Abstract:
A lamp driving circuit is provided for controlling individual block luminances provided by corresponding locally dimmed blocks of a backlight unit of an LCD system where the backlight unit employs high voltage discharge lamps that each need to have an AC excitation signal of at least predetermined minimum high voltage level developed there across in order to generate light. The lamp driving circuit includes a plurality of isolation transformers and corresponding low voltage switch circuits. Each isolation transformer has primary windings and a secondary winding. The secondary winding is interposed between a high voltage AC power source and a corresponding one or more lamps. The equivalent circuit impedance of the secondary winding determines what voltage will develop across its respective lamps. The low voltage switch circuits are operative to alter the equivalent circuit impedances of their respective primary windings, which impedance changes are then reflected by mutual inductance coupling into the secondary windings. Thus control circuits operating at relatively low voltages can be used to control the ON/OFF states of the lamps.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application relies for priority upon Korean Patent Application No. 2009-114174 filed on Nov. 24, 2009, the contents of which are herein incorporated by reference in their entirety. 
       BACKGROUND 
       [0002]    1. Field of Disclosure 
         [0003]    The present disclosure of disclosure relates generally to light-emitting discharge tubes such as cold cathode fluorescent lamps used for light sources of liquid crystal displays, and more specifically to a lamp driving circuit used to selectively turn on and off the discharge tubes, and to a backlight unit having the lamp driving circuit, and to a liquid crystal display (LCD) using the same. 
         [0004]    2. Description of Related Technology 
         [0005]    Conventionally, cold cathode fluorescent lamps (CCFLs) are used as backlighting light source for liquid crystal displays (LCDs), and inverter circuits are used within the LCD electronics to generate high voltage AC power for turning-on the CCFLs. Recently, a scanning control scheme has been proposed for inverter circuits so as to reduce power consumption of the backlight unit. According to the proposed scanning control scheme, a plurality of CCFLs are grouped into block units, and the on/off operation of each CCFL block is controlled through a time division scheme so that light which is not needed is not wastefully generated. 
         [0006]    The backlight unit employing the scanning control scheme includes CCFL blocks and a plurality of inverter circuits connected with the CCFL blocks. The high voltage lamps driving circuits are driven through a time division scheme according to control signals provided from an external control circuit to control the timing of turning on and off of lamps in the CCFL block. 
         [0007]    However, since the backlight unit employing the conventional scanning scheme requires as many individually controlled inverter circuits as there are in number the individually controllable lamps of the CCFL blocks, the manufacturing cost of the LCD is increased, and a mounting area for The high voltage lamps driving circuits is increased in proportion to the number and complexity of The high voltage lamps driving circuits used. In addition, the backlight unit employing the scanning control scheme requires additional circuits for the synchronization of operating frequencies of the high voltage lamps driving circuits and the phase synchronization of the CCFL blocks. Accordingly, a method of driving the backlight unit in this way becomes complicated and expensive and more prone to break down as complexity of the control circuits increases. 
       SUMMARY 
       [0008]    Embodiments in accordance with the disclosure provide a lamp driving circuit capable of simplifying a configuration of a switching circuit. Embodiments in accordance with the disclosure provide an LCD using a backlight unit with the simplified controllable inverter circuit to reduce size, power consumption and price of the backlight unit. 
         [0009]    According to embodiments, a lamp driving circuit includes a high frequency isolation transformer and a low voltage switch circuit. A secondary winding of the isolation transformer is connected in series between a high voltage, high frequency power source and a load composed of one or more high voltage discharge tubes. Depending on the AC impedance provided by the secondary winding, a lamps igniting high frequency, high voltage AC signal will be applied or not applied to the discharge tubes and the lamps will light up or not light up accordingly. The switch circuit switches a state of a primary winding of the isolation transformer between first and second different impedance states, for example between an open circuit state and a short circuited state. The switch circuit responds to a low voltage control signal supplied from a controller that determines when and at what duty cycle the lamps will be driven. Since the switch circuit operates at low voltages, it can be made of relatively small and inexpensive circuit components. 
         [0010]    According to embodiments disclosed herein, a backlight unit includes a power source, a plurality of discharge tube blocks, a plurality of switch circuits, and a control circuit. Each discharge tube block has a plurality of discharge tubes. The isolation transformers are installed in correspondence with the discharge tube blocks, respectively. Secondary windings of the isolation transformers are connected in series between the high voltage power source and input terminals of the discharge tube blocks. The isolation transformers supply high AC voltage to the discharge tube blocks when the tubes are to be lit. The switch circuits are connected to primary windings of the isolation transformers, respectively, to switch a state of the primary windings for example between an open circuit state and a shorted circuit state according to a control signal. The control circuit generates the control signal to control a switching operation of the switch circuits. 
         [0011]    According to embodiments, a liquid crystal display includes a liquid crystal display panel and a backlight. The liquid crystal display panel includes a plurality of liquid crystal devices divided into a plurality of display regions to display an input image. The backlight is provided at a rear of the liquid crystal display panel. The backlight includes a power source, a plurality of discharge tube blocks, a plurality of isolation transformers, a plurality of switch circuits, and a control circuit. The discharge tube blocks include a plurality of discharge tubes and correspond to the display regions. The isolation transformers are installed corresponding to the discharge tube blocks, respectively. Secondary windings of the isolation transformers are connected in series between the power source and input terminals of the discharge tube blocks. The isolation transformers selectively supply high frequency AC voltage signals to the discharge tube blocks. The switch circuits are connected to primary windings of the isolation transformers, respectively, to switch a state of the primary windings to an open state or a short state according a control signal. The control circuit generates the control signal to control a switching operation of the switch circuits. 
         [0012]    As described above, the configuration of a circuit used to switch a plurality of CCFL blocks at a high speed is simplified to provide a lighting system driving circuit having small size, low power consumption, and low price and The high voltage lamps driving circuit is employed in the backlight unit and the LCD having a scanning control function for the CCFL blocks or a time control function for turn-on/turn-off operation of each CCFL block such that the size, power consumption and price of the backlight unit and the LCD can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above and other advantages of the present disclosure will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
           [0014]      FIG. 1  is a circuit diagram schematically showing a backlight unit according to an embodiment of the present disclosure; 
           [0015]      FIG. 2A  is a circuit diagram showing an equivalent circuit and its resonant frequency as it appears on the secondary winding side of the isolation transformer of  FIG. 1  when the primary winding of the isolation transformer is in the open circuit state; 
           [0016]      FIG. 2B  is a circuit diagram showing an equivalent circuit and its resonant frequency as it appears on the secondary winding side of the isolation transformer when the primary winding is shorted by an electronically controlled and low voltage switching circuit; 
           [0017]      FIG. 2C  is a graph showing the variation in resonant frequency and inductance the secondary winding side equivalent circuit when the primary winding is switched from the opened state to the shorted state; 
           [0018]      FIG. 3A  is a timing and waveforms view showing voltage waveforms at nodes N A  and N B  of  FIG. 1  when the primary side switch is turned off (open circuit state); 
           [0019]      FIG. 3B  is a timing and waveforms view showing voltage waveforms at the nodes N A  and N B  of  FIG. 1  when the switch is turned on (closed circuit state); 
           [0020]      FIG. 4  is a table showing the relation between the variation in impedance values of the primary and secondary windings and the operating state of a CCFL block when the switch of  FIG. 1  is turned on or off; 
           [0021]      FIG. 5  is a circuit diagram showing a backlight unit using series resonance occurring by an LC series resonant circuit in detail; 
           [0022]      FIG. 6A  is a view showing the relation between variation in inductance values of the primary and secondary windings of the isolation transformer and the operating state of the CCFL block when FETs of  FIG. 5  are turned on or off; 
           [0023]      FIG. 6B  is a view showing variation in voltage and current when a backlight section of  FIG. 5  is turned on/off; 
           [0024]      FIG. 7  is a circuit diagram showing a structure in which a TRIAC is connected as the switch of the backlight unit shown in  FIG. 1 ; 
           [0025]      FIG. 8  is a circuit diagram showing a structure in which a photo-responsive TRIAC is connected as the switch of the backlight unit shown in  FIG. 1 ; 
           [0026]      FIG. 9  is a circuit diagram showing a structure in which MOSFETs are connected as the switch of the backlight unit shown in  FIG. 1 ; 
           [0027]      FIG. 10  is a circuit diagram showing a backlight unit according to another embodiment of the present disclosure in detail; 
           [0028]      FIG. 11  is a circuit diagram showing a backlight unit according to another embodiment of the present disclosure in detail; 
           [0029]      FIG. 12  is a circuit diagram showing a backlight unit according to another embodiment of the present disclosure in detail; 
           [0030]      FIG. 13  is a circuit diagram showing a backlight unit according to another embodiment of the present disclosure in detail; 
           [0031]      FIG. 14  is a circuit diagram showing a backlight unit according to another embodiment of to the present disclosure in detail; 
           [0032]      FIG. 15  is a circuit diagram showing a backlight unit according to another embodiment of the present disclosure in detail; 
           [0033]      FIG. 16  is a block diagram showing an LCD according to another embodiment of the present disclosure; and 
           [0034]      FIG. 17  is an exploded perspective view showing the structure of the LCD shown in  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Hereinafter, embodiments in accordance with the present disclosure of disclosure will be described in more detail with reference to accompanying drawings. 
         [0036]      FIG. 1  is a circuit diagram schematically showing a backlight unit  100  according to an embodiment of the present disclosure. 
         [0037]    Referring to  FIG. 1 , the backlight unit  100  includes a AC power source  101 , an isolation transformer  102  having a primary winding  102   a  and a secondary winding  102   b  that is DC wise isolated from the primary, an electronically controlled switch SW 1 , a plurality of capacitors (also hereafter, condensers circuit  103 ), and a plurality of cold cathode fluorescent lamps (CCFL) block  104 . The condensers circuit  103  includes a plurality of balancing capacitors (BCs) structured and arranged to uniformly distribute the high voltage AC power which has been output from the AC power source  101  and through the isolation transformer  102  to the plurality of CCFLs in the CCFL block  104 . 
