Abstract:
The present invention relates to a choke with two coils and a core for interleaved applications in step-up or step-down circuits or power factor compensation circuits. The core comprises several core sections with several lateral legs and a middle leg, whereby the core is designed such, that a coupling factor k of the two coils is smaller than 3%-5%. Furthermore, the core is designed such, that the core section form two loops with the middle leg as a common section, whereby each of the two coils lies on different loops outside of the common section. The lateral legs have a cross section A 1  and the middle leg for the common section has a cross section A 2 &lt;2×A 1.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a choke with two coils and one core, optimized to be used in step-up or step-down circuits or power factor compensation (PFC) filters in an interleaved configuration. Furthermore, the present invention relates to an optimized double coil core for interleaved applications in step-up and step-down or power factor compensation (PFC) circuits. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the following, the term “choke” relates to a configuration from one or several coils placed on a common core. 
         [0003]    A step-up or step-down circuit refers to a circuit, which can increase or decrease a direct-current voltage. Step-up and Step-down circuits operate according to similar principles like power factor compensation filters and partially use the same components. 
         [0004]    A power factor correction is imposed in Germany for electric loads over 75 watt since 1 Jan. 2001 by the electromagnetic compatibility norm (EMC). Power factor describes the rate between the value of the effective power and the apparent power. A value less than 1 means that the apparent power, which is drawn from the power grid, is larger than the effective power, so that the power grid is additionally loaded by the apparent power, which has to be provided and transported and which partially has to flow back through the power grids. Hereby greater losses occur in the grid and the grid has to be dimensioned larger than actually necessary. Power factor correction filters make sure that the power factor is as close as possible to 1, i.e. only pure effective power is drawn from the power grid. In an active power factor correction (PFC) the drawn current is readjusted to the time dependent sinus shape voltage of the power grid. 
         [0005]    A central component of step-up, step-down circuits and of PFC is a choke, which is in principle used to temporarily store Energy and release it on requirement. The following explanations confine on the use of the choke in PFC filters. However, similar reasoning is also true for step-up and step-down circuits. 
         [0006]    A switch connected downstream of the choke which can adjust the coil output to a reference potential, is opened and closed by a controlling device so as on the one hand to deliver sufficient power to an electric load, but on the other hand so that the current of the grid voltage curve drawn from the grid is in-phase. 
         [0007]    In a further development the input power voltage is divided between two coils which can be operated independently from one another. In general the switches are operated inverse to one another, i.e. if one switch is opened, the other switch is closed. In such an “interleaved” operational mode a choke branch (master) is directly controlled by the regulation circuit, i.e. the switching times for the choke are directly controlled by the regulation. The second choke branch (slave) generally follows the master with a phase shift of 180 degrees. Such an interleaved working arrangement has the advantage, that a more efficient power factor correction can be achieved. Since each choke has to cope with only half of the output power, smaller components can be dimensioned, so as to improve the power loss and heat generation and allow for smaller PFC-circuits. It is to be noted, that a correct functioning is possible also at other phase shifts &lt;180°. That is, in general, the phasing can be variable. However, the majority of applications operate with a phase shift of 180°. 
         [0008]    Active PFC circuits usually consist of a rectifier with a step-up convertor directly attached downstream with a coil and a switch, which charges a large capacitor to a voltage above the peak voltage of the grid network alternating current.  FIG. 1A  schematically shows the principles of a step-up circuit in interleaved technology. At the input the input voltage V IN  is applied to the two choke coils L 1  and L 2  and the input current I IN  is divided between the two chokes. At the output of each coil or choke L 1  or L 2  a switch S 1  or S 2 , respectively, can set the output L 1  or L 2  controlled by a regulation circuit (not shown) to a reference potential. The outputs of coils L 1  and L 2  are connected through a diode to the capacitor C OUT , which, in interaction with the coils L 1  and L 2  increases the voltage (step-up circuit) and smoothes the voltage, so that it can be delivered to the load resistance R LOAD . 
