Abstract:
Methods and apparatus for cooling systems for cryogenic power conversion electronics are provided. The invention includes systems having electronic power conversion apparatus comprising at least one cryogenically operated semiconductor switch and at least one cryogenically operated capacitor and cooling means for cryogenically cooling the at least one cryogenically operated semiconductor switch and the at least one cryogenically operated capacitor to a temperature between 90K to 236K. The systems can also include input/output means for supplying power to said power conversion apparatus and receiving power from said power conversion apparatus. In alternative embodiments, liquid cryogens can be used in heat exchange systems in conjunction with refrigeration cold heads or heat pipes. In these embodiments, the cryogen can be recondensed if boiled.

Description:
This application claims the priority to and is a continuation-in-part of U.S. patent application Ser. No. 09/218,836, filed Dec. 22, 1998, now abandoned, which application is a continuation of U.S. patent application Ser. No. 08/698,806, filed Aug. 16, 1996, now U.S. Pat. No. 6,023,834, both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to methods and apparatus for cooling systems for use with cryogenic power conversion electronics. The invention more particularly relates to methods and apparatus for cooling systems for use with cryogenic power conversion electronics in which the cryogenic power conversion electronics are maintained in a temperature range of 90K to 236K within the cooling system. 
     BACKGROUND OF THE INVENTION 
     Cryogenics relates to the production and maintenance of very low temperatures, often using cryogenic fluids such as hydrogen, helium, oxygen, nitrogen, air or methane. Various discussions concerning cryogenic systems can be found in literature. See, e.g., Barron,  Cryogenic Systems,  2d Ed., Oxford University Press (1985); Bell, Jr.,  Cryogenic Engineering,  Prentice Hall, Inc. (1963); Vance,  Cryogenic Technology,  John Wiley &amp; Sons, Inc. (1963); and Timmerhaus et al,  Cryogenic Process Engineering,  Plenum Press (1989). 
     U.S. Pat. Nos. 3,320,755; 3,714,796 and 3,728,868 disclose cryogenic refrigeration systems (i.e., cryostats). U.S. Pat. No. 4,237,699 relates to cryostats for producing cryogenic refrigeration by expansion of a working fluid through a Joule-Thomson orifice. The cryostat disclosed in U.S. Pat. No. 4,237,699 can be placed in a dewar so that the liquefied working fluid can be maintained to cool an object such as an infrared detector. U.S. Pat. Nos. 3,021,683 and 3,048,021 relate to gas liquefiers. U.S. Pat. No. 4,653,284 discloses a Joule-Thomson heat exchanger and cryostat. U.S. Pat. No. 4,781,033 discloses a heat exchanger for a fast cooldown. 
     Prior to and as a result of the discovery of high temperature superconductors (HTS), a significant amount of time and money has been spent to evaluate the operating characteristics of circuit components at low temperatures. For example, advantages have been observed when operating power MOSFETs at 77K. These advantages include a reduction of the on-resistance of the MOSFETs by as much as a factor of 30 at 77K. 
     The implementation of cryocooled electronic power conversion apparatus incorporating MOSFETs and HTS (high temperature superconductor) magnetics, however, has been directed at operational temperatures of 77K and lower. This is due in part to operational features of the HTS wire. A temperature of 77K has been achieved by operating the electronic circuitry in a bath of liquid nitrogen. 
     U.S. Pat. No. 5,347,168 to Russo discloses a high performance, cryogenically cooled circuit. The entire circuit, as opposed to for example only the superconducting portions of the circuit, are refrigerated to cryogenic temperatures. In addition to the improved operational characteristics of the superconducting based components, the diodes and the gating elements such as MOSFETs provide a circuit capable of operating a switching power supply at lower frequency using larger inductor values. The entire contents of U.S. Pat. No. 5,347,168 are incorporated herein by reference. 
     While the cryogenic electronics power supplies and power sinks disclosed in U.S. Pat. No. 5,347,168 represent a significant improvement over the prior art, the economics of large scale commercial products employing cryogenic power conversion electronics suggest that overall system and cost efficiency can be more easily met by operating the power electronics at temperatures in the range of 90K to 236K. 
