Abstract:
A cold-cathode tube driving circuit for driving a plurality of substantially-C-shaped cold-cathode tubes, includes: a first transformer  2  having primary windings which induce voltages, and secondary windings which apply voltages of opposite polarities (−V, +V) to respective ends of the respective substantially-C-shaped cold-cathode tubes; and a second transformer  3  having third and fourth primary windings which respectively induce voltages, and third and fourth secondary windings which apply voltages of opposite polarities (−V, +V) to respective ends of the respective substantially-C-shaped cold-cathode tubes. In the cold-cathode tube driving circuit, the first and second transformers  2, 3  apply an in-phase voltage to adjacent ends of the substantially-C-shaped cold-cathode tubes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cold-cathode tube driving circuit, more particularly, to a driving circuit for driving a plurality of substantially-C-shaped cold-cathode tubes. 
     2. Description of the Related Art 
     JP-A-8-45675 describes a discharge lamp lighting apparatus of current feedback type having a switching circuit which causes the entirety or a portion of an electric current flowing through one or a plurality of discharge lamps to flow into an impedance device to thus change the impedance of the impedance device in accordance with the number of lighting circuits. According to the discharge lamp lighting apparatus, a tube current flowing through a single discharge lamp can be automatically maintained constant even when the number of discharge lamps or lighting circuits is arbitrarily changed within a preset range. Lighting of the discharge lamp is less susceptible to the influence of stray capacitance. 
     In JP-A-59-201398, an oscillation transformer having two secondary coils wound around a single iron core is used to light two discharge lamps connected in parallel, and is connected such that a d.c. current flows into the secondary coils in opposite directions, and first and second inductance coils are connected in opposite polarities. In the discharge lamp lighting apparatus, when one discharge lamp (the first discharge lamp) is first lighted, the current flows into the first inductance coil, whereby an induced voltage develops in the secondary inductance coil wound in opposite polarity. A high voltage is applied to the second discharge lamp by means of the induced voltage, to thus light the second discharge lamp without fail. Specifically, even when one discharge lamp has been lighted first, the voltage of the secondary coil used for supplying a voltage to another discharge lamp is prevented from dropping to the lamp voltage, thereby preventing failure to light a discharge lamp which is to be lighted later. 
     SUMMARY OF THE INVENTION 
     A cold-cathode tube of substantially-C-shaped type (hereinafter called “substantially-C-shaped cold-cathode tube”) is frequently employed as a cold-cathode tube to be used for a backlight of a liquid crystal display device (LCD), which entails a necessity to ensure a maximum lighting area within a limited area. In the substantially-C-shaped cold-cathode tube, supply of a cathode ray to corners becomes difficult as compared with supply of the cathode ray to the other area, so that the corners become dark. For this reason, a method for double-feeding power from both ends of the substantially-C-shaped cold-cathode tube has been employed. In practice, a plurality of substantially-C-shaped cold-cathode tubes are arranged side by side; an in-phase voltage (+V) is applied to upper portions of the substantially-C-shaped cold-cathode tubes; and a voltage (−V) of opposite phase is applied to lower portions of the same. A lower portion of one cold-cathode tube is adjacent to an upper portion of the next cold-cathode tube, to thus generate large stray capacitance, and a large electric potential difference arises between these two portions. Accordingly, a leakage current flows by way of stray capacitance, to thus flicker lighting of the cold-cathode tube. In such a case, there may arise a problem of a flicker arising in a screen of a liquid crystal display device, or the like. 
     JP-A-8-45675 states that an electric current flowing into a plurality of discharge lamps is made uniform when a change has arisen in the number of the discharge lamps, but does not state a countermeasure against the influence of stray capacitance in adjacent portions of the plurality of discharge lamps. JP-A-59-201398 also fails to state the arrangement of discharge lamps, nor does it state a measure against the influence of stray capacitance when a plurality of discharge lamps are provided adjacent to each other. 
     The object of the present invention is to prevent occurrence of a flicker in an substantially-C-shaped cold-cathode tube even when power is fed to both sides of the substantially-C-shaped cold-cathode tube. 
     [Means for Solving the Problem] 
     A cold-cathode tube driving circuit according to a first aspect of the present invention is a cold-cathode tube driving circuit for driving a plurality of substantially-C-shaped cold-cathode tubes, including: 
     a first transformer having first and second primary windings which respectively induce voltages, and first and second secondary windings which apply voltages of opposite polarities to respective ends of the respective substantially-C-shaped cold-cathode tubes; 
     a second transformer having third and fourth primary windings which respectively induce voltages, and third and fourth secondary windings which apply voltages of opposite polarities to respective ends of the respective substantially-C-shaped cold-cathode tubes; and 
     a driving circuit for driving the first and second transformers, wherein 
     an in-phase voltage is supplied to adjacent ends of the substantially-C-shaped cold-cathode tubes by means of controlling a direction of an electric current flowing through the second and third primary windings. 
