1. Field of the Invention
The present invention relates to a technique for lighting a discharge lamp.
2. Description of the Related Art
FIGS. 19A and 19B illustrate a technique disclosed in Japanese Patent Application Publication H05-217682. FIG. 19A shows a discharge lamp lighting apparatus. The discharge lamp lighting apparatus comprises an AC power supply 1, a primary voltage power supply unit 2, a primary voltage controller 7, a secondary voltage lighting circuit 3, a transformer 4, a discharge lamp 5, a primary current detector 6 and a CPU 8. FIG. 19B shows discharge lamp voltage applied to the discharge lamp 5. As shown in FIG. 19B, a secondary voltage is applied in addition to a primary voltage, which is necessary to maintain lighting, to temporally increase a voltage applied to the discharge lamp 5 in order to turn on the discharge lamp 5. During a stable period after the discharge lamp 5 is lit, the CPU 8 observes increase and decrease in electric current while it carries out control for applying the first voltage having a fixed frequency.
The discharge lamp lighting apparatus disclosed in Japanese Patent Application Publication H05-217682, however, has the following problems. First, applying a high voltage consisting of the primary voltage and the secondary voltage in lighting easily causes increase in radiant noise or error-causing noise. Accordingly, it has been necessary to take measures such as providing a protection countermeasure circuit or controlling software. Further, it is not guaranteed that onetime application of the high voltage turns on the discharge lamp 5, and in some cases, the high voltage consisting of the primary voltage and the secondary voltage should be applied several times. Moreover, a temperature of the discharge lamp 5 just after extinguishing the discharge lamp 5 is high, so that application of the high voltage is likely cause breakage of the lamp. Therefore, it has been necessary to forbid relighting of the discharge lamp 5 while the temperature of the discharge lamp 5 is high.
In addition, a discharge gap in a discharge tube always changes as time passes and a discharge environment according to a discharge temperature always changes, so that a resonance frequency is different, while control of discharge is always set fixedly. This causes a problem that in often case the discharge lamp is not operating under an optimum condition.