GB2211636A - Controlling the brightness of a fluorescent lamp - Google Patents

Abstract

An apparatus for regulating the brightness of a fluorescent lamp by supplying a variable frequency, variable pulse width and variable level current consists of a current transformer (500) supplying the discharge current to the lamp (300), a transformer driver (600) which provides an alternating primary path for transformer primary current, and a pulse controlled current supply (800) supplying a pulsed primary current for the transformer (500). Frequency and pulse width characteristics are controlled by a controller (700) which receives a feedback signal from an optical feedback circuit (900) in response to the intensity of the light from the lamp (300). <IMAGE>

Description

METHOD AND APPARATUS FOR CONTROLLING THE BRIGHTNESS OF A FLUORESCENT LIGHTING SYSTEM BACKGROUND OF THE INVENTION This invention generally relates to flat panel displays and more particularly relates to displays using fluorescent lamps as a back light and even w > re particularly is concerned with dimmable fluorescent back lights for flat panel displays.

With the recent upsurge in the use of LCD displays in avionics equipment, it is becoming quite desirable to provide a back light which produces a relatively constant light color while concomitantly being capable of brightness control or dimming over an extended range.

Incandescent lamps typically exhibit relatively easy brightness control over a wide range, however the output light color is often not constant over the dimming range. Conversely, fluorescent lamps have been known to typically provide a relatively constant output light color with varying levels of brightness, but achieving a wide brightness range has been difficult to implement.

One method that has been used in the past is disclosed in U.S.

Patent No. 4,277,728 to Carlile Stevens which is entitled "Power Supply for a High Intensity Discharge or Fluorescent Lamp", which patent is incorporated herein by this reference. The Stevens lamp utilizes a switch regulator and a voltage inverter to control the voltage supplied to the lamp. Furthermore, this design utilizes a resonant network interposed between the voltage inverter and the lamp.

While this design has been used in the past it has numerous serious drawbacks. The lamp voltage manipulation approach of this design involves the use of a switching regulator, voltage inverter, and a resonant network which can be difficult to implement and may be of large size and excessive weight. Furthermore, the dimming ratio is often insufficient for certain avionics applications.

Consequently, a need exists for improvement in flat panel display back lights which result in the ability to provide a constant light color output over an extended brightness range in a relatively easily implemented design.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a backlight for a flat panel display which provides a relatively easily implementable light which provides an extended brightness range in a relatively constant color output.

It is a feature of the present invention to utilize a frequency variable current to drive a fluorescent lamp.

It is an advantage of the present invention to allow for brightness control of a fluorescent lamp over an extended range.

It is another object of the present invention to provide a further extension in the dimming control of a fluorescent lamp.

It is another feature of the present invention to provide a self regulating current level supplied to the fluorescent lamp.

It is another advantage of the present invention to achieve a further increase in the dimming control range.

The present invention provides an extended dimming range constant color output flat panel display which is designed to satisfy the aforementioned needs, fulfill the earlier propounded objects, contain the above described features and produce the previously stated advantages. The invention is carried out in a "direct manipulation" and a "non-resonant" design in the sense that no resonant networks are present. Instead, the current is controlled directly without a resonant network.

Accordingly, the present invention relates to an apparatus for regulating the brightness of a fluorescent lamp over a wide range which includes a pulse controlled current supply and a frequency and pulse width current controller.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be more fully understood by reading the following description of the preferred embodiments of the invention in conjunction with the appended drawings wherein: Fig. 1 is a schematic -representation of a dimming circuit of the prior art, which utilizes a resonant network disposed between the current inverter and the lamp.

Fig. 2 is a schematic representation, in block form, of a preferred embodiment of the present invention.

Fig. 3 is a more detailed schematic representation of the dimming control apparatus together with the fluorescent lamp of the present invention.

Fig. 4 is a schematic representation of an electronic circuit which may be utilized in the present invention for chip 702.

DETAILED DESCRIPTION Now referring to the drawings, and more particularly to Figure 1 there is shown a typical dimming control apparatus of the prior art generally designated 100. Apparatus 100 has a fluorescent lamp 110 and an inverter 130 with a resonant network 120 disposed between the inverter 130 and the lamp 110.

Now referring to Figure 2 there is shown a block diagram of the present invention generally designated 200 having a lamp 300, filament supply 400, a current transformer 500, transformer driver 600, frequency and pulse width current controller 700, pulse controlled current supply 800 and an optical feedback controller 900.

In operation, the pulse controlled current supply 800 provides a DC current with its current level, pulse width and pulse height determined by the frequency and pulse -width current controller 700 and the inductor 802. At high frequencies the supply 800 has a self current level adjuster. The current supplied by supply 800 is converted to an AC current by current transformer 500 which is driven by transformer driver 600, which essentially performs a divide by two function so that the alternating paths in the current transformer each is activated 50% of the time. An optical feedback controller is provided to directly monitor the light output of the lamp and maintain a constant brightness setting over variable temperature extremes.

