AU702177B2 - Discharge lamp circuit and ballast - Google Patents

Description

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-1- DISCHARGE LAMP CIRCUIT AND) BALLAST The present invention relates to gas discharge lamps which term includes mercury vapour, metal halide, high pressure sodium and fluorescent lamps. In particular, the present invention relates to an improved ballast for such discharge lamps and an operating circuit for such discharge lamps.

Hitherto it has been known to operate discharge lamps, and in particular metal halide lamps, by means of a lead peak ballast. The main function of such a ballast is to provide a current regulating function. The nominal mains supply voltage is able to vary typically plus or minus six per cent from the supply authority, but due to voltage 1 o drops throughout an installation the voltage variation at the actual lamp fitting may be substantially greater than this. The lead peak ballast reduces the fluctuation in lamp output with fluctuations in the supply voltage so that, for example, a ten per cent change in the supply voltage results in about a ten per cent change in lamp wattage and a somewhat greater change in light output. This compares with about a 20% change compared with a conventional ballast.

The conventional lead peak ballast and operating circuits incorporating same suffer from a number of disadvantages including having an "off-time" in the lamp current just after ignition. This "off-time" is the period each half cycle during which :°'current is essentially zero and adversely affects lamp re-ignition each half cycle during 20 the early stages of lamp warm-up. This can cause some lamps to extinguish before ireaching full operating temperature.

Also this type of ballast is very substantial in size. A typical prior art lead peak ballast for a 400W metal halide lamp has a lamination stack which has a length of 143 mm, a width of 116 mm and a stack height of 100 mm. The lamination stack and 25 the copper windings wound thereon together weigh approximately 8 Kg.

It is the object of the present invention to provide a ballast and an operating circuit which will substantially overcome or ameliorate the above disadvantages and, in S "particular, provide a ballast which is more efficient, smaller and more economical to manufacture.

30 In accordance with the first aspect of the present invention there is disclosed a ballast for gas discharge lamps, said ballast comprising first and second windings 1 wound on a common magnetic core having three magetic paths, a sole path for each winding not coupling the other one of said first and second windings and a joint 1 magnetic path coupling both said first and second windings, said sole magnetic paths including a common saturable portion which, when saturated, diverts magnetic flux from said sole paths to said joint path, and said first and second windings on said joint path being magnetically opposed.

In accordance with the second aspect of the present invention there is disclosed an operating circuit for a gas discharge lamp, said circuit comprising a first winding 4i ia a j aq-r 'S -2connected across an AC mains supply, and a series connection of a capacitor, said lamp and a second winding, said series connection also being connected across said AC supply with the magnitude of the impedance of said capacitor at the frequency of said supply being greater than the magnitude of the inductance of said second winding, said first and second windings being wound on a common magnetic core having three magnetic paths, a sole path for each winding not coupling the other one of said first and second windings and a joint magnetic path coupling both said first and second windings, said sole magnetic paths including a common saturable portion which, when saturated, diverts magnetic flux from said sole paths to said joint path, and said first and second windings on said joint path being magnetically opposed.

Several of the embodiments of the present invention will now be described with reference to the drawings in which: Fig. 1 is a circuit diagram of the prior art lead peak ballast operating circuit, Fig. 2 is a schematic view of the magnetic core of the ballast of Fig. 1, Fig. 3 is a graph of the open circuit lamp voltage produced by the circuit and ballast of Figs. 1 and 2, S° Fig. 4 is a circuit diagram of a first embodiment of the present invention, Figs. 5 and 6 are respectively first and second embodiments of magnetic cores able to be used in the circuit of Fig. 4, 20' Fig. 7 is a modified circuit similar to that of Fig. 4 but showing the 3• "modification required in those regions where the supply voltage is less than desired, ~Fig. 8 is a similar circuit to Fig. 7 but illustrating the modification required in those regions where the supply voltage is greater than desired, Fig. 9 is a circuit similar to Fig. 4 but illustrating the use of an ignitor, 25 Fig. 10 is a circuit similar to Fig. 9 but illustrating an alternative ignitor arrangement, Fig. 11 is a circuit similar to Fig. 4 but utilising a fluorescent lamp, and t .;,Figs. 12 and 13 are views similar to Figs. 5 and 6 but each illustrating a further core embodiment.

