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1. Field of the Invention
The present invention is related to the field of lamp devices. In particular, the present invention is related to a method and apparatus for canceling ripple current in a projector lamp.
2. Description of the Related Art
In order to make for a pleasant viewing experience, an image projected by a data or movie projector must be steady. A data projector is a projector that is used to display still (non-moving) images whereas a movie projector is a projector that is used to display moving images. The stability of the projected image is dependant upon the stability of the current that drives the projector""s lamp. Unfortunately, the current that drives a projector lamp has an alternating current (AC) component (ripple current). Depending on the magnitude and the frequency of the ripple current, the ripple current excites the quartz envelope of the projector lamp and causes the projector lamp to resonate. A resonating projector lamp causes the gases within the lamp to swirl, and the swirling gases in a projector lamp causes the light emitted by the lamp to flicker, which in turn causes the image projected by the projector to be unsteady and to flicker. The greater the magnitude of the ripple current, the more pronounced is the flicker.
Therefore, in order to eliminate a projector lamp""s flicker it is necessary to reduce or to virtually eliminate the ripple current that flows through the lamp. Traditionally, a filter circuit as illustrated in FIG. 1 has been employed to reduce the ripple current. The filter circuit as illustrated, is typically made up of passive components like inductors and capacitors. In order to virtually eliminate the ripple current, one solution is to increase the size of the filter components. However, large inductors and capacitors in the filter circuit is not the optimum solution for a projector wherein the weight of the projector needs to be controlled.
An alternate solution to reducing the ripple current in a projector lamp is to increase the switching frequency of the input current. However, the present state of the art restricts the upper switching frequency limit since, beyond a certain critical frequency, the switching circuit becomes inefficient and generates a larger amount of heat. In a projector, excess heat generation is also undesirable.
What is needed, therefore, is a method and apparatus to virtually eliminate the ripple current in a projector""s lamp without generating heat, and without increasing the size of the filtering components.