Patent Publication Number: US-2010110665-A1

Title: Lighting device for display device and display device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lighting device for a display device and a display device including a lighting device. 
     2. Description of the Related Art 
     In a display device having non-luminous optical elements as typified by a liquid crystal display device, a backlight device is provided on the backside of a display panel such as a liquid crystal panel, so as to illuminate the display panel (as shown in JP-A-2006-66360, for example). 
     JP-A-2006-66360 discloses a backlight assembly that includes lamps and a housing member for holding the lamps. In the backlight assembly thus including lamps and a housing member for holding the lamps, beat tones may be generated during dimming control of the lamps, due to the second and third harmonics of a dimming control frequency. 
     There are various theories as to how the beat tones are generated. For example, one of the theories suggests involvement of current leakage from the lamps to the housing member. That is, the beat tones may be generated by vibration of the housing member caused by leakage current from the lamps. 
     JP-A-2006-66360 discloses that bulging portions or recessed portions corresponding to the lamps are formed on the housing member in order to prevent current leakage between the lamps and the housing member. However, the beat tones cannot be reduced adequately even according to this construction. Moreover, if bulging or recessed portions are provided, the housing member may be prone to having increased thickness due to the bulging or recessed portions. This is a significant problem, particularly where thinning of liquid crystal display devices is being required. 
     SUMMARY OF THE INVENTION 
     Accordingly, preferred embodiments of the present invention provide a lighting device for a display device having a simple construction capable of preventing or suppressing beat tones generated on a lamp housing member, without increasing the thickness of the device. Preferred embodiments of the present invention also provide a high-quality and highly-reliable display device including the lighting device. 
     A lighting device for a display device according to a preferred embodiment of the present invention includes a light source and a chassis arranged to cover the light source, in which the chassis includes an opening section located directly below the light source. 
     The inventor of the present application has repeatedly considered measures for beat tones, and consequently the beat tones can be substantially eliminated when the chassis includes an opening section located directly below the light source. This may be due to major reduction of current leakage from the light source to the chassis. That is, when the chassis thus includes an opening section, the distance between the light source and the chassis can be infinitely large at the opening section. Accordingly, the leakage current may be substantially eliminated, which is expressed by the following formula (I): 
         I= 2 πfεCV= 2 πf ε( S/d ) V   formula (I) 
     where “I” is the amount of leakage current, “C” is the stray capacitance, “V” is the potential difference between the light source and the chassis, “S” is the area of the chassis, and “d” is the distance between the light source and the chassis. 
     JP-A-2006-66360 discloses a construction, also as a measure against the current leakage, in which bulging portions or recessed portions corresponding to the lamps or light sources are formed on the housing member or chassis. However, the beat tones cannot be sufficiently eliminated, when the above bulging or recessed portions are provided as a measure for beat tones. This may be because the slightly longer distance between the light sources and the chassis, caused by the bulging or recessed portions, fails to result in sufficient elimination of the beat tones. Further, in the construction thus including bulging or recessed portions, the chassis can vibrate at the bulging or recessed portions. 
     In contrast, according to a preferred embodiment of the present invention, the opening section is provided directly below the light source, so that the chassis originally has no tangible portion directly below the light source. Accordingly, the chassis cannot vibrate at the position directly below the light source, and therefore the beat tones are substantially eliminated. That is, preferred embodiments of the present invention provide the opening section but not merely as a measure for current leakage. The beat tones are originally generated at an area directly below the light source, and therefore the area is removed from the chassis so as to form an opening section in order to achieve elimination of the beat tones. Particularly, preferred embodiments of the present invention contribute greatly to reduction in size of the lighting device for a display device, because bulges on the chassis, which are generated due to the bulging or recessed portions as in JP-A-2006-66360, are prevented. 
     Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view showing the general construction of a liquid crystal display device according to a preferred embodiment of the present invention. 
         FIG. 2  is a sectional view of the liquid crystal display device shown in  FIG. 1 . 
         FIG. 3  is a perspective view showing the general construction of a chassis included in the liquid crystal display device shown in  FIG. 1 . 
         FIG. 4  is a perspective view separately showing sheets and the like to be attached to the chassis. 
         FIG. 5  is a plan view showing the general construction of the chassis. 
