Abstract:
A backlight module and a liquid crystal display (LCD) device thereof are proposed. The backlight module includes a DC/DC converter producing a positive/negative voltage level. The backlight module can output the positive and negative levels of driving voltage from an output end of the inverter to drive LEDs through alternately switching. Besides, a constant current can be obtained with designs of power on/off periods and energy-storing inductors to prevent LEDs employing current limiting resistors from having too low current efficiency and from being burnt out.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a backlight module, and more particularly, to a backlight module and an LCD using the backlight module. 
     2. Description of Prior Art 
     With a rapid development of monitor types, novel and colorful monitors with high resolution, e.g., liquid crystal displays (LCDs), are indispensable components used in various electronic products such as monitors for notebook computers, personal digital assistants (PDAs), digital cameras, and projectors. The demand for the novelty and colorful monitors has increased tremendously. 
     Backlight modules are one of key components of liquid crystal display panels (LCD panels). Liquid crystals do not emit light, so backlight modules provide an evenly distributed light source providing abundant brightness. The light source is transformed into a high-brightness planar light source having a uniform luminance distribution via a simple, effective opto-mechanism to provide LCD panels with a backlight source. LCD panels have been extensively employed in electronic devices having growth potential, such as monitors, notebook computers, digital cameras, projectors, etc., and particularly in large-sized panels like notebook computers and LCD monitors. Thus, demand for backlight units is growing gradually as well. 
     In addition to cold cathode fluorescent lamps (CCFLs), backlight modules also use light emitting diodes (LEDs) as a light source. LEDs have become the mainstream backlight source for LCD televisions in recent years, because they are mercury-free and thus environmentally-friendly. In addition, LEDs respond fast. Current LEDs are mostly driven by a direct current (DC). But, since LED lighting gets gradually popular, alternative current (AC)-driven LEDs have been more and more common as well. The advantage of AC-driven LEDs is that LEDs only turn on and off in half of the time during an entire circuit period; it greatly reduces heat dissipation from LEDs and further prolongs life of LEDs. For AC-driven LEDs under ideal operating conditions, current limiting resistors have to be connected in series with LEDs to limit the amount of current flowing through the LEDs. But, the conversion efficiency of the circuit may be thus lowered due to a voltage drop caused by the current flowing through the LEDs. 
     A reduction in the number of current limiting resistors may cause LEDs to be burnt out easily. AC voltage fluctuations vary with different areas and environments, and LED forward voltage (VF) changes with temperature and technologies of manufacturers. Thus, problems like the burn-out of LEDs or the brightness inadequacy of LEDs may occur if the number of current limiting resistors is deliberately reduced or the resistance is deliberately lowered for increasing efficiency. 
     SUMMERY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a backlight module comprising a DC/DC converter producing a positive/negative voltage level. The backlight module can output an AC voltage from an output end of the converter through alternate on/off states of a switch to drive LEDs. In this way, LEDs using current limiting resistors can be prevented from having too low current efficiency and from being burnt out. 
     In one aspect, an LCD device comprises a power end for supplying a power supply voltage; a LCD panel for displaying an image; a converter electrically connected to the power end, comprising a switch signal generator for generating a switch signal, the converter for converting the power supply voltage to a positive voltage level of a driving signal in response to a first voltage level of the switch signal, and for converting the power supply voltage to a negative voltage level of a driving signal in response to a second voltage level of the switch signal, the positive and negative voltage levels being generated alternately; and a lighting device electrically connected to the converter, for generating light based on the positive and negative voltage levels of the driving voltage from the converter. 
     According to the present invention, the lighting device is an AC-driven LED. 
     According to the present invention, the lighting device comprises a first lighting unit, a second lighting unit, a third lighting unit, a fourth lighting unit, and a fifth lighting unit. The first lighting unit comprises a positive end and a negative end coupled to a first node and a second node, respectively. The second lighting unit comprises a positive end and a negative end coupled to the second node and a third node, respectively. The third lighting unit comprises a positive end and a negative end coupled to the third node and a fourth node, respectively. The fourth lighting unit comprises a positive end and a negative end coupled to the fourth node and the second node, respectively. The fifth lighting unit comprises a positive end and a negative end coupled to the third node and the first node, respectively. 
