Winding for planar transformer

Abstract

The winding (12) for a planar power transformer comprises at least one stack (50) of conductive tracks (52, 53, 54A, 55, 56, 57, 58). The conductive tracks comprise a first group of primary turns forming the winding (12) and a second group of secondary turns forming the winding (12), the first group being composed of at least one conductive track referred to as the primary conductive track (52, 53) and the second group being composed of at least one conductive track referred to as the secondary conductive track (55, 56, 57, 58). The first group comprises at least one pair (134) of primary conductive tracks consisting of primary conductive tracks (52, 53) electrically connected to each other by at least one first electrical connection (136). At least one secondary conductive track (55, 56, 57, 58) extends perpendicular to the first electrical connector (136).

Description

Technical Field

[0001] The invention relates to a coil for a planar power transformer of this type comprising at least one stack of conductive tracks comprising: a first group forming a primary winding of the coil, the first group consisting of at least one so-called primary conductive track; and a second group forming a secondary winding of the coil, the second group consisting of at least one so-called secondary conductive track.

[0002] Such a coil can be combined with a magnetic circuit to form a planar power transformer. Such a transformer has the advantage of being very compact and capable of supplying high amplitude and high frequency currents.

[0003] The invention also relates to a method for producing such a coil. Background Art

[0004] The power requirements of modern aircraft require high performance and reliable power conversion between the power source and the equipment (such as lighting, pressurization and avionics components) on the aircraft that consumes the power. Specifically, while the power source usually produces AC power at a voltage of 115 volts or DC power at a voltage of 28 volts, new equipment components usually require a supply current at a higher voltage (usually 540 volts). Static power transformers are then used to convert the voltage generated by the energy source to a voltage corresponding to the requirements of the equipment components.

[0005] These transformers usually consist of a copper coil associated with a magnetic circuit. The coil consists of two types of windings: a winding of primary turns, usually called the primary winding; and a winding of secondary turns, usually called the secondary winding. The first of the windings (usually the primary winding) receives electrical energy and generates a magnetic field. The other winding is crossed by the magnetic field generated by the first winding and provides an alternating current with the same frequency but a voltage that can be greater or less than the voltage across the terminals of the first winding.

[0006] Two main transformer technologies are known.

[0007] A conventional transformer technology is the so-called coil transformer, in which the primary and secondary windings are made of enameled copper wire. These transformers are relatively heavy and have a large volume and high production costs. In addition, these transformers show an inconvenient dispersion in their electrical properties, which is related to the manufacturing method of these transformers. Finally, these transformers have high so-called AC (alternating current) losses. These AC losses include losses caused by the alternating nature of the current flowing through the transformer, and these AC losses are particularly caused by the skin effect, the proximity effect and the stacking effect. Compared with the real impedance, these AC losses increase with the increase of the operating frequency of the transformer. Therefore, these AC losses limit the use of high-frequency wound transformers.

[0008] The most recent transformer technology is the planar transformer, in which the turns of the winding are formed by conductive tracks of small thickness. These conductive tracks can be made of conductive layers of a PCB (printed circuit board), or of "foils" cut into sheets of conductive material and pressed flat against an insulating backing.

[0009] Typically, the primary turns and the secondary turns of these planar transformers are stacked, i.e., they are placed in different planes (or layers). Typically, the turns are then electrically connected to each other by electrical connections extending over the entire height of the stack. Each conductive track then comprises a turn-forming portion, and an offset portion electrically connecting the turn-forming portion to the connection hole.

[0010] These planar transformers have the advantage of being compact, in particular by having a reduced height and a small volume, and have high efficiency and good power density. In addition, these planar transformers have reduced AC losses compared to coil transformers. However, even if the AC losses of planar transformers are already small, it is desirable to further reduce these AC losses so that the performance of these transformers can be improved, in particular so that these transformers can be used at increasingly higher frequencies. Summary of the invention

[0011] An object of the present invention is to reduce the AC losses of a planar transformer by utilizing stacked turns.

[0012] To this end, according to a first aspect, the invention relates to a coil for a planar power transformer, the coil comprising at least one stack of conductive tracks, the stack consisting of a printed circuit consisting of a plurality of printed circuit boards, the conductive tracks comprising a first group forming a primary winding of the coil and a second group forming a secondary winding of the coil, the first group consisting of at least one so-called primary conductive track and the second group consisting of at least one so-called secondary conductive track, wherein the first group comprises at least one pair of primary conductive tracks consisting of primary conductive tracks formed in the same printed circuit board and electrically connected to each other by at least one first electrical connection, and wherein at least one secondary conductive track of the second group is formed in the same printed circuit board and belongs to an outer layer of the printed circuit board and extends perpendicularly to the first electrical connection, the first electrical connection connecting the primary conductive tracks of the pair of primary conductive tracks.

[0013] According to a specific embodiment of the present invention, the coil also has one or more of the following features, which may be adopted individually or in any technically feasible combination:

[0014] - the coil comprises at least one sintered metal disk which is inserted between two printed circuit boards of the stack and electrically connects two conductive tracks belonging to the outer layers of the stacked printed circuit boards to each other;

[0015] - the conductive tracks electrically connected to each other by the sintered metal disk are secondary conductive tracks, at least one primary conductive track extending perpendicularly to the sintered metal disk;

[0016] - the conductive tracks electrically connected to each other by the sintered metal disks are secondary conductive tracks, each of said secondary conductive tracks forming a turn;

[0017] - each primary conductive track constituting the pair of primary conductive tracks forms a turn;

[0018] - for the pair of primary conductive tracks or at least one pair of primary conductive tracks, the second group includes a pair of secondary conductive tracks, the pair of secondary conductive tracks being composed of secondary conductive tracks formed in the same printed circuit board as the pair of primary conductive tracks, the pair of secondary conductive tracks sandwiching the pair of primary conductive tracks, the secondary conductive tracks respectively forming one turn and being electrically connected to each other by at least one second electrical connection member crossing the printed circuit board;

[0019] - each of the primary conductive tracks constituting the pair of primary conductive tracks forms a turn, the turn comprising two longitudinal branches spaced transversely from each other, each longitudinal branch having a first longitudinal end and an opposite second longitudinal end, and an end transverse bar connecting the two longitudinal ends of the two branches to each other, each of the pair of primary conductive tracks further comprising an intermediate transverse bar connecting the two branches to each other at a distance from the respective longitudinal ends of the two branches and being interposed between the or each first electrical connector and the or each second electrical connector, each secondary conductive track constituting the pair of secondary conductive tracks extending perpendicularly to the intermediate transverse bar;

[0020] - each of the pair of primary conductive rails further comprises a bend extending from a first longitudinal end of a first longitudinal branch to a second longitudinal branch, wherein a free end of the bend is spaced apart from the second longitudinal branch, thereby leaving a passage between the second longitudinal branch and the bend;

[0021] - a second group comprising a plurality of pairs of secondary conducting tracks, with insulating layers interposed between second electrical connectors electrically connecting different pairs of secondary conducting tracks;

[0022] - the stack is formed by a printed circuit, the first electrical connection or each first electrical connection being formed by a non-through connection hole;

[0023] - the or each second electrical connection consists of a connection hole; and

[0024] The first electrical connection element or each first electrical connection element is constituted by a recessed connection hole.

