Patent Publication Number: US-2006012958-A1

Title: Electronic apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-255544, filed Aug. 30, 2002, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an electronic apparatus housing therein a heat-generating component, such as a semiconductor package, and more particularly, to an electronic apparatus having a cooling structure for enhancing the cooling performance of the heat-generating component.  
      2. Description of the Related Art  
      Portable electronic apparatuses, such as notebook-type portable computers and mobile communications equipment, are provided with microprocessors for processing multimedia information. Higher processing speeds and the development of highly multifunctional versions of the microprocessors of this type have entailed rapid increase of the heat release value during operation. In order to ensure stable operation of the microprocessors, therefore, the heat radiating capability of the microprocessors must be enhanced.  
      To cope with this, a conventional electronic apparatus is furnished with an air-cooling device for compulsorily cooling the microprocessor. The cooling device comprises a heat sink that absorbs heat from the microprocessor and an electric fan that blows air over the heat sink.  
      The heat sink has a heat receiving portion that receives heat from the microprocessor, a plurality of radiating fins, and an air passage. The air passage is defined extending along the heat receiving portion and the radiating fins. An electric fan blows air through the air passage. The air compulsorily cools the heat sink as it flows between the radiating fins. Thus, the heat from the microprocessor transmitted to the heat sink is removed by the flow of air and discharged to the outside of the electronic apparatus through the lower-stream end of the passage.  
      According to this conventional cooling system, the air that flows through the air passage serves as a cooling medium that removes heat from the microprocessor. Thus, the cooling performance of the microprocessor substantially depends on the amount of airflow, and the area of contact between the airflow and the heat sink.  
      If the airflow is increased to improve the cooling performance of the microprocessor, however, the rotational frequency of the electric fan must be increased, so that substantial noises are produced inevitably. If the radiating fins are increased in number or in size, moreover, the heat sink becomes bulky and requires a wide installation space in the electronic apparatus. Therefore, this configuration cannot be applied to small-sized electronic apparatuses, such as portable computers.  
      Microprocessors for electronic apparatuses are expected to be further speeded up and given more functions in the near future. Accordingly, the amount of heat released from the microprocessors is expected to increase drastically. Presumably, therefore, microprocessors cannot be sufficiently cooled by conventional forced air-cooling systems.  
      To solve this problem, a so-called liquid-cooling system is described in Jpn. Pat. Appln. KOKAI Publication No. 7-142886, for example. In this system, a liquid that is much higher than air in specific heat is used as a coolant to enhance the ability to cool efficiency the microprocessor.  
      According to this novel cooling system, a heat receiving head is set in a casing that contains the microprocessor, and a radiating header is set in a display unit that is supported on the casing. The heat receiving head is thermally connected to the microprocessor. A passage through which a liquid coolant flows is defined in the heat receiving head. The radiating header is thermally connected to the display unit, and a passage through which the liquid coolant flows is also defined in the radiating header. A circulation path through which the coolant is circulated connects the respective passages of the heat receiving head and the radiating header to each other.  
      According to this cooling system, heat from the microprocessor is transmitted form the heat receiving head to the coolant and then transferred to the radiating header as the coolant flows. The heat transferred to the radiating header is diffused by thermal conduction as the coolant flows through the passage, and is discharged from the radiating header into the atmosphere through the display unit.  
      Thus, the heat from the microprocessor can be efficiently transferred to the display unit by utilizing the flow of the coolant. In consequence, the cooling performance of the microprocessor can be made higher than in the case of the conventional forced air-cooling, and there is no noise problem.  
      If the radiating header is set in the display unit, in an electronic apparatus that uses the cooling system described above, it is situated adjacent to a liquid crystal display panel. If the radiating header is heated to a temperature higher than the display panel can tolerate, the display panel is thermally damaged, thus the reliability of the electronic apparatus inevitably lowers. Thus, the radiating header should be able to discharge heat efficiently without adversely affecting other components of the apparatus.  
      In the cooling system described above, moreover, the coolant circulates in the electronic apparatus. If water or an antifreeze solution for use as the coolant leaks out, therefore, it may damage electronic components in the apparatus. If the leaked coolant further leaks out of the apparatus, it may cause alarm to the user. In some cases, the coolant may contact the user&#39;s body or clothes, thereby causing problems.  
