EP1921640A1 - Spiral-shaped closed magnetic core and integrated micro-inductance comprising such a closed magnetic core - Google Patents

Abstract

The core (1) has a set of parallel branches (4, 5) arranged in different parallel planes, where the core is in the form of a rectangular spiral comprising two ends (2) joined to one another by a closing segment (3). The parallel branches (4) are perpendicular to the parallel branches (5), where the branches have different thicknesses. Air-gaps having reduced dimensions are arranged in the branches.

Description

Technical field of the invention

The invention relates to a closed magnetic core for integrated micro-inductance.

State of the art

The invention is part of the theme of integrated micro-inductors for applications in power electronics. It can, more generally, apply to all inductive systems (inductors, transformers, magnetic recording heads, actuators, sensors, etc.) requiring a high density of electrical power.

Micro-inductances of various types have existed for many years, using coils of the spiral or solenoid type. However, the discrete components remain very predominantly used in applications using high power densities because they offer a better compromise between inductance and saturation current.

A spiral winding with magnetic plane is easy to integrate and allows to work with strong currents. However, this type of device becomes very cumbersome when one targets high values of inductance (L of the order of μH), because it takes a number of high spiral turns. In addition, the resistance of such devices is important.

Toroidal integrated micro-inductors with solenoid windings, as well as their meander improvements (see the article "Integrated Electroplated Micromachined Magnetic Devices Using Low Temperature Manufacturing Processes" by JYPark et. al., IEEE Transactions on Electronics Packaging Manufacturing, Vol. 23, No. 1, 2000 ), are directly inspired by discrete components and offer the best possible compromise between resistance and inductance level, because we approach the ideal case of the infinite solenoid. However, simulations show that the magnetic flux inside the nucleus is distributed very inhomogeneously. The magnetic field is more intense along the shorter field lines. The zones of the magnetic core subjected to the most intense fields are very quickly saturated, causing a decrease of the inductance from weak currents, whereas other zones are subjected to much weaker fields and participate little or not in inductive phenomenon, that is to say they have no contribution to the value of the inductance. The useful areas of the magnetic core are therefore very quickly saturated while other areas remain unsolicited.

In addition, the maximum power passing through an inductor is determined by the volume of magnetic material used in the case of an integrated component. This volume is determined by the thickness of magnetic material (thicknesses less than 100 microns for integrated components) and the area occupied by this magnetic core.

Transformers and inductors with E-shaped or E-1 magnetic core are widely used in electrical engineering, mainly in discrete transformers (and in discrete DC / DC type devices) to facilitate assembly and winding of inductors , or to be able to play on the conversion factors between the three windings of each branch, or on the effects of mutual inductances between the separate windings of each branch (see the article «New Magnetic Structures for Switching Converters by S.Cuk, IEEE Transactions on Magnetics, Vol. MAG-19, No. 2, 1983 ). In these devices, the winding is not continuous from one branch to the other, but made by separate wires.

Most of the micro-inductances used on the market are discrete components manufactured by micromechanical processes of micro-machining, gluing, micro-winding, etc. These processes are heavy to implement, individual treatment, not very flexible in terms of design and greatly limit the miniaturization of power circuits. In particular, the thickness of the discrete micro-inductors (typically greater than 0.5 mm) does not allow appropriate packaging in the power supply circuits currently used for mobile telephony, for example.

The manufacturing techniques used in microelectronics allow a much greater flexibility in the implementation of different designs, provide a collective treatment and are compatible with the idea of miniaturization because the thickness (including substrate) can easily be less than 300 μ m). However, they are poorly suited to deposition of high thicknesses (greater than 10 μm ) of conductive, magnetic or dielectric materials and to their etching after photolithography.

For integrated components, there are constraints of technological achievement. Indeed, deposits of conductive layers having a thickness greater than 100 microns are currently not feasible in a standard industrial process.

Article Numerical Inductor Optimization by A. von der Weth et al. (Japan Magnetic Trans., Vol.2, No.5, pp.361-366, 2002). ) describes a micro-inductance with an open multi-branch type magnetic circuit. A plurality of turns disjoined from each other constitutes a winding around the branches of the magnetic core. For these devices, it is sought to increase the level of inductance and to minimize losses.

The integrated micro-inductors generally have an inductance which decreases greatly when the current applied to the turns of the micro-inductor is increased, even for weak currents, which makes it necessary to use unintegrated discrete inductors in certain cases.

Microelectronic chips of small dimensions (a few millimeters squared) are generally square. The integration of inductances therefore imposes constraints that are unknown for discrete components. The proposed solutions are therefore often complex. For inductances, in particular, it is sought to minimize the area occupied, especially since the use of thin film deposition techniques greatly limits the useful thicknesses. Indeed, the power of an inductance Ll _sat ² (where L is the inductance and I _sat the saturation current) depends directly on the volume of available magnetic material.

