Patent Abstract:
The present invention discloses a backlight module with a related liquid crystal display (LCD) which activates light emitting diodes (LEDs) by utilizing an alternate control method. The present invention utilizes two inverters to individually activate two sets of LEDs through an alternate method. During the same switching cycle period, the two sets of LEDs take turns turning on/off; that is, the two set of LEDs are in a closed state in a duty cycle of 50 percent. Since each set of the LEDs are in a closed condition in half the time during a switching cycle period, both of excess temperature produced by all of the LEDs when lightened simultaneously and thermal power generated during the lighting of the LEDs can be effectively reduced.

Full Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a backlight module and a liquid crystal display (LCD) employing such a backlight module, and more particularly, to a backlight module for alternately driving lighting device and an LCD employing such a backlight module. 
         [0003]    2. Description of Prior Art 
         [0004]    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. 
         [0005]    A backlight module is a key component of a liquid crystal display (LCD). The purpose of the backlight module is to provide a sufficient-brightness and an even-distribution light surface to the LCD panel. Because the LCD is widely used in various electronic products such as a monitor, a notebook computer, a digital camera, and a projector, the demand for the backlight module has increased tremendously. 
         [0006]    In addition to cold cathode fluorescent lamps (CCFLs), backlight modules also utilize light emitting diodes (LEDs) as a light source. And in recent years, LEDs have gradually become the mainstream backlight light source for LCD televisions, because they are mercury-free and thus environmentally friendly and fast responding. However, some physical properties of LEDs also influence luminous efficiency and lifespan of LEDs. Temperature is such a physical property that affects LEDs most. So, a variety of radiating materials and relevant techniques start to be applied to LED backlighting. The application of such heat dissipation techniques, undoubtedly, attempts to reduce the influence of temperature on LEDs effectively. Referring to  FIG. 1 ,  FIG. 1  shows that LEDs are activated by a traditional converter. A backlight module  1  comprises a power end  12 , a plurality of LEDs  14 , and a converter  16 . The converter  16  comprises an inductor element L, a transistor T, a diode D, and a capacitor element C. The power end  12  supplies the converter  16  with a direct current (DC) supply voltage V DC , and the transistor T switches to output a driving signal to the LEDs  14  in response to a switch signal V G . The LEDs  14  produce light based on the voltage difference of the driving signal. However, the traditional LED backlight module  1  merely utilizes a single converter  16  to simultaneously activate all of the LEDs  14 , which means that the converter  16  has to produce large current outputs to simultaneously activate all of the LEDs  14 . But, large currents may also cause some potential problems, such as excess temperature, which not only shortens the lifespan of the LEDs  14  but also reduce the luminous efficiency of the LEDs  14 . 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an object of the present invention to provide a backlight module and an LCD employing such a module by means of an alternate driving lighting device to reduce thermal power generation. 
         [0008]    In another aspect of the present invention, a liquid crystal display comprises a power end for generating a supply voltage, a liquid crystal display panel comprising a liquid crystal layer for displaying images, a switch signal generator for generating a first switch signal and a second switch signal, a first inverter electrically connected to the power end for generating a first driving signal based on the first switch signal, a second inverter electrically connected to the power end for generating a second driving signal based on the second switch signal, a first lighting device for producing light based on the voltage difference of the first driving signal transmitted from the first inverter, a second lighting device for producing light based on the voltage difference of the second driving signal transmitted from the second inverter. The phase difference between the first driving signal and the second driving signal is 180 degrees. 
         [0009]    In another aspect of the present invention, a backlight module comprises a power end for generating a supply voltage, a switch signal generator for generating a first switch signal and a second switch signal, a first inverter electrically connected to the power end for generating a first driving signal based on the first switch signal, a second inverter electrically connected to the power end for generating a second driving signal based on the second switch signal, a first lighting device for producing light based on the voltage difference of the first driving signal transmitted from the first inverter, and a second lighting device for producing light based on the voltage difference of the second driving signal transmitted from the second inverter. A phase difference between the first driving signal and the second driving signal is 180 degrees. 
         [0010]    According to the present invention, the first lighting device or the second lighting device comprises a light emitting diode (LED) or a plurality of LEDs connected in serial. 
         [0011]    According to the present invention, the first inverter comprises a capacitor element connected in parallel to the first lighting device, an inductor element comprising a first end electrically connected to a first electrode of the power end, a diode electrically connected between a second end of the inductor element and the first lighting device, and a first transistor comprising a first end electrically connected between the inductor element and the diode and a second end electrically connected to a second electrode of the power end for conducting upon receiving the first switch signal. 
