Patent ID: 7145293

Claim:
An electronic ballast for powering a high-pressure discharge lamp (Lp), the ballast comprising: a half-bridge inverter (S 1 ,S 2 ,C 4 ,C 5 ) adapted to receive an intermediate circuit voltage (U 1 ) provided between first and second supply branches, and operable to provide an alternating inverter output voltage having a fundamental frequency, the half-bridge inverter including first and second inverter switches (S 1 ,S 2 ); a resonant circuit (L 1 ,C 1 ,C 2 ,C 3 ) coupled to the inverter, the lamp, and the first and second supply branches, the resonant circuit having a resonant frequency; and a starting circuit (G 1 ) coupled between the resonant circuit and the lamp, and operable to provide a high voltage starting pulse for igniting the lamp during a starting phase; wherein the inverter and resonant circuit are further operable such that: (a) during at least a transfer phase following the starting phase but preceding a continuous operation phase, the inverter operates at a first fundamental frequency that is substantially less than the resonant frequency of the resonant circuit, whereby the inverter and the resonant circuit operate during the transfer phase to provide a lamp voltage across the lamp that is characterized by having: (i) a peak amplitude that is distinctly greater than one half of the intermediate circuit voltage, but substantially lower than the peak amplitude of the high voltage starting pulse; and (ii) a substantially sinusoidal waveshape having a substantially zero average value; and (b) during the continuous operation phase following completion of the transfer phase, the inverter operates at a second fundamental frequency that is substantially lower than the first fundamental frequency, whereby the inverter and the resonant circuit operable to provide a lamp voltage across the lamp that is characterized by having: (i) a peak amplitude that is distinctly less than one half of the intermediate circuit voltage; and (ii) a substantially squarewave waveshape having a substantially zero average value.