Abstract:
A transformer comprises a printed circuit board having elongated conductors printed thereon, a ferrite core having a bottom mounted onto the printed circuit board and a flex circuit. The flex circuit comprises a dielectric sheet and elongated conductors printed on both faces of the sheet. The flex circuit is contoured around a top and sides of the core. The conductors of the flex circuit are surface bonded to respective conductors of the printed circuit board to form a series of primary windings and a series of secondary windings around the core. Provision of the upper portions of the windings by means of the flex circuit is economical because it does not require handling of discrete conductor portions.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to transformers, and deals more particularly with a transformer formed in conjunction with a printed circuit board. 
     Transformers are widely used today for power supplies, power converters and other circuits where electrical/ground isolation, impedance matching or voltage transformation are required. Known transformers comprise a ferrite core and primary and secondary windings wrapped around the core. Typically the core is toroidal in shape, either round or square. The primary and secondary windings are wrapped around the sidewalls of the core and either interlaced with each other or wrapped around separate sections of the core. There are various known techniques to form the windings. For example, the windings can be simple copper wires wound around the core with the free ends joined and soldered to other circuitry as required by the application. 
     U.S. Pat. No. 4,847,986 discloses another transformer which is formed with a printed circuit board. The core is mounted onto a printed circuit board. Underneath the core, elongated printed conductors form segments of respective windings. Metallic wires are contoured over the top and two sides of the core, and are wire bonded to ends of respective printed conductors to form continuous strings of primary and secondary windings which surround the core. While this technique has the advantages of low profile and conjunction with a printed circuit board, further improvements are desirable to lower the cost of construction. 
     European patent application #91309527.9 also discloses a core mounted onto a printed circuit board. Underneath the core, elongated printed conductors form segments of respective windings. Metallic strips are contoured over the top and two sides of the core, and are surface bonded to ends of respective printed conductors to form continuous strings of primary and secondary windings which surround the core. While this technique has the advantages of low profile and conjunction with a printed circuit board, further improvements are desirable to lower the cost of construction. 
     Accordingly, an object of the present invention is to provide a transformer (or inductor) formed in conjunction with a printed circuit board but with lower cost of construction than and comparable electrical and thermal performance as the foregoing prior art. 
     SUMMARY OF THE INVENTION 
     The invention resides in an electromagnetic device comprising a printed circuit board having elongated conductors printed thereon, a ferrite core having a bottom mounted onto the printed circuit board and a flex circuit. The flex circuit comprises a dielectric sheet and elongated conductors printed on the sheet. The flex circuit is contoured around a top and sides of the core. The conductors of the flex circuit are surface bonded to respective conductors of the printed circuit board to form a series of windings around the core. Provision of the upper portions of the windings by means of the flex circuit is more economical than by the discrete conductor portions of the foregoing prior art. 
     According to one embodiment of the present invention, the flex circuit comprises primary winding portions printed on one side of the sheet and secondary winding portions printed on the other side of said sheet. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a cross-sectional view of a transformer according to the present invention. 
     FIG. 2 is a top view of the printed circuit board of the transformer of FIG.  1 . 
     FIG. 3 is a top view of a flex circuit which is subsequently used to form part of the windings of the transformer of FIG. 1, but not yet bent/contoured onto the core. 
     FIG. 4 is a side view of the flex circuit of FIG.  3 . 
     FIG. 5 is a top view of the core of the transformer of FIG. 1 with the flex circuit bent/contoured thereon. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures in detail, wherein like reference numbers indicate like elements throughout, FIG. 1 illustrates a transformer generally designated  20  according to the present invention. Transformer  20  comprises a square toroidal ferrite core  22 , a printed circuit board  24  which supports the core and includes printed conductors which form part of the windings, and a flex circuit  26  which is bent/contoured over the core and includes printed conductors that form the remaining parts of the windings. 
     The square toriodal ferrite core  22  is well known in the industry and is formed by well known ceramic forming techniques such as ram pressing or extrusion followed by high temperature sintering of the ferrite material. The length and width of the core are sufficient to accommodate the required number of primary and secondary windings. The cross-sectional area of the core is sufficient to keep the flux density and heat dissipation low enough for the intended application. 
