Magnetic core structure, electronic device having same, method of using same, and method of providing a nanocrystalline material

Abstract

There is provided, in one example aspect, a magnetic core structure comprising a plurality of core portions that are made of a nanocrystalline material with a reduced magnetic permeability and that are stacked to reduce eddy currents. There is provided, in another aspect, a method of manufacturing a nanocrystalline material comprising: reducing a magnetic permeability of a nanocrystalline material; and forming a plurality of separate flakes in the nanocrystalline material.

Description

MAGNETIC CORE STRUCTURE, ELECTRONIC DEVICE HAVING SAME, METHOD OF USING SAME, AND METHOD OF PROVIDING A NANOCRYSTALLINE MATERIAL TECHNICAL FIELD

[0001] The present disclosure generally relates to a magnetic core structure, an electronic device having same, a method of using same, and a method of providing a nanocrystalline material. BACKGROUND

[0002] Cryogenically cooled power electronics have the potential to significantly enhance efficiency and density in large-scale electric transport systems. In electric aircrafts, operating an inverter at cryogenic temperatures (CT) for a superconducting motor can help offset the additional weight introduced by a cryogenic cooler. However, there remains a need to improve the performance of existing arrangements, particularly in the design of magnetic structures for cryogenic power converters.

[0003] Calderon-Lopez, G., Wang, Y., & Forsyth, A. (2018). Mitigation of Gap Losses in Nanocrystalline Tape-Wound Cores. IEEE Transactions on Power Electronics. DOI: 10.1109 / TPEL.2018.2863665 discloses an inductor configuration consisting of a group of three stacked U-core halves spaced from and opposing another group of three stacked U-core halves. The U-Core halves are wound in copper windings in use and are formed from wound nanocrystalline ribbons (or foils) of high permeability. Each of the three opposing pairs of U-core halves is separated by a respective air gap serving to prevent saturation. The U-core halves in each group are stacked to reduce gap losses attributed to eddy currents, which are concentrated toward the air gaps. Due to the high permeability of the wound nanocrystalline ribbons, the air gaps are employed to avoid core saturation. The gapped U-core structure is also particularly prone to eddy current losses, as inter-ribbon short-circuit paths can be inadvertently formed during the U- core cutting process. SUMMARY

[0004] It is an objective of the present disclosure to ameliorate or overcome at least one problem of the existing arrangements, or to provide the public with a useful choice.

[0005] In afirst aspect, there is provided a magnetic core structure comprising a plurality of core portions that are made of a nanocrystalline material with a reduced magnetic permeability and that are stacked to reduce eddy currents.

[0006] The structure achieves a negative relation between the eddy current density and the number of core portions. Through increasing the number of core portions, the eddy current density can be significantly reduced.

[0007] In a second aspect, there is provided a cryogenic magnetic core structure comprising a core portion made of a nanocrystalline material with a reduced magnetic permeability.

[0008] In a third aspect, there is provided a magnetic core structure comprising a plurality of toroidal core layers stacked to reduce eddy currents.

[0009] In a fourth aspect, there is provided an electronic device comprising the core structure of any one of thefirst to third aspects, and a plurality of windings wound onto the core structure.

[0010] In afifth aspect, there is provided a method of using a magnetic core structure, comprising using the magnetic core structure of any one of thefirst to third aspects in a cryogenic environment.

[0011] In a sixth aspect, there is provided a method of manufacturing a nanocrystalline material comprising: reducing a magnetic permeability of a nanocrystalline material; and forming a plurality of separateflakes in the nanocrystalline material.

