Abstract:
A coaxial push pull transformer is an improved matrix transformer. A number of magnetic cores each contain a pre-wired secondary circuit. The secondary windings are tubular and extend through the core, and the ends of the tubular secondary windings are terminated to make connections to a secondary circuit, such as rectifiers. The cores are placed end to end with the tubular secondary windings aligned and the primary winding is then threaded through all of the cores, so that it is coaxial with the secondary windings when installed, for very low leakage inductance. In the design of the coaxial push pull transformer, care is taken to arrange the terminations of the transformer such that each termination is paired with another termination having a counter-flowing current, to cancel part of the field caused by the flowing currents so as to reduce the overall inductance of the terminals and interconnections. To keep the interconnections to the associated circuitry as short as possible, the associated circuitry may be on circuit boards sandwiched between the transformer cores and directly connected to its terminations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This patent application is a continuation in part of a patent application entitled Cellular Transformers, Ser. No. 10/708,846, filed 27 Mar., 2004 now U.S. Pat. No. 7,023,317 and a provisional patent application entitled Cellular Transformers, Ser. No. 60/460,333 filed 3 Apr., 2003. Priority to these dates is claimed. 
   This patent application is also a continuation in part of a patent application entitled Switched-Current Power Converter, Ser. No. 10/709,484, filed 8 May 2004, which issued as U.S. Pat. No. 6,979,982 on 27 Dec., 2005; a provisional patent application entitled Switched-current Power Converter, Ser. No. 60/473,075 and filed 23 May, 2003; and a provisional patent application entitled Parallel Current Sources for Switched-Current Power Converters, Ser. No. 60/479,706, and filed 19 Jun., 2003. Priority to these dates is claimed. These patent applications are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   To make power converters and like circuits using transformers smaller and more responsive, there is a trend toward using higher and higher frequency excitation. A major obstacle is the parasitic impedances of the transformers, particularly the leakage inductance, both of the windings and of the leads, interconnections and connected circuitry. 
   The prior art “matrix transformer”, sometimes called a “flat transformer”, significantly reduced the leakage inductance of the windings, but there is a need for yet more improvement. A matrix transformer may have a single turn primary which passes through a number n of interdependent “elements”. The elements are separate magnetic cores with their associated secondary windings, and often the elements are assembled as “modules” with through holes through which the primary is threaded at final assembly. 
   A matrix transformer with a single turn primary and n elements will have a ratio of n to one. Because the primary is a single winding passing through all the elements, the currents are constrained to be equal in each element. Usually the secondary windings of the elements are connected in parallel, either directly or at the output of associated rectifier circuits, so the voltages in the elements (and thus the fluxes) are also constrained to be equal. 
   The patent applications cited above for cellular transformers teach an embodiment of the cellular transformer having cellular metal inserts through which a multiple turn primary winding is wound. With a hole for each turn, each active section of the primary winding has a coaxial location within the hole, for very good coupling and low leakage inductance. 
   The patent applications cited above for switched-current power converters show diagrammatically matrix transformers wherein the coaxial winding is applied to matrix transformer elements having a single turn primary. 
   SUMMARY OF THE INVENTION 
   In a transformer, the magnetic core must be excited with alternating voltage so that the integral of the flux over time is zero. This may be accomplished with a single winding in which the polarity alternates positive and negative with equal volt-seconds. In a push-pull transformer the same polarity voltage is used, but it is applied alternately to separate windings having opposite phasing. In a conventional transformer this may be a winding with a center-tap or a split winding, but in as much as only one section of the winding is conducting at any one time, it is the turns of the section that determines the ratio of the transformer. Thus a two turn center-tapped or split winding used in a push-pull transformer is a “single turn push-pull winding”. 
   This invention teaches a coaxial push pull transformer having two coaxial windings, the outer conductor of each being a secondary winding and the inner conductor of each being a primary winding. The coaxial relationship between the primary and secondary windings with the secondary windings surrounding the primary winding has very good coupling for minimal leakage inductance. This invention teaches if that both the primary winding and the secondary windings are push-pull windings, the windings are phased such that when one of the primary conductors is conducting, the secondary winding that surrounds it coaxially will also be conducting so that the currents therein are very closely coupled. 
