Patent Publication Number: US-2005140487-A1

Title: Inductive components

Description:
This invention relates to inductive and inductive-capacitative components, such as transformers, inductors and LC-resonators, to principles of construction of such components, and to methods of making such components.  
      Transformers serve to transfer an electrical current from one circuit to another, (through an electrical induction effect between adjacent current carrying coils. Transformers can carry large currents, in which case they are often referred to as power transformers, or small currents intended to transmit signals rather than power, and such transformers are referred to as signal transformers. The present invention is concerned with all types of transformers and any other inductive components such as inductors, electromagnetic interference chokes or coreless inductive components.  
      Conventional transformer construction requires the winding of coils of wire and placing these coils adjacent one another with appropriate insulation and isolation between the respective coils.  
      It is also possible to produce windings by a printed method using multi-layer printed circuit boards (PCB). With a multi-layer PCB one needs to connect layers together using vias. The number of layers that can be incorporated into a PCB is around  12 , as it would be prohibitively expensive to add more layers than this.  
      Recently it ha been proposed to form the coils by printing serpentine conductive tracks onto a flexible, foldable substrate and then folding the substrate backwards and forwards onto itself so that the individual folds form a stack. The stack is then assembled with ferrite cores (which generally pass through preformed holes in the substrate) to form a transformer. Such transformers are known as Z-folded transformers.  
      Examples of Z-fold transformers are described in U.S. Pat. No. 4,959,630. Such transformers are manufactured from a flat, foldable substrate of insulating material on which serpentine conductive tracks are printed. After printing these tracks, the substrate (referred to as a flex strip) is folded about predetermined fold lines, so that sequential parts of primary and secondary conductive cracks overlie one another to form a stack of interleaved primary and secondary windings.  
      An advantage of Z-folded transformers over conventional wound wire transformers is that they are easy to manufacture and take up relatively little space and can be designed to have a low profile. Another advantage is that those transformers can have very high efficiency (low losses) at high frequency.  
      An advantage of Z-folded transformers over conventional layered PCB transformers is that the Z-fold system is not limited in its layers and can have buried vias to produce connections between tracks on different leaves. Connections between leaves can be made around the folds.  
      A disadvantage of known Z-folding techniques is that the area of the conductive layer on the substrate which actually conducts electricity is limited, and this limits the performance of the transformer. Other problems relate to the completion of the transformer as a stable, rigid package which is required to enable the transformer to be mounted with other components on a circuit board, and the presentation of the transformer terminals in a position where it is easy to make electrical connections to a circuit board or to other electrical components.  
      According to a first aspect of the invention, there is provided an inductive component comprising an insulating substrate with conductive tracks laid down on the substrate and covered by a layer of insulation, wherein the substrate is folded into a plurality of connected, overlapping leaves and is combined with a ferrite core to form the inductive component, and wherein parts of the tracks have conductive surfaces exposed through the insulation, which parts of the track are in electrical contact with other exposed conductive surfaces on adjacent leaves.  
      The invention also provides a flexible, foldable insulating substrate which has conductive tracks laid down on the substrate, holes through the substrate for accepting a ferrite core, and wherein parts of the tracks have exposed conductive surfaces which, when the substrate is folded into a plurality of connected, overlapping leaves, are in electrical contact with other exposed surfaces on adjacent leaves.  
      By allowing facing conductive surfaces to make contact between one layer and another, at selected positions, a much greater area of the conductive layer can participate in the conduction of current, leading to higher electromagnetic performance.  
      The substrate (flex strip) preferably used for Z-folded transformers is known as Kapton which is a flexible electrical insulating polymide film. (KAPTON is a registered trademark of E. I. Du Pont de Nemours and Company). The film is supplied precoated with a layer of conductive copper on both sides, To form the desired conductive tracks in the desired pattern, a resist is applied in an appropriate pattern to the copper surfaces to protect that part of the copper which will take part in the conduction of electricity. The unprotected part of the copper is then removed using known techniques, to leave the resist protected copper which follows a serpentine path across the substrate. Holes will be made through the substrate which line up with each other when the substrate is folded, to accept ferrite cores. The substrate is then folded on itself and combined with ferrite cores to form the transformer. To prevent there being electrical contact between tracks on surfaces which are in contact with one another after this folding, either the resist is left on the copper (if the resist is non-conductive), and/or the tracks are coated with an insulating lacquer of the like or insulator tape attached or laminated on the top of copper.  
      There are however many other methods for producing conductive tracks on a substrate. For example conductive tracks can be stamped out from sheets of conductive material and applied to tape which is then laminated onto a substrate; or printing the substrate in areas which a track is to be placed, and then electroplating a conductive layer on to the printed area.  
