Patent Description:
Various electrical devices, for instance Switch-Mode-Power-Supplies, are typically emitting electromagnetic radiation during their normal operation. In order to reduce that electromagnetic radiation at least down below values defined in relevant electrical standards an EMI-filter typically is implemented within the device. Thus, the EMI-filter is a necessary component to ensure an Electro-Magnetic-Compatibility (EMC) of the device during its operation. The EMI filter comprises a filter-choke which, dependent on the radiation mode to be filtered, can be configured as a differential mode filter-choke or as a common-mode filter-choke.

The filter-choke can comprise a magnetic core and at least two coils, each coil comprising an electric conductor that is arranged around a leg of the magnetic core in form of one or more windings. Since it is a frequent requirement that the magnetic core comprises a low magnetic reluctance, the core is designed as a closed magnetic core in form of a one-piece component. In that case, the coil is typically produced by manually winding the electrical wire around the magnetic core. However, the manual production process is relatively labor intensive, in particular, when an electrical wire with a large cross section area is to be used, which typically is the case when a low ohmic resistance of the electrical conductor is required. In addition, the manual production process results in a relatively poor reproducibility within multiple filter-chokes produced one after the other and theoretically comprising the same design. It also leads to an increase in component size. Therefore, it is desired to provide an optimized design for a filter-choke.

The printed document <CIT> discloses a choke comprising an iron core set, two winding frames, a partition piece, and a winding. The iron core set comprises a first iron core and a second iron core. The two ends of the first iron core and the two ends of the second iron core are connected in a separable mode to form a closed shape. Each winding frame comprises a winding frame body which is provided with an iron core passageway allowing the iron core set to penetrate through. The partition piece can be fixed to and combined with the two winding frames. The winding is wound on the two winding frames. Filter chokes involving modular bobbin assemblies that involve the stacking of at least two bobbins are known from <CIT> and <CIT>.

It is the object of the present invention to provide a filter-choke for use in an EMI filter addressing and eliminating or reducing at least one of the abovementioned drawbacks. Particularly, the filter-choke shall comprise highly reproducible magnetic properties within multiple filter-chokes manufactured according to the same design in combination with an easy way of assembly.

According to the present disclosure, the object of the invention is solved by a filter-choke comprising the features of the independent claim <NUM>. Dependent claims <NUM> to <NUM> are directed to preferred embodiments of the filter-choke according to the present invention. Claim <NUM> is directed to an electrical device comprising the filter-choke of the present invention. Claims <NUM> is directed to a production method of the filter-choke.

According to the present invention, a filter-choke to be used in an EMI filter comprises:.

It goes without saying that the opening of the tubular section - particularly the opening of each tubular section - is also present at a respective position within the base flange, such that a continuous through hole in an axial direction of each tubular section is provided, which not only is present within the tubular section, but also penetrates the base flange. The filter choke may as well comprise more than two bobbins, for instance three or even more bobbins, which are arranged in a stacked manner one above the other and wherein adjacent bobbins are releasably fixed to each other.

The feature that one of the two core-legs is extending through the coaxially arranged openings of the stacked bobbins can imply, that a single core-segment also penetrates these openings. However, this is only one option for realization of the respective feature. In an alternative embodiment it is also possible that in an assembled sate of the filter-choke a gap between two core-segments forming the magnetic-core is arranged between the two bobbins. In that embodiment each one of both core-segments only penetrate a single one of the openings. In a further alternative embodiment, it is also possible, that in an assembled sate of the filter-choke one and/or two of the core-segments only partially penetrate(s) an opening, such that a gap formed between them is localized inside the tubular section of a particular bobbin.

