Patent Publication Number: US-2010118230-A1

Title: Backlight device and liquid crystal display device

Description:
TECHNICAL FIELD 
     The present invention relates to a backlight device, and to a liquid crystal display device provided with the backlight device. 
     BACKGROUND ART 
     In general, a liquid crystal display device, except for a reflection type that forms images by reflecting external light, is provided with a backlight device. Some of such backlight devices utilize fluorescent tubes or light-emitting diodes as light sources. The fluorescent tube is mostly used as a light source for the backlight device, with a light amount and a service life being taken into consideration. 
     Further, in the liquid crystal display device, dimming of the backlight device is performed automatically or manually in accordance with the ambient light intensity, the contrast of a display image, and the like. As the dimming method adopted when a fluorescent tube is used as a light source of the backlight device, the current dimming method and the PWM (Pulse Width Modulation) dimming method are known. 
     The current dimming method, in which the dimming is performed by increasing or decreasing current passing through the fluorescent tube, has a difficulty in widening a dimming range. Therefore, the PWM dimming method is adopted in many backlight devices. In the PWM dimming method, the dimming is performed in such a manner that the output of an inverter circuit is switched on/off forcibly, and a ratio between the on-time and the off-time (duty ratio) is modulated. The PWM dimming method allows a wider dimming range to be achieved compared to the current dimming method. 
     However, even if the PWM dimming method is adopted, there is a limit on the widening of the dimming range. On the other hand, in recent years, a wider dimming range has been demanded for improving the image quality of display images. In view of this, the dimming performed by the PWM dimming method and the current dimming method in combination has been proposed (for example, refer to JP 2003-359097 A). 
     In a backlight device disclosed by JP 2003-359097 A, when an especially high contrast ratio is required, a pulse signal for performing the PWM dimming and a voltage signal for performing the current dimming are multiplied together, and the signal obtained by the multiplication (drive pulse) is fed to the inverter circuit. In this case, since the level of the drive pulse becomes higher than that in the case in which only the PWM dimming is performed, the value of the current passing through the fluorescent tube is increased, whereby brightness of a display screen is increased. Therefore, a wider dimming range (an improved contrast ratio) can be obtained compared to the backlight device in which the dimming is performed by the PWM dimming method alone. 
     DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
     However, in the backlight device disclosed by JP 2003-359097 A, although the increased current passing through the fluorescent tube allows the dimming range (contrast ratio) to be wider compared to the backlight device in which only the PWM dimming is adopted, it causes the service life of the fluorescent tube to decrease. 
     Therefore, it is an object of the present invention to solve the above-described problem, and to provide a backlight device capable of improving a contrast ratio while suppressing a decrease in a service life of a light source, and a liquid crystal display device using the backlight device. 
     Means for Solving Problem 
     In order to achieve the aforementioned object, a backlight device according to the present invention includes: an optical layer having one of main surfaces thereof as a light outgoing surface; a main light source that is disposed at a position facing the other main surface of the optical layer and emits light toward the optical layer; a light guide plate that is disposed on a side of the other main surface of the optical layer, so as to be parallel to the optical layer, and has a main surface on an optical layer side as a light outgoing surface; a sub-light source that is disposed at a position facing a side surface of the light guide plate and emits light toward the side surface; and a driving portion for driving the main light source and the sub-light source, wherein the main light source includes a plurality of fluorescent tubes disposed parallel to the optical layer, and the driving portion outputs drive pulses with respect to the plurality of the fluorescent tubes, respectively, and adjusts light amounts of the plurality of the fluorescent tubes by modulating pulse widths of the drive pulses. 
