WO2017103075A1

WO2017103075A1 - Monolithic inductance cores comprising a heat sink

Info

Publication number: WO2017103075A1
Application number: PCT/EP2016/081388
Authority: WO
Inventors: Gérard DELETTE
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2015-12-17
Filing date: 2016-12-16
Publication date: 2017-06-22
Also published as: FR3045923B1; FR3045923A1

Abstract

The invention relates to an inductance core comprising: a magnetic body (400) comprising an exposed surface (406) and forming a median magnetic path (401a, 401b); and a heat sink (410) comprised in the magnetic body (400). The heat sink (410) is a strip of a certain thickness, said strip having a first conductivity according to the thickness direction thereof, and a second conductivity perpendicularly to the thickness direction thereof, the first conductivity being lower than the second conductivity, the direction according to the thickness being perpendicular to the median magnetic path (401a, 401b) in the volume of the magnetic body (400), the strip being arranged in such a way as to drain the heat towards the exposed surface (406).

Description

MONOLITHIC INDUCTANCE CORES INTEGRATING THERMAL DRAIN

DESCRIPTION

TECHNICAL FIELD AND PRIOR ART

The invention relates to an inductance core comprising a magnetic core integrating a heat sink.

FIG. 1 represents an inductance core 10 known from the state of the art and described in US Patent No. 2009/0146769 A1 [1], comprising:

a magnetic body 10a,

at least one heat sink 11.

The inductance core, illustrated in Figure 1, is generally implemented in an inductance circuit. To do this, a magnetic excitation coil 13 is formed on a part of the magnetic body so as to generate, when traversed by an electric current, a magnetic induction in said part. Such inductive circuits may, for example, be used in power converters, the function of which is to adapt the voltage and the current delivered by a source of electrical power to supply an electrical system. A power converter also comprises electronic components functioning as switches (active components) switching at a given frequency f, and thus make it possible to supply the magnetic excitation coil. In the case of DC / DC converters for example, the active components are transistors that are used to "cut" the input voltage in regular cycles. In order to deliver a DC output voltage, inductance circuits are used to store and destock the electrical energy on each cycle and to smooth the output voltage to its average value.

As soon as it is crossed by an alternating current of frequency f, the coil creates an alternating magnetic induction, in the magnetic body, of the same frequency. This phenomenon is accompanied by magnetic losses which result in a heating of the magnetic body. In particular, a larger increase may be observed in the area of the magnetic body where the magnetic induction takes place. It is, moreover, known that the magnetic losses evolve rapidly with the temperature of the magnetic body, which can exacerbate the disparities from one area to the other of the magnetic body, and also create runaway phenomena.

It is furthermore envisaged to use the power converters at higher frequencies f (for example greater than 1 MHz). Switching frequencies greater than 1 MHz are now accessible using active components including transistors made on Gallium Nitride (GaN). In this respect, those skilled in the art will be able to consult document A. M. LEARY [2]. The main advantage of this increase in frequency is to be able to reduce the volume of inductor circuits for applications, particularly in the fields of aeronautics and the automobile. However, the magnetic losses also increase with the switching frequency of the active components.

It is then imperative to dissipate the heat produced in the magnetic core, and more particularly to maintain a relatively homogeneous temperature throughout its volume.

For this purpose, US patent 2009/0146769 Al [1] proposes to segment the magnetic core and to intercalate between each segment 12 a heat sink 11 made of an electrically insulating material such as aluminum nitride (AlN). Each heat sink allows a heat exchange between the magnetic core and the external environment.

However, this solution is not satisfactory.

Indeed, this approach leads to important air gaps that modify the inductance of the inductance circuit. It is then necessary to add turns on the magnetic excitation coil to maintain the inductance of the circuit at the desired value. This results in an increase in the volume of the inductance circuit. However, for certain applications, it is desirable, even primordial, that the inductor circuits occupy the smallest possible volume. Moreover, the volume reduction permitted by the increase in switching frequency of the active components is then at least partially neutralized by the thermal drains according to the prior art.

In addition, the implementation of the circuit, according to the aforementioned prior art, imposes cutting and assembly operations which have a significant impact on the manufacturing cost of the magnetic core.

Similarly, US Pat. No. 7,573,362 B2 [3] proposes thermal drains made, this time, of an electrically conductive material, for example copper or aluminum.

This solution is not satisfactory either.

Indeed, in addition to presenting the drawbacks of the patent application [1], the thermal drains of the patent [3] are the seat of induced currents that generate additional losses.

An object of the invention is then to provide an inductance circuit having heat dissipation means for cooling, effectively, at a relatively homogeneous temperature the magnetic core, while limiting the increase in the volume of said magnetic core, and the induced current losses.

Another object of the invention is also to propose an inductance circuit having heat dissipation means having a limited impact on the value of the inductance of said circuit.

