Abstract:
Certain types of displays such as electrophoretic displays tend to deteriorate if their pixel units are persistently driven by currents flowing in only one direction for the purpose of maintaining (i.e. refreshing) a relatively constant optical state. A first method pulses the pixel unit with a drive pulse of opposed polarity but duration too short (i.e. less than 1/25 second) for a viewer to notice. A second method pulses the pixel unit with a drive pulse of opposed polarity but magnitude to small to effect change in optical state.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0095861 filed in the Korean Intellectual Property Office on Sep. 29, 2006, the entire disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    (a) Field of Invention 
         [0003]    The present disclosure of invention relates to methods for driving pixel areas of an electrophoretic display. 
         [0004]    (b) Description of Related Art 
         [0005]    Recently, for replacing the conventional CRT tubes; flat panel displays, such as liquid crystal displays, organic light emitting diode displays, electrophoretic displays, and so on, have been developed. 
         [0006]    Among the above flat panel displays, the typical electrophoretic display includes: (1) a thin film transistors (TFT) array supporting panel including pixel electrodes each connected to a thin film field effect transistor; (2) a common electrode supporting panel having a common electrode included thereon, and (3) a layer of electrophoretic particles that have positive or negative charges which are positioned in respective pixel regions, and move between the pixel electrodes of the respective pixel areas and the common electrode. A combination of a single pixel electrode and its opposed area in the spaced-away common electrode and its TFT and the eletrophoretic particles disposed between the pixel electrode and common electrode may be considered as a pixel unit that can be driven by electromotive forces at least between first and second different optical states (i.e. red and black). 
         [0007]    When different data voltages are applied to the pixel electrodes relative to and a common voltage applied to the common electrode, the differences between the data voltages and the common voltage can be sufficient to generate electromotive forces that rearrange the electrophoretic particles that are disposed in the respective pixel regions so as to provide a desired optical state. A first polarity set of the electrophoretic particles having positive or negative charges are attracted to move adjacent to the pixel electrodes while the opposed, second polarity set of particles are attracted to move adjacent to the common electrode by use of a first polarity of driving voltage. Along with the voltage-mediated re-arrangement of the electrophoretic particles, an external light applied to the electrophoretic display may be absorbed by or reflected by the electrophoretic particles, to thereby display the corresponding pixel area as being of a respective black or white or other colored attribute to a user who is looking at the display. 
         [0008]    While some parts of a displayed image may be constantly changing between opposed states (e.g., first displaying white and then black), it is often the case that other parts of the displayed image persistently remain in a same state (e.g., displaying just black as a background color for example) for prolonged periods of time (e.g., 3 seconds or more). In these relatively unchanging areas, it is conventional to apply the same positive or negative driving voltage constantly to the electrophoretic particles for the duration of the time that the user is intended to perceive the area as having a constant black or white or other color. However, when a data voltage having the same polarity and magnitude is re-applied periodically to the pixel electrode of the corresponding pixel region through the thin film transistor for a long period of time, the electrical current that flows through the corresponding thin film transistor (TFT) in order to charge the corresponding pixel-electrode to the desired constant voltage flows in only one direction. (The capacitance of the pixel unit discharges internally after a while and needs to be refreshed in order to maintain the desired persistent optical state.) When current flows through some thin film transistors in only one direction, a current-induced deterioration of the thin film transistor (i.e., due to electromigration) is accelerated as compared to the case in which current flows alternately in opposite directions through a TFT of same structure. 
       SUMMARY 
       [0009]    One or more methods for driving pixel units of an electrophoretic display are disclosed here for preventing or reducing deterioration of the electrophoretic display due to persistent unidirectional flow of current through display circuitry such as through the thin film transistors of the display. 
         [0010]    A first method in accordance with the disclosure for driving an electrophoretic display comprises briefly driving a pixel unit whose pixel area is to appear as persistently having a same color (or as persistently black) to its opposed state for a time period too short for an average human viewer to notice (e.g., for a duration of less than 1/25 of a second) and then driving the pixel unit back to its desired persistent state so that the viewer perceives the desired persistent color (or persistent black state). 
         [0011]    In one embodiment, the magnitude of a first threshold driving voltage (the voltage which flips the charge on pixel area towards its opposed state) may be substantially the same as the magnitude of the second threshold driving voltage (the voltage which charges the pixel area towards its desired, more persistent state). 
