Patent Publication Number: US-7218062-B1

Title: Protective device and a CCFL driving system used thereon

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a protective device and a CCFL driving system used thereon, and more particularly to a protective signal outputted to a control circuit according to a high AC power detected in the high-voltage terminal of a CCFL of the claimed system. The outputted protective signal is used for protecting the control circuit, the protective device of the CCFL, and the CCFL driving system. 
   2. Description of Related Art 
   A CCFL (Cold Cathode Fluorescent Lamp) is used as a light source for the backlight module in a LCD panel. CCFLs are driven by a driving circuit such as an inverter. The size of LCD panels are increasing due to progressive technical developments and consumer demand, so that now it is common for two or more CCFLs to be used for illuminating the panel. 
   In general, a set of the inverters can only light one or two CCFLs, but more inverters are required for driving a plurality of CCFLs as used for a large-sized LCD panel or a large TV LCD screen. Correspondingly, the plural protective circuits are required to protect the inverters and the CCFLs. As such, the cost to manufacture the LCD panel is increasing as well. Additionally, the protective circuit of the conventional scheme is complicated and the circuitry thereof is becoming increasingly complex. 
   SUMMARY OF THE INVENTION 
   Other than the illustrated prior art, the present invention provides a protective device and a CCFL driving system, wherein the protective device improves upon the mentioned drawback. The claimed invention uses high AC power of the high-voltage terminal of the CCFL for protection in an open/short state and under a lighting voltage-limited situation. Furthermore, the claimed invention effectively provides suitable protection if the CCFL is broken. 
   The protective device of the present invention has a short detecting circuit, a rectified diode, a first charging capacitor, an open detecting circuit, and a lighting voltage-limited circuit. In the protective device, the short detecting circuit couples with a high terminal of the CCFL and a power supply through a step-down capacitor, wherein the short detecting circuit receives a high AC power and outputs a short-circuit protective signal to the control circuit when a shorting event occurs to the CCFL; the rectified diode couples with the high terminal of the CCFL through a step-down capacitor, wherein the rectified diode rectifies the high AC power and outputs a high DC power; the first charging capacitor couples with the rectified diode, wherein the first charging circuit generates a detecting voltage based on the high DC power; the open detecting circuit couples with the first charging capacitor and has a threshold, wherein the open detecting circuit outputs an open-circuit protective signal to the control circuit when the detecting voltage is greater than the threshold; and the lighting voltage-limited circuit couples with the first charging capacitor and receives the detecting voltage, and outputs an over-voltage protective signal to the control circuit. 
   The CCFL driving system of the present invention has a lamp current equalizer, a lighting voltage-limited circuit, an open detecting circuit, a short detecting circuit, an overload detecting circuit, a brightness regulating circuit, a current detecting circuit, and an ON/OFF control device, wherein the CCFL driving system of a LCD panel drives a plurality of CCFLs simultaneously by utilizing an active electro-stabilizer or a DC/high-frequency AC converter through a transformer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic view of a CCFL driving system of the present invention; 
       FIG. 2  is a schematic circuitry used on the CCFL driving system of the first preferred embodiment of the present invention; 
       FIG. 3  is a circuit block diagram of the CCFL driving system of the second preferred embodiment of the present invention; 
       FIG. 4  is a detailed circuitry of the second preferred embodiment of the present invention; 
       FIG. 5  is a detailed circuitry of a multiple-CCFL system of the present invention; and 
       FIG. 6  shows a schematic diagram of a protective device being utilized in the CCFL driving system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is illustrated with a preferred embodiment and attached drawings. However, the invention is not intended to be limited thereby. 
   Reference is made to  FIG. 1 , which shows a schematic diagram of a CCFL (Cold Cathode Fluorescent Lamp) driving system provided in the present invention. The CCFL driving system includes a control circuit  1 , a CCFL  3  and a protective device  5  of the claimed invention. The protective device  5  outputs a variety of protective signals to the control circuit  1  as the CCFL  3  reaches a point of failure, in the meantime, the control circuit  1  can protect the CCFL  3  itself according to the various protective signals. 
     FIG. 2  shows a circuitry block diagram of the CCFL driving system of the first preferred embodiment. In this first embodiment, the protective device  5  has a short detecting circuit  501 , a rectified diode  506 , a first charging capacitor  507 , an open detecting circuit  502 , and a lighting voltage-limited circuit  503 . 
