Drivers Provide DC Balanced Update Sequence for Color Electrophoretic Displays

Abstract

The present application relates to a method for driving an electro-optical display having a front electrode, a backplane and a display medium, the display medium comprising particles of at least three different colors, wherein the medium is located between the front electrode and the backplane. The method comprises subjecting the display to a reset phase and a color transition phase, such that the sum of all pulses results in an offset that maintains a DC balance throughout the display medium. The invention further comprises a controller for executing the method.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application Serial No. 15 / 454,276 filed on March 9, 2017. This application also claims priority to U.S. Provisional Application Serial No. 62 / 509,512 filed on May 22, 2017. The entire contents of the above applications are hereby incorporated herein by reference. Technical Field

[0003] The present invention relates to a method for driving an electro-optical display, and more particularly, but not limited to, an electrophoretic display capable of presenting more than two colors using a single layer of electrophoretic material comprising a plurality of colored particles (e.g., white, cyan, yellow and magenta particles), wherein two of the particles are positively charged, two of the particles are negatively charged, and one of the positively charged particles and one of the negatively charged particles have a thick polymer shell. Background Art

[0004] The term "color" as used herein includes black and white. White particles are generally light scattering.

[0005] The term "gray state" is used herein in its conventional sense in imaging technology to refer to a state between two extreme optical states of a pixel, and does not necessarily imply a black-white transition between the two extreme states. For example, several of the E Ink patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and dark blue, such that the intermediate gray state is actually light blue. More specifically, as described, the change in optical state may not be a color change at all. The terms "black" and "white" may be used below to refer to the two extreme optical states of a display, and should be understood to generally include extreme optical states that are not completely black and white, such as the aforementioned white and dark blue states.

[0006] The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to a display including display elements having first and second display states that differ in at least one optical property, and such that any given element, when driven by an addressing pulse of finite duration, assumes either its first or second display state, and that state persists after termination of the addressing pulse for at least a number of times, e.g., at least four times, the minimum duration required for the addressing pulse to change the state of the display element. U.S. Patent No. 7,170,670 shows that some particle-based electrophoretic displays with grayscale capability can remain stable not only in their extreme black and white states, but also in their intermediate gray states, as can some other types of electro-optical displays. Such displays may properly be referred to as multi-stable rather than bistable, but for convenience, the term "bistable" may be used herein to cover both bistable and multistable displays.

[0007] The term "impulse" when used in reference to the driving of an electrophoretic display is used herein to denote the integral of the applied voltage with respect to time during which the display is driven.

[0008] Particles that absorb, scatter or reflect light in a broad band or at a selected wavelength are referred to herein as colored or pigment particles. Various materials other than pigments (a term used in the strict sense to denote insoluble colored materials) that absorb or reflect light, such as dyes or photonic crystals, may also be used in the electrophoretic media and displays of the present invention.

[0009] Particle-based electrophoretic displays have been the subject of intensive research and development for many years. In such displays, a variety of charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays have the properties of good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, problems with the long-term image quality of these displays have hindered their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in insufficient useful life for these displays.

[0010] As described above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but gaseous fluids can be used to produce electrophoretic media; see, for example, Kitamura, T., et al., "Electrical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y., et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4). See also U.S. Patents 7,321,459 and 7,236,291. When such media are used in an orientation that allows such sedimentation (e.g., in a display in which the media is configured in a vertical plane), such gas-based electrophoretic media are susceptible to particle sedimentation like liquid-based electrophoretic media. More specifically, particle sedimentation appears to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media because the lower viscosity of the gaseous suspending fluid compared to the liquid suspending fluid allows electrophoretic particles to sediment more rapidly.

[0011] Many patents and applications assigned to or under the names of Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used in capsule electrophoresis and other electro-optical media. Such capsule media include a plurality of small capsules, each capsule itself including an internal phase of electrophoretically mobile particles contained in a fluid medium and a capsule wall surrounding the internal phase. Typically, the capsule itself is held in a polymer adhesive to form a coherent layer located between two electrodes. The techniques described in these patents and applications include:

[0012] a) electrophoretic particles, fluids, and fluid additives; see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814;

[0013] (b) Capsules, adhesives and encapsulation processes; see, e.g., U.S. Pat. Nos. 6,922,276 and 7,411,719;

[0014] (c) microcell structures, wall materials, and methods of forming microcells; see, e.g., U.S. Pat. Nos. 7,072,095 and 9,279,906;

[0015] (d) Methods of filling and sealing microlocations; see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088;

[0016] (e) Films and sub-assemblies containing electro-optical materials; see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564;

[0017] (f) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624;

[0018] (g) Color formation and color adjustment; see, e.g., U.S. Patent Nos. 6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875; 6,914,714; 6,972,893; 7,038,656; 7,038,670; 7,046,228; 7,052,571; 7,075,502***; 7,167,155; 7,385,751; 7,492,505; 7,667,684; 7,684,108; 7,791,789; 7,800,813; 7,821,702; 7,839,564***; 7,910,17 5; 7,952,790; 7,956,841; 7,982,941; 8,040,594; 8,054,526; 8,098,418; 8,159,636; 8,213,076; 8,363,299; 8,422,116; 8,441,714; 8,441,716; 8, 466,852; 8,503,063; 8,576,470; 8,576,475; 8,593,721; 8,605,354; 8,649,084; 8,670,174; 8,704,756; 8,717,664; 8,786,935; 8,797,634; 8,810, Nos. 8,899; 8,830,559; 8,873,129; 8,902,153; 8,902,491; 8,917,439; 8,964,282; 9,013,783; 9,116,412; 9,146,439; 9,164,207; 9,170,467; 9,170,468; 9,182,646; 9,195,111; 9,199,441; 9,268,191; 9,285,649; 9,293,511; 9,341,916; 9,360,733; 9,361,836; 9,383,623; and 9,423,666; and U.S. Patent Application Publication Nos. No. 2008 / 0043318; 2008 / 0048970; 2009 / 0225398; 2010 / 0156780; 2011 / 0043 543;2012 / 0326957;2013 / 0242378;2013 / 0278995;2014 / 0055840;2014 / 0 078576; 2014 / 0340430; 2014 / 0340736; 2014 / 0362213; 2015 / 0103394; 2015 / 0118390; 2015 / 0124345; 2015 / 0198858; 2015 / 0234250; 2015 / 0268531;2015 / 0301246; 2016 / 0011484; 2016 / 0026062; 2016 / 0048054; 2016 / 0116816; 2016 / 0116818; and 2016 / 0140909;

[0019] (h) Methods for driving displays; see, e.g., U.S. Patent Nos. 5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,061,166; 7,061,662; 7,116,466; 7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242,514; 7,259,744; 7,304,787; 7, 312,794; 7,327,511; 7,408,699; 7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251; 7,602,374; 7,612,760; 7,679,599; 7,679,813; 7,683 ,606; 7,688,297; 7,729,039; 7,733,311; 7,733,335; 7,787,169; 7,859,742; 7,952,557; 7,956,841; 7,982,479; 7,999,787; 8,077,141; 8,125,501 ; 8,139,050; 8,174,490; 8,243,013; 8,274,472; 8,289,250; 8,300,006; 8,305,341; 8,314,784; 8,373,649; 8,384,658; 8,456,414; 8,462,102; 8, 514,168; 8,537,105; 8,558,783; 8,558,785; 8,558,786; 8,558,855; 8,576,164; 8,576,259; 8,593,396; 8,605,032; 8,643,595; 8,665,206; 8,681 ,191;8,730,153;8,810,525;8,928,562;8,928,641;8,976,444;9,013,394;9,019,197;9,019,198;9,019,318;9,082,352;9,171,508;9,218,77 3; 9,224,338; 9,224,342; 9,224,344; 9,230,492; 9,251,736; 9,262,973; 9,269,311; 9,299,294; 9,373,289; 9,390,066; 9,390,661; and 9,412,314;and U.S. Patent Application Publication Nos. 2003 / 0102858; 2004 / 0246562; 2005 / 0253777; 2007 / 0091418; 2007 / 0103427; 2007 / 0176912; 2008 / 0024429; 2008 / 0024482; 2008 / 0136774; 2008 / 0291129; 2008 / 0303780; 2009 / 0174651; 2009 / 0195568; 2009 / 032272 1; 2010 / 0194733; 2010 / 0194789; 2010 / 0220121; 2010 / 0265561; 2010 / 0283804; 2011 / 0063314; 2011 / 0175875; 2011 / 0193840; 2011 / 0193841; 2011 / 0199671; 2011 / 0221740; 2012 / 0001957; 2012 / 0098740; 2013 / 0063333; 2013 / 019 4250; 2013 / 0249782; 2013 / 0321278; 2014 / 0009817; 2014 / 0085355; 2014 / 0204012; 2014 / 0218277; 2014 / 0240210; 2014 / 0240373; 2014 / 0253425; 2014 / 0292830; 2014 / 0293398; 2014 / 0333685; 2014 / 0340734; 2015 / 0070744; 2015 / 0097877; 2015 / 0109283; 2015 / 0213749; 2015 / 0213765; 2015 / 0221257; 2015 / 0262255; 2015 / 0262551; 2016 / 0071465; 2016 / 0078820; 2016 / 0093253; 2016 / 0140910; and 2016 / 0180777 (these patents and applications may be referred to below as MEDEOD (Method for Driving Electro-Optical Display) applications);

[0020] (i) Display applications; see, e.g., U.S. Pat. Nos. 7,312,784 and 8,009,348; and

[0021] (j) Non-electrophoretic displays, such as those described in U.S. Patent No. 6,241,921; and U.S. Patent Application Publication No. 2015 / 0277160; and U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / 0012710.