     It would therefore be desirable to provide methods and apparatus for cryogenic cooling systems that allow operation in a temperature range of 90K to 236K, thereby overcoming the shortcomings associated with the prior art. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide methods and apparatus for cryogenically cooling electronic power conversion apparatus in the temperature range of 90K to 236K, and preferably in the range of 150K to 170K. 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a chemically inert liquid cryogen. 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a nontoxic liquid cryogen. 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include an environmentally attractive liquid cryogen. 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a dielectric cryogenic liquid heat transfer medium exhibiting acceptable heat transfer properties. 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a non-flammable liquid cryogen. 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a non-ozone deleting liquid cryogen. 
     It is yet another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include octafluoropropane (perfluoropropane). 
     It is yet another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include fully fluorinated 5-carbon branched and straight-chain molecules (e.g., perfluoro n-pentane, 2-trifluoromethyl-1,1,1,2,3,3,4,4,4-nonfluorobutane (trifluoromethyl perfluoro n-butane), tetra(trifluoromethyl)methane (perfluoroisopentane), and other isomers). 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90 K to 236K and that include fully fluorinated 6-carbon branched and straight-chain molecules (e.g., tetradecafluorohexane (perfluoro n-hexane), trifluoromethyl perfluoro n-pentane, di(trifluoromethyl) perfluoro n-butane, and other isomers). 
     It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include fully fluorinated 7-carbon branched and straight-chain molecules (e.g., perfluoro n-heptane, trifluoromethyl perfluoro n-hexane, di(trifluoromethyl) perfluoro n-pentane, tri(trifluoromethyl) perfluoro n-butane, and other isomers). 
     It is yet another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a mixture of fully fluorinated straight or branched-chain alkanes. 
     These and other objects of the invention are provided by methods and apparatus that include a cryogenic cooling system employing as a liquid cryogen an inert, dielectric, nonflammable, non-ozone depleting material operating in the temperature range of 90K to 236K. In preferred embodiments of the invention, the liquid cryogen is a fluorocarbon such as a fluoroalkane operating at temperatures between 90K and 236K. Preferably, the fluorocarbon does not contain any chlorine. Exemplary fluoroalkanes suitable for use in the invention include, but are not limited to, octafluoropropane (perfluoropropane), decafluoro n-butane (perfluoro n-butane), decafluoro isobutane (perfluoro isobutane), fluoroethane (e.g., between its boiling and melting points), hexafluoropropane, heptafluoropropane (e.g., 1,1,1,2,3,3,3-heptafluoropropane and 1,1,1,2,2,3,3-heptafluoropropane) and isomers and mixtures thereof. Preferably, the liquid cryogen is saturated and completely halogenated such that the formation of hydrogen fluoride (HF) in the event of an electrical arc is minimized. The temperature of the liquid cryogen can be maintained by a cryogenic refrigeration system with associated controls, cold heads, and heat exchangers. 
     The liquid cryogen may not exhibit nucleate boiling unless a critical heat flux is reached. Under normal conditions below boiling, the liquid will act as a thermal convective heat transfer medium for dissipative power electronic assemblies and components, and is suitable for a number of heat exchange configurations in which the average bulk temperature can be controlled. In a heat exchange configuration, heat may be extracted from the perfluoropropane or perfluoropropane mixture by immersion of a cold head from a cryogenic cooler or by immersion of a conduit through which passes a cryogenic liquid or vapor at a suitably lower temperature. 
     In another embodiment, the liquid cryogen can be used as the heat transfer medium in a heat pipe structure. In this embodiment, heat extracted from the dissipative electronic assembly is extracted at the boiling point temperature of the liquid cryogen and the cryogen is recondensed by a cold head sink at the other end of the heat pipe. 
     The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner of modifying the invention as will be described. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the following Detailed Description of the Preferred Embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the present invention, reference is had to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1A illustrates a cryogenic cooling system suitable for use in accordance with the present invention; 
     FIG. 1B illustrates a cryogenic cooling system suitable for use in accordance with an alternative embodiment of the present invention; 
     FIG. 1C illustrates a cryogenic cooling system suitable for use in another alternative embodiment of the present invention; 
     FIG. 2 shows an alternative cooling system arrangement for use according to the present invention; 
     FIG. 3 illustrates another alternative embodiment of a cooling system suitable use with the present invention; and 
     FIG. 4 illustrates a distribution manifold for use with the cooling systems shown in FIGS. 1B,  1 C,  2  and  3 . 
