MATRIX TRANSFORMER HAVING HIGH DIELECTRIC ISOLATION

Abstract: A matrix transformer with high dielectric having the capability of withstanding very high potentials between its primary (102) and secondary (103a, b) and is well adapted for use as a safety transformer in switch mode power supplies. It can also have the primary and the secondary physically separated from each other by an intermediary winding (606) which is at ground potential.

Description: HATRIX TRAHSFORHER HAVIHG HIGH DIELECTRIC ISOLATION BACKGROUND OF THE INVENTION This invention relates to transformers, and in particular to transformers which have a requirement for high dielectric isolation, such as safety transformers. High dielectric insulation requirements are frequently applied to transformers which must be safe from shock hazard. Adding insulating features to a conventional transformer tends to degrade it performance very considerable, particularly transformers for higher power and high frequency use. A matrix transformer comprises a plurality of interdependant magnetic elements, interwired as a transformer, as taught by U. S. Patent 4,665,357, "Plat Matrix Transformer", Herbert, May 12, 1987. Because of the small size of the interdependant magnetic elements, and the close proximity and intermingling of its windings, it is not apparent that the flat matrix transformer could have much dielectric isolating capability. SUHHARY OF THE INVENTION It is an object of this invention to teach improvements to the matrix transformer which provide high dielectric isolation. It is a further object of this invention to teach various techniques of achieving high dielectric isolation. It is a further object of this invention to teach embodiments of the matrix transformer incorporating a safety shield. It is a further object of this invention to teach an embodiment of the matrix transformer having an intermediate low voltage winding, which can be grounded, for additional protection against the consequences of breakdown. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a matrix transformer having high dielectric insulation. Figure 2 shows the transformer of figure 1, without the wiring, to more clearly show the insulating system. Figure 3 shows a section through one of the magnetic elements of figure 1. Pigure 4 shows a section through one magnetic element of a matrix transformer similar to the transformer of figure 1, but incorporating a safety shield. Figure 5 shows a matrix transformer having an intermediate, low voltage winding to further isolate the secondary circuit from the primary circuit. Figure 6 shows the a transformer similar to the one of figure 5, without the primary and secondary windings to more clearly show the intermediate low voltage winding. The ends of the low voltage intermediate winding are shown grounded. PREFERRED EMBODIMENT OF THE INVENTION The matrix transformer is made of many magnetic elements, which are arranged and interwired to behave collectively as a transformer. The art of designing and manufacturing matrix transformers is adaptable to a very wide variety of shapes, sizes and configurations. Transformers belong to a broad family of static devices in which electric currents in conductors interact by means of magnetic induction with changing fluxes in magnetic cores. These include potential transformers, current transformers, flyback transformers, induction coils, "constant current output" transformers, multiple winding inductors and inductors. "Matrix transformer" is used herein as a generic term including any of these devices when they are built using an array of smaller interdependent magnetic elements interwired as a whole. The matrix transformer designed in this way functions as an ordinary transformer, but because of the manner in which the various elemental parts cooperate interdependently, it has some unique characteristics which can be used to advantage in many applications. Matrix transformers can also be designed which have characteristics which no single core device could have. The magnetic elements can be small cores of ordinary design, such as C cores, E-I cores, pot cores or toroids, but alternatively can be one of several new geometries having multiple magnetic return paths such as two parallel plates bridged by a multitude of posts, a plurality of modified cross cores, or a plate of magnetic material having a plurality of holes therein. Different types of interdependent magnetic elements can be inter¬ mixed in an interdependent matrix array as long as the rules of transformers are followed. The matrix transformer can be very flat, and the electrical circuits can be made using printed wiring board techniques. There is a high degree of flexibility and discretion in the design of matrix transformers, including, but not limited to, the number of magnetic elements, the detailed design of the elements, and the physical arrangements of the elements. Also, the windings of the matrix transformer can be arranged in different ways, and there is flexibility in choosing how and where a particular winding enters and exits the transformer. The voltages and the currents in the matrix transformer have a definite relationship, one to another, and this information can be exploited to optimize the design. Pigure 1 shows a matrix transformer which has been built incorporating several features for high dielectric isolation. Typical requirements