Optimal inductor

Abstract

The present invention relates to a coil (1) for an inductor (6), comprised by metal wire (2) wound circular around a centre axis (C), wherein the wire has an electrically insulating layer (3) insulating each turn of the wire in the winding from neighbouring turns, the shape of the complete winding, building up the coil (1), is substantially toroidal having a substantially elliptic cross section, wherein the thermal heat conductivity is above 1 W/m*K more preferably above 1.2 and most preferably above 1.5. The invention further relates to a magnetic core (7) suitable for an inductor (6), where in the core is made of a soft magnetic composite material made of metallic particles and a binder material, said particles are in the range of 1 m-1000 m, particles that are larger than 150 m are coated with a ceramic surface to provide particle to particle electrical insulation, wherein the volume of magnetic, metallic particles to total core volume is 0.5-0.9. The invention still further relates to an inductor (6) being a combination of said coil (1) and core (7), wherein the substantially all of said particles in the core are magnetically aligned with the magnetic field of the coil. The invention still further relates to the manufacturing methods of such a coil (1) and core (7).

Claims

1. A coil for an inductor, the coil comprising: a metal wire wound circular around a center axis; wherein the wire has an electrically insulating layer insulating each turn of the wire in the winding from neighboring turns; wherein the wire includes a plurality of electrically insulated strands that are twisted 360, 90, for the complete wound coil; wherein the shape of the complete winding, building up the coil, is toroidal having an elliptic cross section in a plane perpendicular to the wire winding direction; and wherein the wound coil has a metal volume to a total volume at a level so that the thermal heat conduction of the coil is above 0.8 W/m*K.

2. The coil according to claim 1, wherein the toroidal shape is a ring torus having a circular cross section.

3. The coil according to claim 1, wherein the strands are electrically insulated by cured resin or cured and semi-cured resin.

4. The coil according to claim 1, wherein the cross section of each strand at each position is shaped to fit tightly to adjacent strands, reducing voids in the wire.

5. An inductor comprising a coil according to claim 1, wherein the coil is embedded in a core; wherein the core is made of a soft magnetic composite material made of metallic particles and a binder material; wherein the coil has an electrically insulating layer covering its surface area; and wherein core particles are magnetically aligned with the H-field of the coil.

6. The inductor according to claim 5, wherein the core has a toroid shape covering the coil.

7. The inductor according to claim 5, wherein the coil is arranged in an optimal position to provide the same magnetic flow in the core material in all directions seen from the coil surface (the same volume in all directions), by having the same cross sectional area of the core on the inside of the coil towards the center axis as on the outside of the core, seen in a cross section along a plane perpendicular to the center axis through the center of the coil.

8. The inductor according to claim 5 wherein the core comprises surface increasing structures modifying the toroidal shape to increase the surface area.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above objects, as well as additional objects, features and advantages of the present invention, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, when taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 is a perspective view of a coil for an inductor.

(3) FIG. 2a is a cross sectional view of the coil in FIG. 1.

(4) FIG. 2b shows an enlarged view of the cross sectional view of FIG. 2b showing the strands of the wire.

(5) FIG. 3 is a perspective view of an inductor including a coil according to FIG. 1 and FIG. 2, integrated in a core according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(6) FIG. 1 shows a perspective view of a coil 1 for an inductor. The coil 1 is torus shaped and is built up by a wounded wire 2, better seen in the cross section of the coil shown in FIG. 2a. The coil is coated or wound with an insulated layer 11. In FIG. 2a it can be seen how the wire 2 has an insulating layer 3, and how the wire laps in the coil 1 have been compressed so that the shape of each inner wire lap is hexagonal, filling substantially all space, so that voids are reduced substantially. FIG. 2a further shows how the external wire layer of the coil is formed after the desired toroidal shape of the total coil so that the external wire layer follows the smooth toroidal torus shape of the coil 1. FIG. 2b shows an enlarged view of the cross sectional view of FIG. 2a showing the strands 4 of the wire 2. The strands 4 of the wire 2 are coated with a thin layer 5 of e.g. a polymer or resin to insulate the strands from one another.

