Patent Publication Number: US-7902772-B2

Title: Circuit and method for sensing open-circuit lamp of a backlight unit and display device with circuit for sensing open-circuit lamp of backlight unit

Description:
This application is a Divisional of application Ser. No. 11/169,783 filed on Jun. 30, 2005 now U.S. Pat. No. 7,298,096, and for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 10-2004-0096762 filed in Korea on Nov. 24, 2004 under 35 U.S.C. §119; the entire contents of all are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to liquid crystal display (LCD) devices. More particularly, the present invention relates to a circuit and a method for sensing an open-circuit lamp of a backlight unit, and a display device with a circuit for sensing an open-circuit lamp of a backlight unit. 
     2. Discussion of the Related Art 
     Cathode ray tube (CRT) devices have been widely used as display devices such as televisions or monitors. However, the CRT devices have the drawbacks of heavy weight and big size. 
     To substitute the CRT devices, liquid crystal display (LCD) devices have been researched and developed. The LCD devices are advantageously light-weight, dimensionally compact, and have low power consumption during operation. Recently, the LCD devices have been widely used as display devices such as monitors for desktop computers, outdoor monitors of more than 30 inches, and hang-on-the-wall televisions as well as monitors for laptop computers. 
     Generally, LCD devices display images by controlling transmittance of external light source. Thus, the LCD devices need an external light source such as backlight units. 
     Backlight units are classified into an edge type and a direct type according to the position of a light source with respect to a display panel. In direct-type backlight units, a light source is disposed directly under a display panel. Since the direct-type backlight units can provide high luminance, the direct-type backlight units are widely used for large LCD devices of more than 30 inches. 
     A direct-type backlight unit uses a plurality of lamps as a light source. The lamp may include a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL). However, if one lamp is open-circuit, a higher voltage will be applied to the other lamps. Accordingly, this may decrease the lifespan of the lamps or affect the operation of the lamps. 
     To solve this problem, a circuit for sensing an open-circuit lamp may be added to an inverter. However, since the related art circuit shuts down the power only when a plurality of lamps are open-circuit, the problems of stability of the device still exist. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a circuit an a method for sensing an open-circuit lamp of a backlight unit, and a display device with a circuit for sensing an open-circuit lamp that substantially obviate one or more of the problems due to limitations and disadvantages of the related art. 
     An advantage of the present invention is to provide a circuit and a method for sensing an open-circuit of a backlight unit, and a display device with a circuit for sensing an open-circuit lamp that effectively protect the backlight unit and increase a lifespan of the backlight unit. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. These and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a circuit for sensing an open-circuit lamp includes an enable signal output unit outputting an enable signal, a reference voltage output unit outputting a reference voltage, a sensing voltage output unit outputting a sensed voltage for deciding whether the lamp is open-circuit, a voltage comparison unit comparing the sensed voltage with the reference voltage and then outputting a decision signal according to a result of comparing the sensed voltage with the reference voltage, and an enable signal control unit controlling an output of the enable signal according to the decision signal. 
     In another aspect, a display device includes a display panel, a panel driving circuit for driving the display panel, a lamp unit including at least one lamp and providing light the display panel, the at least one lamp having electrodes at both ends thereof, a circuit for sensing an open-circuit lamp and a system power control unit controlling power supply according an enable signal. The circuit for sensing the open-circuit lamp includes an enable signal output unit outputting an enable signal, a reference voltage output unit outputting a reference voltage, a sensing voltage output unit outputting a sensed voltage for deciding whether the lamp is open-circuit, a voltage comparison unit comparing the sensed voltage with the reference voltage and then outputting a decision signal according to a result of comparing the sensed voltage with the reference voltage, and an enable signal control unit controlling output of the enable signal according to the decision signal. 