[0012] In another aspect, there is provided a magnetic core structure comprising a plurality of stacked nanocrystalline core portions configured with respective eddy current reduction areas and a reduced magnetic permeability to guide magneticflux through the eddy current reduction areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate examples of the present disclosure and, together with the description, serve to explain principles of the disclosure. In the drawings: Figure 1 shows a partial perspective view of an example magnetic core structure of the present disclosure; Figure 2 shows an enlarged cross-sectional view taken along area “AA” in Figure 1; Figure 3A shows an enlarged isometric view of some laminate elements of the structure of Figure 1, indicating magneticflux in relation to eddy currents; Figure 3B shows a fringeflux in association with some laminate elements of the structure of Figure 1; Figure 4 shows an example device including the structure of Figure 1; Figure 5 shows a table of inductor designs and parameters thereof; Figure 6 shows core losses of the inductor designs of Figure 5 at 50 kHz; Figure 7 shows core losses of the inductor designs of Figure 5 at 100 kHz, Figures 8 to 11 show BH loops of four of the inductor designs of Figure 5, respectively; Figure 12 shows a line chart of ribbon stacking factor (Fr) versus μp / μ0ratio, withlines representing different core stacking factors and temperatures; Figure 13 shows a line chart of normalised perpendicular conductivity σ^versus core stacking factor Nc; Figure 14 shows a line chart of error rate in percentage versus core stacking factor Nc; Figure 15 show a line chart of energy loss per unit volume Evversus magneticflux density at room temperature, obtained in a comparison of an unstacked configuration of 1 × NC2 and a stacked configuration of 6 × NC1; Figure 16 show a line chart of energy loss per unit volume Evversus magneticflux density at a cryogenic temperature, obtained in the comparison of Figure 15; Figure 17 shows parameters of a stacked configuration of 6 × NC1 and a stacked configuration of 2 × NC3; Figure 18 shows a line chart of peak stored energy EL versus core loss P, obtained for the stacked configurations of Figure 17; Figure 19 shows a circuit diagram of a buck converter in an exemplary cryogenic application, with an inductor component implementable using any stacked configuration; Figures 20 and 21 show voltage and current waveforms of the buck converter of Figure 19, respectively; Figure 22 shows a line chart of power versus loss at 50 kHz, comparing two stacked configurations for the inductor component of the buck converter of Figure 19; Figure 23 shows a line chart of power versus loss at 100 kHz, comparing the two stacked configurations for the inductor component of the buck converter of Figure 19; Figure 24 shows a line chart of output power versus efficiency at 50 kHz, comparing the two stacked configurations, as well as unstacked configurations, for the inductor component of the buck converter of Figure 19 at different duty cycles; Figure 25 shows a line chart of output power versus efficiency at 100 kHz, comparing the two stacked configurations, as well as the unstacked configurations, for the inductor component of the buck converter of Figure 19 at the different duty cycles; Figure 26 shows a bar chart of output power versus loss at four output powers at 50 kHz and 50% duty cycle for the buck converter of Figure 19 with one of the stacked configurations; Figure 27 shows a bar chart of output power versus loss at four output powers at 100 kHz and 50% duty cycle for the buck converter of Figure 19 with said one of the stacked configurations; and Figure 28 shows an image of a nanocrystalline material withflakes separated by cracks. DETAILED DESCRIPTION

[0014] Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the samereference numbers will be used throughout the drawings to refer to the same or like parts.

[0015] Figure 1 shows, in one aspect, a sectional view of an example embodiment of a magnetic core structure 100 suitable for cryogenic (e.g., in a cryogenic environment) power conversion applications and non-cryogenic (e.g., room temperature) power conversion applications. The core structure 100 may be used to implement, for example, power converters, wireless power transfer systems, LCfilters, resonant circuits, and other power electronics converters, whether cryogenic or non-cryogenic. For example, the core structure 100 may be used to provide one of an inductor, an inverter and a buck converter, afilter and a resonant circuit. Figure 2 shows a cross section of the core structure 100 taken along the area marked as “AA” in Figure 1.

[0016] The core structure 100 comprises a nanocrystalline material of reduced magnetic permeability, more particularly a plurality of core portions 110 that are made of a nanocrystalline material with a reduced magnetic permeability and that are stacked along an axis (marked as the z-axis in Figure 1) to reduce eddy currents. The core portions 110 of this example are respective core layers 110 oriented in a same direction, more particularly in a same direction with respect to the axis, with each neighbouring pair being spaced from each other. The stacking of the core layers 110 along the axis can be seen as splitting an otherwise monolithic magnetic core structure 100 to reduce eddy currents, which may be regarded as afirst effect of eddy current reduction. By splitting the core structure 100 into the core layers 110 separated by inter-layer air gaps 112, areas for and loss attributed to eddy current circulations are reduced along the entirety of each core layer 110. The inter-layer air gaps 112 have a same height in this example and may have different heights in other examples.

