Conductive film magnetic components

A conductive film magnetic component such as an inductor or transformer includes a conductive film winding having a generally serpentine configuration when disposed in a plane. This film is folded to form a stack of layers with each "layer" comprising part of a winding turn and with successive "layers" connected at the folds via the continuous conductive film. The conductive film may be self-supporting and coated with a dielectric layer or may be disposed on a dielectric membrane. The film and membrane are preferably patterned photolithographically.

FIELD OF THE INVENTION 
The present invention relates to the field of magnetic components, that is, 
inductors and transformers, and more particularly, to the field of 
conductive film magnetic components. 
BACKGROUND INFORMATION 
As the frequency of operation of a magnetic component such as an inductor 
or transformer increases, the depth to which current penetrates the 
transformer's conductors decreases. This penetration depth is referred to 
as the "skin depth". At room temperature, copper has a skin depth .delta. 
equal to 2.60.sqroot.1/f where f is in Hertz. Thus, at a frequency of 
about 1 megahertz, the current penetration in copper is only on the order 
of 2.6 mils. Consequently, if the conductors are more than several skin 
depths thick, then any portion of the conductors which is further than 
3.delta. from the exterior surface is not involved in carrying the 
inductor or transformer currents. For high frequency operation, magnetic 
components are made as small as possible and, therefore, inactive 
conductive material adds to the weight and volume of the component without 
enhancing its operational characteristics. Consequently, for high 
frequency magnetic components, it has become commonplace to use a planar 
conductive film having a thickness on the order of twice the skin depth at 
the intended operating frequency as the magnetic component's conductors. 
These conductive films are normally disposed on a dielectric membrane and 
patterned to provide the desired winding configuration. Multi-turn 
windings normally comprise either a single layer spiral or a stack of 
layers of individual conductive films on dielectric substrates which are 
interconnected layer-to-layer with soldered connecting bars to provide a 
continuous winding. Spiral windings are limited in the number of turns 
they can provide for high currents and multilayer windings have the 
disadvantage of requiring a number of layer-to-layer connections which 
increases with the number of turns in a winding. Connecting thin 
conductive layers layer-to-layer with connecting bars which are soldered 
to the edge of a conductor is an exacting process which tends to have a 
poor yield since the solder can easily short out layers or fail to connect 
to a layer, which results in an inoperative winding. 
Consequently, there is a need both for an improved method of fabricating 
multi-turn, multilayer thin film magnetic windings and for an improved 
structure which ensures good operating characteristics and which is easily 
fabricated. 
OBJECTS OF THE INVENTION 
A primary object of the present invention is to provide a thin film, 
multi-turn magnetic component winding structure which ensures uniformity 
from magnetic component to magnetic component. 
Another object of the present invention is to provide a method of 
fabricating a thin film, multi-turn multilayer winding in a simplified 
manner which obviates the need for individual connections separately 
applied to each layer. 
Another object of the present invention is to provide an easily fabricated 
and assembled conductive film transformer structure. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a multi-turn, multilayer thin 
conductive film magnetic component winding comprises a conductive film 
having a pattern which when folded upon itself, produces a multi-turn 
winding in which the layer-to-layer connections are built into the pattern 
of the conductive film in combination with the pattern of the folding of 
that film. Dielectric material spaces apart adjacent layers of the folded 
film conductor to prevent unintended connections between the different 
layers. 
A multilayer, two-turn, center-tapped secondary winding is preferably 
provided by patterning a conductive film on a dielectric substrate in 
manner in which folding the dielectric substrate creates a stack of 
two-turn centertapped windings which are easily connected in parallel by 
soldering external terminals to exposed folds of the conductive film. 
Alternatively, a secondary winding may comprise a stack of individual 
conductive films.

DETAILED DESCRIPTION 
Each winding of a transformer is itself an inductor. The two (or more) 
windings of a transformer are coupled by their mutual inductance. Thus, 
each of the windings described herein may serve as a stand alone inductor 
if it is not coupled to other windings or may serve as one of the windings 
of a transformer if it is inductively coupled to one or more other 
windings. The term "magnetic component" is used herein as encompassing 
inductors, transformers and other components which include a winding. 
Different embodiments of windings in accordance with the present invention 
have been given reference numerals in different blocks of one hundred. 
Elements identified by reference numbers which end in the same two digits 
in different embodiments serve similar functions and in the interest of 
conciseness, are sometimes not discussed in connection with the later 
presented embodiments. In those situations, the reader is referred to the 
discussion of those elements in connection with earlier presented 
embodiments for an explanation of the element's function and structure. 
