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3 0 T/ 8-68s 551 c7_ oo t'-, o _3 t_ I o I PARTIAL CORE PULSE TRANSFORMER O_ mo C _ O_ INVENTORS ROBERT N. LAWSON 6317 Esther Ave. N.E. Albuquerque, New Mexico D o_ GERALD J ROHWEIN U _ 9209 Evangeline N.E. 0 Albuquerque, New Mexico Thisreportwas preparedas an accountof worksponsored.byan agencyof the UnitedStates Government.Neitherthe UnitedStates Governmentnoranyagencyth,_reof,norany of their employees,makesanywarranty,expressor implied,or assumesanylegal liabilityor responsi- bilityfor the accuracy,completeness,or usefulnessof any information,apparatus,product,or ) LO f'4 processdisclosed,or representsthat its use wouldnot infringeprivatelyownedrights.refer- _3 ence hereinto any specificcommercialproduct,process,or serviceby tradename,trademark, t'- manufacturer, or otherwisedoes no:.necessarilyconstituteor implyits endorsement,recommendation,or favoringby the UnitedStates Governmentor anyagencythereof.the views and opinionsof authorsexpressedhereindo not necessarilystate or reflect those of the UnitedStatesGovernmentor anyagencythereof. [tls"i'btbuti_nof THI,_ DOCUMENTI$ UNLIMITEO

4 S-68,551 PARTIAL CORE PULSE TRANSFORMER The United States Government has rights in this invention pursuant to Contract No. DE-AC04-76DP00789 between the United States Department of Energy and American Telephone and Telegraph Company. Field of the Invention. 5 The invention relates to transformers and in particular to a pulse transformer capable of generating high voltages and particularly adapted for use in space, airborne and mobile ground applications. Backzround of the Invention A pulse transformer functions to step-up the voltage between a pulse-forming 10 network, which generates a rectangular waveform, and a load. To avoid distorting the rectangular waveform, the pulse transformer requires a wide-band frequency

5 characteristic, i.e., the transformer must have a high-coupling coefficient, a low leakage inductance, and a low winding capacitance. For applications requiring less than 50 kilovolts, a conventional full-core transformer, comprising a conductive winding wound around an iron-core, adequately meets these requirements and provides a distortion free 5 step-up in voltage. However, the operating voltage achieved by a conventional full-core transformer is a direct function of weight of the transformer. Therefore, for applications requiring a voltage greater than 50 kilovolts and where weight considerations are important, the weight of a conventional transformer becomes prohibitive, particularly for air, space, or mobile ground uses. For example, a conventional 1000-KV transformer 10 with a one-microsecond pulse width, requires an iron core weighing over 1 ton. Various attempts have been made to provide a light-weight pulse transformer capable of operating at high voltage with low distortion. However, such attempts have been generally unsatisfactory. One candidate for such applications is transformer wherein no iron core is provided. Such "air-core" transformers comprise only two concentric or 15 nested conductive windings. However, to achieve adequate coupling with the pulseforming network, the air-core transformer requires much larger diameter conductive windings than that of an iron-core transformer and therefore has a correspondingly larger capacitance and leakage inductance. Although the air-core transformer is much lighter than the full-core equivalent, the increased capacitance distorts the output pulse. Further, 20 leakage inductance inherent in large diameter air-core transformers also contributes to pulse distortion by degrading the rise time. Finally, since the air-core transformer

6 requires a large winding, the overall size of the transformer is large and thus is not suited for air, space or mobile ground applications. Patents directed to transformers exemplifying various features of the prior art include the following. U.S. Patent No. 4,536,733 (Shelly) discloses a full-core 5 transformer with a unique secondary winding. U. S. Patent No. 4,719,422 (de Walle et al) discloses an eddy current probe having multiple ferrite slugs placed inside two nested windings. The arrangement of ferrite slugs is flat and flexible and the device is intended for scanning irregular surfaces to detect flaws. U. S. Patent No. 4,486,731 (Wescott) discloses a communications device including pancake coil with ferrite strips interleaved 10 within the coil. The ferrite strips are provided to minimize the directional sensitivity of the coil when placed in magnetic fields linking the coil from different directions. U.S. Patent No. 4,518,941 (Harada) discloses a full-core transformer with electrostatic shields in the form of biased foils between the winding layers. The shields are provided to reduce capacitive noise coupling caused by the winding. U. S. Patent No. 4,496, (Richardson) discloses a high voltage, full-core transformer with a slotted shield around the core legs inside the windings. U. S. Patent No. 4,342,976 (Ryser) discloses a closed toroidal core having a winding in the form of flat laminated strips soldered together. U. S. Patent No. 4,092,621 (Nyswander) discloses an ordinary full-core transformer wound with copper strips rather than wire. 20 An earlier publication of interest here is R.N. Lawson and G. J. Rohwein "A Study of Compact, Lightweight, High Voltage Inductors with Partial Magnetic Cores,"