     The printed circuit board  24  comprises a metal back plate  30  (which is optional), dielectric layers  32 ,  34  and  36 , printed primary winding portions  40   a-f  and  42   a-f  (shown collectively in FIG. 1 as  40  and  42  and individually in FIG. 2) and printed secondary winding portions  44   a-c  and  46   a-c  (shown collectively in FIG. 1 as  44  and  46  and individually in FIG.  2 ). Twelve conductive vias  340  are provided through dielectric layer  34 , and openings are provided in dielectric layer  36  to allow electrical connection from pads  240  to primary winding portions  40 . Similarly, twelve conductive vias  342  are provided through dielectric layer  34 , and openings are provided in dielectric layer  36  to allow electrical connection from pads  242  to primary winding portions  42 . Four openings are provided in dielectric layer  36  to allow electrical connection from pads  244  to secondary winding portions  44 . Similarly, four openings are provided in dielectric layer  36  to allow electrical connection from pads  246  to secondary winding portions  46 . As described in more detail below, the winding portions of the printed circuit board mate with respective winding portions in the flex circuit  26  to form a set of complete primary windings and a set of complete secondary windings. A suitable fabrication process for printed circuit board  24  is described in more detail in U.S. patent application Ser. No. 08/429,612, filed by J. M. Lauffer, et al on Apr. 27, 1995 and below. This is the preferred method due to its thermal properties. However, any printed circuit board, fabricated by methods well known in the industry, may be substituted. 
     FIGS. 3 and 4 illustrate the flex circuit  26  before being pressed/contoured onto the core  22 . Flex circuit  26  comprises a central dielectric layer  50 , for example, polyimide or Teflon (trademark of E. I. DuPont de Nemours Inc.), 0.002 inches thick. Elongated copper conductors  140   a-f  and  142   a-f  are printed on a top face of the dielectric layer  50 , and elongated copper conductors  144   a-b  and  146   a-b  are printed on a bottom face of the dielectric layer  50 . By way of example, the conductors  140   a-f ,  142   a-f ,  144   a-b  and  146   a-b  are all 0.002 inches thick and standard printed circuit photo and etch operations are used to form the conductors  140   a-f ,  142   a-f ,  144   a-b  and  146   a-b  on dielectric layer  50 . After these conductors are so printed on the dielectric layer  50 , rectangular windows  440   a-b  and  442   a-b  are cut through the dielectric layer  50  by etching or laser ablation to expose conductors  140   a-f  and  142   a-f  to the underside of the flex circuit  26 . As an alternate to the window, the conductors  140   a-f  and  142   a-f  can be exposed to the underside of the flex circuit by conventional plated through holes, i.e. punched or drilled holes, an electroless plating of copper on the inner walls of the holes, an electrical plating of copper on the electroless plate and a fill of the resultant holes with solder. 
     After the flex circuit  26  is made as described above, the next step in the fabrication process is to press/bend the flex circuit  26  onto the core  22 . First, an adhesive layer, for example, thermally enhanced epoxy, is spread onto the top surface of the core  22 . Then, the flex circuit is brought into alignment with the core. Then, the flex circuit is pressed/bent over the core by a die press with for example, 20 psi pressure. Then, the die press is retracted. Because of the thicknesses and mechanical properties of the dielectric layer  50  and the conductors  140   a-f ,  142   a-f ,  144   a-b  and  146   a-b , the resultant flex circuit  26  remains formed around the top and exposed sides of the core  22  and thereafter maintains the resultant contoured shape as illustrated in FIG.  5 . 