     BRIEF SUMMARY OF THE INVENTION  
      An electronic apparatus according to an embodiment of the invention comprises: a heat-generating component; a heat receiving portion thermally connected to the heat-generating component; a heat radiating portion to radiate heat generated by the heat-generating component; and a circulation path to circulate a liquid coolant between the heat receiving portion and the heat radiating portion. The heat radiating portion includes a circulation passage defined therein, a first region provided with a coolant inlet port and a second region provided with a coolant outlet port. The circulation passage of the heat radiating portion has a first circulation passage in the first region and a second circulation passage in the second region, the first circulation passage extending from the coolant inlet port to a position distant from the circulation inlet port and reaching the second region after passing beside the coolant inlet port again, and the second circulation passage extending between the first circulation passage and the coolant outlet port in the second region.  
      An electronic apparatus according to another embodiment of the invention comprises: a first casing; a heat-generating component arranged in the first casing; a heat receiving portion located in the first casing and thermally connected to the heat-generating component; a second casing connected to the first casing; a heat radiating portion to radiate-heat generated by the heat-generating component, the heat radiating portion being arranged in the second casing; and a circulation path to circulate a liquid coolant between the heat receiving portion and the heat radiating portion. The radiating plate includes a first region provided with a coolant inlet port and a second region provided with a coolant outlet port. The circulation passage in the radiating plate has a first circulation passage in the first region and a second circulation passage in the second region. The first circulation passage extends from the coolant inlet port to a position distant from the coolant inlet port and reaching the second region after passing beside the coolant inlet port again, and the second circulation passage extends between the first circulation passage and the coolant outlet port in the second region. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  is a perspective view of a portable computer according to a first embodiment of the invention with its display unit in an open position;  
       FIG. 2  is a sectional view showing the configuration of a cooling unit of the portable computer;  
       FIG. 3  is an exploded perspective view of a heat receiving head of the cooling unit;  
       FIG. 4  is a sectional view showing the positional relation between a semiconductor package and the heat receiving head of the portable computer;  
       FIG. 5  is a sectional view of the portable computer showing a path for a circulation pipe bestriding the boundary between the body and a display unit of the portable computer;  
       FIG. 6  is a sectional view of a radiator taken along line VI-VI of  FIG. 2 ;  
       FIG. 7  is a sectional view of the radiator of the cooling unit showing a circulation passage configuration of the radiator;  
       FIG. 8  is a graph showing the relation between the area ratio between first and second regions of the radiator, cooling performance, and coolant temperature;  
       FIG. 9  is a sectional view showing a radiator of a portable computer according to a second embodiment of the invention;  
       FIG. 10  is a sectional view showing a radiator of a portable computer according to a third embodiment of the invention; and  
       FIG. 11  is a sectional view showing a portable computer according to a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Portable computers as electronic apparatuses according to embodiments of the present invention will now be described with reference to the accompanying drawings.  
      As shown in  FIGS. 1 and 2 , a portable computer  1  comprises an apparatus body  2  and a display unit  3  supported on the apparatus body  2 . The apparatus body  2  is provided with a first casing  4  of a plastic material. The first casing  4  is a flat box that has bottom wall  4   a , top wall  4   b , left- and right-hand sidewalls  4   c , front wall  4   d , and rear wall  4   e . The top wall  4   b  of the first casing  4  has a keyboard mounting area  5  and a projection  6 . A keyboard  7  is set in the keyboard mounting area  5 . The projection  6  projects upward from the rear end portion of the top wall  4   b  and extends in the width direction of the first casing  4 . The projection  6  has a pair of display support portions  8   a  and Bb, which are spaced in the width direction of the first casing  4 .  
      As shown in  FIGS. 1 and 5 , the display unit  3  is provided with a display housing  10  for use as a second casing and a liquid crystal display panel  11  that is set in the housing  10 . The display housing  10  is formed of a thermally conductive plastic material. The display housing  10  is a flat box that has a front wall  13 , in which a display window  12  is formed, and a rear wall  14  for use as an outer wall. The rear wall  14  is opposed to the display window  12  and the front wall  13 . The liquid crystal display panel  11  has a display screen (not shown) that displays characters and images. The display screen is exposed to the outside of the display housing  10  through the display window  12 .  