         [0009]    The opening and closing times of switch S 1  are set by a controller (master), which ensures that on the one hand the load R LOAD  is provided with sufficient current I OUT  and on the other hand the input voltage I IN  is following in-phase the input voltage V IN . The switch S 1  follows the switch S 1  phase shifted by 180 degrees (slave). This causes in principle a pulse width modulation of the input current, in which the pulse width is controlled by a controller.  FIG. 1B  shows the switching characteristics of the switches S 1  and S 2 . The time in which switch S 1  is closed is denoted as T on  and is variable according to the controller. In the time in which switch S 1  is closed, switch S 2  is opened (180 degree phase shifted). The overall time, which consists from the sum of the time T on , in which the switch S 1  is closed and the time T off , in which the switch is opened, is denoted as period T and is constant. The duty cycle D=T on /T is variable and dependent on the controller. In  FIG. 1   b  a constant duty cycle D of 0.5 is shown. 
         [0010]      FIG. 1C  shows the currents I 1  and I 2  through the coils L 1  and L 2 . The current I 1  through coil L 1  consists from a direct current component Idc 1  and a ripple component Iac 1 , generated by the switching processes. Accordingly, the current I 2  through the coil L 2  consists from a direct current component Idc 2  and a ripple component Iac 2  (alternating current component caused by switching processes). Since the switches are connected with a phase shift by 180 degrees, the phase shift between Iac 1  and Iac 2  is 180 degrees. On the capacitor C OUT  the currents I 1  and I 2  are added. I.e. the complete direct current component results in Idc=Idc 1 +Idc 2 . From Idc 1 =Idc 2 =I IN /2 follows for Idc that Idc=I IN . For the complete ripple current component (alternating current component) follows Iac=Iac 1 −Iac 2 , since Iac 1  and Iac 2  are phase shifted by 180 degrees. This, however, is only true for a duty cycle of D=0.5, i.e. for t on =t off . I.e. for a duty cycle of D=0.5 the ripple current components mutually compensate. At different duty cycles the ripple current components do not precisely compensate each other. In any case, on the whole, in the interleaved design the ripple current component is reduced giving a smoother current curve. 
         [0011]    It should be noted, that at a phase shift of 180° the ripple current maximum in the middle leg is reached at a duty cycle of D=0.5. The interleaved choke, however, also functions at other phasings &lt;180°. Hereby only the duty cycle D, at which the maximum of the alternating current ripple occurs, is shifted. I.e. that in general the phasing can be variable. However, the majority of applications operates at a phase shift of 180°. 
         [0012]    Chokes for use in interleaved step-up circuits and PFC steps are known from state of the art. In the simplest case two coils are wound on a common core, like for example shown in U.S. Pat. No. 6,362,986 B1 of the Volterra company.  FIG. 2  schematically shows the coil arrangement of this patent with switches  40  for the interleaved operation mode. The two coils  20  and  30  are arranged on a common ring-shaped core  10 , i.e. the pair of coils  20 ,  30  works with a strong magnetic coupling, similar to a transformer. Since the magnetic flows from the coils sum up, the core geometries are correspondingly large, so as to reach a high magnetic conductivity and at the same time not to stress the core up to the saturation magnetization. 
         [0013]    U.S. Pat. No. 8,217,746 B2 describes a further development of a choke coil for interleaved PVC circuits, in which the coil core for the two coils is designed such that the two coils are only weakly magnetically coupled.  FIG. 3  shows a schematic view of the coil- and core configuration of U.S. Pat. No. 8,217,746 B2. The core consists from two E-shaped parts  110  and  120 , which are separated from one another by an I-shaped part  130 . The coils  20  and  30  are wound on the middle legs of the E-shaped parts  110  and  120 . Since the magnetic flows Φ 1  and Φ 2  in the coils  20  and  30  from the middle legs of the E-shaped parts divides between the lateral legs of the E-shaped parts, the cross section A 2  of the lateral legs can be half the size of the cross section A 1  of the middle legs. Since the coils  20  and  30  are wound or connected in phase opposition, the direct current components of the magnetic flows Φ 1  or. Φ 2  of the coils  20  and  30  in the I-shaped portion of part  130  extent compensate one another to a large, so that the cross section of the I-shaped part  130  can be designed smaller than the cross section A 1  of the middle legs of the E-shaped parts  110  and  120 . By connecting the two E-shaped parts  110  and  120  and of the I-shaped part  130  air gaps  140  are formed at the joints. 
