Improved planar litz coil; associated manufacturing method and computer program product

Abstract

This coil results from stacking at least two bifacial carriers, each bifacial carrier comprising two conductive layers etched to have tracks. This coil comprises a bundle comprising a plurality of strands, each strand (50) comprising at least one wire, each wire comprising one filament or a pair of filaments, each filament resulting from connection of a plurality of tracks. Each strand results from association of unitary elements (51, 52, 53, 54) that are identical to one another, a unitary element comprising an active central region (51-2), an interconnection region (51-1) and an exchange region (51-3), two unitary elements being electrically connected by interconnection vias (21, 26) between either the interconnection regions when the two unitary elements do not belong to the same bifacial carrier or the exchange regions when the two unitary elements belong to the same bifacial carrier.

Description

[0001] DESCRIPTION

[0002] TITLE: Improved planar Litz coil; Manufacturing process and associated computer program product.

[0003] The present invention relates to the field of electromagnetic coils in printed circuits, also known as planar coils.

[0004] In particular, the present invention relates to the field of planar coils intended for use in high, or even very high, frequency systems. By "very high frequencies," we mean electrical currents with a frequency above 100 kHz, preferably between 100 kHz and 600 kHz.

[0005] An electromagnetic coil is a primarily inductive electrical device.

[0006] However, as the frequency increases, the coil resists the flow of current, causing a loss of power in the circuit.

[0007] This resistive effect can be modeled by a resistor in series with a perfectly inductive coil.

[0008] This resistor has a resistance whose value varies according to the frequency of the current flowing through the coil. Two regimes can be distinguished in this evolution of resistance as a function of frequency.

[0009] In the first regime, corresponding to frequencies below a critical frequency characteristic of the coil, the resistance has a low value, essentially constant with respect to frequency. In this first regime, the resistance is then called "DC resistance" to emphasize that it is the low-frequency resistance.

[0010] In a second regime, corresponding to frequencies above the critical frequency, the resistance value increases significantly with frequency (in fact, it increases exponentially with frequency). In this second regime, the resistance is then called "alternating resistance" to emphasize that it is the high-frequency resistance.

[0011] The presence of such resistance is problematic, especially when one wants to use the coil at high frequencies, i.e. in the second regime.

[0012] The main cause of this resistive effect is a lack of homogeneity in the flow of currents through each of the wires that together form the coil.

[0013] To remedy this problem, and push the critical frequency towards higher frequency values, we know of the so-called Litz coils, which use a bundle of twisted conducting wires so that the different wires of the coil have the same length and a homogeneous spatial distribution.

[0014] The applicant has notably developed a planar Litz coil structure, enabling integration into a printed circuit board. Such a planar Litz coil is, for example, described in French patent application FR No. 22 14588. This Litz coil is characterized in that a loop (or turn) of the coil wires is formed on both copper layers of a single double-sided printed circuit board. This Litz coil thus exhibits a two-dimensional winding – 2D – constrained within the thickness of a single double-sided board.

[0015] Each layer (forming the faces of the double-sided printed circuit board) is etched to present a plurality of tracks. Each track extends between an outer end (located on a periphery radially outside the coil) and an inner end (located on a periphery radially inside the coil).

[0016] The end of a first track carried by a first layer is connected to the opposite end of a second track carried by the second layer of the double-sided printed circuit board.

[0017] A coil wire is obtained by connecting several tracks successively in this way. A wire travels between its two end terminals, passing from the inner to the outer periphery of the coil. This ensures that the different wires of the coil have a roughly identical length. This also ensures that, even though the magnetic induction is not uniform, each wire "sees" a magnetic induction along its entire length that is roughly the same from one wire to the other on the coil.

[0018] By defining a strand as a twist of wires, and a bundle as a set of strands, the coil according to the prior art presents a bundle comprising a single strand, which is engraved on a single biface.

[0019] The article by LOPE IGNACIO et al. "Frequency — Dependent Resistance of Planar Coils in Printed Circuit Board With Litz Structure", IEEE TRANSACTIONS ON MAGNETICS, IEEE, USA, vol. 50, no. 12, December 1, 2014 (2014-12-01), pages 1-9, discloses the series connection of a plurality of planar Litz coils.

[0020] Documents WO 2024 / 016262 A1 and us 2023 / 352235 A1 disclose different ways of electrically connecting tracks engraved on opposite faces of a single biface.

[0021] The aim of the present invention is to propose an improvement to planar Litz coils, to further reduce the value of the alternating resistance.

[0022] To this end, the invention relates to a three-dimensional planar Litz coil according to the attached claims. The invention also relates to equipment comprising a magnetic component integrating a magnetic core and the aforementioned planar Litz coil, the printed circuit board supporting the planar Litz coil having a geometry conjugate to a geometry of the magnetic core.

[0023] The invention also relates to a computer-implemented method for designing the previous planar Litz coil.

[0024] The invention also relates to a computer program comprising software instructions which, when executed by a computer, implement the previous design process adapted to produce a planar Litz coil design.

[0025] The invention and its advantages will be better understood upon reading the following detailed description of a particular embodiment, given solely by way of non-limiting example, this description being made with reference to the accompanying drawings in which:

[0026] - Figure 1 is a schematic cross-sectional representation of the integrated circuit carrying the Litz coil functionality according to the invention;

[0027] - Figure 2 is a schematic perspective representation of a strand on a lathe composed of four unit elements connected in series and engraved on the layers of the two bifaces of a circuit similar to that of Figure 1;

[0028] - Figure 3 is a schematic perspective representation of a three-dimensional planar Litz coil, incorporating three strands;

[0029] - Figure 4 is a schematic representation of a first embodiment of a unit element;

[0030] - Figure 5 is a schematic representation of a second embodiment of a unit element;

[0031] - Figure 6 is a schematic representation of a third embodiment of a unit element;

[0032] - Figure 7 illustrates the interconnection at the level of their respective exchange zones of two unit elements conforming to the third embodiment of Figure 6;

[0033] - Figure 8 illustrates the interconnection at the level of their interconnection zones of two unit elements conforming to the third embodiment of Figure 6;

[0034] - Figure 9 is a schematic axial section illustrating a superimposed configuration and a staggered configuration;

[0035] - Figure 10 is a block representation of a method for automatically generating the plan of a Litz coil according to the invention; and, - Figure 11 is an exploded perspective view of a plan of a Litz coil according to the invention.

