Abstract:
A backlight driving system is provided for a liquid crystal display device. The backlight driving system comprises at least one backlight having at least one terminal, an inverter and at least one transformer. The inverter supplies a voltage to the backlight and has first and second output terminals. The transformer has a first input coil and a second input coil that are connected to the first and the second output terminals of the inverter. The transformer transforms the voltage outputted from the inverter and applies a transformed voltage to the backlight via the terminal of the backlight.

Description:
PRIORITY CLAIM  
       [0001]     This application claims the benefit of Korean Application No. P2004-17365, filed on Mar. 15, 2004. The disclosure of the above application is incorporated herein by reference.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The invention relates to a liquid crystal display device, and more particularly, to a backlight driving system for a liquid crystal display device.  
         [0004]     2. Related Art  
         [0005]     Generally, display devices are compact and lightweight. Although Cathode Ray Tubes (“CRT”) have been widely used for television monitors, a measuring system and an information terminal, they do not provide a compact and light display device due to their inherent size and weight. Accordingly, CRTs have been replaced by other display devices such as a liquid crystal display (“LCD”) device, a plasma display panel (“PDP”) and an electroluminescence display (“ELD”) device. Among those display devices, LCD devices use an electric field optical effect and can provide advantages such as low power consumption and a slim, lightweight structure. As a result, applications of LCD devices range from monitors for personal computers, including desktop and laptop computers, to large size display devices.  
         [0006]     Some LCD devices control light transmittance from ambient light to display images. Others use an additional light source, such as a backlight unit, in an LCD panel.  FIG. 1  illustrates a circuit diagram of a backlight driving system  1  for a LCD device. Referring to  FIG. 1 , a backlight is a lamp  10  that emits light to a liquid crystal display panel (not shown). The lamp  10  may be a cold cathode fluorescent lamp (CCFL). The backlight driving system includes a first inverter  11 , a second inverter  12 , a first transformer  13  and a second transformer  14 . The first inverter  11  outputs a driving voltage to a first terminal  2  of the lamp  10  in accordance with a control signal of a timing controller  15 . Likewise, a second inverter  12  outputs a driving voltage to a second terminal  4  of the lamp  10  in accordance with a control signal of the timing controller  15 . Then, the first transformer  13  transforms an output voltage of the first inverter  11  and supplies a transformed output to the first terminal  2  of the lamp  10 . In the same manner, the second transformer  14  transforms an output voltage of the second inverter  12  and supplies a transformed output to the second terminal  4  of the lamp  10 . Each input coil  5 ,  5 ′ of the first and the second transformers  13  and  14  is connected to output terminals  6 ,  8 ,  6 ′,  8 ′ of the first and second inverters  11  and  12 , respectively. Each output coil  7 ,  7 ′ of the first and the second transformers  13  and  14  are connected to the first terminal  2  and the second terminal  4  of the lamp  10 .  
         [0007]     The first inverter  11  includes a first transistor, a second transistor, a third transistor, and a fourth transistor M 1 , M 2 , M 3 , and M 4 . The third transistor M 3  and the first transistor M 1  are connected in series between a voltage terminal (VCC) and a ground terminal (GND). The fourth transistor M 4  and the second transistor M 2  are connected in series between the voltage terminal (VCC) and the ground terminal (GND). The first output terminal  6  is formed between the third transistor M 3  and the first transistor M 1 , and the second output terminal  8  is formed between the fourth transistor M 4  and the second transistor M 2 . Thus, the first and second output terminals  6 ,  8  are each connected to the input coil  5  of the first transformer  13 .  
         [0008]     The second inverter  12  has the same structure as the first inverter  11  as described above. Specifically, the first output terminal  6 ′ is formed between the third transistor M 3  and the first transistor M 1 , and the second output terminal  8 ′ is formed between the fourth transistor M 4  and the second transistor M 2 . Thus, the first and second output terminals  6 ′,  8 ′ are each connected to the input coil  5 ′ of the second transformer  14 .  
         [0009]     A dot (•) marked on the input coils  5 ,  5 ′ of the transformers  13 ,  14  indicates a starting point of the input coil  5 ,  5 ′. Volts Alternating Current (“VAC”) is a sine wave that is outputted from the first and the second transformers  13 ,  14 . A VAC outputted from the second transformer  14  has an inverted phase from a VAC outputted from the first transformer  13 .  
