Driving of electrowetting display device

A method of driving an electrowetting display device including at least one pixel, the method comprising: applying a first pixel voltage to the pixel during a first portion of the display period; and applying a second pixel voltage to the pixel during a second portion of the display period, the first and second pixel voltages corresponding to a display state of the pixel, and the first pixel voltage and the second pixel voltage having different polarities.

BACKGROUND

A known electrowetting display device has a plurality of picture elements or pixels and a method of driving a pixel. The pixel includes a first, non-conducting fluid and a second, conductive fluid. The position of the first fluid in the pixel is controlled by the applied voltage and causes a display effect providing a display state of the pixel. When driving the pixel, the polarity of the voltage applied to a pixel changes between each subsequent frame or scan period. The known method, combined with application of an alternating voltage on the second fluid and a constant voltage on one of the plates of a storage capacitor of the pixel, is stated to solve the problem of a dependence of a threshold voltage of the pixel on the specific display device and first fluid used therein. The known display device operated by the known method shows an unsatisfactory display of images.

It is desirable to provide a method for operating an electrowetting display device having an improved display of images.

DETAILED DESCRIPTION

The contents of the following applications are incorporated by reference herein:

Embodiments described herein relate to a method of driving an electrowetting display device, a display driving system adapted for using the method and a display apparatus including a display driving system and an electrowetting display device.

In accordance with first embodiments, there is provided a method of driving an electrowetting display device including at least one pixel, the method comprising the following steps for maintaining a display state of the pixel during a display period:applying a first pixel voltage to the pixel during a first portion of the display period, the first pixel voltage corresponding to the display state; andapplying a second pixel voltage to the pixel during a second portion of the display period, the second pixel voltage corresponding to the display state and the first pixel voltage and the second pixel voltage having different polarities.

The images displayed by the known display device are unsatisfactory in that the display state is not maintained at longer display periods, i.e. at low frame rates, in spite of the fact that the pixel voltage is kept at the same level during the display period. The method according to embodiments inverts the polarity of the pixel voltage during a display period. The inversion of the polarity during the display period improves the maintenance of the display state during the display period.

A pixel voltage corresponds to a desired display state if a prolonged application of the pixel voltage to a pixel results in the pixel showing the display state relating to display input data. The desired display state is usually obtained after a short settling time; the settling time for common electrowetting pixels is of the order of a few milliseconds and usually less than 5 ms. A display period is the period during which the desired display state is to be maintained; its duration is equal to a frame for images displayed in frames.

When the display period of a display device is relatively long, e.g. 100 ms or 1 s, the gradual deterioration of the display state can be countered by inverting the polarity of the voltage two or more times during the display period, thereby maintaining substantially the same display state of the pixel. The method may also include an inversion of the polarity at the start of a display period. The ability to maintain the display state during long display periods makes embodiments suitable for driving an electrowetting display device at low frequencies.

Other embodiments relate to a method of driving an electrowetting display device including at least one pixel for providing a display state in response to a pixel voltage applied to the pixel, the pixel voltage corresponding to display input data, the method comprising the following steps:applying a first pixel voltage during a first period for providing a first display state using a first conversion from display input data to pixel voltage magnitude; andapplying a second pixel voltage during a second period for providing a second display state using a second, different conversion from display input data to pixel voltage magnitude, the first pixel voltage and the second pixel voltage having different polarities.

The images displayed by the known display device are unsatisfactory in that the display state of a pixel has been discovered to be different when being driven with a positive pixel voltage and a negative pixel voltage of the same magnitude. Hence, subsequent frames show different display states when driven by the same magnitude of the pixel voltage. The method according to embodiments compensates for this difference by using different conversions from display input data to magnitude of pixel voltage for positive and negative pixel voltages. The magnitude of a voltage is its absolute value or modulus. The two periods may be consecutive. Embodiments also relate to a display driving system using two conversions.

Further embodiments relate to a method of driving an electrowetting display device includingat least one pixel having a first electrode and a storage capacitor anda second electrode, the storage capacitor being directly connected between the first electrode and the second electrode,the method comprising the following steps:applying a first pixel voltage between the first electrode and the second electrode during a first period for providing a first desired display state; andapplying a second pixel voltage between the first electrode and the second electrode during a second period for providing a second desired display state, the first pixel voltage and the second pixel voltage having different polarities.

The pixel may include a first support plate and a second support plate and a space between the first support plate and the second support plate, the space including at least one first fluid and a second fluid immiscible with each other, the second fluid being electroconductive or polar. The first electrode may be arranged in the first support plate and the second electrode is in contact with the second fluid. The desired display state is a display state to be maintained during a display period.