         [0012]    In another embodiment, the magnitude of the first threshold driving voltage may be smaller than the magnitude of the second threshold driving voltage and in one specific variation, the first threshold driving voltage has such a magnitude that its electromotive force is insufficient to change the relative positions of the positive and negative electrophoretic particles relative to the user&#39;s line of view so that the human viewer does not perceive a change of state even though the pixel area is being periodically driven towards an opposed state for purposes of periodically alternating the direction of current flowing through the TFT of that pixel area. In this alternate embodiment where the state flipping first voltage is of insufficient magnitude, the first time may be substantially equal to the second time. In one further variation, the first time is longer than the second time. In one embodiment, the integral over time for the positive and negative drive pulses are substantially the same. For example, in one variation using rectangular pulses, the product of multiplication of the first threshold driving voltage and its corresponding first on time may be substantially equal to the product of multiplication of the second threshold driving voltage and its corresponding second on time. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a layout view illustrating a structure of an electrophoretic display that may be driven by a method for driving an electrophoretic display according to the present disclosure, 
           [0014]      FIG. 2  is a cross-sectional view taken along line II-II′ of the electrophoretic display of  FIG. 1 , 
           [0015]      FIGS. 3 and 4  are cross-sectional views illustrating four pixel regions including a first pixel region and a second pixel region, respectively, of the electrophoretic display of  FIG. 1 , 
           [0016]      FIG. 5  is a view showing a driving voltage, which is time-dependently applied to electrophoretic particles positioned in the first pixel region for explaining a method for driving an electrophoretic display according to one exemplary embodiment, 
           [0017]      FIGS. 6 and 7  are cross-sectional views of an electrophoretic display showing a different behavior state of the electrophoretic particles positioned in the first pixel region by the application of the driving voltage of  FIG. 5 , 
           [0018]      FIG. 8  is a view showing a driving voltage, which is time-dependently applied to electrophoretic particles positioned in the first pixel region for explaining a method for driving an electrophoretic display according to another exemplary embodiment, 
           [0019]      FIG. 9  is a cross-sectional view of an electrophoretic display showing a behavior state of the electrophoretic particles positioned in the first pixel region by the application of the driving voltage of  FIG. 8 , and 
           [0020]      FIG. 10  is a view showing a driving voltage, which is time-dependently applied to electrophoretic particles positioned in the first pixel region for explaining a method for driving an electrophoretic display according to another exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In the drawings, the thickness of layers, films, panels, regions, etc., may be exaggerated for clarity and thus not to scale. Like reference numerals generally designate like elements throughout. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
         [0022]    Methods for using electrophoretic displays according to one or more of various exemplary embodiments will be described with reference to the accompanying drawings. 
         [0023]    First, a structure of an exemplary electrophoretic display will be described in detail with reference to  FIGS. 1 and 4 .  FIG. 1  is a layout view illustrating a structure of an electrophoretic display that may be driven by one more of the current alternating methods disclosed herein.  FIG. 2  is a cross-sectional view taken along line II-II′ of the electrophoretic display of  FIG. 1 , and  FIGS. 3 and 4  are cross-sectional views illustrating four pixel regions (pixel areas) including a first pixel region and a second pixel region, respectively, of the electrophoretic display of  FIG. 1 . 
         [0024]    The illustrated electrophoretic display includes a thin film transistor array containing panel  100 , a common electrode containing panel  200  facing the thin film transistor array panel  100 , and an electrophoretic particles containing member  320  positioned in pixel regions such as A and B, respectively, between the panels  100  and  200 . 
         [0025]    First, a thin film transistor array containing panel  100  will be described. 
         [0026]    As shown in  FIGS. 1 and 4 , a plurality of gate lines  121  for transmitting gate signals are formed on an electrically insulative substrate  110  made of transparent glass or another electrically insulative and light passing material. The gate lines  121  extend substantially in a transverse direction to the gate lines, and each gate line  121  includes a plurality of gate electrodes  124  and an extended end portion  129  for contact with another layer or an external device. 
         [0027]    The gate lines  121  may be made of aluminum-containing metal such as aluminum and/or aluminum-based alloys, silver-containing metals such as silver and silver-based alloys, copper-containing metals such as copper and copper-based alloys, molybdenum-containing metals such as molybdenum and molybdenum-based alloys, chromium, titanium, tantalum, and so forth. Each gate line  121  may include two conductive films having different physical characteristics, i.e., a lower film (not shown) and an upper film (not shown). The upper film may be made of a low resistivity metal such as an Al-containing metal for reducing signal delay or voltage drop in the gate lines  121 . On the other hand, the lower film may be made of an interface material such as Mo, a Mo alloy, and Cr, which has good contact characteristics with other conductive materials such as indium tin oxide (ITO) and indium zinc oxide (IZO). A good example of a combination of the lower film material and the upper film material is Cr and an Al—Nd alloy. 
