Inductor

Abstract

The invention discloses an inductor. The inductor comprises a magnetic body, a plurality of one-piece coils, an electrode structure and a protection structure. The magnetic body has a bottom surface. The plurality of single-piece coils are embedded in the magnetic body and are arranged at intervals. Each one-piece coil has a first end portion and a second end portion exposed from the bottom surface. The electrode structure is arranged on the bottom surface of the magnetic body and comprises a plurality of first electrodes and a plurality of second electrodes. The plurality of first electrodes are arranged at intervals and electrically coupled to the first ends of the plurality of one-piece coils, respectively. The plurality of second electrodes are arranged at intervals and electrically coupled to the second ends of the plurality of one-piece coils, respectively. The protection structure is arranged on the bottom surface of the magnetic body. The protection structure separates the plurality of first electrodes from the plurality of second electrodes.

Description

Technical Field

[0001] The present invention relates to a circuit component, and in particular to an inductor. Background Art

[0002] An inductor is a circuit component that generates an electromotive force by changing the current, thereby resisting the change in current. Therefore, inductors are often installed in electronic devices. As electronic devices have a high demand for computing power (for example, AI application servers), the power consumption of the circuits of electronic devices has also doubled. Therefore, improving the power density of inductors and making them more efficient is an urgent issue at present.

[0003] Therefore, the inventors believe that the above defects can be improved, and have devoted themselves to research and applied scientific principles, and finally proposed the present invention which has a reasonable design and effectively improves the above defects. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide an inductor to address the deficiencies of the prior art.

[0005] One embodiment of the present invention discloses an inductor, comprising: a magnetic body having a bottom surface; a plurality of single-piece coils embedded in the magnetic body, the plurality of single-piece coils being arranged at intervals, each single-piece coil having a first end and a second end, the first end and the second end being exposed from the bottom surface; an electrode structure disposed on the bottom surface of the magnetic body, the electrode structure comprising: a plurality of first electrodes disposed at intervals and electrically coupled to the first ends of the plurality of single-piece coils respectively; and a plurality of second electrodes disposed at intervals and electrically coupled to the second ends of the plurality of single-piece coils respectively; and a protection structure disposed on the bottom surface of the magnetic body, the protection structure separating the plurality of first electrodes from the plurality of second electrodes.

[0006] Optionally, the magnetic body has a first direction and a second direction perpendicular to the first direction, and the magnetic body has a bottom surface in the first direction; multiple single-piece coils are arranged at intervals along the second direction, and two adjacent single-piece coils have a first minimum spacing distance in the second direction.

[0007] Optionally, the magnetic body has a first direction, a second direction perpendicular to the first direction, and a third direction perpendicular to the first direction and the second direction, and the magnetic body has a bottom surface in the first direction; multiple single-piece coils are arranged at intervals along the third direction, and two adjacent single-piece coils have a second minimum spacing distance in the third direction.

[0008] Optionally, the magnetic body has a first direction, a second direction perpendicular to the first direction, and a third direction perpendicular to the first direction and the second direction, and the magnetic body has a bottom surface in the first direction; a plurality of single-piece coils are arranged at intervals along the second direction and the third direction; two single-piece coils adjacent to each other in the second direction have a first minimum spacing distance in the second direction, and two single-piece coils adjacent to each other in the third direction have a second minimum spacing distance in the third direction.

[0009] Optionally, the magnetic body has a first direction and a second direction perpendicular to the first direction, and the magnetic body has a bottom surface in the first direction; a groove is formed between adjacent first electrodes and second electrodes.

[0010] Optionally, the first ends and the second ends of the plurality of single-piece coils extend toward each other.

[0011] Optionally, the first ends and the second ends of the plurality of single-piece coils extend in opposite directions.

[0012] Optionally, the first ends and the second ends of a portion of the multiple single-piece coils extend toward each other; and the first ends and the second ends of another portion of the multiple single-piece coils extend in opposite directions.

[0013] Optionally, the magnetic body includes a top surface opposite to the bottom surface and a ring side surface connecting the top surface and the bottom surface, and the protection structure extends from the bottom surface to the ring side surface and the top surface, so that the protection structure and the electrode structure jointly cover the magnetic body.