      The electrically conductive connection between tracks on adjacent leaves can connect the tracks on the leaves either in series or in parallel. Each track may extend across only one leaf, across a pair of adjacent leaves, or across all the leaves of a flex strip. The connections between adjacent leaves can be located anywhere on the leaves, and/or at their edges. There may be more than one connection between any particular pair of adjacent leaves.  
      According to a second aspect of the invention, there is provided a substrate for use as part of a Z-folded transformer, the substrate comprising a base web of a non-conductive plastics material; a layer of copper on at lease on face of the base web, and strips of a different plastics material along both longitudinal edges of the base web, the different plastic having a higher melting point than the material of the base web.  
      The invention also provides a method of preparing a flex strip for use in manufacturing a Z-folded transformer, the method comprising cutting the strip form a substrate as set forth above by cutting the substrate transverse to its length, to separate from the substrate a strip having a dimension which is greater in the direction transverse to the web than in the direction of the length of the web.  
      Preferably the base web is of polyester. Preferably the longitudinal edge strips are of polyimide.  
      The invention also provides a flex strip for use in manufacturing a Z-folded transformer, the strip having an elongate web of a first plastics material and, at the ends of the web, portions of a different plastics material which has a higher melting point that then material of the elongate web.  
      According to a third aspect of the invention, there is provided a transformer assembly comprising a Z-folded flex strip and a ferrite core, wherein the Z-folded strip is mounted between two ferrite bodies, and the ferrite bodies are clipped together to secure the Z-folded strip between the bodies.  
      The ferrite bodies may be clipped together by a C- or U-shaped clip which engages around both bodies, or the assembly of ferrite bodies and flex strip can be clipped into a housing which holds the components in the correct relative positions.  
      Preferably terminal ends of the flex strip wrap around the ferrite bodies and are held against a surface of one or other of the bodies by the clip or by the housing.  
      According to a fourth aspect of the invention, there is provided a Z-folded transformer comprising an insulating substrate with conductive tracks laid down on the substrate end covered by a layer of insulation, wherein the substrate is folded into a plurality of connected, overlapping leaves and is combined with a ferrite core to form a transformer, and wherein parts of the tracks have conductive surfaces exposed through the insulation, which parts of the track are in electrical contact with other exposed conductive surfaces on adjacent leaves.  
      According to a fifth aspect of the invention, there is provided a Z-folded transformer wherein a insulating substrate has a plurality of separate conductive tracks laid down on the substrate, the substrate is folded into a plurality of connected, overlapping leaves with leaves carrying one conductive track interleaved with leaves carrying another conductive track, the substrate being combined with a ferrite core to form a transformer. 
    
    
      The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  shows a substrate with a conductive track thereon, ready for folding to form a transformer in accordance with the prior art.  
       FIG. 2  is a view similar to  FIG. 1 , but showing a substrate in accordance with the invention;  
       FIG. 3  shows the substrate of  FIG. 2 , with annotations;  
       FIG. 4  is a schematic cross-section through part of a transformer formed in accordance with the invention;  
       FIGS. 5   a  and  5   b  show opposite sides of a second embodiment of a substrate in accordance with the invention;  
       FIG. 6  shows part of a third embodiment of substrate in accordance with the invention;  
       FIG. 7  shows the embodiment of  FIG. 6 , partly folded to form a transformer,  
      FIGS.  8  to  12  show further embodiments of substrates in accordance with the invention;  
       FIG. 13  illustrates a scheme for manufacturing a substrate web;  
       FIG. 14  shows a cross-section through a web manufactured in accordance with  FIG. 13 ;  
       FIG. 15  is a plan view of the web of  FIG. 14 , showing how strips will be cut from the web;  
       FIG. 16  is a side view of a transformer in accordance with the invention;  
       FIG. 17  is a side view similar to FIG,  16  of an another transformer in accordance with the invention;  
       FIG. 18  is an end view on the transformer of  FIG. 17 ;  
       FIG. 19  shows a housing for containing a transformer;  
       FIG. 20  is a cross-section through a transformer housed in the housing of  FIG. 19 ;  
       FIGS. 21 and 22  show alternative constructions in which transformer components are secured together;  
       FIGS. 23 and 24  show two alternative ways in which the transformer terminals can be brought to the same face of the completed transformer;  
       FIG. 25  is a perspective view of the transformer of FIG,  23 ;  
       FIG. 26  shows an outline of a flex strip;  
       FIG. 27  shows the arrangement of terminals on a ferrite body;  
       FIG. 28  is an exploded view of a transformer in accordance with the invention;  
       FIG. 29  shows the transformer of  FIG. 28  assembled;  
       FIG. 30  is an underneath view both the transformer or  FIGS. 28 and 29 ;  
