Patent ID: 8049432
Filing Date: 2011-11-01
Classification: H05B

Abstract:
1. An electronic ballast for driving a gas discharge lamp having first and second electrodes, such that a lamp current flows from the first electrode to the second electrode the lamp and a lamp voltage is produced across the lamp, the ballast comprising: an inverter circuit operable to convert a substantially DC bus voltage to a high-frequency AC voltage; a resonant tank circuit having an output and operable to couple the high-frequency AC voltage to the lamp; a current transformer having first and second primary windings, and first and second secondary windings magnetically coupled to the first and second primary windings, the first primary winding adapted to be coupled in series electrical connection between the output of the resonant tank circuit and the first electrode of the lamp, the second primary winding adapted to be coupled in series electrical connection between the output of the resonant tank circuit and the second electrode of the lamp, the first and second primary windings coupled such that differential-mode currents in the electrodes are added and common-mode currents in the electrodes are subtracted, the first and second secondary windings operable to conduct respective first and second currents representative of the lamp current; and a lamp current measurement circuit coupled to the first and second secondary windings of the current transformer, the lamp current measurement circuit comprising a capacitor coupled such that the first current of the first secondary winding of the current transformer flows into the capacitor when the magnitude of the lamp current is positive, and the second current of the second secondary winding of the current transformer flows out of the capacitor when the magnitude of the lamp current is negative, the capacitor only conducting the first and second currents of the first and second secondary windings during every other half-cycle of the lamp voltage; wherein a voltage produced across the capacitor is representative of the magnitude of the lamp current that is in-phase with the lamp voltage.