   This invention teaches that in a transformer having a coaxial push-pull secondary winding that is used in a topology usually having one winding primary, such as a half-bridge or full-bridge power converter primary circuit, it is preferred, none-the-less, to use two primary windings in parallel, one passing through the secondary windings of one phase and the other passing through the secondary windings of the other phase. 
   While the coaxial relationship of the primary and the secondary windings ensures a very low leakage inductance within the coaxial push-pull transformer, care must be taken to ensure that the interconnections and external circuits also have low leakage inductance or the benefits of the coaxial push-pull transformer may be swamped. This invention teaches that the various terminations of the coaxial push-pull transformer should be arranged and disposed such that each conductor is closely proximate to another conductor in which an equal current flows in the opposite direction (counter-flows) for field cancellation to reduce the inductance therein. 
   This invention teaches that the external connections should be minimized to avoid undue inductance in the external connections. This invention teaches that the switching components (solid state switches and rectifiers) may be incorporated within a modular design very close to the transformer windings and the magnetic core. The module may incorporate two elements with their secondary windings terminated on a common circuit board that is sandwiched between them. Further, if there is a first switching component on one side of the board that conducts in one direction when its associated phase is conducting, it is preferred to locate the complementary (same phase) switching component on the opposite side of the board so that when the first switching component is conducting, the one on the opposite side is also conducting but with current flow in the opposite direction (counter-flowing) to reduce the inductance of the circuit. 
   This invention teaches a transformer having extended insulation between the primary and secondary winding terminations for dielectric isolation and to meet creepage requirements wherein the primary windings returns in a coaxial outer conductor that surrounds the extended insulation and returns the current to a plane so that the current therein can be located closely proximate to a counter-flowing current in the secondary circuit. 
   This invention teaches push-pull secondary windings (center-tapped or split) with a rectifying means incorporated into a modular design so that the connections to the external circuits carry only a dc current. 
   This invention also teaches alternate embodiments of the invention employing symmetrical push-pull windings, either in the primary circuit or the secondary circuit or both. 
   This invention teaches a coaxial secondary winding that uses very simple stamped and formed parts. 
   A variant of the push-pull transformer is the double forward transformer, which can be explained as a push-pull transformer in which the two halves of a push-pull winding are in separate cores. In a push-pull transformer the alternate voltage of the same polarity but opposite phasing ensures that the integral of the flux over time is zero, but when the windings are in separate cores, this mechanism is lost. Accordingly, a means for resetting the flux must be incorporated into the design of such power converters. A variety of such circuits are well known to one skilled in the art of power converters. As long as the circuit does not depend upon high leakage inductance within the transformer, such circuits can be used to energize a double forward coaxial transformer. 
   Another embodiment of the invention uses two single triaxial windings, each in a separate core, to implement a double forward transformer topology. The triaxial winding uses the outer conductor as a secondary winding, the next conductor as a primary winding and the innermost conductor as a reset winding. 
   This invention also teaches the use of a “folded” element comprising two cores of half the length, so that odd integer ratio transformers may be fabricated. This invention also teaches that a module incorporating a symmetrical push-pull winding is inherently “folded” so that an odd number can be used for an odd integer ratio of the transformer. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  shows a coaxial push-pull module comprising a magnetic core and the secondary windings of an element of a coaxial push pull transformer. 
       FIG. 2  shows a magnetic core and the outer coaxial conductors of the module of  FIG. 1 . 
       FIG. 3  shows an exploded view of a module of a coaxial push pull transformer. 
       FIG. 4  shows two elements of a coaxial push pull transformer with segments of a primary winding installed in two modules of the coaxial push-pull transformer. 
       FIG. 5  shows the two elements of the coaxial push pull transformer of  FIG. 4 , less the magnetic cores, with segments of a primary winding installed therein, and with current flow indicated by arrows. 
       FIG. 6  shows two elements of a coaxial push pull transformer, less the magnetic cores, with segments of a primary winding installed therein and terminated. Arrows indicate the current flow. 
       FIG. 7  shows a coaxial push pull transformer having four elements. Arrows indicate the current flow. 
       FIGS. 8 and 9  show a method of terminating a primary winding of a coaxial push-pull transformer for minimum leakage inductance while still providing the creepage distance required for dielectric isolation. 
       FIG. 10  shows a coaxial push pull transformer having a symmetrical push pull primary winding, exaggerated for clarity. 
