Patent Application
20110064594
1. Field of the Invention
The present invention relates to a cooling device for ejecting heat from the inside of an electronic apparatus to the outside thereof, the cooling device using a piezoelectric fan.
2. Description of the Related Art
Recently, for a portable electronic apparatus, in particular, with the advancement of miniaturization and high-density mounting, devices for reducing heat inside the electronic apparatus are desired.
One example of a portable electronic apparatus in which the above issue is especially important is a portable personal computer. For portable personal computers, both miniaturization and increased central processing unit (CPU) speeds that improve performance of information processing are advancing. As a result, high-density mounting of parts deteriorates the ventilation inside an electronic apparatus, while heat generated by a CPU increases. This makes it more difficult to dissipate the heat to the outside of the electronic apparatus and thereby reduce or prevent a temperature increase inside the electronic apparatus.
A traditional radiator in which a movable element having an air surrounding structure is disposed between many heat dissipating fins that are aligned at desired intervals on a heat sink that is in contact with a heating portion of a heating element that sends cool air in between the heat dissipating fins by causing the movable elements to rotate or swing to eject warm air between the heat dissipating fins is disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796.
A piezoelectric fan that includes an air generating oscillator including a piezoelectric oscillator and an outlet and an inlet disposed on the same surface is disclosed in Japanese Unexamined Patent Application Publication No. 2002-339900. The piezoelectric fan including a pair of partition walls extending from a hole of the main body of a casing to an inner portion is disposed so as to sandwich both sides of the air generating oscillator, holes between the partition walls and both sides of the main body of the casing are provided as intakes, and holes sandwiched by the both partition walls are provided as exhausts.
Here, the configuration of the piezoelectric fan illustrated in Japanese Unexamined Patent Application Publication No. 2002-339900 is described with reference to
However, the use of the radiator illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796 in the portable electronic apparatus without being processed is inconvenient in terms of miniaturization. One approach to address this is to use the piezoelectric fan illustrated in Japanese Unexamined Patent Application Publication No. 2002-339900, in place of the movable elements illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796.
When the piezoelectric fan is used, its capability to generate air depends on the amount of displacement of the piezoelectric oscillator in the air generating oscillator. The amount of displacement of the piezoelectric oscillator is less than the movement of the movable elements illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796.
As a result, it is necessary to cool the inside of an electronic apparatus as efficiently as possible. Japanese Unexamined Patent Application Publication No. 2002-339900 discloses that it is desired that the distance between both partition walls is as close as possible to the width of the air generating plate, that is, that the gap between the air generating plate and each of the partition walls is as small as possible.
The movable elements for ejecting warm air between the heat dissipating fins in the radiator illustrated in Japanese Unexamined Utility Model Registration Application Publication No. 02-127796 are arranged to rotate or swing using a strong driving source, such as a motor, even if air resistance to the movable elements is present. Accordingly, the movement of the movable elements is not inhibited by the influence of the air resistance. In contrast, for the air generating oscillator used in the piezoelectric fan of Japanese Unexamined Patent Application Publication No. 2002-339900, if the distance between the heat dissipating fins corresponding to both partition walls and the width of the air generating plate are close to each other, air resistance caused by movement of the air generating oscillator would inhibit displacement.
The air resistance is assumed to be substantially proportional to air density and the air density is assumed to be proportional to pressure, such that this experiment is performed by examining the pressure and the amplitude of the blade end when the blade subjected to a predetermined reduced pressure environment is driven by the piezoelectric oscillator. As shown in
As described above, a problem exists in that, even if the distance between the heat dissipating fins corresponding to both partition walls and the width of the blade are as close as possible to each other, the air resistance caused by movement of the blade inhibits displacement, and thus, increasing the amplitude of the blade is difficult.
To overcome the problems described above, preferred embodiments of the present invention provide a cooling device having an improved cooling capability by increasing the amplitude of a blade, and thus improving the capability to move air and improving the heat dissipation effect produced by heat dissipating fins.
Warm air to be ejected is air that is warmed by heat generated by heat dissipating fins. Therefore, a temperature distribution of a space between the heat dissipating fins is not uniform, and a high-temperature portion is concentrated in the vicinity of the walls of the heat dissipating fins.
