PATENT DOCUMENT

Publication Number: US-8905566-B2
Application Number: US-201213534170-A
Country: US
Kind Code: B2

Title: Thermal mitigation of flat-panel displays

Abstract:
A flat-panel display (such as a liquid crystal display) includes one or more features that reduce temperature changes in an array of light valves disposed on a substrate. In particular, heat generated in a light source and/or electronics is conducted away from the light values. For example, a light-source holder, which contains the light source, may include holes that increase the thermal resistance between the light source and the substrate. Alternatively, the light-source holder may include a material having a thermal conductivity that is higher than that of stainless steel. In addition, the flat-panel display may include materials and/or a geometry that increases the thermal resistance along a direction in a plane of the substrate. By reducing temperature changes in the flat-panel display, these features can reduce or eliminate color changes and other visual artifacts that can degrade the quality of displayed images.

Claims:
What is claimed is: 
     
       1. A display, comprising:
 a substrate with a front surface and a back surface having a two-dimensional array of light valves disposed on the front surface; 
 a light-source holder with an outer surface and an inner surface, wherein the outer surface is thermally coupled to the back surface of the substrate; and 
 a light source contained within a cavity defined by the inner surface of the light-source holder, wherein the light-source holder includes holes defined by edges between the outer surface and the inner surface to increase a thermal resistance between the light source and the substrate. 
 
     
     
       2. The display of  claim 1 , wherein the holes in the light-source holder are disposed in a region of the outer surface of the light-source holder which is thermally coupled to the back surface of the substrate. 
     
     
       3. The display of  claim 1 , wherein the light-source holder includes a material having a thermal conductivity greater than that of stainless steel. 
     
     
       4. The display of  claim 1 , wherein the display further comprises:
 a circuit board with a top surface and a bottom surface, wherein the top surface is thermally coupled to the outer surface of the light-source holder; 
 a radio-frequency shield enclosing integrated circuits disposed on the bottom surface of the circuit board; 
 an integrated circuit disposed on the front surface of the substrate; and 
 thermal tape thermally coupling the integrated circuit to the radio-frequency shield. 
 
     
     
       5. The display of  claim 1 , wherein the display further includes:
 a circuit board with a top surface and a bottom surface, wherein the top surface is thermally coupled to the outer surface of the light-source holder; and 
 a material between the outer surface of the light-source holder and the top surface of the circuit board which decreases a thermal resistance between the light-source holder and the circuit board. 
 
     
     
       6. The display of  claim 1 , wherein the display further includes:
 a circuit board with a top surface and a bottom surface, wherein the top surface is thermally coupled to the outer surface of the light-source holder; 
 a plate adjacent to the circuit board and thermally coupled to the outer surface of the light-source holder; and 
 a material between the outer surface of the light-source holder and the plate to decrease a thermal resistance between the light-source holder and the plate. 
 
     
     
       7. The display of  claim 6 , wherein the material is at least in part surrounded by another material. 
     
     
       8. The display of  claim 1 , wherein the display further includes a light pipe optically coupled to the light source and the back surface of the substrate; and
 wherein the light pipe has a higher thermal conductivity along a symmetry axis of the light pipe than perpendicular to the symmetry axis. 
 
     
     
       9. The display of  claim 1 , wherein the display further includes:
 a light pipe optically coupled to the light source and the back surface of the substrate; and 
 an optical reflector optically coupled to the light pipe on an opposite side of the light pipe than the substrate, wherein the optical reflector includes an optical component and a structural component, 
 wherein the structural component has a higher thermal conductivity than the optical component. 
 
     
     
       10. A portable electronic device, comprising:
 a display, wherein the display includes:
 a substrate with a front surface and a back surface having a two-dimensional array of light valves disposed on the front surface; 
 a light-source holder having an outer surface and an inner surface, wherein the outer surface is thermally coupled to the back surface of the substrate; and 
 a light source contained within a cavity defined by the inner surface of the light-source holder, wherein the light-source holder includes holes defined by edges between the outer surface and the inner surface to increase a thermal resistance between the light source and the substrate. 
 
 
     
     
       11. The electronic device of  claim 10 , wherein the holes in the light-source holder are disposed in a region of the outer surface of the light-source holder which is thermally coupled to the back surface of the substrate. 
     
     
       12. The electronic device of  claim 11 , wherein the light-source holder includes a material having a thermal conductivity greater than that of stainless steel. 
     
