WO2016198278A1

WO2016198278A1 - Foil based inductor assembly made out of a single continuous foil strip

Info

Publication number: WO2016198278A1
Application number: PCT/EP2016/062216
Authority: WO
Inventors: Milosz HANDZEL; Marek Rylko; Mariusz Walczak; Marcin CIECIORKO; Dorota Kusmierczyk-Lew
Original assignee: Sma Solar Technology Ag
Priority date: 2015-06-08
Filing date: 2016-05-31
Publication date: 2016-12-15
Also published as: DE102015108969A1

Abstract

The invention relates to an inductor assembly (1) comprising: two foil based inductors (2, 3) each one comprising an electrically conductive foil (4) wound around an insulating bobbin (5, 6), wherein the two foil based inductors (2, 3) are electrically connected in series characterized in that the two foil based inductors (2, 3) are formed from a single continuous foil strip (7) of the electrically conductive foil (4), wherein the two foil based inductors (2, 3) are electrically connected in series via an electrical connection (9) between outermost layers (10) of the electrically conductive foil (4) of the foil based inductors (2, 3), and wherein each foil based inductor (2, 3) comprises an electrical terminal (8) electrically connected to an innermost layer (11) of the electrically conductive foil (4) of the foil based inductor (2, 3).

Description

FOIL BASED INDUCTOR ASSEMBLY MADE OUT OF A SINGLE

CONTINUOUS FOIL STRIP

FOIL BASED INDUCTOR ASSEMBLY MADE OUT OF A SINGLE CONTINUOUS FOIL STRIP

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an inductor assembly comprising two foil based inductors that are electrically connected in series. Particularly, the present invention relates to an inductor assembly formed from a single continuous foil strip. The invention also relates to a process of manufacturing an inductor assembly. Whereas the inductor assembly may be used in various appliances, it is particularly intended for use in transformers or EMI filters.

BACKGROUND

Within the scope of the present invention and if not explicitly stated otherwise the expression "foil based inductor" in particular is used for - and refers to - a "foil based inductor coil" without necessarily including a magnetic core. Further the expression "inductor assembly" in particular is used for - and refers to - an assembly comprising two "foil based inductor coils" without necessarily including a magnetic core.

Foil based inductors are well known in various electric applications. In a foil based inductor respective inductor windings are made of a thin electrically conductive foil instead of an electrically conductive wire as it is the case for a wire based inductor. Foil based inductors typically are advantageous compared to wire based inductors particularly within electric applications in which the inductors are operated at - or their function is related to - relatively high frequencies. This for instance often is the case for inductors used within switch mode p_ower supplies (SMPS) or electro-magnetic interference (EMI) filters. Due to the Skin-Effect foil based inductors when operated at higher frequencies typically provide a higher utilization of the conductors^' cross-section compared to wire based inductors having the same mass-related amount of conductive winding material. Therefore also the filtering performance of a foil based inductor used within an EMI filter often is advantageous compared to a wire based inductor having the same mass-related amount of conductive winding material.

Foil based inductors in general are manufactured by winding an electrically conductive foil around an insulating bobbin. In order to electrically insulate radially adjacent layers of the electrically conductive foil an insulating layer is disposed between the radially adjacent layers. This structure can be achieved by winding the insulating layer together with the electrically conductive foil around the bobbin.

Often it is the aim to maximize a number of windings of a foil based inductor - and therefore to maximize its respective inductance value - by only using a minimum amount of electrically conductive foil material. As one option this can be achieved by providing an inductor assembly having two foil based inductors which are electrically connected in series. Within the inductor assembly both foil based inductors are wound around two bobbins of substantially the same size. By winding a given amount of electrically conductive foil material on two bobbins the number of windings can be maximized compared to a structure, wherein the same amount of electrically conductive foil material is wound around a single bobbin of the same size. This is due to the fact that by winding the electrically conductive foil around the bobbin a respective winding radius is increasing with every additional winding turn. Additionally, the generated heat during an operation of the inductor assembly is advantageously spread between two coils. The inductor assembly comprising two foil based inductors that are electrically connected in series provides a larger surface area and therefore leads to an improved cooling performance compared to a single foil based inductor with the same inductance value. The improved cooling performance in particular is necessary, because the insulating layers between the radially adjacent layers of the electrically conductive foil typically increases an overall thermal resistance of each foil based inductor and therefore jeopardizes its cooling performance. That jeopardizing effect is only due to the presence of the insulating layers and therefore also occurs at a single foil based inductor. However, for an inductor assembly comprising two foil based inductors the decreased cooling performance generated by the insulating layers is at least partially compensated by the larger surface area provided by the two foil based inductors.

A particular inductor assembly 51 known from prior art is shown in Fig. 5. The inductor assembly 51 comprises two foil based inductors 52, 53, each wound around a respective insulating bobbin 55, 56. In each foil based inductor 52, 53 radially adjacent layers of an electrically conductive foil 54 are electrically insulated from each other by an insulating layer 57 disposed between them. Each foil based inductor 52, 53 is manufactured by using a separate strip 58, 59 of an electrically conductive foil 54, for instance a Copper foil or an Aluminum foil. The two foil based inductors 52, 53 are electrically connected in series via Copper wires or Copper braids 60. Each Copper braid 60 is connected to an innermost layer 63 of the electrically conductive foil 54 of the respective foil based inductor 52, 53 via a soldered joint 61. The other ends of the copper braids 60 are connected to each other via a crimped joint 64. The inductor assembly 51 has two electrical terminals 62 to an outer electric circuit. Each electrical terminal 62 is represented by a Copper Braid 60 wherein one end of each Copper Braid 60 is connected to an outermost layer 65 of the electrically conductive foil 54 of each foil based inductor 52, 53. This connection is provided via a soldered joint 61. Within the inductor assembly 51 in total five joints - four soldered joints 61 and one crimped joint 64 - are present.

