Abstract:
The present invention provides an electromagnetic component formed from adjacent conducting layers of a multi-layer PCB and two additional conducting layers in contact with the PCB. The inventive component includes one or more winding turns formed by connecting the multiple layers of the multi-layer PCB with conductive vias and by connecting the additional conducting layers to respective top and bottom surfaces of the PCB. In one embodiment, one of the conducting layers is soldered to a top conducting layer of the PCB and the other of the conductive layers is soldered to a bottom conducting layer of the PCB, effectively increasing the cross-sectional area of the top and bottom winding layers. In another embodiment, the additional conducting layers are separated from the adjacent conducting PCB layers by a layer of insulation, permitting the additional conducting layers to form separate winding turns. The inventive winding stack can be surface mounted to a PCB, and can be used as an inductor, or in other electromagnetic devices. The winding thus constructed is capable of accepting larger currents with lower resulting temperature increases than windings formed only from PCBs, and are less expensive to manufacture than PCB-only windings.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to electromagnetic components for electric circuits, such as inductors and transformers and, in particular, to the formation of one or more winding turns of an inductor using a multi-layer printed circuit board.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electromagnetic components such as inductors and transformers have traditionally been constructed by winding one or more conductors about a cylindrical or torroidal core. This method of construction requires that a conductor, such as a wire, be wrapped around the outer surface of the core. The resulting components are expensive and time consuming to manufacture, and do not readily lend themselves to miniaturization or automated assembly.  
         [0003]     More recently, electromagnetic components have been constructed using printed circuit board (PCB) manufacturing techniques, where windings and individual winding turns are formed from one or more conducting layers patterned on the surface of an insulating PCB layer, or on one or more layers of a multilayer PCB. The use of PCB conductive traces as windings has several advantages over conventional, wound windings. First, the assembled PCB winding has a smaller mounting footprint than a conventional winding, since it does not need extra leads or soldering pads. Second, the PCB winding assembly is much simpler than conventional windings, since the winding and other components in the winding circuit of a multilayer PCB can be board mounted using the same reflow and automation processes used to mount other components. Third, a multi-layer PCB winding has improved reliability since the likelihood of shorting across adjacent turns of the winding is greatly reduced or substantially eliminated. It is a well known problem of prior art power chokes formed using layers of stacked metal foils separated by insulators that shorting between layers is much more likely to occur.  
         [0004]     In a multi-layer PCB, a PCB winding is formed from a plurality of patterned conductive traces, typically of copper, each formed on a separate insulating layer of the multi-layer PCB. Each trace forms a nearly closed typically circular pattern, so as to create the electromagnetic equivalent of one turn or loop of a prior art wire formed winding. Terminal points are formed at the ends of each trace for making connections to other traces, so as to form the individual turns of the winding. For example, the pattern can be a “C” shape with a terminal point at each of the two extreme points of the C. The PCB winding is formed by connecting the traces from different layers of the PCB through the intervening insulating PCB layers. These connections are typically plated through holes or vias in the PCB insulating layers. The traces can be connected in various ways. The traces can all be connected in series to form a winding where each trace is a separate turn of the winding. In this example, the terminal ends of each trace are offset from the traces on the adjacent levels, so that the plated through holes in each level do not intersect. Two or more traces can also be connected in parallel to decrease the impedance of a particular turn of the winding. The resultant winding is a function of the way in which the conductive traces on each layer of the multi-layer PCB are connected together and coupled to external circuits.  
         [0005]     The inductance of a winding formed using a multi-layer PCB can be increased by introducing a core of a magnetic material through an aperture formed in the PCB layers that extends through a central non-conducting region of each layer. The core is typically included as part of a housing for the multi-layer PCB winding.  
         [0006]     Conductive leads or vias are included on one or more layers of the multi-layer PCB to enable the efficient electrical connection of such components to an external circuit, for example by surface mounting and reflow soldering of the component to other components mounted on the same PCB or to another PCB having such other circuit components. This use of a multi-layer PCB to fabricate electromagnetic components results in smaller, more easily manufactured, and more reproducible components than is possible using a winding formed from a wire wrapped about a core.  
