WO2007141562A1

WO2007141562A1 - Ballast

Info

Publication number: WO2007141562A1
Application number: PCT/GB2007/050247
Authority: WO
Inventors: Alexander Rollo Spowart; Robert Goodman
Original assignee: Uv Energy Limited
Priority date: 2006-06-09
Filing date: 2007-05-10
Publication date: 2007-12-13
Also published as: GB0611408D0

Abstract

A frequency adaptive ballast is disclosed for operation of a glow discharge lamp. An optimum operation frequency for a ballast is found that gives maximum radiation intensity of the lamp, namely by measuring the intensivity of emitted radiation at constant tube power for varying ballast frequencies, and selceting an operating frequency that corresponds to a maximum radiation intensivity.

Description

Ballast

The present invention relates to a ballast, and in particular to an electronic ballast to control the start up and operation of a fluorescent or gas discharge lamp.

The basic operation of a gas discharge lamp is well known. A metal vapour is suspended in a gas and then excited by an electrical power supply to produce a plasma which produces radiation. Common metals used are mercury or sodium, while the gas is usually a noble gas such as argon or neon.

A fluorescent lamp is a gas discharge lamp that uses a mercury dopant in an argon or neon gas, so that ultraviolet (UV) radiation is emitted. An inner surface of the lamp is coated with a phosphor, which fluoresces upon incidence of the UV radiation to produce light in the visible spectrum. Fluorescent lamps can be considered as negative resistance devices, because as the current flow through the lamp is increased, more gas in the lamp becomes ionised, which drops the electrical resistance of the lamp and allows mor current to flow. Therefore, a gas discharge lamp is provided with a ballast to regulate the current that is supplied to the lamp.

The simplest form of ballast is a resistor, but this is very energy inefficient and so is used in only a very few applications. A magnetic ballast uses an inductor which improves efficiency. However, the most common form of ballast is an electronic ballast, which uses electronic circuitry to give more advanced control of the current regulation. Electronic ballasts can be supplied which provide for different methods of starting a lamp, so that the most energy efficient ballast can be chosen based on the prospective use of the lamp which is to be powered. Electronic ballasts are generally smaller, lighter and more efficient than magnetic ballasts.

A lamp connected with a magnetic ballast will illuminate on each half cycle of the AC mains freguency that powers the lamp, thus for a 50Hz mains power supply, a lamp with a magnetic ballast will flicker at 10OHz. An electronic ballast, however, transforms the freguency of the power, typically up to about 2OkHz. 1 According to accepted wisdom, if one wishes to

2 increase the intensity of radiation emitted from the

3 lamp, the power supplied must be increased, that is,

4 a greater current must be supplied to the lamp. 5

6 According to a first aspect of the present

7 invention, there is provided a method of selecting

8 an operating freguency of a ballast for powering a

9 gas discharge lamp, comprising the steps of 0 measuring the intensity of emitted radiation at 1 constant tube power for varying ballast freguencies, 2 and selecting an operating freguency that 3 corresponds to a maximum radiation intensity. 4 5 According to a second aspect of the present 6 invention, there is provided a method of operating a 7 gas discharge lamp comprising operating a ballast at 8 a predetermined selected freguency, said 9 predetermined selected freguency being an operating 0 freguency that corresponds to a maximum radiation 1 intensity for varying ballast freguencies at 2 constant tube power.

'.j 4 Preferably, the operating freguency is chosen to be 5 within a range having its lower bound at the maximum 6 freguency. 7 8 Preferably, the gas discharge lamp comprise a UV 9 tube operable in the glow discharge mode. 0 1 Preferably, the operating freguency is chosen to be 2 between fifty-five and fifty-eight kilohertz. According to a third aspect of the present invention, there is provided a gas discharge lamp comrising an electronic ballast, said electronic ballast being adaptable to operate at a predetermined freguency, said predetermined freguency being an optimum operation freguency that corresponds to a maximum radiation intensity for varying ballast freguencies at constant tube power.

Preferably, said electronic ballast comprises a ballast IC for controlling the operation of the gas discharge lamp, and charge pump means for feeding back a fraction of the tube power when ignited back to the ballast IC.