STATEMENT OF THE INVENTION

The objects of the invention are then achieved by an inductance core comprising:

a magnetic body comprising an exposed surface, and comprising at least a part of a median magnetic path,

at least one heat sink included in the magnetic body,

The heat sink is a lamella having a thickness E, said lamella has a first electrical conductivity in the direction of its thickness E, a second electrical conductivity perpendicular to the direction of its thickness E, the first electrical conductivity being less than the second electrical conductivity, the direction according to the thickness E is substantially perpendicular to the median magnetic path in at least one section of the magnetic body, the lamella being arranged so as to drain heat from the volume of the magnetic body towards the exposed surface.

Thus, the lamella is disposed in the inductance core so that the direction, perpendicular to the direction along the thickness E of the lamella, corresponds to the flow of heat, within the lamella, towards the exposed surface of the magnetic body.

The arrangement of the lamella-shaped heat drains according to the invention makes it possible to obtain a more homogeneous distribution of the temperature in the inductance core.

Indeed, the anisotropic nature of the electrical conductivity of the drain according to the invention, and its arrangement in the magnetic body makes it possible to obtain a better homogeneity of the temperature in the magnetic body while limiting the effect of the induced currents. Indeed, in the configuration proposed by the invention, the energy dissipated under the effect of the induced currents is proportional to the first electrical conductivity and the square of the thickness of the drain. Thus, the lower the first electrical conductivity, the lower the induced current losses are. Moreover, the drain leads the heat all the better in the direction perpendicular to the direction according to the thickness E that its second electrical conductivity (so that its second thermal conductivity) is high. The higher this second electrical conductivity, the smaller the thickness E of the drain can be to evacuate the heat. Consequently, the anisotropic nature of the heat sink makes it possible to obtain a good efficiency in terms of homogenization of the temperature while limiting the losses by induced currents.

Furthermore, according to the invention, the slats are arranged to limit the magnetic flux through the slat when the magnetic body is traversed by a magnetic flux in the direction of the median magnetic path. Thus, the disruption of the inductance of an inductance circuit comprising the core inductance according to the invention is little, if any, affected by the presence of the heat sink.

Finally, advantageously the inductor core according to the invention does not impose, during its manufacture, cutting and assembly steps that affect the manufacturing cost of said inductor core.

According to one mode of implementation, the magnetic body has a toric shape.

According to one embodiment, the magnetic body comprises an axis of revolution, the lamella opens onto the exposed surface of the magnetic body, and is disposed in a median plane of the magnetic body, said plane being perpendicular to the axis of revolution.

According to one embodiment, the magnetic body comprises a bar extending along a longitudinal axis, the lamella being disposed in the volume of the bar, the direction along the thickness E being substantially perpendicular to the longitudinal axis.

According to one embodiment, the magnetic body comprises a first face disposed in the extension of the bar, and into which the lamella opens, the first face advantageously being perpendicular to the longitudinal axis.

According to one mode of implementation, the lamella is disposed in a plane comprising the longitudinal axis.

According to one mode of implementation, the magnetic body forms a closed magnetic loop, advantageously rectangular.

According to one embodiment, the magnetic body comprises a frame, the bar being arranged at the center of the frame so as to form two rectangular magnetic loops, symmetrical with respect to the bar, contiguous at a plane of symmetry of the closed off.

According to one embodiment, the rectangular frame comprises straight portions, at least one straight portion comprises at least one heat sink ,.

According to one embodiment, the core comprises an outer lateral surface, the lamella opening on said outer lateral surface. According to one embodiment, the anisotropy factor of the first and second electrical conductivities of the lamella is greater than 1000, advantageously between 1000 and 100000.

According to one embodiment, the lamella also has a second thermal conductivity perpendicular to the direction of its thickness E, the second thermal and electrical conductivities are, respectively, greater than 1500 W / m / K, and 10 ⁶ (Ω. ΙΎΙ) ¹ .

According to one embodiment, the lamella also has a first thermal conductivity in the direction of its thickness E, the first thermal and electrical conductivities are, respectively, less than 10 W / m / K, and 10 ² (Ω. ΓΪΙ) - ¹ .

According to one mode of implementation, the thickness E is less than 150 μιη, advantageously less than 100 μιη.

According to one embodiment, the coverslip comprises at least one of the elements chosen from: graphite sheet, graphene and carbon nanotubes.

According to one embodiment, the lamella is laminated, and comprises in the direction of its thickness E an alternation of electrically conductive layers and electrically insulating layers.

The invention also relates to an inductance circuit comprising:

the inductance core,

a magnetic excitation coil intended to create a magnetic induction in the inductance core.

The invention also relates to a power converter comprising the inductor core or the inductance circuit.

