Abstract:
An inductive device includes a magnetic core, a conductive winding surrounding the core, a conductive element formed on a selected portion of the surface of the magnetic core, and a termination of the winding mechanically attached and electrically connected to the conductive element. 
     In an assembly that includes a circuit board the conductive element is mechanically attached and electrically connected to a connection pad on the board and the winding termination is connected to the conductive element.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to inductive devices having conductive areas on their surface. 
     A typical inductive device is formed by winding conductive wire around the body of a magnetic core or around a bobbin supporting a magnetic core. Transformers, for example, have primary and secondary windings surrounding the body of the core. The terminations of the primary and secondary windings are connected to input and output circuits, respectively. When used in an electronic circuit, a transformer performs the function of stepping up or down an input voltage and providing an output with a required voltage, frequency, and phase. 
     In a typical electronic assembly the winding terminations of inductive devices are inserted into holes in the printed circuit board and soldered. Electronic components in a typical electronic assembly are often mounted on the surface of a printed circuit board by an automated assembly process. To permit surface mounting of an inductive device the core with the windings typically is attached to a structure (e.g. a box or a frame). The winding terminations are attached to features on the exterior of the structure (contacts or leads), which in turn are attached to a printed circuit board. The structure and interposing attachment features occupy additional volume which would otherwise have been available for circuit elements. The shape of the core used for an inductive device also affects the space otherwise available for other circuit components. Typical inductive devices use cylindrical or ring-shaped annular cores. These toroidal structures do not fit well with the other mostly square electronic components on the printed circuit board. Inductive devices with non-toroidal cores exhibit flux leakage and demagnetization due to their geometry. A more rectangular core shape is shown for example in U.S. pat. No. 5,546,065. That patent describes the use of conductive shields on the surface of the magnetic core to control leakage inductance. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect, the invention features an inductive device that includes a magnetic core, a first conductive winding surrounding the core, a first conductive element formed on selected portion of a surface of the magnetic core, and a first termination of the winding mechanically attached and electrically connected to the first conductive element. 
     Implementations of the invention may include one or more of the following features. The device may include a second conductive element electrically isolated from the first and a second termination of the primary winding mechanically attached and electrically connected to the second conductive element. A second conductive winding may also surround the core, and two additional electrically isolated conductive elements may be formed on selected portions of a surface of the magnetic core, to which may be connected the two terminations of the second winding. 
     In general, in another aspect, the invention features an inductive device assembly that includes a circuit board bearing a first connection pad, a magnetic core, a first conductive winding surrounding the core, and a first conductive element formed on a selected portion of the surface of the magnetic core. The conduction element is mechanically attached to and electrically connected to the connection pad and a first winding termination is mechanically attached and electrically connected to the first conductive element. 
     Implementations of the invention may include one or more of the following features. The windings may be formed from metallic wire, metallic foil, or metallic film lines deposited on the surface of the magnetic core. The conductive element may include layers of a silver-filled epoxy, copper and tin. The magnetic core may have polygonic outside and/or inside perimeters and flat top and bottom surfaces. The dimensions may be chosen to maintain a generally constant cross-sectional area of the core. The core may be a ferrite or iron powder, and may include an electrical insulation layer. The electrical insulation layer may be a para-xylylene polymer. 
     In general, in another aspect, the invention features a method of making an inductive device by covering a selected area of a magnetic core surface with a conductive element, winding a conductive winding around the core and attaching a termination of the conductive winding to the conductive element. 
     In general, in another aspect, the invention features a method of making an inductive device assembly by forming a connection pad on a circuit board, covering a selected area of a magnetic core surface with a conductive element, winding a conductive winding around the core and attaching mechanically and connecting electrically the conductive element to the connection pad on the circuit board. A termination of the winding may also be mechanically attached and electrically connected to the conductive element on the surface of the core. 
     Implementations of the invention may include one or more of the following features. The winding terminations may be mechanically attached and electrically connected to the conductive areas by soldering or thermal compression bonding. The covering of the selected surface areas of the magnetic core with the conductive element may include gravure printing of a silver epoxy, electroplating of copper and electroplating or immersion plating of tin. The inductive device may be connected to the printed circuit board by soldering the conductive surface areas of the core to the contacts on the board. The inductive device may also be attached and connected to the board connection pads via a conductive adhesive. 
     Among the advantages of the invention may be one or more of the following. The invention integrates and combines the function of conductive magnetic flux shields, winding terminations and device mounting contacts on the surface of a magnetic core. The device may be mounted on a printed circuit board by attaching the mounting contacts to the board connection pads, a process suitable for automation and compatible with surface mount printed circuit board technology. In another aspect, an inductive device may be provided, which incorporates windings, winding terminations and mounting contacts on the surface of a magnetic core with any desired geometric configuration. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 and 2 are a perspective view and an exploded perspective view, respectively, of a transformer mounted on a printed circuit board. 