(7) FIG. 3 is a perspective view of an inductor 6 including a coil 1 according to FIG. 1 and FIG. 2a, b, integrated in a core 7 according to the present invention. The ends 8, 9 of the wire that is wound to the coil 1 can be seen. These ends 8, 9 are used for connecting the inductor during operation of the inductor. The core 7 has a surface that is formed to a heat sink 10, to increase the surface are and thereby increase the heat sinking capabilities of the inductor. It is also visible in FIG. 3 that the distance from the coil is not centred in the core, seen in a cross section of the core. The distance D2 of core material from the coil to its central end is longer than the distance D1 from the coil to the peripheral edge of the core. Thereby substantially the same volume of core material is present on the centre side of the coil as on the outside of the coil (away from the central axis of the inductor).

(8) The invention will now be described in detail to explain the function of the optimal inductor design.

(9) Coil

(10) The coil comprises of separately insulated strands of e.g. copper or aluminium. The electrical insulation on each strand is very thin compared to the total cross-sectional area of the strand and can consist of for example a thin polymer coating. This enables a high fill factor of conducting material while maintaining low skin effect losses at high frequencies.

(11) The strands, put together, will form a wire. The wire can consist of one strand or many strands depending on, inter alia, the total current and its frequency content. With smaller diameter strands the skin effect related losses and the proximity effect losses will be reduced.

(12) By putting all strands in parallel and then twisting the package with approximately one complete turn (360 degrees, 90) per coil the proximity effect will be substantially reduced. However when the strands are turned too much that will negatively affect the wire's fill factor and create possible damages to the insulation coating in cases where pressure is applied to the coil.

(13) An electrically insulating layer must be attached around each complete wire. The insulating layer on the wire must be tough enough to withstand mechanical pressures as will be the result when the wire is wound to form a multi-turn, torus shaped, coil. This material prevents dielectric short circuiting between wires and prevents capacitive leakage from wire to wire. To further extend the properties of the coil, especially the heat conduction and the conducting materials fill factor, the coil can be compressed. By using one or more semi-cured resin layers on the strand insulation, it is possible to cure the resin in the coil forming tool and subsequently maintain the optimal shape of the coil after de-moulding it from the tool. The coil is heated, e.g. by running a high current through the coil, so that semi-cured resin will flow into air cavities between strands and wires, enhancing heat conductivity and dielectric and capacitive leakage properties.

(14) A further third insulation layer 11 is also attached to the exterior of the coil to insulate the coil from the outside environment, in this embodiment a moulded core. This ensures that the insulating layer is covering all of the coil, a resin is used in the insulating layer. The resin will also make the outside surface of the coil smooth, following the torus shape of the coil and adapting well with its magnetic field, thereby avoiding hotspots.

(15) Soft Magnetic Core

(16) The soft magnetic core that is moulded around the coil is also essentially torus shaped. The shape of the core can also be equipped with e.g. mounting holes and heat flanges, see FIG. 3.

(17) The essentially torus shape of the core has the benefit from existing technologies of optimally utilizing the exact amount of core material, removing any unnecessary excess material which is not necessary/needed for the coils flux path and the optimal function of the inductor. This reduces material costs as well as the weight and size needed for the inductor.

(18) The permeability of the SM2C can be adjusted to adapt to the design. By running current through the coil, during the moulding and hardening phase of the material, it is possible to enhance its permeability by 10-15%. The H-field of the coil then optimally aligns the surrounding powder particles in the same or similar direction as the flux path of each individual unit. Maintaining the current during hardening ensures that the particles maintain their altered and optimized position. This creates an easier path for the flux to run through which increases the inductance and decreases the inductors losses.

(19) The core would preferably be placed in an axially symmetrical fashion so that the area of the core material, perpendicular to the flux lines, is more or less the same in all parts of the inductor.

(20) The particle size distribution is chosen to provide a good packing of the powder in combination with optimized static and dynamic magnetic properties.

(21) To avoid particle-to-particle electrical conduction in the core, the particles are coated with a thin insulating layer before the moulding process. The insulating layer may e.g. be made of ceramic Nano-particles, which enhances the bulk resistivity of the moulded core and thus reduces the high frequency induced eddy currents.