[0036] STRUCTURE

[0037] Generality

[0038] The planar Litz coil according to the invention is three-dimensional (3D) in that it constitutes a three-dimensional winding within the thickness of a single printed circuit board made up of a stack of a plurality of bifacials. More precisely, a loop (or turn) of the wires constituting the coil results from the connection of traces etched in the different conductive layers of the printed circuit board. The Litz coil according to the invention is planar because it lies within the thickness of the planar printed circuit board. The Litz coil according to the invention is three-dimensional because it extends from the plane of a single bifacial of the printed circuit board, stretching between a lower and an upper layer of the printed circuit board, following a helical geometry around the stacking direction of the bifacials.

[0039] Printed Circuit Board

[0040] Figure 1 is an axial section of a preferred embodiment of a printed circuit integrating a 3D planar Litz coil according to the invention.

[0041] The printed circuit board 10 is annular around a so-called vertical axis Z.

[0042] The circuit 10 thus has a through axial orifice 12 allowing for the accommodation of, for example, a magnetic core.

[0043] Circuit 10 has an inner diameter d1 and an outer diameter d2. It has a height h.

[0044] The circuit 10 comprises, along the vertical direction Z, a central stack 14, and, advantageously but not necessarily, an upper stack 16 and a lower stack 18, respectively located above and below the central stack 14.

[0045] The terms "superior" and "inferior" are relative to an arbitrary orientation of the vertical Z axis.

[0046] The central stack 14 results from the superposition, along the Z direction, of bifaces B 20i to 20B (B being an integer greater than or equal to two).

[0047] Each biface comprises two active layers, an upper conductive layer 22 and a lower conductive layer 24, which are carried by each of the faces of an insulating support layer 23. The layer 23 is, for example, a laminate, in accordance with the state of the art.

[0048] The upper and lower conductive layers, 22 and 24, of each biface are engraved to present a plurality of conductive tracks.

[0049] The conductive tracks of the upper conductive layer and the lower face of the same biface are connected by buried vias, that is to say, which extend vertically only over the thickness of the biface in question, such as the buried via 211 for the biface 20i and the buried via 21 B for the biface 20B in Figure 1.

[0050] Two successive bifaces of the stack 14 are joined by interposing an insulating mounting layer, such as layer 25 interposed between biface 20i and the following biface (not shown) or between the previous biface (not shown) and biface 20B. Layer 25 is for example a prepreg, in accordance with the state of the art.

[0051] The conductive tracks of two different bifaces of the stack 14 can be connected by one or more through vias, that is to say by a via which extends vertically over the entire thickness of the printed circuit 10, like the via 26 of figure 1.

[0052] The upper stack 16 has an upper conductive layer 32 called a closure layer, associated with an insulating mounting layer 35 at the interface with the biface 20B of the stack 14.

[0053] The lower stack 18 has a lower conductive layer 34, called the closure layer, associated by an insulating mounting layer 35, at the interface with the biface 20i of the stack 14.

[0054] Layers 35 are preferably similar to layers 25.

[0055] It should be noted that the upper 16 and lower 18 stacks are optional, or have a different structure. Since PCB construction is often symmetrical, there are either two closing layers or none.

[0056] The upper closure layer 16 and / or the lower closure layer 18 can be etched to present copper patterns, such as a connecting bar (“strap”) between two through vias 26. This bar can integrate one or more surface-mounted electronic component(s) 42, such as a DC blocking and / or resonance capacitor (depending on the intended application for the coil).

[0057] A "strap" has several functions. It contributes to the twisting of the wires within a strand to achieve the Litz effect. It allows the addition of passive components to increase power density. Each strand or pair of strands can be assigned a specific component. Bundle / strands / wires / strands

[0058] Furthermore, the three-dimensional planar Litz coil according to the invention exhibits a hierarchical structure.

[0059] Thus, the coil is a bundle.

[0060] This bundle consists of several strands.

[0061] In turn, each strand contains one or more threads.

[0062] Finally, in turn, a thread is made up of either one strand or a pair of strands.

[0063] A strand results from the series connection of a succession of traces, each trace constituting the same strand being carried by a particular conductive layer of the printed circuit board. In other words, each wire of a strand, and consequently the strand made up of that wire or those wires, travels from layer to layer through the thickness of the printed circuit board between the lower and upper layers of the circuit.

[0064] Modular element

[0065] Finally, the planar 3D-wound Litz coil according to the invention features a modular structure. This approach facilitates case-by-case optimization of the coil configuration based on the desired application.

[0066] More precisely, each strand of the coil results from the connection of a plurality of identical unit elements to each other.

[0067] A unit element occupies a portion of the conductive layer forming an annular sector, delimited by an inner diameter di, an outer diameter d2 and an opening angle 0 (figure 4).

[0068] A unit element can be viewed as the portion of a strand that rests in a particular conductive layer.

[0069] Each unit element of a strand connects all the traces of the wires in that strand, which are etched onto the same layer of the printed circuit board. They are located within the angular sector defined by the opening angle 0.

[0070] The different unit elements constituting the same strand are carried by as many different conductive layers.

