Winding configuration as part of an integrated structure for a medium frequency transformer

Abstract

A method for producing a coil of a medium frequency transformer for a transformer, especially a resonant DC / DC converter or a dual active bridge DC / DC converter is disclosed, the method comprising the following steps: providing a plurality of conductive foil strips, M>1 in number, each conductive foil strip having a first end and a second end; stacking the plurality of conductive foil strips to obtain a foil strip stack having a first end and a second end, wherein an electrical insulation layer is arranged between any two adjacent foil strips; electrically interconnecting the first ends of all the conductive foil strips to a first terminal; for each of the conductive foil strips, providing a connector at the second end of the foil strip; and coiling the foil strip stack from the first end.

Description

Technical Field

[0001] The invention relates to the field of power electronics. The invention relates to a coil of a medium frequency transformer of a resonant DC / DC converter or a dual active bridge DC / DC converter and to a method for producing such a coil according to the independent claim. Background Art

[0002] Transformers handling high currents (hundreds of amperes or more, especially above 200A) at high frequencies of several kilohertz (especially above 3kHz or more) are difficult to build with low cost and / or readily available components due to several effects that are negligible at lower frequencies (especially at frequencies below 1kHz) and / or for low currents (especially currents below 100A). Typical applications involving such high frequencies and / or high currents are as commonly used in solid-state transformers (SSTs, Figure 1 ), a medium frequency transformer (MFT, in particular a SST configured as an AC / DC converter Figure 2 ) for connecting distributed generation to a medium voltage (MV) grid, such as for the following:

[0003] Electric vehicle (EV) fast charging

[0004] Photovoltaic (PV) solar energy

[0005] Battery Energy Storage System (BESS)

[0006] Onshore and offshore wind

[0007] Data Center

[0008] and transformers without MV insulation requirements, such as transformers for high-power low-voltage DC / DC converters with galvanic insulation required in the charging pole of EV fast chargers. Two specific examples of such DC / DC converters are: a dual active bridge converter, as exemplarily described in Swiss Patent Application Publication CH707533A2 or US Patent Application Publication CH2018 / 0159435A1, both of which are incorporated herein by reference in their entirety; and a resonant DC / DC converter, as exemplarily described in PCT Patent Application WO2018 / 141092A1, which is also incorporated herein by reference in its entirety.

[0009] In order to keep high frequency losses in the transformer coils small, one approach is to use Litz wire to form the windings of the coils. Although Litz wire is several times more expensive than solid copper wire, it can be purchased "off the shelf" for AC currents up to 100-200A (root mean square, rms). Litz wire consists of a large number of transposed strands, usually made of copper, and is readily available in total cross-sections up to 0.5cm 2, which allows a maximum current of 100-200A rms In the range (assuming a fill factor of 0.8 and a current density of 2.5...5A / mm 2 ). As is typical in the applications listed above, larger currents require larger cross-sections and will be increasingly difficult to bend. Litz wire is usually not made of aluminum because it is extremely difficult for aluminum to reliably contact all the strands at the wire terminals (e.g., 900 strands of 0.2 mm diameter each in the copper Litz wire mentioned above). Copper Litz wire is at least 2-4 times more expensive than solid copper, and copper is about 3 times more expensive than aluminum. For higher currents (>100A) as is typical in the applications listed above, several copper Litz wires must be connected in parallel, which typically results in circulating currents due to stray flux between the parallel wires, which can significantly increase losses.

[0010] In order to optimize and / or minimize the manufacturing effort and required resources, aluminum foil windings, such as those commonly used in 50Hz transformers, would be the first choice for the coil. At high frequencies, the winding losses in the foil increase significantly due to skin effect and proximity effect. If a single foil is used, the frequency defines the required foil thickness, the desired or required current defines the foil height, and this produces the transformer height. Therefore, for high currents and high frequencies, the shape of the transformer will deviate significantly from the cubic shape, which will result in higher weight, core losses, and increased resource and effort requirements (larger core volume required).

[0011] If parallel foils are used, the foil height can be reduced, but strong circulating currents may be induced due to stray fields between the parallel foils, which will significantly increase losses (the same effect as parallel wires).

[0012] In both designs of copper litz wire and foil, the main problem is the circulating currents between parallel conductors, which usually significantly increase the winding losses, thus reducing the transformer power rating, and / or significantly increase the transformer cost (USD / kW). In forward-looking distributed energy applications such as EV fast charging, PV solar, battery energy storage systems, wind power or data centers, medium frequency transformers (MFT) are key components. For higher currents (especially above 100A), simply scaling up 50 / 60Hz technology and / or adopting off-the-shelf litz wire or low-cost foil wire will lead to huge losses due to high-frequency induced circulating currents, which will significantly reduce transformer performance.

[0013] The generation of circulating currents, in particular in the configuration of two parallel-connected windings, where each winding comprises a plurality of turns, can be understood as follows: each turn is exposed to a stray magnetic field, for example in a winding window formed by the core of the transformer. The parallel Litz wires forming separate windings connected at the input and output terminals of the transformer form a loop exposed to the stray magnetic field. The stray magnetic field varies with the operating frequency of the MFT, thereby producing a voltage that drives the circulating current in the loop. The circulating current is added to the nominal current in the MFT, which may result in one Litz wire carrying more than half of the nominal current and the parallel one Litz wire correspondingly carrying less than half of the nominal current. If the circulating current is large enough, one Litz wire may carry a current exceeding the total nominal current, and then the parallel one Litz wire carries a negative (180° phase shifted) current. In this way, not only is the total available copper cross-section effectively reduced by 50%, but additional losses are introduced and the maximum output power of the MFT is reduced by a factor of two or more.

[0014] Existing solutions to limit circulating currents require additional components, higher manufacturing effort and additional space, and may result in additional problems. One existing solution, transposition of parallel connected wires or foils, such as provided by parallel wires that are twisted or otherwise wound or interlaced with each other, requires additional manufacturing effort, especially for foil windings, leads to an increase in effective wire length, exhibits limited efficiency in MFTs with only a few turns of winding, and may result in high voltage insulation challenges, for example due to geometric inhomogeneities near the transposition location. Alternatively, common mode filters may be added between the parallel wires or foils. However, this requires additional components, and therefore may result in higher costs and higher manufacturing effort, and requires additional space and / or other resources.

[0015] The object of the present invention is to allow the provision of a coil for a transformer allowing efficient suppression of circulating currents, and a method for producing such a coil. Summary of the invention

[0016] This object is achieved by a coil and a method according to the independent claims. Further exemplary embodiments emerge from the dependent claims and the following description in conjunction with the drawings.

[0017] According to the present invention, a method for producing a coil of a medium frequency transformer for a transformer, especially a resonant DC / DC converter or a dual active bridge DC / DC converter, the method comprises the following steps: providing a plurality of conductive foil strips of which the number is M>1, each conductive foil strip having a first end and a second end; stacking the plurality of conductive foil strips to obtain a foil strip stack having a first end and a second end, wherein an insulating layer is arranged between any two adjacent foil strips; electrically connecting the first ends of all conductive foil strips to a first terminal; for each of the conductive foil strips, providing a connector at the second end of the foil strip; and coiling the foil strip stack from the first end.