[0022] Many of the above patents and applications recognize that the walls surrounding discrete microcapsules in encapsulated electrophoretic media can be replaced by a continuous phase, thereby producing a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium contains a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and even though there is no discrete capsule membrane associated with each individual droplet, the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered capsules or microcapsules; see, e.g., U.S. Patent No. 6,866,760. Thus, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0023] A related type of electrophoretic display is the so-called microcell electrophoretic display. In a microcell electrophoretic display, the charged particles and fluids are not encapsulated in microcapsules, but instead are held in a plurality of cavities formed within a carrier medium (usually a polymeric film). See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging Inc.

[0024] Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode," in which one display state is substantially opaque and one display state is transmissive. See, for example, U.S. Pat. Nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays (which are similar to electrophoretic displays, but rely on changes in electric field strength) can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optical displays can also operate in a shutter mode. Electro-optical media operating in grating mode may be used in multilayer structures for full color displays; in such structures at least one layer adjacent to a viewing surface of the display is operated in grating mode to expose or conceal a second layer further from the viewing surface.

[0025] Capsule-type electrophoretic displays generally do not suffer from the failure modes of clustering and settling of conventional electrophoretic devices and offer additional advantages, such as the ability to print or coat the displays on a wide variety of flexible and rigid substrates. (The use of the word printing is intended to include all forms of printing and coating, including but not limited to: pre-metered coatings (e.g., patch die coating, slot or extrusion coating, slide or cascade coating, and curtain coating); roll coating (e.g., knife over roll coating and forward and reverse roll coating); gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air-knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; processes); ink jet printing processes; electrophoretic deposition (see U.S. Pat. No. 7,339,715); and other similar techniques. Thus, the resulting display is flexible. Furthermore, because the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.

[0026] As described above, most simple prior art electrophoretic media display only two colors in essence. Such electrophoretic media use a single type of electrophoretic particles having a first color in a colored fluid having a second different color (in which case the first color is displayed when the particles are adjacent to the viewing surface of the display, and the second color is displayed when the particles are separated from the viewing surface), or use first and second types of electrophoretic particles having different first and second colors in an uncolored fluid (in which case the first type of particles display the first color when adjacent to the viewing surface of the display, and the second color when the second type of particles are adjacent to the viewing surface). Typically, the two colors are black and white. If a full color display is desired, a color filter array can be deposited on the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to produce color stimuli. The available display area is shared between three or four primary colors (e.g., red / green / blue (RGB) or red / green / blue / white (RGBW)), and the filters can be arranged in a one-dimensional (line) or two-dimensional (2×2) repeating pattern. Other choices of primary colors or more than three primary colors are also known in the art. Three (in the case of an RGB display) or four (in the case of an RGBW display) sub-pixels are selected to be small enough so that at the expected viewing distance they visually blend together into a single pixel with uniform color stimulation ("color mixing"). The inherent disadvantage of area sharing is that the colorant is always present, and the color can only be adjusted by switching the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue and white primary colors occupies one quarter of the display area (one of the four sub-pixels), where the white sub-pixel is as bright as the underlying monochrome display white, and each colored sub-pixel is no brighter than one-third of the monochrome display white. The brightness of white displayed by the display as a whole cannot exceed half the brightness of the white sub-pixels (the white area of ​​the display is generated by displaying one white sub-pixel in every four sub-pixels, plus each colored sub-pixel in colored form is equivalent to one-third of a white sub-pixel, so that the combination of three colored sub-pixels contributes no more than one white sub-pixel). The brightness and saturation of the color is reduced by switching the zone-shared color pixels to black. Zone sharing is particularly problematic when mixing yellow because it is brighter than any other color of the same brightness, and saturated yellow is almost as bright as white. Switching the blue pixels (a quarter of the display area) to black would make the yellow too dark.

[0027] Multilayer stacked electrophoretic displays are known in the art; see, e.g., J. Heikenfeld, P. Drzaic, JS Yeo and T. Koch, Journal of the SID, 19(2), 2011, pp. 129-156. In such displays, ambient light is passed through an image in each of the three primary colors in a manner exactly analogous to conventional color printing. U.S. Patent No. 6,727,873 describes a stacked electrophoretic display in which three layers of switchable cells are placed above a reflective background. Similar displays are known in which the colored particles are moved laterally (see, e.g., International Application No. WO 2008 / 065605), or are isolated into microcells using a combination of vertical and lateral motion. In both cases, each layer is provided with electrodes for collecting or dispersing the colored particles on a pixel-by-pixel basis, so that each of the three layers requires a thin film transistor (TFT's) (two of the three TFT's must be substantially transparent) and a light-transmitting counter electrode. This complex electrode arrangement is expensive to manufacture, and it is difficult to provide a sufficiently transparent pixel electrode plate in the current state of the art, especially since the white state of the display must be viewed through several layers of electrodes. Multilayer displays also suffer from parallax issues as the thickness of the display stack approaches or exceeds the pixel size.

[0028] U.S. Patent Application Publication Nos. 2012 / 0008188 and 2012 / 0134009 describe multicolor electrophoretic displays having a single backplane with independently addressable pixel electrodes and a shared light-transmitting front electrode. Multiple electrophoretic layers are disposed between the backplane and the front electrode. The displays described in these applications are capable of presenting any primary color (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, there are disadvantages to the use of multiple electrophoretic layers between a single set of addressing electrodes. The electric field experienced by particles in a particular layer is lower than that of a single electrophoretic layer addressed with the same voltage. In addition, light losses in the electrophoretic layer closest to the viewing surface (e.g., due to light scattering or unwanted light absorption) may affect the appearance of the image formed in the electrophoretic layer below.

[0029] Attempts have been made to provide full-color electrophoretic displays using a single electrophoretic layer. For example, U.S. Patent Application Publication No. 2013 / 0208338 describes a color display comprising an electrophoretic fluid comprising one or two types of pigment particles dispersed in a transparent and colorless or colored solvent, the electrophoretic fluid being disposed between a common electrode and a plurality of pixels or drive electrodes. The drive electrodes are configured to expose a background layer. U.S. Patent Application Publication No. 2014 / 0177031 describes a method for driving a display unit filled with an electrophoretic fluid comprising two types of charged particles with opposite charge polarities and two contrasting colors. The two types of pigment particles are dispersed in a colored solvent, or a non-charged or slightly charged colored particle solvent. This method includes driving the display unit by applying a drive voltage of about 1% to about 20% of the full drive voltage to display the color of the solvent or the color of the non-charged or slightly charged colored particles. U.S. Patent Application Publication Nos. 2014 / 0092465 and 2014 / 0092466 describe an electrophoretic fluid and a method for driving an electrophoretic display. The fluid comprises first, second and third types of pigment particles, all of which are dispersed in a solvent or solvent mixture. The first and second types of pigment particles carry opposite charge polarities, and the third type of pigment particles has a charge level that is less than about 50% of the charge level of the first or second types. The three types of pigment particles have different threshold voltage levels or different mobility levels, or both. The term full color display is used below in a sense not disclosed in these patent applications.

[0030] U.S. Patent Application Publication No. 2007 / 0031031 describes an image processing device for processing image data to display an image on a display medium, wherein each pixel is capable of displaying white, black, and another color. U.S. Patent Application Publication Nos. 2008 / 0151355; 2010 / 0188732; and 2011 / 0279885 describe a color display in which movable particles move through a porous structure. U.S. Patent Application Publication Nos. 2008 / 0303779 and 2010 / 0020384 describe a display medium comprising first, second, and third particles of different colors. The first and second particles can form aggregates, and smaller third particles can move through pores left between the aggregated first and second particles. U.S. Patent Application Publication No. 2011 / 0134506 describes a display device comprising an electrophoretic display component, the electrophoretic display component comprising a plurality of types of particles encapsulated between a pair of substrates, at least one substrate being translucent, and each of the corresponding plurality of types of particles being electrically charged with the same polarity, having different optical properties, and having different migration speeds and / or electric field thresholds for movement, a translucent display side electrode being arranged on a substrate side provided with a translucent substrate, a first back electrode being arranged on another substrate side and facing the display side electrode, and a second back electrode being arranged on another substrate side and facing the display side electrode; and a voltage control portion that controls the voltage applied to the display side electrode, the first back electrode, and the second back electrode, so that particles of the type having the fastest migration speed among the plurality of types of particles or particles of the type having the lowest threshold among the plurality of types of particles move to the first back electrode or the second back electrode in sequence according to each type of different types of particles, and then the particles moving to the first back electrode move to the display side electrode. U.S. Patent Application Publication Nos. 2011 / 0175939, 2011 / 0298835, 2012 / 0327504, and 2012 / 0139966 describe color displays that rely on the aggregation of multiple particles and a critical voltage. U.S. Patent Application Publication No. 2013 / 0222884 describes an electrophoretic particle that includes a polymer containing a charged group and a colorant, and a branched silicone-based polymer attached to the colored particle and containing a reactive monomer as a copolymer component and at least one monomer selected from a specific monomer group. U.S. Patent Application Publication No. 2013 / 0222885 describes a dispersion for an electrophoretic display, which includes a dispersion medium, a group of colored electrophoretic particles dispersed in the dispersion medium and migrating in an electric field, a group of non-electrophoretic particles that do not migrate and have a color different from the electrophoretic particle group, and a compound having a neutral polar group and a hydrophobic group, wherein the compound is contained in the dispersion medium in a proportion of about 0.01 to about 1 mass percent based on the entire dispersion.U.S. Patent Application Publication No. 2013 / 0222886 describes a dispersion for a display comprising floating particles, the floating particles comprising: a core particle containing a colorant and a hydrophilic resin; and a shell covering the surface of each core particle and comprising a hydrophobic resin having a solubility parameter difference of 7.95 (J / cm3)1 / 2 or more. U.S. Patent Application Publication Nos. 2013 / 0222887 and 2013 / 0222888 describe an electrophoretic particle having a specific chemical composition. Finally, U.S. Patent Application Publication No. 2014 / 0104675 describes a particle dispersion comprising first and second colored particles that move in response to an electric field and a dispersion medium, the second colored particles having a larger diameter than the first colored particles and having the same charging characteristics as the first colored particles, and wherein the ratio (Cs / Cl) of the charge amount Cs of the first colored particles to the charge amount C1 of the second colored particles per unit area of ​​the display is less than or equal to 5. Some of the above displays do offer full color, but at the expense of long and cumbersome addressing methods.