Preferably, said charge pump means comprises a resistor and a forward biased diode connected in series between the lamp input and the power supply of the ballast IC.

Preferably, said electronic ballast further comprises a forward biased diode connected between a charging capacitor and the power supply of the ballast IC.

In further aspects, the invention provides a UV sterilisation apparatus comprising the lamp of the third aspect, and a method of sterilising fluid comprising the step of exposing the fluid to radiation emitted by a gas discharge lamp comprising an electronic ballast, wherein the operating frequency of said ballast is selected according to the first aspect; and a similar method where the gas discharge lamp is operated according to the second aspect .

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows general UV tube discharge characteristics, wherein the glow discharge operates in the abnormal glow region 16;

Fig. 2 shows the dominant 254nm peak in the UV emission from a glow discharge tube;

Fig. 3 shows The UV emissions from a modern medium pressure tube;

Fig. 4 shows the spectral power distributions of a glow discharge;

Fig. 5 shows the spectral power distributions of a medium pressure discharge;

Fig. 6 shows the variation of the luminous efficacy of UV generation at 254nm for varying diameters of glow discharge tubes;

Fig. 7 shows an output stage of a ballast circuit; Fig. 8 shows a graph of the variation of UV intensity with ballast freguency;

Fig. 9 shows a sample design ballast circuit; and

Fig. 10 shows a modification to the design of the circuit of Fig. 9.

One particular type of gas discharge lamp is a cylindrical lamp operating in the glow discharge mode that emits UV radiation, and these will now be discussed. However, it is to be appreciated that the invention is in no way limited to any one type of gas discharge lamp, and that the following is only by way of background information and/or for the purposes of describing one or more possible embodiments.

Cylindrical lamps operating in the glow discharge mode are the basis of modern efficient fluorescent lighting. Such tubes first appeared about 1938. For fluorescent tubes for lighting, a doped glass envelope is employed to stop the escape of harmful UV-C at 254nm from the internal glow discharge.

The history of UV generation and the discoveries of its uses are well known. Johann Ritter discovered UV via its chemical reactions in 1801. Niels Finsen over the period 1860-1904 became the father of UV therapy in medicine following the use of UV in the therapeutic treatment of rickets. The first papers on the lethal bacteriological effectiveness of UV appeared in 1880. In 1857 Siemens reported on filamentary UV creation via charge plates, micro discharges, nanosecond discharges, "Entladung", an approach subseguently to dominate ozone production, and well reviewed by Kogelschatz. By 1932 the Coblentz Congress had defined the three regions of the UV action spectrum.

Medium pressure arc lamps (MPAL 's) operate at several atmospheres pressure (as compared with the vacuum pressures of glow discharge tubes) . A typical high guality unit of this type is the Heraeus Amba 5639X. These deliver diffuse UV spectra due to the temperature and pressure broadening of their collision spectra, and reguire massive copper ballasts.

Other long-known methods of UV generation include electrodeless bulbs, where an external microwave generator is used to excite the discharge. This approach was in use for factory lighting by the 1940 's. Amalgam and excimer lamps as well as solid state and gas lasers can also be used to generate UV.

UV generating tube types are defined by their electrical operating characteristics, which in turn are largely determined by the internal gas pressure. All UV generating tubes have to utilise UV transmitting materials because all normal glasses heavily absorb short wave UV radiation. Figure 1 illustrates the well known tube discharge characteristics, with the voltage across the electrodes (in kV) plotted against the current through the tube (in Amperes) . As is well known, the variation of the voltage eguates to different types of discharges, called the Townsend discharge 10, subnormal glow 12, normal glow 14, abnormal glow 16, and finally the arc discharge 18. The usual region for the operation of commercial glow discharge tubes is the abnormal region 16.

The dominant 254nm peak in the UV emission from a glow discharge tube is shown in Figure 2, with the UV emissions from a modern medium pressure tube, specifically optimised for sterilisation are shown in Figure 3, where the germicidally efficient 254nm radiation for sterilisation of fluid is no longer a single dominant peak.

For comparison, the spectral power distributions of a glow discharge is shown in Figure 4, and the spectral power distributions of a medium pressure discharge is shown in Figure 5.