         [0038]    In the backlight unit  100 , a secondary winding  102   b  (at the side of the applied high voltage AC) of the isolation transformer  102  is connected to an output terminal of the AC power source  101  and is connected in series to the condensers circuit  103  and CCFL block  104 . On the other hand, a primary winding  102   a  (at an isolated low voltage side of transformer  102 ) is connected to the switch SW 1 . Since the primary winding  102   a  has been isolated from the secondary winding  102   b  in the above structure, the switch SW 1  having a low voltage stress characteristic can be used to open or short the primary winding  102   a  For this reason, because the relatively low voltage AC signal develops across the secondary  102   b  in the open switch state, the switch SW 1  can be realized in a small size and of a design that does not need to feature resistance to breakdown at high voltage values such that the price and/or size of the switch can be reduced relative to switches that need to avoid breakdown at relatively higher voltage values. The isolation transformer  102  of the backlight unit  100  is a magnetic leakage type transformer in which primary and secondary windings are loosely rather than tightly magnetically coupled, and thus opening of the primary winding  102   a  acts to reduce the value of the high voltage AC signal applied to the CCFL block  104  because a large AC voltage drop (corresponding to a large impedance or Hi-Z state) develops across the secondary  102   b  when the primary  102   a  is open. On the other hand, when the primary  102   a  is shorted, a very small or essentially zero AC voltage drop develops across the secondary  102   b  (corresponding to a low impedance or Low-Z state of the primary) so that driving efficiency is not impaired by the secondary  102   b  being disposed in series between the AC source  101  and the load  103 / 104 . 
         [0039]    Hereinafter, the operating procedure of using the backlight unit  100  shown in  FIG. 1  when the switch SW 1  opens or shorts the primary winding  102   a  of the isolation transformer  102  will be briefly described with reference to  FIGS. 2A ,  2 B,  2 C, and  3 . 
         [0040]      FIG. 2A  is an equivalent circuit diagram showing how the secondary side impedance (Z 1 ) is a function of a resonant frequency of the equivalent RLC circuit and in the case where a roughly 159 KHz AC signal is sourced in the series circuit of the secondary winding  102   b , when the primary winding  102   a  of the isolation transformer  102  is left open (not short circuited), the secondary winding  102   b  side has an equivalent resonant frequency at about 46 KHz and thus presents itself as a high impedance (Hi-Z) in the primary series circuit. On the other hand, in  FIG. 2B , in the case where the primary winding  102   a  is shorted, the equivalent RLC circuit of the secondary winding side  102   b  is about 159 KHz and the secondary side winding thus presents itself as a low impedance (Low-Z) in the primary series circuit.  FIG. 2C  is a graph showing the impedance variation in terms of the effective resonant frequency of the equivalent RLC circuit of the secondary winding side  102   b . In other words, when the switch SW 1  is open, the resonant frequency is low but the equivalent winding inductance (WL) is high. On the other hand, when the switch SW 1  is closed, the resonant frequency is high and the equivalent winding inductance (WL) is low. Referring to  FIG. 2A , when the primary winding  102   a  of the isolation transformer  102  is opened, an equivalent circuit may be constructed as a RLC parallel resonant circuit. In the RLC parallel resonant circuit, a secondary-side inductance value is about 2.8 [H] (in Henrys), a secondary-side capacitance value is about 4.2 [pF] (in picoFarads), and a secondary-side resistance value is about 7.3 [kΩ] (in kilo ohms) Accordingly, a resonance frequency of about 45.7 [kHz] is calculated based on the values of the RLC equivalent circuit components. 
         [0041]    Referring to  FIG. 2B , when the primary winding  102   a  of the isolation transformer  102  is shorted, an equivalent circuit may be constructed as an RLC parallel resonant circuit. In the RLC parallel resonant circuit of the short circuited case, an inductance value is about 0.47 [H], a capacitance value is about 2.2 [pF], and a resistance value is about 10.2 [kΩ]. Accordingly, a resonance frequency of about 158.5 [kHz] is calculated based on the values of the RLC equivalent circuit components. 
         [0042]    In other words, when the switch SW 1  is turned “off” and thus represents an open circuit connected to the primary winding  102   a , the secondary-side equivalent circuit includes a large inductance of about 2.8 [H]. On the other hand, when the switch SW 1  is turned “on” to thus short the primary winding, a substantially smaller inductance of about 0.47 [H] appears as part of the equivalent circuit impedance of the secondary winding. Accordingly, the relation between the resonance frequency of a high frequency AC voltage applied to the CCFL block  104  through the isolation transformer  102  and the equivalent circuit impedance in the secondary winding when the primary winding is opened or shorted varies as is shown in  FIG. 2C . 
         [0043]    More specifically, and as shown in  FIG. 2C , when the primary winding is opened, the equivalent circuit of the secondary winding takes on a relatively low resonant frequency and a relatively large inductance (ωL). On the other hand, when the secondary winding is shorted, the equivalent circuit of the secondary winding takes on a relatively high resonant frequency and a relatively smaller inductance value, where the higher resonant frequency is much nearer to a turn-on frequency of the AC source  101  than is the low resonant frequency. Accordingly, the CCFL  104  is not turned on when the low resonant frequency, high inductance state occurs. Stated otherwise, the high frequency AC voltage of the source is applied to the CCFL block  104  only after having been reduced by a drop across the high inductance presented by the primary-side impedance of the isolation transformer  102  relative to the impedance of the CCFL block  104 . Accordingly, the high frequency AC voltage that develops across to the CCFL block  104  when the switch SW 1  is open, is less than a predetermined high voltage need to turn on the CCFL block  104  (to ignite the gases in the lamps into plasma states), and so that the CCFL block  104  is not turned on. 
         [0044]    By contrast, and as also shown in  FIG. 2C , when the primary winding  102   a  is shorted by a turned “on” state of the switch SW 1 , a resonance frequency of the secondary side equivalent circuit is increased, and the equivalent circuit inductance (ωL) presented by the secondary side of the transformer is decreased at the operating frequency of the CCFL  104 . Accordingly, a large AC voltage drop does not develop across the secondary winding  102   b  and the CCFL block  104  is turned on. In other words, since the secondary-side impedance value is reduced in the switch SW 1  closed state, the high frequency AC voltage applied across the CCFL block  104  becomes greater than or equal to the predetermined voltage needed to initiate the turning on of the lamps in the CCFL block  104 , so that the CCFL block  104  is therefore turned on. 
         [0045]      FIG. 3A  is a view showing voltage waveforms at nodes N A  and N B  relative to common node Nc in the case when the switch SW 1  is turned off (left open).  FIG. 3B  is a view showing voltage waveforms at the nodes N A  and N B  when the switch SW 1  is turned on (placed into a short circuiting state). As shown in  FIGS. 3A and 3B , when the switch SW 1  is turned off to leave open the primary winding  102   a , the voltage value of the node N B  is decreased due to the voltage drop across the secondary winding and as a result, the CCFL block  104  is not turned on. On the other hand, when the switch SW 1  is turned on to thereby provide a short circuit across the primary winding  102   a , the voltage value of node N B  is increased sufficiently so that the CCFL block  104  is turned on. 
         [0046]      FIG. 4  is a table showing the relations between the variation in impedance values of the primary and secondary windings of the isolation transformer  102  and the operating state of the CCFL block  104  when the switch SW 1  is turned on or off. 
         [0047]    As described above, when the switch SW 1  is turned on or turned off, the inductance value of the secondary winding is changed from about 1.67 [H] to about 224 [mH], so that the CCFL block  104  is changed from a turn-off operation state to a turn-on operation state. The CCFL block  104  can be turned on by matching a resonance frequency of a LC series resonant circuit, which is derived from leakage inductance and capacitance of a balance condenser (BC) when the primary winding is shorted, with an inverter frequency. The CCFLs are driven through series resonance occurring by the LC series resonant circuit, so that a capacitance component and an inductance component are offset with each other in the turn-on state, and only a resistance component serves as a load, thereby reducing the value of the high AC voltage applied to the CCFL block  104 . 
         [0048]    Hereinafter, a detailed circuit diagram of a backlight unit  800  using a series of LC resonant circuits will be described with reference to  FIG. 5 . 
         [0049]    Referring to  FIG. 5 , the backlight unit  800  includes a first backlight section  801 , a second backlight section  802 , and a high voltage, high frequency AC power source  803 . The first and second backlight sections  801  and  802  are connected in parallel an out terminal of the AC power source  803 . 
         [0050]    The first backlight section  801  includes a first high frequency isolation transformer  811 , a first pair of MOSFETs  812  forming a first low voltage switching element, a first condensers circuit  813 , and a first CCFL block  814 . The second backlight section  802  includes a corresponding second isolation transformer  821 , a second set of MOSFETs  822  serving as a second switch element, a second condensers circuit  823 , and a second CCFL block  824 . As seen, the first and second backlight sections  801  and  802  have substantially the same circuit configuration except that each is controlled by a respective low voltage ON/OFF control signal. 
         [0051]    Each BC in the condenser circuits  813  and  823  has a capacitance value of 27 [pF], and each CCFL in the CCFL blocks  814  and  824  has a length of 52 inches. Of course, other values may be used in other variations of the basic circuit concept. The capacitance values of the BC&#39;s is a parameter that operates to determine a resonance frequency of an equivalent LC series circuit and the BC value may be set in accordance with consideration of matching impedances based on the inverter operating frequency. Thus, the capacitance values of the BC&#39;s and the lengths of the CCFL&#39;s are not limited to those disclosed for the present embodiment. 
         [0052]    The switch operation of the first FETs  812  is controlled through a first ON/OFF control signal provided from an external operation controller (e.g., a low voltage microcontroller circuit, not shown). Through the switching operation of the first FETs  812 , a primary winding of the first isolation transformer  811  is selectively shorted or opened. The switch operation of the second FETs  822  is similarly controlled through a second ON/OFF control signal provided from the external operation controller. Through the switching operation of the second FETs  822 , a primary winding of the second isolation transformer  821  is selectively shorted or opened. 
         [0053]    Hereinafter, the turn-on operation of the first backlight section  801  will be described with reference to  FIGS. 6A and 6B . 
         [0054]      FIG. 6A  is a table view showing the relations between the variation in inductance values of the primary and secondary windings of the first isolation transformer  811  and the corresponding operating state of the first CCFL block  814  when the first pair of MOSFETs  812  are turned off or on by application of a low voltage control signal to their isolated gate electrodes.  FIG. 6B  is an oscilloscope type view showing high frequency AC voltage waveforms and current waveforms that develop at and through nodes V 0  and VL of  FIG. 5  relative to ground. The illustrated variation in load current ILH of the high voltage circuit is that which is applied to an input terminal of the first CCFL block  814 . On the other hand, the illustrated current ILL is that which is output from an output terminal of the first CCFL block  814 . 
         [0055]    If the first FETs  812  are turned on to thereby substantially short circuit the terminals of the primary winding of the first isolation transformer  811 , a LC series resonant circuit having leakage inductance in the primary winding of the isolation transformer  811  if formed and the capacitance of the BC performs a resonance operation for coupling the power source energy to the lamps. Through such a matched resonance operation of the LC series circuitry, the voltage VL of the load node, as shown in  FIG. 6 , becomes a high frequency AC voltage that is greater than or equal to the voltage needed to initiate the turning-on of the gas lamps in the first CCFL block  814 . 
         [0056]    Referring to  FIG. 5 , on the assumption that the secondary inverter frequency (f) is about 30 [kHz], a resonance frequency (f 0 ) when the primary winding is shorted is calculated from following Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   fo 
                   = 
                   