[0031] U.S. Patent Application Publication Nos. 2012 / 0314273 and 2014 / 0002889 describe an electrophoretic device comprising a plurality of first and second electrophoretic particles contained in an insulating liquid, the first and second particles having different charging characteristics from each other; the device further comprising a porous layer contained in the insulating liquid and formed of a fiber structure. These claims are not full-color displays in the sense described below.

[0032] See also US Patent Application Publication No. 2011 / 0134506 and the aforementioned application Ser. No. 14 / 277,107; the latter describes a full color display using three different types of particles in a colored fluid, but the presence of the colored fluid limits the quality of the white state that the display can achieve.

[0033] In order to achieve a high resolution display, the individual pixels of the display must be addressable without interference from adjacent pixels. One way to achieve this goal is to provide an array of non-linear components, such as transistors or diodes, wherein at least one non-linear component is associated with each pixel to produce an "active matrix" display. The addressing or pixel electrode used to address the pixel is connected to an appropriate voltage source via the associated non-linear component. Typically, when the non-linear component is a transistor, the pixel electrode is connected to the drain of the transistor, and this configuration will be used in the following description, but in essence any pixel electrode can be connected to the source of the transistor. Typically, in a high resolution array, the pixels are arranged in a two-dimensional array of rows and columns, so that any particular pixel can be uniquely identified by the intersection of a particular column and a particular row. The sources of all transistors in each column are connected to a single column electrode, and the gates of all transistors in each row are connected to a single row electrode; again, the assignment of sources to rows and gates to columns is also common, and is essentially arbitrary and can be reversed if desired. The row electrodes are connected to a row driver which essentially ensures that only one row is selected at a given time, i.e., a select voltage is applied to the selected row electrode to ensure that all transistors in the selected row are conducting, and a select voltage is applied to all other rows to ensure that all transistors in these unselected rows remain non-conducting. The column electrodes are connected to a column driver which sets selected voltages on the various column electrodes to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to the shared front electrode which is typically located on the opposite side of the electro-optical medium away from the non-linear array and extends across the entire display). After a preselected time interval called the "line address time", the selected row is deselected, the next row is selected, and the voltage on the column driver is changed to write the next row of the display. This procedure is repeated to write the entire display row by row.

[0034] Typically, each pixel electrode has a capacitor electrode associated therewith, so that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application No. WO 01 / 07961. In some embodiments, the transistor can be formed using an N-type semiconductor (e.g., amorphous silicon), and the "select" and "non-select" voltages applied to the gate electrode can be positive and negative, respectively.

[0035] The attached picture Figure 1An exemplary equivalent circuit for a single pixel of an electrophoretic display is described. As shown, the circuit includes a capacitor 10 formed between a pixel electrode and a capacitor electrode. The electrophoretic medium 20 is represented by a capacitor and a resistor in parallel. In some cases, direct or indirect coupling capacitance 30 between the gate electrode of a transistor associated with the pixel and the pixel electrode (commonly referred to as "parasitic capacitance") may produce unwanted noise to the display. Typically, the parasitic capacitance 30 is much smaller than the parasitic capacitance of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitance 30 may cause a small negative offset voltage, also known as a "kickback voltage," to the pixel electrode, which is typically less than 2 volts. In some embodiments, in order to compensate for the unwanted "kickback voltage," the shared potential V com supplied to the upper plate electrode and capacitor electrode associated with each pixel so that when V com is set equal to the kickback voltage (V KB ), each voltage supplied to the display may be offset by the same amount and no net DC imbalance is experienced.

[0036] However, when V com Problems can arise when the backplane is set to a voltage that is not compensated for the kickback voltage. This can occur when it is desired to apply a higher voltage to the display than can be obtained from the backplane alone. It is well known in the art that if the backplane is supplied with a selection of nominal +V, 0, or -V, for example, and V is supplied at -V com , the maximum voltage applied to the display may be doubled. In this case the maximum voltage experienced is +2V (i.e., below the backplane relative to the topplane), while the minimum is zero. If negative voltages are required, V com The potential must be raised to at least zero. Therefore, the waveform used to address the display with positive and negative voltages using top plane switching must have a voltage distribution for multiple V com A specific frame for each of the voltage settings.

[0037] A set of waveforms for driving a color electrophoretic display having four particles is described in U.S. Application Serial No. 14 / 849,658, which is incorporated herein by reference. In U.S. Application Serial No. 14 / 849,658, seven different voltages are applied to the pixel electrode: three positive voltages, three negative voltages, and zero voltage. However, in some embodiments, the maximum voltage used in these waveforms is higher than the maximum voltage that can be handled by the amorphous silicon thin film transistor. In this case, a suitable high voltage can be obtained by using upper plate switching. When (as described above) V com Deliberately set to V KBHowever, using V com Setting up as many independent power supplies is expensive and inconvenient. Furthermore, it is known that switching on the upper panel will increase the kickback, thereby reducing the stability of the color state. Therefore, it is necessary to use a single power supply for the backplane and V com Of course, any DC offset will result in a longer pulse train and therefore a longer image update time. Summary of the invention

[0038] The present invention relates to a driver configured to send a two-part reset pulse to pixels in a color electrophoretic display. The two-part reset pulse effectively removes the last state information, but does not require more energy or time. As a result, the controller allows a three (or more) particle electrophoretic display to update more quickly while using less energy. Surprisingly, the controller also provides a larger color gamut when the reset pulse is adjusted for individual colors. The present invention further provides a method of driving an electro-optical display that is DC balanced despite the presence of a kickback voltage and voltage variations applied to the front electrode.

[0039] In one aspect, the present invention relates to a method for driving an electrophoretic display having a front electrode, a backplane, and a display medium located between the front electrode and the backplane, the display medium comprising three groups of particles of different colors. The method comprises subjecting the display to a reset phase and a color transition phase. The reset phase comprises applying a first signal to the front electrode, the first signal having a first polarity, a first amplitude varying over time, and a first duration; applying a second signal to the backplane during the first duration, the second signal having a second polarity opposite to the first polarity, a second amplitude varying over time; applying a third signal to the front electrode during a second duration, the third signal having a second polarity opposite to the first polarity, a third amplitude varying over time; and applying a fourth signal to the backplane during the second duration that is equal to the sum of the first and second amplitudes. The color transition phase includes applying a fifth signal to the front electrode, the fifth signal having the second polarity, a fourth amplitude that varies over time, and a third duration that is subsequent to the first and second durations; applying a sixth signal to the back plate, the sixth signal having the first polarity, a fifth amplitude that varies over time, and a fourth duration that is subsequent to the first and second durations; wherein integrating the sum of the first and second amplitudes that vary over time over the first duration and integrating the sum of the first, second, and third amplitudes that vary over time over the second duration and integrating the fourth amplitude that varies over time over the third duration and integrating the fifth amplitude that varies over time over the fourth duration produces a pulse excursion designed to maintain a DC balance of the display medium during the reset phase and the color transition phase. In some embodiments, the reset phase erases a previous optical property presented on the display. In some embodiments, the color transition phase substantially changes the optical property displayed by the display. In some embodiments, the first polarity is a negative voltage. In some embodiments, the first polarity is a positive voltage. In some embodiments, the pulse excursion is proportional to a kickback voltage experienced by the display medium. In some embodiments, the fourth duration occurs during the third duration. In some embodiments, the third duration begins simultaneously with the fourth duration.

[0040] In another aspect, the present invention includes a method for driving an electrophoretic display having a front electrode, a backplane, and a display medium between the front electrode and the backplane, the display medium including three groups of particles of different colors, the method including subjecting the display to a reset phase and a color transition phase. The reset phase includes applying a first signal to the front electrode, the first signal having a first polarity, a first amplitude that varies over time, and a first duration; no signal is applied to the backplane during the first duration; applying a second signal to the front electrode during a second duration, the second signal having a second polarity opposite to the first polarity, a second amplitude that varies over time; and applying a third signal to the backplane during the second duration, the third signal having the first polarity and a third amplitude that varies over time. The color transition phase includes applying a fourth signal to the front electrode, the fourth signal having the first polarity, a fourth amplitude that varies over time, and a third duration after the first and second durations; applying a fifth signal to the back plate, the fifth signal having the second polarity, a fifth amplitude that varies over time, and a fourth duration after the first and second durations, wherein integrating the sum of the first amplitude that varies over time over the first duration and integrating the sum of the second and third amplitudes that vary over time over the second duration and integrating the fourth amplitude that varies over time over the third duration and integrating the fifth amplitude that varies over time over the fourth duration produces a pulse offset designed to maintain a DC balance of the display medium during the reset phase and the color transition phase. In some embodiments, the reset phase erases a previous optical property presented on the display. In some embodiments, the color transition phase substantially changes the optical property displayed by the display. In some embodiments, the first polarity is a negative voltage. In some embodiments, the first polarity is a positive voltage. In some embodiments, the pulse offset is proportional to a kickback voltage experienced by the display medium. In some embodiments, the fourth duration occurs during the third duration. In some embodiments, the third duration begins simultaneously with the fourth duration.