[0014] Optionally, the cross-sections of the first electrodes and the second electrodes are each L-shaped, each first electrode extends from the bottom surface to one side surface of the magnetic body, and each second electrode extends from the bottom surface to the other side surface of the magnetic body.

[0015] In summary, the inductor disclosed in the embodiment of the present invention can have a higher power density in the same volume through the designs of "a plurality of the single-piece coils are buried in the magnetic body, and the plurality of the single-piece coils are arranged at intervals" and "the first end and the second end of each of the single-piece coils are exposed from the bottom surface", thereby further improving the power conversion efficiency.

[0016] To further understand the features and technical contents of the present invention, please refer to the following detailed description and drawings of the present invention. However, such description and drawings are only used to illustrate the present invention and are not intended to limit the protection scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a three-dimensional schematic diagram of an inductor of the present invention.

[0018] Figure 2FIG. 4 is another schematic three-dimensional diagram of the inductor of the present invention.

[0019] Figure 3 For along Figure 1 Schematic cross-sectional view of line III-III.

[0020] Figure 4 FIG. 4 is a schematic top view of an inductor of the present invention.

[0021] Figure 5 FIG. 4 is a bottom view of an inductor according to the present invention.

[0022] Figure 6 This is a schematic diagram of another electrode structure of the inductor of the present invention.

[0023] Figure 7 A schematic diagram of another protection structure for the inductor of the present invention.

[0024] Figure 8 FIG. 4 is a schematic diagram of an inductor according to an embodiment of the present invention.

[0025] Fig. 9 FIG. 4 is a schematic diagram of an inductor according to another embodiment of the present invention.

[0026] Fig.10 for Fig. 9 Schematic diagram of a top view of an inductor.

[0027] Fig.11 for Fig. 9 A bottom-up schematic diagram of an inductor.

[0028] Fig.12 This is a schematic diagram of another embodiment of the present invention in which a single-piece coil is buried in a magnetic body.

[0029] Fig.13 This is a schematic diagram of a single-piece coil embedded in a magnetic body according to another embodiment of the present invention.

[0030] Fig.14 Schematic diagrams of two embodiments of the present invention in which a single-piece coil is buried in a magnetic body.

[0031] Fig.15 for Figure 7 Schematic diagram of the protection structure of another way to set the inductor.

[0032] Fig.16 FIG. 4 is a schematic diagram of an inductor according to another embodiment of the present invention. DETAILED DESCRIPTION

[0033] The following is an explanation of the implementation of the "inductor" disclosed in the present invention through specific embodiments. Those skilled in the art can understand the advantages and effects of the present invention from the contents disclosed in this specification. The present invention can be implemented or applied through other different specific embodiments, and the details in this specification can also be modified and changed based on different viewpoints and applications without departing from the concept of the present invention. In addition, the drawings of the present invention are only simple schematic illustrations and are not depicted according to actual dimensions. It is stated in advance. The following implementations will further explain the relevant technical contents of the present invention in detail, but the disclosed contents are not intended to limit the scope of protection of the present invention.

[0034] It should be understood that, although the terms "first", "second", "third", etc. may be used herein to describe various components or signals, these components or signals should not be limited by these terms. These terms are mainly used to distinguish one component from another component, or one signal from another signal. In addition, the term "or" used herein may include any one or more combinations of the associated listed items depending on the actual situation.

[0035] In addition, in the following description, if it is indicated to refer to a specific figure or as shown in a specific figure, it is only used to emphasize that most of the related content described in the subsequent description appears in the specific figure, but it does not limit the subsequent description to only refer to the specific figure.

[0036] See also Figures 1 to 7 As shown, this embodiment provides an inductor 100. Figure 1 and Figure 2 As shown, the inductor 100 comprises a magnetic body 1, a plurality of single-piece coils 2 embedded in the magnetic body 1, and an electrode structure 3 and a protection structure 4 disposed on the surface of the magnetic body 1. Next, the components of the inductor 100 and their connection relationships are introduced below.

[0037] Re-reference Figures 1 to 3 As shown, the magnetic body 1 in this embodiment is a rectangular body, but the present invention is not limited thereto. For example, in other embodiments not shown, the magnetic body can also be a hexahedron or other three-dimensional structures.