     According to the present invention, the converter comprises a capacitor element, a first switch, a second switch, a third switch, a first inductor element, a second inductor element, a first diode, and a second diode. The capacitor element is coupled to the lighting device in parallel. The first switch electrically coupled to the power end is used for turning on when the switch signal is at the first voltage level. The second switch electrically coupled to the power end is used for turning on when the switch signal is at the first voltage level. The first inductor element is electrically coupled to the first switch. The second inductor element is electrically coupled to the power end, the second switch, and the capacitor element. The third switch electrically coupled to the second inductor element and the power end is used for turning on when the switch signal is at the second voltage level. The first diode is electrically coupled to the capacitor element, the first inductor element, and the second switch. The second diode is electrically coupled to the capacitor element, the second inductor element, the first diode, and the third switch. 
     According to the present invention, the switch generator further comprises an inverter for inverting the switch signal. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of an LCD device according to the preferred embodiment. 
         FIG. 2  shows the converter outputting a positive voltage level of driving voltage when the switches S 1  and S 2  turn on in response to the first switch signal 
         FIG. 3  shows the converter outputting a negative voltage level of driving voltage when the switch S 3  turns on in response to the second switch signal. 
         FIG. 4  illustrates another embodiment of the light device as shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  showing a schematic diagram of an LCD device  20  according to the preferred embodiment, the LCD device  20  comprises a power supply end  21 , an LCD panel  30 , and a backlight module  10 . The backlight module  10  generates light to the LCD panel  30  when the power end  21  supplies to the backlight module  10 . The backlight module  10  comprises a lighting device  22  and a converter  24 . The power end  21  supplies direct current (DC) supply voltage V DC . The LCD panel  30  comprises a liquid crystal layer filled with liquid crystal molecules of which an alignment is varied based on image data to adjust the light from the backlight module  10  and thus to show various grey levels adjust. The lighting device  22  comprises a plurality of AC-driven LEDs  33  driven by AC power supply. The lighting device  22  connected between the converter  24  and a voltage end (e.g. ground end in  FIG. 1 ), is used for generating light based on driving voltage from the converter  24 . 
     Referring to  FIG. 1 , the converter  24  is used for converting the DC supply voltage VDC of 12V from the power end  21  into a high DC voltage signal with alternate positive/negative voltage levels. Actually, the DC voltage signal with alternate positive/negative voltage levels simulates an alternate-current (AC) voltage signal. The converter  24  comprises a switch signal generator  40 , a capacitor element C, a first switch S 1 , a second switch S 2 , a third switch S 3 , a first inductor element L 1 , a second inductor element L 2 , a first diode D 1 , and a second diode D 2 . The capacitor element C is coupled to the lighting device in parallel. The first switch S 1  is electrically coupled to the power end  21 . The first inductor element L 1  is electrically connected to the first switch S 1 . The second inductor element L 2  is electrically coupled to the power end  21 , the second switch S 2 , and the capacitor C. The third switch S 3  is electrically coupled to the second inductor elements L 2 , the power end  21 , and the second diode D 2 . The first diode D 1  is electrically coupled to the capacitor element C, a first inductor element L 1 , and a second switch S 2 . The second diode D 2  is electrically coupled to the capacitor element C, the second inductor L 2 , the first diode D 1 , and the third diode S 3 . All the switches S 1 , S 2 , and S 3  can turn on based on the same polarity voltage. In one embodiment, the switch signal generator  40  is used for generating a first switch signal V G1 . The first switch signal V G1  is inverted to output a second switch signal V G2  by the inverter  41 . In another embodiment, the first switch S 1  and second switch S 2  may be NMOS transistors, while the third switch S 3  may be a PMOS transistor. The first switch S 1 , the second switch S 2 , and the third switch S 3  are controlled by the same switch signal V G1 . When the switch signal V G1  is at high voltage level, the first switch S 1  and the second switch S 2  turn on, but the third switch S 3  turns off. Conversely, when the switch signal V G1  is at low voltage level, the first switch S 1  and the second switch S 2  turn off, but the third switch S 3  turns on. The inductor elements L 1 , L 2  as energy storage element can storage energy when charge flows through. 
