Abstract:
The present invention relates to an external electrode fluorescent lamp which can provide higher degree of and more improved uniformity of brightness than those of conventional ones, and to a LCD back light unit using the external electrode fluorescent lamp. The present invention also relates to an equipment and a driving device to adapt the LCD back light unit. The external electrode fluorescent lamp of the present invention is comprising: an upper panel of lamp which is serpentine shaped; a lower panel which is planar shaped and to be combined with the upper panel to make channels between the upper panel and the low panel; and external electrodes which are located at the two extreme sides at the surface of the upper panel.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to an external electrode fluorescent lamp, a liquid crystal display backlight unit using the same, and a device for driving the external electrode fluorescent lamp, and more particularly to a liquid crystal display backlight unit using an external electrode fluorescent lamp, which can easily produce a surface light source with higher brightness and brightness uniformity than a conventional edge-type or a direct-type backlight unit, reduce a calorific value of a liquid crystal display panel due to electrodes of a fluorescent lamp, prevent breakdown of a fluorescent lamp caused by breakdown of electrodes, and extend the life of a fluorescent lamp, and further particularly, to an external electrode fluorescent lamp, liquid crystal display backlight unit using the same and device for driving an external electrode surface emission fluorescent lamp, which can simplify the manufacturing process and improve the productivity thereof, and which can be easily applied to a large-scale backlight unit. 
   2. Description of Related Art 
   Generally, a liquid crystal display (LCD) used as display means for characters, graphics and moving pictures has been greatly highlighted as a next generation display device for mobile phones or televisions because it causes less fatigue of eyes than a conventional cathode ray tube (CRT) display device, and it can realize miniaturization, light weight, and low power consumption. 
   The construction of a conventional LCD panel in which characters or images are displayed on LCD is described in brief. First, if liquid crystal material is injected between a pair of surface-processed transparent glass plates, and an electrical signal (voltage) is supplied to the injected liquid crystal material using an LCD driving circuit (not shown) for generating a driving signal, phase variation of the liquid crystal material occurs by the electrical signal. The LCD driving circuit applies different voltages to the liquid crystal material to vary distribution of the liquid crystal material, thus enabling specific characters or images to be displayed. 
   However, since an LCD panel on which characters are displayed cannot emit light for itself, a means for assisting in visually recognizing contents (characters or logos) displayed on the panel is required. Currently, a backlight system using lamps which irradiate light from the sides or the back of a LCD panel is generally used as the assisting means. 
   Conventional backlight systems are mainly classified into edge-type backlight units and direct-type backlight units according to positions of fluorescent lamps projecting light. The edge-type backlight units employ a manner in which light sources are positioned beneath both sides of the panel, such that light inputted from the light sources forms a surface light source by a light guide plate and a reflective sheet and the surface light source illuminates cells of the LCD panel. Such an edge-type backlight unit is advantageous in that, since it indirectly guides light radiated from the light sources, brightness uniformity is high. However, it is problematic in that brightness decreases relative to the brightness uniformity. 
     FIG. 1  shows an embodiment of a conventional edge-type backlight unit. Referring to  FIG. 1 , a lamp cover for covering fluorescent lamps, the fluorescent lamps for radiating light by the supply of power, a reflective sheet for reflecting the radiated light in a predetermined direction, a light guide plate for guiding the radiated light, a diffusion sheet for uniformly radiating incident light to prisms, a vertical prism, a horizontal prism, and a protective sheet are layered in order from the bottom. In the above-described edge-type backlight unit, since the fluorescent lamps are positioned at the side of the light guide plate, brightness uniformity increases, while brightness decreases. 
   Further, the direct-type backlight units employ a manner in which light sources (cold cathode fluorescent lamps) are arranged beneath an LCD panel, a diffusion sheet is arranged on the front of the light sources, and a reflective sheet is arranged on the back of the light sources, such that light radiated from the light sources is reflected and diffused to be irradiated onto cells of the LCD panel. Since such a direct-type backlight unit effectively uses light using the reflective sheet and the diffusion sheet, it can obtain high brightness, so it is suitable for backlight units requiring high brightness. However, the direct-type backlight unit is problematic in that it cannot provide sufficient brightness according to the size of LCD panels which become large, and brightness uniformity is also decreased. 
   Moreover, the conventional direct-type backlight unit requires as many inverters as the number of fluorescent lamps used as light sources. That is, characteristics of respective fluorescent lamps used as the light sources are slightly different. Therefore, in the case where the fluorescent lamps are connected in parallel with each other, there occurs a problem that a plurality of fluorescent lamps are not simultaneously turned on due to the difference in discharge properties, if one inverter having a high power supplying capability is mainly used. That is, some of fluorescent lamps may be turned on, and the remaining fluorescent lamps may be turned on late or turned off. In order to solve the problem, inverters are respectively connected to fluorescent lamps to drive the fluorescent lamps. However, there are problems, such as high power consumption, cost increase due to the increased number of inverters, and productivity decrease due to the increased assembly time, degradation of LCD due to heat generated by electrodes, etc. 
   Further, a prior art, plate-type surface emission fluorescent lamp applied by the present applicant, improves brightness uniformity and brightness of conventional light sources (fluorescent lamps) for backlighting.  FIG. 12  is a plan view of a previously applied surface emission fluorescent lamp. As shown in  FIG. 12 , an upper sheet of the lamp is constructed such that serpentine-shaped channels into which discharge gas is injected and which are isolated from the outside are arranged to be adjacent to each other. Further, bent portions are mutually connected, such that a single channel is formed in the upper sheet. At both ends of the single channel, internal electrodes  201  are installed. 
     FIG. 13  is a sectional view by A—A line of the surface emission fluorescent lamp  203  of  FIG. 12 , and shows sections of channels  203   a  formed adjacent to each other. Actually, the channels  203   a  shown to be separated respectively are mutually connected to each other to form a single path. In  FIG. 13 , although the section “A” is depicted by a semicircle, the shape of the channels  203  can be varied to a rectangle, a diamond, etc. 
