Patent Publication Number: US-2023133417-A1

Title: Magnetic element and electronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Chinese Patent Application No. 202111299457.3, filed on Nov. 4, 2021, which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     This application relates to a magnetic element and an electronic device. 
     BACKGROUND 
     With the development of modern science, various electronic and electrical devices provide high efficiency for social production, and greatly facilitate people&#39;s daily life. At the same time, electromagnetic interference and radiation generated in a working process of the electronic and electrical devices affects people&#39;s life and production, and leads to deterioration in an electromagnetic environment of human living space. A magnetic element is used in an electronic device, has a filtering function, can filter an electromagnetic interference signal, and can suppress outward radiation and emission of an electromagnetic wave generated by a high-speed signal cable. How to design a magnetic element for which not only a volume and costs of the magnetic element can be controlled but common-mode impedance of the magnetic element can also be increased is a research direction in the industry. 
     SUMMARY 
     Embodiments of this application provide a magnetic element and an electronic device. 
     According to a first aspect, a magnetic element is provided. The magnetic element is used in an electronic device, and is configured to suppress outward radiation of an electromagnetic wave generated by a high-speed signal cable in the electronic device. The magnetic element includes a composite magnetic core and a winding, the composite magnetic core includes an internal magnet and an external magnetic shell, the internal magnet is formed by winding a strip material, the external magnetic shell partially or entirely covers a periphery of the internal magnet, the external magnetic shell is fixedly connected to the internal magnet, the winding is wound on an outer surface of the external magnetic shell, the external magnetic shell is configured to protect the internal magnet from pulling force in a process of winding the winding, and the external magnetic shell is configured to increase common-mode impedance of the magnetic element. 
     For the internal magnet formed by winding the strip material, a coil cannot be directly wound on an outer surface of the internal magnet, because pulling stress acting on a surface of the internal magnet may be generated in a process of winding the coil, and the stress may destroy the internal magnet and affect a filtering function of the magnetic element. Because the external magnetic shell in the composite magnetic core provided in this application has a magnetic material, as a protective shell covering the periphery of the internal magnet, the external magnetic shell can not only protect the internal magnet from pulling stress in a wire winding process, but also improve the filtering function of the magnetic element. The external magnetic shell in the composite magnetic core has the magnetic material, and the winding is wound on an outer surface of the external magnetic shell. The external magnetic shell has a function of filtering electromagnetic noise, so that the filtering effect of the magnetic element can be improved. If the external magnetic shell has no magnetic material, but has only a function of protecting the internal magnet, and the external magnetic shell is made of a non-magnetic material, such an external magnetic shell in the composite magnetic core cannot participate in the filtering function of the magnetic element. If the external magnetic shell occupies space of the composite magnetic core but has no filtering function, this is not conducive to a miniaturization design of the magnetic element, and consequently, a volume and costs of the magnetic element are increased. 
     In an embodiment, a material of the external magnetic shell includes at least one of ferrite or alloy magnetic powder. In this solution, the material of the external magnetic shell is specifically limited. The external magnetic shell may be a single material, or may be formed by combining a plurality of materials. 
     In an embodiment, a material of the internal magnet includes at least one of amorphous alloy or nanocrystalline. In this solution, the material of the internal magnet is limited, and may be a single material (e.g., an amorphous material or a nanocrystalline material), or may be a combination of an amorphous material and a nanocrystalline material. 
     In an embodiment, a material of the internal magnet is a nanocrystalline strip material, and a material of the external magnetic shell is ferrite. In this solution, the material of the internal magnet and the material of the external magnetic shell are limited, and the internal magnet of the nanocrystalline strip material and the external magnetic shell of the ferrite are combined in one composite magnetic core, so that performance of a common-mode inductor can be improved while a volume and costs of the composite magnetic core are properly controlled. In this way, not only the magnetic element has a relatively good filtering effect, but overall performance of the magnetic element can also be improved in a design condition that a relatively small volume and relatively low costs are controlled. 
     In an embodiment, a material of the internal magnet is a nanocrystalline strip material, and a material of the external magnetic shell is a combination of manganese zinc ferrite, nickel zinc ferrite, and alloy magnetic powder. In this solution, the material of the internal magnet and the material of the external magnetic shell are limited, and the internal magnet of the nanocrystalline strip material matches the external magnetic shell made of the combined material, so that wideband (e.g., 150 kHz to 300 MHz) filtering can be implemented. In other words, in this solution, a frequency range of an electromagnetic filtering signal filtered by the magnetic element is relatively wide, and may be 150 kHz to 300 MHz. In this solution, the volume and the costs of the magnetic element can also be reduced. A filtering range of a magnetic element in a conventional technology is (e.g., 150 kHz to 30 MHz, or 30 MHz to 300 MHz). Compared with the conventional technology, filtering bandwidth is widened in this application. 