[0018] The starting point of the method is a number M>1 of a plurality of conductive foil strips made of conductive foil, in particular aluminium foil. The width of each foil strip, in particular the width in a first or transverse direction, may be w; the length, in particular the length in a second or longitudinal direction, preferably perpendicular to the first direction, may be l; and the thickness, in particular the thickness in a third direction, preferably perpendicular to both the first and second directions, may be d. All foil strips may have at least substantially the same dimensions. Both the width and the length of each foil strip may be much greater than the thickness, i.e. w>>d and l>>d. Thus, each foil strip may be at least substantially rectangular, or the basic form may be at least rectangular. The foil strip may also be elongated, i.e. l>w, preferably l>>w. Each foil strip has a first end and a second end relative to the longitudinal direction. The effective cross-section of each foil strip may be w·d, and the effective cross-section may be at least substantially constant along the length l of the foil strip.

[0019] In a first step, the foil strips are stacked, wherein the first and second ends of the foil strips are located above each other. Electrical insulation is arranged between any two adjacent or neighboring foil strips. In particular, the electrical insulating layer can be provided by a separate and / or additional insulating layer stacked and / or placed between any two adjacent or neighboring foil strips. The width w and length w of the separate and / or additional insulating layer can be at least substantially the same as the foil strips. Other additional insulating layers can be arranged at the bottom and top of the stack. Alternatively or additionally, electrical insulation can also be formed by a part of one or both of the adjacent or neighboring foil strips, and the foil strips can include a conductive layer laminated on the foil insulating layer or laminated between two foil insulating layers.

[0020] The resulting foil strip stack may have a width w and a length l at least substantially the same as the foil strip, and a total thickness D, wherein w>>D and / or l>>D may hold. Thus, the stack may also be at least substantially rectangular, or at least rectangular in basic form. Like the individual foil strips, the stack has a first end and a second end relative to the longitudinal direction, wherein the first end of the stack is located near the first end of the foil strip and the second end of the stack is located near the second end of the foil strip; or in other words, the first end of the stack is closer to the first end of the foil strip than the second end, and vice versa.

[0021] In a further step, the first ends of all conductive foil strips are connected to the first terminal via a first terminal connection (in particular a simple, fast and safe electrical connection of the completed coil to the converter circuit and / or within the converter circuit). Preferably, each of all first ends is directly connected to the first terminal. Alternatively, the first ends may be connected in series, wherein the first end of one foil strip is only directly connected to the first terminal.

[0022] In a further step, provide a plurality of connectors that quantity is M, i.e. a connector for each foil strip. Quantity is that each connector in a plurality of connectors of M is connected to a plurality of conductive foil strips that quantity is different in N. Connector can be formed integrally with each foil strip, for example, by or by the additional foil strip section that extends beyond the second end of foil strip, and has been folded and / or twisted to form a cable or a wire-shaped conductor. More generally, connector can comprise a terminal, especially a terminal, that is arranged or formed on any kind of the second end, and allows to connect a certain conductor, especially a cable or a wire, preferably without any cross-section reduction below one of the corresponding foil strips.

[0023] In a further step, the foil stack is coiled, rolled or rolled up at and / or from the first end in and / or along the longitudinal direction, i.e. preferably at least substantially parallel to the longitudinal direction, to obtain a number N of turns. turn The coil may have a shape at least substantially similar to a cylindrical housing, barrel or sleeve, with a height h at least approximately equal to the width w of the foil strip. The coil may further have or define a central opening, which may have an at least substantially cylindrical shape, wherein a longitudinal axis of the cylindrical shape may extend at least substantially parallel to a transverse direction. In particular, the central opening may be configured to receive a segment of a transformer core.

[0024] The transformer core may be made in particular of a magnetic material with high magnetic permeability, in particular a ferroelectric or ferrielectric material, and comprises a first leg extending in a first direction which may correspond to a transverse direction, a second leg extending in particular in the first direction, a first yoke extending in a second direction in particular perpendicular to the first direction, and a second yoke extending in particular in the second direction. The core window may extend through the core in a third direction perpendicular to both the first and second directions. Preferably, the dimensions D1 and D2 of the core in the first and / or second directions are significantly greater than the dimension D3 of the core in the third direction, i.e. D1>D3 and / or D2>D3, preferably D1>>D3 and / or D2>>D3, so that the core may be considered to be substantially planar; although for a non-zero dimension I3 of the core window in the transverse direction, typically I3≈D3 and often I3=D3. Both the first leg and the second leg may extend from the first yoke to the second yoke, and vice versa, and surround the core window.

[0025] In a transformer design commonly referred to as a shell type, the first leg and the second leg may extend between the first yoke and the second yoke and vice versa and surround the core window. The third leg may extend between the first yoke and the second yoke, in particular in the first direction; and extend between the first leg and the second leg so that the core window is divided into a first sub-window and a second sub-window, preferably both sub-windows extend through the core in the third direction.

[0026] In particular, the core may be a closed core, i.e. no gaps, in particular no air gaps, are present in any leg or yoke, nor between any pair of legs and the yoke, so that the magnetic flux linking the primary winding and the secondary winding travels at least substantially completely within the magnetic material constituting the core, so that at least substantially no losses of the magnetic flux through the air occur. The core, instead of being a closed core, may alternatively comprise one or more core gaps, which are filled with a material having a significantly lower magnetic permeability than the magnetic material of the core, said material being typically air and / or a synthetic material, in particular a plastic. The number of core gaps arranged in one leg is N gap The core gap can be considered as dividing the legs into N gap +1 part. Adjacent parts of the legs may be spaced apart by a distance d core , the distance can be equal for all core gaps but is not required.

[0027] Therefore, the number is N turn The plurality of conductive foil strips form a plurality of windings which are connected in parallel at their first ends and can be considered to be intertwined, in particular intertwined within each other. Alternatively, the windings can be considered to form intersecting spirals which are connected to each other and connected to the first terminal at their inner ends. However, the windings are not transposed, in particular not intertwined.

[0028] In an optional further step, a second terminal and a plurality of impedance elements, in particular M capacitors or M inductors, may be provided, and a different one of the impedance elements may be connected between the second terminal and each connector. In particular, the second terminal may allow a simple, fast and safe electrical connection of the completed coil to and / or within the converter circuit.

[0029] Once the foil stack is coiled, rolled or rolled, the coil thus obtained can be cast into an electrically insulating material, which can then be cured, for example, by polymerization, in particular a resin. If impedance elements are arranged between the second terminal and the connector, these impedance elements may or may not be cast into the insulating material together with the coil. The casting can be formed so that the first terminal, the second terminal and / or the connector are easily accessible, in particular from the periphery of the coil, for a simple, quick and safe electrical connection.