[0041] In another aspect, the present invention includes a controller for an electrophoretic display, the electrophoretic display including a front electrode, a backplane, and a display medium located between the front electrode and the backplane, the display medium including three groups of particles of different colors, the controller being operably connected to the front electrode and the backplane and configured to implement a reset phase and a color transition phase on the display. The reset phase includes applying a first signal to the front electrode, the first signal having a first polarity, a first amplitude that varies over time, and a first duration; applying a second signal to the backplane during the first duration, the second signal having a second polarity opposite to the first polarity, a second amplitude that varies over time; applying a third signal to the front electrode during the second duration, the third signal having a second polarity opposite to the first polarity, a third amplitude that varies over time; and applying a fourth signal to the backplane during the second duration, the fourth signal being equal to the sum of the first and second amplitudes. The color transition phase includes applying a fifth signal to the front electrode, the fifth signal having the second polarity, a fourth amplitude that varies over time, and a third duration after the first and second durations; applying a sixth signal to the backplane, the sixth signal having the first polarity, a fifth amplitude that varies over time, and a fourth duration after the first and second durations, wherein integrating the sum of the first and second amplitudes that vary over time over the first duration and integrating the sum of the first, second, and third amplitudes that vary over time over the second duration and integrating the fourth amplitude that varies over time over the third duration and integrating the fifth amplitude that varies over time over the fourth duration produces a pulse offset designed to maintain a DC balance of the display medium during the reset phase and the color transition phase. In some embodiments, the controller implements different reset phases depending on the color to be displayed by the electrophoretic display. In some embodiments, the display medium includes white, cyan, yellow, and magenta particles. In some embodiments, the display medium includes white, red, blue, and green particles.

[0042] In another aspect, the present invention includes a controller for an electrophoretic display, the electrophoretic display including a front electrode, a backplane, and a display medium located between the front electrode and the backplane, the display medium including three groups of particles of different colors, the controller being operably connected to the front electrode and the backplane and configured to implement a reset phase and a color transition phase on the display. The reset phase includes applying a first signal to the front electrode, the first signal having a first polarity, a first amplitude that varies over time, and a first duration; no signal is applied to the backplane during the first duration; applying a second signal to the front electrode during a second duration, the second signal having a second polarity opposite to the first polarity, a second amplitude that varies over time; and applying a third signal to the backplane during the second duration, the third signal having the first polarity and a third amplitude that varies over time. The color transition phase includes applying a fourth signal to the front electrode, the fourth signal having the first polarity, a fourth amplitude that varies with time, and a third duration after the first and second durations; applying a fifth signal to the back plate, the fifth signal having the second polarity, a fifth amplitude that varies with time, and a fourth duration after the first and second durations, wherein integrating the sum of the first amplitude that varies with time over the first duration and integrating the sum of the second and third amplitudes that vary with time over the second duration and integrating the fourth amplitude that varies with time over the third duration and integrating the fifth amplitude that varies with time over the fourth duration produces a pulse offset designed to maintain a DC balance of the display medium during the reset phase and the color transition phase. In some embodiments, the controller implements different reset phases depending on the color to be displayed by the electrophoretic display. In some embodiments, the display medium includes white, cyan, yellow, and magenta particles. In some embodiments, the display medium includes white, red, blue, and green particles.

[0043] The electrophoretic medium used in the display of the present invention may be any of those described in the aforementioned application serial number 14 / 849,658. Such a medium includes a light scattering particle that is generally white and three substantially non-light scattering particles. The electrophoretic medium of the present invention may take any of the forms discussed above. Thus, the electrophoretic medium may be unencapsulated, encapsulated in discrete capsules surrounded by a capsule wall, or in the form of a polymer dispersion or micro-unit medium. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 is an exemplary equivalent circuit of a single pixel of an electrophoretic display.

[0045] Figure 2It is a cross-sectional schematic diagram showing the positions of various colored particles in the electrophoretic medium of the present invention when displaying black, white, subtractive primary colors, and additive primary colors.

[0046] Figure 3 is a schematic representation of four different types of pigment particles used in multi-particle electrophoretic media;

[0047] Figure 4 It is a schematic representation of the relative strength of the interaction between pairs of particles in a multi-particle electrophoresis medium;

[0048] Figure 5 It is to show the behavior of various particles in an electrophoretic medium when subjected to electric fields of varying strength and duration;

[0049] Figure 6 is an exemplary waveform including a two-part reset phase (A) and a color transition phase (B);

[0050] Figure 7 is a voltage versus time graph showing the voltage obtained across the electrophoretic medium by the waveform used to produce the color in the driving method of the present invention and the front electrode and the pixel electrode as a function of time;

[0051] Fig. 8A is experimental data showing the color gamut produced using various voltage combinations of the two-part reset phase;

[0052] Figure 8B is a display of the total experimental color gamut that can be obtained by implementing a controller that varies the two-part reset phase according to the desired color;

[0053] Fig. 9 is an embodiment showing a DC balance reset pulse;

[0054] Fig.10 is shown as the electrophoretic particles experience Fig. 9 The DC balance reset pulse. DETAILED DESCRIPTION

[0055] As mentioned above, the present invention can be used with an electrophoretic medium comprising one light scattering particle (usually white) and three other particles providing three subtractive primary colors. Such a system is schematically shown in Figure 2 and it can provide white, yellow, red, magenta, blue, cyan, green and black in each pixel.

[0056] The three particles providing the subtractive primary colors can be substantially non-light scattering ("SNLS"). The use of SNLS particles allows color mixing and can provide more color results than the same number of scattering particles. The aforementioned US8,587,859 uses particles with subtractive primary colors, but requires two different voltage thresholds for independent addressing of non-white particles (that is, the display is addressed with three positive voltages and three negative voltages). These thresholds must be sufficiently separated to avoid crosstalk, and this separation requires the use of high addressing voltages for certain colors. In addition, addressing a colored particle with the highest threshold will also move all other colored particles.

[0057] The particles and these other particles must then be switched to their desired positions at a lower voltage. Such a stepwise color addressing method produces unwanted color flickering and long transition times. The present invention does not require the use of such a stepwise waveform, and addressing of all colors can be achieved with only two positive voltages and two negative voltages as described below (i.e., only five different voltages are required in the display, two positive voltages, two negative voltages and zero voltage, but as described below in certain embodiments, more different voltages may be preferably used to address the display).

[0058] As mentioned above, Figure 2 Schematic cross-sectional view showing the positions of various particles in the electrophoretic medium of the present invention when displaying black, white, subtractive three primary colors and additive three primary colors. Figure 2 In the above, it is assumed that the viewing surface of the display is upward (as shown in the figure), that is, the user views the display from this direction and the light is incident from this direction. As mentioned above, in a preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, and Figure 2 In the spectral image, this particle is considered a white pigment. Basically, this light scattering white particle forms a white reflector, and any particles above the white particle can be seen against the white reflector as a background ( Figure 2 ). Light entering the viewing surface of the display passes through these particles, reflects off the white particles, passes back through these particles, and exits the display. Therefore, the particles above the white particles can absorb various colors, and the color presented to the user is produced by the combination of particles above the white particles. Any particles arranged below the white particles (behind the user's viewing angle) are obscured by the white particles and do not affect the displayed color. Because the second, third and fourth particles are essentially non-light scattering, their order or arrangement relative to each other is not important, but their order or arrangement relative to the white (light scattering) particles is critical for the reasons mentioned above.

[0059] More specifically, when the cyan, magenta, and yellow particles are located below the white particles ( Figure 2In the case [A] in Figure 1, there are no particles above the white particles, so the pixel only displays white. When a single particle is above the white particle, the color of that single particle is displayed, and the pixels are displayed as Figure 2 The yellow, magenta, and cyan colors in cases [B], [D], and [F] are shown. When two particles are above a white particle, the color displayed is a combination of the colors of the two particles; Figure 2 In case [C], the magenta and yellow particles appear red, in case [E], the cyan and magenta particles appear blue, and in case [G], the yellow and cyan particles appear green. Finally, when all three colored particles are above the white particle ( Figure 2 In the case [H]), all incident light is absorbed by the subtractive primary color particles, so the pixel appears black.

[0060] It is possible that subtractive primary colors can be presented by particles that scatter light, so the display includes two types of light scattering particles, where one type of light scattering particles is white and the other type of light scattering particles is colored. However, in this case, the position of the light scattering colored particles relative to the other colored particles overlying the white particles will be important. For example, in presenting black (when all three colored particles are located above the white particles), the scattering colored particles cannot be located above the non-scattering colored particles (otherwise, they will be partially or completely hidden behind the scattering particles, and the color presented will be the color of the scattering colored particles, rather than black).

[0061] If more than one type of colored particles scatters the light, it will not be easy to render black.

[0062] Figure 2 An ideal situation is shown in which the colors are free of contamination (i.e., the light scattering white particles completely obscure any particles behind the white particles). In practice, obstruction by white particles may be incomplete, so particles that are ideally to be completely obscured may absorb some light. Such contamination generally reduces the brightness and chromaticity of the displayed color. In the electrophoretic media of the present invention, such color contamination should be minimized so that the resulting colors meet industry standards for color reproduction. A particularly supported standard is SNAP (Standard for Newspaper Advertisement Production), which specifically specifies L*, a*, and b* values ​​for each of the eight primary colors mentioned above. (Hereinafter, "primary colors" will be used to refer to the eight colors, i.e., Figure 2 Black, white, subtractive primary colors, and additive primary colors are shown.)

