Abstract:
A system and method are disclosed for reducing reverse bias in an electrophoretic display. The system and method include the application of varying levels of voltages across an array of electrophoretic display cells of the electrophoretic display to move the cells towards a stable state in a driving cycle. In addition, the system and method disconnect the voltages from the electrophoretic display cells at a time duration prior to reaching step transitions of the voltages during the driving cycle. Pre-driving approaches apply a first pre-driving voltage at a first polarity to the display cells before driving the display cells with a second driving voltage at a second, opposite polarity. Varying the time duration and amplitude of the pre-driving signals produces further beneficial reduction in reverse bias.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS; PRIORITY CLAIM  
       [0001]     This application claims domestic priority under 35 U.S.C. §120 as a Continuation of U.S. application Ser. No. 10/973,810, filed Oct. 25, 2004, the entire contents of which is hereby incorporated into this application by reference for all purposes as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to electrophoretic displays. More specifically, an improved driving scheme for an electrophoretic display is disclosed.  
       BACKGROUND OF THE INVENTION  
       [0003]     The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.  
         [0004]     There are several different types of EPDs. In the partition type of EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. No. 5,961,804 and U.S. Pat. No. 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors. Yet another type of EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent, as disclosed in co-pending application U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000.  
         [0005]     One problem associated with these EPDs is reverse bias. A reverse bias condition could occur when the bias voltage on a particular cell changes rapidly by a large increment or decrement and in conjunction with the presence of a stored charge resulting from the inherent capacitance of the materials and structures of the EPD. The reverse bias condition affects display quality by causing charged pigment particles in affected cells to migrate away from the position to which they have been driven. The following description along with FIG.  FIGS. 1A, 1B , and  2  further illustrate this problem.  
         [0006]      FIG. 1A  shows a sectional view of an example EPD  100 . The EPD  100  includes an upper dielectric layer  108 , an upper electrode  112 , an electrophoretic dispersion layer  102 , a lower dielectric layer  110 , and a lower electrode  114 . The electrophoretic dispersion layer  102  contains a colored dielectric solvent  106  with a plurality of charged pigment particles  104 . In one embodiment, the insulating material of the dielectric layers may comprise a non-conductive polymer. In another embodiment, the insulating material may include a microcup structure or a sealing and/or adhesive layer, as disclosed, for example, in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. Ser. No. 10/222,297, filed on Aug. 16, 2002, U.S. Ser. No. 10/665,898, filed on Sep. 18, 2003 and U.S. Ser. No. 10/762,196, filed on Jan. 21, 2004.  
         [0007]      FIG. 1B  shows a simplified electrical equivalent circuit for EPD  100 . Specifically, C 1  and R 1  represent the combined electrical capacitance and resistance of the upper dielectric layer  108  and the lower dielectric layer  110 , respectively. C 2  and R 2  represent the electrical capacitance and resistance of the electrophoretic dispersion layer  102 , respectively.  
         [0008]     Suppose drive voltage generator  116  applies a square wave V in  to the upper electrode  112  and the lower electrode  114 . The waveform of the voltage applied across the electrophoretic dispersion layer  102 , V ed , has overshooting and undershooting portions as shown in  FIG. 2 . Particularly, when V in  drops to zero, V ed  has a polarity opposite to the drive voltage V in . This “undershooting”, representing the reverse bias condition, causes charged particles to migrate away from a position to which they have been driven and results in degradation of the image-retention characteristics of the EPD  100 .  
         [0009]     One solution to the aforementioned reverse bias problem has been disclosed by Hideyuki Kawai in application U.S. Ser. No. 10/224,543, filed Aug. 20, 2002, US patent publication 20030067666, published Apr. 10, 2003. The solution attempts to address the undershooting phenomenon by applying an input biasing voltage that has a smooth waveform and meets certain time constant requirements. However, this solution is difficult and costly to implement. Therefore, there is a need for an improved driving scheme for an EPD.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1A  illustrates a sectional view of an example electrophoretic display.  