     In order to achieve the aforementioned object, a liquid crystal display device according to the present invention includes: a liquid crystal display panel; and a backlight device for illuminating the liquid crystal display panel from a back surface thereof, wherein the backlight device includes: an optical layer having one of main surfaces thereof as a light outgoing surface; a main light source that is disposed at a position facing the other main surface of the optical layer and emits light toward the optical layer; a light guide plate that is disposed on a side of the other main surface of the optical layer, so as to be parallel to the optical layer, and has a main surface on an optical layer side as a light outgoing surface; a sub-light source that is disposed at a position facing a side surface of the light guide plate and emits light toward the side surface; and a driving portion for driving the main light source and the sub-light source, wherein the main light source includes a plurality of fluorescent tubes disposed parallel to the optical layer, and the driving portion outputs drive pulses with respect to the plurality of the fluorescent tubes, respectively, and adjusts light amounts of the plurality of the fluorescent tubes by modulating pulse widths of the drive pulses. 
     EFFECTS OF THE INVENTION 
     In the present invention, a backlight device is provided with a sub-light source in addition to a main light source, and therefore, can obtain an amount of light that can hardly be obtained by the conventional backlight device that is provided with only a main light source. Besides, in the present invention, this makes it unnecessary to perform current dimming with respect to a fluorescent tube in addition to PWM dimming as in the conventional case, thereby suppressing an increment in a value of current passing through the fluorescent tube (main light source). Therefore, the backlight device according to the present invention is capable of improving a contrast ratio while suppressing a decrease in a service life of a light source. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view three-dimensionally showing a schematic configuration of an optical portion of a backlight device according to Embodiment 1 of the present invention. 
         FIG. 2  is a side view of the optical portion of the backlight device shown in  FIG. 1 . 
         FIG. 3  is a block diagram showing configurations of the backlight device and a liquid crystal display device according to Embodiment 1 of the present invention. 
         FIG. 4  is a block diagram showing a specific configuration of a driving portion constituting the backlight device shown in  FIG. 3 . 
         FIGS. 5(   a ) to  5 ( d ) are diagrams showing exemplary drive pulses for a main light source that have different duty ratios, respectively. 
         FIGS. 6(   a ) to  6 ( d ) are diagrams showing exemplary drive pulses for a sub-light source that have different duty ratios, respectively. 
         FIG. 7  is a block diagram showing configurations of a backlight device and a liquid crystal display device according to Embodiment 2 of the present invention. 
         FIG. 8  is a block diagram showing a specific configuration of a driving portion constituting the backlight device shown in  FIG. 7 . 
         FIG. 9  is a circuit diagram exclusively showing a portion for driving the sub-light source in the driving portion shown in  FIG. 7 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A backlight device according to the present invention includes: an optical layer having one of main surfaces thereof as a light outgoing surface; a main light source that is disposed at a position facing the other main surface of the optical layer and emits light toward the optical layer; a light guide plate that is disposed on a side of the other main surface of the optical layer, so as to be parallel to the optical layer, and has a main surface on an optical layer side as a light outgoing surface; a sub-light source that is disposed at a position facing a side surface of the light guide plate and emits light toward the side surface; and a driving portion for driving the main light source and the sub-light source, wherein the main light source includes a plurality of fluorescent tubes disposed parallel to the optical layer, and the driving portion outputs drive pulses with respect to the plurality of the fluorescent tubes, respectively, and adjusts light amounts of the plurality of the fluorescent tubes by modulating pulse widths of the drive pulses. 
     Further, a liquid crystal display device according to the present invention includes: a liquid crystal display panel; and a backlight device for illuminating the liquid crystal display panel from a back surface thereof, wherein the backlight device includes: an optical layer having one of main surfaces thereof as a light outgoing surface; a main light source that is disposed at a position facing the other main surface of the optical layer and emits light toward the optical layer; a light guide plate that is disposed on a side of the other main surface of the optical layer, so as to be parallel to the optical layer, and has a main surface on an optical layer side as a light outgoing surface; a sub-light source that is disposed at a position facing a side surface of the light guide plate and emits light toward the side surface; and a driving portion for driving the main light source and the sub-light source, wherein the main light source includes a plurality of fluorescent tubes disposed parallel to the optical layer, and the driving portion outputs drive pulses with respect to the plurality of the fluorescent tubes, respectively, and adjusts light amounts of the plurality of the fluorescent tubes by modulating pulse widths of the drive pulses. 
