Patent Publication Number: US-9852685-B2

Title: Pixel circuit and driving method thereof, display apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of PCT/CN2014/087579 filed on Sep. 26, 2014, which claims priority under 35 U.S.C. §119 of Chinese Application No. 201410126737.8 filed on Mar. 31, 2014, the disclosure of which is incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a pixel circuit and a driving method of the same, and a display apparatus. 
     BACKGROUND 
     At present, an active matrix organic light emitting diode (AMOLED) panel uses a thin film transistor (TFT) to drive an organic light emitting diode (OLED) to emit light. 
     An AMOLED pixel circuit adopts generally a 2T1C circuit comprising two TFTs and one capacitor. In this 2T1C circuit, the current I OLED  flowing through OLED is calculated by the equation of: 
               I   OLED     =       1   2     ⁢       μ   n     ·   Cox   ·     W   L     ·       (     Vdata   -   Voled   -   Vthn     )     2               
where μ n , is a carrier mobility, C ox  is a gate oxide layer capacitor, W/L is a width to length ratio of the transistor, Vdata is a data voltage, Voted is an operating voltage of OLED shared by all the pixel units, and Vthn is a threshold voltage of the transistor. Vthn is a positive value for an enhancement type TFT, while Vthn is a negative value for a depletion type TFT.
 