[0017] Each core layer 110 of this example has a plurality of laminate elements 111 stacked radially with respect to the axis to further reduce eddy currents. In Figure 2, the laminate elements 111 of each core layer 110 are shown to be associated with the respective core layer 110. In this embodiment, the laminate elements 111 of nanocrystalline form respective ribbons 111 concentrically arranged with respect to and surrounding the axis. Each core layer 110 is thus looped, discrete and toroidal in shape. Each core layer 110 thus forms a magnetic path uninterrupted by any air gap. The ribbons 111 may be stacked in such a manner using any known technique. The stacking of the laminate elements 111 (or the ribbons 111) of each core layer 110 radially with respect to the axis can be seen as splitting each core layer 110 to reduce eddy currents, which may be regarded as a second effect of eddy current reduction. By splitting each core layer 110 into the corresponding laminate elements 111 separated by inter-element air gaps, areas for and loss attributed to eddy current circulation are further reduced.

[0018] Figure 3A shows some of the laminate elements 111 with indications of magnetic flux relation to eddy currents, where ‘hc’ represents a core layer height, ‘tr’ represents a ribbon width, ‘lc’ represents a core layer length, ‘Φy’ represents aflux perpendicular to the ribbons 111 (i.e., a fringe component of the magneticflux, marked as ‘BB” in Figure 3B), and ‘Φx’ represents aflux parallel to the ribbons 111 (i.e., a confined component of the magneticflux). In a conventional high permeability nanocrystalline core, theflux is substantially confined to the core (i.e., little or no fringe component), resulting in a negligible Φy. The resultant eddy current density ‘pe’ satisfies:where ‘Bm’ represents a peakflux density, ‘^^^^’ represents an angular frequency, and ‘ρ’ represents a resistivity of a conventional nanocrystalline material of the core layers. It can be understood that, with the conventional stacked core layers, the core height hchas a negligible influence on the eddy current density, and stacking of the conventional core layers accordingly has little or no effect of eddy current reduction.

[0019] In contrast, with the example structure 100 of Figure 1, the perpendicularflux Φyand hence the core height hcare nonnegligible. Specifically, referring again to Figure 3A and Figure 3B, it can be understood that respective eddy currents formed in the core layers 110 as a result of the perpendicularflux follow a path marked by A-B-C-D. The eddy currents thus circulate in an area ‘Ay’ satisfying:

[0020] A voltage induced in this area can be calculated using Faraday’s Law:

[0021] With Φyassumed to have a sinusoidal waveform, a peak voltage ‘Vy’ induced by Φycan be defined as ^^^^^^^^ = ^^^^^^^^ℎ^^^^^^^^^^^^^^^^

[0022] Accordingly, a total current loss ‘Py’ attributed to the eddy current path of A-B-C- D can be defined as:

[0023] Further considering the above relation of the peak induced voltage ‘Vy’ as a function of a number of the stacked core layers ‘Nc’, assuming equal division of a total height of the structure 100 by the core layers 110, a ratio between the stacked core layers 110 of Figure 1 to non-stacked core layers of a conventional arrangement can be given as:^^^^^^^^(^^^^^^^^) 1 ^^^^^^^^(1)= ^^^^^^^^

[0024] In practice, with the core layer height hcbeing much smaller than the core layer length lc, the stacked core layer 110 can be regarded as having an equivalent resistance that is Nctimes higher than that of a conventional solid core structure. A ratio of loss of the stacked core structure ^^^^^^^^(^^^^^^^^) to that of a conventional solid core structure ^^^^^^^^(1) can be expressed as:

[0025] Accordingly, based on the model of Figure 3A, the example structure 100 of Figure 1 can be shown to have an eddy current density pe, that satisfies:

[0026] It can be understood that the example structure 100 of Figure 1 advantageously achieves a negative relation between the eddy current density and the number of core layers. Through increasing the number of stacked core layers, the eddy current density can be significantly reduced.

[0027] The core layers 111 assumes the form of a cylindrical structure, with a ratio of a height of each core layer to a combined height of the respective core layer and an associated air gap ranges from 0.7 to 0.9. In this example, each core layer 110 is 30 mm in height. With such a configuration, each core layer 110 is dimensioned to further restricts eddy currents. That is, each core layer 110 is configured with a height smaller than that required by eddy currents to form without loss. In other words, the height is configured to restrict eddy current formation. Such dimensioning of the core layers 110 may be regarded as providing a third effect of eddy current reduction. In some examples, each core layer 110 may range from 2.5 mm to 40 mm in height.

