RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Serial

No. 62/436,972, titled "High Current Toroidal Transformer Construction," filed

December 20, 2016, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to transformers, and in particular to the construction of high current transformers using toroidal magnetic cores.

BACKGROUND

A transformer is a device that transfers Alternating Current (AC) energy between two or more electrical circuits by electromagnetic induction. An AC current in a primary coil generates a varying magnetic field, which induces an AC voltage in a secondary coil. The coils are usually wrapped around a metal core having a high magnetic permeability, such as iron. The ratio of the input voltage to the induced voltage varies according to the ratio of the number of turns of wire in the primary and secondary coils. Transformers are thus used to "step up" and "step down" AC voltages, which are essential steps in the distribution of electrical power. Transformers are widely used in a variety of circuits and devices, e.g. , coupling amplifier stages of RF and audio circuits, impedance matching, converting signals between balanced and single-ended, and the like. However, power-shaping and power distribution transformers present unique design challenges; they must be designed and sized to carry very high currents, and in some cases transfer high-frequency AC energy.

High current transformers must be wound with thick conductors to avoid copper losses. Toroidal core winding machines exist based on a circular shuttle-spool that can be broken at one place to be threaded through the toroid center hole and then reconnected to form a continous loop. The spool is then rotated to fill it with wire.

After this set-up procideure, an end of the wire is attached to the toroidal core at a desired winding starting point, and the spool is rotated by a motor to lay down turns on the toroid. This method cannot, however, be used for the very thick wire needed for high currents. Additionally, the method is not suited for small numbers of turns, as the set-up time is greater than the winding time. Therefore toroids that must be wound with few turns of thick conductors are currently wound by hand.

Another issue with toroids that must be wound with thick wire is that the inside diameter of the toroid is less than the outside diameter, and thus the inside circumference is much less, causing winding congestion in the center hole. Circular cross section wire - the most common, and hence least expensive type - is thus an inefficient way to fill the available winding area. Moreover, at high frequencies, wire of circular cross section does not use the cross section efficiently, due to High Frequency (HF) currents flowing only in a surface layer.

The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section. SUMMARY

The following presents a simplified summary of the disclosure in order to provide a basic understanding to those of skill in the art. This summary is not an extensive overview of the disclosure and is not intended to identify key/critical elements of embodiments of the invention or to delineate the scope of the invention. The sole purpose of this summary is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

According to one or more embodiments described and claimed herein, high current toroidal transformers are provided with conducting turns by placing inner conducting members within the core and outer conducting members outside the core, and selectively electrically connecting the inner and outer conducting members to form turns of one or more transformer windings. At least one of the inner and outer conducting members may comprise U-shaped pieces stamped out of sheet metal by an automatic punch machine.

In one embodiment, the inner conducting members comprise U-shaped pieces stamped out of sheet metal and placed around a magnetic toroid with the open end of the U-shaped piece facing radially outwards from the toroid center, with the legs of the U-shaped piece extending beyond the outer diameter of the toroid. The ends of the legs of different U-shaped pieces are then connected by outer conducting members in the form of strips arranged around the outside diameter of the toroidal transformer, to form complete turns and to form primary and secondary windings of any desired configuration. The outer conducting members may overlap the extended legs of the inner conducting members because the winding congestion at the outside diameter of the toroid is much less than at the inside diameter, thus allowing for the doubling of the conductor thickness at that radius.

In this embodiment, the U-shaped inner conducting members may be set around a selected toroidal core by a jig and encapsulated in position by a plastic moulding machine, leaving the extended legs of the U-shaped inner conducting members protruding from the assembly for later interconnection, such as by outer conducting members, to form primary and seconday windings. The U-shaped pieces may be stamped out of sheet metal by a stamping machine that also forms various angular bends of the legs of the U, out of the plane of the U , so as to facilitate later interconnection by outer conducting members in the shape of flat conducting strips.

In another embodiment, the two legs of a U-shaped piece provide both the inner and outer conducting members, with the ends of each extending below a lower surface of the toroidal core. An interconnection medium, such as a printed circuit board (PCB) having one or more conducting layers, connects to the ends of the U-shaped pieces. The tracks on the PCB layers then interconnect the legs of different U-shaped pieces to form complete turns, and form primary and secondary windings of any desired transformer configuration.

In the this embodiment, the conducting U-shaped pieces may be arranged in the required circular pattern and encapsulated in place using a moulding machine to form an encapsulated part having a circular slot for later insertion of the magnetic toroid into the slot. The same moulded part may be used to make various transformers, with various primary and secondary winding configurations defined by the PCB interconnect pattern, providing that the total number of turns does not exceed the number of encapsulated U-shaped pieces.