for high dielectric isolation will include a creep distance for the primary, a creep distance for the secondary, a creep distance between the primary and the secondary, a required number of layers of insulation, a distance through the insulation, and a dielectric test requirement. The transformer. of figure 1 comprises ten interdependant magnetic elements, shown a toroids 101a, - j. A primary winding 102, shown as a push pull (centertapped) winding, five parallel secondary windings 103a,b, (etc. ) , also shown as push pull (centertapped) windings. Primary insulation 105a,b encloses the primary winding 102, and extends entirely through the matrix transformer, uninterrupted, and extends beyond it on both ends. The amount that it extends on each end is determined by the creepage distance requirements to the secondary. Secondary insulation 104a,b-j encloses the secondary winding where it passes through the magnetic elements, and extends beyond each magnetic element on each side. The amount that it extends is determined by the creepage distance of the secondary to the core. The primary and secondary insulation can be insulating tubing, such as Teflon. Figure 2 shows the transformer of figure 1 without the primary and secondary wires. Primary insulation 205a,b could be one continuous piece of insulation, and would not have to extend beyond the transformer on the closed end. This would be preferred if it were important to minimize primary leakage inductance. In some embodiments of the matrix transformer, the path of the primary winding is quite serpentine. For this, a flexible insulation would be preferred, and it would probably be easier to pull the primary wires through the insulation prior to installing it in the matrix transformer. Figure 3 shows a section through one of the magnetic elements of the transformer of figure 1. All of the are similar. 301 is the magnetic element, illustrated as a toroid, though it could be any type of magnetic core such as a U-I core, one side of an E core, a pot core or a specialized core structure for matrix transformers. The primary winding 302 preferably comprises a winding of insulated wire (preferably not just a film or varnish insulation) such as ordinary Teflon type E hook up wire. The secondary winding 303 is similar to the primary, and can be the same gauge, the currents being equal to the primary as is the nature of matrix transformers. In some applications or embodiments of the invention the secondary might not require the same amount of insulation Secondary and primary insulation 305 and 305 enclose each winding. The dielectric isolation criteria are fulfilled as follows: The primary creepage distance to the core and to the secondary is satisfied by the amount that the primary insulation 104 extends beyond the ends of the matrix transformer. The secondary creepage distance to the core is satisfied by the amount that the secondary insulation 105 extends beyond each magnetic element 101. The spacing through the insulation for the primary winding to the core is satisfied by the combined thickness of the insulation of the primary winding wire 102 and the primary insulation 104. The spacing through the insulation for the secondary winding to the core is satisfied by the combined thickness of the insulation of the secondary winding wire 103 and the secondary insulation 105. The spacing through the insulation from primary to secondary is satisfied by the thickness of the wire insulation of both windings and the thickness of both the primary and the secondary insulations. Some variations of the insulation system would be preferred for various applications. If either the primary or the secondary, or both, were a single wire winding, the use of a wire having sufficient insulation of itself would be consistent with the teachings of this invention. Such an insulation would probably have at least two layers, and its total insulation thickness would be greater than " the specification requirements. In some cases this would be preferred even with windings having more than one wire, particularly if the primary wires followed different paths through the transformer, or if it were interrupted, as for switching elements or other components or the like. In other embodiments, it might be preferred to use a second layer of insulation around the primary or the secondary or both. This might be the case where there were more than one wire in a winding, and the winding factor could be improved by using a wire with insufficient insulation to meet the minimum requirements as a layer. Some magnetic materials are very good insulators, such as nickel ferrites. Also, some magnetic cores are encapsulated or otherwise insulated. In either case, the insulation system, particularly the secondary, might be reduced, or even eliminated, and still provide sufficient dielectric isolation. The primary insulations system might be upgraded. A matrix transformer similar to the one in figure 1 can be made with ferrite cores such as Pair-rite (tm) part no. 2677006301, which are about 3/8" outside diameter. The primary and secondary windings can be made with awg 22 Teflon hook up wire. If the primary and secondary windings are each inside a number 12 Teflon sleeving, the transformer will meet very high dielectric isolation requirements. (In test, there was no dielectric failure to the limit of the available test equipment: 40,000 