     In another aspect, a method for sensing an open-circuit lamp using a circuit for sensing an open-circuit lamp, wherein the circuit for sensing an open-circuit lamp includes an enable signal output unit, a reference voltage output unit, a sensing voltage output unit, a voltage comparison unit, and an enable signal control unit, the method includes outputting an enable signal from the enable signal output unit, outputting a reference voltage from the reference voltage output unit, outputting a sensed voltage from the sensing voltage output unit, comparing the sensed voltage with the reference voltage in the voltage comparison unit and then outputting a decision signal, and controlling an output of the enable signal from the enable signal control unit according to the decision signal. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is a block diagram illustrating a circuit for sensing an open-circuit lamp of a backlight unit according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating a part of a circuit for sensing an open-circuit lamp of a backlight unit according to a first embodiment of the present invention; 
         FIG. 3  is a circuit diagram illustrating a circuit for sensing an open-circuit lamp of a backlight unit including the structure of  FIG. 2  according to the first embodiment of the present invention; 
         FIGS. 4A and 4B  are views illustrating a sensing voltage output unit of a circuit for sensing an open-circuit lamp according to a second embodiment of the present invention; 
         FIGS. 5A and 5B  are views illustrating a sensing voltage output unit of a circuit for sensing an open-circuit lamp according to a third embodiment of the present invention; 
         FIG. 6  is a bottom view illustrating another sensing voltage output unit of a circuit for sensing an open-circuit lamp according to the third embodiment of the present invention; 
         FIG. 7  is a plan view illustrating a sensing voltage output unit of a circuit for sensing an open-circuit lamp according to a fourth embodiment of the present invention; and 
         FIG. 8  is a cross-sectional view illustrating the sensing cable according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     Lamps such as a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), or a flat lamp are used as a light source for a display device. The lamps have infinite impedance when a voltage is applied in an early stage, and then the lamps have impedance of several hundred ohms to several thousand ohms after the voltage is stably applied. However, when one electrode of the lamp is open-circuit, the open-circuit electrode has infinite impedance, and thus a voltage applied to the open-circuit electrode is increased sharply. 
     Accordingly, in the present invention, the infinite impedance of the open-circuit electrode is used for sensing an open-circuit lamp. That is, change of voltage at the open-circuit lamp electrode or change of an induced voltage at the open-circuit lamp electrode is measured and then the measured voltage is compared with a reference voltage to determine whether a lamp is open-circuit. 
       FIG. 1  is a block diagram illustrating a circuit for sensing an open-circuit lamp of a backlight unit according to an embodiment of the present invention. In  FIG. 1 , a circuit  1  for sensing an open-circuit lamp in this embodiment includes an enable signal output unit  10 , a reference voltage output unit  20 , a sensing voltage output unit  30 , a voltage comparison unit  40 , and an enable signals control unit  50 . 
     The enable signal output unit  10  outputs an enable signal ENA for enabling the operation of a backlight unit. The enable signal ENA may be also used to enable the operation of a liquid crystal panel. The enable signal ENA instructs a power source unit (not shown) to continuously apply a voltage to the backlight unit. 
     The reference voltage output unit  20  outputs a reference voltage Vref as a comparison standard to decide whether a lamp is open-circuit. The reference voltage Vref may be provided from an additional external circuit. For example, a voltage at a low voltage terminal of a secondary coil of a transformer in a lamp-driving inverter circuit may be used as the reference voltage Vref. 
     The sensing voltage output unit  30  senses whether electrodes of a lamp are open-circuit. The sensing voltage output unit  30  outputs a sensed voltage Vs to indicate whether the lamp is in a normal state (i.e., a closed-circuit state) or in an abnormal state (i.e., an open-circuit state). When the electrode of the lamp is open-circuit, a voltage or an electric field around the electrode is changed due to the infinite impedance of the electrode. 
     The voltage comparison unit  40 , such as a comparator, receiving the sensed voltage Vs from the sensing voltage output unit  30  and the reference voltage Vref from the reference voltage output unit  20 , compares the sensed voltage Vs with the reference voltage Vref. Thus, the voltage comparison unit  40  outputs a decision signal S when an increased voltage is detected due to an increase in impedance of an open-circuit lamp. In an embodiment, the voltage comparison unit  40  can be an operational amplifier (OP-AMP). 
