Patent Publication Number: US-11037509-B2

Title: Pixel driving circuit and display device having the same for eliminating improper image-displaying of OLED display resulting from drifting of threshold voltage of driving TFT

Description:
RELATED APPLICATIONS 
     The present application is a National Phase of International Application Number PCT/CN2018/076542, filed Feb. 12, 2018, and claims the priority of China Application No. 201711405794.X, filed Dec. 22, 2017. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to display field, and more particularly to a pixel driving circuit and a display device having the same. 
     2. Description of Related Art 
     Organic light-emitting diode (OLED) displays have been popular flat panel display products due to the attributes, such as self-illuminating, wide viewing angle, short response time, high luminous efficiency, wide color gamut, low operating voltage, thin thickness. In addition, the OLED may be adopted in large-scale and flexible displays via simple manufacturing process. Moreover, the cost of OLED displays is low. 
     With respect to OLED displays, thin film transistors (TFTs) often cooperate with capacitor storage signals to control the gray scale of the OLED so as to drive the OLED at a constant current. Each of the pixels at least includes two thin film transistors (TFT)s and a storage capacitor, i.e., each of the pixels has a 2T1C structure.  FIG. 1  is a circuit diagram of a pixel driving circuit of conventional OLED displays. Referring to  FIG. 1 , the pixel driving circuit of the conventional OLED displays may include two TFT and a capacitor. Specifically, the pixel driving circuit of the conventional OLED displays may include a switch TFT T 1 , a driving TFT T 2 , and a storage capacitor C st . Driving current of the OLED is controlled by the driving TFT T 2 . The current may satisfy the equation: I OLED =k(V GS −V th ) 2 , wherein k is an intrinsic conductance factor of the driving TFT T 2 , which is determined by the characteristic of the driving TFT T 2 . V th  is threshold voltage of the driving TFT T 2 . V gs  is a voltage between a gate and a source of the driving TFT T 2 . Due to the long-term operation, the threshold voltage V th  of the driving TFT T 2  may drift, causing the driving current of the OLED change. Thus, the OLED display may not operate properly, which may impacts the quality of the displays. 
     SUMMARY 
     In one aspect, the present disclosure relates to a pixel driving circuit, including: a first thin film transistor (TFT), a second TFT, a third TFT, a fourth TFT, a fifth TFT, a sixth TFT, a seventh TFT, a capacitor, and an organic light-emitting diode (OLED); wherein, during a reset phase, the fourth TFT is turned on to provide a reference voltage to a first end of the capacitor, and the fifth TFT is turned on and then turned off to provide a power supply voltage to a second end of the capacitor, during a threshold voltage compensating phase, the second TFT is turned on to provide a data voltage to a gate electrode of the first TFT, the fourth TFT is maintained to be in a turn-on state, so as to maintain a voltage of the first end of the capacitor to be equal to the reference voltage, and the first TFT and the third TFT are turned on such that the second end of the capacitor is discharged until a voltage equal to a voltage difference between the data voltage and a threshold voltage of the first TFT, and the first TFT is turned off; during an emission driving phase, the fifth TFT is turned on to provide the power supply voltage to the first end of the first TFT, the seventh TFT is turned on to provide a voltage of the capacitor to the gate electrode of the first TFT, the sixth TFT is turned on such that a driving current being provided from the second end of the first TFT to the OLED via the sixth TFT. 
     The first TFT, the second TFT, the third TFT, the sixth TFT, and the seventh TFT are in a turn-off state during the reset phase. 
     The fifth TFT, the sixth TFT, and the seventh TFT are in a turn-off state during the threshold voltage compensating phase. 
     The second TFT, the third TFT, and fourth TFT are in a turn-off state during the emission driving phase. 
     The gate electrode of the first TFT connects to a first node, a first end of the first TFT connects to a second node, and a second end of the first TFT connects to a third node; a gate electrode of the second TFT is configured to receive scanning signals, a second end of the second TFT connects to the first node, and a first end of the second TFT is configured to receive the data voltage; a gate electrode of the third TFT is configured to receive the scanning signals, a second end of the third TFT connects to the third node, and a first end of the third TFT connects to the first node; a gate electrode of the fourth TFT is configured to receive reset signals, a first end of the fourth TFT is configured to receive the reference voltage, and a second end of the fourth TFT connects to a fourth node; a gate electrode of the fifth TFT is configured to receive enabling signals, a first end of the fifth TFT is configured to receive the power supply voltage, and a second end of the fifth TFT connects to the second node; a gate electrode of the sixth TFT is configured to receive the enabling signals, a first end of the sixth TFT connects to the third node, and a second end of the sixth TFT connects to the OLED; a gate electrode of the seventh TFT is configured to receive the enabling signals, a first end of the seventh TFT connects to the fourth node, and a second end of the seventh TFT connects the first node; a first end of the capacitor connects to the fourth node, and a second end of the capacitor connects to the second node. 
