Abstract:
A heat dissipating device includes a heat sink ( 10 ), a heat pipe ( 20 ), a heat reservoir ( 30 ) thermally connecting with the heat sink through the heat pipe, and a fan ( 40 ) generating an airflow through the heat sink. The heat pipe includes an evaporating portion ( 202 ) attached to the heat sink and a condensing portion ( 204 ) attached to the heat reservoir. The heat reservoir is made of metal containing working medium, such as water, therein. The heat reservoir stores or releases heat based on the amount of heat generated by the CPU to realize a compensation to the increase or decrease of temperature of the CPU, whereby the change rate of the temperature of the CPU from idle to busy condition and vice versa can be more stable.

Description:
BACKGROUND  
       [0001]     1. Field  
         [0002]     The present invention relates to a heat dissipating device to dissipate heat generated by an electrical component in a computer system, and more particularly to a heat dissipating device which has a heat reservoir for storing heat received from the electronic component such as a Central Processing Unit (CPU).  
         [0003]     2. Prior Art  
         [0004]     As computer technology continues to advance, electronic components such as central processing units (CPUs) of computers are made to provide faster operational speeds and greater functional capabilities. When a CPU operates at a high speed in a computer, its temperature frequently increases greatly. Cooling is especially important for the CPU of the computer. Without proper cooling, the heat generated by the CPU can quickly cause the CPU to overheat and damage the CPU. It is desirable to dissipate the heat quickly, for example using a heat sink system assembled within the computer, so that the CPU of the computer operates in normal temperature range.  
         [0005]     A conventional heat sink system mounted on CPUs includes a heat sink and a cooling fan.  
         [0006]     Currently, the rotational speed of the cooling fan is controlled by BIOS (Basic Input Output System) according to the CPU temperature. Higher the CPU temperature, faster the speed of the cooling fan runs. When a computer system is busy, the CPU operates at high speed and large amounts of heat is generated by the CPU; the speed of the cooling fan is raised rapidly according to commands from the BIOS in order to timely dissipate the heat, whereby noise of the fan is raised simultaneously. When a computer system is idle, the heat generated by the CPU is reduced enormously, the temperature of the CUP drops very quickly and the speed of the cooling fan is decreased abruptly according to the commands from the BIOS. The sudden change of the temperature of the CPU is harmful to the lifespan of the CPU. The abrupt variation of the speed of the fan according to the change of temperature of the CPU leads the fan to generate a suddenly varied noise level which is very annoying to a user of the computer.  
         [0007]     Consequently, there is a need in the art to maintain the CPU temperature within a slightly changed range and to stabilize the cooling fan speed when the cooling fan operates.  
       SUMMARY  
       [0008]     Accordingly, what is needed is a heat dissipating device which has a substantially uniform heat dissipation effect.  
         [0009]     A heat dissipating device in accordance with the present invention comprises a heat sink, at least one U-shaped heat pipe, a heat reservoir for mounting on the heat sink, a fan mounted horizontally to the heat sink and a pair of clips. The heat sink comprises a heat spreader and a plurality of fins attached on the heat spreader. The heat pipe comprises an evaporating portion attached to the heat spreader and a condensing portion parallel to the evaporating portion. The heat reservoir is made of a material with a high thermal conductivity, and has a sealed chamber containing working medium having higher specific heat, such as water. When the heat generated by the CPU is increased rapidly, the heat reservoir may absorb the heat to drop the temperature of the CPU in a short time. The heat absorbed by the heat reservoir is reserved in the heat reservoir. When the heat generated by the CPU is decreased sharply, the heat reserved in the heat reservoir is released to prevent the temperature of the CPU from being great lowered. Thus, the heat reservoir may reserve or release heat based on the amount of heat generated by the CPU to realize a substantially uniform heat dissipation effect and to maintain the temperature of the CPU at a slightly changed range.  
         [0010]     Other advantages and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is an exploded, isometric view of a heat dissipating device in accordance with a first preferred embodiment of the present invention;  
         [0012]      FIG. 2  is an assembled view of  FIG. 1 ;  
         [0013]      FIG. 3  is an exploded, isometric view of a heat dissipating device in accordance with a second preferred embodiment of the present invention; and  
         [0014]      FIG. 4  is an assembled view of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]      FIGS. 1-2  show a heat dissipating device in accordance with a first preferred embodiment of the present invention. The heat dissipating device comprises a heat sink  10 , three heat pipes  20 , a heat reservoir  30  mounted on the heat sink  10 , a fan  40  mounted to a side of the heat sink  10  and a pair of clips  50 .  
         [0016]     The heat sink  10  comprises a heat spreader  102  and a plurality of fins  104 . The heat spreader  102  is preferably made of copper or aluminum material, and has a bottom mating surface for contacting a heat source such as a CPU (not shown). The heat spreader  102  defines a recess  106  for partially receiving the fan  40  and three parallel adjoining grooves  108  in a top surface thereof, opposing the mating surface. The heat spreader  102  defines mounting holes  107  at two lateral sides, near the recess  106 . The heat spreader  102  forms ears  110  extending outwardly at four comers thereof. The heat spreader  102  is attached on the CPU by screws  112  extending through the ears  110  and engaging with a retainer (not shown) surrounding the CPU. Each screw  112  is provided with a spring  114  therearound, for ensuring a secure engagement between the screws  112  and the retainer. The fins  104  are attached on the spreader  102 . The fins  104  define three adjoining slots  116  in a bottom portion thereof corresponding to the grooves  108  of the heat spreader  102 , and three spaced slots  118  in a top portion thereof. The grooves  108  and the slots  116 ,  118  each have a semicircle cross-section.  
         [0017]     Each heat pipe  20  is generally U-shaped, and forms a capillary structure therein. Opposite ends of each heat pipe  20  respectively form an evaporating portion  202  and a condensing portion  204  parallel to the evaporating portion  202 . The evaporating portions  202  of the heat pipes  20  are fixedly received in holes formed by the corresponding grooves  108  of the heat spreader  102  and slots  116  of the fins  104  by soldering means or by other conventional means such that the heat pipes  20  and the heat sink  10  (inclusive of the fins  104  and the heat spreader  102 ) are thermally connected together.  
         [0018]     The heat reservoir  30  is a block made of a material with a high thermal conductivity, such as copper, and has a sealed chamber therein containing working medium having a high specific heat, such as water. The heat reservoir  30  has a square configuration. Three slots  302  corresponding to the slots  118  of the fins  104  is defined in a bottom portion of the heat reservoir  30 . The condensing portions  204  of the heat pipes  20  are fixedly received in holes formed by the slots  302  of the heat reservoir  30  and the slots  118  of the fins  104  such that the fins  104 , the heat pipes  20  and the heat reservoir  30  are thermally connected together, which means that heat received by the spreader  102  is transferred to the heat reservoir  30  and the fins  104  by the heat pipes  20 . When the CPU operates, heat is generated by the CPU and transferred to the heat sink  10 , the raised temperature of the heat sink  10  can be calculated by following equation: 
 