[0025] According to a second aspect, the invention also relates to a planar transformer comprising a coil according to the first aspect.

[0026] According to a third aspect, the invention also relates to a method for manufacturing a coil for a planar power transformer, the coil being a coil according to any one of the preceding claims, the method comprising the step of producing a printed circuit board, the production step comprising the following sub-steps:

[0027] - providing a substrate made of a dielectric material,

[0028] - producing a first turn forming a conductive track on a first face of the substrate,

[0029] - producing a second turn forming a conductive track on a second face of the substrate,

[0030] - producing at least one first electrical connection through the substrate to electrically connect a first conductive track and a second conductive track to each other, said first conductive track forming a pair of primary conductive tracks of a first set, said first set forming a primary winding of the coil,

[0031] - depositing a first insulating layer on the first face of the substrate, the first insulating layer covering the or each first electrical connection, and

[0032] - producing, on a face of the first insulating layer opposite to the substrate, a third turn-forming conductive track, said third conductive track belonging to a second group forming a secondary winding of the coil, said second group extending perpendicularly to the or each first electrical connection connecting a primary conductive track of said pair of primary conductive tracks.

[0033] According to a specific embodiment of the present invention, the manufacturing method further has one or more of the following features, which may be adopted individually or in any technically feasible combination:

[0034] - The method comprises the following additional steps:

[0035] o depositing a second insulating layer on the second side of the substrate, the second insulating layer covering the or each connecting hole, and

[0036] o Produce a fourth turn on a face of the second insulating layer opposite to the substrate to form a conductive track;

[0037] - a fourth conductive track extending perpendicularly to the or each first connection hole;

[0038] - the manufacturing method comprises the additional step of producing at least one second connecting hole through the substrate and the two insulating layers to electrically connect the third and fourth conductive tracks to each other; and

[0039] - repeating the production steps to produce at least a first printed circuit board and a second printed circuit board, the manufacturing method further comprising the steps of:

[0040] ○ stacking the first printed circuit and the second printed circuit board, and

[0041] o Sintering and laminating the first and second printed circuit boards to form an electrical connection between one of the turn-forming conductive tracks of the first printed circuit board and one of the turn-forming conductive tracks of the second printed circuit board. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Other characteristics and advantages of the invention will become apparent on reading the following description given by way of example only and with reference to the accompanying drawings, in which:

[0043] - Figure 1 is a perspective view of a planar power transformer according to an exemplary embodiment of the present invention,

[0044] - Figure 2 yes Figure 1 The wiring diagram of the transformer,

[0045] - Figure 3 yes Figure 1 A perspective view of the coil of a planar transformer,

[0046] - Figure 4 yes Figure 3 A top view of a coil made of dielectric material. The layers of dielectric material that make up the coil remain transparent, allowing certain production details to become apparent.

[0047] - Figure 5 yes Figure 3 The coil is along Figure 4 The layers of dielectric material that make up the coil remain transparent, allowing certain production details to become apparent.

[0048] - Figure 6 is with Figure 5 A similar view showing only the first set of conducting tracks making up the coil,

[0049] - Figure 7 is with Figure 5 A similar view showing only the second set of conductive tracks making up the coil,

[0050] - Figure 8 It is composed of Figure 3a top view of a first type of conductive track of a first winding of a coil,

[0051] - Fig. 9 It is composed of Figure 3 a top view of a second type of conductive track of a first winding of a coil,

[0052] - Fig.10 It is composed of Figure 3 a top view of a third type of conductive track of a first winding of a coil,

[0053] - Fig.11 It is composed of Figure 3 a top view of a first type of conductive track of a second winding of a coil,

[0054] - Fig.12 It is composed of Figure 3 a top view of a second type of conductive track of a second winding of the coil,

[0055] - Fig.13 It is composed of Figure 3 a top view of a third type of conductive track of a second winding of a coil,

[0056] - Fig.14 It is composed of Figure 3 a top view of a fourth type of conductive track of a second winding of a coil,

[0057] - Fig.15 is shown for manufacturing Figure 1 A diagram of the planar transformer method, and

[0058] - Fig.16 It shows Fig.15 FIG. 1 is a diagram of steps in a manufacturing method for manufacturing a printed circuit board. DETAILED DESCRIPTION

[0059] Figure 1 The planar power transformer 10 shown is typically equipped with an isolated DC-DC voltage converter (not shown) operating at a cut-off frequency between 100 kHz and a few MHz. The voltage converter is, for example, an independent voltage converter. In a variant, the voltage converter belongs to a conversion line intended to convert an AC voltage into a DC voltage (or vice versa), the conversion line comprising a DC-AC voltage converter located upstream or downstream of the DC-DC converter.

[0060] The isolated DC-DC voltage converter is, for example, part of a plasma propulsion engine (not shown); typically, the isolated DC-DC voltage converter supplies power to an anode (not shown) of the plasma engine. In a variant, the isolated DC-DC converter is embedded on an aircraft and supplies power to at least one equipment component of the aircraft, such as a lighting system, a pressurization system or an avionics system.

[0061] The planar power transformer 10 comprises a coil 12 and contact pads 14, 15, 16, 17. In the example shown, the planar power transformer further comprises a magnetic circuit 18.

[0062] like Figure 2 As can be seen in FIG. 1 , the coil 12 includes a primary winding 20 electrically connected to the first contact pad 14 and the second contact pad 15 of the contact pads 14, 15, 16, 17. The coil also includes a secondary winding 22 electrically connected to the third contact pad 16 and the fourth contact pad 17 of the contact pads 14, 15, 16, 17.

[0063] Typically, the primary winding 20 is connected to an electrical energy source (e.g., a battery), and the secondary winding 22 is connected to an electrical energy consumer. The primary winding 20 is preferably connected to the electrical energy source via a low-voltage DC bus (e.g., between 70 volts and 100 volts), advantageously having a high-frequency inverter, such as a full-bridge or half-bridge type, providing a conversion of a DC voltage to an AC voltage to power the primary winding 20. The secondary winding is preferably connected to the electrical energy consumer component via a high-voltage DC bus (e.g., between 300V and 600V), advantageously having a rectifier, such as a controlled or uncontrolled type, with a single AC or a dual AC, providing a conversion of an AC voltage to a DC voltage, for powering the high-voltage DC bus using the current leaving the secondary winding 22.

