WO2014183986A1

WO2014183986A1 - Distributed air gap inductor

Info

Publication number: WO2014183986A1
Application number: PCT/EP2014/058641
Authority: WO
Inventors: Bernd Ackermann; Reinhold Elferich; David Llewellyn JOHN
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-05-17
Filing date: 2014-04-29
Publication date: 2014-11-20

Abstract

The proposed invention relates to a distributed air gap inductor at least comprising a magnetic core formed of one or several first portions (7) of a high magnetically permeable material and at least one second portion (8) of a low magnetically permeable material, said second portion (8) being at least partly surrounded by one or several windings (5) of an electrical conductor. The second portion (8) of the proposed distributed air gap inductor is made of a material of small electrical conductivity and is designed to form a bobbin carrying the one or several windings (5) of the electrical conductor. The proposed distributed air gap inductor has a compact design and can be manufactured at low costs.

Description

Distributed air gap inductor

BACKGROUND OF THE INVENTION

The present invention relates to a distributed air gap inductor at least comprising a magnetic core formed of one or several first portions of a high magnetically permeable material and at least one second portion of a low magnetically permeable material, said second portion being at least partly surrounded by one or several windings of an electrical conductor. Inductors are used in a large variety of applications in electronic circuits, in particular as transformers, e.g. in switch-mode converters. This also applies to the proposed inductor according to the invention.

Conventional inductors comprise a magnetic core, an electrical conductor like a copper wire wound around a portion of the magnetic core, and a bobbin for carrying the windings of the electrical conductor. Magnetic cores are available in a wide variety of shapes, sizes and magnetic materials. They usually consist of two core halves separated by air gaps. In magnetic cores comprising two outer legs and a center leg, the air gaps in the outer legs are negligibly small while the air gap in the center leg is intentionally made larger. This large air gap is usually referred to as a concentrated air gap. The electrical conductor is usually made from copper or aluminum and wound on the bobbin made from plastic. Solid wires, litz wires and foils are used for the electrical conductor. The bobbin comprises also metal pins to which the terminals of the electrical conductor are soldered. These metal pins are used to mount the inductor on a printed circuit board via surface-mount or through-hole connection. Bobbins are also available in a wide variety of shapes and sizes.

Due to the large air gap in the center leg fringing flux is created that extends to the electrical conductor and produces eddy current losses in the winding. The losses can be reduced by using low permeability cores or distributed air gaps. Inductors with a low permeability core made from plastic ferrite are known in the art. Using this low magnetically permeable material only for the center leg of the magnetic core of an inductor whilst the remainder of the core is still made from high permeability material represents one form of a distributed air gap inductor, in which the term "air gap" is not used to indicate a literal air gap; rather, the "distributed air gap" is represented by the portion of low magnetically permeable material. Another manner of forming a distributed air gap inductor is to provide discrete air gaps, i.e. several small air gaps between individual parts of the center leg and/or between the center leg and the remaining portions of the magnetic core instead of one single large air gap. In any case, the bobbin and the electrical conductor are placed around the leg or portion comprising the concentrated or distributed air gap.

US 5,533,249 A discloses an inductor having an inner pole and an outer pole formed of magnetic steel, wherein the bobbin carrying the solenoid windings is mounted or molded over the inner pole core. In order to increase the cross section of the magnetic circuit, the bobbin is made of an insulated magnetic material contributing to the magnetic circuit. This prior art document, does not deal with inductors having distributed air gaps.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a distributed air gap inductor which has a compact design and can be manufactured with reduced costs.

The object is achieved with the distributed air gap inductor according to claim 1. Advantageous embodiments of the proposed inductor are subject matter of the dependent claims or are described in the subsequent portions of the description and preferred embodiments.

The proposed distributed air gap inductor at least comprises a magnetic core formed of one or several first portions of a high magnetically permeable material and at least one second portion of a low magnetically permeable material, said second portion being at least partly surrounded by one or several windings of an electrical conductor. The electrical conductor may be a metallic solid wire, for example made of copper or aluminum, but may also be a litz wire or foil and/or made of other electrically conductive materials. In the proposed distributed air gap inductor the second portion is made of a material of small electrical conductivity and designed to form a bobbin carrying the one or several windings of the electrical conductor. Since the second portion of the magnetic core thus directly carries the one or several windings of the electrical conductor, there is no need for a separate bobbin carrying this component. The second portion of the magnetic core thus takes over all functions of a conventional bobbin so that the latter is no longer required. This results in considerable savings of manufacturing cost, material costs, size and weight of the distributed air gap inductor. Such an inductor may form part of a transformer and can for example be used in switch-mode power supplies or LED drivers in which components of low cost are essential. In the present description and claims the high magnetically permeable material represents a magnetic material having a relative magnetic permeability μ_Γ of preferably > 200. The term low magnetically permeable material means a soft magnetic material with a relative magnetic permeability μ_Γ of preferably < 100. The small electrical conductivity of the low magnetically permeable material means that the electric conductivity σ of this material is preferably < 0.01 S/m. The terms first portion and second portion are used to differentiate between the portions or sections of the magnetic core which are at least partly surrounded by the windings and provide the distributed air gaps on the one hand (second portion(s)) and the remaining portions on the other hand (first portion(s)). The term distributed air gap inductor relates to inductors which comprise several small air gaps in the portions of the magnetic core at least partly surrounded by the windings or in which these portions are formed of a low magnetically permeable material, wherein the remaining portions are formed of a high magnetically permeable material.

