Abstract:
The present invention provides a protection means for a driving circuit which drives a set of external electrode fluorescent lamps (EEFL). The driving circuit with protection function for driving a set of EEFLs consistent with the present invention includes: a transformer connected to the set of EEFLs; a switching network connected to the transformer which delivers power to the transformer; a sensing circuit connected to the set of EEFLs which detects disconnection if one light source is disconnected; and a controller connected to the switching network which controls the switching network to reduce the total current supplied to the EEFLs which remain connected, if the sensing circuit detects that one EEFL is disconnected. Appropriate protection can therefore be implemented when the EEFL is disconnected on one end or both ends.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a protection means, and more specifically, to a protection means for a driving circuit which drives a plurality of external electrode fluorescent lamps.  
       BACKGROUND OF THE INVENTION  
       [0002]     Large-size LCD panels usually use cold cathode fluorescent lamp (CCFL) as conventional backlight source. Each CCFL has two electrode terminals where hard-ware wires are connected to a power source, usually a DC/AC inverter. Due to the increased panel size and number of lamps used in large LCD panel applications, the wiring becomes a complex burden both in manufacturing and cost.  
         [0003]     External electrode fluorescent lamp (EEFL), on the other hand, has both electrodes exposed at both ends of the lamp, does not require any hard-ware wiring. Such that EEFL is used in in large LCD panel applications. Multiple EEFLs are placed in parallel on the back of the LCD panel. There are two metal plates for electrical connections. One plate connects to one end of all the EEFLs and the other plate connects to the other end of all the EEFLs. The power source, generally a DC/AC inverter, simply provides power to both metal plates for operating the EEFLs.  
         [0004]      FIG. 1  is a conventional EEFL driving circuit  10  plus its current sensing circuit  260  in accordance with the prior art. As shown in  FIG. 1 , a conventional driving means plus its current sensing circuit  260  is illustrated. Generally, the driving circuit  10  includes transformer  220  and  222 , which are configured with 180 degree off phase to each other. Thus the quasi-sinusoidal waveforms generated by transformer  220  and  222  are of 180 degree phase difference. Since all the EEFLs  40  are connected in parallel by its nature, the regulation of the EEFL current is implemented by sensing and controlling the sum of the current flowing through each EEFL (EEFL( 1 ), EEFL( 2 ) . . . EEFL(n)). The sensing circuit  260  includes two sensing resistor  17  and  18 . As known by those skilled in art, sensing circuit  260  can sensing the total current by sensing the voltages on the sensing resistor  17  and  18 . One drawback occurs when one of the EEFLs  40  is not connected properly. The rest of the EEFLs will be overdriven since the controller controls the total current only. For example, if one of the EEFLs, EEFL(n) gets open, then EEFL( 1 ) to EEFL(n− 1 ) will be overdriven by excess current flowing through. This will degrade the life time of the EEFL.  
       SUMMARY OF THE INVENTION  
       [0005]     A driving circuit with protection function for driving a set of cold cathode flourescent lamps in accordance with the present invention includes: a transformer connected to the set of cold cathode flourescent lamps; a switching network connected to the transformer which delivers power to the transformer; a sensing circuit connected to the set of cold cathode flourescent lamps which detects disconnection if one cold cathode flourescent lamp is disconnected; and a controller connected to the switching network which controls the switching network to reduce the total current supplied to the cold cathode flourescent lamps which remain connected, if the sensing circuit detects that one cold cathode flourescent lamp is disconnected.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated and better understood by referencing the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0007]      FIG. 1  is a conventional driving circuit plus its current sensing circuit in accordance with the prior art.  
         [0008]      FIG. 2  is a circuit diagram of a driving circuit with protection function in accordance with the present invention.  
         [0009]      FIG. 3  is a circuit diagram of a driving circuit with protection function when the EEFL is disconnected on one end.  
         [0010]      FIG. 4  shows the voltage-current characteristic of the EEFL.  
         [0011]      FIG. 5  is a circuit diagram of a driving circuit with protection function when the EEFL is disconnected on both ends.  
         [0012]      FIG. 6  is a flowing chart of a protection method provided by one end sensing circuit in accordance with the present invention when the EEFL is disconnected on one end.  
