Abstract:
An improved planar magnetic structure in which the voltage gradient between core and windings is reduced by shields disposed between the one or more legs of the core and the windings and extending through the PWB layers; vias are offset to permit them to be contained within the path of the winding; and the induced magnetic and eddy currents intrinsic to interstitial shield layers are reduced by configuring the shield conductors with pairs of courses with opposite and offsetting current propagation.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an improved planar structure with reduced voltage gradient between windings and core, compact placement of vias within the winding path, and improved shields to reduce induced magnetic fields. 
       BACKGROUND OF THE INVENTION 
       [0002]    Using planar magnetics allows the reduction in height of magnetic components and increase in power density for state-of-the-art DC/DC converters. However, conventional structures suffer from excessive copper losses and rely on the increased spacing between windings and the core to prevent corona inception and insulation breakdown. This conventional approach has several problems. 
         [0003]    Corona discharge and eventual insulation breakdown can be caused by voltage concentration across the air gap between the magnetic core and the printed wiring board (PWB). Insulation that supports AC voltages includes air (the gap between the core and the edge of the board) and solid material inside the PWB. When voltage is applied across two dissimilar materials such as air and a solid dielectric, material with the lower permittivity (air) will receive higher stress. The fact that voltage breakdown of air is sensitive to changes in humidity and altitude farther complicates this problem. In addition, all air gaps in the planar assembly can fluctuate due to assembly tolerances. 
         [0004]    Interconnect vias increase component area. Individual winding turns and sections located on different layers are connected by PWB vias placed outside the immediate winding path. This arrangement requires additional area and increases winding resistance. 
         [0005]    Added capacitance and increased winding losses can be caused by electrostatic shields. The shields reduce coupling between transformer windings thereby reducing common-mode noise currents. However, they increase transformer capacitance and eddy current losses. 
       SUMMARY OF THE INVENTION 
       [0006]    This invention features a planar magnetic structure including a printed wiring board having at least one winding segment, an inner clearance through the printed wiring board within the winding segment, and at least one outer clearance through the printed wiring board external to the winding segment. There is a core having an inner leg extending through the inner clearance and at least one outer leg extending through the outer clearance defining a gap occupied by the winding segment. An inner shield is disposed between the inner clearance and the inner core leg. The inner shield surrounds the inner leg but is less than one turn defining a shield gap. The shields reduce the voltage gradient between the core legs and the winding segment. There is at least one outer electrostatic shield between the outer clearance and the at least one core outer leg, the outer shield is disposed between the outer leg and inner leg and a guard barrier proximate the shield gap and between the shield gap and the winding segments reduces the voltage gradient between the inner shield end at the gap and the winding segment. 
         [0007]    In preferred embodiments there may be at least two outer clearances, two outer core legs and two outer shields. The printed wiring board may have a number of winding segments in a stacked array and the clearances, core legs and shields may extend through the printed wiring board coextensive with all of the number of winding segments. The shields and the core legs, and the guard barrier may be at the same, fixed voltage potential. The fixed potential may be ground. The winding segments may form the windings of a transformer. 
         [0008]    This invention also features a planar magnetic structure including a printed wiring board having a plurality of layers, a core having a central leg and at least one external leg spaced from the central leg and extending through the layers of the printed wiring board and a winding segment on each layer, each winding segment having a generally spiral path about the central leg between the central leg and the one or more external legs. The winding segments are connected together from layer to layer. There are a plurality of vias extending through the layers within the boundaries of the generally spiral path. Each of the winding segments except the last winding segment has its output connected to the input of the next winding segment through a via which is within the boundaries of the generally spiral path and the vias unconnected at any particular winding segment passing through that winding segment without electrical contact. 
         [0009]    In preferred embodiments the spiral path may be curvilinear. The spiral path may be rectilinear. All of the winding segments may be wound in the same direction. All of the winding segments may be wound in the same direction alternately inwardly and outwardly. All of the winding segments may be wound in the same direction alternately outwardly and inwardly. The winding segments may be connected in series. The vias may be offset with respect to one another within the boundaries of the generally spiral path. The vias may be offset longitudinally along the direction of the generally spiral path. The vias may be offset laterally in the generally spiral path. The winding segments may have a whole number of turns. The windings segments may have a fractional number of turns. 