Furthermore, toroids of the same size but having different magnetic permeabilities may be used to increase the range of transformers that may be made with the same pre-manufactured part. The U-shaped pieces may also be stamped out with various angular bends out of the plane of the U, to facilitate later interconnection by a heavy-copper PCB pattern.

In both embodiments, it is advantageous to interconnect alternate U-shaped pieces, thereby forming interleaved primary and secondary windings, to achieve tighter coupling. The use of U-shaped pieces stamped out of sheet metal provides a much better shape than wire of round cross section for achieving an efficient fill of the available winding area, and furthermore provides much lower resistance at high frequencies for the same copper volume, due to skin- effect.

More generally, embodiments of the present invention comprise forming inner conducting members and outer conducting members out of a conducting sheet material, in complementary shapes, placing them within and around a toroidal core, respectively, and interconnecting them to form complete turns around the core, thus providing primary windings and secondary windings of any desired configuration.

One embodiment relates to a transformer. The transformer includes a magnetic core comprising a closed loop of magnetic material. The transformer also includes a plurality of inner conducting members disposed within the loop of the magnetic core, in a direction generally perpendicular to a plane of the loop; and a plurality of outer conducting members disposed outside of the loop of the magnetic core, in a direction generally perpendicular to a plane of the loop. Selected ones of the inner and outer conducting members are electrically connected so as to form turns of one or more of a first or second winding around the magnetic core.

Another embodiment relates to a method of manufacturing a transformer. A magnetic core, comprising a closed loop of magnetic material, is provided. A plurality of inner conducting members is disposed within the loop of the magnetic core, in a direction generally perpendicular to a plane of the loop. A plurality of outer conducting members is disposed outside of the loop of the magnetic core, in a direction generally perpendicular to a plane of the loop. Selected ones of the inner and outer conducting members are electrically connected so as to form turns of one or more of a first or second winding around the magnetic core.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

Figure 1 is a perspective view illustrating a first placement of a conducting U-shape around a toroidal core.

Figure 2 is a plan view illustrating an arrangment of multiple U-shapes around a toroidal core.

Figure 3 is an elevation view illustrating interconnecting planar U-shapes by means of off-set interconnecting strips.

Figure 4 is an elevation view illustrating interconnecting off-set U-shapes by planar strips.

Figure 5 is a perspective view illustrating the off-set geometry of a U-shape for the embodiment of Figure 4.

Figure 6 are plan and perspective views illustrating mitering U-shapes and

interconnecting strips using a solder-retaining sleeve.

Figure 7 is a perspective view illustrating a second placement of a conducting U-shape around a toroidal core.

Figure 8 is a plan view illustrating one layer of a printed circuit board interconnect pattern.

Figure 9 is a perspective view illustrating a moulded subassembly that can be fabricated before the final transformer configuration is determined.

Figure 10 are plan and perspective views illustrating the use of ?-shapes.

Figure 1 1 is a perspective view illustrating one low cost construction for a toroidal transformer.

DETAILED DESCRIPTION

For simplicity and illustrative purposes, the present invention is described by referring to an exemplary embodiment thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention.

However, it will be readily apparent to one of ordinary skill in the art that the present invention may be practiced without limitation to these specific details. In this description, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

According to embodiments of the present invention disclosed and claimed herein, a high-current transformer is formed by placing inner conducting members inside a magnetic core in the form of a loop, and outer conducting members outside of the loop. The inner and outer conducting members are then selectively connected so as to form turns of one or more primary or secondary windings. In some embodiments, the primary and secondary winding turns are interleaved by connecting every other adjacent inner or outer conductive member together, such that physically adjacent members are part of different transformer windings. In some embodiments, the outer conducting members selectively connect different ones of the inner conducting members together. In some embodiments, the inner and outer conducting members comprise different portions of the same part, such as the legs of a U-shaped member, and different ones of the U-shaped members are selectively connected, at least at one end, via an interconnection medium, such as a printed circuit board (PCB). In some embodiments, the inner and outer conducting members are distinct parts, and are connected via interconnection media at both ends.

For pedagogical and explanatory reasons, various components may be described herein with reference to an orientation - such as an upper or lower surface or the like. Of course, if the part were inverted, these descriptions would reverse. Hence, such descriptive terms are understood to be related to a presumed orientation, and are not absolute.

