Claims
- 1. An electronic ballast for gas discharge lamps having a filament at each end thereof, said ballast comprising:two line inputs and a neutral input; a converter section coupled to said inputs for providing high voltage direct current; a first inverter and a second inverter coupled to said converter section, wherein each inverter includes an AC output providing high voltage alternating current through a lamp and low voltage alternating current for heating the filament at each end of a lamp; wherein the first inverter provides low voltage alternating current at the AC output of the second inverter and the second inverter provides low voltage alternating current at the AC output of the first inverter.
- 2. The ballast as set forth in claim 1 wherein each AC output includes a series resonant inductor and capacitor and a plurality of filament windings, wherein the filament windings in the first inverter are magnetically coupled to the resonant inductor in the second inverter and the filament windings in the second inverter are magnetically coupled to the resonant inductor in the first inverter.
- 3. The ballast as set forth in claim 2 wherein each AC output includes a capacitor in series with each filament winding.
- 4. The ballast as set forth in claim 3 wherein each capacitor in series with a filament winding has a reactance of at least 100 Ω at the operating frequency of the ballast.
- 5. The ballast as set forth in claim 3 wherein each filament winding includes three to twenty turns.
- 6. The ballast as set forth in claim 2 wherein the filament windings are connected to the outputs with a polarity to minimize terminal current.
- 7. The ballast as set forth in claim 1 wherein either said first inverter or said second inverter or both inverters are enabled, depending upon which of said line inputs receives power.
- 8. A method for operating an electronic ballast having at least two line inputs, a neutral input, and two series resonant outputs, the ballast providing power to gas discharge lamps having a filament at each end thereof, said method comprising the steps of:applying power to at least one line input; initially providing high frequency voltage at both outputs for a predetermined period to provide low voltage AC for heating the filaments in the lamps coupled to the outputs; providing low frequency, high voltage for a predetermined time for starting a lamp, said low frequency voltage being applied to any output for which the corresponding line input receives power; and providing high frequency, high voltage to one output while providing high frequency, low voltage to a second output.
- 9. The method as set forth in claim 8 and further including the steps of:removing the power from one line input; and providing high frequency, low voltage to the corresponding output.
- 10. The method as set forth in claim 9 and further including the steps of:restoring power to the one line input; waiting a predetermined period; providing low frequency, high voltage to the corresponding output.
- 11. The method as set forth in claim 8 wherein said applying step includes the step of applying an alternating current to at least one line input.
- 12. The method as set forth in claim 8 wherein said applying step includes the step of applying a direct current to at least one line input.
- 13. The method as set forth in claim 12 wherein said step of providing low frequency, high voltage includes the step of providing low frequency, high voltage to a single output regardless of which line input receives power.
- 14. A method for operating an electronic ballast having a first output and a second output and including two inverters, wherein each inverter produces lamp current and heater current, said method comprising the steps of:coupling lamp current from a first inverter to the first output; and coupling filament current from a first inverter to the second output.
- 15. The method as set forth in claim 14 and further comprising the steps of:coupling lamp current from the second inverter to the second output; and coupling filament current from the second inverter to the first output.
CROSS-REFERENCES TO RELATED APPLICATIONS
This application relates to application Ser. No. 09/372,201, filed Aug. 11, 1999, entitled “Electronic Ballast with Selective Load Control,” now U.S. Pat. No. 6,137,239 and to application Ser. No. 09/447,333, filed Nov. 22, 1999, entitled “Electronic Ballast with Selective Power Dissipation,” now U.S. Pat. No. 6,177,769, both of which are assigned to the assignee of this invention and both of which are incorporated herein by reference.
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