     In an embodiment, the external magnetic shell is an integrated structure, and is formed on the outer surface of the internal magnet through packaging or coating. The integrated structure facilitates a design of a small size of the composite magnetic core and reduces plate space occupied by the magnetic element. 
     In an embodiment, the external magnetic shell includes a first shell and a second shell, and the first shell and the second shell are connected and jointly surround the internal magnet. In this solution, the external magnetic shell is designed as two parts: the first shell and the second shell, and the first shell and the second shell are connected and jointly surround the internal magnet, so that manufacturing costs of this solution are relatively low, and this helps reduce costs of the magnetic element. 
     In an embodiment, the first shell forms first space, the second shell forms second space, the first space and the second space are connected to each other and jointly accommodate the internal magnet, and a gap is disposed between the internal magnet and an inner surface of the external magnetic shell. Advantages of this solution are: Assembly is easy, and in an assembling process, only the internal magnet needs to be fastened in the first shell or the second shell, and then the second shell and the second shell are fastened to each other. 
     The first shell and the second shell may be of a same structure. A connection between the first shell and the second shell may be planar connection, and the first shell and the second shell are connected and fastened by using glue. Advantages of this solution are: The first shell and the second shell do not need to be distinguished in a process of assembling the composite magnetic core, and because the first shell and the second shell are of the same structure, efficiency is high in an assembling and fastening process. In another embodiment, the connection between the first shell and the second shell may be connected in a concave-convex coordination manner, or the first shell and the second shell are mutually coordinated and connected by using a step structure. 
     In an embodiment, the first shell includes a first wall and a second wall that is bent and extended from an edge of the first wall, the second shell includes a third wall and a fourth wall that is bent and extended from an edge of the third wall, the first wall and the third wall are disposed opposite to each other, the second wall and the fourth wall are disposed opposite to each other, the first shell and the second shell are connected to form accommodating space of different sizes, and the accommodating space is used to accommodate the internal magnet. In this solution, an application range of the external magnetic shell is improved. One external magnetic shell may match internal magnets of different sizes, to form composite magnetic cores of different models. In addition, the external magnetic shell and the internal magnet in the composite magnetic core provided in this solution may be seamlessly connected, and the internal magnet is jointly located by using an inner surface of the first shell and an inner surface of the second shell. Another fastening medium is not needed between the internal magnet and the external magnetic shell, for example, glue does not need to be dispensed between the internal magnet and the external magnetic shell. In this solution, a volume of the composite magnetic core can be reduced, and this is conducive to a design of a small size of the magnetic element. 
     In an embodiment, the external magnetic shell is of an annular structure and forms annular accommodating space used for accommodating the internal magnet, the external magnetic shell includes an inner wall and an outer wall that are stacked in a radial direction, the inner wall forms a through hole, the composite magnetic core further includes a magnetic sheet, the magnetic sheet is located in the through hole and is connected to the inner wall, and the magnetic sheet separates the through hole into two sub-holes. The magnetic sheet has a magnetic conduction function, and can increase differential-mode inductance. In an embodiment, in the magnetic element, air passes through a differential-mode magnetic path, and magnetic permeability of the air is 1. In this solution, the magnetic sheet is added, and magnetic permeability of the magnetic sheet is far greater than that of air, and the magnetic permeability of the magnetic sheet may reach tens to several thousand. Magnetic sheets of different materials have different magnetic permeability. Because the magnetic sheet is disposed in the through hole in this application, it may be understood as that a part of the air is replaced by the magnetic sheet, so that the inductance is increased. Therefore, in this solution, the differential-mode inductance can be increased. 
     In an embodiment, a material of the magnetic sheet is the same as that of the external magnetic shell. 
     In an embodiment, the magnetic sheet and the external magnetic shell are in an integrated structure. 
     In an embodiment, the magnetic sheet and the external magnetic shell are in a separated structure. 
     In an embodiment, the magnetic sheet is entirely accommodated inside the through hole. 
