Abstract:
An electrophoretic display (EPD) with thermal control is disclosed for controlling and maintaining an image in extreme temperature environments. Techniques are also disclosed for maintaining the EPD cell threshold voltage for EPD cells comprising an EPD display media at or above a desired level in an environment in which the EPD may be subjected to an extreme temperature. The techniques comprise sensing a sensed temperature associated with the EPD display media, determining whether the sensed temperature satisfies a criterion established to ensure that the display media temperature remains at a level associated with an acceptable EPD cell threshold voltage, and in the event it is determined that the sensed temperature does not satisfy the criterion, controlling the EPD display media temperature as required to bring the sensed temperature to a level that satisfies the criterion.

Description:
[0001]    This application is a continuation-in-part of U.S. application Ser. No. 11/414,635, filed on Apr. 27, 2006; which is a divisional application of U.S. application Ser. No. 10/956,367 filed on Oct. 1, 2004, now U.S. Pat. No. 7,061,662; which claims the benefit of U.S. Provisional Application No. 60/509,401 filed on Oct. 7, 2003; the entire contents of all the above-identified applications are incorporated herein by reference in their entirety. 
       RELATED APPLICATIONS 
       [0002]    Both U.S. Provisional Patent Application No. 60/505,340, entitled “Passive Matrix Electrophoretic Display Driving Scheme,” filed May 16, 2003; and co-pending U.S. Patent Application Publication No. US 2004-0246562, entitled “Passive Matrix Electrophoretic Display Driving Scheme,” filed Apr. 30, 2004; are incorporated herein by reference in their entirety. 
     
    
     FIELD OF INVENTION 
       [0003]    The present invention relates generally to display devices. More specifically, an electrophoretic display with thermal control is disclosed. 
       BACKGROUND OF THE INVENTION 
       [0004]    The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. The EPD was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference. 
         [0005]    There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of particles such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. No. 5,961,804 and U.S. Pat. No. 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors. 
         [0006]    An improved EPD technology was disclosed in co-pending applications, U.S. patent Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. patent Ser. No. 09/759,212, filed on Jan. 11, 2001, U.S. patent Ser. No. 09/606,654, filed on Jun. 28, 2000 and U.S. patent Ser. No. 09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference. The EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. 
         [0007]      FIG. 1  illustrates a typical EPD cell  100  comprising a quantity of electrophoretic dispersion  102 , the dispersion comprising a plurality of charged pigment particles  104  dispersed in a colored dielectric solvent  106 . The dispersion  102  is contained by a top layer of insulating material  108  and a bottom layer of insulating material  110 . In one embodiment, the insulating material may comprise a non-conductive polymer. In the cells described in the above-incorporated co-pending patent applications, the insulating layer may comprise a sealing and/or adhesive layer, or the microcup structure. The dispersion and associated insulating materials are positioned between an upper electrode  112  and a lower electrode  114 . 
         [0008]    An EPD may be driven by a passive matrix system. For a typical passive matrix system, there are column electrodes on the top side (viewing surface) of the display and row electrodes on the bottom side of the cells (or vice versa). The row electrodes and the column electrodes are perpendicular to each other. 
         [0009]    Cross bias is a well-known problem for a passive matrix display. The voltage applied to a column electrode not only provides the driving bias for the cell on the scanning row, but it also affects the bias across the non-scanning cells on the same column. This undesired bias may force the particles of a non-scanning cell to migrate to the opposite electrode. This undesired particle migration causes visible optical density change and reduces the contrast ratio of the display. 
         [0010]    Conventional EPD devices, such as those described in U.S. Patent Application No. 60/417,762 filed Oct. 10, 2002, which is incorporated herein by reference, are sensitive to environments where temperature ranges may be extreme such as an outdoor environment. When an EPD is used in an outdoor environment, it may experience temperature extremes rising to more than 80° C. or less than −20° C. When the environmental temperature exceeds 60° C. or falls below 0° C., the performance of a conventional EPD can degrade quickly. Although conventional EPDs may work well in controlled, moderate indoor environments, the outdoor temperature extremes can affect the threshold effect exhibited by EPD cells such as those described in the &#39;762 application. 
         [0011]    Heating devices are used in display systems to control temperatures in extreme environments. A heating device for a flat panel display should be thin, compact, and light in weight. Typically micro-wire and thin film heating devices have been used for flat panel displays. Micro-wire heaters exhibit satisfactory transparency and can be applied to the viewing side of a display. However, the costs for micro-wire heaters are generally high. The costs for thin film heaters are also high and such heaters often filter desired light from the display, thus degrading displayed images. 