      FIGS.  31  to  35  show views similar to those of FIGS.  26  to  30 , but of an alternative embodiment;  
       FIG. 36  shows an alternative form or flex strip according to the invention;  
       FIGS. 37 and 38  show different interleaving arrangements for Z-folded transformers;  
       FIG. 39  shows a flex strip prior to folding;  
       FIG. 40  shows the strip of  FIG. 39  after a first stage of folding;  
      FIGS.  41  to  45  show sequential stages in the assembly of a transformer from the strip of  FIGS. 39 and 40 ;  
       FIG. 46  is a perspective, exploded view of a Z-folded transformer winding with a two-part bush for the ferrite core hole;  
       FIG. 47  shows a section through the assembled transformer winding of  FIG. 36 ;  
       FIG. 48  shows a flex strip in accordance with the prior art;  
       FIGS. 49 and 50  show alternative flex strip layouts;  
       FIG. 51  is an exploded view of a transformer with a ferrite core and a strip as shown in  FIG. 49  or  50 ;  
       FIGS. 52 and 53  show two further alternative flex strip layouts;  
       FIG. 54  shows a side view of a flex strip support;  
       FIG. 55  and  56  show the support of FIGS.  54  after a flex strip has been mounted on the support;  
       FIGS. 57, 58  and  59  show details of the assembly of a transformer according to the invention;  
       FIGS. 60, 61  and  62  show details of the assembly of another transformer according to the invention;  
       FIG. 63  shows and interleaving component, in the flat state;  
       FIGS. 64   a ,  64   b ,  64   c  and  64   d  show respectively copper and resist layers applied to opposite sides of a substrate; and  
       FIG. 65  shows a typical cross section through the substrate of  FIG. 64 . 
    
    
       FIG. 1  shows a simple prepared substrate which comprises a Kapton sheet which has a rectangularly outline  10  (indicated by dotted lines). Initially, substantially the entire surface of each face of the sheet has a coating of electrically conductive copper applied to it. To prepare the sheet, the copper is selectively removed, for example by an etching process. The copper track left once the substrate has been prepared is indicated at  12 . Fold lines in the substrate are indicated at  14  and holes through which a ferrite core will be positioned in the finished transformer (the holes will all register with one another once the substrate has been folded on itself) are shown at  16 . It will be seen that (a) more than 50% of the initial copper coating has been removed to form the serpentine track  12 , and that (b) the width of the track  12  is greater at the positions where it crosses the fold lines  14  tan it is between the fold lines.  
      This transformer component is also sometimes referred to as the ‘flex strip’.  
       FIG. 2  shows a flex strip, modified in accordance with the first aspect of the invention. It will be seen that additional areas of copper  12   a  are present on the strip  10  to almost completely encircle the core positions  16 . In order to bring those areas  12   a  into the electrical circuit, at the positions marked X (see  FIG. 3 ), conductive surfaces are exposed so that, when the flex strip is folded on itself, the two areas X marked A make electrical contact with one another and the two areas X marked B also make electrical contact with one another. As a result, the area of conductive copper forming the track is doubled.  
       FIG. 4  shows a cross section through a folded flex strip at one of these positions X.  FIG. 4  shows only two leaves, but in practice there will be a much greater number overlying one another and forming a stack of leaves. The thickness of the copper layers  12  are very much exaggerated in the Figure, to explain the construction.  
      Each of the flex strip leaves  10  has a copper track  12  on each side. In the Figure, the copper tracks on the upper and lower side of each leaf are all coincident, but this does not have to be the case. The copper tracks  12  are protected by an insulating layer  10  which can be a layer of solder resist. At the point X, the solder resist is removed on both upper and lower tracks, and electrical conductive contact is made between the tracks. To achieve this contact, solder may be flowed over the areas where resist has been removed. The surface of the copper tracks may be electroplated with tin to ensure good contact between the solder and the copper.  
      In this way, a larger area of copper track is available to carry current around the ferrite core which will be fitted through the holes  16 . This will improve the transformer capacity and will also mechanically stabilise the winding stack.  
       FIGS. 5   a  and  5   b  show two opposites sides of a flex strip. The strip is generally indicated at  30  and has hoes  32  through which the ferrite core will be passed. The continuous area indicated by reference numeral  34  is all copper coated, with the copper only being removed in the areas indicated by  36 . Copper is also removed in the areas marked  38 , as these areas are coincident with the desired fold lines and this will therefore assist in ensuring that the flex strip folds at the correct positions. After folding, electrical contact can be achieved in the appropriate areas of two facing surfaces of the leaves. One pair of facing surfaces has marks ‘X’ to show where contact can be made.  
      In the embodiment shown, it is necessary to add some insulation in the shaded areas  39 , to prevent shorting out between the conductive tracks on outside folds.  
      In comparison with  FIG. 2 , the embodiment of FIGS.  5  has a much greater conductive area.  
