Patent Application
20100142154
1. Technical Field
This invention relates generally to housings for electrical systems having heat generating components and more particularly to a housing system for handheld devices that facilitates heat dissipation through the housing to the external environment while providing shock absorbing properties to sensitive components disposed within the housings.
2. Background Art
The manufacturing and design technology for electronic devices has advanced significantly in recent years. Modern electronic devices are often portable and offer increased functionality and performance in smaller packages. For instance, in the field of image projection systems, image projectors were once large, bulky, noisy devices that required a sturdy table upon which to rest while in operation. Today, advances in technology provide projection systems that are easily portable and that can be connected to a portable computer or handheld device.
In short, today's devices “accomplish more in less space.” Portable computers, mobile data devices, gaming devices, and multimedia players are incorporating more processing power, more functionality, and more components into smaller mechanical form factors. One issue associated with this trend towards miniaturization is that of heat dissipation. Electronic components must be kept cool to function properly. When these components overheat, their reliability can be compromised. One technique used for cooling compact electronic devices is dissipating heat into the surrounding environment.
In many systems, designers must focus additional thermal management attention on a few specific components. For instance, power supplies, microprocessors, and optical components such as image projection devices tend to produce large amounts of heat. Consequently, they are more difficult to keep cool. Further compounding the issue is the fact that these components are often more sensitive to temperature changes. As a result, improper thermal management can compromise their performance.
Turning now to
Thermal heat sinks 106,107, which are generally manufactured from a rigid material such as an aluminum alloy, are mounted directly to the heat generating electronic component 101. These heat sinks 106,107 generally include a set of fins 108 that extend outwardly from the heat sink 107 so as to increase the overall surface area of the heat sink 107. In some devices, the fins 108 protrude through the airflow perforations 105 in an attempt to deliver more of the heat to the air outside the housing 103.
The problem with these rigid heat sinks 106,107 is four-fold: First, to be effective the surface of heat generating electronic component 101 coupling to the rigid heat sinks 106,107, as well as the heat sinks 106,107 themselves, must have a relatively large surface area. In today's compact electronics, this is seldom the case. Second, in addition to increasing the overall cost of the system 100, the attachment of heat sinks 106,107 to the heat generating electronic component 101 effectively increases the mass of that electronic component. When the system 100 is subjected to mechanical shock, such as in the drop testing commonly required for certification of consumer electronics, the reliability of sensitive devices like optical projection components can be compromised due to the excessive forces being applied to those components when the system collides with a hard surface.
Third, heat sink mounting systems can be unreliable due to the difficulties associated with mechanical adhesion systems. Further, the bulk and weight of most heat sinks can make coupling even more difficult. When adhesives and clips are used to mount heat sinks to components, the attachment may not be stable or reliable. Further, it may impair the operation of components like optical projection elements.
Fourth, heat sinks take up large amounts of room within the housing. Consumers today are demanding smaller and smaller electronics. There is often simply not enough real estate within a device to include bulky, metal heat sinks.
Note that in some other prior art systems, in an attempt to reduce the size of heat sinks that are required, fans are added within the housing to improve airflow. Such fans, working in conjunction with the airflow perforations, attempt to move heat from the interior of the housing to the exterior of the device by forcing air through the airflow perforations. The problems associated with fans are reliability, size, and power consumption. When a fan fails, it is easy for a device to quickly overheat. Second, fans use relatively large amounts of energy. In a portable electronic device that operates with a battery as an energy source, the inclusion of a fan means a much shorter run time on a single charge cycle. Additionally, fans are large devices that often require larger housings to accommodate them.
There is thus a need for an improved system for dissipating heat to the surrounding environment without the need for a fan or air-flow perforations, and that offers sufficient shock absorption so as not to impair component operation when the system is dropped.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed the apparatus components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such apparatus components with minimal experimentation.
Embodiments of the invention are now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” Relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, reference designators shown herein in parenthesis indicate components shown in a figure other than the one in discussion. For example, talking about a device (10) while discussing figure A would refer to an element, 10, shown in figure other than figure A.