     The enable signal control unit  50  receives the enable signal ENA from the enable signal output unit  10  and controls the output of the enable signal according to the decision signal S from the voltage comparison unit  40 . In an embodiment, the enable signal control unit  50  may include a transistor. If the output of the enable signal is cut off due to the open-circuit lamp, the power supply stops providing power and the display device is shut down. 
       FIG. 2  is a circuit diagram illustrating a part of a circuit for sensing an open-circuit lamp of a backlight unit according to a first embodiment of the present invention. In  FIG. 2 , voltage-dividing circuits S 1  and S 2  are connected to both electrodes P 1  and P 2  of a lamp L as the sensing voltage output unit  30  of  FIG. 1 . Each of the voltage-dividing circuits S 1  and S 2  includes division resistors R 1  and R 2  or R 1 ′ and R 2 ′ connected in series and outputs a sensed voltage Vs 1  or Vs 2 . 
     A fluorescent lamp is driven by boosting an inputted power from about 220V AC (alternating current) voltage to about 1000 to 1500 V AC voltage through the transformers Tx 2  and Tx 2  which apply the boosted power to the electrodes P 1  and P 2  of the lamp L. In an embodiment, the division resistors R 1  and R 2  or R 1 ′ and R 2 ′ may be formed such that the sensed voltages divided by the voltage-dividing circuits S 1  and S 2  are about several volts (V). In addition, a low voltage L at a secondary coil of one transformer Tx 1  may be used as the reference voltage Vref of the reference voltage output unit  20 . 
     If one electrode P 1  or P 2  of the lamp L is open-circuit, a voltage at the electrode P 1  or P 2  is increased due to the infinite impedance of the electrode P 1  or P 2 . Therefore, the sensed voltages Vs 1  and Vs 2  outputted from the voltage-dividing circuit S 1  or S 2  are also increased, and the open-circuit lamp is detected by comparing the sensed voltages Vs 1  and Vs 2  with the reference voltage Vref. As a result, appropriate measures such as cutting off voltages are carried out. 
       FIG. 3  is a circuit diagram illustrating a circuit for sensing an open-circuit lamp of a backlight unit including the structure of  FIG. 2  according to the first embodiment of the present invention. In  FIG. 3 , the backlight unit includes a plurality of lamps. A plurality of sensing voltage output units are respectively connected to the lamps, and output sensed voltages Vs_Lamp 1  to Vs_LampN (N is a natural number). The sensed voltages Vs_Lamp 1  to Vs_LampN are inputted to an operational amplifier OP-AMP, which is a comparator and receives a reference voltage Vref as a comparison standard. If there is an increased voltage among the sensed voltages Vs_Lamp 1  to Vs_LampN, the operational amplifier OP-AMP outputs a decision signal. Accordingly, a transistor TR, as an enable signal control unit in this embodiment, receives the decision signal and blocks the output of the enable signal. 
     The circuit for sending an open-circuit lamp may be used for a display device, which includes a display panel, a panel-driving circuit unit for driving the display panel, a lamp unit providing light to the display panel, a lamp-driving circuit unit for driving the lamp unit, and a system power control unit controlling power supply according to an enable signal. Here, if an open-circuit lamp is sensed, the power supply is cut off. Accordingly, the stability of the device is increased. 
       FIGS. 4A and 4B  are views illustrating a sensing voltage output unit of a circuit for sending an open-circuit lamp according to a second embodiment of the present invention.  FIG. 4A  is a plan view of the sensing voltage output unit, and  FIG. 4B  is a cross-sectional view along the line IV-IV. In the second embodiment, the open-circuit lamp is detected by sensing the change of a voltage induced according to the change of a voltage inputted to an electrode of a lamp. That is, when one electrode of the lamp is open-circuit, the voltage at the electrode is increased due to the infinite impedance of the electrode. Thus, if a conductor is disposed in an electric field of a voltage supplying line for providing the voltage, a voltage is induced in the conductor, and the induced voltage is used as a sensed voltage Vs to determine whether the lamp is open-circuit. 