S 30 In relation to Figs. 1 and 2, the prior art is a lead peak ballast circuit in which two windings W1 and W2 are wound in magnetically aiding fashion on a single magnetic core as illustrated in Fig. 2. Magnetic shunts MS are provided in order to increase the leakage inductance of the ballast. This has the effect of decreasing the mutual coupling of the windings Wi and W2 during lamp operation. At the frequency of the mains supply S, the magnitude of the impedance of the capacitor C is much greater than the inductive reactance of the ballast and the ballast core centre limb adjacent to the slot operates at a saturated oi near saturated condition and the result is a 1 degie of non-linearity in the magnetic circuit. As a consequence, if the mains supply S voltage should increase, then the current increases and the self inductance of the -o -2a winding W2 is decreased. Thus a smaller inductive impedance is subtracted from the capacitive impedance and therefore the total impedance increases. As a consequence, the increase in the lamp current brought about by the increase in the supply voltage S is

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reduced. As a consequence of this arrangement, the entire operating circuit has a relatively constant operating current.

Prior to ignition, the lamp L is essentially an open circuit and the voltage applied to the lamp L is increased by the transformer action of the windings W1 and W2. The distortion in the voltage applied to the lamp prior to ignition, as illustrated in Fig. 3, is caused by the saturation of the magnetic core of Fig. 2. The increase in the voltage applied to the lamp over and above the supply voltage S is normally sufficient to ignite the lamp L. However, some lamps require very high volts (in the order of a few thousand volts) and consequently require an electronic ignitor.

In particular, from Figs. 1 and 2 it will be apparent that the windings W1 and W2 are wound on the centre limb of the core in magnetically aiding arrangement so that the voltage induced in winding W2 by the flux produced by the current in winding Wl is in the same direction as the applied voltage. This is necessary in order to achieve the voltage boosting required prior to ignition of the lamp L.

5 Turning now to Fig. 4, the circuit of the first embodiment of the present invention is superficially similar to that of the prior art in Fig. 1, save that the windings Wi and W2 are opposed. However, the magnetic arrangement of the core on which S the windings W1 and W2 are wound is substantially different from that of Fig. 2 and Sthis results in a substantially different performance.

20 As seen in both Figs. 5 and 6, the magnetic core on which the windings W1i and W2 are wound has three magnetic paths. Winding W1 has a sole magnetic path indicated by P1 which does not couple winding W2. Winding W2 has a sole magnetic path indicated by P2 which does not couple winding W1. In addition, there is a joint path indicated by P3 which couples both windings. As indicated in Fig. 5 the limb which carries both paths P1 and P2 preferably includes air gaps. A cut out portion (indicated by broken lines in Fig. or some other reluctance increasing arrangement, is included in the portion of the sole magnetic paths P1 and P2 which is common. Thus this portion, or part of it, in operation saturates. As a consequence, this saturation diverts flux into the joint magnetic path P3 where it will be seen that the windings W1 and W2 are magnetically opposing. The saturation there also introduces a non-linearity in the inductive reactance of winding W2. A similar arrangement applies in relation to Fig. 6 which is configured from two Es and a single I. Other reluctance increasing Sarrangements include air gaps and/or portions of reduced cross-sectional dimensions, and hence area.

With reference to Figs. 4 to 6, prior to ignition the lamp L is essentially an open circuit, the mains voltage is applied across winding Wi, however, essentially no induced voltage appears across winding W2 since the flux caused by the mains voltagei does not link the turns of winding W2. However, the mains voltage is applied across fN:.JBOI0000 255:cg

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I -4the series connection of winding W2, the capacitor C and the lamp L and this voltage is sufficient to ignite the lamp.

Once the lamp starts, it initially acts as if it were substantially a short circuit.

During this condition, the flux in winding W2 created by the lamp current opposes the flux in winding W1 created by the mains voltage. As a consequence, the common path saturates because the sum of the two fluxes in the "common" sections have combined to saturate those parts and part of the fluxes tend to flow in the joint magnetic path P3 rather than flow through the sole magnetic paths P1 and P2 The saturation of the joint magnetic path P3 causes the inductive reactance of winding VWi2 to decrease.