       SUMMARY OF THE INVENTION 
       [0014]    In view of new power safe technologies, such as in automotive engineering in the domain of hybrid and electro vehicles there is a growing demand for chokes for interleaved PFC circuits with low weight and high efficiency so as to safe energy on the one hand (weight) and at the other hand to efficiently transport energy, for instance, if motion energy in electric or hybrid vehicles is retrieved with a generator and supplied into the on-board electrical grid. It is therefore a task of the present invention to provide a choke with an optimized core geometry for a choke coil pair for use in interleaved PFC-application, which is compact and has small losses and a low weight. 
         [0015]    The task is solved with a choke with two coils and one core according to the present invention. 
         [0016]    In particular this task is solved by a choke with two coils and one core, wherein the core contains several core section with several lateral legs and one middle leg, wherein the core is designed such, that the core section form two loops with the middle leg as a common section, wherein each of the two coils lies on different loops outside of the common section, so that the lateral legs have a cross section A 1 , and that the middle leg for the common section has a cross section A 2 &lt;2×A 1 . 
         [0017]    With this arrangement a coupling factor k of the two coils smaller than 5%, preferably smaller than 3%, and more preferably smaller than 1% is realizable, so that the core cross section can be kept small in the lateral legs, since the magnetic fields of the coils no longer overlap in the lateral legs. Furthermore, the magnetic flux, which corresponds to the direct current component, compensates in the common section, so that the cross section of the common section can be designed small so as to save material. Since the coils are not arranged coaxially like in U.S. Pat. No. 8,217,746 B2, but rather are placed on the lateral legs, less material is required for the core, which saves weight. This is for instance reached by arranging the two coils on two opposing lateral legs. 
         [0018]    In another embodiment the cross section A 1  lies in a range between 0.5 A 1  and 0.2 A 1 , so that more weight can be saved. 
         [0019]    In order to reach a coupling factor of less than 5%, 3% or 1%, the core is designed such, that the magnetic resistance in at least one of the lateral legs R MA  is larger than the magnetic resistance of the middle leg R MI , wherein R MA &gt;20 R MI  (5%), R MA &gt;33 R MI  (3%) or R MA &gt;100 R MI  (1%). 
         [0020]    In another embodiment, for the core section in the middle section a material with a high permeability is used to keep the coupling between the two windings or their flows low. In the lateral legs a material with a high saturation flow density is preferably used so as to keep the magnetic cross section of the lateral legs low. 
         [0021]    This embodiment with different materials for lateral legs and middle legs is only advantageous in specific cases, in which a too strong coupling between the two windings and high losses should be avoided. A high permeability is not necessary in general, since the core is sheared. Common power ferrites which can be used generally have an initial permeability μi between 1000 and 3000. A high permeability in the middle leg is advantageous since it reduces the coupling. The influence of the permeability of the lateral legs on the coupling is negligible, since the air gap dominates the magnetic resistance. For this reason rather a highly permeable magnetic material is used in the middle leg. Since losses dominate due to the increased exchange flow duty cycle caused by the reduction of the cross section, the cross section of the middle leg cannot be reduced up to the saturation limit, so that in this case preferably a material with lower losses having a slightly lower saturation flow density is used. In the lateral leg a material with a high duty cycle is used—like in a regular choke. In most applications the entire core can consist of one material. Only in specific cases (too high coupling, high losses in the middle leg) one will use different materials for the lateral legs and the middle leg. 
         [0022]    In order to reach a 100 fold increased magnetic resistance in one of the lateral legs as compared to the middle leg, in one embodiment the lateral leg can feature an air gap, which is preferably arranged in the areas of the coils. 
         [0023]    In order to reach the core geometry according to this invention, different embodiments are possible. 
         [0024]    In one embodiment the core section are formed by two E-shaped parts, which are combined, such that their free ends meet, so that the connected middle legs of the two E-shaped parts form the common section. With this configuration a shorter and thus more compact design than for instance shown in U.S. Pat. No. 8,217,746 is possible. 
         [0025]    In another embodiment the lateral legs are formed by two U-shaped parts, which are connected such, that their free ends meet, creating a magnetic circuit, whereby the middle leg for the common section has a T-shape and is inserted such between the two coils into the magnetic circuit, that the magnetic circuit is short-circuited, so that the magnetic circuit is divided into the two magnetically weakly coupled loops. Besides the compact design this embodiment has the advantage, that when connecting the two formed components, only two surfaces meet, contrarily to the E-shaped forming component, in which three surfaces meet. If three surfaces meet the formed components have to be manufactured with a very high precision so as to avoid uncontrolled air gaps. Due to manufacturing tolerances these air gaps are virtually unavoidable. If two surfaces meet like for the U-shaped components and the T-shaped component, this effect does not occur, so that with this embodiment chokes with lower tolerances can be manufactured. 