The at least two bobbins are typically made out of an insulating material, e.g. made out of a synthetic resin material. The production process of the bobbins can comprise an injection molding process. Due to the injection molding process, small tolerances regarding the dimensions of the bobbins can be ensured, that is advantageous with regard to the reproducibility of the magnetic properties of the filter-choke. The magnetic properties of the filter-choke can be varied by changing the number of bobbins - and therefore by changing the number of pre-wound coils - actually present in the filter-choke in a modular manner. In particular, if a filter-choke requires an increased inductance value, the number of stacked bobbins - and also the number of pre-wound coils - can be increased. Although it is not necessary that the at least two bobbins comprise substantially the same design, it is however advantageous, because then only a single injection mold is necessary keeping the investment at a relatively low level. However, in some application it can be advantageous to use different designs of the at least two bobbins. In this case, the different bobbin designs, in particular, can be different regarding a length of their tubular sections from bobbin to bobbin, such that pre-wound coils with different winding numbers can be placed on the different tubular sections. However, it is advantageous to keep the lateral dimensions of the bobbin and specifically their base flanges constant.

Since the closed magnetic core is configured to be assembled out of at least two core segments, a labor-intensive manual production method isn't required any more. Thus, according to the invention - and due to the design of the filter-choke - a production method of the filter-choke comprises the following acts:.

Due to the design of the filter-choke a highly automatized production method can be used. In particular, an automatic winding technology can be used for winding the electrical conductor, which is advantageous for producing the coils in a highly reproducible and cost-efficient manner. This is particularly advantageous if the electric conductor comprises a large cross section, which manually would be very difficult and irreproducible to wind. Since the winding of the coils is done prior to its placement on the bobbins and on the core-legs, a mechanical stress and its jeopardizing effects implied on the magnetic core during winding - typically present when using a manual winding process - can be eliminated. Although the magnetic core is configured to be assembled out of at least two core segments and therefore comprises at least one gap, the detrimental effect of the gap on the magnetic properties of the assembled magnetic core (here: an increase of the magnetic reluctance of the magnetic core) can be reduced by designing the core segments such that they are providing a bypass path for the magnetic flux around the at least one gap in an assembled state of the magnetic core. This will be explained in more detail in <FIG> and <FIG>.

In a preferred embodiment of the filter-choke, each bobbin comprises two tubular sections extending in perpendicular direction from the base flange, wherein each of the tubular section contains an opening for receiving a different one of the two core legs. In this case a coil formed by an electric conductor and comprising multiple windings is arranged around each tubular section of each bobbin. The two tubular sections can comprise axial directions, which are oriented substantially parallel relative to each other. This specifically can be the case if the magnetic core comprises a closed rectangular design with parallel oriented core legs on opposing sites of the magnetic core. It is also possible that the magnetic core comprises a toroidal form in its assembled state.

In one embodiment, each bobbin of the filter-choke can contain guiding elements arranged on a bottom surface and/or on a top surface of its base flange. The guiding elements can be configured to position and/or align at least one terminal - or each terminal - of the pre-wound coils formed by the electric conductor and arranged on the tubular section in an assembled state of the filter-choke. Preferably, each bobbin may contain guiding elements such that each terminal of each coil present in the assembled filter-choke is aligned relatively to each other and arranged at predefined positions relative to the assembled filter-choke. An easy assembly of the filter-choke with other electrical components in an electric device, in particular an assembly of the filter-choke on a Printed-Circuit-Board (PCB), can be supported by the guiding elements.

According the invention, the at least two first fitting elements of the first bobbin are configured such that they are forming a snap fitting with the respective first fitting elements of the adjacent second bobbin in an assembled state of the filter-choke. Each first fitting element of the first bobbin - and also of the second bobbin - contains a part and a corresponding counterpart of the snap fitting. The snap fitting can comprise a hook/loop design wherein a "hook" as the part engages with "a loop" as the corresponding counterpart of the snap fitting. For each first fitting element the snap fitting part and the snap fitting counterpart are designed such that they are extending perpendicularly relative to the base flange and in opposite axial directions relative to each other from the edge of the base flange. In a preferred embodiment of the filter-choke, the at least two bobbins can be designed substantially identical to each other by providing such designed parts and counterparts within each first fitting element of the bobbins. By designing the at least two bobbins substantially identical to each other, the investment in the required injection molds for producing the bobbins can be minimized. This even is the case if the at least two bobbins are differing only in the length of their respective tubular sections. Therefore, the expression "substantially identical" is meant here to also cover a design of the bobbins, which is identical for the at least two bobbins apart from different lengths of their tubular sections.

