CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. 119 to Korean Patent Application No. 10-2008-0106176, filed on Oct. 28, 2008, which is hereby incorporated by reference in its entirety for all purposes. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Disclosure 
         [0003]    This disclosure is related to a backlight unit, and particularly to a light driver capable of preventing a current deviation between a plurality of lamps which are connected to one another in the backlight unit. 
         [0004]    2. Description of the Related Art 
         [0005]    A cathode ray tube (CRT) is one among a wide number of display devices and is mainly employed in the monitors of television receivers, measuring instruments, and information terminals. (I don&#39;t understand what you mean by ‘information terminals’) It is difficult to apply the CRT to small and light electronic products, because of its weight and size. In other words, the CRT has a limit due to its weight and size while the trend for electronic products is to be light-weight and small in size. 
         [0006]    To address this matter, a liquid crystal display (LCD) device using an electro-optical effect, a plasma display panel (PDP) using a gas discharge, and an electro-luminescent display (ELD) device using an electro-luminescent effect are expected to substitute for the CRT. Among these devices, the LCD device has actively been developed. 
         [0007]    The LCD device controls an amount of incident light from the exterior in order to display a picture. The LCD device necessarily requires a separate light source, such as a backlight unit, irradiating on the LCD panel because it is a light receiving device. The backlight unit employed in the LCD device as the light source can be classified as either an edge type or a direct type in accordance with the disposition of a cylindrical emission lamp. 
         [0008]    The edge type backlight unit includes a lamp unit on the side surface of a light guide panel guiding light. The lamp unit includes a light emitting lamp, lamp holders receiving both ends of the lamp in order to protect the lamp, and a lamp reflection plate reflecting light emitted from the lamp toward the light guide panel. The lamp reflection plate surrounds the outer circumferential surface of the lamp and has an edge portion which is inserted in the side surface of the light guide panel. 
         [0009]    Such an edge type backlight unit with the lamp unit installed on the side surface of the light guide panel is mainly applied to comparatively small-sized LCD devices such as the monitors of laptops and desk-top computers. The edge type backlight unit has good light uniformity, a lengthened lifespan, and the advantage of thinning the LCD device. 
         [0010]    The direct type backlight unit has begun to be concentrically developed as the LCD device is enlarged to a size above 20 inches. The direct type backlight unit forces light to be irradiated onto the entire surface of the LCD panel. To this end, the direct type backlight unit includes a plurality of lamps arranged in a row (or side by side) on the inner surface of a bottom cover. 
         [0011]    Since the direct type backlight unit has a higher light efficiency than the edge type backlight unit, it is mainly used for LCD devices of a large size which require a high brightness. 
         [0012]    In such a direct type backlight unit, the plural lamps arranged at a constant distance are electrically connected to an external inverter, which is installed on the outside of the backlight unit, via a common electrode. In other words, the plural lamps are connected parallel to one another. 
         [0013]    The inverter includes a transformer applying an electric power of alternating current to an output terminal and a balance capacitor disposed between the secondary terminal of the transformer and the end terminals of the lamps. The balance capacitor controls an electric current to be applied to each lamp and uniformly balances the electric current. Also, the balance capacitor matches the lamps and the output side of the inverter in impedance. 
         [0014]    However, the electric current applied to each of the lamps is not uniform when the related art backlight unit is driven by the inverter. This results from an unbalance between the impedance components of the balance capacitor and an equivalent capacitor of the lamp. In other words, although the related art backlight unit includes the balance capacitor, it does not maintain a uniform brightness in each region. 
       SUMMARY OF THE INVENTION 
       [0015]    Accordingly, the present embodiments are directed to a backlight unit that substantially obviates one or more of problems due to the limitations and disadvantages of the related art. 
         [0016]    An object of the present embodiment is to provide a backlight unit driver that is adapted to prevent a deviation between (or among) electric currents applied to plural lamps which are connected parallel to one another. 
         [0017]    According to a general aspect of present embodiment, a backlight unit driver includes: first and second lamps connected parallel to each other; a DC/AC inversion portion inverting a DC voltage into an AC voltage to apply the AC voltage to the lamps; a transformer transforming the AC voltage from the DC/AC inversion portion; a positive polarity AC signal compensator compensating an electric current difference between the first and second lamps using positive polarity AC signals from the first and second lamps; and a negative polarity AC signal compensator compensating the electric current difference between the first and second lamps using negative polarity AC signals from the first and second lamps. 