     
       13. The electronic device of  claim 11 , wherein the display includes a circuit board with a top surface and a bottom surface;
 wherein the top surface is thermally coupled to the outer surface of the light-source holder; and 
 wherein the light-source holder has a height between the back surface of the substrate and the top surface of the circuit board which is less than 2 mm. 
 
     
     
       14. The electronic device of  claim 11 , wherein the light-source holder has a thickness between the inner surface and the outer surface which is less than 0.2 mm. 
     
     
       15. A display, comprising:
 a substrate having a front surface and a back surface with a two-dimensional array of light valves disposed on the front surface; 
 a light source; and 
 a light pipe optically coupled to the light source and the back surface of the substrate, 
 wherein the light pipe has a higher thermal conductivity along a symmetry axis of the light pipe than perpendicular to the symmetry axis. 
 
     
     
       16. The display of  claim 15 , wherein the light pipe includes polymers that are at least partially aligned along the symmetry axis. 
     
     
       17. The display of  claim 15 , wherein the light pipe includes metal particles. 
     
     
       18. The display of  claim 15 , wherein the display further includes an optical reflector optically coupled to the light pipe on an opposite side of the light pipe than the substrate;
 wherein the optical reflector includes an optical component and a structural component; and 
 wherein the structural component has a higher thermal conductivity than the optical component. 
 
     
     
       19. The display of  claim 18 , wherein the structural component includes a material other than plastic. 
     
     
       20. A portable electronic device, comprising:
 a display, wherein the display includes:
 a substrate having a front surface and a back surface with a two-dimensional array of light valves disposed on the front surface; 
 a light source; and 
 a light pipe optically coupled to the light source and the back surface of the substrate, 
 wherein the light pipe has a higher thermal conductivity along a symmetry axis of the light pipe than perpendicular to the symmetry axis.

Description:
BACKGROUND 
     1. Field 
     The described embodiments relate to techniques for controlling the temperature of flat-panel displays. More specifically, the described embodiments relate to techniques for reducing the absolute temperature and temperature gradients in flat-panel displays in portable electronic devices. 
     2. Related Art 
     Flat-panel displays are typically lighter, thinner and consume less power than traditional displays, such as cathode ray tubes. As a consequence, flat-panel displays are widely used in portable electronic devices. 
     Many flat-panel displays are active-matrix addressed displays in which two-dimensional arrays of light valves are used to modulate light from a light source. For example, in liquid crystal displays, a liquid crystal is sandwiched between two electrodes. By applying an electrical voltage between the electrodes, the light diffusing or polarizing properties of the liquid crystal can be modified so that light is transmitted or blocked. The spatial modulation of the light across a two-dimensional array creates an image. 
     However, the light sources (such as light-emitting diodes) and electronics used to illuminate and apply electrical signals to liquid crystal displays generate heat. This heat can cause temperature changes in a liquid crystal display. Because the optical properties of liquid crystals are often a function of temperature, these temperature changes can cause color changes and other visual artifacts that can degrade the quality of the displayed image. 
     SUMMARY 
     The described embodiments include a display that includes a substrate with a front surface and a back surface having a two-dimensional array of light valves disposed on the front surface. The display also includes a light-source holder with an outer surface and an inner surface, where the outer surface is thermally coupled to the back surface of the substrate. Moreover, a light source (such as a light-emitting diode) is contained within a cavity defined by the inner surface of the light-source holder. In order to increase the thermal resistance between the light source and the substrate (and, thus, to control heating of the substrate by the light source), the light-source holder includes holes defined by edges between the outer surface and the inner surface. 
     For example, the holes in the light-source holder may be disposed in a region of the outer surface of the light-source holder which is thermally coupled to the back surface of the substrate. Additionally, the light-source holder may include a material having a thermal conductivity greater than that of stainless steel. 
     In some embodiments, the display includes a circuit board with a top surface and a bottom surface, where the top surface is thermally coupled to the outer surface of the light-source holder. Moreover, the display may include a radio-frequency shield enclosing integrated circuits disposed on the bottom surface of the circuit board. Furthermore, an integrated circuit may be disposed on the front surface of the substrate, and thermal tape may thermally couple the integrated circuit to the radio-frequency shield. 
     The display may include a material between the outer surface of the light-source holder and the top surface of the circuit board that may decrease the thermal resistance between the light-source holder and the circuit board. Alternatively or additionally, the display may include a plate adjacent to the circuit board and thermally coupled to the outer surface of the light-source holder, and a material between the outer surface of the light-source holder and the plate may decrease the thermal resistance between the light-source holder and the plate. For example, the material may be at least in part surrounded by another material. 
     In some embodiments, the display includes a light pipe optically coupled to the light source and the back surface of the substrate, where the light pipe has a higher thermal conductivity along a symmetry axis of the light pipe than perpendicular to the symmetry axis. Alternatively or additionally, the display may include an optical reflector optically coupled to the light pipe on an opposite side of the light pipe than the substrate. The optical reflector may include an optical component and a structural component, where the structural component has a higher thermal conductivity than the optical component. 
     The light-source holder may have a height between the back surface of the substrate and the top surface of the circuit board which is less than 2 mm. Furthermore, the light-source holder may have a thickness between the inner surface and the outer surface which is less than 0.2 mm. 
     Another embodiment provides a display that includes: the substrate, the light source, and the light pipe. The light pipe may have a higher thermal conductivity along a symmetry axis of the light pipe than perpendicular to the symmetry axis. For example, the light pipe may include polymers that are at least partially aligned along the symmetry axis and/or metal particles. Alternatively or additionally, the display may include the optical reflector with the optical component and the structural component. The structural component may include a material other than plastic. 
     Another embodiment provides a portable device that includes one of the embodiments of the display. 
     Another embodiment provides a method for controlling the temperature of a display. During operation, the light source, contained within the cavity defined by the inner surface of the light-source holder, provides light to the two-dimensional array of light valves disposed on a surface of the substrate. Moreover, the thermal resistance between the light source and the substrate is increased by holes defined by edges between the outer surface of the light-source holder and the inner surface, where the outer surface is thermally coupled to the back surface of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram illustrating a cross-sectional view of a display in accordance with an embodiment of the present disclosure. 
         FIG. 2  is a block diagram illustrating a top-view of a light-source holder in the display of  FIG. 1  in accordance with an embodiment of the present disclosure. 
         FIG. 3  is a block diagram illustrating a cross-sectional view of a thermal-coupling material in the display of  FIG. 1  in accordance with an embodiment of the present disclosure. 
         FIG. 4  is a block diagram illustrating a portable electronic device that includes the display of  FIG. 1  in accordance with an embodiment of the present disclosure. 
         FIG. 5  is a flowchart illustrating a method for controlling the temperature of a display in accordance with an embodiment of the present disclosure. 
     
