JP5533115B2 - Electrophoretic display device - Google Patents

Description

  The present invention relates to a display device for driving a display panel that performs display using an electrophoretic display material in which charged particles are dispersed in a dispersion solvent, a display panel driving method, and the like. In particular, the present invention relates to a display device and a display panel driving method for increasing the contrast when using a display panel that performs segment driving.

  2. Description of the Related Art A reflective display device driven by a segment is known when displaying information such as numbers and characters on electronic price tags / shelf labels and table clocks (see, for example, Patent Documents 1 and 2). In particular, in a display device using an electrophoretic display material (electrophoretic display device), unlike a display device using liquid crystal, a polarizing plate is not required, and a memory that can maintain a display even when power supply is stopped. Display is possible. For this reason, the electrophoretic display device has a feature of wide viewing angle and low power consumption, and is widely applied to fields such as electronic shelf labels and digital signage.

  In general, an electrophoretic display material is composed of one or more kinds of charged particles that are positively or negatively charged and dispersed in a dispersion solvent. For this reason, in order to drive an electrophoretic display device using an electrophoretic display material, a DC voltage is applied, and charged particles migrate in the direction of the electrode according to the polarity of the voltage. For example, when the charged particles are composed of two types of negatively charged white charged particles and positively charged black charged particles, when a voltage is applied, the white charged particles and the black charged particles are Run in the opposite direction. As a result, the distribution of white charged particles and black charged particles changes, and when observed from the transparent electrode side, if the distribution density of white charged particles near the transparent electrode is high, “white” is displayed. Conversely, if the distribution density of the black charged particles near the transparent electrode is high, “black” is displayed. Thus, by controlling the distribution state of the charged particles near the transparent electrode by selecting the polarity of the voltage, the contrast of the monochrome display can be controlled. The same applies when the charged particles are composed of one kind of charged particles that are positively or negatively charged. However, in this case, the display can be performed by coloring the dispersion solvent with a dye or the like in a color different from the color of the charged particles. Further, when the dispersion solvent is transparent, display can be performed by coloring the side of the electrode facing the transparent electrode.

  Thus, by applying a DC voltage to the electrodes, the distribution state of, for example, white charged particles and black charged particles can be controlled according to the polarity of the voltage. In particular, in segment driving used for electronic shelf labels and the like, driving is performed by applying a DC voltage.

JP-A 52-70791 JP 2009-69411 A

  However, when the inventor of the present application has studied the conventional electrophoretic display technology, it has been found that there are the following problems. That is, in a conventional general electrophoretic display material, it takes usually several hundred milliseconds to apply voltage until the display is completed by causing charged particles to migrate by applying a DC voltage. . This length of time causes a problem that it is difficult to obtain a display without an afterimage. Further, when the DC voltage is repeatedly applied, a minute separation region is generated in the white display and black display portions, which causes a problem of being visually recognized as display unevenness.

  Therefore, the present invention provides an electrophoretic display device that solves the above-described problems and obtains good display characteristics.

As one embodiment of the present invention, an electrophoretic display material is sandwiched between two electrodes, and a display panel capable of displaying a first color having the maximum reflectance and a second color having the minimum reflectance , after applying a voltage pulse between the two electrodes, have a control circuit for applying a write voltage pulse between at least two times the two electrodes, at least two times the control circuit between the two electrodes applied The sum of the time widths of the write voltage pulses is the time required to apply a DC voltage to the electrophoretic display material and change the color displayed on the display panel from the first color to the second color. Less than a certain response time, after the alternating voltage pulse is applied between the two electrodes, the write voltage pulse is applied between the two electrodes at least twice, and then the reflectivity and the first color in the second color The ratio of the definitive reflectance discloses an electrophoretic display device comprising 20 or more.