       FIG. 11  shows that the switch end of the transformer of  FIG. 10  may be implemented using MOSFET dice on a circuit board that is directly connected to the primary winding. 
       FIG. 12  shows that the connection end of the transformer of  FIG. 10  may be implemented with power and ground planes that are directly connected to the primary winding. 
       FIG. 13  shows the use of a folded element, to make a transformer with odd integer turns ratio. 
       FIG. 14  shows the module of  FIG. 1  further comprising a circuit board directly connected to its secondary terminations. 
       FIG. 15  shows that a pair of the modules of  FIG. 14  may be used “back to back” with a common circuit board. 
       FIGS. 16 and 17  show how the module of  FIG. 15  may be terminated with power and signal surface mount pads. All of the ac currents are confined to the module. 
       FIG. 18  shows a coaxial push pull module having a symmetrical push pull secondary connection, with the components shown in exaggerated detail. 
       FIG. 19  shows that the switching end of the module of  FIG. 18  may be implemented with a circuit board that is directly connected to the secondary windings. 
       FIG. 20  shows that the dc connection end of the module of  FIG. 18  may be implemented with power and ground planes that are directly connected to the secondary windings. 
       FIGS. 21 through 24  show that an module for a coaxial push pull transformer may be made using a magnetic core with folded metal inserts as the secondary windings. 
       FIG. 25  shows a module comprising a single core with a coaxial secondary winding. This arrangement might be used for a forward converter. 
       FIG. 26  shows the winding of the module of  FIG. 25  without the magnetic core, for clarity. 
       FIG. 27  shows two single core modules, each with a tubular secondary winding. A portion of a primary winding is shown, the whole being a triaxial winding. This arrangement might be used for a forward converter having a separate reset winding. 
       FIG. 28  shows a pair of single turn modules. This arrangement might be used for a double forward converter. 
       FIG. 29  shows two pairs of single turn modules with triaxial windings. This arrangement might be used for a double forward converter with separate reset windings. 
       FIG. 30  shows a schematic of a transformer with symmetrical push pull primary and secondary windings. 
       FIG. 31  shows a schematic of a transformer having double forward windings on separate cores. The windings may be triaxial windings with the center conductor used for reset. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a module  1 , which will become an “element” of a coaxial push pull transformer once a primary winding is installed. A coaxial push pull transformer is an improved matrix transformer (sometimes known as a “flat transformer). The prior art matrix transformer is well described in a tutorial written by the inventor entitled “Design and Application of Matrix Transformers and Symmetrical Converters”, for a seminar presented at the Fifth International High Frequency Power Conversion Conference &#39;90 Santa Clara, Calif., May 11, 1990. 
   The design of the module  1  is best introduced by  FIG. 2 , which shows a magnetic core  2  with two tubular secondary windings  3  and  4  passing through a hole therein.  FIG. 1  shows that two insulating pieces  9  and  10  may be used to insulate and locate the tubular secondary windings  3  and  4 . The tubular secondary windings  3  and  4  may then be terminated by surface mount terminations  5 ,  6 ,  7  and  8  for making an electrical connection to external circuitry. Like reference designator indicate the same part in the different figures. 
     FIG. 3  shows in an exploded view that the tubular secondary windings  3  and  4  may be further insulated by insulation sleeves  11  and  12 .  FIG. 3  also shows alternative terminations  5 A,  6 A,  7 A and  8 A, modified for through-hole installation in a printed wiring board. Various means for making a connection to the secondary windings  3  and  4  may be used, and all would be equivalent for this invention. As an alternative to using insulating sleeves  11  and  12 , the tubular secondary windings  3  and  4  may be coated or jacketed with insulation which may be stripped at the ends to enable connection to the terminations A,  6 A,  7 A and  8 A. As another alternate yet, the tubular secondary windings may be un-insulated but with the separation between them maintained by a physical spacer and the magnetic core  2  may be coated with insulating material. 
   In  FIGS. 1 and 3 , the terminations of the module are undedicated and may be connected externally to comprise a center-tapped or split push-pull winding. For example, if terminal  6  is connected externally to terminal  7 , the connection is the center-tap, and terminals  4  and  8  are the “start” and “end”, to borrow terminology from the art of conventional wound transformers. 