         [0011]      FIG. 1B  illustrates a simplified electrical equivalent circuit for a portion of the EPD  100 .  
         [0012]      FIG. 2  illustrates the induced reverse bias effect.  
         [0013]      FIG. 3  illustrates one example characterization of the electrical connectivity between the drive voltage generator  116  and a 3×3 array portion  300  of the EPD  100  in an active matrix implementation.  
         [0014]      FIG. 4A  illustrates one example characterization of the electrical connectivity between the drive voltage generator  116  and an EPD  100  with seven segments.  
         [0015]      FIG. 4B  illustrates a plain view of an embodiment of the EPD  100  with seven segments.  
         [0016]      FIG. 5A  illustrates a block diagram of an example embodiment of the drive voltage generator  116  in an active matrix implementation.  
         [0017]      FIG. 5B  illustrates a block diagram of an example embodiment of the drive voltage generator  116  in a direct drive implementation.  
         [0018]      FIG. 6  shows a timing diagram of a driving cycle of two phases of an example embodiment of the drive voltage generator  116 .  
         [0019]      FIG. 7  illustrates a timing diagram of a single driving cycle employed by an example embodiment of the drive voltage generator  116 .  
         [0020]      FIG. 8A  illustrates a timing diagram of a driving cycle in a uni-polar direct drive implementation employed by an example embodiment of the drive voltage generator  116 .  
         [0021]      FIG. 8B  illustrates a timing diagram of a driving cycle in a bi-polar direct drive implementation employed by an example embodiment of the drive voltage generator  116 .  
         [0022]      FIG. 8C  illustrates a timing diagram of applying a pre-drive voltage in a bi-polar direct drive implementation employed by an example embodiment of the drive voltage generator  116 .  
         [0023]      FIG. 9  illustrates one example system that includes the EPD  100  and the drive voltage generator  116 .  
         [0024]      FIG. 10  is a block diagram of an example electrophoretic display (EPD) device.  
         [0025]      FIG. 11  is a schematic diagram of a circuit network that is electrically equivalent to the EPD device of  FIG. 10 .  
         [0026]      FIG. 12  is a time-versus-voltage plot diagram showing how a white pixel is degraded due to reverse bias.  
         [0027]      FIG. 13  is a time-versus-voltage plot diagram showing how a black pixel is degraded due to reverse bias.  
         [0028]      FIG. 14  is a time-versus-voltage plot diagram showing the use of separate pre-driving and driving phases for a black pixel with the same voltage amplitude and duration.  
         [0029]      FIG. 15  is a time-versus-voltage plot diagram showing the use of separate pre-driving and driving phases with a longer duration for the pre-driving phase, as used for a black pixel.  
         [0030]      FIG. 16  is a time-versus-voltage plot diagram showing the use of separate pre-driving and driving phases with a higher driving amplitude for the pre-driving phase, as used for a black pixel.  
         [0031]      FIG. 17  is a time-versus-voltage plot diagram showing the use of separate pre-driving and driving phases with a longer duration for the pre-driving phase, as used for a white pixel.  
         [0032]      FIG. 18  is a time-versus-voltage plot diagram showing the use of separate pre-driving and driving phases with a higher driving amplitude for the pre-driving phase, as used for a white pixel.  
         [0033]      FIG. 19  is a time-versus-voltage plot diagram showing the use of separate pre-driving and driving phases with both a higher driving amplitude and a longer driving duration for the pre-driving phase, as used for a black pixel.  
         [0034]      FIG. 20  is a time-versus-voltage plot diagram showing the use of separate pre-driving and driving phases with both a higher driving amplitude and a longer driving duration for the pre-driving phase, as used for a white pixel.  
         [0035]      FIG. 21  is a signal pulse timing diagram for a first driving scheme.  
         [0036]      FIG. 22  is a signal pulse timing diagram for a second driving scheme.  
