Electronic ballast for hot cathode discharge lamps

Abstract

An electronic ballast for hot cathode discharge lamp comprises a DC voltage source, a chopper, and an inverter. The chopper includes a switching element connected in series with an inductor across the DC voltage and driven to turn on and off for providing a periodically interrupted voltage which is smoothed by a capacitor to produce a smoothed DC voltage as a chopper output. The inverter includes at least one switching element which is coupled to the chopper output to produce therefrom a high frequency AC voltage as an inverter output to be applied to cathodes of the discharge lamp for lighting the discharge lamp. The ballast is characterized to include an inverter controller and a delay circuit. The inverter controller controls the inverter to operate selectively in a normal mode of providing the inverter output of a first level and in a limited mode of providing the inverter output of a second level which is lower than the first level and is determined to give a preheating current to the cathodes. The delay circuit delays the start-up of the inverter for a time period from the start-up of the chopper. The inverter controller controls the inverter to operate in the limited mode after the start-up of the inverter for a predetermined period for preheating the discharge lamp after which the inverter controller allows the inverter to operate in the normal operation mode for lighting the discharge lamp.