         [0026]    In one of its embodiments the height H 2  of a vertical part of the T-shaped middle leg corresponds to a height H 1  of the lateral leg. Furthermore, a width B 2  of the vertical part of the T-shaped middle leg corresponds to a clear distance between the two coils and a depth T 2  of the vertical part of the T-shaped core section corresponds to an inner distance T 1  of the opposite lateral legs of the magnetic circuit. This contributes to a compact design, since the space between the coils within the magnetic circuit is filled free of clearance and is, thus, completely usable for the magnetic flow. In a further embodiment, a horizontal part of the T-shaped middle leg is supported by a lateral leg. Overall, the T-shaped design of the middle leg allows a simple and precise positioning of the magnetic short-circuit between the two coils. By the supporting surfaces, formed by the horizontal parts of the T-shaped middle leg, the middle leg is precisely inserted up to the correct depth into the magnetic circuit. 
         [0027]    In a further embodiment the lateral legs are formed by two U-shaped parts, whose free ends oppose each other and are separated without play from one another by a straight elongated core section, which serves as a middle leg, so that the two legs are formed with the middle leg as a common section, which form two jointly weakly coupled magnetic circuits. A straight elongated core section, which serves as middle leg, has in comparison to a T-shaped core section the advantage, that micro air gaps created between the T-part and the lateral legs are avoided. Hereby the coupling between the magnetic circuits is reduced. At the same time this arrangement can compensate for the tolerance in the lateral air gaps or lateral legs, respectively, since the middle leg can now be flexibly glued to the lateral legs or side plates. Small excess ends of the middle leg are of no problem, but also a slightly shorter middle leg only insignificantly influences the current flow. In one embodiment each U-shaped part or E-shaped part is composed from several straight parts. These straight parts can be, for example, glued, so that tolerances due to uncontrolled micro air gaps are reduced. 
         [0028]    In an embodiment the core sections are manufactured from a plate stack from a magnetically soft material. With this technique arbitrary core shapes can be realized with little technical effort. 
         [0029]    The above-mentioned task can also be solved with a choke core, which comprises several core sections consisting from several lateral legs and a middle leg. Hereby the core sections are arranged such, that the core section form two loops with the middle leg as a common section, so as to form two weakly coupled magnetic circuits, whereby the lateral legs have a cross section A 1  and whereby the middle leg for the common section has a cross section A 2  smaller than 2×A 1 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    In the following, exemplary embodiments, modification, advantages and application examples of the invention using the enclosed figure are described. Hereby all described and/or depicted features alone or in any combination are in general subject matter of the invention, independent of their summary in the claims or their back reference. Also the content of the claims is made part of the description. It is shown in the figures: 
           [0031]      FIGS. 1A-C  the principle of a step-up convertor in interleaved-technology; 
           [0032]      FIG. 2  a first example for a choke for use in a PFC-device according to state of the art; 
           [0033]      FIG. 3  a second example for a choke for use in a PFC-device according to state of the art; 
           [0034]      FIG. 4  the principle of a choke for PFC-devices according to the present invention; 
           [0035]      FIG. 5  a first embodiment of a choke for PFC-devices according to the present invention; 
           [0036]      FIG. 6A  a top-view of a second and third embodiment of a choke for PFC-devices according to the present invention; 
           [0037]      FIG. 6B  a perspective view of a second embodiment of the choke according to the present invention; 
           [0038]      FIG. 6C  another perspective view of the second embodiment of the choke according to the present invention; and 
           [0039]      FIG. 6D  a perspective view of a third embodiment of the choke for PFC-devices according to the present invention 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    The present invention was made to provide chokes with an optimized compact core geometry for PFC-devices with interleaved-topology. Especially in growing electromobility, technology with electric vehicles (EV) and hybrid electric vehicles (HEV), new, compact, i.e. having low weight, chokes and choke cores are needed, which can be used at frequencies above 100 kHz. 