[0071] Two successive unit elements are carried by two successive conductive layers of the stack.

[0072] Two successive unit elements of the same strand are angularly offset from each other around the Z-axis of layer stacking. In this way, each strand exhibits a helical shape around the Z-axis, throughout the entire printed circuit board, from the bottom layer to the top layer. The covered 0° opening angle of the unit elements of a given strand is adjusted by taking into account the number of strands (i.e., the number of modular elements on a single conductive layer) and the number of unit elements constituting the same strand.

[0073] Figure 2 illustrates, for example, one possible embodiment of a strand of a reel.

[0074] In this figure, strand 50 forms a 360° turn around the Z axis.

[0075] Strand 50 is composed of four unit elements, respectively 51, 52, 53 and 54.

[0076] These four unit elements are identical to each other, but one out of every two unit elements is arranged in an upside-down position.

[0077] They are electrically connected in series.

[0078] Such a strand is made in a circuit whose central stack is made of two bifaces.

[0079] Two successive unit elements which are engraved on the conductive layers of the same biface are connected to each other by one or more buried vias 21. This is notably the case between the first and second unit elements 51 and 52 on the one hand and the third and fourth unit elements 53 and 54 on the other hand.

[0080] Two successive unit elements, which are engraved on conductive layers of two different bifaces, are connected to each other by one or more through vias 26 This is notably the case between the second and third unit elements 52 and 53 in Figure 2.

[0081] Figure 3 illustrates one possible embodiment of a spool. In this figure, only the unit elements constituting the spool are shown.

[0082] Coil 100 consists of a bundle of three strands, respectively 60, 70 and 80. In the figure, each strand is represented with different types of hatching.

[0083] Each strand forms a single 360° turn around the Z axis.

[0084] Each strand is offset by an angle of 120° around the Z axis relative to the previous strand.

[0085] In this embodiment, a strand consists of nine unit elements.

[0086] The unit elements of a strand are identical to each other and to those of another strand of reel 100.

[0087] Such a coil is formed on the four bifacials 120i, 1202, 1203, and 1204 of the central stack and the lower closing layer 134 of the circuit. In Figure 3, only the buried vias 121 between the two successive unit elements of the same strand that are carried by the same bifacial are shown. The through vias have not been shown for the sake of clarity.

[0088] The winding is here made in three dimensions rather than two dimensions, which allows the winding to reduce the amount of circulating current related to the component of the magnetic force along the Z-axis. Another advantage is the increase in the volume density of copper, which contributes to reducing the DC resistance of the Litz coil.

[0089] Zones

[0090] The internal structure of a unit element will now be described with several levels of detail.

[0091] According to a first level of detail, the internal structure of a unit element can be described as having an active central zone, an external interconnection zone and an internal exchange zone.

[0092] This is illustrated for example in Figure 2. The unit element 51 thus presents an external interconnection zone 51-1, an active central zone 51-2, and an internal exchange zone 51-3.

[0093] The unit element 52 has an external interconnection zone 52-1, an active central zone 52-2, and an internal exchange zone 52-3.

[0094] The unit element 53 has an external interconnection zone 53-1, an active central zone 53-2, and an internal exchange zone 53-3.

[0095] The unit element 54 has an external interconnection zone 54-1, an active central zone 54-2, and an internal exchange zone 54-3.

[0096] The active central zone of a modular element, such as zone 51-2 of element 51, consists of a set of tracks, each track consisting of one or two strands.

[0097] This set of tracks follows a geometric pattern which, when the coil is considered as a whole, ensures the most efficient possible current flow.

[0098] Three types of geometric patterns will be presented successively below.

[0099] The interconnection along the z-axis of the active zones of two unit elements is achieved either by the external interconnection zones of these two unit elements, or by the internal exchange zones of these two unit elements.

[0100] External interconnection zones ensure electrical continuity between two active zones of two unit elements belonging to different bifaces, by means of one or more through-vias. For example, in Figure 2, zones 52-1 and 53-1 allow connection by means of through-vias 26 between unit elements 52 and 53, which belong to two different bifaces.

[0101] An interconnection zone is arranged tangentially on the outer periphery of the coil.

[0102] An external interconnection zone of a unit element is advantageously slightly offset from the outer perimeter of that unit element's active zone so that through-vias do not interfere with the active zones of other unit elements. In other words, an external interconnection zone protrudes slightly radially towards the outer periphery of the coil.

[0103] The external interconnection zones which are vertically aligned can be used to establish links between the different strands, preferably to connect them in parallel.

[0104] It should be noted that some through-vias are "removed" or spaced out to create the input / output terminals of the strand. This will be described in more detail below with reference to Figure 11.

[0105] The external interconnection areas also allow connection to a closure layer and any surface-mounted electronic components on that closure layer.

[0106] As will become clearer later in this description, the external interconnection zones also allow the position of the strands or strand pairs of tracks to be swapped from one active zone to another.

[0107] To achieve this, a connecting strap is used at the level of the closure layer(s) to connect a given through-hole via. A connecting strap contributes to increasing the power density of the component, and, due to the planar structure of the Litz coil, each strand or pair of strands has its own capacitance with lower electrical stresses than in state-of-the-art designs. This allows for a more compact design.

[0108] Incidentally, such a connecting strip can also be used to insert a passive electronic component, for example a capacitor, particularly a DC blocking capacitor, in series with each strand / pair of strands of the coil tracks. This embodiment is particularly well-suited for the case of a coil intended for a conversion device with a predominant AC magnetic induction component. This contributes to increasing the power density of the converter.

[0109] The internal exchange zones ensure electrical continuity between two active zones of two unit elements belonging to the same biface, thanks to vertical metallized buried vias running solely through the thickness of the biface in question. For example, in Figure 2, zones 51-3 and 52-3 allow the connection between unit elements 51 and 52, which belong to the same biface, by means of buried vias 21. Similarly, zones 53-3 and 54-3 allow the connection between unit elements 53 and 54, which belong to the same biface, by means of buried vias 21.