[0038] In addition, the magnetic body 1 has a first direction D1, a second direction D2 perpendicular to the first direction D1, and a third direction D3 perpendicular to the first direction D1 and the second direction D2. For ease of understanding, the first direction D1 is defined as the direction in which the magnetic body Figure 1 The Z-axis direction in the second direction D2 is defined as the magnetic body Figure 1 The third direction D3 is defined as the direction of the magnetic body in the Y-axis direction. Figure 1The magnetic body 1 is parallel to the X-axis direction in the XY plane, and one of the side surfaces of the magnetic body 1 parallel to the XY plane is defined as a bottom surface BM, that is, the magnetic body 1 has the bottom surface BM in the first direction D1. In addition, the magnetic body 1 further includes a top surface TP relative to the bottom surface BM and a ring side surface SP connecting the bottom surface BM and the top surface TP.

[0039] Optionally, the material of the magnetic body 1 may include metal magnetic powder and an adhesive component (ie, the magnetic body 1 is a mixture of metal magnetic powder and an adhesive component), and the magnetic body 1 may be manufactured by compression molding.

[0040] The metal magnetic powder may further include at least one of crystalline metal magnetic powder and amorphous metal magnetic powder. For example, the crystalline metal magnetic powder may be iron silicon alloy powder, iron silicon chromium alloy powder, iron silicon aluminum alloy powder, iron nickel alloy powder, carbonyl iron powder, iron powder, iron nickel molybdenum alloy powder, or iron cobalt vanadium alloy powder, including but not limited to the above materials; the amorphous metal magnetic powder may be iron silicon boron carbon powder or iron silicon chromium boron phosphorus carbon powder, including but not limited to the above materials. In addition, the adhesive component may be epoxy resin, acrylic resin or silicone, including but not limited to the above materials.

[0041] Cooperate Figure 1 and Figure 2 As shown, a plurality of the single-piece coils 2 are arranged in a spaced relationship within the magnetic body 1. For the convenience of description, the following first introduces the arrangement relationship of the plurality of the single-piece coils 2 within the magnetic body 1, and then introduces the components of each single-piece coil 2 and their connection relationship.

[0042] Re-reference Figure 1 and Figure 3 As shown, each of the single-piece coils 2 is an open coil having an opening OP, and the openings OP of the single-piece coils 2 face the same direction (ie, the bottom surface BM). Thus, two adjacent single-piece coils 2 can generate electromotive forces in the same direction.

[0043] In addition, in this embodiment, the plurality of single-piece coils 2 are arranged spaced apart from each other along the second direction D2, and two adjacent single-piece coils 2 have a first minimum spacing distance S1 (eg, Figure 1 and Figure 4 shown).

[0044] Wherein, under the requirement of miniaturization of the inductor 100, the first minimum spacing distance S1 can be optionally between 0.01 mm and 10 mm to avoid unexpected voltage or current changes between two adjacent single-piece coils 2. Optionally, the first minimum spacing distance S1 can be between 0.1 mm and 1 mm in the best embodiment, but the present invention is not limited thereto.

[0045] It is worth noting that the plurality of single-piece coils 2 of the present invention are not limited to being arranged spaced apart from each other along the second direction D2. Fig.16 As shown, a plurality of the single-piece coils 2 may also be arranged at intervals along the third direction D3 according to designer requirements.

[0046] Re-reference Figure 3 and Figure 5 As shown, in one embodiment, the cross-section of each of the single-piece coils 2 along the first direction D1 is a semi-closed geometric shape and has a center point CP, and the center points CP of the plurality of single-piece coils can be passed through by a configuration line L parallel to the second direction D2, but the present invention is not limited thereto. For example, the center points CP of the plurality of single-piece coils 2 can also be arranged adjacent to the configuration line L but not pass through it.

[0047] It should be noted that Figure 1 The plurality of single-piece coils 2 in the embodiment are two as an example, but in practice the number of the plurality of single-piece coils 2 can be increased along the second direction D2 as required by the designer (for example: Figure 8 The inductor 100').

[0048] In addition, the number of the plurality of single-piece coils 2 may also be increased along the third direction D3 (for example: Fig. 9 In other words, the plurality of single-piece coils 2 are arranged in a spaced relationship along the second direction D2 and the third direction D3. Two adjacent single-piece coils 2 along the third direction D3 also have a second minimum spacing distance S2. The second minimum spacing distance S2 may be between 0.01 mm and 10 mm (e.g. Fig.11 Optionally, the second minimum spacing distance S2 may be between 0.1 mm and 1 mm in the best embodiment, but the present invention is not limited thereto.