[0030] Instead of casting a single coil into the electrically insulating material, a plurality of coils can also be produced according to one of the method variants described above and stacked on top of each other to form a coil stack, wherein the central openings of all coils can be at least substantially aligned and in particular configured to receive a segment or portion of a transformer core, which segment or portion can be substantially straight and / or elongated such that the segment or portion extends through the central openings of all coils.

[0031] The heights h1, h2, h3 of the individual coils can be different. This allows different total effective heights h to be achieved from a limited number of foil strips of discrete widths w1, w2, w3. eff (corresponds to the sum of all individual heights).

[0032] Two adjacent coils may be separated by a distance d coil , in particular in at least substantially lateral direction, the distance may be in particular significantly smaller than the heights h1, h2 of two adjacent coils, so that a coil gap is produced between the adjacent coils. coil The stack of multiple coils may include a number N gap -1 multiple coil gaps. It has been unexpectedly found that when the total effective height is h eff The coil stack with the same height h=h (corresponding to the sum of all individual heights of the coils in the stack) eff The presence of one or more inter-coil gaps may help reduce circulating currents when compared to a single coil.

[0033] One or more gaps between adjacent coils may be at least substantially aligned with one or more core gaps, wherein the distance d coilCan be slightly larger than the corresponding core gap distance d core , in particular between 1.5 times and 10 times, or between 2 times and 5 times. Further, in particular, for a core gap extending between coordinates x1<x2 along a first direction and a coil gap extending between x'1<x'2 in a transverse direction (in particular at least substantially corresponding to the first direction), x1>x'1 and x2<x'2 may hold. Alternatively, x1<x'1 and x2>x'2 may hold. Further, x1≈x'1 and / or x2≈x'2 may hold. In other words, a first leg portion of a leg may extend through a central opening of a first coil in one of two adjacent coils, and a second leg portion (in particular a different portion of the same leg) may extend through a central opening of a second coil in one of two adjacent coils.

[0034] The first terminals of all coils can be connected together, in particular at the first stack terminal. The entire coil stack can be cast into an electrically insulating material, which can subsequently be cured, for example by polymerization, in particular a resin. If impedance elements are arranged between the second terminals and the connectors of the coils, these impedance elements may or may not be cast into the insulating material together with the coils. The casting can be formed so that the first stack terminals, the second terminals and / or the connectors are easily accessible, in particular from the periphery of the coils, for a simple, quick and safe electrical connection.

[0035] The coil or winding configuration of the coil according to the present invention for a medium-frequency transformer for a transformer, especially a resonant DC / DC converter or a dual active bridge DC / DC converter, includes a winding pair, which includes: a first winding, which is made of a conductive foil strip and includes a first plurality of turns surrounding each other; a second winding, which is made of a conductive foil strip and includes a second plurality of turns surrounding each other, wherein each turn of the first plurality of turns is adjacently surrounded by a turn of the second plurality of turns; wherein the innermost ends of the first winding and the second winding are electrically connected at a first terminal, wherein the first connector and the second connector are respectively arranged at the outermost ends of the first winding and the second winding and contact them.

[0036] The coil or winding arrangement may further include a second terminal; a first connector (T 21 ) between the first impedance element; and connected between the second terminal and the second connector (T 22 ) between the first impedance element.

[0037] The coil or winding arrangement may comprise a further winding intertwined with the first winding and the second winding and comprising a further plurality of turns around each other, wherein the innermost end of at least one further winding is electrically connected to the first terminal, and wherein a further connector is provided for the further winding at and in contact with the outermost end of the further winding. The coil or winding arrangement may further comprise at least one further impedance element, wherein the or each further impedance element is connected between the second terminal and the further connector.

[0038] The first winding, the second winding and any further windings may be electrically connected, in particular directly electrically connected only at the innermost end, in particular via the first terminal, and otherwise electrically isolated from each other, or in other words, the first winding and the second winding may be electrically isolated from each other except for the connection at the innermost ends of the windings, which connection may in particular include the first terminal.

[0039] In particular, such a winding configuration can be produced according to the method described above or some of its method variations, wherein the first winding and the second winding are formed by a first conductive foil strip and a second conductive foil strip among a plurality of conductive foil strips, a number of which is M>1, wherein the innermost ends of the first winding and the second winding are formed by the first ends of the foil strips, and the outermost ends of the first winding and the second winding are formed by the second ends of the foil strips.

[0040] These and further aspects of the invention as described above will become apparent from the embodiments described hereinafter and will be elucidated with reference to the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The subject matter of the invention will be explained in more detail hereinafter with reference to exemplary embodiments illustrated in the drawings.

[0042] Figure 1 A basic, general prior art DC / DC converter is illustrated.

[0043] Figure 2 a) illustrates a basic prior art DC / DC dual active bridge (DAB) converter.

[0044] Figure 2 b) illustrates a basic prior art resonant DC / DC converter.

[0045] Figure 3 Shown from Figure 1 A more detailed schematic diagram of a possible embodiment of a DC / DC converter.

[0046] Figure 4 A coil according to the invention is schematically illustrated.

[0047] Figure 5 The figure illustrates a stack of three coils according to the invention.

[0048] Figure 6 A schematic diagram of an exemplary resonant DC / DC converter is shown.

[0049] Figure 7 A schematic diagram of another exemplary resonant DC / DC converter is shown.

[0050] In principle, the same reference symbols in the figures denote the same features or elements. DETAILED DESCRIPTION

[0051] For background information, Figure 1 a) illustrates a basic, general prior art DC / DC converter 1, as part of which the present invention may be used. The DC / AC converter 11 is configured to convert a DC voltage and / or current from a DC source, preferably comprising a DC link capacitor connected to its input terminals, into an AC voltage and / or current of medium frequency, i.e. preferably in the frequency range between 500 Hz and 500 kHz. The AC voltage and / or current is fed into an AC intermediate circuit 12, which comprises a transformer 141, in particular a medium frequency transformer (MFT), comprising a primary side and a secondary side and providing galvanic insulation between the primary side and the secondary side. The transformer may in particular consist of a coupling inductor L m and L m’ And the stray inductance L s characterization, wherein one or more primary windings of the transformer are connected via an impedance Z 1 is connected to the DC / AC converter. The inductance element 1 can also be a parasitic inductance, in particular a parasitic inductance of a conductor or other connection. The transformer transforms the voltage and / or current at its primary side into a secondary side voltage and / or current in a known manner. The secondary side voltage and / or current is then converted into a DC voltage and / or current by an AC / DC converter 16, in particular a rectifier, at the output of the AC / DC converter 16. In particular, the DC / AC converter 12 may include a plurality of semiconductor switches, which are arranged as follows: Figure 1 a half-bridge configuration, or arranged as Figure 1 b) is a full-bridge configuration as shown. Also in particular, the AC / DC converter 16 may include a plurality of semiconductor switches arranged in parallel with Figure 1 a corresponding to the half-bridge configuration shown in a, or arranged to correspond to the half-bridge configuration shown in a Figure 1 c) corresponds to the full-bridge configuration shown.