[0028] In this example, the nanocrystalline material is annealed, more particularly stress annealed. During the stress annealing process, a mechanical stress is applied to the material which induces magnetic anisotropy depending on the direction of applied stress. This process is described in the document titled “Stress-annealing induced anisotropy in FeCrCuNbSiB nanocrystalline wires”, DOI: 10.1109 / INTMAG.2015.7157152. The induced anisotropy can be applied to reduce magnetic permeability. The annealed laminate elements 111 of this example are further processed to comprise or form formations for reducing eddy currents. The core layers 110 can thus be understood to be configured with respective eddy current reduction formations, with the formations comprising a plurality offlakes separated by cracks (or inter-flake air gaps). Suchflakes may be referred to herein as “separateflakes”. Theflakes can, for example, range in width from 50 µm to 2.5 mm and serve to confine eddy currents therewithin, with aflake area proportional to an eddy current magnitude. Such formations offlakes separated by cracks may be regarded as providing a fourth effect of eddy current reduction. Figure 28 shows an image of a nanocrystalline material with suchflakes. Theflakes may, in some examples, range in width from 40 µm to 500 µm.

[0029] Eddy currents can circulate within but not acrossflakes (i.e., from oneflake to another). Thus, eddy current loss can be reduced by reducing the area or size of each flake. By splitting a cracked material (or crushed material) into the core layers 110, or by forming cracks in an originally uncracked material of the core layers 110, theflakes are limited in width by the ribbon 111 (or the layers 110), and the resultantflake areas for eddy current circulations are reduced. That is, as theflakes width cannot exceed the ribbons height, the limitedflake width serves to reduce eddy current circulations. Moreover, the inter-flake air gaps also provide an effect of permeability reduction and enhance eddy current reduction. The nanocrystalline material of this example has a permeability not exceeding 1000 H / m. In other arrangements, the dimensions of the flakes may be similarly limited by those of the core portions, thereby achieving a similar effect of eddy current reduction.

[0030] In another aspect, referring to Figure 4, an example device 200 is shown to comprise the magnetic core structure 100 and a plurality of windings 210 wound onto the core structure 100. The windings 210 are shown to be equiangularly and equidistantly arranged with respect to the axis of the magnetic core structure 100. Depending on applications, the device 200 may be configured to form, either partially or wholly, one of an inductor, an inverter, afilter, a resonant circuit and a buck converter, for example. The device 200 may also be employed in the implementation of a cryogenic power converter, a wireless power transfer system (such as in its resonant circuit or to implement a pad), or a powerfilter, for example.

[0031] Figure 5 shows a table showingfive inductor designs and parameters thereof, where the acronyms of “MFA” and “SA” stand for “magneticfield annealed” and “stress annealed”, respectively. “F” stands for standing factor, calculated in accordance withASTM A719 / A719M-14 as ^^^^ =× 108. The design “NC U-Core” is a core producedusing MFA and including two opposing U-core halves separated by a 2-mm air gap. The design NC0 is a conventional monolithic (i.e., non-stacked) toroidal core of a high permeability.

[0032] The designs NC1 and NC2 are both produced using MFA with a permeability of approximately 15,000 H / m, and are further crushed using a dot crushing pattern. Each ribbon of NC1 and NC2 has a permeability of approximately 800 H / m, and is constructed by sandwiching three nanocrystalline sheets of 18 μm between two PET films. Each neighbouring pair of the sheets are bonded together by an adhesive of 3 to 5 μm, with each outermost sheet bonded to the respective PETfilm by an adhesive of 3 to5 μm. With such a configuration, the nanocrystalline sheets and the adhesives have a total thickness of approximately 64 μm. Each ribbon of NC1 has a width of 10 mm, while that of NC2 has a width of 65 mm.

[0033] NC3 is stress annealed and has a magnetic permeability of approximately 200 H / m.

[0034] Figure 6 shows core losses of the inductor designs at 50 kHz and Figure 7 shows core losses of the inductor designs at 100 kHz, where “CT” and “RT” represent a “cryogenic temperature” (e.g., -196 °C) and “room temperature”, respectively. Figures 8 to 11 show BH loops of NC0, NC1, NC2 and NC3, respectively, with lines 810, 910, 1010, 1110 representing respective CT performances and lines 820, 920, 1020, 1120 representing respective RT performances.

