WO2007135594A1

WO2007135594A1 - Electrophoretic display devices

Info

Publication number: WO2007135594A1
Application number: PCT/IB2007/051739
Authority: WO
Inventors: Evgueni Boiko
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-05-16
Filing date: 2007-05-09
Publication date: 2007-11-29
Also published as: TW200802228A

Abstract

A method of driving an electrophoretic display device uses a reset phase in which half of the pixels in each row are reset to one extreme display state (e.g. black) and the other half of the pixels in each row are reset to the other extreme display state (e.g. white). This reduces the maximum voltage shift which can be capacitively coupled to the rows by means of the row/column crossovers, during the reset phase or the pixel drive phase.

Description

Electrophoretic display devices

This invention relates to electrophoretic display devices.

Electrophoretic display devices are one example of bistable display technology, which use the movement of particles within an electric field to provide a selective light scattering or absorption function.

In one example, white particles are suspended in an absorptive liquid, and the electric field can be used to bring the particles to the surface of the device. In this position, they may perform a light scattering function, so that the display appears white. Movement away from the top surface enables the color of the liquid to be seen, for example black. In another example, there may be two types of particle, for example black negatively charged particles and white positively charged particles, suspended in a transparent fluid. There are a number of different possible configurations.

It has been recognized that electrophoretic display devices enable low power consumption as a result of their bistability (an image is retained with no voltage applied), and they can enable thin display devices to be formed as there is no need for a backlight or polarizer. They may also be made from plastics materials, and there is also the possibility of low cost reel-to-reel processing in the manufacture of such displays.

If costs are to be kept as low as possible, passive addressing schemes are employed. The most simple configuration of display device is a segmented reflective display, and there are a number of applications where this type of display is sufficient. A segmented reflective electrophoretic display has low power consumption, good brightness and is also bistable in operation, and therefore able to display information even when the display is turned off. However, improved performance and versatility is provided using a matrix addressing scheme. An electrophoretic display using passive matrix addressing typically comprises a lower electrode layer, a display medium layer, and an upper electrode layer. Biasing voltages are applied selectively to electrodes in the upper and/or lower electrode layers to control the state of the portion(s) of the display medium associated with the electrodes being biased.

Another type of electrophoretic display device uses so-called "in plane switching". This type of device uses movement of the particles selectively laterally in the display material layer. When the particles are moved towards lateral electrodes, an opening appears between the particles, through which an underlying surface can be seen. When the particles are randomly dispersed, they block the passage of light to the underlying surface and the particle color is seen. The particles may be colored and the underlying surface black or white, or else the particles can be black or white, and the underlying surface colored. An advantage of in-plane switching is that the device can be adapted for transmissive operation, or trans flective operation. In particular, the movement of the particles creates a passageway for light, so that both reflective and transmissive operation can be implemented through the material. This enables illumination using a backlight rather than reflective operation. The in-plane electrodes may all be provided on one substrate, or else both substrates may be provided with electrodes.

Active matrix addressing schemes are also used for electrophoretic displays. Monochrome electrophoretic display systems are used for electronic reading devices, whilst color versions are being developed for signage and billboard display applications, and as (pixilated) light sources in electronic window and ambient lighting applications.

This invention can be applied to all of these various implementations, and concerns the driving signals used to drive the display. Electrophoretic displays are typically driven by complex driving signals. For a pixel to be switched from one grey level to another, often it is first switched to white or black as a reset phase and to then to the final grey level. Grey level to grey level transitions and black/white to grey level transitions are slower and more complicated than black to white, white to black, grey to white or grey to black transitions.

This reset operation is used in order to achieve satisfactory grey level accuracy. Typically, the display pixels are reset to either the positive or the negative rail depending on the final image (i.e. to either black or white). The pixel is reset to black if the target grey level is closer to black than to white, and vice versa. This results in a visually more attractive image transition compared to the more simply example of always resetting to black or to white, because the result of such a reset sequence is to produce momentarily a black and white image of the final greyscale image. It has also been proposed to provide additional control signals before the transition to the final grey level, to implement a so-called shaking phase, which functions as a preparatory drive phase before the particles are moved to implement new grey levels. This is used in order to speed up the subsequent grey level transition phase, and to reduce the dependency of the response of the display to the previous history.