[0049] It should be noted that when the inductor 100 is in the most miniaturized form, in the embodiment of the present invention, the second minimum spacing distance S2 will be greater than the first minimum spacing distance S1. In other embodiments not shown, the second minimum spacing distance S2 will be smaller than the first minimum spacing distance S1.

[0050] The above is the arrangement relationship of the plurality of the single-piece coils 2 in the magnetic body 1. Next, the components of each of the single-piece coils 2 and their connection relationship are introduced below.

[0051] Re-reference Figures 1 to 3 As shown, in this embodiment, each of the single-piece coils 2 is a C-shaped structure made of a single material with a rectangular cross-section, and each of the single-piece coils 2 is not a coupled coil, but the present invention is not limited thereto. For example, in other embodiments of the present invention not shown, each of the single-piece coils 2 may also be a circular, triangular or other polygonal structure, and the cross-section of the single-piece coil 2 may also be circular or other shapes.

[0052] More specifically, each of the single-piece coils 2 is an open coil, and the single-piece coil 2 has a body portion 21 that is substantially C-shaped, and a first end portion 22 and a second end portion 23 that connect the two ends of the body portion 21. The first end portion 22 and the second end portion 23 of each of the single-piece coils 2 are spaced apart from each other in the third direction D3 and are not connected, so that the single-piece coil 2 forms an open loop, and the single-piece coil 2 has an opening OP between the first end portion 22 and the second end portion 23.

[0053] In practice, each of the single-piece coils 2 can be selected to have a symmetrical structure, and the first end 22 and the second end 23 of each of the single-piece coils 2 can be adjusted in direction according to designer requirements.

[0054] In one embodiment, if Figure 3 As shown, two ends of each of the single-piece coils 2 can be bent toward each other, so that the first end 22 and the second end 23 extend from the main body 21 along the third direction D3 and face each other.

[0055] In another embodiment, if Fig.12 As shown, the two ends of each of the single-piece coils 2A can be bent away from each other, so that the first end 22 and the second end 23 extend from the main body 21 along the third direction D3 and away from each other.

[0056] In yet another embodiment, Fig.13 As shown, both ends of each of the single-piece coils 2B extend along the first direction D1 without being bent, so that the first end 22 and the second end 23 extend from the main body 21 along the first direction D1.

[0057] In addition, it should be emphasized that the magnetic body 1 can have multiple single-piece coils 2 of different or the same type according to the needs. For example, in the two single-piece coils 2 in the magnetic body 1, the first end 22 and the second end 23 of the two single-piece coils 2 are facing each other, facing away from each other, or not bent. For another example, Fig.14 As shown, among the two single-piece coils in the magnetic body 1, the first end 22 and the second end 23 of one of the single-piece coils 2 face each other, and the first end 22 and the second end 23 of the other single-piece coil 2A face away from each other.

[0058] Re-reference Figures 1 to 3 As shown, the first end 22 and the second end 23 of each of the single-piece coils 2 can be exposed from the bottom surface BM of the magnetic body 1, that is, part of the first end 22 and part of the second end 23 are not covered by the magnetic body 1. In addition, the first end 22 and the second end 23 will not protrude from the side of the magnetic body 1 in the third direction D3, or be exposed from the side of the magnetic body 1 in the third direction D3. Thus, each of the single-piece coils 2 can be electrically connected to the electrode structure 3 located on the surface of the magnetic body 1.

[0059] It should be additionally explained that, each of the single-piece coils 2 can adjust the distance between the first end 22 and the second end 23 according to the characteristic requirements of the impedance value, which will not be specifically described here.

[0060] Re-reference Figure 2 and Figure 3 As shown, the electrode structure 3 is disposed on the bottom surface BM of the magnetic body 1, and the electrode structure 3 includes a plurality of first electrodes 31 and a plurality of second electrodes 32 spaced apart from each other. The plurality of first electrodes 31 are electrically coupled to the first ends 22 of the plurality of single-piece coils 2, respectively, and the plurality of second electrodes 32 are electrically coupled to the second ends 23 of the plurality of single-piece coils 2, respectively.

