Abstract:
A display device includes a display panel; a plurality of lamps facing the display panel and arranged in parallel; and at least one transformer connected to the plurality of lamps, wherein the at least one transformer is disposed at a lower portion of the display device adjacent to a ground. A transformer may include a driving transformer and a compensation transformer, where the compensation transformer may be connected to one or more lower lamps and the driving transformer may be connected to the remainder of the lamps. The output voltages of the compensation transformer and the driving transformer may differ.

Description:
The present invention claims the benefit of Korean Patent Application No. 2005-0056097, filed in Korea on Jun. 28, 2005, which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present application relates to a display device and, more particularly, to a display device including a lamp driving circuit. 
     BACKGROUND 
     Display devices have typically used cathode-ray tubes (CRT). Presently, much effort has been made to study and develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDP), field emission displays, and electro-luminescence displays (ELD), as a substitute for CRT. Of these, flat panel displays and LCD devices have advantages, such as high resolution, light weight, thin profile, compact size, and low power supply requirements. 
     In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. Alignment of the liquid crystal molecules in the liquid crystal material changes in accordance with the intensity of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field. LCD devices are non-luminous display devices in that they cannot display images without some light source (e.g., ambient light or a backlight). 
     A backlight unit for the LCD device is categorized as a direct type or an edge type. The direct type backlight unit has a plurality of lamps directly below a liquid crystal panel, and the edge type backlight unit has a lamp at a side of the liquid crystal panel. The direct type backlight unit uniformly supplies light to the liquid crystal panel and, in particular, the large sized liquid crystal panel. 
     As illustrated in  FIG. 1 , a liquid crystal display module includes a liquid crystal panel  13 , a backlight unit  20 , a main support  17 , a bottom cover  27  and a top cover  11 . 
     The backlight unit  20  includes a plurality of lamps  23  arranged in parallel below the liquid crystal panel  13 , a reflector  21  and a plurality of optical sheets  19  including prism and diffusion sheets. The plurality of lamps  23  are retained by a pair of lamp guides  25  combined with the bottom cover  27 . The main support  17  surrounds peripheral portions of the backlight unit  20  and is combined with the bottom cover  27 . The liquid crystal panel  13  is laid on the backlight unit  20 . The top cover  11  is combined with the bottom cover  27  such that the backlight unit  20  and the liquid crystal panel  13  are retained thereby. The lamp  23  includes a CCFL (cold cathode fluorescent lamp) having electrodes at both ends. The electrodes of the CCFL are supplied with driving voltages to supply light to the liquid crystal panel  13 . 
       FIG. 2A  is a plan view illustrating lamps and a driving circuit for lamps of a direct-type backlight unit, and  FIG. 2B  is a plan view illustrating lamps and another driving circuit for lamps of a direct-type backlight unit. 
     As illustrated in  FIG. 2A , a plurality of lamps  23  are arranged in parallel, and the lamp  23  has a glass tube. The glass tube is filled with a discharge gas including an inert gas and mercury (Hg). A fluorescent material layer is formed on an inner surface of the glass tube. First and second (left and right) electrodes are formed on ends of the glass tube. 
     The lamp  23  of  FIG. 2A  is operated by a high-high method. In other words, each of the both electrodes is supplied with a high voltage. When the lamp  23  is operated by a high-low method i.e., high and low voltages are supplied to the electrodes, as a length of the lamp  23  increases, a power operating the lamp  23  increases. Accordingly, when the lamp  23  is operated with the high and low voltages, the lamp  23  is problematic for the large sized LCD device. Therefore, high voltages are supplied to the both electrodes of the lamp  23  to operate the large sized LCD device. To operate the lamp  23  with high voltages, a ground portion (ground terminal) is disposed at a center portion of the lamp  23 . The high voltages supplied to the lamp  23  are AC (alternating current) voltages. 
     To supply the AC high voltage, a driving circuit is used. The driving circuit includes a plurality of inverters  33  and a plurality of transformers  31  each connected to each electrode of the plurality of lamps  23 . The inverter  33  converts a source voltage into an AC voltage, and the transformer  31  increases the AC voltage from the inverter  33  so as to output a AC high voltage. The AC high voltage outputted from each transformer  31  is supplied to each electrode of the lamps  23 , and thus the lamps  23  are operated independently from each other. As the lamps  23  are operated separately, transformers  31  and inverters  33  corresponding to the both electrodes of the lamps  23  are required. Accordingly, as a size of the LCD device increases, and a number of lamps  23  increases, and also, a number of the transformer  31  and the inverters  33  increase. Therefore, fabrication cost increases. 
     As illustrated in  FIG. 2B , a plurality of lamps  23  are arranged in parallel and operated by a high-high method, as similarly in  FIG. 2A . Each lamp  23  has first and second (left and right) electrodes. Each of the first and second electrodes is supplied with high voltages. The first electrodes are supplied with the high voltage from one or two transformers and inverters  31  and  33 , and the second electrodes are supplied with the high voltage from one or two transformers and inverters  31  and  33 . In other words, the driving circuit of  FIG. 2B  includes the fewer transformers  31  and inverters  33  than the driving circuit of  FIG. 2A , and thus fabrication cost is reduced. 
     However, lamp currents flowing in the outermost lamps  23  (that is, the lamps at the top and the bottom of a plurality of lamps arranged in parallel) are distorted due to structure and thermal distribution of the LCD device. With regard to the structure of the LCD device, since upper and lower portions of the bottom cover ( 27  of  FIG. 1 ) are made of metal are bent toward a front, the bent upper portion forms a parasitic capacitor with the uppermost lamps  23  adjacent to the bent upper portion, and the bent lower portion forms a parasitic capacitor with the lowermost lamps  23  adjacent to the bent lower portion. Since the parasitic capacitor causes leakage of the lamp currents, the lamp currents in the outermost lamps are distorted. 
     In addition, with regard to the thermal distribution of the LCD device, there is difference of temperature between the upper portion and the lower portion of a space where the lamps are arranged. When a user looks at the LCD device displaying an image, the lowermost lamps face a ground and the uppermost lamps face a sky. When the lamps  23  are operated, heat is generated by the transformers  31  at the center portion of the LCD device. The generated heat moves toward an upper portion of the space where the lamps are arranged. Therefore, the upper portion of the space has a higher temperature and the lower portion of the space has a lower temperature. Non-uniformity of the thermal distribution in the space causes distortion of the lamp currents in the lamps. 
     As a result, due to the structure and thermal distribution of the LCD device, the lamp currents in the outermost lamps and, in particular, in the lowermost lamps, are distorted. 
     SUMMARY 
     A display device is disclosed, which includes a display panel; a plurality of lamps facing the display panel and arranged in parallel; and a transformer connected to the plurality of lamps, wherein the transformer is disposed at a lower portion of the display device. 
     In another aspect, a display device includes a display panel; a plurality of lamps facing the display panel and arranged in parallel; and a transformer connected to the plurality of lamps, wherein the transformer is arranged in a space inside the display device such that thermal distribution is substantially uniform in the space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a liquid crystal display module including a direct-type backlight unit according to the related art; 
         FIG. 2A  is a schematic plan view illustrating lamps and a driving circuit for lamps of a direct type backlight unit according to the related art; 
         FIG. 2B  is a schematic plan view illustrating lamps and another driving circuit for lamps of a direct type backlight unit according to the related art; and 
         FIG. 3  is a schematic plan view illustrating lamps and a driving circuit for lamps of a direct type backlight unit according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments may be better understood with reference to the drawings, but these examples are not intended to be of a limiting nature. Like numbered elements in the same or different drawings perform equivalent functions. 
       FIG. 3  is a schematic plan view illustrating lamps and a driving circuit for lamps of a direct type backlight unit according to an exemplary embodiment of the present invention. 
     As illustrated in  FIG. 3 , a plurality of lamps  123  of a backlight unit are arranged in parallel, and the lamp  123  has a glass tube. The lamps  123  directly face a rear surface of a liquid crystal panel ( 13  of  FIG. 1 ) such that they uniformly supply light to the entire liquid crystal panel. The lamps  123  are arranged in a space defined between the liquid crystal panel and the bottom cover ( 27  of  FIG. 1 ). When a user looks at the LCD device displaying an image, a lower portion of the lamps-arranged space is closer to a floor and an upper portion of the lamp-arranged space faces a ceiling. The lamps  123  are divided into, for example, two lowermost lamps  125   a  and  125   b  and the rest of the lamps  124 . 