     FIG. 3 is a cross-sectional view at  3 — 3  on FIG.  1 . 
     FIG. 4 is a top view of the magnetic core. 
     FIG. 5 is a bottom view of the transformer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a transformer  12  is mounted directly on a top surface  25  of a printed circuit board  10 , with other electronic components (not shown). The transformer  12  includes a primary winding  16   a , a secondary winding  16   b , and metal shields  18   a  through  18   d , formed on the surface of a one-piece annular ferrite core  11 . As seen in FIG. 4, the shape of annular core  11  is defined by a square inner peripheral wall  21 ; an octagonal outer peripheral wall  22  that has four segments  22   a ,  22   b ,  22   c , and  22   d  parallel to the four sides of inner wall  21  and four segments  24   a ,  24   b ,  24   c , and  24   d  that “cut off the corners” of the outer wall; and top and bottom surfaces  27  (FIG. 2) and  29  (FIG.  5 ), respectively. The shields  18   a  through  18   d , respectively, cover the top  27  and bottom  29  surfaces and segments  24   a  through  24   d  of the outer wall at the four quadrants of the core, leaving the inner wall  21  and gaps  23   a  through  23   d  uncovered. The geometry and placement of the shields are chosen so that they serve as magnetic flux shields, to reduce demagnetization and flux leakage occurring at the sharp edges and corners of the core. 
     The shields also provide attachment points  20   a  through  20   d  (FIG. 5) for winding terminations  19   a  through  19   d , respectively. The terminations  19   a  and  19   b  of the primary winding  16   a  are soldered to the bottom of the adjacent shields  18   a  and  18   b  at attachment points  20   a  and  20   b , which are on the bottom surface  29  of core  11  (FIG.  5 ). Similarly, the terminations  19   c  and  19   d  of the secondary winding  16   b  are soldered to the adjacent shields  18   c  and  18   d  at attachment points  20   c  and  20   d  at the bottom of core  11 , respectively. The shields  18   a  through  18   d  also provide connection surfaces  17   a  through  17   d  for mounting the transformer  12  on the surface  25  of the board  10  via solder connections  15   a  through  15   d  (FIG. 2) to board connection pads  14   a  through  14   d , respectively. 
     Referring to FIG. 3, an insulating layer  13  covers the entire surface of the magnetic core  11 . The windings  16   a  and  16   b  also have an insulation layer  30  and together with the shields  18   a  through  18   d  lie on the insulating layer  13  of the core. The insulating layer  13  has uniform thickness, covers both the flat surfaces and sharp edges and corners of the core, insulates even at low thicknesses, and can withstand high operating temperatures. The geometry and dimensions of the inner and outer peripheral walls  21 ,  22  are chosen to maintain a generally constant cross sectional area at all positions around the core  11 . Referring to FIG. 4, the cross sectional areas along the lines  4 ,  5  and  6  are approximately equal to each other. In one example, the transformer has outer dimensions  30 ,  32  of 0.211″×0.2″, inner dimensions  34 ,  36  of 0.07″×0.07″ and a height  38  (FIG. 3) of less than 0.07″. 
     To make the transformer, the core is first coated with para-xylylene polymer by thermal polymerization to a thickness of about 0.5 mils. The shields are then formed. The shields comprise several layers, including silver-filled epoxy, copper, and tin. The silver-filled epoxy is deposited with a thickness in the range of 0.1 to 0.3 mils by gravure pad printing on the insulating layer  13 . Copper is electroplated to a thickness of about 2 mils on the silver-filled epoxy. Tin is electroplated on the copper with a thickness in the range of 0.25 to 0.5 mils. The windings  16   a ,  16   b  are then wound on the coated and shielded core  11 , the wire insulation  30  is removed from the terminations  19   a  through  19   d , and the terminations are soldered to the shields  18   a  through  18   d , respectively. The finished transformer is mounted on the printed circuit board by soldering the shields  18   a  through  18   d  to the connection pads  14   a  through  14   d  of the board, via the surface contacts  17   a  through  17   d  and solder contacts  15   a  through  15   d , respectively. 
     Other embodiments are within the scope of the following claims. For example, the same techniques could be used for any kind of inductive device, including inductors and chokes, with any number of windings and any number of turns in each winding. The windings may be formed using material other than wound wire, such as metallic foil or metallic film. Other shield patterns may be used. The core could be made of pressed iron powder and may have a different geometry, including toroidal and bar type. Paraxylylene could be replaced by other insulating materials. The wire winding terminations could be attached to the shields by thermal compression bonding. Tin may be deposited by immersion plating. The inductive device could be attached to the board contacts via a conductive adhesive.