     During the reset phase, the reset signals are maintained to be at a low potential, the scanning signals are maintained to be at a high potential, the enabling signals are maintained to be at the low potential during a first predetermined time period, and the enabling signals transits from the low potential into the high potential when the first predetermined time period expires. 
     During the threshold voltage compensating phase, the enabling signals are maintained to be at the high potential, the reset signals are maintained to be at the low potential during a second predetermined time period, the reset signals transits from the low potential into the high potential when the second predetermined time period expires, the scanning signals are maintain to be at the low potential during a third predetermined time period, and the scanning signals transits from the low potential into the high potential when the third predetermined time period expires. 
     During the emission driving phase, the enabling signals are maintained to be at the low potential, and the reset signals and the scanning signals are maintained to be at the high potential. 
     The first TFT, the second TFT, the third TFT, the fourth TFT, the fifth TFT, the sixth TFT, and the seventh TFT are P-trench TFTs. 
     In another aspect, the present disclosure relates to a display device including the pixel driving circuit. 
     In view of the above, the pixel driving circuit may adopt the pixel structure of 7T1C to compensate the threshold voltage of the driving TFT in the OLED. As such, the current passing through the OLED may not be related to the threshold voltage of the driving TFT, so as to eliminate the improper image-displaying of the OLED display resulting from the drifting of the threshold voltage of the driving TFT. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a pixel driving circuit of a conventional organic light-emitting diode (OLED) display. 
         FIG. 2  is a schematic view of an OLED display in accordance with one embodiment in the present disclosure. 
         FIG. 3  is a circuit diagram of a pixel driving circuit in accordance with one embodiment in the present disclosure. 
         FIG. 4  is a timing diagram of each of signals in accordance with one embodiment in the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Following embodiments of the invention will now be described in detail hereinafter with reference to the accompanying drawings. However, there are plenty of forms to implement the present disclosure, and the invention should not be construed as limitation to the embodiments. Rather, these embodiments are provided to explain the principles of the invention and its practical application, thereby enable other person skilled in the art to understand each of the embodiments in the invention and various modifications being suitable for the particular application. 
     In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. Same reference numerals refer to the same components throughout the specification and the drawings. 
       FIG. 2  is a schematic view of an OLED display in accordance with one embodiment in the present disclosure. 
     Referring to  FIG. 2 , the present disclosure relates to an organic light-emitting diode (OLED) display, including: a display panel  100 , a scanning driver  200 , and a data driver  300 . It is noted that the OLED display in the present disclosure may further include other proper components, such as a timing controlling device configured to control the scanning driver  200  and the data driver  300 , and a power supply voltage generator configured to provide a positive voltage of a power supply and a negative voltage of the power supply. 
     Specifically, the display panel  100  may include: a plurality of pixel PX arranged in a matrix, n number of scanning lines G 1  to G N , m number of data lines D 1  to D M . The scanning driver  200  connects to each of the scanning lines G 1  to G N  and drives each of the scanning lines G 1  to G N . The data driver  300  connects to each of the data lines D 1  to D M  and drives each of the data lines D 1  to D M . 
     The scanning driver  200  may provide at least one signal to each of the pixels PX, which may be described in detail later. The data driver  300  may provide data signals to each of the pixels PX, which may also be described in detail later. 
     Each of the pixels PX may include a pixel driving circuit. The pixel driving circuit in the present disclosure may be described in detail as below. 
     Specifically, the display panel  100  may include: a plurality of pixel PX arranged in a matrix, n number of scanning lines G 1  to G N , m number of data lines D 1  to D M . The scanning driver  200  connects to each of the scanning lines G 1  to G N  and drives each of the scanning lines G 1  to G N . The data driver  300  connects to each of the data lines D 1  to D M  and drives each of the data lines D 1  to D M . 
     The scanning driver  200  may provide at least one signal to each of the pixels PX, which may be described in detail later. The data driver  300  may provide data signals to each of the pixels PX, which may also be described in detail later. 
     Each of the pixels PX may include a pixel driving circuit. The pixel driving circuit in the present disclosure may be described in detail as below. 
       FIG. 3  is a circuit diagram of a pixel driving circuit in accordance with one embodiment in the present disclosure. 
     Referring to  FIG. 3 , each of the pixels PX of the OLED display may include a 7T1C pixel structure. The 7T1C pixel structure includes an OLED, a first thin film transistor (TFT) T 1 , a second TFT T 2 , a third TFT T 3 , a fourth TFT T 4 , a fifth TFT T 5 , a sixth TFT T 6 , a seventh TFT T 7 , and a capacitor “C st .”. 
