Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
1999-12-17
2001-08-21
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S2090SC, C315S224000, C315SDIG005
Reexamination Certificate
active
06278244
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a circuit arrangement for operating a high pressure discharge lamp comprising:
input terminals for connection to a supply voltage source;
lamp connection terminals for connection of the lamp,
power supply means coupled to the input terminals for supplying a current to the lamp consisting of successive periods of current and comprising control means suitable for generating a current pulse superpositioned on periods of the current to the lamp.
Such a circuit arrangement is known from U.S. Pat. No. 5,608,294. The known circuit arrangement provides a measure to suppress flickering of a high pressure discharge lamp and is in particular suitable for operating a high pressure discharge lamp in a projection system like a projection television apparatus. In the known circuit arrangement, the lamp is supplied with a current of successive block shaped periods of opposite polarity. The suppression of flickering is reached by supplying to periods of the lamp current additional current pulses with the same polarity at the end of a predetermined fraction of such a period of a lamp current. By means of the thus reshaped current period, the temperature of the electrode is raised to a relatively high value, which high temperature increases the stability of the discharge arc, because the discharge arc originates from the same place on the electrode in each cathodic phase and so flickering is substantially suppressed. The additional current is supplied in a regular sequence, preferably at each successive period. This flickering suppression is of utmost importance as any instability in the discharge position in translated in an optical system like a projection system into a annoying flickering on the projection screen.
In circumstances where the operated lamp forms a part of a projection system with a liquid crystal display (further called LCD display), it has occurred that irregular brightness fluctuations can appear in the projected image. These are a nuisance for the spectator, in particular when those fluctuations have such rate of appearance that they gave the impression of a moving spot of different brightness.
SUMMARY OF THE INVENTION
The invention aims to provide a circuit arrangement for operating a high pressure discharge lamp in a way which substantially overcomes the mentioned drawback and simultaneously is maintaining the substantial suppression of flickering of the lamp during its operation.
According to the invention, a circuit arrangement of the kind mentioned in the opening paragraph is for this purpose characterized in that the circuit arrangement is adapted for synchronizing the control means with an image writing signal. An advantage, whatever might be the precise cause of the perceived brightness differences, they appear to be hardly distinguishable anymore to the human eye, while at the same time arc stability is maintained.
As image writing signals generally are provided with a synchronizing signal, this synchronizing signal of the image writing signal is preferably used for synchronizing the control means. In a preferable embodiment according to the invention the control means incorporates a microcontroller which triggers the generating of a current pulse on receiving at an input the image writing synchronizing signal. In a further preferred embodiment, in which the power supply means incorporates commutating means for supplying a lamp current of successive periods of current having opposite polarity, the image writing synchronizing signal is used for defining a commutating frequency. In a further preferred embodiment the microcontroller of the control means is provided with a threshold frequency value for comparing this value with a detected image writing synchronizing signal frequency and defining the commutating frequency equal to the synchronizing signal frequency if the latter is above the threshold value. In case the synchronizing frequency is below the threshold value the microcontroller defines the commutating frequency to be twice the synchronizing frequency. For any skilled person it will be evident that functions performed by the microcontroller, as just described, can be equally well realized in and performed by a circuit which is constructed without a microcontroller. However using an embodiment incorporating the microcontroller has the advantage that for use in different types of projection systems only adaptation of the software of the microcontroller is required.
REFERENCES:
patent: 5608294 (1997-03-01), Derra et al.
patent: 0406116A1 (1991-01-01), None
patent: WO9714275 (1997-04-01), None
Hevinga Michel A.
Monch Holger
Riederer Xaver
U.S. Philips Corporation
Vu David
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