Abstract:
Power electronics for electric traction motors used to drive automotive vehicles are cooled in a closed system by spraying a dielectric liquid coolant directly onto inverter circuitry. The liquid coolant changes phase and vaporizes as it absorbs heat from power transistors in inverter circuitry comprising the power electronics. The resultant vapor is condensed back to a liquid in a heat exchange arrangement having pipes carrying a second coolant from a radiator used to cool an engine or fuel cell stack in the automotive vehicle. Overspray coolant, which remains liquid, can also be cooled by the heat exchange arrangement. By utilizing the latent heat of evaporation of the dielectric coolant and increasing the rate recycling of the coolant as power output increases, temperature increases in the power electronics are controlled.

Description:
RELATED PATENT APPLICATIONS 
     This application is a continuation in part of U.S. patent application Ser. No. 11/054,483, filed Feb. 9, 2005 now U.S. Pat. No. 7,210,304 having the title, “Cooling Arrangements For Integrated Electric Motor Inverters.” 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to arrangements for and methods of phase change cooling of power electronics. More particularly, the present invention is related to such arrangements and methods for cooling power electronics which include inverter circuitry, wherein the inverter circuitry provides current to traction motors used to drive electric vehicles such as, but not limited to, battery powered vehicles, gas-electric hybrid vehicles and fuel cell powered electric vehicles. 
     BACKGROUND OF THE INVENTION 
     Vehicles which utilize electric traction motors to drive wheels of a vehicle, whether the electric motor is in a gas-electric hybrid vehicle or a fuel cell powered vehicle, typically use a three-phase AC motor coupled with inverter circuitry that converts direct current from a power source to alternating current. Currently, inverter circuitry generally comprises power transistors mounted on a DBC (direct bonded copper) substrate with integrated bus bars. 
     As automotive vehicles start, change cruising speeds, accelerate and brake, power demands of electric traction motors driving the vehicles fluctuate over a wide range. Fluctuations in power demand cause temperature changes in power electronics connected to the traction motors. The power electronics include inverter circuitry comprised of different materials with various coefficients of expansion. Accordingly, heat fluctuations can degrade inverter circuitry as the integrated components thereof expand at different rates tending to shift slightly with respect to one another as the components respond to temperature variations. It is necessary to control temperature to keep expansions and contractions of the components within acceptable levels. Currently, this is accomplished by circulating fluids through heat sinks associated with the DBC or by flowing air thereover to absorb and carry away heat. While these approaches currently appear satisfactory, there remains a need to more precisely control the temperature of inverter circuitry over the life of vehicles utilizing traction electric motors in order to sustain reliability, as well as to control power consumption. 
     SUMMARY OF THE INVENTION 
     In view of the aforementioned considerations, a cooling arrangement is provided for cooling components of power electronics connected to deliver current to an electric traction motor for driving at least one traction wheel of an automotive vehicle. The arrangement comprises a housing having compartment with a space containing the components. The compartment has an inlet opening and an outlet opening for cooling fluid communicating with the space. The cooling fluid is a non-corrosive dielectric cooling fluid which is dispensed in liquid phase into the space and onto the components of the inverter circuitry by a pump provided for recycling the dielectric coolant from a reservoir that collects the dielectric coolant from the components. The dielectric fluid has a phase change point selected to absorb a substantial quantity of heat at the boiling temperature of the coolant before the coolant vaporizes. The reservoir uses a second coolant in a liquid-fluid heat exchanger to condense the recycling fluid prior to reapplying the recycled fluid in liquid phase onto the power electronics components. 
     In a further aspect of the cooling arrangement the dielectric cooling fluid has a boiling point in a range of 90° C. to 120° C. 
     In a further aspect of the cooling arrangement, the dielectric cooling fluid has a boiling point below 100° C. 
     In a further aspect of the cooling arrangement, the dielectric cooling fluid has a boiling point of about 98° C. 
     In a further aspect of the cooling arrangement, the dielectric coolant fluid is a mixture of polypropylene glycol methyl ether and hexamethyldisiloxane. 
     In a further aspect, a method for cooling power electronics is provided in which heat is absorbed fro power electronics used to drive vehicles by recirculating coolant fluid which has changed phase upon absorbing heat from the power electronics; is condensed and resprayed as a liquid onto the power electronics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein: 
         FIG. 1  is a schematic view of an automotive vehicle having a gas-electric hybrid drive; 
         FIG. 2  is a schematic view of an automotive vehicle that uses fuel cell power to drive an electric traction motor; 
         FIG. 3  is a schematic diagram of a heat exchanger loop for cooling and condensing power electronics coolant fluid utilized with the vehicles of  FIGS. 1 and 2 ; 
         FIG. 4  is a front view of a spray cooled inverter circuit configured in accordance with the second embodiment of the present invention shown in  FIG. 3 ; 
         FIG. 5  is a schematic diagram of another embodiment of a heat exchanger loop for cooling and condensing vaporized coolant used to cool power electronics utilized in the vehicles of  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , there is shown an example of a gas-electric drive  10  for powering a vehicle  12  utilizing an internal combustion engine  14  and a three-phase electric traction motor  16  to drive, through a transmission  17 , wheels  18  of the vehicle. A power splitter  22  determines whether the internal combustion engine  14  or the electric motor  16  drives the transmission  17 , or whether the transmission or internal combustion engine drives an electric generator  24 . In an alternative embodiment (not shown), the generator  24  is mounted next to the electric traction motor  16  and cooled with the same arrangement as the traction motor. The electric generator  24  charges a battery  26  and/or provides direct current to inverter circuitry  28  that provides alternating current for the electric traction motor  16 . Since the inverter circuitry  28  generates heat, the inverter circuitry requires a cooling arrangement  32 . In accordance with one aspect of the present invention, the cooling arrangement  32  is connected to a sealed cooling loop  33  which is coupled thermally to a radiator  34  which cools the internal combustion engine  14 . The cooling arrangement  32  has at least a portion that is integral with the inverter circuitry  28  to form a module  35 . 