Object of the invention

The object of the invention is to increase the compactness of a core of an integrated micro-inductance and, for a given size, to increase the value of the inductance.

According to the invention, this object is achieved by a magnetic core according to the appended claims and more particularly by the fact that the magnetic core has a spiral shape having two ends connected to one another by a closing segment.

The invention also aims an integrated micro-inductance comprising a magnetic core according to the invention.

Brief description of the drawings

• la figure 1 représente, en vue de perspective, un mode de réalisation particulier d'un noyau magnétique fermé selon l'invention,
• les figures 2 à 4 illustrent respectivement, en vue de dessus, deux noyaux magnétiques fermés selon l'art antérieur et un mode de réalisation particulier du noyau magnétique fermé selon l'invention,
• la figure 5 représente, en coupe selon l'axe A-A de la figure 4, un mode de réalisation particulier de l'invention,
• la figure 6 représente, en vue de dessus, un mode de réalisation particulier d'un noyau magnétique fermé selon l'invention,
• la figure 7 illustre un mode de réalisation particulier d'une micro-inductance intégrée selon l'invention.

Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given by way of non-limiting example and represented in the accompanying drawings, in which:

• FIG. 1 represents, in perspective view, a particular embodiment of a closed magnetic core according to the invention,
• FIGS. 2 to 4 respectively show, in plan view, two closed magnetic cores according to the prior art and a particular embodiment of the closed magnetic core according to the invention,
• FIG. 5 represents, in section along the axis AA of FIG. 4, a particular embodiment of the invention,
• FIG. 6 represents, in top view, a particular embodiment of a closed magnetic core according to the invention,
• FIG. 7 illustrates a particular embodiment of an integrated micro-inductor according to the invention.

Description of a preferred embodiment of the invention

The magnetic core 1, shown in Figure 1, has a spiral shape. The spiral has two ends 2 connected to each other by a closing segment 3. Thus, the magnetic core 1 is closed.

In FIG. 1, the magnetic core 1 consists of a first set 4 of five parallel branches and a second set of four parallel branches, substantially perpendicular to the branches of the first set 4. The spiral constituted by all the branches of the two sets 4 and 5 is thus rectangular. The connection constituted by the closing segment 3 is added to the spiral to form the magnetic core 1.

As illustrated by means of FIGS. 2 to 4, the magnetic core 1 makes it possible to maximize the occupation of the space in the center of the core 1 and the corresponding micro-inductance.

$${\mathrm{I}}_{\mathrm{sat}}\mathrm{\sim }\mathrm{I}\mathrm{/}\mathrm{N\; et}$$

$${{\mathrm{LI}}_{\mathrm{sat}}}^2\sim \mathrm{I}\mathrm{.}$$

We define a length I of the magnetic core, corresponding to the developed length of the magnetic circuit, and the number N of turns of the coil surrounding the magnetic core 1. It is possible to demonstrate, by means of the reluctance model, the following expressions (L being the inductance and I _sat the saturation current): $$\mathrm{The}\mathrm{~}{\mathrm{<figure-callout\; id="2"\; label="NOT"\; filenames="imgf0001.png"\; state="\{\{state\}\}">NOT</figure-callout>}}^{\mathrm{2}}\mathrm{/}\mathrm{I}\mathrm{,}$$

$${\mathrm{I}}_{\mathrm{sat}}\mathrm{~}\mathrm{I}\mathrm{/}\mathrm{N\; and}$$

$${{\mathrm{LI}}_{\mathrm{sat}}}^2~\mathrm{I}\mathrm{.}$$

Thus, to increase the saturation power P _sat = LI _sat ² of the inductor, it is sought to increase the length I of the magnetic core. The inductance L and the saturation current I _sat thus result from a compromise on the number of turns N, which is all the greater as the length I of the core is large.

An annular inductor according to the prior art, shown in Figure 2, adapts particularly well to a square-shaped chip. The length of the developed ring depends on the outer perimeter of the chip. This geometry does not exploit the central part of the chip.

FIG. 3 represents an improvement in the annular inductance, the meandering inductance described in the aforementioned Park article. The meandering inductance makes it possible to use the central zone by stretching one of the four branches of the ring so as to constitute one or more meanders covering the central part. This solution makes it possible to increase the length I of the constant surface core. By using usual design rules, the occupation of the central zone by the meandering core (FIG. 3) makes it possible to obtain a gain on the length I of the core of the order of 33%, with respect to the annular core ( Figure 2). By increasing the number N of turns according to the length I of the core, a compromise is obtained with a gain on the inductance L of about 20% and a gain on the saturation current I _sat of about 10%.

