Patent Publication Number: US-7221345-B2

Title: Liquid crystal display and apparatus of driving light source therefor

Description:
BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to a backlight driving apparatus of a liquid crystal display, and more in particular to a circuit capable of preventing an arc of an inverter of the backlight. 
   (b) Description of the Related Art 
   Display devices used for monitors of computers and television sets include self-emitting displays such as light emitting diodes (LEDs), electroluminescences (ELs), vacuum fluorescent displays (VFDs), field emission displays (FEDs) and plasma panel displays (PDPs) and non-emitting displays such liquid crystal displays (LCDs) requiring light source. 
   An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer with dielectric anisotropy interposed therebetween. The field-generating electrodes supplied with electric voltages generate electric field in the liquid crystal layer, and the transmittance of light passing through the panels varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are obtained by adjusting the applied voltages. 
   The light may be emitted from a light source equipped in the LCD or may be natural light. When using the equipped light source, the total brightness of the LCD screen is usually adjusted by regulating the ratio of on and off times of the light source or regulating the current through the light source. 
   A light device for an LCD, i.e., a backlight unit usually includes a light source and an inverter for driving the light source. The light source includes a plurality of fluorescent lamps and the inverter converts a DC (direct current) input voltage from an external device into an AC (alternating current) voltage, and then applies the voltage to turn on the lamps. 
   For obtaining good image quality, the current flowing in the lamps is required to be uniform such that the luminescence of the lamps uniform. In order to obtain uniform current, a current detector for detecting the current in the lamps is provided and the current is feedback-controlled depending on the detected current. 
   A conventional technique senses a feedback current and shuts down an inverter when there is no detected current due to change of loads such as disconnection of lamps or separation of an output connector, etc. However, there are some cases that the feedback current is detected even though abnormal operation occurs. For example, the disconnection in a transformer, loose connection of the output connector and so on may generate arcs, which in turn generate the feedback current. Since the inverter is not shut down in this condition, the arc generation continues to burn the transformer or the output connector. 
   SUMMARY OF THE INVENTION 
   A motivation of the present invention is to provide a lighting unit capable of shutting down an inverter when arcs are generated. 
   To accomplish the motivation, an embodiment of the present invention performs shut-down operation after sensing a voltage of a neutral point of two transformers. 
   An apparatus of driving a liquid crystal display according to an embodiment of the present invention includes first and second lamp units and first and second transformers connected thereto. Each of the first and the second transformers includes a primary side and a secondary side. The secondary side of the first transformer has a first terminal connected to the first lamp unit and a second terminal, and the secondary side of the second transformer includes a first terminal connected to the second terminal of the secondary side of the first transformer and a second terminal connected to the second lamp unit. The apparatus further includes a driver converting a DC signal into an AC signal and supplying the AC signal to the primary sides of the first and the second transformer, and a voltage sensor for sensing a voltage at a middle point between the second terminal of the secondary side of the first transformer and the first terminal of the secondary side of the second transformer. 
   The driver is preferably shut down when the voltage sensed by the voltage sensor is larger than a reference voltage. 
   The apparatus may further include a voltage divider for dividing the voltage at the middle point and providing the divided voltage for the voltage sensor. The voltage divider preferably includes first and second resistors serially connected to the middle point. 
   Preferably, the apparatus further includes an on/off controller supplying an off signal to the driver in response to the voltage sensed by the voltage sensor, and/or a feedback controller detecting a current flowing through the first and the second lamp units and controlling the on/off controller based on the detected current. 
   Each of the first and the second lamp units may include a single lamp, or a plurality of lamps connected in series. The primary sides of the first and the second transformers are preferably connected in parallel to the driver. 
   It is preferable that the apparatus further includes first and second resistors connected to the first and the second lamp units, respectively, and the first and the second resistors are commonly connected to a ground. 
