Low profile high power surface mount transformer

A low profile transformer comprises a support member for supporting a coil winding, a first elongated conductive tape forming at least one conductor having lead terminals formed integral therewith formed into a first coil of multiple turns, a second elongated conductive tape having lead terminals formed integral therewith forming at least one conductor formed into a second coil of at least one turn interleaved with the first coil, and each terminal lead of each of the tapes extending from the support member and having a portion disposed at a base of the transformer for engagement and surface bonding to a PC board.

BACKGROUND OF THE INVENTION 
The present invention relates to electronic components and construction, 
and pertains particularly to an improved coil and method of manufacturing. 
For many years, electronic circuit boards have been fabricated by 
interconnecting a plurality of electronic components, both active and 
passive, on a planar printed circuit board. Typically, this printed 
circuit board has comprised an Epoxy/fiberglass laminate substrate clad 
with a sheet of copper, which has been etched to delineate the conductive 
paths. Holes were drilled through terminal portions of the conductive 
paths for receiving electronic component leads, which were subsequently 
soldered thereto. 
More recently, so-called surface mount technology has evolved to permit 
more efficient automatic mass production of circuit boards with higher 
component densities. With this approach, certain packaged components are 
automatically placed at preselected locations on top of a printed circuit 
board, so that their leads are registered with, and lie on top of 
corresponding solder paths or pads. The printed circuit board is then 
processed by exposure to infrared or vapor phase soldering techniques to 
re-flow the solder, and thereby establish a permanent electrical 
connection between the leads and their corresponding conductive paths on 
the printed circuit board. 
The increasing miniaturization of electrical and electronic elements and 
high density mounting thereof has created increasing problems with 
construction of electrical components as well as electrical isolation and 
mechanical interconnection. Demand for even greater miniaturization 
increase the need for better and more efficient components and techniques 
of construction. In particular, it creates more difficulty in providing 
adequate power from smaller components and establishing reliable and 
efficient connection between packaged components and terminals. Presently 
known construction and interconnect methods severely limit the ability to 
provide more compact and powerful components and high density and reliable 
components and electrical and mechanical isolation between components 
distinct terminal points due to space limitations. 
Among the electrical and electronic elements that must be made more compact 
and efficient and surface mounted on PC boards are coils, such as 
transformers, inductors and the like. These must be constructed to be low 
profile, be high powered and efficient. 
The current technique of construction of transformers and other coils is to 
wind round or square copper wires on a somewhat flat bobbin or pole piece. 
Layers of tape are wrapped between the layers of wire to provide high 
voltage insulation. A problem with round wire is that at high frequencies, 
the current penetrates only a small depth, called skin depth, on the wire 
surface. To overcome this, some manufactures have used a bundle of small 
wires, called litz wire. This provides more surface area, but a large 
portion of the cross-sectional area is unused because of the space between 
the wires. This is an inefficient use of the space taken up by the bundle 
of wires. 
Conductive tapes have been proposed to reduce the above density problem. A 
conductive tape is wound alternately with an insulating tape on a 
conventional bobbin, with round wires soldered at the ends of the tape for 
terminal lead connections. However, this terminal lead structure adds 
thickness to the assembly and defeats efforts to miniaturize the 
transformer. For example, a 20 mil wire soldered to a 2.5 mil tape will 
typically result in a 30 mil thickness. 
Another approach to miniaturization has been to go to a planar magnetic 
transformer. This structure has a circuit board type construction wherein 
sheets of conductive plates are formed with a center hole wherein the core 
extends perpendicular to the surface of the plates or circuit boards. 
Leads for the planar magnetic construction extend down through holes in 
the printed circuit boards. This provides a low profile, but is expensive 
and low powered. 
It is, therefore, desirable that an improved transformer construction with 
high power, low profile and with improved lead form for termination and 
surface mounting be available. 
SUMMARY AND OBJECTS OF THE INVENTION 
It is the primary object of the present invention to provide an improved 
electrical transformer coil and method of making. 
