Planar inductor

[0002] This application is a continuation of application Ser. No. 07/414,455, filed on Sep. 29, 1989, now abandoned.

[0003] 1. Field of the Invention

[0004] The present invention relates to a planar inductor applied to, e.g., a DC-to-DC converter.

[0005] 2. Description of the Related Art

[0006] A conventional ferrite troidal coil has been used as a choke coil on the output side of, e.g., a DC-to-DC converter. In contrast to this, a planar inductor has been recently studied in order to achieve miniaturization of an apparatus.

[0007] For example, a planar inductor with a structure having a spiral or meander planar coil, insulating layers stacked on both surfaces of the planar coil, and ferromagnetic layers stacked on the insulating layers is known.

[0008] In order to obtain high inductance, an amorphous alloy ribbon having a high permeability is used as a ferromagnetic layer. Note that many amorphous alloys have a positive saturation magnetostriction. Thus, when an amorphous alloy having a saturation magnetostriction is used as a normal troidal magnetic core, complicated magnetic anisotropy occurs during a heat treatment for eliminating strain by an inverse magnetostrictive effect due to a flexural stress, and soft magnetic properties such as an effective permeability are degraded. On the other hand, when an amorphous alloy is applied to a planar inductor, a ribbon of the alloy is used in a planar state. Therefore, the above-mentioned degradation of soft magnetic property due to an inverse magnetostrictive effect is small, and the soft magnetic property of the alloy can be sufficiently utilized. Therefore, in the troidal magnetic core and the planar inductor, a ferromagnetic ribbon need not be treated in the same manner.

[0009] When the planar inductor is applied to a choke coil on the output side of, e.g., a DC-to-DC converter, a high-frequency current superposed with DC current is supplied to the planar inductor. Therefore, excellent DC superposition characteristics are required.

[0010] The conventional planar inductor, however, undesirably has poor DC superposition characteristics. This problem is caused because the magnetic characteristics of a ferromagnetic ribbon which has been conventionally used are inadequate. More specifically, in the planar inductor, a magnetic flux flows in a plane of a surface of the ferromagnetic ribbon. When the saturation magnetization of the ferromagnetic ribbon is low, however, even if a small DC magnetic field is superposed, a magnetic flux density is saturated. Although the ferromagnetic ribbon having a high permeability is used in order to obtain higher inductance, an inductance is reduced, thus degrading DC superposition characteristics. For example, a ferromagnetic ribbon having a high permeability consisting of a Co-based amorphous alloy is known, and its saturation magnetization is higher than that of a ferrite. However, this saturation magnetization is insufficient to prevent a reduction in inductance, and the DC superposition characteristics are degraded.

[0011] Assume that a Co-based amorphous alloy is used as a ferromagnetic ribbon. If the Co-based amorphous alloy ribbons are stacked, the DC superposition characteristics can be improved to some extent. However, if a large number of amorphous alloy ribbons are stacked, the thickness of the planar inductor is increased. Therefore, in consideration of an object to obtain a thin planar inductor, stacking a large number of amorphous alloy ribbons is not preferable.

[0012] If the DC superposition characteristics of the planar inductor are poor, a inductance is reduced, and a control becomes difficult. Accordingly, the efficiency of a DC-to-DC converter is lowered. Thus, it is inadequate to apply the planar inductor directly to, the DC-to-DC converter and the like. Therefore, in order to improve the DC superposition characteristics, a high saturation magnetization of a ferromagnetic ribbon having a high permeability is required.

[0013] Even if the DC superposition characteristics on the inductance can be improved, an improvement of the efficiency of the DC-to-DC converter to which the planar inductor is applied is limited due to a high-frequency loss of the ferromagnetic ribbon. Therefore, in order to obtain a high efficiency equivalent to that of a conventional ferrite troidal coil, a high-frequency loss of the ferromagnetic ribbon must be decreased.