      As shown in  FIGS. 1 and 2 , the display housing  10  has a pair of legs  15   a  and  15   b  that protrude from its one end portion. The legs  15   a  and  15   b  are hollow and spaced in the width direction of the display housing  10 . The legs  15   a  and  15   b  are fitted in the display support portions  8   a  and  8   b , respectively, of the first casing  4 . The one leg  15   a  is rockably connected to the one display support portion  8   a , while the other leg  15   b  is connected to the first casing  4  by means of a hinge unit  16 .  
      Thus, the display unit  3  is rockable between a closed position where it is leveled to overlie the keyboard  7  and an open position where it rises behind the keyboard  7 . The first casing  4  and the second casing constitute a casing according to the present invention.  
      As shown in FIGS.  2  to  5 , the first casing  4  contains a circuit board  20  for use as a system substrate. A semiconductor package  21  (circuit component), which is a heat-generating component, is mounted on the upper surface of the circuit board  20 . The package  21  constitutes a microprocessor that serves as the CPU for the portable computer  1 . The package  21  has a rectangular base  22  and an IC chip  23  that is soldered to the upper surface of the base  22 . Owing to its increased processing speed and multifunctional performance, the IC chip  23  generates very high heat during operation. Thus, maintenance of its stable operation requires cooling.  
      As shown in  FIG. 2 , the portable computer  1  is mounted with a liquid-cooling unit  25  for cooling the semiconductor package  21 . The cooling unit  25  is provided with a heat receiving head  26  that serves as a heat receiving portion, a radiator  52  that serves as a heat radiating portion, a circulation path  54 , and a centrifugal pump  63 . The pump  63  serves as circulating means for circulating a liquid coolant through these elements.  
      The heat receiving head  26  is located in the first casing  4  and thermally connected to the semiconductor package  21 . More specifically, the head  26  has a heat conduction case  27 , which is a flat box having a plane configuration greater than that of the semiconductor package  21 , as shown in FIGS.  2  to  4 .  
      The heat conduction case  27  is composed of a passage plate  28  and a flat lid plate  30 . The passage plate  28  has a recess that is formed by pressing, etching, or cutting. The lid plate  30  is superposed and fixed on the passage plate  28  by welding, brazing, or adhesive bonding. The passage plate  28  and the lid plate  30  have the same external shape. A plurality of fins  32  are arranged parallel to one another at spaces in the recess of the passage plate  28 . Thus, a plurality of circulation passages  33  are defined side by side in parallel relation in the heat conduction case  27 . This configuration can realize a thin heat receiving portion. Although the lid plate  30  is a flat plate, the outer surface thereof may be formed having-irregularities.  
      The heat conduction case  27  has a coolant inlet port  34  and a coolant outlet port  35 . The coolant inlet port  34  opens in the sidewall portion of the case  27  and communicates with the respective upper-stream ends of the circulation passages  33 . The coolant outlet port  35  opens in the sidewall portion of the case  27  and communicates with the respective lower-stream ends of the circulation passages  33 .  
      On the opposite sides of the coolant inlet port  34 , a pair of first slits  36  are formed in the sidewall portion of the heat conduction case  27 . These first slits  36  constitute a first junction  37  to which a pipe joint can be connected. A first pipe joint  45  is fitted in the first slits  36  of the first junction  37  and communicates with the coolant inlet port  34 .  
      On the opposite sides of the coolant outlet port  35 , a pair of second slits  38  are formed in the sidewall portion of the heat conduction case  27  in like manner. These second slits constitute a second junction  39  to which a pipe joint can be connected. A second pipe joint  46  is fitted in the second slits  38  of the second junction  39  and communicates with the coolant outlet port  35 .  
      By forming the first and second slits  36  and  38  in this manner, the first and second junctions  37  and  39  can be formed directly on the heat conduction case  27 . Accordingly, the configuration of the heat receiving head  26  can be made simpler than in the case where a separate junction is fixed to the heat conduction case by adhesive bonding or welding. Thus, the manufacturing cost can be lowered, and the heat receiving head can be thinned.  
      The heat conduction case  27  constructed in this manner is pressed against the IC chip  23  of the semiconductor package  21  by means of a cross-shaped plate spring  40 . The case  27  is positioned with respect to the semiconductor package  21  by four screws  41 . Apertures  42  are formed individually in the four corner portions of the heat conduction case  27 . An aperture  43  is formed in the distal end of each of arm portions of the spring  40 . A sleeve-type spacer  44  is passed through each of the apertures  42  and  43 . Each screw  41  is passed through a spacer  44  from above and fastened to the circuit board  20 . Thus, the central portion of the spring  40  elastically presses the heat conduction case  27  against the IC chip  23  under desired pressure.  