         [0013]      FIG. 7  is a flowing chart of a protection method provided by another end sensing circuit in accordance with the present invention when the EEFL is disconnected on both ends.  
         [0014]      FIG. 8  is a liquid crystal display system of an embodiment of the invention.  
         [0015]      FIG. 9  is a liquid crystal display system of an embodiment of the invention.  
         [0016]      FIG. 10  is a liquid crystal display system of an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0017]     Turning to  FIG. 2 , a circuit diagram of a driving system  10 ′ with protection function in accordance with the present invention is depicted. Generally, system  10 ′ includes a plurality of EEFLs  40  (EEFL( 1 ), EEFL( 2 ) . . . , EEFL(n)); transformer  220  and  222 ; switching network  210  and  212 ; EEFL inverter controller  230 ; first end sensing circuit  240  and second end sensing circuit  250 .  
         [0018]     Transformer  220  and  222  which delivery power to EEFLs  40  (EEFL( 1 ), EEFL( 2 ) . . . , EEFL(n)) are connected to the two plates of the EEFLs  40  respectively. As shown in  FIG. 2 , switching network  210  is connected to transformer  220 . Similarly, switching network  212  is connected to transformer  222 . Switching network  210  and  212  can include a DC/AC converter, such as the push-pull, half-bridge, full-bridge type DC/AC converter. EEFL inverter controller  230  sends control signals to both switching networks  210  and  212 , thus regulate the current supplied to the EEFLs. Purposely, transformer  220  and  222  are configured with 180 degree off phase to each other. Each secondary side of the transformers  220  and  222  generates a quasi-sinusoidal waveform to the EEFLs. Therefore the quasi-sinusoidal waveforms generated by transformer  220  and  222  are of 180 degree phase difference. First end sensing circuit  240  mainly includes a first current sensing circuit  101 , a second current sensing circuit  102 , a third current sensing circuit  103  and a forth current sensing circuit  104 . The first current sensing circuit  101  includes diode  27  and sensing resistor  17 ; the second sensing circuit  102  includes diode  23  and sensing resistor  13 ; the third sensing circuit  103  includes diode  28  and sensing resistor  18 ; the forth sensing circuit  104  includes diode  25  and sensing resistor  15 . Current sense means is implemented with both positive and negative half cycles. During normal operation, take a positive half for example, current flows from top side of transformer  220  into “left plate” of the EEFLs  40 , through EEFLs to the “right plate”, then into the top of transformer  222 , flowing out of the bottom of transformer  222  into the first current sensing circuit  101  (diode  27  and sensing resistor  17 ) to ground and into the second current sensing circuit  102  (sensing resistor  13 , diode  23 ), then flows into the bottom of transformer  220 . The diode  27  and  23  direct the current supplied to the EEFLs flowing to transformer  220 . This forms a complete current loop for the positive half cycle. Sensing resistor  17  therefore senses a positive half-wave quasi-sinusoidal current while sensing resistor  13  senses a negative half-wave current. In another word, the first current sensing circuit  101  senses a positive current and the second current sensing circuit  102  senses a negative current. Such that the first voltage on sensing resistor  13  indicates the current through a first end (left plate) of the plurality of EEFLs  40  and the second voltage on sensing resistor  17  indicates the current through a second end (right plate) of the plurality of EEFLs  40 . However, the present invention will not be limited by only sensing the positive or negative current. It can be modified to sense the current during half of a current cycle. Assume that the resistance of sensing resistor  17  is equal to that of sensing resistor  13 , by summing the sensed voltages of sensing resistor  13  and sensing resistor  17  through resistor  35 ,  36  and capacitor  6 , the sensed voltage at node V 3  is approximately 0 volt during normal operation, that is, no EEFL is disconnected. Similarly, in the next half cycle, the current flows from the top of transformer  222  into the “right plate” of the EEFLs  40 , through EEFLs to the “left plate”, then into the top of transformer  220 , flows out of the bottom of transformer  220  into the third current sensing circuit  103  (diode  28  and sensing resistor  18 ) to ground and into the forth current sensing circuit  104  (sensing resistor  15 , diode  25 ), then into the bottom of transformer  222 . The diode  28  and  25  direct the current supplied to the EEFLs flowing to transformer  222 . This forms a complete current loop for the negative half cycle. Sensing resistor  18  therefore senses a positive half-wave quasi-sinusoidal current while sensing resistor  15  senses a negative half-wave current. In another word, the third current sensing circuit  103  senses a positive current and the forth current sensing circuit  104  senses a negative current. Such that the first voltage on sensing resistor  18  indicates the current through a first end (left plate) of the plurality of EEFLs  40  and the second voltage on sensing resistor  15  indicates the current through a second end (right plate) of the plurality of EEFLs  40 . However, the present invention will not be limited by only sensing the positive or negative current. Rather, it can be modified to sense the current during any half of a current cycle. Likewise, assume that the resistance of sensing resistor  18  is equal to that of sensing resistor  15 , by summing the sensed voltages of sensing resistor  15  and sensing resistor  18  through resistor  33 ,  34  and capacitor  5 , the sensed voltage at node V 2  is approximately 0 volt during normal operation, that is, no EEFL is disconnected.  