The invention also relates to a method of manufacturing an inductance core comprising a magnetic body and a heat sink, the method comprising the steps of:

- forming the magnetic body by powder injection molding, the powder injection molding being performed so as to provide a cavity in the magnetic body adapted to receive the heat sink, the body comprising an exposed surface, and forming at least a portion of a median magnetic path,

- Introduction and sealing of the heat sink in said cavity with a thermal paste, the heat sink being a lamella having a thickness E, said lamella has a first electrical conductivity in the direction of its thickness E, a second electrical conductivity perpendicular to the direction of its thickness E, the first electrical conductivity being less than the second electrical conductivity, the direction according to the thickness E is substantially perpendicular to the median magnetic path in at least one section of the magnetic body, the strip being arranged so as to drain heat from the volume of the magnetic body to the exposed surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will become apparent in the following description of embodiments of the inductor core according to the invention, given by way of non-limiting examples, with reference to the appended drawings in which:

FIG. 1 is a view from above of a schematic representation of an inductance core known from the prior art,

FIGS. 2a to 2c are top views of representations of median magnetic paths in inductance cores according to the prior art,

FIGS. 3a to 3c are schematic representations of an inductor core according to a first embodiment of the invention,

FIG. 4 is a schematic representation of the inductance core according to a second embodiment of the invention,

FIGS. 5a and 5b are diagrammatic representations of the inductance core according to a third embodiment of the invention,

FIGS. 6a to 6d are diagrammatic representations of the inductance core according to a fourth embodiment of the invention,

FIG. 7 is a view from above of a schematic representation of an inductance core known from the prior art, FIG. 8 represents the evolution of the temperature (on the horizontal axis) in the vicinity of the magnetic excitation coil in an inductance core and a function of the thickness E of the lamellae according to the invention,

FIG. 9 is a schematic representation of the inductance core according to the fourth embodiment of the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Definitions:

Lamella: we mean by lamella an object having a small thickness, comprising two faces of larger area, substantially parallel, interconnected according to their contour. The slat has a thickness E, in a direction substantially perpendicular to both faces, which corresponds to the distance between the two faces of the slat. The faces, although generally flat, may also have a curved surface so as to follow the shape of a magnetic path, where appropriate, itself curved. We also mean, by the direction according to the thickness E of the lamella a direction perpendicular to the two faces of the lamella. We also note "L" the largest dimension of the faces of the lamella (for example the length of said faces, if the latter are rectangular in shape). Thus, according to the present invention, the ratio L / E is advantageously greater than 50.

Median magnetic path: we mean by median magnetic path a closed contour traversing the whole magnetic circuit and on which the circulation of the magnetic field H is equal to N * 1, that is to say the magnetomotive force (N being the number of turns the coil which will be described in the following description, and I the current flowing through said coil). Moreover on this contour, the magnetic field vector is tangent at every point of the contour. The magnetic path materializes the direction of the magnetic flux traversing the magnetic circuit. In other words, the median magnetic path is the median path between the smallest and the largest magnetic path in the magnetic body. The magnetic body forms at least one closed geometric loop, for example a torus 100 (FIG. 2a), a rectangular or square frame 300 (FIG. 2b), two rectangles 400 contiguous to one another on their side and in the same plan (Figure 2c). The dashed lines 101, 301 and 401 represent the median magnetic paths in each of these three configurations.

Cross Section: cross section means the section resulting from the intersection of a plane perpendicular to the longitudinal axis of an elongated element.

Exposed surface: we mean by exposed surface, a surface of a body exposed to the external environment.

Essentially: the term "essentially", as soon as it is used in relation to the orientation of the direction of the thickness E with respect to a given direction, means that a deviation of +/- 5 ° with respect to the direction given direction can be tolerated.

The inductor core comprises a magnetic body. The magnetic body forms a magnetic path, advantageously closed. The magnetic body comprises a median magnetic path.

A magnetic excitation coil comprising a number N of turns of a conductive material may be present around a portion of the magnetic body to form an inductance circuit. The inductance circuit has an inductance value LM defined by the geometrical characteristics of the magnetic body, and the number of turns N.

The magnetic body may comprise a magnetic material of magnetic permeability greater than 50 (μ _Γ > 50).

For example, the magnetic material may comprise a ferrite oxide of spinel crystallography structure. Indeed, the magnetic permeability of such materials is stable in the high frequency range. The most common magnetic materials respond to the formulation:

Mn- _x Zn _x Fe ₂ O ₄ and Ni- _x Zn _x Fe ₂ O ₄ .

For example, the magnetic body comprises Mn- _x Zn _x Fe20 ₄ , with x between 0.3 and 0.6, the magnetic permeability μΓ changes with x, and is between 500 and 1000. A preferred embodiment for the magnetic body, which will be presented in the remainder of the description, comprises the injection molding of NiZn ferrite powder or MnZn ("PIM" or "Powder Injection Molding" according to the English terminology). Saxon).

Advantageously, the ferrite type materials also have high electrical resistivity values, which makes it possible to limit the losses due to induced currents.

The materials Mn- _x Zn _x Fe20 ₄ and Ni- _x Zn _x Fe ₂ O ₄ also have the advantage of being available on an industrial scale.