     The backlight device and the liquid crystal display device according to the aforementioned present invention may be configured so that the sub-light source is a fluorescent tube disposed along the side surface of the light guide plate, and the driving portion further outputs a drive pulse with respect to the fluorescent tube constituting the sub-light source and adjusts a light amount of the fluorescent tube constituting the sub-light source by modulating a pulse width of the drive pulse. 
     Further, the backlight device and the liquid crystal display device according to the present invention described above may be configured so that the sub-light source is a light-emitting diode disposed along the side surface of the light guide plate, and the driving portion adjusts a light amount of the light-emitting diode constituting the sub-light source by modulating a value of current supplied to the light-emitting diode constituting the sub-light source. 
     In the backlight device and the liquid crystal display device according to the present invention described above, the main light source preferably is disposed between the optical layer and the light guide plate. In this case, a decline in output efficiency of the main light source can be suppressed. 
     The liquid crystal display device according to the present invention described above preferably further includes a control portion that causes the driving portion to adjust the light amounts of the main light source and the sub-light source in accordance with a configuration of an image displayed on a display screen of the liquid crystal display panel. In this case, the image quality of the display image can be improved. 
     Embodiment 1 
     Hereinafter, a backlight device and a liquid crystal display device in Embodiment 1 of the present invention will be described with reference to  FIGS. 1 to 6 . First, a schematic configuration of an optical portion of the backlight device in the present Embodiment 1 will be described with reference to  FIGS. 1 and 2 .  FIG. 1  is a perspective view three-dimensionally showing a schematic configuration of an optical portion of a backlight device according to Embodiment 1 of the present invention.  FIG. 2  is a side view of the optical portion of the backlight device shown in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , a backlight device  1  in the present Embodiment 1 includes the following as an optical portion: an optical layer  2 , a main light source  3 , a light guide plate  4 , and a sub-light source  5 . Although it is not shown in  FIG. 1  or  2 , the backlight device  1  further includes a driving portion (see  FIG. 3 ) for driving the main light source  3  and the sub-light source  5 . 
     The optical layer  2  uniforms brightness by diffusing light from the main light source  3  and light from the sub-light source  5  emitted via the light guide plate  4 . One main surface  2   a  of the optical layer  2  serves as a light outgoing surface of the backlight device  1 . The optical layer  2  is formed by, for example, laminating a diffusion sheet, a prism sheet, a reflection/polarization sheet and the like in order. 
     The main light source  3  is disposed at a position facing the other main surface  2   b  of the optical layer  2 , and emits light toward the optical layer  2 . In the present Embodiment 1, the main light source  3  is formed of a plurality of fluorescent tubes. The plurality of the fluorescent tubes constituting the main light source  3  are disposed in parallel between the optical layer  2  and the light guide plate  4 . The main light source  3  functions in the same manner as that in the conventional direct-type backlight device. 
     The light guide plate  4  is disposed on the main surface  2   b  side of the optical layer  2 , so as to be parallel to the main surface  2   b . The sub-light source  5  is disposed at a position facing one side surface of the light guide plate  4 , and emits light toward the side surface. In the present Embodiment 1, a fluorescent tube is used as the sub-light source  5 . The light guide plate is formed of a transparent acrylic plate or the like, and a reflection sheet  7  is provided on a bottom surface  4   b  of the light guide plate  4 . Although it is not shown, a reflection sheet also is provided on a side surface of the light guide plate  4  opposite to the side facing the sub-light source  5 . 
     Further, a lamp reflector  6  is attached so as to surround the fluorescent tube, which serves as the sub-light source  5 . Thus, the light that has been emitted by the sub-light source  5  and enters the light guide plate  4  through the side surface thereof is reflected repeatedly inside the light guide plate  4  and goes out through a main surface (top surface)  4   a  of the light guide plate  4  on the optical layer  2  side to the outside. The main surface  4   a  serves as a light outgoing surface. The light guide plate  4  and the sub-light source  5  function in the same manner as those in the conventional sidelight-type backlight device. 