The present inventors examined a range of low pressure glow discharge tubes with gas fill pressures of around 7 Torr, and it was found the average electrical performance rating was 0.46W per centimetre of tube length. To be commercially useful in, for example, water sterilisation uses, we need an assembly of low pressure glow discharge tubes capable of generating say IkW of UV at 254nm, as this wavelength has a useful germicidal effect. Clearly, from the above measurements, we need to use low pressure glow discharge tubes about 90cm long, giving an average electrical performance rating of about 75W per tube.

A range of low pressure glow discharge tubes of varying diameter were examined. The luminous efficacy of generating UV at 254nm was measured for each tube diameter by measuring the UV output in the 254nm peak as integrated counts and then normalising this to electrical power consumption in watts, the electrical power consumption having been measured by secondary standard grade instrumentation. The experimental results are shown in Figure 6, where the peak in efficiency can clearly be seen to occur at a tube diameter (egual to the inner diameter of the guartz tube) of around 15mm. Relatively small changes in plasma tube diameter are seen to give substantial reductions in 254nm UV luminous efficacy.

As mentioned above, a gas discharge lamp such as (but not limited to) a UV glow discharge tube needs to be powered by a ballast, and the operation of electronic ballasts for these purposes will now be discussed. Figure 7 shows a simplified form of an output stage of an electronic ballast. It can be thought of as being eguivalent to a simple LCR circuit fed by input voltage 36, comprising inductor 30, capacitance 32 and the lamp represented as resistance 34. A high performance ballast should provide the lamp with preheat current in the cathodes for a specified time to bring them to the correct temperature before ignition. During the preheat time the lamp voltage has to be low enough to ensure that ignition will not occur prematurely. At the end of preheat a high voltage is reguired to ignite the lamp and from then on the reguired current should be supplied to the lamp for operation at the correct power. These reguirements are satisfied by selecting appropriate values for the freguency and magnitude of the input voltage, and for L and C. For preheat and ignition, the lamp is not conducting and the circuit is reduced to a series L-C. During its operation, that is, after ignition, the lamp is conducting and the circuit is an L in series with a parallel R-C.

As mentioned above, the intensity of the radiation output from a gas discharge lamp is known to depend only on the input power. However, we have now realised that in fact, the intensity of radiation does actually vary according to the ballast freguency and that therefore, a particular lamp will have an associated ballast freguency at which the intensity of emitted radiation will be optimised.

Individual electrons in the plasma are subjected to an AC field accelerating them first in one direction and then another. Some will experience bouncing off very heavy ions, others capture etc. Some will reach mercury ions and interact in complex ways - mercury has a number of isotopes, and standard tubes contain argon with a trace of neon to assist ignition. There are therefore many interlocking energy levels available for excitation within the plasma. The efficiency of UV creation must depend on how many electrons arrive at a time to excite the mercury ions. In a very low freguency field the electrons have considerable time to accelerate in a given direction before the field reverses . In a very high freguency field they are much more static because the field reverses so guickly. Hence the electron mean free path must be freguency dependent, and so the UV creation efficiency may be time and freguency dependent.

A specific example is illustrated in Figure 8. We investigated the UV intensity at the 254nm wavelength emitted from a glow discharge tube. This wavelength is of interest as having a germicidal effect and is important for UV sterilisation of fluid such as water or air. The tube used for this example was a GH036T5 (16mm diameter) UV tube. The power was held at a constant level, and the UV intensity was measured as a count rate in units of 10³ counts/sec, and plotted against varying ballast freguencies .

It can clearly be seen from this example that a maximum UV intensity is obtained for a ballast freguency of just over 56.25kHz. It is to be noted from the graph that the UV intensity drops sharply as this value is decreased, but drops slowly as this value is increased. Therefore, to take into account any variations in the supplied ballast frequency, an ideal operating range is defined with a lower bound at the ideal frequency, say from 55 to 58 kHz, as it is preferable to err on the side of having too high a frequency rather than risk the frequency dropping too low.

Current thinking does not accept that a relationship exists between the ballast frequency and the intensity emitted from a gas discharge tube, that is, the graph of Figure 8 would be expected if asked to simply be a straight line.