         [0019]     Sensed current signals for both half cycles are sent to the EEFL inverter controller  230 . Based on the state of the sensed current signals which indicate whether the EEFL is disconnected on one end or not, controller  230  will generate corresponding control signals to switch network  210  and  212  in order to regulate the current supplied to the EEFLs  40 . For example, if the sensing signals indicate that the EEFL is disconnected on one end, the control signals will control the switching network  210  and  212  to reduce the total current supplied to the EEFLs which remain connected. The overdriven event is thus avoided.  
         [0020]     In addition, the voltage sense for the secondary windings of transformer  220  and  222  is implemented through second end sensing circuit  250 . Second end sensing circuit  250  generally includes a first voltage sensing circuit  111  and a second voltage sensing circuit  112 . The first voltage sensing circuit  111  includes resistor  11 ,  14  and diode  30 ; the second voltage sensing circuit  112  includes resistor  12 ,  16  and diode  29 . Second end sensing circuit  250  senses the voltage across the EEFLs  40 . The operation of second end sensing circuit  250  will be described below in detail in  FIG. 5 . Similarly, the sensed voltage signals are sent to the EEFL inverter controller  230 . Based on the state of the sensed voltage signals which indicate whether the EEFL are disconnected on both ends or not, controller  230  will generate corresponding control signals to both switching network  210  and  212  to regulate the voltage across the EEFLs. For example, if the sensing signals indicate that both ends of one EEFL are open, the control signals will control the switching network  210  and  212  to reduce the total current across the EEFLs which remain connected. The overdriven event thus can be prevented.  
         [0021]      FIG. 3  is a circuit diagram of a driving circuit  10 ″ with protection function when the EEFL is disconnected on one end. Many elements of  FIG. 3  are similar to those of the  FIG. 2 , as such, are labeled similarly. Hence any repetitive description of similar elements that was already detailed with respect to  FIG. 2  is omitted herein for clarity, and rather the differences between  FIG. 2  and  FIG. 3  are detailed herein. As shown in  FIG. 3 , the “right end” of EEFL(n) is disconnected from transformer  222 , i.e., open. Such that the current flowing in the positive half cycle from top of transformer  220  into “left plate” of the EEFLs  40  will flow through EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ) to transformer  222 , and flow through EEFL(n) body to chassis (ground). Therefore all EEFLs are lit. EEFL(n) is lit by the leakage current flowing through capacitor  8  to ground. In general, the leakage current flows from the top of transformer  220  into the “left plate” of EEFL(n), then through the parasitic capacitor  8  of EEFL(n) to chassis ground, second current sensing circuit  102  (sensing resistor  13 , diode  23 ) and back into the bottom of transformer  220 . The diode  23  directs the current supplied to the EEFLs flowing to transformer  220 . Therefore, this portion of the current is not detected via the first current sensing circuit  101  (sensing resistor  17 ) which only senses the amount of the current flowing through EEFL( 1 ), EEFL( 2 ) . . . and EEFL(n− 1 ). The current flowing through sensing resistor  13  of second current sensing circuit  102  is equal to the total current of the current flowing through sensing resistor  17  of first current sensing circuit  101  plus the leakage current. As described with respect to  FIG. 2 , sensing resistor  17  senses a positive half-wave quasi-sinusoidal current while sensing resistor  13  senses a negative half-wave current. That is, the first current sensing circuit  101  senses a positive current and the second current sensing circuit  102  senses a negative current. Assume that the resistance of sensing resistor  17  is equal to that of sensing resistor  13 , by summing the sensed voltage across sensing resistor  13  and sensing resistor  17  through resistor  35 ,  36  and capacitor  6 , the sensed voltage at node V 3  will be lower than 0 volt. That&#39;s because the positive current flowing through sensing resistor  17  of first current sensing circuit  101  is less than the negative current flowing through sensing resistor  13  of second current sensing circuit  102 . Therefore, the sensed voltage at node V 3  is lower than 0 Volt while the sensed voltage at node V 2  is greater than 0 Volt compared to the normal operating condition, if the right end of one of the EEFLs is disconnected. In a word, the absolute value of the voltage at node V 3  and V 2  will exceed a predetermined level which is 0 Volt during the normal operation if the right end of an EEFLs is disconnected.  