         [0037]      FIG. 23  is a signal pulse timing diagram for a third driving scheme.  
         [0038]      FIG. 24  is a signal pulse timing diagram for a fourth driving scheme.  
         [0039]      FIG. 25  is a signal pulse timing diagram for a fifth driving scheme.  
     
    
     DETAILED DESCRIPTION  
       [0040]     The present invention can be implemented in numerous ways, including as a process, an apparatus, a system, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. The order of the steps of disclosed processes may be altered within the scope of the invention.  
         [0041]     A detailed description of one or more preferred embodiments of the invention is provided below with drawing figures that illustrate by way of example the principles of the invention. While the invention is described in connection with such embodiments, it should be understood that the invention is not limited to any embodiment. On the contrary, the scope of the invention is limited only by the appended claims and the invention encompasses numerous alternatives, modifications and equivalents. For the purpose of example, numerous specific details are set forth in the following description in order to provide a thorough understanding of the present invention. The present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured.  
         [0042]     The whole content of each document referred to in this application is incorporated by reference into this application in its entirety for all purposes as if fully set forth herein.  
         [0043]     A. Overview of the Electrical Connectivity Between the Drive Voltage Generator and the EPD  
         [0044]     In an active matrix implementation of the EPD  100  as shown in  FIG. 1A ,  FIG. 3  illustrates one example characterization of the electrical connectivity between the drive voltage generator  116  and a 3×3 array portion  300  of this EPD  100 . Each one of the nine cells, cells  302 ,  304 ,  306 ,  308 ,  310 ,  312 ,  314 ,  316 , and  318 , in the array portion  300  is connected to the drive voltage generator  116  via source lines  334 ,  336 ,  338 , gate lines  328 ,  330 ,  332 , and a common line. Each cell also represents a pixel and includes a pixel electrode, which is a part of the upper electrode  112  of the EPD  100 , a common electrode, which is a part of the lower electrode  114 , and a dispersion layer, which is a part of the electrophoretic dispersion layer  102 . For example, cell  302  includes a pixel electrode  320 , a dispersion layer  322 , and a common electrode  324 . Although  FIG. 3  shows a separate common electrode  344  for the cell  304 , one can implement the cells with a single common electrode.  
         [0045]     In addition, the pixel electrode  320  is connected to the drain terminal of a transistor  326 , which is configured to control the application of biasing voltages to the pixel electrode  320 . In one alternative embodiment, a switching component other than a transistor, such as a diode, is used in place of the transistor  326 . The gate terminal of transistor  326  is connected to a gate line  328 , or G  328 . The source terminal of the transistor  326  is connected to a source line  334 , or S  334 . As shown in  FIG. 3 , the first, second, and third rows of pixels in the array portion  300  are associated with a gate line  328  (G  328 ), gate line  330  (G  330 ), and gate line  332  (G  332 ), respectively. Similarly, the first, second, and third columns of pixels in the array portion  300  are associated with a source line  334  (S  334 ), source line  336  (S  336 ), and source line  338  (S  338 ), respectively.  
         [0046]     Alternatively, in a direct drive implementation of the EPD  100 ,  FIG. 4A  illustrates one example characterization of the electrical connectivity between drive voltage generator  116  and an EPD  100  with seven segments. The seven segments,  418 ,  420 ,  422 ,  424 ,  426 ,  428 , and  430  are connected to the drive voltage generator  116  via segment lines  402 ,  404 ,  406 ,  408 ,  410 ,  412 , and  414 , respectively. In addition, the background  432  of this EPD  100  is associated with a background line  416 .  FIG. 4B  illustrates a plain view of this embodiment of the EPD  100 .  
