Patent ID: 7436123

Claim:
A discharge lamp ballast device comprising: a rectifier configured to rectify an AC voltage from an AC power supply; a chopper configured to include an inductor, a smoothing capacitor, and a switching element to convert an output voltage from the rectifier into a DC voltage; an inverter configured to include at least one switching element and turn on and off said switching element at a high frequency for converting an output of the chopper into an AC power; a resonance circuit configured to include at least one inductor and a capacitor to resonate the AC power output from said inverter to apply the same to a discharge lamp; an inverter controller configured to selectively drive said at least one switching element at one of a preheating frequency, a starting frequency, and lighting frequency which are different from each other, to give a preheating mode in which the inverter provides a preheating voltage for preheating filaments of said discharge lamp, a starting mode in which the inverter provides a starting voltage for staring the discharge lamp, and a lighting mode in which the inverter provides a lighting voltage for stably lighting said discharge lamp; a lamp abnormality detection circuit configured to detect abnormality of said discharge lamp; reset means configured to the output voltage supplied from said chopper to said inverter, and to cause said inverter controller to operate at said starting mode or preheating mode when the output voltage from said chopper is lowered below a first threshold; inverter stop means configured to operate said inverter controller to stop said inverter when said lamp abnormality detection circuit detects the abnormality; and a timer configured to provide to said inverter controller a signal determining the start of said preheating mode, said starting mode, and said lighting mode, and to generate a reset disable signal Rdis disabling said reset means and a inverter stop disable signal Sdis disabling said inverter stop means, respectively; wherein said inverter controller includes a frequency sweep means which varies the switching frequency gradually from said starting frequency to said lighting frequency, said timer is configured to generate said reset disable signal only during a period (t 1 to t 4 ) starting upon selection of said preheating frequency and ending at a time when said switching frequency is caused by said frequency sweep means to reach said lighting frequency, thereby disabling said reset means during said period; said timer is configured to generate said inverter stop disabling signal only during a period (t 1 to t 3 ) starting upon selection of said preheating frequency and ending at a time when said switching frequency is caused by said frequency sweep means to begin varying from the starting frequency to said lighting frequency, thereby disabling said inverter stop means during this period.