[0014] In addition, the planar inductor is used in practice while being coated with a mold resin. For this reason, if the amorphous alloy ribbon has a positive saturation magnetostriction, when the surface of the planar inductor is coated with a liquid mold resin and the resin is hardened, a compressive stress is applied to the ferromagnetic ribbon upon contraction of the mold resin. An effective permeability is then decreased due to an inverse magnetostrictive effect, thus reducing an inductance.

[0015] It is an object of the present invention to provide a planar inductor having excellent DC superposition characteristics. It is another object of the present invention to provide a planar inductor which suppresses a high-frequency loss of a ferromagnetic layer, and does not decrease in efficiency even when applied to a DC-to-DC converter. It is still another object of the present invention to provide a planar inductor which can prevent a reduction in inductance even if it is covered with a mold resin.

[0016] According to the present invention, there is provided a planar inductor having a planar inductance element, an insulating layer stacked on the inductance element, and a ferromagnetic layer stacked on the insulating layer, the ferromagnetic layer having a saturation magnetization 4πM_S≧10 kG, and a thickness of 100 μm or less. In such a planar inductor according to the present invention, DC superposition characteristics are improved. This planar inductor can be effectively applied to, e.g., a DC-to-DC converter.

[0017] In the planar inductor according to the present invention, the ferromagnetic layer is preferably two-dimensionally divided into a plurality of portions. If the ferromagnetic layer which constitutes the planar inductor is two-dimensionally divided into a plurality of portions, a high-frequency loss can be decreased, and the efficiency of the DC-to-DC converter to which such a planar inductor is applied can be improved.

[0018] When the planar inductor according to the present invention is used in practice, a relaxation layer for contraction of a mold resin is preferably formed on a surface of the ferromagnetic layer, and the entire members are coated with a mold resin. Thus, if the relaxation layer is stacked on the surface of the ferromagnetic layer, contraction generated when the mold resin is hardened and contracted can be relaxed, and transmission of the contraction to the ferromagnetic layer can be prevented, thus preventing a reduction in inductance due to an inverse magnetostrictive effect.

[0019] FIG. 1A is a plan view of planar inductors according to Examples 1 to 3 and Comparative Example 1 of the present invention;

[0020] FIG. 1B is a sectional view taken along the line of A-A′ of FIG. 1A;

[0021] FIG. 2 is a sectional view of planar inductors according to Example 4 and Comparative Example 2 of the present invention;

[0022] FIG. 3 is a sectional view of planar inductors according to Examples 5 and 6 and Reference Examples 1 to 3 of the present invention;

[0023] FIG. 4 is a plan view of planar inductors according to Examples 5 and 6 and Reference Example 2 of the present invention;

[0024] FIG. 5 is a plan view of the planar inductor according to Reference Example 1;

[0025] FIG. 6 is a sectional view of a planar inductor according to Example 7 of the present invention;

[0026] FIG. 7 is a graph showing a relationship between a superposed DC current and an inductance of each planar inductor according to Example 1 and Comparative Example 1 of the present invention;

[0027] FIG. 8 is a graph showing a relationship between a superposed DC current and an inductance of each planar inductor according to Example 2 and Comparative Example 1 of the present invention;

[0028] FIG. 9 is a graph showing a relationship between a superposed DC current and an inductance of each planar inductor according to Example 3 and Comparative Example 1 of the present invention;

[0029] FIG. 10 is a graph showing a relationship between a superposed DC current and an inductance of each planar inductor according to Example 4 and Comparative Example 2 of the present invention;

[0030] FIG. 11 is a graph showing a relationship between a saturation magnetization of a ferromagnetic ribbon which constitutes the planar inductor according to the present invention and an efficiency of a noninsulated voltage-drop type DC-to-DC converter to which the planar inductor is applied;

[0031] FIG. 12 is a graph showing a relationship between a superposed DC current and an inductance of each planar inductor according to Examples 5 and 6 of the present invention;

[0032] FIG. 13 is a graph showing a relationship between a superposed DC current and an iron loss of each planar inductor according to Examples 5 and 6 of the present invention;