   Referring to the  FIG. 2  again, the short detecting circuit  501  of the protective device  5  couples with a high terminal of the CCFL  3  and a power supply Vcc through a step-down capacitor  509 . The short detecting circuit  501  outputs a short-circuit protective signal S 1  to the control circuit  1  as the high terminal of the CCFL  3  receives a high AC power (AC) and a shorting event occurs to the CCFL  3 . 
   Moreover, the rectified diode  506  couples with the high terminal of the CCFL  3  via a step-down capacitor  509 , wherein the rectified diode  506  is used to rectify the high AC power (AC), and outputs a high DC power (DC). The first charging capacitor  507  couples with the rectified diode  506 , and the first charging capacitor  507  generates a detecting voltage based on the above high DC power. The open detecting circuit  502  couples with the first charging capacitor  507  and the control circuit  1 . The open detecting circuit  502  has a threshold, and the open detecting circuit  502  outputs an open-circuit protective signal S 2  to the control circuit  1  when the detecting voltage is greater than the threshold. The lighting voltage-limited circuit  503  couples with the first charging capacitor  507  and the control circuit  1 . Furthermore, the lighting voltage-limited circuit  503  receives the detecting voltage, and outputs an over-voltage protective signal S 3  to the control circuit  1 . 
     FIG. 2  also shows a protective device  5  further having an overload detecting circuit  504  and a current detecting circuit  505 . The overload detecting circuit  504  outputs an overload protective signal S 4  to the control circuit  1  in response to the current flowing across the CCFL  3 . The current detecting circuit  505  outputs a power regulating signal S 5  to the control circuit  1  in response to the current flowing across the CCFL  3 . Moreover, the signals, such as the previously mentioned S 1 , S 2 , S 3 , S 4  and S 5 , outputted from the short detecting circuit  501 , the open detecting circuit  502 , the lighting voltage-limited circuit  503 , the overload detecting circuit  504  and the current detecting circuit  505  of the protective device  5  of the present invention are transmitted into the control circuit  1  via an electro-optical coupler (not shown in the diagram) or a PCB copper-foil (not shown in the diagram). 
   In view of  FIG. 2 , and referring to  FIG. 3 , which shows the second embodiment of the present invention illustrating the circuitry block diagram of the CCFL driving system, compared with the first preferred embodiment, the protective device  5  of the second preferred embodiment further includes a signal-hybrid circuit  508 . The signal-hybrid circuit  508  couples with the control circuit  1  and a contact between the short detecting circuit and the open detecting circuit. Next, the signal-hybrid circuit  508  receives the short-circuit protective signal S 1  and the open-circuit protective signal S 2 , and outputs the open/short-circuit protective signal S 6  to the control circuit  1 . In the second embodiment of the present invention, the signals, such as the previously mentioned S 3 , S 4 , S 5  and S 6 , outputted from the lighting voltage-limited circuit  503  of the protective device  5 , the overload detecting circuit  504 , the current detecting circuit  505  and the signal-hybrid circuit  508  can be transmitted to the control circuit  1  via the electro-optical coupler (not shown in the diagram) or the PCB copper-foil (now shown). 
   Reference is made to the second preferred embodiment shown in  FIG. 4  in view of  FIG. 3 , wherein the short detecting circuit  501  includes a transistor Q 1 , a second charging capacitor C 5 , a charging resistance RI, and a first forward diode D 1 . The transistor Q 1  further has an emitter, a collector and a base, wherein the emitter couples to a high terminal of the CCFL  3 , the base couples to a reference terminal G, and the second charging capacitor C 5  couples to the reference terminal G and the collector of the transistor Q 1 , and the charging resistance RI couples to the power supply Vcc and the collector of the transistor Q 1 , and the first forward diode D 1  couples to the collector of the transistor Q 1 . 
   The above-mentioned transistor Q 1  of the short detecting circuit  501  is turned off as the CCFL  3  shorts, in the meantime, the power supply Vcc charges the second charging capacitor C 5  via the charging resistance RI. After that, the short detecting circuit  501  outputs the short-circuit protective signal S 1  to the signal-hybrid circuit  508  since the voltage depressed on the second charging capacitor C 5  is bigger than the conducting voltage Vp of the first forward diode D 1 . 