     FIG. 4  shows two elements of a coaxial push pull transformer  21  comprising two of the modules  1  of  FIG. 1  mounted on a printed wiring board  24 . Primary windings  22  and  23  pass through the modules  1 ,  1 . Having such long exposed portions of the primary windings  22  and  23  is not preferred, and it is shown here for illustration purposes only. Long exposed portions of the primary winding  22  and  23  would increase the leakage inductance unacceptably. 
     FIG. 5  shows two elements of a coaxial push pull transformer  31  which is the transformer  21  of  FIG. 4  with the magnetic cores removed, their absence from the modules  1 ,  1  of  FIG. 4  being indicated by the new references  1 A,  1 A. In as much as the transformer will not operate without its magnetic cores, this is for illustration only, to better show the conductors and the current flow therein, indicated by arrows and the magnitude I. A current I enters the left end of the primary  23  and continues to flow from the right end to other elements of the transformer or other circuitry. Because the net ampere turns in a transformer must equal zero (neglecting magnetization currents), an equal and opposite current flows in the secondary windings  4 ,  4  and their terminations  6 ,  6 ,  8  and  8 , as indicated by the arrows and the magnitude I. It is assumed that the current is blocked from flowing in the primary winding  22  and the secondary windings  3 ,  3 , as by open switches or reverse biased rectifiers or the like, as illustrations, not limitations, as is usually the case with push-pull windings. 
   At the central part of the drawing  FIG. 5  of the portion of the coaxial push pull transformer  31 , note that the currents flowing in the termination means  6  and  8  are equal and opposite, or “counter flowing”, a condition that significantly reduces the leakage inductance in those conductors by partly canceling the magnetic field associated with the flowing current. This is not the case at the ends of the transformer  31 , as there is no adjacent elements at those locations.  FIG. 6  shows that counter flowing currents can be achieved at the ends of the transformer as well by using parallel conductors  42 ,  43 ,  44  and  45  as terminations for the primary windings  22  and  23 . Having wide parallel conductors with counter flowing currents is preferred for reducing the leakage inductance of the terminations. 
     FIG. 7  shows a complete coaxial push pull transformer  51  having four elements. The turns ratio will be four to one. It is based upon the partial coaxial push pull transformer  41  of  FIG. 6 , changed as follows: The modules  1 — 1  showing the magnetic cores  2 — 2  (with reference to  FIGS. 1 through 3 ) in place to replace the modules  1 A,  1 A of  FIG. 6 . Two additional modules  1 — 1  have been included, two additional termination means  46  and  47  have been added, and the primary windings  22  and  23  are extended so that the currents therein pass through all of the modules  1 — 1  in series. The external connecting link  45  for the primary winding  23  can be seen on the right. It can be seen that in all of the conductors of the coaxial push pull transformer  51  each current is balanced by an equal and opposite counter flowing current, for low leakage inductance. A coaxial push-pull transformer of any even integer turns ratio may be made by using more or fewer elements in pairs. 
   Note that the coaxial push pull transformer  51  is “folded” so that if one follows the primary windings  22  and  23  through the entire transformer, they form a closed loop, returning so that their termination means  42 ,  43 ,  46  and  47  are in a tight cluster and have counter flowing currents therein. Connections are preferable made to external circuits very close to the termination means  42 ,  43 ,  46  and  47 , and may include a “center-tap” connection to a power source and connections to two switching means as push pull switches to return, as an illustration, not a limitation. 
   The coaxial push pull transformer  51  of  FIG. 7  has the limitation that the closely spaced parallel terminations of the primary winding and the secondary winding may not provide adequate creepage distance for safety isolation requirements unless it is potted or otherwise sealed to block the through-the-air creepage paths.  FIGS. 8 and 9  show a transformer  61 , which is a modification of the transformer  51  of  FIG. 7  to achieve as long a creepage path as is necessary. In  FIG. 9 , the transformer is designated  61 A, to distinguish the exploded view. It is assumed that the primary winding wires  22  and  23  are suitably insulated, probably double or triple insulated, as an illustration, not a limitation. The ends of the wires of the primary windings  22  and  23  may be extended beyond the secondary conductors as far as is necessary to meet the creepage specification from the stripped end of the wires of the primary winding  22  and  23  to the secondary winding terminations. 
   Insulating means  64 ,  64  are then placed over the ends of the wires of the primary windings  22  and  23 , one at each end. The insulating means  64 ,  64  may be molded plastic parts, as an illustration, not a limitation, having sufficient thickness and mechanical integrity to meet the dielectric insulation requirements. The insulating means has cylindrical extensions rising from a plane surface to surround the ends of the wires of the primary windings  22  and  23  and insulate them. 