However, the meandering inductance is optimal only in special cases where the width of the ring and the width of the branches satisfy certain geometry conditions. Indeed, the central zone must be large enough to allow the insertion of an integer number of meanders.

As shown in FIG. 3, the core has an overall width T, the branches have a width W and the spacing between two adjacent branches must be greater than a minimum spacing S. Thus, for a given number Nm of meanders, the width T global of the core must fulfill the condition: $$\mathrm{T}\mathrm{\ge }\mathrm{2}\mathrm{W}\mathrm{+}\mathrm{nm}\mathrm{*}\mathrm{2}\mathrm{W}\mathrm{+}\mathrm{(}\mathrm{2}\mathrm{nm}\mathrm{+}\mathrm{1}\mathrm{)}\mathrm{*}\mathrm{S}\mathrm{.}$$

The ratio of the number Nm of meanders on the surface of the central zone is maximized when the left part and the right part of the equation are equal: $$\mathrm{T}=\mathrm{2}\mathrm{W}\mathrm{+}\mathrm{nm}\mathrm{*}\mathrm{2}\mathrm{W}\mathrm{+}\mathrm{(}\mathrm{2}\mathrm{nm}\mathrm{+}\mathrm{1}\mathrm{)}\mathrm{*}\mathrm{S}\mathrm{.}$$

où T/W est le rapport de la largeur globale T sur la largeur W des branches. Pour T/W=7, 11, 15..., le noyau en méandres permet donc de remplir la zone centrale de façon optimale. Pour T/W=9, 13, 17... cependant, une partie importante de la zone centrale reste inutilisée. La mise en oeuvre de noyaux en méandres est donc restrictive dans la pratique puisque la taille de la puce et la largeur des branches sont en général imposées de façon indépendante. Une partie de la zone centrale peut ainsi rester inutilisée.Assuming that the width W of the branches and the minimum spacing S are equal (S = W) the condition is simplified: $$\mathrm{T}\mathrm{/}\mathrm{W}\mathrm{\ge }\mathrm{3}\mathrm{+}\mathrm{4}\mathrm{nm}\mathrm{,}$$

where T / W is the ratio of the overall width T to the width W of the branches. For T / W = 7, 11, 15 ..., the meandering core makes it possible to fill the central zone optimally. For T / W = 9, 13, 17 ... however, some significant amount of the central area remains unused. The implementation of meander cores is therefore restrictive in practice since the size of the chip and the width of the branches are in general imposed independently. Part of the central area can remain unused.

• Lorsque le rapport T/W équivaut essentiellement à la partie de droite de l'équation ci-dessus, c'est-à-dire lorsque $$\mathrm{T}\mathrm{/}\mathrm{W}\mathrm{\approx }\mathrm{3}\mathrm{+}\mathrm{4}\mathrm{Nm}\left(\mathrm{=}\mathrm{7}\mathrm{,}\mathrm{11}\mathrm{,}\mathrm{15}\mathrm{\dots }\right),$$
  
  
  le noyau en spirale fermé et le noyau en méandres sont comparables, car le gain sur la longueur et le gain sur la puissance sont comparables.
• Lorsque l'équation ci-dessus n'est pas vérifiée, le noyau en spirale fermée permet d'obtenir un gain en longueur I et un gain en puissance plus importants que le noyau en méandres, par exemple pour T/W compris entre 8 et 10 (8<T/W<10) ou pour T/W compris entre 12 et 14 (12<T/W<14).

The spiral-shaped closed magnetic core 1 has a greater independence with respect to the dimensional constraints, and thus makes it possible to optimize the length I of the core, the inductance L and the saturation current I _sat for a given surface. any. As before, the gain on the core length I and the power gain of the spiral core (FIG. 4) can be evaluated with respect to the annular reference structure (FIG. 2). It is then necessary to distinguish two cases:

• When the T / W ratio is essentially equivalent to the right-hand part of the equation above, ie when $$\mathrm{T}\mathrm{/}\mathrm{W}\mathrm{\approx }\mathrm{3}\mathrm{+}\mathrm{4}\mathrm{nm}\left(\mathrm{=}\mathrm{7}\mathrm{,}\mathrm{11}\mathrm{,}\mathrm{15}\mathrm{...}\right),$$
  
  
  the closed spiral core and the meandering core are comparable because the gain in length and the gain in power are comparable.
• When the above equation is not satisfied, the closed spiral core makes it possible to obtain a greater gain in length I and power gain than the meandering core, for example for T / W between 8 and 10 (8 <T / W <10) or for T / W between 12 and 14 (12 <T / W <14).

