of ​​the transformer can be reduced.

[0207] The transformer of aspect 8 is the transformer according to any one of aspects 1 to 7, wherein the secondary winding 32 is formed by a plurality of coil patterns in layers provided on a multilayer circuit board 70 and a plurality of via conductors that connect the coil patterns between layers. The coil patterns of the primary winding 31 and the secondary winding 32 provided on the multilayer circuit board 70 may be arranged alternately in a first direction x1 in which the first base portion 11 and the second base portion 12 face each other.

[0208] In this way, by alternately arranging the coil patterns of the primary winding 31 and the secondary winding 32, it is possible to reduce the magnetomotive force generated in the transformer, thereby reducing the winding resistance of the primary winding 31 and the secondary winding 32.

[0209] The transformer of aspect 9 is the transformer according to any one of aspects 1 to 8, wherein the via conductors may be arranged inside a center line mL1 passing through the center of the coil pattern in the width direction.

[0210] According to this configuration, the via conductor can be disposed near the first leg 21 or the second leg 22, thereby reducing the area of ​​the wiring pattern and, therefore, the area of ​​the transformer.

[0211] The transformer of aspect 10 is a transformer according to any one of aspects 1 to 9, in which two secondary windings 32 are wound around each of the first leg 21 and the second leg 22, and the two secondary windings 32 are arranged on either side of the primary winding 31 in the first direction x1 in which the first base 11 and the second base 12 face each other, and a bus bar 60 may be connected to the secondary windings 32.

[0212] With this configuration, the secondary winding 32, which is prone to heat generation due to the large current flowing therethrough, is arranged on the outside, thereby improving the heat dissipation of the transformer. Also, by connecting the bus bar 60 to the secondary winding 32, the wiring resistance of the wiring connected to the secondary winding 32 can be reduced.

[0213] The transformer of aspect 11 is a transformer described in any one of aspects 1 to 10, wherein the first leg 21 has a first recessed portion 21a recessed toward the central axis of the first leg 21, the second leg 22 has a second recessed portion 22a recessed toward the central axis of the second leg 22, and the multiple via conductors may be arranged in the first recessed portion 21a and the second recessed portion 22a.

[0214] In this way, by providing recesses in the first leg portion 21 and the second leg portion 22 and arranging via conductors in the recesses, the area of ​​the transformer can be reduced.

[0215] A transformer according to a twelfth aspect is the transformer according to any one of the first to eleventh aspects, wherein a gap g1 of a predetermined distance is provided between the first leg 21 and the second leg 22 and the second base 12. The secondary winding 32 (including the bus bar 60 joined to the secondary winding 32) may be located on the first base 11 side, in the first direction x1 in which the first base 11 and the second base 12 face each other, away from each of the end E of the first leg 21 facing the second base 12 and the end E of the second leg 22 facing the second base 12, by a distance L1 greater than the predetermined distance.

[0216] In this way, by separating the secondary winding 32 from the end E, it is possible to reduce eddy current loss in the secondary winding 32 caused by leakage flux from the core 10 .

[0217] The transformer of aspect 13 is a transformer according to any one of aspects 1 to 12, wherein the coil pattern is spiral, and the width of the pattern line located on the outermost side of the coil pattern may be wider than the width of the pattern line located on the innermost side.

[0218] This makes it possible to make the wiring resistance uniform according to the number of turns of the coil pattern, thereby reducing copper loss.

[0219] A DC-DC converter according to a fourteenth aspect includes the transformer according to any one of the first to thirteenth aspects, and a resonant capacitor Cs connected in series to the transformer.

[0220] As described above, by using a transformer with a large leakage inductance, the gain of the DC-DC converter can be made to have a wide range.

[0221] (Other Examples) The transformer of the present disclosure has been described above based on the embodiments and examples, but the present disclosure is not limited to the above-described embodiments and examples.

[0222] In the above-described embodiment and examples, the primary winding 31 has an even number of layers, but this is not limiting. For example, if the area of ​​the transformer can be made sufficiently small, the primary winding 31 may have an odd number of layers.

[0223] In the above-described embodiment and examples, an example has been described in which the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side are directly connected, but this is not limiting. For example, the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side may be connected via a connecting pattern line arranged between the coil pattern on the first leg 21 side and the coil pattern on the second leg 22 side.

[0224] In the above-described embodiment and examples, the first leg 21 and the second leg 22 are connected to the first base 11, but this is not limiting. The first leg 21 and the second leg 22 may be connected to the second base 12 instead of the first base 11. The first leg 21 may be connected to the first base 11, and the second leg 22 may be connected to the second base 12. Furthermore, the first leg 21 and the second leg 22 do not necessarily have to be directly connected to the first base 11 or the second base 12. For example, the first leg 21 and the second leg 22 may be connected to the first base 11 and the second base 12 via a non-magnetic material portion that does not contain a magnetic material. Furthermore, when the first leg 21 and the second leg 22 are held by other support members, a gap may be provided between the first leg 21 and the second leg 22 and the first base 11, or between the first leg 21 and the second leg 22 and the second base 12.

[0225] Furthermore, the first leg 21 and the second leg 22 may be divided into two in the first direction x1. For example, when the first leg 21 and the second leg 22 are divided into two in the first direction x1, one of the divided portions may be connected to the first base 11, and the other portion may be connected to the second base 12. In this case, the legs protrude from both the first base 11 and the second base 12 so as to face each other. A gap may be provided in the opposing region of the two opposing legs.