                     1 
                     
                       2 
                        
                       
                           
                       
                        
                       
                         
                           π 
                            
                           
                             ( 
                             
                               L 
                               × 
                               C 
                             
                             ) 
                           
                         
                         
                           1 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0057]    In Equation 1, L refers to a secondary-side inductance value, and C refers to a secondary-side capacitance value. In this case, when the primary winding is shorted, the secondary side equivalent circuit inductance, L becomes 551 [milliH] (see  FIG. 6A ), and the secondary side equivalent circuit capacitance C becomes 27 [pF]×2 (because the two BC&#39;s are basically in parallel with one another once their lamps ignite). The resonance frequency (f 0 ) of the secondary side LC series resonant circuit then becomes about 29.2 [kHz] as shown in following Equation 2 to thereby substantially match the fundamental operating frequency of the power source  803 . 
         [0000]    
       
         
           
             
               
                 
                   fo 
                   = 
                   
                     
                       1 
                       
                         
                           2 
                            
                           
                               
                           
                            
                           
                             
                               π 
                                
                               
                                 ( 
                                 
                                   551 
                                    
                                   
                                       
                                   
                                    
                                   mH 
                                   × 
                                   27 
                                    
                                   
                                       
                                   
                                    
                                   pF 
                                   × 
                                   2 
                                 
                                 ) 
                               
                             
                             
                               1 
                               2 
                             
                           
                         
                          
                         
                             
                         
                       
                     
                     = 
                     
                       29.2 
                        
                       
                           
                       
                       [ 
                       kHz 
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0058]    In order to allow the impedance of the first CCFL block  814 , which is loaded as the LC series resonant circuit is operated at the resonance frequency (f 0 ) of about 30 kHz to appear only as resistance component (R), voltage applied to the first CCFL block  814  is obtained based on following Q factor Equation 3. 
         [0000]    
       
         
           
             
               
                 
                   Q 
                   = 
                   
                     
                       
                         Z 
                          
                         
                             
                         
                          
                         L 
                       
                       R 
                     
                     = 
                     
                       
                         1 
                         ZCR 
                       
                       = 
                       
                         
                           
                             ( 
                             
                               2 
                                
                               
                                   
                               
                                
                               π 
                               × 
                               f 
                               × 
                               L 
                             
                             ) 
                           
                           R 
                         
                         = 
                         
                           1 
                           
                             2 
                              
                             
                                 
                             
                              
                             π 
                             × 
                             f 
                             × 
                             C 
                             × 
                             R 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0059]    In Equation 3, ‘f’ refers to the high voltage lamps driving frequency, and ‘R’ refers to a resistance component of the CCFL block  814 . In this case, if the resistance component (R) has a value of 92 [kΩ], and the L becomes 551 [mH] (see  FIG. 6A ), where the latter is the secondary-side inductance value when the primary winding is shorted, then accordingly, the AC operating voltage applied to the CCFL block  814  becomes 2.14 by the LC series resonant circuit operating at the resonance frequency (f 0 ) as shown in Equation 4. 
         [0000]    
       
         
           
             
               
                 
                   Q 
                   = 
                   
                     
                       
                         ( 
                         
                           2 
                            
                           
                               
                           
                            
                           π 
                           × 
                           
                             30 
                              
                             
                                 
                             
                             [ 
                             kHz 
                             ] 
                           
                           × 
                           
                             551 
                              
                             
                                 
                             
                             [ 
                             mH 
                             ] 
                           
                         
                       
                       
                         
                           ( 
                           
                             
                               92 
                                
                               
                                   
                               
                                
                               k 
                                
                               
                                   
                               
                                
                               Ω 
                             
                             2 
                           
                           ) 
                         
                          
                         
                             
                         
                       
                     