The images displayed by the known display device are unsatisfactory in that the desired display state is not maintained during a display period. This is caused in part by a storage capacitor in the pixel being connected to the first electrode and to a constant reference voltage. According to embodiments, the storage capacitor is not connected to a constant reference voltage but to the second electrode, thereby assuring that the desired display state is maintained during the display period. Embodiments further relate to a display apparatus having said connection of the storage capacitor.

Embodiments also relate to a display driving system for driving an electrowetting display device having an input for display input data and an output for providing a pixel voltage and a processor for converting the input data to the pixel voltages, wherein the processor is adapted for carrying out the method according to embodiments.

Examples of embodiments will now be described in detail.

FIG. 1shows a schematic cross-section of part of an electrowetting display device1. The display device includes at least one picture element or pixel2, one of which is shown in the Figure. The lateral extent of the pixel is indicated in the Figure by two dashed lines3,4. The display device comprises a first support plate5and a second support plate6. The support plates may be separate parts of each pixel, but the support plates may be shared in common by the plurality of pixels. The support plates may include a glass or polymer substrate7,8and may be rigid or flexible.

The display device may be of the transmissive, reflective or transflective type. The display device may be of a segmented display device type in which the image is built up of segments and each segment may include one or more pixels. The display device may be an active matrix driven type or a passively driven type. The plurality of pixels may be monochrome. Alternatively, for a full colour display device, the pixels shown in the Figure may be sub-pixels, each sub-pixel having a different colour; alternatively, a different individual pixel may be able to show different colours using colour filters and/or coloured fluids.

A space11between the support plates includes two fluids: a first fluid12and a second fluid13, wherein the fluids may, for example, be liquids. The second fluid is immiscible with the first fluid. The second fluid is electrically conductive or polar, and may be, for example, water or a salt solution such as a solution of potassium chloride in a mixture of water and ethyl alcohol. The second fluid may be transparent, but may be coloured. The first fluid is electrically non-conductive and may, for instance, be an alkane like hexadecane or (silicone) oil. The first fluid absorbs at least a part of the optical spectrum. The first fluid may be transmissive for a part of the optical spectrum, forming a colour filter. For this purpose the first fluid may be coloured by addition of pigment particles or dye. Alternatively, the first fluid may be black, i.e. absorbing substantially all parts of the optical spectrum.

A hydrophobic layer14is arranged in the support plate5and may be transparent or reflective. The hydrophobic layer may be an uninterrupted layer extending over a plurality of pixels2, as shown in the Figure, or it may be an interrupted layer, each part extending only over one or more pixels2. The layer may be for instance an amorphous fluoropolymer layer such as AF1600 or another low surface energy polymer. The hydrophobic character causes the first fluid12to adhere preferentially to the first support plate5.

The first support plate5includes a first electrode15for each pixel. The electrode is separated from the fluids by an electrically insulating cover layer, which may be the hydrophobic layer14or an additional insulating layer, not shown in the Figure. Further layers may be arranged between the hydrophobic layer and the electrode. The first electrode15can be of any desired shape or form and in this example is planar; it is made of an electrically conducting material and may be transparent or reflective. The electrodes of neighbouring pixels are separated by an insulating layer16. The first electrode15is connected to a circuit17, schematically indicated in the Figure, for supplying the first electrode with a voltage. A capacitor18is part of the circuit.

A second electrode19is in contact with the conductive second fluid13; this electrode may pertain to one pixel or, as in the embodiment of the Figure, may be common to a plurality of or all pixels. The display state of the pixel2can be controlled by a pixel voltage Vpapplied between the first electrode15and the second electrode19. The electrode19and, for each pixel, the electrode15are each coupled to a display driving system. In a display device having the elements arranged in a matrix form, the electrodes can be coupled to a matrix of control lines in the first support plate.

The first fluid12is confined to a single pixel by walls20that follow the extent of the pixel. The walls may extend from the first to the second support plate but may also extend partly from the first support plate to the second support plate. Although the walls are shown as structures of the first support plate5that protrude from the planar surface of the hydrophobic layer14, they may instead be a surface layer of the first support plate that repels the first fluid, such as a hydrophilic layer. The extent of the pixel, indicated by the dashed lines3and4, is defined by the centre of the walls20. The area of the hydrophobic layer14between the walls of a pixel is a display area22over which a display effect occurs. The display area22lies in the plane of a surface23of the hydrophobic layer14.