       FIG. 6  shows how a single turn, centre tapped winding can have the turns connected in parallel using this method. The Figure shows a flex strip  20  which has holes  22  for receiving a ferrite core, fold lines  24  and printed copper tracks  26 . The copper tracks are shown in the Figure with rectangular enlargements  28  at the ends and the middle of each track, where they cross the fold lines  24 .  
       FIG. 7  shows the strip of  FIG. 6  partly folded. No tracks are shown on the reverse side of the strip, but tracks can be and are likely to be provided there also. It will be seen from this figure that when the strip is folded, the holes  22  come to lie above on another so that a ferrite core can be inserted though them, and that the enlargements on the next adjacent fold line. By removing the insulating covering on the tracks at the fold lines, the tracks can be electrically connected to one another at these points. Thus all the tracks  26  (which in the flat state of  FIG. 5  are independent of one another) are connected with each other in parallel, with the centre enlargement  28  forming a centre tap. The position on the fold lines where contact is made between adjacent turns does not have to be enlarged as shown; the width of the track could be uniform all along its length and the necessary contact could still be made.  
      It is also possible with this arrangement to make contact between adjacent tracks at some of the fold lines but not at others.  
      In another arrangement ( FIG. 8 ) the same principle is used to connect windings in series. This arrangement has the advantage that a spiral track can be formed on each folded section of the substrate, thus allowing a greater number of ‘turns’ per ford in the finished transformer.  
      In  FIG. 8 , the flex strip  40  has holes  42  for the ferrite core and, on each leaf of the strip a spiral conductive track is formed. On the leaf  40   a , the track  44  starts from a terminal  46  and ends at a point  48  on the leaf  40   a . On the next track  40   b , the spiral starts from the point  50  and then follows a spiral path outwards and then crosses onto the next leaf  40   c  where the track continues on a spiral path, now spiralling inwards to finish at a point  52 . One the leaf  40   d , the track extends from a point  54  to another terminal  56 . At lease on of the facing faces of the strip will be coated with a resist, so that they do not short out between the coils on facing surfaces. However the resist will be removed at points  40 ,  50 ,  52 ,  54 .  
      When this flex strip is folded up, about the fold lines indicated by dotted lines, contact will be made between the track on leaf  40   a  and the track on leaf  40   b , because the pints  48  and  50  will come into contact with one another and will, in the manner shown in  FIG. 4 , be brought into electrical contact with one another. The same happens between the points  52  and  54  on the leaves  40   c  and  40   d . Alternatively, contact may be achieved by ‘bumping’ the points  48  and  50  or  52  and  54  so that they extend slightly out of the plane of the strip, to come into contact with one another. This will assist in ensuring that the thickness of the resist does not prevent the desired contact.  
      In this way a single winding is effectively formed throughout the whole of the transformer, and it is now possible to place multiple turns on each of the strip, and then to connect the turns on one strip with the turns on another strip, so that all the turns are in series. This may be particularly useful for an inductor with many turns.  
       FIG. 9  shows an arrangement similar to that in  FIG. 8 , but this time there are two coils formed on the same flex strip  60 . The track  62  forms half a turn on each leaf  60   a ,  60   v  to  60   f . The track  64  forms two turns on the leaf  60   a , one turn on the lead  60   d , one turn on the leaf  60   e  and half a turn on the leaf  60   f . Where there is more than half a turn on a leaf, then the turns are connected to the turns on the adjacent leaf by making electrical contact in the folded strip such as takes place between points  66  and  68 , and between points  70  and  72 .  
      In  FIG. 10  an arrangement effectively the same as that in  FIG. 8  is shown. However, in this case the tracks are spiral wound, and the connections are made between the points A-A and B-B.  
      In  FIG. 11 a  more complex mixture of track patterns is illustrated. One track  82  extends the full length of the flex strip, similar to the track  62  of  FIG. 9 . However, the coil  84  takes the form of a spiral on the leaf  80   b , half a turn across the next leaf  80   c  and a spiral on the leaf  80   d . When the flex strip is folded, there will be contact between the coil  84  where it appears on the leaf  80   d  and a coil  86  where it appears on the leaf  80   c  (B-B). Similarly there will be contact between the coil  84  where it appears on the leaf  80   b , and another coil  85  on the leaf  80   a  (A-A). Thus the current path travels (always in the same rotational direction) spiralling in and out on different leaves and upwards and downwards between the leaves in the completed transformer.  
      It will be clear that the ability to form in this way a via between tracks on two adjacent surfaces allow a very wide variety of different turn patterns to be produced.  
       FIG. 12  shows one other embodiment of the invention. In this embodiment an additional area of copper is left on the flex strip at  90 ,  92 . This area of copper takes no part in the electrical characteristics of the finished transformer (it is isolated from the current path), but instead these two areas  90 ,  92  are set up so that they can be soldered to one another in the finished strip to physically fix the leaves of the strip in their folded configuration.  
