Abstract:
A low profile magnetic component such as an inductor or transformer includes a core and a planar magnetic winding body having a dense, rigid structure composed of a stack of individual winding patterns separated by insulating layers, and a binder/filler material. The input and output termini of the individual winding patterns are revealed in a side face of the winding body, where they are interconnected with a plated metallization. Such structures may be mounted onto a PC board, and are useful, for example, in electronic ballasts for the lighting industry.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation divisional of application Ser. No. 08/874,171, filed Jun. 13, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to low profile magnetic components, and more particularly relates to such components including planar magnetic winding structures, such as inductors and transformers, in which the windings are composed of stacks of interconnected layers of conductor patterns. 
     The main use for such planar magnetic components is in electronic circuitry destined for use in a volume restricted space, ie, reduced height and/or reduced total volume. 
     Such structures consist of a stack of layers each containing part of the total winding structure, an insulating layer to prevent electrical contact between turns in adjacent layers, and a contacting structure that permits electrical contact between turns in adjacent layers. The winding structures are optimized with respect to winding losses, and usually are made by etching or stamping, and sometimes by folding. Contacts are usually made by soldering or using plated vias. 
     For example, the winding patterns may be formed by selectively etching a copper layer having a thickness of about 3 mils, from a PC board having a thickness of about 4 mils. The etched PC boards are then stacked to form the winding structure. 
     As such components are reduced in size to meet new miniaturized device requirements, the surface-to-volume ratio becomes smaller and the temperature due to heat dissipation quickly rises with the amount of dissipated heat. In present planar winding structures, such heat dissipation is hindered by the presence of voids between the layers and the windings of each layer, as well as by irregular outer surfaces of the structure, which prevents good thermal contact with surrounding structures. In addition, the layer-to-layer contacts become more difficult to achieve. 
     In DE 44 22 827 A1, to which U.S. Pat. No. 5,652,561 corresponds, the voids between winding layers of a planar magnetic winding structure are filled with glue, but the interconnections are achieved using vias. Such vias constitute a larger proportion of the total winding structure, which can contribute significantly to eddy current losses and other magnetic winding losses. 
     In U.S. Pat. No. 5,598,135, some of the interconnections between winding layers of a planar winding structure are achieved by brazing over connector portions of the windings which terminate in the outer surface of the stack. Such brazing is enabled by the use of a rigid ceramic composition as the insulating portion of the winding layers. However, vias must still be used to establish interconnections in the interior of the stack. Such a complex structure tends to be difficult and costly to manufacture. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a planar magnetic winding structure for a low profile magnetic component which is compact. 
     It is another object of the invention to provide such a the winding layers are readily interconnected. 
     It is another object of the invention to provide such a planar magnetic winding structure which can be readily connected to external circuitry. 
     It is yet another object of the invention to provide such a planar magnetic winding structure which is readily manufacturable, and does not deform during manufacturing. 
     It is yet another object of the invention to provide a low profile magnetic component incorporating such a planar winding structure. 
     According to the invention, there is provided a planar winding body for a low profile magnetic component, the winding body having upper and lower surfaces, an outer sidewall, and an inner sidewall defining an aperture the body comprising a stack of substantially planar layers of an electrically insulating material, each layer bearing a winding pattern formed by a continuous track of electrically conductive material, the input and output termini of the individual tracks revealed in sidewalls of the winding body, an electrically insulating binding material filling the spaces between turns of the tracks, and metal pattern on the side walls of the body, the metal providing interconnection of the input and output termini of the winding patterns as well as contacts for external electrical connections. 
     Preferably, the metal paterns are plated, most preferably, electroless plated. A typical and preferred winding material is copper, while the insulating material may be a dielectric polymer such as a polyimide, and a typical suitable filler/binder material is a dielectric thermosetting resin such as epoxy. 
     In order to maintain planarity of the device during manufacture and thereafter, pads of conductive material of the same thickness as the winding pattern are preferably positioned between the winding pattern and the edges of the layers. Such pads provide support during filling of the voids in the stack and thus prevent loss of filler due to deformation during pressing and curing of the stack to densify and rigidify the structure. 
     In accordance with another aspect of the invention, there is provided a low profile magnetic component comprising a core and the winding body of the invention. 
     Preferably, the core comprises two or more core components having mutually facing planar surfaces. In one embodiment, the core comprises a first lower core component having a planar portion and two or more spaced-apart upstanding portions having planar upper surfaces, the upstanding portions defining a space to accommodate the winding body, the core also comprising a second upper core component having a planar lower surface. 
     In an especially preferred embodiment of the invention, two opposite sides of the winding body have indented portions for accommodating the upstanding portions of the core, and for establishing a predetermined distance between the body and the core, thereby to insure a minimum distance between the contacts on the inner face of the winding body and the adjacent core surface, for electrical isolation purposes. 
     The planar winding structures of the invention are useful in a variety of applications, such as transformers, inductors, motor windings, planar engines, antennas and detectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 a  is a side elevation view of a low profile magnetic component including a planar magnetic winding structure of the invention, mounted on a circuit board; 
     FIG. 1 b  is a side view of a portion of the planar magnetic winding structure of FIG. 1 a;    
     FIG. 1 c  is a top view of the planar magnetic component of FIG. 1 a ; and 
     FIG. 2 is a plan view of a copper foil sheet having two sets of sixteen windings for two different winding structures of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be further elucidated by a detailed description of certain preferred embodiments of the invention, in conjunction with the drawings, in which the same reference numerals are used to indicate similar features or elements in different figures. 