[0033] FIG. 14 is a graph showing a relationship between a superposed DC current and an effective resistance component of an impedance of each planar inductor according to Examples 5 and 6 of the present invention;

[0034] FIG. 15 is a graph showing a relationship between an output current and an efficiency of the noninsulated voltage-drop type DC-to-DC converter constituted by each planar inductor according to Examples 5 and 6 of the present invention;

[0035] FIG. 16 is a graph showing a relationship between a superposed DC current and an inductance of each planar inductor according to Reference Examples 1 to 3;

[0036] FIG. 17 is a graph showing a relationship between a superposed DC current and an iron loss of each planar inductor according to Reference Examples 1 and 2;

[0037] FIG. 18 is a graph showing a relationship between a superposed DC current and an effective resistance component of an impedance of each planar inductor according to Reference Examples 1 to 3;

[0038] FIG. 19 is a graph showing a relationship between an output current and an efficiency of a noninsulated voltage-drop type DC-to-DC converter constituted by each planar inductor according to Reference Examples 1 to 3;

[0039] FIG. 20 is a graph showing a relationship between superposed DC current and an inductance before and after molding of a planar inductor according to Examples 7 and 8 of the present invention; and

[0040] FIG. 21 is a graph showing a relationship between superposed DC current and an inductance after molding of the planar inductor according to Example 7 and Comparative Example 3 of the present invention.

[0041] In the present invention, a planar inductance element consists of, e.g., a spiral or meander coil. The spiral coil normally has a two-layered structure obtained by forming spiral conductors on the front and rear surfaces of an insulating layer, and connecting the conductors via a through hole. Note that if a terminal can be extracted without a problem, a spiral coil having only one layer of a spiral conductor can be used.

[0042] The planar inductance element may be formed by stacking a plurality of spiral or meander coils. When these coils are stacked, an inductance is increased. In this case, a ferromagnetic layer is not preferably inserted between the coils, but only an insulating layer is inserted. This is because even if a ferromagnetic layer is inserted between the coils, it hardly contributes to an increase in inductance, but increases the thickness of the entire planar inductor to reduce an inductance per unit volume.

[0043] In the present invention, the insulating and ferromagnetic layers may be stacked on one or both surfaces of the planar inductance element.

[0044] In the present invention, one or a plurality of ferromagnetic layers may be stacked.

[0045] A saturation magnetization 4πM_Sof the ferromagnetic layer is set to be 10 kG or more because if the saturation magnetization 4πM_Sis less than 10 kG, DC superposition characteristics of the planar inductor are degraded.

[0046] The thickness of the ferromagnetic layer is 100 μm or less for the following reasons. Assume that the planar inductor is applied to, e.g., a DC-to-DC converter, and it is used in a frequency band of 10 kHz or more. If the thickness of the ferromagnetic layer exceeds 100 μm, a generated magnetic flux does not enter inside the layer due to a surface effect. Thus, an inductance is not increased in proportion to an increase in thickness of the ferromagnetic layer, and an inductance per unit volume is reduced. Note that the thickness of the ferromagnetic layer is preferably 4 μm or more. If the thickness of the ferromagnetic layer is less than 4 μm, a sectional area required for passing all the magnetic fluxes generated by supplying a current to a coil cannot be obtained. Therefore, leaked magnetic fluxes are increased, and the inductance is considerably reduced, thus reducing an inductance per unit volume.

[0047] When a plurality of ferromagnetic layers are stacked, each ferromagnetic layer must satisfy the above-mentioned conditions.

[0048] In the present invention, the ferromagnetic layer preferably has an effective permeability μ10 k of 1×10⁴or more at a frequency of 10 kHz. When such a ferromagnetic layer is used, a planar inductor having high inductance can be obtained.

[0049] For example, an amorphous alloy ribbon represented by the following formula is used as a ferromagnetic layer in the present invention:

[0050] (Fe_1-aM_a)100-b^Xb

[0051] where M is at least one of Ti, V, Cr, Mn, Co, Ni, Zr, Nb, Mo, Hf, Ta, W, and Cu, and X is at least one of Si, B, P, C, Ge, and Al, and 0≦a≦0.15, and 12≦b≦30).