      Instead of the rectangular area shown in  FIG. 12 , it might for example be possible to leave a small area of copper at each corner of each leaf of a rectangular flex strip, with the copper at these areas being exposed so that they can be joined, for example soldered, to corresponding areas on the adjacent leaf in the manner shown in  FIG. 4 .  
       FIG. 13  shows a possible construction for a substrate to be used in a Z-folded transformer. The substrate is formed as a continuous web, with a polyester ‘core’  100  provided on each side with a layer of copper  102 , 104  (which will form the tracks  12 ,  26 ) and along each edge, a ribbon  106  of a polyimide material such as Kapton. The completed laminate is indicated at  108 .  
      Flex strips  10 ,  20  will be cut transversely from this web (see  FIG. 15 ), such that they have polyimide  106  at the two ends of the strip (from which connections will be made to external circuit components) and low cost polyester for the major part of the structure. Polyester is considerably cheaper than polyimide and has better moisture absorption characteristics than polymide. However, polyester does not have the necessary heat resistance to allow external components to be soldered to tracks on the substrate, and the higher grade polyimide material is advantageous at points where soldering is to take place.  
      The polyimide films may have adhesive coatings to attach them to the polyester.  
      Another problem inherent in Z-folded transformers is that of completing the assembly as a rigid component which can be mounted for example on a circuit board, or to which other components can be attached.  
      In the following FIGS.  16  to  47 , various constructions are shown in which a folded transformer substrate and associated ferrite cores are joined together to form a component which can be regarded as a single rigid unit and which can therefore be easily mounted on a circuit board in the same way as a conventional wirewound inductive component.  
       FIG. 16  shows a side view of an assembled transformer with a Z-folded flex strip  120  sandwiched between two ferrite bodies  122 ,  124 . Although not shown in this Figure, the ferrite bodies will have projections which extend through holes in the flex strip  120 , as previously described.  
      The ferrite bodes  122  and  124  are solid, rigid bodies. Two end terminal portions  126  and  128  of the conductor formed on the flex strip  120  are brought out from where the flex strip is folded on itself and exposed on the lower face of the body  124 , as indicated at  126  and  128 . By mounting these two end portions on the rigid surface of the body  124 , terminal  126  and  128  are rigidly fixed in space. The terminals  126 ,  128  can be glued or otherwise fixed in place of the body  124 , but in  FIG. 17  an additional component in the form of a clip  130  is applied to the assembly. The clip  130  engages around both the ferrite bodies  122 ,  124   a  to hold the two bodies together and to hold the flex strip in its folded configuration between the bodies. Additionally the clip  130  retains ends  125   a ,  128   a  of the flex strip  120  so that these extremities of the strip are held captive.  FIG. 18  shows this in end view, where the clip  130  has flanges  132  which engage over the top of the ferrite body  122 , and a continuous limb  134  which extend beneath the body  124   a.    
      In this embodiment the body  124   a  is shaped with bulbous projections from its lower surface around which the terminal ends ( 126   a ,  128   a ) of the flex strip are passed. The actual terminal of the transformer, for connection to other components, will be formed at the lowermost parts of these bulbous projections.  FIG. 17  shows how the tree ends of those terminal portions  126   a ,  128   a  are trapped beneath a continuous lower limb  134  of the clip  130 .  
       FIGS. 19 and 20  show another embodiment where a substantially continuous housing  140  is used in place of a clip  130 . FIGS.  19  shows the housing before the transformer components have been mounted inside it, and  FIG. 20  is a section through the housing with a transformer in place.  
      The housing has resilient lugs  142  near its top edge, end cut outs  144  and base slots  146 . The transformer to be mounted within this housing is similar to that depicted in  FIG. 16 , and the same reference numerals will be used for its components.  
      The size of the housing  140  is such that when the transformer is completely inserted, the top surface of the upper ferrite body  122  will snap beneath the lugs  142 , and thus the lugs will effectively keep the ferrite bodies pressed into the housing and will keep the flex strip  120  compressed between them.  
      The terminal ends  126 ,  128  of the flex strips pass out of the housing through the cut-outs  144  at each end, pass around the outer surface of the housing and then back into the housing through the slots  146 . Once they are back inside the housing, the upper ferrite body can be finally snapped down to trap the free ends of the terminals  126 ,  128  between the lower ferrite body and the base  148  of the housing. It will be noted in this Figure that the bulbous shape present on the lower ferrite body  124   a  of  FIG. 17  is in this case provided by shaping of the housing  140 , rather than shaping of the lower ferrite body.  
       FIG. 21  shows a similarly arrangement where a transformer consisting of ferrite bodies  150 ,  152  and flex strip  154  is snap-fitted into a rectangular box  156 . The box  156  has lugs  158 , and when the transformer is correctly inserted, the lugs  158  will be accommodated within a slot  160  on the upper face of the upper ferrite body  150 . The manner in which the terminal ends  162  of the flex strip are connected to external components is not illustrated in this Figure.  