     A gate electrode of the first TFT T 1  electrically connects to a first node “g”. A first end of the first TFT T 1  electrically connects to a second node “s”. A second end of the first TFT T 1  electrically connects to a third node “d”. 
     A gate electrode of the second TFT T 2  is configured to receive scanning signals “Scan”, which are provided by the scanning driver  200 . A first end of the second TFT T 2  is configured to receive a data voltage “V data ”, which are provided by the data driver  300 . A second end of the second TFT T 2  electrically connects to the first node “g”. In one example, the data voltage “V data ” is configured to be at a high potential. 
     A gate electrode of the third TFT T 3  is configured to receive the scanning signals “Scan”. A first end of the third TFT T 3  electrically connects to the first node “g”. A second end of the third TFT T 3  electrically connects to the third node “d”. 
     A gate electrode of the fourth TFT T 4  is configured to receive reset signals “Reset”. A first end of the fourth TFT T 4  is configured to receive a reference voltage “V ref ”. A second end of the fourth TFT T 4  electrically connects to a fourth node “a”. 
     A gate electrode of the fifth TFT T 5  is configured to receive enabling signals “Em”. A first end of the fifth TFT T 5  is configured to receive the positive voltage of the power supply “V dd ”, which is usually generated and provided by a power generator (not shown) of the OLED display. A second end of the fifth TFT T 5  electrically connects to the second node “s”. 
     A gate electrode of the sixth TFT T 6  is configured to receive the enabling signals “Em”. A first end of the sixth TFT T 6  electrically connects to the third node “d”. A second end of the sixth TFT T 6  connects to an anode of the OLED. 
     A gate electrode of the seventh TFT T 7  is configured to receive the enabling signals “Em”. A first end of the seventh TFT T 7  electrically connects to the fourth node “a”. A second end of the seventh TFT T 7  connects the first node “g”. 
     A first end of the capacitor “C st ” electrically connects to the fourth node “a”, and a second end of the capacitor “C st ” electrically connects to the second node “s”. 
     A cathode of the OLED is configured to receive the negative voltage of the power supply “V ss ” which is usually generated and provided by the power generator (not shown) of the OLED. In one example, the positive voltage of the power supply “V dd ” is configured to be at the high potential, the negative voltage of the power supply “V ss ” is configured to be at a low potential, and the positive voltage of the power supply “V dd ” is greater than the negative voltage of the power supply “V ss ”. 
     In one example, the first TFT T 1  is a driving TET. 
     The first ends of each of the first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the fourth TFT T 4 , the fifth TFT T 5 , the sixth TFT T 6 , and the seventh TFT T 7  may be a source electrode or a drain electrode. The second ends of each of the first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the fourth TFT T 4 , the fifth TFT T 5 , the sixth TFT T 6 , and the seventh TFT T 7  may be an electrode different from the first end. 
     For example, when the first end is the drain electrode, the second end is the source electrode. When the first end is the source electrode, the second end is the drain electrode. 
     The first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the fourth TFT T 4 , the fifth TFT T 5 , the sixth TFT T 6 , and the seventh TFT T 7  may be the TFTs having the same trench type. 
     For example, the first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the fourth TFT T 4 , the fifth TFT T 5 , the sixth TFT T 6 , and the seventh TFT T 7  may be P-trench TFTs. 
     The first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the fourth TFT T 4 , the fifth TFT T 5 , the sixth TFT T 6 , and the seventh TFT T 7  may adopt polysilicon TFTs, amorphous silicon TFTs, or oxide TFTs. 
     Operation principles of the pixel driving circuit in the present disclosure are described in detail as follow. In one example, the pixel driving circuit, having the structure of 7T1C, may conduct a reset operation, (i.e., during a reset phase), a threshold voltage compensating operation (i.e., during a threshold voltage compensating phase), and an emission driving operation (i.e., during an emission driving phase).  FIG. 4  is a timing diagram of each of signals in accordance with one embodiment in the present disclosure. 
     During the reset phase, referring to  FIG. 3  and  FIG. 4 , the reset signals “Reset” are maintained to be at the low potential. The scanning signals “Scan” are maintain to be at the high potential. The enabling signals “Em” are maintained to be at the low potential during a first predetermined time period A 1 . The enabling signals “Em” transits from the low potential into the high potential when the first predetermined time period A 1  expires. That is, the enabling signals “Em” and the reset signals “Reset” are maintained to be at the low potential during the first predetermined time period A 1 . 