The inductor core also includes at least one heat sink. The heat sink is included in the volume of the magnetic body. The heat sink is a lamella with a thickness E. The lamella has a first electrical conductivity, in the direction of its thickness E, and a second electrical conductivity, c _m a _X , perpendicular to the direction of its thickness E. The first Electrical conductivity is lower than the second electrical conductivity.

In other words, the electrical conductivity of the coverslip is anisotropic. We then define an electrical conductivity anisotropy factor as the ratio between the second electrical conductivity and the first electrical conductivity. The anisotropy factor of electrical conductivity is advantageously greater than 1000, more particularly between 1000 and 100000.

By second electrical conductivity, c _m _X , perpendicular to the direction of the thickness E of the lamella, we mean a conductivity according to the surfaces defined by the faces of the lamella (also called plane of the lamella). The second electrical conductivity Omax is not necessarily the same in all directions of the plane of the lamella.

Under these conditions, the electrical conductivity along the plane of the lamella is also anisotropic, and two second electrical conductivities c _m axi and c _m a _X 2 can be defined. Thus, since the electrical conductivity in the plane of the lamella is anisotropic, the first electrical conductivity is less than the second electrical conductivity, it is understood that is less than c _m axi and c _m a _X 2. In all the following the description, we will confuse c _m axi and c _m a _X 2, and consider only being understood that the skilled person with this general knowledge can adapt the various embodiments to a slat having anisotropic conductivity according to its plan. The skilled person can also adapt these teaching to the second thermal conductivity presented in the following description.

The first electrical conductivity < _mm may be less than 10 ²

(Ω. ΓΪΙ) - ¹

The second electrical conductivity _m ax may be greater than 10 ⁶ (Ω.ιτη) ^"

1

The lamella may have a first thermal conductivity, _mm , in the direction of its thickness E, and a second thermal conductivity, _m ax, perpendicular to the direction of its thickness E. The first thermal conductivity is less than the second thermal conductivity. In other words, the thermal conductivity of the lamella is anisotropic. We then define a factor of anisotropy of thermal conductivity as the ratio between the second thermal conductivity and the first thermal conductivity. The thermal conductivity anisotropy factor is advantageously greater than 100, more particularly between 100 and 1000.

Particularly advantageously, the direction along the thickness E is essentially perpendicular to the median magnetic path in at least one section of the magnetic body (we will see in the various embodiments the arrangement of the lamella with respect to the median magnetic path ). The section of the magnetic body in which the coverslip can also be called volume section. In the case of a magnetic body comprising a frame and / or bracket (that is, straight portions as discussed in the following description), the section may be a straight portion of said frame and / or the bar. In the case of a magnetic body of toric shape, the section may be a section of said magnetic body.

Thus, the combination of the electrical conductivity properties of the lamella and its positioning in the magnetic body enables the hot zones to be drained from the heat towards the exposed surface of the magnetic body without necessarily necessarily leading to said exposed surface. The first thermal conductivity, _mm , may be less than 10

W / m / K.

The second thermal conductivity, _m ax, can be greater than 1500

W / m / K.

Advantageously, the lamella comprises at least one of the elements chosen from: graphite foil, high orientation graphite foil, graphene and carbon nanotubes. In this respect, the skilled person will find in reference [4] the thermal properties of graphene.

The graphite may, for example, be high orientation pyrolitic graphite ("Highly Oriented Pyrolitic Graphite" in Anglo-Saxon terminology). This type of graphite is generally in lamellar form, and can be obtained by techniques known from the state of the art. In this respect the skilled person can consult the reference [5] cited at the end of the description.

Alternatively, the lamella is laminated, and comprises in the direction of its thickness E an alternation of conductive layers and insulating layers, for example a stack comprising alternating layers of copper and aluminum nitride layers.

We will now present various particular examples of implementations of the invention.

First example:

According to a first particular example of implementation of the invention set forth in FIGS. 3a to 3c, the magnetic body 100 has a toric shape, and comprises a surface 102 exposed to the external environment. The magnetic excitation coil 103 is formed around at least a part of the magnetic body 100.

When traversed by a current, the magnetic excitation coil 103 creates lines of magnetic inductions in the magnetic body 100. The magnetic induction lines are then, essentially, confined in the torus, and form concentric circles. . The body comprises an axis of revolution 104 and a median plane 105 perpendicular to the axis of revolution 104. Two inner concentric circles 101a and 101b outer form the intersection of the torus with the median plane. The median concentric circle 101 of the two inner concentric circles 101a and outer 101b then corresponds to the median magnetic path 101. In this particular example, the lamella 110 may be flat, and disposed in the median plane 105 of the body (Figure 3a).

Alternatively, as shown in FIGS. 3b and 3c, the lamella 110 can match the curvature of the median magnetic path 101 and be perpendicular to the median plane 105.

