Patent Publication Number: US-6342778-B1

Title: Low profile, surface mount magnetic devices

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to magnetic devices such as inductors and transformers. Specifically, the invention relates to magnetic devices that can be assembled as low profile surface mount devices on a printed circuit board or a metallized substrate. 
     BACKGROUND OF THE INVENTION 
     Magnetic devices, such as inductors and transformers, are employed in many different types of electrical devices including communications equipment and power supplies. In practice, most magnetic devices are fabricated of one or more windings, formed by an elongated electrical conductor, such as a wire of circular or rectangular cross-section, or a planar electrical conductor wound about or mounted to a bobbin composed of a dielectric material, such as plastic. In some instances, the electrical member is soldered to terminations on the bobbin. Alternatively, the electrical member may be threaded through the bobbin for connection directly to a metallized area of an underlying circuit board. A magnetic core may be disposed about the bobbin to impart a greater reactance to the magnetic device and thereby alter its operating characteristics. The use of a bobbin, however, generally results in a magnetic device with a large profile, which not only takes up valuable space on the circuit board, but also results in a large height for the overall electrical device. 
     In addition to being formed with bobbins, magnetic devices can be formed with a magnetic core, such as ferrite or iron, wound with conductive coils. These devices are sometimes referred to as wire-wound core devices. One major difficulty with wire-wound core devices is that they have been difficult to miniaturize. While components such as resistors, diodes, capacitors and transistors have been drastically reduced in size, magnetics, including bobbin and wire-wound core devices, remain bulky. 
     One attempt at a low profile magnetic device is described in U.S. Pat. No. 5,574,420 issued Nov. 12, 1996 to Roy et al. The device described in Roy et al. is a magnetic component formed by a plurality of conductive elements surrounding a magnetic core. The conductive elements pass through holes or channels in the magnetic core and then are bent outwards to allow surface mount connection to a printed wiring board or the equivalent. Unfortunately, the magnetic component described by Roy et al. suffers from a number of deficiencies. First, the device is incapable of carrying large amounts of current because the small area of the magnetic core that is surrounded by the conductive elements tends to saturate quickly. Second, the bent out ends of the conductive elements make poor surface mount conductors because they are very difficult to make coplanar. Finally, the magnetic components of Roy et al. can be difficult to manufacture due to the shape of the magnetic core and the arrangement of the conductive elements. 
     Accordingly, what is needed is a low profile magnetic component that is capable of handling larger currents, has more consistently coplanar conductor elements, and is more easily manufactured. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention include providing for a low profile magnetic component formed from a magnetic core and a plurality of conductive elements, also referred to as conductors. The magnetic core includes a bottom, a top, end surfaces and side surfaces. The side surfaces include portions that are angled inward from the bottom to the top thereby forming a plurality of channels. The magnetic core further includes a recess in the top adjacent to the channels. 
     The plurality of conductors surround the magnetic core and pass through a corresponding channel from the plurality of channels. The top of the conductors are adjacent to the recess in the magnetic core and the ends are bent inward against the bottom of the core. The ends of the conductors form contact surfaces which are coplanar and surface mountable. In order to form the conductors tightly around the magnetic core and to ensure that the contact surfaces formed by the ends are coplanar, during manufacture the tops of the conductors are loaded causing the ends to bend inward in to the recess in the magnetic core. While the conductors are loaded the ends are bent inward toward the center of the bottom. After bending, the conductors are unloaded and the spring tension in the conductors causes them to fit tightly around the magnetic core and causes the ends to fit snugly against the base. 
     The magnetic components can be formed into a magnetic device such as an inductor by placing two or more in close proximity and using conductive traces on a printed wiring board or other insulated substrate to form the conductors into windings. This magnetic device can then be utilized in a power supply as, for example, the inductor in an output filter or as transformers in groups of two or more. 
     The foregoing has outlined, rather broadly, preferred and alternative features of embodiments of the invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a low profile magnetic component according to a conventional arrangement; 
     FIG. 2 a  is a perspective view of a low profile magnetic component according to an embodiment of the invention; 
     FIG. 2 b  is a cross-sectional view of the magnetic component from FIG. 2 a;    
     FIG. 3 is plan view of a magnetic device formed from an insulating substrate with conductive traces and two of the magnetic components from FIG. 2; 
     FIG. 4 is a circuit diagram showing a power supply incorporating the magnetic device from FIG. 3; and 
     FIG. 5 is a flow chart describing a process for making a low profile magnetic device in accordance with the principles of the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring now to FIG. 1, a conventional surface mount magnetic component  10  is shown. Magnetic component  10  is formed by body  11  of magnetic material that is surrounded by a plurality of conductive elements  12  distributed along the major dimension of body  11 . Each conductive element  12  is formed with four right angle bends, the first and second bends allowing conductive element  12  to pass through channels  16  to surround a portion of the body, and the third and fourth bends to form a pair of contact surfaces  14 . Magnetic component  10 , however suffers from a variety of drawbacks. First, by having conductive elements  12  pass at right angles through channels  16 , a large portion of the cross-section of body  11  is not surrounded by conductive elements  12 . By limiting the cross-section of body  11  surrounded by conductive elements  12 , the magnetic flux able to be carried by body  11  is limited. Next, the third and fourth bends that form contact surfaces  14  are formed by bending conductive element  12  outward in free space. By forming contact surfaces  14  in this manner making them coplanar for surface mounting within specific tolerances is very difficult. Finally, magnetic component  10  is manufactured by taking the preformed conductive elements and placing them around body  11 . This type of manufacturing is difficult and only exacerbates the problem of trying to make contact surfaces  14  for all conductive elements  12  coplanar. If one conductive element  12  is misplaced by as little as a few thousands of an inch the coplanarity of the entire device is unacceptable. 
