Patent Publication Number: US-2009236081-A1

Title: Device for preheating a component cooled by  conduction and/or by convection

Description:
The present application claims the benefit of French Patent Application Serial No. 0801483, filed Mar. 18, 2008, which is hereby incorporated by reference in its entirety. 
     The present invention relates to a device for preheating a component cooled by conduction and/or by convection. The invention applies notably to the starting of components subjected to low temperatures, for example components installed in systems on board aircraft. 
     Electronic components, for example computer processors, are designed to operate in a limited temperature range, for example between 0° C. and 100° C. Therefore, when these components operate, they give off a quantity of heat which must be cleared away in order to avoid exceeding the authorized top temperature limit. The calories generated by the component are then usually cleared away thanks to means of cooling by conduction or by convection. 
     In certain situations, notably in a large number of onboard systems, the components are placed in environments at low temperatures—for example −40° C. The starting of these components at temperatures below their bottom operating temperature limit poses a problem. In order to raise the temperature beyond this bottom limit, it is possible for example to place a heating resistor close to the component. But the cooling means that are present in order to clear away the excess calories will oppose the action of this heating resistor, thereby slowing, or even totally nullifying its effect. There is therefore a contradiction between the need, when cold, to preheat the component before starting it, and the need, when hot, to clear away the calories generated by the component. 
     As an example, taking as the hypothesis a conduction-cooled 15 W dissipation processor, it is necessary to preheat the processor with 74 W of power for 9 minutes in order to bring its temperature from −40° C. to 0° C. This waiting time is often too long and this required power is usually too high to be acceptable in onboard equipment. 
     An object of the invention is to allow a component associated with cooling means to start in an environment the temperature of which is below its minimum operating temperature. Accordingly, the subject of the invention is a device for preheating a component in contact with a heat-conducting part, the device includes means for heating the said part, the device being characterized in that it includes at least one heat pipe connecting a heat dissipater with the said part, the said dissipater and the said part furthermore being thermally insulated from one another. 
     By preventing the clearing away of the calories by an effect of freezing the fluid inside the heat pipe, the device according to the invention makes it possible to rapidly raise the temperature of the component in order to bring it to an acceptable level before it is started. Preferably, the volume of the heat-conducting part is small, so as to limit the energy used to preheat the component. 
     The heat-conducting part may be formed by an excrescence of the heat pipe, so as to reduce the volume of material to be heated and to enhance the efficiency of conduction between the heat pipe and the heat-conducting part. 
     According to an embodiment of the device according to the invention, each end of the heat pipe is placed in contact with the heat dissipater, the central portion of the heat pipe being in contact with the heat-conducting part. This embodiment makes it possible to generate substantially symmetrical heat-clearing paths, which facilitates the heat conduction, notably when the device sustains accelerations hampering the movement of the fluid contained in the heat pipe. 
     The heating means may include at least one heating resistor, the said resistor being placed adjacent to the heat-conducting part. 
     Advantageously, the heat-conducting part and the heat pipe are insulated in order to limit the heat loss. 
     In addition, the heat-conducting part may be rounded on the surface in order to reduce the heat loss by radiation. 
     Preferably, the heat-conducting part and the heat pipe are made of low specific heat materials, such as, for example, aluminium or copper. 
     According to an embodiment of the device according to the invention, the heat-conducting fluid used in the heat pipe may be chosen so that its solidification temperature is at least equal to or slightly below (for example a few degrees Celsius less) the preheating temperature to be achieved. 
     The device according to the invention may for example be installed in an onboard system greatly limited in available power and subjected to low temperatures. 
    
    
     
       Other features will appear on reading the following detailed, non-limiting description given as an example with respect to the appended drawings which represent: 
         FIG. 1 , a top view of an embodiment of the device according to the invention; 
         FIG. 2 , a view in longitudinal section of the embodiment of  FIG. 1 ; 
         FIG. 3 , a view in cross section of the embodiment of  FIG. 1 ; 
         FIG. 4 , a view of the heat-conducting part used in the embodiment of  FIG. 1 . 
     
    
    