         [0041]      FIG. 4  shows the principle of the present invention. Two coils  20  and  30  are placed on an optimized compact core  200 , which consists from several lateral legs  230  and  240  with a middle leg  250 . The lateral legs consist from two lateral legs  230  with coils and two lateral legs  240 , which serve as connection elements for the lateral legs  230  carrying the coils that form a magnetic circuit. The middle leg  250 , which runs in parallel to the coils carrying lateral legs  230 , and which connects approximately the middles of the lateral legs  240 , causes a magnetic short-circuit between the connection elements  240  and divides the magnetic circuit in two loops  200 -A and  200 -B. The lateral legs have a cross section A 1 . The middle leg has a cross section A 2  which is smaller than 2×A 1 . In the PFC-application the coils  20  and  30  are connected such that the direct current component of the magnetic flux in the middle leg  250  runs in opposite direction and thus compensates itself. Thanks to the compensated DC-flow (direct current) the cross section of the middle leg can be significantly reduced. However, the alternating current of the coils  20  and  30  in general adds in the middle leg, since the alternating current amplitudes sum up in the middle leg due to the inversed poling of the coils  20  and  30 . At a duty cycle of D=0.5, i.e. T on   32  T off , the maximal alternating current is FiAC max=Φac 1  (alternating current through coil  20 )+Φac 2  (alternating current through the coil  30 ). At other duty cycles the maximal alternating current amplitude through the middle leg  250  is reduced. If the lateral legs  230  and  240  are operated up to a saturation current density B satt  of common ferrite materials of 350-400 mT, the relation between the ripple current Iac and the total current Iac+Idc is adjusted to a value between 0.1 and 0.5, the minimal cross section A 2  of the middle leg  240  can become 0.2 to 1 fold of the cross section A 1  of the lateral legs. Preferably, the PFC-steps are adjusted such, that A 1  is in the range between 1×A 1  to 0.2×A 1 . 
         [0042]    In order to reach a coupling between the coils  20  and  30 , the magnetic resistance R MA  in the lateral legs should be 100 times the magnetic resistance R MI  in the middle leg. The coupling factor results from k=R MI /R MA , wherein R MI  is the magnetic resistance in in the middle leg and R MA  is the magnetic resistance in the lateral legs. Despite a small cross section A 2  of the middle leg  250  this is reached through the air gaps L, which can be, for example, incorporated into the lateral legs  230  in the area of the coils  20  and  30 , so as to avoid that a relatively large direct current portion through the coils makes the core in the lateral legs reach saturation. Through the small magnetic coupling the magnetic fields of the coil  20  do not penetrate into the core section of the lateral legs of the coil  30  and inversely, as it would be the case for a strong coupling. For a strong magnetic coupling the magnetic fields of the coils would at least partially enhance each other, so that the saturation magnetization in the lateral legs would be reached faster, i.e. for a strong magnetic coupling of the coils  20  and  30  the cross section of the lateral legs would have to be dimensioned larger. However, in general it is advantageous to use materials with a low magnetic resistance (high permeability) in the middle leg and a high saturation magnetization in the lateral leg. 
         [0043]      FIG. 5  shows an implementation of the present invention according to a first embodiment with two E-shaped formed components  210  and  220 , which are connected such that their free ends meet. To illustrate the E-shape in  FIG. 5  larger gaps L 1 , L 2  and L 3  between the free ends of the two E-shaped parts  210  and  220  are shown. When implementing, as far as possible, no gaps L 1 , L 2  and L 3  should occur in order to avoid undefined air gaps, i.e. the ending surfaces on the free ends of the E-shaped parts have to be manufactured so precisely, that they lie in one plane, so that no air gaps occur. Air gaps are exemplarily selectively introduced in the lateral legs  230  in the area of the coils  20  and  30 . The cross section A 2  of the middle leg  250  is, as explained in detail above, smaller than the cross section A 1  of the lateral legs  240  and  230 . 
         [0044]    In order to avoid a situation, as it can occur with E-shaped parts with the three gaps L 1 , L 2  and L 3 , two U-shaped core parts  260  and  270  can be used, which are connected such, that their free ends meet. 
         [0045]      FIG. 6A  shows a schematic top view of a core from two U-shaped parts  260  and  270  with a middle leg  250  according to a second and third embodiment of the present invention. In  FIG. 6A  the meeting edges of the free ends of the U-parts  260  or  270  are not visible. The coils  20  and  30  are positioned on the coils carrying lateral legs  230  of the core. The middle leg  250  fills the gap between the coils  20  and  30  and also forms a magnetic short-circuit between the lateral legs  240 , so that two magnetic loops are formed. The air gap L in the lateral legs  230  in the area of the coils  20  and  30  leads to an operation outside of the magnetic saturation in the lateral legs and, at the same time, to a lower coupling of less than one percent between the two loops, respectively loops  20  and  30 . 