         [0018]    Additional features and advantages of the embodiments will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the embodiments. The advantages of the embodiments will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
         [0019]    Other systems, methods, features and advantages will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. Nothing in this section should be taken as a limitation on those claims. Further aspects and advantages are discussed below in conjunction with the embodiments. It is to be understood that both the foregoing general description and the following detailed description of the present disclosure are exemplary and explanatory and are intended to provide further explanation of the disclosure as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The accompanying drawings, which are included to provide a further understanding of the embodiments and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the disclosure. In the drawings: 
           [0021]      FIG. 1  is a view schematically showing an LCD device according to an embodiment of the present disclosure; 
           [0022]      FIG. 2  is a view showing the configuration of the inverter of  FIG. 1 ; 
           [0023]      FIG. 3  is a view showing alternating current signals which are applied from the inverter of the related art backlight unit to first and second lamps; and 
           [0024]      FIG. 4  is a view showing alternating current signals which are applied from the inverter of the backlight unit according to the embodiment of the present disclosure to first and second lamps. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. These embodiments introduced hereinafter are provided as examples in order to convey their spirits to the ordinary skilled person in the art. Therefore, these embodiments might be embodied in a different shape, so are not limited to these embodiments described here. Also, the size and thickness of the device might be expressed to be exaggerated for the sake of convenience in the drawings. Wherever possible, the same reference numbers will be used throughout this disclosure including the drawings to refer to the same or like parts. 
         [0026]      FIG. 1  is a view schematically showing an LCD device according to an embodiment of the present disclosure.  FIG. 2  is a view showing the configuration of the inverter of  FIG. 1 . 
         [0027]    Referring to  FIGS. 1 and 2 , the LCD device according to the embodiment of the present disclosure includes: a LCD panel  110  on which gate lines GL 1  to GLn and data lines DL 1  to DLm cross each other; a gate driver  120  applying scan pulses to the gate lines GL 1  to GLn on the LCD panel  110 ; a data driver  130  applying data signals to the data lines DL 1  to DLm on the LCD panel  110 ; and a timing controller  150  controlling the gate driver  120  and the data driver  130 . The LCD panel  110  includes thin film transistors TFT each formed at intersections of the gate lines GL 1  to GLn and the data lines DL 1  to DLm. The thin film transistors TFT drive liquid crystal cells Clc, respectively. 
         [0028]    The LCD device further includes a backlight unit  180  applying light to the LCD panel  110  in accordance with a control signal from the timing controller  150 , and an inverter  160  driving the backlight unit  180  in response to another control signal from the timing controller  150 . 
         [0029]    Although it is not shown in the drawings, the LCD device also includes a common voltage generator outputting a common voltage Vcom and a power supply unit applying a power supply voltage to each of the elements as described above. 
         [0030]    The thin film transistors TFT on the LCD panel  110  are formed opposite the liquid crystal cells Clc and function as switching elements. To this end, each thin film transistor TFT includes a gate electrode connected to the respective gate line GL, a source electrode connected to the respective data line DL, and a drain electrode connected to a pixel electrode of the respective liquid crystal cell Clc and one side electrode of respective storage capacitor Cst. The common voltage Vcom is applied to a common electrode which is generally employed in the liquid crystal cells Clc. The storage capacitor Cst charges the data signal on the respective data line DL upon the turning on of the respective thin film transistor, thereby stably maintaining a voltage charged in the respective liquid crystal cell Clc. 
         [0031]    Also, each of the thin film transistors TFT is turned on and forms a channel between its source and drain electrodes when the scan pulse is applied to the respective gate line GL. At this time, the data voltage on the data line DL is applied to the pixel electrode of the respective liquid crystal cell Clc via the formed channel. Accordingly, the liquid crystal molecules of the liquid crystal cell Clc are aligned by an electric field between the pixel electrode and the common electrode in a different shape, and modulate incident light. 
         [0032]    The gate driver  120  derives the sequential scan pulses from a gate drive control signal GCS which is applied from the timing controller  150 . The gate pulses are sequentially supplied to the gate lines GL 1  to GLn. In this case, the gate drive control signal GCS may include a gate start pulse GSP, at least one gate shift clock GSC, and a gate output enable signal GOE. 
         [0033]    The data driver  130  responds to a data drive control signal DCS and applies the data signals to the data lines DL 1  to DLm. To this end, the data driver  130  samples and latches image data R, G, and B input from the timing controller  150 , opposite to the data lines DL 1  to DLm, and converts the image data R, G, and B into an analog data signal using gamma reference voltages. The gamma reference voltages are generated in a gamma reference voltage generator (not shown) and are applied to the data driver  130  through a gamma reference voltage selector (not shown). The analog data signal may be displayed in a variety of gradations by the liquid crystal cell on the LCD panel  110 . The data drive control signal DCS may include a source start pulse SSP, a source shift clock SSC, a source output enable signal SOE, a polarity inversion signal POL, and so on. 