By using separate strips 58, 59 of the electrically conductive foil 54 the winding process for each foil based inductor 52, 53 is easy to automatize. However, the fabrication of the electrical connections between both foil based inductors 52, 53 and to their respective terminals 62 by using soldering and crimping techniques is relatively complex and often cannot be automatized. It is typically performed in a manual and time consuming manner. In addition the Copper braids providing the series connection between both foil based inductors 52, 53 together with the crimped joint 64 has to be sufficiently insulated in a separate manner, in order not to short- circuit parts of the inductor assembly. This insulation, which requires additional material, is a significant additional effort because it is typically performed via a manual process, too. In total a manufacturing of the inductor assembly 51 is relatively work and cost intensive which in particular is disadvantageous when a high volume production is desired. Another issue arises if instead of a Copper foil an Aluminum foil is used as the electrically conductive foil 54. In this case a soldering of the Copper braid 60 to the Aluminum foil is a technically complex and costly procedure, because a certain soldering flux has to be used in order to dissolve an Aluminum- Oxide layer which normally is present on the surface of the Aluminum foil. If the oxide layer is not sufficiently removed the technical performance of the soldered joint in particular its conductivity decreases which in turn jeopardizes the electrical performance of the inductor assembly 51 and may lead to a hotspot.

An alternative inductor assembly 71 also known from prior art is shown in Fig. 6. Again the inductor assembly 71 comprises two foil based inductors 72, 73 electrically connected in series. Each foil based inductor 72, 73 is mechanically supported by an insulating bobbin 75, 76. Similar to the prior art inductor assembly 51 shown in Fig. 5 the inductor assembly 71 according to Fig. 6 is produced by two separate strips 78, 79 of an electrically conductive foil 74, one strip 78, 79 per each foil based inductor 72, 73. However, the number of electrical joints is reduced compared to the embodiment in Fig. 5. This is achieved by using a folding technique applied to one end-section of each strip 78, 79 of the electrically conductive foil 74. The folded end-section of each strip 78, 79 is located at the innermost layer 77 of each foil based inductor 72, 73 and protrudes underneath the inductor windings in an axial direction of the insulating bobbins 75, 76. The protruding end-section of each strip 78, 79 comprises an additional folding 80. The free ends of the additionally folded end-sections of each strip 78, 79 are joined together in an overlapping manner by using an ultrasonic welding process. This leads to only one joint 82 which provides the electrical connection between both foil based inductors 72, 73. The other end-section of each strip 78, 79, which belongs to an outermost layer 83 of the electrically conductive foil 74 of each foil based inductor 72, 73 is connected via a soldered joint 84 to a Copper Braid as an electrical terminal 81 to an outer circuit.

Within the inductor assembly 71 according Fig. 6 the total number of electrical joints is reduced down to three, compared to five electrical joints within the inductor assembly 51 according to Fig. 5. Therefore the work amount for fabrication of the inductor assembly 71 according Fig. 6 is reduced. Additionally, the reliability and technical performance of the inductor assembly 71 increase due to its reduced number of electrical joints.

Instead of a soldered joint the series connection of the foil based inductors 72, 73 is provided by an ultrasonically welded joint 82. The joining of the electrically conductive foils via an ultrasonic welding process is also possible if instead of a Copper Foil an Aluminum foil is used as electrically conductive foil 74. However, the folding of the end-section of each strip 78, 79 typically cannot be automatized and therefore has to be conducted manually. In addition the folded end-sections of each strip 78, 79 together with the ultrasonically welded joint 82 have to be sufficiently insulated in a separate manner, in order not to short-circuit parts of the inductor assembly. The insulation, which requires additional material, is typically performed via a manual process, too. Each manual related process is typically more work intensive and more fault- prone than an automatized process. Therefore this is still disadvantageous with regard to a high-volume production of the inductor assembly. Keeping this in mind, there is still a need to optimize an inductor assembly of the above mentioned kind in order to overcome the drawbacks.

Document JP 2013-131684 A shows a reactor comprising coils formed by winding a belt-like separator made of an insulation material and a conductor foil overlapping the separator into a roll shape. The coil components have terminals respectively connected to the winding starting side and the winding terminal side of the conductor foil. The reactor also includes magnetic cores to which the coil components are attached.

Document US 2008 / 0068120 A1 discloses an inductive element comprising at least two core- parts including a magnetically permeable material and at least one winding of an electrical conductor which can be a foil winding, a stranded wire (litz) winding or a conventional wire winding. Each core-part has an elongated center piece with an outer winding surface. At each of its longitudinal ends, the center piece has a contact element with a lateral contact surface. The winding is wound directly on the core-parts without a bobbin or the-like. The core-parts of the inductive element are arranged with their longitudinal axes essentially in parallel in a manner that the lateral contact surfaces of each contact element abut on a lateral contact surface of another core-part. Such an inductive element can be manufactured by co-axially arranging the core-parts and using them as a roll-shaft. After the windings have been applied to the core- parts, they can be rearranged, i.e. "flipped over," in a stack-like arrangement in order to form an inductive element. Although a processing of the inductive element with a foil winding using a single continuous foil strip of the electrical conductive foil directly on the core-part is possible, it leads to a plurality of kinks of the foil strip jeopardizing the local electrical resistance at the kinked area of the foil. In addition the foil guidance leads to a higher risk of electric short circuits at the abutting surface of the foil windings.

In US 4,327,311 , an improved inductor-capacitor device and a power regulating circuit incorporating said improved device is shown. An inductor-capacitor device comprises at least two strips of conducting foil which are separated by a thin layer of dielectric material. These strips are rolled together to form a coil-like unit. Electrical terminals are affixed at the start end of one foil and the finish end of the other foil. The improvement of the inductor-capacitor device includes integrally formed terminals on the conducting foil which are supported by the dielectric insulating material that separates the strips of conducting foil. Additionally the setting of the foil windings in a rigid structure is improved by interlaying between each foil strip an insulating strip of dielectric material that is coated with a high temperature thermosetting or thermoplastic bonding cement having physical properties similar to the dielectric material.

Document US 2009 / 0219126 A1 discloses an alpha-turn coil including a plurality of coil main bodies made by winding a wire rod having a desired diameter; and lead wire having a start part and a stop part formed integrally with outer peripheries of the coil main bodies, and connecting the coil main bodies to each other. The stop part of the lead wire of a forward coil main body is extended to the start part of the lead wire of a backward coil main body, and the lead wire is extended to compose the coil main body. An alpha-turn coil has a pair of spiral parts facing each other, wherein the inner peripheries of the spiral parts are connected to each other. Document JP 2014-056970 A discloses a reactor with soft magnetic cores, around which coils are wound. Each coil comprises an insulating separator and a conductive foil. For each coil a lead to an innermost layer and a lead to an outermost layer are provided. The outermost peripheral parts of the coils are at least partially made proximate.