   Then, termination means  62 ,  63 ,  65  and  66  may be installed over the insulating means  64 ,  64 . Hollow cylindrical extensions extend from the plane of the termination means  62 ,  63 ,  65  and  66  to engage the stripped ends of the wires of the primary windings  22  and  23  and are connected thereto as by soldering, as an illustration, not a limitation. The hollow cylindrical extensions then return the current to the plane surfaces of the termination means  62 ,  63 ,  65  and  66  as coaxial, counter flowing currents for low leakage inductance. The plane surfaces of the termination means  62 ,  63 ,  65  and  66  are now close to the secondary conductors as in  FIG. 7  so that counter flowing currents therein will minimize the leakage inductance therein. 
     FIG. 10  shows a coaxial push pull transformer  71  having four elements and comprising four modules  1 — 1  of  FIG. 1 . A primary winding comprising conductors  72  through  75  passes through the modules  1 — 1  and is connected as a symmetrical push pull winding. The power input connections + and − are on the opposite ends of the coaxial push pull transformer  71  from two switching means  75  and  76 . The primary windings  72  through  75  are shown much longer than is preferred, and the connections of the power input connections + and − and the switching means  75  and  76  are show in exaggerated scale for illustration only, to better show how the windings are connected. 
     FIGS. 11 and 12  show a coaxial push pull transformer  81  in which the switching means  76  and  77  of  FIG. 10  have been replaced with MOSFET dice  76 A and  77 A. The MOSFET dice  76 A and  77 A may be mounted on a circuit board  82  to provide optimally short direct connections to the primary winding  72 ,  73 ,  74  and  75 . “Floating capacitors”  78  and  79  may also be on the circuit board  82 . The design and application of symmetrical converters and floating capacitors is explained in the tutorial referenced above, “Design and Application of Matrix Transformers and Symmetrical Converters”.  FIG. 30  shows a schematic diagram of the symmetrical push-pull transformer, and is discussed further below. In some instances, the area of the end of the transformer may not be large enough to have a conforming circuit board as shown. In that case, the circuit board may extend in one or more directions beyond the edge of the transformer. The circuit board may contain the switches, their drivers, perhaps logic and control circuitry, as an example, not a limitation. 
   The coaxial push pull transformer  81  may be connected to the power source + and − through power and ground planes  83  and  84  which connect directly to the primary winding  72 ,  73 ,  74  and  75 , as shown in  FIG. 12 . 
   The modules  1 — 1  may sandwich a printed wiring board  85  that may contain the secondary connections and circuitry. These are not shown here but are discussed in more detail below. In the coaxial push pull transformer of  FIGS. 11 and 12 , only the primary winding is a symmetrical push pull winding. The four elements comprising the secondary circuits may be connected as a push-pull or spit winding. 
   Note that the symmetrical push-pull primary does not have to be terminated in a circuit board as shown in  FIG. 11  nor in ground and power planes as shown in  FIG. 12 . An option would be to use the transformer of  FIG. 7  with modified terminals, all similar to the terminals  42 ,  43 ,  46  and  47  on both ends. A single circuit board could be sandwiched in the transformer, extending beyond the sides and ends as necessary, and all of the circuit connections and associated components could be installed on the single circuit board. 
   A problem of “folded” matrix transformers, including coaxial push pull transformers, is that an equal number of modules may be used on each side, tending to limit the effective turns ratio to even numbers. For example, the coaxial push pull transformer  71  of  FIG. 10  has four modules  1 — 1  and therefor an effective turns ratio of four to one. Adding or removing one module  1  to make a five to one or a three to one transformer respectively would make it difficult to have neat terminations with counter-flowing currents. One solution is to have cores that are longer than necessary to fill up the extra space. The extra flux capacity is beneficial though the conduction losses would be increased. The need for special parts is not desirable, but it would not be a serious limitation if the need were there. 
