Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2000-02-09
2001-03-27
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S307000, C315S360000, C315SDIG002, C315SDIG007
Reexamination Certificate
active
06208088
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method and a ballast for starting a discharge lamp, particularly a high intensity discharge lamp (HID) such as a metal halide lamp.
2. Description of the Prior Art
HID lamps are known to be slow in reaching a stable operation of emitting a destined light output when starting the lamp at a cold state. Particularly, when the lamp is used for a vehicular headlamp or a light source for an LCD projector, it is highly desired to enable a cold start with prompt rise in the light output. To this end, Japanese Patent Laid-open Publication Nos. 4-141988 and 9-82480 propose a ballast which provides, at the start of the lamp, a run-up power greater than a normal power rating required for maintaining the operation of lamp. Then, the run-up power is made to decrease with time from a maximum power rating to the normal power rating over a transition period along a particular curve. The curve of the run-up power is derived from a single charging curve of a capacitor, as shown in
FIG. 15A
, and is represented as a reversal of the charging curve, as shown in FIG.
15
B. Since the ballast has a fixed maximum power rating for the discharge lamp, a portion of the run-up power curve above the maximum power rating should be limited to the maximum power rating, resulting a composite curve in which the maximum power rating is maintained for an initial start time period and then decrease with time to the normal power rating. As the initial start time is required to be longer for attaining a more rapid start of the lamp as indicated by solid lines in the above figures relative to those indicated by dotted lines, both curves, i.e., the charging curve and the run-up curve are made more moderate. Thus, the run-up power decreases along a more moderate slope, applying a more amount of power to the lamp during a transition period from the start to the stable lamp operation, resulting in an overshoot of the light output, as shown in FIG.
16
. In order to avoid this problem, it is desired to separately control the initial start time and the curve shape in the transition period which cannot be made in the above prior art, thereby decreasing the run-up power along a moderate slope to exclude a possibility of the overshoot in the light output.
SUMMARY OF THE INVENTION
In view of the above problem, the present invention has been accomplished to provide a method and a ballast for starting a discharge lamp which is capable of making a rapid start while restraining the overshoot of the light output. More specifically, the present invention enables to separately give the initial start time period of applying a maximum power rating to the lamp and the subsequent curve along which the power decreases to a normal power rating of the lamp in an optimized manner. The method in accordance with the present invention utilizes a ballast having a power converter capable of varying a power being applied to the discharge lamp within a range between the maximum power rating given to the lamp and the normal power rating given to the lamp. The method comprises varying the power along a particular run-up curve so as to apply the maximum power rating and subsequently apply the power decreasing to the normal power rating. The run-up curve is derived from a reference curve having a power level decreasing with time from the energization of the ballast. The reference curve has a maximum value exceeding the maximum power rating, and has an inflection point near the maximum power rating so as to define a first reference curve above the inflection point and a second reference curve below the inflection point, respectively. The first reference curve has a first average slope for a first reference time period from a point of the maximum value to the inflection point. The second reference curve has a second average slope for a second reference time period which starts from the inflection point and lasts for the same time interval as the first reference time period. The second average slope is greater than the first average slope. The run-up curve is a continuous composite curve of a straight line of the maximum power rating defined by a portion thereof below the reference curve and the remainder of the reference curve defined between the maximum power rating and the normal power rating. Thus, the initial start time period defined by the straight line of the run-up curve can be determined by the first reference curve, while the subsequent curve along which the power decreases to the normal power rating can be determined substantially by the second reference curve below the inflection point. With this result, the initial start time and the subsequent curve can be designed separately from each other in order to give a sufficient initial start time period for rapid start of the lamp and at the same time to give an optimum configuration to the subsequent curve for assuring a stable transition from the start to the normal operation of the lamp without causing an overshoot or insufficient light output. Accordingly, it is a primary object of t e present invention to provide a method of starting the discharge lamp with an optimum power characteristic to enable a rapid start with sufficient light output.
Most preferably, the inflection point is set to lie on the maximum power rating so that the second reference curve can defines itself the decreasing curve of applying the decreasing power to the discharge lamp after the initial start time period.
The present invention also provides the ballast which is specifically designed to realize the above method. The ballast includes a power converter capable of applying a varying power to discharge lamp, and a power commander which generates the run-up curve of the power with reference to time and is connected to the power converter to vary the power along the run-up curve.
Preferably, the power commander includes a function generator having a capacitor, a power source and a regulator for charging the capacitor by the power source at different rates to give a charging curve. The reference curve is obtained as a reversal of the charging curve so that the inflection point is given on the reference curve where the charging rate changes critically.
As will be seen in the detailed description of the embodiments of the present invention, various and advantageous configurations are made for the function generator to obtain the inflection point on the reference curve. These and still other object and advantageous features of the present invention will become more apparent from the following description of the embodiments when taken in conjunction with the attached drawings.
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patent: 5212428 (1993-05-01), Sasaki et al.
patent: 5442261 (1995-08-01), Bank et al.
patent: 5680015 (1997-10-01), Bernitz et al.
patent: 5806055 (1998-09-01), Zinda, Jr.
patent: 5828187 (1998-10-01), Fischer
patent: 6040661 (2000-03-01), Bogdan
patent: 6069457 (2000-05-01), Bogdan
patent: 6124681 (2000-09-01), Choi
patent: 6133697 (2000-10-01), Nagai et al.
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patent: 6140779 (2000-10-01), Kanazawa et al.
patent: 4-141988 (1992-05-01), None
patent: 9-082480 (1997-03-01), None
Kidera Kazunori
Konishi Hirofumi
Nakamura Toshiaki
Matsushita Electric & Works Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Philogene Haissa
Tran Thuy Vinh
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