Dimming method and device for fluorescent lamps used for backlighting of liquid crystal screens

• US 5,105,127 A
• Filed: 06/21/1990
• Issued: 04/14/1992
• Est. Priority Date: 06/30/1989
• Status: Expired due to Fees

• Current Assignee: Thomson-CSF
• Original Assignee: Thomson-CSF
• Inventors: Bouron, Jean P., Lavaud, Georges
• Primary Examiner(s): LaRoche, Eugene R.
• Assistant Examiner(s): RATLIFF, REGINALD A
• Application Number:
  US07/541,766
• Time in Patent Office: 663 Days
• Field of Search: 315/291, 315/307, 315/DIG. 4, 340/784, 350/345
• US Class Current: 315/291
• CPC Class Codes: G09G 2320/0626 for control of overall brightness
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: H05B 41/3927 by pulse width modulation H05B41/3922 takes precedence
• CPC Class Codes: Y10S 315/04 Dimming circuit for fluorescent lamps

Abstract

A dimming device, with a brightness dimming ratio of 1 to 1000, for a fluorescent lamp used for the backlighting of a liquid crystal screen comprises a periodic signal generator for delivering rectangular pulses with an adjustable duty cycle. The pulses are synchronized with the image synchronizing signal of the liquid crystal screen. An alternating voltage generator provides power to the lamp only during the pulses. The decrease in tube efficiency for very short pulses allows the required dimming intensity to be achieved without image flickering.

Claims

1. A dimming device for fluorescent lamp used for the backlight of liquid crystal screen with an image synchronizing signal applied to the screen, comprising:

a switching generator for producing switching signals, at a fixed frequency, the switching signals being rectangular periodic signals comprising pulses having adjustable widths;

synchronizing means for synchronizing the switching signals with at least some of the image synchronizing signals;

an alternating voltage supply oscillator, connected to a first supply voltage, for applying an alternating voltage to the fluorescent lamp; and

blocking means, controlled by the switching signals, to allow the alternating voltage supply oscillator to function only during the duration of the pulses of the rectangular periodic signals, the blocking means comprising switching means for applying a second supply voltage, opposite in polarity to the first supply voltage, to the alternating voltage supply oscillator, the second supply voltage temporarily blocking application of the first supply voltage to the alternating voltage supply oscillator.

2. A dimming device according to claim 1, where, the image synchronizing signal comprising pulses, the switching signals used to obtain a minimum brightness value for the fluorescent lamp are the pulses of the image synchronizing signal.

7. A dimming device according to claim 1, further comprising means for varying the magnitude of the alternating voltage.

8. A dimming device according to claim 1, wherein the alternating voltage supply oscillator is a transformer having a primary winding with a center tap and a feedback winding with a further center tap, wherein the first supply voltage is connected to the center tap of the primary winding and wherein the switching means selectively applies the second supply voltage to the feedback winding to block operation of the alternating voltage supply oscillator.

3. A method for dimming a fluorescent lamp used for the backlighting of a liquid crystal screen to which image synchronizing signals are applied, the method comprising the steps of:

generating switching signals at a fixed frequency, the switching signals being rectangular periodic signals comprising pulses having adjustable widths;

synchronizing the switching signals with at least some of the image synchronizing signals;

applying an alternating voltage, via a transformer connected to a primary supply voltage, to the fluorescent lamp; and

selective connecting a blocking supply voltage, opposite in polarity to the primary supply voltage, to the transformer, which temporarily blocks application of the alternating voltage to the lamp, via the transformer, for adjustable periods of time determined by an absence of the pulses of the rectangular periodic signals.

4. A method according to claim 3, wherein the step of selectively connecting is performed in synchronism with integral numbers of the synchronizing signals.

5. A method according to claim 3, wherein the adjustable periods of time have a predetermined maximum duration to assure that the alternating voltage is always applied to the lamp for at least a predetermined minimum period of time.

6. A method according to claim 3, further comprising the step of varying the magnitude of the alternating voltage.