A conductive film winding in accordance with the present invention is 
illustrated generally at 10 in FIG. 1 in a plan view prior to folding to 
form the multi-turn winding. Structurally, as illustrated, the winding 
comprises a dielectric membrane 12 having a plurality of apertures 14 
therein and having an undulatory exterior boundary 16. A number of dashed 
lines 18 and 19 indicate the locations at which the membrane 12 is 
accordion or z-folded to form the winding. These fold lines are at the 
narrow portions of the membrane 12. A conductive film 20 disposed on the 
upper surface of the membrane 12 is of uniform width, has a terminal end 
22 at the bottom of the figure and meanders the length of the substrate 12 
in a serpentine manner to weave the conductive film alternately to the 
left of and then to the right of the successive apertures 14 in the 
substrate 12. This uniform width results in a current density within the 
film which is constant along the length of the film. The film 20 is 
illustrated with bold boundaries to indicate its location on top of the 
dielectric membrane or substrate 12. 
A similar conductive film 30 having the same uniform width as film 20 is 
disposed on the lower surface of the dielectric membrane 12 and meanders 
in a serpentine or woven pattern around the apertures 14 in the membrane 
in the opposite sense to that in which the film 20 meanders. The film 30 
is shown in dashed lines because it is behind the membrane 12. Film 30 has 
a terminal end 32 at the lower portion of the figure. The terminal ends 22 
and 32 may be made wider than the remainder of the films 20 and 30 if that 
is considered beneficial in connecting the winding to other circuitry. 
At the upper end of the figure, a connection 28 is provided between the 
film 20 and the film 30. The connection 28 may take any one of a number of 
forms. It is at this time considered preferable to solder, to both the 
upper conductive film 20 and the lower conductive film 30, a thin 
conductive foil which is folded on itself and which is the same thickness 
and width as or slightly wider than the films 20 and 30 in order to 
provide a continuous winding of uniform width. However, other techniques 
may be used if desired. These include, without limitation thereto, (1) 
depositing or laminating the films 20 and 30 as a single continuous film 
which is continuous at the connection 28 and (2) forming a via hole or 
holes through the dielectric membrane 12 at the location of the connection 
28 and plating that through hole to connect the two conductive films 20 
and 30. 
With the connection 28 in place, the two films 20 and 30 form one 
continuous, uniform width, conductive strip having ends 22 and 32. As will 
be seen in the subsequent figures, the pattern in which the films 20 and 
30 are disposed with respect to the apertures 14 and the manner of folding 
the membrane result in a single continuous winding 10 which encircles the 
apertures 14 in a manner in which, after folding, current flowing in the 
film from terminal 22 to terminal 32 continuously circles the apertures 14 
in the same direction. 
The winding 10 may be folded so that the terminals 22 and 32 are either at 
the top of the stack or the bottom of the stack. The following portion of 
this discussion assumes that winding 10 is folded with the terminals 22 
and 32 at the top of the stack with conductor 20 exposed on top of 
membrane 12. This may be accomplished by z-folding the winding 10 at the 
fold lines 18 and 19 by pulling the fold lines 19 upward from the plane of 
the paper and pushing the fold lines 18 down below the plane of the paper 
in a manner to make an accordion-pleated stack of the segments of the 
winding 20 which are disposed between adjacent fold lines thereby forming 
a "stack" of winding layers. It is preferred to not fold the winding at 
the fold line 18 which is directly adjacent to the terminal ends 22 and 32 
so that the terminal ends will protrude beyond the side of the stack of 
layers to facilitate formation of external electrical connections thereto. 
The winding 10 differs from prior art conductive film windings in that no 
connections between adjacent layers of the winding need to be made after 
"stacking" of the layers. Current flowing in film 20 from terminal 22 to 
connection 28 circles the apertures 14 (the axis of the winding) in a 
clockwise direction, when viewed from the top of the stack, as shown by 
the arrows in FIG. 2, as a result of the manner of folding and the manner 
in which serpentine film 20 meanders among the apertures 14. The 
serpentine film 30 meanders in the opposite sense, which combined with 
flow of current from connection 28 toward terminal end 32 also results in 
that current circling apertures 14 in a clockwise direction when viewed 
from the top of the stack. 