         FIG. 6  is an exploded perspective view showing the general construction of a liquid crystal display device as a modification. 
         FIG. 7  is a plan view showing the general construction of a chassis applied to the liquid crystal display device shown in  FIG. 6 . 
         FIG. 8  is an explanatory diagram showing a driving scheme for cold cathode tubes, which is applied to the liquid crystal display device shown in  FIG. 1  or  6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be hereinafter explained with reference to the drawings. 
       FIG. 1  is an exploded perspective view showing the general construction of a liquid crystal display device according to a preferred embodiment of the present invention.  FIG. 2  is a sectional view showing the general construction of the liquid crystal display device.  FIG. 3  is a perspective view showing the general construction of a chassis or backlight chassis included in the liquid crystal display device of the present preferred embodiment.  FIG. 4  is a perspective view separately showing sheets and the like to be attached to the chassis.  FIG. 5  is a plan view showing the general construction of the chassis.  FIG. 6  is an exploded perspective view showing the general construction of a liquid crystal display device as a modification.  FIG. 7  is a plan view showing the general construction of a chassis applied to the liquid crystal display device shown in  FIG. 6 .  FIG. 8  is an explanatory diagram showing a driving scheme for cold cathode tubes, which is applied to the liquid crystal display device shown in  FIG. 1  or  6 . 
     The general construction of the liquid crystal display device  10  according to the present preferred embodiment will be explained first. Referring to  FIGS. 1 and 2 , the liquid crystal display device  10  includes a liquid crystal panel  11  having a rectangular or substantially rectangular shape, and a backlight device  12  (lighting device for a display device) as an external light source, which are integrally held by a bezel  13  and the like. The liquid crystal panel  11  includes a pair of glass substrates, which are attached to each other so as to face each other while a gap of a predetermined size is kept therebetween. Liquid crystal is sealed between the glass substrates. On one of the glass substrates, components such as switching elements (e.g., TFTs) connected to source wiring lines and gate wiring lines running at right angles to each other, and pixel electrodes connected to the switching elements are provided. On the other of the glass substrates, components such as a counter electrode, a color filter having R, G, and B color sections arranged in a predetermined pattern are provided. 
     Next, the backlight device  12  will be explained. The backlight device  12  preferably is a so-called direct-light type backlight device that includes a plurality of linear light sources (e.g., cold cathode tubes (tubular light sources)  17  as high-pressure discharge tubes, in the present preferred embodiment), which are positioned directly below the back surface of the liquid crystal panel  11  (i.e., the panel surface on the opposite side of the display side), and are arranged along the panel surface. 
     The backlight device  12  includes a metallic backlight chassis  14  having a substantially box-like shape with an opening on its upper side, and a plurality of optical members  15  (e.g., a diffuser plate, a diffusing sheet, a lens sheet and an optical sheet, in order from the lower side of the figure) which are arranged to cover in the opening of the backlight chassis  14 . Further included are a frame  16  arranged to hold the optical members  15  on the backlight chassis  14 , cold cathode tubes or light sources  17  contained in the backlight chassis  14 , rubber (e.g., silicon rubber) holders  18  arranged to hold the end portions of the cold cathode tubes  17 , lamp holders  19  arranged to collectively cover the cold cathode tubes  17  and the holders  18 , and lamp clips  20  arranged to mount and hold the cold cathode tubes  17  on the backlight chassis  14 . Note that the optical member  15  side of the cold cathode tubes  17  corresponds to the light emitting side of the backlight device  12 . 
     Each of the cold cathode tubes  17  defines a tubular shape elongated in one direction. A number (e.g., sixteen in  FIG. 1 ) of cold cathode tubes  17  are contained in the backlight chassis  14  so that the longitudinal direction (or axial direction) thereof conforms with the longitudinal direction of the backlight chassis  14 . On the other hand, the lamp clips  20 , arranged to mount the cold cathode tubes  17  to the backlight chassis  14 , function as clip members for holding light sources, and are preferably made of synthetic resin (e.g., polycarbonate). The plurality of lamp clips  20  are mounted on the backlight chassis  14  so as to support each of the cold cathode tubes  17  at two or three points spaced along the longitudinal direction thereof. 