When a flow velocity distribution when air is blown between the heat dissipating fins is taken into account, the velocity of air flow is relatively fast in the central portion between the heat dissipating fins, whereas the velocity of air flow decreases towards the surface of the wall of each of the heat dissipating fins because viscous drag of air is caused by the wall of the heat dissipating fin.
That is, causing air to flow alone can eject relatively low-temperature air existing in the central portion between the heat dissipating fins, but cannot sufficiently eject high-temperature warm air existing in the vicinity of the walls of the heat dissipating fins.
The inventors of the present invention have determined, from various experiments and simulations, that moving a blade so as to sweep warm air in the vicinity of the walls of the heat dissipating fins and thereby moving the warm air towards the central portion between the heat dissipating fins to facilitate ejecting the warm air enables heat inside the electronic apparatus to be efficiently ejected to the outside without trying to eject all of the air existing between the heat dissipating fins.
A preferred embodiment of the present invention provides a cooling device that includes a piezoelectric fan including a piezoelectric oscillator that bends in accordance with an application of a voltage and a blade that is connected to or provided integrally with the piezoelectric oscillator and that is arranged to swing via the piezoelectric oscillator and a heat sink including at least two heat dissipating fins,
The blade preferably has an elongated shape that extends from the piezoelectric oscillator, the piezoelectric oscillator and the blade are preferably arranged at a position that enables the blade to swing without coming into contact with the heat dissipating fins in a space between the neighboring heat dissipating fins.
The blade preferably has a hole or a cut provided therein. With this structure, air resistance is reduced by the hole or the cut, and the amplitude of the blade is increased. Although the overall amount of generated air is reduced by the hole or cut, the effect of sweeping warm air in the vicinity of the walls of the heat dissipating fins is not reduced, such that the overall cooling capability is improved along with the increase in the amplitude. Additionally, a current of warm air in the vicinity of the heat dissipating fins separates and moves in the direction of the central portion in an undulating manner, and the warm air in the vicinity of the heat dissipating fins is transferred outward. Therefore, the heat dissipation effect and the cooling capability are improved.
In the cooling device, a weight may preferably be disposed at or in the vicinity of an end of the blade that is remote from the piezoelectric oscillator.
With this structure, a moment of inertia is increased by the weight, and driving the blade at a resonant frequency of the blade with the weight increases the amplitude of the blade. Thus, the cooling capability is improved.
The blade may preferably have a bent shape such that the blade is shortened in its longitudinal direction.
With this structure, the overall length of the blade is increased, and the amplitude is increased. Thus, the cooling capability is improved.
The piezoelectric oscillator may preferably be arranged so as to sandwich an end of the blade from both sides, and the piezoelectric oscillator and the blade may preferably define a bimorph oscillator.
With this configuration, the amount of bending displacement with respect to an applied voltage is increased, and the amplitude is increased. Thus, the cooling capability is further improved.
The cooling device may preferably include a fan that generates a current of air that flows between walls of the heat dissipating fins.
With this configuration, a current of warm air separating from the vicinity of the walls of the heat dissipating fins and moving toward the direction of the central portion while undulating by the presence of the hole or cut efficiently flows outward by the additional fan, and the overall cooling capability is improved.
The hole may preferably have a long shape extending along a longitudinal direction of the blade, and a dimension from a longitudinal-direction side of the blade to a side of the hole that is parallel or substantially parallel to the longitudinal-direction side may preferably be greater than a dimension of a gap between the blade and one of the heat dissipating fins.
With this configuration, even greater cooling capability is obtained.
With preferred embodiments of the present invention, the amplitude is increased, and the cooling capability is improved. In addition, the heat dissipation effect from the heat dissipating fins and the cooling capability are improved.
Other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.
The blade 21 preferably includes a hole 22 having a rectangular or substantially rectangular shape, for example, that is punched in a stainless steel plate, and an end connected to an end of the piezoelectric oscillator 20.
The dimensions of each portion of the piezoelectric fan 31 illustrated in
L1: about 12 mm
L2: about 18 mm
W: about 6 mm
t: about 50 μm
The dimensions of the hole 22 are preferably about 12 mm by about 2 mm, for example. An end of the hole 22 near the base is aligned with the end of the piezoelectric oscillator 20.