In one embodiment of the filter-choke, the base flange of the bobbins can comprise two second fitting elements for releasably engaging with corresponding fitting elements of a core clip, wherein the core clip is configured to fix at least one - preferably more than one - of the multiple core segments of the magnetic core within the assembly of the filter-choke. The assembly of the filter-choke, specifically the fixing of the magnetic core segments within the subassembly of the magnetic core can easily be realized by means of the core clip(s).

Preferably, the filter-choke can further comprise a lid made of insulating material and having substantially the same shape as the base flange of the bobbins. The lid acts as an isolating measure by which an unwanted electrical contact between the outer coil(s) and the magnetic core securely can be prevented. The lid can be configured to be releasably fixed to a top one of the first bobbin and the second bobbin within the assembly of the filter-choke. The fixing relative to the respective bobbin can be realized by also providing respective snap fitting parts on one side of the lid, which are configured to engage with the free snap fitting counterparts of the adjacently arranged bobbin. The lid can also comprise the same number of openings at similar positions compared to each one of the bobbins within the filter-choke, such that the lid is also configured to receive the at least one core-leg - or two core-legs - of the magnetic core. However, contrary to the design of the bobbins, the lid on one side comprises a substantially flat surface design without a tubular section extending in that respective direction. On that side, however, second fitting elements can be present, which are configured to engage with corresponding fitting elements of a further core clip of the filter-choke. By providing two core clips on opposing sides of the filter-choke, the whole assembly can be securely fixed in an easy manner.

Since the magnetic core of the filter-choke is configured to be assembled from multiple core segments, a gap is present in the magnetic core, which typically increases its magnetic reluctance. Since a magnetic core having a low reluctance is often required within the relevant applications of the filter-choke the increase in magnetic reluctance is an unwanted effect. That detrimental impact is, conventionally, only slightly reduced by polishing outer surfaces of the several core segments, which in turn comprises an additional labor-intensive process. However, according to the present application, the magnetic core of the filter-choke can comprise a low reluctance bypass-path around the gap, optionally around each gap. Therefore, the detrimental effect can be minimized, or even eliminated, without a labor intensive polishing.

Specifically, the magnetic core of the filter-choke can comprise a gap having a gap plane normal nGap that is oriented substantially parallel to a magnetic flux direction during operation of the filter-choke. The bypass path can then comprise an overlapping region between two core segments that is arranged adjacent to the gap, wherein the overlapping region is oriented with its interface plane normal nOL substantially perpendicular to the magnetic flux during operation of the filter-choke. A close contact of that core segments is provided by the overlapping region between the core segments through which the magnetic flux can easily penetrate from one core segment to the other core segment. In addition, a tendency of penetration of the magnetic flux from one core segment to the other core segment is dependent on a size of the overlapping region. Specifically, said penetration is easier with an increasing area of the overlapping region. Thus, the magnetic reluctance of the bypass path, and also of the magnetic core, decreases with an area increase of the overlapping region.

Therefore, by designing the geometry of the core segments, in particular their overlapping regions, a specific magnetic reluctance of the magnetic core can be designed.

The overlapping region can be formed between the two core segments that are also forming the gap (this is the case for the magnetic core disclosed in <FIG>). However, it is also possible that the magnetic core comprises at least three or more core segments. In that case, it is possible that the gap is formed by a first core segment and a second core segment, wherein two overlapping regions adjacent to the gap are formed, from which one overlapping region is formed between the first core segment and the third core segment, and the other overlapping region is formed between the third core segment and the second core segment (see <FIG>).

In one embodiment, the magnetic core - in an assembled state of the filter-choke - can comprise at least four core segments arranged in at least two layers arranged one above the other. Each layer can contain a closed magnetic sub-core with substantially the same geometry formed out of at least two core segments, wherein the closed sub-cores are coaxially aligned to each other. In that case the core segments of the sub-cores in the different layers are arranged such that all gaps having a gap plane normal oriented substantially parallel to the magnetic flux are arranged offset to each other in the different sub-cores. A low reluctance bypass path around each gap in the first layer is directed along a core segment in the adjacent second layer and arranged above or below the gap in the first layer via this design feature. Furthermore, a low reluctance bypass path around each gap in the second layer is directed along a core segment in the adjacent first layer and arranged above or below the gap in the second layer.