         [0028]    The gate lines  121  may alternatively have a single-layered structure or a triple-layered structure. 
         [0029]    A gate insulating layer  140  such as one made of a silicon nitride (SiNx) is formed on the gate lines  121 . 
         [0030]    A plurality of semiconductor stripes  151  such as those made of hydrogenated amorphous silicon are formed on the gate insulating layer  140 . Each semiconductor stripe  151  extends substantially in the longitudinal direction and has a plurality of projections  154  branched out toward the gate electrodes  124 . The width of each semiconductor stripe  151  becomes large near the gate lines  121  such that the semiconductor stripe  151  covers large areas of the gate lines  121 . 
         [0031]    A plurality of ohmic contact lines and islands  161  and  165  such as made of a silicide or n+ hydrogenated a-Si heavily doped with an n-type impurity are formed on the semiconductor stripes  151 . Each ohmic contact line  161  has a plurality of projections  163 , and the projections  163  and the ohmic contact islands  165  are located in pairs on the projections  154  of the semiconductor stripes  151 . 
         [0032]    A plurality of data lines  171  and a plurality of drain electrodes  175  are formed on the ohmic contacts  161  and  165  and the gate insulating layer  140 , respectively. 
         [0033]    The data lines  171  are used for transmitting data voltages. They extend substantially in the longitudinal direction and intersect the gate lines  121 . Each data line  171  includes a plurality of source electrodes  173  projecting toward the gate electrodes  124  and curved like a character “J” and an extended end portion  179  having a larger area for contact with another layer or an external device. Each pair of the source electrodes  173  and the drain electrodes  175  are separated from each other and opposite each other with respect to a channel region overlapped by a gate electrode  124 . 
         [0034]    The data lines  171  and the drain electrodes  175  may be made of a refractory metal such as chromium, a molybdenum-containing metal, tantalum, and titanium, and may have a multi-layered structure including a lower film (not shown) made of Mo, a Mo alloy, or Cr, and an upper film (not shown) located thereon and made of an Al-containing metal. 
         [0035]    A gate electrode  124 , a source electrode  173 , and a drain electrode  175  along with a projection  154  of a semiconductor stripe  151  form a TFT having a channel formed in the projection  154  disposed between the source electrode  173  and the drain electrode  175 . 
         [0036]    The ohmic contacts  161  and  165  are interposed between the underlying semiconductor stripes  151  and the overlying the source electrode  173  and the overlying drain electrodes  175  thereon, and reduce the contact resistance therebetween. 
         [0037]    The semiconductor stripes  151  include a plurality of exposed portions, which are not covered with the data lines  171  and the drain electrodes  175 , such as portions located between the source electrodes  173  and the drain electrodes  175 . Although the semiconductor stripes  151  are narrower than the data lines  171  at most places, the width of the semiconductor stripes  151  becomes large near the gate lines as described above, to enhance the insulation between the gate lines  121  and the data lines  171 . 
         [0038]    A passivation layer  180  is formed in a single-layered or multi-layered structure on the data lines  171 , the drain electrodes  175 , and the exposed portions of the semiconductor stripes  151 . The passivation layer  180  may be made of a photosensitive organic material having a good flatness characteristic (planarity), a low dielectric insulating material such as a-Si:C:O and a-Si:O:F formed by plasma enhanced chemical vapor deposition (PECVD), and/or an inorganic material such as a silicon nitride. For example, if the passivation layer  180  is formed of an organic material, to prevent the organic material of the passivation layer  180  from contacting with the semiconductor strips  151  exposed between the data line  171  and the drain electrode  175 , the passivation layer  180  can be structured in such a way that an insulating layer (not shown) made of SiNx or SiO 2  is additionally formed under the organic material layer. 
         [0039]    The passivation layer  180  has a plurality of contact holes  181 ,  185 , and  182  exposing the end portions  129  of the gate lines  121 , at least a portion of the drain electrodes  175 , and the end portions  179  of the data lines  171 , respectively. 
         [0040]    A plurality of pixel electrodes  190  and a plurality of contact assistants  81  and  82 , which may be made of ITO or IZO, are formed on the passivation layer  180 . 
         [0041]    The pixel electrodes  190  are physically and electrically connected to the drain electrodes  175  through the contact holes  185  such that the pixel electrodes  190  receive the data voltages from the drain electrodes  175  to apply a data voltage to respective electrophoretic members  320 ,  321 , and  322 . 