OBJECT OF THE INVENTION It is therefore an object of the present invention to provide an inductor assembly comprising two foil based inductors that are electrically connected in series having a reduced number of electrical joints in combination with a reduced number of necessary foil kinks. The inductor assembly provides a reduced risk of internal short circuits, particularly at the abutting surfaces of the foil based inductors, and is easy to manufacture. A further object of the present invention is to provide a manufacturing process of said inductor assembly.

SOLUTION

According to the present invention, the object of the invention is solved by an inductor assembly comprising the features of the independent claim 1. Dependent claims 2 to 11 are directed to preferred embodiments of the inductor assembly according to the present invention. Claim 12 is directed to a process of manufacturing of that inductor assembly. Claims 13 to 19 are directed to preferred embodiments of the manufacturing process. DESCRIPTION OF THE INVENTION

According to the present invention an inductor assembly comprises:

two foil based inductors each one comprising an electrically conductive foil wound around an insulating bobbin,

- wherein the two foil based inductors are electrically connected in series

characterized in that

the two foil based inductors are formed from a single continuous foil strip of an electrically conductive foil

wherein the two foil based inductors are electrically connected in series via an electrical connection between outermost layers of the electrically conductive foil of the foil based inductors, and

wherein each foil based inductor comprises an electric terminal electrically connected to an innermost layer of the electrically conductive foil of the foil based inductor.

Because both foil based inductors of the inductor assembly according to the invention are formed from a single continuous foil strip of an electrically conductive foil it is not necessary to provide an explicit electrical connection - further called a middle joint - between them. Moreover the electrical connection of the middle joint is provided implicitly by the single continuous foil strip of the electrically conductive foil itself. In addition the insulation of the middle joint is provided by the coil construction itself and therefore no separate insulation of the middle joint is required. This is due to the fact that the conductive foil section providing the series connection between the foil based inductors is at a location, where a short circuit of parts of the inductor assembly is not possible. In particular it is precluded by its location between both foil based inductors that said conductive foil section contacts an abutting surface of the foil based inductors. Due to this, a risk of internal short circuits of the foil based inductors, particularly at their abutting surfaces, is minimized.

Because of the absence of a further potentially fault-prone joint the inductor assembly increases its reliability. It also enhances its technical performance due to the fact that each joint - as well as each foil fold or foil kink - in general comprises a region of inhomogeneity and may add a certain amount of ohmic resistance to the inductor assembly. That ohmic resistance in general is not ideally homogeneous over the joint and/or kinked area, moreover it can comprise a localized inhomogeneity generated by microscopic regions having a relatively large ohmic resistance compared to their surrounding regions. The microscopic regions with higher resistance then may act as hotspots within the total joint and/or within kink, and depending on their extent may damage or jeopardize the technical performance of the inductor assembly. Since the total number of electrical joints is reduced by elimination of the middle joint, the risk of fault prone joints is eliminated. The series connection of the foil based inductors is provided integrally by a section of the continuous foil strip itself, and particularly without the necessity of a kink or a fold at that section. The absence of kinks or folds at that section also prevents the kink- or fold-driven generation of a localized inhomogeneity with an increasedohmic resistance at that section. This also enhances the technical performance of the inductor assembly. Additionally it is not necessary to manufacture that joint which is benefiting with regard to the production of the inductor assembly. Since the manufacturing process is less work intensive, a cheaper product price can be achieved.

According to the invention the two foil based inductors are electrically connected in series via an electrical connection between outermost layers of the electrically conductive foil of the foil based inductors, and each foil based inductor comprises an electric terminal connected to an innermost layer of the electrically conductive foil of the foil based inductor. Via the electric terminal each foil based inductor can be connected to an external circuit. By using the above mentioned layers for the electrical connections it is possible to apply a substantially automatized manufacturing process, which will be explained later in more detail.

The electrically conductive foil typically is a metal foil for instance a Copper or an Aluminum foil having a thickness in the range of 50 to 1000 μηη. The electric terminal electrically connected to an innermost layer of the electrically conductive foil of the foil based inductor can comprise a section - in particular an end-section - of the continuous foil strip of the electrically conductive foil. In that case the end-section is folded in order to protrude out of the foil based inductor, preferably in an axial direction. This means that the axial direction is oriented substantially perpendicular to the winding orientation of the continuous foil strip. In this case the electrical connection between the terminal and the innermost layer of the foil based inductor is comprised by the electrically conductive foil without an explicit or separate electrical joint. In other words the end-section of the electrically conductive foil can be used as electric terminals itself and can be connected to an external circuit. Alternatively or additionally to the folded end-sections it is possible to electrically connect a wire based terminal - for instance a metal wire or a metal braid comprising Copper or Aluminum - to the innermost layer of the foil based inductor. The metal wire or the metal braid is configured to be joined to the innermost layer of the foil based inductor by one of an ultrasonic welding process, a soldering process, a laser assisted welding process and a crimping process or a combination thereof. Because for each foil based inductor the respective joint is made to its innermost layer, both joints are significantly secured against damaging influences resulting from outside. In that construction the inductor windings following in a radially outward direction are covering and therefore protecting the joint. This in particular is not the case, if the respective joints are connected to the radially outermost layer of the electrically conductive foil.

In a preferred embodiment each foil based inductor of the inductor assembly comprises an insulating layer disposed between its radially adjacent layers of the electrically conductive foil. The insulating layer can be made of paper or of a synthetic material, for instance polyethylene terephthalate (PET) or polyester (for instance Mylar®). Alternatively also an aramid paper (for instance Nomex®) can be used as the insulating layer. In order to prevent a direct contact between radially adjacent layers at their edges, the width of the insulating layer can be larger than the width of the continuous foil strip of the electrically conductive foil. This ensures an electric insulation between the radially adjacent layers. Within the scope of the invention it is also possible that the electrically conductive foil comprises a surface coating, for instance a surface coating by an insulating lacquer. In this case the insulating layer can be comprised by the insulating surface coating itself and therefore a separate insulating layer is not necessarily needed.

In a preferred embodiment the insulating bobbins supporting the windings of the foil based inductors comprise assembly means in order to pre-assemble the bobbins together in a detachable manner. Due to that preassembly it is ensured that although both foil based inductors are not mounted on a magnetic core, a fixation between them is provided via the assembly means on the insulating bobbins that prevents an unwinding of the previously wound foil based inductors. Sufficient assembly means providing that function are commonly known and therefore do not need to be explained in here in detail.