Independent of whether the magnetic core comprises one of more layers and independent of whether it comprises two or more core-segments, the multiple core segments can comprise different core materials. By choosing different core materials in the core-segments, e.g. material A in a first core segment and material B in a second core segment etc., a larger degree of freedom for achieving a targeted value for a specific magnetic property of the filter-choke can be achieved. Accordingly, the inductance of the filter choke can better be adjusted to a required value than this would be the case for a magnetic core with the same material in each core segment.

In one embodiment of the filter-choke, the electric conductor used for winding the at least two coils comprises a flat wire. Optionally, the at least two coils are formed by winding the flat wire around its thin edge. By winding a flat wire, and particularly by winding a flat wire around its thin edge, a coil (also called "edge wound coil") with a relatively low ohmic resistance combined with a relatively large winding density can be produced. This in turn results in an advantageous decrease of component size for the filter-choke. Since the DC-resistance of such "flat wire coils", and in particular, "edge wound coils" can be kept low, they are preferably used in filter-chokes within power electronic electric devices, e.g. DC/DC-converters or DC/AC-inverters. When using the traditional manual and labor intensive winding technology, it is often not possible to produce such edge wound coils. However, this is not a problem for the filter choke according to the present application, since the method for producing said filter-choke comprises the act of forming the coils by use of an automatic winding technology.

An electrical device according to the invention is characterized by including an EMI filter, which EMI filter comprises a filter-choke according to the invention. The filter-choke within the device can be configured to operate as one of a "differential-mode filter-choke and a "common-mode" filter-choke. The electrical device can be a power electronic device, in particular, a DC/AC-inverter or a DC/DC-converter. The effects resulting for the electrical device are similar to those already described in combination with the filter-choke and the method of its production. Therefore, regarding the effects associated with the electrical device reference is made to the relevant sections.

The invention is further explained and described with respect to preferred exemplary embodiments illustrated in the drawings, wherein.

In the following, the design of an exemplary embodiment of a bobbin <NUM> that can be used in an embodiment of the filter-choke <NUM> according to the invention is explained in more detail. The explanation refers to <FIG> and <FIG>, wherein <FIG> illustrates the bobbin <NUM> in a front view and <FIG> illustrates the same bobbin <NUM> in a back view.

The bobbin <NUM> comprises a planar base-flange <NUM> and two tubular sections 32a, 32b extending from a front surface 36b of and in a direction perpendicular to the base flange <NUM>. Each tubular section 32a, 32b comprises an inner opening <NUM>. Also, the base flange <NUM> comprises respective openings corresponding to and aligned with the inner opening <NUM> of the tubular sections 32a, 32b, such that for each tubular section a continuous through-hole is provided, which also extends through the base flange <NUM>. Each opening <NUM> is configured to receive a different core leg <NUM>, <NUM> of the magnetic core <NUM> (see: <FIG>, <FIG>). Although in <FIG> and <FIG> each opening is divided by a partition wall <NUM>, said partition wall <NUM> is only optional. Therefore, other embodiments of the bobbin <NUM> may not comprise said partitioning walls <NUM> inside the openings <NUM>.

The bobbin <NUM> further comprises a plurality of first fitting elements <NUM> arranged on opposite edges <NUM> of the base flange <NUM>. By example, the bobbin <NUM> displayed in <FIG> and <FIG> comprises in total four first fitting elements <NUM>, two on each opposing edge <NUM>. The bobbin <NUM> is configured to releasably fix another substantially identical further bobbin relative to the bobbin <NUM> via the first fitting elements <NUM>. Each first fitting element <NUM> includes a part 38a and a corresponding counterpart 38b of a snap fitting <NUM>, which is formed when the first fitting elements <NUM> of the bobbin and the further bobbin <NUM> are engaging. By example, the snap fitting <NUM> is formed as a "hook - loop" snap fitting. That is why each part 38a is formed as hook, whereas the corresponding counterpart 38b is formed as a loop. However, other designs of the snap fittings are also possible, for instance a "hook - undercut" snap fitting design.