         [0046]      FIG. 6B  shows a perspective view of the scheme of  FIG. 6A  according to a second embodiment with an extracted middle leg  350 .  FIG. 6B  shows only the core arrangement without the coil windings  20  and  30  according to the second embodiment. The core is composed of two U-shaped parts  260  and  270 , which meet at the face surfaces of the free ends of the U-shaped parts, as shown by line  280  in  FIG. 6B . Since there are only two face surfaces  280 , it is easier to avoid uncontrolled air gaps in the lateral legs. The lateral legs have a cross section of A 1 =B 1 ×H 1 . The distance of the lateral legs  240 , which connect the coils-wearing lateral legs  230  is T 1 . The middle leg  350 , which is extracted in the illustration of  FIG. 6B  from the ring structure, is designed in a T-shape with a vertical part  350 - 1  and a horizontal part  350 - 2 . The vertical part  350 - 1  has a height H 2 , a length T 2  and a width B 2 . In order to optimally fill the air space between coils  20  and  30  (see  FIG. 6A ), the width B 2  of the vertical part of the T-shaped middle leg  350  corresponds to the clear distance of the space in-between the coils. The length T 2  of the T-shaped middle leg  350  corresponds to the distance T 1  between the lateral legs  240  and the height H 2  of the vertical part of the T-shaped middle leg  350  corresponds to the height H 1  of the lateral legs  230  and  240 . The excess ends of the horizontal part  350 - 2  of the T-shaped middle leg  350  have a length L 1  and are supported by the lateral legs  240 . The maximal length L 2  of the horizontal part  350 - 2  of the T-shaped middle leg  350  is maximally T 1 +2×B 1 , or the length L 1  of the excess ends of the excess part  350 - 2 , supported by the lateral legs  240 , are about B 1 . The air gaps L in the U-shaped parts  260  or  270  can be realized through filling materials like CEM1 or FR4. The thickness B 2  of the T-shaped middle leg  350  is smaller than the width B 1  of the lateral legs  230  and  240 . 
         [0047]      FIG. 6C  shows a perspective view of the core according to  FIG. 6B  in composite form. The reference signs in  FIG. 6C , which are identical to the reference signs in  FIG. 6B , denote the same technical features so that the explanations are not repeated at this place. The air gaps between the vertical part  350 - 1  of the middle leg  350  and the coil-carrying lateral legs  230  (the coils are not shown in  FIG. 6C ) are almost completely filled by the windings of the coils. In  FIG. 6C  the horizontal part  350 - 2  of the T-shaped middle leg  350  is flush with the lateral edges of the lateral legs  240 . However, small deviations, i.e. excess ends and shorter ends do not affect the magnetic behavior of the whole core. 
         [0048]      FIG. 6D  shows a perspective view of a core according to a third embodiment of the present invention. The core is composed like in  FIG. 6D  from two U-shaped parts  260  and  270 . Between the U-shaped parts  260  and  270  a middle leg  450  is positioned, so that the free surfaces  280 - 1  and  280 - 2  of the opened ends of the U-shaped parts  260  and  270  meet with the opposite sides of the middle leg  450 , which is rectangular-shaped with a height H 3 , width B 3  and a length of T 1 +2 B 1 . The size indications T 1 , B 1 , and H 1  correspond to the size indications in  FIG. 6B . Otherwise, the U-shaped parts in  FIG. 6B  can be identical with the U-shaped part in  FIG. 6D . In  FIG. 6D  reference signs, which are identical to those in the previous figures, denote the same technical features so that a repetition is waived here.  FIG. 6D  shows a schematic three-dimensional arrangement in which the single elements  260 ,  270  and  450  are shown pulled apart. In the assembled state the middle leg  450  is flexibly glued to the lateral legs  240 -A,  240 -B,  240 -C and  240 -D, which can compensate for the tolerances in the lateral air gaps L. Small excess ends of the middle leg or slightly shorter middle legs only insignificantly influence the current flow from the lateral legs in the middle legs.