     The glass tube of the lamp  123  is filled with a discharge gas including an inert gas and mercury (Hg). A fluorescent material layer is formed on an inner surface of the glass tube. First and second (left and right) electrodes of each lamp  123  are formed on ends of the glass tube. 
     The lamp  123  is operated by a high-high method. In other words, each of the electrodes is supplied with a high voltage. To operate the lamp  123  with high voltages, a ground portion is disposed at a center portion of the lamp  123 . The high voltages supplied to the lamp  123  are AC (alternating current) voltages. 
     When the high AC voltages are applied to the first and second electrodes, the discharge gas enters a plasma state. In other words, due to the high voltages, electrons are generated in the lamp  123 , then the electrons collide with the discharge gas, and thus electrons abruptly increases in the lamp  123 . The generated electrons make the discharge gas excited and emit UV (ultraviolet) rays. The UV rays collide with the fluorescent material layer and emit a visible ray. 
     To supply the AC high voltage, a driving circuit is used. The driving circuit includes at least one driving inverter, at least one compensation inverter, at least one driving transformer and at least one compensation transformer for the first electrodes of the lamps  123 , and at least one driving inverter, at least one compensation inverter, at least one driving transformer and at least one compensation transformer for the second electrodes of the lamps  123 . In this example, two driving inverters  133   a  and  133   b , two compensation inverters  137   a  and  137   b , two driving transformers  131   a  and  131   b  and two compensation transformers  135   a  and  135   b  are used for the first electrodes of the lamps  123 . Two driving inverters  133   c  and  133   d , two compensation inverters  137   c  and  137   d , two driving transformers  131   c  and  131   d  and two compensation transformers  135   c  and  135   d  are used for the second electrodes of the lamps  123 . In particular, the driving and compensation transformers  131   a  to  131   d  and  135   a  to  135   d  are arranged in a lower portion of the LCD device. 
     The first to fourth driving inverters  133   a  to  133   d  are connected to the first to fourth driving transformers  131   a  to  131   d , respectively, and the first to fourth compensation inverters  137   a  to  137   d  are connected to the first to fourth compensation transformers  135   a  to  135   d , respectively. 
     The first and second driving transformers  131   a  and  131   b  are connected to the first electrodes of the rest of the lamps  124 , and the third and fourth driving transformers  131   c  and  131   d  are connected to the second electrodes of the rest of the lamps  124 . Connection relations between the first electrodes of the rest of the lamps  124  and the first and second driving transformers  131   a  and  131   b  and connection relations between the second electrodes of the rest of the lamps  124  and the third and fourth driving transformers  131   c  and  131   d  may be made in a number of alternative arrangements. For example, a group of the first electrodes may be connected to the first driving transformer  131   a  and the remaining first electrodes may be connected to the second driving transformer  131   b , and similarly, a group of the second electrodes may be connected to the third driving transformer  131   c  and the remaining second electrodes may be connected to the fourth driving transformer  131   d.    
     The first and second compensation transformers  135   a  and  135   b  are connected to the first electrodes of the two lowermost lamps  125   a  and  125   b , and the third and fourth compensation transformers  135   c  and  135   d  are connected to the second electrodes of the two lowermost lamps  125   a  and  125   b . Connection relations between the first electrodes of the two lowermost lamps  125   a  and  125   b  and the first and second compensation transformers  135   a  and  135   b  and connection relations between the second electrodes of the two lowermost lamps  125   a  and  125   b  and the third and fourth compensation transformers  135   c  and  135   d  may be made in a number of differing arrangements. For example, the first electrodes of the first and second lowermost lamps  125   a  and  125   b  may be connected to the first and second compensation transformer  135   a  and  135   b , respectively, and similarly, the second electrodes of the first and second lowermost lamps  125   a  and  125   b  may be connected to the third and fourth compensation transformer  135   c  and  135   d , respectively. 