On the backside 36a of the base flange <NUM> two second fitting elements 39a are arranged, which - in an assembled state of the filter-choke <NUM> (see <FIG>) - are configured to engage with corresponding fitting elements 39b of a core clip <NUM>. In addition, the front side 36b and/or the backside 36a can comprise guiding elements configured to guide and / or align coil terminals 42a, 42b of the pre-wound coils <NUM> within the assembled filter-choke <NUM>, the function of which will be explained in more detail in <FIG>.

<FIG> illustrates an embodiment for a preassembly of a first bobbin <NUM> and a second bobbin <NUM>' substantially identical to the first bobbin <NUM>. Both bobbins are arranged in a stacked manner one above the other and are releasably fixed to each other via the first fitting elements <NUM>. Specifically, each part 38a of the first fitting elements <NUM> of the second bobbin <NUM>' engages with its corresponding counterpart 38b of the first fitting elements <NUM> of the first bobbin <NUM>. Two pre-wound coils <NUM> - formed by an electric conductor <NUM> and arranged onto the tubular sections 32a, 32b of the first bobbin <NUM> prior to preassembly - are pressed and thereby fixed between the bobbins via the fixation of the bobbins <NUM>, <NUM>'. Each coil <NUM> comprises two terminals 42a, 42b, that are positioned and aligned within the preassembly via guiding elements 43a - 43c and 43d - 43f extending from the front and/or from the backside of each bobbin <NUM>, <NUM>'. Due to small tolerances which typically can be achieved by injection molding of the bobbins <NUM>, <NUM>' the precise alignment and positioning of the terminals 42a, 42b is ensured.

The assembly of an exemplary embodiment of the filter-choke is now explained with reference to <FIG> and <FIG>, wherein <FIG> illustrates the filter-choke <NUM> in an exploded view and <FIG> shows the filter-choke <NUM> of <FIG> in an assembled state. The filter-choke <NUM> comprises three substantially identical bobbins <NUM>. By example each bobbin is also designed identical to the bobbin <NUM> shown in <FIG> and <FIG>. The filter-choke <NUM> further comprises six pre-wound coils <NUM>, each comprising an electrical conductor <NUM> formed by a flat wire. By example the pre-wound coils <NUM> are formed as so-called "edge-wound coils" which are produced by winding the flat wire <NUM> around its thin edge. In addition, the filter-choke <NUM> comprises a lid <NUM> configured to be attached and releasably fixed to an outer bobbin <NUM> (in <FIG> the bobbin <NUM> on the left side). The design of the lid <NUM> is similar to the design of the bobbin <NUM>, at least with regard to its lateral dimensions as well as the geometry of its backside. The front side of the lid <NUM>, however, is substantially planar. The lid <NUM> also comprises two openings <NUM> on corresponding positions like this is the case for the bobbins <NUM>. Each opening <NUM> of the lid <NUM> is configured to receive a different one of the core legs <NUM>, <NUM> of the magnetic core <NUM>. The magnetic core <NUM> of the filter-choke <NUM> is formed by four U - shaped core segments <NUM> arranged in two adjacent layers. Each layer contains two U - shaped core segments <NUM>, such that in an assembled state of the filter-choke <NUM> each layer comprises a closed magnetic sub-core 28a, 28b. The U - shaped core segments <NUM> are of two different lengths, such that in the assembled state of the filter-choke <NUM> the gaps <NUM> formed within the magnetic sub-core 28a in the first layer are arranged offset relative to the gaps <NUM> formed within the magnetic sub-core 28b in the second layer. By means of these offset gaps and the overlapping regions formed by the core segments <NUM> in the different layers, a low reluctant bypass-path extends across each of the gaps <NUM> of the magnetic core <NUM>. Thus, the magnetic reluctance of the magnetic core <NUM> can be kept relatively low, although the magnetic core comprises in total four gaps <NUM>, each gap having a gap plane normal nGap oriented substantially parallel to the magnetic flux within the magnetic core <NUM> during operation of the filter-choke <NUM>.