     In the cold-cathode tube driving circuit, when a plurality of substantially-C-shaped cold-cathode tubes are arranged side by side, an in-phase voltage is applied to adjacent end portions of the adjacent substantially-C-shaped cold-cathode tubes, and hence no potential difference arises between mutually-adjacent horizontal portions of the substantially-C-shaped cold-cathode tubes, and occurrence of a leakage current, which would otherwise arise by way of stray capacitance, can be prevented. As a result, occurrence of a flicker in lighting of the substantially-C-shaped cold-cathode tubes can be prevented. Moreover, when the substantially-C-shaped cold-cathode tubes are used for a backlight of a liquid crystal display device, occurrence of a flicker in the screen of the liquid crystal display device can be effectively prevented. 
     In the cold-cathode tube driving circuit, voltages applied to adjacent ends of the substantially-C-shaped cold-cathode tubes can be made in phase with each other without changing the winding direction of the primary and secondary windings, by means of controlling the direction of an electric current flowing through second and third primary windings which drive the adjacent ends of the substantially-C-shaped cold-cathode tubes. Consequently, the voltages applied to the adjacent ends of the substantially-C-shaped cold-cathode tubes can be brought in phase with each other without making modifications to the structure of the first and second transformers. 
     A cold-cathode tube driving circuit according to a second aspect of the present invention is a cold-cathode tube driving circuit for driving a plurality of substantially-C-shaped cold-cathode tubes, including a first transformer, a second transformer, and a driving circuit for driving the first and second transformers. The first transformer has first and second primary windings which respectively induce voltages, and first and second secondary windings which apply voltages of opposite polarities to respective ends of the respective substantially-C-shaped cold-cathode tubes. The second transformer has third and fourth primary windings which respectively induce voltages, and third and fourth secondary windings which apply voltages of opposite polarities to respective ends of the respective substantially-C-shaped cold-cathode tubes. The first and second transformers are characterized by applying an in-phase voltage to adjacent ends of the substantially-C-shaped cold-cathode tubes. 
     In the cold-cathode tube driving circuit, when a plurality of substantially-C-shaped cold-cathode tubes are arranged side by side, an in-phase voltage is applied to adjacent end-portions of the adjacent substantially-C-shaped cold-cathode tubes, and hence no potential difference arises between mutually-adjacent horizontal portions of the substantially-C-shaped cold-cathode tubes, and occurrence of a leakage current, which would otherwise arise by way of stray capacitance, can be prevented. As a result, occurrence of a flicker in lighting of the substantially-C-shaped cold-cathode tubes can be prevented. Moreover, when the substantially-C-shaped cold-cathode tubes are used for a backlight of a liquid crystal display device, occurrence of a flicker in the screen of the liquid crystal display device can be effectively prevented. 
     A cold-cathode tube driving circuit according to a third aspect of the present invention is based on the cold-cathode tube driving circuit according to the previous aspect and characterized in that an in-phase voltage is supplied to adjacent ends of the substantially-C-shaped cold-cathode tubes by means of controlling a direction in which an electric current is caused to flow through the second and third primary windings. 
     In the cold-cathode tube driving circuit, voltages applied to adjacent ends of the substantially-C-shaped cold-cathode tubes can be brought in phase with each other without changing the winding direction of the primary and secondary windings, by means of controlling the direction of an electric current flowing through second and third primary windings which drive the adjacent ends of the substantially-C-shaped cold-cathode tubes. Consequently, the voltages applied to the adjacent ends of the substantially-C-shaped cold-cathode tubes can be brought in phase with each other without making modifications to the structure of the first and second transformers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electric circuit diagram of a cold-cathode tube driving circuit according to an embodiment of the present invention; and 
         FIG. 2  is a descriptive view for describing a reduction in the influence of stray capacitance existing between substantially-C-shaped cold cathode tubes. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Overall Configuration 
       FIG. 1  is an electric circuit diagram of a cold-cathode tube driving circuit  1  according to an embodiment of the present invention. 
     The cold-cathode tube driving circuit  1  has transistors Tr 1  and Tr 2 , a capacitor C, and oscillation transformers  2  and  3 . The transistors Tr 1  and Tr 2  and the capacitor C function as a driving circuit for driving the transformers  2  and  3 . 
