Patent Application
20080142195
This relates generally to vapor chambers for use in cooling electronic components.
Electronic components generate heat during operation. Ideally, that heat may be dissipated in passive fashion. By passive, it is intended to refer to heat dissipation without a pump, compressor, or fluid moving device other than a fan. For example, the use of a heat sink of thinned heat exchanger is a passive technique for removing heat. This passivity is advantageous since no power is needed to implement the transfer and there may be less reliability concerns.
Unfortunately, in some cases, passive techniques are insufficient for many advanced heat generating semiconductor technologies such as microprocessors. Because of the amount of heat generated, the relatively small spaces involved and the relatively small size of the components, cooling may become a more complex issue.
The hotter that the electronic component becomes during operation, the less efficiently it continues to operate. Thus, not only may it operate more slowly, but the component may also operate less efficiently from an energy consumption standpoint.
In accordance with some embodiments of the present invention, a vapor chamber may be utilized with what is called an alternate working fluid. Fluid is considered an alternate working fluid because it is an alternative to existing perfluorinated compounds. Unfortunately, perfluorocarbons have high global warming potentials and hydrofluorocarbons are being eliminated in many parts of the world because of their contribution to global warming. Thus, alternative chemistries, such as perfluorinated materials that do not create global warming adverse effects, are considered alternative working fluids. The most promising of these are the segregated hydrofluoroethers.
One problem with segregated hydrofluoroethers is that while they have relatively good evaporator performance, they have relatively poor condensation performance. Evaporation resistance indicates the temperature difference caused by transferring a unit of heat. Thus, using hydrofluoroethers may be advantageous compared to some working fluids, such as water, but their efficiencies are still somewhat insufficient because of their poor condensation resistance.
Particularly in cases where the alternate working fluid is one which has a condensation resistance that constitutes more than 50 percent of the resistance of the overall system, there is a need for ways to reduce the degradation of condensation performance so that these alternate working fluids may improve evaporation resistance, while still providing acceptable environmental effects.
Referring to
The vapor chamber 22 includes a boiling surface 24. In some embodiments, the boiling surface may be plasma sprayed, microporous boiling surface. See, for example, Tehver, J., et al. “Heat Transfer and Hysteresis Phenomena in Boiling on Porous Plasma-sprayed Surface,” Experimental Thermal and Fluid Science, volume 5, 1992, pp. 714-727.
The combination of the plasma sprayed microporous boiling enhancement surface and the use of relatively low evaporation resistance, alternate working fluids, such as segregated hydroethers, results in a situation where the evaporator resistance decreases significantly and condenser resistance increases so that condenser resistance becomes more than half of the overall resistance of the vapor chamber 22.
The vapor chamber interior may be defined by angled side walls 26. These inclined side walls help fluid return to the evaporator and prevent sub-cooling in dead zones. Thus, fluid evaporated from the evaporator may condense on the condensation surface 28 at the upper side of the vapor chamber. The condensation surface 28 may be thermally coupled to a finned heat exchanger 30. Other heat exchanger designs may be utilized as well including thermoelectric coolers. These heat exchangers may be passive or active heat exchangers.
The efficiency of the overall vapor chamber 22 may be improved by decreasing the condensation resistance. The condensation resistance with alternate working fluids may be reduced by using a system that decreases the thickness of the liquid film on the condensation surface 28. Specifically, in the process of condensation, particularly with alternate working fluids, such as hydrofluoroethers, a a condensed liquid film thickness builds up on the condensation surface and introduces a thermal resistance across the thickness of the film due to the thermal conductivity of the fluid.
The surface tension of the segregated hydrofluoro-ethers is relatively low, leading to film-wise condensation which has an order of magnitude lower performance than drop-wise condensation. In addition, due to the low latent heat of the fluid, the condensate film is relatively thicker and the thicker liquid film, together with the relatively low thermal conductivity of the fluid, acts as an insulator.
In order to reduce the film thickness and, therefore, the condensation resistance, the film may be periodically wiped using a mechanism to simply remove the condensed liquid. The wiped, condensed liquid then falls as droplets D back to the evaporation surface.
Various mechanisms may be utilized for this purpose, including a piezoelectric actuator. The piezoelectric actuator, in one embodiment, may include a patch 32 on one side, a patch 32 on the opposite side, and blades 34 in between and connected to one of the patches 32. The piezoelectric operation of the blades 34 causes them to flex and rotate through a sweeping arc W across the condensation surface 28, removing the liquid therefrom and returning it as droplets to the evaporation surface as shown in
Electrical power may be supplied to the piezoelectric actuators by lines 38. Thus, a small amount of electric power may be consumed. Since the chamber operates in a vacuum in one embodiment, that power may be transferred wirelessly to the piezoelectric actuator, in one embodiment. Otherwise, the wire passages may be sealed.
Other systems for removing the condensed liquid may also be used, including microelectromechanical actuators, electroosmotic operators, and other low power consuming moving devices.
The periodicity of the wiping action W may be set to a desired operating point and may be continuous so long as the system is operating. However, in other embodiments, sensors may be utilized, controlled in some cases by the electronic circuit being cooled, to adjust the wiping action, based on either the performance of the vapor chamber 22 or the operating conditions of the electronic component, to mention two examples. For example, in some cases based on the loads on the electronic circuit, its time of operation or even its current temperature, the efficiency of the system may be increased by increasing the rapidity of the wiping action or, when no longer needed, to save power, may be reduced. For example, a feedback controller may monitor noise, power consumption of the cooled component, and temperatures on the hot and cold ends of the vapor chamber.
In some cases, the wiping action need not actually touch the evaporation surface since all that is needed is to reduce the thickness of the condensed liquid. This reduction in thickness decreases the condensation resistance.
It is desired that the condensation resistance be decreased below 0.10° C. per Watt with a 50 micron liquid thickness and, particularly, it is desirable that the condensation thickness be below 0.05° C. per Watt and, most advantageous, approximately 0.01° C. per Watt.
In some embodiments, the piezoelectric system may consume minimal power. For example, power consumption on the order of milli-Watts may be achieved. The wiring may be embedded in the base of the heat sink 30 in some cases.
Thus, particularly where the condensation resistance is a substantial portion of the overall resistance of the system, active condensation enhancement techniques may dramatically improve system heat transfer performance with minimal power consumption.
In some embodiments, a similar wiping action may be applied to the evaporation surface to reduce bubbling on the evaporation surface.
Referring to
In some embodiments of the present invention, any processor-based system may be formed. Thus, the embodiment shown in
References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