     Referring now to FIGS. 1 a  through  1   c , there is illustrated a low profile inductor component  10  of the invention, mounted on a circuit board  11 . The component  10  includes a composite ferrite core made up of a lower “E” core  13 , so named for the E-shape resulting from the upstanding portions  14 ,  15  and  16  on the base portion  12 , and a top “I” core  17 , having a planar configuration. 
     Arranged in the spaces between the upstanding portions  14 ,  15  and  16  of the core is a winding body  18 , having a central aperture consisting of a stack of winding layers, each layer being made up of a polyimide substrate  19 , and an electrically conductive winding pattern  20 . Filling the spaces between the turns of continuous conductive track  20  of the winding pattern and binding the stack into a dense, rigid body is a binder/filler material such as an epoxy  21 . 
     As best seen in FIG. 1 b , at least a terminal portion of the conductive track  20  in each layer extends into the outer sidewall  22  of winding body  18 , where they are interconnected by means of a metal pattern, eg., electroless plated metal contacts  23 - 34  covering the ends of the tracks  20  in the outer sidewall  22  and extending partially onto the upper and lower surfaces of the body  10 . One such contact  23  is shown in the FIG. 1 b , while the remaining contacts  24  through  34  are shown in FIG. 1 c . These plated contacts  23 - 34  also are used for external connection to the winding body  18 . Additional plated contacts  35   a - 35   e  and  36   a - 36   e  are located on the inner sidewall  4  defining the aperture of body  18  adjacent the end walls  5  and  6  of center leg  15  of core  13 . These contacts also serve to interconnect the winding layers, as well as to provide external connections. 
     Preferably, all layers contain an identical pattern of contact pads  40 - 57  around the inner and outer periphery of the winding layers, as shown in FIG. 2, of which only two in each layer are used to provide interconnection to other layers. The remaining pads provide structural support to prevent deformation of the layers during pressing and curing of the filler material to densify and rigidify the stack during manufacturing. 
     The space “d” between the end walls  5 ,  6  of core center leg  15  and the inner sidewalls  4  of the winding body  18  contains a dielectric potting compound  37 , which may also be epoxy, and which fixes the space d, thus preventing creep and insuring against electrical discharges between the coil and the core. 
     The layers of insulating material and winding patterns may be conveniently provided by starting with a sheet of commercially available flex foil, consisting of a 1 mil thick polyimide sheet supporting a copper foil approximately 4 to 5 mils thick. If the desired thickness of copper is not readily available, additional copper may be deposited, for example, by electroplating, to build up the layer to the desired thickness. The compactness and rigidity of the final structure enables such thicknesses, which in turn enables formation of conductive tracks having a sufficient cross section to carry the current needed for high power applications. 
     The winding patterns made up of the conductive tracks are formed by selectively etching the foil to remove the unwanted portions of the copper layer. FIG. 2 shows such a flex foil sheet containing two exemplary sets, of sixteen winding patterns, one set for a first inductor, and a second set for a second inductor. 
     The individual winding layers are then cut from the sheet, and assembled into a stack using the alignment holes “H” in the corners of the layers. In each of the two sets, the first 8 winding layers are stacked in the sequence  1 ,  2  . . .  8 , after which the last 8 layers are rotated 180 degrees in the plane of the sheet as shown in FIG. 2, before being stacked in the sequence  9 ,  10  . . .  16 . 
     Prior to stacking, each winding layer is coated with a binding fluid, eg., dipped in epoxy. After stacking, the binder-coated stack is pressed to remove excess liquid. Ideally, only a very thin layer of binder should remain between the upper surfaces of the conductive tracks of the winding pattern and the lower surface of the insulating sheet above it, to insure maximum density of the stack. In the case of epoxy as the binder, the stack is then cured by heating to about 60 degrees C. for about 1 hour. 
     As will be appreciated, an alternate assembly method would involve laying out the individual winding layers in each sheet in a manner so that the sheets could be stacked, and then densified as described above, and then the stack of sheets could be cut to form individual winding bodies. 
     The resultant winding body is then machined to size, as a result of which the alignment holes are removed, and the input and output termini of the individual winding patterns are revealed in the sidewall of the body. Contacts are then applied, eg, by electroless plating, to interconnect the winding patterns of individual layers, and to provide for external connection as well. Plating contacts onto the exterior surface is much simpler to accomplish than internal via plating and soldering, and occupies little space, thus maintaining the desired density and low profile of the device. 
     During machining, slots  38  and  39  are formed in two opposite sides of the winding body, of a dimension to accommodate outer legs  14  and  16  of the core body. The slots have dimensions and placement to result in a predetermined core-winding spacing d, thereby to insure a minimum distance between the interior contacts  35  and  36  and the end walls  5  and  6  of the core center leg  15 . 
     As will be appreciated from studying FIGS. 1 c  and  2 , in this embodiment each plated contact usually interconnects no more than two winding layers. These layers need not be directly adjacent to one another. 
     The completed winding body is then placed between the upstanding legs of an “E” core, in a manner to maintain the required distance d between the core legs and the body, after which the upper “I” core is glued or clamped to the lower “E” core, and the spaces between the core and coil are filled with a potting compound, eg, epoxy. 
     The completed circuit component may be mounted on a PC board as shown in FIG. 1 a , for example, by inserting the core into a cut-out in the PC board, and then soldering the component input and output contacts to pads (not shown) on the PC board. Due to the planarity and the low profile of the device including the contacts, as well as to the solder connections, significant areas of intimate contact exist between the component and the board, resulting in an enhancement of the conduction of heat from the component to the PC board. 
     The invention has been necessarily described in terms of a limited number of embodiments and variations. Other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be encompassed within the scope of the appended claims. For example, while the core has been described herein as having a rectilinear shape resulting from the assembly of “E” and “I” sections, it could also have other shapes consistent with a planar structure, such as a cylindrical shape.