[0035] Figure 12 shows a line chart of ribbon stacking factor (Fr) versus μp / μ0 ratio, where afirst line 1210 represents a core stacking factor (Fc) of 0.5 at a cryogenic temperature (CT), a second line 1220 represents a core stacking factor (Fc) of 0.9 at the CT, a third line 1230 represents a core stacking factor (Fc) of 0.5 at room temperature (RT), and a fourth line 1240 represents a core stacking factor (Fc) of 0.9 at RT. The simulation results of Figure 12 are obtained based on measurements of the NC3 configuration.

[0036] Figure 13 shows a line chart of normalised perpendicular conductivity σ^ (i.e., conductivity attributed to eddy currents) versus core stacking factor Nc, where substantially overlapping lines 1310, 1320 represent theoretical and ANSYS simulation results, respectively. Figure 14 shows a line chart of error rate in percentage versus core stacking factor Nc. The error rate represents an error between a homogenisation method and a ANSYS simulation.

[0037] The results of Figures 13 and 14 relate to a configuration similar to that of NC3.

[0038] In a comparison with an unstacked configuration of 1 × NC2, a stacked configuration of 6 × NC1 is found to be advantageous by virtue of the layer stacking technique.

[0039] Figure 15 show a line chart of energy loss per unit volume Evversus magneticflux density at room temperature: where dashed lines 1510, 1520 represent results of the 6 × NC1 configuration at 50 kHz and 100 kHz, respectively; and where solid lines 1530, 1540 represent results of the 1 × NC2 at 50 kHz and 100 kHz, respectively. Reductions of 30% and 37% in energy loss per unit volume can be achieved at 50 kHz and 100 kHz, respectively.

[0040] Figure 16 show a line chart of energy loss per unit volume Evversus magneticflux density at a cryogenic temperature: where dashed lines 1610, 1620 represent results of the 6 × NC1 configuration at 50 kHz and 100 kHz, respectively; and where solid lines 1630, 1640 represent results of the 1 × NC2 configuration at 50 kHz and 100 kHz,respectively. A reduction of 20% in energy loss per unit volume can be achieved at 100 kHz.

[0041] In a comparison of a configuration with 6 × NC1 and a configuration with 2 × NC3, the latter configuration is found to have a better performance. Figure 17 shows parameters of these configurations in a table. Figure 18 shows a line chart of peak stored energy ELversus core loss P, where solid and dashed lines 1810, 1820 represent results of the former configuration (6 × NC1) at 50 kHz and 100 kHz, respectively; and where solid and dashed lines 1830, 1840 represent results of the latter configuration (2 × NC3) at 50 kHz and 100 kHz, respectively. It can be observed that latter configuration (2 × NC3) achieves a better performance by having a higher ribbon stacking factor.

[0042] Figure 19 shows a circuit diagram of a buck converter in an exemplary cryogenic application, where “L” represents an inductor based on either the NC1 configuration or the NC3 configuration. Figures 20 and 21 show voltage and current waveforms of the buck converter, respectively: where lines 2010, 2020 represent input and output voltages of the inductor L, respectively; and where lines 2110, 2120 represent input and output currents of the inductor L, respectively.

[0043] Figure 22 shows a line chart of power versus loss at 50 kHz: where lines 2210, 2220 represent core losses of the NC1 and NC3 configurations for the inductor L of the buck converter, respectively; and lines 2230, 2240 represent winding losses of the NC1 and NC3 configurations for the inductor L of the buck converter, respectively

[0044] Figure 23 shows a line chart of power versus loss at 100 kHz: where lines 2310, 2320 represent core losses of the NC1 and NC3 configurations for the inductor L of the buck converter, respectively; and lines 2330, 2340 represent winding losses of the NC1 and NC3 configurations for the inductor L of the buck converter, respectively.

[0045] Figure 24 shows a line chart of output power versus efficiency at 50 kHz: where solid, dotted and dashed lines of circles 2410, 2420, 2430 represent results of the NC3 configuration at duty cycles of 0.4, 0.5 and 0.6, respectively; where solid, dotted and dashed lines of triangles 2440, 2450, 2460 represent results of a conventional ferrite configuration at duty cycles of 0.4, 0.5 and 0.6, respectively; and where solid, dotted and dashed lines of rhombuses 2470, 2480, 2490 represent results of the NC U-Core configuration at duty cycles of 0.4, 0.5 and 0.6, respectively.