   Further,  FIG. 14  is a sectional view by B—B line of  FIG. 12 , and shows the installation of the internal electrodes  201  of the surface emission fluorescent lamp  203 . As shown in  FIG. 14 , one end of each of the internal electrodes  201  is inserted into the surface emission fluorescent lamp  203 . Therefore, many manufacturing processes are added to insert and fix the internal electrodes  201 . 
   As described above, the previously applied surface emission fluorescent lamp  203  uniformly radiates light over the entire surface area, thus supplementing the disadvantages of the conventional edge-type and direct-type backlight units to provide high brightness and high brightness uniformity. Especially, since the surface emission fluorescent lamp  203  has serpentine-shaped channels, the brightness and the brightness uniformity are remarkably improved. Further, the shape of the surface emission fluorescent lamp  203  can be changed to “L”, “W”, etc. In this case, the upper sheet of the lamp is typically formed in the shape of “L” or “W”, and the lower sheet thereof is formed in the shape of a plate, such that the upper and lower sheets are manufactured to be combined with each other, or to be integrated. 
   However, as the construction of the fluorescent lamp is varied as described above, there are inconveniences in that installation positions of the internal electrodes  201  for supplying power to the fluorescent lamp are frequently varied, so manufacturing equipment must be changed. Moreover, the internal electrodes  201  are fixedly inserted into the surface emission fluorescent lamp  203 , thus causing several problems, such as increase of the manufacturing process of the fluorescent lamp and the deterioration of productivity due to breakdown of internal electrodes, etc. Further, the surface emission fluorescent lamp is problematic in that, if a plurality of fluorescent lamps are connected in parallel to apply the fluorescent lamps to a large-scale backlight unit, wiring is complicated to connect inverters to respective electrodes, so the volume of the backlight unit increases. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art. The present invention is an LCD backlight unit using an external electrode fluorescent lamp, which can easily produce surface light sources, reduce a calorific value of an LCD panel caused by electrodes, prevent breakdown of the external electrode fluorescent lamp due to the breakdown of electrodes, and extend the life of the external electrode fluorescent lamp. 
   The present invention is also an LCD backlight unit using an external electrode fluorescent lamp, which can simplify the manufacture of a backlight unit, and improve driving characteristics of a fluorescent lamp by changing a power supplying manner for a fluorescent lamp used in a conventional direct-type backlight unit. 
   The present invention is also an LCD backlight unit using an external electrode fluorescent lamp, in which a backlight unit is produced as a module, thus greatly reducing time required for assembly, and consequently improving productivity. 
   The present invention is also an external electrode surface emission fluorescent lamp for LCDs and backlight unit using the same, in which electrodes of a surface emission fluorescent lamp used for LCD backlighting are constructed as external electrodes, thus simplifying the manufacturing process of the surface emission fluorescent lamp, improving productivity, enabling a large-scale backlight unit to be easily produced, and enabling produced LCD panels to be miniaturized and thinned. 
   The present invention is also an external electrode surface emission fluorescent lamp for LCDs and backlight unit using the same, which has a getter housing for supplying mercury to the inside of channels of the surface emission fluorescent lamp and absorbing several impurities. 
   The present invention is also a device for driving a surface emission fluorescent lamp for LCDs, which applies an initial lighting voltage for generating charged particles to a surface emission fluorescent lamp for LCDs having main electrodes and auxiliary electrodes, and applies a lighting maintaining voltage to the surface emission fluorescent lamp before generated charged particles disappear, thus maintaining a lit condition of the surface emission fluorescent lamp. 
   The present invention is also a device for driving a surface emission fluorescent lamp for LCDs, which can operate stably even at a low voltage and reduce continuous stress of a transformer and losses of switching devices due to a high voltage during initial lighting, and which controls a surface emission fluorescent lamp having main electrodes and auxiliary electrodes. 
   More specifically, the present invention is an LCD backlight unit for radiating light used to read characters displayed on an LCD panel from below, comprising an inverter for generating first and second voltages using a direct current (DC) voltage and supplying the first and second voltages through first and second output lines, respectively; a plurality of external electrode fluorescent lamps each having a first end electrode connected to one of the first and second output lines for supplying the first and second voltages received from the inverter, and a second end electrode arranged opposite to the first end electrode and connected to the ground, the external electrode fluorescent lamps being sequentially arranged on the same plane; and a base for accommodating the external electrode fluorescent lamps by allowing the sequentially arranged external electrode fluorescent lamps to be fixed. 
   Further, the present invention is an external electrode surface emission fluorescent lamp for LCD backlighting, comprising a serpentine-shaped upper sheet having a section for maximizing brightness uniformity within a predetermined distance from its surface; a plate-shaped lower sheet combined with the upper sheet to form mutually connected channels; and main electrodes and auxiliary electrodes installed on surfaces of both ends of the upper sheet. 
   Further, the present invention is an external electrode surface emission fluorescent lamp having the construction in which channels mutually adjacent to each other are isolated to prevent discharge gas from passing through the channels, and a plurality of gas paths, whose thickness, installation position and number are variable, are installed between adjacent channels. 
   Further, the present invention is a device for driving an external electrode surface emission fluorescent lamp for LCDs having main electrodes and auxiliary electrodes, the driving device supplying power to the external electrode surface emission fluorescent lamp, comprising a main controller for lighting the surface emission fluorescent lamp in response to a control signal from an LCD controller, a priming circuit for supplying power to the auxiliary electrodes for a predetermined period of time so as to generate initial charged particles in the external electrode surface emission fluorescent lamp in response to the control signal from the main controller; and a lighting maintaining circuit for supplying power to the main electrodes so as to apply a lighting maintaining voltage using the charged particles generated by the priming circuit. 
