Electronic ballast for high-intensity discharge lamp and method of driving high-intensity discharge lamp

摘要

The electronic ballast includes a power source circuit for transforming a conventional AC power source into a stepped-up DC voltage and a half-bridge type inverted, in response to a pair of switching driving signals, for switching the stepped-up DC voltage to make a high- intensity discharge lamp operate. The ballast also includes a digital controller for generating the pair of the switching driving signals, which have a predetermined dead time respectively between a switching-ON time interval of the switching driving signals and a phase difference of 180 degrees therebetween, in a way that frequencies of the pair of the switching driving signals are gradually shifted into some other stepped levels in turn within a predetermined bandwidth by a predetermined time interval and such a frequency shifting operation is cyclically repeated, to be supplied to the half-bridge type inverter. For the prevention of an audible noise of a high-frequency and stabilization of a flame, the high- intensity discharge lamp is driven in a reformed White Noise Modulation method where a driving signal is repeated with a first identical frequency several to several tens of times during a present period and the driving signal is varied into a second identical frequency and is also repeated several to several tens of times during the next period.