    
    
     Note that like reference numerals refer to corresponding parts throughout the drawings. Moreover, multiple instances of the same part are designated by a common prefix separated from an instance number by a dash. 
     DETAILED DESCRIPTION 
       FIG. 1  presents a block diagram illustrating a display  100 . This display includes a substrate  110  with a front surface  112  and a back surface  114  having a two-dimensional array of light valves  116  disposed on front surface  112 . For example, display  100  may include a liquid crystal display, and light valves  116  may be driven using thin-film transistors. Display  100  also includes a light-source holder  118  with an outer surface  120  and an inner surface  122 , where outer surface  120  is thermally coupled to back surface  114 . Moreover, a light source  124  (such as a light-emitting diode or a light-emitting-diode array) is contained within a cavity  126  defined by inner surface  122  of the light-source holder  118 . 
     During operation of display  100 , heat generated by light source  124  is conducted to substrate  110 . As noted previously, this heat can result in a change in the temperature and/or temperature gradients in light valves  116 , and thus in color and visual artifacts in images displayed on display  100 . Because heat conduction is governed by Fourier&#39;s law of heat conduction, in a one-dimensional analog of display  100  the thermal resistance (R) between light source  124  and substrate  110  can be expressed as 
                     R   =     L     κ   ·   A         ,           (   1   )               
where L is a length of a thermal path (in this case, a distance along light-source holder  118 ), K is the thermal conductivity of light-source holder  118 , and A is a cross-sectional area of light-source holder  118 . Note that in applications such as portable electronic device  400 , which is described below with reference to  FIG. 4 , space and weight constraints may restrict the material(s) and the geometry of light-source holder  118 .
 