Passively cooling, i.e., cooling without a fan, forced liquid, or other powered cooling device, small form factor, high-power electronics provides a significant design challenge. For example, many consumer electronics devices have strict requirements regarding “touch temperature.” A particular device may be required to be able to operate continually in a warm environment with the surface temperature of the device never exceeding a predetermined limit, such as 50 or 60 degrees centigrade, even though the components may be operating at 60 or 65 degrees centigrade within the device. For example, some standards set forth momentary contact temperature exposure limits with which devices must comply. Exemplary standards include MIL-STD-1472F and IEC 60950-1. These standards also set forth continuous contact temperature exposure limits in some cases. In optical devices, such as laser-based projection systems, the design constraints can be especially daunting given the limited types of available enclosure materials.
By way of example, metal enclosures—while working well to transfer heat from the interior of the enclosure to the outer environment—generally become too hot to handle at relativelty low surface temperatures limiting the delta temperature with the ambient and therefore the total heat transfer Plastic enclosures tend to stay cooler to the touch due to their relatively lower thermal conductivity. However, they can tend to have “hot spots” and also tend not to effectively deliver heat from the components through enclosure to the environment.
To remedy these issues, embodiments of the present invention employ a hybrid housing that includes a rigid housing material, such as a thermoplastic, used in conjunction with a compliant thermally conductive heat spreader that passes along an interior of the housing. In addition to spreading heat across the surface of the enclosure, the compliant heat spreader also permits the rigid housing to retain its shock absorbing properties. In other words, the heat spreader is chosen to be compliant so that it will not interfere with the shock absorbing properties of the housing. Rather than using a rigid heat spreader—such as a piece of metal—within the device that would cause the rigidity of the overall housing to increase, embodiments of the present invention use a compliant material such as graphite to permit the thermoplastic housing to still absorb shock when the system is dropped. As such, sensitive electronics disposed within the housing are not subject to increased forces during drop due to the incorporation of the internal heat spreader. Further, enclosures in accordance with embodiments of the present invention provide comfortable touch temperatures, along with comparable heat removal properties exhibited by metal enclosures, while being easy and inexpensive to manufacture.
Embodiments of the present invention include a rigid housing manufactured from a shock absorbing, high emissivity, low-thermally conductive material such as thermoplastic along with a compliant, thermally conductive heat spreader disposed along an interior of the housing. The overall system facilitates heat transfer via radiation, convection and conduction from components within the system to the external environment, while still providing greater shock absorbing properties than prior art thermal management systems.
The compliant thermally conductive material, which in one embodiment is a die-cut, graphite sheet having a thickness of between 100 um and 500 um, acts as a heat spreader. The heat spreader provides a thermally conductive component that spreads heat along an interior surface area of the housing so as to make the enclosure appear to be effectively isothermal. In one embodiment, for instance, the heat spreader passes across at least fifty percent of the interior surface area of the housing. Embodiments of the invention constructed in this fashion deliver emissivitiy of between 0.6. and 1.0. Experimental testing has shown that an enclosure emissivity of 0.8 works well with embodiments of the invention.
The housing material, which in one embodiment is a polycarbonate-ABS plastic blend, provides a more comfortable touch temperature for the user. Further, when using a thermoplastic as the housing material, the touch temperature can be higher—while still being comfortable—than it can with other materials. For instance, while testing has shown that metal housings are comfortable only to forty degrees centigrade, plastic housings can still “feel” comfortable at sixty degrees centigrade when using thermoplastic housings with a heat spreader disposed beneath. Additionally, the housing material absorbs mechanical energy when the overall device is dropped, thereby insulating components within the housing from excessive drop forces.
The high-emissivity, low-thermally conductive housing material also allows the overall enclosure to deliver heat to the environment by radiation in addition to convection and conduction. Radiant heat transfer significantly improves the overall thermal performance of the system, in that it can account for nearly half the total heat dissipation in certain applications.
Turning now to
A rigid housing 203, shown in
Thermoplastics, in one embodiment, are chosen as materials for the rigid housing 203 for a variety of reasons. First, they are relatively inexpensive and easy to manufacture. Plastic housing members can be manufactured, for instance, by injection molding. Second, plastic housings are relatively impervious to shock. For instance, when they are dropped from a height of four or five feet to tile, wood, carpet, or concrete—as is sometimes required during consumer product drop testing—they generally withstand the fall without breaking. Third, plastic housings have good energy absorption benefits for components disposed within the housing. When dropped, the housing will absorb substantial portions of the energy delivered at impact, thereby insulating components disposed within the plastic housing from some of these forces. One other reason for selecting thermoplastics is that they can be easily molded into complex, thin-walled, organic shapes.