         [0047]     B. Overview of the Drive Voltage Generator  
         [0048]      FIG. 5A  illustrates a block diagram of an example embodiment of the drive voltage generator  116  in an active matrix implementation. The generator  116  includes a power supply  500 , a controller interface  502 , a data register  504 , a data latch  506 , and a bank of drivers including source driver  508 , common driver  510 , and gate driver  512 . An alternative embodiment of the generator  116  uses an external power supply as opposed to the illustrated power supply  500 . Either of the mentioned power supplies includes circuitry to generate multiple-level voltages. The controller interface  502  mainly relays the various voltage levels, control signals, and display data to the appropriate components of the generator  116 . An alternative embodiment of the generator  116  includes an internal controller that generates the control signals. The data register  504  mainly stores the display data, and the data latch  506  mainly relays the stored data to the drivers, such as source driver  508 , common driver  510 , and gate driver  512 . In one embodiment, based on the display data, drivers  508 ,  510 ,  512  deliver appropriate levels of voltages to the source lines, common line, and gate lines, respectively, of EPD  100 .  
         [0049]     One example process for the drive voltage generator  116  to drive display data to the EPD  100  involves a number of different control signals. For example, to transfer a certain level of voltage to the source lines, control signal  524  and control signal  526  are involved. Specifically, the control signal  524  enables the data register  504  to store the display data that are on a data line  522 . Then, after the control signal  526  reaches a certain state, such as the falling edge of the signal, the data latch  506  transfers a portion of the stored display data to the drivers, such as the source driver  508 . Based on certain bits in the display data, one embodiment of the source driver  508  transfers one of the multiple-level voltages  520  from the power supply  500  to the source lines. In addition, depending on the state of the driving cycle, the control signal  528  may cause the gate driver  512  to turn off the transistors on its gate lines, such as transistor  326  and transistor  346  on the gate line  328 .  
         [0050]      FIG. 5B  illustrates a block diagram of an example embodiment of the drive voltage generator  116  in a direct drive implementation. The generator  116  includes a power supply  530 , a controller interface  532 , a data register  534 , a data latch  536 , a bank of drivers including segment driver  538 , common driver  540 , and background driver  542 , and a bank of switches including segment switch  544 , common switch  546 , and background switch  548 . The operations of this generator are similar to the aforementioned generator in the active matrix implementation, except for the addition of the bank of switches. For example, depending on the state of the driving cycle, the control signal  560  may cause the segment switch  544  to be turned off. In other words, the segment driver  538  becomes disconnected from the segment lines.  
         [0051]     C. Use of Switches to Mitigate Effect of Reverse Bias  
         [0052]     1. Active Matrix Implementation  
         [0053]     The display states of the pixels shown in the array portion  300  of  FIG. 3  may be controlled in any number of ways. Two typical approaches are the uni-polar or common switching approach and the bipolar approach. Under the uni-polar approach, all the pixels of the array are driven to their destined states in two driving phases. In phase one, selected pixels are driven to a first color state. In phase two, the other pixels are driven to a second color state that contrasts with the first. For example, in phase one, selected pixels may be driven in one embodiment to a first display state in which the charged pigment particles in the dispersion layers have been driven to a position at or near the pixel electrodes on the non-viewing side of the display. In phase two, the other pixels may then be driven to a second display state in which the charged pigment particles are in a position at or near the common electrode on the viewing side of the display. Alternatively, the opposite approach may involve first driving the charged pigment particles of the selected pixels to the viewing side of the display and then driving the particles of the other pixels to positions at or near the non-viewing side.  
         [0054]     Under the bipolar approach, a driving biasing voltage of a first polarity drives the cells to a first display state, and a second biasing voltage of the opposite polarity drives those cells to a second state. For example, a positive bias voltage may be applied to the cells so that a state in which the charged pigment particles are at or near the viewing surface of the display is reached. A negative bias voltage may also be applied to those cells so that the charged pigment particles are in a position at or near the non-viewing side of the display.  