[0110] An exchange zone can be arranged tangentially on the inner periphery of the coil, radially along a radius of the coil, or in an oblique position between these two extreme positions.

[0111] An internal exchange zone preferably protrudes slightly radially towards the inner periphery of the coil relative to the central active zone.

[0112] One or more underground vias lead to an interchange area.

[0113] The internal exchange zones also allow the position of the strands or pairs of strands of the tracks to be swapped from one active zone to another.

[0114] Thus, the permutation of the strands from one layer to another of the PCB, using internal exchange zones and external interconnection zones, makes it possible to obtain the twisting effect characteristic of a Litz coil. This decomposition into "three zones" of a unit element makes it easy to meet industrial constraints.

[0115] The three types of geometric patterns that a unit element can adopt will now be described. This represents a second level of detail regarding the internal structure of a unit element.

[0116] Type 1

[0117] As illustrated in Figure 4 (which is a view in a plane transverse to the Z axis coinciding with a conductive layer), a unit element 151 of the first type has a single track formed of a single strand.

[0118] The track is a plate essentially in the form of an annular sector with an opening of 0. It should be noted that the angle 0 is adjusted, that is to say the angular footprint of a unit element, to maximize, on each layer, the copper surface area given the number of strands.

[0119] It extends radially between an inner peripheral edge 161 and an outer peripheral edge 163. It extends tangentially between a first lateral edge 162 and a second lateral edge 164. The inner peripheral edge 163 essentially coincides with the inner periphery of the coil. A margin is advantageously reserved between the outer peripheral edge 161 and the inner periphery of the coil. The active area 151-2 of the unit element 151 constitutes the body of this single track.

[0120] The external interconnection zone 151-1 of the unit element 151 projects radially from the external peripheral edge 163. The margin between the external peripheral edge 163 and the outer periphery of the coil accommodates the external interconnection zone 151-1. A row of through-interconnection vias 126 is shown in the zone 151-1.

[0121] In the embodiment of Figure 4, the internal exchange zone is arranged radially rather than tangentially. Thus, the internal exchange zone 151-3 coincides with the second lateral edge 164. More generally, the exchange zone 151-3 extends along a direction making an angle with respect to a radius of the planar Litz coil, this angle being greater than or equal to 0 and less than TT / 2.

[0122] A row of buried vias 121 is shown in zone 151-3. Alternatively, the vias in the exchange and interconnection zones are arranged in several rows.

[0123] The strand resulting from the assembly of several unit elements identical to the unit element 151 consists of only one wire, this wire having a single strand.

[0124] The applications considered for this type 1 transformer are those requiring high coupling achieved through the intertwining of different strands, which can be assigned to different windings within the transformer's construction. Alternatively, applications with intentionally reduced coupling are achieved by creating a three-dimensional twist in the bundle. This allows for an improvement in frequency resistance. It should be noted that these applications are also feasible for types 2 and 3, described below.

[0125] It should be noted that at least two interconnection zones of each strand are not connected to all through vias in order to create input and output ranges for the strand. This will be described in more detail below with reference to Figure 11.

[0126] For this type of unit element, the role of any connection strips is to allow the connection of all the vias of two interconnection zones of two separate PCBs in order to achieve a series connection of two corresponding strands.

[0127] Type 2

[0128] As illustrated in Figure 5 (which is a view in a plane transverse to the z-axis coinciding with a conductive layer), a unit element 251 of the second type has a plurality of tracks, each track being formed of a single strand.

[0129] The resulting strand, formed by combining several unit elements of the second type, therefore comprises several wires, each wire consisting of a single strand. The unit element 251 has an angular footprint adapted to maximize the copper surface area on each layer, taking into account the number of strands.

[0130] The unit element 251 is delimited between an internal peripheral edge 261 and an external peripheral edge 263. It extends tangentially between a first lateral edge 262 and a second lateral edge 264.

[0131] The active zone 251-2 of the unit element 251 consists of the different tracks, numbered 271 to 276.

[0132] The external interconnection zone 251-1 protrudes radially from the external peripheral edge 263. The margin reserved between the external peripheral edge 263 and the external periphery of the coil allows the external interconnection zone 251-1 to be accommodated.

[0133] The external interconnection zone 251-1 is equipped with a row of eight through vias 281 to 288. Six of them are respectively connected to the ends of tracks 271 to 276.

[0134] Alternatively, several rows of vias can be provided, each row can have a total number of vias other than eight and a number of vias connected to tracks other than six.

[0135] The method of connecting the through-hole vias on the upper and / or lower outer surface of the PCB allows for the permutation of the leads from one unit element 251 to the next. Optionally, this also allows for the insertion of electronic components, such as a DC blocking capacitor, connected in series, and / or a resonance capacitor depending on the application, between two successive unit elements.

[0136] The internal exchange zone 251-3 of the unit element 251 protrudes radially from the internal peripheral edge 261. The margin reserved between the internal peripheral edge 261 and the internal periphery of the coil allows the exchange zone 251-3 to be accommodated.

[0137] The internal interchange area 251-3 is equipped with a row of six buried vias 291 to 296. They are connected to the ends of runways 261 to 266 at the internal interchange area 251-3. Alternatively, several rows of vias may be provided, each row having a total number of vias other than six.

[0138] Each strand consists of curved segments (with the concavity facing inwards or outwards) and straight segments. Circular arcs can be added to create the "offset" version.

[0139] The length of the segments is adjustable by the algorithm in order to optimize the filling of the n strands present on the PCB layer.

