Patent Publication Number: US-2013249413-A1

Title: Light emitting element pixel driving circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light emitting element pixel driving circuit, and more particularly, to a light emitting element pixel driving circuit with large capacitance for reducing update frequency of driving voltage. 
     2. Description of the Prior Art 
     Please refer to  FIG. 1 .  FIG. 1  is a diagram showing a light emitting element pixel driving circuit of the prior art. As shown in  FIG. 1 , the light emitting element pixel driving circuit of the prior art comprises a control switch  110 , a capacitor  120 , a driving switch  130 , and a light emitting element  140 . The control switch  110  is enabled or disabled according to a scan signal Vs. A first end of the capacitor  120  is electrically connected to the control switch  110 . A second end of the capacitor  120  is electrically connected to a voltage level VSS. The capacitor  120  can receive and store a predetermined driving voltage Vd when the control switch is enabled. After the capacitor  120  stores the predetermined driving voltage Vd, the control switch  110  then will be disabled immediately. A first end t 1  of the driving switch  130  can be electrically connected to a voltage source VDD, a second end t 2  of the driving switch  130  can be electrically connected to the first end of the capacitor  120 , and a third end t 3  of the driving switch  130  can be electrically connected to the light emitting element  140  for instance. The driving switch  130  is for controlling a driving current Id flowing through the driving switch  130  according to the predetermined driving voltage Vd stored in the capacitor  120 . For example, when the predetermined driving voltage Vd stored in the capacitor  120  is larger, a voltage difference between the second end t 2  and the third end t 3  of the driving switch  130  is larger as well, so as to increase the driving current Id flowing through the driving switch  130 . When the driving voltage Vd stored in the capacitor  120  is smaller, the voltage difference between the second end t 2  and the third end t 3  of the driving switch  130  is smaller as well, so as to decrease the driving current Id flowing through the driving switch  130 . The light emitting element  140  is for emitting light according to the driving current Id flowing through the driving switch  130 . When the driving current Id becomes larger, brightness of the light emitting element  140  is higher; and when the driving current Id becomes smaller, the brightness of the light emitting element  140  is lower. 
     Please refer to  FIG. 2 , and refer to  FIG. 1  as well.  FIG. 2  is a diagram showing a layout of the light emitting element pixel driving circuit  100  in  FIG. 1 , wherein the layout of the light emitting element pixel driving circuit  100  represents a layout of a single pixel circuit. As shown in  FIG. 2 , the layout of the light emitting element pixel driving circuit  100  comprises a capacitor area A 1  and a circuit area A 2 . The capacitor area A 1  is a layout area of the capacitor  120 . The circuit area A 2  is a layout area of other circuit elements (such as the control switch  110  and the driving switch  130 ). In the light emitting element pixel driving circuit  100  of the prior art, the layout area of the capacitor area A 1  is less than 10% of total area of the light emitting element pixel driving circuit  100 . The layout area of the capacitor area A 1  relates to capacitance of the capacitor  120 . When the layout area of the capacitor area A 1  is larger, the capacitance of the capacitor  120  is larger; and when the layout area of the capacitor area A 1  is smaller, the capacitance of the capacitor  120  is smaller. 
     According to the above arrangement, when the light emitting element pixel driving circuit  100  of the prior art displays images, the predetermined driving voltage Vd stored in the capacitor  120  is getting lower due to leakage current when the control switch  110  is disabled. In order to prevent the leakage current affecting quality of displaying images, a display period of a frame of the light emitting element pixel driving circuit  100  of the prior art is shorter, so as to turn on the control switch  110  more frequently per unit time for updating the driving voltage Vd stored in the capacitor  120 . However, when enabling the control switch  110  more frequently not only the service life of the control switch  110  is shortened, but also the power consumption of the light emitting element pixel driving circuit  100  is increased. 
     SUMMARY OF THE INVENTION 
     The present invention provides a light emitting element pixel driving circuit. The light emitting element pixel driving circuit comprises a control switch module, a capacitor, a driving switch, and a light emitting element. The control switch module comprises a plurality of series control switches for being simultaneously enabled or disabled according to a scan signal. The capacitor is electrically connected to the control switch module for receiving and storing a predetermined driving voltage when the control switch module is enabled. The driving switch is for controlling a driving current flowing through the driving switch according to the predetermined driving voltage. The light emitting element is electrically connected to the driving switch for emitting light according to the driving current. 