         [0012]    Thus, there is a need for an EPD that can adjust for environmental temperature extremes such as those found in an outdoor setting. There is also a need for an EPD that under extreme temperature conditions can maintain satisfactory EPD performance. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention is directed to an EPD module structure, comprising (i) an EPD panel comprising EPD cells filled with EPD display media; and (ii) a thermal control pad which is capable of controlling the temperature associated with the EPD display media to ensure that the threshold voltage of the EPD cells is equal to or greater than one third of a driving voltage used to drive the EPD cells to a desired display state. The EPD module structure optionally further comprises a clear front plate, a metal plate, an air gap, a thermal barrier sheet, or an electric power source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings. 
           [0015]      FIG. 1  illustrates a typical EPD cell; 
           [0016]      FIG. 2  illustrates a top view of a 2×2 passive matrix; 
           [0017]      FIGS. 3A-3D  illustrates a lateral view of a 2×2 passive matrix; 
           [0018]      FIG. 4  is a graph illustrating the relationship between threshold voltage and temperature, in accordance with an embodiment of the present invention; 
           [0019]      FIG. 5A  illustrates an EPD module structure with a heating pad in accordance with an embodiment of the present invention; 
           [0020]      FIG. 5B  illustrates an EPD module structure with a cooling pad in accordance with an embodiment of the present invention; 
           [0021]      FIG. 5C  illustrates an EPD module structure with a thermal control pad in accordance with an embodiment of the present invention; 
           [0022]      FIG. 6A  illustrates a thermal control pad in accordance with an embodiment of the present invention; 
           [0023]      FIG. 6B  illustrates a thermal control pad in accordance with an alternative embodiment of the present invention; 
           [0024]      FIG. 7A  illustrates a DC voltage  710  applied to an electrode of an EPD in one embodiment; 
           [0025]      FIG. 7B  illustrates an AC signal  720  applied to an electrode layer in one embodiment to achieve the same effect on the EPD cells as the DC voltage of  FIG. 7A  while also generating heat as a product of the AC current passing through the electrode; 
           [0026]      FIG. 8A  illustrates an EPD thin film heater in accordance with an embodiment; 
           [0027]      FIG. 8B  illustrates an AC voltage applied to a common electrode of an exemplary EPD thin film heater; 
           [0028]      FIG. 8C  illustrates an AC voltage applied to an EPD thin film heater; 
           [0029]      FIG. 9A  illustrates a passive matrix EPD  900  used in one embodiment; 
           [0030]      FIG. 9B  illustrates a DC voltage  920  that can be applied to non-selected rows during scanning of EPD  900  in one embodiment; 
           [0031]      FIG. 9C  illustrates a DC voltage  925  of 30V DC applied to the scanning row of EPD  900 ; 
           [0032]      FIG. 9D  illustrates an AC voltage  930  that can be applied to passive matrix EPD  900 ; 
           [0033]      FIG. 10A  is a state diagram illustrating a thermal control algorithm  1000  used in one embodiment; 
           [0034]      FIG. 10B  is a state diagram illustrating a thermal control heating algorithm  1010  used in one embodiment; 
           [0035]      FIG. 10C  is a state diagram  1020  illustrating a thermal control cooling algorithm  1020  used in one embodiment; and 
           [0036]      FIG. 11  illustrates an EPD thermal control system in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The invention can be implemented in numerous ways, including as a process, an apparatus, a system, a composition of matter, a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. 
         [0038]    A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured. 
         [0039]    The term “threshold voltage” (Vth), in the context of the present disclosure, is defined as the maximum bias voltage that does not cause the particles in a cell to move between electrodes. The term “driving voltage” (Vd), in the context of the present disclosure, is defined as the bias voltage applied to change the color state of a cell, such as by driving the particles in the cell from an initial position at or near one electrode to an end position at or near the opposite electrode. The driving voltage Vd used in a particular application must be sufficient to cause the color state of the cell to change within the required performance parameters of the application, including as measured by such parameters as the time it takes for the state transition to be completed. 
         [0040]    A “scanning” row in a passive matrix display is a row in the display that is currently being updated or refreshed. A “non-scanning” row is a row that is not currently being updated or refreshed. A “positive bias”, in the context of the present disclosure, is defined as a bias that tends to cause positively charged particles to migrate downwards (i.e., upper electrode at higher potential than lower electrode). A “negative bias”, in the context of the present disclosure, is defined as a bias that tends to cause positively charged particles to migrate upwards (i.e., lower electrode at higher potential than upper electrode). 
         [0041]    For a typical passive-matrix, the row electrodes are on the top, and the column electrodes are on the bottom and perpendicular to the row electrodes.  FIG. 2  and  FIGS. 3A-3D  illustrate a 2×2 passive matrix.  FIG. 2  shows the top view of a general 2×2 passive matrix. In this figure, voltage A drives the top, non-scanning row and voltage B drives the bottom, scanning row. 