         [0055]     a. Uni-Polar Approach  
         [0056]     Using the cells  302  and  304  shown in  FIG. 3  as an illustration, one example embodiment of the common electrodes  324  and  344  are transparent and are on the viewing side of the display. As mentioned above, one embodiment of the array portion  300  shares a single common electrode. Thus, the common electrodes  324  and  344  are the same common electrode. The dispersion layers  322  and  342  include a dielectric solvent and a number of charged pigment particles suspended in the solvent. For discussion purposes, assume that the positively charged pigment particles are white, and the solvent is black. Thus, when the particles are driven to the common electrodes  324  and  344 , the color of the particles, white, will be displayed. When the particles are driven to the pixel electrodes  320  and  340 , the color of the solvent, black, will be displayed. Black and white pixels or particles are not required; other embodiments may use any two contrasting colors.  
         [0057]      FIG. 6  shows a timing diagram of a driving cycle of two phases of an example embodiment of the drive voltage generator  116 . During the first driving phase  600 , the gate driver  512  as shown in  FIG. 5A  applies a high voltage to the gate line  328  and turns on the transistors  326  and  346 . Also, the common driver  510  and the source driver  508  apply a positive voltage to the common line and the source line  336 , respectively. The source line  334  is held at ground potential. Under such conditions, the cell  302  is driven to the state in which the color of the dielectric solvent in the dispersion layer  322 , in this case black, is visible at the viewing surface of the display, because the white charged pigment particles have been driven to a position at or near the pixel electrode  320  on the non-viewing side of the display. Then the gate driver  512  applies a low voltage to the gate line  328  and in effect turns off the transistor  326 . After a time period  603 , the common line and the source line  334  are held at ground potential. This allows the charge on the cell  302  to be slowly discharged to 0 volt through the high impedance of the off transistor.  
         [0058]     During the second driving phase  602 , selected cells are driven to the white state. In one example case, the color of the dielectric solvent in the dispersion layer  342  is driven to the white state. The common line and source line  334  are held at ground potential and the source line  336  at a positive voltage level. The gate driver  512  applies a high voltage to the gate line  328  and turns on the transistor  346  to transfer the voltage on the source line  336  to the drain of the transistor  346  and to the pixel electrode  340 . As a result, the white charged pigment particles in the dispersion layer  342  are driven to the position at or near the common electrode  344  on the viewing side of the display. Then the gate driver  512  applies a low voltage to the gate line  328  and in effect turns off the transistor  346 . After a time period  605 , the source line  336  is set to 0 volt. This also allows the charge on the cell  304  to be slowly discharged to 0 volt through the off transistor. The duration of the switch off time  604  and  606  depends on the characteristics of the electrophoretic dispersion, dielectric material, and the thickness of each layer.  
         [0059]     b. Bipolar Approach  
         [0060]      FIG. 7  illustrates a timing diagram of a single driving cycle employed by an example embodiment of the drive voltage generator  116 . In particular, the drive voltage generator  116  in a bipolar type active matrix EPD may drive the charged particles using either positive or negative drive voltage.  
         [0061]     Using the cell  302  as shown in  FIG. 3  in conjunction with  FIG. 7 , an appropriate level of voltage is applied to the gate line  328  in a driving cycle  700  to insure that the switching element, such as the transistor  326 , is in a conducting, or on, state. In one implementation, if the display data indicate a showing of a white color, the common electrode  324  is held at ground potential, the source line  334  at a positive voltage level, and the source line  336  at a negative voltage level as shown in  FIG. 7 . This biasing condition causes the charged particles to move towards the common electrode  324  on the viewing side of the display. The source line  336  is held at a negative voltage level during the driving cycle  700  and results in the movement of the particles to the pixel electrode  340 .  
         [0062]     Similar to the uni-polar approach discussions above, one embodiment of the drive voltage generator  116  turns off the transistors  326  and  346  after all the cells are driven to the designated states. After time duration  702 , all source lines are then set to ground (0 volt). The charge at each cell is then slowly discharged through the high impedance of the off transistor. The switch off duration of the transistor switch off time  704  depends on the characteristics of the electrophoretic dispersion, dielectric material, and the thickness of each layer.  