Now referring to Figure 3 there is shown a more detailed schematic representation of the preferred embodiment of the present invention providing a fluorescent lamp 300 having a fluorescent tube 302 with filaments 304 and 306.

Pulse controlled current supply 800 provides a pulsed DC current through inductor 802 and DC current line 804 to current transformer 500. Current supply 800 is controlled by a frequency and pulse width controlled pulse applied to field effect transistor 806 having its gate 808 connected to frequency and pulse width current controller 700 through capacitor 810. A voltage reference is applied at input 812 and along line 814 which is connected to the source 816 of field effect transistor 806. Source 816 is separated from gate 808 by zener 818 and resistor 820 in parallel. Drain 822 of field effect transistor 806 is separated from reference voltage 824 by diode 826. Drain 822 being connected directly with inductor 802.Current supply line 804, supplying the direct current to the current transformer 500 is converted to an alternating current by transformer driver 600 which provides alternating current paths through the current transformer 500 through line 502 and 504 which are connected with field effect transistors 506 and 508 respectively. Line 502 is connected source 510 of transistor 506 while line 504 is connected with source 512 of transistor 508.

Drain 514 of transistor 506 is connected to reference voltage 516 while drain 518 of transistor 508 is connected directly to reference voltage 520. Gate 522 of field effect transistor 506 is connected with emitter 524 of transistor 526 and collector 528 of transistor 530. Collector 532 of transistor 526 is connected to a reference voltage 534, while emitter 536 of transistor 530 is connected directly to reference voltage 538 while gates 540 and 542 of transistors 530 and 526 respectively are interconnected to line 544 and thereby connecting with pin 1 of the clocking chip 546, which can be a dual D flip flop.Gate 548 of field effect transistor 508 is connected with pin 2 of clock chip 546 through a transistor network when the emitter 550 of transistor 552 and the collector 554 of transistor 556 are connected with gate 548 of transistor 508 while emitter 558 of transistor 556 is connected reference voltage 560 and collector 562 of transistor 552 is connected to reference voltage 564 while gate 566 and gate 568 of transistors 552 and 556 respectively are connected to pin 2 of clock chip 546. Chip 546 has its pin to interconnected with pin 5 while pins 4, 6, and 7 are connected with reference voltage 570. Pin 14 is connected with reference voltage 572 while pin 3 is connected to Line 808.

Frequency and pulse width current controller 700 is connected with pulse controlled current supply 800 by line 808. The function of optical feedback controller 900 and frequency and pulse width current controller 700 is to control the frequency and pulse width of the pulse supplied to the field effect transistor 806 and pulse controlled current supply 800. Numerous different and configurations may be utilized to perform this combined function and the following being merely a preferred or exemplary embodiment thereof. Controller 700 includes a chip 702, which is a timer for monitoring the voltage on two of its pins and developing a pulsed output in response thereto. A LM555 timer available from National Semiconductor, shown in Figure 4, may be used to provide this capability.Chip 702 has its pin number 3 connected to line 704 which connects to bases 706 and 708 of transistors 710 and 712, respectively. Collector 714 of transistor 710 being connected with a reference voltage 716 while its emitter 718 is connected to line 808. Collector 720 of transistor 712 is connected with line 808 while emitter 722 of transistor 712 is connected to reference voltage 724. Pin number 1 of chip 702 is connected to reference voltage 726 while pin number 5 is connected through capacitor 728 to reference voltage 730. Pin number 2 is connected through capacitor 732 to reference voltage 734 and further separated from collector 736 of transistor 738 by resistor 740. Pin 6 is connected with pin 2. Resistor 740 and 742 in parallel separate pin 7and pin 2.

Resistor 742 is preferably a thermister. The purpose of the thermister is to vary the width of the pulse due to ambient temperature. The colder the ambient the wider the pulse width, hence more current to the lamp. Pin 8 is connected with reference voltage 744 and separated from pin 4 by resistor 746. Pin 4 connects through capacitor 748 to reference voltage 750. Base 752 of transistor 738 is connected through resistor 754 to reference voltage 756. Base 752 is separated from collector 758 of transistor 738 by zener 760 diode 762 node 764 and resistor 766 in series.

Node 764 connecting with reference voltage 768 and through capacitor 770 to reference voltage 772. Collector 758 of transistor 738 connecting through resistor 774 to line 776. Line 776 extends toward optical feedback controller 900.

Optical feedback controller 900 includes a first integrater Op Amp 902 having its pin 1 connected with line 776 and separated from its pin 2 by capacitor 904. Pin 2 further connected through resistors 906 and 908 in series to potentiometer 910. Pin 4 connecting to reference voltage 912 and through capacitor 914 to reference voltage 916. Pin 11 connecting to reference voltage 918 while pin 3 connects through resistor 920 to reference voltage 922.

