Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
2000-10-04
2004-11-09
Lee, Wilson (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C315S224000
Reexamination Certificate
active
06815906
ABSTRACT:
The present invention relates to an apparatus and method for driving a gas discharge lamp, and in particular for dimmably or non-dimmably driving fluorescent lamps or tubes.
Fluorescent lamps or tubes are widely used in the home, office and in industry to provide lighting. Such lamps generally consist of a tubular glass envelope, up to 2.44 m (8 feet) long, filled with an inert gas such as krypton or argon which when electrically excited in a gas discharge irradiates a fluorescent coating, such as a powder comprising a (Tb,Ce,Gd,Mg) borate, a (Eu,Ba,Mg) aluminate and a (Y,Eu) oxide, on the inside of the glass. An example of such a tube, 1.22 m (4 feet) long, is the model ‘TL’D 36 Watt sold under the trade names “Super 80 (/840) New Generation” and “Standard (/33)” by Philips Electronic and Associated industries Limited.
All gas discharge lamps, including fluorescent lamps, require a ballast to operate. The ballast provides a high initial voltage to initiate the discharge, then rapidly limits the lamp current to safely sustain the discharge. Ballasts are manufactured for three main classes of fluorescent lamp: preheat, rapid start and instant start.
Preheat operation lamp electrodes are heated prior to initiating the discharge. A starter switch closes, permitting a current to flow through each electrode. The starter switch rapidly cools down, opening the switch, and triggering the supply voltage across the arc tube, initiating the discharge. No auxiliary power is applied across the electrodes during operation.
Rapid start operation lamp electrodes are heated prior to and during operation. A transformer has two special secondary winding to provide the proper low voltage to heat the electrodes.
Instant start operation lamp electrodes are not heated prior to operation. Ballasts for instant start lamps are designed to provide a relatively high starting voltage, as compared with preheat and rapid start lamps, to initiate the discharge across the unheated electrodes.
Prior art document U.S. Pat. No. 4,464,606 discloses a circuit for dimmably controlling a pair of fluorescent lamps, in which a push-pull transistor pair is pulse width modulated to vary the duty cycle of a pulsed current supply to the primary of a transformer to the lamps.
It is desirable to be able to dim fluorescent tubes in order achieve increased energy efficiency when full, lighting is not needed. It is known that such tubes up to 1.83 m (6 feet) long can be dimmed with appropriate control circuitry. For example, the above-mentioned 1.22 m fluorescent tube may be dimmably controlled with high frequency regulating ballast sold by Philips Lighting Limited as model number BPL136R.
With reference to Philips Lighting data sheet PL 3322, such known ballasts suffer from a number of limitations. First, it is only possible to achieve adequate control over the dimmable light output for fluorescent tubes up to 1.83 m (6 feet) in length. Secondly, it is only possible to dim down to about 10% of full light output before the tube flickers out. Thirdly, the lighting efficiency of such dimming ballasts drops steadily as the light output falls, the efficiency being 56% at 25% light output and 27% at 10% output, as a result of increased thermal losses in the tube and ballast circuitry. Thus, the benefit of decreased electricity consumption is not fully realised at low power levels.
The reason for these limitations in performance appears to stem from the way conventional non-dimmable high frequency (hf) ballasts have been adapted for use as dimmable ballasts. A conventional hf ballast generates a pulsed voltage, typically at either 28 kHz or 35 kHz, modulated on and off at a low frequency (50 Hz or 100 Hz), with an on/off ratio of 50% so that there is no hf signal during each half-cycle. A conventional dimmable hf ballast reduces the on/off ratio so that the hf pulsed voltage becomes progressively less than 50% of the duty cycle. The hf pulses are therefore applied to the fluorescent tube for a lower average duty cycle and as fewer hf pulses are applied to the tube, the tube dims.
In general, a number of limitations have been noted with such dimmable systems. First of all, because conventional fluorescent ballasts include a choke with a substantial inductance, proportionately greater amounts of energy are lost in ohmic heating of the choke as the tube is dimmed. Secondly, as the tube is dimmed, a point is reached where the tube fails to strike properly owing to the increasingly large proportion of time when the hf voltage is not applied to the tube. The tube therefore tends suddenly to flicker off before it has been fully dimmed, owing to the increasingly discontinuous nature of the pulse train applied to the tube. These problems become worse for increased length of fluorescent tube and consequently it is believed that there are no commercially available dimmable or non-dimmable ballasts for 2.44 m tubes, and the dimmable ballasts available for 1.83 m tubes do not work as well as those for 1.22 m tubes. See, for example, the comprehensive online database to be found on the internet at http://light-light.com/ which lists all commercially available fluorescent lamps and ballasts. This database lists no commercially available dimmable or non-dimmable ballasts for fluorescent tubes longer than 1.83 m.
The fact that 2.44 m non-dimmable hf fluorescent tube ballasts are not commercially available is surprising, since there has been a trend since at least 1981 to use non-dimmable hf ballasts for improved energy efficiency whenever possible. High frequency ballasts are, however, known to suffer from various problems.
One problem results from the relatively greater power and hence current and voltage requirements of 2.44 m fluorescent tubes as compared with shorter tubes. Inefficiencies in the ballast circuitry, including transformers, result in excess heating within the ballast unit, which can be damaging to solid state circuit elements. The space within a typical fluorescent tube fitting is quite limited, and it is believed that the build up of heat owing to the relatively greater power requirements has meant that it has not been possible or economic to manufacture a high frequency ballast for a 2.44 m fluorescent tube with a commercially acceptable failure rate, e.g. of less than IA in the first year after installation.
Another problem is that the circuitry conventionally used generates what are known as “harmonics” and to transmit these harmonics back into the power supply grid. This is a particular problem in certain industrial situations where, for example, a factory may have many hundreds of 2.44 m tubes on a number of lighting circuits supplied through a local step down transformer. In such a situation, harmonics can lead to overloading of transformers, adding of current to the neutral in three phase electrical distribution systems, current/voltage surges or spikes due to circuit resonances with one or more of the harmonic frequencies, and interference with other electronic equipment.
As a result, standards have been introduced to limit the amount of harmonic distortion produced by high frequency ballasts.
It is an object of the invention to provide a circuit for a high frequency ballast for a gas discharge lamp that addresses these problems and which may be dimmable, and which may be used with certain types of gas discharge lamp such as high output 2.44 m fluorescent lamps which to date have not benefited from the increased efficiencies possible with high frequency operation.
SUMMARY OF THE INVENTION
According to the invention, there is provided an electronic circuit for controlling a gas discharge lamp, comprising generation means for generating a high frequency pulse train that may be applied to the electrodes of the lamp to light the lamp, means for connecting the means for generating a high frequency pulse train to an electrical power source, a choke to limit the current drawn by the lamp, characterized in that the circuit comprises means for producing a first series of pulses and means for producing a second series of pulses ind
Aarons David John
Mullenger John
Hathaway Todd N.
Lee Wilson
LandOfFree
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