       FIG. 22  shows a sequence of steps in the assembly of a transformer in accordance with the invention. In  FIG. 22   a , the three main components are shown in exploded view, namely the upper ferrite body  150 , the Z-folded flex strip  154  and the lower ferrite body  152 . In this Figure it can be seen that the lower ferrite body has core pieces  164  which extend up from the internal face of the body and which will extend through holes in the folded flex strip (e.g. the holes  22  in  FIG. 7 ). These core pieces may extend right through the folded flex strip to meet a flat lower face of the upper body  150 , or the upper body  150  may have similarly core pieces and the core pieces of upper and lower bodies may meet half way through the folded flex strip.  
       FIG. 22   b  shows the three components assembled together with a terminal end  162  of the flex strip  154  exposed. Although not shown in this Figure there will be a similar exposed terminal end at the opposite (hidden) end of the assembly. This terminal end is folded under the lower ferrite body  152  ( FIG. 22   c ) in a manner similar to that shown in  FIG. 16 . In order to retain the assembly in this condition, a U-shaped clip  166  engages in corresponding grooves  168 ,  170  on the upper and lower faces of the bodies  150 ,  152 .  FIG. 22   e  shows the finished condition of the transformer.  
       FIGS. 23 and 24  show two alternative configurations in which both ends of the flex strip are brought out of the component on the same face, for connection to external terminals. In these figures, the ferrite bodes  322  are shown spaced apart, to enable the folding pattern of the flex strips  410 ,  412  to be displayed.  
      In  FIG. 23 , the points of the strip  410  to which connection will be made are in the form of ‘bumps’  411 ,  413  pressed out from the plane of the strip. The bumps are initially pressed out in opposite directions, and then when the strip is folded, the bump  413  projects through a hole  415  in the strip, so that both the ‘bumps’ are projected in the same direction. Tracks at one edge of each leaf are in electrical contact (at  419 ) with tracks on an end region  417  of the strip  410 .  
      In  FIG. 24 , a different folding pattern is shown for the strip  412 . Here the ‘bumps’ are at opposite ends of the strip, but are presented adjacent one another after folding of the strip.  
      It will be clear that the pattern of tracks on the strip will be set up so that any desired conductive array can be produced, with the tracks, and their terminals, being arranged as desired across the width of each face of the strip.  
       FIG. 25  shows an underneath view of the component of  FIG. 23 .  
       FIG. 36  shows a modified flex strip  200 . This strip  200  is similar to the strip  20  of  FIG. 6 , but has termination areas  202 . When the strip  200  is folded, the areas  202  will make contact (as described with reference to  FIG. 6 ) with the enlargements  23 . Because the terminations areas  202  are relatively large and uniform in shape, they can present a large area to which external connections can be made.  
      FIGS.  26  to  30  and FIGS.  31  to  35  show two alternative arrangements whereby the terminal ends of a flex folded transformer (with centre taps) can be reliably and accurately connected to for example a printed circuit board.  
       FIG. 26  shows a flex strip  170  with termination areas indicated at  172  at both ends and at positions alongside each elongate edge of the strip.  FIG. 26  shows the strip  170  before it is folded, with some of the lines along which it will be folded indicated at  175 .  FIG. 27  shows the underside of the lower ferrite core once the strip has been folded up and mounted between upper and lower ferrite bodies. It will be seen that the terminal portions  172 ,  174 ,  176 ,  187  are now presented, spaced apart from one another on the lower face of the body  152 . Next, the assembled transformer is inserted into a housing  180  which has a ball grid array in the base, so that the terminals  172 ,  174 ,  176  and  178  make contact with respective ones of the balls.  FIG. 30  shows the underside of the housing  100  with a 4×4 array of balls. For example the terminal  178  may be in contact with the four balls  172   a , the terminal  172  may be in contact with the four balls  178   a , the terminal  174  may be in contact with the four balls  174   a  and the terminal  176  may be in contact with the four balls  176   a . Appropriate solder connections will be made within the housing between the terminals and the balls. Once the component is assembled, ie, in the condition indicated in  FIG. 29 , it can be accurately soldered to a circuit board by defining which of the balls in the array should be soldered to which connections on the circuit board.  
       FIG. 31  shows a similar arrangement but in this case the balls  190  are soldered to the terminals, and there are only balls present where there are connections to be made. This  FIG. 35  shows an array similar to that of  FIG. 30  but with only some of the ball sites populated. Final connections need to be made only between the populated ball sites and the appropriate connections on the circuit board.  