In its final application the inductor assembly often may be mounted on a magnetic core in such a manner that the foil based inductors are located on different legs of the magnetic core. Preferably the different legs of the magnetic core are oriented in parallel to each other. This often is the case if the inductor assembly is used within an EMI filter, for instance an EMI filter such as the one disclosed within the yet unpublished EP patent application EP2014173136.4. Depending on desired directions of the magnetic fluxes within the different legs of the magnetic core, it is possible that the foil based inductors either comprise an equal or an opposite winding orientation relative to each other from radially inward to radially outward or vice versa. In this meaning the winding orientation is to be understood in particular as a clockwise or a counterclockwise winding orientation from radially inward to radially outward and is to be registered in particular when looking at an abutting surface of the foil based inductors in direction of their winding axis. In a preferred embodiment, also - but not solely - used within an EMI filter application, both foil based inductors comprise substantially the same number of windings in order to generate a substantially equal magnetic flux amount in the two legs of the magnetic core, presumed that both legs are substantially identical regarding their material and dimensions.

A process of manufacturing an inductor assembly according to the invention comprises the steps:

- providing a continuous foil strip of an electrically conductive foil, a first insulating bobbin and a second insulating bobbin

winding the continuous foil strip of the electrically conductive foil up to a length L on the first insulating bobbin

unwinding a length L_part with L_part < L of the continuous foil strip of the electrically conductive foil from the first insulating bobbin, and

winding the unwound part of the continuous foil strip of the electrically conductive foil on the second insulating bobbin.

By using the above mentioned steps the process of manufacturing the inductor assembly according to the invention can be conducted in a substantially fully automatized manner which reduces operator related faults that otherwise easily would result, i.e. when using a manual process. A resulting process capability typically is much greater for an automatized process than for a manual process. In addition the time for manufacturing the inductor assembly can significantly be reduced. This in turn leads to lower production costs and therefore also lower product costs. The continuous foil strip of an electrically conductive foil can be provided at an automatic winding machine from a decoiler as storage medium on which a large amount, in particular a large length of the continuous foil strip of the electrically conductive foil is stored. By providing the continuous foil strip on the decoiler as storage medium a cutting of the continuous foil strip is conducted between the winding and the unwinding of the continuous foil strip. However, it is also possible that the continuous foils strip of the electrically conductive foil is precut to length L prior to its winding on the first bobbin. The first insulating bobbin and the second insulating bobbin are placed on different driving axles of the winding machine. An end section of the continuous foil strip of the electrically conductive foil is fixed on the first bobbin. The winding machine rotates the first bobbin and the continuous foil strip is wound around that first bobbin. When a certain number of windings is achieved and/or a desired strip length is reached the winding machine stops the rotation of the first bobbin. Preferably at this stage of the process the continuous foil strip of the electrically conductive foil is cut to the length L, if this is not already done prior to the winding on the first bobbin. The remaining free end-section of the electrically conductive foil wound around the first bobbin then is fixed on the second bobbin. After fixing that free end-section on the second bobbin, the winding machine rotates the second bobbin. This leads to a winding of the continuous foil strip of the electrically conductive foil on the second bobbin.

Preferably the step of winding of the continuous foil strip on the second insulating bobbin and the step of unwinding of the continuous foil strip from the first insulating bobbin are conducted substantially simultaneously to each other. However, within the scope of the invention it is also possible that the operations of winding the continuous foil strip on the second insulating bobbin and unwinding the continuous foil strip from the first insulating bobbin are conducted sequentially one after the other. Although all winding and unwinding operations preferably are conducted on the same automatic winding machine this is not a mandatory requirement. Within the scope of the invention it is also possible, that the winding of the continuous foils strip on the first insulating bobbin and the winding of the continuous foil strip on the second insulating bobbin are conducted on different automatic winding machines.

The continuous foil strip can be cut to length during the process of winding and unwinding. In this case the cutting to length L is preferably performed after the winding of the continuous foil strip on the first bobbin and prior to its unwinding from the first bobbin. However, within the scope of the invention it is also possible that the cutting to length is performed prior to the winding of the continuous foil strip on the first bobbin. Then the winding and the unwinding is performed using a separate piece of the continuous foil strip, which prior to winding may also be preassembled with electric terminals at both of its end-sections. Within the process of manufacturing an inductor assembly the electrically conductive foil can be a metal foil with a thickness in the range of 50 μηη to 1000 μηη, for instance a Copper foil or an Aluminum foil.

In a preferred embodiment of the manufacturing process according to the invention an insulating layer is wound and unwound on the first and the second insulating bobbin together with the continuous foil strip in order to insulate radially adjacent layers of the continuous foil strip on the first as well as on the second bobbin. During winding on and unwinding from the first insulating bobbin the insulating layer is oriented substantially parallel to the continuous foil strip of the electrically conductive foil. The material of the insulating layer can be paper or a synthetic material, for instance polyethylene terephthalate (PET), polyester (for instance Mylar®) or aramid paper (for instance Nomex®). The insulating layer can be supplied in form of a continuous strip that is stored on a decoiler as storage medium at the automatic winding machine. However, it is also possible to provide the insulating layer in separate pieces which are cut to length prior to their winding on the first insulating bobbin. Regardless of whether the insulating layer is supplied in form of a continuous strip or in form of separate pieces that are cut to length prior to their winding the insulating layer can be also self-adhesive. The cutting of the insulating layer is similar to the cutting of the continuous foil strip of the electrically conductive foil and therefore similar process features and process timings as specified above for the continuous foil strip of the electrically conductive foil can be applied to the insulating layer, too. In one embodiment the process of manufacturing an inductor assembly according to the present invention comprises folding a section - preferably an end-section - of the continuous foil strip prior to winding it on the first insulating bobbin. The folded end-section can be used for fixing the continuous foil strip on the first insulating bobbin. However, alternatively or additionally it also can be used at least as a part of the electrical terminal of the inductor assembly. The folding operation is performed at least at one end-section of the continuous foil strip of the electrically conductive foil prior to its winding on the first insulating bobbin. After that winding on and prior to its unwinding from the first insulating bobbin, but after the cutting, the other end-section of the continuous foil strip of the electrically conductive foil can be folded, too. Similar to the first folded end-section the second folded end-section can also be used just for fixing the continuous foil strip on the second insulating bobbin. However, alternatively or additionally it is possible to use the second folded end-section at least partly in order to form the other electrical terminal. In particular in case the continuous foil strip of the electrically conductive foil is cut to length prior to its winding on the first insulating bobbin, both folding operations can be applied to both end- sections prior to winding it on the first insulating bobbin.