Therefore, when designing a ballast circuit for a gas discharge lamp, the desired operating frequency can be determined based on the investigation of these parameters. This holds true for all gas discharge lamps, with the specific example of a UV glow discharge lamp given as an example only.

The operation of a gas discharge tube with this frequency optimised ballast provides for a more energy efficient gas discharge lamp. Returning to the example of Figure 8, two frequency optimised tubes were tested versus an industry standard UV tubes, the Philips T8 Germicidal. The output of our tubes with the new ballast system was found to be 2.7 times greater than the Philips T8 group at equivalent distances with the same wall plug current, and the actual UV irradiance under the 253.7 nanometre peak was 27,500 i_WcπT² at 3mm from the outer quartz sleeve of the tube in shell geometry (a 15 mm tube in a 25mm quartz envelope) .

As mentioned above, an electronic ballast typically transforms the frequency of the power up to about 2OkHz. However, the frequency optimised ballast of the present invention invloves operation at much higher frequencies - 55-58kHz in the above example alone. But the use of this high operating frequency requires more circuit power than can currently be handled by existing ballast ICs in order to enable full-power operation of the gas discharge tubes at the optimised frequency on a stable long-term basis .

This was illustrated by the shortcomings of trying to come up with a ballast circuit design for the new frequency optimised ballast using existing circuit design theory. International Rectifier are a supplier of ballast ICs. They also provide ballast design software, which reduces design time for a ballast IC by performing the complex iterative procedure required to optimise the operating points and component values of the ballast circuit. Given an input of various lamp operational parameters, the software produces a schematic, a bill of materials listing all component values, and winding specifications for the inductors.

We used this ballast design software to model a circuit for a frequency optimised ballast according to the abovementioned specific example, and the results are shown in Figure 9. The design is based around International Rectifier's IR2156 ballast control IC, with a power factor correction IC L6561 from STMicroelectronics .

However, this circuit was found not to give satisfactory performance. The standard way to power a ballast chip is by dropper resistors (RBUS and RSUPPLY) feeding a capacitor (CVDC), so the charging capacitor charges up slowly to around 12 volts, at which point the chip fires, but as the tube fires this route does not provide enough power.

We therefore modified the design of Figure 9, the results of which are shown in Figure 10. This circuit again uses the IR2156 as its basis, but incorporates a charge pump circuit to enable a fraction of the tube power when ignited back to be fed back to the ballast IC, in order to provide the power from the circuit that enables operation at higher freguencies.

With reference to Figure 10, the standard way to power a ballast chip is by dropper resistors (R19, R20) feeding a capacitor (C8) . The power consumption of the circuit is lessened by the addition of resistor R23 and diodes D4 and D5 so power is provided back from the tube via R23 and D4 acting as a charge pump. D5 is added to pull charge off C8. If anything goes wrong with the tubes, this loop acts as a short circuit to protect the tubes from excessive current supply. The frequency optimised ballast circuit needs a stabilized power supply rail at about 480V. This cannot be obtained directly from the rectified 240V and so power factor correction is required.

The power factor correction (PFC) part of the circuit shown in Fiq. 10 principally comprises a forward biased diode (D2) (whose purpose is to isolate the PFC part of the circuit to the left from the ballast IC and tube drive output) connected between a charqinq capacitor (C6) and the power supply of the ballast IC, with a MOSFET and a PFC inductor (L2) .

The PFC circuit as a whole consists of the L6561 Power Factor Controller IC (UI), the PFC inductor (L2), forward biased diode (D2), PFC voltaqe output capacitor (C6) and various other sensinq and compensatinq components, and the PFC MOSFET (Ql) which with its very hiqh input impendence (>10¹² Ohm) is the ideal voltaqe amplifier component.

For this boost voltaqe to work, we need the power factor to be 1, that is the phase difference between voltaqe and current vectors must be 45° so that power factor = cosG = cos45°=l, so the conducted current throuqh our voltaqe boost phase is in phase with the AC line voltaqe.

The PFC circuit starts with a full wave bridqe rectifier followed by C2 providinq some smoothinq, followed by the further steps to reduce total harmonic distortion via the PFC inductor (L2) and the other features of the L6561 PFC chip.