The conductor 20 of winding 10 is shown in perspective view from the top of 
the stack in a partially folded form in FIG. 2 to more clearly illustrate 
the manner in which folding of the conductive film of FIG. 1 results in a 
multi-turn winding which encircles a central core 24 in a single 
(clockwise) direction. In the event that these figures do not enable the 
reader to visualize the winding in its folded form, we suggest that the 
reader photocopy FIG. 1 and cut out the illustration along the outer 
boundary of membrane 12, cut or punch out the apertures 14 and fold this 
"paper doll" at the fold lines 18 and 19 to form the winding. This will 
enable the reader to visualize the configuration of the winding in its 
completed form. 
Prior to folding the dielectric membrane 12 and the accompanying films 20 
and 30, it is considered preferable to dispose dielectric layers over the 
exposed surfaces of the films 20 and 30 so that when folded as a stack, 
the conductive films do not form unintended electrical connections. The 
dielectric overlayer also provides additional environmental protection for 
the conductive film which helps to prevent chemical reactions between the 
film and the ambient environment in which it is used. 
The winding 10 has been described as being comprised of the dielectric 
membrane 12 with the conductive films 20 and 30 disposed thereon. However, 
if desired, the conductive films 20 and 30 may comprise a single, 
continuous, self-supporting conductive film which has twice the length of 
the film 20 and a pattern in which, prior to folding, the serpentine 
configuration of the film 20 continues uninterrupted at the location of 
the "connection 28". The film in that configuration may then be coated 
with a dielectric layer as by dipping or spraying and after the dielectric 
layer has dried, may be folded at the connection 28 into the configuration 
illustrated in FIG. 1. Thereafter, it may be folded in an accordion 
fashion as has been described to provide a single winding with both its 
terminals at the same end of the multilayer stack. Alternatively, rather 
than folding such a long serpentine winding in the middle at the 
"connection 28" prior to the remainder of the folding, the entire winding 
may be accordion folded to place the two terminal ends of the winding at 
opposite ends of the completed stack. Finally, the long conductive film of 
this configuration may be disposed on a self-supporting dielectric 
membrane and folded in either of the manners just described. 
Thus, either the dielectric membrane 12 may be a self-supporting layer or 
the conductive film may be self-supporting or both as may be desired in 
accordance with the desired thicknesses of the conductive film and the 
dielectric membrane. 
The winding 10 of FIGS. 1 and 2 may serve by itself as an inductor for use 
at high frequency and is capable of carrying high currents. Alternatively 
and preferably, the winding 10 may serve as one winding of a transformer 
for use at high frequencies. Where a 1:1 transformer ratio is desired, a 
second similar winding may be interleaved with the winding 10 to serve as 
the other winding of the transformer. Alternatively, the films 20 and 30 
may each be divided into two films by lengthwise extending gaps 21 and 31 
to form films 20a and 30b and 30a and 30b as shown in FIG. 3. Films (20a 
and 30a) and (20b and 30b) form two separate windings 10a and 10b, 
respectively, having terminal ends (22a and 32a) and (22b and 32b), 
respectively. These windings 10a and 10b may serve as the primary and 
secondary, respectively, of a 1:1 turns ratio transformer. Alternatively 
the windings 10a and 10b may be connected in series by connecting terminal 
end 22b to terminal end 32a to thereby provide an overall winding having 
twice as many turns, but half the width and thus half the current carrying 
capacity of winding 10. 
Where a high turns ratio is desired, the winding 10 preferably serves as 
the winding having the larger number of turns and a separate winding 
having a different configuration serves as the winding having the smaller 
number of turns. 
A center tapped, multilayer, secondary winding 40 in accordance with the 
present invention in which each half of the secondary winding comprises a 
single turn is illustrated in plan view in FIG. 4 prior to folding. After 
folding, this winding may be interleaved with the winding 10 of FIG. 1 to 
form a step-down transformer having a centertapped secondary winding. 
Those skilled in the transformer art will recognize that either winding 10 
or winding 40 may serve as the primary, but that normally in a transformer 
having one two-terminal winding and one center-tapped (three-terminal) 
winding, the center-tapped winding is thought of as the secondary winding. 