         [0042]    Initially, as shown in  FIGS. 3A-3D , the particles in cells W, Y and Z are at the top of the cells, and the particles in cell X are at the bottom of the cell. Assume the scanning row B is to be modified such that the particles in cell Y are moved to the bottom electrode while the particles in cell Z are to be maintained at their current position at the top electrode. The particles in the cells of the non-scanning row should, of course, remain at their initial positions—W at the top electrode and X at the bottom electrode—even if a cross-biasing condition is present. 
         [0043]    Because Cells W and X are in a non-scanning row, the goal is to ensure that the particles remain at the current electrode position even when there is a cross bias condition affecting the row. The threshold voltage of the cell is an important factor in these two cases. Unless the threshold voltage is equal to or greater than the cross bias voltage that may be present, the particles in these cells will move when such a cross bias is present, thereby reducing the contrast ratio. 
         [0044]    In order to drive the particles in cell Y from the top electrode to the bottom electrode within a specific time period, a driving voltage Vd must be applied. The driving voltage used in a particular application may be determined by a number of factors, including but not necessarily limited to cell geometry, cell design, array design and layout, and the materials and solvents used. In order to move the particles in cell Y without affecting the particles in cells W, X and Z, the driving voltage Vd applied to change the state of cell Y must also be of a magnitude, and applied in such a way, so as not to result in the remaining cells being cross biased in an amount greater than the threshold voltage Vth of the cells. 
         [0045]    To determine the minimum threshold voltage needed to avoid unintended state changes in the basic passive matrix illustrated in FIGS.  2  and  3 A- 3 D under these conditions, the following inequality conditions should be satisfied: 
         [0000]    
       
      
       A−C≦Vth  
      
     
         [0000]    
       
      
       D−A≦Vth  
      
     
         [0000]    
       
      
       B−C≧Vd  
      
     
         [0000]    
       
      
       B−D≦Vth  
      
     
         [0046]    This system of equations may be solved by summing the three inequalities involving Vth, to yield the inequality (A−C)+(D−A)+(B−D)≦Vth+Vth+Vth, which simplifies to B−C≦3Vth, or 3Vth≧B−C. Combining this inequality with the remaining inequality B−C≧Vd, we conclude that 3Vth≧B−C≧Vd, which yields 3Vth≧Vd or Vth≧⅓ Vd. That is, for the passive matrix illustrated in FIGS.  2  and  3 A- 3 D, the cells must have a threshold voltage equal to or greater than one third of the driving voltage to be applied to change the state of those cells in which a state change is desired in order to avoid changing as a result of cross bias the state of those cells in which a state change is not desired. Referring further to FIGS.  2  through  3 A- 3 D, if the driving voltage Vd is applied to the scanning row B, then solution of the above inequalities indicates that to ensure that the driving bias voltage is applied to cells to be programmed and that no more than the threshold voltage is applied to other cells (i.e., non-programming cells in the scanning row and all cells in the non-scanning row) the voltage applied to the non-scanning row A should be equal to ⅓Vd, the voltage applied to the column electrode associated with a cell in the scanning row to be programmed (i.e., display state changed), such as column electrode C, should be 0 volts, and the voltage applied to the column electrode associated with a cell in the scanning row that is not to be programmed (i.e., retain the initial or reset state) should be equal to ⅔Vd. For example, in one embodiment the driving voltage required to achieve acceptable performance is 30V. If the driving voltage Vd=30V in the passive matrix display illustrated in FIGS.  2  and  3 A- 3 D, then the minimum threshold voltage that would be required to retain the initial state of cells W, X, and Z while changing the state of cell Y by applying a driving voltage of 30V to cell Y would be Vth=10V. Assuming B=30V, the solution to the above equations is A=10V, C=0V and D=20V. By reference to FIGS.  2  and  3 A- 3 D, one can see that under these conditions the bias applied to each of cells W, X, and Z would in fact be less than or equal to the minimum threshold voltage Vth=10V. For proper operation and performance, therefore, the cell threshold voltage must be quite high relative to the driving voltage to be applied to change the electrophoretic display cell state to avoid unwanted state changes or display performance degradation due to cross bias. 
         [0047]    As demonstrated above, the EPD cells must have a threshold voltage greater than or equal to ⅓ of the driving voltage applied to change the cell display state in order for a passive matrix EPD to function properly. By material selection and structural design, a 5V to 50V threshold effect can be achieved. Assuming the EPD can operate at an environmental temperature up to 80° C. and the driving voltage is 30V, the EPD cell material and structural design can be selected such that the cells exhibit a threshold voltage of 10V at 80° C. 
         [0048]    However, as noted above, the threshold voltage also varies with temperature. In general, threshold voltage is low at high temperature and threshold voltage is high at low temperature.  FIG. 4  illustrates the inverse relationship between cell threshold voltage and temperature. 