         [0010]     The backlight driving system  1  described above has the following disadvantages. The system  1  requires the first inverter  11 , the second inverter  12 , the first transformer  13  and a second transformer  14  to supply a desired voltage to the first and the second terminals  2 ,  4  of the lamp  10 . Accordingly, the system  1  is large in size and the power consumption increases. Also, fabrication cost substantially increases. In addition, due to a difference in impedance generated between each load of the first inverter  11 ⇄first transformer  13 ⇄lamp  10  and the second inverter  12 ⇄second transformer  14 ⇄lamp  10 , non-uniform voltage may be transmitted to each end terminal  2 ,  4  of the lamp  10 . This non-uniform voltage reduces product reliability.  
         [0011]     Use of only one inverter and one transformer may not provide the desired uniformity or equally divide and output the voltage. This, a single inverter/transformer backlight driving system provides non-uniform and unequal voltages that may be transmitted to each end terminal of a lamp. This non-uniform and unequal voltage results in non-uniform brightness of the lamp.  
       SUMMARY  
       [0012]     A backlight driving system is provided for a liquid crystal display device that includes a plurality of lamps, an inverter and first and second transformers. The lamps have a first terminal and a second terminal. The inverter outputs a voltage to be supplied to the plurality of lamps and has first and second output terminals. The first and the second transformers have first and second input coils connected to the first and the second output terminals of the inverter, respectively. The first and the second transformers transform a voltage outputted from the inverter and apply a transformed voltage to at least the first terminal of each lamp. A controller outputs control signals for controlling the inverter.  
         [0013]     In one embodiment, each first input coil of the first transformer and the second transformer may be formed by a first wire, and each second input coil of the first transformer and the second transformer may be formed by a second wire.  
         [0014]     A backlight driving system may further include a first common electrode line commonly connecting the first terminals of each lamp, a second common electrode line commonly connecting the second terminals of each lamp, and a plurality of capacitors connected between the first and the second common electrode lines and each lamp. In one embodiment, an output of the first transformer may be connected to the first common electrode line, and an output of the second transformer may be connected to the second common electrode line. Alternatively or additionally, both outputs of the first transformer and the second transformer may be connected to the first common electrode, and the second common electrode line may be grounded. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views.  
         [0016]      FIG. 1  illustrates a circuit diagram of a related art backlight driving system for a liquid crystal display device;  
         [0017]      FIG. 2  illustrates a circuit diagram of a first embodiment of a backlight driving system; and  
         [0018]      FIG. 3  illustrates a circuit diagram of a second embodiment of a backlight driving system. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0019]      FIG. 2  illustrates a circuit diagram of a backlight driving system  30  according to a first embodiment. In the first embodiment, the backlight driving system  30  drives a backlight by supplying a high voltage at each end terminal of a lamp unit  20 . The lamp unit  20  includes a plurality of lamps  28  aligned in one direction. An inverter  21  outputs a voltage for turning on the plurality of lamps  28  in accordance with a control signal. First and second transformers  22  and  23  have first and second input coils  32 ,  32 ′ and first and second output coils  34 ,  34 ′. The first and second input coils  32 ,  32 ′ are wound between the first and second output terminals A and B to cross one another. By using this structure, the transformers  22 ,  23  transform the voltage outputted from the inverter  21  and supply a transformed voltage to first and second end terminals  36 ,  38  of the lamp unit  20 . A control unit  24  outputs control signals for controlling the inverter  21 .  
         [0020]     The lamp unit  20  includes a first common electrode line  25   a  commonly connecting a first end terminal  36  of the plurality of lamps  28  and a second common electrode line  25   b  commonly connecting a second end terminal  38  of the plurality of lamps  28 . A plurality of first capacitors  26  are connected between the first common electrode line  25   a  and the first end terminal  36  of lamps  28 , and a plurality of second capacitors  27  are connected between the second common electrode line  25   b  and the second end terminal  38  of the plurality of lamps  28 . The lamp  28  used here may be a cold cathode fluorescent lamp (CCFL). Alternatively or additionally, an external electrode fluorescent lamp (EEFL) having an electrode on each external end of a tube may be used for the lamp  28 .  
