Current-preheat electronic ballast and resonant capacitor adjusting circuit thereof

A current-preheat electronic ballast for driving at least one lamp group, said current-preheat electronic ballast comprising:

• an AC-to-DC converter connected with a DC bus for converting an AC input voltage into a high DC voltage and outputting said high DC voltage;
• a controlling unit for controlling operations of said current-preheat electronic ballast;
• an auxiliary voltage generator for generating an auxiliary voltage;
• and
• an inverter connected with said DC bus for converting said high DC voltage into an AC output voltage and generating a resonant current and a lamp filament current to said lamp group, wherein said inverter comprises:
• a resonant circuit connected with said lamp group for providing electric energy required to preheat said lamp group, and comprising a resonant inductor and a plurality of resonant capacitors;
• and
• a resonant capacitor adjusting circuit connected with said resonant circuit and a detecting element, wherein said resonant capacitor adjusting circuit judges whether said inverter is enabled according to said detecting element, wherein after said inverter has been enabled for a delayed time, two high-voltage switching terminals of said resonant capacitor adjusting circuit are correspondingly conducted or shut off, so that an equivalent resonant capacitance value of said resonant circuit is changed and a voltage drop across two ends of a lamp filament of said lamp group is changed.