In one embodiment of the proposed distributed air gap inductor the magnetic core comprises two outer legs and a center leg, wherein the outer legs are part of the first portion(s) and the center leg corresponds to the second portion. In another embodiment of the proposed invention, the magnetic core comprises two outer legs corresponding to the second portions and a center leg being part of the first portion(s). In the latter embodiment, the inductor comprises at least two separate windings, one on each of the outer legs. In a further embodiment the magnetic core comprises an U-shaped first portion and a second portion extending between two legs of the first portion.

In order to carry the electrical conductor, the second portion is preferably formed having one or more recesses for supporting the electrical conductor. The second portions may have one or several protrusions or protruding edges on both ends, e.g. designed in its outer shape like a conventional bobbin.

In a further embodiment, the magnetic core is formed of several individual parts which are assembled to form said core. Said individual parts are preferably cubic and/or cylindrical parts. These parts represent simple building blocks which allow realizing a large number of core variants or core shapes using a relatively small number of such building blocks. This allows a cost effective manufacturing of the magnetic core of such an inductor which does not require any grinding actions as is often the case for providing the gaps in the center leg in conventional cores.

The major parts of a conventional inductor are a magnetic core, a bobbin and a winding, usually made from a copper or aluminum wire. Of these, the magnetic core and the wire contribute to its primary electromagnetic function whilst the bobbin provides only auxiliary mechanical functions. It supports the wire and the metal pins used for mounting purposes. Shaping a low permeability leg in an appropriate way and furthermore make it from a material with a small electrical conductivity according to the present invention, this leg takes over the mechanical functions of the bobbin so that a separate bobbin is no longer needed. An example of a material for this low permeability leg or second portion of the present invention is plastic ferrite. The second portion of the present invention is uniformly made of this material having a low magnetic permeability and a low electrical conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

The proposed distributed air gap inductor is described in the following by way of examples in connection with the accompanying figures. The figures show:

Fig. 1 an example of a conventional inductor used in switch-mode converters;

Fig. 2 an example of a conventional distributed air gap inductor; Fig. 3 a first example of the proposed distributed air gap inductor;

Fig. 4 a second example of the proposed distributed air gap inductor;

Fig. 5 a third example of the proposed distributed air gap inductor; and

Fig. 6 a fourth example of the proposed distributed air gap inductor.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows the construction of a conventional inductor used in switch-mode converters. This inductor comprises a soft magnetic core 1 having outer legs 2 and a center leg 3. The center leg 3 comprises an air gap 4 and is surrounded by windings 5 of a copper wire. The windings 5 of the copper wire are carried by a plastic bobbin 6 in order to support the windings. Such an inductor is also referred to as a concentrated air gap inductor due to the large air gap 4 in the center leg 3 of the magnetic core.

In order to avoid a fringing flux that extends from the air gap to the windings, distributed air gap inductors like that of Fig. 2 may be used. This inductor provides a core having high permeability portions 7 and a low permeability portion 8. The low permeability portion 8 forms the center leg. A bobbin 6 of a plastic material surrounds the inner leg, i.e. the low permeability portion 8, and carries the winding 5 of the copper wire. Furthermore, small air gaps 4 are provided between the low permeability portion 8 and the high

permeability portions 7. The upper and lower halve of the high permeability core touches at the outer legs in order to make the corresponding air gaps negligibly small. Somewhat larger air gaps 4 can be tolerated adjacent to low permeability parts as shown in Fig. 2. Practically, these somewhat larger air gaps 4 can be used to reduce requirements on mechanical tolerances in order to reduce manufacturing costs. Nevertheless these smaller air gaps 4 between the low and high permeability portions 7, 8 can also be negligibly small as shown in the following figures.

