Patent Publication Number: US-7723924-B2

Title: Backlight inverter and liquid crystal display using the same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
   This application claims priority to Korean Patent Application No. 2007-0030821, filed Mar. 29, 2007, the contents of which are herein incorporated by reference in their entirety. 
   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present disclosure is directed to a backlight inverter and, more particularly, to a backlight inverter for a liquid crystal display device. 
   2. Discussion of Related Art 
   A liquid crystal display (“LCD”) device includes a thin-film transistor substrate, a color filter substrate facing the thin-film transistor substrate, and a liquid crystal layer between the thin-film transistor substrate and color filter substrate. The LCD device displays an image by applying an electric field to electrodes of the substrates to cause the liquid crystal to flow in the liquid crystal layer. 
   The liquid crystal layer does not emit light by itself and only adjusts the transmittance of incident light to display an image. Accordingly, the LCD device requires a backlight device to emit light sufficient to view an image. 
   The backlight device includes a plurality of fluorescent lamps and a backlight inverter for driving the fluorescent lamps. The backlight inverter converts a low direct current (DC) voltage into an alternating current (AC) voltage, boosts it using a transformer, and then supplies it to the fluorescent lamps in order to drive the fluorescent lamps. 
   The backlight inverter outputs a high level voltage to drive the fluorescent lamps and, therefore, requires a protection circuit to prevent the backlight inverter from operating in an abnormal driving state, such as the occurrence of electrical arcs, or open and short circuits. 
   An electrical arc created in the backlight inverter may burn out the transformer, which may result in a fire. The occurrence of an electrical arc may be difficult to detect because a current may flow in the backlight inverter even though the backlight inverter has an open circuit. 
   SUMMARY OF THE INVENTION 
   An exemplary embodiment of the present invention provides a backlight inverter that is capable of detecting the occurrence of an electrical arc by causing a phase delay between two output voltages from a transformer, and a LCD device using the backlight inverter. 
   According to an exemplary embodiment of the present invention, a backlight inverter comprises: a transformer boosting an input voltage at a ratio of a primary winding to a secondary winding of the transformer and outputting it as a first voltage and a second voltage; a protection level signal generator causing a phase delay between the first voltage and the second voltage, and generating a protection level signal, the protection level signal being a sum of the phase reversed first voltage and the second voltage; and an arc state detector comparing the protection level signal with a reference voltage and generating a detection signal; and a driving controller keeping or stopping the supplying of an input voltage to the transformer in response to the detection signal, wherein in addition to the phase delay, another phase delay occurs caused by an open circuit in the transformer, so that a maximum value of the protection level signal is larger than the reference voltage. 
   A phase of the first voltage may be the reverse of a phase of the second voltage. 
   The protection level signal generator may comprise a first capacitor receiving the first voltage; a second capacitor receiving the second voltage; a third capacitor connected between the first capacitor and a ground to form a detection point where the protection level signal is output; and a delay line connected between the first capacitor and the second capacitor to cause a phase delay between the first voltage and the second voltage. 
   The delay line may be wired in a zig-zag manner, so that the length of the delay line may be adjusted. 
   The arc state detector may comprise a reference voltage generater generating the reference voltage from a voltage of a power supply, and a comparator generating the detection signal in response to the reference voltage and the protection level signal. 
   The comparator can enable the detection signal when the protection level signal is larger than the reference voltage, and can disable the detection signal when the protection level signal is smaller than the reference voltage. 
   The arc state detector may further comprise a protection level signal receiver, the protection level signal receiver rectifying the protection level signal and supplying it to the comparer. 