     Referring now to  FIG. 2 , a fuel cell drive system  10 ′ utilizes a fuel cell  40  to power a three-phase electric traction motor  16  which drives the wheels  18  through a transmission  17 ′. The fuel cell  40  is connected either directly or through a battery pack  26 ′ to supply direct current to inverter circuitry  28 ′ which converts direct current to alternating current for the three-phase electric motor  16 . As with the gas-electric hybrid of  FIG. 1 , the inverter circuitry  28 ′ has a cooling arrangement  32 ′ that is coupled thermally by a cooling loop  33 ′ to a radiator  34 ′ used to cool the fuel cell  40 . As with the first embodiment of  FIG. 1 , in the second embodiment of  FIG. 2  the cooling arrangement  32 ′ at least in part is integral with the inverter circuitry  28 ′. 
     Referring now to  FIG. 3 , the inverter cooling arrangements  32  or  32 ′ shown in  FIGS. 1 and 2 , respectively, are usable with either the gas-electric hybrid drive  10  or the fuel cell drive  10 ′. The gas-electric hybrid drive  10  and the fuel cell drive  10 ′ are merely exemplarily of various configurations for such drives. For example, the gas-electric hybrid drive  10  can be configured as a parallel arrangement, a series arrangement or any other effective arrangement, as can the fuel cell drive  10 ′. The gas-electric hybrid drive  10  may use a gasoline engine, a diesel engine, a turbine engine or any other engine configuration. 
     By using a coolant dispenser  60 , liquid coolant  64  is applied directly to the source of heat in the power electronics; i.e., the power transistors  52  that convert direct current to alternating current. Cooling allows the power density (power per unit volume) of the inverter circuitry  28  to be increased. To be cooled by the liquid coolant  64 , heat generated by the power transistors  52  need not travel through multiple layers of materials, a few of which materials have low thermal conductivity. Rather, a direct thermal path that is provided by spray cooling reduces the temperature of the power transistors  52 . With a lower temperature for the power transistors  52 , increased power is available through the inverter circuitry  28  to the three-phase traction motor  16 . Alternatively, with improved cooling, smaller inverter circuitry  28  may be utilized, producing substantially the same available power to the traction motor  16 , while consuming less space. 
     The spray cooling provided by the spray nozzles  62  is also usable on other components associated with the inverter circuitry  28 , such as capacitors, transformers and integrated circuits that are temperature sensitive. Moreover, the spray cooling provides cooling to wire bonds between the elements of the inverter circuitry  28  and prevents wire bonds from overheating, thus minimize failure. Accordingly, along with the resulting reduction of component temperatures, improved reliability is provided. 
     Because spray cooling provides increased cooling capacity, spray cooling improves resistance of the inverter circuitry  28  to transient power fluctuations. Transient power fluctuations exist on the input to the inverter circuitry  28  due to sudden increases in power demanded by the vehicle  12  ( FIGS. 1 and 2 ) for short periods of time. The fluctuations can be caused by increased resistance to the output of the motor  16  that in turn cause temperature increases in the power transistors  52 . By having direct application of the cooling media  64  to the power transistors  52 , temperature change is reduced in both time duration and temperature increase. 
     In order that the coolant  64  does not electrically interact with or degrade the components of the inverter circuitry  28 , the coolant is a dielectric coolant, preferably having a boiling point in the range of about 90° C. to 120° C. A suggested coolant is a mixture of methylsiloxane and an organic compound such as polypropylene glycol methyl ether, wherein the coolant has minimal instability and reactivity. An example of such a liquid is OS-120 available from Dow Corning Corporation, which is a mixture of hexmethyidisiloxane and propylene glycol methyl ether, the hexmethyidisiloxane having a percentage by weight greater than 60% and the propyleneglycol methyl ether having a percentage by weight in a range of 10% to 30%. OS-120 has a boiling point of about 98° C. and is a dielectric material that does not degrade when used to cool interconnected electrical components. The dielectric liquid coolant  64  continues to absorb heat at about 98° C. without changing phase to its vapor form  64 ′ until the heat capacity of the coolant reaches its boiling point, at which time the liquid coolant vaporizes to carry away heat generated by the power transistors  52  and emanating from other components of the power electronics package. Other dielectric coolants, which have minimal instability and minimal reactivity with the electrical components of the inverter, may be used as alternatives to OS-120. 