[0063] Used for Figure 2Methods for electrophoretically arranging a plurality of differently colored particles in "layers" as shown have been described in the prior art. The simplest method is to cause pigments with different electrophoretic mobilities to "compete"; see, e.g., U.S. Patent No. 8,040,594. Such competition is more complex than might be initially appreciated because the movement of the charged pigments themselves changes the electric field experienced locally in the electrophoretic liquid. For example, as positively charged particles move toward the cathode and negatively charged particles move toward the anode, their charges shield the electric field experienced by the charged particles midway between the two electrodes. It is believed that although pigment competition is involved in the electrophoresis of the present invention, it is not intended as a method of electrophoresis. Figure 2 The reason for the unique phenomenon of the arrangement of particles shown.

[0064] A second phenomenon that can be used to control the motion of multiple particles is hetero-aggregation between different pigment types; see, for example, the aforementioned US 2014 / 0092465. Such aggregation may be charge mediated (Coulomb's law) or may arise due to, for example, hydrogen bonding or van der Waals interactions. The strength of the interaction may be affected by the method of surface treatment of the pigment particles. For example, when the closest distance for oppositely charged particles to approach is maximized by a steric barrier (typically a polymer grafted or adsorbed onto one or both particle surfaces), the Coulomb interaction may be weakened. In the present invention, as described above, such polymer barriers are used for the first and second types of particles, and may or may not be used for the third and fourth types of particles.

[0065] As described in detail in aforementioned application Ser. No. 14 / 277,107, a third phenomenon that can be used to control the motion of multiple particles is voltage or current dependent mobility.

[0066] Figure 3 Schematic cross-sectional views showing four pigment types (1-4) used in preferred embodiments of the present invention. The polymer shell adsorbed onto the core pigment is represented by dark shading, while the core pigment itself is represented by no shading. A variety of shapes can be used for the core pigment: spherical, needle-shaped or other anisometric, aggregates of smaller particles (i.e., "grape bunches"), composite particles comprising small pigment particles or dyes dispersed in a binder, etc. are well known in the art. The polymer shell can be a covalently bonded polymer made by grafting or chemical adsorption methods well known in the art, or can be physically adsorbed on the particle surface. For example, the polymer can be a block copolymer comprising insoluble and soluble segments. Some methods for attaching the polymer shell to the core pigment are described in the examples below.

[0067] In one embodiment of the invention the first and second particle types preferably have a more rigid polymer shell than the third and fourth particle types. The light scattering white particles are of the first or second type (negatively or positively charged). In the following discussion it is assumed that the white particles are negatively charged (i.e., of type 1), but it will be apparent to one skilled in the art that the general principles described will apply to a group of particles in which the white particles are positively charged.

[0068] In the present invention, the electric field required to separate aggregates formed by a mixture of particles of types 3 and 4 in a suspension solvent containing a charge control agent is greater than the electric field required to separate aggregates formed by any other combination of the two types of particles. On the other hand, the electric field required to separate aggregates formed between the first and second types of particles is smaller than the electric field required to separate aggregates formed between the first and fourth particles or the second and third particles (of course, smaller than the electric field required to separate the third and fourth particles).

[0069] Figure 3 The core pigments comprising the particles are shown to be of approximately the same size, and the zeta potential of each particle (although not shown) is assumed to be approximately the same. The thickness of the polymer shell surrounding each core pigment varies. Figure 3 As shown, particles of types 1 and 2 have thicker polymer shells than particles of types 3 and 4 - and this is actually the preferred situation for certain embodiments of the invention.

[0070] In order to understand how the thickness of the polymer shell affects the electric field required to separate the aggregates of oppositely charged particles, it may be helpful to consider the balance of forces between the particle pairs. In practice, the aggregates may be composed of a large number of particles, and the situation will be much more complicated than the case of simple paired interactions. However, the particle pair analysis does provide some guidance for understanding the present invention.

[0071] The force in the electric field acting on one of the pair of particles is given by:

[0072]

[0073] Among them, F App is the force exerted on the particle by the applied electric field, F C is the Coulomb force exerted on the particle by a second particle of opposite charge, F VW is the van der Waals force exerted by the second particle on the particle, F D It is the attractive forces exerted on the particle pairs by depletion flocculation caused by the suspending solvent containing the stabilizing polymer (optional).

[0074] The force F exerted on the particle by the applied electric field AppCalculated by the following formula:

[0075]

[0076] where q is the charge of the particle, which is related to the zeta potential (ζ) as shown in equation (2) (approximately within the Huckel limit), where α is the core pigment radius, s is the thickness of the solvent-swollen polymer shell, and the other symbols have conventional meanings known in the art.

[0077] The magnitude of the force exerted on one particle by another particle due to the Coulomb interaction is approximately given by the following equation for particles 1 and 2:

[0078]

[0079] Note that the F applied to each particle App The force F acts to separate particles, while the other three forces act to attract particles. According to Newton's third law, if F acting on a particle App The force is greater than F acting on the other particle App force (because the charge on one particle is higher than the charge on the other particle), the force separating the pair of particles is composed of two F App It is provided by the weaker of the powers.

[0080] From (2) and (3), it can be seen that the difference between the attractive and the separating Coulomb terms is given by:

[0081]

[0082] If the particles have the same radius and zeta potential, making (α+s) smaller or zeta larger will make the particles more difficult to separate. Therefore, in one embodiment of the present invention, it is preferred that particles of type 1 and 2 are larger and have a relatively low zeta potential, while particles 3 and 4 are smaller and have a relatively large zeta potential.

[0083] However, if the thickness of the polymer shell increases, the van der Waals forces between the particles can also change significantly. The polymer shell on the particle is swollen by the solvent and pushes the surfaces of the core pigments that interact via van der Waals forces further apart. For spherical core pigments whose radius (α1, α2) is much larger than the distance between them (s1+s2),

[0084]

[0085] where A is the Hamaker constant. As the distance between the core pigments increases, the equations become more complicated, but the effect remains the same: increasing s1 or s2 has a significant effect on reducing the attractive van der Waals interactions between the particles.

[0086] With this background, we can understand Figure 3 Logical basis for the particle types shown. Particles of type 1 and 2 have a strong polymer shell swollen by a solvent, which further separates the core pigments and thus reduces the van der Waals interactions between them more than particles of type 3 and type 4, which have a smaller or no polymer shell. Even if the particles have approximately the same size and magnitude of zeta potential, according to the present invention, the strength of the interactions between paired aggregates can be arranged to meet the above requirements.

[0087] about Figure 3 For more details on preferred particles for use in displays, the reader is referred to aforementioned application serial number 14 / 849,658.

[0088] Figure 4 The electric field strength required to separate paired aggregates of the particle types of the present invention is shown in a schematic manner. The interaction between particles of type 3 and 4 is stronger than the interaction between particles of type 2 and 3. The interaction between particles of type 2 and 3 is approximately equal to the interaction between particles of type 1 and 4, but stronger than the interaction between particles of type 1 and 2. All interactions between pairs of particles with the same charge sign are as weak as or weaker than the interaction between particles of type 1 and 2.

[0089] Figure 5 Show how these interactions can be used to form images Figure 2 All primaries are generally discussed (subtractive primaries, additive primaries, black and white).

[0090] When low electric field addressing is used ( Figure 5 (A)), particles 3 and 4 aggregate without separation. Particles 1 and 2 can move freely in the electric field. If particle 1 is a white particle, the color seen from the left is white, and the color seen from the right is black. Reversing the polarity of the electric field can switch between the black state and the white state. However, the instantaneous color between the black state and the white state is colorful. The aggregate of particles 3 and 4 moves very slowly relative to particles 1 and 2 in the electric field. It can be found that particle 2 has moved past particle 1 (to the left), while the aggregate of particles 3 and 4 has not moved significantly. In this case, particle 2 will be seen from the left, while the aggregate of particles 3 and 4 will be seen from the right. In certain embodiments of the present invention, the aggregate of particles 3 and 4 is weakly positively charged and is therefore located near particle 2 at the beginning of such a transition.

[0091] When high electric field addressing is used ( Figure 5(B)), particles 3 and 4 are separated. When viewed from the left, which of particles 1 and 3 (each of which has a negative charge) is visible will depend on the waveform (see below). As shown, particle 3 can be seen from the left, and the combination of particles 2 and 4 can be seen from the right.

[0092] Figure 5 Starting from the state shown in (B), a low voltage of opposite polarity will cause the positively charged particles to move to the left and the negatively charged particles to move to the right. However, positively charged particle 4 will encounter negatively charged particle 1, and negatively charged particle 3 will encounter positively charged particle 2. The result is that the combination of particles 2 and 3 will be seen from the left, while particle 4 can be seen from the right.

[0093] As described above, preferably, particle 1 is white, particle 2 is cyan, particle 3 is yellow, and particle 4 is magenta.

[0094] As is well known in electrophoretic display technology, the core pigment used for white particles is generally a metal oxide having a high refractive index. Examples of white pigments are described in the following examples.

[0095] As described above, the core pigments used to make particles of types 2-4 provide three subtractive primary colors: cyan, magenta, and yellow.

[0096] The electrophoretic fluid of the present invention can be used to construct a display device in several ways known in the art. The electrophoretic fluid can be encapsulated in microcapsules or incorporated into a microcell structure which is then sealed with a polymeric layer. The microcapsules or microcell layer can be coated or embossed onto a plastic substrate or film with a transparent coating of conductive material. This assembly can be laminated to a backplane with pixel electrodes using a conductive adhesive.

[0097] Now let's describe the implementation Figure 2 A first embodiment of a waveform for each particle arrangement is shown. In this discussion, it is assumed that the first particle is white and negatively charged, the second particle is cyan and positively charged, the third particle is yellow and negatively charged, and the fourth particle is magenta and positively charged. Those skilled in the art will understand how the color transition will change if the distribution of the particle colors is changed when one of the first and second particles is assumed to be white. Similarly, the polarity of the charge on all the particles can be reversed, and the electrophoretic medium will still function in the same manner if the polarity of the waveform used to drive the medium (see next paragraph) is also reversed.