The winding 40 is similar to the winding 10 in being formed on a dielectric 
membrane 42 having a plurality of apertures 44 therein along with an 
undulatory exterior boundary 46. Fold lines for the membrane 42 are 
illustrated by the dashed lines 48 and 49. The winding 40 comprises a 
plurality of conductive films 50, each of which comprises a two-turn, 
center-tapped winding. As illustrated, the film 50 at the top of the 
figure has a terminal end 52 which is disposed on the right just below the 
fold line 49 and from there extends upward to the right of, around and 
across above the upper aperture 44 in the membrane 42, back down the left 
side of that aperture, back across the fold line 49, down around the right 
side of the next aperture in the membrane 42, around underneath that 
aperture, back up to and just across the fold line 49 at the left side of 
the membrane 42 in the figure to a terminal end 56. A portion 54 of the 
film 50 which spans the fold line 49 and is separated from the terminal 
ends 52 and 56 by gaps 53 and 55 in the conductive film 50, serves as the 
center-tap terminal of this winding. It is preferred to provide a 
dielectric overlayer on top of the winding 50. This is necessary for 
electrical isolation where the fold line 49 is pulled up out of the plane 
of the paper and the fold lines 48 are pushed down below the plane of the 
paper in folding the winding 40 in order to insulate adjacent windings 
from each other since in the preferred manner of interleaving the primary 
and secondary windings, the top half of the next lower film 50 faces the 
bottom of the top film 50 without other insulation therebetween. As 
illustrated in FIG. 4, the terminal portions 52, 54 and 56 of the winding 
40 are each narrower than the portion of the films which extend around an 
aperture 44. These widths may be made substantially equal by making the 
"waist" of the FIG. 8 wider, i.e. making the membrane 42 and the film 50 
wider at the horizontal centerline of the FIG. 8 (which coincides with the 
fold line 49). 
FIG. 5 illustrates, in perspective view, one of the films 50 in its folded 
configuration in which the terminal portions 52, 54 and 56 of the film 50 
are exposed at the side of the stack. The dielectric membrane 42 is not 
shown in FIG. 5. The folds facilitate connection of the separate films 50 
in parallel, since a first terminal strip 52T (FIG. 8) may be soldered to 
the terminal portion 52 of each of the films 50, a second terminal strip 
54T may be soldered to the terminal portion 54 of each of the films 50 to 
serve as the center-tap terminal and a third terminal strip 56T may be 
soldered to the terminal portion 56 of each of the windings 50 to serve as 
the other outer terminal of the multilayer, two-turn, center-tapped 
secondary winding. This connects the films 50 in parallel to provide a 
high current capacity two-turn center-tapped winding. This is more clearly 
illustrated in FIG. 8. 
In the event it were desired to double the number of layers in the 
secondary winding 40, a plurality of identical conductive films could be 
formed on the opposed surface of the membrane 42. In order to connect 
those additional films to the terminal strips 52T, 54T and 56T, those 
additional films should be connected in the vicinity of the terminal 
portions 52, 54 and 56 to the films 50 via plated through via holes in the 
dielectric membrane 42 or by other techniques such as having two tab 
portions of the backside film extending laterally in the vicinity of the 
fold line 49 as terminal ends 52 and 56. Those two tabs can then be folded 
over onto the front side winding and soldered to terminal ends 52 and 56 
of film 50. Then only the center tap terminal of the backside winding 
needs to be connected to the front side film by a via. The provision of 
the additional films on the back of membrane 42 increases the current 
carrying capacity of the overall secondary winding. 
FIG. 6A illustrates slightly more than one complete cycle of the serpentine 
path of conductor films 20 and 30 and for that portion is identical to a 
corresponding period of the FIG. 1 structure. FIG. 6B, directly below FIG. 
6A, is a cross-section through FIG. 6A taken along the line 6B--6B which 
extends parallel to the length of the winding and extends through the 
center of each of the apertures 14 therein. FIG. 6B is vertically aligned 
with respect to FIG. 6A such that the fold line 19 and two apertures 14 
are aligned in FIGS. 6A and 6B. In FIG. 6B, the vertical or thickness 
direction of the layers is greatly exaggerated relative to the horizontal 
or lengthwise dimension of the layers in order to clearly illustrate the 
layers. In a particular physical embodiment of the winding of FIG. 6A 
intended for use at about 2 MHz, the distance between the fold lines 18 
and 19 is about 0.52 to 1.00 inch and the maximum width of the dielectric 
membrane 12 is about the same. In such a physical embodiment, the membrane 
12 is preferably about 1-3 mils thick, each of the conductive films 20 and 
30 is preferably about 2.5 mils thick, and the dielectric overcoats 26 and 
36 are preferably about 0.5-2 mils thick. Thus, the horizontal dimension 
between a fold line 18 and an adjacent fold line 19 is about 50-100 times 
the entire thickness of the cross-section shown in FIG. 6B. Thus, the 
manner of folding the structure of FIG. 6B is not visually apparent in 
FIG. 6B. In such a physical embodiment, the apertures 14 are preferably 
about 1/4 inch in diameter. FIGS. 6C and 6D are cross-sections through the 
winding 10 whose cut lines are oriented perpendicular to the length of the 
winding. The cut lines for FIGS. 6C and 6D extend through the centers of 
successive apertures 14 along the length of the winding 10. FIGS. 6C and 
6D are above FIG. 6A in the drawings and are aligned with their associated 
section lines in FIG. 6A. 