In another embodiment of the present invention, an electrophoretic display material is sandwiched between two electrodes, and an electrophoretic display capable of displaying a first color having a maximum reflectance and a second color having a minimum reflectance. A method for driving a panel, wherein an alternating voltage pulse is applied between the two electrodes, and then a write voltage pulse is applied at least twice between the two electrodes, and is applied at least twice between the two electrodes. The total time width of the write voltage pulse is the time required to apply a DC voltage to the electrophoretic display material and change the color displayed on the electrophoretic display panel from the first color to the second color. Less than the response time, and after applying the alternating voltage pulse between the two electrodes, the write voltage pulse is applied between the two electrodes at least twice, and the reflectance in the second color and the The ratio of the reflectance in one color is characterized by comprising 20 or more, discloses a driving method of the electrophoretic display panel.

  According to the disclosure of the present invention, display can be performed with high contrast. In addition, writing time can be shortened by applying an alternating voltage pulse and two or more writing voltage pulses.

1 is a functional block diagram of an electrophoretic display device according to an embodiment of the present invention. 1A and 1B are a schematic view of a cross section of a display panel and an example of an appearance of a display panel according to an embodiment of the present invention. It is an example figure for demonstrating the response time of the electrophoretic display material which concerns on one Embodiment of this invention. It is a flowchart explaining the process of the electrophoretic display device which concerns on one Embodiment of this invention. It is an example figure of the pattern of the voltage applied to the electrode of the display panel of the electrophoretic display device which concerns on one Embodiment of this invention. It is a graph which shows the change of the black reflectance and contrast with respect to the application frequency of the writing voltage pulse in the electrophoretic display device which concerns on one Embodiment of this invention. 4 is a graph showing changes in black reflectance and contrast with respect to a time width of a write voltage pulse in an electrophoretic display device according to an embodiment of the present invention. 6 is a graph showing changes in black reflectance and contrast with respect to a pause time width of a write voltage pulse in an electrophoretic display device according to an embodiment of the present invention. It is a graph which shows the change of the black reflectance and contrast with respect to the application frequency of the alternating voltage pulse in the electrophoretic display device which concerns on one Embodiment of this invention.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described as embodiments with reference to the drawings. Note that the present invention is not limited to the embodiments described below. The present invention can be implemented by adding various modifications to the embodiments described below.

  FIG. 1 illustrates a functional block diagram of an electrophoretic display device according to an embodiment of the invention. The electrophoretic display device 100 includes a display panel 101 and a control circuit 102.

  The display panel 101 has an electrophoretic display material sandwiched between two electrodes. Examples of the electrophoretic display material include materials containing charged particles. In this case, the charged particles are dispersed in the dispersion solvent of the electrophoretic display material. As the material of the charged particles, colored or colorless (or white) inorganic pigment particles or organic pigment particles can be used.

  FIG. 2A is a diagram schematically showing a cross section between two electrodes of the display panel 101. A substrate electrode 202 and a pixel electrode 203 facing the substrate electrode 202 are disposed on the substrate 201 of the display panel 101. An electrophoretic display material is sandwiched between the substrate electrode 202 and the pixel electrode 203. In FIG. 2A, charged particles 204 and 205 of two colors are dispersed in a dispersion solvent of an electrophoretic display material. The electrophoretic display material may be enclosed in a capsule or, for example, may be enclosed in a lattice-shaped partition wall. Further, when the electrophoretic display material is sealed in a capsule, one or a plurality of the capsules may be disposed between the substrate electrode 202 and the pixel electrode 203. In the following description, in FIG. 2A, the substrate electrode 202 and the substrate 201 are made of a transparent material so that the display can be seen from the substrate electrode 202 side. And Further, the substrate electrode 202 and the pixel electrode 203 may be referred to as “two electrodes”.

  The charged particles are charged by electric charges. Charged particles of the same color are charged to the same sign, and charged particles of different colors are charged to different signs. Therefore, for example, if the white charged particle 204 is negatively charged and the black charged particle 205 is positively charged, the charged particle 204 is made higher by making the potential of the substrate electrode 202 higher than the potential of the pixel electrode 203. Moves to the substrate electrode 202 side, and the charged particles 205 move to the pixel electrode 203 side. Thus, when the electrophoretic display material is observed from the substrate electrode 202 side, it appears white. Further, when a potential having a reverse polarity is applied to the substrate electrode 202 and the pixel electrode 203, the electrophoretic display material is observed black when viewed from the substrate electrode 202 side.