     As illustrated in  FIG. 4A , a plurality of lamps L 1 , L 2 , L 3  and L 4  are connected to lamp connectors CNT 1  and CNT 2  that are formed on a first side A of a printed circuit board PCB. Lamp-driving voltage lines  81 ,  82 ,  83  and  84  extend from a main power line  80  and are connected to the lamp connectors CNT 1  and CNT 2 . The lamp-driving voltage lines  81 ,  82 ,  83  and  84  provide lamp-driving voltages to the respective lamps L 1 , L 2 , L 3  and L 4 . First patterns C 1 , C 2 , C 3  and C 4  of any shapes and sizes are formed on the lamp-driving voltage lines  81 ,  82 ,  83  and  84 , respectively. The first patterns C 1 , C 2 , C 3  and C 4  are formed on the first side A of the printed circuit board PCB. The first patterns C 1 , C 2 , C 3  and C 4  are formed of a conductive material, and the first patterns C 1 , C 2 , C 3  and C 4  receive the lamp-driving voltages from the lamp-driving voltage line  81 ,  82 ,  83  and  84 . 
     Referring to  FIG. 4B , second patterns C 11 , C 21 , C 31  and C 41  are formed on a second side B of the printed circuit board PCB opposite to the first side A. The second patterns C 11 , C 21 , C 31  and C 41  correspond to the first patterns C 1 , C 2 , C 3  and C 4 , respectively. The second patterns C 11 , C 21 , C 31  and C 41  are also formed of a conductive material. 
     Induced voltages are induced by the second patterns C 11 , C 21 , C 31  and C 41  due to the lamp-driving voltages applied to the first patterns C 1 , C 2 , C 3  and C 4 . The induced voltages of the second patterns C 11 , C 21 , C 31  and C 41  are used as the sensed voltages Vs. Then, the induced voltages, as the sensed voltages Vs, are compared with the reference voltage Vref through the voltage comparison unit  40  of  FIG. 1 . If the lamp is determined as an open-circuit lamp as a result of the comparison, the enable signal control unit  50  of  FIG. 1  will block the output of the enable signal to protect the system. 
       FIGS. 5A and 5B  illustrate a sensing voltage output unit of a circuit for sensing an open-circuit lamp according to a third embodiment of the present invention.  FIG. 5A  is a bottom view of the sensing voltage output unit, and  FIG. 5B  is a plan view of the sensing voltage output unit. In the third embodiment, a conductive pattern is disposed in an electric field of a lamp, and an induced voltage induced due to the conductive pattern is used as the sensed voltage. When the lamp is open-circuit, a higher induced voltage, which is proportional to a higher voltage at an electrode of the lamp due to the infinite impedance, is induced by the conductive pattern. Thus, the open-circuit lamp is sensed by an increase in the inducted voltage. 
     As illustrated in  FIGS. 5A and 5B , printed circuit boards PCB 1  and PCB 2  are disposed on an outer surface of a cover bottom  110 , of which a plurality of lamps L 1 , L 2 , . . . , Lm−1, and Lm are disposed on an inner surface. The printed circuit boards PCB 1  and PCB 2  are disposed at both ends of the lamps L 1 , L 2 , . . . , Lm−1, and Lm. A plurality of conductive patterns C 1 , C 2 , . . . , Cm−1, and Cm or C 1 ′, C 2 ′, . . . , Cm−1′, and Cm′ are formed on each of the printed circuit boards PCB 1  and PCB 2 . The conductive patterns C 1 , C 2 , . . . , Cm−1, and Cm or C 1 ′, C 2 ′, . . . , Cm−1′, and Cm′ correspond to the respective lamps L 1 , L 2 , . . . , Lm−1, and Lm. 