[0064] The primary winding 20 includes a plurality of primary turns 24, preferably an even number of primary turns. Here, these primary turns 24 are distributed between a plurality of branches 26A, 26B, 26C connected in parallel to the first contact pad 14 and the second contact pad 15. Each branch 26A, 26B, 26C includes a plurality of primary turns 24 (here two primary turns) connected in series. In the example shown, the number of these branches 26A, 26B, 26C is three.

[0065] The secondary winding 22 comprises a plurality of secondary turns 28, preferably an even number of secondary turns, which are connected in series between the third contact pad 16 and the fourth contact pad 17. In the example shown, the number of secondary turns 28 is equal to the number of primary turns 24.

[0066] like Figure 3It can be seen in FIG. 1 that the primary turns 24 and the secondary turns 28 are stacked one above the other in the stacking direction Z. In particular, each primary turn 24 and each secondary turn 28 extends around a recess 30 oriented in the stacking direction Z and common to said turns 24 , 28 .

[0067] Here and in the rest of the text, the terms of orientation are to be understood with reference to the directly orthogonal reference systems X, Y, Z shown in the drawings, in which, in addition to the stacking direction Z, it is possible to discern:

[0068] - a longitudinal direction X orthogonal to the stacking direction Z, and

[0069] A transverse direction Y, which is orthogonal to the longitudinal direction X and to the stacking direction Z and which forms with the longitudinal direction U a so-called horizontal plane.

[0070] Each turn 24 , 28 extends substantially in a horizontal plane.

[0071] In the example shown, the primary turns 24 and the secondary turns 28 are staggered, i.e. the coil 12 has an alternation of primary turns 24 and secondary turns 28 along the stacking direction Z. In particular, each pair of primary turns 24 belonging to the same branch 26A, 26B, 26C is sandwiched between a pair of secondary turns 28. The turns 28 in the pair of secondary turns 28 are specific to the pair, i.e. each turn 28 in the pair of secondary turns 28 is different from any other turn 28 belonging to the pair of secondary turns 28 sandwiching the other pair of primary turns 24. Furthermore, the two turns 24 constituting the pair of primary turns 24 are arranged consecutively along the stacking direction Z, i.e. no secondary turn 28 is interposed between the two primary turns 24.

[0072] The stack of turns 24, 28 thus comprises the following sequence of turns:

[0073] - a secondary turn 28,

[0074] - two primary turns 24 connected in series,

[0075] - two secondary turns 28,

[0076] - two primary turns 24 connected in series,

[0077] - two secondary turns 28,

[0078] - two primary turns 24 connected in series, and

[0079] - a secondary turn 28 .

[0080] This alternation of primary turns 24 and secondary turns 28 achieves good coupling of the primary winding 20 to the secondary winding 22 and minimizes losses due to minimization of the H-field.

[0081] Back to Figure 1 , the magnetic circuit 18 is formed in a manner known to those skilled in the art by a block 32 of ferromagnetic material, generally made of ferrite, traversed by two longitudinal windows 34, 36 through which the turns 24, 28 of the coil 12 pass. The magnetic circuit 18 thus has an upper plate 38 and a lower plate 40, as well as two outer legs 42 and a central leg 44 connecting the upper plate 38 and the lower plate 40 to each other. The central leg 44 engages in the recess 30 and around which each turn 24, 28 surrounds.

[0082] In the example shown, the block 32 has a topology of the double E type, that is, it is formed by two superimposed bodies 46, 48, each in the shape of an E, the branches of which partially form the legs 42, 44 and the vertical line of which forms one of the plates 38, 40. In variants, the block 32 has a topology of the EI, CI, CC type or any other topology known to a person skilled in the art.

[0083] refer to Figure 5 , the coil 12 is constituted by a stack 50 of conductive tracks 52, 53, 54A, 54B, 55, 56, 57, 58, which are insulated in pairs by a layer of dielectric material 59. Here, this stack 50 is constituted by a printed circuit 51, the conductive tracks 52, 53, 54A, 54B, 55, 56, 57, 58 being constituted by a conductive layer of said printed circuit 51. In a variant (not shown), the conductive tracks 52, 53, 54A, 54B, 55, 56, 57, 58 are respectively constituted by a foil of conductive material.

[0084] In particular, the printed circuit 51 is constituted by a plurality of multilayer PCBs (Printed Circuit Boards) 61 sintered one above the other. For example, each printed circuit board 61 has a thickness substantially equal to 3 mm measured along the stacking direction Z. In practice, this parameter is adjusted according to the application.

[0085] The conductive tracks 52, 53, 54A, 54B, 55, 56, 57, 58 are made of copper, for example. In a variant, these conductive tracks are made of silver or aluminium.

[0086] The conductive tracks comprise a first group 60A ( Figure 6 ), the first group 60A consists of so-called primary conductive tracks 52, 53, 54A, 54B (track 54B, see Figure 3 The conductive track also includes a second group 60B ( Figure 7 ), the second group 60B consists of so-called secondary conductive tracks 55, 56, 57, 58.

[0087] The primary conductive tracks include conductive tracks 52 of a first type, conductive tracks 53 of a second type, and conductive tracks 54A, 54B of a third type. The secondary conductive tracks include, with respect to the stacking direction Z, end conductive tracks 55 of a first type, end conductive tracks 56 of a second type, intermediate conductive tracks 57 of a first type, and intermediate conductive tracks 58 of a second type. Conductive tracks of the same type are substantially identical to one another.

[0088] refer to Figure 8 , each primary conductive track 52 of the first type forms a turn. To this end, the primary conductive track of the first type comprises two longitudinal branches 62, 63 spaced apart from each other transversely, each longitudinal branch having a first longitudinal end 64 and an opposite second longitudinal end 66, and an end transverse bar 68 connecting the two longitudinal ends 66 of the two branches 62, 63 to each other. The primary conductive track of the first type also comprises a bend 70 extending from the first longitudinal end 64 of the first longitudinal branch 62 of the longitudinal branches 62, 63 to the second longitudinal branch 63. The free end 71 of the bend 70 is spaced apart from the second longitudinal branch 63, so that a channel 72 is left between the second longitudinal branch 63 and the bend 70.

[0089] The length of the bend 70 measured along the transverse direction Y is greater than half, in particular greater than two thirds, of the transverse spacing between the longitudinal branches 62 , 63 . In particular, the length of the bend 70 is substantially equal to three quarters of the transverse spacing between the longitudinal branches 62 , 63 .