[0052] Figure 2a) illustrates a basic prior art DC / DC dual active bridge (DAB) converter 1', which can be considered as Figure 1 a) is shown as an embodiment of a DC / DC converter 1 and is another potential starting point for the present invention. The DC / AC converter 11 is configured to convert a DC voltage and / or current from a DC source, preferably comprising a DC link capacitor connected to its input terminals, into an AC voltage and / or current of medium frequency, i.e. preferably in a frequency range between 500 Hz and 500 kHz. The AC voltage and / or current is fed into an AC intermediate circuit 14', which comprises a transformer 141', in particular a medium frequency transformer (MFT), which comprises a primary side and a secondary side and provides galvanic insulation between the sides. The transformer can in particular consist of a coupling inductor L m and L m’ And the stray inductance L s Characterization, wherein one or more primary windings of the transformer are connected to the DC / AC converter via an inductor as an impedance element, wherein the inductor is sometimes referred to as an inductor L DAB 1 The transformer transforms the voltage and / or current at its primary side into a secondary side voltage and / or current in a known manner. The secondary side voltage and / or current is then converted into a DC voltage and / or current at the output of the AC / DC converter 16 by an AC / DC converter 16 ′ (in particular a rectifier). The inductance of the optional inductor connected between the secondary side of the transformer and the AD / DC converter is preferably L DAB 2 , the inductance is preferably at least substantially the same as L DAB 1 In particular, the DC / AC converter 12 may include a plurality of semiconductor switches arranged to Figure 1 b) or arranged to correspond to the half-bridge configuration shown in FIG. Figure 1 c) corresponds to the full-bridge configuration shown in FIG. Also in particular, the AC / DC converter 16 may include a plurality of semiconductor switches arranged in parallel with Figure 1 b) or arranged to correspond to the half-bridge configuration shown in FIG. Figure 1 c) A full-bridge configuration corresponding to the full-bridge configuration shown in FIG. A dual active bridge converter is also exemplarily described in Swiss patent application publication CH707533A2 or US patent application publication US2018 / 0159435A1.

[0053] Figure 2 b) illustrates a basic prior art resonant DC / DC converter 1", which can be considered as Figure 1a) is another embodiment of a DC / DC converter 1 and is yet another potential starting point for the present invention. The DC / AC converter 11 is configured to convert a DC voltage and / or current from a DC source, preferably comprising a DC link capacitor connected to its input terminals, into an AC voltage and / or current of medium frequency, i.e. preferably in a frequency range between 500 Hz and 500 kHz. The AC voltage and / or current is fed into an AC intermediate circuit 14", which comprises a transformer 141", in particular a medium frequency transformer (MFT), which comprises a primary side and a secondary side and provides galvanic insulation between the sides. The transformer can in particular consist of a coupling inductor L m and L m’ And the stray inductance L s Characterization, wherein one or more primary windings of the transformer are connected to the DC / AC converter via a capacitor as an impedance element, wherein the capacitance of the capacitor is C res1 The capacitor together with the stray inductance is part of a resonant tank formed by the AC intermediate circuit which can store electrical energy and which is characterized by a resonant frequency which in turn depends on L s and C res1 The capacitor is therefore often referred to as a resonant capacitor. The transformer transforms the voltage and / or current at its primary side into a secondary side voltage and / or current in a known manner. The secondary side voltage and / or current is then converted into a DC voltage and / or current by an AC / DC converter 16 (in particular a rectifier) ​​at the output of the AC / DC converter 16. In particular, the DC / AC converter 12 may include a plurality of semiconductor switches, which are arranged in parallel with each other. Figure 1 b) or arranged to correspond to the half-bridge configuration shown in FIG. Figure 1 c) corresponds to the full-bridge configuration shown in FIG. Also in particular, the AC / DC converter 16 may include a plurality of semiconductor switches arranged in parallel with Figure 1 b) or arranged to correspond to the half-bridge configuration shown in FIG. Figure 1 c) corresponds to the full-bridge configuration shown in FIG. As an alternative to a variant comprising an active bridge as described above and allowing bidirectional power flow, the AC / DC converter 16 may be embodied in particular without semiconductor switches and comprising diodes arranged only in a half-bridge or full-bridge configuration when only unidirectional power flow is required. A resonant DC / DC converter is exemplarily described in PCT patent application publication WO2018 / 141092A1.

[0054] Figure 3 Shown from Figure 1a) is a more detailed schematic diagram of a possible embodiment of a DC / DC converter, wherein two parallel windings are arranged on the primary side 1001 and the secondary side 1002 of the transformer, and the impedance element Z1 and Z2 has been split and distributed between the parallel windings. For background information only, a voltage source connected to the first DC link 10, connected to the second DC link 18 and connected by a resistor R load The resistive load is represented, as well as the reluctance network 19 (grey) of the core and the stray flux of the transformer.

[0055] Figure 4 The coil 100 according to the invention is schematically illustrated. The coil 100 comprises a first winding 101 made of a first conductive foil strip 111 and a second winding 102 made of a second conductive foil strip 112, which have been stacked and coiled to form a number N1=3 turns around a central opening 109 which can receive a Figure 3 The innermost ends of the first winding and the second winding are at T 10 The first connector T is electrically connected to each other and is electrically connected to / is electrically connected to the first terminal T1. 21 and the second connector T 22 The first winding and the second winding are respectively arranged at the outermost ends of the first winding and the second winding and are in contact with them. The electrical insulation layer 113 is arranged between the first conductive foil strip 111 and the second conductive foil strip 112. Another insulating layer 114 is arranged on both sides of the first conductive foil strip 111 and the second conductive foil strip 112. The first winding and the second winding are only connected to each other at T 10 The coil further includes a second terminal T2, a first connector T2 connected to the second terminal and a second connector T3 connected to the innermost end of the winding. 21 and a capacitor 121 connected between the second terminal and the second connector T 22 The second capacitor 122 is disposed between the first and second conductive foil strips. The height h of the coil 100 corresponds to the width w of the first conductive foil strip and the second conductive foil strip.

[0056] For illustration purposes only, Figure 4 Also shown is a DC link 10 and a DC / AC converter 12 ′ comprising an active half-bridge including two semiconductor switches S1 , S2 for applying an AC voltage to the coil 100 .

[0057] Figure 5 A stack of three coils is shown, two first coils 100' and one second coil 100", each of which is substantially identical to the first coil 100 from FIG. Figure 3', although for height the first two coils 100' are h' and the second coil 100" is h. By choosing different values ​​for the capacitances 121', 122' and 121", 122", the current in coil 100' will be different from the current in coil 100". This can be used to advantageously make the current density in the three coils at least approximately the same by choosing appropriate capacitances, including compensating for any possible increased current in the outermost foil 100' due to distortions of the transformer stray fields, in particular the winding window stray fields.