Δ t=Q /( m 1× C 1+ m 2× C 2) 
 
 where 
    Δt is the raised temperature of the heat sink  10 ;     Q is the amount heat of the CPU operating;     m 1  is the mass of the heat sink  10 ;     m 2  is the mass of the water in the heat reservoir  30 ;     C 1  is the specific heat of the heat sink  10 ;     C 2  is the specific heat of the water in the heat reservoir  30 . 
 
 In a comparison between the prior art and the present invention, suppose that the amount heat generated by the CPU is a constant value, the specific heat of the heat sink in the prior art is the same as C 1  of the present invention, and the mass of the heat sink in the prior art is a combination of m 1  and m 2  of the present invention. The heat sink  30  is made of copper, the specific heat of water is bigger than the specific heat of copper, which means C 2  is bigger than C 1 , so the increased or decreased speed of the temperature of the heat sink  10  with the heat reservoir  30  is much lower than the heat sink without the heat reservoir in the prior art when they are used to absorb or dissipate a determined amount of heat. Because the heat sink  10  and the heat reservoir  30  are thermally connected with the CPU, the large degree of fluctuation of the temperature of the CPU between busy and idle conditions can be improved. Thus, the speed of the fan does not need to be varied abruptly due to the sudden increase or decrease of the temperature of the CPU. 
   