         [0012]    According to the present invention, the second inverter comprises a capacitor element connected in parallel to the second lighting device, an inductor element comprising a first end electrically connected to a first electrode of the power end, a diode electrically connected between a second end of the inductor element and the second lighting device, a second transistor comprising a first end electrically connected between the inductor element and the diode and a second end electrically connected to a second electrode of the power end for conducting upon receiving the second switch signal. 
         [0013]    According to the present invention, a phase inverter for inverting a switch signal generated by the switch signal generator to generate another switch signal, the two switch signals act as the first switch signal and the second switch signal. 
         [0014]    According to the present invention, the first transistor is a PMOS transistor and the second transistor is a NMOS transistor. 
         [0015]    Compared with the prior art, the backlight module with the related LCD in the present invention activates LEDs by using an alternate control method. If a duty cycle is set at 50 percent during a switching cycle period, the LEDs in the same string will be in a closed state in a duty cycle of 50 percent. And, all of the switching frequencies are above 1 kHz, so human eyes cannot detect variations in brightness of the LEDs. Besides, excess temperature produced by the LEDs when lightened simultaneously and thermal power generated during the lighting of the LEDs can be effectively reduced for the reason that the LEDs are in a closed condition in half or even more of the time during the switching cycle period. 
         [0016]    These and other objects of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows that LEDs are activated by a traditional converter. 
           [0018]      FIG. 2  is a schematic diagram of a liquid crystal display according to a first embodiment of the present invention. 
           [0019]      FIG. 3  is a schematic diagram of an LCD according to the second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring to  FIG. 2 ,  FIG. 2  is a schematic diagram of a liquid crystal display (LCD)  20  according to a first embodiment of the present invention. The LCD  20  comprises a power end  21 , an LCD panel  30 , and a backlight module  10 . The backlight module  10  produces light that the LCD panel  30  requires with a voltage provided by the power end  21 . The backlight module  10  comprises a first lighting device  22 , a second lighting device  24 , a switch signal generator  25 , a first inverter  26 , and a second inverter  28 . The power end  21  provides a DC supply voltage V DC . The LCD panel  30  comprises a liquid crystal (LC) layer for displaying images. The first lighting device  22  and the second lighting device  24  comprise a single LED  32  or a plurality of LEDs  32  in serial. The first lighting device  22  comprises one end electrically connected to the first inverter  26  and the other end electrically connected to a voltage end (a ground end in  FIG. 2 ) for producing light based on the voltage difference of a first driving signal emitted by the first inverter  26 . The second lighting device  24  comprises one end electrically connected to the second inverter  28  and the other end electrically connected to the voltage end (the ground end in  FIG. 2 ) for producing light based on the voltage difference of a second driving signal emitted by the second inverter  28 . The switch signal generator  25  generates a switch signal V G1 . 
         [0021]    Please continue referring to  FIG. 2 . The first inverter  26  and the second inverter  28  convert a DC voltage (12V) of the power end  21  into an alternating current (AC) high voltage. The first inverter  26  comprises a capacitor element  40 , an inductor element  42 , a diode  44 , and a first transistor  46 . The capacitor element  40  and the first lighting device  22  are connected in parallel. The inductor element  42  comprises a first end electrically connected to a first electrode of the power end  21 . The diode  44  is electrically connected between a second end of the inductor element  42  and the first lighting device  22 . The inductor element  42  is an charge storage element for reserving a DC supply voltage from the power end  21 . The first transistor  46  comprises a first end electrically connected to the inductor element  42  and to the diode  44  and a second end electrically connected to a second electrode of the power end  21 . In the present embodiment, the first transistor  46  is an N-type metal-oxide-semiconductor (MOS) transistor, having a gate connected to a first switch signal V G1  output by a square wave. When the first switch signal V G1  is at a high voltage level, the first transistor  46  conducts to make the first transistor  46 , the first lighting device  22 , and the diode  44  form a current loop. Meanwhile, the first lighting device  22  receives a first driving signal (i.e., a voltage level of an output end of the diode  44 ). The first lighting device  22  emits light because of the voltage difference of the first driving signal. When the first switch signal V G1  is at a low voltage level, the first transistor  46  is turned off. Meanwhile, the voltage level of the output end of the diode  44  is lowered to be identical to that of the ground end. So, the first driving signal is not transmitted to the first lighting device  22  at this time, causing that the first lighting device  22  cannot produce light due to no voltage difference of the first driving signal. 
