JP2015088513A - Inductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve both of the thermal conductivity and the electrical insulation properties of an inductor.SOLUTION: The inductor includes a winding 5, a first resin 7 and a second resin 8. The winding 5 is wound around a bobbin 2. The winding 5 is impregnated with the first resin 7 and the first resin 7 is structured without including filler. The second resin 8 includes inorganic filler and is formed to cover the winding 5 from the outside together with the bobbin 2.

Description

  Embodiments described herein relate generally to an inductor.

  There is known a wireless charging device that charges a battery such as an electric vehicle in a contactless manner. This wireless charging device uses, for example, electromagnetic induction between a power transmission coil (primary coil) installed on the ground side and a power reception coil (secondary coil) in an inductor mounted on the electric vehicle side to power the battery. Supply.

  By the way, for example, a receiving coil in an inductor through which such wireless power is transmitted is applied with a twisted wire such as a litz wire in order to reduce the influence of eddy current (influence of skin effect) caused by transmission of high-frequency power or the like. ing.

  Here, in the general casting method for resin molding of the outer part of the inductor, the resin is not properly impregnated between the strands of the litz wire, the problem that the heat dissipation effect of the litz wire is reduced, and the resin impregnation There is a problem that partial discharge occurs during use in the void portion caused by the shortage. On the other hand, when applying vacuum heating casting in order to realize void free, an increase in cost becomes a problem.

JP 2011-229202 A JP 2008-87733 A

  More specifically, when a void is generated in the resin impregnated in the litz wire, heat is not diffused uniformly, so that a so-called hot spot (heat storage location where heat is not dissipated) may be formed. On the other hand, when a method of diffusing heat to a vehicle body such as an electric vehicle is applied, if the thermal conductivity of the molding resin of the inductor body is not sufficient, the resin may be deteriorated due to overheating.

  Furthermore, in the above-described inductor that integrally molds internal electronic components, stress concentrates on the corners and the like of the electronic components. Therefore, cracks may occur in the corners due to the generation of internal stress caused by heating. is there. Further, when a large amount of filler is filled in the molding resin in order to improve thermal conductivity, generation of voids due to insufficient impregnation of the power receiving coil is induced. In particular, when a filler having a high relative dielectric constant is used, electric field concentration occurs in the void portion and discharge occurs, which may cause dielectric breakdown.

  As described above, although the above-described inductor is expected to improve the cooling property by the resin mold, it is difficult to achieve both the improvement of the thermal conductivity and the improvement of the electric insulation, and the generation of cracks due to heating and the void portion There is concern about the occurrence of dielectric breakdown due to discharge.

  That is, the problem to be solved by the present invention is to provide an inductor capable of improving both thermal conductivity and electrical insulation.

  The inductor according to the embodiment includes a winding, a first resin, and a second resin. The winding is wound around the bobbin. The first resin is formed without impregnating the winding and containing a filler. The second resin contains an inorganic filler and is molded so as to cover the winding from the outside together with the bobbin.

The perspective view shown typically in the state which saw through the internal structure of the inductor which concerns on 1st Embodiment. FIG. 2 is a cross-sectional view schematically showing the structure of the inductor in FIG. 1. The figure which shows the relationship between the filling amount of the filler with which it fills in 2nd resin, and thermal conductivity. Sectional drawing which shows the structure of the inductor of a comparative example typically. Sectional drawing which shows typically the structure of the inductor which concerns on 2nd Embodiment. The figure which shows the relationship between the filling amount of carbon black further contained (added) in the filler (boron nitride) with which it fills in 2nd resin, and thermal conductivity. Sectional drawing which shows typically the structure of the inductor which concerns on 3rd Embodiment.

Hereinafter, embodiments will be described with reference to the drawings.
<First Embodiment>
As shown in FIGS. 1 and 2, the inductor 10 of the present embodiment is used, for example, for wireless power transmission, and uses electromagnetic induction from a power transmission coil (primary coil) installed on the ground or the like. It is a device that receives power without contact. The inductor 10 for wireless power transmission is attached to the bottom of a vehicle body 6 such as an electric vehicle that receives power from a battery via an aluminum member or the like.

  Specifically, as shown in FIGS. 1 and 2, the inductor 10 includes a bobbin 2, a ferrite core 3, a winding 9 as a power receiving coil (secondary coil), a first resin 7, and a second resin 8. Etc. The ferrite core 3 is for aligning the magnetic field generated around the winding 9 and is inserted into the axial center of the bobbin 2 formed in a rectangular tubular shape.