         [0049]    Controlling EPD cell temperature to maintain the cell threshold voltage within a desired range, regardless of the ambient temperature, is disclosed. In one embodiment, a temperature control system is configured to maintain the cell temperature in a range that results in the threshold voltage being maintained at a value between ⅓ to ⅔ of the driving voltage. Using the example illustrated in  FIG. 4 , in one embodiment a heating pad and associated thermal control are provided to maintain the cell temperature between 40° C. and 80° C., resulting in the cell threshold voltage being maintained in the range of 10V to 20V (i.e., ⅓ to ⅔ of the 30V driving voltage in the illustrative example described above). In one embodiment, the EPD cell temperature is maintained at or near a fixed temperature, e.g., 50° C.+/−5° C. The temperatures and voltages described above are presented for sake of example only, and the temperatures and voltages may be different depending on such factors as the cell structure, materials used, and the relationship between temperature and threshold voltage for a particular EPD cell. 
         [0050]      FIG. 5A  illustrates an EPD module structure with a heating pad used in one embodiment. The EPD module structure includes frame  502 , clear front plate  504 , EPD panel  506 , front air gap  508 , metal plate  510 , heating pad  512 , thermal barrier sheet  514 , rear air gap  516 , and solar cell panel  518 . Although shown in this embodiment, metal plate  510 , solar cell panel  518 , and thermal barrier sheet  514  may be excluded in other embodiments. 
         [0051]    Clear front plate  504  may be implemented using polycarbonate, plexi-glass, or another transparent thermal barrier material. In one embodiment, EPD panel  506  is laminated to clear front plate  504 . Heating pad  512 , which may be composed of an individual or multiple pads, can be placed behind EPD panel  506  between front air gap  508  and heating pad  512 . Metal plate  510  can be laminated to the front of heating pad  512  to evenly distribute heat. Thermal barrier sheet  514 , which may or may not be present in other embodiments, can be laminated to the rear side of heating pad  512  or the rear side of solar cell panel  518 . Solar cell panel  518  provides electric power for heating pad  512  and the control and driver circuit, as described below in connection with  FIG. 11 . In other embodiments, sources of electric power other than a solar cell panel may be used. Thermal barrier sheet  514  can be used to prevent heat loss from the rear side of the EPD module structure as well as thermal insulation against an external (e.g., outdoors) environment. Rear air gap  516  between solar cell panel  518  and heating pad  512  can be included to provide additional protection as an extra thermal barrier. 
         [0052]    The EPD panel in  FIGS. 5A ,  5 B and  5 C comprises EPD cells which are filled with EPD display media. 
         [0053]    In one embodiment in which an EPD temperature above 10° C. is to be maintained in an environment in which the temperature is −30° C., the EPD module structure is constructed using the following materials and dimensions: 
         [0054]    Clear front plate: 0.25″ polycarbonate sheet 
         [0055]    Frame: G10 fiberglass 
         [0056]    Front air gap: 0.1″ 
         [0057]    Metal plate: 0.125″ aluminum 
         [0058]    Heating pad: Floor heating roll, 10 W/ft 2    
         [0059]    Thermal barrier: none 
         [0060]    Rear air gap: 0.2″ 
         [0061]      FIG. 5B  illustrates an EPD module structure with a cooling pad used in one embodiment. Again, the EPD module structure includes frame  502 , clear front plate  504 , EPD panel  506 , front air gap  508 , metal plate  510 , thermal barrier sheet  514 , rear air gap  516 , and solar cell panel  518 . However, instead of heating pad  512  ( FIG. 5A ), a cooling pad  520  is provided. Also, metal plate  510  and thermal barrier sheet  514  may be excluded in other embodiments. 
         [0062]    Constructed similarly to the EPD module structure in  FIG. 5A , the EPD module structure includes cooling pad  520 , which can be implemented as an individual pad or multiple pads or segments. Implementation of cooling pad  520  can be provided using typical cooling elements such as refrigerants or other coolants in both gas and liquid forms. Cooling pad  520  can be placed behind EPD panel  506 . Front air gap  508  can be included between EPD panel  506  and cooling pad  520 . Optional metal plate  510  may be laminated to the front of cooling pad  520  in order to evenly absorb heat. Optionally, thermal barrier sheet  514  laminated to the rear side of cooling pad  520  or the rear side of solar cell panel  518 . Solar cell panel  518  provides electric power for cooling pad  520  and the control and driver circuit, as described below in connection with  FIG. 11 . In other embodiments, sources of electric power other than a solar cell panel may be used. Optional thermal barrier sheet  514 , if included, prevents heat transfer from the rear side of the EPD module structure as well as thermal insulation against an external (e.g., outdoors) environment. Rear air gap  516  may be placed between solar cell panel  518  and cooling pad  520  to provide protection as an extra thermal barrier. 