There are many other design changes between the two circuits of Figures 9 and 10, which will be apparent from study of the diagrams .

It will be appreciated again that the examples shown in Figures 8 to 10 are not intended to limit the scope of the invention.

The freguency optimised ballast of the invention brings with it many advantages. The concept is applicable to any type of gas discharge tube used for any application. One example industry that would benefit from this design is the water purification industry.

In the water purification industry, various directives control the degree of sterilisation which must be achieved, for example in Europe the EEC Bathing Waters Standard, EU 1976, sets a limit of 2000FC/100ml, and a further drinking water standard of total inorganic carbon (TOC) of 3mg/l.

Currently, it is many times cheaper for a water utility provider to purify water through a chlorination process, but this brings with it health and safety issues, particularly following inadvertent high chlorine dosing into the network. Because of its high costs, commercial pressure means that UV sterilisation is only generally used when absolutely necessary.

However, if its running costs could be decreased and the electrical efficiency increased, UV sterilisation would become increasingly attractive, because it does not bring with it the health and safety issues to do with monitoring the levels of chlorine in water. Indeed, as health and safety guidelines are continually being monitored and modified and as the effects of chlorine in the water supply is still an area of active research, a move away from chlorine may even in the future become imperative.

Various improvements and modifications can be made to the above without departing from the scope of the invention.

Claims

1 CLAIMS

2

3

4 1. A method of selecting an operating freguency of a

5 ballast for powering a gas discharge lamp,

6 comprising the steps of measuring the intensity of

7 emitted radiation at constant tube power for varying

8 ballast freguencies, and selecting an operating

9 freguency that corresponds to a maximum radiation 0 intensity. 1 2 2. The method of claim 1, wherein the operating 3 freguency is chosen to be within a range having its 4 lower bound at the maximum freguency. 5 6 3. The method of claim 1 or claim 2, wherein the gas 7 discharge lamp comprise a UV tube operable in the 8 glow discharge mode. 9 0 4. The method of any preceding claim, wherein the 1 operating freguency is chosen to be between fifty- 2 five and fifty-eight kilohertz.

'.j 4 5. A method of operating a gas discharge lamp 5 comprising operating a ballast at a predetermined 6 selected freguency, said predetermined selected 7 freguency being an operating freguency that 8 corresponds to a maximum radiation intensity for 9 varying ballast freguencies at constant tube power 0

6. The method of claim 5, wherein the operating frequency is chosen to be within a range having its lower bound at the maximum frequency.

7. The method of claim 5 or claim 6, wherein the gas discharge lamp comprise a UV tube operable in the glow discharge mode.

8. The method of any of claims 5 to 7, wherein the operating frequency is chosen to be between fifty- five and fifty-eight kilohertz.

9. A gas discharge lamp comrising an electronic ballast, said electronic ballast being adaptable to operate at a predetermined frequency, said predetermined frequency being an optimum operation frequency that corresponds to a maximum radiation intensity for varying ballast frequencies at constant tube power.

10. The lamp of claim 9, wherein said electronic ballast comprises a ballast IC for controlling the operation of the gas discharge lamp, and charge pump means for feeding back a fraction of the tube power when ignited back to the ballast IC.

11. The lamp of claim 10, wherein said charge pump means comprises a resistor and a forward biased diode connected in series between the lamp input and the power supply of the ballast IC.

12. The lamp of any of claims 9 to 11, wherein said electronic ballast further comprises a forward biased diode connected between a charging capacitor and the power supply of the ballast IC.

13. The lamp of any of claims 9 to 12, comprising a UV tube operable in the glow discharge mode.

14. A UV sterilisation apparatus comprising the lamp of any of claims 9 to 13.

15. A method of sterlising fluid, comprising the step of exposing the fluid to radiation emitted by a gas discharge lamp comprising an electronic ballast, wherein an operating freguency of said ballast is selected according to the method of any of claims 1 to 4.

16. A method of sterilising fluid, comprising the step of exposing the fluid to radiation emitted by a gas discharge lamp comprising an electronic ballast, wherein the gas discharge lamp is operated according to the method of any of claims 5 to 8.