Figure 1 illustrates the construction of a high-current, toroidal transformer according to one embodiment of the present invention. Inner conducting members (100) are pre-stamped out of sheet metal in great quantity by a stamping machine, which can use a die that produces tens or even hundreds at each stamping. Copper is a superior electrical conductor, but thanks to the efficient fill of the winding area when embodiments of the invention is used, aluminum may sometimes suffice, with resulting significant cost reduction. Thus, although conductors are sometimes described herein as being formed from copper, embodiments of the present invention also encompass the use of aluminum conductors, or indeed any metal or electrically conducting material.

In the embodiment of Figure 1 , the inner conducting members (100) are generally U- shaped - that is, comprising a base and two legs, the legs generally parallel to each other and generally normal to the base. The pre-formed inner conducting members (100) are arranged around magnetic toroid (1 10) with the legs of the U-shaped pieces facing radially outwards. Figure 1 depicts the legs of the inner conducting members (100) extending beyond the outer diameter of toroid (1 10) for later interconnection by outer conducting members, e.g. , in the form of strips, to form complete turns. In other embodiments, one or both legs of the inner conducting members (100) may not extend beyond the outer diameter of the toroid (1 10) - in this case, the outer conducting members comprise a shape other than a simple rectangular strip.

As illustrated in Figure 2, one advantage of the legs of the U-shaped inner conducting members (100) facing outwards is that the outer conducting members attached to the legs to interconnect different inner conducting members (100) together to form winding turns, may overlap the legs of the U-shaped piece. This is possible since there is room for the resulting double conductor thickness at the outer diameter, while perhaps not at the inside diameter of the toroid (1 10).

In Figure 2, an exemplary 24 copper U-shaped inner conducting members (100) are arranged at angular intervals Θ around the toroid (1 10). For 24 U-shaped inner conducting members (100), 0= 15° yields equi-angular spacing. For any other number N of U-shaped inner conducting members (100) , equi-angular spacing is obtained by a value of Θ=(360/Ν) degrees.

Strip-shaped outer conducting members (120) overlap the legs of the U-shaped inner conducting members (100) extending beyond the toroid outer diameter r2 to an outside diameter r4. It may be seen that, while the U-shaped inner conducting members (100) approach each other closely at the inner diameter r1 and even more closely at the inside diameter r3, there is a much greater gap at radius r2 to r4, to accomodate the double thickness of copper caused by the overlapping strip-shaped outer conducting members (120) .

One advantage of connecting U-shaped inner conducting members (100) with overlapping strip-shaped outer conducting members (120) is to obtain a high area of contact and thus low ohmic resistance. The overlapping material can be solder-sweated together or possibly spot-welded to obtain a low-resistance junction. Of course other junction methods could be used, even without overlap, for example a laser-welded butt-joint, or a rivet or cleat, as will be depicted herein. A manufacturing process engineer, with the teachings of the present disclosure, may choose an appropriate connection method based on the required production rate and volume, to achieve the easiest method to automate and/or the minimum recurring cost.

Optionally, insulating spacers may be inserted between the U-shaped inner conducting members (100), for example at the inner diameter where they approach most closely. Moreover, the whole assemby can be potted in a suitable insulating material, using a poured mould or injection moulding machine. Another alternative is to contain the assembly immersed in cooling oil and sealed in a shielding can, particularly in the case of very high power transformers.

Figure 3 illustrates a geometry for the strip-shaped outer conducting members (120), according to one embodiment.

When the U-shaped inner conducting members (100) are planar, the interconnecting outer conducting members (120) must be bent at the ends, and exhibit an offset along their length through twice the angle Θ defined in Figure 2, in order for the ends of outer conducting members (120) to be in the same plane as the U-shaped inner conducting members (100) to which they will be joined, by for example solder-sweating. The reason for an offset of 2Θ is that the strips may connect alternate U-shaped inner conducting members (100) - for example, the upper leg of U1 is shown connected to the lower leg of U3; the upper leg of U2 is connected to the lower leg of U4; etc. Thus two separate, unconnected, and interleaved windings are formed by this interconnection pattern. For example, a 3-turn winding can start at the lower leg of U1 and end at the upper leg of U5. Any number of interleaved primary and secondary windings can be formed, providing that the total number of turns is less or equal to the total number of U- shaped inner conducting members (100) that can be accomodated without touching. Triple or higher-order interleaving is also possible, but the geometry of the outer conducting members (12) then must be adjusted accordingly.