         [0007]     Windings constructed from two or more conducting layers of a multi-layer PCB have many advantages over conventional wire windings, but have problems that result from the structure of PCBs. One problem with multi-layer PCB windings results from their having thin conducting layers separated by insulating material. The high current carrying capacity required for some types of inductors, such as power chokes, can result in excessive heating and thus a reduced lifetime for the component. Current carrying capacity of the winding can be increased by increasing the number of PCB layers in the multi-layer PCB and connecting the conductive traces on these new layers in parallel with pre-existing conductive layers on other layers of the PCB, but this is an expensive option since the cost of an inductor formed in a multi-layer PCB is proportional to the number of layers and the weight of the copper used in each layer. To handle a high current of over 40 amps with a two or three turn winding with low loss, a PCB having eight to ten layers will require approximately 4 ounces of copper.  
         [0008]     What is needed is an improved winding for an inductor that is formed from a multi-layer PCB and that allows for higher current flow without a corresponding increase in temperature, or alternatively allows for fewer layers in the PCB, and which provides increased manufacturing and layout efficiencies. The resulting device should be compatible with PCB surface mounting manufacturing techniques and should be less expensive than prior art devices whose windings are formed solely from multi-layer PCBs.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention solves the above-identified problems of windings formed by multi-layer PCBs. In particular, a winding is provided for an electromagnetic component that is formed from a combination of multi-layer PCB conductive traces and two additional conducting layers, each preferably comprising a metal foil, that are adjacent to the PCB winding and electrically integrated into the winding. This combination of a PCB winding and two additional conducting layers provides for winding designs that can accommodate higher currents with greater efficiency.  
         [0010]     It is one aspect of the present invention to provide an electromagnetic component formed from a multi-layer PCB. The electromagnetic component may be an inductor, a transformer, or a like device. The PCB includes a plurality of conductive traces having a curved shape and two terminal ends. Each conductive trace is formed on an insulating layer of said PCB and is positioned with respect to the other conductive traces such that the conductive traces form a stack. A plurality of conductors are used to interconnect the terminal ends of each conductive trace to form at least one turn of a winding. A conductive layer is attached to an outer surface of said PCB in a position at the top of said stack. The conductive layer has two terminal ends and approximately the same shape as said conductive traces. An additional conductor is used to connect at least one of the conductive layer terminal ends to a terminal end of at least one of the conductive traces. A second conductive layer is attached in a similar fashion to the PCB in a position at the bottom of said stack. The second conductive layer has two terminal ends and approximately the same shape as the conductive traces. At least one second conductor is also used to connect at least one of the terminal ends of the second conductive layer to one of the conductive traces in the PCB.  
         [0011]     In one embodiment of the invention, the additional conductive layer and the adjacent conductive trace of said PCB are in conductive contact along a substantial portion of their respective surfaces as by the soldering of the conductive layer to the conductive trace. In another embodiment of the present invention, an insulator is disposed between the outer conductive trace of said PCB and the adjacent conductive layer. The conductive traces and adjacent conductive layers can be connected in various configurations, including where a plurality of conductive traces are connected by the conductors to form a first turn of the winding and wherein at least one of the plurality of conductive traces is connected by said conductors to form a second turn of said winding. Additional turns of the winding can be formed, as desired, using selected groupings of conductive traces to form the winding turns, up to a winding having a number of turns equal to the number of conductive traces and conductive layers.  
         [0012]     It is yet another aspect of the present invention to provide an electromagnetic component wherein a core is positioned in an aperture formed in the PCB such that the core is substantially surrounded by each said conductive trace and conductive layer. Specifically, each said insulating layer of the PCB defines an aperture, wherein each said conductive trace is in the shape of a loop positioned adjacent to the perimeter of a respective one of said apertures, and wherein said conductive layer is shaped to define an aperture that corresponds to the shape of the apertures formed in said insulating layers. The core is positioned in the space defined by said apertures.  