         [0022]     Similarly, if the “left end” of EEFL(n) is disconnected from transformer  220 , the current flowing in the negative half cycle from top of transformer  222  into “right plate” of the EEFLs will flow through EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ) to transformer  220 , and flow through and EEFL(n) body to chassis (ground). All EEFLs are lit. EEFL(n) is lit by the leakage current flowing through capacitor  8  to ground. The leakage current flows from the top of transformer  222  into the “right plate” of EEFL(n), then through the parasitic capacitor  8  of EEFL(n) to chassis ground, forth current sensing circuit  104  (sensing resistor  15 , diode  25 ) and the bottom of transformer  222 . The diode  25  directs the current supplied to the EEFLs flowing to transformer  222 . This portion of the current is not detected via the third current sensing circuit  103  (sensing resistor  18 ) which only senses the amount of the current flowing through EEFL( 1 ), EEFL( 2 ) . . . and EEFL(n− 1 ). The current flowing through sensing resistor  15  of forth current sensing circuit  104  is equal to the total current of the current flowing through sensing resistor  18  of third current sensing circuit  103  plus the leakage current. Likewise, the sensed voltage at node V 2  will be lower than 0 Volt while the sensed voltage at node V 3  is greater than 0 Volt compared to the normal operating condition, if the left end of an EEFLs is disconnected. That is, the absolute value of the voltage at node V 3  and V 2  will exceed a predetermined level which is 0 Volt during the normal operation if the left end of one of the EEFLs is disconnected.  
         [0023]     Therefore, the first end sensing circuit  240  can detect the the disconnection by summing the voltage at node V 2  and V 3 , if one end of an EEFL is disconnected from the transformer. If the absolute value of the voltage at node V 3  and V 2  exceeds a predetermined level which is 0 Volt during the normal operation, first end sensing circuit  240  will detect that the EEFL is disconnected on one end.  
         [0024]     Discussed above is the situation when only one EEFL gets disconnected. But the present invention will not be limited by such situation when only one EEFL gets disconnected and will be applicable to multiple EEFLs disconnections. When one or more right ends of EEFLs get disconnected, the sensed voltage at V 2  will be greater than 0 Volt while V 3  is lower than 0 Volt. When one or more left ends of EEFLs get disconnected, the sensed voltage at V 3  will be greater than 0 Volt while V 2  is lower than 0 Volt.  
         [0025]     By measuring the voltage at node V 2  and V 3 , first end sensing circuit  240  will generate corresponding sensing signals to EEFL inverter controller  230  based on the sensed voltage at node V 2  and V 3 . EEFL inverter controller  230  in turn provides appropriate control signals to both switching network  210  and  212  based on those sensing signals. The control signals are used for controlling the switching network  210  and  212  to regulate the current supplied to the EEFLs to avoid the over-current condition. For example, if the sensing signals indicate that an EEFL is open on one end, the control signals will control the switching network  210  and  212  to reduce the total current supplied to the EEFLs which remain connected. Appropriate protection therefore can be implemented by sensing V 2  and V 3  to the EEFL inverter controller  230  when the EEFL is disconnected on either end.  
         [0026]     Turning to  FIG. 4 , the voltage-current characteristic of the EEFL is depicted. The voltage-current characteristics in  FIG. 4  shows that the voltage and current of the EEFL are approximately in linear relationship. That is, the higher the current flowing through EEFL is, the higher the voltage across the EEFL will be. Advantageously, the invention circuit takes the advantage of the characteristics to provide proper protection functions as will be detailed below.  