         [0042]    The contact assistants  81 / 82  are connected to the exposed end portions  129 / 179  of the gate lines  121 /the data lines  171  through the contact holes  181 / 182 . The contact assistants  81  and  82  protect the exposed portions of the gate lines  121  and the data lines  171  and complement the adhesion between the exposed portions and external devices such as a driving integrated circuit. 
         [0043]    Next, the common electrode containing panel  200  will be described. 
         [0044]    The common electrode panel  200  is opposed to the thin film transistor array containing panel  100 , and includes a transparent insulation substrate  210  and a common electrode  220  formed on the insulation substrate  210  and facing the pixel electrodes  190 . 
         [0045]    The common electrode  220  may be a transparent electrode made of ITO or IZO and it is used to apply a common voltage for thereby creating an electric field interacting with respective electrophoretic particles  323 ,  324 ,  325 , and  326  of the electrophoretic members  320 ,  321 , and  322 . 
         [0046]    When the common electrode  220  has a common voltage applied to it, it may change the position of the electrophoretic particles  323 ,  324 ,  325 , and  326  relative to the respective electrophoretic particles  323 ,  324 ,  325 , and  326  if a countering driving voltage of sufficient magnitude is applied to the opposed pixel electrodes  190 , thereby displaying images of desired black and white luminance or of different colors. 
         [0047]    Next, the electrophoretic members  320 ,  321 , and  322  will be described. 
         [0048]    Each of the electrophoretic members  320 ,  321 , and  322  is sandwiched between the pixel electrodes  190  and the common electrode  220  by use of a binder  310 , and is thus positioned in the pixel regions A and B between the pixel electrodes  190  and the common electrode  220 . 
         [0049]    Each of the electrophoretic members  320 ,  321 , and  322  may alternately and repetitively be disposed in a plurality of pixel regions A and B different from each other. 
         [0050]    The first electrophoretic member  320  includes red electrophoretic particles  323 , black electrophoretic particles  326 , a dispersion medium  327  having the respective electrophoretic particles  323  and  326  dispersed therein, and a capsule  329  enclosing the elements  323 ,  326 , and  327 . 
         [0051]    The red electrophoretic particles  323  are electrification particles that show a red color and have negative charges. 
         [0052]    The black electrophoretic particles  326  are electrification particles that show a black color and have positive charges. 
         [0053]    The red electrophoretic particles  323  and the black electrophoretic particles  326  may have positive charges and negative charges, respectively, contrary to the above. 
         [0054]    The second electrophoretic member  321  and the third electrophoretic member  322  are the same as the first electrophoretic member  320 , except that they include green electrophoretic particles  324  and blue electrophoretic particles  325 , respectively, instead of red electrophoretic particles  323 . The green electrophoretic particles  324  are electrification particles that show a green color and have negative charges. The blue electrophoretic particles  325  are electrification particles that show a blue color and have negative charges. 
         [0055]    The green electrophoretic particle  324  and black electrophoretic particles  326  of the second electrophoretic member  321  may have positive charges and negative charges, respectively, contrary to the above. The blue electrophoretic particles  325  and black electrophoretic particles  326  of the third electrophoretic member  326  may have positive charges and negative charges, respectively, as above. 
         [0056]    Meanwhile, the red electrophoretic particles  323 , the green electrophoretic particles  324 , and the blue electrophoretic particles  325  may be replaced with electrophoretic particles having a yellow color, electrophoretic particles having a magenta color, and electrophoretic particles having a cyan color. 
         [0057]    The red electrophoretic particles  323 , the green electrophoretic particles  324 , and the blue electrophoretic particles  325  all may be replaced with white electrophoretic particles. In this case, unlike the above, the electrophoretic display can represent a luminance of only black and white without other colors. 
         [0058]    The dispersion medium  327  may disperse the respective electrophoretic particles  323 ,  324 ,  325 , and  326 , and have a transparent or black color. If the dispersion medium  327  shows a black color, the black electrophoretic particles  326  contained in the respective electrophoretic particles  320 ,  321 , and  322  may be omitted since the black color can be represented by using the dispersion medium  327  alone. 
         [0059]    The capsule  329  encloses the respective electrophoretic particles  323 ,  324 ,  325 , and  326  and the dispersion medium  327 , and accordingly, the respective electrophoretic particles  323 ,  324 ,  325 , and  326  are movable for color representation only within the capsule  329 . 
         [0060]    Among the pixel regions A and B, it is assumed here that the first pixel region A is a region that represents a relatively constant image having no change for a relatively long time during the driving process of the electrophoretic display. That is, the first pixel area A is a region that represents a luminance of either black or white or represents one color image. 