[0058] For illustration purposes only, Figure 5 Also shown is a DC link 10 and a DC / AC converter 12 ′ comprising an active half-bridge including two semiconductor switches S1 , S2 for applying an AC voltage to the coils 100 ′, 100 ″.

[0059] Figure 6 Shown as from Figure 3 Schematic diagram of an exemplary resonant DC / DC converter of an embodiment of a DC / DC converter of FIG. A coil 100 including a pair of M=2 windings and a pair of capacitors 121 , 122 is connected between the output of a DC link 10 and a neutral terminal of the DC link 10 .

[0060] Figure 7 A schematic diagram of another exemplary resonant DC / DC converter is shown, as part of which various embodiments of the coil according to the invention or produced according to the invention can be used. The converter comprises a first DC link 10, a DC / AC converter 212 comprising a plurality of semiconductor switches S1, S2, S3, ..., S6, an AC intermediate circuit 214, an AC / DC converter 216, and a second DC link 18. The converter comprises a plurality of active half-bridges connected to a single first DC link 10, while each output of the plurality of active half-bridges is connected via a first plurality (N=3) of capacitors C resA A single capacitor and a common node C are connected to the primary coil of the medium frequency transformer 2141, which in particular provides galvanic isolation between the primary side and the secondary side of the transformer. A coil 100 comprising a pair of M=2 windings and a pair of capacitors 121, 122 is connected between the common node C and the neutral terminal of the DC link 10. For background information only, a voltage source connected to the first DC link 10, connected to the second DC link 18 and connected by a resistor R load The resistive load is represented, as well as the core's reluctance network 19 (grey) and the stray flux of the transformer 2141.

[0061] Preferred embodiments of the present invention, in particular the embodiments described above, can be implemented in detail in numbered variants and / or embodiments according to the items listed below, advantageously in combination with one or more features detailed above, or according to the claims further presented below.

[0062] 1) A method for producing a coil (100, 100') for a transformer, in particular a medium frequency transformer of a resonant DC / DC converter 1" or a dual active bridge DC / DC converter 1', the method comprising the following steps:

[0063] a) providing a plurality of conductive foil strips (111, 112, 111', 112', 111", 112"), in particular elongated conductive foil strips, with a number M>1, each conductive foil strip having a first end and a second end;

[0064] b) stacking the plurality of conductive foil strips to obtain a foil strip stack having a first end and a second end, wherein the electrical insulation layer 113 is disposed between any two adjacent conductive foil strips;

[0065] c) electrically connecting the first ends of all conductive foil strips to the first terminals (T1, T1');

[0066] d) for each of the conductive foil strips, providing a connector (T 21 , T 22 , T 21’ , T 22’ , T 21” , T 22” );as well as

[0067] e) coiling said foil stack from said first end.

[0068] 2) A method according to variant 1, wherein the foil stack is coiled to form a coil (100, 100') having a central opening 109, in particular a coil extending in a transverse direction.

[0069] 3) The method according to any of the above variations further comprises the following steps:

[0070] a) Provide a second terminal (T2, T 2’ );

[0071] b) providing a plurality of impedance elements, M in number,

[0072] c) For each connector (T 21 , T 22 , T 21’ , T 22’ , T 21” , T 22”), connecting different impedance elements among the impedance elements between the connector and the second terminal.

[0073] 4) The method as described in any of the above variants further comprises the step of pouring insulating material around the coil (100, 100') and preferably around the impedance element, wherein a passage extending through the central opening 109 is provided in the insulating material.

[0074] 5) A method as in any of the previous variants, wherein the impedance elements are capacitors (121, 122, 121', 122', 121", 122"), preferably all capacitors having at least approximately the same capacitance.

[0075] 6) The method as described in any of the above method variants 1 to 4, wherein the impedance elements are inductors, preferably all inductors have at least approximately the same inductance.

[0076] 7) A coil (100, 100') or a winding arrangement for a medium frequency transformer, in particular a resonant DC / DC converter 1" or a dual active bridge DC / DC converter 1', in particular a coil or a winding arrangement produced according to one of the method variants 1 to 6, the coil comprising

[0077] a) A winding pair 101, 102 wound around a central opening 109, the winding pair comprising:

[0078] i) a first winding 101 made of a first conductive foil strip (111, 111', 111") and comprising a first plurality of turns surrounding each other;

[0079] ii) a second winding 102, the second winding being made of a second conductive foil strip (112, 112', 112") and comprising a second plurality of turns surrounding each other, wherein each turn of the first plurality of turns is adjacently surrounded by a turn of the second plurality of turns; wherein,

[0080] iii) the innermost ends of the first winding and the second winding are electrically connected to a first terminal T1 and / or to the first terminal, in particular to provide an output terminal;

[0081] in,

[0082] b) First connector T 21 and the second connector T 21 In particular, the input terminals are respectively arranged at the outermost ends of the first winding and the second winding and are in contact with the outermost ends.

[0083] 8) The coil as described in Example 7, wherein the first connector T 21does not contact the second winding, and the second connector T 21 No contact with the first winding.

[0084] 9) A coil as described in one of embodiments 7 or 8, wherein the winding pairs 101, 102 are wound around each other.

[0085] 10) The coil as described in any one of embodiments 7 to 9, wherein the winding pair 101, 102 is not twisted.

[0086] 11) A coil as described in any one of embodiments 7 to 10, wherein the winding pairs 101, 102 are not transposed.

[0087] 12) The coil as described in any one of embodiments 7 to 11 is further characterized in that the coil further comprises

[0088] a) a second terminal T2;

[0089] b) a first impedance element, the first impedance element is connected between the second terminal and the first connector T 21 between; and

[0090] c) a second impedance element, the second impedance element is connected between the second terminal and the second connector T 22 between.

[0091] 13) The coil as described in the previous embodiment, wherein the impedance elements are capacitors (121, 122, 121', 122', 121", 122"), preferably all capacitors have at least approximately the same capacitance.

[0092] 14) A coil as described in Example 12, wherein the impedance elements are inductors, and preferably all inductors have at least approximately the same inductance.

[0093] 15) The coil of any one of embodiments 7 to 14, wherein the first winding and the second winding are electrically isolated from each other except for the connection at the innermost ends of the windings, in particular via the first terminal T1.

[0094] 16) The coil as described in any one of Examples 7 to 15 is further characterized in that the first winding and the second winding are electrically connected only via the first terminal and are electrically isolated from each other in other ways.

[0095] 17) The coil as described in any one of embodiments 7 to 16 includes at least one additional winding intertwined with the first winding and the second winding and includes a plurality of additional turns surrounding each other, wherein:

[0096] a) the innermost end of the at least one further winding is electrically connected to the first terminal T1;

[0097] b) For each further winding of the at least one further winding at the outermost end of the further winding and in contact therewith, a further connector is provided, in particular as an input terminal.