     The present invention further provides a light emitting element pixel driving circuit. The light emitting element pixel driving circuit comprises a control switch module, a variable capacitor module, a driving switch, and a light emitting element. The control switch module is enabled or disabled according to a scan signal. A first end of the variable capacitor module is electrically connected to the control switch module, and a second end of the variable capacitor module is electrically connected to a voltage level. The variable capacitor module is for receiving and storing a predetermined driving voltage when the control switch module is enabled. The variable capacitor module changes its capacitance according to a capacitor control signal. The driving switch is electrically connected to the variable capacitor module and a voltage source for controlling a driving current flowing through the driving switch according to the predetermined driving voltage stored in the variable capacitor module. The light emitting element is for emitting light according to the driving current flowing through the control switch. 
     In contrast to the prior art, the light emitting element pixel driving circuit of the present invention is capable of reducing switching times of the driving switches, and further extending the service life of the driving switch as well as reducing the power consumption of the light emitting element pixel driving circuit. Moreover, the light emitting element pixel driving circuit of the present invention can flexibly adjust the capacitance for adaptively displaying the dynamic images or static images. 
     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 that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a light emitting element pixel driving circuit of the prior art. 
         FIG. 2  is a diagram showing a layout of the light emitting element pixel driving circuit in  FIG. 1 . 
         FIG. 3  is a diagram showing a light emitting element pixel driving circuit of the present invention. 
         FIG. 4  is a diagram showing a layout of the light emitting element pixel driving circuit in  FIG. 3 . 
         FIG. 5  is a diagram showing another embodiment of the light emitting element pixel driving circuit of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 3 .  FIG. 3  is a diagram showing a light emitting element pixel driving circuit  200  of the present invention. As shown in  FIG. 3 , the light emitting element pixel driving circuit  200  of the present invention comprises a control switch module  210 , a capacitor  220 , a driving switch  230 , and a light emitting element  240 , wherein the light emitting element  240  can be an electroluminescent element, such as an organic light emitting diode (OLED), an inorganic light emitting diode, etc. The control switch module  210  comprises a plurality of series control switches  212 . The control switches  212  of the control switch module  210  are simultaneously enabled or disabled according to a scan signal Vs. A first end of the capacitor  220  is electrically connected to the control switch module  210 , and a second end of the capacitor  220  is electrically connected to a voltage level VSS. The capacitor  220  is for receiving and storing a predetermined driving voltage Vd when the control switches  212  of the control switch module  210  are enabled. After the capacitor  220  stores the predetermined driving voltage Vd, the control switches  212  of the control switch module  210  then will be disabled immediately. A first end t 1  of the driving switch  230  can be electrically connected to a voltage source VDD, a second end t 2  of the driving switch  230  can be electrically connected to the capacitor  220 , and a third end t 3  of the driving switch  230  can be electrically connected to the light emitting element  240  for instance. The driving switch  230  is for controlling a driving current Id flowing through the driving switch  230  according to the predetermined driving voltage Vd stored in the capacitor  220 . For example, when the predetermined driving voltage Vd stored in the capacitor  220  is larger, a voltage difference between the second end t 2  and the third end t 3  of the driving switch  230  is larger as well, so as to increase the driving current Id flowing through the driving switch  230 . When the driving voltage Vd stored in the capacitor  220  is smaller, the voltage difference between the second end t 2  and the third end t 3  of the driving switch  330  is smaller as well, so as to decrease the driving current Id flowing through the driving switch  230 . The light emitting element  240  is for emitting light according to the driving current Id flowing through the driving switch  230 . When the driving current Id becomes larger, brightness of the light emitting element  240  is higher; and when the driving current Id becomes smaller, the brightness of the light emitting element  240  is lower. 
     Please refer to  FIG. 4 , and  FIG. 3  as well.  FIG. 4  is a diagram showing a layout of the light emitting element pixel driving circuit  200  in  FIG. 3 , wherein the layout of the light emitting element pixel driving circuit  200  represents a layout of a single pixel circuit. As shown in  FIG. 4 , the layout of the light emitting element pixel driving circuit  200  comprises a capacitor area A 1 ′ and a circuit area A 2 ′. The capacitor area A 1 ′ is a layout area of the capacitor  220 . The circuit area A 2 ′ is a layout area of other circuit elements (such as the control switch module  210  and the driving switch  230 ). In the light emitting element pixel driving circuit  200  of the present invention, the layout area of the capacitor area A 1 ′ is more than 20% area of the light emitting element pixel driving circuit  200 . That is, capacitance of the capacitor  220  of the light emitting element pixel driving circuit  200  of the present invention is more than double of capacitance of the capacitor  120  of the light emitting element pixel driving circuit  100  of the prior art. 