     The substantially box-like backlight chassis  14  is preferably defined by a metallic plate. A light reflecting sheet  14   a  is provided on the inner surface side (light source side) of the backlight chassis  14 , which defines a light reflecting surface. The backlight chassis  14  thus includes the light reflecting sheet  14   a , and thereby the light from the cold cathode tubes  17  can be reflected to the optical members  15  such as the diffuser plate (hereinafter, sometimes referred to as “the diffuser plate  15  and the like”). The light reflecting sheet  14   a  can be formed of a resin sheet having light reflectivity, for example. 
     On the chassis  14 , opening sections  55  are formed to be located directly below the respective cold cathode tubes  17 . Each opening section  55  is formed preferably by partially removing the chassis  14 , so as to have an elongated shape along the axial direction of a cold cathode tube  17  (See  FIG. 3 ). The opening sections  55  are arranged to defined a striped configuration formed of strips along the array of the cold cathode tubes  17 . Referring to  FIG. 5 , each opening section  55  is formed preferably to have a width larger than the line-width of the cold cathode tube  17 , so that it appears to overlap with the cold cathode tube  17  when viewed planarly. Specifically, the outer diameter of the cold cathode tube  17  is preferably between about 3.4 mm and about 4.0 mm, while the width of the opening section  55  preferably is between about 10 mm and about 20 mm, for example. 
     The opening sections  55  are provided on the inner side of the light reflecting sheet  14   a , and therefore are shown by broken lines in  FIG. 1 . The opening sections  55  are preferably made during the sheet processing of the chassis  14 , in the present preferred embodiment. 
     The light reflecting sheet  14   a  is provided on the inner surface side of the chassis  14  as described above, while a light blocking sheet  14   b  is provided on the outer surface side of the chassis  14  as shown in  FIGS. 2 and 4 . The light blocking sheet  14   b  is arranged to cover at least the opening sections  55  of the chassis  14 , and is bonded or screwed to the chassis  14 . The light blocking sheet  14   b  can be formed of a polycarbonate-resin or acrylic-resin sheet as a molded piece to which lightproof coating material is applied, for example. Further preferably, the light blocking sheet  14   b  also has resistance to high temperatures, because the cold cathode tubes  17  generate heat. 
     An inverter board  50  arranged to supply a drive voltage to the cold cathode tubes  17  is mounted to the chassis  14 , or specifically, mounted on the opposite side of the cold cathode tubes  17  (i.e., on the opposite side of the light emitting surface). The inverter board  50  includes an inverter circuit that generates a high-frequency voltage for lighting the cold cathode tubes  17 . Specifically, in the present preferred embodiment, the inverter circuit is connected to one of two end portions of each cold cathode tube  17 , and therefore the one end portion is subjected to high voltage during lighting. Referring to  FIG. 8 , in the present preferred embodiment, the cold cathode tubes  17  are driven by pulse-width modulation (PWM), for example. Thereby, the dimming control is performed in a predetermined cycle. 
     The liquid crystal display device  10  thus constructed according to the present preferred embodiment can provide the following operational effects. In the liquid crystal display device  10  of the present preferred embodiment, the chassis  14  of the backlight device  12  includes opening sections  55  located directly below the respective cold cathode tubes  17 . According to this unique construction, the chassis  14  is less likely to generate beat tones, which can be caused by vibration thereof. 
     The beat tones generated on the chassis  14  may be caused by vibration of the chassis  14 . The vibration may result from various factors, and the factors include current leakage from the cold cathode tubes  17 . 
     The chassis  14  is preferably formed of a conductive metal plate, and therefore a capacitor may be formed between the cold cathode tube  17  and the chassis  14 . Accordingly, an ordinary construction (not including opening sections  55 ) may be prone to current leakage from the cold cathode tubes  17  to the chassis  14 . A force acting on the chassis  14  can be generated due to the leakage current, which causes the chassis  14  to vibrate resulting in beat tones. Particularly, in the case of pulse-width modulation, the leakage current can be periodic, and therefore a periodic force acts on the chassis  14  so as to generate beat tones. 