   According to an exemplary embodiment of the present invention, a LCD device comprises: a voltage convertion portion generating a common voltage, a gamma voltage, and gate-on and gate-off voltages using an input direct current voltage; a display panel portion displaying data using the common voltage, the gamma voltage, and the gate-on and gate-off voltage; an inverter boosting and converting the input direct current voltage into a first voltage and a second voltage having a phase that is a reverse of a phase of the first voltage, causing a phase delay between the first voltage and the second voltage, generating a protection level signal, the protection level signal being a sum of the phase reversed first voltage and the second voltage, comparing the protection level signal with a reference voltage and generating a detection signal, and keeping or stopping the supplying of an input signal depending on a result of the comparison; and a lamp portion illuminating light on the display panel portion in response to the first voltage and the second voltage, wherein in addition to the phase delay, another phase delay occurs by an open circuit in the inverter, so that a maximum value of the protection level signal is larger than the reference voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention will be understood in more detail from the following descriptions taken in conjunction with the accompanying drawings, in which exemplary embodiments of the present invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set for the herein: 
       FIG. 1  is a block diagram of an LCD device according to an exemplary embodiment of the present invention; 
       FIG. 2  is a circuit diagram of an exemplary embodiment of the inverter of  FIG. 1 ; 
       FIGS. 3A ,  3 B, and  3 C are graphs for illustrating an operation of the inverter of  FIG. 2 ; and 
       FIG. 4  is a circuit diagram of the inverter of  FIG. 1  according to an exemplary embodiment of the present invention. 
   

   DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  is a block diagram of an LCD device according to an exemplary embodiment of the present invention. Referring to  FIG. 1 , an LCD device  100  includes a display panel portion  110 , a DC (Direct Current)-DC (Direct Current) voltage conversion portion  120 , a DC (Direct Current)-AC (Alternating Current) inverter  130 , and a lamp portion  140 . 
   The display panel portion  110  includes a data driver  114 , a gate driver  116 , and an LCD panel  112 . The data driver  114  supplies the LCD panel  112  with a gray level display voltage corresponding to a data gray level using a gamma voltage VGMA from the DC-DC voltage conversion portion  120 . The gate driver  116  supplies gate-on and gate-off voltages VON and VOFF, respectively, from the DC-DC voltage conversion portion  120  to the LCD panel  112 . 
   The LCD panel  112  supplies liquid cells with the gray level display voltage from the data driver  114  and a common voltage VCOM from the DC-DC voltage conversion portion  120  in response to the gate-on and gate-off voltages VON and VOFF, respectively, from the gate driver  116 . 
   The DC-DC voltage conversion portion  120  includes a DC-DC converter  122 , a common voltage generator  124 , and a gamma voltage generator  126 . The DC-DC converter  122  converts the level of an externally supplied DC voltage VIN, and supplies it to the common voltage generator  124  and gamma voltage generator  126 . The common voltage generator  124  generates the common voltage VCOM using a DC voltage from the DC-DC converter  122 , and supplies it to the display panel portion  110 . The gamma voltage generator  126  generates a gamma voltage VGMA using a DC voltage from the DC-DC converter  122 , and supplies it to the display panel portion  110 . 
   The DC-AC inverter  130  converts the externally supplied DC voltage VIN into an alternating voltage, boosts it to a high level voltage using a transformer, and supplies it to the lamp portion  140  in order to drive the lamp portion  140 . 
   The lamp portion  140  adjusts the amount of output light in response to the voltage from the DC-AC inverter  130 , and illuminates the output light to the rear surface of the LCD panel  112 . 
     FIG. 2  is a circuit diagram of an exemplary embodiment of the inverter shown in  FIG. 1 . Referring to  FIG. 2 , the inverter  130  includes a driver  131 , a driving controller  132 , a transformer  134 , a protection level signal generator  138 , and an arc state detector  136 . 
   The driver  131  maintains or stops the supplying of the externally supplied voltage VIN, after having been converted to an alternating current voltage, to the transformer  134  in response to a control signal CNTL supplied from the driving controller  132 . The driver  131  can include any well-known DC to AC inverter circuit (no shown). 
   The driving controller  132  generates the control signal CNTL in response to a detection signal DET supplied from the arc state detector  136  in order to control the driver  131 . The driver  131  includes an inverter (not shown) and a switching means (not shown) for transmitting or not the externally supplied voltage VIN to the transformer  134  under control of the control signal CNTL generated in response to the detection signal DET fed to the driving controller  132 . The switching means may include a transistor (not shown). The transistor (not shown) of the driver  131  may include a control terminal for receiving the control signal CNTL, an input terminal for receiving the externally supplied voltage signal VIN, and an output terminal connected to a primary winding L 11  of the transformer  134 . 