   According to an embodiment of the present invention, a liquid crystal display is provided, which includes a lighting unit and a liquid crystal panel assembly. The lighting unit includes first and second lamps, first and second transformers respectively connected to the first and the second lamps, including primary sides and secondary sides, and transmitting an AC signal for driving the first and the second lamps, and a driver supplying a signal to the primary sides of the first and the second transformers. A liquid crystal panel assembly includes liquid crystal for displaying images by adjusting transmittance of light generated from the lighting unit. The secondary sides of the first and the second transformers are connected to each other to form a neutral point, and the driver is shut down when a voltage divided from the voltage at the neutral point is larger than a reference voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or the similar components, wherein: 
       FIG. 1  is a block diagram of an LCD according to an embodiment of the present invention; 
       FIG. 2  is an exploded perspective view of an LCD according to an embodiment of the present invention; 
       FIG. 3  is an equivalent circuit diagram of a pixel of an LCD according to an embodiment of the present invention; 
       FIG. 4  is a schematic circuit diagram of an apparatus of driving light source for a liquid crystal display according to an embodiment of the present invention; and 
       FIG. 5  is a circuit diagram of an apparatus of driving light source according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the inventions invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
   Now, LCDs and methods of driving light source therefor according to the present invention will be described in detail with reference to accompanying drawings. 
   An LCD according to an embodiment of the present invention will be described with reference to  FIGS. 1–3 . 
     FIG. 1  is a block diagram of an LCD according to an embodiment of the present invention,  FIG. 2  is an exploded perspective view of an LCD according to an embodiment of the present invention, and  FIG. 3  is an equivalent circuit diagram of a pixel of an LCD according to an embodiment of the present invention. 
   Referring to  FIG. 1 , an LCD according to an embodiment of the present invention includes a LC panel assembly  300 , a gate driver  400  and a data driver  500  which are connected to the panel assembly  300 , a gray voltage generator  800  connected to the data driver  500 , a lighting unit  900  for illuminating the panel assembly  300 , and a signal controller  600  controlling the above elements. 
   In structural view, the LCD according to an embodiment of the present invention includes a display unit  330  and a backlight unit  340  as shown in  FIG. 2 . 
   The display unit  330  includes the LC panel assembly  300 , a plurality of gate flexible printed circuit (FPC) films  410  and a plurality of data FPC films  510  attached to the LC panel assembly  300 , and a gate printed circuit board (PCB)  450  and a data PCB  550  attached to the associated FPC films  410  and  510 , respectively. 
   The LC panel assembly  300 , in structural view shown in  FIGS. 2 and 3 , includes a lower panel  100 , an upper panel  200  and a liquid crystal layer  3  interposed therebetween while it includes a plurality of display signal lines G 1 –G n  and D 1 –D m  and a plurality of pixels connected thereto and arranged substantially in a matrix in circuital view shown in  FIGS. 1 and 3 . 
   The display signal lines G 1 –G n  and D 1 –D m  are provided on the lower panel  100  and include a plurality of gate lines G 1 –G n  transmitting gate signals (called scanning signals) and a plurality of data lines D 1 –D m  transmitting data signals. The gate lines G 1 –G n  extend substantially in a row direction and are substantially parallel to each other, while the data lines D 1 –D m  extend substantially in a column direction and are substantially parallel to each other. 
   Each pixel includes a switching element Q connected to the display signal lines G 1 –G n  and D 1 –D m , and an LC capacitor C LC  and a storage capacitor C ST  that are connected to the switching element Q. The storage capacitor C ST  may be omitted if unnecessary. 
   The switching element Q such as a TFT is provided on the lower panel  100  and has three terminals: a control terminal connected to one of the gate lines G 1 –G n ; an input terminal connected to one of the data lines D 1 –D m ; and an output terminal connected to the LC capacitor C LC  and the storage capacitor C ST . 