Alternatively or additionally to the folding of the end-sections of the continuous foil strip of an electrically conductive foil the process of manufacturing the inductor assembly according to the invention can comprise the step of joining a metal braid, preferably a Copper braid or an Aluminum braid to the continuous foil strip prior to winding it on the first insulating bobbin. The joining of the metal braid can be performed by one of an ultrasonic welding process, a soldering process, a laser assisted welding process and a crimping process or a combination thereof.

The winding operation on the first insulating bobbin and the winding operation on the second insulating bobbin can comprise an equal or an opposite rotation direction. The rotation direction in particular can be clockwise or counter-clockwise, when looking on the first and the second insulating bobbins as they are mounted on and rotated by the automatic winding machine.

According to the process of manufacturing an inductor assembly a length L_part of that part of the continuous foil strip that is unwound from the first insulating bobbin is selected to be

substantially half of the length L of the continuous foil strip that was wound on the first insulating bobbin. This ensures that both foil based inductors comprise substantially the same winding numbers.

Advantageous developments of the invention result from the claims, the description and the drawings. The advantages of features and of combinations of a plurality of features mentioned at the beginning of the description only serve as examples and may be used alternatively or cumulatively without the necessity of embodiments according to the invention having to obtain these advantages. Without changing the scope of protection as defined by the enclosed claims, the following applies with respect to the disclosure of the original application and the patent: further features may be taken from the drawings, in particular from the illustrated designs and the dimensions of a plurality of components with respect to one another as well as from their relative arrangement and their operative connection. The combination of features of different embodiments of the invention or of features of different claims independent of the chosen references of the claims is also possible, and it is motivated herewith. This also relates to features which are illustrated in separate drawings, or which are mentioned when describing them. These features may also be combined with features of different claims. Furthermore, it is possible that further embodiments of the invention do not have the features mentioned in the claims.

The number of the features mentioned in the claims and in the description is to be understood to cover this exact number and a greater number than the mentioned number without having to explicitly use the adverb "at least". For example, if a plurality of conductor windings is mentioned, this is to be understood such that there is exactly one plurality of conductor windings or there are two pluralities of conductor windings or more pluralities of conductor windings. Additional features may be added to these features, or these features may be the only features of the respective product. The reference signs contained in the claims are not limiting the extent of the matter protected by the claims. Their sole function is to make the claims easier to understand.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is further explained and described with respect to embodiments illustrated in the drawings.

Fig. 1 illustrates a first embodiment of an inductor assembly according to the present invention from a side view

Fig. 2 illustrates the first embodiment of an inductor assembly according to the present invention from a top view.

Fig. 3a, 3b illustrate different embodiments of a continuous foil strip, which is cut to length and preassembled prior to its winding on the first insulating bobbin.

Fig. 4a, 4b, 4c illustrates a manufacturing process of the inductor assembly according to the present invention by using an automatic winding machine.

Fig. 5 illustrates an embodiment of an inductor assembly known from prior art having two foil based inductors connected in series; Fig. 6 illustrates an alternative embodiment of an inductor assembly known from prior art.

DESCRIPTION OF THE DRAWINGS

Fig. 1 and Fig. 2 are illustrating a first embodiment of an inductor assembly 1 according to the present invention in a side view (Fig. 1 ) and in a top view (Fig. 2). The inductor assembly 1 comprises two foil based inductors 2, 3 wherein each foil based inductor 2, 3 has an electrically conductive foil 4 that is wound on an insulating bobbin 5, 6 in form of a plurality of layers. The two foil based inductors 2, 3 are formed from a single continuous foil strip 7 of the electrically conductive foil and are electrically connected in series via their outermost layers 10 of the electrically conductive foil 4. The innermost layer 11 of each foil based inductor 2, 3 is electrically connected to an electrical terminal 8, which is used to connect the inductor assembly 1 to an external circuit. Within the illustrated embodiment, the electrical terminal 8 for each foil based inductor 2, 3 comprises a folded end-section of the electrically conductive foil 4. The folding is performed such that the electrical terminal 8 protrudes out of the windings in an axial direction of the insulating bobbin 5, 6, which in the illustrated view is perpendicular to the paper plane. By providing that construction the electrical terminal 8 is easily accessible in order to connect it to an external circuit. Alternatively or additionally to folding an end-section of the electrically conductive foil 4 it is also possible to join a metal braid - for instance a Copper braid or an Aluminum braid - via one of an ultrasonic welding, a soldering process, a laser assisted welding process and a crimping process or a combination thereof to the end-section of the electrically conductive foil 4 in order to provide an electrical terminal 8 of the inductor assembly 1.

In order to insulate radially adjacent layers of the electrically conductive foil 4 from each other an insulating layer out of polyethylene terephthalate (PET), polyester (for instance Mylar®) or an aramid paper (for instance Nomex®) is disposed between them. The insulating layer is slightly wider than the continuous foil strip 7 of the electrically conductive foil 4 in order to prevent a contact - and therefore a short circuit - of the radially adjacent layers of the electrically conductive foil 4 at their edges. Alternatively and / or additionally it is also possible to insulate radially adjacent layers of the continuous foil strip 7 of the electrically conductive foil 4 via an insulating surface coating provided on the surface of the electrically conductive foil 4. The electrical connection 9 between both foil based inductors 2, 3 is formed by the electrically conductive foil 4 itself. In particular the electrically conductive foil 4 connects the outer layer 10 of the electrically conductive foil 4 of the first foil based inductor 2 to the outer layer 10 of the electrically conductive foil 4 of the second foil based inductor 3. This is due to the fact that both foil based inductors 2, 3 of the inductor assembly 1 are formed from the same single continuous foil strip 7. Therefore no additional or separate joint is needed in order to electrically connect both foil based inductors 2, 3 to each other. Because the electrical connection 9 between both foil based inductors 2, 3 is formed by the electrically conductive foil 4 itself, there is no difference between electrical and physical properties of the electrical conductors and the electrical connection 9. In other words, the electrical connection 9 within the disclosed inductor assembly 1 does not lead to disadvantageous effects resulting from an inhomogeneity within the joint like this is typically the case for a separate electrical connection between two conductors as known from prior art. Within a separate electrical connection different materials are in a close contact and fixed relative to each other, which fixing in combination with external influences - for instance temperature - generates mechanical stress. All these influences are jeopardizing electrical performance and lifetime of the separate electrical connection. Since for the inductor assembly 1 according to the present invention electrical connections provided by explicit and separate manufactured joints are minimized in number, electrical performance and lifetime of the inductor assembly 1 significantly increases. In the illustrated embodiment both foil based inductors 2, 3 comprise an equal winding orientation when starting from an innermost layer 11 in a radially outward direction, which here is represented by a counter-clockwise direction. However, within the scope of the invention it is also possible that the first foil based inductor 2 comprises an opposite winding orientation relative to the second foil based inductor 3. As illustrated both foil based inductors 2, 3 also comprise a substantially equal number of windings. However, it is also possible that the first foil based inductor 2 comprises a different number of windings compared to the second foil based inductor 3. The winding orientation and/or the number of windings of each foil based inductor 2, 3 substantially depend on a desired magnetic flux of each foil based inductor 2, 3 within its later application. The first foil based inductor 2 is detachably assembled with - and therefore fixed to - the second foil based inductor 3 via detachable assembly means 13 provided at their insulating bobbins 5, 6. By fixing both foil based inductors 2, 3 relative to each other a decoiling of each foil based inductor 2, 3 is prevented. Additionally the foil based inductors 2, 3 can be easier assembled with a magnetic core not shown, when fixed to each other. Within the assembly with a magnetic core each foil based inductor 2, 3 is located on different legs of the magnetic core. The magnetic core can be of any commonly known shape, for instance a C-l shape, an E-l shape or a more complex shape like it is disclosed within the yet unpublished European patent application EP2014173136.4.

In Fig. 3a and Fig. 3b two different embodiments of a continuous foil strip 7 of an electrically conductive foil 4 together with an insulating layer 12 are illustrated in a preassembled status. In the preassembled status the continuous foil strip 7 of electrically conductive foil 4 is cut to a length L-i prior to its winding on the insulating bobbin 5, 6. Also the insulating layer 12 is cut to a desired length prior to its winding on the insulating bobbin. The length L-i can have any value depending on the application of the inductor assembly 1. This is illustrated in Fig. 3a and Fig. 3b by an interrupted middle section.

In the first embodiment shown in Fig. 3a the continuous foil strip 7 of the electrically conductive foil 4 comprises two electrical terminals 8 which are provided by folding a length s of both end- sections such that they are oriented perpendicular to a length direction "x" of the continuous foil strip 7. Via that folding it is achieved that each end-section of the electrically conductive foil 4 protrudes underneath the windings in an axial direction of the insulating bobbin 5, 6 after the winding process. The insulating layer 12 in the first embodiment may be a paper layer which - regarding its width - is somewhat larger than the electrically conductive foil 4. It is oriented parallel to the electrically conductive foil 4. It is possible to fix the insulating layer 12 to the electrically conductive foil 4 - for instance by gluing - in order to prevent a location mismatch during the winding and handling operation and to ease the manufacturing process. It is sufficient to provide the insulating layer on one side of the electrically conductive foil 4, only, since this is sufficient for preventing a contact and therefore a short circuit of radially adjacent layers of the foil based inductor 2, 3 after the winding process. However, within the scope of the invention it is also possible to provide an insulating layer 12 on both sides of the electrically conductive foil 4. Alternatively or additionally to a separate paper layer, it is also possible that the insulating layer 12 is provided by a coating layer - for instance an insulating lacquer - on the surface of the electrically conductive foil 4. Th e second embodiment shown in Fig. 3b is similar to the first embodiment shown in Fig. 3a. In difference to the first embodiment the second embodiment comprises an electrical terminal 8 at each end-section of the continuous foil strip 7 that comprises a metal braid or a metal wire of substantially a length s. The metal braid or the metal wire can be made of Copper or Aluminum and can be joined to the electrically conductive foil 4 by one of an ultrasonic welding process, a soldering process, a laser assisted welding process, a crimping process or a combination thereof.

Fig. 4a and Fig. 4b the manufacturing process of the inductor assembly 1 according to the present invention is schematically shown in two subsequent process situations. Whereas the process starts with the situation illustrated in Fig. 4a, a later process situation is shown in Fig. 4b. The manufacturing process is explained in the following by way of example by using a large length of a continuous foil strip 7 of an electrically conductive foil 4 coiled up on a decoiler 41 used as a storage medium. This is to be understood only exemplarily - but not limiting - since instead of a large length of the continuous foil strip 7 stored on a decoiler 41 , it is also possible to use a plurality of separate preassembled continuous foil strips 7 of an electrically conductive foil 4, which are cut-to-length and optionally prefabricated - for instance folded and / or joined with a metal braid at their end-sections - prior to the winding process.

On an automatic winding machine 40 a large length of a continuous foil strip 7 of an electrically conductive foil 4 is coiled on a first decoiler 41 which is used as a storage medium for the continuous foil strip 7. In addition to the electrically conductive foil 4 a large length of an insulating layer 12 is stored on a second decoiler 42, which also is used as a storage medium for the insulating layer 12. The first insulating bobbin 5 of the inductor assembly 1 is mounted on a driving axle 43 of the automatic winding machine 40. A first free end-section of the electrically conductive foil 4 is prefabricated - in particular folded and / or joined to a metal braid - in order to provide an electric terminal 8. In order to fold the free end-section of the electrically conductive foil 4 - and optionally also the insulating layer 12 - in a reproducible manner the automatic winding machine 40 comprises sufficient means, for instance a folding gauge 47.

After its prefabrication the first free end-section is fixed on the first insulating bobbin 5. The first insulating bobbin 5 is now rotated by the automatic winding machine 40 in order to wind a certain amount of the electrically conductive foil 4 and simultaneously the insulating layer 12 on it. This can be defined for instance via a predefined length L or a predefined number of rotation turns. The rotation of the first insulating bobbin 5 is depicted in Fig. 4a by an arrow 48 which - by way of example, only - is oriented in a clockwise direction. That rotation generates a plurality of layers of the continuous foil strip 7 of the electrically conductive foil 4 wound around the first insulating bobbin 5. The first free end-section comprising the previously fabricated electrical terminal 8 is therefore connected to an innermost layer 11 of the plurality of layers on the first insulating bobbin 5.

While rotating the first insulating bobbin 5 the electrically conductive foil 4 and also the insulating layer 12 is unwound from their decoilers 41 , 42. On their way to the first insulating bobbin 5 the continuous foil strip 7 of the electrically conductive foil 4 is aligned and assembled to the insulating layer 12 via pressing rolls 45. The alignment between both is preferably fixed via a glue applied to the surface of either the insulating layer 12 or the electrically conductive foil 4.