   The transformer  134  includes the primary winding L 11  and secondary windings L 21  and L 22 . The transformer  134  boosts a transformer driving voltage from the driver  131  in a ratio of the primary winding L 11  and the secondary windings L 21  and L 22 , and supplies the boosted voltage to the lamp portion  140 . In the exemplary embodiment, the transformer  134  includes one primary winding L 11  and two secondary windings L 21  and L 22 , but the transformer is not limited thereto. 
   Output terminals CH 1 , CH 2 , CH 3 , and CH 4  of the secondary windings L 21  and L 22  are connected to the lamps  141 ,  142 ,  143 , and  144 , respectively. The other terminals of the lamps  141 ,  142 ,  143 , and  144 , which are not connected to the transformer  134 , are commonly connected to a ground. 
   The output terminals CH 1  and CH 2  of the secondary winding L 21  supply a first alternating current (AC) voltage and a second alternating current (AC) voltage to the lamps  141  and  142 , respectively, wherein a phase of the first alternating current voltage is the reverse of the phase of the second alternating current voltage. The output terminals CH 3  and CH 4  of the secondary winding L 22  supply a first alternating current voltage and a second alternating current voltage to the lamps  143  and  144 , respectively, wherein a phase of the first alternating voltage is the reverse of the phase of the second alternating voltage. A phase of the first alternating current voltage from the terminal CH 1  is identical to the phase of the first alternating current voltage from the terminal CH 3 , and a phase of the second alternating current voltage from the terminal CH 2  is identical to the phase of the second alternating current voltage from the terminal CH 4 . 
   The protection level signal generator  138 , which is located between the lamp portion  140  and output terminals CH 1 , CH 2 , CH 3 , and CH 4  of the transformer  134 , generates a plurality of protection level signals PL 1  and PL 2 , which indicate an occurrence of an electrical arc in the transformer  134 , and supplies them to the arc state detector  136 . 
   The protection level signal generator  138  includes a plurality of protection level signal generating units  138   a  and  138   b . The protection level signal generating unit  138   a  includes, for example, a first capacitor C 11 , a second capacitor C 12 , a third capacitor C 13 , and a delay line L 1 . 
   One end of the first capacitor C 11  is connected to the output terminal CH 1 , and one end of the second capacitor C 12  is connected to the output terminal CH 2 . One end of the third capacitor C 13  is connected to the other end of the second capacitor C 12 , and the other end of the third capacitor C 13  is connected to a ground. A node connecting the second capacitor C 12  and third capacitor C 13  operates as a detection point A for the protection level generating unit  138   a . The first protection level signal PL 1  is taken from node A. 
   The delay line L 1  connects the detection point A to the other end of the first capacitor C 11 . The delay line L 1  causes a phase delay of an output voltage from the output terminal CH 1 . The phase delay may occur by increasing the resistance of the delay line L 1 , for example, by wiring the delay line in a zigzag manner so as to lengthen the delay line L 1 . 
   The phase delay may be performed so that a maximum value of an amplitude of a summed wave is smaller than a reference voltage VREF of the arc state detector  136 . The summed wave is generated by adding amplitudes of the first and second voltages applied from the output terminals CH 1  and CH 2  to the detection point A. 
   The protection level signal generating unit  138   b  includes a first capacitor C 21 , a second capacitor C 22 , a third capacitor C 23 , and a delay line L 2 . The protection level signal generating unit  138   b  has the same construction as that of the protection level signal generating unit  138   a  and, therefore, the detailed description will be omitted. The second protection level signal PL 2  is taken from a node. B at the connection of the second capacitor C 22  and the third capacitor C 23 . 
   The arc state detector  136  compares the reference voltage VREF with the protection level signals PL 1  and PL 2  from the protection level signal generator  138 , and generates the detection signal DET to indicate the occurrence of an electrical arc. 
   The arc state detector  136  includes a protection level signal receiver  136   b , a reference voltage generator  136   a , and a comparator CMP. The protection level signal receiver  136   b  rectifies the protection level signals PL 1  and PL 2  from the protection level signal generator  138 , performs an OR operation of the rectified protection level signals PL 1  and PL 2 , and supplies the result of the OR operation to the non-inverting terminal (+) of the comparator CMP. 
   The protection level signal receiver  136   b  includes first diodes D 1  and D 3 , which are located between the non-inverting terminal of the comparator CMP and the protection level signal input terminals  136   c  and  136   d , respectively, and second diodes D 2  and D 4 , which are located between ground and protection level signal input terminals  136   c  and  136   d , respectively. 