   An alternative embodiment is shown in  FIG. 13 , where the coaxial push pull transformer  90  is the coaxial push pull transformer  81  of  FIG. 12  modified by removing two of the modules  1 — 1  and adding a “folded” module  91 . The folded module  91  is equivalent to one of the modules  1  of  FIG. 1 , so the effective turns ratio of this transformer is three to one. The folded module  91  comprises two cores  92 ,  92 , each of which is the same cross section but half the length of the core  2  of  FIG. 1 , so the total flux capacity is the same. The two halves of the folded module  91  are bridged by wide flat connection means  93  and  94 , and the other secondary connections are not changed. The modules  1 ,  1  and  91  of the coaxial push pull transformer  90  may sandwich a printed wiring board  85 A that may contain the secondary connections and circuits. 
     FIG. 14  shows a module  100  for a coaxial push pull transformer that is the module  1  of  FIG. 1  (inverted) further comprising a circuit board  101  upon which are mounted two rectifier dice  102  and  103 . While the connections of a transformer are somewhat discretionary, so long as phasing is observed, a representative connection, as an example, not a limitation, may use diagonally opposite terminations as a center-tap, perhaps terminals  6  (see  FIG. 1  for the reference designators of the terminals) and  7 , preferably connected together in a ground or power plane and perhaps terminated in a positive output terminal (not shown in  FIG. 14 , but an example is shown in  FIG. 16  and discussed below). The other two terminals,  8  and  5  (see  FIG. 1  for the reference designators of the terminals), may connect respectively to the rectifier dice  102  and  103 , and they may in turn connect to another power or ground plane that may also be terminated as a negative output terminal (not shown in  FIG. 14 ). In this manner, all of the ac circuits are confined to the module  100  with optimally short connections. 
     FIG. 15  shows this concept extended to a double module  110  which comprises two of the modules  1  of  FIG. 1  sandwiching a circuit board  111  on which may be mounted four rectifier dice, one die  112  of which is shown. The arrows show that the circuit is well coupled with equal counter flowing currents for reduced leakage inductance.  FIG. 15  also shows primary windings  113  through  116  in place within the module  110 . This double module comprises two elements of a coaxial push pull transformer. Note in particular that the currents flowing in the circuit board  111  and through the rectifier die  112  are counter-flowing, left to right on the top and right to left on the bottom. Thus the cancellation of the magnetic field may be achieved in the switching devices as well, if they are carefully placed. 
   Note, however, that the module  110  of  FIG. 115  is not constrained to any particular connection of its terminals nor any particular circuit upon its circuit board  111 . As an example of an alternative winding arrangement and circuit, the module  110  of  FIG. 15  may be connected as a symmetrical push-pull module, and switches and capacitors may be put on the circuit board  111 . Another example of a symmetrical push-pull module is shown below in  FIGS. 18 through 20  and a schematic diagram is shown in  FIG. 30 . If necessary, the circuit board  111  may extend beyond the edges of the modules  1 ,  1 . 
     FIGS. 16 and 17  show the module  110  of  FIG. 15  modified with an alternative circuit board  111 A having power and return terminals  121  and  122 , shown, as examples, not limitations, as fairly large surface mount terminals. The circuit board  111 A may contain synchronous rectifiers (not shown) that may or may not include drivers. Regardless of the details of the circuit, which will vary from application to application as would be well known by one skilled in the art of power converters and transformers, the circuit board  111 A may require timing and control signals from external circuits, and these may be brought to the circuit board by signal terminals, shown representatively as terminals  123  and  124 . Fewer, more or no signal terminals may be needed for a particular application and the circuit board  111 A may be modified accordingly. 
     FIGS. 18 through 20  show modules  140  and  160  which are secondary modules arranged and connected so that the secondary windings thereon are symmetrical push-pull secondary windings. The symmetrical push-pull secondary winding may be seen in the schematic of  FIG. 30 , which is discussed further below. The modules  140  and  160  comprises two magnetic cores  141 ,  141  surrounding tubular secondary windings  142  through  145 . The tubular secondary windings are terminated on one end of the element  140  by four termination plates  146  through  149 . The tubular secondary windings  142  through  145  are connected at the other end of the element by a power plane  171  and a ground plane  172 , which are in turn the positive + and negative − secondary power output connections for the modules  140  and  160 , as shown in  FIGS. 18 through 20 , but more particularly in  FIG. 20 . 
   In  FIG. 18 , the first tubular secondary winding  142  is connected to the secondary tubular winding  143  when a first switching means  150  is closed, and the third tubular secondary winding  144  is connected to the fourth tubular secondary winding  145  when a second switching means  151  is closed. The element  140  may also have floating capacitors  163  and  164 . 