     Similar reference characters refer to similar parts throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As discussed above, implementation of cryocooled electronic power conversion apparatus including for example MOSFETs and HTS (high temperature superconductor) magnetics has been directed at operational temperatures of 77K and lower. This has been due in part to operational features of the HTS wire. A temperature of 77K has been achieved by operating the electronic circuitry in a bath of liquid nitrogen. 
     Overall system efficiency and cost effectiveness, however, can be more easily achieved by operating electronic power conversion apparatus (with or without HTS) at temperatures in the range of 90K to 236K, and preferably in the range of 150K to 170K. This may be attributable to the higher COP (coefficient of performance) and lower cost of high capacity heat pumps operating at these higher temperatures as compared with those operating at  77 K. A significant problem in being able to operate at these higher temperatures has been the selection of a suitable liquid cryogen that is inert, nontoxic and environmentally safe at these temperatures. In addition, the cryogen should be a dielectric and exhibit acceptable liquid heat transfer medium characteristics. The most common available substances which exhibit liquid behavior in the temperature range between 90K and 200K are corrosive, flammable, or toxic, e.g., methane, ethane, fluorine, nitric oxide, ethylene, CClF 3  and restricted refrigerants exhibiting ozone-depleting properties. 
     The present invention includes a cryogenic cooling system for use with electronic power conversion apparatus. The system employs a liquid cryogen that is operable in the temperature range of 90K to 236K and that is non-flammable, dielectric, non-toxic, stable, and chemically inert and exhibits near zero ozone depletion potential. In preferred embodiments of the invention, the liquid cryogen is a fluorocarbon such as a fluoroalkane operating at temperatures between 100K and 236K. Preferably, the fluorocarbon does not contain any chlorine. Exemplary fluoroalkanes suitable for use in the invention include, but are not limited to, octafluoropropane (perfluoropropane), decafluoro n-butane (perfluoro n-butane), decafluoro isobutane (perfluoro isobutane), fluoroethane, hexafluoropropane, heptafluoropropane (e.g., 1,1,1,2,3,3,3-heptafluoropropane and 1,1,1,2,2,3,3-heptafluoropropane), perfluoro n-pentane, trifluoromethyl butane, perfluoro isopentane, perfluoro n-hexane, trifluoromethyl perfluoro npentane, di(trifluoromethyl) perfluoro n-butane, perfluoro n-heptane, trifluoromethyl perfluoro n-hexane, di(trifluoromethyl) perfluoro n-pentane, tri(trifluoromethyl) perfluoro n-butane, and isomers and mixtures thereof. The prefix “perfluoro” indicates that the only substituents on the carbon chain are fluorines and other alkyl groups described in the molecule&#39;s name. 
     Preferably, the liquid cryogen is saturated and completely halogenated such that the formation of hydrogen fluoride (HF) is minimized. The temperature of the liquid cryogen can be maintained by a cryogenic refrigeration system with associated controls, cold heads, and heat exchangers. It will be appreciated, however, that the invention is not limited to such cryogens. 
     For example, other cryogens and mixtures of cryogens that exhibit the characteristics mentioned above are suitable for use in accordance with the invention. In addition, appropriate combination in a mixture allows both the boiling point and the freezing point to be adjusted to suit the physical property requirements of a cooling system in a specified temperature range. 