When no voltage is applied between the electrodes, the first fluid12forms a layer between the walls20, as shown in the Figure. This layer may have a typical thickness of 4 micrometer. The layer of second fluid13may have a typical thickness in the order of 50 micrometer. A typical size of the display area22is 160 micrometer by 160 micrometer. Application of a voltage between the first electrode15and the second electrode18will contract the first fluid, for example against a wall as shown by the dashed shape21in the Figure. The shape of the first fluid is controllable by controlling the applied voltage, and is used to operate the picture element as a light valve, providing a display effect over the display area22. Further details of features of the display device are described in international patent publication no. WO2003/071346, the contents of which is incorporated herein by way of reference.

FIG. 2shows a schematic circuit diagram of a display apparatus25including a display driving system and an electrowetting display device having an active matrix configuration. Display input data26represents images to be displayed on the display device. The data is input to a controller27, which processes the data. It outputs a signal28connected to the electrode19, common to all pixels of the display device. The signal28determines the voltage applied to the second fluid13. The signal28may be a DC signal or an AC signal.

The controller27also outputs a control signal29with timing information for controlling the gates of the pixels. The control signal29is connected to a display row driver30, which, in dependence on the control signal29sends gate pulses over gate control lines31,31′, also called row control lines. The display driver also includes a driver stage for each gate control line.

The controller27further outputs a signal33representing the display states for each of the pixels. A controller34switches the signal33between two outputs35and36, depending on the display cycle, as will be explained below. Output35is connected to a first convertor37and output36to a second convertor38. The convertors convert the display input data to voltages corresponding to the voltage to be applied to a pixel, i.e. the pixel voltage, to show the display state corresponding to a current value of the display input data. The conversion takes into account the response characteristics of a pixel. Since the characteristics of the pixel may depend on the polarity of the pixel voltage, the conversion carried out by the first convertor37for a first polarity of the pixel voltage differs from the conversion carried out by the second convertor38for a second polarity inverse to the first polarity. A combiner39combines the outputs of the two convertors to a single control signal40. Each convertor may have the form of a look-up table. The elements34,37,38and39may be combined in a single convertor having two look-up tables, the selection between the two tables being made in dependence on the display cycle.

The control signal40is connected to a display column driver41, which includes a distributor for distributing the voltages in the control signal40over source control lines42,42′, also called column control lines. The display column driver also includes a driver stage for each source control line.

FIG. 2also shows a circuit diagram of four pixels in the display device, arranged in a matrix configuration. Circuit diagrams for further pixels can be added in a known way. The pixels in the Figure are arranged in horizontal rows and vertical columns.

The circuit of the pixel connected to the control lines31and42includes a TFT50having a gate51connected to one of the gate control lines31, a source52connected to one of the source control lines42, and a drain53connected to an element54, drawn as a capacitor. The lower plate of the capacitor is the electrode15of the pixel2inFIG. 1. The second fluid13is represented by the top plate of the capacitor. The sharing of the second fluid between pixels in the embodiment ofFIGS. 1 and 2is represented by a connection of the top plate of the capacitor of all pixels in the matrix. The top plate is connected to the electrode19and set by signal28. The voltages on the gate control line31and the source control line42are Vgand Vs, respectively. The electrode19is at a common voltage Vc. The pixel voltage, i.e. the voltage across the capacitor54, is Vp.

In an embodiment of the display device, the circuit also includes a storage capacitor55, one plate of which is connected to the drain53and the other plate is connected to the corresponding plates of the other pixels by storage capacitor lines56in the first support plate5. The storage capacitor increases the time a pixel can hold a voltage. In a further embodiment the storage capacitor lines56are directly connected to the second electrode19. The phrase ‘directly connected’ means that the top plate of the storage capacitor55and the second fluid13have substantially the same voltage during operation of the display device.

The display apparatus shown inFIG. 2comprises a display driving system and a display device. The display driving system includes a display controller with the elements27,34,37,38and39and a display driver with the display row driver30and the display column driver41. The display device includes elements51to56. The display driving system may be integrated on the first support plate5of the display device. The elements27,34,37,38and39may be implemented in one or more processors.

The operation of the display apparatus will now be explained with reference toFIG. 3. It shows voltages V for a pixel inFIG. 2as a function of time t for two frames F1and F2. A frame is a still image of a video to be displayed. The frame rate refers to the speed at which the image is refreshed. At the start of frame F1the display controller will address all pixels in the matrix of the display device within a sub-frame SF1from time t1to t4and load the pixels with pixel voltages pertaining to the image of frame F1of the display input date26to be displayed. In a subsequent sub-frame SF2from time t4to t7the same pixel voltages with inverted polarity are loaded. In the following sub-frame SF3, from t7to the end of frame F1, t8, the pixel voltages are held in the pixels. In the next frame, F2, the next image is displayed.

