Abstract:
A valved heat pipe and an adaptive circuit cooling system using it are disclosed. The valved heat pipe includes a sealed tube, liquid in the tube, and at least one valve in the tube. This valve operates as a function of temperature to selectively obstruct circulation of the liquid from one end of the tube to the other. The adaptive cooling system includes heat generating equipment, e.g., electronic circuitry such as radio frequency transmitters and/or recievers, a heat sink and such a valved heat pipe that thermally connects the circuitry to the heat sink.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is directed toward the field of heat pipes, and more particularly to the field of valved heat pipes as well as adaptive cooling systems (especially for electrical circuits) including the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    Heat pipes are known cooling devices. A heat pipe is a pipe that has been sealed at both ends and then evacuated. A small amount of working fluid is introduced into the pipe and the pipe is resealed. If heat is applied to one end of the heat pipe, which is referred to as the evaporator end, then the fluid vaporizes and carries the heat of vaporization in the vapor very quickly to the other end, known as the condenser end. At the condenser end, the latent heat of vaporization is released as the vapor condenses back into liquid form on the condenser end, which it was at a lower temperature then the evaporator end prior to the condensation. The condensed liquid is then carried back to the evaporation end by capillary action and/or gravity.  
           [0003]    Thus, heat pipes have very high thermal conductivity because they use convection in addition to conduction.  
           [0004]    Typically, a source of heat at the evaporator end is an electrical device, such as a transistor mounted to an internal base. The condenser end is typically thermally connected to a heat sink such as a finned metal external structure. The heat sink is either actively cooled by a fan or passively cooled by convection on the surface of the package. There is a temperature differential between the internal base plate and the external heat sink that drives the heat pipe&#39;s convection process. FIG. 1 depicts a Background Art heat pipe-based cooling system.  
           [0005]    The Background Art heat pipe-based cooling system  100  of FIG. 1 includes an electrical circuit  102  mounted on a thermally conductive base plate  104 . A heat pipe  108  is connected to the base plate  104  and also to a finned heat sink  106 . The heat pipe  108  has a 90° bend  110 . Typically, the base plate  104  is maintained in a horizontal orientation. As such, a foot portion  112  of the heat pipe  108  is also horizontal while a leg portion  114  is substantially vertical. This orientation uses gravity to promote the return of condensed liquid from the condenser end  116  to the evaporator end  118 .  
           [0006]    The heat pipe  108  is depicted in more detail in Background Art FIG. 2. FIG. 2 is a front view of the pipe  108 , in contrast to the three-quarter perspective view of FIG. 1. As such, the 90° bend  110  of the heat pipe  108  is less discernable in FIG. 2 than in FIG. 1. In FIG. 2, the heat pipe  108  contains liquid  202  that is pooled in the foot portion  112 . Some gaseous molecules  204  of the liquid  202  are depicted as having moved, or moving, toward the condenser end  116  of the leg portion  114 .  
           [0007]    For optimal performance of the cooling system  100 , the type of liquid  202  and the internal pressure within the heat pipe  108  are chosen carefully with respect to the operational temperature of the circuitry  102 . Ideally, the change of state temperature (from liquid to vapor) is engineered such that it is between the temperature of the base plate  104  and the temperature of the heat sink. The liquid  202  boils at the base plate or evaporator end  118  of the heat pipe  108 , absorbing heat. The vapor rises towards the heat sink or condenser end  116 , and then condenses at the lower external heat sink temperature, releasing the heat. Then the liquid falls to recycle or pool at the evaporator end  118 . The phase state change improves the cooling efficiency of the heat pipe  108 .  
           [0008]    The performance of the heat pipe will vary as a function of temperature. At very low temperatures, the liquid  202  may freeze into a solid, leaving the conductivity of the external surface of the heat pipe  108  as the only heat flow mechanism, thus lowering the effective conductivity. At low temperatures, most of the liquid  202  may remain in the liquid state, leaving only a small portion of gaseous material to perform convection.  
           [0009]    At higher temperatures, a greater portion of the liquid  202  may be involved in the phase change cycle. This is the maximum cooling rate. At an even higher temperature, all of the liquid  202  may remain in the gaseous state, thus lowering the efficiency of the heat pipe  108 , as there is no phase state change. Thus, the range of operational temperature and the chemical make-up of the fluid may be constrained. Also, the effective conductivity of the heat pipe  108  may vary as a function of temperature, but not in a very controlled manner.  
           [0010]    Regular thermal conduction through a metal works best when there is a large temperature differential between the base plate (the source) and the heat sink. Thus, the fastest (best) cooling rate occurs when the ambient temperature is low and the base plate  104  temperature is high. Likewise, the slowest (worst) cooling rate occurs when the heat sink  106  temperature (external ambient) is high, approaching that of the internal base plate  104  temperature, leaving a small differential. In order to achieve adequate cooling at these high ambient temperatures, the conduction path or heat pipe  104  size must be made large enough to function adequately at the low temperature differential. Thus, there is excess cooling capacity at lower ambient temperatures, and the base plate  104  temperature will vary widely as a function of the external ambient temperature. Temperature differentials drive the phase state cycle in a heat pipe  108 .  