     The transistors Tr 1  and Tr 2  are field-effect transistors (FETs), and drive the transformers  2  and  3  when being alternately turned on and off. Gates of the transistors Tr 1  and Tr 2  are connected to an unillustrated power source, and a switch used for activating the cold-cathode tube driving circuit  1  is interposed between the transistors Tr 1 , Tr 2 , and the power source. Specifically, at the time of activation of the cold-cathode tube driving circuit  1 , the switch is turned on and, subsequently, turned off, thereby causing a circuit, which comprises the transistors Tr 1 , Tr 2 , the capacitor C, and the oscillation transformers  2 ,  3 , to oscillate. By means of oscillation of the cold-cathode tube driving circuit  1 , substantially-C-shaped cold-cathode tubes  10  and  20  connected to an output side of the cold-cathode tube driving circuit  1  are lighted. Drains of the transistors Tr 1  and Tr 2  are connected to respective sides of the capacitor C, as well as being connected to the primary sides of the oscillation transformers  2  and  3 . The drain of the transistor Tr 1  is connected to a coil end of a primary winding L 1  of the oscillation transformer  2 , and the drain of the transistor Tr 2  is connected to a coil start of a primary winding L 2  wound in the same direction in which the primary winding L 1  is wound. The drain of the transistor Tr 2  is connected to a coil end of a primary winding L 3  of an oscillation transformer  3 , and the drain of the transistor Tr 1  is connected to a coil start of a primary winding L 4  wound in the same direction in which the primary winding L 3  is wound. 
     The oscillation transformer  2  has primary windings L 1 , L 2 , and L 9 ; secondary windings L 5  and L 6 ; and a core  4  around which the primary windings L 1 , L 2 , and L 9  and the secondary windings L 5  and L 6  are wound. The primary windings L 1 , L 2 , and L 9  and the secondary windings L 5  and L 6  are wound in the same direction. The coil end of the primary winding L 1  is connected to the drain of the transistor Tr 1 ; the coil start of the primary winding L 2  is connected to the drain of the transistor Tr 2 ; and the coil start of the primary winding L 1  and the coil end of the primary winding L 2  are connected to a power source VDD. The coil start of the primary winding L 9  is connected to the gate of the transistor Tr 1  by way of a resistor R 1 , and the coil end of the primary winding L 9  is connected to the gate of the transistor Tr 2  by way of a resistor R 2 . The coil start of the secondary winding L 5  is connected to the ground, and the coil end of the secondary winding L 5  is connected to an upper end portion  14  of the substantially-C-shaped cold-cathode tube  10 . The coil start of the secondary winding L 6  is connected to a lower end portion  15  of the substantially-C-shaped cold-cathode tube  10 , and the coil end of the secondary winding L 6  is connected to the ground. 
     The oscillation transformer  3  has primary windings L 3 , L 4 , and L 10 ; secondary windings L 7  and L 8 ; and a core  5  around which the primary windings L 3 , L 4 , and L 10  and the secondary windings L 7  and L 8  are wound. The primary windings L 3 , L 4 , L 10  and the secondary windings L 7  and L 8  are wound in the same direction. The coil end of the primary winding L 3  is connected to the drain of the transistor Tr 2 ; the coil start of the primary winding L 4  is connected to the drain of the transistor Tr 1 ; and the coil start of the primary winding L 3  and the wind end of the primary winding L 4  are connected to the power source VDD. In the above connection, an electric current flows in opposite directions in the primary windings L 2  and L 3 . Consequently, voltages of opposite polarities are induced in the primary windings L 2  and L 3 , and voltages of opposite polarities are induced in the secondary windings L 6  and L 7 , as well. The primary winding L 10  is not used and is opened. The coil start of the secondary winding L 7  is connected to the ground, and the coil end of the secondary winding L 7  is connected to an upper end portion  24  of the substantially-C-shaped cold-cathode tube  20 . The coil start of the secondary winding L 8  is connected to a lower end portion  25  of the substantially-C-shaped cold-cathode tube  20 , and the coil end of the secondary winding L 8  is connected to the ground. 
     The cold-cathode tubes  10  and  20 , which are shown in  FIG. 1 , are mounted on, e.g., a liquid crystal display device and used as backlights. The cold-cathode tubes  10  and  20  are substantially-C-shaped cold-cathode tubes. When the cold-cathode tubes are mounted on a liquid crystal display device, a plurality of cold-cathode tubes are usually arranged as shown in  FIG. 1 . Here is described a case where the cold-cathode tubes  10  and  20  are arranged vertically. However, the present embodiment can be applied similarly to a case where the cold-cathode tubes  10  and  20  are arranged side by side; namely, where the cold-cathode tubes  10  and  20  shown in  FIG. 1  are rotated clockwise or counterclockwise through 90°, so long as a mutual positional relationship between the cold-cathode tubes  10  and  20  is maintained. 
     The cold-cathode tube  10  has horizontal portions  11  and  12 , and a vertical portion  13  for coupling together the horizontal portions  11  and  12 . The coil end of the secondary winding L 5  of the oscillation transformer  2  is connected to the upper end portion  14  of the cold-cathode tube  10 . The coil start of the secondary winding L 6  of the oscillation transformer  3  is connected to the lower end portion  15  of the cold-cathode tube  10 . 