[0140] Adjusting the number of segments, their type and their characteristics allows them to circulate essentially parallel to each other in the active zone, while giving them a suitable length to connect an exchange zone and an interconnection zone.

[0141] Preferably, the strands have an identical cross-section to facilitate manufacturing.

[0142] The construction of the pattern, in particular the choice of the number of wires, number of bifaces, number of vias, results from constraints recalled below in order to obtain an optimal geometry to ensure maximum performance of the coil in the chosen application:

[0143] The number of wires, and therefore strands, is equal to K times the number of bifaces, with K between 1 and 5, knowing that the value K=2 has been identified as the optimal value;

[0144] The number of vias in each interconnection zone is equal to K times the number of bifaces plus two units;

[0145] The number of vias in each exchange zone is equal to K times the number of bifaces.

[0146] These interconnection management constraints ensure a uniform distribution of currents across the coil strands while guaranteeing the highest possible copper density on each biface.

[0147] The applications envisaged for type 2 preferentially concern medium power transformers, typically less than 500W.

[0148] In Figure 5, interconnecting bars have been schematically shown extending beyond the cross-section of the coil to make them visible, although they are not actually visible in such a view, as they are etched onto the coil's closing layer and therefore within the coil's cross-sectional footprint. The first connecting bar 241 links through-vias 281 and 284, while the second connecting bar 242 links through-vias 285 and 288.

[0149] Type 3

[0150] As illustrated in Figure 6 (which is a view in a plane transverse to the z-axis coinciding with a conductive layer), a unit element 351 of the third type (type 3) has a plurality of tracks, each track being formed of a pair of strands.

[0151] The strand resulting from the association of several unit elements of the third type therefore comprises several wires, each wire being made up of two strands.

[0152] The unit element 351 has an adapted angular footprint, to maximize, on each layer, the copper surface area given the number of strands.

[0153] The unit element 351 is delimited between an inner peripheral edge 361 and an outer peripheral edge 363. It extends tangentially between a first lateral edge 362 and a second lateral edge 364. The active area 351-2 of the unit element 351 is made up of the different tracks, numbered 371 to 276. A track, such as track 371, consists of a first strand 371-1 and a second strand 371-2.

[0154] The external interconnection zone 351-1 protrudes radially from the external peripheral edge 363. The margin reserved between the external peripheral edge 363 and the external periphery of the coil allows the external interconnection zone 351-1 to be accommodated.

[0155] The external interconnection area 351-1 is equipped with a row of eight through vias 381 to 388. Six of them are connected to the ends of tracks 371 to 376.

[0156] Alternatively, several rows of vias can be provided, each row can have a total number of vias other than eight and a number of vias connected to tracks other than six.

[0157] Both strands of a trace are connected to the same through-hole via. The way through-hole vias are connected on the upper and / or lower outer surface of the PCB allows for swapping the traces from one 351 unit to the next. Optionally, this also allows for the insertion of one or more electronic components, such as DC blocking capacitors and / or resonance capacitors depending on the application, between two successive unit elements.

[0158] The internal exchange zone 351-3 protrudes radially from the internal peripheral edge 361. The margin reserved between the internal peripheral edge 361 and the internal periphery of the coil allows the exchange zone 351-3 to be accommodated.

[0159] The internal interchange area 351-3 is equipped with a row of six pairs of buried vias 291-1 to 296-2. The first via of a pair of vias is connected to one of the strands of a track, the second strand of said pair of vias being connected to the other strand of said track.

[0160] Alternatively, several rows of vias can be provided, each row can have a total number of vias other than six.

[0161] Each track consists of alternating segments in the shape of a circular arc (with a concavity facing inwards or outwards) and straight segments.

[0162] The geometric characteristics of the strands are adjustable to adapt the total angular footprint of the pattern.

[0163] In Figure 6, the interconnection bars are not shown. However, through-vias 381 and 384 are connected by a first bar, while through-vias 385 and 388 are connected by a second bar.

[0164] Compared to configuration type 2, configuration type 3, by moving from a single strand to a pair of strands, results in the same number of vias in the interconnection zone but twice as many vias in the exchange zone. This also allows for the introduction of a sub-level, below that of the wires, such as can be found in the state of the art for "Litz wires".

[0165] The type 3 configuration allows the number of parallel strands to be doubled.

[0166] This geometry makes it possible to reduce the current density per strand in applications where the type 2 configuration would have reached its geometric limit with an interconnection zone leading to filling the entire perimeter of the coil with vias.

[0167] The construction of the pattern, in particular the choice of the number of wires, number of bifaces, number of vias, results from formulas recalled below in order to obtain an optimal geometry to ensure maximum performance of the coil in the chosen application:

[0168] The number of wires, and therefore pairs of strands, is equal to K times the number of bifaces, with K between 1 and 5, knowing that the value K=2 has been identified as the optimal value;

[0169] The number of vias in each interconnection zone is equal to K times the number of bifaces plus two units;

[0170] The number of vias in each exchange zone is equal to K times the number of strand pairs, i.e., 2K times the number of bifaces.

[0171] These interconnection management constraints ensure a uniform distribution of currents across the coil strands while guaranteeing the highest possible copper density on each biface.

[0172] The applications envisaged for type 3 preferentially concern high power transformers, typically greater than 500W.

[0173] Here again, an interconnection zone of a strand is devoid of connections to some of the through-vias in order to create input / output connection ranges. These through-vias can then be removed. All strands / pairs of strands in a strand are connected between the input range and the output range.

[0174] Figure 7 illustrates the series interconnection of two successive unit elements 451 and 551 at the level of their respective exchange zones, 451-3 and 551-3, on either side of the same biface.

[0175] In Figure 7, two unit elements of type 3 are considered as an example.