[0090] The longitudinal branches 62, 63, the rod 68 and the bend 70 have substantially equal widths. The width of the longitudinal branches 62, 63 is measured along the transverse direction Y. The width of the rod 68 and the bend 70 is measured along the longitudinal direction X.

[0091] In particular, the second branch 63 is longer than the first branch 62 so that the first longitudinal end 64 of the second branch protrudes longitudinally relative to the first longitudinal end 64 of the first branch 62. Therefore, the first longitudinal end 64 of the second branch 63 has a through hole 73 for passing the first interconnection pad 14.

[0092] In the example shown, each primary conductive track 52 of the first type also comprises an intermediate transverse bar 74 connecting the two branches 62 , 63 to one another at a distance from their respective longitudinal ends 66 .

[0093] The intermediate transverse rod 74 is inserted between the bent portion 70 and the end transverse rod 68. In particular, the intermediate transverse rod is closer to the end transverse rod 68 than the bent portion 70.

[0094] The middle transverse bar 74, together with the longitudinal branches 62, 63 and the bent portion 70, defines a cross section of the recess 30. The middle transverse bar also defines a window 76 together with the longitudinal branches 62, 63 and the end bar 68.

[0095] refer to Fig. 9 , each second type of primary conductive track 53 is constituted by a symmetrical image of a first type of primary conductive track 52 with respect to the longitudinal plane XZ. Therefore, the above description of the first type of primary conductive track 52 applies mutatis mutandis, except for the detail that the through-hole 73 is here adapted to pass the second interconnection pad 15 (instead of the first interconnection pad 14, as was the case with the first type of primary conductive track 52).

[0096] refer to Fig.10 Each third type of primary conductive track 54A, 54B has a thin strip shape 80 perforated at the first longitudinal end 82 of its longitudinal ends 82, 84, and has a through hole 86 for passing the first interconnection pad 14 or the second interconnection pad 15. The transverse dimension of the thin strip 80 is substantially equal to the width of the longitudinal branches 62, 63 of the first type of primary conductive track 52 and the second type of primary conductive track 53. The longitudinal dimension of the thin strip 80 is smaller than the difference in length between the longitudinal branches 62, 63 of the first type of primary conductive track 52 and the second type of primary conductive track 53.

[0097] refer to Fig.11 , the first type of end secondary conductive track 55 forms a turn. To this end, the first type of end secondary conductive track comprises two longitudinal branches 90, 92 spaced apart from each other transversely, each longitudinal branch having a first longitudinal end 94 and an opposite second longitudinal end 96, and a transverse bar 98 connecting the first longitudinal ends 94 of the branches 90, 92 to each other. The first type of end secondary conductive track also comprises a bend 100 extending from the second longitudinal end 96 of the first longitudinal branch 90 of the longitudinal branches 90, 92 to the second longitudinal end 92. The free end 101 of the bend 100 is spaced apart from the second longitudinal end 192, so that a channel 102 is left between the second longitudinal branch 92 and the bend 100.

[0098] The lateral spacing between the longitudinal branches 90 , 92 of the secondary conductive track 55 is substantially equal to the lateral spacing between the longitudinal branches 62 , 63 of the primary conductive tracks 52 , 53 of the first and second types.

[0099] The length of the bend 100 measured along the transverse direction Y is greater than half, in particular greater than two thirds, of the transverse spacing between the longitudinal branches 90 , 92 . In particular, the length of the bend 100 is substantially equal to three quarters of the transverse spacing between the longitudinal branches 90 , 92 .

[0100] The longitudinal branches 90 , 92 , the rod 98 and the bend 100 together define a cross section of the recess 30 .

[0101] The longitudinal branches 90, 92, the rods 98 and the bends 100 have substantially equal widths. The width of the longitudinal branches 90, 92 is measured along the transverse direction Y. The width of the rods 98 and the bends 100 is measured along the longitudinal direction X. This width is substantially equal to the width of the longitudinal branches 62, 63 of the first type 52 and the second type 53 of the primary conductive track and the width of the rods 68 and the width of the bends 70.

[0102] The second branch 92 is longer than the first branch 90 such that a second longitudinal end 96 of the second branch protrudes longitudinally relative to the second longitudinal end 96 of the first branch 90. The difference in length between the longitudinal branches 90, 92 is greater than the length of the branch extensions 74, 76 of the first and second type of primary conducting rails 52, 53 measured along the longitudinal direction X.

[0103] Here, the first longitudinal end portion 94 of the second branch 92 has a through hole 103 for passing the third interconnection pad 16 therethrough.

[0104] The first type of secondary conductive track 55 further comprises a protruding portion 104 protruding longitudinally outwardly from the bend 100. The protruding portion 104 is spaced apart from the second branch 92. The longitudinal dimension of the protruding portion is smaller than the length difference between the longitudinal branches 90, 92. The longitudinal dimension is typically between 1.5 and 2 times the width of the bend 100.

[0105] In the example shown, the first longitudinal branch 90 extends as a tapered edge 106 filling the space between the protruding portion 104 and the second end 96 of said first longitudinal branch 90 .

[0106] refer to Fig.12 , the end secondary conductive track 56 of the second type is constituted by a symmetrical image of the end secondary conductive track 55 of the first type with respect to the longitudinal plane XZ. Therefore, the above description of the end secondary conductive track 55 of the first type applies mutatis mutandis, except for the detail that the through-hole 103 is here adapted to pass the fourth interconnection pad 17 (instead of the third interconnection pad 16, as was the case with the end secondary conductive track 55 of the first type).

[0107] refer to Fig.13, each first type of intermediate secondary conductive track 57 forms a turn. To this end, each first type of intermediate secondary conductive track comprises two longitudinal branches 110, 112 spaced apart from each other transversely, each longitudinal branch having a first longitudinal end 114 and an opposite second longitudinal end 116, and a transverse bar 118 connecting the first longitudinal ends 114 of the two branches 110, 112 to each other. Each first type of intermediate secondary conductive track also comprises a first bend 120 extending from the second longitudinal end 116 of the first longitudinal branch 110 of the longitudinal branches 110, 112 to the second longitudinal branch 112. The free end 121 of the first bend 120 is spaced apart from the second longitudinal branch 112, so that a channel 122 is left between the second longitudinal branch 112 and the bend 120.

[0108] The lateral spacing between the longitudinal branches 110 , 112 is substantially equal to the lateral spacing between the longitudinal branches 62 , 63 of the primary conductive tracks 52 , 53 of the first and second types.