     In an embodiment, thickness of the magnetic sheet is greater than or equal to 1 mm. In this solution, the thickness of the magnetic sheet is limited to be greater than 1 mm, so that a relatively good effect of increasing common-mode inductance can be implemented. 
     In an embodiment, thickness of the magnetic sheet is the same as thickness of the external magnetic shell, and the thickness of the external magnetic shell is greater than or equal to 1 mm. 
     In an embodiment, the magnetic sheet may be a rigid material. In this solution, thickness of the magnetic sheet may be set to be relatively thick, and may be greater than or equal to 1 mm, to ensure that magnetic permeability of the magnetic sheet can meet a filtering requirement of the magnetic element. 
     In an embodiment, the magnetic sheet may alternatively be a flexible sheet material. In this solution, thickness of the magnetic sheet may be relatively thin, for example, is less than 1 mm, and the magnetic sheet may be bent and extended in the through hole. A volume occupied by the magnetic sheet in the through hole is set based on a size of the winding and a size of the through hole by using a characteristic that the flexible sheet material is easily bent. In this way, a volume occupied by the magnetic sheet may be as large as possible, to obtain relatively large common-mode inductance. 
     In an embodiment, the magnetic element is a common-mode inductor, and may be a two-phase common-mode inductor or a three-phase common-mode inductor. 
     According to a second aspect, an electronic device is provided. The electronic device includes a circuit board and the magnetic element provided in any one of the embodiments of the first aspect. The magnetic element is disposed on the circuit board, and the winding is electrically connected to the circuit board. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       To describe technical solutions in embodiments of this application or in the background more clearly, the following describes the accompanying drawings used in embodiments of this application or in the background. 
         FIG.  1    is a schematic diagram of an electronic device according to an embodiment; 
         FIG.  2    is a schematic diagram of a magnetic element according to an embodiment; 
         FIG.  3    is a schematic three-dimensional decomposed diagram of the magnetic element shown in  FIG.  2   ; 
         FIG.  4    is a schematic diagram of structures of an external magnetic shell and a magnetic sheet in a composite magnetic core in a magnetic element according to an embodiment; 
         FIG.  5    is a schematic three-dimensional decomposed diagram of an external magnetic shell in a composite magnetic core in a magnetic element according to an embodiment; 
         FIG.  6    is a schematic diagram of a cross section of an external magnetic shell in a composite magnetic core in a magnetic element according to an embodiment; 
         FIG.  7    is a schematic diagram of a state in which an internal magnet is mounted inside the external magnetic shell shown in  FIG.  6   ; 
         FIG.  8    is a schematic three-dimensional decomposed diagram of an external magnetic shell in a composite magnetic core in a magnetic element according to an embodiment; 
         FIG.  9    is a schematic diagram of a cross section in a state in which an internal magnet is mounted inside the external magnetic shell in the composite magnetic core in the magnetic element shown in  FIG.  8   ; 
         FIG.  10    is a schematic diagram of a cross section in another state in which an internal magnet is mounted inside the external magnetic shell in the composite magnetic core in the magnetic element shown in  FIG.  8   ; and 
         FIG.  11    is a schematic three-dimensional decomposed diagram of an external magnetic shell in a composite magnetic core in a magnetic element according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of this application with reference to the accompanying drawings in embodiments of this application. 
     As shown in  FIG.  1   , a circuit board  200  is disposed in an electronic device  100  in an embodiment, and a magnetic element  300  is disposed on the circuit board  200 . The magnetic element  300  may be a common-mode inductor, and plays a role of EMI filtering to suppress outward radiation of an electromagnetic wave generated by a high-speed signal cable in the electronic device. In an embodiment, the electronic device  100  may be a computer, a router, another communication device, a terminal device, or the like, or the electronic device  100  may be a mobile phone, a tablet computer, a vehicle-mounted computer, an intelligent wearable product, or the like. A specific type of the electronic device is not specifically limited in this embodiment of this application, and the electronic device needs to be a device with the magnetic element. In an embodiment, a switch-mode power supply circuit is disposed in the electronic device  100 . The magnetic element  300  provided in this application may be disposed in the switch-mode power supply circuit to filter a common-mode electromagnetic interference signal. The magnetic element  300  may be a two-phase common-mode inductor, or may be a three-phase common-mode inductor. In an embodiment, a winding of the magnetic element  300  is electrically connected to the circuit board  200 . 