                     ≈ 
                     2.14 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0060]    When the voltage is applied to the first CCFL block  814  by the LC series resonant circuit operating at the resonance frequency (f 0 ), the voltage V 0  at the node V 0  and the current ILH flowing through the first CCFL block  814 , which are shown in  FIG. 5 , are substantially in phase as shown in  FIG. 6B , and the high AC voltage applied to the first CCFL block  814  by the LC series resonant circuit can be lowered. Accordingly, the driving efficiency can be improved. Meanwhile, the second backlight section  802  has the same turn-on/turn-off operation as that of the first backlight section  801 . 
         [0061]    In addition, according to the present disclosure, time to turn on the first CCFL block  814  can be controlled through the switching operation of the switch SW 1  to open or short the primary winding of the isolation transformer  811 . Hereinafter, detailed possible structures for the switch SW 1  will be described with reference to  FIGS. 7 to 9 . Meanwhile, the same reference numerals will designated to elements of  FIGS. 7 to 9  identical to those of  FIG. 1 . 
         [0062]      FIG. 7  is a circuit diagram showing a structure in which an AC triode switch (a TRIAC)  105  is provided as the switch SW 1 . 
         [0063]    In this case, an ON/OFF control signal can be input to a trigger terminal of the TRIAC  105  from an external control circuit to turn on/turn off the TRIAC  105 , thereby changing the turn-on/turn-off state of the CCFL block  104 . 
         [0064]    In the embodiment of  FIG. 8  a photo-coupled TRIAC  106  is provided as the switch SW 1 . The photo-TRIAC  106  includes a light-sensitive TRIAC part  106   a  and a light emitting diode (LED)  106   b  (e.g., IR emitting diode) that is optically coupled to the TRIAC trigger layers instead of by way of direct trigger electrodes. The optical coupling of the light emitting diode  106   b  to the photosensitive part  106   a  is understood to be a high voltage isolation coupling. 
         [0065]    An ON/OFF signal is input to the LED  106   b  from an external control circuit to turn on or off the light-sensitive TRIAC part  106   a , thereby turning on or off the photo-TRIAC  106  such that the turn-on/turn-off state of the CCFL block  104  can be changed. 
         [0066]      FIG. 9  is a circuit diagram showing a structure in which two MOSFETs  107  are connected as shown to form the switch SW 1 . Here, each of MOSFETs  107  takes half the voltage stress when the primary side is in the open circuit state. Also, the control voltage applied to the gates of the MOSFETs  107  to switch them into the conductive state can be relatively low. In this case, an ON/OFF signal can be input to gate terminals of the FETs  107  from an external control circuit to turn on or off the FETs  107 , thereby changing the turn-on/turn-off state of the CCFL block  104 . 
         [0067]    Each switching operation of the TRIAC  105 , or the photo-TRIAC  106 , or the tandem FETs  107  shown in respective  FIGS. 7 to 9  and serving as the switch SW 1  is controlled by the ON/OFF control signal having a relatively low voltage and being well isolated from the high voltage side of the circuitry. In other words, in order to prevent high AC voltage from being applied to the primary winding of the isolation transformer  102 , the backlight unit  100  according to one embodiment of the present disclosure can employ a semiconductor switching device operable at low voltage as the switch SW 1 . 
         [0068]    Thus, since the backlight unit  100  can employ the semiconductor switching device operable at low voltage, the switch SW 1  can be realized in a small size, and low-voltage operation can be realized. In addition, a higher-speed switching operation can be realized as compared with a switching operation of a high voltage switch. Accordingly, the backlight unit  100  can perform a switch function (scanning control function or dynamic local dimming function for each block) at a high speed suitable for controlling the luminance of the displayed image portion of that backlighting block upon the turn-on/turn-off operation for each CCFL block. 
         [0069]    Hereinafter, a circuit configuration of a backlight unit  200  according to one embodiment of the present disclosure will be described in detail with reference to  FIG. 10 . 
         [0070]    Referring to  FIG. 10 , the backlight unit  200  includes an inverter circuit section  201 , a switch circuits section  202 , and a CCFL block groups section  203 . The inverter circuit section  201  includes a power transformer  211  to provide supply high frequency, high voltage AC signal to the switch circuits section  202 . The CCFL block groups section  203  includes respective CCFL blocks  203   a  to  203   f . Each of the CCFL blocks  203   a  to  203   f  includes three CCFLs. 
         [0071]    The switch circuits section  202  includes respective isolation transformers denoted as  221   a  to  221   f , corresponding switching transistors (or other switching elements)  222   a  to  222   f,  and condenser circuits  223   a  to  223   f , which correspond to the CCFL blocks  203   a  to  203   f  in number, and a control circuit  224  operatively coupled to the switching elements  222   a  to  222   f.    
         [0072]    Secondary windings of the isolation transformers  221   a  to  221   f  are connected between an output terminal of the power source transformer  211  and input terminals of the condenser circuits  223   a  to  223   f , respectively, in series. First ends of primary windings of the isolation transformers  221   a  to  221   f  are grounded at one end, and second ends of the primary windings of the isolation transformers  221   a  to  221   f  are connected to the corresponding switching transistors  222   a  to  222   f,  respectively. A base terminal of each of the illustrated bipolar switching transistors  222   a  to  222   f  is connected to the control circuit  224 . (But as mentioned, other forms of switching elements may be used for items  222   a  to  222   f .) The switching transistors  222   a  to  222   f  perform a switching operation by an ON/OFF signal input through the base terminal connected to the control circuit  224  so that the primary windings are shorted or opened. 
         [0073]    The condenser circuits  223   a  to  223   f  include a plurality of BCs to uniformly distribute the high frequency AC voltage signals which are output through the isolation transformers  221   a  to  221   f , to the plurality of CCFLs provided in the CCFL blocks  203   a  to  203   f.    
         [0074]    The control circuit  224  generates the ON/OFF signal used to time-division multiplex-wise control the turn-on/turn-off operation modes and phases of the CCFL blocks  203   a  to  203   f  based on a PWM (pulse width modulated) scan control signal provided from an external operation control circuit (e.g., microcontroller, not shown) for the backlight unit, and outputs the ON/OFF signal to the base terminal of each of the switching transistors  222   a  to  222   f.    
         [0075]    If the respective switching transistors  222   a  to  222   f  are in corresponding OFF states, the respective primary windings of the isolation transformers  221   a  to  221   f  attain an opened circuit state. On the other hand, if the switching transistors  222   a  to  222   f  are in an ON state, the corresponding primary windings of the isolation transformers  221   a  to  221   f  become shorted. Accordingly, the control circuit  224  can time-division wise control the ON/Off states of the individual CCFL blocks  203   a  to  203   f  by controlling ON/OFF operation time of the switching transistors  222   a  to  222   f  based on the PWM scan signals applied to respective input terminals of control circuit  224 . (In an alternate embodiment, the input terminals of control circuit  224  receive digital control signals indicated duty cycles to be attained for respective ones of the individual CCFL blocks  203   a  to  203   f  and the control circuit  224  generates corresponding PWM control signals for application to switching elements  222   a  to  222   f .) 
         [0076]    As described above, in the backlight units  100  and  200  according to one embodiment of the present disclosure, the primary windings (at the side of low-voltage) of the isolation transformers  102  and  221   a  to  221   f  are opened/shorted through the switching operation of the switches SW 1  and the switching transistors  222   a  to  222   f,  thereby allowing for duty cycle or other time-division controlling of the turn-on/turn-off operation time of the high frequency driven CCFL blocks  203   a  to  203   f  and thus controlling the apparent luminance of the respective CCFL blocks  203   a  to  203   f . In addition, an LC series resonant circuit is constructed by leakage inductance of the isolation transformers  102  and  221   a  to  221   f  and capacitance of a BC, and the CCFL blocks  104  and  203   a  to  203   f  are turned on through the series resonance of the LC series resonant circuit. Accordingly, the value of high AC voltage applied to the CCFL blocks  104  and  203   a  to  203   f  can be lowered by employing only a resistance component as a load in turning on the CCFL blocks  104  and  203   a  to  203   f.    
         [0077]    Accordingly, the voltage stress of a switch circuit can be reduced. In addition, the isolation transformers  102  and  221   a  to  221   f , the switch SW 1 , and the switching transistors  222   a  to  222   f  having a low voltage stress characteristic are used, so that small-size and low-price switch circuits having low power consumption can be realized. The cost of a backlight unit employing the switch circuit can be reduced. 
         [0078]    In particular, since high AC voltage is not directly applied to the switch SW 1  and more specifically, to the collectors or drains of the switching transistors  222   a  to  223   f  to open or short the corresponding primary windings of the isolation transformers  221   a  to  221   f , then semiconductor switching devices operating at low voltage can be used, a small-size and low-price backlight unit having low power consumption can be realized. 
