Patent Publication Number: US-2011075360-A1

Title: Electronic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2009-228908 filed on Sep. 30, 2009, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     One embodiment of the invention relates to an electronic apparatus having a heat dissipation structure. 
     2. Description of the Related Art 
     In related art electronic apparatuses such as video recorders and personal computers, a heat generating element such as an LSI is mounted on a circuit board. To cool such a heat generating element, a cooling system including, for example, a heat pipe and a heat sink is used. 
     In the heat sink described in JP-A-2009-150561, an end portion of a first heat pipe or a second heat pipe is thermally connected to respective heat generating elements, and the other end portions of the first and second heat pipe are thermally connected to a plurality of fins. The first heat pipe and the second heat pipe have different areas of contact to the fins to efficiently cool the heat generating elements. 
     However, in the related art, no consideration is made to prevent reduction of cooling efficiency with respect to a sloped heat pipe. For example, in a case in which a heat pipe is arranged such that its heat receiving portion is positioned higher than its heat releasing portion, circulation of working fluid enclosed therein is obstructed as the slope of the heat pipe becomes steep. When a plurality of heat pipes are arranged on top of each other for high-density mounting, cooling efficiency may decrease in each of the heat pipes. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. 
         FIG. 1  is an exemplary perspective view of a Set-top box according to an embodiment of the invention: 
         FIG. 2  is an exemplary sectional view schematically illustrating an internal configuration of the Set-top box according to the embodiment of the invention; 
         FIG. 3  is an exemplary plan view of a fourth circuit board having first and second heat receiving blocks which are thermally connected to a heat sink. 
         FIG. 4  is an exemplary perspective view of the fourth circuit board having the first and second heat receiving blocks which are thermally connected to the heat sink and a heat dissipating plate which is thermally connected to FETs in the embodiment of the invention; 
         FIG. 5  is an exemplary rear view of the Set-top box schematically illustrating a relative positional relationship between first to fourth circuit boards, the heat sink, and an exhaust fan, which are arranged inside a housing in the embodiment of the invention; 
         FIG. 6  is an exemplary sectional view of the Set-top box schematically illustrating a positional relationship between the fan and the fourth circuit board which are arranged inside the housing in the embodiment of the invention; and 
         FIG. 7  is an exemplary sectional view of heat pipes used in the embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, an electronic apparatus includes a housing, a heat dissipating member disposed inside the housing, a first heat generating element mounted on the circuit board, a second heat generating element mounted on the circuit board, a first heat pipe, and a second heat pipe. The first heat pipe includes a first heat receiving portion thermally connected to the first heat generating element, and a first heat releasing portion thermally connected to the heat dissipating member. The second heat pipe includes a second heat receiving portion thermally connected to the second heat generating element, a second heat releasing portion thermally connected to the heat dissipating member, and a fluid capturing structure configured to temporarily hold a working fluid enclosed inside the second heat pipe. 
       FIG. 1  illustrates a Set-top box  1  which is an example of an electronic apparatus. The Set-top box  1  is connected to a liquid crystal TV receiver in use, and has, for example, a function of receiving various TV programs and a function of recording a plurality of programs simultaneously or recording a long program. 
     The Set-top box  1  is has a flat box-shaped main body  2 . The main body  2  includes a metal housing  4  which is covered with a decorative cover  3 , and left and right front doors  5   a ,  5   b  which cover a front face of the decorative cover  3 . 
     The housing  4  serves as a frame of the main body  2 . As shown in  FIGS. 2 ,  5 , and  6 . The housing  4  includes a bottom wall  6 , left and right side walls  7   a ,  7   b , a front wall  8 , a back wall  9 , and a top wall  10 . The bottom wall  6  has a rectangular shape having four corner portions. Legs  6   a  are attached to the corner portions of the bottom wall  6 , respectively, and are placed, for example, on a TV receiver rack. A rear half portion of the bottom wall  6  is formed with a plurality of air inlets  11  through a central portion thereof. 
     The side walls  7   a ,  7   b , the front wall  8 , and the back wall  9  are arranged upright from a perimeter of the bottom wall  6 . The left side wall  7   a  has first to third intake holes  12   a ,  12   b ,  12   c . The first to third intake holes  12   a ,  12   b ,  12   c  are arranged in line in the front-rear direction of the housing  4  at intervals, and communicate with the outside of the main body  2  via a plurality of vents  13  of the decorative cover  3 . 
     A right half portion of the back wall  9  has a plurality of first air outlets  14   a  and a plurality of second air outlets  14   b . The top wall  10  bridges over upper edges of the side walls  7   a ,  7   b , the front wall  8 , and the back wall  9 , and is opposed to the bottom wall  6 . 
     As shown in  FIG. 2 , the housing  4  has a first accommodation space  15  and a second accommodation space  16 . The first accommodation space  15  has a front section which extends in the right-left direction of the housing  4  along the front wall  8  of the housing  4 , and a rear section which extends in the front-rear direction of the housing  4  along the right side wall  7   b . The first intake hole  12   a  of the side wall  7   a  communicates with a left part of the front section of the first accommodation space  15 . The first air outlets  14   a  of the back wall  9  communicate with a rear part of the rear section of the first accommodation space  15 . 
     The second accommodation space  16  is surrounded by the left side wall  7   a  and the back wall  9  of the housing  4 , and is located behind the front section of the first accommodation space  15 . The air inlets  11  of the bottom wall  6  communicate with a right part of the second accommodation space  16 . The second intake hole  12   b  and the third intake hole  12   b  of the left side wall  7   a  communicate with a left part of the second accommodation space  16 . 
     As shown in  FIG. 2 , a first information storage module  17 , a second information storage module  18 , a card connection device  19 , and a power module  20  are disposed inside the first accommodation space  15  of the housing  4 . 
     The first and second information storage modules  17 ,  18  serve to record TV programs and to play back the recorded TV program with quick search. The first information storage module  17  has, for example, two 5-inch hard disk drives. The second information storage module  18  has, for example, two 3.5-inch hard disk drives. 
     The card connection device  19  has, for example, six card slots into which six B-CAS cards for receiving ground-wave digital broadcasts, BS digital broadcasts, etc. are to be inserted. The first and second information storage modules  17 ,  18  and the card connection device  19  are disposed in the front section of the first accommodation space  15 , and are arranged in line in the right-left direction of the housing  4 . 
     The power module  20  has a first circuit board  22  which is a power board. The first circuit board  22  is fixed to a right end portion of the bottom wall  6  of the housing  4 . The first circuit board  22  is mounted with a plurality of circuit components  23  forming a power circuit. The circuit components  23  include components that generate heat during operation. The circuit components  23  are disposed in the rear section of the first accommodation space  15 . 
     A first axial flow fan  24  is disposed in the left end part of the front section of the first accommodation space  15 . The first axial flow fan  24  serves to take air forcibly into the first accommodation space  15  from outside the housing  4 , and is arranged to face the first intake hole  12   a.    
     A second axial flow fan  25  is disposed in the rear end part of the rear section of the first accommodation space  15 . The second axial flow fan  25  is an example of an exhaust fan which primarily serves to forcibly discharge air from the first accommodation space  15  to the outside of the housing  4 , and is arranged to face the first air outlets  14   a.    
     When the first axial flow fan  24  and the second axial flow fan  25  are driven, air is introduced into the front section of the first accommodation space  15  from outside the housing  4  through the first intake hole  12   a . At the same time, air is discharged from the rear section of the first accommodation space  15  to the outside of the housing  4  through the first air outlets  14   a.    
     As a result, as indicated by an arrow X in  FIG. 2 , the air flow is created inside the first accommodation space  15  from the front section towards the rear section, whereby the first and second information storage modules  17 ,  18 , the card connection device  19 , and the power module  20  are forcibly cooled. 
     The power module  20  generates more heat than the first and second information storage modules  17 ,  18  and the card connection device  19 . Thus, the power module  20  is disposed in the downstream side of the air flow X in the first accommodation space  15 . Accordingly, even when the power module  20  generates a large amount of heat, the first and second information storage modules  17 ,  18  and the card connection device  19  are prevented from being thermally affected by the power module  20 . 
     As shown in  FIGS. 5 and 6 , second to fourth circuit boards  27 ,  28 ,  29  are disposed in the second accommodation space  16  of the housing  4  so as to be stacked at intervals in the heightwise direction of the housing  4 . 
     The second circuit board  27  is an image processing board, and is horizontally supported above the bottom wall  6  of the housing  4 . The second circuit board  27  is mounted with a chip component  30  for image processing. A heat sink  31  is attached to the chip component  30 . 
     The third circuit board  28  is a tuner board, and is horizontally supported above the second circuit board  27  via a bracket (not shown). The third circuit board  28  is mounted with six tuner modules  33  for receiving TV signals and one distributor  34  which is connected to the tuner modules  33 . 
     The fourth circuit board  29  is a main board, and is horizontally supported above the third circuit board  28  via a bracket (not shown). The fourth circuit board  29  has a first surface  29   a  and a second surface  29   b . The first surface  29   a  is arranged to face the third circuit board  28 . The second surface  29   b  is located on the opposite side of the first surface  29   a , and is arranged to face the top wall  10  of the housing  4 . A high-performance processor  36  and an I/O controller  37  are mounted on the first surface  29   a.    
     The high-performance processor  36  and the I/O controller  37  are examples of heat generating elements. According to one embodiment, the heat generated by the high-performance processor  36  and the I/O controller  37  is transferred to a heat sink  39  (a heat dissipating member), and is then forcibly dissipated from the heat sink  39  to the outside the housing  4 . 
     More specifically, as shown in  FIGS. 4 to 6 , a first heat receiving block  40  is thermally connected to the high-performance processor  36 . The first heat receiving block  40  is made of a metal material having high thermal conductivity such as copper. The first heat receiving block  40  is held by the first surface  29   a  of the fourth circuit board  29  via a cruciform pressing spring  41 . The pressing spring  41  presses the first heat receiving block  40  against the high-performance processor  36  with prescribed pressure. 
     Likewise, a second heat receiving block  42  is thermally connected to the I/O controller  37 . The second heat receiving block  42  is made of a metal material having high thermal conductivity such as copper. The second heat receiving block  42  is held by the first surface  29   a  of the fourth circuit board  29  via an N-shaped pressing spring  43 . The pressing spring  43  presses the second heat receiving block  42  against the I/O controller  37  with prescribed pressure. 
     The heat sink  39  has a plurality of heat radiation fins  44 , which are arranged parallel to each other at intervals. Two heat pipes  45   a ,  45   b  are arranged to extend between the heat sink  39  and the first heat receiving block  40 . 
     One end portion of each of the heat pipes  45   a ,  45   b  is fixed to the first heat receiving block  40  by, for example, crimping so as to be thermally connected to the first heat receiving block  40 . The other end portion of each of the heat pipes  45   a ,  45   b  penetrates through the heat radiation fins  44 , and is thermally connected to the heat radiation fins  44 . 
     Accordingly, the heat generated by the high-performance processor  36  is transmitted to the first heat receiving block  40 , and is then transferred from the first heat receiving block  40  to the heat sink  39  via the heat pipes  45   a ,  45   b.    
     Likewise, a heat pipe  46  is arranged to extend between the heat sink  39  and the second heat receiving block  42 . One end portion of the heat pipe  46  is fixed to the second heat receiving block  42  by, for example, crimping so as to be thermally connected to the second heat receiving block  42 . The other end portion of the heat pipe  46  penetrates through, and is thermally connected to the heat radiation fins  44 . 
     Accordingly, the heat generated by the I/O controller  37  is transmitted to the second heat receiving block  42 , and is then transferred from the second heat receiving block  42  to the heat sink  39  via the heat pipe  46 . 
     As shown in  FIGS. 2 and 6 , a fan  60  is disposed inside the second accommodation space  16  of the housing  4 . The fan  60  includes a fan casing  61  and an impeller  62 . The fan casing  61  has an outer casing  63  and an inner casing  64 . The rear edge of a top plate  67  of the fan casing  61  and a rear opening of the outer casing  63  form an air outlet  73  of the fan  60 . The air outlet  73  is opened toward the rear side of the housing  4  so as to be perpendicular to a first air inlet  66  and a second air inlet  69 , and is disposed to face the heat sink  39  and the end portions of the heat pipes  45   a ,  45   b ,  46 . According to this configuration, cooling air that is sent out from the fan  60  directly towards the heat sink  39  and the heat pipes  45   a ,  45   b ,  46 , whereby the cooling efficiency can be increased. 
     The heat pipes  45   a ,  45   b ,  46  hold the heat sink  39  such that the heat sink  39  is placed near a rear end portion of the first surface  29   a  of the fourth circuit board  29 . Therefore, when the fourth circuit board  29  is horizontally supported above the third circuit board  28 , the heat sink  39  is disposed in a rear end part of the second accommodation space  16  of the housing  4  so as to face the second air outlets  14   b  of the housing  4 . 
     As shown in  FIGS. 4 to 6 , according to one embodiment, the heat pipes  45   a ,  45   b ,  46  are connected to the heat sink  39  in a row. More specifically, the heat pipe  46  is connected to the heat sink  39  at a position that is closest to the first surface  29   a  of the fourth circuit board  29 . The heat pipe  45   a  is connected to the heat sink  39  at a position that is second closest to the first surface  29   a  of the fourth circuit board  29 . The heat pipe  45   b  is connected to the heat sink  39  at a position that is farthest from the first surface  29   a  of the fourth circuit board  29 . That is, in the order of the heat pipe  45   b , the heat pipe  45   a  and the heat pipe  46 , a distance between a position of connection to the heat receiving block  40 ,  42  and the position of connection to the heat sink  39  becomes longer, and a slope thereof becomes gradual. 
     Next, configurations of the heat pipes  45   a ,  45   b ,  46  according to one embodiment will be described with reference to  FIG. 7 .  FIG. 7  is an exemplary sectional view of the heat pipes  45   a ,  45   b ,  46 . 
     A working fluid W is enclosed in each of the heat pipes  45   a ,  45   b ,  46 . The working fluid W evaporates and vaporizes upon receipt of heat from the first heat receiving block  40  or the second heat receiving block  42 . The vaporized working fluid W condenses and liquefies as it releases the heat to the heat sink  39 . In this way, inside each of the heat pipes  45   a ,  45   b ,  46 , the working fluid W circulates by repeating the evaporation and the liquefaction. 
     However, in a case in which a heat pipe is steeply sloped or in a case in which the length of the heat pipe itself is long, cooling efficiency may decrease due to stagnation of the working fluid circulation. In particular, where the position of the heat releasing portion is lower than the position of the heat receiving portion in a state in which a TV-received-associated apparatus is set in place, what is called a top-heat state may occur. As the slope of the heat pipe becomes steeper, the working fluid circulation becomes more likely to stagnate, which results in a remarkable decrease of the cooling efficiency. 
     In view of the above, according to one embodiment, wicks  45   b   1  are provided on an inner side of the heat pipe  45   b  having a steep slope, in order to suppress the stagnation of the circulation of the working fluid W. The wicks  45   b   1  provide a fluid capturing structure to temporarily hold the working fluid W. For example, the wicks  45   b   1  is made of a porous material or has projections that project from the inner surface of the heat pipe  45   b . The wicks  45   b   1  serves to increase the surface area of the inner surface of the heat pipe  45   b , and exerts capillary force to the working fluid W. 
     According to the embodiment described above, the heat pipe  45   b  having a large inclination has the fluid capturing structure to hold the operation fluid W. With the heat pipe  45   b  having this structure, even when the position of the heat releasing portion is lower than that of the heat receiving portion in a state in which the TV-received-associated apparatus  1  is set in place, stagnation of the circulation of the working fluid W is suppressed to prevent the cooling efficiency from remarkably being lowered. 
     As shown in  FIG. 7 , the wicks  45   b   1  may have a first region A and a second region B which is closer to the first heat receiving block  40  than from the first region A. The second region B has more pores than the first region A. That is, the second region B has a structure that is more adapted for holding of the working fluid W than the first region A. According to this configuration, even when the position of the heat releasing portion is lower than the position of the heat receiving portion, stagnation of the circulation of the working fluid W is suppressed more effectively to prevent the cooling efficiency from remarkably being lowered. 
     The upper limit of the rated temperature range of the high-performance processor  36  is higher than the upper limit of the rated temperature range of the I/O controller  37 . According to one embodiment, the distance from the high-performance processor  36  to the heat sink  39  is shorter than the distance from the I/O controller  37  to the heat sink  39 . That is, the lengths of the heat pipes  45   a ,  45   b  thermally connected to the high-performance processor  36  are shorter than the length of the heat pipe  46  thermally connected to the I/O controller  37  which generates a smaller amount of heat than the high-performance processor  36 , whereby heat exchange efficiency can be made higher for the component that has a larger heat generation amount. 
     As shown in  FIG. 6 , the fourth circuit board  29  has an extension  29   c  which projects toward the first circuit board  22  than the second circuit board  27  and the third circuit board  28 . The lower surface of the extension  29   c  of the fourth circuit board  29  is mounted with a plurality of field-effect transistors (FETs)  48 . The FETs  48  are examples heat generating circuit components, and are located next to the first heat receiving block  40  so as to be arranged in line in the front-rear direction of the housing  4 . 
     The lower surface of the extension  29   c  of the fourth circuit board  29  is also mounted with a heat dissipating plate  49  which is made of a metal material having high thermal conductivity such as aluminum. The heat dissipating plate  49  is configured and arranged to extend in the direction in which the FETs  48  are arranged, and is fixed to the fourth circuit board  29  with screws  50  at its respective ends in the longitudinal direction of the heat dissipating plate  49 . The heat dissipating plate  49  is thermally connected to the FETs  48  so as to cover the FETs  48  from below, so that the heat dissipating plate  49  dissipates the heat generated by the FETs  48  toward an internal space of the housing  4 . 
     As shown in  FIGS. 3 and 6 , an upper surface of the extension  29   c  of the fourth circuit board  29  is mounted with a back plate  51  which is made of a metal material having high thermal conductivity such as aluminum. The back plate  51  is fixed to the fourth circuit board  29  with the screws  50 . Thus, the fourth circuit board  29  is sandwiched between the back plate  51  and the heat dissipating plate  49 . The back plate  51  reinforces, from above the fourth circuit board  29 , the portion of the fourth circuit board  29  to which the heat dissipating plate  49  is attached. 
     The back plate  51  is thermally connected to the fourth circuit board  29  on a side opposite to the FETs  48 . Therefore, a part of the heat generated by each FET  48  is transmitted to the back plate  51  indirectly, that is, via the fourth circuit board  29 . 
     As such, the back plate  51  placed on the fourth circuit board  29  also has a function of to indirectly dissipate the heat generated by the FETs  48 . By virtue of the back plate  51 , the heat that is transmitted to the heat dissipating plate  49  is reduced, and the temperature increase of the heat dissipating plate  49  is suppressed. 
     As shown in  FIGS. 2 and 6 , the fan  60  is disposed in the second accommodation space  16  of the housing  4 . The fan  60  sends a cooling air toward the heat sink  39 , and is disposed between the bottom wall  6  of the housing  4  and the extension  29   c  of the fourth circuit board  29 . 
     The fan  60  has the fan casing  61  and the impeller  62 , and the fan casing  61  has the outer casing  63  and the inner casing  64 . The outer casing  63  has a rectangular box shape that is opened at the top and on the rear side. 
     The outer casing  63  has a cylindrical duct portion  65  at the bottom. The duct portion  65  projects from the bottom of the outer casing  63  toward the bottom wall  6  of the housing  4 , and its bottom end portion is fixed to the bottom wall  6  with a plurality of screws. 
     The duct portion  56  surrounds that portion of the bottom wall  6  which is formed with the air inlets  11 . As such, the duct portion  56  constitutes the first air inlet  66  which communicates with the outside of the housing  4  via the air inlets  11 . 
     The inner casing  64  is fitted in the outer casing  63  and has the top plate  67 . The top plate  67  is attached to the top edges of the outer casing  63  so as to cover the top opening of the outer casing  63 . The top plate  67  has an impeller attachment portion  68  and the second air inlet  69 . 
     As shown in  FIG. 2 , the impeller attachment portion  68  is a central portion of the top plate  67 . The second air inlet  69  has first to fourth openings  70   a ,  70   b ,  70   c ,  70   d . The first to fourth openings  70   a ,  70   b ,  70   c ,  70   d , each of which is curved like an arc, surround the impeller attachment portion  68 . More specifically, the first to fourth openings  70   a ,  70   b ,  70   c ,  70   d  are arranged at intervals along a circle that is concentric with the impeller attachment portion  68 . 
     As shown in  FIG. 6 , the impeller  62  is supported by the bottom surface of the impeller attachment portion  68  with a flat motor  72  interposed in between. The impeller  62  is disposed between the bottom of the outer casing  63  and the top plate  67  of the inner casing  64  with its rotation axis O 1  extending in the vertical direction. Therefore, the first air inlet  66  and the second air inlet  69  are opposed to each other with the impeller  62  disposed between them and are arranged in the direction of the rotation axis O 1  of the impeller  62 . 
     As shown in  FIG. 2 , the rear edge of the top plate  67  of the fan casing  61  and the rear opening of the outer casing  63  constitute the air outlet  73 . The air outlet  73  is formed near the rear plate  9  of the housing  4  so as to be perpendicular to the first air inlet  66  and the second air inlet  69 , and is opposed to the heat sink  39 . 
     When the impeller  62  is driven by the flat motor  72 , air outside the housing  4  is taken into the rotation center portion of the impeller  62  through the air inlets  11  and the first air inlet  66  (indicated by arrows in  FIG. 6 ). At the same time, air inside the housing  4  is taken into the rotation center portion of the impeller  62  through the first to fourth openings  70   a ,  70   b ,  70   c ,  70   d  of the second air inlet  69 . 
     As shown in  FIGS. 2 and 6 , about a half of the fan  60  is located below the extension  29   c  of the fourth circuit board  29 . In one embodiment, about a half of the first opening  70   a , the second opening  70   b  and the third opening  70   c  of the second air inlet  69  of the fan  60  face a gap  75  between the extension  29   c  of the fourth circuit board  29  and the top plate  64  of the fan casing  61 . 
     The heat dissipating plate  49  which is thermally connected to the FETs  48  also faces the gap  75 , and is opposed to most of the second opening  70   b  and a part of the third opening  70   c  via the gap  75 . 
     Of the second air inlet  69  of the fan  60 , the remaining half of the first opening  70   a  and most of the fourth opening  70   d  are located in the housing  4  but do not overlap with the extension  29   c  of the fourth circuit board  29 . In other words, the remaining half of the first opening  70   a  and most of the fourth opening  70   d  do not face the gap  75 , and are opposed to the top wall  10  of the housing  4 . 
     The fan  60  is disposed next to the second axial flow fan  25 . When the second axial flow fan  25  is in operation, the air inside the housing  4  is suctioned by the second axial flow fan  25 . As a result, as indicated by an arrow in  FIG. 2 , an air flow path  76  toward the second axial flow fan  25  is created inside the housing  4 . 
     In one embodiment, of the second air inlet  69  of the fan  60 , the remaining half of the first opening  70   a  and most of the fourth opening  70   d  are located in the air flow path  76  to take in the air that flows along the air flow path  76 . 
     In the Set-top box  1  having the above configuration, the FETs  48  which are mounted on the fourth circuit board  29  generate heat during operation. A large part of the heat generated by each FET  48  is directly transmitted to the heat dissipating plate  49  and radiated from the heat dissipating plate  49  to the gap  75  between the fourth circuit board  29  and the top plate  67  of the fan casing  61 . The remaining part of the heat generated by each FET  48  is transmitted, via the fourth circuit board  29 , to the back plate  51  which is disposed on the back side of the FETs  48 , and radiated to the inside space of the housing  4  from the back plate  51 . 
     When the first and second axial flow fans  24 ,  25  are operated during use of the Set-top box  1 , air outside the housing  4  is taken into the first accommodation space  15  of the housing  4  through the first intake hole  12   a . Furthermore, the air inside the rear section of the first accommodation space  15  is sent out of the housing  4  through the first air outlets  14   a , whereby the air flow path  76  toward the second axial flow fan  25  is created inside the housing  4 . 
     When the fan  60  is operated during use of the Set-top box  1 , air outside the housing  4  is taken into the rotation center portion of the impeller  62  through the air inlets  11  and the first air inlet  66  of the fan casing  61 . At the same time, since the half of the first opening  70   a , the second opening  70   b , and the third opening  70   c  of the second air inlet  69  of the fan casing  61  face the gap  75  which is formed in the housing  4 , the air in the gap  75  is taken into the rotation center portion of the impeller  62  through the first to third openings  70   a ,  70   b ,  70   c . As a result, an air flow toward the second air inlet  69  is created in the gap  75 . 
     The air that has been sectioned into the rotation center portion of the impeller  62  is sent toward the inside space of the fan casing  61  through the periphery of the impeller  62 , and is then sent toward the heat sink  31  through the air outlet  73  of the fan casing  61 . As a result, the heat generated by each of the high-performance processor  36  and the I/O controller  37  is emitted to outside the housing  4  being carried by the air that passes the heat sink  31 . 
     According to one embodiment of the invention, the heat generated by each FET  48  and radiated to the gap  75  from the heat dissipating plate  49  is carried by an air flow created in the gap  75  and is taken into the first to third openings  70   a ,  70   b ,  70   c  of the second air inlet  69 . 
     Furthermore, since most of the second opening  70   b  and part of the third opening  70   c  are opposed to the heat dissipating plate  49  via the gap  49 , the heat generated by each FET  48  and radiated from the heat dissipating plate  49  is taken into the fan casing  61  together with the air through the second and third openings  70   b ,  70   c  before being dispersed over the gap  75 . 
     A half of the first opening  70   a  and the fourth opening  70   d  of the second air inlet  69  are formed in the housing  4  so as not to face the gap  75  and to be located in an air flow path  76  toward the second axial flow fan  25 . 
     Therefore, in addition to the air in the gap  75 , the second air inlet  69  also positively takes in a part of the air flowing along the air flow path  76 . Therefore, no strong resistance is likely to occur when air is taken in through the second air inlet  69 . 
     As a result, good ventilation is attained in the housing  4  including the gap  75 , which prevents a phenomenon that the heat that is radiated from the heat dissipating plate  49  stays in the gap  75 . Therefore, the heat dissipation performance of the FETs  48  can be enhanced and overheating and an operation failure of the FETs  48  can be prevented in a reliable manner. 
     The invention is not limited to the embodiment described above, and various changes and modifications can be made therein without departing from the spirit and scope of the invention. 
     For example, although in the embodiment the heat dissipating plate  49  which is thermally connected to the FETs  48  is opposed to the second and third openings  70   b  and  70   c  of the second air inlet  69 , the invention is not limited to such a case. For example, the FETs  48  may be exposed to the gap  75  (the heat dissipating plate  49  is omitted) and opposed to the second and third openings  70   b ,  70   c  of the second air inlet  69 . 
     Furthermore, the heat generating circuit component to which the heat dissipating plate  49  is connected is not limited to the FET and may be other circuit components such as a semiconductor package. 
     In addition, an electronic apparatus according to an embodiment of the invention is not limited to the Set-top box, and may other apparatuses such as a personal computer or a server.