COUPLED INDUCTOR

Abstract

A coupled inductor comprises an annular core 1 and coils 2a, 2b wound around the core. The annular core 1 includes a sendust core having a maximum differential permeability that is equal to or greater than 30.

Claims

1. A coupled inductor comprising: an annular core including a sendust core having a maximum differential permeability that is equal to or greater than 30; and a coil wound around the core, wherein the coil includes two coils wound around the core such that magnetic fluxes generated from the two coils are oriented in opposite direction to each other, wherein a coupling coefficient of the coupled inductor formed by the two coils is equal to or smaller than 0.8.

2. The coupled inductor according to claim 1, wherein the annular core is formed by combining a plurality of cores.

3. The coupled inductor according to claim 2, wherein the annular core comprises two U-shaped core members abutting end faces thereof with each other.

4. The coupled inductor according to claim 2, wherein the annular core comprises a gap formed between opposing end faces of respective cores.

5. The coupled inductor according to claim 4, wherein the gap is formed by disposing a spacer made of ceramic plate between the opposing end faces of the respective cores.

6. The coupled inductor according to claim 1, wherein the coil comprises an edgewise winding.

7. A coupled inductor according to claim 1, wherein the two coils are disposed in parallel to each other in the same axis direction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a perspective view illustrating a coupled inductor according to a first embodiment;

[0017] FIG. 2 is a perspective view illustrating a core according to the first embodiment;

[0018] FIG. 3 is a perspective view illustrating an edgewise winding utilized according to the first embodiment;

[0019] FIG. 4 is a graph illustrating a relationship between a frequency and a core loss of a sendust core according to this embodiment;

[0020] FIG. 5 is a graph for comparing a DC superimpose characteristic of a sendust core with that of a ferrite core;

[0021] FIG. 6 is a graph for comparing a current waveform of the sendust core and that of the ferrite core when a duty is 29%; and

[0022] FIG. 7 is a graph for comparing a current waveform of the sendust core with that of the ferrite core when a duty is 50%.

DETAILED DESCRIPTION OF THE EMBODIMENTS

1. FIRST EMBODIMENT

[0023] A structure according to a first embodiment of the present disclosure will be explained below in detail with reference to FIGS. 1 to 3.

(1) Structure

[0024] As illustrated in FIG. 1, a coupled inductor of this embodiment has two coils 2a, 2b wound around an annular core 1, and currents are allowed to flow through the respective coils in such a way that magnetic fluxes generated from the two coils 2a, 2b are in the opposite directions. In other words, by winding the two individual coils 2a, 2b around the annular core 1, two coils 2a, 2b are magnetically coupled and generate the magnetic fluxes in mutual opposite directions to cancel the magnetic fluxes with each other. In this case, it is preferable that the coupling coefficient of the coupled inductor formed by the two coils should be equal to or smaller than 0.8. As illustrated in FIG. 2, as the annular core 1, two U-shaped core members 1a, 1b combined annularly by abutting the end faces thereof with each other are used. Gaps 3a, 3b are formed between the opposing faces of the U-shaped core members 1a, 1b.

[0025] Sendust cores are utilized as the core members 1a, 1b. In this embodiment, a sendust core is formed by adding a binder of silicon resin and a lubricant to aqueous atomized powders with an average particle diameter of 40 μm, shaping and calcinating the material. A magnetic condition of the present disclosure is that the maximum differential permeability is equal to or greater than 30. In general, it is ideal that the effective permeability of a reactor be substantially 30. Hence, it is necessary that the permeability of the core alone should be equal to or greater than 30 at minimum. That is, when the maximum differential permeability p of the core alone becomes equal to or greater than 30, the effective permeability becomes 30 at maximum relative to the reactor. When the gaps 3a, 3b are formed under such a circumstance, the effective permeability of the reactor further decreases, and becomes close to an ideal value.

[0026] As to other magnetic characteristics of the sendust core of this embodiment, when the volume of the core is 1 m.sup.3, the saturated flux density at 15000 A/m is equal to or greater than 0.5 T, the core loss at 10-kHz-100-mT is equal to or smaller than 50 kW/m.sup.3, the core loss at 30-kHz-100-mT is equal to or smaller than 180 kW/m.sup.3, and the core loss at 50-kHz-100-mT is equal to or smaller than 340 kW/m.sup.3.

[0027] FIG. 4 illustrates a relationship between a loss and a frequency when the operation flux density of the sendust core of the present invention is 100 mT. It is preferable that the core loss should be lower than the graph in FIG. 4. A value in FIG. 4 is a value of the core loss when the operation flux density is 100 mT and the volume of the core is 1 m.sup.3. The core loss of the reactor varies depending on the operation flux density and the core volume. Hence, in FIG. 4, as a representative value of the operation flux density, 100 mT is adopted, and in an actual reactor, the operation flux density varies depending on the cross-sectional area of the core and the number of turns of winding, etc.

[0028] The gaps 3a, 3b are not always necessary according to the present disclosure, but in this embodiment, spacers each formed of a ceramic sheet with a thickness of substantially 1 mm are disposed between end faces of the U-shaped core members 1a, 1b to form the gaps 3a, 3b in an appropriate size. As explained above, such gaps 3a, 3b set the effective permeability of the reactor to be a further appropriate value relative to a circuit used with this coupled inductor, and thus the effective permeability can be reduced in comparison with a gap-less reactor.