[0176] A symmetry with respect to a median plane M of the exchange zones is sought by connecting, by two buried vias, the pair of strands of track 452 of rank n of the element 451 with the pair of strands of track 552 of rank N+1-n of the element 551 (with n an integer between 1 and N, N being 6 for the embodiment of figure 7).

[0177] The individual tracks that together make up a wire gradually move from the inside of the coil to its outside. Thus, over the length of these two individual elements, the different wires that make up the strand "see" a virtually identical magnetic field.

[0178] Note that the vias of an exchange zone (but also of an interconnection zone) can be arranged on more than one row, for example two rows.

[0179] Figure 8 illustrates the series interconnection of the tracks of two unit elements 610 and 660 at their interconnection zones, 611 and 661. The interconnection zones 611 and 661 belong to two bifaces.

[0180] In Figure 8, two unit elements of type 3 are considered as an example. In Figure 8, the interconnection zones, 611 and 661, are seen one above the other.

[0181] Thus, the pair of strands of track 612-1 is connected to the pair of strands of track 662-

[0182] 3 by a through via 681, a first connection strip 691 (provided on the closing layer of the coil), and a through via 683.

[0183] Thus, the pair of strands of track 612-2 is connected to the pair of strands of track 662-

[0184] 4 by a via crossing 682.

[0185] Thus, the pair of strands of track 612-3 is connected to the pair of strands of track 662-

[0186] 1 by a through via 684, a second connection strip 692 (provided on the coil closure layer), and a through via 686.

[0187] Thus, the pair of strands of track 612-4 is connected to the pair of strands of track 662-

[0188] 2 by a via traversing 685.

[0189] The different tracks that together make up a wire thus gradually pass from the inside of the coil to the outside of it.

[0190] More generally, whether we consider type 2 or type 3, a symmetry with respect to a median plane P of the interconnection zones is sought by connecting, at the same through via, the pair of strands 612-n of the track 612 of rank n of the element 610 with the pair of strands 662-m of the track 662 of rank m of the element 660.

[0191] A generic formula can be derived linking the ranks n and m. With Q the number of traversing vias (assuming one via per strand or pair of strands), we have: when n takes the value 1 or the value Q / 2+1, the strand / pair of strands 612-n is connected to the strand / pair of strands 662-m with m equal to Q / 2 + 2 - n;

[0192] For all other values ​​of n, the strand / pair of strands 612-n is connected to the strand / pair of strands 662-m with m equal to Q - n +2.

[0193] This method of connecting the tracks ensures the electrical continuity of a wire across two successive layers, while also creating a crossing of the wires that allows their positions within the strand to be swapped. Figure 9 shows two possible configurations in an axial cross-section.

[0194] In a first configuration A, the tracks of two unit elements (651 and 652 or 653 and 654) which are etched in the conductive layers of the same biface (620i or 62O2) run essentially opposite each other. This is a superimposed configuration of the tracks.

[0195] In configuration B, each track of the unit element (752 or 754) engraved on the upper layer of a biface (720i or 72O2) is positioned radially between two tracks of the unit element (751 or 753) on the lower layer of the same biface. This is an offset configuration. If W p is a track width and Wj n t being the width of the gap between two tracks, the offset step e from one layer to the other is: (W p + Wj n t).

[0196] This configuration reduces industrial manufacturing constraints (particularly during the pressing of the prepreg between two bifacials). It reduces the equivalent capacity between two adjacent strands of a unit element.

[0197] We can choose which layers (even or odd) to apply the offset to.

[0198] The superimposed configuration ensures a reduction in industrial constraints and a reduction in inter-strand capacity.

[0199] AUTOMATIC DESIGN PROCESS

[0200] Figure 10 schematically illustrates a method 800 for designing a coil according to the invention.

[0201] The implementation of this process results from the execution of a program by a computer.

[0202] A computer includes means of calculation, such as a processor, and means of storage, such as memory.

[0203] The memory stores, in particular, the instructions of the computer program whose execution enables the implementation of the design process of a three-dimensional Litz coil.

[0204] This program allows the automatic generation of the plan of a three-dimensional Litz coil from a number of configuration parameters.

[0205] These configuration parameters include: the shape and dimensions of the magnetic core to determine the available volume and calculate the inner and outer radii of the coil (advantageously taking into account an isolation distance between the conductive coil and the magnetic core); the number of turns or spirals in the bundle; the number of strands; the type of the different strands; the number of wires (i.e., per unit element, the number of tracks); the position of the bundle's inputs and outputs; physical characteristics such as the size of the strands and vias, the nature of the insulators, the precision of the patterns, the isolation distance between each conductive element, etc.; and the presence or absence of blocking capacitors.

[0206] These configuration parameters are entered, in a first step 810 of process 800, by the operator according to the desired application. For example, if the coil to be manufactured is intended to be used as a transformer, the number of turns of the primary winding and the number of turns of the secondary winding, the current level, etc., are entered by an operator.

[0207] The program is advantageously presented in a modular format, with each possible type of unit element associated with a module. Therefore, choosing the strand type when entering configuration parameters allows you to select and launch the corresponding module.

[0208] The program execution then continues with step 820, which verifies the consistency of the parameters entered in step 810, particularly with regard to the constraints imposed by the chosen strand type, i.e., the geometry of the unit element. If some information is missing or inconsistent, the operator is prompted to complete the information in an iteration of step 810.

[0209] Once the verification step is completed, the next step 830 is to calculate the structural parameters of the Litz coil.

[0210] For example, the number of wires (i.e. tracks) is determined automatically, as well as the number of PCB layers, the coordinates of the vias, the angles between the side edges and the internal and external peripheral edges of the elementary pattern to be repeated, etc.