         [0013]     In a preferred embodiment of the present invention, the conductors used to connect the conductive traces to one another and to the conductive layers comprise plated through holes formed in the various insulating layers of said PCB.  
         [0014]     In another embodiment of the present invention, the electromagnetic component is formed from a multi-layer PCB having a plurality of conductive traces, a first conductive layer conductively attached to the top conductive trace, and a second conductive layer conductively attached to the bottom conductive trace. Each conductive trace is formed on an insulating layer of said PCB, has a curved shape and two terminal ends, and is positioned such that said conductive traces form a stack. A plurality of conductors are used to interconnect the terminal ends of each said conductive trace to form at least one turn of a winding.  
         [0015]     It is another aspect of the present invention to provide an electromagnetic component that conserves layout area on a PCB, is low profile and provides high power density, is compatible with printed circuit board assembly techniques, is more reliable than prior art components formed from stacked metal foils and insulators, and is less expensive than prior art devices.  
         [0016]     A further understanding of the invention can be had from the detailed discussion of the specific embodiment below. For purposes of clarity, this discussion refers to devices, methods, and concepts in terms of specific examples. It is intended that the invention is not limited by the discussion of specific embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]     The foregoing aspects and the attendant advantages of the present invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0018]      FIGS. 1A-1C  are several views of an inductor according to the present invention, where  FIG. 1A  is a side view,  FIG. 1B  is a top view, and  FIG. 1C  is a sectional view taken along the line  1 C- 1 C of  FIG. 1 ;  
         [0019]      FIG. 2  is an exploded perspective view the embodiment of  FIGS. 1A-1C ;  
         [0020]      FIG. 3  is an exploded perspective view of an embodiment of an inductor according to the present invention wherein the PCB has six layers and the inductor has two turns;  
         [0021]      FIG. 4  is a sectional view of the embodiment of  FIG. 3 ;  
         [0022]      FIG. 5  is a circuit diagram of the embodiment of  FIG. 3 ;  
         [0023]      FIG. 6  is a graph showing the effect of the addition of a copper foil layer on the temperature rise in a two-turn inductor;  
         [0024]      FIG. 7  is an exploded perspective view of an embodiment of an inductor according to the present invention wherein the PCB has six layers and the inductor has three turns;  
         [0025]      FIG. 8  is a sectional view of the embodiment of  FIG. 7 ;  
         [0026]      FIG. 9  is a circuit diagram of the embodiment of  FIG. 7 ;  
         [0027]      FIGS. 10A and 10B  are partially exploded perspective views of an exemplary PCB according to the present invention illustrating the reflow soldering process used to connect a copper foil layer to the multi-layer PCB  
         [0028]      FIGS. 11A, 11B , and  11 C are partially exploded perspective views of an exemplary PCB according to the present invention wherein the PCB has six layers and wherein two conductive layers are attached to the PCB, with  FIG. 11A  showing the conductive layers before attachment to the PCB and  FIG. 11C  showing the conductive layers after attachment;  
         [0029]      FIG. 12  is a sectional view of the embodiment of  FIG. 11 ;  
         [0030]      FIG. 13  is a circuit diagram of the embodiment of  FIG. 1 , showing an inductor winding having three turns;  
         [0031]      FIG. 14  is an exploded perspective view of an embodiment of an inductor according to the present invention wherein the PCB has four layers and the inductor winding has four turns;  
         [0032]      FIG. 15  is a sectional view of the embodiment of  FIG. 14 ; and  
         [0033]      FIG. 16  is a circuit diagram of the embodiment of  FIG. 14 .  
     
    
       [0034]     Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     To facilitate its description, the invention is described below in terms of inductors having windings whose turns are formed by traces, each of which are patterned on the surface of a different insulating layer of a multi-layer PCB, and wherein at least one winding turn includes two conductive layers that are not a PCB trace. In general, the present invention provides an electromagnetic component that is formed using a multi-layer PCB, where the component can comprise an inductor, including but not limited to power chokes, or the like.  
