Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of priority of Korean Patent Application No. 10-2006-0011809 filed on Feb. 7, 2006. 
   FIELD OF THE INVENTION 
   The present invention relates to a lamp driving apparatus for a display and, more particularly, to a lamp driving apparatus capable of more stably driving a plurality of lamps in parallel. 
   DESCRIPTION OF THE RELATED ART 
   Back-lighted displays often employ one or more cold cathode fluorescent lamps as the light source. In recent years, large-sized LCDs with high luminance have required an increased number of lamps, increasing the size and cost of the driving apparatus.  FIG. 1  is a schematic view showing a prior art lamp driving apparatus in which a DC/AC inverter  21 , a transformer  23  and a controller  25  supply power to a parallel-connected plurality of lamps  10 . Because the load characteristic of certain lamps is affected by temperature, current may flow only to lamps having low resistance resulting in only some of the lamps being illuminated. Accordingly, there is a need for a driving apparatus in which current is applied so that all lamps are illuminated. 
   SUMMARY OF THE INVENTION 
   The present invention provides a lamp driving apparatus wherein a plurality of two-winding balance circuits are connected to a plurality of parallel-connected lamp groups so that all of the lamps may be illuminated even though ambient conditions cause some of the lamps to have low resistance. Capacitors are connected to first electrodes and certain of the balance coil windings are connected to second electrodes of one of the lamp groups while, in a second lamp group, others of the balance coil windings are connected to the first electrodes and capacitors are connected to the second electrodes thereof, current flowing through balance coil windings to some lamps being affected by current flowing through balance coil windings to other lamps. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention may become more apparent from a reading of the ensuing description together with the drawing, in which: 
       FIG. 1  is a schematic view showing a configuration of a conventional lamp driving apparatus of a liquid crystal display (LCD); 
       FIG. 2  is a schematic view showing a configuration of a lamp driving apparatus for a display according to the present invention; 
       FIG. 3  is a schematic view showing configurations of balance circuit units of a lamp driving apparatus according to a first embodiment of the present invention; 
       FIG. 4  is a schematic view showing configurations of balance circuit units of a lamp driving apparatus according to a second embodiment of the present invention; 
       FIG. 5  is a schematic view showing a configuration of a lamp driving apparatus according to a third embodiment of the present invention; 
       FIG. 6  is a schematic view showing a configuration of a lamp driving apparatus according to a fourth embodiment of the present invention; and 
       FIG. 7  is an exploded perspective view of a direct type LCD having a lamp driving apparatus according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  is a schematic view showing a configuration of a lamp driving apparatus for a back-lighted display such as a liquid crystal display (LCD). Referring to  FIG. 2 , the lamp driving apparatus comprises a plurality of lamps  410 , an inverter unit  450  for applying a driving voltage to the plurality of lamps  410 , and balance circuit units  460  and  470  for supplying a uniform current to the plurality of lamps  410 . 
   Each of the plurality of lamps  410  comprises a lamp tube and electrodes formed at both ends of the lamp tube. Further, the lamp tube comprises a body, a fluorescent substance layer, and a discharge gas. If a voltage is applied to the electrodes of the lamp, invisible light generated when the discharge gas is changed into plasma in the lamp tube excites the fluorescent substance layer so that visible light can be emitted to the outside, each of the lamps  410  may be a cold cathode fluorescent lamp (CCFL), a hot cathode fluorescent lamp (HCFL), an external electrode fluorescent lamp (EEFL) or an external and internal electrode fluorescent lamp (EIFL). 
   The inverter unit  450  comprises a DC/AC inverter  453  for converting a DC voltage supplied from the outside into an AC voltage, a transformer  455  for converting the level of the AC voltage output from the DC/AC inverter  453 , and a controller  457  for controlling the operation of the DC/AC inverter. 
   The transformer  455  comprises a first winding T 1  connected to output terminals of the DC/AC inverter  453 , a second winding T 2  for providing a first phase voltage, and a third winding T 3  for providing a second voltage with a second phase, the first and second voltages having phases opposite to each other. 
