Patent Publication Number: US-8982020-B2

Title: Pixel driving circuit of organic-light emitting diode

Description:
RELATED APPLICATIONS 
     This application claims priority to Taiwanese Application Serial Number 102129156, filed Aug. 14, 2013, which is herein incorporated by reference. 
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to a pixel driving circuit, and more particularly, to the pixel driving circuit of an organic light-emitting diode 
     2. Description of Related Art 
     In the conventional display device, the power source uses the wire to provides the voltage to the driving circuit; however, since the wire itself has the impedance, hence, the terminal end of the wire will inevitably have the problem of voltage degradation, and this phenomenon will cause the decrease of the driving current of the pixel of the organic light-emitting diode, such that the display device provides images with uneven brightness. With the development of technology, display devices with larger sizes are being developed, and the above-mentioned phenomenon is particularly severe in larger display devices. 
     Moreover, since the transistors used in the pixel driving circuit of the organic light-emitting diode are not exactly the same, hence, the manufacturing processes and component characteristics thereof are also different, and when the manufacturing processes are different or when other factors are different, it will result in the difference of the threshold voltages of the transistors; in this way, it also causes the uneven brightness of the display device. 
     Further, the life cycles of the components of the organic light-emitting diode are limited, and hence, the characteristics of the organic light-emitting diode will gradually degrade with the passage of the display time, which phenomenon will also affect the brightness of the organic light-emitting diode, thereby resulting in the uneven brightness of the display device. 
     SUMMARY 
     The present invention provides a pixel driving circuit of an organic light-emitting, which addresses the problem existed in the prior art. 
     For achieving the foregoing goal, one aspect of the present invention is related to a pixel driving circuit of an organic light-emitting diode. The pixel driving circuit of the organic light-emitting diode comprises a first transistor and a capacitor, in which the first transistor comprises a first terminal, a control terminal and a second terminal, and the capacitor comprises a first terminal and a second terminal. The first terminal of the first transistor is electrically coupled to power source, the second terminal of the first transistor is electrically coupled to the organic light-emitting diode, the first terminal of the capacitor is electrically coupled to the first terminal of the first transistor at a first node, and the second terminal of the capacitor is electrically coupled to the control terminal of the first transistor at a second node. In a first period, the power source does not provide a power supply voltage to the first node, data voltage is written in the first node, and a variable voltage is written in the second node. In a second period, the power source provides the power supply voltage to the first node, such that the voltage of the first node is pulled to the power supply voltage, and the voltage of the second node is correspondingly pulled to the sum of the variable voltage and the power supply voltage subtracting the data voltage, and the first transistor provides a driving current to the organic light-emitting diode based on the voltages of the first node and the second node. 
     In one embodiment of the present invention, the variable voltage is adjusted to compensate the driving current. 
     In another embodiment of the present invention, the data voltage is adjusted to compensate the driving current. 
     In yet another embodiment of the present invention, the driving current is calculated according to the equation as follows,
 
 I   OLED   =K ( V   data   −V   R   −|V   TH | 2 );
 
wherein I OLED  is the driving current, K is the conductivity coefficient of the first transistor, V data  is the data voltage, V r  is the variable voltage, and V TH  is the threshold voltage of the first transistor.
 