[0098] 18) The coil as described in the previous embodiment, wherein all windings are electrically connected only via the first terminal and are electrically isolated from each other in other ways.

[0099] 19) A coil as described in any one of Examples 7 to 18, wherein the winding pair or the multiple windings are formed by a stack of coils of conductive foil strips, each conductive foil strip having a first end and a second end; and wherein an insulating layer is arranged between any two adjacent conductive foil strips.

[0100] 20) A transformer, in particular a medium frequency transformer of a resonant DC / DC converter or a resonant solid-state transformer unit, comprising

[0101] a) a core, the core preferably having a core gap;

[0102] b) At least a first coil (100, 100', 100") according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6, wherein the winding pair surrounds at least one section of the core, wherein the section extends through a central opening 109 of the winding.

[0103] 21) The transformer as described in the above embodiment, wherein the first winding 101 and the second winding are primary windings of the transformer.

[0104] 22) The transformer as described in the previous embodiment further includes at least a second coil (100, 100', 100") described in one of embodiments 7 to 19 or produced according to one of method variants 1 to 6, wherein the winding pair of the second coil surrounds at least another section of the core, wherein the another section extends through the central opening of the winding, and wherein the winding pair of the second coil is the secondary winding of the transformer.

[0105] 23) In the transformer as described in the above embodiment, the first winding 101 and the second winding 102 of the second coil are secondary windings of the transformer.

[0106] 24) A transformer as described in one of embodiments 20 to 21, comprising a plurality of coils (100, 100', 100") described in one of embodiments 7 to 19 or produced according to one of method variants 1 to 6, wherein the first terminals of all coils are connected together.

[0107] 25) A transformer as described in the above embodiment, wherein:

[0108] a) the height of the first coil is h1, wherein the impedance elements are capacitors having at least approximately the same capacitance C1;

[0109] b) the height of the second coil is h2, wherein the impedance elements are capacitors having at least approximately the same capacitance C2;

[0110] c) wherein at least approximately C1 / h1=C2 / h2.

[0111] 26) A transformer as described in Example 24, wherein:

[0112] a) the height of the first coil is h1, wherein the impedance elements are all inductors having at least approximately the same inductance L1;

[0113] b) the height of the second coil is h2, wherein the impedance elements are capacitors having at least approximately the same inductance L2;

[0114] c) In which, at least approximately, L1·h1=L2·h2.

[0115] 27) A transformer as described in any one of embodiments 24 to 26, wherein:

[0116] a) two adjacent coils are separated by a coil gap between the adjacent coils;

[0117] b) the core has a core gap, in particular an air gap;

[0118] c) The coil gap is aligned, overlaps and / or coincides with the core gap, in particular with respect to a transverse direction.

[0119] 28) A transformer as described in one of embodiments 20 to 27, wherein the or second coil surrounds the first coil, preferably wherein the first coil extends within and / or extends through the central opening 109 of the winding of the first coil.

[0120] 29) A transformer as described in one of the embodiments, wherein the winding of the first coil is the primary winding of the transformer, and the winding of the second first coil is the secondary winding; or vice versa.

[0121] 30) A resonant DC / DC converter or a resonant solid-state transformer unit, comprising a transformer as described in one of the above embodiments.

[0122] 31) A DC / DC converter, comprising

[0123] a) a first DC link, said first DC link preferably comprising a first DC link capacitor;

[0124] b) an inverter bridge connected to the first DC link;

[0125] c) a transformer, preferably a medium frequency transformer, in particular a transformer as described in one of embodiments 20 to 29, the transformer preferably having a primary side and a secondary side;

[0126] d) The primary side of the transformer comprises at least one coil according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6.

[0127] 32) A DC / DC converter, comprising

[0128] a) a first DC link, said first DC link preferably comprising a first DC link capacitor;

[0129] b) a plurality of inverter bridges, the number of which is N>1, connected in parallel to the first DC link;

[0130] c) a transformer, in particular a transformer as described in one of embodiments 20 to 29, preferably a medium frequency transformer, the transformer preferably having a primary side and a secondary side;

[0131] d) The primary side of the transformer comprises at least one coil according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6; wherein,

[0132] e) The converter further comprises a first number N of a plurality of further impedance elements, wherein:

[0133] f) For each converter bridge, a different impedance element from the first plurality of impedance elements is connected between the converter bridge and the coil, in particular between the converter bridge and a second terminal of the coil.

[0134] 33) A DC / DC converter comprising

[0135] a) a first DC link, said first DC link preferably comprising a first DC link capacitor;

[0136] b) a DC / AC converter having an input connected to the first DC link and comprising:

[0137] i) a converter bridge connected to the first DC link;

[0138] c) an AC intermediate circuit having an input connected to the output of the DC / AC converter and comprising

[0139] i) a transformer, preferably a medium frequency transformer, the transformer having a primary side and a secondary side;

[0140] ii) the primary side comprises at least one coil according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6;

[0141] d) an AC / DC converter having an input connected to the secondary side of the AC intermediate circuit,

[0142] e) A second DC link, preferably a second DC link capacitor, connected to the output of the AC / DC converter.

[0143] 34) A DC / DC converter, comprising

[0144] a) a first DC link, said first DC link preferably comprising a first DC link capacitor;

[0145] b) a DC / AC converter having an input connected to the first DC link and comprising:

[0146] i) a first number N>1 of converter bridges connected in parallel to the first DC link;

[0147] c) an AC intermediate circuit having an AC input connected to the output of the DC / AC converter enter end and includes

[0148] i) a transformer, preferably a medium frequency transformer, the transformer having a primary side and a secondary side;

[0149] ii) the primary side comprises at least one coil according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6;

[0150] d) an AC / DC converter having an input connected to the secondary side of the C intermediate circuit,

[0151] e) a second DC link, preferably a second DC link capacitor, connected to the output of the AC / DC converter; wherein,

[0152] f) The AC intermediate circuit further comprises a first number N of impedance elements, wherein:

[0153] g) For each converter bridge, a different impedance element from the first plurality of impedance elements is connected between the converter bridge and at least one coil, in particular between the converter bridge and a second terminal of the at least one coil.

[0154] 35) A DC / DC converter as described in any one of Examples 28 to 31, wherein each of the converter bridges is an inverter half-bridge, the inverter half-bridge comprising a first input terminal and a second input terminal, and an inverter bridge output terminal (alternatively, conductively connected to the first input terminal or the second input terminal via a plurality of semiconductor switches), wherein, for each inverter half-bridge, different impedance elements among the first plurality of impedance elements are connected in series with the inverter bridge output terminal.

[0155] 36) A DC / DC converter as described in any one of the preceding embodiments, wherein:

[0156] a) the first DC link has a positive terminal and a negative terminal,

[0157] b) the first input terminals of all inverter half-bridges are connected to the positive terminal, and

[0158] c) The second input terminals of all inverter half-bridges are connected to the negative terminal.

