Patent ID: 6501225
Filing Date: 2002-12-31
Classification: H05B

Abstract:
A ballast for powering at least one gas discharge lamp having heatable filaments, comprising:an inverter having a pair of inputs and an output, and operable to receive a substantially direct current (DC) voltage and to provide an alternating voltage at the inverter output; first, second, third, and fourth output connections adapted for connection to the lamp, wherein the first and second output connections are coupled to a first filament of the lamp, and the third and fourth output connections are coupled to a second filament of the lamp; a resonant inductor coupled between the inverter output and the first output connection; a resonant capacitor coupled between the first output connection and a first node; a direct current (DC) blocking capacitor coupled between the fourth output connection and circuit ground; a filament heating and protection circuit coupled to the first node and the first, second, third, and fourth output connections, and operable to provide: (i) a filament preheating mode wherein a voltage across each filament is maintained at a preheat level, and a voltage between the first and fourth output connections is maintained at a pre-ignition level, in order to preheat the filaments prior to attempting to ignite the lamp; (ii) an ignition mode wherein the voltage between the first and fourth output connections is increased to an ignition level that is greater than the pre-ignition level; (iii) a normal operating mode wherein the voltage across each filament is maintained at an operating level that is substantially less than the preheat level; and (iv) a fault mode wherein the filament preheating mode and the ignition mode are repeated in response to a lamp fault condition.