         [0061]    Among the pixel regions A and B, the second pixel region B is a region that represents an image portion having a relatively high degree of change during the same period in the driving of the electrophoretic display. That is, the second pixel region B is a region that represents a different black and white luminance or a different color image. The above will be described again with reference to  FIGS. 3 and 4 . In the first pixel region A, there is relatively no change in the position of the red electrophoretic particles  323  and the black electrophoretic particles  326  over a relatively long period of time (e.g., 3 seconds of more). Accordingly, an external light is continuously incident on the red electrophoretic particles  323  positioned on the common electrode  220  and then reflected to a user&#39;s eyes, so the first pixel region A appears to the user only as a constant red image area. Meanwhile, if there is no change in position when the positions of the red electrophoretic particles  323  and the black electrophoretic particles  326  are opposite to each other, an external light is continuously incident on the black electrophoretic particles  326  positioned on the common electrode  220  and then absorbed, so the first pixel region A displays only as a relatively constant black image. Meanwhile, in the second pixel region B, the positions of the red electrophoretic particles  323 , the green electrophoretic particles  324 , the blue electrophoretic particles  325 , and the black electrophoretic particles  326  are being interchanged at a relatively fast rate in so far as what the user experiences. 
         [0062]    As shown in  FIG. 3 , the external light is reflected by the green electrophoretic particles  324 , the blue electrophoretic particles  325 , and the red electrophoretic particles  323  positioned on the common electrode in order of increasing distance from the first pixel region A. Due to this, three second pixel regions B display green, blue, and red images in the order of increasing distance from the first pixel region A. However, in  FIG. 4 , the external light is absorbed by the black electrophoretic particles positioned on the common electrode  220  due to a change in the position of the electrophoretic particles  323 ,  324 ,  325 , and  326 , so all of the three second pixel regions B display an image changed into black. 
         [0063]    Hereinafter, a method for driving an electrophoretic display according to various exemplary embodiments will be described in detail with reference to  FIGS. 5 to 10 . 
         [0064]      FIG. 6 ,  FIG. 7 , and  FIG. 9  illustrate only a first pixel region A so that the first electrophoretic member  320  including the red electrophoretic particles  323  is positioned therein. However, practically, a plurality of similarly situated first pixel regions A exist, and the second electrophoretic member  321  including the green electrophoretic particles  324  and the third electrophoretic member  322  including the blue electrophoretic particles  325  are positioned in each of the other of first pixel regions A. In addition, each of the electrophoretic members  320 ,  321 , and  322  may be constructed such that they may include white electrophoretic particles instead of the red, green, and blue electrophoretic particles  320 ,  321 , and  322 . That is, the first pixel region A may continuously represent any one of green, blue, and white rather than red, green and blue alternating with black. 
         [0065]    First, a method for driving an electrophoretic display according to one exemplary embodiment will be described in detail with reference to  FIGS. 5 to 7 . 
         [0066]      FIG. 5  is a view of a voltage versus time graph showing a driving voltage that is time-dependently applied to pixel areas of relatively constant coloration (in so far as what the user sees) so as to thereby position the electrophoretic particles mostly in the desired orientation for providing the apparent coloration that the user sees.  FIGS. 6 and 7  are cross-sectional views of the electrophoretic display showing the different behavioral states of the electrophoretic particles positioned in the first pixel region after charge refreshing period T 2  and during the current flipping period T 1  as provided by the application of the driving voltage of  FIG. 5 . 
         [0067]    In addition, the driving voltage to be mentioned with respect to  FIG. 5  means a value obtained by subtracting a data voltage applied to the pixel electrode from a common voltage applied to the common voltage, which is defined as follows. 
         [0068]    The first threshold driving voltage (V 1 ) that is applied during first time intervals T 1  is a negative (−) voltage of sufficient magnitude to allow the red electrophoretic particles  323  to overcome fluid resistance caused by the dispersion medium  327  and to move the red particles to proximity with the pixel electrodes  190 , and to also allow the black electrophoretic particles  326  to overcome the fluid resistance caused by the dispersion medium  327  and to move to proximity with the common electrode  220 . 
         [0069]    The second threshold driving voltage (V 2 ) that is applied during second time intervals T 2  is a positive (+) voltage having substantially the same absolute magnitude as the first threshold driving voltage (V 1 ) and that allows the red electrophoretic particles  323  to overcome fluid resistance caused by the dispersion medium  327  and to move to proximity with the common electrode  220 , and it also allows the black electrophoretic particles  326  to overcome fluid resistance caused by the dispersion medium  327  and to move to proximity with the pixel electrode  190 . 