     Based on Eqn. 1, R between light source  124  and substrate  110  can be increased (and, thus, heating of substrate  110  by light source  124  can be reduced) by decreasing the effective cross-sectional area and/or K. For example, light-source holder  118  may include holes defined by edges (such as hole  108  defined by edges  128 ) between outer surface  120  and inner surface  122  (such as 10×1 mm 2  oblong holes staggered approximately every 5 mm, which are illustrated in  FIG. 2 ). These holes may be disposed in a region  130  which is thermally coupled to back surface  114 , and may increase R without reducing a mechanical strength of light-source holder  118 . 
     Additionally, light-source holder  118  may include a material having K greater than that of stainless steel. For example, light-source holder  118  may include a copper alloy such as C18080 (from Olin Brass of Louisville, Ky.) that has a K twenty-times higher than that of stainless steel. 
     Display  100  may include: a circuit board  132  with a top surface  134  and a bottom surface  136 , where top surface  134  is thermally coupled to outer surface  120 ; a radio-frequency (RF) shield  138  enclosing integrated circuits (ICs)  140  disposed on bottom surface  136 ; and an integrated circuit (IC)  142  (such as a display driver) disposed on front surface  112 . During operation of display  100 , heat may also be generated by IC  142 . Therefore, IC  142  may be thermally coupled to RF shield  138  to reduce the heating of substrate  110  (and, thus, light valves  116 ). For example, optional thermal tape  144  (such as copper tape) may thermally couple IC  142  to RF shield  138 . 
     In addition to the thermal properties, light-source holder  118  may be designed to have particular mechanical properties or a geometry needed for use in applications such as portable electronic devices. For example, light-source holder  118  may have a height  146  between back surface  114  and top surface  134  which is less than 2 mm (such as 1.37 mm). Furthermore, light-source holder  118  may have a thickness  148  between outer surface  120  and inner surface  122  which is less than 0.2 mm (such as 0.15 mm). 
     Alternatively or additionally to the aforementioned features in light-source holder  118 , heat conduction along a direction  150  perpendicular to a plane of substrate  110  in display  100  may be increased to allow heat generated by light source  124  and/or electronics (such as IC  142 ) to conduct away from substrate  110  (and, thus, light valves  116 ). For example, an optional material  152  (such as a thermally conducting grease, a thermal adhesive, a metallic tape, etc.) may be positioned between outer surface  120  and top surface  134 , which may decrease R between light-source holder  118  and circuit board  132 . Furthermore, display  100  may include a metal plate  154  adjacent to circuit board  132 . This plate may be thermally coupled to outer surface  120  by an optional material  156  between outer surface  120  and plate  154 , thereby decreasing R between light-source holder  118  and plate  154 . For example, optional material  156  may include: solder, a pressure-sensitive adhesive or a thermal grease (such as silicone grease). However, it can be difficult to control the gap or spacing between outer surface  120  and plate  154  using a pressure-sensitive adhesive. Note that, because thermal greases can be hard to handle or contain during assembly or rework of display  100  (i.e., they can be messy), optional material  156  may, at least in part, be surrounded by another optional material  158  (as shown in  FIG. 3 ). For example, optional material  156  may be rolled in a thermal fabric or contained in a tube. 
     Display  100  may include a light pipe  160  optically coupled to light source  124  and back surface  114 . In particular, light pipe  160  may convey light generated by light source  124  and may illuminate light valves  116 . To facilitate heat conduction along direction  150 , light pipe  160  may have a higher K along a symmetry axis  162  of light pipe  160  than perpendicular to symmetry axis  162 . For example, light pipe  160  may include polymers that are at least partially aligned along symmetry axis  162  (such as D or E-series polymers from Cool Polymers, Inc. of North Kingstown, R.I.) and/or a thermally enhanced polycarbonate that includes metal particles. These features in light pipe  160  may increase its heat spread capability so that heat is conducted away from substrate  110  and uniformly conducted across light values  116  (i.e., heat is uniformly transported across light values  116 ). 
     Furthermore, display  100  may include a reflector  164  optically coupled to light pipe  160  on an opposite side of light pipe  160  than substrate  110 . An optical component  166  (such as a so-called ‘white’ reflector or a specular reflector from 3M, Inc. of Minneapolis, Minn.) in reflector  164  may reflect light along direction  150 . In addition, a structural component  168  in reflector  164  may provide mechanical support for optical component  166 . To facilitate heat diffusion along direction  150 , structural component  168  may have a higher K than optical component  166 . For example, structural component  168  may include a material other than plastic, such as copper foil or a graphite film instead of polyethylene terephthalate. These features in reflector  164  may increase its heat spread capability so that heat is conducted away from substrate  110  and uniformly conducted across light values  116  (i.e., heat is uniformly transported across light values  116 ). 
