Patent ID: 8232741

Claim:
An electronic ballast comprising: a power converting circuit for receiving a DC power input and converting the DC power input into an AC power output; a starting circuit coupled between output terminals 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 dependent on a mode of operation; and a symmetry detecting circuit configured to determine a positive-negative symmetrical state of the output power provided to the discharge lamp with respect to ground, said symmetry detecting circuit comprising: a current transformer having a primary winding operable to connect in series with the lamp and a secondary winding with a center tap coupled to ground; a first capacitor coupled to a first end of a secondary winding of the current transformer, a second capacitor coupled to a second end of the secondary winding, and the symmetry detecting circuit configured to compare voltages across the capacitors with a predetermined threshold value to determine the positive-negative symmetrical state of the output power provided to the discharge lamp, wherein the control circuit further comprises a starting mode wherein the discharge lamp is triggered to start with a high voltage generated by the starting circuit, an electrode heating mode wherein the AC power output of the power converting circuit is controlled to a first frequency for heating each lamp electrode, and a steady-state mode wherein the AC power output of the power converting circuit is controlled to a second frequency for maintaining lighting of the discharge lamp, wherein the control circuit shifts from the electrode heating mode to the steady-state mode when the symmetry detecting circuit determines that the AC power output is in a positive-negative symmetrical state in the electrode heating operation.