         [0027]      FIG. 5  is a circuit diagram  10 ′″ of a driving circuit with protection function when the EEFL is disconnected on both ends. Many elements of  FIG. 5  are similar to those of the  FIG. 2  and  FIG. 3 , as such, are labeled similarly. Hence any repetitive description of similar elements that was already detailed with respect to  FIG. 2  and  FIG. 3  is omitted herein for clarity, and rather the differences between  FIG. 5  and  FIG. 2, 3  are detailed herein. As shown in  FIG. 5 , both “right end” and “left end” of EEFL(n) are disconnected from transformer  222  and transformer  220 . The current will flow only through EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ), since EEFL(n) is not conducted. The total current will be supplied to EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ), except EEFL(n). Since the total current is fixed and EEFL(n) is not conducted, EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ) will be overdriven. However, the second end sensing circuit  250  will have higher sensed voltage when one of the EEFLs, e.g. EEFL(n) is not conducted since all other EEFLs (EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 )) are driven with higher current. Second end sensing circuit  250  generally includes a first voltage sensing circuit  111  and a second voltage sensing circuit  112  and a resistor divider  50  which comprises resistor  29 ,  30 . The first voltage sensing circuit  111  includes resistor  11 ,  14  and diode  30 ; the second voltage sensing circuit  112  includes resistor  12 ,  16  and diode  29 . Second end sensing circuit  250  senses the voltage across the EEFLs  40 . Resistor divider  50  is used to minimize the effect of voltage variation to voltage sense. As shown in the  FIG. 5 , the first voltage sensing circuit  111  senses the voltage during one half of the voltage cycle while the second voltage sensing circuit  112  senses voltage during the other half of the voltage cycle. The voltage sensed by the second end sensing circuit  250  at node V 1  is proportional to the voltage across the EEFLs  40 . By differentiating the voltage at node V 1  under normal operation and abnormal operation conditions, EEFL inverter controller  230  can provide an appropriate protection to the inverter circuit. For example, if one of the EEFLs is not conducted, the sensed voltage at node V 1  will be higher than the normal operation. In normal operation, when no EEFL is disconnected, the sensed voltage at node V 1  is set as a predetermined level. Once the sensed voltage at node V 1  is detected that exceeds a predetermined level which is under normal operation, a disconnection will be detected. And the sensed signals will be sent to controller  230 , which in turn generates the corresponding control signals to control the switching network  210  and  212 , thus provide appropriate protection to the inverter circuit. For example, if the sensing signals indicate that one EEFL is disconnected on both ends, the control signals will control the switching network  210  and  212  to reduce the total current applied to the EEFLs  40 , such that the overdriven protection is provided.  
         [0028]      FIG. 6  shows a flowing chart  600  of protection method provided by the first end sensing circuit  240  in accordance with the present invention. Generally, first end sensing circuit  240  detects and provides protection function to a driving circuit if the EEFL is disconnected on one end. As shown in step  602  of  FIG. 6 , first end sensing circuit  240  senses a first voltage across the first sensing resistor and a second voltage across the second sensing resistor by sensing the current during a first half of a current cycle and a second half of a current cycle in step  601 . Take  FIG. 2  for reference, the first voltage is the voltage across sensing resistor  13  which indicates the current through a first end (left plate) of the plurality of EEFLs  40  and the second voltage is the voltage across sensing resistor  17  which indicates the current through a second end (right plate) of the plurality of EEFLs  40 . In  FIG. 2 , during normal operation, take a positive half for example, current flows from top side of transformer  220  into “left plate” of the EEFLs  40 , through EEFLs to the “right plate”, then into the top of transformer  222 , flowing out of the bottom of transformer  222  into the first current sensing circuit  101  (diode  27  and sensing resistor  17 ) to ground and into the second current sensing circuit  102  (sensing resistor  13 , diode  23 ), then flows into the bottom of transformer  220 . The diode  27  and  23  direct the current supplied to the EEFLs flowing to transformer  220 . This forms a complete current loop for the positive half cycle. Sensing resistor  17  therefore senses a positive half-wave quasi-sinusoidal current while sensing resistor  13  senses a negative half-wave current. In