         [0063]     2. Direct Drive Implementation  
         [0064]     As an illustration, the direct drive implementation of the EPD  100  described in this section involves white positively charged pigment particles and either black or some other contrasting background color dielectric solvent. Also, as shown in  FIG. 4A , this implementation includes a common electrode in an upper layer of the display, above an array of cells with electrophoretic dispersion layers, on the viewing surface side of the EPD and a number of segment electrodes in a lower layer of the display, below the array of the cells, on the non-viewing side of the display. Thus, the white pigment particles in the dispersion layers of the cells that are associated with segments can be driven towards the viewing surface to display a white color in those segments. Alternatively, the particles can also be driven to a position at or near the segment electrodes to display a black color or other background color in those segments.  
         [0065]     a. Uni-Polar Approach  
         [0066]      FIG. 8A  illustrates a timing diagram of a driving cycle in a uni-polar direct drive implementation employed by an example embodiment of the drive voltage generator  116  as shown in  FIG. 5B . Using the segments  426  and  430  as shown in  FIGS. 4A and 4B  and also in conjunction with  FIGS. 5B and 8A , a uni-polar driving cycle comprises two driving phases. During phase  800 , with the common switch  546  turned on, the common driver  540  drives the common electrode with a positive voltage. The segment electrode of the segment  426  is driven by the segment line  410  with 0 volt and with the segment switch  544  turned on. The background electrode of the background  432  is driven by the background line  416  with 0 volt and with the background switch  548  turned on. During this phase of the driving cycle, both the segment  426  and the background  432  show the background color, or black in this example. On the other hand, because the segment line  414  is driven to a positive voltage, which is the same as the voltage being applied to the common electrode, the color state of the segment  430  does not change.  
         [0067]     After the segments reach their desired color states, the segment switch  544 , the common switch  546 , and the background switch  548  are turned off. After a time period  803 , the drivers, such as  538 ,  540 , and  542 , set 0 volt on the lines. This allows the charges on the segments and the background to be slowly discharged to 0 volt through the high impedance of the off switches.  
         [0068]     During phase  802 , the common remains at 0 volt. The segment electrode of the segment  426  is driven by the segment line  410  with 0 volt and with the segment switch  544  turned on. The background electrode of the background  432  is driven by the background line  416  with also 0 volt and with the background switch  548  turned on. During this phase of the driving cycle, both the segment  426  and the background  432  show the color of the solvent (background), or black in this example. On the other hand, the segment line  414  is driven to a positive voltage. The segment  430  instead shows the color of the particles, or white in this example. After the segments reach their desired color states, the segment switch  544 , the common switch  546 , and the background switch  548  are turned off. After a time period  805 , the drivers, such as  538 ,  540 , and  542 , set 0 volt on the lines. This allows the charges on the segments and the background to be slowly discharged to 0 volt through the high impedance of the off switches. The switch off duration of the transistor switch off time  804  and  806  depends on the characteristics of the electrophoretic dispersion, dielectric material, and the thickness of each layer.  
         [0069]     b. Bi-Polar Approach  
         [0070]      FIG. 8B  illustrates a timing diagram of a driving cycle in a bi-polar direct drive implementation employed by an example embodiment of the drive voltage generator  116  as shown in  FIG. 5B . Using the segments  426  and  430  as shown in  FIGS. 4A and 4B  and also in conjunction with  FIGS. 5B and 8B , during a bi-polar driving cycle, with the common switch  546  turned on, the common driver  540  drives the common electrode with 0 volt. The segment electrode of the segment  426  is driven by the segment line  410  with a negative voltage and with the segment switch  544  turned on. The segment electrode of the segment  430  is driven by the segment line  414  with a positive voltage and with the segment switch  544  turned on. The background electrode of the background  432  is driven by the background line  416  with 0 volt and with the background switch  548  turned on. In this driving cycle, both the segment  426  and the background  432  show the background color, or black in this example. The segment  430 , on the other hand, shows the color of the particles, or white in this example. After the segments and the background are driven to the designated states, the switches, such as  544 ,  546 , and  548 , are turned off. After a time period  820 , the drivers, such as  538 ,  540  and  542 , set 0 volt on the lines. This allows the charges on the segments and the background to be slowly discharged to 0 volt through the high impedance of the off switches. The switch off duration of the transistor switch off time  830  depends on the characteristics of the electrophoretic dispersion, dielectric material, and the thickness of each layer.  