         [0036]     The inventive PCB winding includes a plurality of conductive layers or traces wherein each conductive trace is formed on an insulating layer of said PCB and is positioned with respect to the other conductive traces such that the conductive traces form a stack. An additional conductive layer, such as a metal foil, is attached to an outer surface of the PCB. The additional conductive layer can form a separate loop of the winding, or can be connected in parallel with a PCB layer to form a single winding loop of greater cross-sectional area. The connection of an additional conductive layer to the conductive PCB layers allows for improved performance since it enables the use of low profile multi-layer PCBs having a fewer number of conducting layers while maintaining the same or better current carrying capacity. The inventive winding can include any number of turns, as is known in the art. The scope of the invention is therefore not limited by the following embodiments and examples.  
         [0037]     The present invention will now be described in more detail with reference to the Figures.  FIGS. 1A-1C  and  2  are several views of an inductor  100  of the present invention that is shown in one example as being mounted on a separate main PCB  160 , where  FIG. 1A  is a side view,  FIG. 1B  is a top view,  FIG. 1C  is a sectional view, and  FIG. 2  is an exploded perspective view. As shown in  FIG. 10 , described below, in the preferred implementation of the multi-layer PCB inductor according to the present invention, the PCB is an integral part of the PCB used to mount and interconnect the other components of the circuit in which the inductor is one component. Thus, no separate PCB  160  is needed.  
         [0038]     The inductor  100  includes a winding  110  having one or more turns that is formed from a stack  120  of conducting and insulating elements, as described below, a housing  130 , and terminals  140  and  150  providing electrical connections from the stack to PCB  160 . Inductors according to the present invention can be incorporated into circuits, including but not limited to power converter circuits, or the like.  
         [0039]      FIG. 2  is an exploded perspective view of an inductor according to the present invention. As shown in  FIGS. 1C and 2 , stack  120  includes a multi-layer PCB  122  having a top surface  203  and a bottom surface  205 , and an adjacent layer  124 , which includes a conducting material, connected to top surface  203 . Specifically, layer  124  includes a conducting layer, preferably a copper foil. As will be discussed below, layer  124 , which may also include an insulating layer between the layer and PCB  122 , is connected to the traces in PCB  122  so as to form one of the turns in winding  110 , and thereby increase the current carrying capability of inductor  100 .  
         [0040]     An aperture  207  is formed through stack  120 , and includes a central opening through the multi-layer PCB  122  and the adjacent layer  124 . As best seen in  FIG. 1A , stack  120  also has a side  201  where terminals  140  and  150  are provided for connecting winding  110  to an external circuit. In a preferred embodiment, a second layer of conducting material, as described below, corresponding to layer  124  and also having an opening that corresponds to the dimensions of aperture  207  is connected to bottom surface  205  of PCB  122  to provide a second additional conductive layer to further enhance the current carrying capacity of winding  110 .  
         [0041]     In general, the one or more turns that form winding  110  are formed from individual or interconnected ones of conducting layers of multi-layer PCB  122  and layer  124 . Specifically, a plurality of conducting layers of multi-layer PCB  122 , the topmost conducting layer indicated as a conductive layer  211 , as seen in  FIG. 2 , and a conducting layer  124  on top of conducting layer  211  are electrically interconnected in a manner dictated by the type of winding  110  a given user desires.  
         [0042]     As shown in  FIGS. 1A and 1C , a housing  130  surrounds stack  120  and forms a core  133  that is sized to fit in aperture  207 . Housing  130  also includes an outer shell  131  having a bottom surface  139  that is designed to mount on PCB  160 . A preferred embodiment of housing  130  is shown in greater detail in  FIGS. 1C and 2  as including an upper core member  132  and a lower core member  134  that each have a central leg  136  and a pair of corresponding outer legs  138 . The central legs  136  of members  132  and  134  form core  133 , and the outer legs  138  of member  132  and  134  meet on the outside of stack  120 , to form outer shell  131  of housing  130 .  