[0052] A function and a composition ratio of each element which constitutes the amorphous alloy ribbon will be described hereinafter.

[0053] The element M is a component which contributes to an improvement of a permeability in a high-frequency region and an increase in crystallization temperature. Even if a small amount of the component M is added, it exhibits the above-mentioned function. In practice, preferably, a≧0.01. When a>0.15, it is not preferable in practice since a Curie temperature is extremely lowered.

[0054] The element X is necessary to obtain an amorphous state. In consideration of heat stability in practice, a combination of elements Si and B is preferable. Note that when b<12 and b>28, it is difficult to obtain an amorphous state, and hence preferably, 12≦b≦28. More preferably, 15≦b≦25. Si is preferably added in an amount of 2 to 13%, and preferably, 2 to 8%.

[0055] Most amorphous alloys with the above composition have saturation magnetizations of 10 kG or more. By performing an optimal heat treatment for eliminating strain, an effective permeability of 1×10⁴or more can be obtained.

[0056] In order to achieve an object of the present invention, in particular, a ferromagnetic layer having an extremely high saturation magnetization and permeability is preferably used. For example, a hyperfine grain alloy ribbon obtained by thermally treating an amorphous alloy ribbon having a composition of Fe_73.5Cu₁Nb₃Si_13.5B₉at a temperature higher than a crystallization temperature is used as a ferromagnetic layer having the above excellent characteristics (see EP 271,657). This magnetic alloy ribbon has a high permeability (an effective permeability μ10k=5×10⁴at a frequency of 10 kHz), and a high saturation magnetization (4πM_s=13.5 kG). When such a magnetic alloy ribbon is used, a planar inductor having a high inductance and excellent DC superposition characteristics can be obtained.

[0057] In the present invention, the ferromagnetic layer which constitutes the planar inductor is preferably two-dimensionally divided into a plurality of portions. When the ferromagnetic layer is two-dimensionally divided into a plurality of portions, a high-frequency loss can be decreased, and the efficiency of a DC-to-DC converter manufactured using such a planar inductor is improved for the following reasons. That is, an effective resistance component R of an impedance Z is represented as follows:

[0058] R=2πf·L·tan δ

[0059] where f is the frequency, L is the inductance, and tan δ is the high-frequency loss. As is apparent from the above equation, R is in proportion to the high-frequency loss tan δ. When the ferromagnetic layer is divided into a plurality of portions, an eddy current loss tan δ is decreased so that R is decrease. For example, an efficiency η of a noninsulated voltage-drop type DC-to-DC converter having an inductance on its output side is approximately represented by η=100R_L/(R_L+R)(%) (where RL is the load resistance). Therefore, when the value of R is smaller, the efficiency of the DC-to-DC converter is improved.

[0060] When the planar inductor according to the present invention is incorporated and used in an apparatus in practice, the entire inductor is coated with a mold resin, as described above. In this case, e.g., an organic polymer film having a thermal deformation temperature higher than a hardening temperature of the mold resin is preferably stacked on a surface of the ferromagnetic layer as a relaxation layer for contraction of the mold resin. While the side surfaces of the planar inductor are sealed with an adhesive, the entire inductor is coated with the mold resin. Thus, if the organic polymer film having a thermal deformation temperature higher than a hardening temperature of the mold resin is stacked on the surface of the ferromagnetic layer, contraction generated when the mold resin is hardened and contracted can be relaxed, and transmission of the contraction to the ferromagnetic ribbon or its stacked body is prevented, thus preventing a reduction in inductance due to an inverse magnetostrictive effect.