Further discussion of known drive schemes can be found in WO 2005/071651 and WO 2004/066253.

Conventionally, data signals (including preset, reset and drive phase signals) are provided to column conductors, and row selection signals are provided to row conductors. These are otherwise known as gate signals, as they are applied to TFT gates in active matrix arrays. This invention is based on the recognition that the reset phase provides a significant cause of parasitic voltage coupling from the columns to the gate (row) driver circuit.

According to the invention, there is provided a method of driving an electrophoretic display device, comprising an array of rows and columns of display pixels, the method comprising: resetting the display pixels to a reset condition in which each pixel is driven to one of two extreme display states; and driving the display pixels from their respective reset condition to the desired grey level for an image to be displayed, wherein in the reset condition, approximately half of the pixels in each row are reset to one extreme display state and the other approximately half of the pixels in each row are reset to the other extreme display state. By providing each row with approximately equal numbers of pixels reset to each extreme drive state (black or white), this method reduces the maximum voltage shift which can be capacitively coupled to the rows by means of the row/column crossovers, when the columns undergo transitions between signals for different rows in the reset or drive phases. This method thus can be considered to provide the advantages associated with inversion drive schemes of active matrix liquid crystal displays. However, electrophoretic materials respond to the polarity of an applied driving voltage, so that conventional inversion driving schemes as applied to liquid crystal displays cannot be applied to electrophoretic displays. All of the display pixels can be reset before any display pixels are driven to the desired grey levels, and this reset phase may comprise multiple frames, with each frame comprising the resetting of pixels row by row.

A presetting (shaking) drive phase may be used before the resetting of the display pixels, during which the display pixels are driven alternately to the two extreme display states during the presetting drive phase. This presetting phase may also comprise multiple frames, with each frame comprising the driving of pixels row by row.

In one embodiment, in the reset condition, the display pixels are reset to a checkerboard pattern of the two extreme display states. With sufficiently small pixels, the pattern may not be discernible to the viewer, and the reset phase then becomes less visually disturbing than a reset to black or white. Alternatively, in the reset condition, the display pixels can be reset to a random or pseudo-random pattern of the two extreme display states. The invention also provides an electrophoretic display device, comprising: an array of rows and columns of display pixels; and control means for supplying drive signals to the pixels to drive the pixels to predetermined optical states corresponding to an image to be displayed, wherein the control means comprises a row driver and a column driver, wherein the control means is adapted to: reset the display pixels to a reset condition in which each pixel is driven to one of two extreme display states; and drive the display pixels from their respective reset condition to the desired grey level for an image to be displayed, wherein in the reset condition, approximately half of the pixels in each row are reset to one extreme display state and the other approximately half of the pixels in each row are reset to the other extreme display state. The invention also provides a control circuit for an electrophoretic display device which comprises an array of rows and columns of display pixels, the control circuit comprising a row driver and a column driver for supplying drive signals to the pixels to drive the pixels to predetermined optical states corresponding to an image to be displayed, and adapted to: reset the display pixels to a reset condition in which each pixel is driven to one of two extreme display states; and drive the display pixels from their respective reset condition to the desired grey level for an image to be displayed, wherein in the reset condition, approximately half of the pixels in each row are reset to one extreme display state and the other approximately half of the pixels in each row are reset to the other extreme display state.

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows schematically one type of device to which the invention can be applied; Fig. 2 shows in schematic form the electric circuit control for the device of

Fig. 1;

Fig. 3 shows an example of voltage transitions of the column conductors which can arise in known addressing schemes;

Fig. 4 shows a first example of voltage transitions of the column conductors which can arise in the addressing scheme of the invention;

Fig. 5 shows a second example of voltage transitions of the column conductors which can arise in the addressing scheme of the invention; and

Fig. 6 shows the steps of the addressing method of the invention.

The same references are used in different Figures to denote the same layers or components, and description is not repeated.