However, as the capacitive reactance of the capacitor C is linear (ie independent of lamp current or applied voltage) the overall impedance in series with the lamp (being the difference between the capacitive reactance and the inductive reactance of winding W2) increases. This acts to reduce the lamp current change at starting and during 1 operation whenever there is a supply voltage change.

e When the lamp L warms up the voltage drop across the lamp L increases.

Under these circumstances the phase relationship between the flux generated in winding to W1 and that in winding W2 changes. However, the overall magnetic path linking J .i winding W2 continues to operate in a non-linear part of its magnetic characteristic.

SThe overall effect is to keep the current through the lamp L fairly constant irrespective of any changes in the supply voltage S.

In addition, when the common magnetic path starts to saturate, then windings W1 and W2 start to become magnetically mutually coupled. As the saturation increases so does the degree of coupling. However, the polarity of the voltage induced in winding W2 by the flux produced by winding W1 is in opposition to the supply voltage. In effect, this induced voltage opposes any change in the supply voltage S thereby tending to keep the voltage actually applied to the lamp circuit more constant.

Fig. 7 illustrates a modification to the winding W1 required in order to increase the effective supply voltage S. Such an arrangement is used in those localities where the supply voltage S is lower than desired (particularly to initially ignite the lamp L) and/or to optimise ballast design. Fig. 8 illustrates the analogous position for those localities where the supply voltages is greater than desired. Along side each of windings W1 and W2 are two parallel solid lines which indicate the magnetic core of that winding. The solid lines are connected by means of dotted lines to indicate that under lamp operating conditions there is a small degree of mutual coupling between the two magnetic cores.

The circuits of Figs. 4, 7 and 8 do not incorporate any ignitor. The circuits of Figs. 9 and 10 illustrate two ignitor circuits utilising an ignitor IG. The arrangements of thewindings W1 and W2 is essentially the same. In Fig. 9.the capacitor C is illustrated connected between the windings W2 and W1. However, as indicated by ir~l ,onn~Mlc~.r, dotted lines in Fig. 9 the capacitor C could be placed in alternative positions indicated by letters Cl and C2. These alternative positions apply to all the other circuits.

In the circuits of Figs. 4 and 7 to 10, a high intensity discharge lamp L is utilised. However, the present invention is also applicable to fluorescent lamps and Fig. 11 illustrates a typical circuit. Here the fluorescent lamp Li has filaments at each end which are each connected to a corresponding filament heating winding FH which derives its power by transformer action from winding Wi1 as indicated in Fig. 11.

Turning now to Fig. 12, here a further magnetic core, and hence ballast, embodiment is illustrated. The core is formed from I-shaped laminations which can all be easily stamped from strip steel. There is an outer periphery 20 of rectangularconfiguration having a central strip 21 aligned with the longitudinal axis of the periphery 20. The two windings Wi and W12 are A ound on the central strip so as to4 -9oppose each other. Between the windings Wi and W2 are located two shunts 22 which *0 ***are again formed from strip and aligned with the transverse axis of the rectangular periphery 20. Between each end of the central strip 21 and the periphery 20 is located one of a pair of air gaps 23. Preferably the shunts 22 include slots 24 which, since they 0 are optional, are indicated by broken lines.

As indicated in dotted lines in Fig. 12, the various strip components making up the periphery 20 can be end abutted or v-abutted. Also the proportions of the periphery 20 can be adjusted so that the periphery 20 is square rather than rectangular, or has its transverse axis (as the term is used above) longer than its longitudinal axis. That is, the 99 rectangle is elongated in the opposite direction, this makes the shunts 22 longer than as 9999 illustrated.

A still further core and ballast embodiment is illustrated in Fig. 13. Here the 25 core is made from two E-laminations 30, 31 and two I-laminations 32, 33. The Elaminations 30, 31 are opposed and back-to-back abutted. The E-laminations 30, 31 have the windings Wi and W2 respectively wound on their centre limbs 35, 36 which are slightly shorter than the outer limbs thereby creating air gaps 38, 39 between the centre limbs 35, 36 and the corresponding I-laminations 32, 33.

In the backs of the E-laminations are located slots 40 and 41 which increase the magnetic reluctance of the magnetic path formed by the abutting backs.