  The winding 9 is wound around the outer peripheral surface of the bobbin 2 with a predetermined interval. The winding 9 is constituted by a litz wire 5 in which a plurality of strands 5a are bundled as a stranded wire. Copper (Cu) is applied to the material of the litz wire 5. The litz wire 5 constituting the winding 9 is applied in order to reduce the influence of the skin effect when transmitting high-frequency power or the like.

  Next, the first resin 7 and the second resin 8 will be described. The winding 9 is impregnated with the first resin 7. That is, the first resin 7 is impregnated in the gaps between the plurality of strands 5 a constituting the litz wire 5. The first resin 7 is a resin that does not contain a filler such as an inorganic filler, and is made of, for example, an epoxy resin.

  Here, if a resin containing a filler is used as the resin for impregnating the litz wire 5, the resin may not enter the litz wire 5 because the viscosity of the resin is high. Therefore, in the winding 9 (gap in the litz wire 5), the space in which the resin is present and the space in which the resin is not present are unevenly distributed. As a result, when the inductor 10 is used, the temperature in the winding 9 becomes uneven. Such variations in temperature distribution cause internal stress in the winding 9 and cause undesirable results such as generation of cracks.

  Therefore, the inductor 10 of the present embodiment improves heat transfer between the strands 5a of the litz wire 5 by impregnating the litz wire 5 (winding 9) with the first resin 7 that does not contain a filler. be able to. As a result, the temperature distribution of the winding 9 can be leveled, and the occurrence of hot spots (partial heat storage locations) can be reduced. The first resin 7 may be a polyester resin that does not contain a filler.

  On the other hand, the second resin 8 contains an inorganic filler (inorganic particles), and as shown in FIG. 2, the ferrite core 3 is molded so as to cover the winding 9 from the outside together with the bobbin 2 inserted in the shaft center. Has been. Specifically, the second resin 8 is made of a material in which an inorganic filler is filled in an epoxy resin such as a bisphenol A type epoxy resin. Further, the second resin 8 is integrally molded together with the bobbin 2 and the winding 9 impregnated with the first resin 7 by casting, for example.

  Here, for example, when a large current flows through the winding 9, the conductor temperature rises, so the outer portion of the winding 9 (the litz wire 5) quickly transfers the heat generated by the winding 9 to the vehicle body 6 side. It is desirable to let it escape. Therefore, a material having higher thermal conductivity than the first resin 7 by applying an inorganic filler to the second resin 8 is applied.

Further, the linear expansion coefficient of copper (17 × 10 ⁻⁷ m / m · ° C.) and the linear expansion coefficient of epoxy resin (37 × 10 ⁻⁷ m / m · ° C.) constituting the winding 9 (Litz wire 5). ) Is large, and when the conductor temperature of the winding 9 is increased, the inductor 10 may cause interface peeling or cracking due to the stress caused by the difference in the linear expansion coefficient (linear expansion coefficient) described above. .

Therefore, for example, silicon dioxide (SiO ₂ ) as an inorganic filler is filled in the second resin 8 by 30% by volume in a normal temperature environment, so that the linear expansion coefficient ^becomes 27 × 10 ⁻⁷ m / m · ° C., for example. It becomes possible to do. Thereby, the difference in mutual linear expansion coefficient between the copper winding 9 (Litz wire 5) and the second resin 8 is reduced, and the generation of internal stress in the inductor 10 can be reduced. In addition, by reducing the linear expansion coefficient of the second resin 8 as described above, the interface between the inductor 10 joined to the vehicle body 6 via the aluminum member and the aluminum member is peeled off. Can be suppressed.

That is, if the linear expansion coefficient of the second resin 8 exceeds, for example, 30 × 10 ⁻⁷ m / m · ° C., the probability of occurrence of the above-described interface peeling or cracking increases. An inorganic filler such as silicon dioxide, boron nitride or aluminum nitride is contained so that the coefficient is 30 × 10 ⁻⁷ m / m · ° C. or less.