another word, the first current sensing circuit  101  senses a positive current and the second current sensing circuit  102  senses a negative current. Such that the first voltage on sensing resistor  13  indicates the current through a first end (left plate) of the plurality of EEFLs  40  and the second voltage on sensing resistor  17  indicates the current through a second end (right plate) of the plurality of EEFLs  40 . Thus the current difference between the first end (left plate) of EEFLs  40  and the second end (right plate) of EEFLs  40  will be calculated in step  604  of  FIG. 6 . Therefore, still in  FIG. 6 , first end sensing circuit  240  sums the voltage on node V 2  and V 3  respectively in step  605 , that is, to sum the sensed first voltage on sensing resistor  13 / 18  and the second voltage on sensing resistor  17 / 15 . The sum of the first voltage on sensing resistor  13 / 18  and the second voltage on sensing resistor  17 / 15  indicates the current difference between the current through the left plate and right plate of EEFLs  40 . Back to  FIG. 2 , assume that the resistance of sensing resistor  17  is equal to that of sensing resistor  13 , by summing the sensed voltages of sensing resistor  13  and sensing resistor  17  through resistor  35 ,  36  and capacitor  6 , the sensed voltage at node V 3  is approximately 0 volt during normal operation, that is, no EEFL is disconnected. So in step  606  of  FIG. 6 , if V 2 =0 and V 3 =0, then first end sensing circuit  240  will determine that the driving circuit is under normal operation, i.e., no one end of EEFL is disconnected (step  607 ). If not, e.g., V 2 &gt;0 and V 3 &lt;0 or V 2 &lt;0 and V 3 &gt;0, then first end sensing circuit  240  will determine that the EEFL is disconnected on one end. (in step  608 ). Take  FIG. 3  for reference, the “right end” of EEFL(n) is disconnected from transformer  222 , i.e., open. Such that the current flowing in the positive half cycle from top of transformer  220  into “left plate” of the EEFLs  40  will flow through EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ) to transformer  222 , and flow through EEFL(n) body to chassis (ground). Therefore all EEFLs are lit. EEFL(n) is lit by the leakage current flowing through capacitor  8  to ground. In general, the leakage current flows from the top of transformer  220  into the “left plate” of EEFL(n), then through the parasitic capacitor  8  of EEFL(n) to chassis ground, second current sensing circuit  102  (sensing resistor  13 , diode  23 ) and back into the bottom of transformer  220 . The diode  23  directs the current supplied to the EEFLs flowing to transformer  220 . Therefore, this portion of the current is not detected via the first current sensing circuit  101  (sensing resistor  17 ) which only senses the amount of the current flowing through EEFL( 1 ), EEFL( 2 ) . . . and EEFL(n− 1 ). The current flowing through sensing resistor  13  of second current sensing circuit  102  is equal to the total current of the current flowing through sensing resistor  17  of first current sensing circuit  101  plus the leakage current. As described with respect to  FIG. 2 , sensing resistor  17  senses a positive half-wave quasi-sinusoidal current while sensing resistor  13  senses a negative half-wave current. That is, the first current sensing circuit  101  senses a positive current and the second current sensing circuit  102  senses a negative current. Assume that the resistance of sensing resistor  17  is equal to that of sensing resistor  13 , by summing the sensed voltage across sensing resistor  13  and sensing resistor  17  through resistor  35 ,  36  and capacitor  6 , the sensed voltage at node V 3  will be lower than 0 volt. That&#39;s because the positive current flowing through sensing resistor  17  of first current sensing circuit  101  is less than the negative current flowing through sensing resistor  13  of second current sensing circuit  102 . Therefore, the sensed voltage at node V 3  is lower than 0 Volt while the sensed voltage at node V 2  is greater than 0 Volt compared to the normal operating condition, if the right end of one of the EEFLs is disconnected. Therefore, back to  FIG. 6 , If V 2 &gt;0 and V 3 &lt;0 or V 2 &lt;0 and V 3 &gt;0, then first end sensing circuit  240  will determine that the EEFL is disconnected on one end. (in step  608 ). First end sensing circuit  240  will further provide sensing signals to EEFL inverter controller  230  indicating that the EEFL is disconnected on one end in step  609 . In step  610 , EEFL inverter controller  230  will send corresponding control signals to both switching network  210  and  212 . Thus the total current supplied to the EEFLs which remain connected to the transformer will be reduced in step  611 . Therefore the protection means of providing protection function to the EEFL driving circuit when the EEFL is disconnected on one end is implemented.  