         [0010]    This invention also features an electrostatic shield for a multilayer electronic device including at least one interstitial shield layer and a shield on the shield layer including a serpentine conductor made of a series of courses, each pair of courses in the serpentine conductor propagating current in opposite directions for offsetting the induced magnetic fields and resulting currents. 
         [0011]    In preferred embodiments the serpentine conductor may be arranged in a circumferential path of less than one turn. The courses may extend radially. 
         [0012]    This invention also features an electrostatic shield for a multilayer electronic device including a first set of conductors including at least two spaced courses and a second set of conductors including at least two spaced courses interdigitated with the first set of conductors; each of the conductors including a barrier section which separates the courses of the other set of conductors and is connected to a fixed potential. 
         [0013]    In preferred embodiments the courses may be curvilinear. The courses may be rectilinear. The courses may be less than one turn. The device may include a magnetic structure having a core and the courses may surround the core. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
           [0015]      FIG. 1  is an exploded three dimensional view of a full stacking of primary, secondary and shield layers of a planar transformer constructed according to the teachings of the present invention in which connecting vias are not shown for ease of illustration; 
           [0016]      FIG. 2  is a circuit diagram of the transformer of  FIG. 1 ; 
           [0017]      FIG. 3  is a three dimensional rear view showing in greater detail, a single layer of the transformer of  FIG. 1  illustrating the layer winding path, leg and center clearance hole edge plating and the construction of unconnected and global ground barrier vias according-to the teachings of the present invention; 
           [0018]      FIG. 4  illustrates in greater detail, the edge plating for the center, left and right ferrite core clearance holes of  FIG. 1  according to the teachings of the present invention; 
           [0019]      FIG. 5  is an exploded three dimensional front view of the upper shield layer and top four layers of the primary winding of the transformer of  FIG. 1  illustrating interconnecting and global ground vias constructed according to the teachings of the present invention; 
           [0020]      FIG. 6  is a two dimensional top view of the primary winding and shield layers of the transformer of  FIG. 1  showing the winding path details, vias and clearance hole of each such layer constructed according to the teachings of the present invention; 
           [0021]      FIG. 7  is a two dimensional top view of the secondary winding and shield layers of the transformer of  FIG. 1  showing the winding path details, vias and clearance holes of each such layer constructed according to the teachings of the present invention; 
           [0022]      FIG. 8  illustrates a serpentine conductor pattern for the shield layers of the transformer of  FIG. 1  constructed according to the teachings of the present invention; 
           [0023]      FIG. 9  illustrates an interdigitated conductor pattern included in the shield layers of the transformer of  FIG. 1  constructed according to the teachings of the present invention; and 
           [0024]      FIG. 10  illustrates an alternative interdigitated conductor pattern for use in the shield layers of the transformer of  FIG. 1  constructed according to the teachings of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
         [0026]    There is shown in  FIG. 1  an embodiment of this invention in a transformer  10  having a primary winding  10   a  and secondary winding  10   b  constructed on a multi-layer circuit board  12  formed of a stacked array of twelve layers  14 ,  16 ,  18 ,  20 ,  22 ,  24 , 26 ,  28 ,  30 , 32 ,  34 . Layers  14 - 24  are associated with primary winding  10   a  while layers  26 - 34  are associated with secondary winding  10   b.  Layers  14 ,  24 ,  26 , and  34  are shield layers and contain on them shields  36 ,  38 ,  40 , and  42 , respectively. Layers  16 - 22  are winding layers and contain winding segments  44 ,  46 ,  48 , and  50 , which combine to form the primary winding  10   a,  layers  28 - 32  are winding layers containing winding segments  52 ,  54 ,  56  and  58  which are associated with secondary winding  10   b.  Shield layers  14 ,  24 ,  26 , and  34  and their associated shields  36 ,  38 ,  40 , and  42  conventionally reduce coupling between transformer windings thereby reducing common mode noise currents. However, these shields in accordance with this invention additionally decrease transformer capacitance and eddy current losses intrinsic to conventional shields and are discussed more fully in  FIGS. 8-10 . Each transformer core  60  includes an upper section  62  and lower section  64 . Core  60  includes an inner leg such as formed by center leg sections  66  and  68  and at least one external leg such as formed by external leg sections  70 ,  72 . There may be a number of external leg sections in addition such as formed by external leg sections  74  and  76 . Each layer,  14 - 34  includes an inner clearance hole  80  and outer clearance holes  82  and  84  which have been numbered only on layer  14  for the sake of clarity. Inner clearance holes  80  accommodate the center leg formed by center leg sections  66  and  68  while the external clearances  82  and  84  accommodate the external legs formed by external leg sections  70 ,  72 , and  74 ,  76 , respectively. To reduce the voltage gradient and therefore the probability of voltage breakdown or corona discharge between the inner leg sections  66 ,  68  and the winding segments  44 - 50  and  52 - 58  on layers  14 - 22  and  28 - 32 , respectively, a shield  90  is plated on the inside of clearances  80  and shields  92  and  94  are plated on the inner walls of clearances  82  and  84 . Shield  80  reduces the voltage gradient between center leg section  66 ,  68  and winding segments  44 - 50  and  52 - 58 , while shields  92  and  94  reduce the voltage gradient between windings  44 - 50  and  52 - 58  and the external core leg sections  70 ,  72  and  74 ,  76 , respectively. 