         [0063]      FIG. 5C  illustrates an EPD module structure used in one embodiment with a thermal control pad for heating and cooling. As with  FIGS. 5A and 5B , an EPD module structure is provided, including frame  502 , clear front plate  504 , EPD panel  506 , front air gap  508 , optional metal plate  510 , optional thermal barrier sheet  514 , rear air gap  516 , and solar cell panel  518 . However, instead of heating pad  512 , a thermal control pad  522  is provided. The solar cell panel, as stated above, may be replaced with other sources of electric power. 
         [0064]    Thermal control pad  522  provides temperature control and maintenance for the EPD module structure. Implemented as either an individual pad or multiple segments/pads, thermal control pad  522  can provide heating and cooling of EPD module structure, as needed. In one embodiment, the thermal control pad  522  is used to raise or lower the temperature of the EPD panel  506  to maintain the EPD cell threshold voltage at a desired value or within a desired range of values. As stated above, the desired range of the threshold voltage for the EPD cells is equal to or greater than one third of a driving voltage used to drive the EPD cells to a desired display state. 
         [0065]    The thermal control pad  522  may comprise a heating or cooling element. In one embodiment, the heating element may be a radiant heater or a convection heater. The radiant heater or convection heater may be placed behind the EPD panel. In another embodiment, the heating element may be a resistive heater. The resistive heater may be embedded in a thermally conductive holder on which the EPD module structure is mounted. In a further embodiment, the cooling element may be a refrigerant or coolant in either gas or liquid form. 
         [0066]      FIG. 6A  illustrates a thermal control pad used in one embodiment. In this embodiment, thermal control pad  600  comprises both heating and cooling elements. In this example, a striped configuration is created by integrating column shaped heating pad segments  604  into a cooling pad  602 . In one embodiment, the heating pad segments  604  are placed on top of cooling pad  602 . In other embodiments, the individual heating pad segments  604  may be placed under or within cooling pad  602 . Other arrangements can also be implemented. 
         [0067]      FIG. 6B  illustrates a thermal control pad used in one embodiment. In this embodiment, thermal control pad  610  is implemented in a “checkerboard” pattern. In one embodiment, cooling pad  612  is interwoven with individual heating pads  614 . In other embodiments, a single heating pad can be interwoven with individual cooling pads. In still other embodiments multiple individual heating pads can be interwove with multiple individual cooling pads. Other embodiments of thermal control pad  610  may be implemented using a variety of patterns of cooling and heating pads and is not limited to only those embodiments listed herein. 
         [0068]    In one embodiment, the heating required to maintain the threshold voltage of the EPD cells at the desired level or within the desired range is provided at least in part by applying to an electrode of the EPD an AC signal that generates the required heat without interfering with the operation of the EPD. In one embodiment, using this approach eliminates the need to include in the EPD a separate heating pad, such as the heating pad  512  of  FIG. 5A . A typical EPD is driven by applying a DC voltage to generate an electric field to cause the charged pigment particles to migrate to a desired position. In a typical EPD, the particles on average do not move very fast, and may have a response time on the order of 5 ms to 500 ms. As a result, when a fast switching AC signal is applied, for example a square waveform at 10 kHz (100 μs cycle), the particles cannot react to the fast switching waveform and as a result react only to the DC voltage of the waveform. In one embodiment, this characteristic is used to generate heat by using an electrode of the display as a heating element. Heat is generated by applying to the electrode a fast switching AC signal selected so as to generate the required heat as a product of the AC current passing through the electrode while not interfering with the operation of the display by selecting a driving signal that has a DC voltage equal to the DC voltage required to be applied to the electrode under the applicable EPD driving scheme. 
         [0069]      FIG. 7A  illustrates a DC voltage  710  applied to an electrode of an EPD in one embodiment. In this example, a DC voltage of 20V is applied to the electrode, such as may be required under the driving scheme of the EPD. In one embodiment, the electrode may be a thin film electrode. 
         [0070]      FIG. 7B  illustrates a driving signal  720  applied to an electrode layer in one embodiment to achieve the same effect on the EPD cells as the DC voltage of  FIG. 7A  while also generating heat as a product of the AC current passing through the electrode. In this example, a square wave at 10 KHz (100 us cycle) has the same effect on the EPD cells as the DC voltage shown in  FIG. 7A . 
