Electronic ballasts

Details Electronic ballasts Electronic ballasts

2001-06-13

2003-04-01

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Periodic switch in the supply circuit

Impedance or current regulator in the supply circuit

C315S291000

Reexamination Certificate

active

06541923

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
REFERENCE TO A “MICROFICHE APPENDIX”
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical lighting systems and to electronic ballasts therefor for High intensity discharge lamps.
2. Description of Related Art
Such lamps have a high impedance before they are lit, and a low impedance while they are lit. High intensity discharge lamps are characterised by a short arc length, typically less than 20 mm for a 70 watt lamp, and have a sealed envelope containing at least two electrodes for an electrical discharge, and typically have a high internal pressure when hot.
High intensity discharge lamps are in widespread use. A disadvantage of these lamps is that additional components are required for their proper operation. In particular a means of limiting or controlling the current drawn by the lamp is required, together with a means of starting an arc discharge in the lamp.
The arrangements of components associated with the control of lamp current are commonly referred to as a “ballast”, whilst those components performing the function of starting the lamp are collectively referred to as the “ignitor”.
Traditionally the ballast function was performed by a large and heavy inductor operating at the power line frequency, whilst the ignitor function was performed by the generation of high (2-5 kV) voltage impulses superimposed onto the supply terminals of the lamp.
More recently electronic means have been devised for operation of discharge lamps in order to overcome some of the disadvantages associated with traditional methods of lamp operation. These disadvantages included size, weight, and a lack of any means of accurately controlling lamp power with variations in supply voltage or frequency.
The electronic ballast means employed to date have been successful in overcoming the above mentioned disadvantages, but as a result of their complexity, new disadvantages of cost and reliability have prevented their widespread use. It is convenient to describe such electronic ballasts as utilising “square wave technology” because of their output waveforms.
One of the reasons for the complex design of square wave technology ballasts, (which operate lamps at relatively low frequencies 50-400 Hz for example), is that discharge lamps exhibit undesirable instabilities when operated in the frequency range of 1 khz-300 kHz. Elaborate electronic topologies are required to generate low frequencies with power levels and control characteristics suited to discharge lamps.
Should the operating frequency (or some harmonic or sub harmonic of the operating frequency) be such as to excite standing waves of pressure within a lamp then undesirable movement or even extinction of the arc can occur. This can be damaging to the lamp since arc movement can cause the arc to impinge upon the burner walls with consequent lamp failure. At the very least these movements of the arc spoil the quality of illumination obtained.
The above mentioned instability and standing waves of pressure are manifestations of a phenomenon known as “acoustic resonance”. Acoustic resonance arises as a result of pressure variations in the lamp caused by the operating frequency or some harmonic or sub harmonic of the operating frequency.
BRIEF SUMMARY OF THE INVENTION
According to the invention there is provided a lighting system comprising at least:
a) a high intensity discharge lamp,
b) an electronic ballast having a regulated alternating current power output,
c) a joint operating circuit included in and between the ballast and the lamp,
d) the ballast having a variable frequency generator which is arranged to vary the frequency of the ballast output over a range of frequencies, with a minimum frequency of 400 kHz,
e) the circuit having a reactance such that the circuit is arranged to be resonant to provide a starting voltage for the lamp at or above the minimum frequency.
A benefit of such a ballast is that it may be manufactured at low cost which when operating the discharge lamp has a high efficiency and reliability. A benefit of operating at a high frequency greater than 400 kHz and greater than a maximum acoustic resonant frequency of the lamp is that the life of the lamp and the quality of the illumination is improved by the avoidance of acoustic resonance.
Preferably a maximum frequency of the range is such that the power output is sufficient to maintain operation of the lamp.
A benefit of restricting the frequency range is that stable operation of the lamp is ensured.
Preferably the ballast of the invention is arranged to regulate the power output to the lamp by varying the frequency of the ballast output.
A benefit of arranging the ballast of the invention to control the power to the lamp is that the lamp may be operated at its optimal rating, and the effect of variations arising from manufacturing tolerances may be minimised.
Preferably the high intensity discharge lamp comprises a sealed envelope containing at least two electrodes for an electrical discharge.
Preferably the ballast or frequency generator of the invention operates in a frequency range of between 400 kHz to 30 MHz. More preferably the frequency generator operates in a frequency range of between 400 kHz and 1,500 kHz.
Preferably the ballast is arranged to provide a constant power to the lamp when it is running in a steady state, and a higher current during a starting phase of the lamp. Preferably the starting phase of the lamp is transient, and of a short duration.
A benefit of providing a higher current during starting is that the lamp reaches its operating condition more quickly and reliably.
Preferably the ballast is arranged to limit the higher current to a permissible value within a safe rating of the lamp.
Preferably the alternating current output wave form is of a substantially sinusoidal waveform with a harmonic distortion of less than 40%.
More preferably the substantially sinusoidal waveform has a harmonic distortion of less than 15%.
A benefit of an electrical output with a sinusoidal wave form is that a sine wave has only one fundamental frequency, compared with a square wave where a Fourier analysis shows the square wave form to comprise a very large range of frequencies. Hence in avoiding the excitation of acoustic resonance, with an output with a sinusoidal wave form it is only necessary to avoid the resonances that may be excited by one frequency.
A further benefit of a sine wave is that the rate of change of voltage with time dv/dt is minimised. This reduces the stress on electrical and electronic components in the control, reducing a cause of failure. A benefit of this is that the size of components may be reduced, both reducing cost and size.
Another benefit of a sine wave is that the electrical efficiency of the control is improved and hence the light output from the lamp per power input is improved.
Preferably the frequency of the output may be varied while the lamp is in a lit state.
A benefit of this is that by varying the frequency of the output any excitation of acoustic resonance by a sub-harmonic of the output frequency may be reduced.
Preferably the frequency of the output is varied at a rate greater than 50 Hz. A benefit of this is that any acoustic resonance that may occur does not have an opportunity to develop to an amplitude that would cause an arc within the lamp to impinge on an internal surface of a burner of the lamp. A further benefit is that the perception of flicker caused by the variation of light output arising from the change in lamp power arising from such frequency variation is minimised.
An additional benefit of this is that although any acoustic resonance is undesirable, lamps may withstand such resonance provided that it is not allowed to develop to such an amplitude that causes the arc to be deflected onto the internal surface of the burner. When the arc is deflected so that it impinges on the internal surface of the burner damage is caused to the burner, and the life of the lamp

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