7 Sandia National Laboratories, Albuquerque, New Mexico 87185, which discloses the use of partial cores comprised of symmetrical arrangements of ferrite bars in raising the inductance of high voltage windings. These partial cores are disclosed as being more efficient in terms of inductance gain per weight of material than either full-flux path cores 5 or solid-slug type cores. Summary. of the Invention In accordance with the invention, a partial-core pulse transformer is provided which produces high-voltage output pulses with low distortion and generally overcomes the disadvantages of the prior art discussed above. A partial core pulse transformer 10 constructed in accordance with the invention comprises a pair of high frequency windings arranged in a generally cylindrical configuration defining an inside and an outside surface and including a plurality of pairs of circumferentially spaced ferromagnetic bars disposed so as to extend longitudinally along the winding. Each pair of bars includes an inside bar disposed in spaced relation to the inside surface of the winding and an outside bar 15 disposed in spaced relation to the outside surface of the winding. Preferably, for each pair of bars, the inside bar and the outside bar are disposed immediately opposite each other along the winding, and further, the pairs are equispaced around the circumference of the winding and the bars have a length at least equal to the width of the winding. 20 The pulse transformer of the invention permits stepping up of the voltage of a low voltage square wave pulse without significant distortion of the pulse waveshape because of

8 the greatly improved magnetic coupling provided between the windings as compared with an air core transformer. In fact, the coupling provided is comparable to certain slug cores and full-core transformers but without the very serious weight penalty associated with the latter. Coupling coefficients slightly greater than 0.99 can be obtained with partial ferrite 5 cores. Further, the pulse transformer of the invention also provides increased band width as compared with an air-core transformer which as noted above, is important in preventing distortion of the pulse. The pairs of ferrite bars disposed along the winding allows the transformer of the invention to achieve the necessary inductance, and thus mutual coupling, for high voltage operation without either the very high weight penalty of 10 a full-core transformer or the high capacitance distortion of a air-core transformer. The invention thus provides a light-weight, small-size pulse transformer for generating high voltages for use, in particular, to drive lasers, microwave generators, and various particlebeam devices. Other features and advantages of the invention will be set forth in, or be apparent 15 from, the detailed description of the preferred embodiments which follows. Brief Description of the Drawings _,.,,9 V_lie,. -_. "Jff-'. Figure 1 is a schematic diagram of a/lstep-up pulse transformer circuit including a,g_'z.. _.Jb-9/ pulse forming network and a pulse transformer in accordance with the invention; Figure 2 is a transverse cross sectional view of a partial core pulse transformer 20 constructed in accordance with a preferred embodiment of the invention;

9 Figure 3 is a longitudinal cross sectional view taken generally along line II-III of Figure 2; Figure 4 is a graph of inductance as a function of the number of ferrite bars for three exemplary embodiments of the invention; 5 Figure 5a is a transverse cross-sectional view, similar to that of Figure,/', of a /_ 0-tt.-qt partial core transformer constructed in accordance with an exemplary embodiment of the invention, wherein five ferrite bars are arranged symmetrically;..._'f 7'"f6" _ Figure 5b is a transverse cross sectional view, similar to that of Figures/t / and 5a, of a partial core transformer constructed in accordance with an exemplary embodiment of 10 the invention, wherein five ferrite bars are arranged in close proximity; Figure 6 is a graph of inductance as a function of weight for three exemplary embodiments of the invention; and Figure 7 is a graph of inductance gain per ferrite bar weight as a function of bar length for three exemplary embodiments of the invention. 15 Detailed Description of the Preferred Embodiments Preferred embodiments of the invention will now be described in connection with the drawings, and referring to Figure 1, a partial core pulse transformer, indicated at 10 and including windings 12, is used for stepping up the voltage o_"an output pulse provided by a conventional pulse-forming network 11 connected to a load As shown in Figures 2 and 3, transformer 10 includes conventional cylindrical windings (winding-pair) 12 with a plurality of pairs of ferromagnetic bars 14 or like

