Patent ID: 8222835

Claim:
An electronic ballast comprising: a power converting circuit for receiving DC power and outputting AC power at output ends; a starting circuit connected between the output ends of the power converting circuit, and configured to generate a high voltage for starting a discharge lamp; a control circuit configured for controlling the power converting circuit to generate AC power output to the lamp; and a symmetry determining circuit configured to determine a positive-negative symmetrical state of the AC power output to the lamp with respect to ground; the control circuit further configured to after startup of the discharge lamp, enter a filament heating operation in which the output frequency of the power converting circuit is controlled to a first frequency, at one or more predetermined current detection points during the filament heating operation, obtain the symmetrical state from the symmetry determining circuit, end the frequency heating operation and enter a steady state operation when the symmetry determining circuit determines that the output current is in a symmetrical state at the current detection point, the steady state operation comprising an output frequency of the power converting circuit controlled to a second frequency for maintaining the lighting of the discharge lamp, and increase the output current to the lamp and continue the frequency heating operation when the symmetry determining circuit determines that the output current is in an asymmetrical state at the current detection point; and wherein the symmetry determining circuit is configured to detect a positive peak signal and a negative peak signal provided to the lamp with respect to ground, compare the peak signals to a symmetry reference voltage, determine the output power provided to the lamp to be in a symmetrical state when both of the positive and negative peak signals exceed the symmetry reference voltage, and determine the output power provided to the lamp to be in an asymmetrical state when at least one of the positive and negative peak signals do not exceed the symmetry reference voltage.