VDC) . A transformer similar to the transformer of figure 1 was installed in a bread board of a push-pull pulse width modulated switch mode power supply. The input voltage was 40 to 60 volts, with 5 volts out. The primary was driven with a Unitrode (tm) part no. 3825 integrated circuit, buffered to improve the output fall time. The primary switches were n-channel power MOSFET's (metal- oxide-silicon field effect transistors). The individual secondaries were rectified with dual Schottky rectifiers. The individual secondary centertaps were connected to ground through small inductors. The anodes of each pair of rectifiers were connected together and to small ceramic capacitors, which was returned to ground, the whole being kept very tight. The outputs were then paralleled. No primary snubbers were used, the drain-source capacitance of the FET's apparently being more than adequate to absorb the stored energy in the primary. An auxiliary winding was used to damp ringing (which was in the order of 20MHz). The bread board was operated a 1 MHz (500 kHz primary frequency) with 250 watt output (5 volt at 50 amperes). The transformer obviously had much greater capacity, and was limited by the test circuit. Figure 4 shows a section of a magnetic element of a transformer similar to the transformer of figure 1, except that a safety shield 406 has been added. All other features are the same as the corresponding features of figure 3. The safety shield would preferably be terminated to a ground plane (which could also be a heat sink) immediately when it left the core. The shield could more fully, or completely enclose the primary or the secondary, or both, (but preferably not both together in one shield) . Figure 5 teaches an embodiment of the matrix transformer having an intermediate low voltage winding to further isolate the primary from the secondary. A primary winding 502 couples a first set of ten interdependant magnetic elements 501k-u. A primary insulator 503a,b extends through the transformer. Pairs of magnetic elements 501k-u couple a pairs of a second set-of magnetic elements 501a-j through intermediate low voltage windings 506a-e. The second set of magnetic elements 501a-j can have any suitable matrix transformer secondary, shown here as five parallel secondaries 503a-e. Figure 6 shows the intermediate low voltage windings more clearly, and shows that they are preferably grounded to a ground plane at both ends. In this manner, it can be seen that no dielectric failure will allow primary voltage to appear on the secondary as long as the ground plane can carry fault currents away.

Claims: I CLAIM 1. A matrix transformer having at least one primary insulator passing through the matrix transformer, and extending beyond the matrix transformer sufficiently to meet primary creepage requirements. 2. A matrix transforemr having a secondary insulator for each of magnetic element of the matrix transformer, each secondary insulator passing through the magnetic element adn extending beyond the magnetic element sufficiently to meet secondary creepage requirements. 3. A matrix transformer having at least one primary insulator, and a secondary insulator for each magnetic element. 4. A matrix transformer having a primary insulator comprising at least one insulating means passing through the matrix transformer and extending beyond the matrix transformer by a distance.sufficient for primary creepage, and an insulation on the conductor of the primary winding, the insulating means and the insulation on the conductor of the primary winding together having a sufficient number of layers and a sufficient total thickness to meet primary insulation requirements. 5. A matrix transformer having a secondary insulator comprising at least one insulating means passing through each of the magnetic elements of the matrix transformer and extending beyond the magnetic elements by a distance sufficient for secondary creepage, and an insulation on the conductor of the secondary winding, the insulating means and the insulation on the conductor of the secondary winding together having a sufficient number of layers and a sufficient total thickness to meet secondary insulation requirements. 6. A matrix transformer havin a first set of magnetic elements, a primary winding interconnecting the first set of magnetic elements, a second set of magnetic elements, an intermediate low voltage winding interconnecting the first set of magnetic elements and the second set of magnetic elements, and a secondary winding interconnecting the second set of magnetic elements. 7. The matrix transformer if claim 6 in which the low voltage intermediate winding is grounded to a ground plane. 8. A matrix transformer having a safety shield for each magnetic element of the matrix transformer interposed between the primary winding and the secondary winding in each of the magnetic elements of the matrix transfomrer. 9. The matrix transformer of claim 8 in which the safety shield entirely surrounds the primary winding of each of the magnetic elements of the matrix transformer. 10. The matrix transformer of claim 8 in which the safety shield entirely surrounds the secondary winding of each of the magnetic elements of the matrix transformer.

Inventors: HERBERT EDWARD, HERBERT, EDWARD
Applicants: FMTT INC, FMTT, INC.
Application year: 1989
Main classification: H01F17
IPC codes: H01F17, H01F27, H01F38