         [0034]    The timing controller  150  receives a vertical synchronous signal Vsync, a Horizontal synchronous signal Hsync, a clock signal clk, a data enable signal DE, and the image data R, G, and B from an external system. Also, the timing controller  150  generates the control signals GCS and DCS controlling the gate and data drivers  120  and  130 , using the vertical synchronous signal Vsync, the Horizontal synchronous signal Hsync, the clock signal clk, and the data enable signal DE. 
         [0035]    The backlight unit  180  applies light on the LCD panel  110 . To this end, the backlight unit  180  includes a plurality of cold cathode fluorescent lamps (CCFLs) or external electrode fluorescent lamps (EEFLs). 
         [0036]    The inverter  160  inverts a direct current electric power from the exterior into the alternating current (AC) electric power of fixed frequency and voltage level which is adapted to drive the lamps of the backlight unit  180 . To this end, the inverter  160  may include a DC/AC inversion portion  161 , a transformer  165 , a frequency controller  163 , a positive polarity AC signal compensator  190 A, and a negative polarity AC signal compensator  190 B. 
         [0037]    The DC/AC inversion portion  161  inverts the DC electric power Vin from the exterior into the AC electric power. The inverted AC electric power is applied to a primary coil of the transformer  165 . To this end, the DC/AC inversion portion  161  may include two switching elements which are turned on and off alternately and complementarily to each other. 
         [0038]    The transformer  165  includes a primary coil connected to the DC/AC inversion portion  161  and a secondary coil connected to one end of the first and second lamps  181   a  and  181   b . Such a transformer  165  transforms the AC voltage from the DC/AC inversion portion  161  into a high AC voltage and drives the first and second lamps  181   a  and  182  using the transformed AC voltage. More specifically, the transformer  165  boosts the AC voltage at its first coil as a winding ratio of the first and second coils, so that the boosted AC voltage is induced at its secondary coil. 
         [0039]    The frequency controller controls the DC/AC inversion portion  161  to stably output the AC voltage of a fixed frequency. 
         [0040]    The positive and negative polarity AC signal compensators  190   a  and  190   b  are commonly connected to the other ends of the first and second lamps  181   a  and  181   b  in order to maintain the AC signals (i.e., electric currents) flowing through the first and second lamps  181   a  and  181   b.    
         [0041]    The positive polarity AC signal compensator  190   a  includes first and second diodes D 1  and D 2  connected to the other ends of the first and second lamps  181   a  and  181   b , a first transistor Q 1  connected to the first diode D 1 , and a second transistor Q 2  connected to the second diode D 2 . In this case, the first and second diodes D 1  and D 2  are shorted when a positive polarity AC signal is input. Also, the first and second transistors Q 1  and Q 2  may be N-type transistors. 
         [0042]    The first transistor Q 1  includes a collect electrode connected to the first diode D 1 , and an emitter electrode connected to a first resistor R 1 . The collect and base electrodes of the first transistor Q 1  are connected to each other. The first resistor R 1  is connected to a ground electric current source. 
         [0043]    The second transistor Q 2  includes a collect electrode connected to the second diode D 2 , the base electrode connected to the base electrode of the first transistor Q 1 , and an emitter electrode connected to a second resistor R 2 . The second resistor R 2  is connected to the ground electric current source. 
         [0044]    If the positive polarity AC signal is applied to the first and second lamps  181   a  and  181   b , the first and second diodes D 1  and D 2  included in the positive polarity AC signal compensator  191   a  are shorted so that the first and second transistors Q 1  and Q 2  are turned on. In this case, an electric current difference between the positive polarity AC signals flowing through the first and second lamps  181   a  and  181   b  is minimized or is not generated. This results from the fact that the collect and base electrodes of the first transistor Q 1  are connected with each other and the base electrodes of the first and second transistor Q 2  are connected with each other. 
         [0045]    In this way, the positive polarity AC signal compensator  190   a  operates as a current mirror, by means of the shorted first and second diodes D 1  and D 2 , when the positive polarity AC signal is applied to the first and second lamps  181   a  and  181   b . Accordingly, the electric current difference between the positive polarity AC signals through the first and second lamps  181   a  and  181   b  of a parallel connection configuration can be prevented or minimized. 
         [0046]    On the other hand, the negative polarity AC signal compensator  190   b  includes third and fourth diodes D 3  and D 4  connected to the other ends of the first and second lamps  181   a  and  181   b , a third transistor Q 3  connected to the third diode D 3 , and a fourth transistor Q 4  connected to the fourth diode D 4 . In this case, the third and fourth diodes D 3  and D 4  are shorted on when a negative polarity AC signal is input. Also, the third and fourth transistors Q 3  and Q 4  may be P-type transistors. 