The plurality of heat dissipating fins 30 extending in parallel or substantially in parallel with one another is disposed in a heat sink 40. In this example, a heating element (heating component) 110, e.g., a CPU, is mounted on the upper portion of a circuit board 120, and the heat sink 40 is arranged such that its bottom is thermally coupled to the upper surface of the heating element 110.
As described above, the heat sink 40 and the plurality of piezoelectric fans 31 define a cooling device 100.
As illustrated, the amplitude of the blade end when the heat dissipating fins exist on both sides of the blade is less than that when the space is open. For example, a comparison when an input voltage of about 30 V is applied and no hole exists is discussed below. The amplitude of the blade end is about 9.2 mm when no heat dissipating fins exist, whereas the amplitude of the blade end is reduced to about 5.5 mm when the heat dissipating fins exist, for example. When the hole 22 is disposed, the amplitude of the blade end is increased to about 7.5 mm, for example.
This is because the presence of the hole 22 reduces a substantial area of the blade 21, and facilitates air to escape through the hole 22, which is described below, and thus, air resistance is reduced correspondingly.
As described above, it is clear that the hole 22 in the blade enables the blade to swing with a relatively large amplitude even in a space between the heat dissipating fins.
As described above, the hole 22 not only increases the amplitude of the blade 21 by reducing air resistance but also acts so as to separate a current of warm air in the vicinity of the walls of the heat dissipating fins 30 and sweep it together with the currents of air sandwiched by the two neighboring heat dissipating fins 30 toward the outside. (Hereinafter, this operational effect is referred to as “sweep effect.”) Accordingly, the heat dissipation effect is improved.
Three-dimensional simulation is required calculate a temperature distribution of a current of air flowing in a space between the heat dissipating fins in a cooling device having the structure illustrated in
As illustrated, when the current of air flowing in the space sandwiched by the heat dissipating fins 30 is a laminar flow, the velocity of flow of air increases toward the central portion of the space, and, in theory, it is zero at the walls of the heat dissipating fins 30 due to viscous drag of air. Accordingly, the quantity of cool air that is supplied from the left end and then flows toward the right end through the central portion in the space sandwiched by the heat dissipating fins 30 while remaining cool is relatively large, so the heat dissipation effect of the heat dissipating fins 30 is relatively low.
As described above, the warm air (high-temperature air layer) distributed over the walls of the heat dissipating fins 30 undulates and moves towards the central portion of the space, and is swept out by a current of air flowing through the entire space. Accordingly, the overall heat dissipation effect is improved.
In the first preferred embodiment, the blade 21 includes the single hole 22 extending along the longitudinal direction of the blade.
The example shown in
In the example shown in
Differences in operational effect depending on the location of each of the holes and the number of holes in the blade 21 are described below.
First, as illustrated in
In contrast, as illustrated in
As described above, there is, to some extent, a trade-off between the performance of sending air and the sweep effect, so the shape, location, and size of the hole can be determined so as to achieve a maximum cooling capability.
Also, when a plurality of holes are provided, as illustrated in
It is noted that, when the hole is disposed near the base of the blade (adjacent to the piezoelectric oscillator 20), although the substantial width of the section at which the bending stress of the blade 21 is relatively large is small, the shape of the hole 22 extending in the longitudinal direction of the blade 21, not in the width direction, reduces the concentration of the bending stress, such that reliability in long-time driving can be improved.
The weights 24a and 24b are preferably made of the same or substantially the same stainless steel as the blade 21 and are joined thereto by adhesive, for example. The dimensions L3 and d of the weights 24a and 24b illustrated in the drawing are preferably about 2 mm and about 0.5 mm, respectively, for example. The thickness dimension of the blade 21 preferably is about 100 μm, for example. The other dimensions L1, L2, and W are the same or substantially the same as in the first preferred embodiment illustrated in
As described above, the weights 24a and 24b provided to the end of the blade 21 increases moment of inertia, and driving the blade with the weights at a resonant frequency increases the amplitude of the blade 21 even when the piezoelectric fan 34 is arranged in a space sandwiched by the heat dissipating fins, as illustrated in
The addition of the weights and the formation of the hole have a synergistic effect. This is because, due to the addition of the weights and the formation of the hole, the center of mass of the blade is moved closer to the end, and thus, the moment of inertia per mass of the blade is increased.