[0159] 37) A DC / DC converter as described in any one of Embodiments 31 to 36, wherein the first DC link further has a neutral terminal, and the second terminals of all multiple primary windings are connected to the neutral terminal.

[0160] 38) A DC / DC converter as described in any one of Examples 31 to 37, wherein the converter is a resonant converter and each of the plurality of impedance elements is a capacitor.

[0161] 39) A DC / DC converter as described in any one of Examples 31 to 38, wherein the converter is a dual active bridge converter and each of the multiple impedance elements is an inductor.

[0162] 40) A DC / DC converter as described in any one of embodiments 31 to 3639, further comprising

[0163] a) A second DC link, which preferably comprises a second DC link capacitor

[0164] b) a rectifier bridge connected to the second DC link;

[0165] c) The secondary side of the transformer comprises at least one coil according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6.

[0166] 41) The DC / DC converter as described in any one of Embodiments 31 to 40, further comprising

[0167] a) a second DC link, said second DC link preferably comprising a second DC link capacitor;

[0168] b) a plurality of rectifier bridges, the number of which is N'>1, the rectifier bridges being connected in parallel to the second DC link;

[0169] c) The secondary side of the transformer comprises at least one further coil according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6; wherein,

[0170] d) The converter further comprises a third number N' of further impedance elements, wherein:

[0171] e) For each converter bridge, a different impedance element from a third plurality of impedance elements is connected between the converter bridge and the coil, in particular between the converter bridge and the second terminal of the coil.

[0172] 42) A coil stack comprising a plurality of coils according to one of embodiments 7 to 19 or produced according to one of method variants 1 to 6.

[0173] 43) A coil stack as described in Example 42, wherein the central openings of all coils are aligned relative to each other and / or aligned relative to a lateral direction.

[0174] 44) A coil stack as described in Example 42 or 43, wherein the first terminals of all coils are connected.

[0175] 45) A coil stack as described in one of embodiments 42 to 45, wherein the stack is cast or molded into an insulating material, which surrounds, in particular completely surrounds all coils, in particular is at least substantially aligned with the central opening of all coils, and is in particular configured to receive a coil of a segment or part of a transformer core, wherein the segment or part can be substantially straight and / or elongated so that the segment or part extends through the central opening of all coils.

[0176] 46) A transformer, in particular a medium frequency transformer of a resonant DC / DC converter or a resonant solid-state transformer unit, comprising

[0177] a) a core, the core preferably having an air gap;

[0178] b) A coil stack as described in any one of embodiments 42 to 45, wherein

[0179] c) A section of the core extends through the central opening 109 of all coils of the stack.

[0180] 47) A transformer as described in the above embodiment, wherein:

[0181] a) two adjacent coils in the coil stack are separated by a coil gap between the adjacent coils;

[0182] b) the core has a core gap, in particular an air gap;

[0183] c) The coil gap is aligned, overlaps and / or coincides with the core gap, in particular with respect to a transverse direction.

[0184] Unless otherwise specified, a connection, in particular a connection between any two entities (in particular any two entities including nodes, points, terminals, elements, devices, etc. or combinations thereof) refers to a conductive connection, in particular a conductive connection established by, for example, a wire, cable, busbar, conductive track, trace or line on a (printed) circuit board, solder, etc. The conductive connection is preferably at least substantially direct, in particular without any discrete components, in particular resistors, capacitors, inductors or other passive or active components or devices connected between the connected entities. Thus, the conductive connection has at least substantially negligible resistance, capacitance and inductance, preferably at least substantially zero resistance, capacitance and inductance. In particular, the resistance, capacitance and inductance of the conductive connection are essentially completely parasitic. Further, the resistance, capacitance and inductance of the conductive connection are respectively significantly smaller than the resistance, capacitance and impedance of the resistor, capacitor or inductor connected by the conductive connection and / or by the circuit or network including the conductive connection (preferably smaller by a factor of 1 / 100, 1 / 1000 or 1 / 10000).

[0185] Unless otherwise stated, the electrical connections or electrical connections are the same as those defined above.

[0186] Unless otherwise stated, if two entities, especially including nodes, points, terminals, elements, devices, etc. or a combination thereof, are referred to as being connected, electrically connected or (electrically) connected together, then there is a connection as defined above between the two entities.

[0187] Unless otherwise stated, if a first and a second entity, in particular a first and a second node, point, terminal, element, device, etc. or a combination thereof, is referred to as being connected via a third entity (in particular a third entity comprising a third node, point, terminal, element, device or having such a third entity between them), then the connection as described above existing between the first entity and the third entity also exists between the third entity and the second entity. However, there is no connection as described above between the first entity and the second entity, in particular there is no at least substantially direct connection. If explicitly specified, the third element may also be a connection, in particular a conductor, a wire, a cable, a busbar, etc. In this case, it can be assumed that there is no connection as described above except for the specified connection.

[0188] Unless otherwise stated, it is assumed throughout this patent application that the statement a≈b means |ab| / (|a|+|b|)<10, preferably |ab| / (|a|+|b|)<100, wherein a and b may represent any variable described and / or defined anywhere in this patent application, or any variable as otherwise known to a person skilled in the art. Further, the statement that a is at least approximately equal to or at least approximately identical to b means a≈b, preferably a=b. Further, unless otherwise stated, it is assumed throughout this patent application that the statement a>>b means a>10b, preferably a>100b; and the statement a<<b means 10a<b, preferably 100a<b. Further, unless otherwise stated, it is assumed throughout this patent application that the statement a>>b or a is significantly greater than or much greater than b means a>10b, preferably a>100b; and the statement that a<<b or a is significantly less than or much less than b means 10a<b, preferably 100a<b. Furthermore, a statement that two values ​​a and b substantially deviate from one another or are significantly different means that a≈b does not hold, in particular a>>b or a<<b.

[0189] The present specification and drawings illustrating various aspects and embodiments of the present invention should not be considered as limiting the claims defining the protected invention. In other words, although the present invention has been illustrated and described in detail in the drawings and the preceding description, such illustration and description should be considered illustrative or exemplary, rather than restrictive. Various mechanical, compositional, structural, electrical and operational changes may be made without departing from the spirit and scope of the present specification and claims. In some instances, well-known circuits, structures and techniques are not shown in detail to avoid obscuring the present invention. Therefore, it should be understood that ordinary technicians can make changes and modifications within the scope of the attached claims. In particular, the present invention encompasses additional embodiments having any combination of features from different and / or separate embodiments as described above and below. Embodiments according to the present invention may include additional and / or additional features, elements, aspects, etc. that are not shown in the drawings or the above description.

[0190] The method steps listed in the description, especially in the claims, are preferably performed in the order listed, but may alternatively be performed in any other order as long as it is technically and practically feasible.