In accordance with a primary aspect of the present invention, a low profile 
transformer comprises a support member for supporting a coil winding, a 
first elongated conductive tape forming at least one conductor formed into 
a first coil of multiple turns having terminal ends, a second elongated 
conductive tape forming at least one conductor formed into a second coil 
of at least one turn interleaved with said first coil, and each terminal 
end of each of said tapes extending from said support member and having a 
portion disposed at said base for engagement and surface bonding to a PC 
board.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to the drawings, particularly FIG. 1, there is illustrated an 
exemplary embodiment of a compact, low profile transformer constructed in 
accordance with a preferred embodiment of the invention, designated 
generally by the numeral 10. The transformer 10 has a very low height and 
comprises a coil assembly 12 contained within a double E type core 14, 
with terminals projecting from beneath or at the base of the frame of the 
core assembly 14 for surface bonding to a PC board. 
Referring to FIG. 2, major components of the transformer assembly, 
including a pair of conductive tapes, each designated generally by the 
numerals 16 and 18, respectively, are illustrated. The tape 16 is 
constructed of a conductive material and preferably a material such as 
copper and a dielectric laminate. However, it is apparent that separate 
conductive and insulation tapes can be used. The strip or tape 16 is 
formed of two conductive paths 20 and 22 and forms the primary coil or 
winding of the transformer. The tape is constructed in two main sections, 
A for forming the A winding of the primary coil and B for forming the B 
section of the primary coil. These sections are offset from one another to 
position them in a non-interfering position with respect to the bobbin or 
support structure on which it is wound. 
The tape is provided with terminal leads 24 and 26 for conductive strips 
20, and terminal leads 28 and 30 for conducting the strip 22. The winding 
will thus have built-in, self-leaded terminals. The lead terminals 24 and 
28 on one end adjacent section A are offset from the section A to permit 
winding of the coil sections A and B onto the bobbin or other support 
structure. The conductive strip for the illustrated embodiment is 
preferably on the order of about one-thousandths of an inch in thickness. 
The insulation layer preferably overlaps the edge of the conductive layer. 
The secondary coil or winding is formed by the conductive tape 18 which is 
precision formed, preferably of a material similar to that of the 
conductive strip or tape 16, however with a preferred thickness of about 
six-thousandths of an inch. This conductive tape is formed with laterally 
extending terminal leads or ends 32 and 34 and a central laterally 
extending terminal lead 36. The central terminal 36 is on the opposite 
side of the terminals 32 and 34 and is directly opposite a notch or 
cut-out 38 which, as will be subsequently described, provides a recess for 
a portion of the terminals 32 and 34 for maintaining a low profile. 
The coil is formed or wound by selecting suitable support means for the 
tapes, such as a bobbin or merely a winding support bar. For example, the 
conductive tapes may be wound on a support mandrel and then removed for 
final assembly on a suitable core. For the purposes of illustration, a 
bobbin or similar support member 40 is selected and properly positioned at 
one end of the section A in alignment therewith for winding that section 
onto the bobbin or other support structure. The tape is positioned and 
wound onto the bobbin such that when the first section is wound, the 
assembly appears as shown in FIG. 3, with the offsets of tabs 24 and 28 
positioned on the same side of the bobbin and substantially aligned with 
the offset of the section B portion of the tape. 
After the section A of the tape has been wound on the support member 40, it 
will appear as shown in FIG. 3, with both sections B and the terminals 
ends of section A offset axially to either side of the support member 40. 
As shown in FIG. 3, both tab sections 24, 28 and section B are on top of 
the support member. The assembly is then turned over, that is, rotated 
one-hundred eight degrees about the axis of the support member 40, and the 
tape 18 defining the secondary winding is started at what is now the top 
of the support member, as shown in FIG. 4. The tape 18 is wound clockwise 
about the support member 40 until the terminal end 34 is positioned 
adjacent terminal end 32 over, and received in the slot 38. This places 
the terminal ends 32, 34 and 36 in a position which is now on top of the 
support or bobbin 40. They also extend outward axially to the side of the 
bobbin or support member. 