In FIG. 6B, the conductive film 20 passes behind the righthand aperture 14 
as may be seen more clearly in FIG. 6C. This causes the dielectric 
overcoat 26 in FIG. 6B, at a distance behind the aperture 14, to be flush 
with the portions of the dielectric overcoat 26 at the cross section cut 
where the dielectric 26 overlies the film 20. At the lefthand aperture 14, 
the film 20 passes in front of the aperture as may be seen more clearly in 
FIG. 6D. Consequently, in FIG. 6B, the dielectric overcoat 26 behind the 
lefthand aperture 14 is at a lower level than (depressed relative to) the 
portion of the overcoat 26 which overlies the film 20 at the cross-section 
cut. This is because of the absence of the thickness of the film 20 behind 
the lefthand aperture 14. In a similar manner, the dielectric overcoat 36 
on the lower surface is flush behind the lefthand aperture and depressed 
inward behind the righthand aperture. 
FIG. 7 is a schematic illustration of a preferred manner of interleaving 
the primary and secondary windings. The fold lines 18 and 19 of winding 10 
are disposed perpendicular to the fold lines 48 and 49 of secondary 
winding 40. Successive folds in the winding 10 are identified by capital 
letters beginning with "A" at the bottom of the figure. Successive folds 
in the winding 40 are identified by lower case letters beginning with "a" 
at the bottom of the figure. There are essentially twice as many layers of 
the secondary winding as there are layers of the primary winding. The 
secondary winding at the fold lines 49 where the terminal ends 52, 54 and 
56 are located preferably has a layer of the primary winding disposed 
"inside" it. This is illustrated in the vicinity of fold "c" in the 
secondary winding where the metal of that winding has been included in the 
figure. The metal is omitted elsewhere for drawing clarity. This manner of 
interleaving increases the vertical length of the exposed surfaces of the 
terminals 52, 54 and 56 which is available for soldering to external 
terminal strips as is shown in FIG. 8 to be discussed subsequently. It 
will be noted in FIG. 7 that the secondary winding films 50 face each 
other separated only be a dielectric overcoat, but are spaced from the 
primary winding by the dielectric membrane 42 plus any dielectric overcoat 
on the primary winding. Only a low voltage is present across adjacent 
secondary winding turns, while higher voltages are present between some of 
the secondary winding turns and the adjacent primary winding turns. The 
membrane 42 is selected to withstand these higher voltages. 
With twice as many folds (and layers) in the secondary windings as the 
primary and with uniform interleaving as shown in FIG. 7, the folds in the 
two windings cannot be disposed parallel to each other. If they were 
disposed parallel, some of the folds would interfere with the uniform 
interleaving. The windings are shown with their folds perpendicular for 
clarity of illustration. With the windings having the shape shown in FIGS. 
1 and 4, the windings can be interleaved with their folds in between 
parallel and perpendicular. However, perpendicular folds are preferred 
since that provides a more uniform structure. 
For a transformer in which there are essentially the same number of folds 
and layers in each winding, the interleaving can be done with the fold 
lines of the different windings disposed parallel. However, this is not 
preferred because inserting one folded edge directly into another folded 
edge results in a thicker winding stack than when the fold lines of the 
windings are disposed perpendicular. 