Several lamellae 110 may be arranged in the magnetic body 100.

According to the aforementioned arrangements, the or lamella 110 is arranged to conform to the flow of the flow lines in the body, ie the direction along the thickness E of the lamella is substantially perpendicular to the magnetic flux lines flowing in the magnetic body 100. Furthermore, the strip 110 opens onto the exposed surface 102 of the magnetic body 100.

According to the configuration thus described, the magnetic flux, created by the coil 103, through the lamella 110 is minimal, or even zero.

Second example:

According to a second particular example of implementation of the invention set out in FIG. 4, the magnetic body 200 comprises a bar 200a. The bar 200a comprises a longitudinal axis 204 coincides with the median magnetic path 201 in said bar 200a. The cross section of the bar 200a is constant and may be, without limiting the invention, circular, square or rectangular. The magnetic body 200 comprises a first face (not shown) exposed to the external environment, and in the extension of the bar 200a along its longitudinal axis 204. The magnetic excitation coil 203 is formed around a portion of the bar , and is intended to create magnetic induction lines in the bar 200a. The magnetic induction lines are essentially parallel to the longitudinal axis 204 of the bar 200a. In this example, the strip 210 is disposed in the bar 200a, so that the direction along the thickness E is perpendicular to the longitudinal axis 204 of the bar 200a. The lamella 210 also opens onto the first face of the bar 200a. Several lamellae 210 may be arranged in the bar 200a.

In this example, the magnetic flux through the lamella is minimal or zero.

Third example:

According to a third particular example of implementation of the invention set forth in FIGS. 5a and 5b, the magnetic body 300 has the shape of a rectangular frame 300a comprising straight portions of constant cross section, each straight part comprising a longitudinal axis.

The rectangular frame 300a forms a rectangular magnetic loop.

The rectangular frame 300a includes an outer side surface 306. The rectangular frame 300a includes a first surface 306a and a second surface 306b parallel to each other, perpendicular to the outer side surface 306, and connected by said outer side surface 306.

The magnetic excitation coil 303 is formed around a first right portion of the rectangular frame 300a, which we will now call bar 300b. The magnetic excitation coil 303 is intended to create magnetic induction lines in the bar 300b.

Two concentric rectangles 301a and 301b form the intersection of the frame with a median plane 305, the median plane 305 being parallel to the first 306a and second 306b surfaces of the rectangular frame 300a. The median magnetic path 301 (shown in broken lines in FIG. 5a) is then the median rectangle of the two concentric rectangles 301a and 301b.

Each side of the median rectangle is collinear with the longitudinal axis of the right part in which it is located.

The lamella 310 may be flat and arranged in the bar 300b of the magnetic frame 300 so that the direction along the thickness E of the lamella 310 is perpendicular to the median magnetic path 301 in said bar 300b.

More particularly, as shown in FIG. 5b, the lamella 310 may be disposed in the median plane 305. For example, the lamella 310 extends from the axis longitudinal of the bar 300b, parallel to the median plane 305, and opens out from the outer lateral surface 306 of the rectangular frame 300a. The rectangular frame 300a comprises a first face 307 disposed at the surface 306. The first face is also disposed in the extension of one end of the bar 300b, and perpendicular to the longitudinal axis of said bar 300b. The coverslip can then extend also in the direction of the longitudinal axis and lead to the first face 307.

Thus, the magnetic flux created by the magnetic excitation coil 303 is minimal or even zero.

In addition, several slats can be arranged in the rectangular frame 300a, and more particularly in each of the straight portions of the rectangular frame 300a.

Fourth example:

According to a fourth particular example of implementation of the invention set forth in FIGS. 6a to 6d, the bar 400b described in the second embodiment of the invention is disposed in the center of a rectangular frame 400a so as to form two rectangular magnetic loops, symmetrical with respect to the bar, and contiguous at a plane of symmetry of the bar 400b. This configuration is known as E or E-E.

The rectangular frame 400a comprises straight portions of surface cross section A. The cross section of the bar 400b has a surface area of 2xA (the area of the cross section of the bar is thus twice the cross sectional area of the parts right of the frame).

The rectangular frame 400a includes an outer side surface 406.

The rectangular frame 400a comprises a first surface 406a and a second surface 406b parallel to each other, perpendicular to the outer lateral surface 406, and connected by said outer lateral surface 406 (seen in section ZZ 'of the magnetic body, in Figure 6b).

The bar 400b includes an exposed surface 406c. The magnetic excitation coil 403 is formed around a portion of the bar 400b. The magnetic excitation coil 403 is intended to create a magnetic induction in the bar 400b. The magnetic flux originates in bar 400b along the longitudinal axis of said bar 400b, and flows symmetrically from one end of bar 400b into rectangular frame 400a to loop back into the other end of the bar. bar 400b. Each rectangular magnetic loop therefore comprises a median magnetic path 401a and 401b (broken line in Figure 6a). Thus two median magnetic paths 401a and 401b circulate in the bar 400b, parallel to the longitudinal axis of the bar 400b.