In particular, in the case of a ratio T / W = 9, the spiral core (FIG. 4) makes it possible to obtain 53% gain on the length I and on the power, with respect to the ring (FIG. ).

The branches and the closure segment 3 have a preferential direction of propagation of the magnetic flux in dynamics. The magnetic axes of the branches and the closing segment 3 are oriented relative to the other, so as to obtain a flow in the form of a closed loop as represented in FIG. 4 by the arrows 6.

The branches can be arranged in different parallel planes. Thus, as shown in FIG. 5, the first set of parallel branches is arranged in a first plane and the second set of parallel branches is arranged in a second plane, parallel to the first plane and greater than the first plane in FIG. In addition, the branches can have different thicknesses. Thus, in FIG. 5 the branches of the first set 4 are less thick than the branches of the second set 5. This makes it possible in particular to adapt the core to the local constraints of the chip used and the adjacent electronic components.

One or more air gaps may optionally cut the magnetic core 1 to increase the reluctance of the magnetic circuit. The magnetic core 1 shown in FIG. 6 comprises several air gaps 11 of small size (at least a factor 1/10 between the dimension of the gap and the total length of the magnetic circuit). The gaps can be arranged in one or more branches.

As represented in FIGS. 1, 4 and 6, the branches constitute a spiral of rectangular type, or at least substantially rectangular, having two turns forming part of two concentric rectangles. However, as needed, more complex spirals may be considered. The forms involved may be arbitrary, for example the geometry of the spiral is rectangular, round, square or octagonal. The person skilled in the art determines the particular form by using simulation software such as the Flux software from Cedrat or the Maxwell software from Ansoft.

FIG. 7 illustrates a micro-inductance comprising the magnetic core 1 according to the invention. A plurality of disjointed turns 9 constitute a winding around the magnetic core 1. All the branches of the core may comprise winding turns. Preferably, the turns envelop substantially all of the surface of the magnetic core 1, a minimum isolation gap separating the adjacent turns. Each turn may comprise a lower plane section in a lower plane, an upper plane section in an upper plane and two rising sections. The winding preferably comprises a single electrical input and a single electrical output. The closing segment 3 preferably has no turns 9.

For integrated components using conventional micro-fabrication techniques, the micro-inductance presents no additional manufacturing difficulties compared to conventional pre-existing systems.

For the magnetic core 1, high permeability magnetic materials (greater than 10), typically iron (Fe) and / or nickel (Ni) and / or cobalt (Co) alloys, which can contain one or more of aluminum, Al, Si, Tantalum, Hf, N, O, and B The core may be heterogeneous and consist of several ferromagnetic and conductive or dielectric (non-magnetic) or antiferromagnetic layers. In particular, the core may consist of an alternation of magnetic layers and intermediate layers, for example a stack comprising two magnetic layers separated by an intermediate layer. The intermediate layers may, for example, be metal (Cu copper, titanium Ti or ruthenium Ru, for example) or an insulating material such as silicon oxide SiO ₂ or aluminum oxide Al ₂ O ₃ , for example. example. The intermediate layers may also consist of antiferromagnetic materials such as nickel oxide NiO or alloys of manganese (Mn) comprising nickel (NiMn), iridium (IrMn) or platinum (PtMn).

The micro-inductance is not limited in its frequency of use, and may be suitable for high frequency uses, which always demand more power. One can then very well imagine such components working in the range of microwaves and replacing integrated or discrete inductances, with or without magnetic material, which are usually used. We then find applications such as filtering, impedance matching, etc.

Claims (9)

Magnetic core (1) closed for integrated micro-inductance, characterized in that it has a spiral shape having two ends (2) connected to one another by a closing segment (3). Magnetic core (1) according to claim 1, characterized in that it has a spiral shape of rectangular type. Magnetic core (1) according to one of claims 1 and 2, characterized in that the magnetic core (1) is constituted by a plurality of branches. Magnetic core (1) according to claim 3, characterized in that at least two branches are arranged in different parallel planes. Magnetic core (1) according to claim 4, characterized in that a first set (4) of parallel branches is arranged in a first plane and a second set (5) of parallel branches is arranged in a second plane. Magnetic core (1) according to claim 5, characterized in that the branches of the first set (4) of parallel branches are substantially perpendicular to the branches of the second set (5) of parallel branches. Magnetic core (1) according to one of Claims 3 to 6, characterized in that at least two branches have different thicknesses. Magnetic core (1) according to any one of claims 1 to 7, characterized in that it comprises at least one gap (11). Integrated micro-inductor, characterized in that it comprises a magnetic core (1) according to any one of Claims 1 to 8.

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