In FIG. 8, the resulting interleaved stack of layers of the primary winding 
10 with layers of the secondary winding 40 is shown in an exploded, 
stylized view with a magnetic cup core 70. In FIG. 8, the layers of the 
winding 10 visible at the front of the drawing are illustrated 
schematically as extending between two successive layers of dielectric 
membrane 42. This is to avoid cluttering FIG. 8 with so much detail as to 
become meaningless. As has been discussed just above, the layer of the 
winding 10 which is inserted between the two layers of membrane 40 which 
are associated with a single film 50 of the secondary winding has a total 
thickness of from about 5 mils to about 11 mils. For a dielectric membrane 
42 thickness of 1-3 mils and a conductive film 50 thickness of 1-3 mils, a 
single thickness of the secondary has a thickness of 2-6 mils, and a 
"folded" thickness of 4-12 mils, which, with the 5-11 mil thick primary 
winding layer inside the fold provides the folded edge of the film 50 at 
terminal portions 52, 54 and 56 with a total height of from about 12 mils 
to about 23 mils. The manner of soldering the terminal strip 52T to the 
terminal portions 52 of successive layers of the secondary winding is 
illustrated at the far righthand portion of FIG. 8. The solder layer 58 
which connects the terminal strip 52T to the terminal portions 52 of the 
layers of the secondary winding bonds to the folded edges of these layers 
and extends between the layers in a manner which provides a solder bond 
having a vertical extent along each fold which is substantially equal to 
the 12-23 mil overall distance between the uppermost surface of the film 
50 and its lowermost surface. The actual length of the solder bond is 
greater than that because of the curved nature of the fold and the manner 
in which the solder follows that fold to the point where the dielectric 
overcoat (not shown) on the top film 50 contacts the dielectric overcoat 
(not shown) on the next lower film 50 or the foldover portion of terminal 
52 stops, or where the fold-over portion of terminal 52 contacts the 
non-fold-over portion of the next lower terminal 52. The terminal strips 
54T and 56T are soldered in a similar manner, but are shown cut away in 
FIG. 8 for drawing clarity. This 12-23 mil "thickness" of the solderable 
edge 52 of a 1-3 mil film is 12-8 times the thickness of the edge of a 
single 1-3 mil film to which a solder connection is made in the primary 
winding of a prior art thin-film transformer made of successive layers 
which are connected by soldered-on minuscule connector bars. Thus, the 
present invention provides a much better surface for making low resistance 
solder connections between the terminal portions of the film 50 and the 
terminal strips than is the case in the prior art structures. 
Further, in the prior art, each of the connector strips is separately 
connected to the ends of two adjacent turns of the primary winding. Thus, 
each connector strip has to be isolated from other layers. In contrast, in 
the structure shown in FIGS. 1, 2 and 4-8, each of the three terminal 
strips 52T, 54T and 56T (FIG. 8) is much longer and connects to each of 
the layers and is properly isolated by avoiding solder bridges to the 
other terminal strips and wrong terminal ends of the individual winding 
layers. Further, the folded terminal ends are held in fixed relation to 
each other by the dielectric membrane 42, thereby obviating any need to 
hold individual films in place during soldering. Thus, attaching the 
terminal strips 52T, 54T and 56T is much simpler than attaching the 
connector "bars" of prior art layered primary windings. 
The magnetic cup core 70 has a lower disk-shaped cap 72, a vertically 
extending sidewall 74 and an upper disk-shaped cap 76. The cup core 70 
includes a central post 78 which extends through each of the apertures 14 
and 44 in the dielectric membranes 12 and 42 on which the films 20 and 50 
are disposed. As illustrated, the sidewall 74 has a first aperture therein 
through which the terminal ends 22 and 32 of the primary winding extend 
and a second aperture through which the terminal portions 52, 54 and 56 of 
the secondary winding extend. When the cover 76 is in place in direct 
contact with the sidewalls 74 and the central post 78, a single magnetic 
structure encloses the entire transformer winding. 
It will be noted that the winding 10 and each half of the secondary winding 
40 each comprise the same number of physical turns encircling the 
apertures in the conductive membranes. By interleaving the folds of these 
windings as illustrated in FIG. 7, a secondary winding conductive film 50 
brackets each layer of primary winding 10 (a half turn in film 20 and a 
half turn in film 30) in a manner to provide direct, close coupling 
between the primary and secondary windings. In this manner, the current 
carrying capacity of the secondary winding and the current which can be 
extracted therefrom are both rendered relatively high and losses are 
significantly reduced. 
While FIG. 8 illustrates a circular cup core and the terminals of the 
windings disposed at 90.degree. to each other, it will be recognized that 
other core shapes and other terminal locations may be employed if desired. 
As the size of the transformer shrinks with increasing frequency, use of a 
rectangular core having "corner" or end "posts" connecting the upper and 
lower caps at the outside becomes preferable because it allows for 
protrusion of the folds in the dielectric in the spaces between the 
"posts" while maintaining the magnetic structure as close to the 
conductors of the windings as possible. This rectangular "post" core is 
especially useful with transformers in which both the primary and the 
secondary are z-folded and their fold lines are oriented perpendicular to 
each other as shown in FIG. 7. 
In this way, a conductive film transformer is provided in which only the 
multiple layers of the secondary winding are soldered to external 
terminals in a stacked configuration, but at folded edges of the 
conductive films. Consequently, the present transformer is substantially 
less complex to manufacture and assemble than is a prior art conductive 
film transformer and is substantially more reliable. 