     In still another embodiment of the present invention, the variable voltage is adjusted to compensate the threshold voltage of the first transistor. 
     In yet another embodiment of the present invention, the data voltage is adjusted to compensate the threshold voltage of the first transistor. 
     In still another embodiment of the present invention, the organic light-emitting diode is electrically coupled to a reference voltage terminal, wherein the reference voltage terminal, in the first period, does not provide a reference voltage to the organic light-emitting diode, and the reference voltage terminal, in the second period, provides the reference voltage to the organic light-emitting diode. 
     In still another embodiment of the present invention, the pixel driving circuit of the organic light-emitting diode further comprises a second transistor and a third transistor. The second transistor and third transistor both comprise a first terminal, a control terminal and a second terminal. The first terminal of the second transistor is electrically coupled to the first node, the control terminal of the second transistor is electrically coupled to a scan line, and the second terminal of the second transistor is electrically coupled to a data line. The first terminal of the third transistor is electrically coupled to second node, the control terminal of the third transistor is electrically coupled to the scan line, and the second terminal of the third transistor is electrically coupled to a variable power source. In the first period, the scan line transmits a scan voltage to the control terminal of the second transistor and the control terminal of the third transistor, such that the second transistor is turned on and writes the data voltage in the first node, and the third transistor is tuned on and writes the variable voltage in the second node. 
     In yet another embodiment of the present invention, the first transistor is a P-type transistor, and the second and third transistors are N-type transistors. 
     In still another embodiment of the present invention, the first, second and third transistors are all P-type transistors. 
     In view of the foregoing, the embodiments of the present invention provide a driving circuit so as to improve the problem of uneven brightness of the display device arises from the voltage degradation, variation of threshold voltages of transistors, and degradation of characteristics of the organic light-emitting diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the following detailed description of the embodiments, with reference made to the accompanying drawings as follows: 
         FIG. 1  shows a schematic diagram of a pixel driving circuit of an organic light-emitting diode according to one embodiment of the present invention. 
         FIG. 2  shows a schematic diagram of the driving waveform of the pixel driving circuit of the organic light-emitting diode according to  FIG. 1  of the present invention. 
         FIG. 3  shows a schematic diagram of the validation of the pixel driving circuit of the organic light-emitting diode according to  FIG. 1  of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic diagram of a pixel driving circuit  100  of an organic light-emitting diode according to one embodiment of the present invention. As illustrated in the drawing, the pixel driving circuit  100  of the organic light-emitting diode is used to drive the organic light-emitting diode  200 . The pixel driving circuit  100  of the organic light-emitting diode comprises a first transistor M 1  and a capacitor C s ; the first transistor M 1  comprises a first terminal, a control terminal and a second terminal; the capacitor C s  comprises a first terminal and a second terminal. The first terminal of the first transistor M 1  is electrically coupled to a power source V H , the second terminal of the first transistor M 1  is electrically coupled to the organic light-emitting diode  200 , the first terminal of the capacitor C s  is electrically coupled to the first terminal of the first transistor M 1  at a first node N 1 , and the second terminal of the capacitor C s  is electrically coupled to the control terminal of the first transistor M 1  at a second node N 2 . 
     Further, to facilitate the understanding of the present invention further, reference is now made to  FIG. 2  for illustratively explaining the present invention.  FIG. 2  shows a schematic diagram of the driving waveform of the pixel driving circuit of the organic light-emitting diode according to  FIG. 1  of the present invention, wherein V h  is the power supply voltage outputted by the power source V H . As illustrated in the drawing, in the first period T 1 , the power source V H  does not provide the power supply voltage V h  to the first node N 1 ; at the same time, a data voltage V data  is written in the first node N 1 , and a variable voltage V r  is written in the second node N 2 . 
     In a second period T 2 , the power source V H  provides the power supply voltage V h  to the first node N 1 , such that the voltage of the first node N 1  is pulled up to the power supply voltage V h , and the voltage of the second node N 2  is correspondingly pulled up to the sum of the variable voltage V r  and the power supply voltage V h  subtracting the data voltage V data . Thereafter, the first transistor M 1  can, based on the voltage of the first node N 1  and the voltage of the second node N 2 , provide a driving current I OLED  to the organic light-emitting diode  200 . 
     In this way, since the variable voltage V r  can be adjusted depending on the user&#39;s need, hence, when the power supply voltage V h  provided by the power source V H  via a wire experiences a voltage degradation, it is possible to adjust the variable voltage V r  to compensate the degraded voltage; moreover, when the degradation of the characteristic of the organic light-emitting diode  200  results in the uneven brightness of the display device, it is possible to adjust the variable voltage V r  to compensate the characteristic degradation of the organic light-emitting diode  200 . 
     In conclusion, when the electronic components of the display device have different parameters or degrade, it is feasible to use the driving circuit  100  of the embodiments of the present invention to adjust the variable voltage V r  so as to perform compensation, thereby improving the problem of uneven brightness of the display device, so as to enhance the display quality of the display device. 
     In the present embodiment, in addition to adjusting the variable voltage V r  to compensate the driving current I OLED , the data voltage V data  can also be adjusted to compensate the driving current I OLED ; in this way, the level of the driving current I OLED  can be maintained, thereby maintaining the brightness of the display device, and enhancing the display quality of the display device. 
     Regarding the driving current I OLED , the original equation thereof is:
 