         [0071]     c. Pre-Drive Approach  
         [0072]     In a typical EPD, the charge property of the particles relates to the field strength that the particles experience. For instance, after the particles are under a strong field for a period of time, the reverse bias effect is greatly reduced. Due to the capacitance characteristics of an EPD cell, the field strength is the strongest during the transition from a positive driving voltage to a negative driving voltage or vice versa. In  FIG. 8C , a pre-drive voltage is applied to a pixel before the actual driving voltage is applied. Using a bi-polar direct drive system as an illustration, the segment line  410  is first set at a positive voltage for a period of time, and then it is set to a negative voltage in a normal driving cycle. It has been observed that even without turning off the segment switch  544  and the common switch  546 , this pre-drive approach greatly reduces the reverse bias effect. It should be apparent to one with ordinary skill in the art to apply this pre-drive approach to a uni-polar direct drive EPD system, bi-polar active matrix EPD system, and uni-polar active matrix EPD system.  
         [0073]     A plurality of pre-drive driving approaches for EPDs are now described with reference to  FIG. 10  through  FIG. 26 , respectively.  
         [0074]     To provide background,  FIG. 10  is an example of an electrophoretic display (EPD) device. An EPD, especially a Microcup®-based EPD, usually comprises three layers, namely, an insulating layer ( 11 ), an electrophoretic fluid (i.e., dispersion layer  12 ) comprising charged pigment particles dispersed in a dielectric solvent or solvent mixture and a sealing layer ( 13 ). In  FIG. 10 , the sealing layer ( 13 ) is the non-viewing side whereas the insulating layer ( 11 ) is the viewing side. The insulating layer  11  may be formed from a material used for the formation of the microcup structure as described in co-pending application U.S. Ser. No. 09/518,488, the entire contents of which are incorporated herein by reference in its entirety for all purposes as if fully set forth herein.  
         [0075]      FIG. 11  shows a circuit network that is electrically equivalent to the EPD device. This type of display devices often will experience the reverse bias problem as shown in  FIG. 12  and  FIG. 13 .  
         [0076]     In  FIGS. 12-20 , the solid line denotes the applied voltage and the dotted line denotes the voltage experienced by the particles in the dispersion layer. For illustration purpose, the particles, in  FIGS. 12-20 , are white and carry a positive charge and the dielectric solvent or solvent mixture in which the particles are dispersed is black. The use of white and black colors is not required; alternate embodiments may use any contrasting colors.  
         [0077]     According to  FIG. 12 , the particles in the dispersion layer would be moved to the viewing side (i.e., the white state) in Phase A and then experience an opposite voltage (i.e., reverse bias voltage) in Phase B, after the power is turned off. Such reverse bias effect causes degradation of the quality of the image shown (i.e., a degraded white state) because the particles at the top of the dispersion layer are dragged down by the opposite voltage.  
         [0078]     The reverse bias phenomenon is caused by the capacitor charge holding characteristics of the insulating layer and the sealing layer. At any bias voltage transition, these layers, functioning as a capacitor, will not charge or discharge instantly. Without a special driving waveform design, a reverse polarity bias voltage will apply to the dispersion layer and cause particles migrate to the opposite direction of the desired state.  
         [0079]     A similar degradation of the quality may also be observed with a black pixel, according to  FIG. 13 , due to the reverse bias effect.  