[0061] The material of the electrode structure 3 may be at least one of copper, nickel, and tin, including but not limited to the above materials. For example, the material of the electrode structure 3 may be a copper-nickel alloy, a tin-copper alloy, or a copper-nickel-tin alloy, or the electrode structure 3 may include a copper layer, a nickel layer, and a tin layer arranged in sequence, and the copper layer is in contact with the first end 22 or the second end 23.

[0062] In one embodiment, each of the first electrodes 31 and each of the second electrodes 32 is a rectangular sheet structure (i.e., the cross-section is a rectangle), and any adjacent first electrodes 31 and first electrodes 31, any adjacent first electrodes 31 and second electrodes 32, and any adjacent second electrodes 32 and second electrodes 32 may be separated by a gap D greater than or equal to 0.1 mm, but the shapes of the first electrodes 31 and second electrodes 32 of the present invention are not limited to this.

[0063] For example, if Figure 6 As shown, in another embodiment, the cross-sections of the plurality of first electrodes 31 and the cross-sections of the plurality of second electrodes 32 are each L-shaped, and the plurality of first electrodes 31 and the plurality of second electrodes 32 are arranged facing each other. That is, each of the first electrodes 31 is extended from the bottom surface BM to one side surface of the magnetic body 1, and each of the second electrodes 32 is extended from the bottom surface BM to the other side surface of the magnetic body 1. In this embodiment, the height H31 of the cross-sections of the plurality of first electrodes 31 along the first direction D1 and the height H32 of the cross-sections of the plurality of second electrodes 32 along the first direction D1 are lower than or equal to the height H1 of the magnetic body 1 along the first direction D1.

[0064] Optionally, a height H31 , H32 of each of the first electrodes 31 and each of the second electrodes 32 along the first direction D1 is greater than or equal to 0.01 mm.

[0065] Therefore, the L-shaped first electrodes 31 and the second electrodes 32 can provide more contact areas to increase the bonding strength of the inductor 100 on the printed circuit board (PCB).

[0066] In practice, the thickness of each first electrode 31 and the thickness of each second electrode are less than or equal to the thickness of the magnetic body 1. In this embodiment, the thickness of each first electrode 31 and the thickness of each second electrode 32 can be greater than or equal to 0.01 mm, but the present invention is not limited thereto.

[0067] In addition, a groove G is formed between the adjacent first electrodes 31 and the second electrodes 32, and the positions of the plurality of grooves G correspond to the openings OP of the plurality of single-piece coils 2. In other words, the number of the plurality of grooves G may be positively correlated with the number of the plurality of single-piece coils 2, and the plurality of grooves G may be arranged along the second direction D2. Thus, the plurality of grooves G may provide solder buffer space to improve the bonding between the inductor 100 and other components.

[0068] In practice, the depth of each of the grooves G along the first direction D1 may be greater than or equal to 0.1 mm.

[0069] It is worth noting that in order to avoid short circuit, the first electrodes 31 and the second electrodes 32 can be electrically isolated by the protection structure 4 .

[0070] Specifically, if Figure 2 , Figure 3 and Figure 5 As shown, the protection structure 4 is disposed on the bottom surface BM of the magnetic body 1, and the protection structure 4 is located between the plurality of first electrodes 31 and the plurality of second electrodes 32. In this embodiment, the material of the protection structure 4 can be epoxy resin, acrylic resin or silicone. Of course, the inductor 100 can also further avoid the situation of "the plurality of first electrodes 31 and the plurality of second electrodes 32 being connected to each other during the manufacturing process" by virtue of the property that metal cannot be deposited on the protection structure 4.

[0071] Optionally, the thickness of the protective structure 4 is less than the thickness of the electrode structure 3, so that the electrode structure 3 can protrude relative to the protective structure 4 to facilitate subsequent electrical connection with other components, but the present invention is not limited thereto. For example, in other embodiments of the present invention not shown, the thickness of the protective structure 4 can also be greater than the thickness of the electrode structure 3, so that the electrode structure 3 is sunken into the protective structure 4 (that is, the first electrode 31 and the second electrode of the electrode structure 3 are surrounded by the protective structure 4).