Therefore, with the increase in amplitude of the blade caused by the weights 24a and 24b, the sweep effect caused by the hole 22 can be further enhanced.
In this example, the blade 21 is bent into three segments indicated by 21a, 21b, and 21c, the segment 21a near the piezoelectric oscillator 20 does not include a hole, and the segments 21b and 21c include the holes 22b and 22c, respectively. The holes 22b and 22c do not overlap the bent portions. With this structure, the base adjacent to the piezoelectric oscillator is more elastic, the end is less elastic, and the amplitude of each of the segments 21b and 21c (in particular, the amplitude of the segment 21c) is increased. Accordingly, swinging is similar to movement of a round fan, such that a high cooling capability is obtained.
The stress does not concentrate on the hole, so the reliability in long-time driving can be improved.
It is noted that a weight as illustrated in
Each of the piezoelectric elements 20a and 20b includes an electrode film provided on its surface. Applying a driving voltage corresponding to the polarization direction of each of the piezoelectric elements 20a and 20b between the blade and each of the electrodes expands and contracts the piezoelectric elements 20a and 20b in opposite directions, thus driving the piezoelectric elements 20a and 20b as a bimorph piezoelectric oscillator.
As described above, the bimorph type increases the displacement of flexure of the blade 21 with respect to an applied voltage by the piezoelectric elements 20a and 20b, so the amplitude of the blade 21 can be more efficiently increased.
It is noted that the spacers 28 and 29 are not necessary.
Here, an end of the lower piezoelectric oscillator 26b is fixed, such that an end of the upper piezoelectric oscillator 26a can swing at an angle that is approximately twice as large as when a single piezoelectric oscillator is used. Accordingly, the amplitude of the blade 21 illustrated in
It is noted that, although
Here, an end of each of the piezoelectric oscillators 27a and 27b is fixed, such that an end of the central piezoelectric oscillator 27c can swing at an angle that is approximately twice as large as when a single piezoelectric oscillator is used. Accordingly, the amplitude of the blade 21 illustrated in
It is noted that, although
The blade 21 including the hole 22 and the piezoelectric oscillator 20 define the piezoelectric fan 31, and the piezoelectric fan 31 is substantially the same as the piezoelectric fan illustrated in
A component that faces a current of air caused by the blower fan 50 is increased, and the conditions of the hole 22 are similar to those of the simulation illustrated in
It is noted that, although
As illustrated in
As illustrated in
As described above, the cooling device 102 including the piezoelectric fan in which the plurality of blades can be arranged to swing by the single piezoelectric oscillator is configured.
In the example of
Similarly, in the example of
With these configurations, the blade 21 in any region can independently swing depending upon the desired purpose. For example, in
As described above, oscillation of a member to which the support member 41 is attached can be suppressed and prevented, so noise can be minimized and eliminated.
It is noted that the number of regions and the number of blades in each region are not limited to the ones illustrated in
A positional relationship and dimensional relationship among portions of a cooling device according to the above preferred embodiments are discussed below.
A piezoelectric fan includes a blade that is disposed between heat dissipating fins of a heat sink and the blade pulls hot air on the surface of the heat dissipating fins and sweeps it to facilitate cooling. To facilitate cooling, an increase in the area from which hot air is pulled is important. To this end, it is necessary to increase the surface area of the heat dissipating fins of the heat sink and to increase the amplitude of the blade, and preferably, the elongated blade may preferably be disposed in the gap between the neighboring heat dissipating fins.