     As mentioned above, octafluoropropane (C 3 F 8 ) and other fluorinated alkanes are suitable for use as a cryogen in accordance with the present invention. Fully fluorinated alkanes are inert, dielectric, nonflammable, and non-ozone depleting. Octafluoropropane is a gas which at 1 atmosphere of pressure exhibits a boiling point of 236K and exhibits a pour point of 90K. When octafluoropropane is utilized as the cryogen, it is preferred that the operating temperature of the system is at least 100K. Decafluoro n-butane (C 4 F 10 ) exhibits similar properties to those of octafluoropropane, but exhibits a boiling point of 271K and a pour point of 145K at 1 atmosphere. Octafluoropropane (C 3 F 8 ) and decafluoro n-butane (C 4 F 8 ) are commercially available from 3M™ under the trade names PF-5030 and PF-5040 (available as a mixture of isomers), respectively. Octafluoropropane (C 3 F 8 ) is sometimes referred to as R218 or FC218. Perfluoropentane (C 5 F 12 ) exhibits a boiling point of 30° C. and a pour point of −115° C. (158K) at 1 atm. pressure. It is available as a mixture of isomers from 3M™ as PF-5050 and is also known as FC-87 Fluorinert. Perfluorohexane (C 6 F 14 ) and perfluoroheptane (C 7 F 16 ) are both liquids at room temperature. Perfluorohexane exhibits a boiling point of 58° C. and a pour point of −90° C. (183K) and is sold by 3M™ as a mixture of isomers as PF-5060 and is also known as FC-72 Fluorinert. Perfluoroheptane exhibits a boiling point of 80° C. and a pour point of −95° C. (178K) and is sold as a mixture of isomers by 3M™ as PF-5070 and is also know as FC-84 Fluorinert. In the following description, octafluoropropane is sometimes used to refer to the cryogen. This is intended for purposes of illustration and is not to be construed as limiting. It will be appreciated that other cryogens having the characteristics discussed above can be utilized in accordance with the present invention. 
     Referring now to FIG. 1A, a cryogenic cooling system suitable for use in accordance with the present invention is illustrated. Cooling system  10  includes an insulated pressure vessel  12 , refrigeration apparatus  14  and electronic power conversion apparatus  16 . Cryogenic electronics power supplies and power sinks suitable for use in accordance with the present invention include those disclosed in commonly owned U.S. Pat. Nos. 5,347,168; 5,612,615 and 5,801,937. The entire contents of U.S. Pat. Nos. 5,347,168; 5,612,615 and 5,801,937 are incorporated herein by reference. Other electronic power conversion apparatus that are cryogenically operated can be used in the present invention. For example, electronic power conversion apparatus can contain at least one cryogenically operated semiconductor switch and at least one cryogenically operated capacitor, although it may not always be necessary to include the capacitor. System  10  also includes cooling coils  22 , feed through connectors  24 , power source  26  and power sink(s)  28 . 
     Vessel  12  can be a dewar or the like. Liquid cryogen  20  is contained within vessel  12  such that sufficient vapor space  18  is provided. Electronic power conversion apparatus  16  is cryogenically cooled and maintained at a temperature between 90K and 236K in vessel  12  by liquid cryogen  20 . 
     Liquid cryogen  20  may not exhibit nucleate boiling unless a critical heat flux is reached. Under normal conditions below the normal boiling point, liquid cryogen  20  will act as a thermal convective heat transfer medium for dissipative power electronic conversion apparatus  16  and is suitable for various heat exchange configurations in which the average bulk temperature can be controlled. 
     Under some circumstances, cryogen  20  may reach a boiling point during operation such that vapor forms in vapor space  18 . Cryogen  20  is cooled by refrigeration apparatus  14 . For example, cooling coils  22  can be provided to cool cryogen  20  in vessel  12 . Preferably, cooling coils  22  are finned to increase the surface area for heat transfer from the cryogen to the refrigeration coil. It will be appreciated that alternative configurations for the cooling coils can be employed so long as the desired heat transfer characteristics from the cryogen to the refrigeration coil can occur. As also shown in FIG. 1A, coils  22  are connected to refrigeration apparatus  14  as indicated by lines  22   a  and  22   b . In this manner, cryogen  20  can be maintained within a temperature range of 90K to 236K and preferably in a range of 140K to 170K. 
     As further shown in FIG. 1A, a portion of the coils  22  is positioned to be above the cryogen surface level  20   a , i.e. in the vapor space  18 . When the system  10  is not in use, cryogen  20  is a gas and vapor thus forms in vapor space  18 . During start-up and the like, the portion of coils  22  in vapor space  18  provide cooling of the cryogen vapor. 
     Feed through connectors  24  are connected to electronic power conversion apparatus  16  in a manner such that a power source  26  can deliver power to apparatus  16  and such that connections for sinking power to a load  28  are provided. Feed through connectors  24  have pressure seals contained therein to accommodate temperature changes. Such changes can occur for example during shipping or the like. More specifically, when a system  10  is shipped, the cryogen may be in a gaseous state. The pressure seals are configured to accommodate vapors from the cryogen. 