   Lastly, the present invention is a driving device further comprising a feedback circuit for detecting a current flowing into the surface emission fluorescent lamp, converting the current into a voltage and outputting the voltage to the main controller, thus enabling an input voltage for brightness control to be controlled through the lighting maintaining circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a conventional edge-type backlight unit; 
       FIG. 2  is a view showing the wiring of a backlight unit according to the present invention; 
       FIG. 3  is a perspective view of a base unit according to the present invention; 
       FIGS. 4 to 6  are side views of a fluorescent lamp into which electrodes and rubber holders are inserted; 
       FIGS. 7 and 8  are views showing the connection of output lines for supplying power to the fluorescent lamp; 
       FIG. 9  is a block diagram of an inverter for supplying power to the fluorescent lamp; 
       FIG. 10  is a detailed circuit diagram of the inverter of  FIG. 9 ; 
       FIG. 11  is a waveform diagram showing driving pulses used by the inverter; 
       FIG. 12  is a plan view of a conventional surface emission fluorescent lamp; 
       FIG. 13  is a sectional view by A—A line of  FIG. 12 ; 
       FIG. 14  is a sectional view by B—B line of  FIG. 12 ; 
       FIG. 15  is a plan view of an external electrode surface emission fluorescent lamp according to a preferred embodiment of the present invention; 
       FIG. 16  is a plan view of another external electrode surface emission fluorescent lamp according to another preferred embodiment of the present invention; 
       FIG. 17  is a view showing the installation of auxiliary electrodes of the external electrode fluorescent lamp of the present invention; 
       FIG. 18  is a view showing another installation of auxiliary electrodes of the external electrode fluorescent lamp of the present invention; 
       FIG. 19  is a sectional perspective view of a part “B” of  FIG. 15  for showing the installation of a getter; 
       FIG. 20  is a sectional perspective view showing another installation of the getter; 
       FIG. 21  is a side sectional view of a backlight unit according to the present invention; 
       FIG. 22  is a plan view of a conventional surface emission fluorescent lamp having external electrodes; 
       FIG. 23  is a block diagram of a device for driving a fluorescent lamp according to a preferred embodiment of the present invention; 
       FIG. 24  is a block diagram of another device for driving a fluorescent lamp according to another preferred embodiment of the present invention; 
       FIG. 25  is a detailed circuit diagram of the fluorescent lamp driving device of  FIG. 23 ; 
       FIG. 26  is a detailed circuit diagram of the fluorescent lamp driving device of  FIG. 24 ; and 
       FIG. 27  is a flowchart showing the operation of a device for driving a surface emission fluorescent lamp of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a view showing the wiring of a backlight unit according to the present invention. The backlight unit of the present invention comprises an inverter  90  and a base  100 . The inverter  90 , which is used to supply lighting voltages to a plurality of external electrode fluorescent lamps,  110 , generates first and second voltages and supplies the first and second voltages to the external electrode fluorescent lamps  110  through first and second output lines  120  and  122 . The base  100  includes the plurality of fluorescent lamps  110  lit by receiving the first and second voltages from the inverter  90 . 
   The fluorescent lamps  10  are external electrode fluorescent lamps having external electrodes, not internal electrodes formed at both ends of a fluorescent lamp like a conventional fluorescent lamp. Even though the external electrodes are not indicated by a specific reference numeral in  FIG. 2 , they are formed to be protruded from both ends of each of the fluorescent lamps  110 , wherein, a contact point  118  is formed on any one of the both-end external electrodes of each of the fluorescent lamps  110 . 
   The external electrode fluorescent lamps  110  are divided into two parts by odd and even orders, such that they are supplied with lighting voltages through the first output line  120  or the second output line  122  connected to the inverter  90 . Referring to  FIG. 2 , the voltages supplied to the fluorescent lamps  110  are supplied through the contact points  118  connected to the first or second output line  120  or  122  every other lamp. The contact points  118  are electrically connected to the electrodes formed on end portions of the fluorescent lamps  110  by soldering, which will be described later. Therefore, the fluorescent lamps  110  on which the contact points  118  coming into contact with the output lines are formed are supplied with power through corresponding output lines. 
   In this way, stable supply of power can be achieved by supplying voltages using the first and second output lines. An electrode  112  of the other end of each of the fluorescent lamps  110 , which is not supplied with power, is connected to the ground. Connection manners for the first and second output lines  120  and  122 , which can be considered as another characteristics of the present invention, will be described later. 
   Referring to  FIG. 3 , the base  100  for accommodating the external electrode fluorescent lamps  110  is formed in the shape of an open rectangular box in which a pair of isolation plates  102  longitudinally extended and arranged opposite to each other, and a pair of lamp fixing plates  104  having both ends connected to the isolation plates  102  and being opposite to each other, are constructed to form four sides of a rectangle, and a reflective sheet  106  is further installed on the bottom. 
   Even though not shown in  FIG. 3 , after the external electrode fluorescent lamps  110  are horizontally arranged, a diffusion sheet is arranged over the fluorescent lamps  110 , thus enabling the base  100  to be produced in the form of a full rectangular box. Further, terminals for connecting the first and second output lines  120  and  122  are formed on the side of the base  100 . In this way, if the base  100  is produced as a box-shaped module, the assembly of the backlight unit can be promptly accomplished. 
   On the pair of lamp fixing plates  104 , a plurality of lamp fixing holes  108  are oppositely formed at regular intervals. The lamp fixing holes  108  are used to fix the external electrode fluorescent lamps  110  in a line. The lamp fixing holes  108  allow the external electrode fluorescent lamps  110  to be fixed thereto using rubber holders. 
   The reflective sheet  106  reflects light radiated from the external electrode fluorescent lamps  110  arranged thereon. A reflective sheet with a coating-processed surface and with high reflection efficiency can be used as the reflective sheet  106 . 
     FIGS. 4 and 5  show the electrodes  112  formed on circumferences of both ends of the external electrode fluorescent lamp  110 . The electrodes  112  serve to supply a high voltage to the fluorescent lamp  110  so as to sufficiently excite the fluorescent lamp  110  when high voltage pulses are applied to the fluorescent lamp  110  from the outside. The electrodes  112  are produced using materials with excellent conductivity, for example, copper, aluminum tape, etc. 