     The fourth TFT T 4  is turned on to provide the reference voltage “V ref ” to the fourth node “a”, i.e., the first end of the capacitor “C st ”. Thus, a voltage “Va” of the fourth node “a” may be equal to “V ref ” (Va=V ref ). The fifth TFT T 5  is turned on and then turned off to provide a power supply voltage “V dd ” to the second node “s”, i.e., the second end of the capacitor “C st ” Thus, a voltage “Vs” of the second node “a” may be equal to “V dd ” (Vs=V dd ). It is noted that a starting point of the first predetermined time period A 1  coincides with a starting point of the reset phase, therefore, the enabling signals “Em” is configured to be at the low potential and transits to the high potential to guarantee the voltage “Vs” of the second node “s” being equal to “V dd ”, i.e., (Vs=V dd ). 
     The first TFT T 1 , the second TFT T 2 , the third TFT T 3 , the sixth TFT T 6 , and the seventh TFT  17  are in a turn-off state during the reset phase. 
     During the threshold voltage compensating phase, the enabling signals “Em” are maintained to be at the high potential. The reset signals “Reset” are maintained to be at the low potential during a second predetermined time period A 2 . The reset signals “Reset” transits from the low potential into the high potential when the second predetermined time period A 2  expires. The scanning signals “Scan” are maintain to be at the low potential during a third predetermined time period A 3 , and the scanning signals “Scan” transits from the low potential into the high potential when the third predetermined time period A 3  expires. A starting point of the second predetermined time period A 2  is the same with a starting point of the third predetermined time period A 3 , and the second predetermined time period A 2  is greater than the third predetermined time period A 3 . 
     The second TFT T 2  is turned on to provide the data voltage “V data ” to the first node “g”, i.e., the gate electrode of the first TFT T 1 . The fourth TFT T 4  is maintained to be in a turn-on state, so as to maintain a voltage of the first end of the capacitor “C st ” to be equal to the reference voltage “V ref ”, i.e., the voltage “Va” of the fourth node “a”, (Va=V ref ). The first TFT T 1  and the third TFT T 3  are turned on such that the second end of the capacitor “C st ” is discharged until a voltage equal to a voltage difference between the data voltage “V data ” and a threshold voltage “V th ” of the first TFT T 1  via a path of the first TFT T 1  and the third TFT T 3 , and the first TFT T 1  is turned off. That is, the voltage “Vs” of the second node “s” is equal to (V data −V h ), wherein “V th ” is the threshold voltage of the first TFT T 1 . 
     The fifth TFT T 5 , the sixth TFT T 6 , and the seventh TFT T 7  are in the turn-off state during the threshold voltage compensating phase. 
     During the emission driving phase, the enabling signals “Em” are maintained to be at the low potential, and the reset signals “Reset” and the scanning signals “Scan” are maintained to be at the high potential. The fifth TFT T 5  is turned on to provide the power supply voltage “V dd ” to the second node “s”, i.e., the first end of the first TFT T 1 . The voltage “Vs” of the second node “s” is equal to “V dd ” (Vs=V dd ). The seventh TFT T 7  is turned on to provide a voltage of the capacitor “C st ” to the first node “g”, i.e., the gate electrode of the first TFT T 1 . The voltage “Vg” of the first node “g” is equal to (V dd −V data +V th +V ref ), i.e., (Vg=V dd −V data +V th +V ref ). The sixth TFT T 6  is turned on such that a driving current being provided from the second end of the first TFT T 1  to the OLED via the sixth TFT T 6 . 
     As such, the driving current “I” passing through the OLED may be represented as below:
 
 I=k ( Vgs−Vth ) 2   =k ( V ref+ Vth−V data− Vth ) 2   =k ( V ref− V data) 2   (1)
 
     A voltage difference “Vgs” between the first node “g” and the second node “s” is equal to (Vg−Vs), i.e., (Vgs=Vg−Vs=V dd −V data +V th +V ref −V dd =V ref −V data +V th ), wherein “k” is an intrinsic conductance factor of the first TFT T 1 , which is determined by characters of the first TFT T 1 . 
     It can be noted that, in the equation (1), the driving current “I” passing through the OLED is not related to the threshold voltage “V th ” of the first TFT T 1 . As such, the improper image-displaying of the OLED display resulting from the drifting of the threshold voltage of the driving TFT may be eliminated. 
     The second TFT T 2 , the third TFT T 3 , and fourth TFT T 4  are in the turn-off state during the emission driving phase. 
     In view of the above, the current passing through the OLED may not be related to the threshold voltage of the driving TFT. So as to eliminate the improper image-displaying of the OLED display resulting from the drifting of the threshold voltage of the driving TFT. 
     The above description is merely the embodiments in the present disclosure, the claim is not limited to the description thereby. The equivalent structure or changing of the process of the content of the description and the figures, or to implement to other technical field directly or indirectly should be included in the claim.