     The cover bottom  110  includes at least one sensing hole  112  in order to increase an induced voltage induced by the conductive patterns C 1 , C 2 , . . . , Cm−1, and Cm or C 1 ′, C 2 ′, . . . , Cm−1′, and Cm′ at a bottom side thereof. In an embodiment, the sensing hole  112  is disposed at each side of the lamps L 1 , L 2 , . . . , Lm−1, and Lm and is adjacent to an electrode at each side of the lamps L 1 , L 2 , . . . , Lm−1, and Lm. As illustrated in  FIG. 5A , the lamps L 1 , L 2 , . . . , Lm−1, and Lm have a minimum distance from the conductive patterns C 1 , C 2 , . . . , Cm−1, and Cm or C 1 ′, C 2 ′, . . . , Cm−1′, and Cm′ through the sensing hole  112 . The sensing hole  112  extends along a direction crossing the lamps L 1 , L 2 , Lm−1, and Lm. 
       FIG. 6  is a bottom view illustrating another sensing voltage output unit of a circuit for sensing an open-circuit lamp according to the third embodiment of the present invention. As illustrated in  FIG. 6 , a plurality of sensing holes  112  are formed only in the respective regions corresponding to the lamps L 1 , L 2 , . . . , Lm−1, and Lm and the conductive patterns C 1 , C 2 , . . . , Cm−1, and Cm or C 1 ′, C 2 ′, . . . , Cm−1′, and Cm′. 
     In the third embodiment, the cover bottom  110  and the conductive patterns C 1 , C 2 , . . . , Cm−1, and Cm or Cm′, C 2 ′, . . . , Cm−1′, and Cm′ are used as the sensing voltage output unit  30  of  FIG. 1 . When one electrode of a lamp is open-circuit, a voltage at the electrode is increased due to the infinite impedance, and an induced voltage is induced from an electric field around the lamp. The induced voltage is changed along with the change of the electric field. Thus, the induced voltage is used as the sensed voltage Vs. A plurality of sensed voltage Vs are compared with the reference voltage Vref through the voltage comparison unit  40  of  FIG. 1 . If the lamp is determined as an open-circuit lamp as a result of the comparison, the enable signal control unit  50  of  FIG. 1  will block the output of the enable signal to protect the system. 
       FIG. 7  is a plan view illustrating a sensing voltage output unit of a lamp open sensing circuit according to a fourth embodiment of the present invention. In the fourth embodiment, the change of the induced voltage is detected through a cable including at least one conductive line to determine whether the lamp is open-circuit. In  FIG. 7 , at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm is disposed on a cover bottom  130   a , and a sensing cable  140  is disposed between the cover bottom  130  and the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm. The sensing cable  140  senses the induced voltage as the sensed voltage. The sensing cable  140  may be one of a flexible printed circuit (FPC) and a flexible flat cable (FFC). 
     The sensing cable  140  extends along a direction crossing the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm. The sensing cable  140  includes at least one conductive line, which includes a metallic core and an insulator surrounding the metallic core. The insulator is removed at a region facing the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm by a minimum distance from the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm to form an exposed portion  142  exposing the metallic core. In this embodiment, only one exposed portion  142  is formed in each conductive line. The exposed portion  142  functions as same as the patterns of the third embodiment. In this embodiment, the exposed portion  142  is disposed directly under the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm. 
     Desirably, to detect the open-circuit lamp at both sides of the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm, the sensing cable  140  is disposed at both sides of the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm. The sensing cable  140  is adjacent to an electrode at each side of the at least one lamp L 1 , L 2 , . . . , Lm−1, or Lm. 
       FIG. 8  is a cross-sectional view illustrating the sensing cable according an embodiment of the present invention. In  FIG. 8 , the sensing cable  140  further includes a conductive cover  144  on the exposed metallic core of the exposed portion  142  so that an induction effect of the induced voltage is increased. 
     In the fourth embodiment, a sensed voltage Vs is outputted from the sensing cable  140  due to the induced voltage at each exposed portion  142  and is compared with the reference voltage Vref through the voltage comparison unit  40  of  FIG. 1 . If the lamp is determined as an open-circuit lamp as a result of the comparison, the enable signal control unit  50  of  FIG. 1  will block the output of the enable signal to protect the system. 
     In an embodiment of the present invention, lamps may be sensed individually, and appropriate measures can be carried out. Accordingly, the system is effectively protected and a lifespan of the system is increased. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.