Patent ID: 8593078

Claim:
An electronic ballast comprising: first and second output branches, each including associated first and second connection terminals; an inverter effective to provide an output signal across each of the first and second output branches in accordance with a dimming control input signal; a filament voltage control block effective to provide a filament heating voltage across the first and second connection terminals associated with each of the first and second output branches; a control block effective to control the inverter and the filament voltage control block in accordance with a preheat operating mode during which the control block is effective to provide a first control signal to disable the inverter and to provide a second control signal to enable the filament voltage control block, and a normal operating mode during which the control block is effective to adjust the first control signal to enable the inverter and to modulate the second control signal to define a pulse width modulated (PWM) control signal having a duty ratio based on a detected output signal from the inverter and; the filament voltage control block further comprises first and second filament heating switches coupled in series between a DC rail voltage and ground, a driver circuit effective to drive the first and second filament heating switches, a filament heating transformer having a primary winding coupled on a first end to a node between the first and second filament heating switches and on a second end to ground, and the control block is effective during the preheat operating mode to provide the second control signal to enable the driver circuit to drive the first and second filament heating switches at a predetermined frequency.