  FIG. 2B illustrates an example of a plan view when part of the display panel 101 is viewed from the pixel electrode 203 side. In the example of FIG. 2B, seven pixel electrodes 203 having the shape of the pixel portion 206 are arranged so as to draw a letter “8” in order to display the numbers 0 to 9. By appropriately applying a voltage between the pixel electrode 203 corresponding to each pixel portion 206 and the substrate electrode 202, characters including the numbers 0 to 9 are displayed. The substrate electrode 202 may be disposed in common with respect to a plurality (seven) of pixel electrodes 203, or each substrate electrode 202 may be disposed with respect to each pixel electrode 203. .

  The control circuit 102 applies an alternating voltage pulse between the two electrodes, and then applies a write voltage pulse between the two electrodes at least twice. Here, the alternating voltage is a potential at which a positive voltage and a negative voltage alternately alternate at a predetermined frequency, and generally, the absolute values of the positive voltage and the negative voltage are substantially equal. The alternating voltage is, for example, a potential at 10 Hertz, in which 15 V and −15 V are alternately repeated. However, the absolute values of the positive voltage and the negative voltage may be different. The alternating voltage may be applied to all of the pixel electrodes almost simultaneously. The alternating voltage pulse is an alternating voltage that changes in time in a pulse shape. Here, the frequency of the alternating voltage is preferably in the range of 10 to 200 hertz, and more preferably in the range of 20 to 100 hertz. If the frequency of the alternating voltage is small, for example, less than 10 Hz, it may be visually recognized that flicker is displayed on the display panel, and display quality may be impaired. In addition, since the ionic substance that is an additive contained in the dispersion solvent of the electrophoretic display material responds even at a high frequency of about several tens of kilohertz, the frequency of the alternating voltage is high. For example, when the frequency exceeds 200 Hz, the charged particles Although there is an effect of improving the dispersibility of the charged particles, the followability of the charged particles to the alternating voltage is reduced. It may be difficult to obtain.

  In one embodiment of the present invention, by applying the alternating voltage pulse, the time width of one write voltage pulse can be set to a time width that is equal to or shorter than the response time of the electrophoretic display material. In addition, the sum of the time widths of the write voltage pulse applied at least twice after the application of the alternating voltage pulse can be set to a time width that is equal to or less than the response time of the electrophoretic display material.

  The “response time” of the electrophoretic display material is defined as follows. That is, a first DC voltage is applied between the two electrodes sandwiching the electrophoretic display material, and the displayed color is set to the first color (for example, white), and then the first DC voltage is set. Cut off. And after letting a state where there is no voltage difference between the two electrodes for a certain time or longer, a second DC voltage is applied between the two electrodes, and the color displayed on the one electrode side is Transition to the second color (for example, black). At this time, the time until the reflectance of the electrophoretic display material sandwiched between the two electrodes takes the maximum value or the minimum value by applying the second DC voltage is referred to as “response time”. Here, the reflectance refers to the reflectance of the color displayed by the electrophoretic display material. In other words, the time until the color displayed by the electrophoretic display material transitions from the first color to the second color by applying the second DC voltage is referred to as “response time”.

  The reason why the control circuit 102 applies the write voltage pulse at least twice between the two electrodes is that, as will be described later, high contrast can be obtained by applying the write voltage pulse twice or more. This is also because the time width of the write voltage pulse can be made shorter than the response time.

  In addition, although described as "alternating voltage pulse" and "write voltage pulse", this is a square when the time change of the voltage is displayed in a graph, and a voltage of a certain level or more is applied for a certain period of time. Means that it is preferable. The overshoot may occur at the rising edge of the voltage pulse, or the rising or falling edge of the voltage pulse may not be perpendicular to the time axis. Further, the maximum absolute value of the “alternating voltage pulse” voltage and the maximum absolute value of the “write voltage pulse” voltage may be the same or different as necessary. The absolute values of the positive and negative polarities of the alternating voltage pulse may not be the same. The voltage of each pulse of the alternating voltage pulse and the write voltage pulse may be different from other pulses.