10 ferromagnetic members disposed symmetrically along the inside and outside surfaces of the winding so as to extend parallel to the axis of cylindrical windings 12. Each pair of ferromagnetic bars 14 includes a first bar disposed adjacent to but spaced from the inside surface of windings 12, and a second bar disposed adjacent to but spaced from the outside 5 surface of windings 12 and positioned immediately opposite the first bar. The ferromagnetic bars 14 are preferably comprised of ferrite or a like ferromagnetic material and each has a length equal or greater to the width of windings 12. Windings 12 are constructed from any conventional technique used for constructing an air-core winding but, as discussed above, windings 12 need not be any larger than a 10 conventional full-core winding. The inductance gain per weight of magnetic material is maximized when bars 14 are placed with at least a two millimeter-wide air gaps between adjacent bars and with the radial thickness of the bars chosen to be the minimum necessary to achieve desired coupling and winding inductance values. 15 It will be evident from the foregoing that the transformer used in the present invention is similar to a conventional full-core transformer but, in contrast to a full iron core, transformer 10 has ferrite bars 14 disposed along the inner and outer surfaces of winding 12 as described above. The provision of bars 14 increases the inductive coupling of transformer 10 to pulse-forming network 11 but does not correspondingly increase 20 either winding capacitance or leakage inductance. Thus transformer 10 provides maximum winding inductance with a minimum weight of ferromagnetic material.

11 The performance characteristics of transformer 10 will now be described with respect to various exemplary embodiments. Figure 4 graphically shows inductance as a function of the number of pairs of bars 14 for three sizes of the bars: in embodiment A, indicated in Figure 4 by "stars," the bars 14 are 1 x 1 x 8 inches in size; in embodiment 5 B, indicated by "circles," the bars 14 are 1 x 1/2 x 8 inches; and in embodiment C, indicated by "squares," the bars 14 are 1/2 x 1/2 x 8 inches. In all three embodiments, inductance is measured for a typical 10:1 spiral-strip winding having an outer diameter of 15.5 inches. The inductance is measured with respect to a 20-turn secondary winding. As can be seen from Figure 4, inductance increases as a generally linear function of the 10 number of pairs of bars. The advantage of symmetrical bar spacing is illustrated with reference to Figures 5a and 5b. Figure 5a shows an embodiment with five pairs of bars 14 arranged symmetrically around winding 12, while Figure 5b shows an embodiment with five pairs of bars 14 arranged in a side-by-side relation in a small sector of winding 12. In both 15 cases the sizes of winding 12 and bars 14 are that of embodiment A discussed above. The inductance for the embodiment of Figure 5a is 729 #H whereas the inductance for the embodiment of Figure 5b is only 507 #H. Referring to Figure 6, the effect on the inductance of transformer 10 of the radial size of bars 14 will now be discussed. Figure 6 is a graph of inductance gain as a 20 function of the weight of bars 14 for a 4-inch wide, 20-turn coil for the three embodiments (A, B, and C) discussed above. As can be seen from Figure 6, in

12 embodiment A, with 1 x 1 x 8-inch bars, the inductance gain is 17.4 #H/kg whereas in embodiment C, with the 1/2 x 1/2 x 8-inch bars, the inductance gain is 43 #H/kg. Inductance gain is also a function of the length of bars 14. Figure 7 shows the gain per unit weight, as a function of length, for four symmetrically spaced embodiments 5 of! x I x 8-inch ferrite bars. The embodiments of Figure 7 include three, six, nine, and twelve pairs of bars represented by "stars", "circles", "squares" and "triangles",.. respectively. Inductance is measured for a 1 inch wide, 18 inch outer-diameter, 20-turn winding. As can be seen from Figure 7, the inductance gain per weight is maximum for a bar length to winding width ratio of about 12 and declines for higher values. 10 It is evident from the foregoing that the inductance of transformer 10 can be maximized, and the weight of the transformer can be minimized by choosing the optimum spacing, radial thickness and length for bars 14. Although the invention has been described with respect to exemplary embodiments thereof, it will be understood by those skilled in the art that variations and modifications 15 can be effected in these exemplary embodiments without departing from the scope and spirit of the invention.

13 ABSTRACT OF,THE DISCLOSURE A light-weight partial-core pulse transformer is provided for generating high voltage output pulses with low distortion. The transformer includes sets of ferrite bars arranged so as to extend longitudinally along the inside and outside surfaces of a high frequency cylindrical coil winding-pair. The ferrite bars are arranged in pairs with the bars of each pair being located on opposite sides of the winding-pair. The bars are preferably disposed in a radially symmetric arrangement around the winding-pair, and each has a length at least equal to the width of the winding-pair. 14

14 10 t0 ".t4 t2 FIG. 50 FIG.5b

15 INDUCTANCE (uh) 70- C 10-0 I I I I I I _ NO. OF BARS FIG. 4 INDUCTANCE (uh) 70-0 m C B A 2O t0 0 I I I I I I I I I I I I I I I I _ I I i I I I I : TOTAL WEIGHT (GRAMS) FIG. 6

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