The magnetic performance of the cores of Figs. 12 and 13 is substantially the same as that described above in relation to Figs. 5 and 6, and will be clear to those skilled in the art.

The above described embodiments of the present invention give rise to a number of advantages. Foremost amongst these is that the magnetic, circuit is able to be operated at a very high flux density and this enables the amounts of iron and copper in t-he ballast to be substantially reduced. Material savings are in the vicinity of twenty to forty per cent over the prior art lead peak ballast. A prototype ballast constructed by I -_Lf I__III~YLI -6the applicant has a magnetic core with a length of 114 mm, a width of 76 mm and a stack height of 70 mm. This is for the magnetic core of Fig. 6. In addition, the weight of such a ballast including its copper winding is only 5 Kg. Typically losses are also reduced, being down by approximately twenty per cent.

In addition to these substantial constructional advantages including the ability to use readily available standard laminations or and cores, there are a number of operational advantages able to be achieved. In particular, the lamp starting current does not have any "off-time" and the rate of change of current as it changes polarity is quite high which is conducive to good lamp re-ignition during its warm-up period. In addition, the operating circuit has a high impedance to audio frequency injection signals (eg 1050 Hz) such as are used in ripple control switching of hot water supplies. As a consequence, there is no need for a blocking inductor as is often required in the prior art circuits when shunt power factor capacitors are used.

Similarly, there is a low harmonic distortion on the current drawn by the operating circuit and this is highly advantageous for the supply authorities. i ~In addition, the operating circuit operates at near unity power factor and again this is an advantageous result.

The foregoing describes only some embodiments of the present invention and 0 modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention.

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Claims (28)

1. A ballast for gas discharge lamps, said ballast comprising first and second windings wound on a common magnetic core having three magnetic paths, a sole path for each winding not coupling the other one of said first and second windings and a joint magnetic path coupling, both said first and second windings, said sole magnetic paths including a common saturable portion which, when saturated, diverts magnetic flux from said sole paths to said joint path, and said first and second windings on said joint path being magnetically opposed.

2. A ballast as claimed in claim 1 wherein said core comprises a first T- shaped portion having a stem and two arms, each of said arms being U-shaped and being arranged to return back to the base of said stem, said sole paths each comprising one of said arms and said stem with said first winding being wound on one of said arms and said second winding being wound on the other of said arms, and said joint C omagnetic path comprising said arms and the base only of said stem. S 15

3. A ballast as claimed in claim 2 and including a pair of air gaps located C C f one at the extremity of each arm and extending to the base of said stem.

4. A ballast as claimed in claim 2 or 3 wherein said stem includes a t reluctance increasing arrangement.

5. A ballast as claimed in claim 4 wherein said reluctance increasing 20 arrangement includes an arrangement selected from the class consisting of a cut-out portion, a reduction in cross sectional area, and an air gap.

6. A ballast as claimed in claim 1 wherein said core has a figure 8 configuration formed from a stack of two opposed E-laminations between which is interposed an I-lamination, said first winding is wound on the centre limb of one of said oc 25 E-laminations and said second winding is wound on the centre limb of the other of said E-laminations, each said sole magnetic path comprises the outer limbs and centre limb of a corresponding one of said E-laminations and extends along said I-lamination, and said joint magnetic path comprises the centre limbs of both said E-laminations, the outer limbs of both said E-laminations and extends across said I-lamination. 30

7. A ballast as claimed in claim 6 and including a pair of air gaps located one at the extremity of the centre limb of each E-lamination and extending to said I- lamination.

8. A ballast as claimed in claim 6 or 7 wherein each said centre limb of said E-laminations includes a reluctance increasing arrangement.

9. A ballast as claimed in claim 8 wherein said reluctance increasing arrangement includes an arrangement selected from the class consisting of a cut-out portion, a reduction in cross sectional area, and an air gap.

A ballast as claimed in claim 1 wherein said core. comprises an outer periphery within which is lokated a central member extending substantially across said II IC.e L I I I II -8- periphery, said first and second windings being located on said central member, and a magnetic shunt means extending from said central member at a location between said windings to said periphery.

