Patent ID: 6426597
Filing Date: 2002-07-30
Classification: H05B,Y02B

Abstract:
A circuit for operating a gas discharge lamp comprising:four controllable switches arranged in a full bridge circuit, to which a direct voltage has been applied; in which a first of said switches being connected in series with a second of said switches, and a third of said switches being connected in series with a fourth of said switches, said first switch being connected to said third switch, and said second switch being connected to said fourth switch, said full bridge circuit having a bridge branch in which a gas discharge lamp can be connected, said bridge branch connecting a nodal point between said first switch and said second switch to a nodal point between said third switch and said fourth switch, and a control circuit which for normal operation of the gas discharge lamp alternately switches between a first and a second state at a first frequency, in which case during said first state causes said first and fourth switches to be open, said second switch to be switched on and off alternately at a second frequency which is higher than said first frequency, and said third switch to be closed at least when said second switch is also closed, and in which case during said second state causes said second and third switches to be open, said first switch to be switched on and off alternately at said second frequency, and said fourth switch to be closed at least when said first switch is also closed, said control circuit being constructed and arranged to monitor a branch current flowing in said bridge branch and during said first state to close said second switch, or in said second state to close said first switch whenever said branch current is at a minimum value.