     After the flex circuit  25  is contoured around and glued to the core  22 , the resultant core assembly  49  is surface mounted to printed circuit board  24  as follows. Solder paste is screened onto pads  240  and  242  and onto pads  244  and  246 . Then, a dot adhesive  151  is dispensed onto the printed circuit board  24  beneath the landing sites of the core  22 , the core assembly is positioned into place, and the solder paste is reflowed to connect the printed conductors  140   a-f  and  142   a-f  to the printed conductors  40   a-f  and  42   a-f  and the printed conductors  144   a-b  and  146   a-b  to the printed conductors  44   a-c  and  46   a-c . To improve thermal dissipation, the dot adhesive is a thermally enhanced type, for example, EPO-TEK H70E-4 or EPO-TEK 115-SMT from Epoxy Technology, Inc., and the conductors printed on the underside of the flex circuit dissipate more heat than the conductors printed on the topside of the flex circuit. The optional metal back plate  30  serves as a heat sink. 
     The following describes in more detail the preferred make-up of, and preferred method for making printed circuit board  24 . This type of printed circuit board technology (without the transformer) is described in pending U.S. patent application Ser. No. 08/429,612. Metal plate  30  comprises copper, copper plated aluminum, copper-invar-copper, aluminum or molybdenum-copper and is thick, for example 0.020″-0.080″. Each of the dielectric layers  32  and  34  comprises a thin, photo imageable solder mask material such as IBM ASMs, or IBM ASMDF material. The dielectric layer  32  is screen printed or coated by other conventional means on the top side of the metal plate  30 . Either dry film apply or flood screen print techniques may be used. A copper foil of appropriate thickness, for example, 2 ounce copper, is laminated to dielectric layer  32  with sufficient heat and pressure to adhere the copper foil to the dielectric layer  32 , and to provide complete cure of dielectric layer  32 . The conductors  40   a-f  and  42   a-f  are then defined in the laminated copper layer by conventional photoresist and subtractive copper etch processes. A second dielectric layer  34  is then applied to the top surface in the same manner as dielectric layer  32 . Likewise, a second copper foil of appropriate thickness is then laminated to dielectric layer  34 . Heat and pressure of this second lamination step are sufficient to flow the coated dielectric and adhere the copper foil to dielectric layer  34 , but not sufficient to cure dielectric layer  34 . Conductors  44  and  46  are then formed by conventional photoresist and subtractive copper etch processes. After defining conductors  44  and  46 , the dielectric layer  34  is photo-imaged and developed to create vias  340  and  342 , through dielectric layer  34 , exposing ends of conductors  40  and  42 . The structure is then heated at sufficient time and temperature to fully cure dielectric layer  34 . Next, protective coating layer  36  is applied in a conventional manner such as flood screen and image or pattern screen printing. Holes  240  and  242  to conductors  40  and  42 , as well as holes  244  and  246  to conductors  44  and  46 , are provided in the protective coating layer  36  by conventional means. Finishing processes such as nickel/gold plating, solder leveling, etc. may be completed in a conventional manner. Finally, the core assembly  49  is surface mounted to the printed circuit board as described above. 
     Based on the foregoing, a transformer formed in conjunction with a printed circuit board according to the present invention has been disclosed. However, numerous modifications and substitutions can be made without deviating from the scope of the present invention. For example, the number of primary and second windings are tailored to the desired application. Also, if desired, only the left or right half of the flex circuit  26  can be made and contoured over one leg only of the core  22 . Also, flex circuit  26  can be pressed over the top and two sides of a round toroidal (donut-shaped) core instead of the square toroidal core as shown using the same die, although in this configuration the flex circuit will not“hug” the lower portions of the sides of the core. Also, the same technology as illustrated for transformer  20  can be used to form an inductor. In this case however, only a single set of conductors such as  140   a-f  and  142   a-f  is printed on the flex circuit and a single set of mating conductors  40   a-f  and  42   a-f  is printed on the printed circuit board  24  and interconnected as illustrated to form one continuous set of windings. Also, other types of printed circuit boards may be substituted for the preferred type of printed circuit board with integrated thermal carrier. The inductor core may contain a physical gap in one side, or it may likewise be constructed of an integrally gapped material such as powdered iron. Therefore, the present invention has been disclosed by way of illustration and not limitation, and reference should be made to the following claims to determine the scope of the present invention.