         [0027]    An electrical schematic circuit of transformer  10  is shown in  FIG. 2  where the fundamental electrical nature of transformer  10  can be more easily seen. Note that the four winding segments  44 ,  46 ,  48  and  50  which constitute primary winding  10   a  result in seven turns, as too with the four secondary winding segments  52 ,  54 ,  56  and  58 . This is so because each winding segment  44 - 58  constitutes 1.75 turns. The configuration of outer shields  92 ,  94  and inner shield  90  which reduces the voltage gradient between winding segment  44  and the core legs which normally are present in clearances  80 ,  82 , and  84  that are omitted for clarity is shown to better advantage in  FIG. 3 . Shield  90  for example, plated about clearance  80  reduces the voltage gradient between the inner edge of winding segment  44  and the inner core leg normally present in clearance  80  in the gap indicated at  100 . Shields  92  and  94  likewise reduce the voltage gradient between the outer edges of winding segment  44  and the outer core legs normally present in clearances  82  and  84  at gaps  102 . In accordance with this invention inner shield  90  surrounds the inner core leg but stops short of completely surrounding it in order to avoid presenting a completed, shorted turn. For this reason shield  90  contains a gap  104 . In creating this gap  104 , however, to prevent a shorted turn, a secondary area near the edges of the gap where a voltage gradient is high is now created. Note that winding segment  44  typically contains a high voltage for example 1,000 or 1,500 volts as opposed to the core. To reduce steep voltage gradients and to prevent a breakdown a guard barrier  106  is provided near gap  104 , between it and the inner edge of winding segment  44 . Thus, the voltage will be applied between the inner edge of winding segment  44  and guard barrier  106  while the voltage between guard barrier  106  and gap  104  will be minimal. Because the guard barrier radius is greater that that of the edge of the gap  90  the steep voltage gradient between the inner edge of winding segment  44  and the gap  90  will be reduced. Guard barrier  106  is typically a via as shown more clearly in  FIG. 4  where all parts have been removed with the exception of shields  90 ,  92 ,  94  and guard barrier  106 . 
         [0028]    In another aspect of the invention,  FIG. 5 , the winding segments and their associated via which interconnect them are kept compact employing a minimum amount of area on the printed wiring board layers. In conventional structures the vias are often grouped in the area of the wiring board between the winding segment conductor turns see U.S. Pat. No. 6,847,284,  FIG. 51 . However, in this invention the vias are always confined within the boundaries of the path of the winding segment. In this case the winding segment has a generally spiral path which is curvilinear. It may as well be rectilinear or any other shape and it need not necessarily be spiral. In the embodiment of this invention as shown in  FIG. 5 , the guard barrier via  106  does not interconnect with any winding segment. The other three vias  110 ,  112 , and  114  do. The remaining via in  FIG. 5  is a global ground via  116 , for example, and there may be more than one of these. There will typically be a number of local vias  110 ,  112 ,  114 , which is one less than the number of winding segment layers. In this case since there are four winding segment layers there are three local vias,  110 ,  112 ,  114 . 
         [0029]    The continuity of the winding segments  44 - 50  and their interconnection using vias  110 ,  112  and  114  are shown to better advantage in  FIG. 6 . There each winding segment  44 - 50  is shown having a generally spiral path about the center leg of the core and confined between the central leg and the one or more external legs which are not shown in  FIG. 6 . The winding segments are connected together from layer to layer using the vias; there are a plurality of vias  110 ,  112 ,  114 . These vias are confined within the boundaries of the generally spiral path of the winding segments. Each winding segment except the last winding segment has its output connected to the input of the next winding segment through a via which is again within the boundaries of the generally spiral path. The vias unconnected at any particular winding segment pass through that winding segment without electrical contact. Although the spiral paths of the winding segments in  FIG. 6  are shown as curvilinear they may be rectilinear or may take other shapes. 