         [0043]     An embodiment of an inductor according to the present invention formed on a six layer PCB and having two winding turns is shown in the exploded perspective view of  FIG. 3 , in the sectional view of  FIG. 4 , and in the circuit diagram of  FIG. 5 . As seen in these figures, a winding  320  includes a conducting layer  324  and a multi-layer PCB  322  that are connected to form a first turn  311  of said winding that includes four PCB layers and a second turn  313  of said winding that includes two PCB layers. Multi-layer PCB  322  has six alternating insulating layers  301  and conducting layers  303 , with layer  324  soldered to one of layers  303 , thus increasing the thickness of winding turn  313 . As illustrated in  FIG. 3  with reference to conducting layer  303   a , each conducing layer  303  has a curved portion  305  that is positioned about aperture  207 . Curved portion  305  terminates in a first end  307  and a second end  309 . Layer  324  also has a curved portion  327  that is positioned about aperture  207  and terminates at a first end  325  and a second end  326 . Ends  307  and  309  are interconnected through the insulating layers  301  in a conventional fashion by one or more plated through holes formed therein, indicated in  FIG. 3  as dashed lines. Specifically, a first plated through hole  315  connects a first subset of layers  303  and is connected to a terminal  150 . A second plated through hole  317  connects a second subset of layers  303 . A third plated through hole  319  connects a third subset of layers  303  and is connected to a terminal  140 . Each of these plated through holes is preferably formed using a large number of plated micro-vias to increase conductivity of the conductor formed between the conductive traces on adjacent layers of PCB  322 . These micro-vias may also accept solder, thereby further increasing the conductivity of the vias.  
         [0044]     More specifically, as shown in  FIGS. 3 and 4 , multi-layer PCB  322  includes: conducting layer  303   a  between insulating layers  301   a  and  301   b ; conducting layer  303   b  between insulating layers  301   b  and  301   c ; conducting layer  303   c  between insulating layers  301   c  and  301   d ; conducting layer  303   d  between insulating layers  301   d  and  301   e ; conducting layer  303   e  between insulating layers  301   d  and  301   f ; and conducting layer  303   f  on top of insulating layer  301   f . Conducting layers  303  and layer  324  are connected as follows: a first plated through hole  315  through insulating layers  301   b - 301   d  connects one end of conducting layers  303   a - 303   d , a second plated through hole  317  through insulating layers  301   b - 301   f  connects the other end of conducting layers  303   a - 303   d  to one end of layers  301   e  and  301   f , and a third plated through hole  319  through insulating layer  301   f  connects the other end of conducting layers  303   e  and  303   f . Layer  324 , which is a copper foil, is soldered directly onto conducting layer  303   f , preferably using a single reflow soldering step. Layer  324  also has a first end  325  soldered to plated through hole  317  and a second end  326  soldered to second plated through hole  319 .  
         [0045]     The conducting layers connected as described above result in a winding  320  according to the circuit diagram of  FIG. 5 , where first turn  311  is formed by conducting layers  303   a - 303   d  wired in parallel, and second turn  313  is formed by conducting layers  303   d - 303   f  and layer  324  wired in parallel. Plated through holes  315 ,  317 , and  319  are also shown schematically in  FIG. 5 , as well as terminals  140  and  150 . The additional layer  324  of turn  313  allows for this turn to accept a greater current even though only two PCB layers are used.  
         [0046]      FIG. 6  is a graph showing the variation of resistance with temperature for a two-turn PCB winding and a two-turn PCB winding having an additional copper foil layer according to the present invention. The PCB windings have a thickness of 0.3 mm, and the copper foil layer has a thickness of 0.6 mm. In general, the temperature of the winding increases with resistance, and the resistance of the PCB traces and foil combination has a lower resistance than the PCB traces alone. Since an increased resistance further increases the winding temperature due to resistive losses, the additional foil layer allows the inductor to operate at a reduced temperature increase for a given current, or to accept a larger current with the same temperature increase, thus increasing its efficiency.  
         [0047]     Specifically, the use of a 0.6 mm foil provides approximately the same inductive effect as two PCB layers. The cost of the foil layer is much less than the cost of two additional layers on a multi-layer PCB assembly, however, resulting in a significant cost saving when the copper foil is used as one turn of the winding. In addition to having a lower cost, the exemplary inductor formed from a 6-layer PCB plus a copper foil has the advantage of being able to operate at a lower temperature, for a given current, or to accept a larger current and operate at the same temperature as an 8-layer PCB inductor.  