[0046] Figure 25 shows a line chart of output power versus efficiency at 100 kHz: where solid, dotted and dashed lines of circles 2510, 2520, 2530 represent results of the NC3 configuration at duty cycles of 0.4, 0.5 and 0.6, respectively; where solid, dotted and dashed lines of triangles 2540, 2550, 2560 represent results of a conventional ferrite configuration at duty cycles of 0.4, 0.5 and 0.6, respectively; and where solid, dotted and dashed lines of rhombuses 2570, 2580, 2590 represent results of the NC U-Core configuration at duty cycles of 0.4, 0.5 and 0.6, respectively. It is worth noting that thebuck converter is able to achieve an efficiency of 99.3% at 100 kHz and at a duty cycle of 0.6.

[0047] Figure 26 shows a bar chart of output power versus loss at four output powers at 50 kHz and 50% duty cycle for the buck converter with the NC3 configuration. Each bar consist of, from top to bottom, afirst component representing a loss of an output capacitor (labelled as “Cout” in Figure 19), a second component representing a loss of an input capacitor (labelled as “Cin” in Figure 19), a third component representing a total loss of switches (labelled as “S1” and “S2”in Figure 19), and a fourth component representing a loss of the inductor L implemented based on the NC3 configuration.

[0048] Figure 27 shows a bar chart of output power versus loss at four output powers at 100 kHz and 50% duty cycle for the buck converter with the NC3 configuration. Each bar consist of, from top to bottom, afirst component representing a loss of the output capacitor (labelled as “Cout” in Figure 19), a second component representing a loss of the input capacitor (labelled as “Cin” in Figure 19), a third component representing a total loss of the switches (labelled as “S1” and “S2”in Figure 19), and a fourth component representing a loss of the inductor L implemented based on the NC3 configuration.

[0049] In yet another aspect, the nanocrystalline material is manufactured in accordance with an example embodiment of a method of manufacture. The method comprises reducing a magnetic permeability of a nanocrystalline material, and forming a plurality of separateflakes in the nanocrystalline material. More particularly, the method comprises, in one example, annealing a nanocrystalline material, and crushing the annealed material. More specifically, thefirst step comprises stress annealing, and the second step comprises feeding the annealed material through a set of rollers.

[0050] Thefirst step is performed to reduce the permeability of the nanocrystalline material. During the annealing process, an external magneticfield or tensile stress can be applied to the material. The annealing process alters the structure of the material by inducing anisotropies depending on the direction of external magneticfield or tensile stress. This means that the magnetic domains are easier to magnetise in some directions than others, resulting in an overall lower permeability.

[0051] The second step is performed to form theflakes separated by cracks. It is worth noting that, either step alone may suffice and any suitable technique may be used in either step, provided that the magnetic permeability of the nanocrystalline material can be sufficiently reduced to achieve a desired effect of eddy current reduction for a desired cryogenic application.

[0052] The reduction of the magnetic permeability in combination with the stacking of the core layers 110 along the axis is advantageous. Specifically, where the core structure 100 is exposed to a magneticflux, the reduced permeability serves to cause an increased amount of the magneticflux to traverses as fringingfields around andalong the core layers 110, which advantageous reduces saturation risks. Moreover, the stacking of the core layers 110 along the axis, which is enabled by the reduction in permeability of the crystalline material, serves to reduce circulation areas for eddy currents resulting from the increased fringingfields, which advantageously reduce eddy current loss. A cryogenic buck converter with an inductor based on the magnetic core structure 100 is able to demonstrate a peak efficiency of 99.3% at 3kW. Additionally, the inductor is only 2% in weight and volume in comparison with a conventional room- temperature ferrite inductor at similar losses. The disclosed technique and configurations can be used to reduce eddy current losses in different known core designs, including cut cores such as U-cores, in similar manners.

[0053] It is worth noting that the annealed nanocrystalline material can also achieve reduced eddy currents without being further processed to form theflakes with cracks. That is, the annealing step alone may suffice. Moreover, in practice, the method may comprise performing any means of magnetic permeability reduction and any means of flake formation for the nanocrystalline material.

[0054] In summary, a nanocrystalline material subject can achieve a significantly improved performance once subjected to a combination of stress annealing and crushing processes. Furthermore, stacking of layers made of such a nanocrystalline material can significantly reduce eddy current formation within or throughout the layers. A buck converter implemented based on the NC3 configuration is capable of achieving an improved efficiency of 99.3% at 3 kW and 100 kHz. An inductor implemented based on, for example, the NC3 configuration can achieve a significantly reduced volume, being 2% in volume and weight of a conventional room-temperature ferrite inductor. Through careful selection of nanocrystalline materials and applying the stacking technique in accordance with the techniques of the present disclosure, it is possible implement a cryogenic power converter or a non-cryogenic one with a significantly improved power density and a significantly improved efficiency.