     The cold-cathode tube  20  has horizontal portions  21  and  22 , and a vertical portion  23  coupling together the horizontal portions  21  and  22 . The coil end of the secondary winding L 7  of the oscillation transformer  3  is connected to the upper end portion  24  of the cold-cathode tube  20 . The coil start of the secondary winding L 8  of the oscillation transformer  3  is connected to the lower end portion  25  of the cold-cathode tube  20 . 
     (2) Working-Effects 
     In the cold-cathode tube driving circuit  1  shown in  FIG. 1 , the transistors Tr 1  and Tr 2  are alternately turned on and off, to thus drive the oscillation transformers  2  and  3 . In the oscillation transformers  2  and  3 , the voltages of opposite polarities are induced in the primary windings L 2  and L 3 , and the voltages of opposite polarities are induced in the secondary windings L 6  and L 7 , as well. The coil start of the secondary winding L 6  is connected to the lower end portion  15  of the cold-cathode tube  10 , whereas the coil end of the secondary winding L 7  is connected to the upper end portion  24  of the cold-cathode tube  20 . Specifically, the voltages of opposite polarities are induced in the secondary windings L 6  and L 7 , whilst the coil start of the secondary winding L 6  and the coil end of the secondary winding L 7  become in phase with each other (−V). Consequently, the lower end portion  15  of the cold-cathode tube  10  and the upper end portion  24  of the cold-cathode tube  20  are in phase with each other (−V), and the horizontal portion  12  of the cold-cathode tube  10  and the horizontal portion  21  of the cold-cathode tube  20  are also in phase with each other. Meanwhile, the voltages of opposite polarities are induced in the primary windings L 1  and L 4 , and the voltages of opposite polarities are induced in the secondary windings L 5  and L 8 . Now, the coil end of the secondary winding L 5  is connected to the upper end portion  14  of the cold-cathode tube  10 , whilst the coil start of the secondary winding L 8  is connected to the lower end portion  25  of the cold-cathode tube  20 . Specifically, the voltages of opposite polarities are induced in the secondary windings L 5  and L 8 , but the coil end of the secondary winding L 5  and the coil end of the secondary winding L 8  become in phase with each other. The voltage appearing in the coil end of the secondary winding L 5  and the voltage appearing in the coil end of the secondary winding L 8  are +V and opposite in polarity to the voltage appearing in the coil start of the secondary winding L 6  and the voltage appearing in the coil end of the secondary winding L 7 . 
     As mentioned above, according to the cold-cathode tube driving circuit  1 , the voltages appearing in the adjacent horizontal portions  12  and  21  of the substantially-C-shaped cold-cathode tubes  10  and  20  are equal in phase to each other, and no potential difference arises between the horizontal portions  12  and  21 . 
       FIG. 2  is a descriptive view for describing the principle that the influence of stray capacitance Cs existing between the substantially-C-shaped cold-cathode tubes  10  and  20  is diminished by the cold-cathode tube driving circuit  10  of the present embodiment. As shown in  FIG. 2 , when the substantially-C-shaped cold-cathode tubes  10  and  20  are arranged vertically, the lower horizontal portion  12  of the cold-cathode tube  10  and the upper horizontal portion  21  of the cold-cathode tube  20  are adjacent to each other and approach each other. In the cold-cathode tubes  10  and  20  arranged as shown in  FIGS. 1 and 2 , the stray capacitance Cs existing between the mutually-adjacent horizontal portions  12  and  22  becomes extremely large. When the voltage appearing in the horizontal portion  12  is not in phase with the voltage appearing in the horizontal portion  22  at that time, a potential difference arises between the horizontal portions  12  and  22 . When a potential difference has arisen between the horizontal portions  12  and  22 , a very large leakage current flows between the horizontal portions  12  and  22 , because the stray capacitance Cs is extremely large, thereby making lighting of the cold-cathode tubes  10  and  20  unstable. As shown in  FIG. 2 , the cold-cathode tube driving circuit  1  of the present embodiment drives, as a measure to prevent instability of lighting of the cold-cathode tubes, the cold-cathode tubes  10  and  20  such that the mutually-approaching horizontal portions  12  and  22  become in phase with each other (−V), to thus be able to prevent occurrence of a potential difference between the horizontal portions  12  and  22  as well as to flow of a leakage current between the horizontal portions  12  and  22  even when the value of the stray capacitance Cs is large. Namely, the cold-cathode tube driving circuit  1  of the present embodiment drives the cold-cathode tubes  10  and  20  such that the mutually-approaching horizontal portions  12  and  22  become in phase with each other (−V), thereby diminishing and preventing the influence of the stray capacitance Cs on lighting of the cold-cathode tubes  10  and  20 . Thus, lighting of the cold-cathode tubes  10  and  20  is made stable, and occurrence of a flicker in the screen of the liquid crystal display device can be prevented.