In order to improve such a construction of a distributed air gap inductor of the prior art, the present invention proposes to form the low permeability portion of the magnetic core of an electrically non-conductive material and to design or shape this portion to fulfill also the function of a bobbin supporting the windings of the copper wire or of similar conductor forming the windings. In the embodiment of Fig. 3, the low permeability portion 8 is shaped in an appropriate way and is made from a material with a small electrical conductivity in such a way that it takes over the mechanical functions of the bobbin, i.e. keeping the winding 5 in place and carrying conductors, e.g. in form of pins (not shown in the figures) to connect the winding to the substrate on which the magnetic component is mounted. A separate bobbin is thus no longer needed. The low permeability portion 8 in this case may be made from a plastic ferrite material, for example. In the embodiment of Fig. 3, the high permeability portion 7 is made of two halves contacting at the outer legs. In this embodiment the low permeability portion 8 also touches the high permeability portions 7. Nevertheless, instead of touching, also small air gaps may be formed.

Fig. 4 shows another example of a distributed air gap inductor according to the present invention. In this embodiment, an E core is formed where both outer legs have been replaced by a low permeability distributed air gap leg with a winding placed around them. The center leg is made of a high permeability material, i.e. forms part of the high

permeability portions 7. The outer legs forming the low permeability portions 8 are made of an electrically low conducting material and shaped to carry the two windings 5 of the electrical conductor. To this end, like in the embodiment of Fig. 3, the second portions (outer legs) comprise a broad recess for supporting the windings 5. The two windings 5 on these outer legs act as independent inductors since the magnetic field of each of them will preferably pass through the high permeability center leg and not through the low permeability leg formed by the other outer leg. The high permeability portions can also be formed using two core halves instead of a single part.

The distributed air gap inductor shown in Fig. 5 comprises an U-shaped core forming the high permeability portions 7. An inner leg made of a low permeability material extends between the two outer legs of the U-shaped core. This inner leg represents the low permeability portion 8. It is made of an electrically low conducting material and shaped to carry the winding 5 of the electrical conductor.

In a further embodiment of the proposed invention the high permeability portions are formed of several individual parts, each having a simple geometrical shape like a cubic shape and/or a cylindrical shape. Fig. 6 shows such an example in which the high permeability portions 7 are formed of three brick-shaped portions I, II and III. This assembling from such simple buildings blocks has the advantage of low manufacturing costs. Furthermore, different forms of the magnetic core may be achieved by only few such building blocks of different sizes and shapes.

While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Other shapes of the magnetic core may be used to form the inductor. The material of the low permeability portion is not limited to plastic ferrite. Also other materials are possible. The second portion(s) carrying the winding preferably include electrical connections, which can electrically connect the windings to an electric circuit of an underlying carrier, e.g. a printed circuit board. Although all the figures show single windings as they are used for inductors, the kind of bobbin described in the invention can also be used for transformers comprising both a primary and one or multiple secondary winding(s). Also more complicated bobbins with e.g. two or more slots are possible, as well as bobbins comprising several separated parts. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The features of claims 4 to 9 can be freely combined with each other and with claims 1 to 3. Any reference signs in the claims should not be construed as limiting the scope of the invention. LIST OF REFERENCE SIGNS:

1 soft magnetic core

2 outer legs

3 center leg

4 air gap

5 winding

6 bobbin

7 high permeability portion

8 low permeability portion

I - III individual parts

Claims

CLAIMS:

1. A distributed air gap inductor at least comprising a magnetic core formed of one or several first portions (7) of a high magnetically permeable material and at least one second portion (8) of a low magnetically permeable material, said second portion (8) being at least partly surrounded by one or several windings (5) of an electrical conductor,

wherein said second portion (8) is made of a material of small electrical conductivity and is designed to form a bobbin carrying the one or several windings (5) of the electrical conductor.

2. The inductor according to claim 1,

wherein the magnetic core comprises two outer legs and a center leg, said outer legs being part of said one or several first portions (7) and said center leg corresponding to said second portion (8).

3. The inductor according to claim 1,

wherein the magnetic core comprises two outer legs and a center leg, said outer legs corresponding to said second portions (8) and said center leg being part of said one or several first portions (7).

4. The inductor according to claim 1,

wherein the magnetic core comprises an U-shaped core forming said one or several first portions (7) and an inner leg extending between two outer legs of said U-shaped core section, said inner leg corresponding to said second portion (8).

5. The inductor according to claim 1,

wherein said second portion (8) is formed of several subportions separated from one another by one or several air gaps.

6. The inductor according to claim 1,

wherein said second portion (8) is separated on one or both ends by an air gap from said one or several first portions (7).

7. The inductor according to claim 1,

wherein the magnetic core is formed of several individual parts (I-III), which are assembled to form said core.

8. The inductor according to claim 7,

wherein the individual parts (I-III) are cubic and/or cylindrical parts.

9. The inductor according to claim 1,

wherein the second portion (8) comprises at least one recess supporting the one or several windings (5).

10. The inductor according to claim 1,

wherein the second portion (8) is uniformly made of the low magnetically permeable material.