         [0030]    All of the winding in  FIG. 6  are wound in the same direction but alternately inwardly and outwardly (or they could be wound outwardly and inwardly). The winding segments are connected in series through the vias which are offset with respect to one another within the boundaries of the generally spiral path of the winding segments. The vias may be offset within the spiral path either longitudinally along the spiral path or laterally to the general path. Each winding segment may have an integer or whole number of turns or may be fractional turns. Thus far in this embodiment each winding segment has a length of 1¾ turns but of course this is not limiting to the invention. 
         [0031]    In  FIG. 6  the vias shown by a single circle in a winding segment indicate vias which are electrically connected to that winding segment while vias indicated by a double circle indicate vias which are not electrically connected to that winding segment. Current is introduced into winding segment  44  at input end  120  and propagates through it in a counterclockwise direction, as indicated by the arrows, passing via  114 , which is not electrically connected, and via  112  which is not electrically connected, until it reaches the output end and via  110  which is electrically connected. From via  110  the current moves down to the input  124  of winding segment  46  where it again moves counterclockwise past via  112 , to which there is no electrical connection, and then to via  114  which is electrically connected to output end  126 . From output end  126  the current moves down via  114  to the input end  128  of winding segment  48  where again it moves in a counterclockwise direction past via  110 , to which it is not electrically connected, until it reaches the output  130  and then passes through via  112  which is electrically connected. Via  112  is connected to the input  132  of winding segment  50 , the current then continues to move past via  110  and  114 , neither of which is electrically connected, to the output end  134 . 
         [0032]    Similarly, with respect to secondary winding  10   b,    FIG. 7 , winding segments  52 ,  54 ,  56  and  58  are interconnected by a second set of vias  140 ,  142 ,  144 . Current introduced at the input end  146  of winding segment  52  moves counterclockwise past vias  144  and  146 , with no electrical contact, to the output end  148  where there is contact with via  140 . The current moves down via  140  to the input end  150  of winding segment  54 . It then moves past via  142 , without electrical contact, and then makes contact with via  144  at output  152 . Via  144  makes contact with input  154  of winding segment  56  and the current moves past via  140 , without electrical contact, and makes electrical contact at output end  156  with via  142 . The current moves into input end  158  of winding segment  58  and moves past vias  140  and  144 , without electrical connection, to reach the output end  160 . 
         [0033]    In another aspect of the invention interstitial shields  36 ,  38 ,  40 ,  42  may be formed on a shield layer with a serpentine conductor made of a series of courses each pair of courses in the serpentine conductor propagating current in opposite directions for offsetting the induced magnetic fields and resulting currents. The serpentine conductor may be arranged in a circumferential path of less than one turn. The courses may extend radially. Such a device is shown in  FIG. 8 , where shield  170  is formed from a serpentine conductor  172  formed from a plurality of courses  174  which run alternately in opposite directions. In this case they extend radially and the current in one course, for example, course  174   a  may run radially outward and in the next one  174   b  radially inward. In the next course  174   c  the current will run radially outward and so on. In this way the oppositely directed currents produce offsetting magnetic fields and resulting currents. The shield may take other forms such as indicated by shield  170   a ,  FIG. 9 , which is an interdigitized shield which has two sets of conductors including at least two spaced courses. The first set of conductors  190  includes two spaced courses  192  and  194 . The second set of conductors  196  includes a first course  198  and second course  200 . Each of the courses  192 ,  194 ,  198 , and  200  stop short of forming a complete turn, to prevent a shorted turn. Each set of conductors  190 ,  196  includes a barrier section  197 ,  199  which separates the courses of the other set of conductors and is connected to a fixed potential. Although the shield in  FIG. 9  is shown as a curvilinear arrangement of courses this is not a necessary limitation of the invention for as shown in  FIG. 10 , the courses may be arranged in a rectilinear fashion as well, for example. 
         [0034]    Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
         [0035]    In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
         [0036]    Other embodiments will occur to those skilled in the art and are within the following claims.