      The bottom wall of the heat conduction case  27  forms a flat heat receiving surface  27   a . The surface  27   a  is in contact with the IC chip  23  of the semiconductor package  21  with a heat conduction sheet  48  between them. Thus, the case  27  is thermally connected to the chip  23  through the sheet  48 .  
      According to the present embodiment, as shown in  FIG. 4 , a plate  50  with high heat conductivity is embedded in the heat receiving surface  27   a  of the heat conduction case  27 . If the passage plate  28  of the case  27  is formed of a material with relatively low heat conductivity, such as SUS304, which cannot be easily corroded by water for use as a liquid coolant, the temperature distribution in the longitudinal direction of each fin  32  in the heat receiving head  26  is uneven. Therefore, it is difficult to effectively use all the fins  32  to cool the liquid coolant. Dispersion of the temperature distribution in the longitudinal direction of each fin  32  can be lessened by embedding a high-conductivity plate material, such as copper, aluminum, or aluminum nitride, in the heat receiving surface  27   a . Thus, the heat receiving head  26  with a high heat transfer capability can be obtained without failing to prevent corrosion by the liquid coolant. Although the fins  32  are formed of SUS304 that has relatively low heat conductivity, in this case, their influence can be reduced by lessening their height (in the direction perpendicular to the passage plate  28 ). In consequence, the fins  32  hardly cause the heat transfer capability of the heat receiving head  26  to lower.  
      As shown in  FIGS. 2, 5  and  6 , the radiator  52  of the cooling unit  25  is set in the display housing  10  and interposed between the rear wall  14  of the housing  10  and the liquid crystal display panel  11 . The radiator  52  is in the form of an oblong plate that is substantially equal to the panel  11  in size. The radiator  52  is provided with first and second radiating plates  55  and  56 . The first and second radiating plates  55  and  56  are formed of a plastic material, such as polypropylene that combines heat conductivity with thermal resistance. The radiating plates  55  and  56  are superposed on each other, and their respective outer peripheral edges are integrally coupled by thermal welding throughout the circumference. A plastic surface layer  58  for liquid leakage prevention covers the respective outer surfaces of the first and second radiating plates  55  and  56 . The plates  55  and  56  may be formed of a metallic material with high heat conductivity such as aluminum alloy, copper, or magnesium.  
      The first radiating plate  55  is rugged and has bulges  59  that project on the side opposite from the second radiating plate  56 . The bulges  59  are formed substantially over the whole surface of the first radiating plate  55  and open to the joint surface of the second radiating plate  56 . The flat second radiating plate  56  closes the respective open ends of the bulges  59 . The bulges  59  define a circulation passage  60  over the second radiating plate  56 . The bulges  59  are formed having a designed pattern, which will be mentioned later.  
      Behind the liquid crystal display panel  11 , the radiator  52  is fixed to the rear wall  14  of the display housing  10  by fitting, adhesive bonding, or screwing. The surface layer  58  is sandwiched between the second radiating plate  56  and the rear wall  14 . Thus, the radiator  52  is connected thermally to the display housing  10 .  
      As shown in  FIG. 2 , the radiator  52  has a coolant inlet port  62  and a coolant outlet port  64 . The inlet  62  is continuous with the upper-stream end of the circulation passage  60  and is situated at the left-hand end portion of the radiator  52 . It adjoins the left-hand leg  15   a  of the display housing  10 . The outlet  64  is continuous with the lower-stream end of the passage  60  and is situated at the right-hand end portion of the radiator  52 . It adjoins the right-hand leg  15   b  of the housing  10 . Thus, the inlet  62  and the outlet  64  are spaced in the width direction of the display housing  10 .  
      The following is a description of the configuration of the circulation passage  60  of the radiator  52 . As shown in  FIG. 7 , the radiator  52  has two regions, a first region A that covers the coolant inlet port  62  and a second region B that covers the coolant outlet port  64 . The passage  60  has a first circulation passage  60   a  in the first region A and a second circulation passage  60   b  in the second region B.  