   The plurality of lamps  410  are connected in parallel with the transformer  455  of the inverter unit  450 . First and second electrodes are formed at both ends of each of the plurality of lamps  410  and the balance circuit units  460  and  470  are connected to the electrodes, respectively. Further, one ends of the second and third windings T 2  and T 3  of the transformer  455  are connected to the balance circuit units  460  and  470 , respectively, and the other ends are connected to the controller  457 . 
   The balance circuit units  460  and  470  are connected to the electrodes formed respectively at the both ends of each of the plurality of lamps  410  so that they can perform the function of controlling load characteristics of the lamps not to vary according to temperature and an ambient environment, thereby adjusting a current balance such that a uniform current flows to the plurality of lamps  410 . A first voltage output of winding T 2  of transformer  455  is applied to one electrodes of the lamps via the balance circuit unit  470 , and the second voltage output of winding T 3  of the transformer  455  is applied to the other electrodes of the lamps via the balance circuit unit  460 , so that the plurality of lamps  410  can be stably driven using the single transformer  455 . 
     FIG. 3  is a schematic view showing a configuration of balance circuit units of a lamp driving apparatus according to a first embodiment of the present invention. 
   Referring to  FIG. 3 , the balance units  460  and  470  of the lamp driving apparatus have a plurality of capacitors C bn  and a plurality of coils BC n . The plurality of lamps  410  comprise eight lamps L 1  to L 8 . As described above, first and second electrodes are formed at both ends of each of the lamps. The capacitor C bn  is connected to any one of the first and second electrodes of the lamp and the balance coil BC n  is connected to the other electrode, thereby constructing the balance circuit units  460  and  470  simultaneously using capacitors and balance coils. 
   The plurality of lamps  410  are composed of a first lamp group L 1 , L 3 , L 5  and L 7 , and a second lamp group L 2 , L 4 , L 6  and L 8 . Further, capacitors C b1 , C b3 , C b5  and C b7  are connected in series to the first electrodes of the lamps belonging to the first lamp group L 1 , L 3 , L 5  and L 7 , and balance coils BC 1 , BC 3 , BC 5  and BC 7  are connected to the second electrodes, any one of first and second coils of each of balance coils BC 1 , BC 3 , BC 5  and BC 7  is connected to the second electrode of the lamp, and the other coil of the first and second coils is connected to form a loop. Balance coils BC 2 , BC 4 , BC 6  and BC 8  are connected in series to the first electrodes of the lamps belonging to the second lamp group L 2 , L 4 , L 6  and L 8 , and capacitors C b2 , C b4 , C b6  and C b8  are connected to the second electrodes, any one of the first and second coils of each of balance coils BC 2 , BC 4 , BC 6  and BC 8  is connected to the first electrode of the lamp, and the other coil of the first and second coils is connected to form a loop. 
   In the plurality of lamps  410 , the lamps belonging to the first lamp group and those belonging to the second lamp group are alternately arranged one by one. Further, the balance circuit units  460  and  470  further include balance coils BC 9  and BC 10 , respectively. Furthermore, the balance coil BC 9  connects capacitors C b2 , C b4 , C b6  and C b8  and balance coils BC 1 , BC 3 , BC 5  and BC 7 , which are connected to the second electrodes of the plurality of lamps  410 , to the output terminal of the third winding T 3  of the transformer  455 , and the balance coil BC 10  connects capacitors C b1 , C b3 , C b7  and C b9  and balance coils BC 2 , BC 4 , BC 6  and BC 8 , which are connected to the first electrodes of the plurality of lamps  410 , to the output terminal of the second winding T 2  of the transformer  455 . Thus, a single transformer, eight capacitors and ten balance coils are required to drive eight lamps. 
   As described above, since the balance circuit units are constructed by compositely using ballast capacitors with a relatively low price and balance coils with superior reliability, the cost of the lamp driving apparatus can be reduced and the reliability thereof can be improved. Although this embodiment has been described in connection with the configuration in which the eight lamps can be simultaneously driven in parallel using the single transformer, this is only for convenience of illustration. The number of lamps and the numbers of capacitors and balance coils depending thereon are not limited thereto. 
     FIG. 4  is a schematic view showing a configuration of balance circuit units of a lamp driving apparatus according to a second embodiment of the present invention. In the balance circuit units of the lamp driving apparatus according to the second embodiment shown in FIG.  4 , the arrangement of capacitors and balance coils is different from that in the first embodiment. A different configuration will be mainly described below. 