   The LC capacitor C LC  includes a pixel electrode  190  on the lower panel  100 , a common electrode  270  on the upper panel  200 , and the LC layer  3  as a dielectric between the electrodes  190  and  270 . The pixel electrode  190  is connected to the switching element Q, and the common electrode  270  covers the entire surface of the upper panel  100  and is supplied with a common voltage Vcom. Alternatively, both the pixel electrode  190  and the common electrode  270 , which have shapes of bars or stripes, are provided on the lower panel  100 . 
   The storage capacitor C ST  is an auxiliary capacitor for the LC capacitor C LC . The storage capacitor C ST  includes the pixel electrode  190  and a separate signal line (not shown), which is provided on the lower panel  100 , overlaps the pixel electrode  190  via an insulator, and is supplied with a predetermined voltage such as the common voltage Vcom. Alternatively, the storage capacitor C ST  includes the pixel electrode  190  and an adjacent gate line called a previous gate line, which overlaps the pixel electrode  190  via an insulator. 
   For color display, either each pixel represents only one of three primary colors (spatial division) such that spatial average of the primary colors makes a desired color, or each pixel represents three primary colors in turn (time division) such that temporal average of the primary colors makes a desired color.  FIG. 3  shows an example of spatial division by providing one of a plurality of red, green and blue color filters  230  in an area occupied by the pixel electrode  190 . The color filter  230  shown in  FIG. 3  is provided in the corresponding area of the upper panel  200 . Alternatively, the color filter  230  is provided on or under the pixel electrode  190  on the lower panel  100 . 
   Referring to  FIG. 2 , the backlight unit  340  includes a plurality of lamps  341  disposed at edges of the LC panel assembly  300 , a plurality of lamp covers  345  for protecting the lamps  341 , a light guide  342  and a plurality of optical sheets  343  disposed between the panel assembly  300  and the lamps  341  and guiding and diffusing light from the lamps  341  to the panel assembly  300 , and a reflector  344  disposed under the lamps  341  and reflecting the light from the lamps  341  toward the panel assembly  300 . 
   The lamps  341  preferably include fluorescent lamps such as CCFL (cold cathode fluorescent lamp) and EEFL (external electrode fluorescent lamp). An LED is another example of the lamp  341 . 
   The lamps  341  shown in  FIG. 2  are included in the lighting unit  900  shown in  FIG. 1 . 
   A pair of polarizers (not shown) polarizing the light from the lamps  341  are attached on the outer surfaces of the panels  100  and  200  of the panel assembly  300 . 
   Referring to  FIGS. 1 and 2 , the gray voltage generator  800  generates two sets of a plurality of gray voltages related to the transmittance of the pixels and is provided on the data PCB  550 . The gray voltages in one set have a positive polarity with respect to the common voltage Vcom, while those in the other set have a negative polarity with respect to the common voltage Vcom. 
   The gate driver  400  preferably includes a plurality of integrated circuit (IC) chips mounted on the respective gate FPC films  410 . The gate driver  400  is connected to the gate lines G 1 –G n  of the panel assembly  300  and synthesizes the gate-on voltage Von and the gate off voltage Voff from the driving voltage generator  700  to generate gate signals for application to the gate lines G 1 –G n . 
   The data driver  500  preferably includes a plurality of IC chips mounted on the respective data FPC films  510 . The data driver  500  is connected to the data lines D 1 –D m  of the panel assembly  300  and applies data voltages selected from the gray voltages supplied from the gray voltage generator  800  to the data lines D 1 –D m . 
   According to another embodiment of the present invention, the IC chips of the gate driver  400  and/or the data driver  500  are mounted on the lower panel  100 , while one or both of the drivers  400  and  500  are incorporated along with other elements into the lower panel  100  according to still another embodiment. The gate PCB  450  and/or the gate FPC films  410  may be omitted in both cases. 
   The signal controller  600  controlling the drivers  400  and  500 , etc. is provided on the data PCB  550  or the gate PCB  450 . 
   Now, the operation of the LCD will be described in detail. 