     However, due to limitation of crystallization process and manufacturing level, non-uniformity always occurs in electrical parameters such as threshold voltage, mobility and so on of the TFT switching circuit manufactured on a large-area glass substrate, such that threshold voltage offset of respective TFTs is inconsistent. It can be known from the above equation that currents flowing through different OLEDs have a difference if Vthn among different pixel units is different. If Vthn of pixels drifts with the time, this may result in that currents flowing through successively the same OLED are different, thereby causing image sticking. Furthermore, because non-uniformity of the OLED devices causes the operating voltages of OLED to be different, it may also result in current difference, thereby causing difference in display brightness of AMOLED. 
     SUMMARY 
     Embodiments of the present disclosure provide a pixel circuit and a driving method of the same, and a display apparatus, which can compensate for the threshold voltage drift of TFT effectively, raise uniformity of light emitting brightness of the display apparatus, and enhance display effect. 
     According to one aspect of an embodiment of the present disclosure, there is provided a pixel circuit, comprising: a first transistor, a second transistor, a third transistor, a storage capacitor and a light emitting device, wherein a gate of the first transistor is connected to a first control signal terminal, and a first electrode thereof is connected to a data signal terminal; a gate of the second transistor is connected to a second electrode of the first transistor, a first electrode thereof is connected to a second electrode of the third transistor, and a second electrode thereof is connected to a first terminal of the light emitting device; a gate of the third transistor is connected to a second control signal terminal, and a first electrode thereof is connected to a first power supply signal terminal; one terminal of the storage capacitor is connected to the gate of the second transistor, and the other terminal thereof is connected to the second electrode of the second transistor; one terminal of a parasitic capacitor formed by the light emitting device is connected to the first terminal of the light emitting device, and the other terminal thereof is connected to a second terminal of the light emitting device; and the second terminal of the light emitting device is further connected to a second power supply signal terminal. 
     According to another aspect of an embodiment of the present disclosure, there is further provided a display apparatus comprising the pixel circuit as described above. 
     According to another aspect of an embodiment of the present disclosure, there is further provided a pixel circuit driving method for driving the pixel circuit as described above, comprising: 
     in a first phase, turning on the first transistor and the third transistor; inputting a first voltage by the first power supply signal terminal, inputting a reset signal by a data signal terminal, turning on the second transistor, and controlling the light emitting device to be in a turn-off state, such that a voltage of the storage capacitor is greater than a threshold voltage of the second transistor; 
     in a second phase, maintaining the first transistor and the third transistor turning on; inputting a second voltage by the first power supply signal terminal, so that the second transistor is turned off, the voltage of the storage capacitor is equal to the threshold voltage of the second transistor, and the light emitting device is in a turn-off state; 
     in a third phase, maintaining the first transistor turning on; turning off the third transistor, inputting a data signal by the data signal terminal, so that the second transistor is turned on, and data is written into the first terminal of the light emitting device through a voltage dividing effect of the storage capacitor and a parasitic capacitor formed by the light emitting device; and 
     in a fourth phase, turning off the first transistor, turning on the third transistor, and driving the light emitting device to emit light by a current flowing through the second transistor and the third transistor. 
     The pixel circuit, its driving method and the display apparatus according to the embodiments of the present disclosure control turning off/on and charging/discharging of the circuit through a plurality of transistors and capacitors, so that the current flowing through the transistors and being used to drive the light emitting device is unrelated to the threshold voltage of the transistors, which compensates for the difference in current flowing through the light emitting device due to inconsistency or offset of the threshold voltage of the transistors, raises uniformity of light emitting brightness of the display apparatus, and greatly enhances display effect. In addition, this pixel circuit has a simple structure and a smaller number of transistors, so as to be able to reduce the size of shading area covering the transistors and increase the aperture ratio of the display apparatus effectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a connecting structure of a pixel circuit provided in an embodiment of the present disclosure; 
         FIG. 2  is a timing diagram of respective signal lines driving the pixel circuit as shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an equivalent circuit of a pixel circuit provided in an embodiment of the present disclosure in a resetting phase; 
         FIG. 4  is a schematic diagram of an equivalent circuit of a pixel circuit provided in an embodiment of the present disclosure in a compensating phase; 
         FIG. 5  is a schematic diagram of an equivalent circuit of a pixel circuit provided in an embodiment of the present disclosure before being ready to write data; 
         FIG. 6  is a schematic diagram of an equivalent circuit of a pixel circuit provided in an embodiment of the present disclosure in a data writing phase; 
         FIG. 7  is a schematic diagram of an equivalent circuit of a pixel circuit provided in an embodiment of the present disclosure before being ready to drive a light emitting device to emit light; 
         FIG. 8  is a schematic diagram of an equivalent circuit of a pixel circuit provided in an embodiment of the present disclosure in a light emitting phase; 
         FIG. 9  is flow schematic diagram of a pixel circuit driving method provided in an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Technical solution in embodiments of the present disclosure will be described clearly and completely by combining with the accompanying figures. 
       FIG. 1  shows schematically a connecting structure of a pixel circuit of an embodiment of the present disclosure. As shown in  FIG. 1 , the pixel circuit comprises: a first transistor T 1 , a second transistor T 2 , a third transistor T 3 , a storage capacitor C 1  and a light emitting device L. 
     A gate of the first transistor T 1  is connected to a first control signal terminal S 1 , and a first electrode thereof is connected to a data signal terminal DATA. 