   The balance circuit units  460  and  470  of the lamp driving apparatus have a plurality of capacitors C bn  and a plurality of coils BC n . The plurality of lamps  410  comprise eight lamps L 1  to L 8 . As described above, first and second electrodes are formed at both ends of each of the lamps. The capacitor C bn  is connected to any one of the first and second electrodes of the lamp, and the balance coil BC n  is connected to the other, thereby constructing the balance circuit units simultaneously using capacitors and balance coils. 
   The plurality of lamps  410  are composed of a first lamp group L 1 , L 2 , L 5  and L 6 , and a second lamp group L 3 , L 4 , L 7  and L 8 . Further, balance coils BC 1  and BC 3  are connected to the first electrodes of the lamps belonging to the first lamp group L 1 , L 2 , L 5  and L 6 , and capacitors C b1 , C b2 , C b5  and C b6  are connected to the second electrodes. Capacitors C b3 , C b4 , C b7  and C b8  are connected in series to the first electrodes of the lamps belonging to the second lamp group L 3 , L 4 , L 7  and L 8 , respectively, and balance coils BC 2  and BC 4  are connected to the second electrodes. That is, the lamps belonging to the first lamp group L 1 , L 2 , L 5  and L 6  and those belonging to the second lamp group L 3 , L 4 , L 7  and L 8  are alternately arranged one by one, and the two lamps belonging to the same lamp group are adjacent to each other. Further, the balance coil BC 5  is connected to the balance coil BC 1  and capacitors C b3  and C b4 , and the balance coil BC 7  is connected to the balance coil BC 3  and capacitors C b7  and C b8 . In addition, the balance coil BC 9  connects balance coils BC 5  and BC 7  to the output terminal of the second winding T 2  of the transformer  455 . Similarly, the balance coil BC 6  is connected to the balance coil BC 2  and capacitors C b1  and C b2 , and the balance coil BC 8  is connected to the balance coil BC 4  and capacitors C b5  and C b6 . In addition, the balance coil BC 10  connects balance coils BC 6  and BC 8  to the output terminal of the third winding T 3  of the transformer  455 . Thus, in the balance circuit units of the lamp driving apparatus according to the second embodiment of the present invention, a single transformer, eight capacitors and ten balance coils are also required to drive eight lamps in the same manner as the balance circuit units according to the first embodiment. 
     FIG. 5  is a schematic view showing a configuration of a lamp driving apparatus according to a third embodiment of the present invention. Referring to  FIG. 5 , a balance circuit unit of the lamp driving apparatus according to the third embodiment shown in  FIG. 5  is different from that of the first embodiment in that a voltage is applied only to any one of the first and second electrodes, and the balance circuit unit is also connected only to any one of the first and second electrodes. Since the configuration and arrangement of the balance circuit unit is identical with that of the first embodiment, a different configuration will be mainly described below. 
   The lamp driving apparatus comprises a plurality of lamps  410 , an inverter unit  450  for applying a driving voltage to the plurality of lamps  410 , and a balance circuit unit  460  for supplying a uniform current to the plurality of lamps  410 . 
   The inverter unit  450  comprises a DC/AC inverter  453  for converting a DC voltage supplied from the outside into an AC voltage, a transformer  455  for converting the level of the AC voltage output from the DC/AC inverter  453 , and a controller  457  for controlling the operation of the DC/AC inverter  453 . 
   The transformer  455  includes a first winding T 1  connected to output terminals of the DC/AC inverter  453 , and a second winding T 2  for inducing a voltage by means of a winding ratio thereof to the first winding T 1 . The plurality of lamps  410  are connected in parallel with the transformer  455  of the inverter unit  450 , and first and second electrodes are formed at both ends of each of the plurality of lamps  410 . The balance circuit unit  460  is connected to any one of the first and second electrodes, and the second winding T 2  of the transformer  455  is connected to the other electrode to which the balance circuit unit  460  is not connected. The balance circuit unit  460  is connected to only any one of the first and second electrodes formed at the both ends of each of the plurality of lamps  410  so that it can perform the function of controlling load characteristics of the lamps not to vary according to temperature and an ambient environment, thereby adjusting a current balance such that a uniform current flows to the plurality of lamps  410 . Accordingly, the plurality of lamps  410  can be more stably driven in parallel. 