     A two-phase common-mode inductor is used as an example for description. In an embodiment, as shown in  FIG.  2    and  FIG.  3   , the magnetic element  300  includes a composite magnetic core  400  and a winding  500 , and the winding  500  is wound around the composite magnetic core  400 . The winding  500  is a coil, and the winding  500  may be formed by winding an enameled copper wire. The enameled copper wire includes a round wire and a flat wire. A material of the winding is not limited in this application. The composite magnetic core  400  includes an internal magnet  41  and an external magnetic shell  42 . The external magnetic shell  42  partially or entirely covers a periphery of the internal magnet  41 . 
     The internal magnet  41  is formed by winding a strip material. In an embodiment, the strip material of the internal magnet  41  in the composite magnetic core  400  provided in this application may be a nanocrystalline material or an amorphous strip material, and the internal magnet  41  may be made by a winder. For the internal magnet formed by winding the strip material, a coil cannot be directly wound on an outer surface of the internal magnet, because pulling stress acting on a surface of the internal magnet  41  may be generated in a process of winding the coil, and the stress may destroy the internal magnet  41  and affect a filtering function of the magnetic element  300 . Therefore, a protective structure needs to be disposed on the outer surface of the internal magnet  41 . Because the external magnetic shell  42  in the composite magnetic core  400  provided in this application has a magnetic material, as a protective shell covering the periphery of the internal magnet  41 , the external magnetic shell  42  can not only protect the internal magnet  41  from pulling stress in a wire winding process, but also increase common-mode impedance of the composite magnetic core  400  and improve the filtering function of the magnetic element  300 . In an embodiment, the external magnetic shell  42  covers the periphery of the internal magnet  41  and is fixedly connected to the internal magnet  41 , and the external magnetic shell  42  may protect the internal magnet  41 . The external magnetic shell  42  in the composite magnetic core  400  provided in this application has the magnetic material, the winding  500  is wound on an outer surface of the external magnetic shell  42 , and the external magnetic shell  42  has a function of filtering electromagnetic noise. Therefore, the composite magnetic core  400  provided in this application can improve a filtering effect of the magnetic element  300 . If the external magnetic shell  42  has no magnetic material, but has only a function of protecting the internal magnet  41 , and the external magnetic shell  42  is made of a non-magnetic material, such an external magnetic shell  42  in the composite magnetic core  400  cannot participate in the filtering function of the magnetic element  300 . If the external magnetic shell  42  occupies space of the composite magnetic core  400  but has no filtering function, this is not conducive to a miniaturization design of the composite magnetic core  400 , and consequently, a volume and costs of a component (that is, the magnetic element  300 ) are increased. 
     In an embodiment, a material of the external magnetic shell  42  is ferrite. In an embodiment, the material of the external magnetic shell  42  is alloy magnetic powder (which may also be understood as an alloy powder magnet). In an embodiment, the external magnetic shell  42  may be a combination of manganese zinc ferrite, nickel zinc ferrite, and alloy magnetic powder. If the non-magnetic material is replaced with a magnetic material, common-mode impedance of the magnetic element  300  can be increased, and this is conducive to miniaturization and costs reduction of the magnetic element  300 . For a magnetic element in a conventional technology, for an electromagnetic wave signal of 100 kHz, common-mode impedance of the magnetic element is 2000Ω. For the magnetic element  300  provided in this application, if ferrite whose magnetic permeability is 7000, for example, is used as a solution of a nanocrystalline protective shell whose magnetic permeability is 30000, for example, that is, the material of the external magnetic shell  42  is ferrite whose magnetic permeability is 7000, and a material of the internal magnet  41  is nanocrystalline whose magnetic permeability is 30000, for an electromagnetic wave signal of 100 kHz, common-mode impedance of the magnetic element  300  is 4060Ω. It can be learned that, in a same size, a filtering effect of the magnetic element  300  provided in this application is significantly improved. 
     In an embodiment, a material of the internal magnet  41  in the composite magnetic core  400  is a nanocrystalline strip material, and a material of the external magnetic shell  42  in the composite magnetic core  400  is ferrite. In this solution, performance of a common-mode inductor of the magnetic element  300  can be improved, and a volume and costs of the magnetic element  300  can be reduced. In an embodiment, in this solution, the material of the internal magnet and the material of the external magnetic shell are limited, and the internal magnet of the nanocrystalline strip material and the external magnetic shell of the ferrite are combined in one composite magnetic core, so that performance of a common-mode inductor can be improved while a volume and costs of the composite magnetic core are properly controlled. In this way, not only the magnetic element has a relatively good filtering effect, but overall performance of the magnetic element can also be improved in a design condition that a relatively small volume and relatively low costs are controlled. 
