Patent ID: 7271552

Claim:
Ballast circuit for operating gas discharge lamp, comprising a half-bridge DC-AC converter having a voltage controlled oscillator for alternating switching two switches of said half-bridge, said oscillator having an input with a control voltage which determines an operating frequency of said half-bridge; a resonance circuit connected to said half-bridge for feeding the lamp; and a feedback circuit connected at a first end to said resonance circuit for adjusting the operating frequency of said half-bridge, characterized in that the other end of said feedback circuit is connected to the input of said voltage controlled oscillator and designed such that during at least a substantial part of a start-up period of the lamp an equilibrium exists wherein the half-bridge frequency is at least nearly equal to a resonance frequency and a half-bridge voltage is forced to operate at last nearly in phase with a half-bridge current; and characterized in that said oscillator input is further connected to a current supply and a capacitor, wherein said equilibrium is determined by said currently supply loading said capacitor, and said feedback circuit at least partially unloading said capacitor each half-bridge switching cycle.