     A gate of the second transistor T 2  is connected to a second electrode of the first transistor T 1 , a first electrode thereof is connected to a second electrode of the third transistor T 3 , and a second electrode thereof is connected to a first terminal of the light emitting device L. 
     A gate of the third transistor T 3  is connected to a second control signal terminal S 2 , and a first electrode thereof is connected to a first power supply signal terminal ELVDD. 
     One terminal of the storage capacitor C 1  is connected to the gate of the second transistor T 2 , and the other terminal thereof is connected to a second electrode of the second transistor T 2 . 
     One terminal of a parasitic capacitor C 2  formed by the light emitting device L is connected to a first terminal of the light emitting device L, and the other terminal thereof is connected to a second terminal of the light emitting device L. 
     The second terminal of the light emitting device L is further connected to a second power supply signal terminal ELVSS. 
     It needs to note that the light emitting device L in the embodiment of the present disclosure may be various current-driven light emitting devices including a light emitting diode (LED) or an organic light emitting diode (OLED). The embodiments of the present disclosure are described by taking OLED as an example. 
     The pixel circuit provided in the embodiment of the present disclosure controls turning off/on and charging/discharging of the circuit through a plurality of transistors and capacitors, so that the current flowing through the transistors and being used to drive the light emitting device is unrelated to the threshold voltage of the transistors, which compensates for the difference in current flowing through the light emitting device due to inconsistency or offset of the threshold voltage of the transistors, raises uniformity of light emitting brightness of the display apparatus, and greatly enhances display effect. In addition, since this pixel circuit has a simple structure and a smaller number of transistors, the size of shading area covering the transistors can be reduce and the aperture ratio of the display apparatus are increased effectively. 
     As an example, the first transistor T 1 , the second transistor T 2  and the third transistor T 3  are N type transistors. First electrodes of the first transistor T 1 , the second transistor T 2  and the third transistor T 3  are drains, and second electrodes thereof are sources. The first terminal of the light emitting device is an anode, and the second terminal thereof is a cathode. 
     It needs to note that the manufacturing technique of adopting N type transistors to integrate a driving circuit has been already mature currently. Therefore, the first transistor T 1 , the second transistor T 2  and the third transistor T 3  in the embodiments of the present disclosure are N type transistors, which can reduce manufacturing cost and realize simple technique. 
     In the operation of the pixel circuit as shown in  FIG. 1 , its operating process can be divided into four phases, i.e., a resetting phase, a compensating phase, a data writing phase, and a light emitting phase respectively.  FIG. 2  is a timing diagram of respective signal lines in the operating process of the pixel circuit as shown in  FIG. 1 . As shown in  FIG. 2 , P 1 , P 2 , P 3  and P 4  are used to represent the resetting phase, the compensating phase, the data writing phase and the light emitting phase in  FIG. 2  respectively. 
     As an example, the phase P 1  is the resetting phase. The equivalent circuit in this phase is as shown in  FIG. 3 . In the resetting phase, the first control signal terminal S 1  and the second control signal terminal S 2  are input a high level, a first power supply signal terminal ELVDD is input a low level (Voted), and the data signal terminal DATA is input a reset signal (Vref) at the low level, wherein Vref−Voled&gt;Vth (Vth is the threshold voltage of the second transistor T 2 ). Now, the first transistor T 1 , the second transistor T 2  and the third transistor T 3  are turned on, the voltage across the storage capacitor C 1  is Vref−Voled, the anode voltage of the light emitting device L is Voted, and the light emitting device L is in a turn-off state. 
     The phase P 2  is the compensating phase. The equivalent circuit in this phase is as shown in  FIG. 4 . In the compensating phase, the first control signal terminal S 1 , the second control signal terminal S 2  and the first power supply signal terminal ELVDD are input the high level, and the data signal terminal DATA is input the reset signal (Vref) at the low level. Now, the first transistor T 1 , the second transistor T 2  and the third transistor T 3  are maintained to be turned on, and the anode voltage of the light emitting device L rises as the second transistor T 2  is charged, until the voltage is equal to Vref−Vth. When the compensating phase ends up, the charge stored at the two terminals of the storage capacitor C 1  is Vth·C ST , where C ST  is capacitance of the storage capacitor C 1 . At this time, the second transistor is turned off, and the voltage at the two terminals of the storage capacitor C 1  is the threshold voltage Vth of the second transistor. 
     The phase P 3  is the data writing phase. It is required to turn off the third transistor T 3  before the pixel circuit is ready to write data. The equivalent circuit in this phase is as shown in  FIG. 5 . The gate voltage of the second transistor T 2  is the reset signal Vref at the low level input by the data signal terminal DATA. Now, the anode voltage of the light emitting device L is Vref−Vth. In the data writing phase, the equivalent circuit is as shown in  FIG. 6 , wherein the first control signal terminal S 1  and the first power supply signal terminal ELVDD are input the high level, the second control signal terminal S 2  is input the low level, and the data signal terminal DATA is input the data signal (Vdata) at the high level. Now, the first transistor T 1  and the second transistor T 2  are turned on, and the third transistor T 3  is turned off. Then, due to the voltage dividing effect of the parasitic capacitor C 2  formed by the storage voltage C 1  and the light emitting device, the anode voltage of the light emitting device L changes now into Vref−Vth+a(Vdata−Vref), where a=C ST /(C ST +C L +C L ), and C L  is capacitance of the parasitic capacitor C 2 . 
     The P 4  phase is the light emitting phase. It is required to turn off the first transistor T 1  before the pixel circuit is ready to drive the light emitting device to emit light. The equivalent circuit in this phase is as shown in  FIG. 7 . In the light emitting phase, the first power supply signal terminal ELVDD and the second control signal terminal S 2  are input the high level, and the first control signal terminal S 1  is input the low level, so that the third transistor T 3  is turned on and the first transistor T 1  is maintained to be turned off. Now, the equivalent circuit is as shown in  FIG. 8 . At this time, the voltage difference between the gate and source of the second transistor T 2  is Vgs=(1−a)Vdata−Vref)+Vth. 
     Now, the current flowing through the third transistor T 3 , the second transistor T 2  and the light emitting device L is: 
     