[0061] For example, polyphenylenesulfide (PPS) is used as an organic polymer film having a high thermal deformation temperature which is used as a relaxation layer. Note that if a similar effect can be obtained, the relaxation layer is not limited to the organic polymer film, as a matter of course. The thickness of such a relaxation layer is preferably 20 μm or more. If the thickness of the relaxation layer is less than 20 μm, wrinkles tend to be formed, and the contraction of the mold resin cannot be relaxed. The contraction is then transmitted to the ferromagnetic ribbon or its stacked body, and a reduction in inductance due to an inverse magnetostrictive effect cannot be prevented.

[0062] The present invention will be described below in detail by way of its examples.

[0063] A planar inductor having a structure showing FIGS. 1A and 1B was manufactured in Examples 1 to 3, and Comparative Example 1. Note that FIG. 1A is a plan view of the planar inductor, and FIG. 1B is a sectional view taken along the line of A-A′ of FIG. 1A.

[0064] Referring to FIGS. 1A and 1B, a spiral coil 1 had a structure obtained by forming spiral conductors 2aand 2bon both surfaces of an insulating layer 3b,and electrically connecting the conductors 2aand 2bvia a through hole 4. A current flowed through the conductors 2aand 2bin the same direction. Solid and broken lines in FIG. 1A denote the center lines of the conductors 2aand 2blocated on the front and rear surfaces of the insulating layer 3b,respectively. Insulating layers 3aand 3cwere respectively stacked on both the surfaces of the spiral coil 1, and ferromagnetic layers 5aand 5bwere respectively stacked on the insulating layers 3aand 3c,thus the planar inductor was constituted. An inductance was formed between terminals 6aand 6bof the planar inductor including the above-mentioned members.

[0065] Such a planar inductor was manufactured in practice, as follows. Cu foils each having a thickness of 35 μm were applied on both surfaces of a polyimide film (the insulating layer 3b) having a thickness of 25 μm, and the Cu foils were connected via the through hole 4 in a central portion to prepare a double-sided FPC board (flexible printed circuit board). The Cu foils on both the surfaces were etched to obtain the conductors 2aand 2beach having an outer size of 20 mm×20 mm, a coil width of 250 μm, a coil pitch of 500 μm, and the number of turns of the coil of 40 (20 turns for each surface), thus manufacturing the spiral coil 1. Polyimide films (the insulating layers 3aand 3c) each having a thickness of 7 μm were stacked on both surfaces of the spiral coil 1, and square ferromagnetic ribbons (the ferromagnetic layers 5aand 5b) each having a side of 25 mm were further stacked on the polyimide films, respectively, thus manufacturing the planar inductor.

[0066] A square sample having a side of 25 mm was prepared from an amorphous alloy ribbon which had a composition of (Fe_0.95Nb_0.05)₈₂Si₆B₁₂, a mean thickness of 16 μm, and a width of 25 mm, and which was manufactured by a single-roll method, and the sample was used as a ferromagnetic layer. In this amorphous alloy ribbon, an effective permeability μ_10k=1×10⁴at a frequency of 10 kHZ, and a saturation magnetization 4πM_S=12.3 kG.

[0067] A square sample having a side of 25 mm was prepared from an amorphous alloy ribbon which had a composition of Fe₇₈Si₉B₁₃, a mean thickness of 16 μm, and a width of 25 mm, and which was manufactured by a single-roll method, and the sample was used as a ferromagnetic layer. In this amorphous alloy ribbon, an effective permeability μ10k=2,000 at a frequency of 10 kHZ, and a saturation magnetization 4πM_S=15.6 kG.

[0068] A square sample having a side of 25 mm was prepared from a hyperfine grain alloy ribbon obtained by thermally treating in a nitrogen atmospher 550° C. for one hour an amorphous alloy ribbon, which had a composition of Fe_73.5Cu₁Nb₃Si_13.5B₉, a mean thickness of 18 μm and a width of 25 mm, and which was manufactured by a single-roll method, and the sample was used as a ferromagnetic layer. In this alloy ribbon, an effective permeability μ_10k=5×10⁴at a frequency of 10 kHZ, and a saturation magnetization 4πMs=13.5 kG.