     It should be noted that, in the example shown in  FIGS. 1 and 2 , the fluorescent tube constituting the sub-light source  5  is a straight tube, but Embodiment 1 is not limited to this configuration. The fluorescent tube constituting the sub-light source  5  may be a U-shaped tube, a L-shaped tube, a pseudo U-shaped tube that is obtained by connecting end parts of two straight tubes with a bridge, an angular U-shaped tube (channel-shaped tube) that is obtained by folding two portions of a tube vertically in the same direction, or the like. Further, the number of fluorescent tubes constituting the sub-light source  5  is not limited; it may be two or more. For example, the example shown in  FIGS. 1 and 2  may have a configuration in which each of the opposing side surfaces is provided with one fluorescent tube. 
     Next, a configuration of an entirety of the backlight device (also including portions other than the optical portion) in the present Embodiment 1 and the liquid crystal display device utilizing the backlight device in the present Embodiment 1 will be described with reference to  FIGS. 3 and 4 .  FIG. 3  is a block diagram showing configurations of the backlight device and a liquid crystal display device according to Embodiment 1 of the present invention. 
     As shown in  FIG. 3 , the liquid crystal display device in the present Embodiment 1 includes the backlight device  1 , a liquid crystal display panel  10 , and a control portion  15 . The liquid crystal display panel  10  is provided with an active matrix substrate  11  on which pixels are formed in matrix, a filter substrate  12  on which color filters corresponding to each pixel are formed, and a liquid crystal layer (not shown) interposed therebetween. The pixels are mainly composed of TFTs and pixel electrodes. A gate driver IC  13  and a source driver IC  14  are mounted on the active matrix substrate  11  in a region where the filter substrate  12  is not placed (in a region surrounding a region where the pixels are formed). 
     External equipment (not shown) connected to the liquid crystal display device inputs a video signal to the control portion  15 . Receiving the video signal, the control portion  15  inputs a control signal and the like corresponding to the video signal to the gate driver IC  13  and the source driver IC  14 . Consequently, the gate driver IC  13  and the source driver IC  14  are activated, and the pixels are driven in accordance with the video signal. At this time, when illumination light is emitted from the backlight device  1 , a video image is displayed on a display screen. 
     As shown in  FIG. 3 , the backlight device  1  in the present Embodiment 1 further includes a driving portion  8  and a dimming signal generating portion  9  in addition to the configuration shown in  FIGS. 1 and 2 . The dimming signal generating portion  9  generates a main light source dimming signal for setting the light amount of each fluorescent tube constituting the main light source  3 , and a sub-light source dimming signal for setting the light amount of the fluorescent tube constituting the sub-light source  5 , and inputs these signals to the driving portion  8  (see  FIG. 4 ). In the present embodiment, each of the main light source dimming signal and the sub-light source dimming signal is a direct voltage signal for setting a light amount by specifying it with a voltage level or pulse signal for setting a light amount by specifying it with a duty ratio. 
     Further, the generation of the main light source dimming signal and the sub-light source dimming signal by the dimming signal generating portion  9  is performed in accordance with an instruction given by the control portion  15 . In the present Embodiment 1, in order for a video image displayed according to the video signal to obtain an optimal brightness, the control portion  15  gives an instruction to the dimming signal generating portion  9  concerning the light amounts of the respective light sources. For example, when the control portion  15  requires the backlight device  1  to provide maximum brightness, the dimming signal generating portion  9  generates a main light source dimming signal for maximizing the light amount of the main light source, and a sub-light source dimming signal for maximizing the light amount of the sub-light source. 
     The driving portion  8  is an inverter. When the main light source dimming signal and the sub-light source dimming signal are input to the driving portion  8  from the dimming signal generating portion  9 , the driving portion  8  drives the main light source  3  and the sub-light source  5  in accordance with these signals. Specifically, the driving portion  8  generates a drive pulse having a duty ratio corresponding to the voltage level or the duty ratio of the main light source dimming signal, and outputs this pulse to the plurality of the respective fluorescent tubes constituting the main light source  3 . Further, the driving portion  8  also generates a drive pulse having a duty ratio corresponding to the voltage level or the duty ratio of the sub-light source dimming signal, and outputs this pulse to the fluorescent tube constituting the sub-light source  5 . 
     In the present Embodiment 1, the driving portion  8  separately drives the plurality of the fluorescent tubes constituting the main light source  3  and the fluorescent tube constituting the sub-light source  5  in accordance with instructions given by the control portion  15 , by a so-called PWM dimming method. 