  FIG. 3 is an example of a diagram for explaining the response time of the electrophoretic display material. In FIG. 3, the first DC voltage is applied between two electrodes, the displayed color is set to the first color (for example, white), and then the first DC voltage is cut off. Then, a state where there is no voltage difference between the two electrodes is allowed to elapse for a certain period of time. Then, after the potential difference between the substrate electrode 202 and the pixel electrode 203 is set to a predetermined magnitude (second DC voltage) from a certain time, when the potential is continued for a predetermined time (for example, 1 second or more), the color difference The time until the reflectance becomes maximum is shown as the response time. In FIG. 3, the response time of the electrophoretic display material is 500 milliseconds in a general electrophoretic display material. FIG. 3 also shows that the reflectance is decreased by applying a potential difference of reverse polarity having a predetermined magnitude to the substrate electrode 202 and the pixel electrode 203 in a state where the reflectance is increased.

  The reason why the potential difference is generated after a certain period of time has elapsed without a potential difference between the substrate electrode 202 and the pixel electrode 203 is that the change in the reflectance of the electrophoretic display material tends to depend on the history of the applied potential difference. Because is seen. “Fixed time” means the time required for the change in reflectance of the electrophoretic display material to no longer depend on the history of the previously applied potential difference.

  FIG. 3 also shows that if the voltage is continuously applied, even if the reflectivity reaches the maximum state, the reflectivity may decrease slightly thereafter, or even if it reaches the minimum state, it may increase slightly thereafter. ing. This phenomenon was discovered when the inventor of the present application measured a change in reflectance with time using an electrophoretic display material according to the prior art. There arises a problem that the contrast is lowered due to the fluctuation of the reflectance due to this phenomenon. The electrophoretic display material is composed of charged particles and a dispersion solvent for dispersing the charged particles. Generally, the dispersion solvent contains an ionic additive. For this reason, when a voltage is applied to display white charged particles and black charged particles for display, the charged particles migrate to the vicinity of the electrode and white charged particles or black charged particles in the vicinity of the electrode. Before forming the distribution state, the additive ions may respond to the voltage and cause convection of the dispersed solvent. When convection of the dispersion solvent is caused, it becomes difficult for the charged particles to be distributed in the vicinity of the electrode due to the convection of the dispersion solvent. In order to stably distribute the charged particles in the vicinity of the electrode, it is necessary to apply a voltage for a long time. This is necessary and the response time will be longer.

  Therefore, it is considered necessary to suppress the influence of convection by shortening the voltage application time. However, simply shortening the voltage application time, for example, if the voltage application time is made shorter than the response time, the moving distance of the charged particles is shortened, and it is difficult to display the charged particles in the vicinity of the electrodes. It becomes. The inventor of the present application has arrived at the present invention based on such consideration.

  That is, the inventor of the present application has found that the contrast of the electrophoretic display material is increased by shortening the voltage application time as much as possible in order to suppress the convection of the dispersion solvent and further providing the writing voltage pause time. During the rest period of the writing voltage, no electric force is applied to the ions, so that convection of the dispersed solvent does not occur. For this reason, there is no adverse effect on the distribution of charged particles. The electrophoretic display device according to an embodiment of the present invention achieves both a short-time response and high contrast by applying such a plurality of intermittent write voltages.

  Furthermore, since the voltage application time cannot be shorter than the time for applying the voltage to the electrodes and moving the charged particles by the distance between the electrodes, it is difficult to make the voltage application time significantly shorter than the response time of the electrophoretic display material. However, the inventors of the present application applied an alternating voltage pulse prior to the application of the write voltage pulse, thereby applying a voltage pulse to the electrodes and moving the charged particles by the distance between the electrodes. As a result, it was found that the electrophoretic display device can be driven by the application of, and the present invention has been reached.