     In order to overcome these limitations a low-profile magnetic component is needed that a) uses as much of the cross-section of the magnetic core as possible, b) has conductive elements with contact surfaces that are tightly coplanar, and c) is easy to manufacture within design tolerances. 
     Referring now to FIGS. 2 a  and  2   b , a magnetic component according to an embodiment of the invention is shown. Low-profile magnetic component  20  shown in FIGS. 2 a  and  2   b  is formed from magnetic core  22  and a plurality of conductive elements  24 . Magnetic core  22  typically is rectangular in shape, having a length l greater than the width w and height h. Conductive elements  24  are located in a center section  26  along the length l of magnetic core  22 . Center section  26  contains a number of features to accommodate conductive elements  24 , including recess  28  in the top  34  of magnetic core  22 , and channels  30 , which further include angled side surfaces  32 . Angled side surfaces  32  form a cross section that increases from top  34  to some distance above bottom  36  of magnetic core  22 . The angled side surfaces allow for better inspection capability of the assembled component. Wrapping the conductive elements  24  around the outside of magnetic core  22  allows more cross-sectional area to be surrounded by conductive elements  24 . This greater crosssectional area increases the amount of flux that can be handled by the core before saturation, and therefore, increases the amount of current that the magnetic device can accommodate. 
     Conductive elements  24  are also formed with coplanar contact surfaces  38 . Unlike magnetic device  10  from FIG. 1, contact surfaces  38  are formed by bending conductive elements  24  inward against the bottom  36  of magnetic core  22 . Bending conductive elements  24  inward to form contact surfaces  38  allows for much greater control over the coplanarity of the contact surfaces. Bottom  36  of magnetic core  22  is used as a stop to ensure consistent coplanarity both between contact surfaces  38  of a specific conductive element  24  as well as between contact surfaces  38  of different conductive elements  24 . Recess  28 , in top  34  of magnetic core  22 , aids in the formation of contact surfaces  38 . During manufacture, the top of conductive element  24  is displaced into recess  28  before it is bent to form contact surfaces  38 . After contact surfaces  38  are bent into place, the top of conductive element  24  is unloaded releasing the spring tension, which causes contact surfaces  38  to curl tightly up against bottom  36  of magnetic core  22 . Use of this loading of conductive element  24  allows a much more consistent formation of contact surfaces  38  which result in very coplanar surface mount contacts. The loading technique also allows conductive elements  24  to fit more tightly around magnetic core  22  to limit any potential movement of the conductive elements  24 . 
     A magnetic device is formed from magnetic component  22  by mounting two or more devices in close proximity on an insulating substrate having conductive traces for interconnecting the conductive elements of the magnetic component into windings. FIG. 3 shows a magnetic device  40  formed from a pair of magnetic components  20 A and  20 B placed side by side to form an air gap  42  between them. Conductive trace  41  on an insulating substrate, such as printed wiring board  44 , is used to interconnect the conductive elements of magnetic devices  20 A and  20 B into windings. The magnetic components  20 A and  20 B, air gap  40  and conductive trace  41  together form a magnetic device  42  such as an inductor. Although magnetic device  40  is formed using two magnetic components, those skilled in the art would understand that similar magnetic devices could be formed using any number of magnetic components. 
     The inductor formed by magnetic device  40  from FIG. 3 is suitable as a magnetic element in a power supply module. The circuit for such a power supply module  50  is shown in FIG.  4 . Power supply module  50  is formed by buck converter  52  with input voltage  54 , power switches  56 , output filter  58  and regulated output voltage  60 . Inductor  62  in output filter  58  is formed from magnetic device  40  from FIG.  3 . The operation of buck converter  50  is well understood in the art and will not be discussed further. Although the magnetic device is shown with reference to a buck-type converter, those skilled in the art would understand that the magnetic device according to embodiments of the invention is suitable for use in any type power supply which utilizes magnetic devices, particularly inductors. 
     Referring now to FIG. 5, a flow chart is shown that generally describes the manufacturing process  60  for making a magnetic component  20  from FIGS. 2 a  and  2   b  according to embodiments of the invention. Manufacturing process  60  begins at step  62  by cutting conductive elements  24  to the required length from a continuous supply of conductive material. Once the conductors are cut to length, the process proceeds to step  64  where the conductors are preformed by bending them into a u-shape such that they will fit around the magnetic core  22 . Step  66  then requires that the conductors be placed around magnetic core  22 . In step  68  the tops of the conductors are loaded forcing them down slightly into recess  28  as was described with reference to FIGS. 2 a  and  2   b . The process then proceeds to step  70  where the ends of the conductors are bent inward against the bottom  36  of magnetic core  22  to form contact surfaces  38 . Finally the tops of conductors, or conductive elements  24 , are unloaded allowing the conductors to fit snugly to the magnetic core  22  and allowing contact surfaces  38  to form coplanar surfaces for surface mounting. 
     Typically, the embodiment magnetic core  22  is a ferrite material. For example, the conductive elements  24  are formed from copper, which is coated for solderability. Although particular references have been made to specific structures, topologies and materials, those skilled in the art should understand that magnetic component  20  could be formed in a multitude of materials and in a multitude of shapes and sizes, all of which are well within the broad scope of the invention. 
     Although embodiments of the invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.