The winding 10 may preferably be formed by providing a dielectric sheet 
having separate conductive films disposed on its upper and lower surfaces. 
The two conductive films 20 and 30 are then defined by photolithographic 
patterning of the conductive films on the top and bottom surfaces, 
respectively, and etching or otherwise removing the portions of those 
continuous films which do not form a part of the respective patterned 
films 20 and 30. Thereafter, the dielectric membrane 12 itself is 
preferably patterned to leave the dielectric membrane 12 protruding 
slightly beyond the conductive films and having the apertures 14 therein 
either by die cutting or by photolithographically patterning and etching 
the dielectric membrane. The winding 40 may be formed in a similar manner 
from a dielectric sheet having a conductive film disposed only on the top 
surface thereof. Once the conductive film pattern and the dielectric 
membrane pattern have been formed, the windings are folded at the fold 
lines into their stacked forms which have been described. Where the 
winding is formed in this manner, it is preferred to have the conductive 
film set back from the edge of the dielectric in order to ensure that the 
conductor is insulated from the magnetic core in which it is inserted. 
Alternatively, where a particularly compact structure is desired, the 
conductive film may be a self-supporting foil which is coated with a thin 
dielectric after patterning. Thin dielectric coatings may preferably be 
applied electrophoretically to ensure adequate coating of edges and 
corners without producing excessive thicknesses on the wide surfaces. 
Electrophoretic deposition is explained more fully in "Electrodeposition 
of Polymers from Nonaqueous Systems. I. Polyimides: Some Deposition 
Parameters" by W.M. Alvins et al., Journal of Applied Polymer Science, 
Volume 27, 1982, pages 341-351, which is incorporated herein by reference. 
Other dielectric coating techniques may be used as desired and include 
dipping in varnish, spraying and so forth. Although not preferred because 
of the spacings required, air dielectrics may also be used. 
An alternative conductive film pattern for a multiturn winding is 
illustrated in plan view prior to folding in FIG. 9 as a winding 110. The 
winding 110 comprises three-quarters of a turn of the winding in each 
layer of the final, stacked, folded configuration. To provide this 
three-quarters of a turn, the fold lines 118 and 119 are disposed at right 
angles to each other rather than parallel to each other as with the 
winding 10. The conductive film 120 is illustrated in the top portion of 
FIG. 9 as having a piece-of-pie-shaped portion missing from an otherwise 
substantially circular, annular conductive pattern on each square of 
membrane 112. In the lower portion of FIG. 9, the conductive film forms a 
complete annulus except for a narrow gap 126 which extends from the outer 
edge of the annulus to its central hole at aperture 114. Either of these 
configurations is effective in controlling the path followed by current 
flowing through the winding to restrict it to flowing in the three-quarter 
turn path which encircles the central apertures 114 in the membrane 112. 
The film 120 is formed into a multi-turn winding by folding the membrane 
at fold line lines 118 and 119 by lifting the fold lines 119 out of the 
plane of the paper and pushing the fold lines 118 down below the plane of 
the paper. 
In FIG. 9, the membrane 112 is illustrated as a stair step pattern of 
square segments which form the separate layers of the stack after folding. 
This is for illustrative purposes to indicate that the dielectric membrane 
can have a different shape than the conductive film. However, it is 
considered preferable to have the dielectric membrane closely follow a 
circle which is just larger than the exterior boundary of the conductive 
film in order to provide a substantially circular stack for insertion into 
a cup core to form the final inductor or transformer while providing 
isolation which prevents contact between the conducting film and the 
enclosing ferrite cup core. 
In FIG. 10, a single turn of one side of a secondary winding 140 is 
illustrated in plan view along with cross-sectional illustrations of 
terminal strips 152T, 154T and 156T to which the individual winding turns 
are attached. It will be noted that the conductive film 150 illustrated in 
FIG. 10 makes contact only to the terminal strip 156T and the terminal 
strip 154T because the conductive film "terminal end" in alignment with 
strip 152T is too short to reach that terminal strip. 
In FIG. 11, two of the films 150 of FIG. 10 are illustrated superimposed 
with the lower one flipped over relative to the upper one. The dielectric 
membrane between the films is omitted for clarity. In this configuration, 
it can be seen that films in the configuration shown in FIG. 10 connect to 
the terminal strips 156T and 154T, while the flipped version of that film 
connects to the terminal strips 154T and 152T to thereby provide turns of 
the two halves of the secondary winding in accordance with the orientation 
in which each film 150 is added to the stack. This orientation may 
preferably alternate. 
