End-of-life detector for gas discharge lamp and the ballast incorporating the same

A ballast for driving at least one first gas discharge lamp, comprising:

• a power circuit for converting an input voltage into at least one first lamp voltage to drive the at least one first gas discharge lamp;
• a control unit for controlling operations of the ballast;
• and
• an end-of-life detector for gas discharge lamp connected to the at least one first gas discharge lamp and the control unit, comprising:
• a lamp state signal detecting circuit connected to the at least one first gas discharge lamp for detecting a lamp state signal of the at least one first gas discharge lamp and in response thereto generating a positive voltage signal during a positive half-cycle of the lamp state signal and a negative voltage signal during a negative half-cycle of the lamp state signal;
• a comparing circuit connected to the lamp state signal detecting circuit for comparing the positive voltage signal with a positive selecting voltage and comparing the negative voltage signal with a negative selecting voltage, and in response thereto generating a positive control signal and a negative control signal;
• and
• a positive/negative duty time interval detecting circuit connected to the comparing circuit for generating a lamp life state signal which is generated by a difference between a positive duty time interval defined by the positive control signal and a negative duty time interval defined by the negative control signal;
• wherein the lamp life state signal is varied along with the difference between the positive duty time interval and the negative duty time interval, the control unit is configured to control operations of the power circuit according to the lamp life state signal, and when the control unit determines that the first gas discharge lamp has entered an end-of-life state by the lamp life state signal, the control unit activates protection functions of the power circuit.