   The reference voltage generator  136   a  supplies the reference voltage VREF to the inverting terminal (−) of the comparator CMP. The reference voltage generator  136   a  includes a plurality of resistors R 1  and R 2  that are connected in series between an input terminal of a power supply (not shown) and a ground. The reference voltage generator  136   a  splits a voltage VCC from the power supply (not shown) to generate the reference voltage VREF. The reference voltage VREF may be selected by adjusting the value of the resistors R 1  and R 2 . 
   The comparator CMP compares the reference voltage VREF with the protection level signals PL 1  and PL 2 , and generates the detection signal DET. For example, when one of the protection level signals PL 1  and PL 2  is larger than the reference voltage VREF, the comparator CMP enables the detection signal DET and supplies it to the driving controller  132 . 
   When both protection level signals PL 1  and PL 2  are smaller than the reference voltage VREF, the comparator CMP disables the detection signal DET and supplies it to the driving controller  132 . Enabling the detection signal DET indicates the occurrence of an electrical arc, and disabling the detection signal DET indicates the non-occurrence of an electrical arc. 
     FIGS. 3A ,  3 B, and  3 C are wavefroms for illustrating an operation of the inverter  130  of  FIG. 2 .  FIG. 3A  shows a waveform of a voltage output from the secondary windings,  FIG. 3B  shows a waveform of a voltage supplied to the detection point by the delay line, and  FIG. 3C  shows a waveform of a voltage supplied to the detection point during the occurrence of the electrical arc. 
   Referring to  FIGS. 2 and 3A , one of the output terminals CH 1  and CH 2  of the secondary winding L 21  outputs a first voltage and a second voltage, wherein a phase of the first voltage is the exact reverse of that of the second voltage. The first voltage is in a trade-off relation with the second voltage and, therefore, the amplitude of a summed wave SUM 0  of the first and second voltages becomes ‘0’. 
   Referring to  FIGS. 2 and 3B , a first voltage of the output terminal CH 1  is delayed from a second voltage of the output terminal CH 2  that is supplied to the detection point A by a prescribed distance D 1  due to the delay line L 1 . The sum of the first voltage and the second voltage yields a summed wave SUM 1  that has prescribed amplitude. The summed wave SUM 1  is input to the arc state detector  136  as a protection level signal PL 1 . The maximum value of the summed wave SUM 1  may be less than that of the reference voltage VREF used in the arc state detector  136 . 
   Referring to  FIGS. 2 and 3C , when an electrical arc is created by an open circuit in the inverter  130 , the first voltage from the output terminal CH 1  may be further delayed from the second output terminal CH 2  by a prescribed distance D 2 . Accordingly, the amplitude of a summed wave SUM 2  of the first voltage and the second voltage becomes larger than that of the summed wave SUM 1 . The summed wave SUM 2  is input to the arc state detector  136  as a protection level signal PL 1 . The maximum value of the summed wave SUM 2  may be larger than that of the reference voltage VREF used in the arc state detector  136 . 
   The output terminals CH 1  and CH 2  of the secondary winding L 21 , which output phase-reversed first and second voltages, respectively, are connected to each other via the delay line L 1 , so that the first voltage from the output terminal CH 1  may be delayed from the second voltage from the output terminal CH 2 , and the delayed first voltage may be further delayed from the second voltage due to the open circuit in the inverter  130 . 
   The maximum value of the summed wave SUM 1 , which is a sum of the first voltage and the second voltage, is smaller than the reference voltage VREF when the first voltage is delayed from the second voltage only by the delay line L 1 , but the maximum value of the summed wave SUM 2 , which is a sum of the first voltage and the second voltage when the delayed first voltage by the delay line L 1  is further delayed by the occurrence of an electrical arc, is larger than the reference voltage VREF. As a result, the inverter  130  may detect whether an electrical arc has occurred by comparing the reference voltage VREF with the protection level signals PL 1  and PL 2 . 
     FIG. 4  is a circuit diagram of an inverter such as used in the system of  FIG. 1  according to another exemplary embodiment of the present invention. Referring to  FIG. 4 , the DC-AC inverter  130 ′ includes a transformer portion  134 ′, a protection level signal generator  138 ′ corresponding to the transformer portion  134 ′, a driver  131 ′, a driving controller  132 ′, and an arc state detector  136 ′. The driver  131 ′ receives the input voltage VIN (not shown) and inverts it to an alternating current voltage. 