11. A ballast as claimed in claim 10 wherein said periphery is rectangular, said central member is straight and is aligned with either the longitudinal or transverse axis of said rectangular periphery, and said shunt means comprises two shunt members extending from said central member to said rectangular periphery along either said transverse or longitudinal axis respectively.

12. A ballast as claimed in claim 10 or 11 and including air gap means between said central member and said periphery.

13. A ballast as claimed in claim 11 or claim 12 when dependent upon claim 11 wherein each said shunt member includes a reluctance increasing arrangement.

14. A ballast as claimed in claim 13 wherein said reluctance increasing arrangement includes an arrangement selected from the class consisting of a cut-out portion, a reduction in cross sectional area, and an air gap.

A ballast as claimed in claim 1 wherein said core is formed from a o stack of two E-shaped laminations and two I-shaped laminations, wherein the three Sparallel limbs of each E-lamination are spanned by a corresponding I-lamination to form two substantially identical EI pairs, each EI pair is arranged with said E- 20 laminations opposed and back-to-back abutting, and said first and second windings are wound on the centre limbs of said E-laminations.

16. A ballast as claimed in claim 15 wherein between each centre limb of said E-lamination and the corresponding I-lamination is located an air gap.

17. A ballast as claimed in claim 15 or 16 wherein a reluctance increasing S: 25 arrangement is located in the abutting backs of said E-laminations.

18. A ballast as claimed in claim 17 wherein said reluctance increasing arrangement includes an arrangement selected from the class consisting of a cut-out r portion, a reduction in cross sectional area, and an air gap. 9

19. A ballast for gas discharge lamps, said ballast being substantially as described with reference to any one of Figs. 5, 6, 12 or 13 of the drawings.

An operating circuit for a gas discharge lamp, said circuit comprising a first winding connected across an AC mains supply, and a series connection of a capacitor, said lamp and a second winding, said series connection also being connected across said AC supply with the magnitude of the impedance of said capacitor at the frequency of said supply being greater than the magnitude of the inductance of said second winding, said first and second windings being wound on a cormmon magnetic core having three magnetic paths, a sole path for each winding not coupling the other i i RA one of said first and second windings and a joint magnetic path coupling both said first and second windings, said sole magnetic paths including a common saturable portion i -9- which, when saturated, diverts magnetic flux from said sole paths to said joint path, and said first and second windings on said joint path being magnetically opposed.

21. A circuit as claimed in claim 20 wherein said AC supply is connected to only some of the windings of said first winding, and said series connection is connected to said AC supply via the remaining windings of said first winding.

22. A circuit as claimed in claim 20 wherein said series connection is connected to said AC supply via a tapping on said first winding.

23. A circuit as claimed in any one of claims 20-22 and including an ignitor for said lamp.

24. A circuit as claimed in claim 23 wherein said ignitor is connected in parallel with said lamp.

A circuit as claimed in claim 23 wherein said ignitor is connected in series with said lamp.

26. A circuit as claimed in any one of claims 20-22 wherein said first winding includes a pair of filament heating windings mutually coupled thereto and which comprise transformer secondary windings for said first winding.

27. An operating circuit for a gas discharge lamp, said circuit be :substantially as described with reference to any one of Figs. 4, or 7-11 of the drawings.

28. An operating circuit for a gas discharge lamp, saia circuit having first i 20 and second windings which are magnetically opposed on a common core, said circuit being as claimed in any one of claims 20-27 and said core forming part of the ballast as claimed in any one of claims 1-19. DATED this Twenty-first Day of December 1998 Trestoto Pty Limited It I t S Patent Attorneys for the Applicant 30 SPRUSON FERGUSON DISCHARGE LAMP CIRCUIT AND BALLAST ABSTRACT The present invention discloses an improved ballast and discharge lamp operating circuit. The ballast has two independent paths (P1 and P2) each of which carries a corresponding winding (W1 and W2). The windings also have a joint magnetic path. The independent paths each have a common portion which is saturable thereby increasing the mutual coupling between the two windings (W1 and W2). In the circuit the winding a capacitor and the lamp are connected in series with the capacitive reactance X C greater than the inductive reactance XW2- This series connection is in parallel with the winding (W1) and across and AC supply. Lamp current remains substantially constant with variations in supply voltage. 4• *a. Q o~ o