Figure 4 depicts an alternative interconnection geometry. In Figure 4, an offset is formed in the U-shaped inner conducting members (100) by the stamping machine, such that when placed on the magnetic toroid (1 10), the upper leg of U1 is substantially directly above the lower leg of U3 and in the same plane - thus U1 (upper) may be joined to U3 (lower) by an outer conducting member (120) in the shape of a simple planar strip. Figure 5 illustrates the required deformation of a U-shaped inner conducting member (100) from a planar shape to a 3-D shape. To deform a U-shaped inner conducting member (100) to the shape shown in Figure 5, the angle of the legs of the U before deformation may have to be greater than 90 degrees. Mechanical CAD programs such as SolidWorks can be used to compute the exact planar shape from the final 3D shape, as well as a program for a numerically controlled tool that makes the punch tool and die.

An alternative method of interconnecting U-shaped inner conducting members (100) similar to Figure 4 is to use a miter joint and right-angle, solder-retaining sleeve as shown in Figure 6. In Figure 6, the legs of the U-shaped inner conducting members (100) and their interconnecting outer conducting members (120) are mitered together in the same plane. A right-angle sleeve (140) is slipped over the miter joint of the inner and outer conducting members (100, 120) for soldering. Solder is applied through the solder flow hole (130) of sleeve (140) and wicks through the solder flow holes (130) in the miter joint to the other side of the pieces to form a large electrical interconnecting area which supports high currents. The illustrated positioning of the solder holes is only exemplary and instead holes may be entirely contained within one metal piece instead of appearing as half-holes in two adjacent metal pieces. Moreover, the use of a straight miter joint is merely exemplary, as other shapes could be used advantageously, such as dovetails or jigsaw-puzzle type bobbles and holes, which would provide an interlock between the inner and outer conducting members (100, 120) for easier handling. Pieces interlocked in this way could be welded by pressure welding.

The right-angle solder-retaining sleeves may be fabricated from a smaller gauge (e.g., 26 gauge) of copper sheet stock (150) by a first punch operation that cuts out a shape (150) and splits it in two, diagonally along cut line (160), to obtain two triangular punched pieces (170), which may then be bent along bending lines (180) by a bending tool to obtain the final 3- D sleeve pieces (140). The sleeve pieces are useful for maintaining the co-planar positioning of the parts being interconnected, as well as retaining solder and facilitating the wicking of solder into the joint. The parts being interconnected may alternatively be held together by for example a dovetail joint or other interlocking shape, and the joint formed by applying a high pressure causing the material to pressure weld.

Figure 7 depicts another embodiment of the present invention. In this embodiment, one leg of a U-shaped piece (190) comprises the inner conducting member, and the other leg of the same U-shaped piece (1 90) comprises the outer conducting member. The inner and outer conducting members are joined across an upper surface of the magnetic toroid (1 10) by the base of the U-shaped piece (190) , and both legs (that is, the ends of both the inner and outer conducting members) extend below a lower surface of the magnetic toroid (1 10) . With the embodiment of Figure 7, a different method of interconnecting U-shaped pieces (190) to form compete turns than the connecting strips (120) of Figure 2 is desirable, as the double strip thickness at the inner diameter of the toroid (1 10) would lead to inefficient copper fill. Instead, a planar interconnection medium can be used, such as a printed circuit board (PCB) , as shown in Figure 8.

Figure 8 depicts a section of one side of a two-layer (e.g. , double-sided) PCB 200 where the pattern of conductive metal, such as copper, is shown as shaded. White areas indicate where metal has been etched away. A design in which the minimum amount of metal is etched away, sufficient to provide electrical isolation between different metal areas, is called a

"maximum metal" layout, as opposed to the more usual design where metal is etched away everywhere except where it is desired to leave thin interconnecting tracks. The white shape (203) indicates where metal has been etched away to leave isolated metal interconnecting (shaded) shapes (202). Slots (201) are routed or milled through the PCB 200 at an outer and an inner radius in the middle of the metal interconnecting patterns (202) to receive the legs of U- shaped pieces (190) of Figure 7 - that is, the inner and outer conducting members - which will be soldered in place. The slots may be plated through to assist solder wicking. It is assumed that the U-shaped pieces (190) have been formed with an offset similar to Figure 5, such that the outer leg of one U-shaped piece (190) is connected by a metal shape (202) to the inner leg of the U-shaped piece (190) two away, so as to complete a spiral conducting turn wrapped around the toroidal core, interleaved with other spiral turns formed by interconnecting the U- shaped pieces (190) in between. The U-shaped pieces (190) in between are connected by a similar pattern on the other side of the PCB 200. Metal areas on different sides of the PCB 200 may overlap without shorting due to the insulating board substrate material between them, allowing metal areas (202) to be almost two U-spacings wide.