FIG. 3shows the voltages for a pixel connected to the control lines31and42inFIG. 2. During the first sub-frame SF1the voltage applied to the second fluid Vcis negative, e.g. −15 V and during the second sub-frame SF2positive, e.g. +15 V. Vcmay be zero or any other voltage such as +15 V or −15 V during the third sub-frame SF3.

The source voltage VsinFIG. 3is the voltage put on the column control line42for providing a pixel voltage for each pixel in the column of the matrix, i.e. one pixel voltage for each row or line. For sake of clarity, the Figure shows only ten voltages, which would correspond to ten lines. In an actual display device there may be for example 480 rows and 640 columns. When in an embodiment the addressing of a line takes 10 microseconds, the duration of sub-frames SF1and SF2will be 4.8 ms each; the duration of sub-frame SF3will be 15.2 ms for a frame duration of 20 ms corresponding to a frame rate of 50 Hz.

InFIG. 3the pixel connected to control lines31and42will close the TFT switch50during the time t2to t3a pulse is on the row control line31, corresponding to the second row of the matrix. The instantaneous voltage Vspresent on the column control line42in the period t2-t3will be set on the electrode15and, if a storage capacitor is present, on a plate of the storage capacitor55.

The pixel voltage Vp, i.e. the voltage applied between the electrode15and the second fluid13, is equal to Vs−Vc. If for example Vsis −10 V and Vcis −15 V, Vpis +5 V. At time t3, i.e. the end of the gate pulse, the TFT switch50opens and the capacitors54and55will be floating. Hence, the voltage on the capacitor54is maintained, except for current leakage. The fluids in the pixel will adjust their configuration to the voltage Vp. The adjustment typically takes a few milliseconds. Hence, the display state of the pixel will be shown within the sub-frame SF1of the sub-frame has a length of 4.8 ms.

The voltage Vpwill be maintained on the pixel till the pixel is addressed at time t5in the second sub-frame SF2. The change of the common voltage Vcat t4will not affect Vp, because the capacitor54is floating at that moment in time. Since the storage capacitor55, if present, is connected in parallel to the capacitor54, a change in Vcwill not affect the pixel voltage. In contrast, the storage capacitor of a known display device is not connected to the electrode19but to a constant voltage. Hence, any change in the common voltage will affect the pixel voltage in the known display device.

At time t5the row control line31is addressed again and the TFT passes the instantaneous voltage Vson the electrode15. At this moment the changed voltage Vctakes effect and will influence the pixel. If for example Vsis +10 V and Vcis +15 V, Vpis Vs−Vc, which is −5 V. If the configuration of the fluids in the space11of the pixel are not sensitive to the polarity of the pixel voltage, the same display state will be maintained in sub-frames SF1and SF2.

During the third sub-frame SF3the TFT50is open and the pixel voltage Vpwill be maintained. Vpmay change when the pixel is addressed again at time t9. Hence, the display state is maintained during a first display period DP1, i.e. from t2to t9. SF3is therefore a hold stage, in which no pixels of the row are addressed. At t9a second display period DP2starts. The duration of a display period DP is the same as the duration of a frame F; the display period and frame coincide for the first row of the matrix and are shifted in time for the other rows.

A similar time diagram can be drawn for the next pixel in the row connected to control line31; however, the source voltages Vswill be different from the ones shown inFIG. 3. The time diagram for the pixel in the next row is also similar toFIG. 3but with the pulse Vgmoved one position to the right, starting at t3instead of t2.

The second frame F2inFIG. 3has also a polarity inversion of Vpat time t12. In addition, the polarity in the Figure changes at the start of the second frame, at time t9. A third pixel voltage Vpin a last portion of the display period DP1(t7-t9), −5 V in the Figure, is changed to a positive fourth voltage in the first portion of the display period DP2(t9-t12), the fourth voltage corresponding to the display state to be shown by the pixel in display period DP2. The two portions may be consecutive, but may also be separated by an intermediate period. The polarity change improves the response of the pixel in frame F2.

The display period DP1inFIG. 3shows a pixel voltage Vpthat can be represented as: + − h, i.e. positive voltage, negative voltage, hold stage. The display period may alternatively have for example: − + h, + h − h, − h + h, + − + h, − + − h, etc. Additionally, the polarity at the transition from one display state to the next may remain the same or change.