   Further, the electrodes can be formed such that they fully enclose the circumferences of both ends of the external electrode fluorescent lamp  110 , as shown in the electrodes  112  of  FIG. 4 , or they partially enclose upper portions or lower portions of circumferences of both ends thereof, as shown in electrodes  114  of  FIG. 5 . The advantage of the former is in that, since the areas of the electrodes  112  are wide, sufficient exciting power can be provided to sufficiently light the external electrode fluorescent lamp  110  when high voltage pulses are applied. On the other hand, the latter has an advantage in that, since electrodes  114  partially cover the surface of the external electrode fluorescent lamp  110 , a large amount of light is radiated, thus improving light radiating efficiency of fluorescent lamps. 
   In left sides of  FIGS. 4 and 5 , side views of the external, electrode fluorescent lamp  110  are simply depicted. Referring to  FIG. 4 , it can be seen that the electrodes  112  are formed to fully enclose circumferences of both ends of the external electrode fluorescent lamp  110 . Further, referring to  FIG. 5 , it can be seen that the electrodes  114  are partially formed on only lower circumferences (or upper circumferences) of both ends of the external electrode fluorescent lamp  110 . Applications of the electrodes  112  and  114  will be described later. 
     FIG. 6  shows the coupling of rubber holders  116  for allowing the external electrode fluorescent lamps  110  to be fixed to the lamp fixing holes  108  formed in the base  100 . The rubber holders  116  are interlocked with both ends of the external electrode fluorescent lamp  110 , and are each comprised of two parts having different diameters. In a first part having a smaller diameter, a hole for accommodating an end of the external electrode fluorescent lamp  110  is formed. A second part having a larger diameter is adhered closely and fixed to the border surface of each of the lamp fixing holes  108 . A left side of  FIG. 6  shows a side view of the rubber holder  116 . As described above, the present invention is advantageous in that it employs the external electrode fluorescent lamps  110 , thus enabling the fluorescent lamps to be easily installed. 
     FIGS. 7 and 8  are views respectively showing the connections of the first and second output lines  120  and  122  for supplying, power to the external electrode fluorescent lamps  110 . 
     FIG. 7  shows the positions of the contact points  118  and the wiring (of first and second output lines) if each of the electrodes  112  circularly encloses the circumference of the external electrode fluorescent lamp  110 . Both the first and second output lines  120  and  122  are coated with covering material, so they are insulated. Therefore, only parts coming into contact with the contact points  118  are exposed to transmit electricity. 
   The first output line  120  passes over an external electrode fluorescent lamp  110   a  which is located first from the left side, and is connected with the electrode  112  through the contact point  118 . After being connected with the electrode  112 , the first output line  120  passes under an external electrode fluorescent lamp  110   b , which is located second from the left side, without any electrical connection. Further, the first output line  120  is connected with the contact point  118  on an external electrode fluorescent lamp  110   c , which is located third from the left side. That is, it can be seen that the first output line  120  is electrically alternately connected to external electrode fluorescent lamps  110   a ,  110   c  and  110   e  which are positioned in odd order. The contact points  118  are electrically connected to both the first and second output lines  120  and  122  by soldering. 
   Contact points can be directly formed by soldering or can be formed in the shape of a ring on the surface of the electrode. That is, ring-shaped contact points are formed, the first output line (or the second output line) is inserted into the contact points, and the contact points are then electrically connected to the first or second output line by soldering, thus simplifying the assembly of the backlight unit. 
   On the other hand, the second output line  122  passes under the first external electrode fluorescent lamp  110   a  (in the opposite direction of the first output line  120 ), and is connected to the contact point  118  on the second external electrode fluorescent lamp  110   b . The second output line  122  connected to the contact point  118  passes under the third external electrode fluorescent lamp  110   c  and extended to the top of the next fluorescent lamp. That is, it can be seen that the second output line  122  is alternately connected to the external electrode fluorescent lamps  110   b ,  110   d  and  110   f , which are positioned in even order. Therefore, if power is supplied to both the first and second output lines  120  and  122 , the first output line  120  supplies power to the external electrode fluorescent lamps  110   a ,  110   c , and  110   e ), and the second output line  122  supplies power to the external electrode fluorescent lamps  110   b ,  110   d  and  110   f.    
   Further, the external electrode fluorescent lamps  110   a  to  110   f  are wired to the first and second output lines  120  and  122  while being enclosed by the first and second output lines  120  and  122 . Especially, it is preferable to adhere the first and second output lines  120  and  122  to the surfaces of the fluorescent lamps, through which the output lines  120  and  122  pass, as closely as possible. Due to this wiring construction, a sufficient electric field is generated when high voltage pulses are applied from the inverter  90 . Due to the sufficient electric field, lighting operations of external electrode fluorescent lamps can be indirectly improved. 
     FIG. 8  shows the wiring mariner fundamentally similar to that of  FIG. 7 , wherein the wiring manner of  FIG. 8  is different from that of  FIG. 7  in positions where contact points  118  of the first and second output lines  120  and  122  are formed. In this case, the contact points  118  are always formed on the lower surfaces of electrodes. In  FIG. 8 , even though the contact points  118  are different from those of  FIG. 7 , it is clear that the contact points  118  can be applied to any of circular electrodes or partially formed electrodes. 
   Further, the wiring of the first and second output lines  120  and  122  is achieved such that the output lines  120  and  122  are adhered closely to the external electrode fluorescent lamps to enclose them, as shown in  FIG. 8 . Therefore, a sufficient electric field is generated when high voltage pulses are applied from the inverter. Due to the sufficient electric field, lighting operations of external electrode fluorescent lamps can be indirectly improved. 
   Further, in the wiring of the output lines of the present invention, the positions of the contact points  118  are not limited by embodiments shown in the drawings. On the other hand, it is clear that the contact points are formed and soldered at convenient positions to connect the external electrode fluorescent lamps with the first and second output lines  120  and  122 . 
   A block diagram of the inverter  90  for supplying power to the plural external electrode fluorescent lamps  110  having the above construction is depicted in  FIG. 9 , and a detailed circuit diagram of the inverter  90  of  FIG. 9  is depicted in  FIG. 10 . The inverter  90  is described in detail with reference to  FIGS. 9 and 10 . 