  That is, by applying an alternating voltage pulse, condensation of charged particles contained in the electrophoretic display material can be suppressed and dispersibility can be improved. As a result, the write voltage pulse applied after the application of the alternating voltage pulse is effectively applied to the charged particles, and the display can be changed at a higher speed. Therefore, according to the present invention, the application time of the write voltage pulse can be shortened by applying the alternating voltage pulse prior to the application of the write voltage pulse. By providing a rest period between the two, it becomes possible to suppress convection of the dispersed solvent. Thereby, high-speed writing and high contrast can be achieved simultaneously.

  Note that as an example of a structure of the control circuit 102, there is a structure using a voltage application circuit 1021, a display control circuit 1022, and an alternating voltage control circuit 1023.

  The voltage application circuit 1021 is a circuit that applies a voltage pulse that generates a potential difference of a predetermined magnitude between the substrate electrode 202 and the pixel electrode 203. The time width of the applied voltage pulse, the time interval between the voltage pulses, and the number of voltage pulses can be controlled by signals from the display control circuit 1022 and the alternating voltage control circuit 1023.

  The display control circuit 1022 applies a write voltage pulse having a time width shorter than the response time at least twice. The application of the write voltage pulse at least twice can be performed by operating the voltage application circuit 1021.

  The alternating voltage control circuit 1023 is a circuit that applies the alternating voltage pulse before the display control circuit 1022 applies the write voltage pulse. The application of the alternating voltage pulse can be performed by operating the voltage application circuit 1021.

  FIG. 4 is an example of a flowchart for explaining processing by the electrophoretic display device according to this embodiment. FIG. 4A shows the order of application of alternating voltage pulses and application of write voltage pulses. In other words, the sequence of operations of the display control circuit 1022 and the alternating voltage control circuit 1023 is mainly shown. FIG. 4B illustrates a process of applying a write voltage pulse. In other words, the processing mainly by the operation of the display control circuit 1022 is described.

  In FIG. 4A, an alternating voltage pulse is applied between the substrate electrode 202 and the pixel electrode 203 as process S401. In step S <b> 402, a write voltage pulse is applied between the substrate electrode 202 and the pixel electrode 203.

  As processing S403, for example, a predetermined natural number of 2 or more is substituted into the variable n. Next, as processing S <b> 404, a writing voltage pulse having a first time width is applied between the substrate electrode 202 and the pixel electrode 203 once. In step S405, an operation for reducing the value of the natural number held in n by 1 is performed, and substitution for n is performed. In step S406, it is determined whether n holds a positive natural number. If n holds a positive natural number, the process proceeds to the process S407, the application of the voltage pulse is stopped for the second time, and then the process proceeds to the process S404. If n does not hold a positive natural number in step S406, the process ends. The relationship between the first time width and the second time width will be described later.

  The above-described processing can be realized by configuring the electrophoretic display device 100 using a CPU, a memory, a microcomputer having an external interface, and the like, and causing the CPU to operate a program that executes the flowchart shown in FIG. . Further, by using a sequential circuit having a state corresponding to the process S401 and the process S402 and a state corresponding to the number of natural numbers assigned to n in the process S403, the above-described process can be performed by dedicated hardware without operating the program. Can be realized.

  FIG. 5 shows an example of a train of voltage pulses applied between the substrate electrode 202 and the pixel electrode 203 when the processing of the flowchart shown in FIGS. 4A and 4B is executed. . For example, when there are white and black charged particles, when the pixel electrode 203 side is white, an alternating voltage pulse is applied to the substrate electrode 202 during the alternating voltage period as illustrated in the upper part of FIG. And the pixel electrode 203. After applying the alternating voltage pulse a predetermined number of times, a voltage pulse is applied between the substrate electrode 202 and the pixel electrode 203, with the first time being the time width of the write voltage pulse and the second time being the rest period. To do.

