Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
1999-06-09
2001-03-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S307000
Reexamination Certificate
active
06198234
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This present invention relates to a power conversion circuit for driving fluorescent lamps, such as, for example, cold cathode fluorescent lamps (CCFLs) and more particularly to the drive topology of such circuits.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate light is limited. One such application is the backlighting for a flat panel computer display, or the like. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube, which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphorescent material coated in the inner wall of the tube, resulting in visible light.
One problem with CCFL tubes is that such tubes do not generate a high level of light output at low temperatures. When these systems are installed where they are exposed to environmental conditions, such as in automobiles, it can take several minutes of operation before the lamp temperature reaches a point to generate an acceptable amount of light output. This makes the backlight systems unusable at low temperatures.
To combat this problem, some manufactures developed “self-heating” lamps. Essentially, these lamps contain two different gases, one optimized to operate at a cold temperature and a second optimized at a normal operating temperature. At low temperatures, the first gas glows and provides an enhanced light output. After the lamp warms up, the second gas takes over and provides the light output. Although these self-heating lamps provide some improvement, the light output at low temperatures is still below the desired range.
Further, it is often desired to control the brightness of the backlight systems Even at low lamp temperatures, it may desirable to have a controllable level of brightness. Dimming of conventional backlight systems is accomplished by adjusting the amplitude of the current. However, the precision of dimming available with amplitude control is limited. Using amplitude controlled dimming, most CCFL tubes are limited to a 3:1 dimming range. Further, decreasing the current amplitude at low temperature may not be feasible.
Because CCFL lamps are installed in a variety of locations, the type of input signal received by a backlight system may vary. The input signals, which may control a variety of functions of the lamp, including but not limited to power on and off, brightness control, contrast control, or the like, may be a digital control signal or a DC voltage. Previously, separate input circuits were necessary depending on the type of input signal to be used.
A power conversion circuit is needed which permits an increased brightness level at low temperatures. Further, the power conversion circuit should be capable of accepting either digital or analog inputs.
SUMMARY OF THE INVENTION
The present invention provides increased light output at low temperatures and also provides a full range of dimming. Both current amplitude control and current duty cycle control are used to more precisely adjust the lamp light output. During low temperatures, the lamp is overdriven using a high amplitude current source. The increased current provides increased light output at low temperatures. When the lamp temperature increases, the amount of current flowing to the lamp is reduced to prevent damage to the lamp. The lamp may be dimmed throughout the entire temperature range by adjusting the duty cycle of the current source. By dimming using the duty cycle, the light output of the lamp may be more precisely controlled. The amplitude and duty cycle may be controlled using either an analog control signal or a digital control signal.
One embodiment of the present invention is a dimmable backlight system. The backlight system comprises a lamp and at least one integrator for converting a control signal into a DC voltage. A controller receives the DC voltage and adjusts either the duty cycle or the amplitude of an output signal based on the DC voltage. A network converts the output signal into a substantially sinusoidal AC current to illuminate the lamp at a plurality of different brightness levels.
Another aspect of the present invention is a method of illuminating a backlight lamp. The method comprises the steps of supplying a current signal to the lamp at a first current level and detecting the temperature of the lamp. It is then determined whether the temperature exceeds a predetermined level. The current level of the current signal is reduced when the signal exceeds the predetermined level.
Another aspect of the present invention is a method of dimming a backlight lamp. The method comprises the steps of receiving a first control signal indicating the desired current duty cycle and receiving a second control signal indicating the desired current amplitude. An AC current having a defined amplitude and duty cycle is then generated.
Another aspect of the present invention is a backlight system. The system comprises a lamp and a current source which provides a drive current to the lamp. A temperature detector determines the temperature of the lamp. A controller then adjusts the amplitude of the current source based on the temperature of the lamp.
Another aspect of the present invention is an integrator for converting an input signal of either a digital pulse train or an analog waveform into a DC voltage. The integrator comprises a first voltage amplifier which receives the input signal and clamps the input signal at a predetermined level. The first voltage amplifier amplifies the input signal to generate an output signal. A second amplifier receives and integrates the output signal to create a DC voltage.
REFERENCES:
patent: 4053813 (1977-10-01), Kornrumpf et al.
patent: 4307441 (1981-12-01), Bello
patent: 4453522 (1984-06-01), Salzgeber
patent: 4480201 (1984-10-01), Jaeschke
patent: 4555673 (1985-11-01), Huijsing et al.
patent: 4585974 (1986-04-01), Stupp et al.
patent: 4626770 (1986-12-01), Price, Jr.
patent: 4682080 (1987-07-01), Ogawa et al.
patent: 4779037 (1988-10-01), LoCascio
patent: 4792747 (1988-12-01), Schroeder
patent: 4812781 (1989-03-01), Regnier
patent: 4885486 (1989-12-01), Shekhawat et al.
patent: 5083065 (1992-01-01), Sakata et al.
patent: 5130635 (1992-07-01), Kase
patent: 5220272 (1993-06-01), Nelson
patent: 5235254 (1993-08-01), Ho
patent: 5289051 (1994-02-01), Zitta
patent: 5327028 (1994-07-01), Yum et al.
patent: 5471130 (1995-11-01), Agiman
patent: 5475285 (1995-12-01), Konopka
patent: 5479337 (1995-12-01), Voigt
patent: 5514947 (1996-05-01), Berg
patent: 5528192 (1996-06-01), Agiman
patent: 5552697 (1996-09-01), Chan
patent: 5563501 (1996-10-01), Chan
patent: 5608312 (1997-03-01), Wallace
patent: 5612594 (1997-03-01), Maheshwari
patent: 5615093 (1997-03-01), Nalbant
patent: 5619402 (1997-04-01), Liu
patent: 5719474 (1998-02-01), Vitello
patent: 5748460 (1998-05-01), Ishikawa
patent: 5818172 (1998-10-01), Lee
patent: 5854617 (1998-12-01), Lee et al.
patent: 5886477 (1999-03-01), Honbo et al.
patent: 5907742 (1999-05-01), Johnson et al.
Knobbe Martens Olson & Bear LLP
Linfinity Microelectronics
Tran Chuc D
Wong Don
LandOfFree
Dimmable backlight system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dimmable backlight system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dimmable backlight system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2492227