Florescent lamp ballast control for zero -voltage switching operation over wide input voltage range and over voltage protection

摘要

A ballast system for at least one dimmable fluorescent lamp includes a resonant switching inverter and a controller which controls the inverter to operate above resonance during starting and normal running operation. After a start delay timer allows time for the lamp filaments to heat up, the controller provides control signals to a gate driver to drive the switching devices of the switching inverter initially at a relatively high frequency and then reduces the frequency until a sufficiently high voltage is reached to start the lamp. Once the lamp is started, the inverter is operated in its normal feedback mode. The ballast system further includes an overvoltage shutdown mechanism. During lamp starting, if either the output of the start delay timer is high or the output voltage is greater than a first overvoltage shutdown threshold, then an overvoltage shutdown timer is activated to shut down operation of the inverter for a predetermined overvoltage shutdown period. After the lamp has started, a second overvoltage shutdown threshold is activated which is lower than the first overvoltage threshold for avoiding operation of the inverter below resonance. Both the frequency and duty cycle of the output voltage from the switching inverter are variable to achieve a wide dimming range over a relatively narrow frequency range.