         [0047]    The third transistor Q 3  includes a collect electrode connected to the third diode D 3 , and an emitter electrode connected to a third resistor R 3 . The collect and base electrodes of the third transistor Q 3  are connected to each other. The third resistor R 3  is connected to the ground electric current source. 
         [0048]    The fourth transistor Q 4  includes a collect electrode connected to the fourth diode D 4 , the base electrode connected to the base electrode of the third transistor Q 3 , and an emitter electrode connected to a fourth resistor R 4 . The fourth resistor R 4  is connected to the ground electric current source. 
         [0049]    When the negative polarity AC signal is applied to the first and second lamps  181   a  and  181   b , the third and fourth diodes D 3  and D 4  included in the negative polarity AC signal compensator  191   b  are shorted so that the third and fourth transistors Q 3  and Q 4  are turned on. At this time, an electric current difference between the negative polarity AC signals flowing through the first and second lamps  181   a  and  181   b  is minimized or is not generated. This results from the fact that the collect and base electrodes of the third transistor Q 3  are not only connected with each other but the base electrodes of the third and fourth transistors Q 3  and Q 4  are also connected with each other. 
         [0050]    In this manner, the negative polarity AC signal compensator  190   b  operates as a current mirror, because the third and fourth diodes D 3  and D 4  are shorted by the negative polarity AC signal. Accordingly, the electric current difference between the negative polarity AC signals through the first and second lamps  181   a  and  181   b  of a parallel connection configuration may be prevented or minimized. 
         [0051]      FIG. 3  is a view showing alternating current signals which are applied from the inverter of the related art backlight unit to the first and second lamps.  FIG. 4  is a view showing alternating current signals which are applied from the inverter of the backlight unit according to the embodiment of the present disclosure to the first and second lamps. 
         [0052]    As shown in  FIG. 3 , an electric current difference between the AC signals flowing through the first and second lamps of the related art backlight unit is caused by the different impedances of the first and second lamps. The electric current difference includes a positive polarity electric current difference in the positive polarity AC signal region and a negative polarity electric current difference generated in the negative polarity AC signal region. The positive polarity electric current difference is greatly generated as shown in PV 1  of  FIG. 3 . Also, the negative polarity electric current difference is greatly developed as NV 1  of  FIG. 3 . Accordingly, in the related art backlight unit, the lightness of the first lamp is different from that of the second lamp due to the positive and negative polarity electric current differences. 
         [0053]    On the other hand, an electric current difference between the AC signals flowing through the first and second lamps of the backlight unit according to the embodiment of the present disclosure is hardly generated as shown in  FIG. 4 . More specifically, when the positive polarity AC signal is applied the first and second lamps, a positive polarity electric current difference is hardly generated due to the compensating operation of the positive polarity AC signal compensator  190   a , as shown in PV 2  of  FIG. 4 . Similarly, a negative polarity electric current difference is hardly developed due to the compensating operation of the negative polarity AC signal compensator  190   b , as NV 2  of  FIG. 4 . Consequently, the backlight unit driver according to the embodiment of the present disclosure can minimize or eliminate the electric current difference between the first and second lamps. 
         [0054]    As described above, the backlight unit driver according to the embodiment of present disclosure can reduce or eliminate effectively and with low-cost the electric current difference between the first and second lamps connected with each other. This results from the fact that the backlight unit driver includes the positive polarity AC signal compensator compensating the difference between the positive polarity AC signals, and the negative polarity AC signal compensator compensating the difference between the negative polarity AC signals. Also, the backlight unit driver can compensate the electric current difference between the lamps, regardless of the polarity of the AC signal. 
         [0055]    It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure. Thus, the present disclosure may not be limited to the above embodiment. Furthermore, it is intended that the present disclosure cover the modifications and variations of this embodiment provided they come within the scope of the appended claims and their equivalents.
A backlight unit, with a parallel configuration of plural lamps, having improved reliability is disclosed. The backlight unit driver includes: first and second lamps connected parallel to each other; a DC/AC inversion portion inverting a DC voltage into an AC voltage to apply the AC voltage to the lamps; a transformer transforming the AC voltage from the DC/AC inversion portion; a positive polarity AC signal compensator compensating an electric current difference between the first and second lamps using positive polarity AC signals from the first and second lamps; and a negative polarity AC signal compensator compensating the electric current difference between the first and second lamps using negative polarity AC signals from the first and second lamps.