   The inverter  90  comprises a line filter  10 , a pulse generator  20 , a pair of driving buffers  30  and  30 ′, a pair of resonance circuits  40  and  40 ′, a pair of step-up transformers  50  and  50 ′, and a feedback circuit  70 . The line filter  10  rectifies a direct current (DC) voltage supplied by a DC power unit, and outputs the rectified DC voltage. The pulse generator  20  generates switching driving pulses. The driving buffers  30  and  30 ′ selectively output driving pulses generated by the pulse generator  20  to the resonance circuits  40  and  40 ′. The resonance circuits  40  and  40 ′, which are driven by the driving pulses outputted from the driving buffers  30  and  30 ′, convert the DC voltage received from the line filter  10  into an alternating current (AC) voltage and output the AC voltage. The step-up transformers  50  and  50 ′ amplify each of the AC voltages outputted from the resonance circuits  40  and  40 ′, respectively, and output the amplified AC voltage to the external electrode fluorescent lamps  110  which are divided into two groups. The feedback circuit  70  detects a secondary voltage used to control brightness of the external electrode fluorescent lamps  110 . High voltages outputted from the transformers  50  and  50 ′ of the inverter  90  are synchronized and operated at the same phase and same frequency, thus enabling the external electrode fluorescent lamps  110  to be synchronously driven. By the synchronous driving of the two transformers, the fluorescent lamps  110  of the present invention can obtain more stable and improved operation characteristics compared with a case where a single transformer is used. 
   The operations of the inverter  90  are described in detail. 
   The line filter  10  receives DC power from the DC power unit (rectifying unit, battery or rechargeable battery), and rectifies the DC power to supply a stable current. Referring to  FIG. 10 , the line filter  10  consists of a coil L 11  and a condenser C 11 , and serves to rectify and output a DC voltage supplied by the DC power unit. The DC voltage outputted from the line filter  10  is inputted to a center tap of each of the step-up transformers  50  and  50 ′. However, the DC voltage is controlled by the resonance circuits  40  and  40 ′ to be modulated and outputted as an AC voltage. 
   The pulse generator  20 , which is used to generate switching driving pulses, transmit the driving pulses to the driving buffers  30  and  30 ′, respectively. Further, the pulse generator  20  controls voltage values supplied to the fluorescent lamps by varying the widths of the generated pulses in response to a signal received from the feedback circuit  70 . The pulse generator  20  supplies a DC voltage for performing a stable operation using a regulator. Referring to  FIG. 10  the pulse generator  20  outputs two pulse signals. 
   The driving buffers  30  and  30 ′ output the driving pulses generated by the pulse generator  20  to the resonance circuits  40  and  40 ′. As shown in  FIG. 10 , the driving buffers  30  and  30 ′ are comprised of NPN transistors Q 1  and Q 3  and PNP transistors Q 2  and Q 4 , and NPN transistors Q 5  and Q 7  and PNP transistors Q 6  and Q 8 , respectively. One pulse outputted from the pulse generator  20  is inputted to a base of each of the transistors Q 1  and Q 2  of the driving buffer  30 . The other pulse outputted from the pulse generator  20  is inputted to a base of each of the transistors Q 3  and Q 4 . These operations are applied to the driving buffer  30 ′ in the same manner as the driving buffer  30 . Therefore, driving points are varied according to waveform of pulses outputted from the pulse generator  20 . 
   The resonance circuits  40  and  40 ′ each convert the DC voltage received from the line filter  10  into a voltage signal of a predetermined frequency and output the voltage signal, in response to the switching pulse signals outputted from the driving buffers  30  and  30 ′. By the switching driving pulses of the pulse generator  20  and operations of the resonance circuits  40  and  40 ′, AC voltage pulses of predetermined frequency are generated. 
   The outputted AC voltages are inputted to the step-up transformers  50  and  50 ′, respectively, and sufficiently boosted. The boosted voltages are supplied to the external electrode fluorescent lamps  110 . The voltages supplied to the external electrode fluorescent lamps  110  are high voltage signals having the, same frequencies and the same phases, as described above, and are used to drive the external electrode fluorescent lamps  110 . The voltages outputted from the step-up transformers  50  and  50 ′ are respectively supplied to the external electrode fluorescent lamps  110  through the first and second output lines  120  and  122  shown in  FIG. 2 . Since the output voltages have the same phases and frequencies, there is no difficulty in operating the fluorescent lamps  110  even though any one of the output lines  120  and  122  is connected to each of the two voltages. 
   The feedback circuit  70  detects a current flowing into the external electrode fluorescent lamps  110  in a secondary side as a voltage using a resistor R 71 , and outputs a control signal so as to vary brightness of the external electrode fluorescent lamps  110  on the basis of the voltage. The widths of the driving pulses generated by the pulse generator  20  are varied due to the signal received from the feedback circuit  70 , thus optimizing the brightness of the external electrode fluorescent lamps  110 . 
     FIG. 11  is a waveform diagram showing the driving pulse signals of the inverter  90  of the present invention, and shows the pulse signals ch 1  and ch 2  generated by the pulse generator  20 . As shown in  FIG. 11 , the driving pulse signals inputted to the driving buffers  30  and  30 ′ have the same frequencies in different phases. The resonance circuits  40  and  40 ′ are operated by the driving pulse signals to generate AC voltages. 
   An external electrode surface emission fluorescent lamp capable of substituting for the above-described bar-shaped external electrode fluorescent lamp and a backlight unit using the same are described later. The construction of such an external electrode surface emission fluorescent lamp is an improved structure of the surface emission fluorescent lamp previously applied by the present applicant. Due to this structure, a plurality of bar-shaped fluorescent lamps can be replaced with a single external electrode surface-emission fluorescent lamp. 
   That is, if an LCD panel is constructed as a large-scale panel, the external electrode surface emission fluorescent lamp as described later can be substituted for the bar-shaped external electrode fluorescent lamps, so a display area can be increased. 