     In an embodiment, a material of the internal magnet  41  in the composite magnetic core  400  is a nanocrystalline strip material, and a material of the external magnetic shell  42  in the composite magnetic core  400  is a combination of manganese zinc ferrite, nickel zinc ferrite, and alloy magnetic powder. In this solution, the material of the internal magnet and the material of the external magnetic shell are limited, and the internal magnet of the nanocrystalline strip material matches the external magnetic shell made of the combined material, so that wideband (150 kHz to 300 MHz) filtering can be implemented. In other words, in this solution, a frequency range of an electromagnetic filtering signal filtered by the magnetic element  300  is relatively wide, and may be 150 kHz to 300 MHz. In this solution, the volume and the costs of the magnetic element  300  can also be reduced. A filtering range of a magnetic element in a conventional technology is (150 kHz to 30 MHz, or 30 MHz to 300 MHz). Compared with the conventional technology, filtering bandwidth is widened in this application. 
     As shown in  FIG.  2    and  FIG.  3   , in an embodiment, the external magnetic shell  42  is of an annular structure and forms annular accommodating space  420  used for accommodating the internal magnet  41  (a reference numeral  420  in  FIG.  3    means space enclosed inside the external magnetic shell  42 , and the external magnetic shell  42  is a hollow structure). In an embodiment, an outer contour of the annular structure formed by the external magnetic shell  42  is square, and an inner contour is also square. The internal magnet  41  and the external magnetic shell have similar structure forms but different sizes. The internal magnet  41  may be mounted inside the external magnetic shell  42  and surrounded by the external magnetic shell  42 . The external magnetic shell  42  includes an inner wall  421  and an outer wall  422  that are stacked in a radial direction, and the inner wall  421  forms a through hole H. The composite magnetic core  400  further includes a magnetic sheet  43 , and the magnetic sheet  43  is located in the through hole H and is connected to the inner wall  421 . In an embodiment, the composite magnetic core  400  is applied to a two-phase common-mode inductor, there are two groups of windings  500 , and there is one magnetic sheet  43 . The magnetic sheet  43  separates the two groups of windings  500  (as shown in  FIG.  2   ), and the magnetic sheet  43  separates the through hole H into two sub-holes H 1  and H 2 . One sub-hole H 1  is used to accommodate one group of windings  500 , and the other sub-hole H 2  is used to accommodate the other group of windings  500 . In this embodiment, the magnetic sheet  43  is disposed in the through hole H enclosed by the external magnetic shell  42 , and the magnetic sheet  43  has a magnetic conduction function, and can increase differential-mode inductance. In an embodiment, in the magnetic element, air passes through a differential-mode magnetic path, and magnetic permeability of the air is 1. In this solution, the magnetic sheet  43  is added, and magnetic permeability of the magnetic sheet  43  is far greater than that of air, and the magnetic permeability of the magnetic sheet  43  may reach tens to several thousand. Magnetic sheets  43  of different materials have different magnetic permeability. Because the magnetic sheet  43  is disposed in the through hole H in this application, it may be understood as that a part of the air is replaced by the magnetic sheet  43 , so that the inductance is increased. Therefore, in this solution, the differential-mode inductance can be increased. 
     A material of the magnetic sheet  43  may be the same as the material of the external magnetic shell  42 . This solution facilitates a manufacturing process of the external magnetic shell  42  and the magnetic sheet  43 . The magnetic sheet  43  and the external magnetic shell  42  may be in an integrated structure. The material of the magnetic sheet  43  may alternatively be different from the material of the external magnetic shell. The magnetic sheet  43  and the external magnetic shell  42  may be designed as a separated structure. For example, the magnetic sheet  43  and the external magnetic shell  42  may be directly fixedly connected through cooperation of a buckle and a slot, or may be connected by using another conversion bracket. A specific position of the magnetic sheet  43  in the through hole H may be adjusted in a separated design, to adjust filtering performance of the magnetic element  300 . 