Another reason thermoplastic materials, such as a polycarbonate-ABS blend, are used with some embodiments of the invention is the thermal conductivity that can be achieved and designed into the material. Turning briefly to
The thermoplastic material can be designed to have any of a range of thermal conductivities. Embodiments of the present invention with laser projection systems have shown that rigid housings (203) having a thermal conductivity with a range 1004 of between 0.1 Watts/meter*Kelvin and 1.0 Watts/meter*Kelvin work well in that they deliver sufficient amounts of thermal energy to the exterior environment when employed with a compliant heat spreader without causing the surface temperature to become too hot. Said differently, this range minimizes the change in temperature between the interior (209) and the exterior (210) while maintaining an acceptable touch temperature of the exterior (210). Specifically, a 118 mm×61 mm×14 mm housing with a battery operated MEMS scanning mirror projector running therein can be kept below 55 degrees centigrade easily. Polycarbonate-ABS blends for some embodiments of the invention are therefore constructed to have thermal conductivities within this range 1004. Further, in accordance with some embodiments of the invention, the thickness (213) of the housing is selected to be between one and two millimeters. While thermoplastic materials are one type of material suitable for use as the rigid housing (203), it will be obvious to those of ordinary skill in the art having the benefit of this disclosure that other materials, including rubber-based materials, resin-based materials, and so forth, having similar shock absorbing properties and thermal conductivities, can also be used.
Turning now back to
One or more of the heat-generating components 201 are disposed along the printed circuit board 202. Heat generating components can include microprocessors, power generation components, or optical components such as projectors and image producing devices. In one embodiment, the heat-generating component 201 is an image projection device, such as a Microelectromechanical System (MEMS) image production device including laser light sources and a MEMS scanning mirror as an image modulation device.
Turning briefly to
In one embodiment, the display engine 1100 comprises a MEMS display engine that employs a MEMS scanning mirror to deliver light from the plurality of light sources 1103, 1104, 1105. MEMS scanning display engines suitable for use with embodiments of the present invention are set forth in US Pub. Pat. Appln. No. 2007/0159673, entitled, “Substrate-guided Display with Improved Image Quality”; which is incorporated by reference herein.
Turning now back to
The amount of interior surface area 211 covered by the compliant heat spreader 215 will vary with application. For instance, it will depend upon the number of heat-generating components 201, the permissible operating temperatures, and power consumption. Additionally, it will depend upon the surface touch temperature that can be tolerated, as well as the overall dimensions of the device. In one embodiment, for example, the rigid housing 203 has dimensions of less than 200×100×20 millimeters. However, a battery and battery door must be accommodated within this small space. Where the heat-generating component 201 comprises a MEMS display engine, experimental testing has shown that a rigid housing 203 manufactured from a polycarbonate-ABS blend, with thermal conductivity of between 0.2 Watts/meter*Kelvin and 1.0 Watts/meter*Kelvin and a thickness of about 1 millimeter, with a compliant heat spreader 215 covering about fifty percent of the internal surface area 211, is sufficient to avoid hot spots, keep the surface temperature below 45 degrees centigrade, and to make the overall enclosure approach being effectively isothermal. This can even be accomplished with no metal exposed from the rigid housing 203 and without airflow perforations in the rigid housing.
The materials that can be used for construction of the compliant heat spreader 215 includes flexible copper sheets such as copper foil, flexible aluminum sheets such as aluminum foil, and flexible graphite sheets. To avoid shorting electrical components, the compliant heat spreader can be encapsulated in an optional electrically insulating material 216, such as Polyethylene terephthalate (PET). In one embodiment, the compliant heat spreader 215 is a flexible graphite fiber sheet having a thickness of between 100 um and 500 um. Such material is generally inexpensive, easily die cut, and easy to work with in a manufacturing environment. Further, such a material does not sufficiently interfere with the shock absorbing properties of the rigid housing 203. For instance, where the heat-generating component 201 is a sensitive component, such as an image projection system, the combination of the polycarbonate-ABS rigid housing 203 and the compliant heat spreader 215 manufactured from graphite fiber will absorb enough energy that the image projection system will be subjected to a shock force of less than 3000 times the earth's gravitational force, “3000 G,” when dropped from four feet to concrete.
