Ballast with filament heating control circuit

A ballast for powering at least one gas discharge lamp having first and second lamp filaments, the ballast comprising:

• an inverter having first and second input terminals and first and second output terminals, wherein the second input terminal and the second output terminal are coupled to a circuit ground, the inverter including an inverter driver circuit having a preheat control output;
• an output circuit coupled to the inverter output terminals, the output circuit having first, second, third, and fourth output connections adapted for connection to the at least one gas discharge lamp, wherein the first and second output connections are adapted for connection to the first lamp filament, and the third and fourth output connections are adapted for connection to the second lamp filament, the output circuit including a first filament heating winding coupled between the first and second output connections, and a second filament heating winding coupled between the third and fourth output connections;
• a dimming control circuit having a pair of input connections adapted to receive a dimming control input;
• and
• a filament heating control circuit coupled to the dimming control circuit and at least one of the inverter and the output circuit, the filament heating control circuit being operable to control the inverter and output circuit such that:
• (i) during a preheat mode that occurs prior to ignition of the lamp, the lamp filaments are heated at a first level;
• (ii) during a full-light mode that occurs after ignition of the lamp, the lamp filaments are not heated, wherein the full-light mode includes operating the lamp at a current level that is substantially equal to a rated normal operating current of the lamp;
• and
• (iii) during a dimming mode that occurs after ignition of the lamp, the lamp filaments are heated at a second level, wherein the dimming mode includes operating the lamp at a current level that is substantially less than the rated normal operating current of the lamp;
• and
• wherein the filament heating control circuit includes a first electronic switch that turns on for controlling heating of the lamp filaments during the preheat mode, and a second electronic switch that turns on for controlling heating of the lamp filaments during the dimming mode.

A ballast for powering at least one gas discharge lamp having first and second lamp filaments, the ballast comprising:

• an inverter, comprising:
• first and second input terminals adapted to receive a source of substantially direct current (DC) voltage;
• first and second output terminals;
• a first inverter transistor coupled between the first input terminal and the first output terminal;
• a second inverter transistor coupled between the first output terminal and the second output terminal, wherein the second input terminal and the second output terminal are coupled to a circuit ground;
• an inverter driver circuit coupled to, and operable to provide substantially complementary commutation of, the first and second inverter transistors, the inverter driver circuit including a preheat control output;
• an output circuit coupled to the inverter output terminals, the output circuit comprising:
• first, second, third, and fourth output connections adapted for connection to the at least one gas discharge lamp, wherein the first and second output connections are adapted for connection to the first lamp filament, and the third and fourth output connections are adapted for connection to the second lamp filament;
• a resonant inductor coupled between the first output terminal of the inverter and the first output connection;
• a resonant capacitor coupled between the first output connection and the second output terminal of the inverter;
• a direct current (DC) blocking capacitor coupled between the fourth output connection and the second output terminal of the inverter;
• a first filament heating winding coupled between the first and second output connections;
• and
• a second filament heating winding coupled between the third and fourth output connections;
• a dimming control circuit having a pair of input connections adapted to receive a dimming control input;
• and
• a filament heating control circuit, comprising:
• a first terminal coupled to the first output terminal of the inverter;
• a second terminal coupled to the preheat control output of the inverter driver circuit;
• a third terminal coupled to the dimming control circuit;
• a first capacitor coupled between the first terminal and a first node;
• a filament heating control winding coupled between the first node and a second node, wherein the filament heating control winding is magnetically coupled to the first and second filament heating windings of the output circuit;
• a first electronic switch having a drain coupled to the second node, a gate coupled to the second terminal, and a source coupled to circuit ground;
• a second electronic switch having a drain, a gate coupled to the third terminal, and a source coupled to circuit ground;
• and
• a second capacitor coupled between the second node and the drain of the second electronic switch.