If the configuration of the fluids depends on the polarity of the pixel voltage, the pixel voltages in the sub-frames SF1and SF2must be set at a different magnitude to compensate for the dependence. The dependence may relate to a difference in offset and/or a different shape of response curve of the pixel, i.e. the display effect versus pixel voltage curve. The offset compensates for a difference in threshold voltage between the response curve of the pixel for positive and negative pixel voltages. For example a pixel voltage of +5.0 V in SF1may give the same display state as −5.1 V in SF2. The different magnitudes can be set by using converter37(seeFIG. 2) in SF1and converter38in SF2. They provide different gamma corrections for positive and negative pixel voltages by applying a different shape of a pixel voltage versus input data curve. This provides an improved solution to offset differences compared to a known solution.

The converters37and38may also take into account the so-called kickback, in which a change in Vgaffects Vpthrough a parasitic gate-drain capacitor of the TFT50. Methods for compensation of the kickback are described in inter alia patents U.S. Pat. No. 6,392,626 and U.S. Pat. No. 7,834,837.

The compensation for the polarity dependence of the fluids configuration can be used in any display device using inversion of the pixel voltage. It can be used in embodiments where the polarity is inverted only at the start of a frame, i.e. in the first sub-frame SF1, or both at the start of a frame and during the frame.

The embodiment of the method shown inFIG. 3has a frame Fl including two consecutive sub-frames SF1and SF2in which the rows of the matrix are being scanned and the polarity of the pixel voltage is inverted, and subsequently a hold period SF3without scanning The first pixel voltage set during SF1is applied to the pixel for a first portion t2to t5of the display period t2-t9. The second pixel voltage with inverse polarity and set during SF2, is applied to the pixel for a second portion t5to t9. InFIG. 3the first portion is shorter than the second portion; the first portion has the duration of a scan period, i.e. the period necessary for subsequently addressing all lines of an active matrix display device.

Alternatively, the frame F1may include the sub-frame SF1, followed by a hold period, the sub-frame SF2, and another hold period. In this case, the first portion and the second portion can be equal. The first and second portions are also equal where the durations of the first and second sub-frames is equal to the duration of the frame.

In the embodiment of the method shown inFIG. 3there is a polarity inversion in the first sub-frame SF1at the start of the frame F1and in the second sub-frame SF2during the frame F1. Alternatively, the first sub-frame may use the same polarity of the pixel voltage as at the end of the preceding frame and sub-frame SF2inverts the polarity during the frame. For longer durations of the frame the number of polarity inversions in a frame may be made larger than two, e.g. three, four or more.

The use of a voltage Vcalternating periodically around zero volt as in the embodiment ofFIG. 3has the effect that a swing of 30 V of the pixel voltage can be achieved with a circuit operating at −15-+15V power supply, which has a lower manufacturing cost than a driving circuit operating at 0-30 V power supply. Another effect is the lower power consumption of the circuit. Alternatively, the display device may be operated using a voltage Vcthat alternates between 0 V and a positive voltage, e.g. 30 V, or between 0V and a negative voltage, e.g. −30 V, or using a DC voltage for Vc. In these alternative cases the voltages Vsmust be changed accordingly.

Other embodiments relate to a display device having storage capacitors55in the pixels connected in parallel with the electrode—second fluid capacitors54as shown inFIG. 2. This display device may be driven using a method in which the polarity is changed one or more times in a frame or once in two or more consecutive frames. The display device may be driven with or without the dual converters37and38shown inFIG. 2.

Further embodiments relate to a display apparatus having the dual converters37and38. This display apparatus may be driven using a method in which the polarity is changed between a first period and a second period. The first and second period may be portions of a single display period or may be display periods. The display device may have storage capacitors connected as shown inFIG. 2.

The polarity inversion of the pixel voltage appears to have an effect on maintenance of the display state during a display period similar to a known reset pulse technique. The polarity inversion requires fewer scans addressing the rows than a reset pulse: a polarity inversion requires one scan and a reset pulse requires two scans. Hence, a polarity inversion can be carried out faster than a reset pulse. Moreover, polarity inversion requires less energy than reset pulses, because the charge of a pixel is changed twice for a reset pulse and only once for a polarity inversion. Note, that the voltage applied to a pixel during a reset pulse is usually a maximum or a minimum voltage; it is not a voltage corresponding to a display state maintained during a display period. The duration of a reset pulse must be short enough to make any change in display state invisible. In contrast, the sub-frames in which the polarity is changed may have a duration long enough to make the change in display state visible.