     Still referring mainly to  FIG. 3 , the coolant  64  is sprayed as a liquid and is collected in a sump portion  70  of the compartment  50  and through a spray return  72  is returned to a reservoir  74  which is connected through a filter  75  to a pump  76 . The pump  76  is connected to the liquid dispenser  60  by line  76   a  that supplies recycled liquid coolant  64  to the spray nozzles  62  for continued cooling of the inverter circuitry  28 . While the liquid coolant  64  is circulating through the reservoir  74 , it is cooled by the inverter cooling loop  33  or  33 ′ (see also  FIGS. 1 and 2 ) with a second liquid coolant  77 . An example of the second liquid coolant  77  is a water ethylene glycol solution. The second coolant  77  flows through tubes  78  in the reservoir  74  and is supplied by the radiator  34 , which cools the internal combustion engine  14  of  FIG. 1  or is supplied by the radiator  34 ′, which cools the fuel cell stack  40  of  FIG. 2 . 
     The cooling arrangement of  FIG. 3  takes advantage of the latent heat of vaporization of coolant  64 . When the liquid coolant  64  is sprayed onto power electronics components that operate at temperatures hotter than the vaporization (i.e., boiling) temperature of the coolant, the coolant changes phase from the liquid coolant  64  to a vaporized coolant  64 ′. The vaporized coolant  64 ′carries waste heat away from the inverter circuitry  28  (or  28 ′) as the vaporized coolant disperses into the chamber  50 ′. The coolant loop  33  or  33 ′ includes a condenser  200  which is separate from the reservoir  74 ′, as well as a separate passage  206  for conveying vaporized coolant  64 ′ from the compartment  50 ′ to the condenser  200 . A second coolant flow  77  from the vehicle radiator  34  or  34 ′is circulated through the coolant pipes  78 ′ to change the phase of the coolant from a vapor  64 ′ back to coolant liquid  64 . The liquid  64  from the condenser  200  mixes with the liquid in the reservoir  74 ′ and is filtered by a filter  75  prior to being pumped by the pump  76  over line  76   a  back to the fluid dispenser  60 , where the coolant  64  is sprayed in liquid form onto the power inverter circuitry  28 . 
     The pump  76  is preferably a variable output pump which is controlled by a controller  79  that is activated by an output current signal over line  80  from the output line  81  from the power transistors  52  to the three-phase electric motor  16 . The controller  79  increases the pumping rate of pump  75  as the output of the power transistors  52  increases. By providing variable spray cooling, temperature control under all operating conditions is achieved. Such an arrangement increases component reliability by minimizing temperature changes so that the inverter circuitry  28  operates under substantially isothermal conditions. By consistently controlling the amount of dielectric liquid coolant  64  sprayed through the atomizer nozzles  62 , sufficient liquid mist is provided at maximum power dissipation conditions. The liquid mist  64  exhibits a phase change converting to a vapor  64 ′ after being sprayed on the inverter circuitry  28 . When the phase change occurs, the power transistors  52  remain at substantially constant temperature regardless of increased power output and increased power dissipation. By varying the flow of the liquid coolant  64  with respect to actual component power dissipation, the phase change region of the fluid comprising the liquid coolant  64  is utilized so that the coolant accommodates all operating conditions. 
     Alternatively, the temperature of the transistors  52  may be monitored with a thermocouple arrangement with the speed of the pump  76  being increased as the temperature of the transistors increases to spray more liquid coolant and thereby decrease the temperature of the transistors. 
     Referring now more specifically to  FIG. 4  in conjunction with  FIG. 3 , the spray cooling arrangement of  FIG. 3  is configured as a module  35  or  35 ′exemplified in  FIG. 4 . Vapor  64 ′ is pulled through passage  206  by negative pressure of the pump  76  and into the condenser  200 . In the arrangement of  FIG. 4 , the condenser  200  is disposed above the reservoir  74  so that the liquid coolant  64 , condensed from the coolant vapor  64 ′, flows down and mixes with overspray coolant liquid  64  which has entered the reservoir via passageway  72 . All of the cooled and condensed coolant is then pulled through the filter  75  and into the pump  76 ; recycled by the pump, and then sprayed as a liquid mist or liquid droplets  64  through the nozzles  62  onto the inverter circuitry  28 . The liquid mist or droplets  64  form a liquid layer on the inverter circuitry  28 , and as previously described, the liquid layer absorbs heat and at least a portion of the liquid vaporizes into vapor  64 ′. The vapor  64 ′ is then pulled through the passageway  206  into the condenser  200  by the pump  76  and mixed with cooling liquid coolant  64  in the reservoir  74  to continue the cooling cycle. 
     Referring now to  FIG. 5 , a second embodiment of the invention is shown where only the vaporized coolant  64 ′ is recycled, the line  72  of  FIG. 3  having been deleted. With this arrangement, the temperature of the power electronics  28  is substantially controlled by the heat of vaporization of the liquid coolant  64 , which in the case of the aforedescribed OS-120 is less than 100° C. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.