         [0079]    Although the switching transformers  222   a  to  222   f  are used in the switching circuit  202  shown in  FIG. 10 , a semiconductor switching device such as the TRIAC  105 , the photo-TRIAC  106 , or the MOSFETs  107  can be used instead of the bipolar switching transistors  222   a  to  222   f  as shown in  FIGS. 7 to 9 . If the semiconductor switching device is employed when the backlight unit  200  according to one embodiment of the present disclosure is adapted to the LCD which will be described later, a function (scanning control function or dynamic local dimming function for each block) to switch the turn-on/turn-off state of CCFL blocks at a high speed in a block unit can be performed to represent the maximum brightness of a displayed image area according to the brightness of an input image for that area. Accordingly, the image quality and/or power consumption efficiency of the LCD can be improved. 
         [0080]      FIG. 11  is a circuit diagram showing a backlight unit  300  according to another embodiment of the present disclosure. 
         [0081]    Referring to  FIG. 11 , the backlight unit  300  includes an inverter circuit section  301 , a switch circuits section  302 , and a CCFL blocks group  303 . The CCFL blocks group  303  includes CCFL blocks  331   a  to  331   f . Each of the CCFL blocks  331   a  to  331   f  includes three CCFLs. A first phase or “normal”-phase high frequency high voltage AC signal is applied to the odd numbered CCFL blocks  331   a  to  331   c  (normal-phase CCFL blocks), and a differently phased, for example inverse-phase high frequency, high voltage AC signal is applied to the interdigitated and even numbered CCFL blocks  331   d  to  331   f  (inverse-phase CCFL blocks). 
         [0082]    The inverter circuit section  301  includes a normal-phase power outputting transformer  311  and an inverse-phase power outputting transformer  312 . The normal-phase power transformer  311  supplies the normal-phase high AC voltage signal to the odd-number wise ordered parts of the switch circuit  302 , and the inverse-phase power transformer  312  supplies the inverse-phase high AC voltage signal to the even-number number wise ordered parts of the switch circuit  302 . 
         [0083]    The switch circuits section  302  thus includes normal-phase isolation transformers  321   a  to  321   c , inverse-phase isolation transformers  321   d  to  321   f , normal-phase switching transistors  322   a  to  322   c , inverse-phase switching transistors  322   d  to  322   f,  condenser circuits  323   a  to  323   f , and a control circuit  324 . 
         [0084]    Secondary windings of the normal-phase isolation transformers  321   a  to  321   c  are connected between an output terminal of the normal-phase power transformer  311  and input terminals of the condenser circuits  323   a  to  323   c , respectively, in series. First ends of primary windings of the normal-phase isolation transformers  321   a  to  321   c  are grounded, and second ends of the primary windings are connected to the normal-phase switching transistors  322   a  to  322   c , respectively. 
         [0085]    Secondary windings of the inverse-phase isolation transformers  321   d  to  321   f  are connected between an output terminal of the inverse-phase power transformer  312  and input terminals of the condenser circuits  323   d  to  323   f , respectively, in series. First ends of primary windings of the inverse-phase isolation transformers  321   d  to  321   f  are grounded, and second ends of the primary windings are connected to the inverse-phase switching transistors  322   d  to  322   f,  respectively. 
         [0086]    A base terminal of each of the normal-phase switching transistors  322   a  to  322   c  is connected to the control circuit  324 . The normal-phase switching transistors  322   a  to  322   c  receive an ON/OFF signal for a normal-phase operation from the control circuit  324  through the base terminals (or alternatively gate electrodes) to perform the desired switching operations at appropriate time points, thereby shorting or opening the primary windings of the normal-phase isolation transformers  321   a  to  321   c.    
         [0087]    A base terminal of each of the inverse-phase switching transistors  322   d  to  322   f  is connected to the control circuit  324 . The inverse-phase switching transistors  322   d  to  322   f  receive an ON/OFF signal for an inverse-phase operation from the control circuit  324  through the base terminal to perform a switching operation, thereby shorting or opening the primary windings of the inverse-phase isolation transformers  321   d  to  321   f.    
         [0088]    The condenser circuits  323   a  to  323   f  include a plurality of BCs to uniformly distribute normal-phase high AC voltage, which is output from the normal-phase isolation transformers  321   a  to  321   c , and inverse-phase high AC voltage, which is output from the inverse-phase isolation transformers  321   d  to  321   f , to a plurality of CCFLs in the CCFL blocks  331   a  to  331   f.    
         [0089]    The control circuit  324  generates the ON/OFF signal used to time-division wise control the duty cycles and the turn on and off times the CCFL blocks  331   a  to  331   f  based on a PWM scan signal provided from an external operation control circuit (not shown) for a backlight unit, and outputs the ON/OFF signal to the base terminal, and outputs the ON/OFF signal to the base terminals of the normal-phase switching transistors  322   a  to  322   c  and the inverse-phase switching transistors  322   d  to  322   f.    
         [0090]    When the normal-phase switching transistors  322   a  to  322   c  are in an off state, the primary windings of the normal-phase isolation transformers  321   a  to  321   c  are opened. When the normal-phase switching transistors  322   a  to  322   c  are in an on state, the primary windings of the normal-phase isolation transformers  321   a  to  321   c  are shorted. Accordingly, the control circuit  324  controls an on/off operation time of the normal-phase switching transistors  322   a  to  322   c  based on the PWM scan signal to time-division control the turn-on/turn-off operation time of the CCFL blocks  331   a  to  331   c.    
         [0091]    When the inverse-phase switching transistors  322   d  to  322   f  are in the off state, the primary windings of the inverse-phase isolation transformers  321   d  to  321   f  are opened. When the inverse-phase switching transistors  322   d  to  322   f  are in the on state, the primary windings of the inverse-phase isolation transformers  321   d  to  321   f  are shorted. Accordingly, the control circuit  324  controls an on/off operation time of the inverse-phase switching transistors  322   d  to  322   f  based on the PWM scan signal to time-division control the turn-on/turn-off operation time of the CCFL blocks  331   d  to  331   f.    
         [0092]    Since the CCFL blocks  331   a  to  331   c  to receive normal-phase high AC voltage are alternately interposed with the CCFL blocks  331   d  to  331   f  to receive inverse-phase high AC voltage in the backlight unit  300  shown in  FIG. 11 , noise components between adjacent CCFL blocks can be offset with each other because one lamp will be receiving a positive going noise spike, if so present in the high voltage power signal and the next adjacent lamp will be receiving a negative going noise spike, if so present. Accordingly, the quality of a display image can be improved by employing out of phase lamp drive signals. 
         [0093]    Since the primary side switches are in the low voltage portions of the isolation transformers, accordingly, in the backlight unit  300  shown in  FIG. 11 , the voltage stress of each switch circuit can be reduced, and the normal-phase isolation transformers  321   a  to  321   c , the inverse-phase isolation transformers  321   d  to  321   f , the normal-phase switching transistors  322   a  to  322   c , and the inverse-phase switching transistors  322   d  to  322   f  having a low voltage stress characteristic can be used, so that small-size and low-price switch circuits having low power consumption can be realized. Accordingly, the power consumption, size, and price of a backlight unit employing the switch circuits can be also reduced. 
         [0094]      FIG. 12  is a circuit diagram showing a backlight unit  400  in which CCFLs of receiving normal-phase high AC voltage are alternately aligned with CCFLs of receiving inverse-phase high AC voltage. Moreover, the balancing condensers (BC&#39;s) in each lamp block are alternatively connected as shown. 
         [0095]    Referring to  FIG. 12 , the backlight unit  400  includes an inverter circuit  401 , a switch circuit  402 , and a CCFL block group  403 . The CCFL block group  403  includes CCFL blocks  431   a  to  431   f.  Each of the CCFL blocks  431   a  to  431  includes four CCFLs. In each of the CCFL blocks  431   a  to  431   f , half the CCFLs are connected to receive the normal-phase high AC voltage signal and the other half are connected to alternately receive the out of phase (e.g., inverse phase) high AC voltage signal. 
         [0096]    The high voltage lamps driving circuit  401  includes a normal-phase power transformer  411  and an inverse-phase power transformer  412 . The normal-phase power transformer  411  supplies normal-phase high AC voltage signal to the switch circuit  402 . 
         [0097]    The inverse-phase power transformer  412  supplies differently phased (e.g., inverse-phase) high AC voltage signal to the switch circuit  402 . 
         [0098]    The switch circuit  402  includes normal-phase isolation transformers  421   a  to  421   f , inverse-phase isolation transformers  423   a  to  423   f , normal-phase switching transistors  422   a  to  422   f,  inverse-phase switching transistors  424   a  to  424   f , condenser circuits  425   a  to  425   f , and a control circuit  426 . 