 I   OLED   =K ( V   SG   −|V   TH |) 2   (1)
 
where I OLED  is the driving current, K is the conductivity coefficient of the first transistor M 1 , V SG  is voltage difference between the first terminal and the control terminal of the first transistor M 1 , and V TH  is the threshold voltage of the first transistor M 1 .
 
     In electrical operation, first, the driving circuit  100 , in the first period T 1 , writes the data voltage V data  in the first node N 1 ; at the same time, writes the variable voltage V r  in the second node N 2 . Next, the first node N 1  of the driving circuit  100 , in the second period, receives the power supply voltage V h  provided by the power source V H , such that the voltage of the first node N 1  is pulled up to the power supply voltage V h , and the voltage of the second node N 2  is correspondingly pulled up to the sum of the variable voltage V r  and the power supply voltage V h  subtracting the data voltage V data . 
     For example, the first terminal of the first transistor M 1  can be a source, and the control terminal of the first transistor M can be a gate. Since the first node N 1  is electrically coupled to the source of the first transistor M 1 , and the second node N 2  is electrically coupled to the gate of the first transistor M 1 , in the second period T 2 , the voltage of the source of the first transistor M 1  is V h , and the voltage of the gate of the first transistor M 1  is V r +V h −V data . Substituting the above-mentioned voltages into the equation (1) would give the following equation:
 
 I   OLED   =K ([V h −( V   r   +V   h   −V   data )−| V   TH |]) 2   (2)
 
wherein V data  is the data voltage, V r  is the variable voltage.
 
     Further simplification of the equation (2) would give the following equation:
 
 I   OLED   =K ( V   data   −V   r   −|V   TH |) 2   (3)
 
