Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
1999-12-15
2001-05-29
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C315S224000, C315SDIG005
Reexamination Certificate
active
06239556
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a circuit arrangement for operating a high pressure discharge lamp having, during operation, an electrode which is in a cathodic phase, comprising
input terminals for connection to a supply voltage source,
output terminals for connecting the high pressure discharge lamp, and
means, coupled to the input terminals, for supplying an alternating lamp current having successive periods of opposite polarity to the high pressure discharge lamp, the lamp current per period having a mean value Im.
Such a circuit arrangement is known from U.S. Pat. No. 5,608,294. In the known circuit the lamp current at each period is provided with a superposition of a current pulse in a latter part of each period. In the case where a high pressure discharge lamp is operated with an AC current, each electrode of the lamp alternatingly functions as a cathode and as an anode during successive periods of the lamp current. During these successive periods the electrode emitting the lamp current is said to be in the cathodic phase and the other electrode to be in the anodic phase respectively. Because the total amount of current through the lamp is increased at the end of each period of the lamp current by means of the current pulse, the temperature of the electrode is sufficiently raised to increase the stability of the discharge arc. Accordingly, flickering of the high pressure discharge lamp can be substantially suppressed. Due to its flickering suppression properties the circuit arrangement is in particular suitable for operating a high pressure discharge lamp in a projection system like a projection television apparatus.
Lamps operated with the known circuit arrangement unfortunately showed a continuous increase of the arc voltage over an operating time of several hundred hours, which voltage increase appeared to continue when the lamp was experimentally operated for several thousand hours. As a stable luminous output of the lamp which is fairly constant over the life of the lamp is of vital importance for use in a projection system, a continuous arc voltage increase forms a serious draw back in reaching a long lamp life.
SUMMARY OF THE INVENTION
The invention aims to provide a circuit arrangement for operating a high pressure discharge lamp in which the mentioned draw back is counteracted.
According to the invention a circuit arrangement of the kind mentioned in the opening paragraph is for this purpose characterized in that the lamp current at the start of each period is lowered with respect to the mean value Im so as to allow for stable diffuse attack on the cathodic phase electrode. The arrangement has the advantage that during lamp operation the discharge arc has a soft start each period by diffusely attaching both electrodes resulting in a lower electrode load, so making it possible to operate the lamp without flickering over a long time together with a very significant reduction in occurrence of an increase of the arc voltage.
The mean value of the current Im over a period corresponds to the power Pla of the lamp according to the relation Pla=Im*Vla, where Vla is the lamp voltage. Experiments have shown that preferably the mean value of the current over a first part of the period Ie is smaller than the value Im and over a second part of the period a mean current value I2 preferably is larger than Im. This further improves a soft current start at the beginning of each period with a diffuse arc attachment over the first part of the period resulting in a stabilized arc attachment. As a consequence of the current over the second part of the period I2 being larger than Im, the need of an additional current pulse near the end of the period is further reduced.
In a preferred embodiment of the arrangement according to the invention, the period has a time duration tp and the first part of the period a time duration t1 which satisfies the relation 0.05≦t1/tp≦0.85. In case of a value of the ratio t1/tp smaller than 0.05 experiments showed no measurable improvement over the prior art. The upper value of the ratio t1/tp depends on a compromise. For the diffuse stable attachment the ratio should be as long as possible. However as the cathode tends to cool during the cathode phase so the current which is emitted with a diffuse stable attachment tends to lower, this comes in conflict with the need for keeping the lamp operating at the requested power rate. So experimentally it was shown that for an increasing current shape in the first part of the period the ratio should preferably be at most 0.5. In the situation of a decreasing current during t1 the maximum ratio can be 0.85. Otherwise it has appeared that for the values of currents Ie and Im a relation should hold which reads 0.3≦Ie/Im≦0.9 and preferable satisfies 0.6≦Ie/Im≦0.8, resulting in a reduced extra load on the electrode in comparison to the prior art. For current values of Ie>0.9Im no measurable effects have been found. If le becomes too low with respect to Im the value of the mean current I2 in the second part of the period must be chosen so high that there exists a serious risk of the occurrence of arc instabilities during the second part of the period, resulting in lamp flickering.
In a further preferred embodiment the current at the start of the period is higher than Ie. In this way there is taken into account in an advantageous way that the cathode temperature lowers during the period and thus correspondingly the current value for which a stable diffuse attachment of the arc holds.
Arc stabilization and therethrough reducing lamp flicker is still further promoted by adding to the lamp current an additional current pulse as known from the prior art. Preferably the lamp current is then provided with a pulse of the same polarity at the end of the period with a value I3 satisfying the relation I3≦2Im.
REFERENCES:
patent: 5608294 (1997-03-01), Derra et al.
patent: 4439885A1 (1996-05-01), None
patent: WO9714275 (1997-04-01), None
Derra Gunther H.
Fischer Hanns E.
Ganser Hans G.
Krücken Thomas V
Moench Holger
U.S. Philips Corporation
Vu David
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