     The same reference numbers in various figures designate the same elements. 
       FIG. 1 ,  FIG. 2  and  FIG. 3  show respectively a top view, a view in longitudinal section and a view in cross section of an embodiment of the device according to the invention. 
     The device  100  of the example includes a component  102  placed on a substrate  104 , which substrate  104  is placed on a printed circuit board  106 . A heat-conducting part  108 , for example an aluminium block, is placed in contact with the component  102 . In the example of  FIGS. 1 ,  2  and  3 , the conducting part  108  is placed adjacent to the top face of the component  102 . 
     Furthermore, a heat dissipater  110 , for example an aluminium drain, is clamped by cold plates  112   a ,  112   b  and placed close to the component, without touching it. In addition, all contact should be avoided between the conducting part  108  and the heat dissipater  110  so that no direct heat path between the conducting part  108  and the heat dissipater  110  is created. In the example, the heat dissipater  110  is a plate that is perforated so that the conducting part  108  can pass through without touching the plate. 
     The heat dissipater  110  should make it possible to clear away a large proportion of the calories generated by the component  102 . Therefore, a heat pipe  114  connects the heat dissipater  110  to the conducting part  108  so that the heat pipe  114  establishes an efficient heat-conducting path between the part  108  and the heat dissipater  110  when the temperature of the heat pipe  114  is high enough to allow the fluid contained in the heat pipe  114  to leave the solid phase and to operate a heat-conducting circuit. On the other hand, when the temperature of the heat pipe  114  is too low to allow the fluid to melt, the heat-conducting circuit inside the heat pipe  114  is impossible, which thermally insulates the conducting part  108  from the heat dissipater  110 . In the example, the top face of the heat dissipater  110  is grooved  111  to the dimensions of the heat pipe  114  and the heat pipe  114  is placed in the groove  111 , the area of contact between the heat pipe  114  and the heat dissipater  110  thereby being maximized. 
     In order to raise the temperature of the component  102 , a heating resistor  116  is placed close to the latter. In the example, the heating resistor  116  is placed adjacent to the conducting part  108 , but in another embodiment, the heating resistor  116  is placed on the heat pipe  114 . 
     The heat-conducting part  108  notably plays a role of a heat interface. Specifically, on the one hand, it carries the calories originating from the heating resistor  116  to the component  102 , and on the other hand, when the component  102  is operating, the heat-conducting part  108  carries the calories originating from the component  102  to the heat pipe  114 . According to one embodiment, the heat-conducting part  108  is only an excrescence of the heat pipe  114 , the said excrescence  108  being fashioned at the time of manufacture of the heat pipe  114 . 
     When, initially, the device  100  is subjected to an ambient temperature that is lower than the solidification temperature of the fluid contained in the heat pipe  114  (at the internal pressure of the heat pipe  114 ), the heating resistor  116  transmits calories to the heat-conducting part  108 , hence to the component  102  and the heat pipe  114 . Since the fluid contained in the heat pipe  114  is solidified, the heat pipe  114  is inoperative; therefore, the calories remain largely confined to the assembly formed from the component  102 , the heat-conducting part  108  and the heat pipe  114 . The temperature of this assembly increases until it reaches the minimum operating temperature of the component  102 . The component can then be started without risk, then when the temperature of the assembly {component  102 , conducting part  108 , heat pipe  114 } reaches or even exceeds the melting temperature of the fluid, then a heat-conducting circuit can be made in the heat pipe  114 , which then carries away the received calories to the heat dissipater  110 , thus preventing the component  102  from overheating. 
     The fluid used in the heat pipe  114  is, for example, water. However, fluids with different solidification temperatures may be chosen in order to suit the conditions of use of the device  100 . For example, for components operating at very low temperatures, alcohol may be a judicious choice for the heat pipe  114 . Whatever fluid is chosen, its solidification temperature should be below the maximum operating temperature of the component  102 . 
     In the example, each end of the heat pipe  114  is placed in contact with the heat dissipater  110 , the central portion of the heat pipe  114  being in contact with the conducting part  108 . Therefore, the heat pipe  114  of the example includes an evaporator on the conducting part  108  and two condensers, one at each end of the heat pipe  114 . This configuration results in obtaining two substantially symmetrical heat clearance paths, which reduces the distance of travel of the fluid from a condenser to the evaporator and consequently makes it easier to establish a heat circuit. Notably, this configuration improves the heat-conducting path brought about by the heat pipe  114  when the device  100  sustains accelerations—including natural gravitation—hampering the movement of the fluid. 
     According to another embodiment, a first end of the heat pipe  114  is placed adjacent to the conducting part  108 , whereas its second end is placed in contact with the heat dissipater  110 , this configuration resulting in a simple evaporator-condenser circuit. 
       FIG. 4  shows a view of the heat-conducting part used in the embodiment of  FIG. 1 . 
     The conducting part  108  of the example is in the form of an arch. In other words, it is a parallelepiped of which one face  401  has been hollowed out to form a groove  410  to the dimension of the heat pipe  114 . The face  402  opposite to the hollowed-out face is placed in contact with the component  102  ( FIGS. 1 ,  2 ,  3 ) and a side face  403  includes a sufficient area to place a heating resistor thereon. 
     According to another embodiment of the device according to the invention, the conducting part  108  may be made in many ways, provided notably that the shape chosen allows good heat conduction between the said part  108 , the heat pipe  114  and the component  102 . Furthermore, several heating resistors may be placed adjacent to the conducting part  108  in order to more rapidly raise its temperature. 
     To take the example cited in the preamble of this application, with a temperature of approximately −40° C. and a conduction-cooled 15 W dissipation processor, the use of a device according to the invention makes it possible to reduce the power needed to be supplied to 20 W and the preheating time to 40 seconds in order to bring its temperature from −40° C. to 0° C., compared with 74 W and 9 minutes that are necessary with a conventional device, which represents approximately a gain (time×power) with a factor of 50. 
     The device according to the invention may be used to bring a component to its minimum specified temperature, for example 0° C., before it is powered up or before it is initialized. Nevertheless, if the available power is very low and/or the acceptable, time for preheating is very short, the device may also used to bring the component to a temperature below the specified temperature, for example −20° C.,—but all the same higher than the ambient cold temperature—40° C. This limited rise in temperature may in effect be sufficient to achieve an acceptable success rate during a cold sorting of the card fitted with the component. 
     The device according to the invention preferably applies to the electronic components present on cards cooled by conduction. However, without departing from the context of the invention, the device may equally apply to cards cooled by convection considering that a cold wall may be formed by a convection radiator associated with the component.