         [0048]     Another embodiment illustrative of the many winding configurations that are within the scope of the present invention is illustrated by winding  720  which is shown in the exploded perspective view of  FIG. 7 , in the sectional view of  FIG. 8 , and in the circuit diagram of  FIG. 9 . As seen in these figures, winding  720  is a three-turn winding wherein the six layers of a multi-layer PCB  722  form two of the turns and where an additional conducting layer forms a third turn. Specifically, winding  720  includes a layer  724  and a multi-layer PCB  722  that are connected to form a first turn  711  having three traces, a second turn  713  having two traces, and a third turn  714  formed by layer  724 .  
         [0049]     Multi-layer PCB  722  has alternating insulating layers  701  and conducting layers  703 , and layer  724  includes a conducting layer  727  and an insulting layer  728 . As illustrated in  FIG. 7  with reference to conducting layer  703   a , each conducting layer  703  has a curved portion  705  that is positioned about aperture  207 . Curved portion  705  terminates in a first end  707  and a second end  709 . Conducting layer  727  also has a curved portion that is similarly positioned about aperture  207  and terminates at a first end  725  and a second end  726 . Ends  707  and  709  are interconnected through the insulating layers  701  in a conventional fashion by one or more plated through holes formed therein, indicated in  FIG. 7  as dashed lines. Specifically, a first plated through hole  715  connects a first subset of conducting layers  703  and is connected to a terminal  150 . A second plated through hole  717  connects a second subset of conducting layers  703 . A third plated through hole  719  connects a third subset of conducting layers  703 . Plated through hole  719  also connects to first end  725  of conductive layer  727 , as shown at  718   a . The second end  126  of conductive layer  727  is connected to a terminal  140 , as shown at  718   b . As in the other embodiment described above, each plated through hole in PCB  722  is preferably formed using a large number of micro-vias.  
         [0050]     More specifically, as shown in  FIGS. 7 and 8 , multi-layer PCB  722  includes: conducting layer  703   a  between insulating layers  701   a  and  701   b ; conducting layer  703   b  between insulating layers  701   b  and  701   c ; conducting layer  703   c  between insulating layers  701   c  and  701   d ; conducting layer  703   d  between insulating layers  701   d  and  701   e ; conducting layer  703   e  between insulating layers  701   d  and  701   f ; and conducting layer  703   f  on top of insulating layer  701   f . Conducting layers  703  and layer  727  are connected as follows: a first plated through hole  715  through insulating layers  701   b - 701   c  connects one end of conducting layers  703   a - 703   c  to terminal  150 , a second plated through hole  717  through insulating layers  701   b - 701   f  connects the other end of conducting layers  703   a - 703   c  to one end of layers  703   d - 703   f , and a third plated through hole  719  through insulating layer  701   e  and  701   f  connects the other end of conducting layers  703   d - 703   f . Layer  724  includes insulating layer  728  on top of conducting layer  701   f , and conducting layer  727  on top of insulating layer  728  to insulate conducting layers  701   f  and  727 . Conducting layer  727 , which is preferably a conducting layer copper foil, is connected through insulating layer  727  to conducting layer  701   f  at a first end  725  preferably by a first plated through hole  718   a . A second plated through hole  718   b  connects second end  726  to terminal  140 .  
         [0051]     The conducting layers connected as described above result in a winding  720  according to the circuit diagram of  FIG. 9 , where the first and second turns ( 711  and  713 ) are formed from the multi-layer PCB  722  and the third turn  714  is formed from the additional layer  724 . Specifically, where first turn  711  is formed by conducting layers  703   a - 703   c  wired in parallel, second turn  713  is formed by conducting layers  703   d - 703   f  wired in parallel, and third turn  714  is formed by layer  727 . Plated through holes  715 ,  717 ,  719 ,  718   a  and  718   b  are also shown schematically in  FIG. 9 , as well as terminals  140  and  150 .  