     The embodiment shown in FIG. 1A is operated at atmospheric pressure such that cryogen  20  exhibits its normal boiling point and heat is transferred primarily by convection and conduction. In this embodiment, liquid cryogen  20  can blanketed by a space of dry nitrogen and sealed at 1 atm. In an alternative embodiment of the invention illustrated in FIG. 1B, system  10  can be operated in a manner such that the contents in cryostat  12  operate under reduced pressure (e.g., 0.1 atm), and in which the apparatus functions primarily as a distillation apparatus. For instance, cryogen  20  is locally boiled at lower than its normal boiling point with the coil  22  above the liquid level  20   a  serving as a reflux condenser. The heat transfer in this embodiment utilizes the latent heat of vaporization (i.e., the change in enthalpy required to boil the liquid to a vapor) at a specified temperature and pressure. The refluxed liquid may be subcooled relative to the boiling point, thereby requiring additional enthalpy change to first heat the liquid to its boiling point. 
     Another alternative embodiment of the invention is shown in FIG.  1 C. The embodiment illustrated in FIG. 1C allows cryogen  20  to act a heat transfer medium in system  30 . In particular, cryogen  20  is contained in vessel  12  which also contains electronic power conversion apparatus  16 . The temperature of cryogen  20  is maintained within a desired range by circulating cryogen  20  through heat exchanger  32  and circulating pump  38  in line  36 . 
     Cryogen  20  is cooled by the cooling medium in line  34 . While not to be construed as limiting, the cooling medium could be a fluoroalkane. As further shown in FIG. 1C, the temperature of the cooling medium in line  34  is maintained by refrigeration unit  14 . At elevated temperatures, for example when refrigeration is not being utilized, the system is under pressure. This is due to the cryogen being a vapor at room temperature. There may be a two phase condition, depending on the pressure, volume, temperature and the quantity of material in the system. It may therefore be desirable to provide vapor space within the circulating loop, although such space is not necessarily limited to the container housing the electronic assembly. 
     During normal operation, such as when refrigeration is utilized, it is desirable for the pump to handle the liquid phase rather than the vapor phase. 
     Another alternative cooling system arrangement for use according to the present invention is shown in FIG.  2 . System  50  includes vessel  12  having electronics power conversion apparatus  16  positioned therein. Cryogen  20  is provided in an amount sufficient to cool apparatus  16  to a desired temperature. 
     System  50 , as shown in FIG. 2, also includes metallic cold fingers  52  and refrigeration coil  54 . Metallic cold fingers  52  are preferably formed of copper. Metallic cold fingers  52 , which cool cryogen  20  in vessel  12 , are cooled by refrigeration coil  54 . Refrigeration coil  54  is cooled by a refrigeration unit  14 . Alternatively, liquid cryogen could be used in place of refrigeration coil  54  to cool metallic cold fingers  52 . 
     As the temperature of cryogen  20  increases, vapor is produced and rises to vapor space  18 . Metallic cold fingers  52  are positioned to provide sufficient cooling to vapor in space  18  and to cryogen  20  within vessel  12 . 
     FIG. 3 illustrates another alternative embodiment of a cooling system suitable use with the present invention. System  60  includes an insulated housing  62  (without cryogen) having electronics power conversion apparatus  16  positioned therein. In this embodiment, apparatus  16  includes a thermally conductive substrate  64  disposed on the exterior surfaces of apparatus  16 . While not to be construed as limiting, thermally conductive substrate  64  can be formed of copper and electrically insulated from the electronic assembly. Alternatively, substrate  64  can be formed of beryllium oxide or other thermally conductive electrical insulators. Thermally conductive substrate  64  can be utilized to enhance the cooling efficiency of system  60 . System  60  also includes metallic cold fingers  66  which may be formed of copper or the like. 
     Metallic cold fingers  66  function in a manner similar to metallic cold fingers  52  described above in connection with FIG. 2, except that metallic cold fingers  66  are configured to physically contact and cool substrate  64  rather than cryogen  20 . In this manner, apparatus  16  is maintained within the desired temperature range. Cryogen  20  (not shown in FIG. 3) can be circulated in a coil (such as coil  54  shown in FIG. 2) to maintain cold fingers  66  within the desired temperature range. It will be appreciated that the apparatus  16  having substrate  64  disposed on the exterior surfaces thereof can also be used in the embodiment shown in FIG.  2 . 