At this stage, the assembly is rotated another one-hundred eighty degrees 
to present the arrangement wherein the second section B is on top, as 
shown in FIG. 3, but with the tape 18 wound about the bobbin. The section 
B of the primary tape 16 is then folded across its offset to place the 
section B in line to be wound on the bobbin, as shown in FIG. 5. The 
section B is then wound on the bobbin, producing a structure as shown in 
FIG. 6, with the terminal ends 26 and 30 extending outward from the bobbin 
40. 
The offset terminals leads 24 and 28 are then folded over across the 
offset, positioning them in line with the bobbin as shown in FIG. 7. The 
terminals 24, 26, 28 and 30 are then pressed downward below the 
rectangular hole for passage through the bobbin. A suitable core assembly, 
such as a double E type core assembly, is selected and inserted in the 
throughhole or bore of the bobbin, such that side frame pieces of the core 
assembly overlie the terminal leads, as shown in FIG. 8. The double E core 
assembly is made up of two identical generally E-shaped members 42 having 
a central core portion 44, end frame member 46 and two side frame members 
48 and 50. These are inserted into the ends of the bobbin facing one 
another so that the side frame members 48 and 50 engage, as shown in FIG. 
8, and overlie the lead terminals. The leads are thus presented below the 
coil assembly in a position for surface bonding to a PC board. 
It may be necessary in some instances to provide greater insulation to 
handle higher voltage requirements. This can be provided by extending the 
insulation layer over and around the edges of the conductive tape so that 
they are completely covered. 
Referring to FIG. 10, an alternate geometric configuration is illustrated 
for the conductive tape assembly. In this embodiment, for example, the 
primary and secondary windings are embodied in the same tape, as 
illustrated in FIG. 10. It is also apparent that the tapes can be 
separate, but geometrically formed to nest together so that they can be 
wound radially on the same support. The tape is selected to have an 
appropriate thickness, and the conductive strips are then formed with 
suitable widths. The conductive tapes can be formed from large sheets of 
copper insulator laminate, and using an etching, such as photo chemical 
etching and photo lithographic technique to form conductive paths. 
Patterns for the conductive strips can be generated by a computer as a 
master for etching windings or paths and pads on the laminate. This 
approach can provide a very high degree of accuracy in the components. 
The tape assembly, as illustrated, provides an in-line arrangement of the 
primary coil windings and the secondary coil winding. The assembly, 
designated generally by the numeral 52, comprises a first primary coil 
section, designated generally at 54, with an adjacent and substantially 
in-line secondary coil winding section 56 and following that, a second 
primary coil winding section 58. In the illustrated embodiment, the 
primary winding is provided with three conductive strips 60, 62 and 64 
having lead terminals 66, 68 and 77 at one end. The second section 58 of 
the primary winding is provided with conductive strips 60', 62' and 64', 
with lead terminals 72, 74 and 76 at the opposite end. The two sections of 
the primary winding are connected by narrow conductive strips 86, 88 and 
90. Disposed substantially at the center of the overall tape assembly is a 
secondary coil winding 78 with lead terminals 80, 82 and 84. 
This in-line arrangement provides a geometry that permits easy winding of 
the overall tape assembly onto a bobbin or other suitable support. This 
construction permits the use of machine winding and assembly. The various 
lead terminals are positioned on the respective conductive strip so that 
upon winding on the bobbin or other support, the lead terminals will be 
positioned as desired with respect to the overall coil structure. The 
primary and secondary conductive paths can be connected in any suitable 
number of parallel or series arrangements to provide the desired number of 
windings and transformer characteristics. Any number of geometrical 
arrangements may be provided for the conductive tape assembly. For 
example, any number of primary windings can be provided, as well as any 
number of secondary windings, e.g. secondary, tertiary, quarternary, etc. 
The terminal leads are formed by an appropriately extended integral piece 
of the tape. 
While I have illustrated and described my invention by means of specific 
embodiments, it should be understood that numerous changes and 
modifications may be made therein without departing from the spirit and 
scope of the invention as defined in the appended claims.