The invention provides a method of driving an electrophoretic display device, using a reset phase in which half of the pixels in each row are reset to one extreme display state (e.g. black) and the other half of the pixels in each row are reset to the other extreme display state (e.g. white). This reduces the maximum voltage shift which can be capacitively coupled to the rows by means of the row/column crossovers, when the columns undergo transitions between signals for different rows in the reset or drive phases.

Before describing the invention in more detail, one example of the type of display device to which the invention can be applied will be described briefly.

Figure 1 diagrammatically shows a cross section of a portion of an electrophoretic display device 1, for example of the size of a few display elements, comprising a base substrate 2, an electrophoretic film with an electronic ink which is present between two transparent substrates 3,4 for example polyethylene. One of the substrates 3 is provided with transparent picture electrodes 5 and the other substrate 4 with a transparent counter electrode 6.

The electronic ink comprises multiple micro capsules 7, of about 10 to 50 microns. Each micro capsule 7 comprises positively charged white particles 8 and negatively charged black particles 9 suspended in a fluid F. When a positive field is applied to the picture electrode 5, the white particles 8 move to the side of the micro capsule 7 directed to the counter electrode 6 and the display element become visible to a viewer.

Simultaneously, the black particles 9 move to the opposite side of the microcapsule 7 where they are hidden to the viewer. By applying a negative field to the picture electrodes 5, the black particles 9 move to the side of the micro capsule 7 directed to the counter electrode 6 and the display element becomes dark to a viewer (not shown). When the electric field is removed, the particles 8,9 remain in the acquired state and the display exhibits a bi-stable character and consumes substantially no power.

Figure 2 shows diagrammatically an equivalent circuit of a display device 1 incorporating the display pixels of Figure 1, and comprising an electrophoretic film laminated on the base substrate 2 provided with active switching elements, a row driver 16 and a column driver 10. Preferably, the counter electrode 6 is provided on the film comprising the encapsulated electrophoretic ink, but it could be alternatively provided on a base substrate in the case of operation using in-plane electric fields. The display device 1 is driven by active switching elements, in this example thin film transistors 19. The display thus comprises a matrix of display elements 18 at the area of crossing of row (selection) electrodes 17 and column (data) electrodes 11.

The row driver 16 consecutively selects the row electrodes 17, while a column driver 10 provides a data signal to the column electrode 11. Preferably, a processor 15 firstly processes incoming data 13 into the data signals. Mutual synchronization between the column driver 10 and the row driver 16 takes place via drive lines 12. Select signals from the row driver 16 select the pixel electrodes 22 via the thin film transistors 19 whose gate electrodes 20 are electrically connected to the row electrodes 17 and the source electrodes 21 are electrically connected to the column electrodes 11. A data signal present at the column electrode 11 is transferred to the pixel electrode 22 of the display element coupled to the drain electrode via the TFT. In the embodiment shown, an additional capacitor 23 is provided at the location at each display element 18, and is connected to one or more storage capacitor lines 24. Instead of TFTs, other switching elements can be applied such as diodes, MIMs devices, etc.

This is only one example of transverse field active matrix device, and many other configurations are possible, including in-plane switching arrangements as well as passive matrix devices.

As outlined above, a known addressing scheme comprises:

(i) A plurality of frames of "shaking" pulses, with each row driven alternately from positive to negative in sequential frames. The frame time is not sufficient for the electrophoretic particles to move to the position corresponding to the drive state, and this functions only as a preset phase.

(ii) A plurality of frames during which a reset signal is applied to each pixel (reset phase).

(iii) A plurality of frames of drive signals to produce the desired grey level (drive phase). After the reset phase, if the desired grey level corresponds to the state of the pixel in its reset condition, the bistability of the electrophoretic material allows a voltage of OV to be applied, and this enables a reduction in the power consumption of the addressing scheme.

This invention concerns the reset phase. In order to explain the invention, it is useful to analyze the set of voltages to be applied to the columns which provide the worst possible capacitive coupling to the row conductors.

Figure 3 shows the column voltages which can result from an addressing scheme in which the pixels are all reset to one extreme optical state. In the example shown, a row of light pixels is followed by a row of dark pixels. Thus, the pixels in row n are all driven to +15 V, representing white (for the purpose of this example). The addressing of the next row corresponds to black, and a 30V voltage step in the same direction is applied to all columns at the same time, as shown in region 30. Some of this voltage swing is capacitively coupled to the row lines.