     Each of the first to fourth driving inverters  133   a  to  133   d  has an integrated circuit including a plurality of switching devices. Each of the first to fourth driving inverters  133   a  to  133   d  converts a source voltage into an AC driving voltage by switching the switching devices according to a control signal. Each of the first to fourth driving transformers  131   a  to  131   d  boosts the AC driving voltage from each driving inverter  133   a  to  133   d  into the AC high driving voltage. The high AC driving voltages outputted from the first and second driving transformers  131   a  and  131   b  are supplied to the first electrodes of the rest of the lamps  124  electrically connected in parallel, and the high AC driving voltages outputted from the third and fourth driving transformers  131   c  and  131   d  are supplied to the second electrodes of the rest of the lamps  124  electrically connected in parallel. 
     Similarly to the driving inverters  133   a  to  133   d , the first to fourth compensation inverters  137   a  to  137   d  convert a source voltage into an AC compensation voltage according to a control signal. Each of the first to fourth compensation transformers  135   a  to  135   d  boosts the AC compensation voltage from each compensation inverter  137   a  to  137   d  into the high AC compensation voltage. The high AC compensation voltages outputted from the first and second compensation transformers  135   a  and  135   b  are supplied to the first electrodes of the two lowermost lamps  125   a  and  125   b , and the AC high compensation voltages outputted from the third and fourth compensation transformers  135   c  and  135   d  are supplied to the second electrodes of the two lowermost lamps  125   a  and  125   b . The AC high compensation voltage has a voltage level such that distortion of the lamp currents in the lowermost lamps  125   a  and  125   b  is reduced. For example, the AC high compensation voltage may have a voltage level higher than the AC high driving voltage to reduce the distortion of the lowermost lamp currents. 
     As explained above, the lowermost lamps  125   a  and  125   b  are operated with the compensation transformers  135   a  to  135   d  and the compensation inverters  137   a  to  137   d , separately from the rest of the lamps  124 . In other words, the compensation transformers  135   a  to  135   d  and the compensation inverters  137   a  to  137   d  supply the high AC compensation voltages which are different from the high AC driving voltages applied to the rest of the lamps  124 . Accordingly, distortion of the lamp currents due to the parasitic capacitor generated between the bent portion of the bottom cover and the lowermost lamps  125   a  and  125   b  can be compensated. 
     In addition, the driving and compensation transformers  131   a  to  131   d  and  135   a  to  135   d  are arranged in the lower portion of the LCD device. Accordingly, when the lamps  123  are operated, the transformers  131   a  to  131   d  and  135   a  to  135   d  generate heat in the lower portion of the LCD device. The generated heats move from the lower portion to the upper portion of the LCD device, and thus thermal convection is generated in the entire space where the lamps  123  are arranged. Therefore, the thermal convection in the entire lamp-arranged space, a substantially more uniform thermal distribution can be obtained. The uniform thermal distribution effectively reduces distortion of the uppermost lamp currents due to a parasitic capacitance generated between the bent portion of the bottom cover and the uppermost lamps. Accordingly, the uppermost lamps may not be operated with the compensation transformers  135   a  to  135   d . In addition, the uniform thermal distribution may substantially reduce distortion of the lowermost lamp currents along with the AC compensation voltage. The amount of reduction in current distortion or temperature gradient which is obtained may depend on the size of the display, the lamp characteristics, and the specific geometry and materials of the housing. In a quantitative sense, therefore, substantially is intended to connote a meaningful change in current or temperature that may be associated with either visibly observable effects of an improvement in lamp lifetime. 
     In the exemplary embodiment, the two driving transformers for each of the first electrodes and the second electrodes of the rest lamps, and the two compensation transformers for each of the first electrodes and the second electrodes of the lowermost lamps are used. However, a number of the transformers is not limited providing that thermal convection is generated and distortion of the lowermost lamp currents is compensated. 
     In addition, the transformers may be arranged in the lower portion of the LCD device. However, the arrangement of the transformers is not limited providing that thermal convection occurs. For example, a few of the transformers may be arranged in the upper portion, and most of the transformers may be arranged in the lower portion such that the thermal convention is generated. 
     In addition, The compensation transformers may operate the lowermost lamps. However, it should be understood that the compensation transformers may also be used to reduce distortion of the uppermost lamp currents. In addition, a number of the outermost lamps requiring the compensation transformers may be varied according to the structure of the LCD device. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the LCD device and the backlight unit for the LCD or other display device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.