     As noted previously, display  100  may be used in a portable electronic device. This is shown in  FIG. 4 , which presents a block diagram illustrating a portable electronic device  400 . Portable electronic device  400  may include: one or more program modules or sets of instructions stored in an optional memory subsystem  410  (such as DRAM or another type of volatile or non-volatile computer-readable memory), which may be executed by an optional processing subsystem  412 . Note that the one or more computer programs may constitute a computer-program mechanism. Moreover, instructions in the various modules in optional memory subsystem  410  may be implemented in: a high-level procedural language, an object-oriented programming language, and/or in an assembly or machine language. Furthermore, the programming language may be compiled or interpreted, e.g., configurable or configured, to be executed by optional processing subsystem  412 . 
     In some embodiments, functionality in these circuits, components and devices may be implemented in one or more: application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), and/or one or more digital signal processors (DSPs). Moreover, the circuits and components may be implemented using any combination of analog and/or digital circuitry, including: bipolar, PMOS and/or NMOS gates or transistors. Furthermore, signals in these embodiments may include digital signals that have approximately discrete values and/or analog signals that have continuous values. Additionally, components and circuits may be single-ended or differential, and power supplies may be unipolar or bipolar. Note that components in display  100  ( FIG. 1 ) and portable electronic device  400  may be directly or indirectly thermally coupled. 
     Portable electronic device  400  may include one of a variety of devices that can include a display, including: a desktop computer, a server, a laptop computer, a media player (such as an MP3 player), an appliance, a subnotebook/netbook, a tablet computer, a smartphone, a cellular telephone, a network appliance, a set-top box, a personal digital assistant (PDA), a toy, a controller, a digital signal processor, a game console, a device controller, a computational engine within an appliance, a consumer-electronic device, a portable computing device, a personal organizer, and/or another electronic device. 
     More generally, the thermal management techniques illustrated in display  100  ( FIG. 1 ) may be used in an electronic device in which the display is optimized for thickness based on size and/or weight constraints. 
     Although we use specific components to describe display  100  ( FIG. 1 ) and portable electronic device  400 , in alternative embodiments different components and/or subsystems may be used. For example, the preceding embodiments illustrated the use of passive thermal techniques to mitigate temperature-related visual artifacts in displayed images. However, in other embodiments display  100  ( FIG. 1 ) may use active techniques to manage the temperature, including a feedback technique based on one or more thermal sensors (such as temperature sensors or sensors that determine the polarization associated with a liquid crystal). Moreover, ICs  140  ( FIG. 1 ) may have increased copper thickness and thermal vias to conduct heat away from substrate  110  ( FIG. 1 ). Furthermore, while a liquid crystal display was used as an illustration, in other embodiments the aforementioned thermal-management techniques may be applied in a wide variety of flat-panel displays, including: an electroluminescent display, a field emission (or nano-emissive) display, an interferometric modulator display, a light-emitting-diode display, an organic light-emitting-diode display, a plasma display, a quantum dot display, or a surface-conduction electron-emitter display. 
     Additionally, one or more of the components may not be present in  FIGS. 1-3 . In some embodiments, display  100  ( FIG. 1 ) and/or portable electronic device  400  include one or more additional components that are not shown in  FIGS. 1-3 . Also, although separate components are shown in  FIGS. 1-3 , in some embodiments some or all of a given component can be integrated into one or more of the other components and/or positions of components can be changed. 
     In the preceding description, we refer to ‘some embodiments.’ Note that ‘some embodiments’ describes a subset of all of the possible embodiments, but does not always specify the same subset of embodiments. 
     We now describe embodiments of a method.  FIG. 5  presents a flowchart illustrating a method  500  for controlling the temperature of a display, such as display  100  ( FIG. 1 ). During operation, the light source, contained within the cavity defined by the inner surface of the light-source holder, provides light to the two-dimensional array of light valves disposed on the front surface of the substrate (operation  510 ). Moreover, the thermal resistance between the light source and the substrate is increased by holes defined by edges between the outer surface and the inner surface of the light-source holder (operation  512 ), where the outer surface is thermally coupled to the back surface of the substrate. 
     In some embodiments of method  500 , there may be additional or fewer operations. Moreover, the order of the operations may be changed, and/or two or more operations may be combined into a single operation. 
     The foregoing description is intended to enable any person skilled in the art to make and use the disclosure, and is provided in the context of a particular application and its requirements. Moreover, the foregoing descriptions of embodiments of the present disclosure have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present disclosure to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Additionally, the discussion of the preceding embodiments is not intended to limit the present disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

Metadata:
Filing Date: 20120627
Publication Date: 20141209
Grant Date: 20141209
Priority Date: 20120627
Inventors: CHOWDHURY IHTESHAM H.
LIANG FRANK F.
HERESZTYN AMAURY J.
FRANKLIN JEREMY C.
RAPPOPORT BENJAMIN M.
WRIGHT DEREK W.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B6/0085", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/0085", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 49777961