[0072] Re-reference Figure 7 ,and Fig.15 As shown, the protective structure 4' can be further provided on other surfaces of the magnetic body 1 other than the bottom surface BM. Specifically, the protective structure 4' can be the bottom surface BM extending to the ring side surface SP and the top surface TP, and the protective structure 4 and the electrode structure 3 can cover the magnetic body 1 together to prevent the magnetic body 1 or each of the single-piece coils 2 from contacting the outside world. Therefore, the protective structure 4' can be used to improve the insulation and rust resistance of the component and prevent electroplating diffusion.

[0073] [Technical Effects of Embodiments of the Invention]

[0074] In summary, the inductor disclosed in the embodiment of the present invention can have a higher power density in the same volume through the designs of "a plurality of the single-piece coils are buried in the magnetic body, and the plurality of the single-piece coils are arranged at intervals" and "the first end and the second end of each of the single-piece coils are exposed from the bottom surface", thereby further improving the power conversion efficiency.

[0075] The above descriptions are only optional feasible embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Therefore, all equivalent technical changes made using the contents of the specification and drawings of the present invention are included in the protection scope of the present invention.

Claims

1. An inductor, characterized in that: The inductor comprises: A magnetic body having a bottom surface; A plurality of single-piece coils are embedded in the magnetic body, the plurality of single-piece coils are arranged at intervals, each of the single-piece coils has a first end and a second end, and the first end and the second end are exposed from the bottom surface; an electrode structure, disposed on the bottom surface of the magnetic body, the electrode structure comprising: a plurality of first electrodes, arranged at intervals from each other and respectively electrically coupled to the first ends of the plurality of the single-piece coils; and a plurality of second electrodes, arranged at intervals from each other and respectively electrically coupled to the second ends of the plurality of single-piece coils; and A protection structure is disposed on the bottom surface of the magnetic body, and the protection structure separates a plurality of the first electrodes from a plurality of the second electrodes.

2. The inductor according to claim 1, characterized in that The magnetic body has a first direction and a second direction perpendicular to the first direction, and the magnetic body has the bottom surface in the first direction; a plurality of the single-piece coils are arranged at intervals along the second direction, and two adjacent single-piece coils have a first minimum spacing distance in the second direction.

3. The inductor according to claim 1, characterized in that The magnetic body has a first direction, a second direction perpendicular to the first direction, and a third direction perpendicular to the first direction and the second direction, and the magnetic body has the bottom surface in the first direction; a plurality of the single-piece coils are arranged at intervals along the third direction, and two adjacent single-piece coils have a second minimum spacing distance in the third direction.

4. The inductor according to claim 1, wherein: The magnetic body has a first direction, a second direction perpendicular to the first direction, and a third direction perpendicular to the first direction and the second direction, and the magnetic body has the bottom surface in the first direction; a plurality of the single-piece coils are arranged at intervals along the second direction and the third direction; two of the single-piece coils adjacent to each other in the second direction have a first minimum spacing distance in the second direction, and two of the single-piece coils adjacent to each other in the third direction have a second minimum spacing distance in the third direction.

5. The inductor according to claim 1, wherein: The magnetic body has a first direction and a second direction perpendicular to the first direction. The magnetic body has the bottom surface in the first direction. A groove is formed between the adjacent first electrodes and the second electrodes.

6. The inductor according to claim 1, characterized in that The first end portions and the second end portions of the plurality of single-piece coils extend toward each other.

7. The inductor according to claim 1, characterized in that The first ends and the second ends of the plurality of single-piece coils extend in opposite directions.

8. The inductor according to claim 1, wherein: The first end and the second end of a portion of the multiple single-piece coils extend toward each other; the first end and the second end of another portion of the multiple single-piece coils extend in opposite directions.

9. The inductor according to claim 1, wherein: The magnetic body includes a top surface opposite to the bottom surface and a ring side surface connecting the top surface and the bottom surface. The protection structure extends from the bottom surface to the ring side surface and the top surface, so that the protection structure and the electrode structure jointly cover the magnetic body.

10. The inductor according to claim 1, wherein: The cross-sectional surfaces of the first electrodes and the second electrodes are each L-shaped. The first electrodes are each extended from the bottom surface to one side surface of the magnetic body, and the second electrodes are each extended from the bottom surface to the other side surface of the magnetic body.

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