To pull hot air on the surface of each of the heat dissipating fins 30 by the blade 21, it is preferable that the gap G between the heat dissipating fin 30 and the blade 21 be relatively small. However, if the gap G is small, air resistance when moving the blade 21 would be large and the amplitude of the blade 21 would be small. In terms of the purpose of pulling air on the surface of the heat dissipating fin 30, it is not overly important to pull air at the central portion between the heat dissipating fins 30 (the region approximately indicated by the letter B illustrated in
It is preferable that the hole 22 be disposed over the overall length of the blade 21. However, in practice, the hole is disposed only at the central portion of the blade 21 (the position indicated by the letter A in
First, when the base of the blade 21 is considered, because the amplitude of the base is relatively small, air resistance is also relatively small, such that there is no need to provide the hole. Next, when the end of the blade 21 is considered, because the blade 21 does not extend beyond the end, pushed air can easily escape to a wide space. As a result, at the end of the blade 21, the effect of reducing air resistance by the hole is relatively small. If the hole extends to the end of the blade 21, two thin slender blades would move in a narrow gap between heat dissipating fins. However small air resistance and an unstable oscillation may occur because of the interaction between the movements of the blades through air. As a result, it is preferable that the end of the blade 21 is defined by a rigid coupled portion. Additionally, to prevent heat build-up between the heat dissipating fins 30 of the heat sink, air flow is necessary to some extent. The end at which the amplitude is the largest greatly affects the effect of air flow, and the hole being disposed adjacent to the base, not at the end, facilitates production of a stable one-way current of air.
Also for these reasons, it is preferable that the hole 22 be disposed only at the central portion of the blade 21.
As for the shape of the hole 22, for the reasons described below, the hole in the longitudinal direction can be configured in a wide range except for the base and the end, whereas the hole in the width direction is required to have a minimum dimension E from the longitudinal-direction side of the blade 21 to the side of the hole 22 that is parallel or substantially parallel to that longitudinal-direction side. Because of this and the fact that the blade 21 is elongated, the hole 22 is also elongated.
When the end-surface effect is neglected, air resistance at a point in the x direction is assumed to be proportional to the square of the speed and the cross-section ratio. That is, where the amplitude is h(x), the frequency is f, and the cross-section ratio is rA, Air Resistance in x∝f2h2rA and the one in which air resistance at each cross section is integrated over the length of the blade is air resistance of the whole.
It is noted that [Cross-section Ratio]=([Blade Width]−[Hole Width])/[Distance between Heat Dissipating Fins].
As described above, because air resistance is proportional to the square of the amplitude, air resistance in the vicinity of the base whose amplitude is relatively small is negligible. As a result, there is no reason to provide the hole in the vicinity of the base.
As for the end, even if no hole is provided, a wide space is present beyond the end. As a result, pushed air can flow toward the end direction without passing through the narrow gap between the blade 21 and the heat dissipating fin (the portion G in
Because of a mechanism of improving the cooling capability, if the difference between the temperatures at both ends of the blade (the position G and the position G+E in
When the distance from the heat dissipating fin 30 is sufficiently small, the temperature distribution is a substantially straight line. Where the slope of this straight line (=temperature gradient) is k and the temperature at the surface of the heat dissipating fin is To, the temperature at a position that is remote by the dimension G from the wall of the heat dissipating fin is To+k*G and the temperature at a position that is remote by the dimension (G+E) is To+k*(G+E). If the temperature at the position of the blade 21 is perfectly uniform, the temperature at this blade 21 is To+k*(G+E/2), so the temperature gradient can be estimated at k*(1+0.5*E/G).
That is, if E=G, the improvement in the cooling capability can be estimated at about 50% at a maximum. In view of the impossibility of perfectly uniform temperature distribution of the blade portion and the possibility that the temperature distribution may deviate from a linear range, setting E>G is effective to achieve distinct improvement in cooling capability.
In some of the preferred embodiments described above, a unimorph piezoelectric fan is preferably arranged such that an end of a piezoelectric oscillator is connected to an end of a blade. However, the entire surface of a piezoelectric oscillator may be connected to an end of a blade.
Some of the preferred embodiments described above illustrate an example in which a weight is connected to an end of a blade. The weight and the blade may be integrally provided. In addition, the weight may be disposed in the vicinity of an end, instead of at the distal end.
In the preferred embodiments described above, other than stainless steel, a highly elastic metal plate, such as one made of phosphor bronze, and a resin plate, for example, may also preferably be used as a blade.
Additionally, in the examples, except for the configuration illustrated in
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-239815 | Sep 2007 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2008/066201 | Sep 2008 | US |
| Child | 12722650 | US |