         [0021]     The inverter  21  includes first, second, third, and fourth transistors M 1 , M 2 , M 3 , and M 4 . The third transistor M 3  and the first transistor Ml are connected in series between a voltage terminal (VCC) and a ground terminal (VSS). Likewise, the fourth transistor M 4  and the second transistor M 2  are connected in series between the voltage terminal (VCC) and the ground terminal (VSS). A first output terminal “A” outputs a first output signal and is connected between the third transistor M 3  and the first transistor M 1 . A second output terminal “B” outputs a second output signal and is connected between the fourth transistor M 4  and the second transistor M 2 . Tank voltage, which is a generally oscillating voltage, is outputted from the first and the second output terminals A and B as shown in  FIG. 2 . The transistors M 1 ˜M 4  may be MOS transistors. For example, the first and second transistors M 1 , M 2  are formed of NMOS transistors, and the third and fourth transistors M 3 , M 4  are formed of PMOS transistors.  
         [0022]     The control unit  24  outputs first, second, third and fourth output signals IN 1 , IN 2 , IN 3 , and IN 4  in order to control the first, second, third, and fourth transistors M 1 ˜M 4  of the inverter  21 , respectively. Volts Alternating Current (VAC) is a sine wave that is outputted from an output coil  40  of the first transformer  22 . As shown in  FIG. 2 , a VAC having an inverted phase is outputted from an output coil  40 ′ of the second transformer  23 . The output coil  40  of the first transformer  22  is connected to the first common electrode line  25   a , and the output coil  40 ′ of the second transformer  23  is connected to the second common electrode line  25   b.    
         [0023]     The first and second input coils  32 ,  34 ,  32 ′,  34 ′ of the first and second transformers  22  and  23  are wound between the first output terminal A and the second output terminal B to cross a first wire W 1  and a second wire W 2 . Specifically, the first wire W 1  extends from the first output terminal A to the second output terminal B of the inverter  21 . The first wire W 1  is connected to the first output terminal A, the first input coil  32  of the first transformer  22 , the first input coil  32 ′ of the second transformer  23 , and the second output terminal B of the inverter  21  (i.e., the first input coil  32  of the first transformer  22 →the first input coil  32 ′ of the second transformer  23 →the second output terminal B of the inverter  21 ). At this point, the first wire W 1  is wound so that the direction of the first input coil  32  of the first transformer  22  and the direction of the first input coil  32 ′ of the second transformer  23  become opposite to one another. A dot (•) marked on each first input coil  32 ,  32 ′ of the first and second transformers  22  and  23  indicates a starting point of winding of the coil  32 ,  32 ′.  
         [0024]     In addition to the first wire W 1 , the second wire W 2  extends from the first output terminal A to the second output terminal B of the inverter  21 . The second wire W 2  is connected to the first output terminal A, the second input coil  34 ′ of the second transformer  23 , the second input coil  34  of the first transformer  22 , and the second output terminal B of the inverter  21  (i.e., the second input coil  34 ′ of the second transformer  23 →the second input coil  34  of the first transformer  22 →the second output terminal B of the inverter  21 ). A dot (•) marked on the second input coil  34 ,  34 ′ of the first and second transformers  22  and  23  indicates a starting point of winding of the coil  34 ,  34 ′.  
         [0025]     As described above, the first input coils  32 ,  32 ′ share the first wire W 1 , and the second input coils  34 ,  34 ′ share the second wire W 2 . As a result, each transformer shares a uniform and equal voltage. Even if a first current transmitted to the first input coils  32 ,  32 ′ is not precisely half of the entire current outputted from the inverter  21 , a second current transmitted to the first and second transformers  22  and  23  through the second input coils  34 ,  34 ′ can compensate the first current. Accordingly, divided currents are uniform and equal, and the first and second transformers  22  and  23  can output uniform signals.  
         [0026]      FIG. 3  illustrates a circuit diagram of a backlight driving system  40  according to a second embodiment. In the second embodiment, the backlight driving system  40  drives a backlight by applying a high voltage to one end terminal of a lamp and a low voltage to the other end terminal of the lamp. As shown in  FIG. 3 , applying a voltage to each end terminal of the lamp through first and second transformers and winding first and second wires W 1  and W 2  to form input coils of the transformers are different from the first embodiment.  
         [0027]     Referring to  FIG. 3 , a lamp unit  20  has a plurality of lamps  28  aligned in one direction. An inverter  21  outputs a voltage for turning on the plurality of lamps  28  in accordance with a control signal. First and second transformers  22 ,  23  have first and second input coils  32 ,  32 ′,  34 ,  34 ′ and first and second output coils  40 ,  40 ′. The first and the second input coils  32 ,  32 ′,  34 ,  34 ′ are connected to the first and second output terminals A and B. The transformers  22 ,  23  transform the voltage outputted from the inverter  21  and supply a transformed voltage to first and second end terminals  36 ,  38  of the lamp unit  20 . A control unit  24  outputs control signals for controlling the inverter  21 .  