The lamella 410 may be flat and arranged in the bar 400b so that the direction along the thickness E of the lamella 410 is perpendicular to the median magnetic path 401a, 401b in said bar 400b.

According to a particularly advantageous example, as shown in FIG. 6a, the lamella 410 extends along a plane comprising the longitudinal axis of the bar 400b, said plane being perpendicular to a median plane 405 of the rectangular frame 400a.

The median plane 405 includes the longitudinal axis of the bar 400b, and is parallel to the plane of the rectangular frame 400a. The plate 410 then opens onto the exposed surface 406c of the bar 400b.

The rectangular frame 400a includes a first face 407. the first face 407 is disposed at the outer lateral surface 406 of the rectangular frame 400a, and in the extension of a first end of the bar 400b.

The lamella 410 can then also extend in the direction of the longitudinal axis and lead to the first face 407.

FIG. 6c shows two lamellae 410 included in the bar 400b, each lamella 410 is disposed symmetrically with respect to the longitudinal axis of the bar 400b, and in a plane perpendicular to the median plane of the rectangular frame 400a comprising the longitudinal axis. of the 400b bar. Each of the two lamellae 410 also extends parallel to the axis of the bar 400b, and opens out on the first face 407. Advantageously, as shown in Figure 6d, the two lamellae 410 previously described, are added two additional lamellae 410. The additional lamellae 410 are arranged symmetrically with the two lamellae previously described with respect to the center C of the bar 400b. The additional strips 410 open on a second face 408 of the rectangular frame, disposed at the outer lateral surface, and in the extension of a second end of the bar.

In the four examples of implementation of the invention, the direction along the thickness E of the lamella is perpendicular to the median magnetic path. In the aforementioned examples, the body may comprise an air gap ("gap gap" in the Anglo-Saxon terminology) of thickness less than 5% of the length of the median magnetic path. In the presence of an air gap, the slats 410 or present in the bar 400b can also lead into the gap.

The first face, and possibly the second face, is advantageously maintained by a temperature control system at a constant temperature, for example a temperature of between 50 and 100 ° C., for example 80 ° C.

Advantageously, the thermal regulation system is a cold source that imposes a constant and uniform temperature on the face on which it is affixed. The cold source may be for example a solid of high inertia attached to the magnetic body or a fluid flowing on said face.

The Applicant has found that the zone for which the greatest rise in temperature is observed is in the vicinity of the magnetic excitation coil. For the second, third and fourth examples of magnetic bodies, the greatest rise in temperature is therefore observed in the bar.

By way of example, Figure 7 shows the temperature distribution in a magnetic body, as described in the fourth example. The temperature distribution is obtained by a numerical simulation based on a finite element solving method. The general characteristics of the magnetic body considered are given in Table 1: Material Ni- _x ZnxFe20 ₄

Total width 46 mm

Thickness of core 11 mm

Total height 28 mm

Central bar diameter 11 mm

4 mm air gap

Core volume 9.8 cm ³

Number of N turns 18

Power 1000 W

Current in the coil 5 A

5 MHz switching frequency

Table 1.

We can see, in Figure 7, for a core devoid of heat drains, that zone A is the hottest zone and has a temperature of 135 ° C, while the first and second faces are maintained at 80 ° C.

The presence of two lamellae 410, positioned symmetrically with respect to the center of the bar 400b. the two lamellae 410 each go right through to bar 410 and lie in a plane perpendicular to the median plane of the rectangular frame 400a, said perpendicular plane comprising the longitudinal axis of the bar 400b. Each leaflet opens on the outer lateral surface 406 of the rectangular frame 400a. Each strip is made of graphite and measures 14 x 11 mm ² . FIG. 8 represents the temperature, in degrees Celsius, of zone A (on the horizontal axis) as a function of the thickness E, in micrometers, of the four lamellae arranged in the bar.

The greater the thickness of the lamellae, the more the temperature of zone A decreases and tends, asymptotically, towards the temperature first and second faces of the rectangular frame, maintained in this example at 80 ° C.

Thus, the lamellae are arranged in the core so that the heat produced in the core flows from the hot zones of the core to the thermalized core faces (maintained at a constant temperature)

Thus, in the example presented, slats 150 μιη thick reduce the temperature difference between the zone A and the first and second faces at about 20 ° C.

Thus, the anisotropic nature of the drain according to the invention, and its arrangement in the magnetic body makes it possible to obtain a better homogenization of the temperature in the magnetic body while limiting the effect of the induced currents. Indeed, in the configuration proposed by the invention, the energy dissipated under the effect of the induced currents is proportional to the first electrical conductivity and the square of the thickness of the drain. Thus, the lower the first electrical conductivity, the lower the induced current losses are. Moreover, the drain leads the heat all the better in the direction perpendicular to the direction according to the thickness E that its second thermal conductivity (so that its second electrical conductivity) is high.

