US 3703677 A
There is disclosed an inverter circuit particularly, but not exclusively, for operating fluorescent lamps. This circuit has a transformer to the primary of which is to be supplied a periodically switched D.C., a load being connected across the secondary. The transformer has an annular core having at least one air gap in it and two limbs projecting towards one another from opposing parts of the annulus and separated by an air gap. The primary and secondary windings surround the annulus on opposite sides of the limbs.
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Description (OCR text may contain errors)
United States Patent Farrow  FLUORESCENT LAMP INVERTER CIRCUIT  Inventor: Victor Farrow, London, England  Assignee: Thorn Electronics Limited, London,
England  Filed: June 19, 1968  Appl. No.: 738,165
'  Foreign Application Priority Data Aug. 11, 1967 Great Britain .;.37,083/67  US. Cl ..32l/45 R, 315/254, 336/165, 336/178  Int. Cl. "H02!" 7/52  Field of Search....315/98, 212, 219, 254,100 U, 315/100 H, 100 T; 321/45; 323/892, 89.8;
 References Cited UNITED STATES PATENTS 3,008,068 11/1961 Wilting et a1.... ..315/100 'l [451 Nov. 21, 1972 1,353,711 9/1920 Bergman ..336/l65 X 2,170,446 8/1939 Edwards ..336/165 X 2,949,565 8/1960 Rohloff et a1 ..315/98 FOREIGN PATENTS OR APPLICATIONS 1,035,405 4/1953 France ..3l5/l00 H 6,605,747 10/1966 Netherlands ..315/100 H Primary ExaminerWilliam M. Shoop, .lr. Att0mey-Laurence Burns  ABSTRACT There is disclosed an inverter circuit'particularly, but not exclusively, for operating fluorescent lamps. This circuit has a transformer to the primary of which is to be supplied a periodically switched DC, a load being connected across the secondary. The transformer has an annular core having at least one air gap in it and two limbs projecting towards one another from opposing parts of the annulus and separated by an air gap The primary and secondary windings surround the annulus on opposite sides of the limbs.
1 Claim, 7jDrawing Figures ACOUTPUT FLUORESCENT LAMP INVERTER CIRCUIT The present invention relates to inverted circuits, particularly, but not exclusively, for use for operating fluorescent lamps. The invention, when applied to the operation of such lamps, is especially concerned with arrangements in which the D.C. supply voltage is between 6 and 50 volts, the commonest supply voltages used are 12 and 24 volts and the most popular wattage is about 40 watts. These parameters imply the use of fairly thick wire on the primary windings of the transformers (for instance 18 or 20 s.w.g.). The frequency of operation-may be in the range of to 50 kc/s, but in most cases is likely to be about 20 kc/s as this is suitably above audible frequency.
A typical inverter circuit of the kind discussed in the preceding paragraph is shown, in schematic form, in FIG. 1 of the accompanying drawings.
In FIG. 1, the terminals 10 and 11 are connected across a suitable D.C. supply. The circuit includes switching transistors V and V capacitors C and C and resistors R and R connected as shown to the primary windings of a transformer T The secondary winding of the transformer across which an A.C. output is developed is connected to a load, in this example a fluorescent lamp -12. I
The transformer T serves two purposes: the first is to step up the primary voltage to a suitable level for operation of the lamp, and the second is to provide adequate leakage inductance to stabilize the lamp current. It must be appreciated that in most other circuits an additional ballast component, either an inductor or a capacitor, is required to perform this stabilizing function.
An additional advantage of the circuit shown is that the cathode heating voltage V is high before the lamp strikes and falls when the lamp is alight. This is desirable because power is conserved when the lamp is in use. To achieve the same result with other circuits involves considerable complication.
FIG. 2 shows, somewhat schematically, a leakage reactance transformer, such as the transformer T in FIG. 1, in use at present. This comprises a core of square or rectangular cross section formed in two E- shaped parts, the center limbs carrying bobbins 1 and 2 respectively. The core material is usually manganesezinc or nickel-zinc ferrite and a gap 13 is provided between the center limbs of the two parts. This gap determines the pre-strike frequency of the A.C. output. Bobbin 1 contains the whole of the primary windings and bobbin 2 contains the whole of the secondary windings. Adequate leakage reactance is obtained by virtue of the two bobbins being adjacent and coaxial.