         [0070]    In the method for driving an electrophoretic display according to one exemplary embodiment, firstly, as shown in  FIG. 5 , the second threshold driving voltage (V 2 ) is applied for duration of the second time interval T 2  to the respective electrophoretic particles  323  and  326  positioned in the first pixel region A of the electrophoretic display. 
         [0071]    Here, the second time T 2  is at least as large as a minimum time required for the red electrophoretic particles  323  and the black electrophoretic particles  326  to be moved and rearranged in proximity with the common electrode  220  and the pixel electrodes  190 , respectively, by the application of the second threshold driving voltage. 
         [0072]    Accordingly, as shown in  FIG. 2 , the red electrophoretic particles  323  that are dispersed in the dispersion medium  327  of the first electrophoretic member  320  positioned in the first pixel region A and have negative charges are moved and arranged in proximity with or on the common electrode  220  as shown in  FIG. 6 . Meanwhile, the black electrophoretic particles  326  having positive charges are moved toward the pixel electrodes  190  to be arranged in proximity with or thereon. 
         [0073]    By this arrangement, an external light incident through the common electrode panel  200  is reflected back to the human user by the respective electrophoretic particles  323 , thereby displaying the red color represented by the red electrophoretic particles  323 . Between the first application of +V 2  and first application of −V 1 , a drive voltage of about zero is maintained and as a result no motive force is applied to the electrophoretic particles. 
         [0074]    The first threshold driving voltage V 1  is applied to the electrophoretic particles  323  and  326  positioned in the first pixel region A during a following first time interval T 1  at a predetermined time interval after the application of the second threshold driving voltage V 2 , and then the second threshold driving voltage V 2  is applied almost immediately thereafter during the next occurrence of second time interval T 2 . 
         [0075]    The first time interval T 1  is at least a minimal time that is required for the red electrophoretic particles  323  and the black electrophoretic particles  326  to be moved and rearranged in proximity with the pixel electrodes  190  and the common electrode  330 , respectively, by the application of the first threshold driving voltage V 1 , and has substantially the same length as the second time T 2 . 
         [0076]    As shown in  FIG. 7 , the red electrophoretic particles  323  are moved from the common electrode  220  to the pixel electrodes  190  and arranged thereon by the application of the first threshold driving voltage V 1  during the first time T 1 . Meanwhile, the black electrophoretic particles  326  having positive charges are moved from the pixel electrodes  190  to the common electrode  220  and arranged thereon. 
         [0077]    By this arrangement, an external light incident through the common electrode panel  200  from the outside is absorbed by the black color electrophoretic particles  326 . Accordingly, the first pixel area A reflects a black color towards the user&#39;s eyes. However, the first pixel region A is a region that should appear as continuously and constantly displaying a red color to the outside. If the first time T 1  for applying the first threshold driving voltage V 1  is lengthened beyond a predetermined sub-blink time, a person may recognize the switch over to the black color. However, if the first time T 1  is kept below the predetermined sub-blink time, for example to less than 1/25 of a second, the average person will not recognize the brief switch over to the black color in the pixel area driven by the waveform of  FIG. 5 . 
         [0078]    Immediately after the first threshold driving voltage V 1  is applied to the electrophoretic particles  323  and  326  during the first time T 1 , the second threshold driving voltage V 2  is applied again during the second time T 2 . 
         [0079]    In one embodiment the application of the second threshold driving voltage V 2  is carried out periodically and continuously immediately after completion of the application of the first threshold driving voltage V 1  (the color reversing voltage). Since the average person cannot recognize the brief flip over to the black color followed by the longer persistence of the intended red color, the first pixel region A will appear to display only the red color due to application of the second threshold driving voltage V 2  during the first time intervals T 2 , followed by longer application of the zero voltage and then brief application of V 1  followed by immediate or almost immediate reapplication of V 2 . 
         [0080]    As shown in  FIG. 6 , the red electrophoretic particles  323  having negative charges are moved to the common electrode  220  and arranged thereon by the application of the second threshold driving voltage V 2  during the second time T 2 . Meanwhile, the black electrophoretic particles  326  having positive charges are moved to the pixel electrodes  190  and arranged thereon. 
         [0081]    By this arrangement, an external light incident through the common electrode panel  200  is reflected by the respective electrophoretic particles  323 , thereby displaying the red color represented by the red electrophoretic particles  323 . 