The individual conductor layers of FIGS. 10 and 11 may also be formed 
directly from a conductive film such as copper foil and then coated with 
insulation by dipping or spraying with a varnish or by electrophoretic 
deposition. 
In FIG. 12, the film patterns of FIGS. 10 and 11 are illustrated in a 
continuous dielectric membrane strip form similar to FIG. 4 in which the 
individual winding layers are defined in the conductive film, the 
dielectric material of membrane 142' is removed at least in the vicinity 
of the terminal connections and the dielectric membrane is then folded to 
form the multilayer stack of winding layers. In FIG. 12, one set of 
terminal strips 152T, 154T and 156T is illustrated for orientation 
purposes. After folding on the fold lines 148 and 149, the winding 140 is 
positioned in contact with the terminal strips 152T, 154T and 156T and 
soldered to each of the terminal strips. 
In FIG. 13, a transformer comprised of the winding 110 and the windings 140 
is illustrated in a closed cup core package ready for installation in a 
system for use. In FIG. 13, the two terminal ends of the primary winding 
are disposed at opposite ends of the stack and extend at 90.degree. to the 
secondary terminal legs. The extension of the secondary terminals in FIG. 
13 outside the core is greatly exaggerated for drawing clarity and in 
physical embodiments is normally minimized. 
Unsupported secondary windings of the type shown in FIGS. 10-12 are most 
useful at frequencies such as from about 250 KHz to about 1.0 MHz. This is 
because at those frequencies the conductor film must be relatively thick 
in order to be approximately 2 skin depths thick, ranging from about 10.4 
mils at 250 KHz to about 5.2 mils at 1 MHz. These relatively thick films 
provide a relatively thick edge for soldering to the terminal posts. 
However, these windings may be used at higher frequencies, if desired. 
Secondary windings of the type shown in FIGS. 4-8 are most useful at 
relatively higher frequencies above about 1 MHz. This is because as the 
operating frequency increases, the required thickness of the conductive 
film decreases thereby complicating edge soldering. However, these 
windings may be used at lower frequencies, if desired. 
FIG. 14 is a plan view illustration of an alternative configuration for the 
conductive film of a multi-turn foldable winding. This winding 210 differs 
from the windings 10 and 110 in that the fold lines 218 and 219 are 
neither parallel nor perpendicular to each other and the winding does not 
form a whole multiple of a quarter turn per layer. The winding 210 may be 
converted into both windings of a 1:1 turns ratio transformer by providing 
a lengthwise-extending gap down the center of the serpentine pattern of 
the film 220 (similar to that in FIG. 3) to divide it into lefthand and 
righthand halves which then comprise separate windings, but are held in 
position by the dielectric membrane 212. The central post of a cup core is 
preferably inserted in the apertures 214 after folding. 
While the specific embodiments illustrated have been described as being 
accordion folded, that is with adjacent folds in opposite senses, these 
films may have successive folds in the same sense if allowance for layer 
thicknesses is made in the film patterns. Mixtures of fold patterns may 
also be used. However, accordion folds are preferred. While a number of 
variations of conductive film windings in accordance with the present 
invention have been illustrated and described herein, those skilled in the 
art will be able to design many variations on the particular conductive 
film patterns employed in these embodiments without departing from the 
true spirit and scope of the present invention. For example, if it were 
desired to do so, the transformer of FIGS. 1, 2 and 4-8 could be formed by 
providing the primary winding on the lower surface only of membrane 12 as 
the conductive film 30 and by providing the secondary windings as 
conductive films 50 disposed on the upper surface of the same membrane 12 
and by folding the membrane in a manner to bring the terminal portions 52, 
54 and 56 of the secondary windings to the outside of the stack in an 
exposed manner for attachment of the external terminal strips. Many other 
pattern variations are possible while still providing layer-to-layer 
connections via the conductive film itself. Further, film patterns with 
more than one aperture in a layer and more than one post in a cup core may 
be used if desired. Substantially any desired turns ratio may be provided 
by appropriate design of the conductive film shapes and the manner of 
their interleaving after folding. Similarly, the relative number of layers 
in the windings of a transformer is a matter of design choice. From about 
1/6 to about 5/6 of a winding turn per layer may conveniently be provided 
in accordance with this invention where a single film is employed in each 
layer. Further, although the tapped windings illustrated are center-tapped 
windings, it will be recognized that tapped windings in which the tap is 
not centered may also be used. 
While the invention has been described in detail herein in accord with 
certain preferred embodiments thereof, many modifications and changes 
therein may be effected by those skilled in the art. Accordingly, it is 
intended by the appended claims to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.