This 30V swing can be partially coupled to the row conductor during the transition between frames in the reset phase or during the transition between frames in the drive phase.

The invention ensures that the reset voltage is not the same for all columns.

Figure 4 shows the reset phase voltages for two successive rows in accordance with the invention. For each row address period, half the columns are reset to the +15V rail and the other half are reset to the -15 V rail. In the example shown, the reset pattern alternates for successive rows, with the result that the reset pattern will be a checkerboard pattern across the display.

The transitional image is then a mid-grey field, provided that the pixels are sufficiently small for the checkerboard pattern to be integrated by the human eye. As can be seen in Figure 4, the transition 30 between reset patterns for sequential rows gives rise to equal and opposite voltage fluctuations coupled to the row lines.

Figure 5 shows an example of the column voltages which can result from an addressing scheme of the invention, in particular at the transition between frames in the reset phase, or at the transition between frames in the drive phase. Again, Figure 5 takes the worst possible example for the row lines, and assumes the use of a OV holding voltage for the case when the column is already at the desired state. It is no longer possible for all columns to have a step voltage change by the maximum rail to rail voltage.

During the resetting phase, the worst case as represented in Figure 5 is that half of all columns move to one of the rails (the step changes from +15 V to -15V shown) whilst the other half of the columns do not change voltage at all. This represents the case where the pixels are already at the desired state and accordingly the columns are held at OV. The inverse situation will arise in the next row.

For a typical picture, the reset frame(s) will have a number of columns moving in both directions thus minimizing the effect on the row electrodes.

During the drive phase, the worst possible case is again that half of all columns move to one of the rails whilst the other half of the columns do not change voltage at all.

It will thus be seen that the invention limits the capacitive coupling to the row electrode and this in turn enables the specification of the gate driver to be relaxed, resulting in smaller circuits and possibly lower power consumption from a reduced supply voltage range.

Figure 6 shows the sequence of operations of the addressing scheme of the invention. In phase 60, the plurality of frames of "shaking" pulses are applied forming a preset phase, with each row driven alternately from positive to negative in sequential frames. In reset phase 62, the reset signal of the invention is applied to each pixel for a plurality of frames, and in drive phase 64 a plurality of frames of drive signals are applied to produce the desired grey level. As explained above, the reset phase may comprise a checkerboard pattern of opposite extreme drive signals (e.g. +15V and -15V).

The invention is applicable to electrophoretic displays generally, and may particularly be applied to flexible displays where the gate driver is integrated on the flexible active matrix substrate (e.g. in amorphous silicon, organic technology, low temperature polysilicon etc).

The term "grey level" has been used above to indicate an intermediate drive state between the extreme optical states. Color implementations can use multiple colored particles which are independently driven, and the term "grey level" should of course not be understood as excluding an intermediate drive state for colored particles. Color displays can be implemented using other approaches, for example color filters or colored backlights.

Electrophoretic display systems can form the basis of a variety of applications where information may be displayed, for example in the form of information signs, public transport signs, advertising posters, pricing labels, billboards etc. In addition, they may be used where a changing non- information surface is required, such as wallpaper with a changing pattern or color, especially if the surface requires a paper like appearance. The display may also be used as a light source.

The specific design of the pixels and the row and column drivers has not been described in detail, as this will be known to those skilled in the art. Further details can be found in the references cited above, and these are incorporated herein by way of reference material.

The number of frames used to form the different phases of the addressing sequence can be selected in routine manner in order to obtain the desired pixel response to the control signals applied. Only one example of reset pattern has been given, in the form of a checkerboard pattern. However, many other patterns are possible which ensure that half the columns in a row have one polarity and the other half have the opposite polarity. Furthermore, there is no absolute requirement for the reset pattern to have exactly half the columns of each drive state. For example, a random or pseudo random pattern may be generated.

In prior art systems, in which the reset pattern is dependent on the image data, it is of course possible that the reset pattern of this invention could be generated in response to corresponding image data. However, the reset pattern described above is independent of the image data. The reset pattern is thus applied in a repeating sequence irrespective of the image data.