Dotted slots (204) indicate the slots that will receive the legs of U-shaped pieces (190) in between, which are interconnected by a similar metal pattern on the opposite side of the board. Shape (203) is widened out around the dotted slots in order to provide clearance between metal areas (202) and the legs of the U-shaped pieces (190) to which they shall not connect.

Only as many U-shaped pieces (190) are interconnected as turns required to form a primary or secondary winding of the transformer. The unconnected ends of the first and last of a chain of interconnected U-shaped pieces (190) then form the terminal connections to the winding. One of these two terminal connections will lie in the center hole, and thus additional PCB layers may be needed to transport it to the outside. For example, the interconnecting pattern of Figure 8 may be a printed circuit component that is part of the transformer, and the whole assembly mounted to a separate PCB for interconnecting additional components.

Interconnect patterns in which all four layers of two double side boards (or more, if multilayer) participate may then be contemplated as a way of getting fatter metal interconnect areas for reduced resistance. If only round plated holes are available in a PCB technology, as opposed to the milled slots of Figure 8, the ends of the legs of the U-shaped pieces (190) may be formed into pins that will fit into a round hole, with some consequent increase in the total winding resistance.

Alternatively, an interconnect pattern can be formed using two, single-sided PCBs where one board is placed over the legs of even-numbered U-shaped pieces (190) and soldered first, while access to its metal traces is available, and then a second board is placed over the first board to interconnect odd-numbered U-shaped pieces (190) and soldered on its metal side.

Single-sided boards may be formed from a thick metal sheet, such as copper, that has been attached to a thin insulating carrier material and then etched to form the conducting shapes (201) to form planar interconnecting patterns of low resistance.

Since the pattern of interconnection of U-shaped pieces (190) defines the number of primary and secondary windings and their turns ratios, the final transformer configuration may be varied by changing only the interconnection medium (e.g. , PCB) patterns. According to one embodiment, a prefabricated part comprises U-shaped pieces (190) that have been pre- moulded into an insulating, encapsulating material, which may serve to make a variety of transformer configurations. In this embodiment, the majority of fabrication work can take place before the transformer is committed to a particular design requirement. Figure 9 illustrates such an uncommitted transformer assembly. U-shaped pieces (190), of which only one planar variant is shown for simplicity, are arranged at equianglular intervals and potted into a puck-shaped assembly by use, for example, of an injection moulding machine. The encapsulating material (250) encases the U-shaped pieces (190) except where their legs protrude for interconnection. A slot (260) is formed by the moulding tool into which the magnetic toroid can later be fitted. Thus neither the choice of magnetic material nor the primary or secondary winding

configurations are as yet determined. Only the maximum total number of turns is limited by the number of U-shaped pieces (190) fitted. An exemplary application of transformers constructed according to Figures 7, 8, and 9 is the inventive DC to AC inverter of U.S. Patent Number 8,937,822 to current Applicant, which is hereby incorporated by reference in its entirety. In one embodiment of the '822 patent, a DC-to- AC converter requires a set of floating DC voltages in the ratio 1 : 1/3: 1/9 : 1/27. These are produced by the high-frequency bidirectional DC-DC converter of Figure 2 of the '822 patent, which uses a high frequency transformer having turns ratios of 1 : 1/3: 1/9: 1/27. The primary may be driven by an H-bridge and therefore does not need to be center tapped. The secondaries are center tapped to allow the use of full-wave synchronous rectifiers, which are simpler than full H- bridges. Since the primary has the largest number of turns, it is worth the extra complexity of an H-bridge to halve the total number of turns for that winding. A suitable toroidal core for this transformer is Magnetics Inc part number 0F44916TC in R-material, which has low loss at a frequency of 60KHz. The number turns on the 120 volt primary is 27 and the center-tapped secondaries have 9+9, 3+3 and 1 +1 turns, respectively. Thus the total number of turns needed is 53.