After the previously defined length L of the continuous foil strip 7 and / or a predefined number of turns of the first insulating bobbin 5 is achieved, the rotation of the first insulating bobbin 5 is stopped. The continuous foil strip 7 of the electrically conductive foil 4 is then cut to length. For performing the cutting operation in a reproducible manner on the electrically conductive foil 4 - and optionally simultaneously also on the insulating layer 12 - the automatic winding machine 40 can comprise a cutting device 46. That cutting operation generates a second free end- section of the continuous foil strip 7 of the electrically conductive foil 4, which previously is coiled up on the first insulating bobbin 5. Similar to the first free end-section, the second free end-section is prepared - i. e. folded and/or joined to another metal braid - in order to provide an electrical terminal 8 at the second free end-section, too.

After preparation of the second free end-section of the continuous foil strip 7 of the electrically conductive foil 4 the second free end-section is fixed on the second insulating bobbin 6, which is mounted on another driving axle 44 of the automatic winding machine 40. After fixing the second free end-section on the second insulating bobbin 6 that second insulating bobbin 6 is rotated by the automatic winding machine 40. By rotating the second insulating bobbin 6 the continuous foil strip 7 of the electrically conductive foil 4 together with the adjacent insulating layer 12 is unwound from the first insulating bobbin 5 while substantially simultaneously wound on the second insulating bobbin 6. After a previously defined length L_part of the continuous foil strip 7 wound on the second insulating bobbin 6 and / or a predefined number of turns of the second insulating bobbin 6 is achieved the rotation of the second insulating bobbin 6 is stopped. At this process situation each one of the first 5 and the second insulating bobbins 6 comprises a plurality of layers of the continuous foil strip 7 of the electrically conductive foil 4 wound around it. Similar to the first free end-section also the second free end-section comprising the previously fabricated electrical terminal 8 is connected to an innermost layer 11 of the plurality of layers on the second insulating bobbin 6.

The plurality of layers of the electrically conductive foil 4 surrounding and mechanically supported by the insulating bobbins 5, 6 are representing the foil based inductors 2, 3 of the inductor assembly 1. After the winding and unwinding operations are finished, the foil based inductors 2, 3 are demounted from the automatic winding machine 40. They are then detachably assembled to each other via the detachable assembly means 13 provided on the insulating bobbins 5, 6. Via that assembly a decoiling of the foil based inductors 2, 3 after the winding operation on the automatic winding machine 40 is prevented. Also the further assembly with a magnetic core is simplified. According to Fig. 4b an arrow 49 characterizing the rotation direction of the second insulating bobbin 6 while winding the continuous foil strip 7 is opposite to the arrow 50 depicting the rotation direction of the first insulating bobbin 5 while substantially simultaneously unwinding the continuous foil strip 7. In addition arrow 49 characterizing the rotation direction of the second insulating bobbin 6 while winding the continuous foil strip 7 is equal to the arrow 48 depicting the rotation direction of the first insulating bobbin 5 while winding the continuous foil strip 7. That embodiment of the manufacturing process leads to two foil based inductors 2, 3 comprising an equal winding orientation relative to each other from radially inward to radially outward. However, by using a slightly modified process it is also possible, to produce an inductor assembly 1 having two foil based inductors 2, 3 comprising an opposite winding orientation relative to each other from radially inward to radially outward. A respective process situation leading to an opposite winding orientation of the two foil based inductors 2, 3 from radially inward to radially outward is schematically illustrated in Fig. 4c. The difference to the process situation of Fig. 4b is within the rotation direction of the second insulating bobbin 6 while winding the continuous foil strip 7 compared to the rotation direction of the first insulating bobbin 5 while substantially simultaneously unwinding the continuous foil strip 7. In Fig. 4b the arrows 49, 50 are oppositely oriented - clockwise for the second insulating bobbin 6 vs. counter-clockwise for the first insulating bobbin 5. According to Fig. 4c these arrows 49, 50 are equally oriented - counter-clockwise for the second insulating bobbin 6 vs. counter-clockwise for the first insulating bobbin 5. In addition the arrow 49 characterizing the rotation direction of the second insulating bobbin 6 while winding the continuous foil strip 7 is oppositely oriented to the arrow 48 depicting the rotation direction of the first insulating bobbin 5 while winding the continuous foil strip 7. This embodiment of the manufacturing process leads to two foil based inductors 2, 3 comprising an opposite winding orientation relative to each other from radially inward to radially outward. Therefore in case an opposite winding orientation is intended for the two foil based inductors 2, 3 relative to each other from radially inward to radially outward the process situation according Fig. 4c has to be used instead of the process situation according to Fig. 4b. In case the continuous foil strip 7 of the electrically conductive foil 4 is coated on its surface with an insulating coating - for example with an insulating lacquer - a separate insulating layer 12 is not needed in order to insulate radially adjacent layers of the foil based inductors 2, 3. In this case the manufacturing process can be performed without an insulating layer 12. Alternatively it is also possible that the continuous foil strip 7 of the electrically conductive foil 4 is covered or wrapped with a self-adhesive insulating layer 12 prior to the winding and unwinding operation.

In Fig. 4a, 4b, 4c the insulating layer 12 is applied on one specific surface side of the continuous foil strip 7 of the electrically conductive foil 4, only. However, within the scope of the invention it is also possible to apply the insulating layer 12 on two surface sides of the continuous foil strip 7 of the electrically conductive foil 4. In particular the insulating layer 12 can be applied at a first surface side of the electrically conductive foil 4 during the step of winding the continuous foil strip 7 of the electrically conductive foil 4 up to a length L on the first insulating bobbin 5 and at an oppositely oriented second surface side during the step of winding the unwound part of the continuous foil strip 7 of the electrically conductive foil 4 on the second insulating bobbin 6. Via cutting solely the insulating layer 12 during the manufacturing process, it can be ensured that along the length L of the electrically conductive foil 4 there will be no significant overlap section at which the insulating layer 12 is present at both sides of the electrically conductive foil 4 concurrently. Furthermore the insulating layer 12 is either present at the first surface side or at the second surface side of the electrically conductive foil 4. By using that embodiment of the manufacturing process it can be ensured that each foil based inductor 2, 3 of the inductor assembly 1 is covered with the insulating layer 12 at its outer circumference regardless whether the two foil based inductors 2, 3 comprise an equal or an opposite winding orientation relative to each other. LIST OF REFERENCE NUMERALS