       
         
           
             
               
                 I 
                 OLED 
               
               = 
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 
                   
                     μ 
                     n 
                   
                   · 
                   Cox 
                   · 
                   
                     W 
                     L 
                   
                   · 
                   
                     
                       ( 
                       
                         Vgs 
                         - 
                         Vth 
                       
                       ) 
                     
                     2 
                   
                 
               
             
             , 
             
               
 
             
             ⁢ 
             then 
           
         
       
       
         
           
             
               I 
               OLED 
             
             = 
             
               
                 1 
                 2 
               
               ⁢ 
               
                 
                   μ 
                   n 
                 
                 · 
                 Cox 
                 · 
                 
                   W 
                   L 
                 
                 · 
                 
                   
                     [ 
                     
                       
                         ( 
                         
                           1 
                           - 
                           a 
                         
                         ) 
                       
                       ⁢ 
                       
                         ( 
                         
                           Vdata 
                           - 
                           Vref 
                         
                         ) 
                       
                     
                     ] 
                   
                   2 
                 
               
             
           
         
       
     
     It can be known from the above equation that the current for driving the light emitting device L to emit light is unrelated to the threshold voltage of TFT and the voltage at the two terminals of OLED, thereby eliminating effectively the influence of non-uniformity and drift of the threshold voltage. 
     It needs to note that in the above embodiments, the transistors is described by taking the enhancement N type TFTs as an example. Or, depletion N type TFTs can also be adopted. The different lies in the threshold voltage Vth for the enhancement type TFTs is a positive value, while the threshold voltage Vth for the depletion type TFTs is a negative value. 
     The pixel circuit with such a structure, regardless of the enhancement type TFTs or the depletion type TFTs, can compensate for the influence of non-uniformity of the threshold voltage, and thus has wider applicability. At the same time, this structure uses a smaller number of TFTs, has a simple control signal, and is applicable to a high-resolution pixel design. 
     An embodiment of the present disclosure further provides a display apparatus comprising an organic light emitting display and other displays and the like. The display apparatus comprises any one of the pixel circuits as described above. The display apparatus can comprise a plurality of pixel unit arrays. Each pixel unit comprises any one of the pixel circuits as described above. 
     As an example, the display apparatus provided in the embodiment of the present disclosure can be a display apparatus having a current-driven light emitting device and including the LED Display or OLED display. 
     The display apparatus provided in the embodiment of the present disclosure comprises the pixel circuit and controls turning off/on and charging/discharging of the circuit through a plurality of transistors and capacitors, so that the current flowing through the transistors and being used to drive the light emitting device is unrelated to the threshold voltage of the transistors, which compensates for the difference in current flowing through the light emitting device due to inconsistency or offset of the threshold voltage of the transistors, raises uniformity of light emitting brightness of the display apparatus, and greatly enhances display effect. In addition, since this pixel circuit has a simple structure and a smaller number of transistors, the size of shading area covering the transistors can be reduced and the aperture ratio of the display apparatus is increased effectively. 
       FIG. 9  shows schematically a flow of a pixel circuit driving method of an embodiment of the present disclosure. The pixel circuit driving method provided in an embodiment of the present disclosure can be applicable to the pixel circuits provided in the embodiments described above. As shown in  FIG. 9 , the method comprises following operating processes. 
     In step S 901 , the first transistor and the third transistor are turned on, the first power supply signal terminal is input a first voltage, and the data signal terminal is input the reset signal, so that the second transistor is turned on, the light emitting device is controlled to be in the turn-off state, and the voltage of the storage capacitor is greater than the threshold voltage of the second transistor. 
     In step S 902 , the first transistor, the second transistor and the third transistor are maintained turning on, the light emitting device is in the turn-off state, and the first power supply signal terminal is input a second voltage until the second transistor is turned off, and the voltage of the storage capacitor is equal to the threshold voltage of the second transistor. 
     In step S 903 , the first transistor is maintained turning on, the third transistor is turned off, and the data signal terminal is input the data signal, so that the second transistor is turned on, and data is written into the first terminal of the light emitting device through the voltage dividing effect of the storage capacitor and the parasitic capacitor formed by the light emitting device. 
     In step S 904 , the first transistor is turned off, the third transistor is turned on, and the current flowing through the second transistor and the third transistor drives the light emitting current to emit light. 