The reset pattern can invert between successive addressing phases, although this is not essential. As explained above, the reset pattern preferably provides an equal number of the two extreme drive conditions to the pixels in each row (namely 50% and 50%). This may deviate from exactly 50%, for example preferably within the range <75% and >25% and more preferably with the range <60% and >40%. A pseudo random pattern can be generated with constraints to ensure the desired ratio is met. The extreme optical states may also be at or near the black and white drive levels, for example the drive voltage for an extreme display state may be at least 80% of the maximum magnitude drive voltage, and preferably at least 90% or even 95%. The term "extreme display state" should thus be understood in this context.

One example of display has been given with row and columns in a particular orientation. The orientation is however somewhat arbitrary. The row is in the example given the conductor to which the pixel address signal is applied and the column is the conductor to which the data signal is applied. These may be switched around, and it should therefore be understood that a "row" may run from top to bottom, and a "column" may run from side to side. The scope of the claims should be understood accordingly. Various modifications will be apparent to those skilled in the art.

Claims

CLAIMS:

1. A method of driving an electrophoretic display device, comprising an array of rows and columns of display pixels, the method comprising: resetting the display pixels to a reset condition in which each pixel is driven to one of two extreme display states; and driving the display pixels from their respective reset condition to the desired grey level for an image to be displayed, wherein in the reset condition, approximately half of the pixels in each row are reset to one extreme display state and the other approximately half of the pixels in each row are reset to the other extreme display state.

2. A method as claimed in claim 1, wherein all of the display pixels are reset before any display pixels are driven to the desired grey levels.

3. A method as claimed in claim 2, wherein the display pixels are reset row by row.

4. A method as claimed in any preceding claim, further comprising a presetting drive phase before the resetting of the display pixels, wherein the display pixels are driven alternately to the two extreme display states during the presetting drive phase.

5. A method as claimed in claim 4, wherein the display pixels are driven row by row during the presetting drive phase.

6. A method as claimed in any preceding claim, wherein in the reset condition, the display pixels are reset to a checkerboard pattern of the two extreme display states.

7. A method as claimed in any one of claims 1 to 5, wherein in the reset condition, the display pixels are reset to a random or pseudo-random pattern of the two extreme display states.

8. An electrophoretic display device, comprising: an array of rows and columns of display pixels; and control means for supplying drive signals to the pixels to drive the pixels to predetermined optical states corresponding to an image to be displayed, wherein the control means comprises a row driver and a column driver, wherein the control means is adapted to: reset the display pixels to a reset condition in which each pixel is driven to one of two extreme display states; and drive the display pixels from their respective reset condition to the desired grey level for an image to be displayed, wherein in the reset condition, approximately half of the pixels in each row are reset to one extreme display state and the other approximately half of the pixels in each row are reset to the other extreme display state.

9. A device as claimed in claim 8, wherein the control means is further adapted to apply a presetting drive phase before the resetting of the display pixels, wherein the display pixels are driven alternately to the two extreme display states during the presetting drive phase.

10. A device as claimed in claim 8 or 9, wherein the control means is adapted, in the reset condition, to reset the display pixels to a checkerboard pattern of the two extreme display states.

11. A device as claimed in claim 8, 9, or 10, comprising an active matrix electrophoretic display device.

12. A control circuit for an electrophoretic display device which comprises an array of rows and columns of display pixels, the control circuit comprising a row driver and a column driver for supplying drive signals to the pixels to drive the pixels to predetermined optical states corresponding to an image to be displayed, and adapted to: reset the display pixels to a reset condition in which each pixel is driven to one of two extreme display states; and drive the display pixels from their respective reset condition to the desired grey level for an image to be displayed, wherein in the reset condition, approximately half of the pixels in each row are reset to one extreme display state and the other approximately half of the pixels in each row are reset to the other extreme display state.

13. A circuit as claimed in claim 12, further adapted to apply a presetting drive phase before the resetting of the display pixels, wherein the display pixels are driven alternately to the two extreme display states during the presetting drive phase.