         [0082]    Afterwards, the first threshold driving voltage V 1  and second threshold driving voltage V 2  are repeatedly applied to the electrophoretic particles  323  and  326 , respectively, during the first time T 1  and second time T 2 , respectively, at a predetermined duty cycle. The duty cycle may be that determined as necessary or sufficient for keeping a capacitance formed by the pixel-electrode and common electrode and the electrophoretic particles interposed therebetween charged to a desired state for retaining the desired orientation of the interposed electrophoretic particles. The duty cycle may be that determined as necessary or sufficient for scanning across the display area and forming a new image frame having some areas thereof retaining a persistent coloration for a substantial length of time (e.g., 3 seconds or greater) while causing other areas to change coloration relatively quickly (e.g., 24, 30 or 60 times every second depending on the vertical frame rate). 
         [0083]    After the application of the second threshold driving voltage V 2 , the red electrophoretic particles  323  are positioned at the common electrode  220  until the first threshold driving voltage V 1  is applied again thereto at a predetermined time interval, and positioned in the pixel electrodes  190  of the black electrophoretic particles  326 . Accordingly, the first pixel region A continues to represent a red color until the first threshold driving voltage V 1  is applied again. 
         [0084]    According to another method for driving an electrophoretic display according to one exemplary embodiment, the first pixel area A is driven to persistently display not a red color but a black color in which case, the first threshold driving voltage V 1  is applied first during the first time T 1  and then reapplied immediately after each strobing with V 2 . 
         [0085]    Accordingly, according to one method for driving an electrophoretic display according to one exemplary embodiment, the first pixel region A is able to represent a relatively constant image. In addition, the first threshold driving voltage V 1  and the second threshold driving voltage V 2  having the same size and the opposite polarity to each other are alternately applied to the electrophoretic particles  323  and  326 , thus the polarity of the data voltage applied to the pixel electrodes  190  positioned in the first pixel region A is reversed. Accordingly, in the thin film transistor and/or other circuitry connected to the pixel electrodes  190 , experiences current flows moving alternately not just to one side but in both directions and by equal magnitudes to both sides. Thus the degree of potential deterioration to parts that can suffer deterioration from persistent unidirectional current flow is reduced as compared to the case where current flows a majority of time in only one direction. 
         [0086]    Meanwhile, in this embodiment, the second threshold driving voltage V 2  that is initially applied before the first threshold driving voltage V 1  is firstly applied to the electrophoretic particles  323  and  326 , the initial application of the second threshold driving voltage V 2  may be omitted, and the first threshold driving voltage V 1  and the second threshold driving voltage V 2  that are periodically consecutive may be applied. 
         [0087]    Hereinafter, a second method for driving an electrophoretic display according to the same or another embodiment will be described with reference to  FIGS. 8 and 9 . 
         [0088]      FIG. 8  is a voltage versus time graph showing another driving voltage that may be time-dependently applied to pixel areas of electrophoretic particles positioned in the first pixel region A.  FIG. 9  is a cross-sectional view of an electrophoretic display showing a behavior state of the electrophoretic particles positioned in the first pixel region A by the application of the driving voltage of  FIG. 8 . 
         [0089]    In addition, the driving voltage to be mentioned with respect to  FIG. 8  means a value obtained by subtracting a data voltage applied to the pixel electrode from a common voltage applied to the common voltage, which is defined as follows. 
         [0090]    The first threshold driving voltage V 3  that is applied during first time intervals T 1  is a negative (−) voltage of sufficiently low magnitude that it allows the red electrophoretic particles  323  and black electrophoretic particles included in the dispersion medium  327  to maintain their original spatial state. In other words, V 3  is of sufficiently small amplitude that it does not provide enough motive force to the electrophoretic particles to cause them to substantially change in their respective positions during time period T 1 . 
         [0091]    The second threshold driving voltage V 2  is a positive (+) voltage of absolute amplitude greater than that of the first threshold driving voltage V 3  and the amplitude of V 2  is sufficiently large that it does provide enough motive force to the electrophoretic particles to cause the red electrophoretic particles  323  to overcome fluid resistance caused by the dispersion medium  327  and move towards the common electrode  220 , and also to cause the black electrophoretic particles  326  to overcome fluid resistance caused by the dispersion medium  327  and move towards the pixel electrode  190  during the time period T 2 . 
         [0092]    The method for driving an electrophoretic display according to this exemplary embodiment is roughly the same as the method for driving an electrophoretic display according to previous exemplary embodiment of the present invention as shown in  FIGS. 5 to 7 , except that the firstly-applied driving voltage V 3  has a magnitude substantially smaller than the secondly-applied driving voltage V 2  and has an opposite polarity. Moreover in the second embodiment, the first time T 1  interval associated with the firstly-applied driving voltage V 3  has substantially the same length as the second time T 2  associated with the secondly-applied threshold driving voltage V 2  where both V 3  and V 2  are applied as sequentially adjacent pairs according to a predetermined cycle after the initial application of just the second threshold driving voltage V 2 . 