Pin 3 further connecting through capacitor 924 to reference voltage 926. Pin 3 connecting through resistor 928 to reference voltage 930. Pin 2 of Op Amp 902 connects with node 932. Node 932 connects through diode 934 to collector 936 of transistor 938 which has its emitter 940 coupled with reference voltage 942. Collector 936 separated from reference voltage 944 by resistor 946. Base 948 of transistor 938 is connected through resistor 950 to reference voltage 952. Base 948 is coupled with resistor 954 node 956 and capacitor 958 to reference voltage 960. Node 956 is separated from reference voltage 962 by diode 964 and resistor 966 in parallel.

Node 932 is coupled through resistor 968 to pin 6 of current to voltage converter Op Amp 970. Pin 4 is coupled to reference voltage 972 while pin 7 is coupled with reference voltage 974 and further coupled through capacitor 976 to reference voltage 978. Pin 3 is coupled through resistor 980 to reference voltage 982. Pin 3 is also coupled through resistor 984 to reference voltage 986. Pin 3 is further coupled to reference voltage 988 through capacitor 990.

Pin 2 is coupled through capacitor 992 and photo diode 994 in parallel, to pin 3. Pin 2 is further coupled through resistor 996 and capacitor 998, in parallel, with pin 6.

In operation, the brightness of the fluorescent lamp is regulated by regulating the rms current supplied to the lamp filaments. A pulsed DC current with variable current level is supplied along DC current line 804 to current transformer 500. The current level and the frequency of the current pulses on line 804 is a function of the gate 808 voltage of field effect transistor 806.

The transformer driver 600 essentially performs a divide by two function to regulate the alternate current paths through the current transformer 500 so that each of the alternate current paths is operative 50% of the time.

The voltage on gate 808 of transistor 806 is coupled with pin 3 of chip 702 which chip essentially converts the variable constant current out of emitter 736 of transistor 738 into a frequency variable voltage pulse. The variable frequency of the voltage pulse on pin 3 is a function of the current level in emitter 736.

Optical feedback controller 900 with potentiometer 910 set the level of the variable constant current through emitter 736.

With the optical feedback controller 900 in place it essentially monitors the light output from the lamp by generating a current through photo diode 994 which is converted to a voltage by Op Amp 970 and its associated circuitry which essentially is a current to voltage converter. The voltage on pin 6, which is a function of the lamp brightness creates a current through resistor 968 and is compared with the current from potentiometer 910 through resistors 908 and 906 by the Op Amp 902 which essentially performs an integrater function or a current comparitor function. The frequency of the voltage pulse supplied to gate 808 of transistor 806 is therefore a function of the variable constant current out of emitter 736 which is regulated by the optical feedback controller which essentially compares the photo diode current with the selected current through resistors 908 and 906.The pulse width of the voltage pulse to gate 808 of transistor'806 is a function of the discharge path of capacitor 732 through resistor 7X0 to pin 7 of chip 702. It is contemplated that a variable pulse width circuit could be easily implemented.

It is thought that the method and apparatus for controlling the brightness of a fluorescent lighting system of the present invention and many of its intended advantages will be understood from the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the parts thereof without departing from the spirit and scope of the invention, or sacrificing all of their material advantages, the forms hereinbefore described being merely preferred or exemplary embodiments thereof. It is the intention of the appended claims to cover all such changes.

Reference should also be made to our copending patent application no. t8 294oS7 entitled "Fluorescent lamp dimmer" filed on the same day claiming priority from the United States patent application serial no 113047 of Wesley G. Runyan.

Claims (9)

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1. A method for regulating the brightness of a fluorescent lamp comprising the steps of: a. supplying a variable frequency current through a fluorescent lamp.

2. A method of Claim 1 wherein supplying a variable frequency current through a fluorescent lamp further comprises: a. supplying a frequency variable pulsed direct current; and b. alternating the direct current through a current transformer.

3. A method of Claim 2 further comprising: a. monitoring the lamp brightness; b. comparing the lamp brightness to a predetermined brightness characteristic; c. regulating the pulse frequency in response to the lamp brightness and the predetermined brightness characteristic.

4. A fluorescent lamp dimming apparatus comprising: a. means for supplying a frequency variable current to a fluorescent lamp.

5. An apparatus of Claim 4 further comprising a. means for supplying a frequency variable pulsed DC current; and b. means for alternating the DC current through a current transformer.

6. An apparatus of Claim 5 further comprising: a. means for monitoring the lamp brightness; b. means for comparing the lamp brightness to a predetermined brightness characteristic; and c. means for regulating the pulsed DC current in response to the lamp brightness and the predetermined brightness characteristic.

7. A flat panel avionics liquid crystal display back lamp comprising: a. a fluorescent lamp; b. a current source operatively associated with the lamp, for supplying a DC current; c. means for pulsing the DC current at a variable frequency; and d. means for pulsing the DC current with a variable pulse width.

8. A back lamp of Claim 7 further comprising: a. means for alternating the DC current through a current transformer.

9. A back lamp of Claim 8 further comprising: a. means for generating an optical feedback current in response to the lamp brightness; b. means for comparing the optical feedback current to a predetermined current corresponding to a predetermined lamp brightness; and c. means for regulating the pulse width and frequency of the DC current in response to the optical feedback current and the predetermined current.