   Reference is made to  FIG. 4 , the open detecting circuit  502  includes a second forward diode D 8  and a Zener diode ZD 1 .  FIG. 4  shows that the second forward diode D 8  couples to the first charging capacitor  507 , and receives a detecting voltage depressed on the first charging capacitor  507 . Moreover, the Zener diode ZD 1  couples to the second forward diode D 8 , wherein the open-circuit protective signal S 2  is outputted to the signal-hybrid circuit  508  since the detecting voltage is bigger than a conducting threshold of the Zener diode ZD 1 . The lighting voltage-limited circuit  503  shown in  FIG. 4  includes a third forward diode D 9  and a voltage divider composed of a serial connection of a first resistance R 5  and a second resistance R 6 . The voltage divider couples to the first charging capacitor  507  and divides the detecting voltage. The third forward diode D 9  couples to a contact of the first resistance R 5  and the second resistance R 6 , when the voltage across the mentioned contact is bigger than the conducting voltage of the third forward diode D 9 , the lighting voltage-limited circuit  503  outputs the over-voltage protective signal S 3  to the control circuit  1 . 
   Please refer to  FIG. 4  in view of  FIG. 3 , the signal-hybrid circuit  508  is composed of the coupled capacitor C 12 , resistances R 8 , R 9  and a MOS switch Q 4 . Moreover, the overload detecting circuit  504  is composed of the coupled diode D 7  and resistance R 4 , and the current detecting circuit  505  is composed of the coupled resistance R 7  and the diodes D 10 , D 11 . 
   Reference is made to  FIG. 5 , which shows a schematic diagram of the detailed circuit of a multiple-CCFL system of the present invention. When the protective device  5  of the present invention is operated on the multiple-CCFL system, each of the CCFLs  3  simultaneously use the first charging capacitor  507 , the open detecting circuit  502 , the lighting voltage-limited circuit  503 , the overload detecting circuit  504 , and the current detecting circuit  505 . However, each CCFL  3  also needs to couple a short detecting circuit  501 , a step-down capacitor  509  and a rectified diode  506  as well. Moreover, each mentioned short detecting circuit  501  couples with the high terminal of each CCFL  3  and the power supply Vcc via the step-down capacitor  509 , and each rectified diode  506  couples with the high terminal of each CCFL  3  via each step-down capacitor  509 . 
   Referring to  FIG. 6  in view of  FIG. 2 ,  FIG. 6  shows a schematic diagram of the CCFL driving system incorporating the protective device  5 . The present invention further provides the shown CCFL driving system used for the protective device  5 , wherein the CCFL driving system includes the protective device  5 , a lamp current equalizer  8 , a brightness regulating circuit  6 , and an ON/OFF control device  7 . Furthermore, the protective device  5  has a lighting voltage-limited circuit  503 , an open detecting circuit  502 , a short detecting circuit  501 , an overload detecting circuit  504 , and a current detecting circuit  505 . In this embodiment, the CCFL driving system further utilizes an active electro-stabilizer  2  or a DC/high-frequency AC converter (not shown) to drive the plurality of CCFLs  3  through a transformer  4 . 
   The aforementioned lamp current equalizer  8  couples to both terminals of the CCFL  3 . One terminal is an inductive element, and the other is a capacitive element. Moreover, the sources of the signals detected from the lighting voltage-limited circuit  503 , open detecting circuit  502  and the short detecting circuit  501  are the same. The signal detected from the overload detecting circuit  504  and the signal detected from the lighting voltage-limited circuit  503  are mixed, and a particular signal after the mixture is transmitted back to the active electro-stabilizer  2  via a feedback line (not shown in the diagram). Moreover, each CCFL  3  has its proprietary lighting voltage-limited circuit  503 . 
   The above-mentioned ON/OFF control device  7  can be a transistor or an electro-optical coupler. The current detecting circuit  505  is used to detect the electrical signal of the CCFL  3 , and the electrical signal is transmitted back to the active electro-stabilizer  2  via the feedback line (not shown in the diagram), so as to regulate the power of loading. The overload detecting circuit  504  is used to detect a current flowing a load, a terminal voltage of the load, or a resistance of the load. 
   From the above description, the feedback line is formed by an electro-optical coupler or a PCB copper foil, and the signal detected by the overload detecting circuit  504  and the signal detected by the lighting voltage-limited circuit  503  are transmitted back to the active electro-stabilizer  2  via the feedback line respectively. 
   To sum up, when the protective device  5  is operated on the multiple-CCFL system, the present invention provides a simplified circuit to protect the plurality of CCFLs. Therefore, the protective device  5  of the present invention efficiently improves the conventional scheme utilizing many protective circuits to protect the inverter and the CCFL of the multiple-CCFL system. The drawback of the conventional scheme includes: 
   1. high cost of manufacturing the LCD panel; 
   2. a complicated design of the protective circuit; and 
   3. a more difficult layout of the circuit. 
   While the invention has been described by means of a specification with accompanying drawings of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims.