Abstract:
A heat dissipation device includes a heat sink ( 20 ) having a plurality of fins ( 26 ), a fan duct ( 50 ), a fan ( 70 ) and a mounting bracket ( 60 ) for mounting the fan duct and the fan to the heat sink. The fan duct is mounted to a front side of the heat sink, and includes an inlet, an enlarged outlet covering the front side of the heat sink and at least two channels ( 56 ). The fan duct is capable of expanding an airflow generated by the fan by the enlarged outlet and dividing the airflow by the at least two channels into at least two sub-airflows. Thus, the fan can blow the airflow through all of the fins to thereby promote a heat dissipating efficiency of the heat dissipation device.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a heat dissipation device, and particularly to a heat dissipation device having a ventilating duct to guide an airflow generated by a fan to a heat sink. 
   DESCRIPTION OF RELATED ART 
   Electronic devices such as central processing units (CPUs) generate large amounts of heat during normal operation, which can destabilize the electronic devices and cause damage to the electronic devices. Oftentimes, a heat dissipation device is used to dissipate heat from an electronic device. The heat dissipation device frequently comprises a heat sink for being attached to the electronic device. 
   Such a heat sink typically includes a base from which fins project. The base of the heat sink conducts heat away from the electronic device and the fins of the heat sink radiate the heat to ambient air. To further promote the heat removal effectiveness, a fan is typically disposed adjacent to a front side the heat sink to blow or otherwise force airflow through the fins of the heat sink. 
   One problem with this approach, however, is that the airflow which the fan can generate to flow through the fins of the heat sink is limited only a portion of the fins, since the heat sink is much larger in volume than that of the fan. In other words, the fan is unproportionately smaller than the heat sink, and can not cover the front side of the heat sink totally. Therefore, the airflow generated by the fan can not flow through all of the fins of the heat sink thoroughly, which, in turn, adversely affects the heat-dissipation effectiveness of the heat sink. 
   SUMMARY OF INVENTION 
   Accordingly, what is needed is a heat dissipation device having a ventilating duct; the duct has an enlarged outlet and a plurality of channels which can effectively disperse and divide an airflow generated by a fan over a total area of a front side of a heat sink; thus, the airflow can flow thoroughly through every fin of the heat sink to effectively remove heat therefrom so as to enhance heat dissipating efficiency of the heat sink. 
   According to a preferred embodiment of the present invention, a heat dissipation device comprises a heat sink assembly having a plurality of fins, a fan duct, a fan and a mounting bracket for mounting the fan duct and the fan to the heat sink assembly. The fan duct mounted to a front side of the heat sink assembly, and comprises an inlet, an enlarged outlet and at least two channels. The fan duct is capable of expanding an airflow generated by the fan to flow over the entire front side of the heat sink. The enlarged outlet covers a total area of the front side of the heat sink, and the at least two channels divide the airflow into two sub-airflows. Thus, the airflow generated by the fan can blow all of the fins through the fan duct, whereby the heat dissipation device can have a better heat dissipating efficiency. 
   Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is an assembled, isometric view of a heat dissipation device according to a preferred embodiment of the present invention; 
       FIG. 2  is a partly exploded view of  FIG. 1 ; 
       FIG. 3  is an assembled view of a heat sink assembly of  FIG. 2  from another aspect; 
       FIG. 4  is an exploded, isometric view of a fan duct  FIG. 2 ; 
       FIG. 5  is an assembled view of  FIG. 4 ; 
       FIG. 6  is a cross-sectional view taken along line VI-VI of  FIG. 5 ; and 
       FIG. 7  is an assembled view of the fan duct and a mounting bracket of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a heat dissipation device mounted on a printed circuit board (PCB)  10 . The heat dissipation device comprises a heat sink assembly having a first heat sink  20  and a second heat sink  30  mounted to the first heat sink  20  via a pair of heat pipes  21 , a fan duct  50  mounted to a front side of the first heat sink  20 , a fan  70  blowing an airflow to the first heat sink  20  via the fan duct  50 , and a mounting bracket  60  for mounting the fan  70  and the fan duct  50  to the first heat sink  20 . 
   Referring to  FIGS. 2-3 , the first heat sink  20  has a width longer than a height thereof. The first heat sink  20  comprises a base  22 , a top plate  24  and a plurality of fins  26  sandwiched between the base  22  and the top plate  24 . A heat spreader  23  is attached to a bottom surface of the base  22  and contacts a CPU (not shown) mounted on the PCB  10 . The heat spreader  23  is made of metal material such as copper, with good heat conductivity. A total size of the heat spreader  23  is smaller than that of the base  22  such that a step  224  is formed on the base  22  relative to the heat spreader  23 . The step  224  defines screw holes  226 . The base  22  forms an ear  220  extending outwardly from each of four corners thereof. The first heat sink  20  is fastened to the PCB  10  by fasteners  28  extending through the ears  220  to engage with a back plate (not shown) on an underside of the PCB  10 . The top plate  24  defines a step  240  at a front edge thereof, opposite to the step  224  of the base  22 . The step  240  defines a pair of screw holes  242  in alignment with the screw holes  226 . 
   The heat pipes  21  are mounted to the first heat sink  20 . Each heat pipe  21  has a U-shaped configuration, and forms a capillary structure therein. Working medium is contained in the heat pipe  21 . Each heat pipe  21  comprises an evaporating portion  210 , a condensing portion  212  parallel to the evaporating portion  210 , and a middle portion  213  interconnecting the evaporating portion  210  and the condensing portion  212 . The evaporating portions  210  of the heat pipes  21  are sandwiched between the heat spreader  23  and the base  22 , for absorbing heat generated by the CPU. The condensing portions  212  of the heat pipes  21  are sandwiched between a top of the fins  26  and the top plate  24 , for dissipating the heat from the heat spreader  23  to the fins  26 . The CPU engages a bottom face of the heat spreader  23  directly below the evaporating portions  210  of the heat pipes  21 . When the evaporating portions  210  absorb the heat from the CPU, the working fluid in the evaporating portions  210  becomes vapor. The vapor flows to the condensing portions  212  of the heat pipes  21  and is cooled and condensed into liquid. The condensed working fluid flows back to the evaporating portions  210  by capillarity effect of the capillary structure in the heat pipes  21  to complete a heat discharging cycle in the heat pipes  21 . Thereafter, the circle is repeated. 