         [0052]      FIGS. 10A and 10B  provide partially exploded perspective views of an exemplary PCB assembly  800  according to the present invention illustrating the reflow soldering process used to connect a copper foil layer  810  to the multi-layer PCB  820 . As seen in  FIG. 10A , the multi-layer PCB is an integral part of a larger multi-layer PCB that includes other components, as shown at  830 , mounted thereon. The metal foil conductive layer  810  is connected to the surface of the PCB, in a position above the stack of conductive traces formed in the PCB, during a conventional reflow soldering process. As seen in  FIG. 10B , after the metal foil  810  is attached to the surface of the PCB  820  in this manner, a ferrite core and housing  840  for the inductor component is installed around the conductive traces and conductive layer, as above described.  
         [0053]      FIGS. 11A, 11B , and  11 C are partially exploded perspective views of an exemplary PCB according to the present invention wherein the PCB has six layers and wherein two conductive layers are attached to the PCB, with  FIG. 11A  showing the conductive layers before attachment to the PCB and  FIG. 11C  showing the conductive layers after attachment.  FIG. 12  is a sectional view of the embodiment of  FIG. 11  and  FIG. 13  is a circuit diagram of the embodiment of  FIG. 1 , showing an inductor winding having three turns.  
         [0054]     As seen in  FIGS. 11-13 , a winding  920  is formed by a PCB  922  and two separate conductive layers attached thereto, as shown in the exploded perspective view of  FIG. 11B , in the sectional view of  FIG. 12 , and in the circuit diagram of  FIG. 13 . As seen in these figures, winding  920  is a three-turn winding wherein the six layers CL 1 -CL 6  of multi-layer PCB  922  form three turns in conjunction with the two additional conducting layers. Specifically, winding  920  includes a first conductive layer  924 , a second conductive layer  926 , and six layers of multi-layer PCB  922 . These layers are connected to form a first turn  911  having one trace CL 1  and layer  924 , a second turn  913  having four traces CL 2 -CL 5 , and a third turn  915  formed by the bottom trace CL 6  of PCB  922  and layer  926 .  
         [0055]     Multi-layer PCB  922  has alternating insulating layers and conducting layers as described above for the other embodiments of an inductor according to the present invention. As also described above, each conductive layer is preferably connected by means of conductors formed as plated through holes in said insulators.  
         [0056]      FIG. 14  is an exploded perspective view of an embodiment of an inductor according to the present invention wherein two conductive layers are attached to the PCB and wherein the PCB has four layers and the inductor winding has four turns.  FIG. 15  is a sectional view of the embodiment of  FIG. 14 , and  FIG. 16  is a circuit diagram of the embodiment of  FIG. 14  showing an inductor winding having four turns.  
         [0057]     As seen in  FIGS. 14-16 , a winding  920  is formed by a PCB  1022  and two separate conductive layers attached thereto, as shown in the exploded perspective view of  FIG. 14 , in the sectional view of  FIG. 15 , and in the circuit diagram of  FIG. 16 . As seen in these figures, winding  1020  is a four turn winding wherein the four layers CL 1 -CL 4  of multi-layer PCB  1022  form four turns in conjunction with the two additional conducting layers. Specifically, winding  1020  includes a first conductive layer  1024 , a second conductive layer  1026 , and four layers of multi-layer PCB  1022 . These layers are connected to form a first turn  1011  having one trace CL 2 , a second turn  1013  having one trace CL 1  and layer  1024 , a third turn  1015  having one trace CL 4  and layer  1026 , and a fourth turn having one trace CL 3 .  
         [0058]     Multi-layer PCB  1022  has alternating insulating layers and conducting layers as described above for the other embodiments of an inductor according to the present invention. As also described above, each conductive layer is preferably connected by means of conductors formed as plated through holes in said insulators.  
         [0059]     The invention has now been explained with regard to specific embodiments. Variations on these embodiments and other embodiments may be apparent to those of skill in the art. It is therefore intended that the invention not be limited by the discussion of specific embodiments. It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.