     FIG. 15  is a plan view of the above-described surface emission external electrode fluorescent lamp  203 . As shown in  FIG. 15 , it can be seen that bar-shaped external electrodes  202  are formed at both ends of the surface emission fluorescent lamp  203  which is serpentine-shaped and is constructed as a single channel. The external electrodes  202  are simply implemented by attaching conduction materials for enabling electricity to easily pass therethrough to both ends of the lamp  203 . Especially, since energy must be provided to the inside of the external electrode surface emission fluorescent lamp  203  through external surfaces of the electrodes  202 , the electrodes  202  have sufficiently wide surface areas so as to provide sufficient excitation energy. 
   The external electrodes  202  serve to supply a high voltage to the surface emission fluorescent lamp  203  so as to sufficiently excite the surface emission fluorescent lamp  203  when high voltage pulses are applied to the fluorescent lamp  203  from the outside. The external electrodes  202  are produced using materials with excellent conductivity, for example, copper, aluminum tape, etc. In this case, the conduction materials are not inserted into the surface emission fluorescent lamp  203  like the internal electrodes  201  of  FIG. 14 , but brought into contact with the surface of the fluorescent lamp  203  and fixed not to be separated from the fluorescent lamp  203  during the operation of the fluorescent lamp  203 , thus completing the attachment of the materials. 
     FIG. 16  shows a modified embodiment in which gas paths  207  are formed between adjacent channels  203   b  of the external electrode surface emission fluorescent lamp  203  of the present invention. This embodiment shows that mutual horizontal channels are connected through the gas paths  207 , compared with the embodiment of  FIG. 15  in which respective bent portions of the external electrode surface emission fluorescent lamp  203  are connected to each other to form a single channel. Such a construction is advantageous in that it can uniformly distribute discharge gas into the surface emission fluorescent lamp  203  at a velocity higher than a moving velocity of discharge gas in the single channel construction shown in  FIG. 12 . Although it is depicted in  FIG. 16  that one gas path  207  is formed in both ends of each of horizontal channels, the thickness, installation position and the number of the gas paths  207  can be varied. Obviously, it is clear that these modifications or variations are included in the scope of the present invention. A distribution speed of gas can be optimized by varying the thickness, installation position and the number of the gas paths  207  according to the length or thickness of the surface emission fluorescent lamp  203 . 
     FIGS. 17 and 18  show the installation of auxiliary electrodes for driving the external electrode fluorescent lamp of the present invention at a low voltage. The external electrode surface emission fluorescent lamp  203  is long, so a high voltage is required to discharge it. Therefore, it is preferable to employ the auxiliary electrodes. 
   That is, if a voltage is applied to auxiliary electrodes connected to additional power, charged particles are generated within the fluorescent lamp  203 . Thereafter, if a voltage is applied to the external electrodes  202  used as main electrodes, the surface emission fluorescent lamp  203  starts discharging even at a low voltage. Therefore, if the auxiliary electrodes are used, power consumption can be greatly reduced compared with an external electrode surface emission fluorescent lamp using only main electrodes. The auxiliary electrodes are installed on the surface of the surface emission fluorescent lamp  203  in the same manner as that of the external electrodes  202 . In this case, the auxiliary electrodes can be installed in the shape of a line, without occupying a wide area. 
     FIG. 17  shows that auxiliary electrodes  202   a  are formed to enclose each of the channels  203   b . On the other hand,  FIG. 18  shows that auxiliary electrodes  202   b  are formed to pass along both ends of each of channels  203   b . The installation position of the auxiliary electrodes can be varied in consideration of the length, area, etc. of the external electrode surface emission fluorescent lamp  203 . That is, if it is required to shorten a response time of the surface emission fluorescent lamp  203 , auxiliary electrodes are installed on the same positions as those of the auxiliary electrodes  202   a  of  FIG. 17 , while if it is permitted to delay a response time to some degree, they are installed on the same positions as those of the auxiliary electrodes  202   b  of  FIG. 18 . In this case, it is preferable that the auxiliary electrodes use a different power source from that of the external electrodes. 
   Further, in the surface emission fluorescent lamp  203  of the present invention, an upper sheet  205  and a lower sheet  206  can be manufactured to be integrated, or separately manufactured and fused later. The former case is disadvantageous in that it is difficult to apply fluorescent materials; while it is advantageous in that it simplifies a manufacturing process because it does not execute a sealing process. On the contrary, the latter case is disadvantageous in that it requires a sealing process for junction portions, while it is advantageous in that the application of fluorescent materials can be easily performed. 
   The surface emission fluorescent lamp  203  further has a getter inserted thereinto. The getter  208  is used to supply mercury into the channels  203   b  of the surface emission fluorescent lamp  203  and absorb several impurities existing in the channels  203   b . The getter  208  is fixed by a getter housing such that it cannot move in the channels  203   b.    
     FIGS. 19 and 20  shows embodiments of the getter housing  209 . The embodiment of  FIG. 19  can be applied to a case where the getter housing  209  is installed in a bent portion, that is, a bent portion B′ of each of channels indicated in  FIG. 15 . The getter housing  209  of  FIG. 19  has a portion exposed to the outside in the shape of a quarter of a circle, and has a center portion dented inwardly on the inner surface of the getter housing  209  to fix the getter  208 . Further, the getter housing  209  can be constructed such that dented portions are formed on both the inner and outer surfaces of the getter housing  209 . 
   On the other hand,  FIG. 20  shows that a getter housing is formed in a portion of a bar-shaped channel, in which both ends of a channel are inwardly dented. It is clear that the fixing position of the getter  208  can be formed in any places within a range without preventing light radiation of the surface emission fluorescent lamp  203 . 
   An embodiment of a backlight unit using the surface emission fluorescent lamp  203  of the present invention having the above construction is shown in  FIG. 21 . 
   As shown in  FIG. 21 , a diffusion sheet  212  is arranged on the upper portion of the backlight unit, a reflective sheet  214  is arranged on the lower portion thereof, and the surface emission fluorescent lamp  203  is inserted therebetween. Even though not shown in detail, the diffusion sheet  212 , the reflective sheet  214  and the surface emission fluorescent lamp  203  are fixed to a frame of the backlight unit. External electrodes  202  are also attached to the surface emission fluorescent lamp  203 , as shown in  FIG. 17 , such that power is supplied from the outside. 