[0029] As the two coils 2a, 2b, as illustrated in FIG. 3, edgewise windings (also called as flat windings) are utilized. In reactors, a conductive wire near the core generates large heat, and according to conventional round winding, the internal generated heat is not likely to be repelled due to the windings turned in multiple layers and unnecessary gaps between conductive wires, and thus the temperature rise is relatively large. Hence, a temperature difference between an internal conductive wire portion and an external conductive wire portion is large. In contrast, according to the edgewise winding, since the cross-section is rectangle, the winding cross-sectional area is large, and the space factor is improved, thereby decreasing the resistance value. In particular, according to the edgewise winding, a monolayer structure is employed relative to the internal diameter of the core, and thus the temperature difference occurs within the same cross-section. As a result, in accordance with the thermal conduction of copper, heat is dissipated to the external side without being blocked. Therefore, a heat dissipation performance is excellent and a temperature rise is small.

(2) Advantageous Effects

[0030] When a saturated flux density and a core loss are compared between a reactor including the sendust core of this embodiment and a reactor including a pure-iron-based dust core and a ferrite core under the same condition as that of the former reactor other than the material of the core, the following results were obtained. In table 1, the value of the pure-iron-based dust core was taken as a criterion value “1” to carry out a relative comparison with other cores. As is clear from table 1, the sendust core satisfies both saturated flux density and core loss, and is suitable for a large-current application.

TABLE-US-00001 TABLE 1 Pure-iron-based dust core Ferrite core Sendust core Saturated flux 1 0.2  0.5 density Excellent Poor Good Core loss 1 0.04 0.4 Poor Excellent Good Pure-iron-based dust core is taken as a criterion

[0031] Likewise, regarding reactors in the same shape, with the same dimension, and with the same coils wound therearound, under the condition in which the frequency was 30 kHz, and the operation flux density was 168 mT, a characteristics comparison was carried out for a ferrite core and a sendust core. The following results were obtained.

TABLE-US-00002 TABLE 2 Characteristic Comparison THERMAL CHARAC- RIPPLE TERISTIC NUM- COU- CURRENT (SIMPLE GAP BER PLING (AVERAGE REACTOR LOSS THERMAL THICK- OF COEFFI- CURRENT): 94 A COPPER IRON ANALYSIS) NESS GAPS CIENT Duty 29% Duty50% LOSS LOSS Total COIL CORE SENDUST 0.0 mm 0 0.72 24.0Ap-p 21.0Ap-p 175.0 W 52.3 W 227.3 W 121.2° C. 123.0° C. FERRITE 3.0 mm 2 0.62 30.6Ap-p 48.2Ap-p 252.0 W  3.8 W 255.8 W 138.2° C. 112.2° C.

[0032] As is clear from this table 2, with respect to the ripple current, the sendust core with a low current value accomplished a good result. With respect to the loss, the smaller loss was a good result, and the sendust core had a large iron loss than the ferrite core, but had a smaller ripple current. The sendust core had a gap width of 0 mm, and thus the copper loss indicates the low value. As a result, the sendust core had a smaller total loss. With respect to the thermal characteristic, the lower characteristic was a good result, and the sendust had a lower result, so that the similar result was accomplished for the sendust core with respect to the thermal characteristic.

[0033] FIG. 5 illustrates a single-sided superimpose characteristic of the ferrite core and that of the sendust core indicated in table 2. As is clear from this graph, the sendust core indicates an excellent characteristic even if no gap is formed in comparison with the ferrite core with two gaps.

[0034] FIGS. 6 and 7 illustrate a comparison result of a current waveform between the ferrite core and the sendust core indicated in table 2. FIG. 6 illustrates a current waveform when the duty is 29%, and FIG. 7 illustrates a current waveform when the duty is 50%. Those current waveforms are the current waveforms of a current flowing through either one of the coils 2a, 2b of the coupled inductor. As is clear from FIGS. 6 and 7, the sendust core of this embodiment has a little change in the current waveform regardless of a change in the duty, and the ripple in the current is little.

2. Other Embodiments

[0035] The present disclosure is not limited to the aforementioned embodiment, and covers the following other embodiments.

[0036] (1) As the annular core, in addition to the combination of the two U-shaped cores, an annular core formed by a single piece as a whole may be used. An annular core including one or multiple leg-portion cores provided between the two U-shaped cores may be used. As the leg-portion cores, for example, cores having I-shape, polygonal column shape, circular column shape, or elliptical shape may be used. Additionally, the cores of a cube or cuboid shape may be used. As a material for the leg-portion cores, The powder magnetic core formed by compression molding of the soft magnetic powder, the laminated core laminating the metal plate, The magnetic powder and the resin mixed core in which the magnetic core is dispersed, or the core formed by winding the thin film of iron-based amorphous alloy may be used. Moreover, an annular core formed by abutting two E-shaped cores with end faces thereof with each other may be used.

[0037] (2) Regarding the gap, gaps may be provided between the right and left core-legs, respectively as illustrated, or a gap-less structure may be employed. A further larger number of gaps may be provided.

[0038] (3) It is preferable that the coil should be formed of an edgewise winding, but a round winging may be applied. Coils may be wound around the right and left core-legs of the annular core, respectively, and two coils may be wound around one core-leg. The coil is not limited to a copper-made coil, and an aluminum-made coil may be applied.