Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2003-04-29
2004-04-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S2090SC, C315S224000, C315SDIG007
Reexamination Certificate
active
06717375
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a discharge-lamp lighting device for lighting a discharge lamp such as a metal halide lamp, and a system such as a liquid crystal projector using the discharge-lamp lighting device.
BACKGROUND ART
Recently, the brightness of projection-type liquid crystal projectors has been improved dramatically by the use of highly efficient high-pressure discharge lamps (hereinafter, may be abbreviated as ‘lamp(s)’). However, fluctuation in brightness on a screen, which is caused by a fluctuation of an arc track of a lamp, or a phenomenon called ‘arc jump’, has been remarkable. A problem associated with this type of lamp is that the discharge arc becomes unstable depending on the temperature of the electrode and the condition of the surface of the electrode. As explained in U.S. Pat. No. 5,608,294, the discharge arc becomes unstable because the origin of the discharge arc jumps to some sharp-pointed projections (hereinafter, referred to as ‘spot(s)’) formed on the surface of the electrode.
The above-identified U.S. Pat. No. 5,608,294 discloses a method for solving the arc jump caused by the above factors. As shown in
FIGS. 8 and 9
, the method includes generating, by using either a bistable multivibrator or a flip-flop, a current pulse Pc in a predetermined fraction of given half periods of the lamp current, supplying a lamp current I obtained by superimposing a homopolar current to a back porch part of an alternating current waveform at a ratio of 0.05-0.15 to the half periods with an amount of current corresponding to 5-15% of the energy supplied to the lamp.
An effect of this method is the prevention of arc jump, which is obtained by preheating the electrode which was an origin of the arc at the time of a polarity inversion of the lamp current with a previous current pulse before the surface temperature of the electrode is lowered, so that a returning point of the arc after the polarity inversion matches with the previous origin of the arc. However, this method alone is insufficient to prevent the arc jump since the method does not addresses some problems including: spot formation that is caused by tungsten halide oxide and that have been remarkable in the recent trend of highly efficient lamps, lowering temperature of the electrode surface caused by the polarity inversion time, furthermore, matters of heat capacity of the lamp electrodes and bulb shapes, aging of the lamp, and a new method of controlling switching power fed to an identical lamp.
A problem concerning the lamp lighting is that, depending on the temperature and a surface condition of the electrode, the discharge arc becomes unstable. As described in U.S. Pat. No. 5,608,294, the instability of the discharge arc tube is caused by jumping of the origin of the discharge arc to some spots formed on the surface of the electrode. The arc jumps occur at the time of the polarity inversion of the lamp current. The reason is that the lamp current is zero-crossed inevitably at the moment of the polarity inversion of the lamp current, whereby a considerable lowering of the electrode surface temperature through the entire periods of the lamp current results.
For lighting a lamp, glow discharge is carried out by applying a high-voltage pulse between the electrodes in order to warm the electrodes gradually, and subsequently, the operation shifts to discharge thermoelectrons, so that the arc will discharge continuously. At that time, the arc origin tends to discharge electrons by selecting electrode spots on the electrode surface, since more electrons will spring out from such spots. The spots are formed, since a metal such as tungsten used as a material of the electrodes is heated to approximately the melting point during the lighting of the lamp, on places where electrons impinge to cause sputtering of the electrode and to deform the electrode slightly.
Moreover, since lamps have bulbs of small diameters in the recent trend for highly efficient lamps, the arc and the quartz glass forming the bulb get much closer to each other. As a result, halogenated tungsten combined with a halide filled in the bulb will be coupled easily with oxygen as a component of the quartz glass, increasing the generation of tungsten halide oxide. The tungsten, which is evaporated and combined with a halogen, is combined also with oxygen substances such as oxides adhering to a molybdenum foil or the like that composes a conductor of the electrodes, thereby forming tungsten halide oxide. Furthermore, since the tungsten halide oxide tends to be separated at a part with a comparatively high temperature, the evaporated tungsten that has been reduced at a part with a comparatively low temperature among the entire electrodes, is reduced intensively at spots as origins of arc at the tips of the electrodes, and further accelerates the growth of the spots.
The grown spot is spattered by an inrush current at the next lighting of the lamp, and thus, many small spots are formed on the electrode surface. While the lamp is lighting, fluctuation of the arc origin is generated on the electrode surface at some parts at which the tungsten halide oxide is reduced, thereby forming many spots. Considering this, the origins of discharge arc can be fixed and stabilized by decreasing considerably the number of the electrode spots and maintaining the fewer spots to have a sharp-pointed shape, where only one of the spots has a high temperature.
DISCLOSURE OF INVENTION
The present invention aims to solve the above-described problems, and an object thereof is to provide a discharge-lamp lighting device with fewer arc jumps and less fluctuation in brightness of a lamp even when an inexpensive, small and highly efficient lamp is used. This object can be achieved by accelerating positively the growth of spots. And a lamp of the present invention has a long life. The present invention provides also a system such as a liquid crystal projector using the discharge-lamp lighting device.
For achieving the above-described objects, a discharge-lamp lighting device according to the present invention comprises: a DC-DC converter for stepping down an input DC voltage in accordance with a current-controlling signal and outputting a desired current; a commutator for commutating a direct current from the DC-DC converter to an alternating current in accordance with a drive-controlling signal with a rectangular waveform; a high-pressure discharge lamp to be fed with the alternating current from the commutator; and a controlling portion for outputting the drive-controlling signal and also for outputting the current-controlling signal on the basis of a value of current flowing in the high-pressure discharge lamp or a value of voltage of the high-pressure discharge lamp in order to make electric energy in the high-pressure discharge lamp constant. The controlling portion sets a frequency of the drive-controlling signal to be within a predetermined frequency range so that sharp-pointed projections (spots) formed by an arc discharge on electrodes composing the high-pressure discharge lamp grow through a cycle of oxidation-reduction of a metal composing the electrode, superimposes a triangular wave signal generated on the basis of the drive-controlling signal onto the current-controlling signal through the entire periods of the drive-controlling signal in order to make a peak value of the current flowing in the high-pressure discharge lamp constant, and adjusts a waveform of the current flowing in the high-pressure discharge lamp.
According to this configuration, the polarity inversion frequency of the lamp current as a frequency of the drive-controlling signal is set to a frequency (e.g., 170 Hz) allowing growth of the spots and the waveform of the current amount can be adjusted to have a constant peak value of the back porch of the lamp current. Thereby, it is possible to preheat gradually the origin of the arc just before polarity inversion of the lamp current, suppress lowering of the electrode surface temperature at the time of polarity inversion, and then war
Kakimoto Yasuhiro
Noguchi Toshiyuki
Dinh Trinh Vo
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Wong Don
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