[0226] In addition, the present disclosure also includes forms obtained by applying various modifications to the above-described embodiments and examples that a person skilled in the art would conceive, and forms realized by arbitrarily combining the components and functions in the embodiments and examples within the scope that does not deviate from the spirit of the present disclosure.

[0227] The transformer of the present disclosure is useful as a transformer used in a power source for an on-vehicle auxiliary device.

[0228] 1, 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 1i, 1J, 1K, 1L, 1M, 1N, 1o Transformer 5, 5A, 5B, 105 LLC converter (DC-DC converter) 6 Primary side circuit 7 Secondary side circuit 10 Core 10a First core 10b Second core 11 First base 12 Second base 21 First leg 21a First recessed portion 22 Second leg 22a Second recessed portion 23, 24 Outer leg 31, 31a, 31b Primary winding 32, 32a, 32b Secondary winding 41, 42 Primary coil 51, 52, 53, 54 Secondary coil 60 Bus bar 70 Multilayer circuit board 71, 72, 73, 74, 75, 76, 77, 78 Sheet layers c11, c11i, c11n, c12, c12n, c21, c22, c31, c32, c41, c42 Coil pattern Cs Resonant capacitor Cr Capacitance E End g Gap ioL Input / output line Lm, Lma, Lmb Excitation inductance Lr, Lra, Lrb Leakage inductance mL1 Center line p11, p12 Primary port p21, p22 Secondary port pL1 Pattern line s11, s12, s13, s14, s21, s22, s23, s24 Switch element va1, va2, va3, va4, vb1, vb2, vc1, vc2, vc3, vc4 Via conductors x1 First direction x2 Second direction x3 Third direction

Claims

1. A transformer comprising: a core having a first base, a second base provided opposite the first base, and a first leg and a second leg provided between the first base and the second base; a primary winding wound around each of the first leg and the second leg; a secondary winding wound around each of the first leg and the second leg; and a multilayer circuit board having multiple layer coil patterns and multiple via conductors connecting the coil patterns between layers, wherein the primary winding is formed by the multiple layer coil patterns and the multiple via conductors, and in at least two layers of the coil patterns of the multiple layer coil patterns, the coil pattern on the first leg side wound around the first leg and the coil pattern on the second leg side wound around the second leg are connected in series on the same layer of the multilayer circuit board.

2. A transformer as described in claim 1, wherein in the coil patterns of all layers among the coil patterns of the multiple layers, the coil pattern on the first leg side and the coil pattern on the second leg side are connected in series on the same layer of the multilayer circuit board.

3. The transformer according to claim 2, wherein the primary winding further has an inner coil pattern wound around the first leg in one of the multiple layers and arranged between the first leg and the coil pattern on the first leg side, and the inner coil pattern is connected via the via conductor to the coil pattern on the first leg side in any other layer of the multiple layers different from the one layer in question.

4. The transformer according to claim 1, wherein the multiple layer coil patterns are three or more layer coil patterns, and in one layer of the multiple layer coil patterns, the coil pattern on the first leg side and the coil pattern on the second leg side are insulated from each other on the same layer of the multilayer circuit board, and in all remaining coil patterns of the multiple layer coil patterns different from the coil pattern on the one layer, the coil pattern on the first leg side and the coil pattern on the second leg side are connected in series on the same layer of the multilayer circuit board.

5. A transformer as claimed in any one of claims 1 to 4, wherein the coil pattern is spiral, and the plurality of via conductors are each connected to a pattern line located at the innermost position of the coil pattern.

6. A transformer as claimed in any one of claims 1 to 4, wherein the coil pattern is spiral, the coil patterns of each layer of the coil pattern arranged on the first leg side have the same number of turns, and the coil patterns of each layer of the coil pattern arranged on the second leg side have the same number of turns.

7. A transformer as claimed in any one of claims 1 to 4, wherein the coil pattern is spiral, and in each of the plurality of layers, the coil pattern arranged on the first leg side and the coil pattern arranged on the second leg side have the same number of turns.

8. A transformer as claimed in any one of claims 1 to 4, wherein the secondary winding is formed by coil patterns of multiple layers provided on the multilayer circuit board and multiple via conductors that connect the coil patterns between layers, and the coil patterns of the primary winding and the coil patterns of the secondary winding provided on the multilayer circuit board are alternately arranged in a first direction in which the first base and the second base face each other.

9. A transformer according to any one of claims 1 to 4, wherein the via conductors are arranged inside a center line passing through the center of the coil pattern in the width direction.

10. A transformer as claimed in any one of claims 1 to 4, wherein two secondary windings are wound around each of the first leg and the second leg, the two secondary windings being arranged on either side of the primary winding in a first direction in which the first base and the second base face each other, and a bus bar is connected to the secondary windings.

11. A transformer as described in any one of claims 1 to 4, wherein the first leg has a first recessed portion recessed toward the central axis of the first leg, the second leg has a second recessed portion recessed toward the central axis of the second leg, and the plurality of via conductors are arranged in the first recessed portion and the second recessed portion.

12. A transformer as claimed in any one of claims 1 to 4, wherein a gap of a predetermined distance is provided between the first leg and the second leg and the second base, and the secondary winding is located on the first base side, in a first direction in which the first base and the second base face each other, away from each end of the first leg facing the second base and each end of the second leg facing the second base, by a distance greater than the predetermined distance.

13. A transformer as claimed in any one of claims 1 to 4, wherein the coil pattern is spiral, and the width of the outermost pattern line of the coil pattern is wider than the width of the innermost pattern line.

14. A DC-DC converter comprising: a transformer according to any one of claims 1 to 4; and a resonant capacitor connected in series to the transformer.

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