   In  FIG. 19 , the switching means  150  and  151  of  FIG. 18  are replaced by solid state switching means  166  and  167 , which may, as examples, not limitations, be rectifier die, Schottky rectifier die or metal oxide silicon field effect transistor (MOSFET) die as synchronous rectifiers. The solid state switching means  166  and  167  may be mounted on a circuit board  165  that in turn is soldered to the tubular secondary windings  142  through  145 . Chip capacitors  168  and  169  may also be mounted on the circuit board  165  and connected as floating capacitors. If the solid state switching means  166  and  167  are synchronous rectifiers, timing and control may be from external circuitry (not shown) which may connect to the module  160  through a plurality of control terminals  173 — 173 . More, fewer or no control terminals may be needed in a particular application. If necessary, the circuit board  165  may extend beyond the face of the magnetic cores  141 ,  141  in one or more directions, to enable the use of larger components, more components and components on both sides of the circuit board  165 , as options. 
   The symmetrical push-pull secondary module is naturally “folded”, and each one will comprise one element of the finished coaxial push pull transformer once it is assembled and the primary windings are installed and terminated. 
     FIGS. 21 through 24  show an alternate embodiment of the invention. A module  200  comprises a magnetic core  201  and first and second secondary windings  202  and  203  that are formed of a sheet metal conductor material such as copper. A rectangular section is shown where the first and second secondary windings are within the magnetic core  201 , as an example, not a limitation, but a “U” shape or round shape would be alternatives. Flat extensions of the first and second secondary windings  202  and  203  may be formed around the edge of the magnetic core  201  at the ends, to make surface mount feet, as shown, as an example, not a limitation. The first and second secondary windings  202  and  203  must be electrically insulated from each other and the magnetic core  201 . An insulating insert  204  may be used between the first and second secondary windings  201  and  203 , as shown, as an example, not a limitation. Alternatively, the first and second secondary windings  102  and  203  may be coated with an insulating film or they may be installed in an insulating sleeve (in the manner of  FIG. 3 ). As a further alternative, the magnetic core may be coated with an insulating coating. 
   A push-pull winding is usually a split or center-taped winding wound on a magnetic core, but the teachings of a coaxial winding and closely coupled terminations with counter-flowing currents may be applied to other windings as well.  FIGS. 25  shows a module  210  comprising magnetic core  211  having therein a single tubular secondary winding  212 , which may be terminated with surface mount terminals  213 ,  213 .  FIG. 26  shows the tubular secondary winding  212  and the surface mount terminals  213 ,  213  without the magnetic core  211 , to show more particularly the design of the secondary winding. Because the magnetic core  211  is necessary for a properly functioning transformer, this drawing  26  is for illustration only. 
     FIG. 27  shows part of a transformer  220  comprising two of the modules  210  of  FIG. 25  placed end to end and further comprising coaxial windings  222  and  221  running through the center hole defined by the tubular secondary winding  212 . With the tubular secondary winding  212 , the whole comprises a triaxial winding. An outer winding  222  may be the primary winding, and the inner winding  221  may be a reset winding. Such an arrangement could be used for a forward converter having a separate reset winding, as an example, not a limitation. 
     FIG. 28  shows that the modules  210  of  FIG. 25  may be used in parallel pairs, and  FIG. 29  shows a representative partial transformer  230 , in which all of the parts can be identified from the above discussions. It is contemplated that this transformer would be completed in the manner of the push-pull transformers above for use as a double forward converter, for example, the manner of  FIGS. 7 ,  8  and  9 ,  10  through  12  or  16  and  17 , as examples, not limitation.  FIG. 31  shows a schematic of a representative double forward transformer, and is discussed further below. A forward converter is usually excited with power pulses of one polarity, and there are a large number of schemes to provide a reset pulse of opposite polarity, any of which can be used with the transformer  230  of  FIG. 29 . However, it is contemplated that the transformer (completed as discussed above) could be energized as a push-pull transformer using the outer windings  222 ,  222  as the primary windings, just as if the windings were in a double core rather than two single cores. Then the inner windings  221 ,  221  could then be cross coupled to the other side to provide a reset excitation, as shown in  FIG. 31 . 