         [0028]     The lamp unit  20  includes a first common electrode line  25   a  commonly connecting the first end terminal  36  of the plurality of lamps  28  and a second common electrode line  25   b  commonly connecting the second end terminal  38  of the plurality of lamps  28 . A plurality of first capacitors  26  are connected between the first common electrode line  25   a  and the first end terminal  36  of each lamp  28 , and a plurality of second capacitors  27  are connected between the second common electrode line  25   b  and the second end terminal  38  of each lamp  28 . The first common electrode  25   a  is connected to output coils  40 ,  40 ′ of the first and second transformers  22  and  23  to receive a sine wave VAC. Unlike the first embodiment, the second common electrode line  25   b  is grounded and the plurality of second capacitors  27  may be omitted. Lamps  28  may be formed by using a cold cathode fluorescent lamp (CCFL). Alternatively or additionally, the lamps  28  may be an external electrode fluorescent lamp (EEFL) having an electrode on each external end of a tube.  
         [0029]     The inverter  21  includes first, second, third, and fourth transistors M 1 , M 2 , M 3 , and M 4 . The third transistor M 3  and the first transistor M 1  are connected in series between a voltage terminal (VCC) and a ground terminal (VSS). The fourth transistor M 4  and the second transistor M 2  are connected in series between the voltage terminal (VCC) and the ground terminal (VSS). A first output terminal “A” outputting a first output signal is connected between the third transistor M 3  and the first transistor M 1 . A second output terminal “B” outputting a second output signal is connected between the fourth transistor M 4  and the second transistor M 2 . The transistors M 1 ˜M 4  may be MOS transistors. For example, the first and second transistors are NMOS transistors, and the third and fourth transistors are PMOS transistors.  
         [0030]     The first and second input coils  32 ,  32 ′,  34 ,  34 ′ of the first and second transformers  22  and  23  are each connected to the first output terminal A and the second output terminal B of the inverter  21 . The first wire W 1  extends from the first output terminal A to the second terminal B of the inverter  21 . The first wire W 1  is wound to form the first input coil  32  of the first transformer  22  and the first input coil  32 ′ of the second transformer  23 . Then, the first wire W 1  is connected to the second output terminal B of the inverter  21 . The first wire W 1  is connected in the following order: the first input coil  32  of the first transformer  22 →the first input coil  32 ′ of the second transformer  23 →the second output terminal B of the inverter  21 . On the other hand, the second wire W 2  extends from the first output terminal A to the second output terminal B of the inverter  21 . The second wire W 2  is wound to form the second input coil  34  of the first transformer  22  and the second input coil  34 ′ of the second transformer  23 . Then, the second wire W 2  is connected to the second output terminal B of the inverter  21 . Specifically, the second wire W 2  is connected in the following order: the first output terminal A of the inverter  21 →the second input coil  34  of the first transformer  22 →the second input coil  34 ′ of the second transformer  23 →the second output terminal B of the inverter  21 . The first input coils  32 ,  32 ′ of the first and second transformers  22 ,  23  are coiled in the same direction. In addition, the second input coils  34 ,  34 ′ of the first and second transformers  22 ,  23  are coiled in the same direction.  
         [0031]     As described above, the first input coils  32 ,  32 ′ of the first and second transformers  22  and  23  share the first wire W 1 , and the second input coils  34 ,  34 ′ of the first and second transformers  22  and  23  share the second wire W 2 . As a result, the transformers  22 ,  23  share a uniform and equal current. Therefore, even if a first current transmitted to the first input coils  32 ,  32 ′ is not equal to the exact half of the entire current outputted from the inverter  21 , a second current transmitted to each transformer through the second input coils  34 ,  34 ′ can compensate the first current. Consequently, the first and second transformers  22  and  23  can output uniform signals.  
         [0032]     The invention provides a backlight driving system having the following advantages. First and second input coils of first and second transformers share first and second wires. Accordingly, equal and uniform amount of current is controlled to be transmitted to each end terminal of a lamp. This results in uniform brightness of the lamp and enhancement in product reliability. In addition, a plurality of lamps can be turned on by using a single inverter, thereby simplifying an entire backlight unit system. Consequently, both power consumption and fabrication cost are substantially reduced.  
         [0033]     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.