[0109] The length of the bend 120 measured along the transverse direction Y is greater than half, in particular greater than two thirds, of the transverse spacing between the longitudinal branches 110 , 112 . In particular, the length of the bend 120 is substantially equal to three quarters of the transverse spacing between the longitudinal branches 110 , 112 .

[0110] The longitudinal branches 110 , 112 , the rod 118 and the bent portion 110 together define a cross section of the recess 30 .

[0111] The second branch 112 is longer than the first branch 110 such that a second longitudinal end 116 of the second branch protrudes longitudinally relative to the second longitudinal end 116 of the first branch 110. The difference in length between the longitudinal branches 110, 112 is substantially equal to the length of the branch extensions 74, 76 of the first type 52 and the second type 53 of primary conducting tracks measured along the longitudinal direction X.

[0112] Each first type of intermediate secondary conductive track 57 further comprises a second bend 124 that protrudes transversely from the second longitudinal end 116 of the second longitudinal branch 112 to the first longitudinal branch 110. The transverse dimension of the second bend 124 is substantially equal to the transverse dimension of the first bend 120. The free end 125 of the second bend is opposite to the junction of the second bend and the second longitudinal branch 112.

[0113] The longitudinal branches 110, 112, the rod 118 and the first 120 and second 124 bends have substantially equal widths. The width of the longitudinal branches 110, 112 is measured along the transverse direction Y. The width of the rod 118 and the two bends 120, 124 is measured along the longitudinal direction X. This width is substantially equal to the width of the longitudinal branches 62, 63 of the first type 52 and the second type 53 of the primary conductive track and the width of the rod 68 and the width of the bend 70.

[0114] Each first-type intermediate secondary conductive track 57 further includes a protruding portion 126 protruding longitudinally outward from the bend portion 120. The protruding portion 126 is spaced apart from the second branch 112. The longitudinal dimension of the protruding portion 126 is smaller than the longitudinal space between the first bend portion 120 and the second bend portion 124. The longitudinal dimension is greater than half of the longitudinal space between the first bend portion 120 and the second bend portion 124, in particular greater than two-thirds of the space, and generally substantially equal to three-quarters of the space.

[0115] Furthermore, the longitudinal dimension of the protruding portion 126 is substantially equal to the longitudinal dimension of the protruding portion 104 of the end secondary conductive tracks 55 , 56 .

[0116] In the example shown, the first longitudinal branch 110 extends as a tapered portion 128 filling the space between the protruding portion 126 and the second end 116 of said first longitudinal branch 110 .

[0117] refer to Fig.14 , each intermediate secondary conductive track 58 of the second type consists of a symmetrical image of an intermediate secondary conductive track 57 of the first type with respect to the longitudinal plane XZ. Therefore, the above description of the intermediate secondary conductive tracks 57 of the first type applies mutatis mutandis.

[0118] The first longitudinal ends 64, 94, 114 of the longitudinal branches 62, 63, 90, 92, 110, 112 of the conductive tracks 52, 53, 55, 56, 57, 58 are located on the same first side 130 ( Figure 3 The second longitudinal ends 66, 96, 116 of the longitudinal branches 62, 63, 90, 92, 110, 112 of the conductive tracks 52, 53, 55, 56, 57, 58 are located on the same second side 132 (opposite to the first side of the recess 30) as the first side of the recess 30. Figure 3 )superior.

[0119] The conductive tracks 52, 53, 55, 56, 57, 58 are arranged such that:

[0120] the intermediate bars 74 of the primary rails 52 , 53 are substantially aligned with each other in the stacking direction Z and with the bends 100 of the end secondary rails 55 , 56 and with the first bends 120 of the intermediate secondary rails 57 , 58 ,

[0121] the first longitudinal branches 62 , 90 , 110 of the primary rails 52 , 53 and of the secondary rails 55 , 56 , 57 , 58 are substantially aligned with one another along the stacking direction Z,

[0122] the second longitudinal branches 63 , 92 , 112 of the primary rails 52 , 53 and of the secondary rails 55 , 56 , 57 , 58 are substantially aligned with one another along the stacking direction Z,

[0123] the bends 70 of the primary rails 52 , 53 are substantially aligned with each other in the stacking direction Z and with the rods 98 , 118 of the secondary rails 55 , 56 , 57 , 58 , and

[0124] The end bars 68 of the primary rails 52 , 53 are substantially aligned with each other in the stacking direction Z and with the second bends 124 of the intermediate secondary rails 57 , 58 .

[0125] Thus, the superposition of the primary turns 24 and the secondary turns 28 is optimized, which enables good electromagnetic coupling of said turns 24 , 28 and reduces AC losses.

[0126] refer to Figure 6 The first type of primary conductive tracks 52 and the second type of primary conductive tracks 53 are grouped into pairs 134 of primary conductive tracks. Each pair 134 consists of a first type of primary conductive track 52 and a second type of primary conductive track 53 electrically connected to each other via a recessed first connection hole 136.

[0127] Each pair 134 forms a branch 26A, 26B, 26C of the primary winding 20 .

[0128] The primary conductive tracks 52, 53 constituting each of these pairs 134 are formed in the same printed circuit board 61, and in particular belong to an inner layer of the printed circuit board 61, in which the first connection holes 136 are formed. The primary conductive tracks 52, 53 of different pairs 134 are formed in different printed circuit boards 61.

[0129] refer to Figure 8 and Fig. 9 The first connection holes 136 are formed at the free end 71 of the bent portion 70 of each of the conductive tracks 52, 53. In the example shown, the number of the first connection holes 136 is four, and the first connection holes 136 are aligned along the width of the bent portion 70 here.

[0130] like Figure 5 It can be seen in FIG. 1 that each of the secondary conductive tracks 55 , 56 , 57 , 58 extends perpendicularly to the connection hole 136 along the stacking direction Z. In particular, the rod 98 , 118 of each of the secondary tracks 55 , 56 , 57 , 58 extends perpendicularly to the connection hole 136 along the stacking direction Z.

[0131] Therefore, the primary conductive tracks 52, 53 of each pair 134 are electrically connected to each other without being offset with respect to any part of the secondary conductive tracks 55, 56, 57, 58, which enables AC losses to be reduced.

[0132] refer to Figure 3 and Figure 6 The third type of primary conductive tracks 54A, 54B comprises a third type of first primary conductive track 54A ( Figure 6 ) and a second primary conductive track 54B of a third type ( Figure 3 ). In particular, the primary conductive tracks of the third type comprise, for each board 61, at least one first primary conductive track 54A of the third type and at least one second primary conductive track 54B of the third type. The first primary conductive tracks 54A of the third type are aligned with each other along the stacking direction Z, and the second primary conductive tracks 54B of the third type are aligned with each other along the stacking direction Z. Each primary conductive track 54A, 54B of the third type belongs to an outer layer (not indicated) of one of the printed circuit boards 61.

