Foil Winding Inserts for Matrix Transformers

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Evan Fletcher
7 years ago
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1 Foil Winding Inserts for Matrix Transformers A preferred construction for matrix transformers is to use metal foil secondary windings inserted into magnetic cores. A number of cores are aligned, and the primary winding is wound through the remaining window area. Figure 1: A push pull secondary winding of metal foil is inserted into a core (shown in phantom) with an insulating separator. For many circuit layouts, it is preferred to have the ends of the ends of the two windings in opposite corners. Electrically and magnetically, it makes no difference how the metal foil windings are terminated, so the terminations are chosen to facilitate the connection to the external circuit. Often it is easier to have the respective ends of a push pull winding on opposite corners, as in figure 1, but they may be terminated on one surface, as in figure 2. Figure 2: A metal foil push pull secondary winding with the ends of the two windings on the same surface. Also, electrically and magnetically, it makes little difference whether the magnetic core is square or round. With foil windings, however, square cores make it easier to fold the metal foil at the edge of the center hole and around the outside corners. Further, the flat surface of a square core is easier to mount on a flat surface and contact to a flat heat sink will be better, being a surface rather than a line.

2 Gap-less ferrite cores are usually used in matrix transformers. They are inexpensive, very rugged, and have good characteristics at high frequencies. Amorphous metal cores are an alternative, and they have a much higher flux capacity than ferrite. Although more expensive, they may be the better choice for lower frequency operation or to make a more compact transformer. Amorphous metal cores can be wound with a square center hole, though some radius in the corner is necessary and the outside corners will be quite rounded. Figure 3: In this matrix transformer core, one end of each winding is folded back around the corner of the core while the other end extends straight out. For ruggedness, the terminations can be folded around the corners of the core and bonded securely. However, for some installations it will be better to have the terminations extend straight out, either on some of the terminations as shown in figure 3 or on all of them, as in figure 4. Figure 4: In this matrix transformer core, all of the windings are terminated with straight tabs. The core and the winding conduct heat out of the core. Whether this is mostly through the core or mostly through the winding depends upon the installation. With a high flux density, most of the loss may be in the core, but at high current, the loss in the winding may dominate. Regardless, it is good practice to make the thermal interface between the core and the winding as good as possible. It is not necessary to provide safety insulation between the secondary winding and the core if the core is considered to be part of the secondary circuit. This concept has been accepted by the safety agencies, and many matrix transformer products have been

3 registered. In a push pull circuit, there may be significant voltages between the two sides of the winding, so it is preferred to insulate the core and/or the winding. However, this need be working insulation only, and can be a thin coating. The metal foil windings can be formed to have a slight outward tension to ensure a tight fit to the inside of the core. The thermal interface can be improved further with a thermal compound or thermal adhesive. Figure 5: A matrix transformer core has a single foil winding comprising a split cylinder of metal strip. When installed, the foil winding is compressed slightly, and presses against the core to ensure good thermal contact. The core may have a metal jacket for heat sinking and for mounting on a flat surface. An outer metal jacket can be used to improve heat sinking from the outside surface of the cores and to provide a mounting means. This is particularly advantageous with round cores. Figure 6: This cross section of a matrix transformer core shows a push pull secondary winding with an insulating separator. The metal foil is shaped to define four conduits for the primary winding, for improved heat sinking and lower leakage inductance. If a number of primary turns are to be wound in a matrix transformer, the problem with the lay of the wire in the through holes can be solved by providing guiding means for the wires. In figure 6, the metal foil inserts are formed to provide guide ways for four primary wires, perhaps a two turn push pull primary winding. In figures 7 and 8, the guide ways are molded or extruded parts inserted within the metal foil secondary. They may be metal, in which case they become part of the secondary circuit and add to the electrical and thermal conduction as well as reducing the leakage inductance between the primary and the secondary windings.

4 For a less critical application, plastic inserts can be used. With modern double and triple insulated wire, additional insulation usually is not needed, so the insert shown in figures 7 and 8 are open on the edges. Figure 7: If a number of primary wires are to be installed in a matrix transformer, an insert to guide the wires may be used. The inserts may extend slightly beyond the core. A close fitting coupling could be used over the extended ends to align the through holes from core to core. Alternatively, with an insulating insert, the insert could extend on one end of the core and be recessed somewhat on the other, so one core would plug into the next, to ensure alignment. Figure 8: An end view of the matrix transformer core of figure 7 showing the wire guides. The wire guides may be metal, to improve the heat transfer and leakage inductance, or they may be plastic, for economy.

5 The inserts occupy some window area, but the net gain in having the wires lay in an orderly arrangement is very worthwhile. When the wires tangle and cross over each other (without guides), it is very difficult to get a good fill and consistent characteristics. Some applications may have particularly stringent dielectric isolation requirements. If so, as many layers of insulation as needed may line the cores, and the insulation may extend as far as necessary for creepage. One experimental matrix transformer used Teflon sleeves extending from end to end. It withstood 40,000 volts (the limit of the available test equipment) without breaking down the insulation. Figure 9 shows a matrix transformer core having a molded or extruded insulating insert. Guide ways for six wires are shown, but a similar part could be made with any number of guide ways for different numbers of wires. The insulators can extend beyond the ends of the cores, for creepage (though with triple insulated wire the extension may not need to be the entire creepage distance and it may serve more as a protective bushing to prevent wire damage). Figure 9: The wire guides may be closed plastic insert, for higher dielectric isolation between the primary and secondary windings. If necessary, the insert can extend on each end to provide a creepage distance. An alternative construction, not shown, could use a metal insert in the general shape of the insert of figure 9 or a wrap of foil, insulated from secondary winding by peripheral insulation and from the primary circuit by using triple insulated wire for the primary winding. Such a floating metal insert could be used as a shield or safety ground if terminated at one end to a ground plane. Because the drain wire would be very short and paralleled in the several cores, it would be much more effective than the shield in a conventional transformer for bypassing noise or fault currents to ground.

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