  Further, as described above, it is important to increase the thermal conductivity of the second resin 8. Here, the thermal conductivity of the epoxy resin is about 0.2 W / m · K. By configuring the second resin 8 so as to have twice the thermal conductivity, the cooling performance of the inductor 10 is improved. Can be greatly improved. Note that when the thermal conductivity of the second resin 8 is, for example, less than 0.4 W / m · K, the heat dissipation in the inductor 10 is significantly reduced. Fig. 3 shows the relationship between the amount of aluminum oxide filled into the epoxy resin and the thermal conductivity (measured by the laser flash method) when aluminum oxide (alumina) is used as the inorganic filler with high thermal conductivity. Is shown. As shown in FIG. 3, the thermal conductivity of 0.4 W / m · K or more can be obtained by setting the aluminum oxide filling amount to about 10% by volume or more at room temperature, for example.

  Here, in the inductor 10 for wireless power transmission intended for an electric vehicle or the like, a voltage applied to the vehicle body 6 is 0 V, while a high voltage of, for example, 5 kV or more is applied to the winding 9. Therefore, the second resin 8 covering the outside of the winding 9 is affected by the high voltage described above. As shown in FIG. 4, the inductor 10 of the comparative example does not contain a filler such as an inorganic filler in the resin 28 that covers the outside of the winding 9. Defects 21 such as cracks may exist. In this case, a high voltage is applied to the site where the defect 21 exists, and partial discharge occurs.

  The partial discharge itself does not immediately lead to a failure of the inductor, but if the partial discharge occurs continuously, it eventually becomes an electric tree (dendritic breakdown trace), and the high voltage and the ground voltage are reduced. There is a high possibility that a short circuit will cause the surrounding electrical equipment including the inductor to malfunction. Here, the aluminum oxide exemplified as the inorganic filler has a relative dielectric constant of about 8, which is twice that of an epoxy resin having a relative dielectric constant of about 4. For this reason, when aluminum oxide is simply filled as an inorganic filler, the voltage applied to the defect increases, and as a result, partial discharge is likely to occur.

Therefore, the inorganic material to be applied is based on the premise that the second resin 8 has a thermal conductivity of 0.4 W / m · K or more and a linear expansion coefficient of 30 × 10 ⁻⁷ m / m · ° C. or less. By using boron nitride or aluminum nitride having a relatively low relative dielectric constant instead of aluminum oxide as a filler, the voltage applied in the defect can be reduced, and the occurrence of partial discharge can be suppressed. .

  As described above, in the inductor 10 of the present embodiment, the first resin 7 that does not include a filler is applied as the resin impregnated in the winding 9, while the resin that is molded so as to cover the winding 9 from the outside. By using the second resin containing an inorganic filler, it is possible to improve both thermal conductivity and electrical insulation. Specifically, according to the inductor 10, the winding 9 is impregnated with the first resin 7 containing no filler, so that the temperature distribution in the winding 9 is made uniform, such as a hot spot. Can be suppressed.

  Further, according to the inductor 10, the internal stress that may occur due to thermal expansion or the like is relieved by adding an inorganic filler to the second resin that covers the winding 9 from the outside, and heat to the vehicle body 6 side is also reduced. The conductivity is improved and the cooling property of the inductor 10 main body is improved, and thereby the generation of cracks and voids can be suppressed.

[Second Embodiment]
Next, a second embodiment will be described based on FIGS. In FIG. 5, the same components as those in the first embodiment shown in FIG. 2 are given the same reference numerals, and descriptions thereof are omitted.

The inductor 30 according to this embodiment includes a second resin 38 instead of the second resin 8 included in the inductor 10 according to the first embodiment. The second resin 38 also has a linear expansion coefficient of 30 × 10 ⁻⁷ m / m · ° C. or less and a thermal conductivity of 0.4 W / m · K or more. Further, the inorganic filler (inorganic particles) filled in the second resin 38 of the present embodiment includes a first containing at least one of boron nitride, aluminum nitride, and aluminum oxide each having an average particle size of 5 μm or more. A particle group and a second particle group having an average particle diameter of 1 μm or less are included.

  FIG. 6 shows, as an example of an inorganic filler to be filled in the second resin 38, boron nitride (first particle group) having an average particle diameter of 5 μm, for example, at 50% by volume and 60% in an environment such as room temperature. % Or 68% by volume of epoxy resin (for example, bisphenol A type epoxy resin) filled with carbon black (second particle group which is fine particles) having an average particle size of 90 nm. The relationship between the filling amount of the carbon black and the thermal conductivity (measured by a laser flash method) is shown.