Moreover, it is particularly advantageous to maintain the lamellae at a thickness E less than 150 μιη, preferably less than 100 μιη. In fact, the arrangement of the lamellae according to the invention makes it possible to limit the effect on the inductance of an inductance circuit comprising an inductance core according to the invention. Thus, unlike the prior art, it is not necessary to add turns to the magnetic excitation coil, and therefore to increase the volume of the inductance circuit.

The invention is not limited to the positioning of heat drains in the bar of the magnetic body. Indeed, in the third and fourth examples, the frame comprises straight portions. At least one straight portion may comprise at least one heat sink 410. Thus, in the example, particularly advantageous, shown in Figure 9, thermal drains 410 are disposed in the rectangular frame 400a.

The rectangular frame includes first sections parallel to the bar 400b, and second sections perpendicular to the bar 400b.

For each first or second section of the rectangular frame 400a, the heat sink 410 may be included in a plane comprising the longitudinal axis of the first or second section comprising said heat sink, and perpendicular to the median plane 405 of the rectangular frame 400a.

Each first section may comprise one or two thermal drains 410. The heat sink 410 may be included in a plane comprising the longitudinal axis of the first section comprising said heat sink, and perpendicular to the median plane 405 of the rectangular frame. The heat sink 410 may open into one of the first 406a or second 406b surfaces. Advantageously, two thermal drains 410 are included in each first section of the rectangular frame 400a, and open respectively into the first 406a and the second 406b surfaces. The second sections of the rectangular frame 400a may also comprise thermal drains 410. In each second section, the heat sink 410 is included in a plane comprising the longitudinal axis of the second section comprising said heat sink, and perpendicular to the median plane 405 of the rectangular frame 400a. The heat sink 410 may open into one of the first 406a or second 406b surfaces.

Advantageously, four thermal drains 410 are included in each second section of the rectangular frame 400a. For each of the two second sections of the rectangular frame 400a, two drains open into the first surface 406a, and the other two into the second surface 406b, and so that two thermal drains 410 opening into the same surface are arranged symmetrically with respect to the plane comprising the longitudinal axis of the bar 400b, and perpendicular to the median plane 405.

As previously mentioned, the formation of the magnetic body can be carried out by injection molding of ferrite powder. In this respect, the man of the trade will find the detail of the powder injection molding method in the patent FR2970194 [6]. This technique comprises a first step of forming a masterbatch comprising an organic material (for example, polyolefins such as polyethylene, polypropylene), and inorganic powders (for example, for the intended application, oxides of the type ferrite, for example Mn- _x Zn _x Fe ₂ O ₄ and Ni- _x Zn _x Fe ₂ O ₄ , and with a particle size of the order of 10 to 20 μιη). The masterbatch is then injected into a mold to give it the desired shape. The molded part is then debinded (Anglo-Saxon terminology) at a temperature between 400 and 700 ° C (for example 500 ° C) to remove organic matter. The debinding step is then followed by a sintering step ("sintering" according to the Anglo-Saxon terminology), conducted at a temperature of between 900 and 1300 ° C. (for example 1220 ° C. for a ferrite oxide Mn). _x Zn _x Fe20 ₄ ) thus making it possible to increase the density of the piece thus formed.

The magnetic body may be made in one piece, or may comprise an assembly of several pieces (for example an assembly of two pieces in E, or an E-piece and an I-piece, or a U-piece and a piece in I). Depending on the embodiment of the magnetic body, one or more molds may be necessary. The mold is also machined to make cavities in the magnetic body for receiving the slats.

After formation of the magnetic body, the lamellae are then introduced into cavities (formed during the molding step) and assembled, by techniques known to those skilled in the art and as described in document [7], to the using a thermal paste that connects with the core. The thermal paste may, for example, be based on polyethylene glycol diluted in ethylcellulose. REFERENCES

[1] US 2009/0146769. [2] A. M. LEARY, "Soft Magnetic Materials in High-Frequency, High Power Conversion Applications," JOM, Volume 64, Issue 7, July 2012, Pages 772-781.

[3] US 7,573,362 B2. [4] Eric Pop, Vikas Varshney, Ajit K. Roy, Thermal properties of graphene: Fundamentals and applications, M RS BULLETI N, VOLUM E 37, DECEM BER 2012,

[5] Carl Zweben, Thermal Materials Solve Power Electronics Challenges, Power Electronics Technology February 2006. [6] FR2970194.