      When the flex strip is folded, it can be folded with or without interleaving.  FIG. 37  shows a non-interleaved folding pattern with ferrite cores  201 , 203  where all of one set  204  of folded leaves are above all of the other set  206  of folded leaves.  FIG. 3A  shows a fully interleaved folding pattern, where a pair of folded leaves from one set  204  are interleaved between each pair of folded leaves from the other set  206 . The ferrite cores are then pressed together to close up the Z-folded leaves.  
       FIG. 39  shows a flex strip  201  which can be folded to from a transformer. The strip has primary winding sections  212  at one end of the strip and secondary winding sections  214  at the other end of the strip. The PCB terminations  216  are located at the middle section. The winding patterns are designed so that the windings start and end at the termination area  216 .  
      The flex  210  is Z-folded as shown in  FIG. 40 . The primary section has a region  218  where leaves are spaced apart and into which the secondary winding stack will be later interleaved. FIGS.  41  to  45  show the assembly of this strip into a transformer. In  FIG. 41 , the termination area  216  is placed against (under) a ferrite bottom plate  220 . The winding sections  212 , 214  are flipped above the ferrite bottom plate. ( FIG. 42 ) and interleaved with each other ( FIG. 43 ). A ferrite top plate  222  is attached with glue, clips or similar ( FIG. 45 ).  
      One attractive benefit of this construction is that there are no ‘ends’ to glue or lock. The ferrite itself, when the two halves are fixed in place, holds the assembly fully captive with the free ends of the flex strip effectively trapped. The location of the termination parts  216  beneath the ferrite bottom plate  220  makes it easy to attach the components to other connections, for example on a PCB.  
      Interleaving of primary and secondary windings can be designed more flexibly than with conventional Z-folded transformers where primary and secondary layers follow each other (complete interleaving) and interwinding capacitance may become excessive. In forward converters two to three times interleaving may be the best choice as a comprise between leakage inductance and interwinding capacitance.  
      A single sided flex strip can be used. Static shields can be placed on the opposite side using a thin copper layer.  
       FIG. 46  shows Z-folded secondary transformer windings  226  together with a two-part bushing or bobbin  224   a ,  224   b . The two halved of this busing fit into a hole  225 , from above and below, and clip together to form a lining to the hole  225 , and to provide a hole  228  through which a ferrite core  230  can be inserted. The bushing  224  builds extended creepage paths  232 ,  234  (for example these paths may be 6 mm long) between primary and secondary windings.  
      The use of the bushing  224  enables certain minimum creepages and clearances to be met. Assume the tracks of the primary winding are close to the ferrite centre pole. Then a minimum creepage distance must be maintained from the centre pole to the secondary winding, and this can easily be achieved along the labyrinthine passage  232  illustrated in  FIG. 47 , and over the flanges  238  of the bushing, as shown at  234 . The bushing will preferably be made of two plastics moulded parts.  
      Alternative winding patterns for Z-folded transformers in accordance with the invention are shown in  FIGS. 49, 50 ,  52  and  53 . With the winding layouts shown here no solder resist, lamination or the insulation is required on the flex strip after arrangement of the tracks and prior to Z-folding.  
       FIG. 48  shows a conventional winding pattern with a track  300  on a flex strip  302  with holes  305  for a ferrite core, and fold lines  304 . An insulation layer (solder resist, paint, epoxy, tape etc.) is needed to prevent the conductor tracks  300  from short circuiting between each other when the strip is folded.  
       FIG. 49  shows an alternative conductor pattern on a flex strip  302  with two rows of core holes  305 . This strip can be Z-folded on the fold lines  304  without any copper conductors being shorted together because the winding turns are completed on Z-folded sections  306  which are separated by the insulating substrate. Folding on the three fold lines  304  produces a concertina with two holes for ferrite cores but no shorted tracks. It is envisaged that this would fit with a two-part ferrite  308  with two centre poles  310 ,  312 , as shown in  FIG. 51 .  
      When folded, the tracks always go clockwise round one pole and anti-clockwise around the other, so that the magnetic circuit is complete. For example, the flux direction can be downwards on one limb and upwards on the other, thus allowing the flus to circuit.  
      The strip of  FIG. 50  has a basic conductor pattern which corresponds to that of  FIG. 49 , but which has added to it additional copper conductor areas  301  as described with reference to  FIGS. 2 and 3  of this specification. When the strip is folded about the lines  304 , there will be contact between points on the track, such as the points A-A and B-B, as described in connection with  FIG. 3 . The extra conductor parts  301  will contact the original conductor track  301  to increase copper thickness.  
       FIGS. 52 and 53  show conductor patterns with four core holes  305  on each leaf. The arrangement of  FIG. 52  will, when folded, produce four turns around the cores in the holes  305 . The arrangement of  FIG. 53  applies the principles of  FIG. 50  to the strip of  FIG. 52 .  