           [0011]    Problems with a wide temperature differential at the base plate are that it stresses both the mechanical packaging technology and the electrical performance of the circuitry  102 . The electrical performance of transistor circuits, especially for RF amplification, will vary widely as a function of temperature. Because many of these RF circuits cannot apply conventional negative feedback, which is commonly used at lower frequencies, excess gain cannot be designed in and used to stabilize the circuit&#39;s performance. Thus other, more complex open loop or closed loop sensing schemes must be used to compensate for device gain changes as a function of temperature. This requires additional temperature or power detector circuits, and can be quite complex, and may require calibration. From an electrical point of view, operation at a constant temperature would simplify the circuit design immensely, as the gain would not vary.  
           [0012]    Mechanically, many failure mechanisms are due to material differentials, such as different rates of thermal expansion and galvanic corrosion. Repetitive thermal cycling is a significant source of aging and may damage many types of connections, such as solder joints, wire bonds, packaging interconnects and other mechanical joints. Elimination of temperature cycling, or lowering the range of differential may significantly lower the failure rate of circuits, particularly power supply and RF amplification circuits. For outdoor mounted equipment, the daytime/nighttime circadian cycle may be the dominant temperature driver.  
         SUMMARY OF THE INVENTION  
         [0013]    The invention, in part, provides a mechanism to control the rate of heat flow to be proportional to the cooling needs of the active devices. Such a mechanism can cool a target at an adaptive cooling rate. At high ambient temperatures, a high thermal effective conductivity is provided to maximize the cooling rate despite the lower temperature differential between base plate and heat sink. At low ambient temperatures, a lower effective thermal conductivity is provided to prevent over cooling the base plate, i.e., to prevent allowing it to drop to a very low temperature. Instead of cooling the circuitry maximally to the lowest differential above the outdoor ambient temperature, the invention limits the cooling rate and allows the base plate temperature to stabilize at a higher temperature, thus limiting the total base plate&#39;s temperature cycle.  
           [0014]    The invention also provides, in part, an adaptive circuit cooling system. Such a cooling system comprises: heat generating equipment, e.g., electronic circuitry; a heat sink; and a heat pipe that thermally connects the circuitry to the heat sink. The heat pipe includes at least one valve that operates as a function of temperature.  
           [0015]    The invention also provides, in part, a valved heat pipe comprising: a sealed tube; liquid in said tube; and at least one valve in the tube that operates as a function of temperature to selectively obstruct circulation of the liquid from one end of the tube to the other.  
           [0016]    Objectives of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus do not limit the present invention.  
         [0018]    [0018]FIG. 1 is a three-quarter perspective view of a cooling system employing a heat pipe according to the Background Art.  
         [0019]    [0019]FIG. 2 is a front view of the heat pipe of FIG. 1.  
         [0020]    [0020]FIG. 3A is a three-quarter perspective view of an embodiment of a cooling system employing a heat pipe according to the invention.  
         [0021]    [0021]FIG. 3B is a front view of the heat pipe of FIG. 3A where the valve is closed.  
         [0022]    And, FIG. 3C is a front view of the heat pipe of FIG. 3A where the valve is open. 
     
    
       [0023]    It is noted that the Drawings are not drawn to scale.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 3A is a three-quarter perspective view of an embodiment of a cooling system employing the heat pipe according to the invention.  
         [0025]    Components common to the Background Art will share the same item numbers.  
         [0026]    In the cooling system  300  of FIG. 3A, heat generating equipment, e.g., electronic circuitry such as radio frequency transmitters and/or receivers,  102  is formed on a base plate  104 . A heat pipe  301  thermally connects the base plate  104  to a heat sink  106 . Similar to the Background Art, the heat pipe  301  has approximately  90  bend  324  that divides the heat pipe  301  into a foot portion  320  and a leg portion  322 . The base plate  104 , and therefore the foot  320 , can be oriented so as to be horizontal, thus making the leg portion  322  substantially vertical. Again, this takes advantage of gravity to assist in the recirculation of fluid condensed at a condenser  328  back to an evaporator and  326 .  
         [0027]    In contrast to the Background Art heat pipe  108 , the heat pipe  301  includes a valve  302  that can take on a closed state as in FIG. 3B or an open state as in FIG. 3C. FIGS. 3B and 3C are front views that contrast with the three-quarter perspective view of FIG. 3A. As such, the  90  bend  324  in FIGS. 3B and 3C is less discernable than in FIG. 3A.  
         [0028]    At its most simple, the valve  302  takes the form of a valve that is either completely open or completely closed based upon a temperature to which the valve is exposed. An example of such a valve is a bimetallic valve. Generally, bimetallic valves are known such that no further discussion of the bimetallic valve technology is needed. In this simple form, the valve  302  is a passive device.  