     The pixel circuit driving method provided in the embodiment of the present disclosure controls turning off/on and charging/discharging of the circuit through a plurality of transistors and capacitors, so that the current flowing through the transistors and being used to drive the light emitting device is unrelated to the threshold voltage of the transistors, which compensates for the difference in current flowing through the light emitting device due to inconsistency or offset of the threshold voltage of the transistors, raises uniformity of light emitting brightness of the display apparatus, and greatly enhances display effect. In addition, since this pixel circuit has a simple structure and a smaller number of transistors, the size of shading area covering the transistors can be reduced and the aperture ratio of the display apparatus is increased effectively. 
     It needs to note that the light emitting device in the embodiments of the present disclosure can be various current-driven light emitting devices including LED or OLED. 
     In the embodiments of the present disclosure, the first transistor, the second transistor and the third transistor are N type transistors. The timing of the control signal can be as shown in  FIG. 2 , and then the control timing corresponding to the step S 901  is: the first control signal terminal and the second control signal terminal are input the high level, the first power supply signal terminal is input the low level, and the data signal terminal is input the reset signal at the low level. 
     The control timing corresponding to the step S 902  is: the first control signal terminal, the second control signal terminal and the first power supply signal terminal are input the high level, and the data signal terminal is input the reset signal at the low level. 
     The control timing corresponding to the step S 903  is: the first control signal terminal and the first power supply signal terminal are input the high level, the second control signal terminal is input the low level, and the data signal terminal is input the data signal at the high level. 
     The control timing corresponding to the step S 904  is: the first power supply signal terminal and the second control signal terminal are input the high level, and the first control signal terminal and the data signal terminal are input the low level. 
     As an example, when the first transistor, the second transistor and the third transistor are N type transistors, the step S 901  can comprise: the first control signal terminal S 1  and the second control signal terminal S 2  are input the high level, the first power supply signal terminal ELVDD is input the low level (Voted), and the data signal terminal DATA is input the reset signal (Vref) at the low level, wherein Vref−Voled&gt;Vth (Vth is the threshold voltage of the second transistor T 2 ). 
     The step S 901  is corresponding to the resetting phase. As shown in  FIG. 2 , in the resetting phase (P 1 ), the first control signal terminal S 1  and the second control signal terminal S 2  are input the high level, the first power supply signal terminal ELVDD is input the low level (Voted), and the data signal terminal DATA is input the reset signal (Vref) at the low level. Now, the first transistor T 1 , the second transistor T 2  and the third transistor T 3  are turned on, the voltage across the storage capacitor C 1  is Vref−Voled, the anode voltage of the light emitting device L is Voted, and the light emitting device L is in the turn-off state. 
     Correspondingly, the step S 902  can comprise: the first control signal terminal S 1 , the second control signal terminal S 2  and the first power supply signal terminal ELVDD are input the high level, and the data signal terminal DATA is input the reset signal (Vref) at the low level. 
     The step S 902  is corresponding to the compensating phase. In this phase, the first transistor T 1 , the second transistor T 2  and the third transistor T 3  are maintained turning on, and the anode voltage of the light emitting device L rises as the second transistor T 2  is charged until the voltage is equal to Vref−Vth. When the compensating phase ends up, the second transistor is turned off, the voltage across the storage capacitor is the threshold voltage Vth of the second transistor, and charges stored at the two terminals of the storage capacitor C 1  is Vth·C ST , where C ST  is the capacitance of the storage capacitor C 1 . 
     Thereafter, the step S 903  can comprise: it is required to turn off the third transistor T 3  before the pixel circuit is ready to write data. The equivalent circuit at this time is as shown in  FIG. 5 . The gate voltage of the second transistor T 2  is the reset signal Vref at the low level input by the data signal terminal DATA. Now, the anode voltage of the light emitting device L is Vref−Vth. 
     The step S 903  is corresponding to the data writing phase. In this phase, the first control signal terminal S 1  and the first power supply signal terminal ELVDD are input the high level, the second control signal terminal S 2  is input the low level, and the data signal terminal DATA is input the data signal (Vdata) at the high level. Since the first transistor T 1  and the second transistor T 2  are turned on at this time, the gate voltage of the second transistor increases from Vref to Vdata, and the potential at the gate of the second transistor T 2  changes by Vdata−Vref. Further, due to the voltage dividing effect of the storage capacitor and the parasitic capacitor formed by the light emitting device, the voltage across the storage capacitor C 1  changes by C L (C ST +C L )(Vdata−Vref), C L  is the capacitance of the parasitic capacitor Cs formed by the light emitting device. The voltage across the parasitic capacitor Cs formed by the light emitting device changes by C ST /(C ST +C L )(Vdata−Vref), that is, the anode voltage of the light emitting device L changes by a(Vdata−Vref), where a=C ST /(C ST +C L ). Then, the anode voltage of the light emitting device L changes into Vref−Vth+a(Vdata−Vref) now, and thus data writing is completed. However, since the third transistor T 3  is turned off, the light emitting device is maintained turning off. 
     In addition, the step S 904  can comprise: it is required to turn off the first transistor T 1  before the pixel circuit is ready to drive the light emitting device to emit light. 
     The step S 904  is corresponding to the light emitting phase. In this phase, the first power supply signal terminal ELVDD and the second control signal terminal S 2  are input the high level, and the first control signal terminal S 1  is input the low level, so that the third transistor T 3  is turned on, and the first transistor T 1  is maintained turning off. Now, the gate voltage of the second transistor T 2  is Vdata, and the source voltage thereof is Vref−Vth+a(Vdata−Vref), then the voltage difference Vgs between the gate and source of the second transistor T 2  is: Vgs=Vdata−[Vref−Vth+a(Vdata−Vref)], then Vgs=(1−a)(Vdata−Vref)+Vth. 
     Now, the current flowing through the third transistor T 3 , the second transistor T 2  and the light emitting device L is: 
     