     FIG. 6  is a schematic view showing a configuration of a lamp driving apparatus according to a fourth embodiment of the present invention. A balance circuit unit of the lamp driving apparatus according to the fourth embodiment shown in  FIG. 6  is different from the second embodiment in that a voltage is applied only to any one of the first and second electrodes formed at the both ends of each lamp and the balance circuit unit is also connected only to any one of the first and second electrodes. The configuration and arrangement of the balance circuit unit is almost identical with those in the first embodiment. 
     FIG. 7  is an exploded perspective view of a direct type LCD having a lamp driving apparatus according to the present invention. 
   Referring to  FIG. 7 , the LCD comprises a top chassis  300 , an LCD panel  100 , driving circuit units  220  and  240 , a mold frame  800 , a plurality of optical sheets  710 , a diffusion plate  720 , a lamp unit, and a bottom chassis  900 . 
   The driving circuit units  220  and  240  are connected to the LCD panel, and comprise a gate-side printed circuit board  224  having a control IC (Integrated Circuit) mounted thereon to apply a predetermined gate signal to gate lines of a TFT substrate  120 , a data-side printed circuit board  244  having a control IC mounted thereon to apply a predetermined data signal to data lines of the TFT substrate  120 , a gate-side flexible printed circuit board  222  having an exposed ground pattern to connect the gate-side printed circuit board  224  to the TFT substrate  120 , and a data-side flexible printed circuit board  242  having an exposed ground pattern to connect the data-side printed circuit board  244  to the TFT substrate  120 . 
   The gate- and data-side printed circuit boards  224  and  244  are connected to the gate- and data-side flexible printed circuit boards  222  and  242  to apply a gate driving signal and an external image signal. The gate- and data-side printed circuit boards  224  and  244  may be integrated into a single printed circuit board. Further, a driving IC (not shown) is mounted on the flexible printed circuit boards  222  and  242  so that it transmits RGB (Red, Green and Blue) signals generated from the printed circuit boards  224  and  244  and digital power to the LCD panel  100 . Although a tape-automated bonding (TAB) mounting method has been described by way of example in the embodiment of the present invention, otherwise, it is also possible to employ a chip on glass (COG) mounting method in which a driving IC is not mounted on the flexible printed circuit boards  222  and  242  but is installed on a thin film transistor substrate. 
   The top chassis  300  is formed to take the shape of a rectangular frame with a plane portion and sidewall portions which are bent perpendicularly to one another so that the LCD panel  100  and the driving circuit units  220  and  240  cannot come out therefrom and can be simultaneously protected against an external impact. 
   The lamp unit comprises lamps  410 , lamp sockets  430  in which the lamps  410  are seated, and a printed circuit board  480  on which the lamp sockets  430  and a lamp driving apparatus (not shown) are mounted. As described above, the lamp driving apparatus mounted on the printed circuit board  480  comprises an inverter unit for applying a driving voltage to the lamps  410 , and a balance circuit unit for supplying a uniform current to the plurality of lamps  410 . Further, the plurality of lamps  410  are connected in parallel with the inverter unit, and the balance circuit unit has a plurality of capacitors and a plurality of balance coils. 
   The plurality of optical sheets  710 , the diffusion plate  720 , at least one lamp unit and a reflection plate (not shown) are sequentially stacked from the bottom of a storage space defined at a lower portion of the mold frame  800 , and the bottom chassis  900  is coupled to the mold frame  800  to support the aforementioned components thereon. 
   As described above, according to the present invention, there is provided a lamp driving apparatus, wherein a balance circuit unit comprising capacitors and balance coils is connected to one end or both ends of each of a plurality of lamps so that the plurality of lamps can be stably driven with a single transformer. As a result, the number of parts required for the lamp driving apparatus is reduced, thereby obtaining an advantage of cost reduction. 
   The foregoing is merely exemplary embodiments of a lamp driving apparatus and a liquid crystal display having the same according to the present invention, and the present invention is not limited thereto. It will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the technical spirit and scope of the present invention defined by the appended claims.

Technology Category: 5