     Referring to  FIGS. 2 and 3 ,  FIG. 2  shows the converter  24  outputting a positive voltage level of driving voltage when the switches S 1  and S 2  turn on in response to the first switch signal V G1 , and  FIG. 3  shows the converter  24  outputting a negative voltage level of driving voltage when the switch S 3  turns on in response to the second switch signal V G2 . When the first switch signal V G1  is at high voltage level while the second switch signal V G2  is at low voltage level, the switches S 1  and S 2  turn on but the switch S 3  turns off, so that the inductor element L 1 , the diode D 1 , and the lighting device  22  form a current loop to feed the driving voltage (i.e. voltage level at the output of the diode D 1 ) to the lighting device  22 . The lighting device  22  may emit light based on the positive voltage level of the driving voltage Vo. When the first switch signal V G1  is at low voltage level while the second switch signal V G2  is at high voltage level, the switches S 1  and S 2  turn off but the switch S 3  turns on, so that the capacitor element L 2  and the lighting device  22  form a current loop. At this moment, the driving voltage Vo becomes negative voltage level due to a reversed flow direction through the capacitor element C. Since the first switch signal V G1  and the second switch signal V G2  are high frequency square wave, the converter  24  is capable of alternately feeding positive/negative voltage levels of the driving voltage to the lighting device  22 . In other words, the converter  24  is a DC/DC converter capable of alternately outputting positive/negative voltage levels of the driving voltage by frequently switching which simulates an AC voltage. 
     Referring to  FIG. 4  illustrating another embodiment of the light device  22  as shown in  FIG. 1 , differing from the AC-driven LED  33 , the lighting device  22  comprises a first lighting unit  22   a , a second lighting unit  22   b , a third lighting unit  22   c , a fourth lighting unit  22   d , and a fifth lighting unit  22   e . The first lighting unit  22   a  having a positive end and a negative end coupled to a first node N 1  and a second node N 2 , respectively. The second lighting unit  22   b  having a positive end and a negative end coupled to the second node N 2  and a third node N 3 , respectively. The third lighting unit  22   c  having a positive end and a negative end coupled to the third node N 3  and a fourth node N 4 , respectively. The fourth lighting unit  22   d  having a positive end and a negative end coupled to the fourth node N 4  and the second node N 2 , respectively. The fifth lighting unit  22   e  having a positive end and a negative end coupled to the third node N 3  and the first node N 1 , respectively. Each of the lighting units  22   a - 22   e  is a single Light Emitting Diode (LED)  32  or multiple LEDs in series. When the lighting device  22  is fed by the positive voltage level of the driving voltage, the light units  22   a ,  22   b , and  22   c  are under forward bias to be driven to light, but the lighting units  22   d  and  22   e  are under reversed bias to be shut down. Conversely, when the lighting device  22  is fed by the negative voltage level of the driving voltage, the light units  22   d ,  22   b , and  22   e  are under forward bias to be driven to light, but the lighting units  22   a  and  22   c  are under reversed bias to be shut down. Therefore, because a number of three light units are always lighting, a twinkle image is not visible. As it is, the converter  24  without using a current limiting resistor and keeping the LED lighting in a long term can upgrade operating efficiency and thus reduce a risk of LED to be burnt out. 
     Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to the ordinary skilled person in the art that the invention is not limited to the embodiments, but rather various changes or modifications thereof are possible without departing from the spirit of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.