  In FIG. 6, in particular, the thermal conductivity of the epoxy resin with 60% by volume or 68% by volume of boron nitride added with 1% by volume of carbon black is dramatically higher than that with no added carbon black. It can be seen that it is improved (increased about 1.5 times). This is presumably because the addition of fine particles to the epoxy resin between the boron nitrides improved the elastic modulus, and as a result, the phonons carrying heat were more easily propagated.

  That is, in the second resin 38, a percolation path is formed in which the first particle group having a large particle size made of boron nitride or the like and the second particle group having a small particle size made of carbon black or the like are continuously connected. Is done. The number of percolation paths increases exponentially when the volume ratio of the first and second particle groups in the resin of the base material exceeds a certain value (percolation threshold).

  The second resin 38 of this embodiment is a so-called two-particle composite. The conductive properties of such a two-particle composite have been described by the inventor, for example, by Tetsushi Okamoto et al. In Proceeding of 2001 International Symposium on Electrical Insulating Materials (2001). 83-P. 86 (Percolation phenomena of field grading materials made of two kind of filler).

  Further, in the case where, for example, aluminum oxide having an average particle diameter of 90 nm is added instead of carbon black added to the second resin 38 as the second particle group, the thermal conductivity can be similarly increased. For example, the thermal conductivity of a composite in which styrene ethylene polymer is filled with 60% by volume of boron nitride particles at room temperature or the like is about 2 W / m · K. However, when 5% by volume of aluminum oxide having an average particle size of 90 nm is added to this composite at room temperature or the like, the thermal conductivity can be increased to about 4.6 W / m · K.

  Thus, thermal conductivity can be significantly improved by adding fine particles to the second resin. As described in the first embodiment, aluminum oxide is a material having a high relative dielectric constant, but since it is added in a small amount, it does not significantly affect the electrical characteristics. Therefore, also in the inductor 30 of this embodiment, heat conductivity and electrical insulation can be improved.

<Third Embodiment>
Next, a third embodiment will be described with reference to FIG. In FIG. 7, the same constituent elements as those in the first embodiment shown in FIG.

The inductor 50 according to this embodiment includes a second resin 58 instead of the second resin 8 included in the inductor 10 according to the first embodiment. The second resin 58 also has a linear expansion coefficient of 30 × 10 ⁻⁷ m / m · ° C. or less and a thermal conductivity of 0.4 W / m · K or more. Here, the second resin 58 further contains liquid rubber or core-shell rubber in addition to the inorganic fillers exemplified in the first and second embodiments. The second resin 58 can improve the fracture toughness value by introducing such a rubber-elastic domain into a resin as a base material such as an epoxy resin.

  Therefore, according to the inductor 50 according to the third embodiment, in addition to the effects obtained in the first and second embodiments, the influence of internal stress that can be generated by the operating heat of the inductor body is reduced by the second resin 58. Since it can relieve | moderate by the rubber elasticity which has, the suppression effect about generation | occurrence | production of a crack etc. can be heightened.

  As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  2 ... bobbin, 3 ... ferrite core, 5 ... litz wire, 5a ... strand of litz wire, 7 ... first resin, 8, 38, 58 ... second resin, 10, 30, 50 ... inductor.

Claims (8)

A winding wound around a bobbin;
A first resin impregnated in the winding and not containing a filler;
A second resin containing an inorganic filler and molded with the bobbin so as to cover the winding from the outside;
Inductor comprising. The second resin contains the inorganic filler so that the linear expansion coefficient is 30 × 10 ⁻⁷ m / m · ° C. or less,
The winding is made of copper,
The inductor according to claim 1. The second resin has a thermal conductivity of 0.4 W / m · K or more.
The inductor according to claim 1 or 2. The inorganic filler is made of boron nitride, aluminum nitride, or silicon dioxide.
The inductor according to any one of claims 1 to 3. The inorganic filler includes a first particle group including at least one of boron nitride, aluminum nitride, and aluminum oxide each having an average particle diameter of 5 μm or more and a second particle group having an average particle diameter of 1 μm or less. ,
The inductor according to any one of claims 1 to 3. The second resin further contains liquid rubber or core-shell rubber,
The inductor according to any one of claims 1 to 3. The second resin has a higher thermal conductivity than the first resin.
The inductor according to any one of claims 1 to 6. A ferrite core inserted in the bobbin axis;
The inductor according to any one of claims 1 to 7.

Images