[0098] In the following discussion, waveforms (voltage versus time) applied to the pixel electrodes of the display backplane of the present invention are described and plotted assuming that the front electrode is grounded (i.e., at zero potential). The electric field experienced by the electrophoretic medium is of course determined by the potential difference between the backplane and the front electrode, and the distance separating them. The display is usually viewed through its front electrode, so it is the particles adjacent to the front electrode that control the color displayed by the pixel, and it is sometimes easier to understand the optical transitions involved if the potential of the front electrode relative to the backplane is considered; this can be done simply by inverting the waveforms discussed below.

[0099] These waveforms require that each pixel of the display can be addressed by five different addressing voltages (with +V high 、+V low , 0, -V low and -V high In practice, a greater number of addressing voltages may be used. If only three voltages are available (i.e., +V high , 0, and -V high ), then it is possible to have a voltage V high However, pulse addressing with a duty cycle of 1 / n is used to achieve the same performance as at lower voltages (e.g., V high / n, where n is a positive integer greater than 1).

[0100] The waveform used in the present invention may include three phases: a DC balance phase, in which a DC imbalance caused by a previous waveform applied to the pixel or caused by a subsequent color reproduction transition is corrected (as is known in the art); a "reset" phase, in which the pixel is returned to at least approximately the same starting configuration regardless of the pixel's previous optical state; and a "color development" phase described below. Depending on the requirements of a particular application, the DC balance and reset phases are optional and may be omitted. If employed, the "reset" phase may be the same as the magenta color development waveform described below, or may involve driving the maximum possible positive and negative voltages continuously, or may be some other pulse pattern as long as it returns the display to a state where subsequent colors can be reproducibly obtained.

[0101] The general principle of generating eight primary colors (white, black, cyan, magenta, yellow, red, green and blue) using the second driving method suitable for the display of the present invention will now be described (e.g., as shown in FIG. Figure 2Assume that the first pigment is white, the second pigment is cyan, the third pigment is yellow and the fourth pigment is magenta. A person skilled in the art will know that if the distribution of the pigment colors is changed, the color presented by the display will change.

[0102] The maximum positive and negative voltage applied to the pixel electrode (at Figure 6 ±V max to represent) produced by the second and fourth particles (cyan and magenta, to produce blue - see Figure 2 [E]) mixture or a third particle (yellow - see Figure 2 [B] - Colors formed by white pigments scattering light and lying between colored pigments. These blues and yellows are not necessarily the best blues and yellows that can be achieved by the display. The mid-level positive and negative voltages applied to the pixel electrodes (in Figure 6 ±V mid to represent) produce black and white colors respectively (but not necessarily the best black and white achievable by the display - see Figure 5 (A)).

[0103] From these blue, yellow, black or white optical states, the other four primary colors can be obtained by moving only the second particles (in this case, the cyan particles) relative to the first particles (in this case, the white particles), which is achieved using the lowest applied voltage (at Figure 6 ±V min Therefore, the cyan is moved out of the blue (by applying -V min to the pixel electrode) to produce a magenta color (see Figure 2 [E] and Figure 2 [D]); move cyan into yellow (by applying +V min to the pixel electrode) can provide green (see for yellow and green respectively Figure 2 [B] and Figure 2 [G]); move cyan out of black (by applying -V min to the pixel electrode) can provide red (see for black and red respectively Figure 2 [H] and Figure 2 [C]), and move the cyan into the white (by applying +V min to the pixel electrode) can provide cyan (see for white and cyan respectively Figure 2 [A] and Figure 2 [F]).

[0104] While these general principles are useful in the construction of waveforms for producing specific colors in displays of the present invention, in practice the ideal behavior described above may not be observed and modifications to the basic approach may be desirable.

[0105] The general waveforms used to address the color electrophoretic display of the present invention are described in Figure 6 In which the horizontal axis represents time (arbitrary unit) and the vertical axis represents the voltage difference between the pixel electrode and the shared front electrode. Figure 6 The three positive voltages used in the driving method shown may be between approximately +3V and +30V, while the three negative voltages used may be between approximately -3V and -30V. In a preferred embodiment, the highest positive voltage +V max +30V, the middle positive voltage is +V mid 15V, minimum positive voltage +V min is 9V. In a similar manner, the negative voltage -V max , -V mid and -V min In a preferred embodiment, this is -30 V, -15 V and -9 V. It is not necessary for the voltage magnitude |+V|=|-V| to be true for any of the three voltage levels, but this may be preferred in some cases.

[0106] exist Figure 6 There are two distinct phases in the general waveform shown. In the first phase, the max and -V max A pulse (wherein a "pulse" means a unipolar square wave, i.e., a constant voltage applied for a predetermined time) is provided for erasing the previous image presented on the display (i.e., "resetting" the display). The length of these pulses (t1 and t3) and the remaining time (i.e., the zero voltage period between them (t2 and t4)) can be selected so that the entire waveform (i.e., Figure 6 The entire waveform shown (integral of voltage over time) is DC balanced (i.e., the integral of voltage over time is substantially zero). DC balance can be achieved by adjusting the length of the pulse and the remaining time in phase A so that the net pulse provided in this phase has the same amplitude and opposite sign as the net pulse provided in phase B, during which the display is switched to a particular desired color.

[0107] Here, the term "frame" means a single update of all rows in the display. A person skilled in the art will clearly know that in the display of the present invention driven by a thin film transistor (TFT) array, Figure 6The available time increments on the abscissa of will typically be quantified in terms of the frame rate of the display. Likewise, it will be apparent that the display is addressed by varying the potential of the pixel electrode relative to the front electrode, and this can be achieved by varying the potential of either the pixel electrode or the front electrode, or both. In the current state of the art, there is typically a matrix of pixel electrodes on the backplane, with the front electrode being shared by all pixels. Therefore, when the potential of the front electrode is varied, this affects the addressing of all pixels. Regardless of whether a varying voltage is applied to the front electrode, the above Figure 6 The basic structure of the waveforms is the same.

[0108] Figure 6 The general waveform shown requires the drive electronics to provide up to seven different voltages to the data lines during the update of a selected row of the display. Although multi-level source drivers capable of delivering seven different voltages are available, many commercially available source drivers for electrophoretic displays only allow for the delivery of three different voltages (typically, positive, zero, and negative) during a single frame. Figure 6 The universal waveform can be adapted to the 3-level source driver architecture as long as the three voltages supplied to the panel (usually +V, 0 and -V) can be changed from one frame to the next (i.e., so that, for example, in frame n, the voltage (+V max , 0, -V min ), and in frame n+1, a voltage (+V mid , 0, -V max )).

[0109] Sometimes it may be necessary to control an electrophoretic display using a so-called "upper-panel switching" driving method. In the upper-panel switching driving method, the upper-panel common electrode can be switched between -V, 0, and +V, and the voltage applied to the pixel electrode can also be changed from -V, 0 to +V, and when the common electrode is at 0 voltage, the pixel transition in one direction is processed, and when the common electrode is at +V, the pixel transition in the other direction is processed.

[0110] When the upper switch is used in conjunction with a 3-stage source driver, the same as above Figure 6 The general principles described above apply. When the source driver cannot supply the optimal V max At equally high voltages, top-plane switching may be better. Methods of driving electrophoretic displays using top-plane switching are well known in the art.

[0111] Typical waveforms of the (E Ink) prior art are shown below in Table 1, where the numbers in brackets correspond to the frame numbers driven with the indicated backplane voltage (relative to the topplane which is considered to be at zero potential).

[0112] Table 1

[0113]

[0114] During the reset phase of this waveform, pulses of maximum negative and positive voltages are provided to erase the previous state of the display. The number of frames for each voltage is offset by an amount (denoted by Δx for color x) that compensates for the net pulses in the high / mid-level voltage and low / mid-level voltage phases that render the color. To achieve DC balance, Δx is chosen to be half of the net pulse. The reset phase need not be implemented exactly as shown in the table; for example, when top-plane switching is used, a specific number of frames must be allocated to the negative and positive drivers. In such cases, it is preferable to provide the maximum number of high voltage pulses consistent with achieving DC balance (i.e., subtract 2Δx from the negative or positive frame, as appropriate).

[0115] In the high / medium voltage stage, as described above, a sequence of N repetitions of a pulse sequence suitable for each color is provided, where N can be 1-20. As shown, this sequence includes 14 frames, which are allocated with a size of V max or V mid The pulse sequence shown is consistent with the discussion given above. It can be seen that in this phase of the waveform, the pulse sequence that presents the colors white, blue, and cyan is the same. Similarly, in this phase, the pulse sequence that presents yellow and green is the same (because starting from the yellow state to achieve green).

[0116] In the low / medium voltage stage, blue and cyan colors are obtained from white, and green color is obtained from yellow.

[0117] The previous discussion of waveforms, especially the discussion of DC balance, ignores the problem of kickback voltage. In fact, as mentioned earlier, each backplane voltage will be offset from the power supply voltage by an amount equal to the kickback voltage V KB Therefore, if the power supply used provides three voltages +V, 0, and -V, the backplane will actually receive a voltage of V+V KB 、V KB and -V+V KB (Note that in the case of amorphous silicon thin film transistors, V KB (usually negative). However, the same power supply will supply +V, 0, and -V to the front electrode without any kickback voltage offset. So, for example, when the front electrode is supplied with -V, the display will see 2V+V KB The maximum voltage and V KB Instead of using a separate power supply to supply V KB to the front electrode (which can be expensive and inconvenient), the waveform can be divided into positive voltage, negative voltage and VKB Supply the front electrode section.

