Patent ID: 8049435

Claim:
A frequency synchronizing method for a discharge tube lighting apparatus having a resonant circuit including a capacitor connected to at least one of primary and secondary windings of a transformer, an output of the resonant circuit being connected to a discharge tube, and a plurality of switching elements of bridge configuration connected to both ends of a DC power source, to pass a current through the primary winding of the transformer and the capacitor, comprising: generating a triangular wave signal whose inclination for charging an oscillator capacitor and inclination for discharging the same are the same and which is used to turn on/off the plurality of switching elements; generating a first drive signal in a period shorter than a half period of the triangular wave signal, the first drive signal having a pulse width corresponding to a current passing through the discharge tube, and configured to drive a first group of one or more switching elements among the plurality of switching elements and provide a current to the discharge tube; generating a second drive signal having a pulse width substantially equal to that of the first drive signal and a phase difference of about 180 degrees with respect to the first drive signal, configured to drive a second group of one or more switching elements among the plurality of switching elements and provide a current to the discharge tube in a direction opposite to the current driven by the first drive signal; and converting a synchronization pulse voltage signal into a pulse current that alternates between positive and negative current values of the same absolute value at a duty of about 50% and superimposing the pulse current on the triangular wave signal of the oscillator for generating a modulated pulse current, wherein the first and second drive signals are generated in synchronization with the frequency of the modulated pulse current.