     In another embodiment, as shown in  FIG.  4   , the composite magnetic core  400  is applied to a three-phase common-mode inductor, there are three groups of windings  500 , there are three magnetic sheets  43 , and the magnetic sheets  43  separate the through hole H into three sub-holes H 1 , H 2 , and H 3 . The three sub-holes H 1 , H 2 , and H 3  are separately used to accommodate different windings. In this embodiment, alternatively, there is one magnetic sheet  43 . A form of the magnetic sheet  43  is different from a form shown in  FIG.  3   . The magnetic sheet  43  is non-straight or non-plate-shaped, and may have a plurality of branches, for example, three branches, and edges of the three branches are fastened together, and an included angle between two branches is 120 degrees. In this way, the through hole may be divided into three sub-holes H 1 , H 2 , and H 3  by one magnetic sheet  43 . In  FIG.  4   , the inner wall  421 , the outer wall  422 , and the magnetic sheet  43  are schematically represented by using lines. In an actual product, structures represented by these lines have specific thickness, and are entity structures. An edge of the magnetic sheet  43  is fixedly connected to an outer surface of the inner wall  421 . 
     In an embodiment, the edge of the magnetic sheet  43  may not exceed a boundary of the through hole H; in other words, the magnetic sheet  43  is entirely accommodated in the through hole H. This solution helps implement a connection between the magnetic sheet  43  and the external magnetic shell  42 , and facilitates manufacture and winding. In another embodiment, an outer contour of the magnetic sheet  43  may alternatively exceed a boundary of the through hole H, and may be implemented in the magnetic element  300 . An orthogonal projection of the winding on the magnetic sheet  43  falls within the magnetic sheet  43 . In this solution, a clearer effect of increasing the differential-mode inductance is implemented. Certainly, an area of an orthogonal projection of the winding in a plane in which the magnetic sheet  43  is located may also be greater than an area of the magnetic sheet  43 . 
     In an embodiment, thickness of the magnetic sheet  43  is greater than or equal to 1 mm. In this solution, the thickness of the magnetic sheet is limited to be greater than 1 mm, so that a relatively good effect of increasing common-mode inductance can be implemented. 
     In an embodiment, thickness of the magnetic sheet  43  is the same as thickness of the external magnetic shell  42 , and the thickness of the external magnetic shell  42  is greater than or equal to 1 mm. 
     In an embodiment, the magnetic sheet  43  may be a rigid material. In this solution, thickness of the magnetic sheet  43  may be set to be relatively thick, and may be greater than or equal to 1 mm, to ensure that magnetic permeability of the magnetic sheet  43  can meet a filtering requirement of the magnetic element. 
     In an embodiment, the magnetic sheet  43  may alternatively be a flexible sheet material. In this solution, thickness of the magnetic sheet  43  may be relatively thin, for example, is less than 1 mm, and the magnetic sheet  43  may be bent and extended in the through hole H. A volume occupied by the magnetic sheet  43  in the through hole H is set based on a size of the winding and a size of the through hole H by using a characteristic that the flexible sheet material is easily bent. In this way, a volume occupied by the magnetic sheet  43  may be as large as possible, to obtain relatively large common-mode inductance. 
     The external magnetic shell  42  in the composite magnetic core  400  provided in this application may be an integrated structure, and is formed on the outer surface of the internal magnet  41  through packaging or coating. For example, the external magnetic shell  42  may be formed on the outer surface of the internal magnet  41  through spraying or electroplating, or the external magnetic shell  42  may be fabricated on the outer surface of the internal magnet  41  by using an integrated injection molding process. The external magnetic shell  42  may be in an entirely closed structure without any gap (or air gap). In another embodiment, a gap (or an air gap) may alternatively be disposed on the external magnetic shell  42 , magnetism of the composite magnetic core is adjusted by disposing the gap (or the air gap), and a filtering effect of the magnetic element  300  is controlled. 
     As shown in  FIG.  5   ,  FIG.  6   , and  FIG.  7   , in another embodiment, the external magnetic shell  42  may alternatively be a separated structure. For example, the external magnetic shell  42  includes a first shell  42 A and a second shell  42 B, and the first shell  42 A and the second shell  42 B are connected to jointly surround the internal magnet  41 . 