         [0099]    Secondary windings of the normal-phase isolation transformers  421   a  to  421   f  are connected between an output terminal of the normal-phase power transformer  411  and input terminals of the condenser circuits  425   a  to  425   f , respectively, in series. First ends of primary windings of the normal-phase isolation transformers  421   a  to  421   f  are grounded, and second ends of the primary windings are connected to the normal-phase switching transistors  422   a  to  422   f,  respectively. 
         [0100]    Secondary windings of the inverse-phase isolation transformers  423   a  to  423   f  are connected between an output terminal of the inverse-phase power transformer  412  and the input terminals of the condenser circuits  425   a  to  425   f , respectively, in series. First ends of primary windings of the inverse-phase isolation transformers  423   a  to  423   f  are grounded, and second ends of the primary windings are connected to the inverse-phase switching transistors  424   a  to  424   f , respectively. 
         [0101]    A base terminal of each of the normal-phase switching transistors  422   a  to  422   f  is connected to the control circuit  426 . The normal-phase switching transistors  422   a  to  422   f  receive an ON/OFF signal for a normal-phase operation from the control circuit  426  through the base terminal to perform a switching operation, thereby shorting or opening the primary windings of the normal-phase isolation transformers  421   a  to  421   f.    
         [0102]    A base terminal of each of the inverse-phase switching transistors  424   a  to  424   f  is connected to the control circuit  426 . The inverse-phase switching transistors  424   a  to  424   f  receive an ON/OFF signal for an inverse-phase operation from the control circuit  426  through the base terminal and perform a switching operation in response to the ON/OFF signal to short or open the primary windings of the inverse-phase isolation transformers  423   a  to  423   f.    
         [0103]    The condenser circuits  425   a  to  425   f  include a plurality of BCs to uniformly distribute normal-phase high AC voltage signal, which is output from the normal-phase isolation transformers  421   a  to  421   f , or the inverse-phase high AC voltage signal, which is output from the inverse-phase isolation transformers  423   a  to  423   f , to a plurality of CCFLs in the CCFL blocks  431   a  to  431   f.    
         [0104]    The control circuit  426  generates the ON/OFF signal used to time-division control time to turn on the CCFL blocks  431   a  to  431   f  based on a PWM scan signal provided from an external operation control circuit (not shown) for a backlight unit, and outputs the ON/OFF signal to the base terminals of the normal-phase switching transistors  422   a  to  422   f  and the inverse-phase switching transistors  424   a  to  424   f.    
         [0105]    When the normal-phase switching transistors  422   a  to  422   f  are in an off state, the primary windings of the normal-phase isolation transformers  421   a  to  421   f  are opened. When the normal-phase switching transistors  422   a  to  422   f  are in an on state, the primary windings of the normal-phase isolation transformers  421   a  to  421   f  are shorted. Accordingly, the control circuit  426  controls an on/off operation time of the normal-phase switching transistors  421   a  to  421   f  based on the PWM scan signal to time-division control the turn-on/turn-off time of the CCFLs that receive the normal-phase high AC voltage signal and are provided in the CCFL blocks  431   a  to  431   f.    
         [0106]    When the inverse-phase switching transistors  424   a  to  424   f  are in the off state, the primary windings of the inverse-phase isolation transformers  423   a  to  423   f  are opened. When the inverse-phase switching transistors  424   a  to  424   f  are in the on state, the primary windings of the inverse-phase isolation transformers  423   a  to  423   f  are shorted. Accordingly, the control circuit  426  controls an on/off operation time of the inverse-phase switching transistors  424   a  to  424   f  based on the PWM scan signal to time-division control the turn-on/turn-off operation time of the CCFLs that receive the inverse-phase high AC voltage signal and are provided in the CCFL blocks  431   a  to  431   f    
         [0107]    Since the backlight unit  400  shown in  FIG. 12  has a circuit configuration in which an even number of (e.g., four) CCFLs are provided in each of the CCFL blocks  431   a  to  431   f  and these are alternatively connected to alternately receive the normal-phase high AC voltage signal and the differently phased (e.g., inverse-phase) high AC voltage signal, noise components between adjacent CCFL blocks may be offset with each other. Accordingly, the quality of a display image can be improved. 
         [0108]    Accordingly, in the backlight unit  400  shown in  FIG. 12 , the voltage stress of a switch circuit can be reduced, and the normal-phase isolation transformers  421   a  to  421   f , the inverse-phase isolation transformers  423   a  to  423   f , the normal-phase switching transistors  422   a  to  422   f,  and the inverse-phase switching transistors  424   a  to  424   f  having a low voltage stress characteristic can be used, so that small-size and low-price switch circuits having low power consumption can be realized. Accordingly, the power consumption, size, and price of a backlight unit employing the switch circuits can be also reduced. 
         [0109]    Although the switch circuits  302  and  402  shown in  FIGS. 11 and 12  employ the switching transistors  322   a  to  322   f,    422   a  to  422   f,  and  424   a  to  424   f , semiconductor switching devices such as the TRIAC  105 , the photo-sensitive TRIAC  106 , and the MOSFET  107  shown in  FIGS. 7 to 9  can be used. If backlight units  300  and  400  employing the semiconductor switching device are adapted to the LCD, the turn-on/turn-off state of CCFL blocks can be switched at a high speed in a block unit suitably for the brightness of an input image, thereby improving image quality. 
         [0110]      FIG. 13  is a circuit diagram showing a backlight unit  500  according to another embodiment of the present disclosure. 
         [0111]    Referring to  FIG. 13 , the backlight unit  500  includes an inverter circuit  501 , a switch circuit  502 , and a CCFL block group  503 . 
         [0112]    The inverter circuit  501  includes an AC power source  511  to provide supply voltage to the switch circuit  502 . The CCFL block group  503  includes CCFL blocks  531   a  to  531   f . Each of the CCFL blocks  531   a  to  531   f  may include an even number of CCFL&#39;s (e.g., two CCFLs). 
         [0113]    The switch circuit  502  includes isolation transformers  521   a  to  521   f , semiconductor switch circuits  522   a  to  522   f,  and condenser circuits  523   a  to  523   f  that correspond to the CCFL blocks  531   a  to  531   f  in number. 
         [0114]    A secondary winding of each of the isolation transformers  521   a  to  521   f  is connected between an output terminal of the AC power source  511  and an input terminal of each of the condenser circuits  523   a  to  523   f , in series. Both ends of a primary winding of each of the isolation transformers  521   a  to  521   f  are connected to each of the semiconductor switch circuits  522   a  to  522   f.  Each of the semiconductor switch circuits  522   a  to  522   f  includes two MOSFETs and two kickback current routing diodes, and base terminals of the two MOSFETs are connected to an input line  524  for a block control signal. The semiconductor switch circuits  522   a  to  522   f  are provided with the input line  524  connected to an external control circuit (not shown). Each of the semiconductor switch circuits  522   a  to  522   f  receives a control signal (ON/OFF signal) for each block from the input line  524  through the base terminal to perform a switching operation, so that the primary winding is shorted or opened. 
         [0115]    The condenser circuits  523   a  to  523   f  include a plurality of BCs to uniformly distribute high AC voltage, which is output from the isolation transformers  521   a  to  521   f , to a plurality of CCFLs provided in the CCFL blocks  531   a  to  531   f.    
         [0116]    When the semiconductor switch circuits  522   a  to  522   f  are in an off state, primary windings of the isolation transformers  521   a  to  521   f  are opened. When the semiconductor switch circuits  522   a  to  522   f  are in an on state, the primary windings of the isolation transformers  521   a  to  521   f  are shorted. The on/off operation time of the semiconductor switch circuits  522   a  to  522   f  is controlled based on the control signal (ON/OFF signal) for each block, thereby time-division control the turn-on/turn-off operation time of the CCFL blocks  531   a  to  531   f.    
         [0117]    Accordingly, the voltage stress of a switch circuit can be reduced, and the isolation transformers  521   a  to  521   f  and the semiconductor switch circuits  522   a  to  522   f  having a low voltage stress characteristic can be used. Accordingly, the cost of a backlight unit employing the switch circuit can be reduced. In particular, high AC voltage, which is applied to CCFL blocks, is not applied to the semiconductor switch circuits  522   a  to  521   f  to switch the open/short state of the primary windings of the isolation transformers  521   a  to  521   f . Accordingly, since the semiconductor switching device to operate at low voltage can be used, a small-size and low-price backlight unit having low power consumption can be realized. 