      After the first circulation passage  60   a  diverges left and right from the coolant inlet port  62 , the resulting branches extend in the height direction of the display housing  10  to positions distant enough from the coolant inlet port  62 , and join again in a position  65  near the inlet  62 . Then, the first circulation passage  60   a  extends again in the height direction of the housing  10  from the position  65  near the inlet  62  to a position distant from the inlet  62 , thereby reaching the second region B.  
      The second circulation passage  60   b  substantially covers the whole area of the second region B. It is composed of a plurality of branch passages  61   a , which extend in the height direction of the display housing  10  and are situated in parallel with one another, and two manifold-shaped passages  61   b , which extend individually on the opposite sides of the branch passages  61   a . The second circulation passage  60   b  extends from the first circulation passage  60   a  to the coolant outlet port  64 .  
      According to the radiator  52  constructed in this manner, the liquid coolant introduced into the radiator  52  through the coolant inlet port  62  flows through the first circulation passage  60   a  in the first region A. After its temperature is lowered in some measure by heat radiation, the liquid coolant exchanges heat with the liquid coolant in the passage in the position  65  near the coolant inlet port  62 . Thus, the maximum value of the liquid coolant temperature attained in the position  65  near the coolant inlet port  62  can be restrained. If the value of heat release from the semiconductor package  21  is about 30 W, for example, the temperature of the liquid coolant that flows into the coolant inlet port  62  of the radiator  52  may reach about 60° C., in some cases. If the first circulation passage  60   a  of the radiator  52  is constructed in the aforesaid manner, however, the liquid coolant near the coolant inlet port  62  can be cooled to, for example, a temperature lower than 50° C., that is the heat resisting temperature of the liquid crystal display panel  11 .  
      If the radiator  52  and the first and second radiating plates  55  and  56  are formed of a resin or the like that has low heat conductivity, the effect of restraining the maximum value of the liquid coolant temperature can be promoted by providing a high-conductivity metallic plate near the coolant inlet port  62 .  
      The area ratio between the first and second regions A and B of the radiator  52  is set in accordance with the necessary cooling performance of the radiator  52 . If the area of the first region A is increased, as shown in  FIG. 8 , the general cooling performance of the radiator  52  lowers correspondingly, although the maximum value of the liquid coolant temperature near the coolant inlet port  62  can be restrained. In  FIG. 8 , dT represents the difference between the liquid coolant temperature near the coolant inlet port  62  and the liquid coolant temperature near the coolant outlet port  64 . Thus, the maximum value of the liquid coolant temperature is adjusted to a level not higher than a given temperature, and the first and second regions A and B are freely set in a ratio such that a desired cooling capacity can be obtained. The respective shapes of the first and second regions A and B are not limited to the rectangular ones shown in  FIG. 7 , and may be selected variously depending on the design.  
      As shown in  FIGS. 2, 4  and  5 , the circulation path  54  of the cooling unit  25  is provided with first and second circulation pipe  66  and  68 . The pipes  66  and  68  bestride the boundary between the first casing  4  and the display housing  10 .  
      The first circulation pipe  66  connects the coolant outlet port  35  of the heat receiving head  26  and the coolant inlet port  62  of the radiator  52 . After the pipe  66  is guided through the interior of the first casing  4  toward the left-hand display support portion  8   a , it is introduced into the display housing  10  through the support portion  8   a  and the left-hand leg  15   a . As shown in  FIGS. 3 and 4 , the first circulation pipe  66  is connected to the second junction  39  of the heat receiving head  26  by means of the second pipe joint  46 .  
      The second circulation pipe  68  connects the coolant inlet port  34  of the heat receiving head  26  and the coolant outlet port  64  of the radiator  52 . After the pipe  68  is guided through the interior of the first casing  4  toward the right-hand display support portion  8   b , it is introduced into the display housing  10  through the support portion  8   b  and the right-hand leg  15   b . As shown in  FIGS. 3 and 4 , the second circulation pipe  68  is connected to the first junction  37  of the heat receiving head  26  by the first pipe joint  45 .  
      Thus, the circulation passages  33  in the heat receiving head  26  and the circulation passage  60  in the radiator  52  are connected to one another by means of the first and second circulation pipes  66  and  68 . The liquid coolant is sealed in the circulation passages  33  and  60  and the circulation pipes  66  and  68 . The liquid coolant may be water or an antifreeze solution formed of water doped with ethylene glycol, for example.  