     The first shell  42 A and the second shell  42 B are connected or fastened to each other to form the external magnetic shell  42 , the first shell  42 A forms first space  420 A, the second shell  42 B forms second space  420 B, the first space  420 A and the second space  420 B are connected to form accommodating space  420 , and the internal magnet  41  is jointly accommodated, and a gap G is disposed between the internal magnet  41  and an inner surface of the external magnetic shell  42 . The internal magnet  41  and the inner surface of the external magnetic shell  42  may be fastened by an adhesive. As shown in  FIG.  7   , two adjacent surfaces of the internal magnet  41  are bonded to and fixedly connected to the inner surface of the external magnetic shell  42 . A gap G may exist between the inner surface of the external magnetic shell  42  and the other two surfaces of the internal magnet  41 . Because there is a tolerance between the internal magnet  41  and the external magnetic shell  42  in a processing and assembling process, or a planeness problem is caused due to a processing procedure of the outer surface of the internal magnet  41  and the inner surface of the external magnetic shell  42 , the internal magnet  41  and the external magnetic shell  42  cannot be entirely bonded in a contact process, and consequently, sizes of the internal magnet  41  and the external magnetic shell  42  do not entirely match each other (when the sizes of the internal magnet  41  and the external magnetic shell  42  entirely match each other, the outer surface of the internal magnet  41  may be entirely bonded to the external magnetic shell  42 ). In this condition, there is the gap G between the internal magnet  41  and the external magnetic shell  42 , but the gap G does not affect normal working of the composite magnetic core  400 . 
     In an embodiment, the first shell  42 A and the second shell  42 B may be of a same structure. A connection between the first shell  42 A and the second shell  42 B may be planar connection, and the first shell  42 A and the second shell  42 B are connected and fastened by using glue. Advantages of this solution are: The first shell  42 A and the second shell  42 B do not need to be distinguished in a process of assembling the composite magnetic core, and because the first shell  42 A and the second shell  42 B are of the same structure, efficiency is high in an assembling and fastening process. In another embodiment, the connection between the first shell  42 A and the second shell  42 B may be connected in a concave-convex coordination manner, or the first shell  42 A and the second shell  42 B are mutually coordinated and connected by using a step structure. In an embodiment shown in  FIG.  5    to  FIG.  7   , a connection surface between the first shell  42 A and the second shell  42 B includes an L-shaped surface or a Z-shaped surface. This connection manner helps improve a sealing effect at the connection between the first shell  42 A and the second shell  42 B. In an embodiment, the first shell  42 A includes a first connection part  42 A 1 , the second shell  42 B includes a second connection part  42 B 1 , and a connection surface of the first shell  42 A and the second shell  42 B is a surface in which the first connection part  42 A 1  and the second connection part  42 B 1  are in contact with each other. A direction extending from the inner surface of the external magnetic shell  42  to the outer surface of the external magnetic shell  42  is a radial direction. In the radial direction, the first connection part  42 A 1  and the second connection part  42 B 1  are stacked. In this embodiment, the first connection part  42 A 1  is disposed around a periphery of the second connection part  42 B 1 , and the first connection part  42 A 1  and the second connection part  42 B 1  may be connected and sealed by using glue. 
     In an embodiment, after being connected, the first shell  42 A and the second shell  42 B may form accommodating space  420  of different sizes. As shown in  FIG.  8   ,  FIG.  9   , and  FIG.  10   , the first shell  42 A includes a first wall  42 A 2  and a second wall  42 A 3  that is bent and extended from an edge of the first wall  42 A 2 , the second shell  42 B includes a third wall  42 B 2  and a fourth wall  42 B 3  that is bent and extended from an edge of the third wall  42 B 2 , the first wall  42 A 2  and the third wall  42 B 2  are disposed opposite to each other, the second wall  42 A 3  and the fourth wall  42 B 3  are disposed opposite to each other, the first shell  42 A and the second shell  42 B are connected to form accommodating space of different sizes, and the accommodating space is used to accommodate the internal magnet  41 . As shown in  FIG.  9   , the external magnetic shell  42  and the internal magnet  41  in the composite magnetic core  400  provided in this solution may be seamlessly connected, and the internal magnet  41  is jointly located by using an inner surface of the first shell  42 A and an inner surface of the second shell  42 B. Another fastening medium is not needed between the internal magnet  41  and the external magnetic shell  42 , for example, glue does not need to be dispensed between the internal magnet  41  and the external magnetic shell  42 . In this way, a volume of the composite magnetic core  400  can be reduced, and this is conducive to a design of a small size of the magnetic element  300 . With reference to  FIG.  9    and  FIG.  10   , a size of the internal magnet  41  in the solution shown in  FIG.  10    is less than a size of the internal magnet  41  in the solution shown in  FIG.  9   . For the external magnetic shell  42 , when a position at which the first shell  42 A and the second shell  42 B are connected is different, a size of the accommodating space inside the external magnetic shell  42  can be adjusted. Therefore, the external magnetic shell  42  provided in this solution can match internal magnets  41  of more sizes, to form a plurality of composite magnetic cores  400  of different sizes, and is widely applied. 