[0069] A square sample having a side of 25 mm was prepared from an amorphous alloy ribbon which had a composition of (Co_0.88Fe_0.06Nb_0.02Ni_0.04)₇₅Si₁₀B₁₅, a mean thickness of 16 μm, and a width of 25 mm, and which was manufactured by a single-roll method, and the sample was used as a ferromagnetic layer. In this amorphous alloy ribbon, an effective permeability μ_10k=2×10⁴at a frequency of 10 kHZ, and a saturation magnetization 4πM_S=6.7 kG.

[0070] Each of FIGS. 7 to 9 shows a relationship between a superposed DC current and an inductance of the planar inductors according to Examples 1 to 3, and Comparative Example 1. The inductance was measured at a frequency of 50 kHz.

[0071] As shown in FIGS. 7 to 9, in the planar inductors in Examples 1 to 3, each DC superposition characteristic was largely improved as compared with that in the planar inductor in Comparative Example 1.

[0072] A planar inductor shown in FIG. 2 was manufactured in Example 4 and Comparative Example 2.

[0073] Five square samples each having a side of 25 mm were prepared from an amorphous alloy ribbon having the composition, the mean thickness, and the width which were equal to those of the ribbon in Example 1, and were stacked. After a heat treatment for eliminating a strain was performed for the stacked body, the resultant body was used as a ferromagnetic layer.

[0074] Five square samples each having a side of 25 mm were prepared from an amorphous alloy ribbon having the composition, the mean thickness, and the width which were equal to those of the ribbon in Comparative Example 1, and were stacked. After a heat treatment for eliminating a strain was performed for the stacked body, the resultant body was used as a ferromagnetic layer.

[0075] FIG. 10 shows a relationship between a superposed DC current and an inductance of the planar inductors in Example 4 and Comparative Example 2. Note that the inductance was measured at a frequency of 50 kHZ.

[0076] As shown in FIG. 10, in the planar inductor in Example 4, the DC superposition characteristic was largely improved as compared with that in the planar inductor in Comparative Example 2.

[0077] An efficiency when the planar inductor with the same structure manufactured using a ferromagnetic ribbon having a different saturation magnetization was applied to a noninsulated voltage-drop type DC-to-DC converter of 5-V output 2-W class will be described hereinafter.

[0078] FIG. 11 shows a relationship between a saturation magnetization 4πM_Sof an amorphous alloy ribbon and an efficiency n of a DC-to-DC converter. The DC-to-DC converter was applied a planar inductor constituted of a spiral coil (thickness: about 1 mm) having an air-core inductance of 54 μH, and a coil resistance of 1.8ω, polyimide films having a thickness of 7.5 μm stacked on both surfaces of the spiral coil, and five-layered bodies of Co-or Fe-based amorphous alloy ribbons (thickness: about 15 μm) stacked on the polyimide films. The efficiency was measured under the conditions of an input voltage of 15 V, an output voltage of 5 V, and an output current of 0.4 A.

[0079] As shown in FIG. 11, the efficiency η obtained when an amorphous alloy ribbon (4πM_s≧10 kG) was used was substantially constant, i.e., about 70%. However, when an amorphous alloy ribbon (4πM_S<10 kG) was used, an inductance was degraded because of the superposed DC current, and the efficiency was decreased.

[0080] In Examples 5 and 6, and Reference Examples 1 to 3, a planar inductor of a multi-layered type shown in FIG. 3 was manufactured.