         [0029]    The simple form of the valve  302  can also be described as a binary valve in the sense that it is either completely open or completely closed. Alternatively, the valve  302  can be a continuously variable valve in the sense that the size of the orifice can vary according to variations in the temperature to which the valve is exposed.  
         [0030]    It is noted that it is unnecessary to provide a fan to force air across the heat sink  106  of the cooling system  300  because of the efficiency of this system. This eliminates moving parts, which greatly improves reliability.  
         [0031]    [0031]FIG. 3B is a front view of the heat pipe  301  of FIG. 3A. In FIG. 3B, the valve  302  is closed, which obstructs the circulation of the liquid  202  between the evaporator end  326  and the condenser end  328 . Gaseous molecules  305  are shown as rising above the  90  bend  324 , but they are blocked by the valve  302  from moving any closer to the condenser end  328 . In contrast, FIG. 3C (again, a front view) depicts the valve  302  in its opened state. There, most of the gas has risen above the valve  302  as depicted by the molecules  307  though there remain some molecules  305  below the valve  302 . In FIG. 3C, the valve  302  no longer obstructs circulation of the gas and liquid from the evaporator end  326  to the condenser end  328  and vice-versa.  
         [0032]    When the ambient temperature is high, the valve  302  will open so that cooling rate is maximized. At a low ambient temperature, the valve  302  closes, lowering the effective thermal conductivity of the heat pipe  301 .  
         [0033]    In FIG. 3A, some optional elements are depicted in broken lines. In particular, FIG. 3A depicts an optional temperature sensor  310  connected to an optional controller  308  via an optional signal line  312 . The controller  308  is connected to the valve  302  by the optional signal line  318 . As an alternative to the temperature sensor  310 , an optional temperature sensor  314  is connected to the controller via an optional signal line  316 .  
         [0034]    When the controller  308  is present, the valve  302  can be an electrically actuated valve that is driven by the signal on the line  318 . Based upon a temperature T 1  sensed by the sensor  310 , or alternatively based upon the temperature T 2  sensed by the temperature sensor  314 , the controller  308  causes the valve  302  to open or close. When the sensed temperature T 1  or T 2  is greater than or equal to a reference temperature, the controller preferably controls the valve  302  to open. When the temperature falls below the reference value, the controller causes the valve  302  to close. By being positioned against the heat sink  106 , the sensor  310  can sense the ambient temperature. The sensor  314  can sense the temperature of the base plate  104 .  
         [0035]    Examples of electrically actuated valves are solenoid valves. This will define the system  300  as an active system. Further in the alternative, the valve can be a hydraulically actuated valve, in which case the signal line from the controller  308  is a hydraulic rather than electrical signal line.  
         [0036]    The base plate  104  is typically metallic. Alternatively, the circuitry  102  could be mounted on a low thermal conductivity substrate. In that case, then the foot portion of the heat pipe  301  would be directly thermally connected to the circuitry  102 .  
         [0037]    Only one valve  302  has been depicted in the heat pipe  301  according to the invention, for simplicity. But it is to be noted that the invention contemplates at least one valve rather than only one valve. For example, a heat pipe might have two or more valves that are either passive or actively controlled. If they are actively controlled by the controller  308 , then these valves can each be controlled according to the same reference temperature or they can each have their own reference temperature. Similarly, more than one such heat pipe can be provided per unit of heat generating equipment.  
         [0038]    The invention has been described in terms of electronic circuitry as an example of the heat generating equipment because such circuitry is well suited to being cooled according to this technology. But the invention can also be applied to other heating generating equipment, e.g., motors or engines.  
         [0039]    The provision of a valve in the heat pipe has the advantage that it gives the thermal designer more degrees of freedom to design the heat flow rate of the heat pipe then merely the choice of fluid, volume and pressure therein.  
         [0040]    This invention has the advantages that it allows simpler electrical circuits to perform adequately over a large range of ambient temperatures, thus lowering the cost of electrical circuits. Also, it improves reliability by reducing thermal stresses caused by repetitive thermal cycling. It allows high efficiency heat pipes to be used in packages that may be exposed to very cold external ambient environments, yet still maintain a usable internal temperature by not overcooling at low temperatures. As such, it can eliminate the use of heaters to allow operation at cold temperatures, as the active devices&#39; own thermal dissipation may be enough to maintain an adequate operational temperature. It also allows the design of totally passive cooling equipment, improving reliability of the cooling system, which is usually one of the dominant limiting factors. Additional advantages of the invention are that it makes possible the use of passively controlled, or actively controlled, heat pipe-based cooling mechanisms. Most thermal regulating alternatives require a heater, a fan, an active heat pump, an air conditioner, or more than one active system, to help maintain a constant temperature. The invention also furthers the development of more complex, temperature compensated systems circuits that can perform adequately over a very large temperature range.  
         [0041]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.