      It is also possible to make on or more longitudinal folds after the transverse folding, so, for example,  FIGS. 49 and 50  would have three transverse folds  304  and a final longitudinal fold  314 . This would leave one single central hole in the stack for a conventional centre pole. The advantage of this is that no insulation is required on the top of the copper to prevent shorting of tracks. The strips of  FIG. 52  and  53  could also be folded longitudinally. It is quite possible also to do the longitudinal folding first, then the transverse folding, though, in these cases, the extra copper segments  301  in  FIGS. 50 and 53  would need to be in different places.  
      Using the winding patterns of  FIGS. 49-50  or  52 - 53  allows the use of a substrate with no additional insulation which is easier to manufacture and has a lower cost.  
      FIGS.  54  to  57  are schematic illustrations of different types of termination which will enable Z-folded transformers to be mounted on a circuit board using conventional mounting technology.  
       FIG. 54  shows a header  500 , typically moulded from an insulating plastics material, with terminal pins  502  standing up from the header. The pins  502  extend beneath the header in a configuration which either is designed for surface mounting technology (full lines) or for through-hole mounting technology (dotted lines). The header is necessarily of insulating material, and is present to ensure that the finished, Z-folded component has a rigid structure.  
       FIG. 55  shows a Z-folded strip  508  mounted on the header, and with electrical terminations between conductive tracks on the strip and the terminal pin  502   b , at  504 . The strip  508  can have holes which can be placed over the pins  502 . Some of the holes may be conductive tracks surrounding them, so that electrical connection can be made between the strip and the tracks, for example by soldering. Other holes may just provide a physical location to ensure that the strip is correctly positioned on the header. To complete the component, a ferrite body has to be mounted, and this will extend over the part of the flex strip indicated by the double-headed arrow  506 . This length corresponds to the reduced thickness area  509  of the header. In order to minimise the height of the finished component above the circuit board on which it is to be mounted, the ferrite fits partly underneath the header, and the header should be as thin as possible at this point, consistent with providing the necessary insulation.  
      It is also possible ( FIG. 56 ), to make electrical connections between tracks on the strip and the terminal pins  502  by butting parts of the strips against the pins and then applying solder to make the connection. This is shown at  510 .  
       FIG. 57  shows a transformer with features as shown in FIGS.  54  to  56 , partly assembled. The conductive tracks  512 , a centre hole  514  for a ferrite core and holes  516  for fitting over the pins  502  can all be seen in this Figure. The strip  508  is Z-folded to form the compact package shown in  FIG. 58 , and this package is fitted to the header with the holes  516  fitting over the pins  502 . A two-part ferrite core  518   a ,  518   b  with a central core  522  is then applied to the assembly with the core  522  passing through the hole  514 , to form a finished product as shown in  FIG. 59 . The two halves of the core can be held together, for example by gluing.  
      The upper leaf  520  of the strip shown in FIGS.  57  to  59  has no conductive tracks. This leaf is folded over on top of the stack, and forms an insulating layer between tracks on the leaf below, and the surface of the ferrite.  
      FIGS.  60  to  62  show a similarly construction, but this time the Z-folded strip is interleaved with other, conducting, leaves  540 . These leaves  540  are stamped from copper sheet and initially have the form shown in  FIG. 63 . They are folded about a fold line  542 , before being interleaved with the flex strip  508 . The leaves  540  will make electrical contact with the posts  502 , as can be seen in particular with  FIG. 6 . Using interleaved conductors in this way expands the range of possible ‘winding’ and tapping configurations which can be achieved.  
       FIG. 62  shows an underneath view of this finished component, with the pins  502  set in positions appropriate for surface mounting the component on a circuit board. This figure also shows recesses  544  in the header  500 , adjacent the position where the ferrite body  518   a  will fit. As a final step in production, these recesses will be filled with a settable compound, such as an epoxy resin, to firmly unite the header  500  with the ferrite body  518   a , to impart rigidity.  
       FIG. 65  shows four views of the same flex strip  600 , before folding. The fold lines are indicated at  602 .  FIG. 65   a  shows the pattern for copper conductors  604  on the top face.  FIG. 65   b  shows the pattern of copper conductors  606  on the bottom face.  FIG. 65   c  shows the pattern of resist  608  to be applied over the copper on the top face, and  FIG. 65   d  shows the pattern of resist  610  to be applied over the copper on the bottom face.  FIG. 66  shows an exemplary section through the strip and illustrates the relative positions of the layers.  
      It will be noted that the fold lines in some cases cross copper areas of the copper tracks, and in some cases cross non-copper areas. Where the fold lines cross copper areas, the copper track will be exposed at the edge of the fold, so that electrical connections can be made at that point. For this purpose, the resist is removed where such connections are to be made, and this can be seen for example by comparing the region circled at A in  FIG. 65   b  with the region circled B in  FIG. 65   d . The resist coating is interrupted in areas where it is not required, so as to avoid unnecessarily increasing the thickness of the folded strip.