         [0118]    Meanwhile, although MOSFETs are used in the semiconductor switch circuits  522   a  to  522   f  of the switch circuit  502  shown in  FIG. 13 , semiconductor switching devices such as the TRIAC  105  or the photo-TRIAC  106  may be used as shown in  FIGS. 7 and 8 . Such a semiconductor switching device is employed, so that a switching operation (scanning control function or local dimming for each block) to switch the turn-on/turn-off operation state of CCFL blocks at a high speed in a block unit suitably for the brightness of an input image can be adapted to an LCD which will be described later. Accordingly, the image quality can be improved. 
         [0119]      FIG. 14  is a circuit diagram showing a backlight unit  600  according to another embodiment of to the present disclosure. The present embodiment is characterized in that a balance coil is used instead of a condenser circuit including a BC. 
         [0120]    Referring to  FIG. 14 , the backlight unit  600  includes an inverter circuit  601 , a switch circuit  602 , and a CCFL block group  603 . 
         [0121]    The inverter circuit  601  includes an AC power source  611  to provide supply voltage to the switch circuit  602 . The CCFL block group  603  includes CCFL blocks  631   a  to  631   f.  Each of the CCFL blocks  631  to  631   f  includes two CCFLs. 
         [0122]    The switch circuit  602  includes isolation transformers  621   a  to  621   f  and semiconductor switch circuits  622   a  to  622   f  which correspond to the CCFL blocks  631   a  to  631   f  in number. 
         [0123]    A secondary winding of each of the isolation transformers  621   a  to  621   f  is divided (e.g., center tapped) in each CCFL provided in the CCFL blocks  631   a  to  631   f  to thereby construct a balanced coil. The central tap point of the secondary winding of each of the isolation transformers  621   a  to  621   f  is connected to an output terminal of the AC power source  611 , and the opposed non-center ends of the secondary windings are connected to a respective one or more CCFLs. In addition, both ends of a primary winding of each of the isolation transformers  621   a  to  621   f  are connected to each of the semiconductor switch circuits  622   a  to  622   f.  Each of the semiconductor circuits  622   a  to  622   f  includes two FETs and two diodes, and base terminals of the two FETs are connected to an input line  624  through which a control signal for each block is input. The semiconductor switch circuits  622   a  to  622   f  receive a control signal (ON/OFF signal) for each block through the base terminal connected to the input line  624  to perform a switching operation so that the primary winding is shorted or opened. 
         [0124]    When the semiconductor switch circuits  622   a  to  622   f  are in an off state, the primary windings of the isolation transformers  621   a  to  621   f  are opened. When the semiconductor switch circuits  622   a  to  622   f  are in an on state, the primary windings of the isolation transformers  621   a  to  621   f  are shorted. The on/off operation time of the semiconductor switch circuits  622   a  to  622   f  is controlled based on the control signal(ON/OFF signal) for each block, so that the turn-on/turn-off operation time of the CCFL blocks  631   a  to  631   f  can be time-division controlled. 
         [0125]    Accordingly, the voltage stress of the switch circuit can be improved, and isolation transformers  621   a  to  621   f  and the semiconductor switch circuits  622   a  to  622   f  having a low voltage stress characteristic can be used, so that a small-size and low-price switch circuit having low power consumption can be realized. Accordingly, the power consumption, size, and price of a backlight unit employing the switch circuit can be also reduced. In particular, since high AC voltage, which is applied to CCFL blocks, is not applied to the semiconductor switch circuits  622   a  to  622   f  to switch the open/short state of the primary winding of the isolation transformers  621   a  to  621   f , a semiconductor switching device operating at low voltage can be used, thereby contributing to the reduction of the power consumption, size, and price of the backlight unit. Further, the secondary winding of the isolation transformers  621   a  to  621   f  is divided to construct a balanced coil structure, so that a distribution-balancing condenser can be omitted, and the price of the inverter driving circuit can be more reduced. Since the resonance frequency is adjusted by using only an inductance component and without concern for the capacitance of balancing condensers (not present), impedance matching with CCFLs can be more easily adjusted so that the turn-on/turn-off operation can be easily controlled. 
         [0126]    Although FETs are used in the switching transformers  622   a  to  622   f  of the switching circuit  602  a shown in  FIG. 14 , a semiconductor switching device such as the TRIAC  105 , or the photo-TRIAC  106  can be used as shown in  FIGS. 7 to 9 . Such a semiconductor switching device is employed, so that a switching operation (scanning control function or local dimming for each block) to switch the turn-on/turn-off operation state of CCFL blocks at a high speed in a block unit suitably for the brightness of an input image can be adapted to an LCD which will be described later. Accordingly, the image quality can be improved. 
         [0127]      FIG. 15  is a circuit diagram showing a backlight unit  700  according to another embodiment of the present disclosure. The illustrated embodiment is again characterized in that a balance coil structure is used instead of a condenser circuit including a BC. Moreover, independent and optionally differently phased signal sources  711 ,  712  are used to power each CCFL block (e.g.,  731   a ). 
         [0128]    Referring to  FIG. 15 , the backlight unit  700  includes an inverter circuit  701 , a switch circuit  702 , and a CCFL block group  703 . 
         [0129]    The high voltage lamps driving circuit  701  includes a normal-phase AC power source  711  and a differently phased (e.g., inverse-phased) power source  712 , and normal-phase supply voltage and inverse-phase supply voltage signals are supplied to the switch circuit  702 . The CCFL block group  703  includes CCFL blocks  731   a  to  731   f . Each of the CCFL blocks  731   a  to  731   f  includes an even number (e.g., two) of CCFLs. 
         [0130]    The switch circuit  702  includes isolation transformers  721   a  to  721   f  and semiconductor switch circuits  722   a  to  722   f  that correspond to the CCFL blocks  731   a  to  731   f  in number. 
         [0131]    A secondary windings of the isolation transformers  721   a  to  721   f  are each divided in each CCFL block among CCFL blocks  731   a  to  731   f  to thereby construct a balanced coil structure. Inner ends of the divided secondary winding of each of the isolation transformers  721   a  to  721   f  are connected to output terminals of the normal-phase power source  711  and the inverse-phase power source  712 , respectively. Outer ends of the secondary winding of each of the isolation transformers  721   a  to  721   f  are connected to the CCFLs. Both ends of a primary winding are connected to each of the semiconductor switch circuits  722   a  to  722   f.  Each of the semiconductor switch circuits  722   a  to  722   f  includes two FETs and two diodes, and base terminals of two FETs are connected to an input line  724  through which a control signal for each block is input. The semiconductor switch circuits  722   a  to  722   f  receive a control signal (ON/OFF signal) for each block through the base terminals connected to the input line  724  to perform a switching operation to short or open the primary windings. 
         [0132]    When the semiconductor switch circuits  722   a  to  722   f  are in an off state, the primary windings of the isolation transformers  721   a  to  721   f  are opened. When the semiconductor switch circuits  722   a  to  722   f  are in an on state, the primary windings of the isolation transformers  721   a  to  721   f  are shorted. Accordingly, an on/off operation time of the semiconductor switch circuit  722   a  to  722   f  is controlled based on the control signal (ON/OFF signal) for each block, so that the turn-on/turn-off operation time of each of the CCFL blocks  731   a  to  731   f  can be time-division controlled. 
         [0133]    Accordingly, the voltage stress of the switch circuit can be reduced, and isolation transformers  721   a  to  721   f  and the semiconductor switch circuits  722   a  to  722   f  having a low voltage stress characteristic can be used, so that a small-size and low-price switch circuit having low power consumption can be realized. Accordingly, the power consumption, size, and price of a backlight circuit employing the above switch circuit can be also reduced. In particular, since high AC voltage, which is applied to CCFL blocks, is not applied to the semiconductor switch circuits  722   a  to  722   f  to switch the open/short state of the primary windings of the isolation transformers  721   a  to  721   f , a semiconductor switching device operating at low voltage can be used, thereby contributing to the reduction of the power consumption, size, and price of the backlight unit. Further, the secondary winding of the isolation transformers  721   a  to  721   f  is divided to construct a balance coil, so that a corresponding balance condenser (BC) can be omitted. Accordingly, the price of the inverter circuit can be more reduced. Since the resonance frequency is adjusted by using only an inductance component, impedance matching with CCFLs can be easily adjusted so that the turn-on/turn-off operation can be easily controlled. 
         [0134]    In the inverter circuit  702  shown in  FIG. 15 , although MOSFETs are used in the semiconductor switch circuits  722   a  to  722   f,  a semiconductor switching device such as the TRIAC  105  or the photo-TRIAC  106  can be used as shown in  FIGS. 7 to 9 . Such a semiconductor switching device is employed, so that a switching operation (scanning control function or local dimming for each block) to switch the turn-on/turn-off operation state of CCFL blocks at a high speed in a block unit suitably for the brightness of an input image can be adapted to an LCD. Accordingly, the image quality can be improved. 