  When the pixel electrode 203 side is black, an alternating voltage pulse is applied between the substrate electrode 202 and the pixel electrode 203 during the alternating voltage period, as illustrated in the lower part of FIG. After applying the alternating voltage pulse a predetermined number of times, a voltage pulse is applied between the substrate electrode 202 and the pixel electrode 203 with the first time as the time width of the write voltage pulse and the second time as the rest period. In this case, a voltage pulse having a polarity opposite to that when the pixel electrode 203 side is white is applied. Further, after the alternating voltage pulse is applied a predetermined number of times, the voltage pulse is applied between the substrate electrode 202 and the pixel electrode 203 with the first time as the time width of the write voltage pulse and the second time as the rest period. A voltage pulse may be applied between the substrate electrode 202 and the pixel electrode 203, with the first time being a pause period and the second time being the time width of the write voltage pulse. good. Furthermore, the order of the write voltage pulse and the pause time may be arbitrarily changed. As shown in FIG. 5, the time variation of the alternating voltage pulse may be opposite to that when the pixel electrode 203 side is white or may have the same polarity. Further, the first time and the second time may be the same length of time.

  In FIGS. 6 to 9, general electrophoretic display materials containing white charged particles and black charged particles are sealed between the substrate electrode 202 and the pixel electrode 203, and the substrate electrode 202 and the pixel are subjected to various conditions. It is a graph which shows the black reflectance and contrast after applying an alternating voltage pulse and a write-in voltage pulse between electrodes. Here, the contrast means a relative ratio between the reflectances of two colors that can be displayed by the electrophoretic display material, and the contrast is defined as a ratio between the black reflectance and the white reflectance. That is, contrast is defined as the magnitude of white reflectance relative to black reflectance.

  Various methods for measuring the black reflectance and the white reflectance are known. For example, the black reflectance and the white reflectance can be measured as Y values (luminous reflectance) in the tristimulus values X, Y, and Z of the XYZ color system. Therefore, various measurement results shown in FIGS. 6 to 9 were obtained by a measurement method suitable for defining the response time. The measurement method is that the diffuse reflection light from the electrophoretic display panel is received by a photomultiplier tube, and the diffuse reflection light intensity in each of the black display state and the white display state is measured to measure the black reflectance and white reflectance. Is a method of calculating In a specific measurement procedure, first, the diffuse reflected light intensity of a white standard plate with a known reflectance is measured, and the result of this measurement is used as a reference value. Next, the reflectance of each of the black state and the white state was calculated as a relative value of the diffused light intensity of the reference value and the white state and the black state.

  Here, the diffuse reflected light intensity was measured by the following method. First, as a sample, the white standard plate is placed on a specimen table of a dark field microscope of an epi-illumination method, and using a dark field objective lens (LMPlan 5 × BD manufactured by Olympus), diffuse reflected light of the sample is reflected on the dark field microscope. It detected with the photomultiplier tube (H7422-20 made from Hamamatsu Photonics) arrange | positioned at the lens-barrel, and the output of the diffuse reflected light intensity was measured with the oscilloscope. Next, the electrophoretic display panel displaying black or white was placed on the sample stage, and the diffuse reflected light intensity was measured by the same method as that for the white standard plate to obtain the reflectance. The diffuse reflected light intensity was measured by detecting a value during a period in which no voltage was applied between the substrate electrode 202 and the pixel electrode 203 after 2 seconds had passed after the DC voltage was cut.

  FIG. 6 is a graph showing the relationship between the number of write voltage pulses, the black reflectance, and the contrast. Before the measurement of any reflectivity, the alternating voltage pulse having a time width of 10 milliseconds is applied 10 times between the substrate electrode 202 and the pixel electrode 203, and then the time width and the rest time width of the writing voltage pulse are applied. A write voltage pulse was applied, both of which were 25 milliseconds. What is different for each measurement is the number of write voltage pulses, and the number of application of the write voltage pulses was changed from 1 to 10. In addition, the number of application of the alternating voltage pulse is such that the application of continuous positive and negative or negative and positive voltage pulses is the number of application of the alternating voltage pulse once.