[0055] In addition to those described above, some alternatives arrangements are also described below.

[0056] In some arrangements, a magnetic core structure with a nanocrystalline material of reduced magnetic permeability may be employed, with or without stacking (e.g., along the axis).

[0057] In some arrangements, the core structure may have a non-toroidal shape, depending on the applications.

[0058] In some arrangements, the laminate elements of the core layer may be segmented such that the core layer comprises a plurality of complementary portions.

[0059] In some arrangements, the structure may comprise any number of core portions of any suitable shape.

[0060] In some arrangements,

[0061] In some arrangements, the core layers in each neighbouring pair may be in contact with (or abut) each other.

Claims

CLAIMS 1. A magnetic core structure comprising a plurality of core portions that are made of a nanocrystalline material with a reduced magnetic permeability and that are stacked to reduce eddy currents.

2. The magnetic core structure of claim 1, wherein the core portions are respective core layers stacked in afirst stacking direction.

3. The magnetic core structure of claim 2, wherein each core layer comprises a plurality of laminate elements stacked in a second stacking direction perpendicular to thefirst stacking direction.

4. The magnetic core structure of claim 2 or 3, wherein each core layer is looped.

5. The magnetic core structure of claim 4, wherein each core layer is toroidal.

6. The magnetic core structure of claim any one of claims 2 to 5, wherein a ratio of a height of each core layer to a combined height of the respective core layer and an associated air gap ranges from 0.7 to 0.

9.

7. The magnetic core structure of claim any one of claims 2 to 6, wherein each core layer ranges from 2.5 mm to 40 mm in height.

8. The magnetic core structure of any one of the preceding claims, wherein the core portions are dimensioned to reduce eddy currents.

9. The magnetic core structure of any one of the preceding claims, wherein the core portions are configured with respective eddy current reduction formations.

10. The magnetic core structure of claim 9, wherein the eddy current reduction formations comprise a plurality offlakes separated by cracks.

11. The magnetic core structure of claim 10, wherein theflakes range in width from 50 µm to 2.5 mm.

12. The magnetic core structure of any one of the preceding claims, wherein the core portions are annealed.

13. The magnetic core structure of claim 12, wherein the core portions are stress annealed.

14. The magnetic core structure of any one of the preceding claims, wherein the core portions are crushed.

15. The magnetic core structure of any one of the preceding claims, wherein the core portions have a magnetic permeability not exceeding 1000 H / m.

16. The magnetic core structure of any one of the preceding claims, wherein the core portions comprise more than two core portions.

17. The magnetic core structure of any one of the preceding claims, wherein each neighbouring pair of the core portions are spaced from each other.

18. The magnetic core structure of any one of the preceding claims, wherein the core portions are oriented in a same direction.

19. The magnetic core structure of any one of the preceding claims, wherein each core portion forms a magnetic path uninterrupted by any air gap.

20. A cryogenic magnetic core structure comprising a core portion made of a nanocrystalline material with a reduced magnetic permeability.

21. A magnetic core structure comprising a plurality of toroidal core layers stacked to reduce eddy currents.

22. An electronic device comprising: the core structure of any one of claims 1 to 21; and a plurality of windings wound onto the core structure.

23. The electronic device of claim 22, the electronic device being one of an inductor, an inverter and a buck converter.

24. A method of using a magnetic core structure, comprising using the magnetic core structure of any one of claims 1 to 21 in a cryogenic environment.

25. A method of manufacturing a nanocrystalline material comprising: reducing a magnetic permeability of a nanocrystalline material; and forming a plurality of separateflakes in the nanocrystalline material.

26. The method of claim 25, wherein reducing the magnetic permeability comprises annealing the nanocrystalline material.

27. The method of claim 26. wherein annealing the nanocrystalline material comprises stress annealing the nanocrystalline material.

28. The method of any one of claims 25 to 27, wherein forming theflakes comprises crushing the nanocrystalline material.

28. The method of claim 27, wherein crushing the nanocrystalline material comprises feeding the nanocrystalline material through a set of rollers.

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