   Numerical values indicated in the left side of  FIG. 21  are values of an optimized manufacturing embodiment of a backlight unit having a size equal to or greater than 15.1″ and having brightness uniformity of 90%. The thickness of the diffusion sheet  212  is 2 mm, the thickness of the surface emission fluorescent lamp  203  is 7.1 mm, the thickness of the reflective sheet  214  is 1 mm, a spaced distance between the diffusion sheet  212  and the surface emission fluorescent lamp  203  is 1.9 mm, and a spaced distance between the reflective sheet  214  and the surface emission fluorescent lamp  203  is 0.1 mm, so the backlight unit is produced to have an entire thickness of 12.1 mm. These numerical values are values optimized under the above conditions satisfying the size equal to or greater than 15.1″ and the brightness uniformity equal to or greater than 90%. Further, a predetermined voltage is applied to the external electrodes  202  of the surface emission fluorescent lamp  203  using a power supply circuit (not shown), thus operating the backlight unit. 
   In the above backlight unit, even though only the external electrodes  202  are attached to the external electrode surface emission fluorescent lamp  203 , the external electrode surface emission fluorescent lamp  203  may further comprise auxiliary electrodes  202   a  and  202   b , and may be manufactured as an integrated type or a separated type. Moreover, it is clear that gas paths can be formed in the external electrode surface emission fluorescent lamp  203 , as shown in  FIG. 16 , and the construction of the getter housing  209  containing the getter  208  therein can be applied to the backlight unit. These applications are included in the scope of the present invention. 
   A driving device usable for the above-described external electrode surface emission fluorescent lamp having main electrodes and auxiliary electrodes will be described later. An embodiment of the external electrode surface emission fluorescent lamp having main and auxiliary electrodes is shown in  FIG. 22 . 
     FIG. 23  is a block diagram of a device for driving an external electrode surface emission fluorescent lamp according to an embodiment of the present invention. Referring to  FIG. 23 , the driving device comprises a main controller  310 , a priming circuit  320 , and a lighting maintaining circuit  330 . The main controller  310  lights a surface emission fluorescent lamp  350  in response to a control signal outputted from an LCD controller  360  which is a higher control unit. The priming circuit  320  supplies power to auxiliary electrodes  305  so as to generate initial charged particles in the external electrode surface emission fluorescent lamp  350  in response to a control signal outputted from the main controller  310 . The lighting maintaining circuit  330  supplies power to the main electrodes  303  so as to supply a lighting maintaining voltage using the charged particles generated by the priming circuit  320 . 
   The external electrode surface emission fluorescent lamp  350  is fundamentally the same as the surface emission fluorescent lamp  301  of  FIG. 22 . For convenience of description, the surface emission fluorescent lamp is designated by reference numeral  350  later. Further, it must be understood that external electrodes represent both the main electrodes  303  and the auxiliary electrodes  305 , and are connected, as shown in  FIG. 22 . 
   The operations of the driving device having the above construction are described in detail. 
   If a driving signal for initiating the operation of the driving device is received from the LCD controller  360  which is a higher controller, the main controller  310  outputs the driving signal to the priming circuit  320 , and turns off the priming circuit  320 . In this case, before charged particles generated in the external electrode surface emission fluorescent lamp  350  disappear, the main controller  310  applies a driving signal to the lighting maintaining circuit  330  and drives the external electrode surface emission fluorescent lamp  350  using the generated charged particles. Such a lit condition is maintained until an OFF control signal is inputted to the main controller  310  from the LCD controller  360 . 
     FIG. 25  is a detailed circuit diagram of the driving device of the present invention. The main controller  310  comprises a timing controller for supplying power, to the auxiliary electrodes  305  for a predetermined period of time in response to a control signal received from the LCD controller  360  to perform an initial lighting operation and thereafter supplying lighting power to the main electrodes  303 . Further, the main controller  310  comprises a pair of resistors R 11  and R 12  connected in series with the priming circuit  320  and the lighting maintaining circuit  330 , respectively, so as to apply driving signals to the priming circuit  320  and the lighting maintaining circuit  330 . 
   Therefore, if a lighting signal is inputted from the LCD controller  360 , the main controller  310  controls the timing to allow the priming circuit  320  to generate charged particles in the external electrode surface emission fluorescent lamp  350  using power supplied through the auxiliary electrodes  305  from the priming circuit  320 . Thereafter, before the generated charged particles disappear, the lighting maintaining circuit  330  supplies power to the surface emission fluorescent lamp  350  through the main electrodes  303 , thus operating the external electrode surface emission fluorescent lamp  350  at a low voltage. The predetermined period of time for driving the priming circuit  320  is determined by a lighting maintaining time of a fluorescent material required to generate sufficient number of charged particles in the surface emission fluorescent lamp  350 . 
   Further, the priming circuit  320  is comprised of a transformer T 1  for self-excited driving, a resonance capacitor C 22 , a pair of switching transistors Q 1  and Q 2 , a transistor Q 3  for receiving a signal from the main controller  310  to drive the switching transistors Q 1  and Q 2 , resistors R 21  and R 22  for controlling a voltage and a current inputted to a gate, and a line filter consisting of a coil L 21  and a capacitor C 21  to stabilize an inputted current, thus enabling a voltage to be applied to the auxiliary electrodes  305  of the external electrode surface emission fluorescent lamp  350 . 
   The lighting maintaining circuit  330  is used to receive a signal from the main controller  310  and apply a high voltage through the main electrodes  303  before charged particles formed in the external electrode surface emission fluorescent lamp  350  by the priming circuit  320  disappear. The lighting maintaining circuit  330  is comprised of a transformer T 2  for self-excited driving, a resonance capacitor C 32 , a pair of switching transistors Q 3  and Q 4 , transistors Q 5  and Q 6  for receiving a control signal from the main controller  310  to drive the switching transistors Q 3  and Q 4 , resistors R 31  and R 32  for controlling a voltage and a current inputted to a gate, and a line filter consisting of a coil L 31  and a capacitor C 31  to stabilize an inputted current, thus enabling a voltage to be applied to the main electrodes  303  of the external electrode surface emission fluorescent lamp  350 . 