In Figure 6 of the '822 patent, a common mode filter having a common mode choke of approximately 1 mH inductance is shown. This choke can be formed by 9+9 turns, preferably interleaved, on a toroidal core of exactly the same size as the above-mentioned Magnetics Inc part, but in the higher permeability W material. Thus a prefabricated puck assembly such as shown in Figure 9 having 54 U-shaped pieces (190) can be used to meet both the requirements of a 60KHz DC-DC converter transformer needing 53 turns in total by using a suitable interconnect pattern to form the required primary and secondary windings, as well as meeting the requirements for a common mode choke needing 9+9 turns, which may be formed from the same total of 54 turns by paralleling three windings of 9+9 turns using a suitable interconnect pattern to obtain minimum copper loss.

Figure 10 depicts another geometry for a transformer, according to another embodiment.

This embodiment has the properties that (a) the inner and outer conductive members, which fit around the magnetic core from the inside and the outside, respectively, are identical, and (b) the ends of the inner and outer conductive members to be interconnected protrude beyond the bottom face of the toroid at the same radius, which is intermediate between the inner and outer radius of the magnetic core, and where there is a larger circumference to ease interconnect congestion. The inner and outer conductive members now generally resemble a question mark and so are designated ?-shapes (290). It may be seen that the inner conductive members are given odd indices ?-shape 290-1 , 3, 5, etc. , while the outer conductive members are given even indices. With this numbering, interconnecting alternate turns to form separate interleaved windings comprises joining an odd-numbered inner conducting member to the next higher even- numbered outer conducting member with a joint (280) on the upper surface of the toroid, while the legs of the inner and outer conducting members that protrude beyond the bottom face of the toroid are joined in the order 2 to 3; 4 to 5; etc. by an interconnection medium, such a PCB board or etched planar copper pattern. Joints (280) can employ a solder retaining sleeve that for this geomtery might be a deep-drawn part.

Yet another geometry of inventive toroidal transformer according to another embodiment employs PCBs to make both the top interconnecting joint (280) as well as the bottom-face interconnects. According to one embodiment, a particularly simple and low-manufacturing-cost example is shown in Figure 1 1 , although this will not have as good current handling due to the use of round wires, which fill the winding space less efficiently. In Figure 1 1 , both the inner and outer conducting members comprise straight rods, or alternatively pins or wires, vertically disposed around the inner and outer diameter of the magnetic toroid, respectively, at regular intervals. The inner and outer conducting members are interconnected at their top ends by a first interconnection medium pattern (301) and at their bottom ends by a second interconnection medium pattern 302, the interconnection medium patterns being chosen such that, together with the inner and outer conducting members, complete loops are formed that encircle the magnetic core in a spiral winding pattern yielding the desired number of primary or secondary windings with desired numbers of turns. The rods may be cut to equal lengths by an automated machine which places them vertically in a jig. The upper interconnection medium, such as a PCB, is then placed over the top ends of the pins and flow soldered while the jig holds the pins in place. The sub-assembly comprising interconnect pattern (301) with inner and outer connection members now attached is placed over the toroid and the inner and outer connection members inserted into the bottom interconnection medium, which may be a motherboard holding other components. One advantage of Figure 1 1 is the elimination of a winding operation, allowing the transformer to be constructed with parts that are easily made by low-cost automated processes. However, it does not achieve as good copper fill as when copper strips are used, rather than round rods, wires, or pins. Of course the same principle of Figure 1 1 may be used with wide copper strips instead of round pins, with the interconnection media preferably having plated through slots to receive the ends of the strips for soldering, or alternatiively the strips may stamped out with the ends whittled down to one or more square pins which will fit into standard round hole sizes of the printed circuit interconnect patterns (301 , 302).

These embodiments can alternatively comprise U-shaped pieces (190) arranged around a magnetic toroid (1 10), the whole assembly being immersed in cooling oil for high-dissipation applications, and contained within a shielding can to contain the oil. The can is then sealed with either all legs of the U-shaped pieces (190) still protruding though insulating seals for external interconnection to form the required windings, or else the interconnection medium is also contained within the oil bath or shielding can, and only the terminal ends of the final windings brought out through insulating seals.

Embodiments of the present invention have been described for the case when the magnetic core has the shape of a toroid with rectangular cross section. However, the toroidal shape and rectangular cross section are exemplary and not intended to be limiting. A magnetic core of any shape forming a closed loop of magnetic material can be used, for examie an elliptic toroid instead of a circular toroid, or a square or rectangular shape sometimes called a

"squaroid." A closed loop of magnetic material with a gap in it is also sometimes used if the current in a winding is expected to have a DC bias; as used herein, the term "closed loop of magnetic material" also encompasses gapped cores. Moreover, the cross section may be square, rectangular, or rounded and the shape of the encircling inner and outer conducting members may be adapted accordingly.

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.