         [0022]    Similarly, the second inverter  28  comprises a capacitor element  50 , an inductor element  52 , a diode  54 , and a second transistor  56 . The capacitor element  50  and the second lighting device  24  are connected in parallel. The inductor element  52  comprises a first end electrically connected to the power end  21 . The diode  54  is electrically connected between a second end of the inductor element  52  and the second lighting device  24 . The inductor element  52  is an energy storage element for reserving a DC supply voltage from the power end  21 . The second transistor  56  comprises a first end electrically connected to the inductor element  52  and to the diode  54  and a second end electrically connected to a second electrode of the power end  21 . In the present embodiment, the second transistor  56  is an NMOS transistor, having a gate connected to a second switch signal V G2  output by a square wave. It is notified that, a phase inverter  58  inverts the first switch signal V G1  to form the second switch signal V G2 , so the phase difference between the first switch signal V G1  and the second switch signal V G2  is 180 degrees. Therefore, when the first switch signal V G1  is at a low voltage level, the second switch signal V G2  is at a high voltage level. When the second switch signal V G2  is at a high voltage level, the second transistor  56  conducts to make the second transistor  56 , the diode  54 , and the second lighting device  24  form a current loop. Meanwhile, the second lighting device  24  receives a second driving signal (i.e., a voltage level of an output end of the diode  54 ). The second lighting device  24  emits light because of the voltage difference of the second driving signal. When the second switch signal V G2  is at a low voltage level, the second transistor  56  is turned off. Meanwhile, the voltage level of the output end of the diode  54  is lowered to be identical to that of the ground end. So, the second driving signal is not transmitted to the second lighting device  24  at this time, causing that the second lighting device  24  cannot produce light due to no voltage difference of the second driving signal. The phase difference between the first switch signal V G1  and the second switch signal V G2  is 180 degrees, which causes that the phase difference between the first driving signal and the second driving signal is 180 degrees, too. In this way, the duration of lighting of the first lighting device  22  and that of the second lighting device  24  are alternate on account of the activations of the first and second driving signals; that is, either the first lighting device  22  or the second lighting device  24  is allowed to emit light at any point of time. 
         [0023]    Referring to  FIG. 3 ,  FIG. 3  is a schematic diagram of an LCD  60  according to the second embodiment of the present invention. The LCD  60  comprises a power end  21 , an LCD panel  30 , and a backlight module  70 . It is notified that, every element in  FIG. 3  marked with the same code shown in  FIG. 2  is given the same function. To simplify the description below, the functions of the same elements are not repeated in the following. Differing from the first embodiment in  FIG. 2 , in this embodiment a second transistor  66  of the second inverter  28  is a p-type metal-oxide-semiconductor (PMOS) transistor; the gate of the second transistor  66  is also controlled by the first switch signal V G1 ; the phase inverter  58  is not needed. Opposite to the NMOS transistor, the PMOS transistor is turned on when the first switch signal V G1  is at a low voltage level and turned off when the first switch signal V G1  is at a high voltage level. In other words, even if both of the first transistor  46  and the second transistor  66  are controlled by the first switch signal V G1  at the same time, the second lighting device  24  will emit light upon receiving the second driving signal (i.e., the voltage level of the output end of the diode  54 ), and the first lighting device  22  will not receive the first driving signal and emit light (and vice versa). This is because the second transistor  66  (PMOS transistor) has opposite polarity of the threshold voltage from the first transistor  46  (NMOS transistor). In this way, the duration of lighting of the first lighting device  22  alternates with that of the second lighting device  24  owing to the activation of the first driving signal. In other words, either the first lighting device  22  or the second lighting device  24  is allowed to emit light at any point of time. 
         [0024]    It is supposed that the one skilled in this art understand that, as long as the polarity of the turn-on voltage of the first transistor  46  is opposite to that of the second transistor  66 , an object of alternately lighting of the first lighting device  22  and the second lighting device  24  can be achieved by only using the same switch signal. It is not necessary to set the first transistor  46  and the second transistor  66  as an NMOS transistor or a PMOS transistor as the above-mentioned approach does. 
         [0025]    Both of the first switch signal and the second switch signal have a 50% duty cycle in the above embodiments. Practically, the duty cycles of the first switch signal and the second switch signal can be adjusted to 60% to 40% or to other ratios depending on actual requirements. And, the duty cycles of the first driving signal and the second driving signal are modified with those of the first switch signal and the second switch signal, too. 
         [0026]    Consequently, the backlight module with the LCD employing such a backlight module activates the first lighting device and the second lighting device by using an alternate method. So, if both of the first switch signal and the second switch signal have a 50% duty cycle during the same switching cycle period, the first lighting device and the second lighting device will be in a closed state in a duty cycle of 50 percent, which can effectively prevent temperature from being too high when the lighting devices are lightened simultaneously and can effectively reduce thermal power generation during the lighting of the lighting devices. 
         [0027]    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.

Technology Classification (CPC): 7