         [0071]      FIG. 8A  illustrates an EPD thin film heater in accordance with an embodiment. The thin film heater  800  comprises a common electrode  802 . In one embodiment, the common electrode  802  comprises a common electrode on the viewing side of a segment display. In one embodiment, the common electrode  802  comprises a common electrode on the viewing side of an active matrix display. The thin film heater  800  comprises a first contact pad  804  connected electrically to one end of the common electrode  802  and a second contact pad  806  connected electrically to the opposite end of common electrode  802 . In one embodiment, DC voltage source  808  is configured to apply a DC voltage to common electrode  802  by supplying DC voltage to the first contact pad  804  and one input of adder circuit  811 . The DC voltage supplied by DC voltage source  808  is a DC voltage applied to the common electrode  802  under the driving scheme used by an EPD in which the thin film heater  800  is used. The thin film heater  800  also comprises an AC voltage source  810  configured to supply an AC signal to another input of adder  811  when heating is desired. When heating is desired, the switch associated with AC voltage source  810  is in the position shown in  FIG. 8A , i.e., the AC signal supplied by source  810  is provided as an input to the adder  811 . If heating is not desired, the switch associated with source  810  is switched to open the connection between the source  810  and the adder  811  and instead connect the AC input line to adder  811  to ground, as can be seen from  FIG. 8A . When the switch associated with AC source  810  is aligned to provide the AC signal supplied by source  810  as input to adder  811 , i.e., when heating is desired, adder  811  adds the DC signal from DC voltage source  808  and the AC signal from  810  together and applies the combined signal to the second contact pad  806 . Driver and control circuits not shown in  FIG. 8A  control the operation of DC voltage source  808  to apply the DC voltage required for driving and the operation of AC voltage source  810  and its associated switch, when needed, to provide heating as required for proper operation of the EPD. 
         [0072]    In one embodiment, the thin film heater  800  of  FIG. 8A  is used in an EPD in which the driving scheme requires that 10 V DC be applied to the common electrode for driving.  FIG. 8B  illustrates a DC voltage supplied by DC voltage source  808  for driving in one such embodiment. The DC voltage  812  may be applied to common electrode  802  alone for driving when heating is not required. However, as described below, DC voltage  812  may also be applied while applying an AC voltage to generate heat for thermal control, as described above. 
         [0073]      FIG. 8C  illustrates an AC voltage applied to an EPD thin film heater in one embodiment. In this example, AC voltage  820  may be applied to common electrode  802  to enable thin film heater  800  to produce heat. In one embodiment, the frequency of the AC signal  820  is such that the EPD cells of the display comprising the EPD thin film heater do not react quickly enough to be affected materially by the AC signal. As a result, the cells do not change state (or remain in substantially the same state) when the AC signal is applied for heating at a time when no DC voltage is being applied to the common electrode for driving. Likewise, when the AC signal is applied to generate heat during driving the stable state of the cells is determined by the DC driving voltage applied to the common electrode and the voltages applied to other electrodes associated with the cells (e.g., pixel or segment electrodes), and not by the AC signal being applied for heating. 
         [0074]      FIG. 9A  illustrates a passive matrix EPD electrode and heater configuration  900  used in one embodiment. In the example shown, the passive matrix EPD electrode and heater configuration  900  comprises a plurality of column electrodes  902  in a first electrode layer positioned on a first side of the EPD cell layer (not shown) and a plurality of row electrodes  904  in a second electrode layer positioned on a second side of the EPD cell layer opposite the first side. In one embodiment, the column electrodes  902  comprise transparent conductive material, such as ITO or other transparent conductive material, and are located on the viewing side of the display. Row electrodes  904  may use thin film conductive material that generates heat when current passes through the electrode. Each of row electrodes  904  has a pair of contact pads  915 - 917  at either end for making electrical contact. In one alternative embodiment, each row electrode may have a contact pad on only one side of the electrode. Each of row electrodes  904  is configured to enable application of one or the other of two DC voltages used in the driving scheme. In the example shown in  FIG. 9A , the DC voltages are supplied by DC voltage source  910  and DC voltage source  912 . A voltage signal is provided to a contact pad at one end of each row electrode via a switch associated with DC supply lines  915 - 917 , respectively, and to a corresponding contact pad at the opposite end of the row electrode via a conductive trace (not shown). Each of the DC supply lines  915 - 917  may be connected via an associated switch with a selected one of the two DC voltage sources  910  and  912 . This enables the DC voltage provided by DC voltage source  910  to be applied to the contact pads of the row electrode when the corresponding switch is aligned to DC voltage source  910 . Alternatively, the DC voltage provided by DC voltage source  912  can be applied instead to the contact pads when the switches associated with DC supply lines  915 - 917  are aligned to DC voltage source  912 . In this example, the DC voltage supplied by DC voltage source  910  is 10V DC and the DC voltage supplied by DC voltage source  912  is 30V DC. 