      As shown in  FIGS. 2 and 5 , those parts of the first and second circulation pipes  66  and  68  which are introduced into the legs  15   a  and  15   b  of the display housing  10  are formed of a flexible bellows tube  70  each. When the display unit  3  is rocked toward the closed or open position, the bellows tube  70  is smoothly deformed to absorb bending force that acts on each of the first and second circulation pipes  66  and  68  as the unit  3  rocks.  
      The centrifugal pump  63  is connected to the middle portion of the second circulation pipe  68  and held in the first casing  4 . The pump  63  is actuated when it is connected to the power supply or when the semiconductor package  21  is heated to a predetermined temperature. It causes the liquid coolant to circulate through the circulation path  54 .  
      In the portable computer  1  constructed in this manner, the IC chip  23  of the semiconductor package  21  generates heat during the operation of the computer. The heat from the chip  23  is transmitted to the heat receiving surface  27   a  of the heat receiving head  26 . Since the head  26  has the circulation passages  33  in which the liquid coolant is sealed, the liquid coolant absorbs much of the heat transmitted to the surface  27   a.    
      When the temperature of the semiconductor package  21  reaches a given value, the centrifugal pump  63  starts to operate. Thereupon, the liquid coolant is forced out from the heat receiving head  26  toward the radiator  52  and compulsorily circulated between the circulation passages  33  of the head  26  and the circulation passage  60  of the radiator  52 .  
      Thus, the liquid coolant heated by heat exchange in the heat receiving head  26  is pressurized by means of the centrifugal pump  63  and guided into the radiator  52  through the first circulation pipe  66 . The liquid coolant enters the radiator  52  through the coolant inlet port  62 , flows through the circulation passage  60 , and is guided to the coolant outlet port  64 . The heat from the IC chip  23  that is absorbed by the liquid coolant in the process of this flow is diffused into the first and second radiating plates  55  and  56 , and discharged from the surface of the radiator  52  into the display housing  10 . In consequence, the heated liquid coolant is cooled by heat exchange in the radiator  52 .  
      As mentioned before, the liquid coolant first passes through the first circulation passage  60   a  and flows in the first region A of the radiator  52 . After it is temporarily cooled, the liquid coolant is returned to the position  65  near the coolant inlet port  62 , and exchanges heat with the liquid coolant near the coolant inlet port. Thus, the liquid coolant temperature that has its maximum near the coolant inlet port  62  is lowered. Thereafter, the liquid coolant passes through the second circulation passage  60   b , flows in the second region B of the radiator  52 , and is cooled by heat exchange in the radiator.  
      The liquid coolant that is cooled as it passes through the radiator  52  flows through the second circulation pipe  68 , and is returned to the circulation passages  33  of the heat receiving head  26  by the centrifugal pump  63 . After the liquid coolant absorbs the heat from the IC chip  23  again as it flows through the circulation passages  33 , it is guided to the radiator  52 . As this cycle is repeated, the heat form the IC chip  23  is discharged to the outside of the portable computer  1  through the display unit  3 .  
      According to this configuration, the radiator  52  is set in the display housing  10  of the display unit  3 , and the liquid coolant is circulated between the radiator  52  and the heat receiving head  26  that receives heat from the semiconductor package  21 . Therefore, the heat from the package  21  can be efficiently transferred to the display unit  3  by utilizing the liquid coolant flow and then discharged into the atmosphere. Thus, the heat radiation performance of the semiconductor package  21  can be enhanced considerably as compared with the case of conventional forced air-cooling.  
      According to the embodiment described above, the radiator  52  has the first region A that includes the coolant inlet port  62  and the second region B that includes the coolant outlet port  64 . The liquid coolant temperature near the coolant inlet port  62  is lowered by running the liquid coolant through the first circulation passage  60   a  in the first region A. Further, the liquid coolant is cooled by radiating heat as it is run through the second circulation passage  60   b  in the second region B. According to the radiator  52  constructed in this manner, the maximum value of the liquid coolant temperature attained in the position near the coolant inlet port  62  can be restrained. If the value of heat release from the semiconductor package  21  is about 30 W, for example, the temperature of the liquid coolant that flows into the coolant inlet port  62  of the radiator  52  reaches about 60° C. If the first circulation passage  60   a  of the radiator  52  is constructed in the aforesaid manner, however, the liquid coolant near the coolant inlet port  62  can be cooled to, for example, a temperature lower than 50° C., the heat resisting temperature of the liquid crystal display panel  11 . Even if the radiator  52  is located adjacent to the liquid crystal display panel  11  in the display housing  10 , therefore, the display panel  11  can be prevented from being thermally damaged by the radiator  52 . Thus, the package  21  can be efficiently cooled, and the reliability of the portable computer  1  can be improved.  