[0081] Cu foils each having a thickness of 100 μm were applied on both surfaces of a polyimide film having a thickness of 25 μm, and the Cu foils were connected via a through hole in a central portion to prepare a double-sided FPC board. The Cu foils on both the surfaces were etched to obtain spiral conductors each having an outer size of 20 mm×20 mm, a coil width of 250 μm, a coil pitch of 500 m, and the number of turns of the coil of 40 (20 turns for each surface), thus manufacturing the spiral coil. Tow spiral coils were stacked with polyimide film having a thickness of 7 μm (the insulating layers 3d) interposed between the coils and the coils were electrically connected in parallel to manufacture a multi-layered coil. In addition, two multilayered coils were stacked with the polyimide film (the insulating layers 3d) having a thickness of 7 μm, interposed between the multi-layered coils and the multi-layered coils were electrically connected in series to manufacture a multi-layered coil (four-layered coil). Polyimide films (the insulating layers 3aand 3c) each having a thickness of 7 μm were stacked on both surfaces of the multi-layered coil, and a square five-layered ferromagnetic ribbon having a side of 25 mm were further stacked on the polyimide films, thus manufacturing the planar inductor. Note that the ferromagnetic ribbon has a square shape having a side of 25 mm obtained by combining a plurality of two-dimensionally divided portions, or without two-dimensionally dividing.

[0082] Five rectangular samples each having sides of 25 mm×12.5 mm were prepared from an amorphous alloy ribbon having the composition, the mean thickness, and the width which were equal to those of the ribbon in Example 1, and were stacked to manufacture a multi-layered body. As shown in FIG. 4, after a heat treatment for eliminating a strain was performed for the multi-layered body 11, two such multi-layered bodies 11 were aligned in a horizontal direction without gaps on a single plane to obtain a square structure having a side of 25 mm, and the square structure was used as a ferromagnetic layer.

[0083] Five square samples each having a side of 25 mm were prepared from an amorphous alloy ribbon having the composition, the mean thickness, and the width which were equal to those of the ribbon in Example 1, and were stacked to manufacture a multi-layered body. After a heat treatment for eliminating a strain was performed for a multi-layered body, the resultant body was used as a ferromagnetic layer.

[0084] Various characteristics of the planar inductors in Examples 5 and 6 were examined. FIG. 12 shows a relationship between a superposed DC current and an inductance. FIG. 13 shows a relationship between a superposed DC current and an iron loss. FIG. 14 shows a relationship between a superposed DC current and an effective resistance component of an impedance. FIG. 15 shows a relationship between an output current and an efficiency η of a noninsulated voltage-drop type DC-to-DC converter of 5-V output 2-W class, which was constituted by the planar inductors.

[0085] As is apparent from FIGS. 12 to 15, in the planar inductor in Example 5 obtained by dividing the ferromagnetic layer into two portions, an inductance was slightly improved as compared with the planar inductor in Example 6 in which the ferromagnetic layer was not divided. In addition, when the iron loss was decreased, an effective resistance component of the impedance was decreased. As a result, a noninsulated voltage-drop type DC-to-DC converter using the planar inductor in Example 5 had an efficiency higher than that of the converter using the planar inductor in Example 6.

[0086] Note that in Examples 5 and 6, the ferromagnetic ribbon which satisfied the condition of 4πM_S≧10 kG was used. When the ferromagnetic ribbon was divided, the above-mentioned effect could be obtained even if a ferromagnetic ribbon which does not satisfy the condition of 4πM_S>10 kG is used. This will be described with reference to Reference Examples 1 to 3 below.

[0087] Five square samples each having a side of 12.5 mm were prepared from an amorphous alloy ribbon having the composition, the mean thickness, and the width which were equal to those of the ribbon in Comparative Example 1, and were stacked to manufacture a multi-layered body 12. As shown in FIG. 5, after a heat treatment for eliminating strain was performed for the multi-layered body 12, four such multi-layered bodies 12 were arranged in a horizontal direction without gaps on a single plane to obtain a square structure having a side of 25 mm, and the square structure was used as a ferromagnetic layer.

[0088] Five rectangular samples each having sides of 25 mm×12.5 mm were prepared from an amorphous alloy ribbon having the composition, the mean thickness, and the width which were equal to those of the ribbon in Comparative Example 1, and were stacked to manufacture a multi-layered body 11. As shown in FIG. 4, after a heat treatment for eliminating strain was performed for the multi-layered body 11, two such multi-layered bodies 11 were arranged in a horizontal direction without gaps on a single plane to obtain a square structure having a side of 25 mm, and the square structure was used as a ferromagnetic layer.

