Electric lamp and discharge devices: systems – Current and/or voltage regulation – Automatic regulation
Reexamination Certificate
1999-12-27
2002-02-19
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
Automatic regulation
C315S244000, C315S291000, C315S094000, C315S324000, C315SDIG005
Reexamination Certificate
active
06348769
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to circuitry for electronic ballasts for use with fluorescent lamps, and more particularly to an electronic ballast designed to extend the the life of lamps by operation of a power controller applying two complementary high frequencies, a first frequency for filament pre-heating under fixed voltage control and a second complementary frequency, applied after a frequency adjustment-based transition period, for lamp ignition.
BACKGROUND OF THE INVENTION
It is known in the prior art to use a ballast circuit to heat the two filaments of a fluorescent lamp to a high temperature, such that when an electric field is applied between the filaments, they more easily emit electrons and ionize the gas in the lamp. Responsive to radiation generated due to the electric current flowing through the gas, phosphors coating the inner surface of the lamp fluoresce, emitting visible light. The ballast typically controls both the initial ignition and the steady-state operation of the lamp.
Electronic ballasts apply this pre-heating circuitry prior to lamp ignition to lengthen lamp filament life, and thus lamp life, by increasing the concentration of electrons with a sufficient energy level to be discharged from the metal filament when a starting or striking voltage is applied to it. Typically, prior art pre-heating circuits operate using a current control technique, to maintain the filament heating current at a constant value. The resulting filament power dissipation which results is according to the standard equation, P=I
2
R where P is the power dissipated in the filament, I is the constant current, and R is the filament resistance.
When the lamp is new, the cold resistance of the filament is significantly lower than its value after the lamp has been in use over a period of time. Thus, according to the filament power dissipation equation above, the ageing of the filament causes it to have an increasing power dissipation. The lamp dims over time as this increasing power dissipation causes the filament to deteriorate more rapidly, until finally the filament breaks and the lamp ceases to ignite.
Examples of electronic ballasts incorporating current-controlled pre-heating circuitry include U.S. Pat. No. 5,656,891 to Luger et al., which discloses a continuously variable heating power, and U.S. Pat. No. 5,500,576 to Russell, which discloses a predetermined lamp warm-up time using current control. PCT Pat. Appln. publication WO 97/13391 discloses the use of low-voltage windings of a transformer to supply sufficient current to pre-heat the filaments.
It is also known in the prior art of electronic ballast design to provide a lamp driving circuit which operates at one frequency during the pre-heating phase of lamp, and at a different steady-state operating frequency. In U.S. Pat. No. 4,553,071 to Boyd, a ballast is disclosed having a tuned circuit which limits the current during warm-up, and when the lamp filament resistance increases, the tuned circuit develops a starting voltage for lamp ignition.
U.S. Pat. No. 5,686,798 to Mattas provides a single driving signal frequency, as contrasted with ballasts which operate at more than one frequency and use feedback circuitry to sense when lamp ignition has occurred so as to determine when to switch between the frequencies.
U.S. Pat. No. 4,641,061 to Munson discloses a ballast operating at a selected frequency high enough to develop a starting voltage, while being below the resonant frequency of the LC circuit coupled to the lamp. After starting conduction through the lamps, the frequency is reduced to a frequency substantially below the selected frequency, to limit the current flow.
U.S. Pat. No. 5,021,714 to Swanson et al discloses a circuit for starting and operating fluorescent lamps from an AC low-frequency power source. A ballast generates a voltage, whose frequencies include a plurality of harmonics of the power-source frequency, which voltage causes a capacitor and a cathode heating transformer to resonate responsive to the harmonics. The resonant voltage is applied across the fluorescent lamps to aid the starting of their discharge, and thereafter the lamps operate at the AC power source frequency.
U.S. Pat. No. 5,723,953 to Nerone et al., discloses a high voltage gas discharge lamp ballast, including a resonant load circuit which incorporates the lamp, and includes two resonant impedances whose values determine the operating frequency of the resonant load circuit. High voltage switches are used to disconnect the lamp's filaments during the pre-heating phase.
U.S. Pat. Nos. 5,208,511 and 5,175,470 to Garbowicz, disclose a fluorescent lamp system which includes a ballast with primary and secondary windings and a switch for each electrode of each of the lamps in the lamp system. Each switch operates in response to the voltage across its associated lamp, such that after the lamp turns on, the switch interrupts the connection of its associated electrode to a heater winding.
Additionally, U.S. Pat. No. 5,015,923 to Nilssen, U.S. Pat. No. 5,563,473 to Mattas et al., and U.S. Pat. No. 5,677,602 to Paul et al., describe other electronic ballasts for use with fluorescent lamps.
As described above, lamp operation is here comprised of a filament pre-heating phase and a lamp ignition phase. In the lamp ignition phase, which follows the filament pre-heating phase, a relatively high voltage is placed across the lamp. Prior art electronic ballasts which rely on a switching arrangement to handle the transition between these phases, such as the pre-conditioner of the above-mentioned Mattas patent, do not account for the actual filament conditions obtaining after the pre-heating phase. That is, if there has been sufficient filament pre-heating, a relatively low starting voltage across the lamp is all that is needed to ignite the lamp and the filament is thereby not overly stressed. Whereas, if the filament pre-heating has been less than sufficient, then a typically higher starting voltage will be required to ignite the lamp, and this higher starting voltage of existing ballast designs will develop a stress on the filaments during ignition, thereby shortening the life of the filament, and thus the life of the lamp.
Once a lamp filament has been broken, the useful life of the lamp is effectively over. The application of starting voltage to such a lamp may be hazardous, as dangerously high voltages may be produced at the lamp socket terminals. In U.S. Pat. No. 5,747,941 to Shackle et al., an electronic ballast is disclosed which prevents a starting cycle if the lamp filaments are not intact.
As can be seen from the above, in order for electronic ballast design to extend lamp life, it must (1) overcome the problems related to constant current control as a means of filament pre-heating, since the resulting filament power dissipation will increase as the lamp ages, and (2) it must provide a transition to lamp ignition in such a way that prevents premature application of the lamp ignition voltage, before completion of the pre-heating phase.
Therefore, it would be desirable to overcome the above-mentioned problems related to pre-heating and ignition associated with existing electronic ballast designs, and provide an efficient, compact and inexpensive electronic ballast design capable of extending lamp life.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved ballast circuit for use in operating a fluorescent lamp in order to extend lamp life.
It is another object of some aspects of the present invention to provide improved devices and methods for pre-heating, igniting, and maintaining efficient steady-state operation of a fluorescent lamp.
It is a further object of some aspects of the present invention to provide improved devices and methods for generating a smooth transition between the pre-heating phase, the ignition phase, and the steady-state phase of fluorescent lamp operation.
In accordance with a preferred embodiment of the present invention, there is provided
Pinchuk Dmitry
Pinchuk Lodmila
Pinchuk Neli
Yoskovich David
Clalight Israel Ltd.
Edward Langer, Pat. Atty.
Philogene Haissa
Pinchuk Lodmila
Pinchuk Neli
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