      The present invention is not limited to the first embodiment described above, and various changes and modifications may be effected therein without departing from the scope or spirit of the invention. According to the first embodiment, heat is not fully radiated from the liquid coolant that flows through the first circulation passage  60   a  in the first region A of the radiator  52 , so that the liquid coolant is at a relatively high temperature. On the other hand, the liquid coolant that flows through the passages  61   b  on the lower-stream side of the circulation passage  60  in the second region. B has its heat fully radiated and is at a low temperature. In a portable computer according to a second embodiment of the invention, as shown in  FIG. 9 , first and second radiating plates  55  and  56  that constitute a radiator  52  are formed having a slit  71  that extends between first and second regions A and B. The slit  71  serves to prevent heat exchange between a higher-temperature portion of the liquid coolant that flows through the first region A and a lower-temperature portion of the liquid coolant that flows through the second region B. Thus, the liquid coolant can radiate heat more efficiently in the second region of the radiator  52 , and so that the cooling performance can be improved.  
      According to a third embodiment shown in  FIG. 10 , a second circulation passage  60   b  in a second region B of a radiator  52  is composed of a plurality of branch portions  72   a  and  72   b  (e.g., two in number) that are arranged side by side and one connecting passage  74  that connects the branch portions  72   a  and  72   b . Each of the branch portions  72   a  and  72   b  has a plurality of branch passages  76  that extend parallel to one another.  
      If the two branch portions  72   a  and  72   b  are connected through the single connecting passage  74 , load that acts on the centrifugal pump  63  can be made lighter than in the case where one second circulation passage is laid spirally. Thus, the power consumption of the portable computer can be lessened without lowering the cooling capacity.  
      According to the cooling system described above, on the other hand, a liquid coolant, such as water or an antifreeze solution, circulates in the portable computer  1 , so that it may leak out into the casing. According to a fourth embodiment shown in  FIG. 11 , therefore, the respective inner surfaces of a first casing  4  and a display housing  10  are provided with a water absorbing material. In this case, a water absorbing polymer sheet  78  is used as the water absorbing material. The polymer sheet  78  is stuck substantially to the whole area of the respective inner surfaces of the first casing  4  and the housing  10 .  
      The water absorbing material need not always cover the whole surface of the housing, and is expected only to be provided at least on the inner surface of the housing near the pipe joints, near the heat receiving head  26  and the radiator  52 , or near the circulation pipes. Besides the water absorbing polymer sheet, water absorbing paper or the like may be used as the water absorbing material. Further, the water absorbing material need not always be a sheet, and may be a gel that is spread over the inner surface of the casing.  
      Even if the liquid coolant leaks from the cooling unit  25  into the first casing  4  or the display housing  10 , according to the configuration described above, the water absorbing polymer sheet  78  can absorb and keep the leaked liquid coolant therein. Accordingly, the possibility of the leaked liquid coolant touching and damaging electronic components in the portable computer  1  can be lowered considerably. At the same time, the leaked liquid coolant can be prevented from leaking out of the computer  1 .  
      The second to fourth embodiments share other configurations with the first embodiment. Therefore, like reference numerals are used to designate like portions throughout the drawings, and a detailed description of those portions is omitted. Any of the second to fourth embodiments can produce the same effects of the first embodiment. Further, each of the first to fourth embodiments may be combined with any other embodiment or embodiments.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.  
      For example, the present invention is not limited to portable computers, and it is also applicable to any other electronic apparatus, such as a desktop computer. In this state, the electronic apparatus is not limited to one that is provided with first and second casings, and may be one that is provided with only one casing. Further, the positions of the components of the cooling unit may be changed as required. For example, the centrifugal pump may be provided in the second casing. Furthermore, the radiator may be provided together with the heat receiving portion in the first casing instead of the second casing.