       
         
           
             
               
                 I 
                 OLED 
               
               = 
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 
                   
                     μ 
                     n 
                   
                   · 
                   Cox 
                   · 
                   
                     W 
                     L 
                   
                   · 
                   
                     
                       ( 
                       
                         Vgs 
                         - 
                         Vth 
                       
                       ) 
                     
                     2 
                   
                 
               
             
             , 
             
               
 
             
             ⁢ 
             then 
           
         
       
       
         
           
             
               I 
               OLED 
             
             = 
             
               
                 1 
                 2 
               
               ⁢ 
               
                 
                   μ 
                   n 
                 
                 · 
                 Cox 
                 · 
                 
                   W 
                   L 
                 
                 · 
                 
                   
                     
                       [ 
                       
                         
                           ( 
                           
                             1 
                             - 
                             a 
                           
                           ) 
                         
                         ⁢ 
                         
                           ( 
                           
                             Vdata 
                             - 
                             Vref 
                           
                           ) 
                         
                       
                       ] 
                     
                     2 
                   
                   . 
                 
               
             
           
         
       
     
     It can be known from the above equation that the current for driving the light emitting device L to emit light is unrelated to the threshold voltage of TFT and the voltage at the two terminals of OLED, thereby eliminating effectively the influence of non-uniformity and drift of the threshold voltage. 
     The pixel circuit with such a structure, regardless of the enhancement type TFTs or the depletion type TFTs, can compensate for the influence of non-uniformity of the threshold voltage, and thus has wider applicability. At the same time, this structure uses a smaller number of TFTs, has a simple control signal, and is applicable to a high-resolution pixel design. 
     Those skilled in the art can understand: all or part of flows that realize the above method embodiments can be completed by computer program instruction related hardwares. The above program can be stored in a computer readable storage medium. Upon being executing, the program executes the steps comprising the above method embodiments. The above storage medium comprises various media that can store program codes such as ROM, RAM, disk or optical disk and the like. 
     The above descriptions are just exemplary embodiments of the present disclosure, but the protection scope of the present disclosure is not limited thereto. Any alternation or replacement that can be easily conceived by those skilled in the art within the technical scope of the present disclosure shall fall into the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure shall be subject to the protection scope of the claims. 
     The present application claims the priority of a Chinese patent application No. 201410126737.8 filed on Mar. 31, 2014. Herein, the content disclosed by the Chinese patent application is incorporated in full by reference as a part of the present disclosure.