         [0075]    In this example, the configuration  900  is used in a passive matrix EPD in which 10 V DC is applied to non-scanning rows during driving and 30 V DC is applied to the scanning row(s). Each of the row electrodes  904  also is configured to have applied to it, as desired, for heating an AC signal supplied by AC voltage source  914 . If heating is not desired, then the switch associated with AC voltage source  914  is aligned to connect the AC supply lines to the adders  911  to ground (i.e., opposite the position as shown in  FIG. 9A ), thus preventing the supply of an AC voltage to adders  911  and, subsequently, row electrodes  904 . For non-scanning rows of the passive matrix EPD comprising configuration  900 , the 10V DC voltage supplied by DC voltage source  910  is applied, and the AC signal supplied by AC voltage source  914  may also be applied via the adders  911  as desired to generate heat. For scanning rows, the 30V DC voltage supplied by DC voltage source  912  is applied, and the AC signal supplied by AC voltage source  914  may also be applied through adder  911  as desired to generate heat. If heating is desired when the display is not being scanned, the AC signal supplied by AC voltage source  914  may be applied without also applying a DC voltage to the row electrodes  904 . As in the embodiments described above in connection with  FIG. 8A , the frequency of the AC signal supplied by AC voltage source  914  is selected such that it does not affect the state of the EPD cells materially and does not interfere materially with the driving of the EPD cells to desired stable states, which is determined instead by the DC voltages applied to the row and column electrodes during driving. 
         [0076]    The frequency of the AC signal used for heating must be selected based on the characteristics of the EPD cells and associated structures. In one embodiment, the frequency is selected based on simulations performed in which the response of the EPD cells to the application of different AC waveforms is simulated and the AC waveform selected for heating is one to which the EPD cells did not react materially. In one alternative approach, a physical embodiment of the EPD cells and associated structures may be constructed and a variety of AC waveforms actually applied to the physical embodiment to observe the response of the EPD cells to the various AC waveforms. 
         [0077]    In the example shown in  FIG. 9A , the rows associated with the top and bottom row electrodes  904  are not being scanned and are therefore shown as being connected to non-scanning row DC voltage source  910 . The row associated with center row electrode  904  is being scanned and is therefore shown, in this example, as being connected to scanning row DC voltage source  912 . The AC voltage source  914  may be enabled to generate heating, if desired, or disabled when heating is not required. 
         [0078]      FIG. 9B  illustrates a 10V DC voltage  920  supplied by non-scanning row DC voltage source  910  of  FIG. 9A  in one embodiment. In this example, 10V DC is the DC voltage that must be applied to non-scanning rows of the passive matrix EPD in order to ensure that the cross bias applied to cells of the non-scanning rows is less than the threshold voltage. 
         [0079]      FIG. 9C  illustrates a 30V DC voltage  925  supplied by scanning row DC voltage source  912  of  FIG. 9A  in one embodiment. In this example, 30V DC is applied to the scanning row, such as the row associated with center row electrode  904  of  FIG. 9A , for driving. 
         [0080]      FIG. 9D  illustrates an AC voltage  930  supplied by the AC voltage source  914  of  FIG. 9A  in one embodiment. AC voltage  930  varies between 10V and −10V and, as described above, switches between the two levels with a frequency selected such that the EPD cells do not have time to react materially to the AC signal  930 . 
         [0081]    For an EPD, (e.g., a passive matrix EPD) similar configurations to those described above can be applied. In other embodiments, different configurations can be implemented including variations to cell and electrode matrices, AC/DC voltages, contact pads, electrode materials as well as other aspects of the thin film heater. 
         [0082]    Significant advantage can occur by using one or more electrodes of an EPD to generate heat, as described above. This eliminates additional costs for an outside heating device, or for incorporating additional materials and structures into the EPD to provide for heating, and improves optical performance of the display by reducing the obstructions to light emitting from an EPD. 
         [0083]    While the thin film heaters of  FIGS. 8A-9D  are described in connection with an EPD, those of ordinary skill in the art will recognize that the techniques described herein may be applied as well to other types of device in which an AC signal may be applied to an existing electrode or other conductive structure to generate heat, as desired, without interfering materially with the normal operation of the device and in particular with the primary function of the structure used for heating. 
         [0084]      FIG. 10A  is a state diagram illustrating a thermal control algorithm  1000  used in one embodiment. In one embodiment, the algorithm shown in  FIG. 10A  is used in connection with a thermal control system comprising a thermal control pad capable of either heating or cooling the EPD, as needed, such as the thermal control pad  522  of  FIG. 5C . The EPD thermal control system, if not heating or cooling, may be in a standby state  1002  in which heating and cooling pads are not actively heating or cooling the EPD. In one embodiment, the thermal control system is deactivated when in standby state  1002 . In one embodiment, the thermal control system is not fully deactivated but instead is running in a standby mode when in standby state  1002 . 