   Further,  FIG. 24  is a block diagram showing another driving device further comprising a feedback circuit  340  according to another embodiment of the present invention. The driving device comprises a main controller  310 , a priming circuit  320 , a lighting maintaining circuit  330  and a feedback circuit  340 . The main controller  310  lights a surface emission fluorescent lamp  350  in response to a control signal outputted from the LCD controller  360 , and outputs a control signal to control the brightness of the external electrode surface emission fluorescent lamp  350  to correspond to brightness set by a user. The priming circuit  320  supplies power to the auxiliary electrodes  305  so as to generate initial charged particles in the external electrode surface emission fluorescent lamp  350  in response to the control signal outputted from the main controller  310 . The lighting maintaining circuit  330  supplies power to the main electrodes  303  so as to apply a lighting maintaining voltage using the charged particles generated by the priming circuit  320 . The feedback circuit  340  detects a current flowing into the external electrode surface emission fluorescent lamp  350 , converts the current into a voltage, and applies the voltage to the main controller  310  so as to control the brightness of the lamp  350  through the lighting maintaining circuit  330 . If the embodiment of  FIG. 24  is compared with that of  FIG. 23 , the driving device further comprises the feedback circuit  340 , and further has a function of controlling the brightness of the external electrode surface emission fluorescent lamp  350  by the main controller  310  using a feedback operation of the feedback circuit  340 . 
   That is, if power is supplied through the lighting maintaining circuit  330 , a current flowing into the external electrode surface emission fluorescent lamp  350  is detected by the feedback circuit  340 , the detected current is converted into a voltage, and the, voltage is inputted to the main controller  310 . Due to this voltage, the main controller  310  controls the brightness of the external electrode surface emission fluorescent lamp  350  by applying a PWM signal to the lighting maintaining circuit  330 . A setting value for brightness control is inputted to the LCD controller  360  by the user, and then inputted to the main controller  310  of the driving device, together with the driving signal of the LCD controller  360 . 
     FIG. 26  is a detailed circuit diagram of the driving device of  FIG. 24 , wherein detailed descriptions are omitted because the circuit construction of  FIG. 26  is similar to that of  FIG. 25 . The feedback circuit  340  is comprised of a rectifier consisting of a pair of diodes D 41  and D 42  to detect the current flowing into the external electrode surface emission fluorescent lamp  350 , a resistor R 41  for converting the detected current into a voltage, and a line filter consisting of a capacitor C 41  and a resistor R 42  to stably transmit the voltage inputted from the resistor R 41  to the main controller  310 . 
   The operations of the device for driving the surface emission fluorescent lamp for LCDs of the present invention having the above construction are described with reference to a flowchart of  FIG. 27  as follows. 
   First, if a user turns on a power lamp so as to operate a device on which an LCD panel is mounted, the LCD controller  360  applies a driving signal to the driving device at step S 1 . The driving signal from the LCD controller  360  is inputted to the main controller  310  of the driving device. If the driving signal is inputted, the main controller  310  outputs a driving signal to the priming circuit  320  at step S 2 . The priming circuit  320  is operated for a predetermined period of time set according to RC time constant. After the operation of the priming circuit  320  is finished at step S 3 , the main controller  310  outputs a driving signal to the lighting maintaining circuit  330  to light the external electrode surface emission fluorescent lamp  350  at step S 4 . That is, after power is supplied to the auxiliary electrodes  305  of the external electrode surface emission fluorescent lamp  350  to generate charged particles in the external electrode surface emission fluorescent lamp  350 , power is supplied to the fluorescent lamp  350  through the main electrodes  303  using the lighting maintaining circuit  330  before the charged particles disappear. If the feedback circuit  340  is not further included in the driving device, the above operations are repeatedly performed. 
   As described above, if power is supplied by the lighting maintaining circuit  330 , the external electrode surface emission fluorescent lamp  350  maintains its lit condition, thereby providing light sources to the LCD screen. Such a lighting maintaining operation is repeatedly performed until an OFF control signal is inputted from the LCD controller  360 . 
   Further, if the feedback circuit  340  is further included in the driving device, as shown in  FIG. 24 , the main controller  310  controls the brightness by comparing a value set by the user with a measured value inputted by feedback using a voltage value inputted from the feedback circuit  340 , and then outputting a voltage signal to the lighting maintaining circuit  330  in a PWM manner at step S 5 . Such a brightness control operation is repeatedly performed until the OFF control signal is inputted from the LCD controller  360 . 
   INDUSTRIAL APPLICABILITY 
   As described above, the present invention is advantageous in that it can easily produce surface light sources with high brightness and high brightness uniformity compared with a conventional edge-type backlight unit or a direct-type backlight unit using cold cathode fluorescent lamps, reduce a calorific value of an LCD panel due to electrodes of the fluorescent lamp, prevent breakdown of lamps due to breakdown of electrodes, and extend the lives of fluorescent lamps. 
   Further, the present invention is advantageous in that electrodes of a surface emission fluorescent lamp used for LCD backlighting are constructed as external electrodes, thus simplifying a manufacturing process of surface emission fluorescent lamps, improving productivity thereof, easily producing a large-scale backlight unit, and enabling produced LCDs to be miniaturized and thinned. 
   Further, the present invention is advantageous in that an initial lighting voltage for generating charged particles is applied to the surface emission fluorescent lamp for LCDs having main electrodes and auxiliary electrodes, charged particles are generated, and a lighting maintaining voltage is applied to the surface emission fluorescent lamp before the generated charged particles disappear, thus maintaining the lit condition of the surface emission fluorescent lamp, stably operating the surface-emission fluorescent lamp even at a low voltage, and reducing continuous stress of a transformer and loss of switching devices due to a high voltage for initial lighting.