The present invention relates to an Electronic
ballast for operating a high-pressure
discharge lamp, comprising a switched-mode power supply circuit (SMPS) for supplying a rail
voltage Vrail to the
high pressure discharge lamp, the power supply circuit comprising a
half bridge commutating forward circuit including an
inductance (Lhbcf), a
capacitor circuit of a first
capacitor (Cs1) and a second
capacitor (Cs2) in series, the lamp (LA) being connectable between the
inductance (Lhbcf) and a midpoint (M) between the first and second capacitor (Cs1,Cs2); a
duty cycle control circuit, connected to the switched-mode power supply circuit, for controlling the
duty cycle of the
half bridge commutating forward circuit; and
overvoltage protection means for protecting said capacitors against
overvoltage, comprising—a midpoint
voltage detection circuit for detecting the
voltage at the midpoint between the first and second capacitor, the midpoint voltage detection circuit providing a first
signal representative of the detected midpoint voltage; voltage difference determining circuit for determining the difference between the first
signal and a second
signal, which is representative of half the rail voltage, wherein the voltage difference determining circuit is connected to the
duty cycle control circuit and the duty
cycle control circuit is operable to change the duty cycle of the
half bridge commutating forward circuit if the determined difference exceeds a maximum voltage difference.