  As shown in FIG. 6, the black reflectance is lower when the write voltage pulse is applied twice than in any other case. The contrast is also maximized. When the write voltage pulse is applied three times or more, the contrast is lower than when the write voltage pulse is applied twice, but the contrast is higher than when the write voltage pulse is applied once. Yes. By applying the write voltage pulse twice or more, the contrast is approximately 25 or more.

  Further, when the number of application of the write voltage pulse is 2, the contrast is maximized. In this case, the sum of the application time and the rest time of the write voltage pulse applied twice is 75 milliseconds, which is shorter than 500 milliseconds (response time) in the case of applying the DC voltage in FIG. This is presumably because the alternating voltage is applied prior to the write voltage pulse, so that the dispersibility of the charged particles is increased and the write voltage pulse can be applied more effectively.

  In addition, although a present Example is an Example about a typical electrophoretic display material, the same tendency is acquired even if it uses another electrophoretic display material. However, the application of the two write voltage pulses as in the present embodiment is not necessarily the optimum number of times. However, if the number of write voltage pulses is about 10 times or less, a sufficient effect can be obtained.

  FIG. 7 is a graph showing the relationship between the time width of the write voltage pulse, the black reflectance, and the contrast. Before the measurement of any reflectivity, an alternating voltage pulse having a time width of 10 milliseconds was applied 10 times, and then a writing voltage pulse having a time width equal to that of the writing voltage pulse was applied 10 times. What differs depending on the measurement time is the time width of the write voltage pulse, which was changed from 10 milliseconds to 100 milliseconds.

  As shown in FIG. 7, the contrast is 20 or more when the write voltage pulse is 15 milliseconds or more and 60 milliseconds or less.

  FIG. 8 is a graph showing the relationship between the pause time of the write voltage pulse, the black reflectance, and the contrast. Before any measurement of the reflectivity, an alternating voltage pulse with a write voltage pulse duration of 10 milliseconds is applied 10 times, and then a write voltage pulse with a write voltage pulse duration of 25 milliseconds is applied 10 times. Applied. The difference was the length of the pause time, which was varied from 10 milliseconds to 40 milliseconds.

  As shown in FIG. 8, the contrast increases as the pause time increases. In particular, the contrast is 25 or more when the pause time is 25 milliseconds or longer, which is the same as or longer than the writing time.

  FIG. 9 is a graph showing the relationship between the number of alternating voltage pulses and the black reflectance and contrast. Before any reflectance measurement, a writing voltage pulse having a time width and a resting time width of the writing voltage pulse of 25 milliseconds was applied. The difference is the number of times of applying the alternating voltage pulse whose pulse width is 10 milliseconds, and shows the measurement results of the black reflectance and contrast when changing from 0 to 10 times (the alternating voltage pulse is Applying 0 times means not applying an alternating voltage pulse).

  As shown in FIG. 9, the contrast generally improves as the number of application of the alternating voltage pulse increases. However, when the number of application of the alternating voltage pulse is 4 or more, a contrast of 25 or more can be obtained. Recognize. Therefore, in the present invention, the number of application of the alternating voltage pulse is not limited to 10 times, and may be appropriately changed from 4 times or more according to the characteristics of the material used as the electrophoretic display material. The number of application of the alternating voltage pulse can be made larger than 20, for example. However, if the time is greater than 20, the time for applying the alternating voltage pulse is as long as 400 ms, so the time until the image is displayed after the writing voltage pulse is applied is longer than the response time of the electrophoretic display material. As a result, the display time is not shortened. Therefore, the number of application of the alternating voltage pulse is preferably 4 or more and 20 or less.

  From the above, it can be seen that the following is preferable in order to obtain a high contrast (for example, a contrast of 20 or more). The application of the write voltage pulse is preferably 2 or 3 or more and 10 or less. If it is 10 or more, the contrast will not be improved even if the number of times of application of the write voltage pulse is increased, and the write time for image display will become longer. In addition, a phenomenon in which an afterimage of image display is noticeable occurs. The time width of the write voltage pulse is preferably 15 milliseconds or more and 60 milliseconds or less. The length of the rest period of the write voltage pulse is preferably equal to the time width of the write voltage pulse or larger than the time width of the write voltage pulse. Moreover, it is preferable that the frequency | count of application of an alternating voltage pulse is 2-10. It is more preferable that the number of application of the alternating voltage pulse is 2 or more because a contrast of 20 or more is obtained. If it is less than 2, the contrast is lowered as described above, and if it is 10 or more, the writing time for image display becomes long and the afterimage of image display becomes conspicuous.