The compliant heat spreader 215 is, in one embodiment, thermally coupled 217 to the rigid housing 203. This can be achieved in several ways. In one embodiment, the compliant heat spreader 215 is adhesively affixed to the rigid housing 203. In another embodiment, the compliant heat spreader 215 is thermally coupled 217 to the rigid housing 203 by an insert molding process. Specifically, the compliant heat spreader 215 can be inserted into an injection-molding tool. The thermoplastic material of the rigid housing 203 can then be injected about the compliant heat spreader 215 such that the compliant heat spreader 215 becomes an integral part of the rigid housing 203. Insert molding allows the parts to be formed in complex three-dimensional shapes. Other advantages of the insert molded embodiment are that integrating the compliant heat spreader 215 into the housing facilitates thinner thermoplastic layers, easier manufacture through part count reduction, increased surface area coverage, and being able to uniquely design the thickness of the thermoplastic layers about the heat spreader layers.
Turning now to
To further distribute the heat, embodiments of the invention may employ an optional thermal management feature. Specifically, a compressible non-electrically conductive material 320—such as compressible foam—may be added to the interior 309 of the rigid housing. A loop 321 of thermally conductive material—such as graphite—can then be disposed about the compressible non-electrically conductive material 320. This thermal management feature can then be compressed between portions of the rigid housing 303 and the heat-generating component 301 so as to transfer heat from the heat-generating component 301 to the interior surface of the rigid housing 303. While this optional thermal management feature is shown only in
Turning now to
Turning now to
In the embodiment of
The lower rigid housing 563 and upper rigid housing 553 can be coupled and sealed together in a variety of ways, including adhesives, sonic welding, or other means. Similarly, the lower compliant heat spreader 565 and upper compliant heat spreader 555 may be thermally and mechanically coupled together by adhesives or mechanical bonding. The lower compliant heat spreader 565 and upper compliant heat spreader 555 are thermally coupled together such that heat can be delivered from the lower compliant heat spreader 565 to the upper compliant heat spreader 555 for more optimal dissipation to the environment. In one embodiment, the lower compliant heat spreader 565 and upper compliant heat spreader 555 overlap each other at their interface 575 and affix to each other such that at least a portion of one of the compliant heat spreader members overlaps and is affixed to at least a portion of another compliant heat spreader member.
Turning now to
The rigid housing 603 comprises two parts—a lower rigid housing 663, and an upper rigid housing 653. To further spread the captured heat, in the embodiment of
Turning now to
A housing 703 is formed from an upper housing 753 and a lower housing 763. The upper housing 753 and lower housing 763 can be sealed together by adhesives, sonic welding, or by mechanical components, such as the screws 790 shown in
In one embodiment, the housing has a rigidity that is greater than that of the compliant thermally conductive material 715 and a thermal conductivity that is less than that of the compliant thermally conductive material 715. In one embodiment, the housing 703 is manufactured from a polycarbonate-ABS blend, while the compliant thermally conductive material is a flexible graphite material.
The projector 701 is powered by a rechargeable battery (not shown) that is replaceable through a battery door 793. Electronics used in projecting images are disposed along the various circuit boards within the device. The substrate 702 is fixed relative to the housing 703 by a mechanical support 712. The compliant thermally conductive material 715 is coupled, for example, to the lower housing 763 by a conductive adhesive film, which may be thermally conductive as well as mechanically adhesive.
The compliant thermally conductive material 715 couples to other heat spreaders 765,785,795 so as to pass along an interior of the housing 703 across a substantial portion of the interior of the housing 703. In the illustrative embodiment of
In one embodiment, the compliant thermally conductive material 715 couples to the other heat spreaders 765,785,795 by a thermally conductive adhesive. For instance, the compliant thermally conductive material 715 overlaps and affixes to the side heat spreaders 785,795. Similarly, the side heat spreaders 785,795 overlap and affix to the upper heat spreader 765. As such, heat generated by the projector 701 is delivered about the interior of the device, through the housing 703, and to the exterior environment.
Turning now to
In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Thus, while preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims.