     When the light emitting element pixel driving circuit  200  of the present invention displays images, leakage current is smaller while the control switch module  210  is disabled due to the control switch module  210  comprising the plurality of series control switches  212 . In addition, since the capacitance of the capacitor  220  of the light emitting element pixel driving circuit  200  of the present invention is more than twice as large as the capacitance of the capacitor  120  of the light emitting element pixel driving circuit  100  of the prior art, influence of the current leakage on the driving voltage Vd stored in the capacitor  220  is further reduced. According to the above arrangement, a display period of a frame can be extended. For example, the display period of the frame can be extended from 16.67 milliseconds to 1 second, such that reset rate for a frame according to the predetermined driving voltage Vd stored in the capacitor  220  can be reduced. That is, switching times of the control switches  212  of the control switch module  210  per unit time can be reduced. Especially when displaying a static image, the control switches  212  of the control switch module  210  are only required to be enabled one time for allowing the light emitting element pixel driving circuit  200  displaying a same image for a longer period. Therefore, service life of the control switches  212  of the light emitting element pixel driving circuit  200  is increased, and power consumption of the light emitting element pixel driving circuit  200  is reduced. 
     In addition, in a display period of a frame, when the light emitting element  240  emits light, the voltage level VSS can be a ground level (such as 0 volt), and when the light emitting element  240  does not emit light, the voltage level VSS can be a negative level (such as −5 volt) for pulling down a voltage level of the first end of the capacitor  220  (that is, a voltage level of the second end t 2  of the driving switch  230 ) to 0 volt, such that the driving switch  230  can be exactly disabled when the light emitting element  240  does not emit light. 
     Please refer to  FIG. 5 .  FIG. 5  is a diagram showing another embodiment of a light emitting element pixel driving circuit  300  of the present invention. As shown in  FIG. 5 , the light emitting element pixel driving circuit  300  of the present invention comprises a control switch module  310 , a variable capacitor module  320 , a driving switch  330 , and alight emitting element  340 . The control switch module  310 , the driving switch  330 , and the light emitting element  340  in  FIG. 5  are respectively identical to the control switch module  210 , the driving switch  230 , and the light emitting element  240  in  FIG. 3 . Therefore, no further illustration is provided. The variable capacitor  320  comprises a first capacitor  322 , a second capacitor  324 , and a capacitor control switch  326 . A first end of the first capacitor  322  can be electrically connected to the control switch module  310 , and a second end of the first capacitor  322  can be electrically connected to a voltage level VSS. A first end of the second capacitor  324  can be electrically connected to the control switch module  310 , and a second end of the second capacitor  324  can be electrically connected to the capacitor control switch  326 . Capacitance of the second capacitor  324  is larger than capacitance of the first capacitor  322 . The capacitor control switch  326  is electrically connected between the second end of the second capacitor  324  and the voltage level VSS for being enabled or disabled according to a capacitor control signal Vc. When the capacitor control switch  326  is disabled, capacitance of the variable capacitor module  320  is equal to the capacitance of the first capacitor  322 . When the capacitor control switch  326  is enabled, the capacitance of the variable capacitor module  320  is equal to a sum of the capacitance of the first capacitor  322  and the capacitance of the second capacitor  324 . 
     According to the above arrangement, when the light emitting element pixel driving circuit  300  is utilized for displaying dynamic images, the capacitor control switch  326  can be disabled in order to reduce the capacitance of the variable capacitor module  320 , and further reduce charging time of the variable capacitor module  320 ; and when the light emitting element pixel driving circuit  300  is utilized for displaying static images, the capacitor control switch  326  can be enabled in order to increase the capacitance of the variable capacitor module  320 , and further extend a display period of a same image displayed by the light emitting element pixel driving circuit  300 . 
     In addition, the plurality of series control switches  212 ,  312  of the control switch modules  210 ,  310  can be indium gallium zinc oxide (IGZO) thin-film transistors. The IGZO thin-film transistors have better current conductivity for allowing larger current passing through when the control switch module is enabled, so as to rapidly charge the capacitors  220 ,  322 ,  324  to a predetermined level. 
     In contrast to the prior art, the light emitting element pixel driving circuit of the present invention is capable of reducing the switching times of the driving switches, and further extending the service life of the driving switch as well as reducing the power consumption of the light emitting element pixel driving circuit. Moreover, the light emitting element pixel driving circuit of the present invention can flexibly adjust the capacitance for adaptively displaying the dynamic images or static images. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.