   The signal controller  600  is supplied with RGB image signals R, G and B and input control signals controlling the display thereof such as a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a main clock MCLK, and a data enable signal DE, from an external graphic controller (not shown). After generating gate control signals CONT 1  and data control signals CONT 2  and processing the image signals R, G and B suitable for the operation of the panel assembly  300  on the basis of the input control signals and the input image signals R, G and B, the signal controller  600  provides the gate control signals CONT 1  for the gate driver  400 , and the processed image signals R′, G′ and B′ and the data control signals CONT 2  for the data driver  500 . 
   The gate control signals CONT 1  include a vertical synchronization start signal STV for informing of start of a frame, a gate clock signal CPV for controlling the output time of the gate-on voltage Von, and an output enable signal OE for defining the width of the gate-on voltage Von. The data control signals CONT 2  include a horizontal synchronization start signal STH for informing of start of a horizontal period, a load signal LOAD or TP for instructing to apply the appropriate data voltages to the data lines D 1 –D m , an inversion control signal RVS for reversing the polarity of the data voltages (with respect to the common voltage Vcom) and a data clock signal HCLK. 
   The data driver  500  receives a packet of the image data R′, G′ and B′ for a pixel row from the signal controller  600  and converts the image data R′, G′ and B′ into the analogue data voltages selected from the gray voltages supplied from the gray voltage generator  800  in response to the data control signals CONT 2  from the signal controller  600 . 
   Responsive to the gate control signals CONT 1  from the signals controller  600 , the gate driver  400  applies the gate-on voltage Von to the gate line G 1 –G n , thereby turning on the switching elements Q connected thereto. 
   The data driver  500  applies the data voltages to the corresponding data lines D 1 –D m  for a turn-on time of the switching elements Q (which is called “one horizontal period” or “1H” and equals to one periods of the horizontal synchronization signal Hsync, the data enable signal DE, and the gate clock signal CPV). Then, the data voltages in turn are supplied to the corresponding pixels via the turned-on switching elements Q. 
   The difference between the data voltage and the common voltage Vcom applied to a pixel is expressed as a charged voltage of the LC capacitor C LC , i.e., a pixel voltage. The liquid crystal molecules have orientations depending on the magnitude of the pixel voltage and the orientations determine the polarization of light passing through the LC capacitor C LC . The polarizers convert the light polarization into the light transmittance. 
   By repeating this procedure, all gate lines G 1 –G n  are sequentially supplied with the gate-on voltage Von during a frame, thereby applying the data voltages to all pixels. When the next frame starts after finishing one frame, the inversion control signal RVS applied to the data driver  500  is controlled such that the polarity of the data voltages is reversed (which is called “frame inversion”). The inversion control signal RVS may be also controlled such that the polarity of the data voltages flowing in a data line in one frame are reversed (which is called “line inversion”), or the polarity of the data voltages in one packet are reversed (which is called “dot inversion”). 
   A lighting unit  900  will be described in detail with reference to  FIGS. 4 and 5 . 
     FIG. 4  is a schematic circuit diagram of a lighting unit for an LCD according to an embodiment of the present invention, and  FIG. 5  is an exemplary detailed circuit diagram of a lighting unit according to an embodiment of the present invention. 
   As shown in  FIG. 4 , a lighting unit  900  according to an embodiment of the present invention includes a plurality of lamps  911  and  912  and an inverter for driving and controlling the lamps  911  and  912 . The inverter includes a driver  920  for driving the lamp  911  and  912 , a controller  930 , a pair of transformers T 1  and T 2 , a pair of ballast capacitors C 1  and C 2 , a plurality of resistors R 1 , R 2 , R 3  and R 4 , and a pair of diodes D 1  and D 2 . The transformers T 1  and T 2  are connected between the driver  920  and the lamps  911  and  912 , respectively, and a driving signal from the driver  920  is supplied to the lamps  911  and  912  through the transformers T 1  and T 2 . The driver  920  is connected to primary coils of the transformers T 1  and T 2 , and secondary coils of the transformers T 1  and T 2  are connected to first terminals of the lamps  911  and  912  through the capacitors C 1  and C 2  for stabilizing currents in the respective lamps  911  and  912 . 