[0211] Arrangement rules are verified, such as: the number of turns must be less than or equal to twice the number of bifacials constituting the PCB; the number of turns and the number of bifacials must be relatively prime; the number of strands and the number of bifacials must be relatively prime; in the case of a Type 1 PCB, the number of wires is equal to twice the number of bifacials minus one; in the case of a Type 2 or Type 3 PCB, the number of wires is equal to twice the number of bifacials. For each strand, the number of internal exchange zones and the number of external interconnection zones must be equal to each other and to the number of bifacials.

[0212] The program execution continues with step 840, which generates the layout of the unit elements and their arrangement.

[0213] For example, a unit element is positioned on the upper layer of a biface. By symmetry with respect to the midpoint of the exchange zone, another unit element is positioned on the lower layer of this biface.

[0214] This operation is iterated by performing an appropriate angular rotation to create the portion of each of the other strands in the biface under consideration. This operation is also iterated to create the next portion of a strand by considering the next biface, for example, the one directly below.

[0215] The geometry of the unit element, which is repeated to form the coil, is subject to optimization.

[0216] In particular, the angular footprint of a unit element is adapted to ensure maximum copper volume in each layer of the PCB and consequently in the total volume of the coil.

[0217] The positioning of interconnection zones and through vias, particularly to create input and output terminals for each strand, is also optimized during this step.

[0218] It is in this step that the electronic components of the or each closure layer are defined.

[0219] Figure 11 illustrates how a pair of interconnection zones of a strand is adapted to define a pair of input / output ranges (or terminals) for that strand.

[0220] The 911 interconnect zone of a 910 unit element is modified to present a range, for example, of input 901.

[0221] The interconnection zone 961 of a unit element 960 is modified to present a range for example of output 902.

[0222] If range 902 is selected as the input range of the strand, then range 901 is the output range of that strand, and vice versa.

[0223] Interconnection zones 911 and 961 are one above the other.

[0224] Four through vias 981, 983, 984 and 986 run vertically.

[0225] The two outermost vias, 981 and 986, are not connected to either beach 901 or beach 902.

[0226] Via 983 is connected to the first range 901 only, and via 984 is connected to the second range 902 only. The first range 901 is connected to the second, third, and fourth wires (pairs of strands) of element 910. The first wire (pair of strands) of element 910 is connected to the third wire (pair of strands) of element 910 by means of through-via 981, a first connecting strip 991 (on the upper closure layer and incorporating an electronic component), and through-via 983.

[0227] The output terminal 902 is connected to the fourth, third and second wires (pair of strands) of the element 960. The first wire (pair of strands) of the element 960 is connected to the third wire (pair of strands) of the element 960, by means of the through via 986, a second connecting strip 992 (on the upper closure layer and incorporating an electronic component), and the through via 984.

[0228] It is observed that, with respect to an interconnection zone used for the continuity of a strand from one bifacial to the next and which comprises six through vias (in the embodiment of Figure 11), the intermediate through vias between vias 981 and 983 on the one hand, and vias 986 and 984 on the other, are not present for the interconnection zones used to define the entry and exit of a strand. The unused vias are removed.

[0229] In step 850, a final plan is generated. It takes into account the chosen magnetic core and gives the external layout of the PCB.

[0230] The Litz 900 coil made from such a plan is shown, for example, in Figure 11. The coil shown in this figure is illustrative of type 3. It has one bundle made up of three strands, each strand consisting of four wires, each wire consisting of a pair of strands. A strand is made by connecting four individual elements in series.

[0231] Advantageously, the process continues with a step 860 of manufacturing the coil from the plan obtained in step 850.

[0232] It should be noted that a reel, such as reel 900 corresponding to the plane of figure 11, for example, has different possible uses.

[0233] For example, if the three strands are connected in parallel, a single-turn coil is obtained. This coil can form the primary winding of a transformer. A second, similar coil can then be used as the secondary winding of this transformer.

[0234] For example, if we connect two PCBs in series, each PCB having the structure of the previous example (single-turn bundle), then we obtain a coil with a two-turn bundle. Thus, depending on the intended application, each strand can be assigned to one or more different bundles, forming one or more sub-coils. This allows us to cover several use cases, such as:

[0235] - inductance

[0236] - transformer with maximum coupling (interleaved configuration); and,

[0237] - transformer with leakage inductance, for resonant type systems for example.

[0238] VARIANTS

[0239] For types 2 and 3, in an interconnection zone and an exchange zone respectively, there is not necessarily a single via per strand or pair of strands. Alternatively, there may be several vias for each strand or pair of strands to address current density issues in high-power applications, for example.

[0240] While the description focuses primarily on an annular construction around a Z-axis, other geometries are possible to conform the coil to the shape of the core, so that the Litz coil has a geometry conjugate to that of the magnetic core. Thus, instead of a core with a cylindrical cross-section, one might prefer to use a core with an elliptical cross-section. In this case, those skilled in the art will know how to extend the teachings of this description of a circular annular coil to an oblong annular coil.

[0241] BENEFITS

[0242] The advantage of such a three-dimensional winding is to offer a Litz coil whose bundle is composed of N helical strands.

[0243] Each layer of each biface carries a unit element of each of these N strands, and the unit elements of a strand are angularly offset from one layer to the next.

[0244] With this helical geometry, all the wires in a single strand are subjected to the non-homogeneous magnetic induction not only radially, i.e., in the plane perpendicular to the Z-axis, but also axially, i.e., along the Z-axis. This extends Litz's principle along the third direction, corresponding to the stacking of the bifacials. This homogeneous exposure of each wire to the magnetic field along its path leads to a reduction in alternating resistance.

[0245] Volumetric copper densification, particularly through adjustment of the angular footprint of each unit element while maintaining a homogeneous current distribution in all directions, also contributes to reducing AC resistance. A power density an order of magnitude greater than that of the prior art is achieved.