         [0135]    The secondary winding of the isolation transformers  621   a  to  621   f  is evenly divided to construct a balance coil as shown in  FIG. 14 , so that a JIN type transformer normally used as a conventional balance coil can be removed. Accordingly, an isolation transformer performing both functions of an inverter transformer and a balance coil is used, thereby more reducing the size and the price of the inverter circuit. 
         [0136]      FIG. 16  is a block diagram showing an LCD  900  including a backlight unit  930  having the structure similar to that of the backlight unit  200  shown in  FIG. 10 . 
         [0137]    As shown in  FIG. 16 , the LCD  900  includes an AC/DC power supply  910 , an LCD module  920 , and the backlight unit  930   
         [0138]    The AC/DC power supply  910  includes an AC power plug  911 , an AC/DC rectifier  912 , and a first DC-to-DC converter  913 . The AC/DC power supply  910  converts external commercial AC supply voltage ( 100 V or  240 V) into DC supply voltage and outputs the DC supply voltage to the LCD module  920  by way of the first DC-to-DC converter  913 . 
         [0139]    The LCD module  920  includes a second DC/DC converter  921 , a common electrode (Vcom) voltage generator  922 , a gamma (γ) voltage generator  923 , an LCD panel  924 , and the backlight unit  930  to display images corresponding to image data provided from an external graphic controller (not shown). The LCD panel  924  includes a plurality of liquid crystal devices connected to each other at a region in which a plurality of data lines and a plurality of gate lines extending from data and gate drivers, respectively, cross each other. The liquid crystal devices are distributed in a plurality of display regions to control the gray scale of each display region. 
         [0140]    The Vcom generator  922  generates common electrode voltage Vcom based on level-converted DC voltage supplied from the second DC/DC converter  921  and outputs the common electrode voltage Vcom to the LCD panel  924 . The γ voltage generator  923  generates γ voltage Vdd based on the level-converted DC voltage in the DC/DC converter  921  to supply the γ voltage to the LCD panel  924 . Although, the Vcom generator  922  and the γ voltage generator  923  are separated from the LCD panel  924  as shown in  FIG. 16 , the Vcom generator  922  and the γ voltage generator  923  may be embedded in the LCD panel  924 . 
         [0141]    The backlight unit  930  includes an inverter section  931  and a backlight section  932 . The inverter section  931  includes the isolation transformers  221   a  to  221   f , the switching transistors  222   a  to  222   f,  and the optional condenser circuits  223   a  to  223   f  provided in the switch circuit  202  such as shown in  FIG. 10 . The backlight section  932  includes the CCFL block group  203  shown in  FIG. 10 . A plurality of CCFL blocks in the CCFL block group  203  correspond to the plural display regions, respectively. The turn-on/turn-off operation time of the CCFL blocks is time-division controlled corresponding to the brightness of each display region when an input image is displayed on the LCD panel  924 . 
         [0142]    Since the inverter section  931  provided in the backlight unit  930  of the LCD  900  includes the isolation transformers  221   a  to  221   f , the switching transistors  222   a  to  222   f,  and the condenser circuits  223   a  to  223   f , the voltage stress of the switch circuit can be reduced, and the isolation transformers  221   a  to  221   f  and the switching transistors  222   a  to  222   f  having a low voltage stress characteristic can be used. Therefore, a small-size and low-price switch circuit having low power consumption can be realized. As a result, the power consumption and cost of a backlight circuit employing the above switch circuit can be also reduced. In addition, a switching function (scanning control function or local dimming function for each block) to switch the turn-on/turn-off operation state of the CCFL blocks at a high speed can be used to control the brightness of a display image according to the brightness of an input image, so that the image quality of the LCD  900  can be improved. Meanwhile, the AC/DC power supply  910  may be embedded in the LCD module  920 . 
         [0143]      FIG. 17  is an exploded perspective view showing an assembly of LCD  1000  having the structure similar to that of the LCD  900  shown in  FIG. 16 . 
         [0144]    As shown in  FIG. 17 , the LCD  1000  includes a backlight assembly  1010 , a display unit  1070 , and a container  1080 . 
         [0145]    The display unit  1070  includes a liquid crystal display panel  1071  to display an image and a data printed circuit  1072  and a gate printed circuit  1073  to output a driving signal used to drive the liquid crystal display panel  1071 . The data and gate printed circuits  1072  and  1073  are electrically connected with the liquid crystal display panel  1071  through a data tape carrier package (TCP)  1074  and a gate TCP  1075 . 
         [0146]    The liquid crystal display panel  1071  includes a first substrate  1076 , a second substrate  1077  opposite to the first substrate  1076 , and a liquid crystal  1078  interposed between the first and second substrates  1076  and  1077 . 
         [0147]    The first substrate  1076  may be a transparent glass substrate in which TFTs (not shown) serving as a switching device are provided in the form of a matrix. Data and gate lines are connected to source and gate terminals of each TFT, and a transparent electrode (not shown) including transparent conductive material is formed at a drain terminal 
         [0148]    The second substrate  1077  may be a substrate in which RGB pixels (not shown) are formed through a thin film process. The second substrate  1077  is provided thereon with a common electrode (not shown) including transparent conductive material. 
         [0149]    The container  1080  includes a bottom surface  1081  and a sidewall  1082  formed along the edge of the bottom surface  1081  to form a receiving space. The container  1080  fixes the backlight assembly  1010  and the liquid crystal display panel  1071  to prevent the backlight assembly  1010  and the liquid crystal display panel  1071  from moving. 
         [0150]    The bottom surface  1081  has an area sufficient to receive the backlight assembly  1010  and has configuration corresponding to that of the backlight assembly  1010 . According to the present embodiment, the bottom surface  1081  and the backlight assembly  1010  have a rectangular plate shape. The sidewall  1082  approximately perpendicularly extends from the edge of the bottom surface  1081  such that the backlight assembly  1010  does not deviate out of the container  1080 . 
         [0151]    According to the present embodiment, the LCD  1000  further includes an inverter circuit  1060  and a top chassis  1090 . 
         [0152]    The inverter circuit  1060  is disposed outside of the container  1080  to generate high voltage discharge signals used to drive the lamps of the backlight assembly  1010 . The discharge voltage generated from the inverter circuit  1060  is applied to the backlight assembly  1010  through first and second power lines  1063  and  1064 . The first and second power lines  1063  and  1064  may be connected with first and second electrodes  1040   a  and  1040   b , which are formed at both side portions of the backlight assembly  1010 , directly or by using another part (not shown). In addition, the switch circuit  202  including the isolation transformers  221   a  to  221   f , the switching transistors  222   a  to  222   f,  and the condenser circuits  223   a  to  223   f  may be embedded in the inverter circuit  1060 . 
         [0153]    The top chassis  1090  is coupled with the container  1080  while surrounding the edge of the liquid crystal display panel  1071 . The top chassis  1090  can protect the liquid crystal display panel  1071  from external shock, and prevent the liquid crystal display panel  1071  from deviating from the container  1080 . 
         [0154]    The LCD  1000  may further include at least one optical sheet  1095  to improve the characteristic of light output from the backlight assembly  1010 . The optical sheet  1095  may include a diffusion sheet to diffuse light or a prism sheet to concentrate light. 
         [0155]    Accordingly, when the inverter circuit  1060 , which performs a scanning control function for the turning-on operation of a CCFLA block group or a control function for the turn-on/turn-off operation time of each CCFL block by shorting or opening the primary windings of the isolation transformers through the ON/OFF operation of switching transistors, is adapted for an LCD including a power supplying inverter, the voltage stress of the inverter circuit  1060  can be reduced, and the low power consumption, small-size, and low price of the inverter circuit  1060  can be realized. In addition, the above-described backlight unit  100 ,  200 , or  300  is adapted to the LCD  1000 , so that a switching function (scanning control function or local dimming function for each block) to switch the turn-on/turn-off operation state of the CCFL blocks at a high speed in a block unit can be performed in order to control the brightness of a display image according to the brightness of an input image. Accordingly, image quality can be improved. 
         [0156]    According to the embodiments of the present disclosure, although the inverter circuit is separated from the switch circuit, the inverter circuit may alternatively be integrated with the switch circuit. 
         [0157]    Although exemplary embodiments of the present disclosure have been described, it is understood that the present teachings should not be limited to these exemplary embodiments but various changes and modifications can be made by one ordinary skilled in the art in view of the foregoing and within the spirit and scope of the present teachings.