[0133] In the example shown, each printed circuit board 61 comprises two first primary conductive tracks 54A of the third type and two second primary conductive tracks 54B of the third type, except for the printed circuit boards 61 arranged at the ends of the stack 50 (one comprising a single first primary conductive track 54A of the third type and the other comprising a single second primary conductive track 54B of the third type). In a variant (not shown), each printed circuit board 61, including the printed circuit boards 61 arranged at the ends of the stack, comprises two first primary conductive tracks 54A of the third type and two second primary conductive tracks 54B of the third type.

[0134] The primary conductive track 52 of the first type of each pair 134 is electrically connected to the or each first primary conductive track 54A of the third type formed in the same printed circuit board 61. The primary conductive track 52 of the first type is electrically connected to the or each first primary conductive track 54A of the third type via a through-connection hole 137A formed through the printed circuit board 61.

[0135] Similarly, each pair 134 of second-type primary conductive tracks 53 is electrically connected to the or each third-type second primary conductive track 54B formed in the same printed circuit board 61. In particular, the second-type primary conductive tracks 53 are connected to the printed circuit board 61 via through-connection holes 137B ( Figure 4 ) is electrically connected to the second primary conductive track 54B of the third type or these second primary conductive tracks 54B of the third type.

[0136] These connection holes 137A, 137B are isolated from corresponding connection holes 137A, 137B formed through the or each adjacent printed circuit board 61 by an insulating layer 59. This insulating layer 59 is usually obtained during the sintering lamination of the printed circuit boards 61 constituting the stack.

[0137] like Figure 4 , Figure 8 and Fig. 9 1 , it can be seen that the connection hole 137A is formed at the first longitudinal end 64 of the second branch 63 of the first type of primary conductive track 52, and the connection hole 137B is formed at the first longitudinal end 64 of the second branch 63 of the second type of primary conductive track 52. In the example shown, the number of connection holes 137A or connection holes 137B is four, and the connection holes 137A or connection holes 137B are aligned here along the width of the second branch 63 of the primary conductive track 52, 53.

[0138] refer to Figure 5 and Figure 7 , the secondary conductive tracks 55, 56, 57, 58 are also grouped into pairs 138 of secondary conductive tracks. Each pair 138 of secondary conductive tracks sandwiches a corresponding pair 134 of primary tracks.

[0139] The secondary conductive tracks 55, 56, 57, 58 constituting each pair 138 are formed in the same printed circuit board 61 as the primary tracks 52, 53 constituting the sandwiched pair 134, and belong to the outer layer of the printed circuit board 61 respectively. The secondary conductive tracks 55, 56, 57, 58 constituting each pair 138 are electrically connected to each other through the second connection holes 140 that cross the printed circuit board 61. Therefore, each printed circuit board 61 includes tracks of a first pair 134, the tracks of the first pair 134 are constituted by the primary conductive tracks 52, 53 electrically connected to each other through the recessed first connection holes 136, and tracks of a second pair 138 are constituted by the secondary conductive tracks 55, 56, 57, 58 electrically connected to each other through the second through connection holes 140, and the second pair 138 sandwiches the first pair 134.

[0140] In particular, each pair 138 consists of an intermediate secondary conductive track 55 of the first type or an end secondary conductive track 57 of the first type and an intermediate secondary conductive track 56 of the second type or an end secondary conductive track 58 of the second type. Figures 11 to 14 It can be seen in FIG. 1 that second connection holes 140 are formed in the protruding portion 104, 126 of each of the conductive tracks 55, 56, 57, 58. In the example shown, the number of second connection holes 140 is four, and the second connection holes 140 are arranged in a square shape here.

[0141] like Figure 5 14, it can be seen that the second connection hole 140 extends through the windows 79 of the primary conductive tracks 52, 53 that make up the clamped pair 134. Thus, the intermediate rod 74 of each of these conductive tracks 52, 53 is inserted between the first connection hole 136 on the one hand and the second connection hole 140 on the other hand. Here and in the rest of the text, the term "insertion" is understood to mean that the rod 74 extends through a straight line connecting the first connection hole 136 to the second connection hole 140.

[0142] Due to this arrangement, the length of the offset portions 104, 126 required for the interconnection of each pair 138 of secondary conductive tracks 55, 56, 57, 58 is minimized. In addition, the surface of the conductive material superposed between the primary winding 20 and the secondary winding 22 in the region of the series interconnection of the secondary turns 28 is maximized. This improves the electromagnetic coupling between the primary winding 20 and the secondary winding 22 and enables a reduction in AC losses.

[0143] return Figure 7 , the secondary conductive tracks 55, 56, 57, 58 of different pairs 138 are formed in different printed circuit boards 61, and an insulating layer 59 is inserted between the second connection holes 140 electrically connected to the secondary conductive tracks 55, 56, 57, 58 of different pairs 138. This insulating layer 59 is usually obtained during the sintering lamination of the printed circuit boards 61 constituting the stack.

[0144] refer to Figure 5 , the coil 12 also includes sintered metal disks 142, 144, which provide electrical connections between the conductive tracks 54A, 54B, 57, 58 belonging to the outer layers of the printed circuit boards 61 that make up the stack. Therefore, each sintered metal disk 142, 144 is inserted between two printed circuit boards 61 that make up the stack.

[0145] Each sintered metal disk is typically made of silver.

[0146] The sintered metal disks 142, 144 include first sintered metal disks 142, each of which connects two primary conductive tracks 54A, 54B of the third type to each other. The sintered metal disks 142, 144 also include second sintered metal disks 144, each of which connects an intermediate secondary conductive track 57 of the first type to an intermediate secondary conductive track 58 of the second type.

[0147] like Figure 6 and Fig.10 8, each first sintered metal disk 142 is arranged perpendicularly to the second end 84 of the thin strip 80 forming the primary conductive track 54A, 54B of the third type, opposite the through hole 86. Therefore, for each of the primary conductive tracks 54A, 54B of the third type connected by the first sintered metal disk 142, a connecting hole 137A, 137B is inserted between the through hole 86 and the first sintered metal disk 142 along the longitudinal direction X.

[0148] The third type of primary conductive tracks 54A, 54B, the connection holes 137A, 137B and the first sintered metal disk 142 together provide a parallel electrical connection of the pair 134. The third type of primary conductive tracks 54A, 54B, the connection holes 137A, 137B and the first sintered metal disk 142 enable the pair 134 to be electrically connected to each other without electrical bonding of the insertion holes of the contact pads 14, 15, which cannot be achieved using current technical means.