     In the embodiment shown in  FIG.  8   , the first wall  42 A 2  of the first shell  42 A and the third wall  42 B 2  of the second shell  42 B form a top wall and a bottom wall of the external magnetic shell  42 , and the first wall  42 A 2  and the third wall  42 B 2  may be the same in shapes and sizes. Both the first wall  42 A 2  and the third wall  42 B 2  may be of a flat plate-shaped structure, and both the first wall  42 A 2  and the third wall  42 B 2  may alternatively be of an arcuate plate-shaped structure. The second wall  42 A 3  of the first shell  42 A forms an outer wall of the external magnetic shell  42 , the fourth wall  42 B 3  of the second shell  42 B forms an inner wall of the external magnetic shell  42 , the fourth wall  42 B 3  forms the through hole H of the external magnetic shell  42 , the second wall  42 A 3  is located on a periphery of the fourth wall  42 B 3 , and an area between the second wall  42 A 3  and the fourth wall  42 B 3  is an annular area. An inner contour and an outer contour of the annular area may be square, circular, or another shape, and the internal magnet  41  is accommodated in the annular area. 
     In an embodiment, as shown in  FIG.  11   , the external magnetic shell  42  includes a first shell  42 A and a second shell  42 B. A structure of the first shell  42 A is the same as that in the embodiment shown in  FIG.  8   . A structure of the second shell  42 B is different from that in the embodiment shown in  FIG.  8    in that the second shell  42 B includes a magnetic sheet  43 . In this embodiment, the magnetic sheet  43  is a flat plate-shaped structure, two opposite ends of the magnetic sheet  43  are connected to a fourth wall  42 B 3  of the second shell  42 B, and the magnetic sheet  43  separates a through hole H formed by the fourth wall  42 B 3  into two sub-holes H 1  and H 2 . In another embodiment, the magnetic sheet  43  may alternatively be arcuate plate-shaped. 
     In the embodiment shown in  FIG.  11   , a first wall  42 A 2  of the first shell  42 A and a third wall  42 B 2  of the second shell  42 B are disposed at intervals in a first direction A, and a size of the magnetic sheet  43  in the first direction A may be less than or equal to a vertical distance between an outer surface of the first wall  42 A 2  and an outer surface of the third wall  42 B 2 . It may also be understood as that a top surface of the magnetic sheet  43  is flush with the outer surface of the first wall  42 A 2 , and a bottom surface of the magnetic sheet  43  is flush with the outer surface of the third wall  42 B 2 . The outer surface of the first wall  42 A 2  is a surface that is of the first wall  42 A 2  and that is away from the third wall  42 B 2 , and the outer surface of the third wall  42 B 2  is a surface that is of the third wall  42 B 2  that is away from the first wall  42 A 2 . 
     The external magnetic shell  42  provided in the foregoing embodiment entirely covers the internal magnet  41 , to provide full protection for the internal magnet  41 . In another embodiment, the external magnetic shell  42  may alternatively partially cover the internal magnet  41 , and covers only a part of the internal magnet  41  corresponding to the winding. For example, as shown in  FIG.  2    and  FIG.  3   , in this embodiment, the composite magnetic core  400  is approximately rectangular in shape, and includes four magnetic pillars, and two magnetic pillars are opposite to each other. The winding  500  is wound around left and right magnetic pillars, and no winding is disposed on peripheries of upper and lower magnetic pillars. Based on this case, the external magnetic shell  42  may be disposed only on peripheries of the left and right magnetic pillars, and no external magnetic shell may be disposed at positions of the upper and lower magnetic pillars. The external magnetic shell can protect the internal magnet and increase the common-mode impedance of the magnetic element provided that the winding is wound on the external magnetic shell. 
     The foregoing embodiments are merely intended for describing the technical solutions of this application, but not for limiting this application. Although this application is described in detail with reference to the foregoing embodiments, a person of ordinary skill in the art should understand that modifications to the technical solutions recorded in the foregoing embodiments or equivalent replacements to some technical features thereof may still be made, without departing from the scope of the technical solutions of embodiments of this application.