   The second heat sink  30  is mounted to the first heat sink  20 , for enhancing heat dissipation efficiency. The second heat sink  30  comprises a base  32  thermally connected to the middle portions  213  of the heat pipes  21 . The base  32  is parallel to a lateral side of the fins  26 . A plurality of fins  34  extends laterally from a side of the base  32 . The fins  34  of the second heat sink  30  are oriented perpendicular to the fins  26  of the first heat sink  20 . 
   Also referring to  FIGS. 4-6 , the fan duct  50  is mounted to a the front side of the first heat sink  20 . The fan duct  50  is made of plastic by injection molding or made of cardboard by folding. The fan duct  50  comprises a first tube  51 , a second tube  52  and a third tube  53 . The first, second and third tubes  51 ,  52 ,  53  have a same length along an axial direction thereof. The first, second and third tubes  51 ,  52 ,  53  each have a funnel-shaped configuration. The first, second and third tubes  51 ,  52 ,  53  each have an inlet  510 ,  520 ,  530  and an enlarged outlet  512 ,  522 ,  532 . A diameter of each outlet  512 ,  522 ,  532  is bigger than that of the corresponding inlet  510 ,  520 ,  530 . The first tube  51  has a gradually increased radius from the inlet  510  to the outlet  512  thereof. The second and third tubes  52 ,  53  are truncated at opposite top and bottom surfaces adjacent to the outlets  520 ,  530  thereof, respectively, such that the second and third tubes  52 ,  53  have a pair of opposite flat surfaces  524 ,  534 , respectively. The second and first tubes  52 ,  51  are in turn mounted into the third tube  53  and the flat surface  524  of the second tube  52  contacts the flat surface  534  of the third tube  53  such that three divided channels  56  are formed by the first, second and third tubes  51 ,  52 ,  53  in the fan duct  50 . Radiuses of the first, second and third tubes  51 ,  52 ,  53  are in turn increased along a horizontal direction of the fan duct  50 . An outlet (not labeled) of the fan duct  50  has a width larger than a height thereof, thereby to fit the shape of the front side of the first heat sink  20 . Edges of the inlets  510 ,  520 ,  530  of the first, second and third tubes  51 ,  52 ,  53  each define four cutouts  5160 ,  5260 ,  5360 . The cutouts  5160 ,  5260 ,  5360  are located on two perpendicularly crossed horizontal and vertical lines through an inlet (not labeled) of the fan duct  50 . When mounted to the first heat sink  20 , the outlet of the fan duct  50  covers substantially a total area of the front side of the first heat sink  20 . 
   Also referring to  FIG. 7 , the mounting bracket  60  has a U-shaped configuration. The mounting bracket  60  comprises a pair of parallel side walls  62  parallel to the flat surfaces  534  of the third tube  53  of the fan duct  50 , for facilitating the fan duct  50  to be sandwiched between the side walls  62 . The mounting bracket  60  further comprises a faceplate  64  interconnecting front edges of the side walls  62 . A rear edge  620  of each of the side walls  62  defines a pair of through holes  622  corresponding to the screw holes  242 ,  226  of the first heat sink  20 . The rear edges  620  of the mounting bracket  60  are respectively positioned on the steps  240 ,  224  of the first heat sink  20  such that the step  240  of the top plate  24  and the step  224  of the base  22  are sandwiched between the rear edges  620  of the mounting bracket  60 . The mounting bracket  60  is fixed to the first heat sink  20  by extending screws (not shown) through the through holes  622  to threadedly engage with the screw holes  242 ,  226  of the first heat sink  20 . The faceplate  64  has a square configuration, and defines a circular opening  640  at a center thereof. A pair of crossed ribs  642  corresponding to the cutouts  5160 ,  5260 ,  5360  of the fan duct  50  is formed on the faceplate  64 . The two ribs  642  are perpendicular each other and divide the opening  640  into four equal quadrants. The ribs  642  engage in the cutouts  5160 ,  5260 ,  5360  of the fan duct  50  such that the inlet of the fan duct  50  corresponds to the opening  640 . The faceplate  64  has four corners  644 . Each corner  644  defines a screw hole  646 . 
   Referring again to  FIG. 2 , the fan  70  has a square configuration, and comprises a fan frame  72  and a impeller  74  mounted in the frame  72 . The fan frame  72  has four corners  76 . Each corner  76  defines a through hole  760  corresponding to the screw hole  646  of the mounting bracket  60 . The fan  70  is mounted to the faceplate  64  of the mounting bracket  60  by extending screws (not shown) through the through holes  760  to threadedly engage with the screw holes  646  of the mounting bracket  60 . An airflow generated by the fan  70  flows through the channels  56  of the fan duct  50  from the opening  640  of the mounting bracket  60 . The airflow can be divided into different sub-airflows by these channels  56  to flow to the fins  26  of the first heat sink  20 . The enlarged outlets  512 ,  522 ,  532  result in an expansion of the airflow such that the airflow can flow through all of the fins  26 . The airflow can dissipate heat absorbed by all of the fins  26 , thereby causing the heat to be quickly dissipated from the CPU and the first heat sink  20 . 
   It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.