[0191] The present disclosure also covers all further features shown individually in the drawings, even though said features may not have been described in the preceding or following description. Furthermore, individual alternatives to the embodiments described in the drawings and the description and individual alternatives to their features may be excluded from the subject matter of the present invention or the disclosed subject matter. The present disclosure includes subject matter consisting of the features defined in the claims or exemplary embodiments as well as subject matter including said features.

[0192] Furthermore, in the claims, the word "comprise" does not exclude further or additional features, elements or steps, etc., and the indefinite article "a" or "an" does not exclude a plurality. A single unit or step may satisfy the functions of several features listed in the claim. The simple fact that certain measures are stated in mutually different dependent claims does not indicate that the combination of these measures cannot be advantageously utilized. As described, the terms "substantially", "about", "approximately", etc. related to attributes, properties or values ​​also particularly precisely include the attributes, properties or values, respectively. In the context of a given numerical value or range, the term "approximately" or "about" refers to a value or range within 20%, within 10%, within 5% or within 2% of a given value or range, for example, and particularly also includes the precise value or range described. The components described as coupled or connected may be directly electrically or mechanically coupled, or they may be indirectly coupled via one or more intermediate components. Any figure mark in the claims should not be interpreted as limiting the scope.

Claims

1. A method for producing a coil (100, 100') of a transformer, the method comprising the steps in the following order: a) providing a plurality of conductive foil strips (111, 112, 111', 112', 111", 112"), the number of which is M>1, each conductive foil strip having a first end and a second end; b) stacking the plurality of conductive foil strips to obtain a foil strip stack having a first end and a second end, wherein: An electrical insulating layer (113) is provided between any two adjacent conductive foil strips; c) electrically interconnecting the first ends of all the conductive foil strips to the first terminals (T1, T1'), so that all the conductive foil strips are connected in parallel at their first terminals; d) for each of said conductive foil strips, providing a connector (T21, T22, T21', T22', T21", T22") at a second end of said foil strip; and e) coiling the foil stack from the first end, The method further comprises the following steps: f) providing a second terminal (T2, T2'); g) providing a plurality of impedance elements, M in number, h) for each connector (T21, T22, T21', T22', T21", T22"), connecting a different one of the impedance elements between the connector and the second terminal; and The impedance values ​​of the plurality of impedance elements, whose number is M, are determined at least based on the height of the coil.

2. The method of claim 1, wherein: The foil stack is coiled to form a coil (100, 100') having a central opening (109).

3. The method according to claim 2, further comprising the step of pouring an insulating material around the coil (100, 100'), wherein: A passage extending through the central opening (109) is provided in the insulating material.

4. The method of claim 2, further comprising the step of pouring an insulating material around the impedance element, wherein: A passage extending through the central opening (109) is provided in the insulating material.

5. The method according to any one of claims 1 to 4, wherein: The impedance elements are capacitors (121, 122, 121', 122', 121", 122").

6. The method of claim 5, wherein: The impedance elements are capacitors (121, 122, 121', 122', 121", 122") having the same capacitance within a range of at least 20%.

7. The method according to any one of claims 1 to 4, wherein: The impedance element is an inductor.

8. The method of claim 7, wherein: All inductors have the same inductance within at least 20%.

9. The method according to any one of claims 1 to 4, wherein: The transformer is a medium frequency transformer for a resonant DC / DC converter (1") or a dual active bridge DC / DC converter (1').

10. A coil (100, 100') for a transformer, the transformer being used as a medium frequency transformer of a resonant DC / DC converter (1") or a dual active bridge DC / DC converter (1'), the coil being produced according to the method according to any one of claims 1 to 9, the coil comprising: a) a winding pair (101, 102), the winding pair comprising: i) a first winding (101) made of a first conductive foil strip (111, 111', 111") and comprising a first plurality of turns surrounding each other; ii) a second winding (102) made of a second conductive foil strip (112, 112', 112") and comprising a second plurality of turns surrounding each other, wherein each turn of the first plurality of turns is adjacently surrounded by a turn of the second plurality of turns; wherein, iii) the innermost ends of the first winding and the second winding are electrically interconnected and connected to a first terminal (T1) and / or connected to a first terminal; and the first winding and the second winding (101, 102) are electrically isolated from each other in addition to being connected; in, b) a first connector (T21) and a second connector (T22) are respectively arranged at the outermost ends of the first winding and the second winding and are in electrical contact with the first winding and the second winding respectively; c) a second terminal (T2), the second terminal having a first impedance element connected between the second terminal (T2) and the first connector (T21), and a second impedance element connected between the second terminal (T2) and the second connector (T22), and The impedance values ​​of the first impedance element and the second impedance element are determined based on at least the height of the coil.

11. The coil according to claim 10, wherein The first impedance element and the second impedance element are capacitors (121, 122, 121', 122', 121", 122"), all capacitors having the same capacitance at least within a range of 20%.

12. The coil according to claim 11, wherein The capacitors (121, 122, 121', 122', 121", 122") have the same capacitance within a range of at least 20%.

13. The coil according to claim 10, wherein: The first impedance element and the second impedance element are inductors.

14. The coil according to claim 13, wherein All inductors have the same inductance within at least 20%.

15. The coil according to any one of claims 10 to 14, wherein At the innermost ends of the windings, the first and second windings (101, 102) are electrically isolated from each other except for a connection via the first terminal (T1).

16. The coil according to any one of claims 10 to 14, wherein The winding pair (101, 102) is formed by a coil stack of conductive foil strips (111, 111', 111", 112, 112', 112"), each conductive foil strip having a first end and a second end; and wherein an insulating layer is provided between any two adjacent conductive foil strips.

17. The coil according to any one of claims 10 to 14, wherein the transformer is a medium frequency transformer for a resonant DC / DC converter (1") or a dual active bridge DC / DC converter (1').

18. A transformer, comprising: a) core; b) at least a first coil (100, 100', 100"), the first coil being a coil according to any one of claims 10 to 17 or produced by a method according to any one of claims 1 to 9, wherein the coil comprises a winding pair surrounding at least one section of the core, wherein the section extends through a central opening (109) of the first coil.

19. The transformer of claim 18, wherein: The transformer is a medium frequency transformer for a resonant DC / DC converter or a resonant solid-state transformer unit.

20. The transformer of claim 18, wherein: The core has an air gap.

21. A transformer, comprising a plurality of coils (100, 100', 100") according to any one of claims 10 to 17 or a plurality of coils (100, 100', 100") produced by the method according to any one of claims 1 to 9, wherein: The first terminals of all coils are connected together.

22. A transformer, wherein: a) the height of the first coil is h1, wherein the first impedance element and the second impedance element of the first coil are capacitors having the same capacitance C1 within a range of at least 20%; b) the height of the second coil is h2, wherein the first impedance element and the second impedance element of the second coil are capacitors having the same capacitance C2 at least within a range of 20%; c) wherein at least approximately C1 / h1=C2 / h2, Wherein, the first coil and the second coil are produced by the coil according to any one of claims 10-17 or the method according to any one of claims 1-9.

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