         [0029]      FIG. 7  is a flowing chart  700  of a protection method provided by the second end sensing circuit  250  in accordance with the present invention. Generally, first end sensing circuit  250  detects and provides protection function to a driving circuit if both ends of an EEFL are disconnected. In step  702  of  FIG. 7 , the second end sensing circuit  250  senses the voltage Vsense across the EEFLs  40  by sensing the voltage during a first half of a voltage cycle and a second half of a voltage cycle in step  701 . The operation of sensing the voltage Vsense across the EEFLs  40  is described in  FIG. 5 . Now take  FIG. 5  for reference, both “right end” and “left end” of EEFL(n) are disconnected from transformer  222  and transformer  220 . The current will flow only through EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ), since EEFL(n) is not conducted. The total current will be supplied to EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ), except EEFL(n). Since the total current is fixed and EEFL(n) is not conducted, EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 ) will be overdriven. However, the second end sensing circuit  250  will have higher sensed voltage when one of the EEFLs, e.g. EEFL(n) is not conducted since all other EEFLs (EEFL( 1 ), EEFL( 2 ) . . . EEFL(n− 1 )) are driven with higher current. Second end sensing circuit  250  generally includes a first voltage sensing circuit  111  and a second voltage sensing circuit  112  and a resistor divider  50  which comprises resistor  29 ,  30 . The first voltage sensing circuit  111  includes resistor  11 ,  14  and diode  30 ; the second voltage sensing circuit  112  includes resistor  12 ,  16  and diode  29 . Second end sensing circuit  250  senses the voltage across the EEFLs  40 . Resistor divider  50  is used to minimize the effect of voltage variation to voltage sense. As shown in the  FIG. 5 , the first voltage sensing circuit  111  senses the voltage during one half of the voltage cycle while the second voltage sensing circuit  112  senses voltage during the other half of the voltage cycle. The voltage sensed by the second end sensing circuit  250  at node V 1  is proportional to the voltage across the EEFLs  40 . Thus the voltage Vsense across the EEFLs  40  is sensed by sensing the voltage during a first half of a voltage cycle and a second half of a voltage cycle. By differentiating the voltage at node V 1  under normal operation and abnormal operation conditions, EEFL inverter controller  230  can provide an appropriate protection to the inverter circuit. For example, if one of the EEFLs is not conducted, the sensed voltage at node V 1  will be higher than the normal operation. In normal operation, when no EEFL is disconnected, the sensed voltage at node V 1  is set as a predetermined level. Once the sensed voltage at node V 1  is detected that exceeds a predetermined level which is under normal operation, a disconnection will be detected. Back to  FIG. 7 , if in step  702  the sensed voltage Vsense across the EEFLs  40  exceeds a predetermined level Vnormal, which represents the voltage level during normal operation, second end sensing circuit  250  will determine that the EEFL is disconnected on both ends at step  705 . If not, it will be determined under normal operation in step  704 . In step  706 , second end sensing circuit  250  will provide sensing signals to EEFL inverter controller  230  indicating that both ends of an EEFL are disconnected. In step  707 , EEFL inverter controller  230  will send corresponding control signals to both switching network  210  and  212 . Thus the total current supplied to the EEFLs which remain connected to the transformer is reduced in step  708 . Therefore the protection means of providing protection function to the EEFL driving circuit when both ends of the EEFL are open is implemented.  