1 Inductor assembly

2 Foil based inductor

3 Foil based inductor

4 Electrically conductive foil

5 Insulating bobbin

6 Insulating bobbin

7 Continuous foil strip

8 Electrical terminal

9 Electrical connection

0 Outermost layer

1 Innermost layer

2 Insulating layer

3 Assembly means

X Direction

S Length

i Length

0 Winding machine

1 Decoiler

2 Decoiler

3 Driving axle

4 Driving axle

5 Pressing rolls

6 Cutting device

7 Folding gauge

8 Arrow

9 Arrow

0 Arrow 1 Inductor assembly

2 Foil based inductor

3 Foil based inductor

4 Electrically conductive foil Insulating bobbin

Insulating bobbin

Insulating layer

Strip

Copper braid

Soldered joint

Electrical terminal Innermost layer

Crimped joint

Outermost layer

1 Inductor assembly

2 Foil based inductor

3 Foil based inductor

4 Electrically conductive foil

5 Insulating bobbin

6 Insulating bobbin

7 Innermost layer

8 Strip

9 Strip

Ό Folding

Λ Electrical terminal i2 Ultrasonically welded joint

;3 Outermost layer

I4 Soldered joint

Claims

1. An inductor assembly (1 ) comprising:

- two foil based inductors (2, 3) each one comprising an electrically conductive foil (4) wound around an insulating bobbin (5, 6),

- wherein the two foil based inductors (2, 3) are electrically connected in series

characterized in that

- the two foil based inductors (2, 3) are formed from a single continuous foil strip (7) of the electrically conductive foil (4),

- wherein the two foil based inductors (2, 3) are electrically connected in series via an electrical connection (9) between outermost layers (10) of the electrically conductive foil (4) of the foil based inductors (2, 3), and

- wherein each foil based inductor (2, 3) comprises an electrical terminal (8) electrically connected to an innermost layer (11 ) of the electrically conductive foil (4) of the foil based inductor (2, 3). 2. The inductor assembly (1 ) of claim 1 , characterized in that each foil based inductor (2, 3) comprises an insulating layer (12) disposed between radially adjacent layers of the electrically conductive foil (4). 3. The inductor assembly (1 ) of claim 1 or 2, characterized in that the electrically conductive foil (4) is a foil comprising Copper or Aluminum. 4. The inductor assembly (1 ) of any of the claims 1 to 3, characterized in that the electrical terminal (8) comprises a metal wire or a metal braid comprising Copper or Aluminum joined to the innermost layer (11 ) of the foil based inductor (2, 3). 5. The inductor assembly (1 ) of claim 4, wherein the metal wire or metal braid comprising Copper or Aluminum is configured to be joined by one of an ultrasonic welding process, a soldering process, a laser assisted welding process, a crimping process or a combination thereof.

6. The inductor assembly (1 ) of any of the preceding claims, characterized in that the electrical terminal (8) comprises a folded end-section of the continuous foil strip (7) protruding out of the foil based inductor (2, 3). 7. The inductor assembly (1 ) of any of claim 1 to 6, characterized in that the insulating bobbins (5, 6) comprise detachable assembly means (13) in order to provide a fixation between them. 8. The inductor assembly (1 ) of any of claim 1 to 7, characterized in that both foil based inductors (2, 3) comprise substantially the same winding number. 9. The inductor assembly (1 ) of any of claim 1 to 8, characterized in that the foil based inductors (2, 3) comprise an equal winding orientation relative to each other from radially inward to radially outward. 10. The inductor assembly (1 ) of any of claim 1 to 8, characterized in that the foil based inductors (2, 3) comprise an opposite winding orientation relative to each other from radially inward to radially outward. 11. The inductor assembly (1 ) of any of claim 1 to 10, characterized in that the foil based inductors (2, 3) are located on different legs (16, 17) of a magnetic core (15). 12. A process of manufacturing an inductor assembly (1 ) comprising the steps:

- providing a continuous foil strip (7) of an electrically conductive foil (4), a first insulating bobbin (5) and a second insulating bobbin (6)

- winding the continuous foil strip (7) of the electrically conductive foil (4) up to a length L on the first insulating bobbin (5)

- unwinding a length L_part with L_part < L of the continuous foil strip (7) of the electrically conductive foil (4) from the first insulating bobbin (5), and

- winding the unwound part of the continuous foil strip (7) of the electrically conductive foil (4) on the second insulating bobbin (6).

The process of manufacturing an inductor assembly (1 ) according to claim 12, wherein an insulating layer (12) is wound and unwound on the insulating bobbins (5, 6) together with the continuous foil strip (7). 14. The process of manufacturing an inductor assembly (1 ) according to any of claim 12 to 13, wherein the electrically conductive foil (4) is a foil comprising Copper or Aluminum. 15. The process of manufacturing an inductor assembly (1 ) according to any of claim 12 to 14, further comprising folding a section of the continuous foil strip prior to winding it on the first insulating bobbin (5).

16. The process of manufacturing an inductor assembly (1 ) according to any of claim 12 to 15, wherein the steps of:

unwinding a length L_part with L_part < L of the continuous foil strip (7) of the electrically conductive foil (4) from the first insulating bobbin (5), and

winding the unwound part of the continuous foil strip (7) of the electrically conductive foil (4) on the second insulating bobbin (6)

are conducted substantially simultaneously. 17. The process of manufacturing an inductor assembly (1 ) according to any of claim 12 to 16, further comprising joining a metal wire or a metal braid comprising Copper or Aluminum to the continuous foil strip (7) prior to winding it on the first insulating bobbin (5) by one of an ultrasonic welding process, a soldering process, a laser assisted welding process, a crimping process or a combination thereof. 18. A process of manufacturing an inductor assembly (1 ) according to any of claim 12 to 17, wherein the winding operation on the first insulating bobbin (5) and the winding operation on the second insulating bobbin (6) comprise an equal rotation direction. 19. A process of manufacturing an inductor assembly (1 ) according to any of claim 12 to 17, wherein the winding operation on the first insulating bobbin (5) and the winding operation on the second insulating bobbin (6) comprise an opposite rotation direction. 20. A process of manufacturing an inductor assembly (1 ) according to any of claim 12 to 19, wherein the length L_part of the continuous foil strip (7) that is unwound from the first insulating bobbin (5) is selected to be substantially half of the length L of the continuous foil strip (7) that was wound on the first insulating bobbin (5).