         [0085]    If when the thermal control system is in the standby state  1002  the EPD display media temperature drops below a heat activation threshold temperature T H1 , the thermal control system transitions as illustrated in  FIG. 10A  to a “heating” state  1004  in which the heating functionality of the thermal control pad is activated to provide heating to the EPD display media. If, when the thermal control system is in the “heating” state  1004 , the EPD display media temperature rises above a “deactivate heat” threshold temperature T H2 , the heating functionality of the thermal control pad is deactivated and the system returns to the standby state  1002 . In one embodiment, the deactivate heat threshold temperature is different from the activate heat threshold temperature. In one embodiment, the activate threshold temperature T H1  is less than the deactivate heat threshold temperature T H2 . In other embodiments, the activate threshold temperature, T H1 , may be the same as the deactivate heat threshold temperature, T H2 . If the system is in the standby state  1002  and the temperature rises above an “activate cooling” temperature T C1 , the thermal control system transitions to a “cooling” state  1006 . In one embodiment, the cooling functionality of the thermal control pad is used to cool the EPD display media when the thermal control system is in cooling state  1006 . If when the system is in cooling state  1006  the temperature drops below a “deactivate cooling” threshold temperature, T C2 , the system transitions to the standby state  1002  and the cooling functionality of the thermal control pad is deactivated. In one embodiment, the activate cooling threshold temperature, T C1 , may be different than the deactivate cooling threshold temperature, T C2 . In one embodiment, the activate cooling threshold, T C1 , is greater than the deactivate cooling threshold, T C2 . In one alternative embodiment, the activate cooling threshold temperature, T C1 , may be the same as the deactivate cooling threshold temperature, T C2 . 
         [0086]      FIG. 10B  is a state diagram illustrating a thermal control algorithm  1010  used in one embodiment. In one embodiment, the algorithm  1010  is implemented to control a thermal control system that comprises a heating pad, such as the heating pad  512  of  FIG. 5A . The EPD thermal control system, if not heating, may be in a standby state  1012  in which the heating pads is not actively heating the EPD. In one embodiment, the thermal control system is deactivated when in standby state  1012 . In one embodiment, the thermal control system is not fully deactivated but instead is running in a standby mode when in standby state  1012 . 
         [0087]    If when the thermal control system is in the standby state  1012  the EPD display media temperature drops below a heat activation threshold temperature T H1 , the thermal control system transitions as illustrated in  FIG. 10B  to a “heating” state  1014  in which the heating pad is activated to provide heating to the EPD display media. If when the thermal control system is in the “heating” state  1014  the EPD display media temperature rises above a “deactivate heat” threshold temperature, T H2 , the heating pad is deactivated and the system returns to the standby state  1012 . In one embodiment, the deactivate heat threshold temperature, T H2 , is different than the activate heat threshold temperature, T H1 . In one alternative embodiment, the deactivate heat threshold, T H2  may be the same as the activate heat threshold temperature, T H1 . 
         [0088]      FIG. 10C  is a state diagram  1020  illustrating a thermal control algorithm  1020  used in one embodiment. In one embodiment, the algorithm  1020  is implemented to control a thermal control system that comprises a cooling pad, such as cooling pad  520  of  FIG. 5B . The EPD thermal control system, if not cooling, may be in a standby state  1022  in which the cooling pad is not actively cooling the EPD. In one embodiment, the thermal control system is deactivated when in standby state  1022 . In one embodiment, the thermal control system is not fully deactivated but instead is running in a standby mode when in standby state  1022 . If when the system is in the standby state  1022  the temperature rises above an “activate cooling” temperature T C1 , the thermal control system transitions to a “cooling” state  1024 . In one embodiment, the cooling pad is used to cool the EPD display media when the thermal control system is in activate cooling state  1024 . If when the system is in “cooling” state  1024  the temperature drops below a “deactivate cooling” threshold temperature, T C2 , the system transitions to the standby state  1022  and the cooling pad is deactivated. In one embodiment, the activate cooling threshold, T C1 , is greater than the deactivate cooling threshold, T C2 . In one embodiment, the “activate cooling” threshold temperature, T C1 , may be the same as the “deactivate cooling” threshold temperature, T C2 . 
         [0089]      FIG. 11  illustrates an EPD thermal control system  1100  used in one embodiment. Thermal control pad  1102  receives driving voltages from thermal control pad driver  1104 . Overall control of thermal control system  1100  is provided by thermal control logic module  1106 , which receives temperature signals from sensor  1108 . In one embodiment, the sensor  1108  is configured to sense the ambient temperature and the thermal control logic  1106  controls the thermal control pad driver circuit as required to maintain the EPD display media temperature at the desired level based on the ambient temperature. In one embodiment, the sensor  1108  is configured to sense the temperature of the EPD display media. In one embodiment, the sensor  1108  comprises a temperature sensor positioned in or near the EPD cell media. In one embodiment, the sensor  1108  comprises a thermocouple embedded in the EPD display media. In one embodiment, the sensor  1108  may comprise multiple sensors configured to sense parameters other than environmental and/or display media temperature, such as humidity or other environmental conditions. 
         [0090]    In one embodiment, the logic implemented on thermal control logic  1106  comprises one of the algorithms illustrated in  FIG. 10A ,  10 B, or  10 C, depending on the type of thermal control pad included in the EPD (i.e., heating/cooling, heating, or cooling). 
         [0091]    Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.