[0089] Five square samples each having a side of 25 mm were prepared from an amorphous alloy ribbon having the composition, the mean thickness, and the width which were equal to those of the ribbon in Comparative Example 1, and were stacked to manufacture a multi-layered body. After a heat treatment for eliminating strain was performed for the multi-layered body, the resultant body was used as a ferromagnetic layer.

[0090] Various characteristics of the planar inductors in Reference Examples 1 to 3 were examined. FIG. 16 shows a relationship between a superposed DC current and an inductance. FIG. 17 shows a relationship between a superposed DC current and an iron loss. FIG. 18 shows a relationship between a superposed DC current and an effective resistance component of an impedance. FIG. 19 shows a relationship between an efficiency Ti and an output current of a noninsulated voltage-drop type DC-to-DC converter of 5-V output 2-W class, which was constituted by the planar inductors.

[0091] As shown in FIGS. 16 to 19, the same tendencies as in FIGS. 12 to 15 according to Examples 5 and 6 described above appear.

[0092] In Examples 7 and 8, an inductance when the planar inductor was covered with a mold resin was examined.

[0093] As shown in F a planar inductor 20 having a four-layered coil and a five-layered ferromagnetic ribbon which had an outer size of 25 mm×25 mm and which was manufactured in Examples 5 and 6 was used. PPS (polyphenylenesulfide resin) films 21 each having an outer size of 30 mm×30 mm, and a thickness of 100 μm were formed on both outer surfaces of the ferromagnetic ribbon. The side surfaces of the multi-layered coil were sealed with an adhesive 22 (Cemedine Super available from CEMEDINE CO., LTD.), so that when the multi-layered coil was dipped into a liquid mold resin in a subsequent step, the mold resin would not be brought into direct contact with the coil and the ferromagnetic ribbon. After the multi-layered coil was dipped into a mold resin 23 (Ceracoat 640-43 available from Hokuriku Toso K.K.), the coil was removed from the resin. After the coil was naturally dried for about one hour, the dried coil was heated at 150° C. for one hour to harden the mold resin 23, thus manufacturing a mold planar inductor.

[0094] A mold planar inductor was manufactured following the same procedures as in Example 7, except for the step of forming PPS films on both outer surfaces of a ferromagnetic ribbon, and the step of sealing the side surfaces of a multi-layered coil with an adhesive.

[0095] A planar inductor in this example had the same structure as that in Example 7, i.e., a structure having a four-layered coil and a five-layered ferromagnetic ribbon. In this planar inductor, the ferromagnetic ribbon consisted of square samples each having a side of 25 mm which were prepared from an amorphous alloy ribbon having a composition of (Co_0.88Fe_0.06Nb_0.02Ni_0.04)₇₅Si₁₀B₁₅, a mean thickness of 16 μm, and a width of 25 mm was used, and a mold planar inductor was manufactured following the same procedures as in Example 7.

[0096] FIG. 20 shows a relationship between a superposed DC current and an inductance before and after molding of the planar inductors in Examples 7 and 8. FIG. 21 shows a relationship between a superposed DC current and an inductance after molding of the planar inductors in Example 7 and Comparative Example 3.

[0097] As is apparent from FIG. 20, in the mold planar inductor without PPS films on both outer surfaces of the ferromagnetic ribbon in Example 8, an inductance after molding is lower than that before molding by about 20%. On the contrary, in the mold planar inductor with PPS films on both outer surfaces of the ferromagnetic ribbon in Example 7, an inductance after molding is lower than that before molding by only about 7%. As is apparent from FIG. 21, the mold planar inductor in Comparative Example 3, which employs the amorphous alloy ribbon having an insufficient saturation magnetization is different from the mold planar inductor in Example 7, as follows. That is, when a superposed DC current is 0.3 A or more, an inductance is considerably reduced.

[0098] Note that although a case wherein a spiral coil is used as a planar inductance element is described with reference to the above embodiments, a coil having another shape such as a meander coil may be used as the planar inductance element.