Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DEVICE FOR PREHEATING A COMPONENT COOLED BY CONDUCTION
AND/OR BY CONVECTION
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 -40r. 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 15W
dissipation processor, it is necessary to preheat the processor with 74W of
power for 9 minutes in order to bring its temperature from -409C 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 comprising means for heating the said part, the device being
characterized in that it comprises at least one heat pipe connecting a heat
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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 necessary 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 comprise 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.
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Other features will appear on reading the following detailed, non-
limiting description given as an example with respect to the appended
drawings which represent:
- Figure 1, a top view of an embodiment of the device according to the
invention;
- Figure 2, a view in longitudinal section of the embodiment of Figure 1;
- Figure 3, a view in cross section of the embodiment of Figure 1;
- Figure 4, a view of the heat-conducting part used in the embodiment
of Figure 1.
The same reference numbers in various figures designate the
same elements.
Figure 1, Figure 2 and Figure 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 comprises 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 Figures 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 112a, 112b and placed close to the
component, without touching it. In addition, all contact must 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 must 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
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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 consisting of 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.
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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
5 temperature must 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 comprises 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.
Figure 4 shows a view of the heat-conducting part used in the
embodiment of Figure 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 (Figures 1, 2, 3) and a side face 403 comprises 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.
. ~ .. . . .. . . . . . . .
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To take the example cited in the preamble of this application, with
a temperature of approximately -409C and a conducti on-cooled 15W
dissipation processor, the use of a device according to the invention makes it
possible to reduce the power needed to be supplied to 20W and the
preheating time to 40 seconds in order to bring its temperature from -409C to
0'C, compared with 74W and 9 minutes that are neces sary with a
conventional device, which represents approximately a gain (time x 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 be used to bring the component to a temperature below the
specified temperature, for example -201C, - but all the same higher than
the ambient cold temperature - 409C. This limited r ise 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.