         [0093]    Because the first threshold driving voltage V 3  is a voltage of such a magnitude as to not change the position of the respective electrophoretic particles  323  and  326 , the electrophoretic particles  323  and  326  maintain the state as shown in  FIG. 9  during the first time T 1 . Accordingly, the first pixel region A of the electrophoretic display can continuously represent a red image. Given this, the first time T 1  does not need to be specifically restricted to a particular length for applying the first threshold driving voltage V 3 . 
         [0094]    Therefore, according to the method for driving an electrophoretic display according to  FIGS. 8 and 9 , the first pixel region A is able to represent a constant image, and reduce the deterioration of the thin film transistor of the electrophoretic display as compared to a situation where the circuitry associated with a pixel area having persistent coloration is persistently subjected to current flows in only one direction. 
         [0095]    Hereinafter, a third method for driving an electrophoretic display will be described with reference to  FIG. 10 . 
         [0096]      FIG. 10  is a view showing a driving voltage that is time-dependently applied to electrophoretic particles positioned in the first pixel region for explaining a method for driving an electrophoretic display according to another exemplary embodiment of the present disclosure. 
         [0097]    In addition, the driving voltage to be mentioned with respect to  FIG. 10  means a value obtained by subtracting a data voltage applied to the pixel electrode from a common voltage applied to the common voltage, which is defined as follows. 
         [0098]    The first threshold driving voltage V 5  is a negative (−) voltage of sufficiently low magnitude that it allows the red electrophoretic particles  323  and black electrophoretic particles included in the dispersion medium  327  to maintain their original spatial state, in other words, without causing a change in their respective positions. 
         [0099]    The second threshold driving voltage V 2  is a positive (+) voltage having an absolute amplitude greater than that of the first threshold driving voltage V 5  and V 2  is of sufficiently large magnitude that it allows the red electrophoretic particles  323  to overcome fluid resistance caused by the dispersion medium  327  and to move towards the common electrode  220 , and it allows the black electrophoretic particles  326  to overcome fluid resistance caused by the dispersion medium  327  and to move towards the pixel electrode  190 . 
         [0100]    The method for driving an electrophoretic display according to this exemplary embodiment is roughly similar to the method for driving an electrophoretic display according to  FIGS. 8 to 9 , except that the first threshold driving voltage V 3  has an absolute magnitude substantially smaller than that of the second threshold driving voltage V 2  and V 3  has an opposite polarity and V 3  is applied to the respective electrophoretic particles  323  and  326  during the first time T 3  that is substantially longer than the second time T 2 , where the V 3  pulse is applied just before the second threshold driving voltage V 2  is applied again thereto during the second time T 2  in a predetermined duty cycle after the initial application of the second threshold driving voltage V 2 . 
         [0101]    The first time T 3  that is longer than the second time T 2  may be set in a manner such that a value S 1  of multiplication of the first time T 3  and the first threshold driving voltage V 5  may be substantially equal to a value S 2  of multiplication of the second time T 2  and the second threshold driving voltage V 2 . Such balancing in the voltage versus time plots of the areas under the positive driving voltages with the areas under the negative driving voltages helps to balance the amount of charge that is driven through the circuitry in the respective positive and negative directions. As such, deterioration of the thin film transistors can be further reduced because it is possible to perform reverse driving of the electrophoretic display in a balanced manner similar to the method for driving an electrophoretic display according to the exemplary embodiment shown in  FIGS. 5 to 7 . 
         [0102]    Therefore, according to the method for driving an electrophoretic display according to  FIG. 10 , the first pixel region A is able to represent a constant image, and at the same time reduce the deterioration of the thin film transistor of the electrophoretic display that may be due to persistent driving of current therethrough in only one direction. 
         [0103]    The above exemplary embodiments have described a number of driving methods for allowing the first pixel region A to appear to be continuously of a red color. However, it is needless to say that the first pixel region A can be similarly driven to instead continuously appear to represent a black color by applying a driving voltage waveform, which has the opposite polarity to the first and second threshold driving voltages mentioned in the above exemplary embodiments and having the same magnitude thereof, to the electrophoretic particles  323  and  326 . 
         [0104]    While examples have been described in connection with what is presently considered to be practical in the current art, it is to be understood that the disclosure is not to be considered as limited to just the disclosed embodiments regarding electrophoretic particles, but, on the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the disclosure including application to other situations where it is advantageous to drive balanced positive and negative going currents through circuitry of similar types of displays while causing the viewing person to perceive certain areas as having a relatively persistent coloration and/or luminosity.