[0118] As described above, in some of the waveforms described in the aforementioned application serial number 14 / 849,658, seven different voltages can be applied to the pixel electrode: three positive voltages, three negative voltages, and zero voltage. Preferably, the maximum voltage used in these waveforms is higher than the maximum voltage that can be handled by amorphous silicon thin film transistors in the state of the art at the time. In such a case, high voltages can be obtained through the use of upper plate switching, and the drive waveform can be configured to compensate for the kickback voltage and can be substantially DC balanced by the method of the present invention. Figure 7 One such waveform for displaying a single color is schematically depicted. Figure 7 As shown, the waveform for each color has the same basic form: that is, the waveform is essentially DC balanced and may include two parts or stages: (1) a preliminary series of frames used to provide a "reset" of the display to reproducibly achieve any color state and during which a DC imbalance equal and opposite to that of the rest of the waveform is provided, and (2) a series of frames that are specific to the color to be presented; see Figure 6 Parts A and B of the waveform are shown.

[0119] During the first "reset" phase, the resetting of the display ideally erases any memory of the previous state, which includes residual voltages and pigment configurations specific to the previously displayed color. Such erasure is most effective when the display is addressed at the maximum possible voltage during the "reset / DC balance" phase. In addition, enough frames can be allocated in this phase to allow balancing of the most unbalanced color transitions. Because some colors require positive DC balance in the second part of the waveform, while other colors require negative balance, the front electrode voltage V is set to 0.05V for approximately half of the frames in the "reset / DC balance" phase. com Set to V pH (allowing for the maximum possible negative voltage between the backplate and the front electrode), and in the remainder, V com Set to V nH (the maximum possible positive voltage allowed between the backplate and the front electrode). Empirically, it has been found that V com =V pH Frame shape in V com =V nH The frame shape was better before.

[0120] exist Figure 7 The "desired" waveform (i.e., the actual voltage versus time curve expected to be applied to the electrophoretic medium) is described below, and its implementation as top-panel switching is shown above, where the voltage applied to the front electrode (V com) and the potential of the backplane (BP). Assume that the column driver is connected to a power supply capable of supplying the following voltages: V pH 、V nH (maximum positive and negative voltage, usually within ±10-15V range), V pL 、V nL (lower positive and negative voltages, usually in the range of ±1-10V) and zero voltage. In addition to these voltages, the kickback voltage V can also be supplied by an additional power supply KB (a small value specific to the particular backplane used, measured as described in U.S. Patent No. 7,034,783) to the front electrode.

[0121] Figure 7 As shown, each backplane voltage will be offset from the voltage supplied by the power supply by V KB (indicated by negative numbers), except that the front electrode is explicitly set to V KB When , the front electrode voltage does not have this shift.

[0122] Although the display of the present invention has been described as producing eight primary colors, in practice, it is preferred to produce as many colors as possible at the pixel level. Full color grayscale images can then be presented by mixing between these colors using techniques well known to those skilled in the art of imaging. For example, in addition to the eight primary colors produced as described above, the display can be configured to present an additional eight colors. In one embodiment, these additional colors are: light red, light green, light blue, dark cyan, dark magenta, dark yellow, and two grayscales between black and white. The terms "light" and "dark" as used in this context refer to colors that have approximately the same hue angle as a reference color in a color space (e.g., CIE L*a*b*) but have a higher or lower L*, respectively.

[0123] In general, light and dark colors are obtained in the same manner, except that waveforms with slightly different net pulses are used in stages B and C. Thus, for example, the light red, light green, and light blue waveforms have a more negative net pulse than the corresponding red, green, and blue waveforms in stages B and C, while the dark cyan, dark magenta, and dark yellow waveforms have a more positive net pulse than the corresponding cyan, magenta, and yellow waveforms in stages B and C. Variations in net pulse can be achieved by varying the length of the pulses, the number of pulses, or the amplitude of the pulses in stages B and C.

[0124] Gray is usually achieved by a pulse train that oscillates between low or medium voltages.

[0125] Those skilled in the art will clearly know that in the display device of the present invention driven by a thin film transistor (TFT) array, Figure 7 The available time increment on the abscissa of will typically be quantified by the frame rate of the display. Likewise, it will be apparent that the display is addressed by varying the potential of the pixel electrode relative to the front electrode, and this can be achieved by varying the potential of either the pixel electrode or the front electrode, or both. In the current state of the art, there is typically a matrix of pixel electrodes on the backplane, with the front electrode being shared by all pixels. Therefore, when the potential of the front electrode is varied, this affects the addressing of all pixels. Whether or not a varying voltage is applied to the front electrode, the above Figure 7 The basic structure of the waveforms described is the same.

[0126] Figure 7 The general waveform shown requires the drive electronics to provide up to seven different voltages to the data lines during the update of a selected row of the display. Although multi-level source drivers capable of delivering seven different voltages are available, many commercially available source drivers for electrophoretic displays only allow for the delivery of three different voltages (typically, positive, zero, and negative) during a single frame. The term "frame" here means a single update of all rows in the display. Figure 7 The universal waveform can be adapted to the 3-level source driver architecture as long as the three voltages supplied to the panel (usually +V, 0 and -V) can be changed from one frame to the next (i.e., so that, for example, in frame n, the voltage (+V max , 0, -V min ), and in frame n+1, a voltage (+V mid , 0, -V max )).

[0127] Reference now Figure 6 , it can be seen from phase A (reset phase) that this phase is divided into two parts of equal duration (indicated by the dashed lines). When switching using the upper plate, the upper plate is maintained at one potential in the first of these parts and at a potential of opposite polarity in the second part. Figure 6 In the specific case of P H, while the backplane will remain at V n H, so that the entire electrophoresis fluid has V n HV P During the second part, the top plate will be held at V n H, while the backplane remains at V p H. As shown, during the second portion, the electrophoretic fluid will be subjected to V pHV n H potential, which is the highest potential available. However, for some color re-rendering, exposure to such high voltages may result in such an initial pigment arrangement that it is difficult to achieve the desired final configuration. For example, as described in the prior art, in order to render cyan, it is necessary to constrain the magenta pigment (which has the same charge polarity as the cyan pigment) with the yellow pigment in an aggregate. Such an aggregate will be separated by the high applied potential, so the magenta will not be controlled and will contaminate the cyan.

[0128] However, it is not necessary to use the maximum possible voltage in both parts of waveform phase A. All that is required for phase A is that the previous color state is erased so that the newly presented color is the same regardless of which color came first, and that the net pulse provided in phase A balances the net pulse in phase B.

[0129] Therefore, an experiment was performed in which phase B of a waveform of the type shown in Table 1 was held constant, while the voltage applied in each of the two parts of phase A was varied (but in each case the same number of frames was allocated to phase A: 120 frames in total, 60 frames in the first part, and 60 frames in the second part). After addressing the display, the CIELab L*, a* and b* values ​​for each primary color were measured.

[0130] Table 2 shows the default case, where the maximum possible negative and positive voltages are applied in the first and second parts of phase A. This can be accomplished using top-plane switching, where the voltages in the first column are applied to the backplane and the voltages in the second column are applied to the topplane. The color gamut, measured as the volume of the convex hull containing the eight points listed in Table 2, is 21,336 ΔE 3 .

[0131] Table 3 shows the case where the backplane is held at zero voltage during the first part of phase A. The applied voltage is lower in this case than in Table 2. The applied voltage in the second part of phase A is the same as in Table 2. The application time of the lower voltage must of course be relatively long in order to maintain DC balance. The color gamut, measured as the volume of the convex hull encompassing the eight points listed in Table 2, is 20,987 ΔE 3 .

[0132] Table 4 shows the case where the backplane is held at zero voltage during the second part of phase A. The voltage applied during the first part of phase A is the same as in Table 2. The color gamut, measured as the volume of the convex hull containing the eight points listed in Table 2, is 20,339 ΔE 3 .

[0133] Table 2

[0134] First reset voltage Second reset voltage color L* a* b* <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> K 24.67 2.68 -12.53 <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> B 37.26 0.97 -14.51 <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> R 43.2 16.16 11.34 <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> M 43.56 21.93 -10.65 <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> G 36.29 -19.89 13.13 <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> C 48.34 -9.82 -6.73 <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> Y 67.99 -10.29 56.06 <![CDATA[V n H–V p H]]> <![CDATA[V p H–V n H]]> W 70.29 -1.24 7.83

[0135] Table 3

[0136] First reset voltage Second reset voltage color L* a* b* <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> K 27.82 2.2 -15.78 <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> B 37.99 0.41 -14.78 <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> R 43.7 17 11.4 <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> M 44.02 22.03 -10.39 <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> G 37.37 -21.57 13.38 <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> C 49.06 -9.96 -7.78 <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> Y 67.73 -10.25 53.71 <![CDATA[0–V p H]]> <![CDATA[V p H–V n H]]> W 70.02 -0.99 6.7

[0137] Table 4

[0138] First reset voltage Second reset voltage color L* a* b* <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> K 27.42 -4.03 -10.77 <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> B 31.99 -7.38 -11.16 <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> R 46.19 8.49 21.11 <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> M 47.46 12.8 -3.05 <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> G 33.33 -24.63 11.2 <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> C 43.03 -19.38 -9.32 <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> Y 67.21 -9.44 59.36 <![CDATA[V n H–V p H]]> <![CDATA[0–V n H]]> W 70.12 -3.49 14.26

[0139] Fig. 8A The results of these experiments are shown as projections on the a* / b* plane: the abscissa represents a* and the ordinate represents b*. It can be seen that some colors (e.g., red, magenta, and blue) appear better through the stage A settings corresponding to Table 2 or 3, while other colors (cyan, green, and yellow) appear better through the stage A settings corresponding to Table 4.