[0149] At the same time, the second sintered metal disk 144 provides a series electrical connection for the pair 138 .

[0150] For this reason, Fig.14 and Fig.15 As can be seen in the figure, each second sintered metal disk 144 is arranged perpendicularly to the second bend 124 of the intermediate secondary conductive tracks 57, 58. Each second sintered metal disk contacts the second bend 124 of the first type of intermediate secondary conductive tracks 57 and contacts the second bend 124 of the second type of intermediate secondary conductive tracks 58.

[0151] Therefore, if Figure 5 As can be seen in the figure, each of the first type of primary conductive tracks 52 and the second type of primary conductive tracks 53 extends perpendicularly to the second sintered metal disk 144 along the stacking direction Z. In particular, the second transverse bar 78 of each of the first type of primary conductive tracks 52 and the second type of primary conductive tracks 53 extends perpendicularly to the second sintered metal disk 144 along the stacking direction Z.

[0152] Therefore, the pairs 138 are electrically connected to each other without any portion being offset relative to the turn-forming primary conductive tracks 52 , 53 , which enables reduction of AC losses.

[0153] Now refer to Fig.15 A method 200 for manufacturing a planar power transformer 10 is described.

[0154] The method 200 comprises a first step 201 of manufacturing the coil 12 .

[0155] Step 201 begins with a first sub-step 202 of producing a printed circuit board 61 .

[0156] refer to Fig.16 , the production step 202 begins with a sub-step 210 of providing a substrate (not shown) made of a dielectric material.

[0157] Sub-step 210 is followed by a sub-step 220 of producing a first inner conductive layer (not indicated) of the printed circuit board 61 on the first side of the substrate. This sub-step 220 comprises producing 222 primary conductive tracks 52 of a first type on said first side of the substrate.

[0158] Sub-step 210 is followed by a sub-step 230 of producing a second inner conductive layer (not indicated) of the printed circuit board 61 on the second side of the substrate. This sub-step 230 comprises producing 232 primary conductive tracks 53 of a second type on said second side of the substrate.

[0159] Sub-steps 220 and 230 are performed in parallel or sequentially.

[0160] Sub-steps 220 and 230 are followed by a sub-step 240 of producing recessed connection holes through the substrate to electrically connect the primary conductive tracks of the first type 52 and the primary conductive tracks of the second type 53 to each other. These recessed connection holes constitute the first connection holes 136.

[0161] After this sub-step 240 , producing 202 the printed circuit board 61 comprises a sub-step 250 of depositing a first insulating layer 59 on the first face of the substrate. This first insulating layer 59 covers the first connection holes 136 .

[0162] Sub-step 250 is followed by a sub-step 260 of producing a first external conductive layer (not indicated) on a face of the first insulating layer opposite to the substrate. This sub-step 260 comprises producing 262 at least one (preferably two) third type of primary conductive track 54A, 54B perpendicular to the first longitudinal end 64 of the second longitudinal branch 63 of at least one of the first type of primary conductive track 52 and the second type of primary conductive track 53. This sub-step 260 also comprises producing 264 a first type of intermediate secondary conductive track 55 or a first type of end secondary conductive track 57, the rods 98, 118 of said tracks 55, 57 extending perpendicularly to each first connection hole 136.

[0163] After sub-step 240 , producing 202 the printed circuit board 61 further comprises a sub-step 270 of depositing a second insulating layer 59 on the second side of the substrate. The second insulating layer 59 covers the first connecting holes 136 .

[0164] Sub-step 270 is followed by a sub-step 280 of producing a second outer conductive layer (not shown) on a face of the second insulating layer opposite to the substrate. This sub-step 280 comprises producing 282 at least one (preferably two) third type of primary conductive track 54A, 54B perpendicular to the first longitudinal end 64 of the second longitudinal branch 63 of at least one of the first type of primary conductive track 52 and the second type of primary conductive track 53. This sub-step 280 also comprises producing 284 a second type of intermediate secondary conductive track 56 or a second type of end secondary conductive track 58, the rods 98, 118 of said tracks 56, 58 extending perpendicularly to each first connection hole 136.

[0165] Sub-steps 270 and 280 are performed in parallel or sequentially with sub-steps 250 and 260 .

[0166] Sub-steps 260 and 280 are followed by a sub-step 290 of producing through-connection holes through the substrate and the two insulating layers 59. This sub-step 290 comprises producing 292 second connection holes 140 to electrically connect the secondary conductive tracks 55, 57 of the first type to the secondary conductive tracks 56, 58 of the second type. This sub-step 290 also comprises producing 294 third connection holes 137A, 137B to electrically connect each of the primary conductive tracks 52, 53 of the first type and the primary conductive tracks of the second type to at least one primary conductive track 54A, 54B of the third type.

[0167] Thereby, a printed circuit board 61 is obtained.

[0168] Back to Fig.15 , repeat sub-step 202 until all the printed circuit boards 61 of the stack have been produced.

[0169] Next, the manufacture 201 of the coil 12 comprises a sub-step 203 of stacking the printed circuit boards 61 thus produced. During this stacking step, a metal paste is inserted between the printed circuit boards 61, perpendicularly on the one hand to the primary conductive tracks 54A, 54B of the third type and on the other hand to the second bends 124 of the intermediate secondary conductive tracks 57, 58. The remaining spaces between the printed circuit boards 61 are filled with a dielectric material.

[0170] Sub-step 203 is followed by a sub-step 204 of sintering-laminating the printed circuit boards 61. During this sub-step 204, the metal inserted between the printed circuit boards 61 becomes sintered and becomes tightly bonded to the conductive tracks of the outer layer, thereby forming an electrical connection between the primary conductive tracks 54A, 54B of the third type on the one hand, and between the second bends 124 of the intermediate secondary conductive tracks 57, 58 on the other hand. In addition, the dielectric material accommodated between the printed circuit boards 61 hardens and forms an insulating layer. The principle of sintering-laminating is well known to those skilled in the art, so that they will know how to implement this step without any particular difficulties.

[0171] The coil 12 is thus obtained.

[0172] Step 201 is followed by a step 205 of mounting the interconnection pads 14, 15, 16, 17. During this step 205, the pads 14, 15, 16, 17 are inserted into the through holes 72, 86, 103 provided therefor and washers are then fitted onto the pads 14, 15, 16, 17 to electrically connect the pads 14, 15, 16, 17 to the conductive tracks 52, 53, 54A, 54B, 55, 56, 57, 58. In particular:

[0173] - fitting a first washer (not shown) on the first pad 14 to provide an electrical connection between said first pad 14 and a first primary conductive track 54A of the third type at the stacking surface,

[0174] - fitting a second washer (not shown) on the second pad 15 to provide an electrical connection between said second pad 15 and a second primary conductive track 54B of the third type at the stacking surface,

[0175] - fitting a third gasket (not shown) on the third pad 16 to provide an electrical connection between said third pad 16 and the end secondary conductive track 55 of the first type, and

[0176] - fitting a fourth washer (not shown) on the fourth pad 17 to provide an electrical connection between said fourth pad 17 and the end secondary conductive track 56 of the second type.