     Here, a specific configuration of the driving portion  8  is described with reference to  FIG. 4 .  FIG. 4  is a block diagram showing a specific configuration of a driving portion constituting the backlight device shown in  FIG. 3 . As shown in  FIG. 4 , the driving portion  8  includes pulse generating portions  21  and  31 , inverter control portions  22  and  32 , transformer driving portions  23  and  33 , transformer portions  24  and  34 , and protection circuits  25  and  35 . 
     Among these, the pulse generating portion  21 , the inverter control portion  22 , the transformer driving portion  23 , the transformer portion  24 , and the protection circuit  25  are used for generating a drive pulse for the main light source. On the other hand, the pulse generating portion  31 , the inverter control portion  32 , the transformer driving portion  33 , the transformer portion  34 , and the protection circuit  35  are used for generating a drive pulse for the sub-light source. 
     The pulse generating portions  21  and  31 , when the dimming signals input thereto are direct voltage signals, generate pulses having duty ratios corresponding to the voltage levels and input the same to the corresponding transformer driving portions  23  and  33 , respectively. Specifically, each of the pulse generating portions  21  and  31  compares a dimming signal with a triangular wave signal as a reference, and generates a pulse as follows: when the level of the dimming signal is higher than that of the triangular wave signal, the pulse has a high level; and when the level of the dimming signal is lower than that of the triangular wave signal, the pulse has a low level. Further, in the case where the dimming signals input thereto are pulse signals, the pulse generating portions  21  and  31  adjust amplitudes thereof, and the like. 
     The inverter control portions  22  and  32  perform basic inverter control with respect to the pulse generating portions  21  and  31 . The inverter control portions  22  and  32  further perform current control of the corresponding pulse generating portions  21  and  31 , respectively, in accordance with feedback signals from current regulator circuits of the transformer driving portions  23  and  33 , which will be described later. The inverter control portions  22  and  32  further perform safety control in accordance with error signals from the protection circuits  25  and  35 , which will be described later. 
     Each of the transformer driving portions  23  and  33  is provided with a current amplifier circuit, a level shift circuit, and a transformer drive circuit, and using these, the transformer driving portions  23  and  33  drive the transformer portions  24  and  34 . In order to enhance the driving ability of pulses generated by the pulse generating portions  21  and  31 , the current amplifier circuits amplify the current values of the pulses. The level shift circuit has a circuit for generating a gate signal of a P-channel type transistor. The transformer drive circuit has an inverter circuit of a full-bridge or a half-bridge type, a push-pull type, or the like. 
     The transformer portion includes a transformer for converting a voltage of an input pulse into a higher voltage suitable for lighting fluorescent tubes; a current regulator circuit for adjusting a lamp current by feedback control; and an output abnormality detection circuit for detecting an abnormality of the output. The feedback signal from the current regulator circuit is input to the inverter control portions  22  and  32 . The signal from the output abnormality detection circuit is input to the protection circuits  25  and  35 . When the output abnormality is detected, the protection circuits  25  and  35  output error signals to the inverter control portions  22  and  32 , respectively. 
     Next, a dimming range in the case where the backlight device in the present Embodiment 1 is used will be described with reference to  FIGS. 5 and 6 .  FIGS. 5(   a ) to  5 ( d ) are diagrams showing exemplary drive pulses for a main light source that have different duty ratios, respectively.  FIGS. 6(   a ) to  6 ( d ) are diagrams showing exemplary drive pulses for a sub-light source that have different duty ratios, respectively. 
     As shown in  FIGS. 5(   a ) to  5 ( d ) and  FIGS. 6(   a ) to  6 ( d ), in the present Embodiment 1, it is assumed that the driving portion  8  outputs, for example, four kinds of drive pulses having different duty ratios with respect to both the main light source  3  and the sub-light source  5 . Here, when the brightness at the light outgoing surface of the backlight device  1 , in the case that the duty ratio of the drive pulse for the sub-light source is 0% and the duty ratio of the drive pulse for the main light source is 100%, is assumed to be 100%, the dimming ranges will be described as shown in Table 1. It should be noted that Table 1 is merely an example showing the case where the light amount of the sub-light source  5  varies in the range of −20% to +20% to the light amount of the main light source. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Duty ratio of the drive pulse 
                   