         [0030]      FIG. 8  illustrates a liquid crystal display system  800  of an embodiment of the invention. Liquid crystal display system  800  comprises thin film transistor screen  801 . Thin film transistor screen  801  is coupled to column driver  802 . Column driver  802  controls columns on thin film transistor screen  801 . Thin film transistor screen  801  is also coupled to row driver  803 . Row driver  803  controls rows on thin film transistor screen  801 . Column driver  802  and row driver  803  are coupled to timing controller  804 . Timing controller  804  controls timing for column driver  802  and row driver  803 . Timing controller  804  is coupled to video signal processor  805 . Video signal processor  805  processes video signals. Video signal processor  805  is coupled to video demodulator  806 . Video demodulator  806  demodulates video signals. Video demodulator  806  is coupled to tuner  807 . Tuner  807  provides video signals to video demodulator  806 . Tuner  807  tunes liquid crystal display system  800  to a particular frequency. Video demodulator  806  is also coupled to microcontroller  808 . Tuner  807  is also coupled to audio demodulator  811 . Audio demodulator  811  demodulates audio signals from tuner  807 . Audio demodulator  811  is coupled to audio signal processor  810 . Audio signal processor  810  processes audio signals from audio demodulator  810 . Audio signal processor  810  is coupled to audio amplifier  809 . Audio amplifier  809  amplifies audio signals from audio signal processor  810 .  
         [0031]     Thin film transistor screen  801  is illuminated by system  10 ′. As described above, system  10 ′ includes a plurality of EEFLs  40  (EEFL( 1 ), EEFL( 2 ) . . . , EEFL(n)); transformer  220  and  222 ; switching network  210  and  212 ; EEFL inverter controller  230 ; sensing circuit  240  and  250 . Once powered on, system  10 ′ starts operation. The detailed operation of the system  10 ′ is similar to the preferred embodiment of  FIG. 2  and will not be fully described herein. Thus the EEFLs  40  are powered and will provide backlight to thin film transistor screen  801 . If one or several EEFLs are disconnected, system  10 ′ can still provide protection and avoid overdriven event.  
         [0032]      FIG. 9  illustrates a liquid crystal display system  900  of an embodiment of the invention. Liquid crystal display system  900  comprises thin film transistor screen  901 . Thin film transistor screen  901  is coupled to column driver  902 . Column driver  902  controls columns on thin film transistor screen  901 . Thin film transistor screen  901  is also coupled to row driver  903 . Row driver  903  controls rows on thin film transistor screen  901 . Column driver  902  and row driver  903  are coupled to timing controller  904 . Timing controller  904  controls timing for column driver  902  and row driver  903 . Timing controller  904  is coupled to video signal processor  905 . Video signal processor  905  processes video signals. In an alternative embodiment, video signal processor  905  could be a scalar device.  
         [0033]     Similarly, thin film transistor screen  901  is illuminated by system  10 ′. As described above, system  10 ′ includes a plurality of EEFLs  40  (EEFL( 1 ), EEFL( 2 ) . . . , EEFL(n)); transformer  220  and  222 ; switching network  210  and  212 ; EEFL inverter controller  230 ; sensing circuit  240  and  250 . Once powered on, system  10 ′ starts operation. The detailed operation of the system  10 ′ is similar to the preferred embodiment of  FIG. 2  and will not be fully described herein. Thus the EEFLs  40  are powered and will provide backlight to thin film transistor screen  801 . If one or several EEFLs are disconnected, system  10 ′ can still provide protection and avoid overdriven event.  
         [0034]      FIG. 10  illustrates a liquid crystal display system  1000  of an embodiment of the invention. Liquid crystal display system  1000  comprises graphics adaptor  1090 . Liquid crystal display system  1000  can also comprise the components of liquid crystal display system  900  described above and illustrated in  FIG. 9  or can also comprise the components of liquid crystal display system  800  described above and illustrated in  FIG. 8 . Graphics adaptor  1090  is coupled to a video signal processor which can be video signal processor  905  described above and illustrated in  FIG. 9  or video signal processor  805  described above and illustrated in  FIG. 8 .  
         [0035]     Graphics adaptor  1090  is coupled to chipset core logic  1091 . Chipset core logic  1091  transfers data between devices coupled to it. Chipset core logic  1091  is also coupled to microprocessor  1092 . Microprocessor  1092  processes data including video data. Chipset core logic  1091  is also coupled to memory  1093 . Memory  1093  can be random access memory and provides short term storage of data. Chipset core logic  1091  is also coupled to hard disk drive  1094 . Hard disk drive  1094  provides long term storage of data. Chipset core logic  1091  is also coupled to optical drive  1095 . Optical drive  1095  retrieves data from a CD-ROM or a DVD-ROM.  
         [0036]     The foregoing descriptions of the preferred embodiment of the present invention are an illustration of the present invention rather than a limitation thereof. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. While the preferred embodiments of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.