[0177] Step 205 is followed by a step 206 of mounting magnetic circuit 18 . During this step 206 , block 32 of ferromagnetic material is positioned around coil 12 .

[0178] Thereby, the planar transformer 10 is obtained.

[0179] Due to the above exemplary embodiment, the stacking of the conductive tracks 52, 53, 55, 56, 57, 58 forming the turns 24, 28 of the primary winding 20 and the secondary winding 22 is optimized, which makes it possible to improve the electromagnetic coupling between the primary winding 20 and the secondary winding 22 and reduce AC losses. In addition, the offset portion of the conductive tracks 52, 53, 55, 56, 57, 58 forming these turns 24, 28 is greatly reduced, which further contributes to reducing AC losses. Therefore, the planar transformer 10 can be easily used at high frequencies.

Claims

1. A coil (12) for a planar power transformer (10), said coil comprising at least one stack (50) of conductive tracks (52, 53, 54A, 54B, 55, 56, 57, 58), said stack consisting of a printed circuit (51), said printed circuit consisting of a plurality of printed circuit boards (61), said conductive tracks comprising a first group (60A) forming a primary winding (20) of said coil (12) and a second group (60B) forming a secondary winding (22) of said coil (12), said first group consisting of at least one so-called primary conductive track (52, 53) and said second group consisting of at least one so-called secondary conductive track (55, 56, 57, 58), wherein: The first group (60A) comprises at least one pair (134) of primary conductive tracks, the at least one pair of primary conductive tracks being composed of primary conductive tracks (52, 53) formed in the same printed circuit board (61) and electrically connected to each other via at least one first electrical connector (136), and at least one secondary conductive track (55, 56, 57, 58) in the second group (60B) being formed in the same printed circuit board (61) and belonging to an outer layer of the printed circuit board (61) and extending perpendicularly to the first electrical connector (136), the first electrical connector connecting the primary conductive tracks (52, 53) in the pair (134) of primary conductive tracks.

2. The coil (12) according to claim 1, comprising at least one sintered metal disk (144) which is inserted between two printed circuit boards (61) of the stack and electrically connects two conductive tracks (52, 53, 54A, 54B, 55, 56, 57, 58) of an outer layer of a printed circuit board (61) belonging to the stack (50) to each other.

3. The coil (12) according to claim 2, wherein: The conductive tracks (52, 53, 54A, 54B, 55, 56, 57, 58) electrically connected to each other through the sintered metal disk (144) are secondary conductive tracks (55, 56, 57, 58), and at least one primary conductive track (52, 53) extends perpendicularly to the sintered metal disk (144).

4. The coil (12) according to claim 2 or 3, wherein: Conductive tracks (52, 53, 54A, 54B, 55, 56, 57, 58) electrically connected to each other through the sintered metal disk (144) are secondary conductive tracks (55, 56, 57, 58), each of which forms a turn.

5. The coil (12) according to any one of the preceding claims, wherein Each primary conductive track (52, 53) of the pair (134) of primary conductive tracks constituting the printed circuit board (61) forms a turn.

6. The coil (12) according to any one of the preceding claims, wherein For the pair (134) of primary conductive tracks or at least one pair of primary conductive tracks, the second group (60B) includes a pair (138) of secondary conductive tracks, the pair of secondary conductive tracks being composed of secondary conductive tracks (55, 56, 57, 58) formed in the same printed circuit board (61) as the pair (134) of primary conductive tracks, the pair of secondary conductive tracks sandwiching the pair (134) of primary conductive tracks, the secondary conductive tracks (55, 56, 57, 58) respectively forming one turn and being electrically connected to each other via at least one second electrical connector (140) crossing the printed circuit board (61).

7. The coil (12) according to claim 6, wherein: Each primary conductive track (52, 53) constituting the pair (134) of primary conductive tracks forms a turn, the turn comprising two longitudinal branches (62, 63) spaced apart transversely from each other, each longitudinal branch having a first longitudinal end (64) and an opposite second longitudinal end (66), and an end transverse bar (68) connecting the two longitudinal ends (66) of the two branches (62, 63) to each other, each of the pair of primary conductive tracks further comprising an intermediate transverse bar (74) connecting the two branches (62, 63) to each other at a distance from the respective longitudinal ends (66) of the two branches, and the intermediate transverse bar is inserted between the first electrical connector (136) or each first electrical connector and the second electrical connector (140) or each second electrical connector, each secondary conductive track (55, 56, 57, 58) constituting the pair (138) of secondary conductive tracks extending perpendicularly to the intermediate transverse bar (74).

8. The coil (12) according to claim 7, wherein: Each of the pair of primary conductive rails further comprises a bend (70) extending from a first longitudinal end (64) of a first longitudinal branch (62) to a second longitudinal branch (63) of the longitudinal branches (62, 63), a free end (71) of the bend (70) being spaced apart from the second longitudinal branch (63) so as to leave a channel (72) between the second longitudinal branch (63) and the bend (70).

9. A planar transformer (10) comprising a coil (12) according to any one of the preceding claims.

10. A method for manufacturing a coil (12) for a planar power transformer (10), the coil being a coil according to any one of the preceding claims, the method comprising a step (202) of producing a printed circuit board (61), the production step (202) comprising the following sub-steps: - providing (210) a substrate made of a dielectric material, - producing (222) a first turn-forming conductive track (52) on a first face of said substrate, - producing (232) a second turn-forming conductive track (53) on a second face of said substrate, - producing (240) at least one first electrical connection (136) through the substrate to electrically connect a first conductive track and a second conductive track (52, 53) to each other, the first conductive tracks (52, 53) forming a pair (134) of primary conductive tracks of a first group (60A), the first group forming a primary winding (20) of the coil (12), - depositing (250) a first insulating layer (59) on the first face of the substrate, the first insulating layer (59) covering the or each first electrical connection (136), and - producing a third turn-forming conductive track (55) on a face of the first insulating layer opposite to the substrate, the third conductive track (55) belonging to a second group (60B) forming a secondary winding (22) of the coil (12), the second group extending perpendicularly to the first electrical connection (136) or each first electrical connection connecting a primary conductive track (52, 53) of the pair (134) of primary conductive tracks.

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