               
               
                   
                 for the main light source [%] 
               
            
           
           
               
               
               
               
               
            
               
                   
                 0 
                 10 
                 55 
                 100 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Duty ratio of 
                 0 
                  0% 
                  5% 
                 50% 
                 100% 
               
               
                 the drive pulse 
                 10 
                  1% 
                  6% 
                 51% 
                 101% 
               
               
                 for the sub- 
                 55 
                 10% 
                 15% 
                 60% 
                 110% 
               
               
                 light source [%] 
                 100 
                 20% 
                 25% 
                 70% 
                 120% 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, the backlight device according to the present Embodiment 1 is capable of increasing the maximum brightness, and thus, widening a dimming range, compared to the conventional backlight device in which only a main light source is provided. Accordingly, the liquid crystal display device according to the present Embodiment 1 increases a contrast ratio of a display screen compared to the conventional type. Further, in the backlight device according to the present Embodiment 1, current dimming is not performed with respect to the main light source  3  and the sub-light source  5 , whereby an increment in a value of current passing therethrough is suppressed. Therefore, a decrease in the service life of these light sources can be suppressed as well. 
     Further, as shown in Table 1, since the light amounts of the main light source and the sub-light source are adjusted separately, the brightness of the entire backlight device can be adjusted finely. This makes it possible to set the brightness optimal for a video image easily, thereby improving the image quality of display images. 
     Embodiment 2 
     Next, a backlight device and a liquid crystal display device in Embodiment 2 of the present invention will be described with reference to  FIGS. 7 to 9 .  FIG. 7  is a block diagram showing configurations of a backlight device and a liquid crystal display device according to Embodiment 2 of the present invention.  FIG. 8  is a block diagram showing a specific configuration of a driving portion constituting the backlight device shown in  FIG. 7 .  FIG. 9  is a circuit diagram exclusively showing a portion for driving the sub-light source in the driving portion shown in  FIG. 7 . 
     In the present Embodiment 2, a backlight device  41  includes a plurality of light-emitting diodes as a sub-light source  42 , which differs from the backlight device  1  in Embodiment 1. Further, since the sub-light source  42  is formed of light-emitting diodes, a configuration of a driving portion  43  also differs from that of the driving portion  8  in Embodiment 1. 
     Except the aforementioned differences, the backlight device  41  and the liquid crystal display device in the present Embodiment 2 are configured in the same manner as the backlight device  1  and the liquid crystal display device in Embodiment 1. Hereinafter, the differences will be described specifically. 
     In the present Embodiment 2, as shown in  FIG. 7 , in place of the fluorescent tube constituting the sub-light source  5  shown in  FIGS. 1 and 2 , a plurality of light-emitting diodes constituting the sub-light source  42  are disposed along a side surface of the light guide plate  4  so as to allow the emitted light to be incident upon the surface side. The driving portion  43  drives the fluorescent tubes constituting the main light source  3 , and the light-emitting diodes constituting the sub-light source  42 . 
     As shown in  FIG. 8 , the driving portion  43 , like the driving portion  8  in Embodiment 1, includes the pulse generating portion  21 , the inverter control portion  22 , the transformer driving portion  23 , the transformer portion  24 , and the protection circuit  25 . The driving portion  43  drives the fluorescent tubes constituting the main light source  3  by the PWM dimming method, like in Embodiment 1. 
     Further, as shown in  FIGS. 8 and 9 , the driving portion  43  includes a pulse generating portion  44 , a light-emitting diode transformer portion  45 , a rectifying portion  46 , and an auxiliary voltage generating portion  47 , and these modulate a value of current that is supplied to the light-emitting diodes constituting the sub-light source  42 . As a result, the light-emitting diodes emit light in an amount instructed by a sub-light source dimming signal. 
     Specifically, the sub-light source dimming signal from the control portion  15  is input to the pulse generating portion  44 . The pulse generating portion  44  generates a pulse from the input sub-light source dimming signal, as the pulse generating portion  31  shown in  FIG. 4  does. 
     The pulse generated in the pulse generating portion  44  is input to the light-emitting diode transformer portion  45 . The light-emitting diode transformer portion  45  boosts the input pulse, and inputs it to the rectifying portion  46 . The rectifying portion  46  rectifies the input pulse, and generates a direct current. Then, the generated direct current is supplied to the light-emitting diodes constituting the sub-light source  42 , so as to lighten the light-emitting diodes. The auxiliary voltage generating portion  47  applies a voltage preliminarily to the light-emitting diode transformer portion  45  for suppressing the occurrence of a time-lag between an instruction of lighting of the sub-light source  42  and a start of the lighting. 
     The current value of the direct current generated by the rectifying portion  46  is proportional to the duty ratio of the pulse that is output by the pulse generating portion  44 . Therefore, the light-emitting diodes have a brightness in accordance with the voltage level or the duty ratio of the sub-light source dimming signal. In the present Embodiment 2, the driving portion  43  drives the light-emitting diodes constituting the sub-light source  42  by the PWM dimming method in accordance with an instruction given by the control portion  15 . 
     As described above, the backlight device of the present Embodiment 2, like the backlight device of Embodiment 1, is capable of increasing the maximum brightness and widening a dimming range compared to the conventional backlight device. Further, in the present Embodiment 2, an increase in current passing through the main light source  3  and the sub-light source  5  can be suppressed, whereby a decrease in the service life of light sources can be suppressed. 
     In Embodiments 1 and 2, although the adjustment of the light amounts of the main light source and the sub-light source is performed in accordance with the contents of a video image displayed on the display screen, the present invention is not limited to the foregoing examples. The present invention, for example, may be configured so that the sub-light source is switched on and off manually by a user of the liquid crystal display device. 
     INDUSTRIAL APPLICABILITY 
     The backlight device and the liquid crystal display device according to the present invention have an industrial applicability as a backlight device configured so that a decrease in a service life of a light source is suppressed and as a liquid crystal display device having an improved contrast ratio, respectively.