[0140] Interestingly, an alternative experiment in which the order of the first and second parts of phase A was reversed gave very poor results, with all colors being contaminated by yellow.

[0141] The best color combination for this experiment is given in Table 5. The color gamut, measured by the volume of the convex hull containing the eight points listed in Table 2, is 28,092 ΔE 3 Thus, by appropriately selecting the voltage applied during the reset phase (phase A) of the waveform, the color gamut is increased by approximately 50%. The results of Table 5 are plotted in Figure 8B middle.

[0142] The method of the invention is particularly important when it is desirable to make the waveform as short as possible. With a fixed voltage in phase A, phase B needs to be made longer in order to compensate for the deviation introduced in phase A for certain colors.

[0143] Although the present invention is described using only two sections in phase A, those skilled in the art will appreciate that any reasonable number of sections may be used. However, when upper plate switching is used, the same configuration of upper plate potentials is fixed regardless of which color is to be presented. In accordance with the present invention, the backplane settings corresponding to each upper plate potential may vary in phase A of the waveform depending on the color to be presented, but without violating the condition that the overall waveform including phases A and B is DC balanced.

[0144] Table 5

[0145] First reset voltage Second reset voltage color L* a* b* <![CDATA[V n H-V p H]]> <![CDATA[V p H-V n H]]> K 24.67 2.68 -12.53 <![CDATA[0-V p H]]> <![CDATA[V p H-V n H]]> B 37.99 0.41 -14.78 <![CDATA[0-V p H]]> <![CDATA[V p H-V n H]]> R 43.7 17 11.4 <![CDATA[0-V p H]]> <![CDATA[V p H-V n H]]> M 44.02 22.03 -10.39 <![CDATA[V n H-V p H]]> <![CDATA[0-V n H]]> G 33.33 -24.63 11.2 <![CDATA[V n H-V p H]]> <![CDATA[0-V n H]]> C 43.03 -19.38 -9.32 <![CDATA[V n H-V p H]]> <![CDATA[0-V n H]]> Y 67.21 -9.44 59.36 <![CDATA[0-V p H]]> <![CDATA[V p H-V n H]]> W 70.02 -0.99 6.7

[0146] Resetting the DC balance of the pulse can be achieved by:

[0147] For the DC balance reset process, a set of voltages must be selected for all transitions in the waveform. Selecting a set of voltages can be problematic because some palette colors require high voltages and others require low voltages. For devices with a large number of simultaneous backplane voltages, this is not a problem because each transition can be balanced individually, but in the case of upper-plane switching, each transition is tied together through the upper-plane, which forces the transitions to align with each other. An additional restriction is imposed by the source driver standard, which currently limits the number of simultaneous backplane voltages to three.

[0148] The conversion is a sequence of voltages applied to the backplane and topplane. in, is the backplane voltage used to switch j at frame i, is the upper plate voltage at frame i. Before implementing DC balance reset, let T j The total pulses, where n j It is T j The update length (in frames), and V KB is the kickback voltage of the display.

[0149] Let σ j is the desired DC balanced pulse offset (time*V), d r is the expected total duration of the DC balance reset. The DC balance reset has two pulses in it, so the upper plate voltage needs to be selected for each pulse, and the backplane voltage needs to be selected for each pulse and each transition. Let For conversion T j The voltage of the kth pulse, where is used to convert T j The backplane voltage of the kth reset pulse, and is the upper plate voltage for the kth reset pulse. It is important to choose the two pulse voltages so that and For each transition there is an opposite sign.

[0150] You need to choose a "zero" voltage, which would ideally be 0V, but this is not always possible

[0151]

[0152] in,

[0153]

[0154] Next, calculate the overall maximum duration of each of the two pulse waves

[0155]

[0156]

[0157] Then, the "ideal" duration of each pulse wave for each conversion is calculated, which is The duration of the case. Define the notation then,

[0158]

[0159]

[0160] We then decompose each pulse into a "valid" part and a "zero" part in order to balance the transitions:

[0161]

[0162]

[0163]

[0164]

[0165]

[0166] Now we are ready to construct the DC balance reset phase of the waveform. To drive a duration Then To drive a duration Fig. 9 As shown, for each conversion T j , we use Drive for a duration Then Drive for a duration Then with Drive for a duration Then Drive for a duration The resulting waveform experienced by the ink is shown in Fig.10 middle.

[0167] At first glance, it would appear that the successive scanning of different rows of an active matrix display might upset the above calculations designed to ensure accurate DC balance of the waveform and drive method, because when the voltage of the front electrode is changed (usually between successive scans of the active matrix), each pixel of the display will experience an "incorrect" voltage until the scan reaches the relevant pixel and the voltage on its pixel electrode is adjusted to compensate for the change in the front electrode voltage, and the period between the change in frontplate voltage and the time when the scan reaches the relevant pixel varies depending on the row in which the relevant pixel is located. However, further investigation will show that the actual "error" in the pulse applied to the pixel is proportional to the change in frontplate voltage multiplied by the period between the change in frontplate voltage and the time when the scan reaches the relevant pixel. Assuming that the scan rate does not change, the latter period is fixed, so that for any series of changes in the frontplate voltage that make the final frontplate voltage equal to the initial frontplate voltage, the sum of the "errors" of the pulses will be zero and will not affect the overall DC balance of the drive method.

[0168] Therefore, the present invention provides a DC balanced waveform for a multi-particle electrophoretic display. Having described several aspects and embodiments of the present invention technology, it will be understood that a person skilled in the art of the invention will easily think of various changes, modifications and improvements. Such changes, modifications and improvements are intended to fall within the spirit and scope of the technology described in the present application. For example, a person skilled in the art of the invention will easily think of various other means and / or structures for performing functions and / or obtaining the results and / or one or more advantages described herein, and each such change and / or modification is considered to be within the scope of the embodiments described herein. Those familiar with the art will recognize or be able to determine many equivalents of the specific embodiments described herein using only routine experiments. Therefore, it will be understood that the aforementioned embodiments are presented only by way of example, and within the scope of the attached claims and their equivalents, the invention embodiments may be implemented in a manner different from the specific description. In addition, if such features, systems, objects, materials, kits and / or methods are not inconsistent with each other, any combination of two or more features, systems, objects, materials, kits and / or methods described herein is included within the scope of the present application.

Claims

1. A method for providing a DC balance reset pulse for an electrophoretic display, the electrophoretic display having a front electrode, a backplane including a first pixel electrode and a second pixel electrode, and a display medium located between the front electrode and the backplane, the display medium including three groups of particles of different colors, the method comprising: applying a first signal to the front electrode, the first signal having a first polarity, a first amplitude and a first duration; No signal is applied to the first pixel electrode during the first duration; applying a second signal to the second pixel electrode during the first duration, the second signal having a second polarity opposite to the first polarity and a second amplitude; applying a third signal to the front electrode during a second duration, the third signal having the second polarity and a third amplitude; applying a fourth signal to the first pixel electrode during the second duration, the fourth signal having the first polarity and a fourth amplitude; as well as No signal is applied to the second pixel electrode during the second duration.

2. The method of claim 1 further comprising applying a pulse excursion to the first pixel electrode during the first duration that is proportional to a kickback voltage experienced by the display medium.

3. The method of claim 1, further comprising applying a pulse offset to the second pixel electrode during the second duration that is proportional to a kickback voltage experienced by the display medium.

4. The method according to claim 1, wherein: The first polarity is a negative voltage. 5 . The method according to claim 1 , further comprising applying a fifth signal to the first pixel electrode during the second duration, the fifth signal having the second polarity and a fifth amplitude. 6 . The method according to claim 1 , further comprising applying a sixth signal to the second pixel electrode during the first duration, the sixth signal having the first polarity and a sixth amplitude.

7. A controller for an electrophoretic display, the electrophoretic display comprising a front electrode, a backplane comprising a first pixel electrode and a second pixel electrode, and a display medium located between the front electrode and the backplane, the display medium comprising three groups of particles of different colors, the controller being operably connected to the front electrode, the first pixel electrode and the second pixel electrode, the controller being configured to apply a DC balance reset pulse to the display, The DC balance reset pulse wave comprises: applying a first signal to the front electrode, the first signal having a first polarity, a first amplitude and a first duration; No signal is applied to the first pixel electrode during the first duration; applying a second signal to the second pixel electrode during the first duration, the second signal having a second polarity opposite to the first polarity and a second amplitude; applying a third signal to the front electrode during a second duration, the third signal having the second polarity and a third amplitude; applying a fourth signal to the first pixel electrode during the second duration, the fourth signal having the first polarity and a fourth amplitude; as well as No signal is applied to the second pixel electrode during the second duration.

8. The controller of claim 7, wherein the DC balance reset pulse further comprises applying a pulse offset proportional to a kickback voltage experienced by the display medium to the first pixel electrode during the first duration.

9. The controller of claim 7, wherein the DC balance reset pulse further comprises applying a pulse offset proportional to a kickback voltage experienced by the display medium to the second pixel electrode during the second duration.

10. The controller according to claim 7, wherein: The first pixel electrode and the second pixel electrode have different color states before the DC balance reset pulse starts.

11. The controller according to claim 7, wherein: Before the DC balance reset pulse starts, the first pixel electrode and the second pixel electrode have the same color state.

12. The controller according to claim 7, wherein: The display medium includes cyan, yellow and magenta particles.

13. The controller according to claim 7, wherein: The display medium includes red, blue and green particles.

14. The controller according to claim 7, wherein: The DC balance reset pulse erases previous optical properties present on the display.

Images