     FIG. 30  shows a representative schematic of a symmetrical push-pull transformer  300 . A primary winding  301  comprising four equal primary winding sections  302  through  306  has two switching means  307 ,  308  symmetrically disposed with respect to a power input Vi and a return. The primary winding sections  301  and  304  may be wound through one or more first cores  311 , and the primary winding sections  302  and  303  may be wound through one or more second cores  312 . Alternatively, all of the windings may be wound on a single core, phased as shown, it being electrically equivalent, but the two or more core example is more representative of the coaxial push-pull transformers discussed above. 
     FIG. 30  also shows a symmetrical push pull secondary winding  320  comprising four equal secondary winding sections  321  through  324 . The secondary winding sections  321  and  324  may be wound through the first core  311  and the secondary winding sections  322  and  323  may be wound through the second core  312 . 
     FIG. 30  also shows “floating capacitors”  307 ,  308 ,  327  and  328 . The physical location of the floating capacitors  307  and  308  is illustrated in  FIG. 10  as capacitors  78  and  79 . The physical location of the floating capacitors  327  and  328  is illustrated in  FIG. 18  as capacitors  163  and  164 . 
   As shown in  FIG. 30 , if each coil represents a single turn, the transformer will have a one to one turns ration. The schematic may be interpreted more generally, however. If more than one first core  311  and second core  312  is used, there may be a plurality of secondary winding sections  321  through  324 , one set for each pair of cores and wired in parallel. A single primary winding may pass through all of the elements. A physical example of a symmetrical push pull primary winding is shown in  FIG. 10 , and a physical example of a symmetrical push pull secondary winding is shown in  FIG. 18 . 
   The use of a symmetrical push-pull primary winding does not require the use of a symmetrical push-pull secondary winding, and vice versa, either could be a conventional push-pull winding or another configuration such as half bride or full bridge. 
     FIG. 31  shows a schematic diagram of a representative symmetrical double forward converter  350 . The double forward converter  350  has two sections, one above the other in the schematic diagram. A first section comprises two magnetic cores  370  and  371  which may have therein triaxial windings. A first secondary section  381  may comprise the outer conductor of a triaxial winding within the first core  370 , a first primary winding section  351  may comprise an intermediate conductor of the triaxial winding within the first core  370 , and a first reset winding section  354  may comprise the inner conductor of the triaxial winding within the first core  370 . 
   A second secondary section  382  may comprise the outer conductor of a triaxial winding within the second core  371 , a second primary winding section  352  may comprise an intermediate conductor of the triaxial winding within the second core  371 , and a second reset winding section  353  may comprise the inner conductor of the triaxial winding within the second core  371 . 
   A third secondary section  383  may comprise the outer conductor of a triaxial winding within the third core  372 , a third primary winding section  356  may comprise an intermediate conductor of the triaxial winding within the third core  372 , and a third reset winding section  359  may comprise the inner conductor of the triaxial winding within the third core  372 . 
   A fourth secondary section  384  may comprise the outer conductor of a triaxial winding within the fourth core  373 , a fourth primary winding section  357  may comprise an intermediate conductor of the triaxial winding within the fourth core  373 , and a fourth reset winding section  358  may comprise the inner conductor of the triaxial winding within the fourth core  373 . 
   A first switching means  360  connects the first primary section  351  to the second primary section  352  when the first switching means  360  is on. A second switching means  361  connects the third primary section  356  to the fourth primary section  357  when the second switching means  361  is on. 
   A third switching means  362  connects the first secondary section  381  to the second secondary section  382  when the third switching means  362  is on. A fourth switching means  364  connects the third secondary section  383  to the fourth secondary section  384  when the fourth switching means  364  is on. 
   To avoid cluttering the schematic, four tie points A, B, C and D have been shown, it being understood that connecting like lettered tie points to each other shows one way in which the reset windings  353 ,  354 ,  358  and  359  may be energized. 
   The separate reset winding has several advantages. One is that the power excitation and the reset excitation can be separately controlled. The power pulses could overlap somewhat without shorting the windings through the transformer coupling. In a conventional transformer, the coupling between the primary winding and the reset winding would have significant leakage inductance, but the triaxial winding arrangement of the present invention would have near perfect coupling for extremely low leakage inductance. 
   The single core with the single tubular winding has the advantage that the core to the winding may not need to be insulated, allowing a tighter fit between the core and the winding, which in turn allows the core volume to be smaller and the thermal coupling to be greater.