         [0080]     To resolve the reverse bias issue, according to one embodiment, driving Phase A is separated into two phases. The first phase is called the pre-driving phase, and the second phase is called the driving phase. The voltage amplitude and duration of the pre-driving phase are higher and longer, respectively, than the amplitude and duration of the driving phase, to overcome the reverse bias effect. Otherwise, the reverse bias effect will be present as illustrated in  FIG. 14 , in which the pre-driving and driving phases have the same voltage amplitude and the same duration. In the case of  FIG. 14 , the particles will experience a reverse voltage of about 5V at the beginning in Phase B.  
         [0081]     The voltage amplitudes and durations of the two phases may be optimized, together or individually, to overcome the reverse bias effect.  
         [0082]      FIG. 15  and  FIG. 16  show how a black pixel is driven. In  FIG. 15 , the pre-driving phase has a longer driving duration than that of the driving phase, but the two phases have the same driving voltage amplitude. The reverse bias voltage is removed and the negative bias voltage in Phase B will help particles stay at the bottom of the dispersion layer. In  FIG. 16 , the driving durations in the pre-driving and driving phases are the same but the pre-driving phase has a higher voltage amplitude than the driving phase. The particles therefore experience a negative bias voltage in Phase B which will keep them staying at the bottom of the dispersion layer.  
         [0083]      FIG. 17  and  FIG. 18  show how a white pixel is driven. The positive bias voltage experienced by the particles in Phase B is helpful to keep the white particles staying at the top of the dispersion layer.  
         [0084]      FIG. 19  and  FIG. 20  show that both the driving voltage amplitude and the duration of the pre-driving phase are adjusted. The driving voltage amplitude of the pre-driving phase is higher and the driving duration of the pre-driving phase is longer, than those of the driving phase in  FIGS. 19, 20 . The bias voltages of Phase B that can maintain the particles at their intended positions in  FIG. 19  and  FIG. 20  are even higher than those in which only one of the driving voltage amplitude and duration is optimized ( FIGS. 15-18 ).  
         [0085]      FIGS. 21-25  present a plurality of alternative approaches that address the foregoing problems.  
         [0086]     In Scheme I as shown in  FIG. 21 , after reset, the display is cleared to its dark state and then white pixels are driven according to the intended image. To show a dark image on a white background, one can swap the voltages applied to V comm  and Segments.  
         [0087]     In Scheme II as shown in  FIG. 22 , resetting the display is optional. The white pixels are driven first and then the dark pixels. Scheme III in  FIG. 23  is the same as Scheme II except that the dark pixels have less pre-drive time. Scheme IV in  FIG. 24  is the same as Scheme II except that the dark pixels are driven first in Scheme IV. Scheme V in  FIG. 25  is the same as Scheme III except the white pixels have less pre-drive time in Scheme V.  
         [0088]     The voltage and duration of each phase of the driving schemes may be adjusted, according to specific display and driver requirements, based on the pre-drive mechanisms disclosed above.  
         [0089]     D. Example Systems and Applications  
         [0090]      FIG. 9  illustrates one example system that includes the EPD  100  as shown in  FIG. 1A  and the drive voltage generator  116  as shown in  FIG. 5 . The system  900  also includes a data collector  902 , a processing engine  904 , a controller  906 , and memory  908 . The data collector  902  is mainly responsible for retrieving display data from various content sources, such as, without limitation, any form of storage medium (e.g., compact disks, DVDs, hard drives, tape drives, memory, etc.) and online content and through various communication channels, such as terrestrial, wireless, and infrared connections. The processing engine  904 , together with memory  908 , can process the retrieved display data, such as decoding, filtering, or modifying. Also, the engine can also work with the controller  906  to issue control signals to the drive voltage generator  116 .  
         [0091]     Numerous applications utilize the illustrated system  900  in one form or another. Some examples include, without limitation, electronic books, personal digital assistants, mobile computers, mobile phones, digital cameras, electronic price tags, digital clocks, smart cards, and electronic papers.  
         [0092]     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing both the process and apparatus of the improved driving scheme for an electrophoretic display. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.