Fixed operating frequency inverter for cold cathode fluorescent lamp having strike frequency adjusted by voltage to current phase relationship

• US 7,294,974 B2
• Filed: 02/16/2005
• Issued: 11/13/2007
• Est. Priority Date: 10/02/2003
• Status: Expired

• Current Assignee: Monolithic Power Systems Incorporated
• Original Assignee: Monolithic Power Systems Incorporated
• Inventors: Moyer, James C., Rust, Timothy J.
• Primary Examiner(s): Philogene; Haissa
• Application Number:
  US11/060,237
• Publication Number:
  US 20050140313A1
• Time in Patent Office: 1,000 Days
• Field of Search: 315/209.R, 315/194, 315/142, 315/224, 315/244, 315/291, 315/276, 315/307, 315/DIG.5, 315/DIG.7, 363/16, 363/17, 363/24, 363/41, 363/132
• US Class Current: 315/291
• CPC Class Codes: H05B 41/2828 using control circuits for the switching elements H05B41/2827 takes precedence
• CPC Class Codes: Y10S 315/05 Starting and operating circuit for fluorescent lamp
• CPC Class Codes: Y10S 315/07 Starting and control circuits for gas discharge lamp using transistors

Abstract

A method of driving a lamp that uses a DC to AC inverter that is connected to a primary winding of a transformer is disclosed. The inverter frequency is variable, and in one embodiment, may be controlled by a voltage controlled oscillator. Circuitry is included that monitors the phase relationship between a voltage across a secondary of the transformer and a current through the primary of the transformer. The circuitry monitors the phase relationship and adjusts the inverter frequency, such as by adjusting voltage controlled oscillator, so that the phase relationship is maintained at a predetermined relationship.

Claims

1. A method of driving a lamp that uses a DC to AC inverter that is connected to a winding of a transformer comprising:

(a) monitoring a phase relationship between a voltage across said winding of said transformer and a current through said winding of said transformer; and

(b) keeping said phase relationship between said voltage across said winding of said transformer and said current through said winding of said transformer at substantially a predetermined relationship.

2. The method of claim 1, wherein said voltage across said winding of said transformer is substantially in phase with said current through said winding of said transformer.

3. The method of claim 1, wherein during ignition of said lamp, the operating frequency of said inverter is increased by maintaining said predetermined relationship between said voltage across said winding and said current through said winding.

4. The method of claim 2, wherein said voltage across said winding of said transformer is maintained substantially in phase by using the zero-crossing information of said current in said winding.

5. An apparatus for driving a lamp comprising

a transformer having a primary and a secondary;

means for converting a DC power into AC power and operating at a frequency, the means for converting driving the primary of said transformer;

means for phase comparison that monitors a phase relationship between a voltage across said primary of said transformer and a current through said primary of said transformer; and

means for frequency control that adjusts the frequency of said means for converting such that said phase relationship between said voltage across said primary of said transformer and said current through said primary of said transformer is maintained at substantially a predetermined relationship.

6. The apparatus of claim 5, further including a voltage controlled oscillator that is responsive to said means for frequency control and to output an oscillation used by said means for converting to generate said frequency.

7. The apparatus of claim 5, wherein said means for phase comparison and said means for frequency control operate to maintain said phase relationship as being substantially in phase.

8. The apparatus of claim 5 wherein said means for phase comparison further includes a zero-crossing detector for monitoring said current through said primary.