     In this way, when the electronic components of the display device have different parameters or degrade, as is apparent from the foregoing equations, it is possible to use the driving circuit  100  of the embodiments of the present invention to adjust the variable voltage V, to perform the compensation, so as to further improve the problem of the uneven brightness of the display device and enhance the display quality of the display device. 
     In the present embodiment, the variable voltage V r  of the equation is adjusted to compensate the of threshold voltage V TH  of the first transistor M 1  moreover, the data voltage V data  is also adjusted to compensate the threshold voltage V TH  of the first transistor M 1 , such that the driving current I OLED  is maintained stable, thereby maintaining the brightness of the display device and enhancing the display quality of the display device. 
     In the present embodiment, referring to both  FIG. 1  and  FIG. 2 , the organic light-emitting diode  200  is electrically coupled to a reference voltage terminal V L , wherein the reference voltage terminal V L , in the first period T 1 , does not provide the reference voltage V I  to the organic light-emitting diode  200 , and said reference voltage terminal V only provides the reference voltage to the organic light-emitting diode  200  in the second period T 2 . 
     With reference to  FIG. 1 , the pixel driving circuit  100  of the organic light-emitting diode is configured to drive the organic light-emitting diode  200  of a display panel, and the display panel comprises a scan line  300  and a data line  400 , wherein the pixel driving circuit  100  of the organic light-emitting diode further comprises a second transistor M 2  and a third transistor M 3 . The second transistor M 2  and the third transistor M 3  both comprise a first terminal, a control terminal and a second terminal. The first terminal of the second transistor M 2  is electrically coupled to the first node N 1 , the control terminal of the second transistor M 2  is electrically coupled to the scan line  300 , and the second terminal of the second transistor M 2  is electrically coupled to the data line  400 . 
     Further, the first terminal of the third transistor M 3  is electrically coupled to the second node N 2 , the control terminal of the third transistor M 3  is electrically coupled to the scan line  300 , and the second terminal of the third transistor M 3  is electrically coupled to the variable power source V R . 
     Similarly, to further facilitate the understanding of the present invention, reference is now made to both  FIG. 1  and  FIG. 2 . In the first period T 1 , the scan line  300  transmits a scan voltage V scan  to the control terminal of the second transistor M 2  and the control terminal of the third transistor M 3 , such that the second transistor M 2  is turned on and writes the data voltage V data  in the first node N 1  and the third transistor M 3  is turned on and writes the variable voltage V r  in the second node N 2 , wherein the data voltage V data  is outputted by the data line  400  and the variable voltage V r  is outputted by the variable power source V R . 
     In this way, the user may, depend on his/her needs, to use the variable power source V R  to adjust the variable voltage V r ; hence, when the power supply voltage V h  provided by the power source V H  via a wire experiences a voltage degradation, it is possible to adjust the variable voltage V r  to compensate the degraded voltage; moreover, when the difference among the threshold voltages of the transistors M 1  to M 3  or the degradation of the characteristic of the organic light-emitting diode  200  results in the uneven brightness of the display device, it is possible to adjust the variable voltage V r  to compensate the characteristic degradation of the organic light-emitting diode  200 . 
     In the present embodiment, with reference to  FIG. 1 , the first transistor M 1  can be a P-type transistor, and the second and third transistors M 2 , M 3  are N-type transistors. However, the present invention is not limited thereto, and suitable transistor types could be flexibly select depending on the actual need. In one embodiment, all of the first, second and third transistors M 1  to M 3  can be P-type transistors. 
     To validate the operation condition of the above-mentioned circuit, the present invention embodiment adopts the built-in Device Model of the Smart-SPICE to validate the driving circuit  100 . The parameters used in the validation include, the W/L of the first transistor is 50/3.84 μm (P-type), the W/L of the second and third transistors is 8 μm/3.84 μm (n-type), Cs=2.5 pF, V TH  of the first transistor is −3, V TH  of the second and third transistors is 1, V data =0˜5 V, V scan =−10˜20 V, V r =0˜2 V, V h =12 V, and V I =0 V, wherein W is the width of the channel, L is the length of the channel, V TH  is the threshold voltage of the transistor, V data  is the data signal outputted by the data line, V scan  is the scan signal outputted by the scan line  300 , V is the variable voltage outputted by the variable power source V R , V h , is the power supply voltage outputted by the power source V H , and V I  is the reference voltage outputted by the reference voltage terminal V L . 
     The result of validation is summarized in  FIG. 3  which shows a schematic diagram of the validation of the pixel driving circuit of the organic light-emitting diode according to  FIG. 1  of the present invention. As illustrated in the drawing, after 30 μs, the voltage at each terminal point of the transistor M 1  tends to be stable. As can be seen in the drawing, the voltage V S  of the source (the first terminal) of the first transistor M 1  is greater than the voltage V G  of the gate (the control terminal) of the first transistor M 1 , and the voltage V G  of the gate (the control terminal) of the first transistor M 1  is greater than the voltage V D  of the drain (the second terminal) of the first transistor M 1 ; since the first transistor M 1  is a P-type transistor, the above-mentioned electrical condition can allow the first transistor M 1  to be in a saturation mode; in this way, it is possible to ensure that the driving circuit  100  of embodiments of the present invention can adjust the variable voltage V r  so as to compensate the decrease of the driving current I OLED  caused by the parameter variation among the components in the circuit, and therefore, improve the problem of the uneven brightness of the display device. 
     In view of the foregoing embodiments of the present invention, many advantages of the present invention are now apparent. The embodiment of the present invention provides a driving circuit to improve the problem of the uneven brightness of the display device caused by the voltage degradation, difference of threshold voltages of transistors and the characteristic degradation of the organic light-emitting diode.