[7] Chia-Ken Leong, D.D.L. Chung, "Carbon black dispersions as thermal pastes that surpass solder providing high thermal contact conductance", Carbon 41 (2003) 2459-

Claims

An inductor core comprising:

a magnetic body (100, 200, 300, 400) comprising an exposed surface (102, 306, 406), and forming at least a portion of a median magnetic path

(101, 201, 301, 401a, 401b),

at least one heat sink (110, 210, 310, 410) included in a section of the magnetic body (100, 200, 300, 400),

the inductance core being characterized in that the heat sink (110, 210, 310, 410) is a lamella having a thickness E, said lamella has a first electrical conductivity in the direction of its thickness E, a second electrical conductivity perpendicularly in the direction of its thickness E, the first electrical conductivity being lower than the second electrical conductivity, the direction along the thickness E is essentially perpendicular to the median magnetic path (101, 201, 301, 401a, 401b) in the body section wherein the lamella is located in such a manner that the lamella is arranged to drain heat from the volume of the magnetic body (100, 200, 300, 400) to the exposed surface (102, 306, 406).

An inductor core according to claim 1, wherein the magnetic body (102) has a toric shape.

An inductor core according to claim 2, wherein the magnetic body (100) comprises an axis of revolution (104), the lamella opens onto the exposed surface (102) of the magnetic body (100), and is disposed in a median plane (105) of the magnetic body (100), said plane being perpendicular to the axis of revolution (104).

An inductor core according to claim 1, wherein the magnetic body (200, 300, 400) comprises a bar (200a, 300b, 400b) extending in a direction longitudinal axis, the lamella being disposed in the volume of the bar (200a, 300b, 400b), the direction along the thickness E being substantially perpendicular to the longitudinal axis (200a, 300b, 400b).

5. An inductor core according to claim 4, wherein the magnetic body (300) comprises a first face disposed in the extension of the bar (300b, 400b), and on which the lamella opens, the first face advantageously being perpendicular to the longitudinal axis.

6. Inductor core according to claim 4 or 5, wherein the lamella is disposed in a plane comprising the longitudinal axis.

An inductor core according to one of claims 4 to 6, wherein the magnetic body (300) forms a rectangular magnetic loop.

An inductor core according to one of claims 4 to 6, wherein the magnetic body comprises a rectangular frame (400a), the bar (400b) being disposed at the center of the rectangular frame (400a) so as to form two loops. Rectangular magnets, symmetrical with respect to the bar (400b), contiguous at a plane of symmetry of the bar (400b).

9. An inductor core according to claim 8, wherein the rectangular frame (400a) comprises straight portions, at least one straight portion comprises at least one heat sink (410).

10. Inductor core according to one of claims 4 to 9, wherein the core comprises an outer side surface, the strip opening on said outer side surface (306, 406).

11. Core inductor according to one of claims 1 to 10, wherein the anisotropy factor of the first and second electrical conductivities of the lamella is greater than 1000, preferably between 1000 and 10000.

12. Inductor core according to one of claims 1 to 11, wherein lamella also has a second thermal conductivity perpendicular to the direction of its thickness E, the second thermal and electrical conductivities are respectively greater than 1500 W / m. / K, and at 10 ⁶ (Ω. Ϊ́Ύΐ) ^"1 .

13. Inductor core according to one of claims 1 to 12, wherein the strip also has a first thermal conductivity in the direction of its thickness E, the first thermal and electrical conductivities are, respectively, less than 10 W / m. / K, and at 10 ² (Ω.ιη) ^Λ .

14. Inductor core according to one of claims 1 to 13, wherein the thickness E is less than 150 μιη, preferably less than 100 μιη.

15. Nucleus of inductance according to one of claims 1 to 14, wherein the lamella comprises at least one of the elements selected from: graphite sheet, graphene and carbon nanotubes.

16. An inductor core according to one of claims 1 to 14, wherein the lamella is laminated, and comprises in the direction of its thickness E an alternating electrically conductive layers and electrically insulating layers.

17. Inductance circuit comprising:

the inductance core according to one of claims 1 to 16 a magnetic excitation coil intended to create a magnetic induction in the inductance core.

Power converter comprising the inductor core according to one of claims 1 to 16 or the inductor circuit according to claim 17.

19. A method of manufacturing an inductance core comprising a magnetic body and a heat sink, the method comprising the steps of:

- forming the magnetic body by powder injection molding, the powder injection molding being performed to provide a cavity in the magnetic body adapted to receive the heat sink, the body comprising an exposed surface (102, 306, 406) and forming at least a portion of a median magnetic path (101, 201, 301, 401a, 401b),

introduction and sealing of the heat sink in said cavity with a thermal paste, the heat sink (110, 210, 310, 410) being a lamella having a thickness E, said lamella has a first electrical conductivity in the direction of its thickness E, a second electrical conductivity perpendicular to the direction of its thickness E, the first electrical conductivity being less than the second electrical conductivity, the direction according to the thickness E is substantially perpendicular to the median magnetic path (101, 201, 301, 401a, 401b) in at least one portion of the magnetic body (100, 200, 300, 400), the lamella being arranged to drain heat from the volume of the magnetic body (100, 200, 300, 400) to the exposed surface (102, 306, 406).