When assembling the filter-choke <NUM>, each coil <NUM> is arranged around a different one of the tubular sections 32a, 32b of the several bobbins <NUM>. After placement of the coils <NUM> onto the tubular sections 32a, 32b, each bobbin <NUM> is releasably fixed to its adjacent bobbin <NUM>, thereby pressing the coils <NUM> between adjacent bobbins and arranging / aligning the terminals 42a, 42b of the coils <NUM> in predefined positions relative to the whole assembly. In addition, the lid <NUM> is releasably fixed onto the outer bobbin <NUM> via corresponding parts 38a and/or counterparts 38b of snap fittings also arranged on opposing edges of the lid <NUM>. Then, the magnetic core <NUM> is assembled by inserting the U-shaped core-segments <NUM> into the openings <NUM> of the lid <NUM> and the bobbins <NUM>. Finally, the core segments <NUM> of the magnetic core are fixed relative to an outer bobbin <NUM> and also relative to the lid <NUM> by releasably fixing one of two core clips <NUM> on each opposing side of the filter-choke <NUM>.

In the following, the operating principle of the low reluctant bypass path <NUM> is explained in more detail. Specifically, <FIG> represents a schematic drawing of a magnetic core <NUM> section according to a first embodiment of the magnetic core <NUM>. The section shows a gap <NUM> that is formed by a first core segments 23a and a second core segment 23b. The gap <NUM> comprises a gap plane normal nGap that is oriented substantially parallel to the magnetic flux Φ, the direction of which is symbolized by bolt arrows in the core segments 23a, 23b. Without countermeasures, said gap <NUM> normally would result in an increase of the magnetic reluctance of the magnetic core <NUM>. However, as depicted in <FIG> both adjacent core segments 23a, 23b are designed such that an overlapping region <NUM> is provided adjacent to the gap <NUM>. The overlapping region <NUM> is oriented such that its plane normal nOL is directed substantially perpendicular to the direction of the magnetic flux Φ during operation of the filter-choke <NUM>. In the overlapping region <NUM> a close contact between both core segments 23a, 23b over a relatively large surface area (at least larger than an area associated with the gap <NUM>) can be provided. Therefore, within the overlapping region <NUM> there is a lower resistance for a penetration of the magnetic flux Φ from the first core segment 23a to the second core segment 23b and vice versa, at least significantly lower as it would be for a penetration through the gap <NUM>. This results in a low reluctance bypass path <NUM> around the gap <NUM> as symbolized in <FIG> by the small arrows and the dotted line.

The resistance for penetration of the magnetic flux Φ in the overlapping region <NUM> is dependent on its lateral dimensions - the resistance typically decreases with increasing lateral dimensions of the overlapping region <NUM> - and thus can easily be varied by a targeted design of the core segments 23a, 23b relative to each other.

<FIG> represents a schematic drawing of a gap <NUM> and its low reluctance bypass path <NUM> in a second embodiment of the magnetic core <NUM>. The situation is similar to the situation illustrated in <FIG>. However, in the embodiment in <FIG> the magnetic core <NUM> comprises two adjacent layers, wherein in each layer a closed magnetic sub-core 28a, 28b is provided, such that the closed sub-cores 28a, 28b are stacked one above the other. Although each sub-core 28a, 28b represents a closed magnetic sub-core, gaps <NUM> are present due to the fact that each sub-core 28a, 28b is formed by at least two core segments 23a, 23b (in the first layer 28a) and 23c, 23d (not illustrated in <FIG>) in the second layer 28b. The gaps <NUM> in the different sub-cores 28a, 28b are arranged offset to each other, such that adjacent to a gap in one of the sub-cores 28a, 28b, there is always a continuous section of a core segment <NUM> of the other of the sub-cores 28b, 28a. <FIG> depicts the situation around such a gap <NUM> in the first layer 28a (in <FIG> the first layer) by example.