  In addition, the time width of the pulse is 10 milliseconds, and the time required to apply the alternating voltage pulse twice is 40 milliseconds. When the write voltage pulse is applied twice, the time width and pause of the write voltage pulse are applied. If the time width is in the range of 15 to 60 milliseconds, 45 to 180 milliseconds are required to apply the write voltage pulse. Therefore, it takes about 85 milliseconds to 220 milliseconds as a whole, and it can be seen that this is less than about half of 500 milliseconds which is a response time of a general electrophoretic display material. That is, the sum of the time width of the alternating voltage pulse and the time width of the write voltage pulse is equal to or shorter than the response time of the electrophoretic display material.

  Therefore, according to the present invention, it is possible to obtain a display with high contrast and no afterimage in a short time, which is useful.

  100 electrophoretic display device, 101 display panel, 102 control circuit

Claims (10)

A display panel capable of sandwiching an electrophoretic display material between two electrodes and displaying a first color having a maximum reflectance and a second color having a minimum reflectance ;
After an alternating voltage pulse is applied between the two electrodes, it has a control circuit for applying a write voltage pulse between at least two times the two electrodes,
The sum of the time widths of the write voltage pulse applied at least twice between the two electrodes by the control circuit applies a DC voltage to the electrophoretic display material to change the color displayed on the display panel to the first color. The response time, which is the time required to change from color to the second color, is less than the response time, and after the alternating voltage pulse is applied between the two electrodes, the write voltage pulse is applied at least twice between the two electrodes. An electrophoretic display device in which the ratio of the reflectance in the second color to the reflectance in the first color is 20 or more after being applied .   2. The electrophoretic display device according to claim 1, wherein a sum of a time width of the alternating voltage pulse and a time width of the write voltage pulse is equal to or shorter than a response time of the electrophoretic display material.   2. The electrophoretic display device according to claim 1, wherein a time width of the write voltage pulse is 15 milliseconds or more and 60 milliseconds or less.   The electrophoretic display device according to claim 1, wherein an interval between the write voltage pulses is equal to or longer than a time width of the write voltage pulse.   The electrophoretic display device according to claim 1, wherein the number of the alternating voltage pulses is two or more. An electrophoretic display panel driving method capable of sandwiching an electrophoretic display material between two electrodes and displaying a first color having a maximum reflectance and a second color having a minimum reflectance ,
After applying an alternating voltage pulse between the two electrodes, a write voltage pulse is applied between the two electrodes at least twice ;
The sum of the time widths of the write voltage pulse applied at least twice between the two electrodes is obtained by applying a DC voltage to the electrophoretic display material to change the color displayed on the electrophoretic display panel from the first color. Less than the response time, which is the time required to change to the second color, and after applying the alternating voltage pulse between the two electrodes, the writing voltage pulse is applied between the two electrodes at least twice. A method for driving an electrophoretic display panel, wherein the ratio of the reflectance in the second color to the reflectance in the first color is later 20 or more . The electrophoretic display panel according to claim 6 , wherein the sum of the time widths of the alternating voltage pulses and the time width of the write voltage pulses is equal to or shorter than the response time of the electrophoretic display material. Driving method. 7. The method of driving an electrophoretic display panel according to claim 6, wherein a time width of the write voltage pulse is not less than 15 milliseconds and not more than 60 milliseconds. The method of driving an electrophoretic display panel according to claim 6 , wherein the interval between the write voltage pulses is equal to or longer than a time width of the write voltage pulse. The method of driving an electrophoretic display panel according to claim 6 , wherein the number of the alternating voltage pulses is two or more.

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