 
         [0025]     When the CPU is busy and the heat generated by the CPU is increased, the heat reservoir  30  can absorb the heat and store the heat therein to drop the temperature of the CPU to an acceptable level. When the CPU is shifted to idle and the heat generated by the CPU is decreased sharply, the heat reservoir  30  can release the heat stored therein to prevent the temperature of the CPU from being lowered too quickly. Thus the temperature of the CPU can have a more stable variation.  
         [0026]     The fan  40  is contained in a bracket  402  having a generally circular configuration, and is mounted to a lateral side  105  of the fins  104  via the clips  50 . The fan  40  is used for producing forced airflow to flow through channels (not labeled) between the fins  104  to thereby promote heat dissipation efficiency of the heat sink  10 . The bracket  402  forms four lugs  404  extending outwardly therefrom. An aperture  406  is defined in each lug  404 . The rotational speed of the fan  40  is controlled by BIOS based on the temperature of the CPU.  
         [0027]     Each clip  50  comprises a body  502  positioned on the lateral side  105  of the fins  104 , and a tab  504  perpendicularly extending from a bottom side of the body  502 . An engaging screw  510  extends through a top end of the body  502  for engaging in a corresponding mounting hole  304 . Each body  502  forms a pair of bulges  506 . A screw hole  507  is defined in each bulge  506 . Each tab  504  defines a locating hole  508  corresponding to one the mounting holes  107  of the heat spreader  102 . In this embodiment, the clips  50  are fastened on the heat sink  10  by engaging screws  113  through the locating holes  508  of the clips  50  and into the mounting holes  107  of the heat spreader  102 , and by engaging the screws  510  into the mounting holes  304  of the heat reservoir  30 . The lugs  404  of the bracket  402  are fastened to the clips  50  by extending screws (not shown) through the apertures  406  of the lugs  404  into the screw holes  507  defined in the bulges  506  of the clips  50 . Thus, the fan  40  is secured on the lateral side  105  of the heat sink  10  and the heat reservoir  30  and blows air flow through the channels of fins  104  to enhance heat dissipating effectiveness of the heat sink  10 .  
         [0028]     In operation of the heat dissipating device, one portion of heat accumulated on the heat spreader  10  can be immediately taken away to the fins  104  of the heat sink  10  where the heat is dissipated to atmosphere. The other portion of the heat accumulated on the heat spreader  10  is transferred to the heat reservoir  30  via the heat pipes  20 . The heat on the heat reservoir  30  is absorbed by the working medium in the heat reservoir  30  and is reserved in the heat reservoir  30 . When the heat generated by the CPU is decreased sharply, the heat can be quickly dissipated by the heat sink, the heat reserved in the heat reservoir  30  is released to prevent the temperature of the CPU from being dropped too quickly. When the heat generated by the CPU is increased rapidly, the heat reservoir  30  may absorb the heat and the fins  104  can dissipate the heat to lower the temperature of the CPU to an acceptable level. Thus, the heat reservoir  30  may store or release heat based on the amount of heat generated by the CPU to realize a compensation to the quick increase or decrease of the temperature of the CPU due to shift between idle and busy conditions, thereby maintaining the temperature of the CPU to have a stable change rate within a predetermined range. Thus, change of the rotation speed of the fan  40  is maintained at a stable rate within a predetermined range since the change of the speed of the fan is proportional to the change of the temperature of the CPU. Accordingly, an abrupt fluctuation of the noise level generated by the operation of the fan  40  can be avoided.  
         [0029]      FIGS. 3-4  show a heat dissipating device in accordance with a second preferred embodiment of the present invention. The heat dissipating device of the second preferred embodiment is similar with that of the first preferred embodiment. However, a heat reservoir  30 ′ replaces the heat reservoir  30  of the first embodiment. The heat reservoir  30 ′ has a U-shaped configuration, and comprises an upper portion  302 ′, a lower portion  304 ′ and a middle portion  306 ′. The upper portion  302 ′ of the heat reservoir  30 ′ is parallel to the lower portion  304 ′, and is connected to the lower portion  304 ′ via the middle portion  306 ′. The upper portion  302 ′ defines three slots  308 ′ in a bottom portion thereof, corresponding to the slots  118  of the fins  104 . A pair of mounting holes  305 ′ is defined in a lateral side of the upper portion  302 ′. In the second preferred embodiment, the heat sink  10  is sandwiched between the upper portion  302 ′ and the lower portion  304 ′. The lower portion  304 ′ is attached to the CPU.  
         [0030]     In operation of the heat dissipating device in accordance with the second embodiment, one portion of heat accumulated on the lower portion  304 ′ is conducted to the heat spreader  102 , and is subsequently transferred to the fins  104  for being dissipated to atmosphere. The other portion of the heat accumulated on the lower portion  304 ′ is absorbed by the working medium in the lower portion  304 ′ and is stored in the lower portion  304 ′. The heat pipes  20  transfer one portion of the absorbed heat from the heat spreader  102  to the upper portion  302 ′ of the heat reservoir  30 ′. The heat on the upper portion  302 ′ is absorbed by the working medium in the upper portion  302 ′ and is stored in the upper portion  302 ′. When the heat generated by the CPU is decreased sharply, the heat stored in the heat reservoir  30 ′ can be released to prevent the temperature of the CPU from being quickly dropped. When the heat generated by the CPU is increased rapidly, the heat reservoir  30 ′ may absorb the heat to lower the temperature of the CPU to an acceptable level. Thus, the heat reservoir  30 ′ may store or release heat based on the amount of heat generated by the CPU to realize a compensation to the quick increase or decrease of the temperature of the CPU, thereby maintaining the temperature of the CPU to have a stable change rate. Therefore, the change of the rotation speed of the fan  40  is maintained at a stable rate, and the abrupt fluctuation of the noise level generated by the operation of the fan  40  is prevented.  
         [0031]     It is believed that the present invention and its advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.