Adjacent to the gap <NUM> and on each of its site overlapping regions <NUM> between two different core segments 23a - 23c are formed. Particularly in <FIG>, on one side of the gap <NUM> an overlapping region <NUM> between the first core segment 23a and the third core segment 23c is formed, whereas on the other site of the gap <NUM> a further overlapping region <NUM> between the second core segment 23b and the third core segment 23c is formed. Based on the explanation provided in conjunction with <FIG>, this results in a low reluctance bypass path around the gap <NUM> starting on one side of the gap <NUM> in the first core segment 23a, through the overlapping region <NUM> via the third core segment 23c and through the further overlapping region <NUM> into the second core-segment 23b, and vice versa. In this embodiment the low reluctance bypass path <NUM> is guided around the gap <NUM> through a core section 23c, 23d of the magnetic sub-core in an adjacently arranged sub-core 28a, 28b, i.e. the sub-cores 28a, 28b in the adjacent layers. If more than two sub-cores 28a 28b or layers are provided within the magnetic core <NUM>, a respective bypass path <NUM> around a gap <NUM> is present in each adjacent sub-core 28a, 28b.

In the following, an alternative embodiment of a magnetic core <NUM> compared to that one illustrated in <FIG> and <FIG> is explained. For the explanation it is referred to the <FIG> and <FIG>, wherein <FIG> illustrates the magnetic core <NUM> in an exploded view and <FIG> in a preassembled state. The magnetic core <NUM> is formed by two core segments <NUM>. Each core-segment 23a, 23b comprises a U-shape design comprising two U-legs and a U-base having a thickened middle region. In a preassembled state of the core segments 23a, 23b a closed magnetic core <NUM> having a rectangular geometry (as seen from a top view) is formed. Further, in the preassembled status of the magnetic core <NUM> in <FIG>, <FIG>, two gaps <NUM> between the two core segments 23a, 23b are formed on each U-base (in other words: on each short side of the magnetic core <NUM>). Further, two overlapping regions <NUM> are formed in the preassembled state, which overlapping regions <NUM> extend along the U-legs (in other words: along the core-legs <NUM>, <NUM>). Keeping in mind the explanation provided in <FIG> this results in a magnetic bypass path <NUM> around each gap <NUM> as schematically illustrated in <FIG> for two (of four) gaps <NUM> on one short side of the magnetic core <NUM>.

Claim 1:
A filter-choke (<NUM>) to be used in an EMI filter, the filter choke (<NUM>) comprising:
- a closed magnetic core (<NUM>) comprising two core-legs (<NUM>, <NUM>), wherein the magnetic core is configured to be assembled out of at least two core-segments (<NUM>),
- at least two bobbins (<NUM>), each bobbin comprising a base flange (<NUM>) and a tubular section (32a, 32b) extending in perpendicular direction from the base flange, wherein the tubular section (32a, 32b) comprises an opening (<NUM>) for receiving one of the two core-legs (<NUM>, <NUM>),
- a coil (<NUM>) formed by an electric conductor (<NUM>) having multiple windings arranged around the tubular section (<NUM>) of each bobbin (<NUM>),
wherein in an assembled state of the filter-choke (<NUM>), the bobbins (<NUM>) are arranged in a stacked manner, such that their openings (<NUM>) are aligned coaxially to each other, one of the core legs (<NUM>, <NUM>) extending through the openings (<NUM>), and
characterized in that each bobbin (<NUM>) comprises at least two first fitting elements (<NUM>) arranged on opposite edges (<NUM>) of its base flange (<NUM>), wherein the first fitting elements (<NUM>) of the first bobbin (<NUM>) are configured to engage with the first fitting elements (<NUM>) of the adjacent second bobbin (<NUM>) for releasably fixing the two bobbins (<NUM>) together,
- wherein the at least two first fitting elements (<NUM>) are each containing a part (38a) and a corresponding counterpart (38b) configured to form a snap fitting (<NUM>) with the adjacent second bobbin (<NUM>), and
- wherein for each first fitting element (<NUM>) the snap fitting part (38a) and the snap fitting counterpart (38b) extend in opposite axial directions from the edge (<NUM>) of the base flange (<NUM>).