[0246] The modular structure of the coil makes it possible to establish generic construction rules, and thus cover a wide variety of possible configurations and applications.

[0247] Once manufactured, the coil exhibits a low variation in these characteristics. In particular, it exhibits:

[0248] - an alternating resistance reduced by up to a factor of 2 over a range of 100kHz to 600kHz compared to a conventional copper loop;

[0249] - a significant reduction in capacitance between neighboring strands;

[0250] - an increase in power density (greater than 15%);

[0251] - a reduction in shear stress facilitating the printing operation of the printed circuit board.

[0252] APPLICATIONS

[0253] The three-dimensional planar Litz coil according to the invention finds application in any electronic device intended for the transfer of electrical energy and integrated into an electronic board.

[0254] It can play the role of a magnetic component without galvanic isolation (like an inductor) or that of a magnetic component with galvanic isolation (like a transformer).

[0255] This magnetic component technology is particularly well suited for aeronautics, due to the reproducibility of the coil's electrical characteristics, its optimized heat dissipation profile, and its low height.

[0256] The repeating aspect of a pattern (modular element) allows both windings or winding combinations with a high number of turns (high voltage application) or a low number of turns (power application), with significant volume densities of copper.

[0257] It can be used as: an energy converter, particularly for converting between a battery and certain functions (screen) of equipment in the field of telephony, for example; a contactless transmission device for various sectors of activity (e.g., aeronautics, automotive, etc.); a battery charger, for example in automobiles;...

Claims

23 DEMANDS 1. A three-dimensional planar Litz coil (100), characterized: in that the three-dimensional planar Litz coil is made within the thickness of a printed circuit board (10), the printed circuit board being made up of a stack of bifaces, the stack comprising at least two bifaces (211, 20B), each biface having two conductive layers, the printed circuit board thus comprising a succession of conductive layers between a lower layer and an upper layer in a direction of stacking of the bifaces, each conductive layer being etched so as to present a plurality of tracks; in that the three-dimensional planar Litz coil comprises a bundle, said bundle (90) comprising a plurality of strands (60, 70, 80), each strand comprising at least one wire, the wire or each wire comprising either a single strand or a pair of strands, each strand resulting from the connection of a plurality of tracks,each track of the plurality of tracks of a strand being etched on different conductive layers of the printed circuit; and, in that each strand of the three-dimensional planar Litz coil results from the connection of a succession of identical unit elements (51, 52, 53, 54), each unit element joining the track or tracks etched on a single layer and located within a portion of said layer forming an annular sector, two successive unit elements of the succession of unit elements forming a strand being carried by two successive layers of the succession of conductive layers and angularly offset from each other around the stacking direction, so that the strand has a helical shape extending through the entire printed circuit, from the bottom layer to the top layer, each unit element having an active central area (51-2),an interconnection zone (51-1) and an exchange zone (51-3), an electrical connection, according to a stacking direction (Z) of the conductive layers of the printed circuit board, of the traces of the active zones of two successive unit elements of a strand being made by interconnection vias between either the interconnection zones when said two successive unit elements do not belong to the same biface, the interconnection vias then being through-vias (26), which pass through the printed circuit board, or the exchange zones when said two successive unit elements belong to the same biface, the vias, interconnection then being buried vias (21), which cross only the same biface.

2. Three-dimensional planar Litz coil according to claim 1, wherein an interconnection zone (51-1) extends over a portion of an external periphery of the three-dimensional planar Litz coil (100) and protrudes radially with respect to the active zone (51-2) of the unit element (51).

3. Three-dimensional planar Litz coil according to any one of claims 1 to 2, wherein an exchange zone (51-3) extends over a portion of an internal periphery of the three-dimensional planar Litz coil (100).

4. Three-dimensional planar Litz coil according to any one of claims 1 to 3, wherein the printed circuit board comprises, in addition to a stack of bifaces forming a central stack, at least one closure layer (32), said closure layer being located above or below the stack of bifaces, the through interconnection vias (26) opening onto said at least one closure layer.

5. Three-dimensional planar Litz coil according to claim 4, wherein said at least one closure layer carries one or more connecting bars (691, 692), a connecting bar connecting a first through-interconnect via (681, 684) and a second through-interconnect via (683, 686), said first and second through-interconnect vias connecting the tracks of two different unit elements (610, 660) of the same strand, a connecting bar preferably incorporating at least one electronic component, such as a DC blocking capacitor or a resistor.

6. Three-dimensional planar Litz coil according to any one of claims 1 to 5, wherein an interconnection zone and / or an exchange zone of a unit element is adapted to connect the tracks of two successive unit elements by introducing a crossing between wires of the corresponding strand allowing to obtain the Litz effect.

7. Three-dimensional planar Litz coil according to any one of claims 1 to 6, wherein each unit element (151) comprises a single track, each strand comprising only a single wire consisting of a single strand.

8. A three-dimensional planar Litz coil according to any one of claims 1 to 6, wherein a unit element (251) comprises a plurality of tracks (271 to 276), each strand comprising a plurality of wires, each wire consisting of a single strand, each track belonging to a strand, or wherein a unit element (351) comprises a plurality of tracks (371-1 to 376-2), each strand comprising a plurality of wires, each wire consisting of a pair of strands, each track belonging to one strand of a pair of strands.

9. Equipment comprising a magnetic component integrating a magnetic core and a three-dimensional planar Litz coil (900) according to any one of claims 1 to 8, the printed circuit supporting the planar Litz coil having a conjugate geometry of a geometry of the magnetic core.

10. A computer-implemented method (800) for designing a three-dimensional planar Litz coil (900) according to any one of claims 1 to 8.

11. Computer program comprising software instructions which, when executed by a computer, implement a method according to claim 10 of designing a three-dimensional planar Litz coil (900) according to any one of claims 1 to 8.

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