     FIG. 4 illustrates a distribution manifold for use with the cooling systems shown in FIGS. 1A,  1 B, IC,  2  and  3 . The arrangement shown in FIG. 4 provides a system  70  that can cryogenically cool a plurality of apparatus  16 . System  70  includes refrigeration unit  72 , distribution manifold  74  having cryogen  76  therein. Cryogen  76  provides the appropriate cooling capabilities to metallic cold fingers  52 , or cold fingers  66  of FIG.  3 . 
     Vessel  12 , as shown in FIG. 4, is similar to the arrangement shown in FIG.  2 . It will be appreciated, however, that any of the previously described embodiments such as those in FIGS. 1A,  1 B, IC and  3  are suitable for use in system  70 . It will also be appreciated that a combination of these embodiments could be implemented for use in system  70 . In some instances, for example, it may be desirable to cool an apparatus in accordance with the embodiment shown in FIG. 1B, to cool another apparatus in accordance with the embodiment FIG. 1C, and to cool yet another apparatus in accordance with the embodiment of FIG.  3 . In addition, alternative system configurations and arrangements are suitable for use in system  70 . 
     As discussed above, the liquid cryogen may not exhibit nucleate boiling unless a critical heat flux is reached. Under normal conditions below boiling, the liquid will act as a thermal convective heat transfer medium for dissipative power electronic assemblies and components, and is suitable for a number of heat exchange configurations in which the average bulk temperature can be controlled. In one heat exchange configuration, heat may be extracted from the perfluoropropane mixture by immersion of a cold head from a cryogenic cooler or by immersion of a conduit through which passes a cryogenic liquid or vapor at a suitably lower temperature. 
     In yet another implementation, the perfluoropropane is the heat transfer medium in a heat pipe structure, in which the heat extracted from the dissipative electronic assembly is extracted at the boiling point temperature and is recondensed by a cold head sink at the other end of the heat pipe. If the pressure is lower than atmospheric in the heat pipe due to for example how the heat pipe was filled, the boiling point of the cryogen will be altered and lowered. For instance, if the heat pipe is first evacuated and then back filled with cold liquid, the sealed pressure will be the vapor pressure of the substance at low temperature. This is a very low pressure for the liquid(s) and the boiling point will thus be lowered relative to that at one atmosphere pressure. Accordingly, it will be appreciated that how the container is filled can determine if the embodiment will function in a conductive (see, e.g., FIG. 1A) or in a distillation (see, e.g., FIG. 1B) manner. It should also be appreciated that the boiling point of the cryogen can thus be altered in this manner. If the boiling point of the cryogen at atmospheric pressure is too high, for example, the heat pipe can be back filled as described above. In this manner, the pressure can be reduced and the boiling point of the cryogen can be reduced relative to that at atmospheric pressure. 
     The optimization of system performance and cost effectiveness suggest that it is desirable to use cryopower circuits at about 150K-180K, particularly in large machines. For example, the coefficient of performance (COP) of coolers (e.g., such as POLYCOLD® coolers available from Polycold Systems, Inc., San Rafael, Calif.) is much greater at 170K than 77K. 
     As discussed above however, a major disadvantage of prior art cryogenic cooling systems has been a lack of a suitable liquid cryogen operable in a wide range, e.g., from 90K-220K. In addition, the cryogen should be: non-flammable, dielectric, non-toxic, stable, and chemically inert and exhibit near zero ozone depletion potential. 
     The use of octafluoropropane, for example, satisfies these characteristics and has been found suitable for use in accordance with the present invention such that electronic power conversion apparatus can be cryogenically cooled. Octafluoropropane is commercially available from 3M™ under the name PF-5030. As discussed more fully above, additional fluoroalkanes are also suitable for use in the invention. For example, mixtures of fluorinated alkanes can be formed to grade boiling point and freezing point for easier handling. Decafluoro n-butane (perfluoro n-butane) is commercially available from 3M™ under the name PF-5040. Fluorinated 2,3 and 4 carbon alkanes are gases at 1 atmosphere unless cooled below room temperature. Longer fluorinated alkanes are liquids at room temperature and 1 atm. 
     It should be appreciated by those skilled in the art that the specific embodiments disclosed above may readily be utilized as a basis for modifying or designing other methods or structures for carrying out the same purpose of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.