   The transformer portion  134 ′ includes two transformers TRANS 1  and TRANS 2 . The first transformer TRANS 1  includes a primary winding L 11  and secondary windings L 21  and L 22 , and the second transformer TRANS 2  includes a primary winding L 12  and secondary windings L 23  and L 24 . 
   The output terminals CH 1 , CH 2 , CH 3 , CH 4 , CH 5 , CH 6 , CH 7 , and CH 8  of the secondary windings L 21 , L 22 , L 23 , and L 24  are connected to lamps  141 ,  142 ,  143 ,  144 ,  145 ,  146 ,  147 , and  148 , respectively of a lamp portion  140 ′. 
   The output terminals CH 1  and CH 2 ; CH 3  and CH 4 ; CH 5  and CH 6 ; and CH 7  and CH 8  of the secondary windings L 21 ; L 22 ; L 23 ; and L 24 , respectively, supply a first alternating current (AC) voltage and a second alternating current (AC) voltage to the corresponding lamps  141  and  142 ;  143  and  144 ;  145  and  146 ; and  147  and  148 , respectively, wherein a phase of the first alternating current voltage is the reverse of the phase of the second alternating current voltage. 
   The phases of the first voltages from the output terminals CH 1 , CH 3 , CH 5 , and CH 7  are identical to one another, and the phases of the second voltages from the output terminals CH 2 , CH 4 , CH 6 , and CH 8  are also identical to one another. 
   The protection level signal generator  138 ′ includes four protection level signal generating units. The first protection level signal generating unit includes a capacitor C 1 , a capacitor C 12 , a capacitor C 13 , and a delay line L 1 . The capacitor C 11  and capacitor C 12  are connected to each other by the delay line L 1 . The second protection level signal generating unit includes a capacitor C 21 , a capacitor C 22 , a capacitor C 23 , and a delay line L 2 . The capacitor C 21  and capacitor C 22  are connected to each other by the delay line L 2 . The third protection level signal generating unit includes a capacitor C 31 , a capacitor C 32 , a capacitor C 33 , and a delay line L 3 . The capacitor C 31  and capacitor C 32  are connected to each other by the delay line L 3 . The fourth protection level signal generating unit includes a capacitor C 41 , a capacitor C 42 , a capacitor C 43 , and a delay line L 4 . The capacitor C 41  and capacitor C 42  are connected to each other by the delay line L 4 . 
   The four protection level signal generating units provide for protection level signals (not shown) that can be taken from nodes A, B, C, and D and fed to the arc state detector  136 ′. In the exemplary embodiment, the inverter  130 ′ includes two transformers TRANS 1  and TRANS 2 . The protection level signal generating unit of the inverter  130 ′ includes a delay line, for example, L 1 , for connecting an output terminal, for example, CH 1 , of the transformer TRANS 1  with an output terminal, for example, CH 6 , of the transformer TRANS 2 . The output terminal, for example, CH 1 , of the transformer TRANS 1  outputs a first voltage, and the output terminal, for example, CH 6 , of the transformer TRANS 2  outputs a second voltage, wherein a phase of the first voltage is the reverse of the phase of the second voltage. The delay time may be shaped depending on the distance between two output terminals, that is, the length. The first protection level signal (not shown) may be taken from the first node A and fed to the arc state detector  136 ′. 
   Other constructions and operations of the inverter  130 ′ may be easily understood by those of ordinary skill in the art, and therefore, the detailed descriptions will be omitted. 
   Although two transformers TRANS 1  and TRANS 2  are included in the inverter  130 ′ in the exemplary embodiment, the number of transformers is not limited thereto and more than three transformers may be employed. 
   The backlight inverter according to exemplary embodiments of the present invention can detect the occurrence of an electrical arc by causing a phase difference between two phase-reversed voltages from the transformer and, therefore, can prevent the inverter from being damaged. 
   Although exemplary embodiments of the present invention have been described herein with reference with the accompanying drawings, it is understood that the present invention is not be limited to these exemplary embodiments, and that various other changes and modifications may be affected therein by one of ordinary skill in the related art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.