In some cases wedges 14 of silicon iron are provided between the bobbins but this is of doubtful value since these iron wedges are very lossy at high frequency and unless very carefully proportioned can substantially reduce the available pre-strike secondary voltage.
In the absence of a secondary load, the main magnetic flux set up by the primary coil follows the path A. When the secondary coil is loaded, i.e., when the lamp strikes, this coil sets up an opposing m.m.f. The two opposing magnetic fluxes meet in the center of the core and the balance of the primary flux is compelled to return by the composite ferrite-air path B. In so doing it induces losses both in the surrounding metal work and in the thick copper primary wires.
One practical effect of this is that it is not always possible to get pro-rata increases in efficiency by increasing the transformer size or wire gauges. For example, for 40 watt inverters in use at present the best performance obtainable, compatible with general allround performance and economic mechanical design, is approximately 65 to 70 percent. A large proportion of the losses are due to the transformer which will overheat unless special precautions are taken.
According to the present invention, there is provided an inverter circuit comprising a transformer and means for applying to the primary winding of the transformer a periodically switched voltage derived from a D.C. supply when this is applied to input terminals of the circuit, an A.C. output being derived, in use, from the secondary winding of the transformer, the transformer having an annular core provided with one or more air gaps and with two limbs projecting toward one another from opposing parts of the annulus and having an air gap between their ends, and primary and secondary windings surrounding the annulus on opposite sides of said limbs.
The invention will be described, by way of example, with reference to FIGS. 3 and 4 (ad) of the drawings in which FIG. 3 shows in elevation a transformer that may be used in a circuit as shown in FIG. 1 in carrying out the present invention, and
FIG. 4 shows at (a),.(b), (c) and (11) views in top plan, front elevation, side elevation and underside plan respectively of one half of thecore of the transformer of FIG. 3.
Referring to FIG. 3, this shows one embodiment of a transformer that can be used in a circuit as shown in FIG. 1 in accordance with the present invention. The
core has a rectangular annular shape and the twobobbins 1 and 2 are arranged on the outer limbs, gaps Z/2 (making a total gap length of Z) are provided in the outer magnetic flux path linking the two bobbins, and gap of length y is provided between two center limbs C and D which extend between opposing parts of the annulus and form a magnetic shunt.,This magnetic shunt causes the opposing m.m.f. magnetic fluxes to by-pass most of the main winding. Some fringing will occur in the center gap y but this will only influence a small section of the coil windings.
The ratio of the gap y to the main circuit gap Z determines how much secondary pre-strike voltage is lost due to the effect of limbs C and D. However, owing to the physical make-up of the core a comparatively large y gap can be used whilst maintaining a good ratio of x/y. This is a measure of the protection which the coils are getting from stray flux.
Arranging the coils on the outer limbs, as shown in FIG. 3, results in a larger cooling surface than with the arrangement of FIG. 2 and the winding depth is smaller. The gaps of length Z/2 determine the prestrike frequency. The gap length y may vary between 0.01 inch and x/2.
In FIG. 4 are shown views of one half of a core of the transformer of FIG. 3 in a practical example. At (a) and (d) are top and underside plan views respectively, (b) is a view in front elevation and (c) is a view in side elevation. It will be noted that the center limb C has been extended in height (dimension h in FIG. 4(a), that is the dimension in a direction perpendicular to the while delivering 40 watts to the load and when powered by a 28-volt D.C. battery.
What is claimed is:
1. An inverter circuit comprising D. C. input terminals, A.C. output terminals, a transformer, a coupling between said input terminals and the primary winding of said transformer, switching means in said coupling, and connections between the secondary winding of said transformer and said output terminals, said transformer having an annular core, at least one air gap in said annular core, and two limbs projecting toward one another from opposing parts ofsaid annular core and having an air gap between their ends, said primary and secondary windings surrounding said annular core on opposite sides of said limbs, and a primaryshunting condenser, wherein said limbs have a dimension greater than said annulus in a direction perpendicular to a plane through the axes of said windings.
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