     In contrast, according to the present preferred embodiment, the opening sections  55  directly below the cold cathode tubes  17  are provided on the chassis  14 , so that the possibility of current leakage described above is minimized. Consequently, beat tones can be prevented or suppressed. That is, when the opening sections  55  are thus provided, the distance between the cold cathode tube  17  and the chassis  14  can be infinitely large at each opening section  55 . Accordingly, the leakage current may be substantially eliminated, which is expressed by the following formula (I): 
         I= 2 πfεCV= 2 πf ε( S/d ) V   formula (I) 
     where “I” is the amount of leakage current, “C” is the stray capacitance, “V” is the potential difference between the cold cathode tubes  17  and the chassis  14 , “S” is the area of the chassis  14 , and “d” is the distance between the cold cathode tubes  17  and the chassis  14 . 
     Particularly, according to the construction in which opening sections  55  are provided directly below the cold cathode tubes  17  as in the present preferred embodiment, the chassis  14  cannot vibrate at the positions directly below the cold cathode tubes  17 , because the chassis  14  originally has no tangible portion directly below the cold cathode tubes  17 . Accordingly, beat tones can be substantially eliminated. 
     Further, the opening sections  55  provided for prevention or suppression of current leakage, as in the present preferred embodiment, enable a simple construction, which contributes to thinning the backlight device  12  and therefore to thinning the liquid crystal display device  10 . 
     Moreover, in the present preferred embodiment, the light blocking sheet  14   b  is attached to the chassis  14  so as to cover the opening sections  55 . Thereby, the light passing through the opening sections  55  can be prevented or suppressed. Consequently, the quality reduction of the backlight device  12 , and therefore of the liquid crystal display device  10 , can be prevented or suppressed. 
     Shown above is a preferred embodiment of the present invention. However, the present invention is not limited to the preferred embodiment explained in the above description made with reference to the drawings. The following preferred embodiments may be included in the technical scope of the present invention, for example, and further the present invention may be embodied in various forms without departing from the scope of the invention. 
     In the liquid crystal display device  10  shown in  FIG. 1 , the opening sections  55  are preferably formed to extend along the longitudinal direction of the cold cathode tubes  17 . However, opening sections may be formed to be located directly below the high voltage areas of the respective cold cathode tubes  17 , for example.  FIG. 6  shows an example of a liquid crystal display device in which each opening section  56  is provided directly below the high-voltage-side end portion of a cold cathode tube  17  that is one of the end portions of the cold cathode tube  17 .  FIG. 7  is a plan view of the chassis  14  thereof. 
     The provision of the opening sections on the chassis  14  may result in disadvantages such as strength reduction of the chassis  14 . In view of this, the opening sections  56  are solely provided on the areas prone to generating beat tones or on the areas directly below the areas to be subjected to high voltage, as shown in  FIGS. 6 and 7 . Thereby, beat tones are effectively prevented while the disadvantages, such as insufficient strength, of providing the opening sections are minimized. Note that the inverter board  50  having an inverter circuit is arranged on the side of the high-voltage-side end portion. That is, the portion, connected to the inverter circuit or to an electric circuit that generates a high-frequency voltage for lighting light sources, is provided as a high-voltage-side end portion. Therefore, it is preferable to provide an opening section on the area of the chassis  14  that corresponds to the end portion connected to the inverter circuit. 
     In the above preferred embodiment, the backlight device, in which one end portion of each cold cathode tube  17  is arranged to be subjected to high voltage, is shown for illustrative purposes. However, the above construction can be employed on a backlight device  12  in which both end portions of each cold cathode tube  17  are arranged to be subjected to high voltage. That is, in this construction, opening sections can be formed on the chassis  14  so as to be located directly below the respective two end portions of each cold cathode tube  17 . 
     In the above preferred embodiment, cold cathode tubes  17  are preferably used as light sources. However, the present invention can include a construction in which another type of light sources such as hot cathode tubes is used, for example. 
     In the above preferred embodiment, TFTs are preferably used as switching elements of the liquid crystal display device. However, the present invention can be applied to a liquid crystal display device that uses another type of switching elements than TFTs (e.g., thin-film diodes (TFDs)). Further, the present invention can be applied to a liquid crystal display device for monochrome display, as well as a liquid crystal display device capable of color display. 
     Moreover, although a liquid crystal display device is shown in the above preferred embodiment, the present invention can be applied to other types of display devices than a liquid crystal type, which use a backlight device. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.