   Second terminals of the lamps  352   a  and  352   b  are commonly connected to a reference voltage Vref through the resistors R 3  and R 4 , respectively, and the lamps  911  and  912  are lightened by the voltage difference between the reference voltage Vref and voltages supplied to the lamps  911  and  912 . The reference voltage Vref is preferably a ground voltage. The total current I FB  from the lamps  911  and  912  passing through the diodes D 1  and D 2  is supplied to the controller  930  as a feedback current. The secondary coils of the transformers T 1  and T 2  are connected to each other, and a node A between the transformers T 1  and T 2  works as a neutral point in that a load formed by the capacitor C 1  and the lamp  911  and a load formed by the capacitor C 2  and the lamp  912  are symmetrical. The node A is connected to a voltage divider for dividing a voltage thereof, which includes the resistors R 1  and R 2  connected in series between the node A and a ground. The controller  930  senses a voltage Vd between the resistors R 1  and R 2  and shuts down the lighting unit  900  by supplying an off signal to the driver  920  if the voltage Vd becomes to have a reference value. 
   In detail, the voltage at the node A is relatively small during a normal operation since the lamps  911  and  912  symmetrically operate. However, when the load of one of the lamps  911  and  912  becomes larger than the other due to an abnormal operation such as arc generation caused by disconnection of the transformers T 1  and T 2 , the neutral point the transformer T 1  and T 2  is moved and such that the voltage Vd at the node A has a value much larger than the reference value. Then, the controller  930  shuts down the driver  920 . 
   A lighting unit according to an embodiment of the present invention will be described in more detail with reference to  FIG. 5 , which illustrates an exemplary circuit of a lighting unit. 
   Referring to  FIG. 5 , the driver  920  includes a MOSFET M 1 , a diode D 3 , an inductor L, a Royer circuit  921  and a switching driver  922 , and the controller includes a feedback controller  931 , a voltage sensor  932  and an on/off controller  933 . 
   The MOSFET M 1  transmits a DC input voltage Vin to the Royer circuit  921  via the inductor L in response to a switching signal of the switching driver  922 . The transformers T 1  and T 2  are connected in parallel to the inductor L. 
   The Royer circuit  921  includes a pair of transistors S 1  and S 2 , a pair of resistors R 5  and R 6 , and a capacitor C 3 . The transistors S 1  and S 2  have respective emitters connected to a ground, bases connected to the inductor L via the resistors R 5  and R 6 , respectively, and collectors connected by the capacitor C 3 . The transformers T 1  and T 2  are connected in parallel to the collectors and to the bases of the transistors T 1  and T 2 . The Royer circuit  921  converts a DC signal from the DC voltage Vin into an AC signal to be supplied to the lamps  911  and  912 . 
   The lamps  911  and  912  are discharged by output voltages of the secondary coils of the transformers T 1  and T 2 , and the currents passing through the lamps  911  and  912  join to become the feedback-current I FB  to be supplied to the feedback controller  931 . The feedback controller  931  supplies a signal to the on/off controller  933  for controlling the driver  920  based on the feedback current I FB . The voltage sensor  932  measures a voltage, which is divided by the resistors R 1  and R 2  from the voltage between the node A and the ground, and makes the on/off controller  933  generate an off signal when the measured voltage has a value larger than the reference level. The switching driver  922  turns off the MOSFET M 1  to shut down the inverter  900  in response to the off signal. 
   According to another embodiment of the present invention, a plurality of lamps connected in series are connected to each transformer T 1  and T 2 . According to another embodiment of the present invention, the transformers T 1  and T 2  are driven by respective drivers. 
   While the present invention has been described in detail with reference to the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the sprit and scope of the appended claims.