Abstract:
A power control system may use power semiconductor devices such as insulated gate bipolar transistors (IGBT&#39;s) in a switching unit to provide motor control. The IGBT&#39;s may be cooled with a system that is configured and sized to provide proper cooling at steady-state operating conditions of the switching unit. The IGBT&#39;s may be placed in thermal communication with a compartment that may contain phase change material (PCM). When and if the switching unit is operated under transient high load conditions, excess heat may be absorbed by melting of the PCM. When steady state operating conditions are restored the PCM may solidify and release its latent heat to a coolant. The PCM may thus act as a thermal buffer for the cooling system and thus may provide that the cooling system may be minimally sized.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to power semiconductor devices and, more particularly, to providing cooling for such devices. 
         [0002]    Power semiconductor devices, such as insulated gate bipolar transistors (IGBT&#39;s), are used in a variety of applications that require high frequency switching of electrical power. These applications may include motor controllers or power converters. In many of these applications, heat may be generated in the device. It may become necessary to remove this generated heat from the device. 
         [0003]    Typically, heat may be removed from the device with a heat sink. Heat sinks may be constructed from heat conductive material which may absorb heat for the device and then transfer the heat to a surrounding environment. For example, a heat sink may comprise a metal plate with cooling fins which allow heat to be dissipated by convection into air. In a more complex example, a heat sinking arrangement may comprise a heat conductor and a fluid flow system for extracting heat from the device. Fluid-flow cooling systems may comprise closed channels through which a cooling liquid may pass. 
         [0004]    Prior-art heat cooling systems are typically designed with a cooling capacity that may accommodate a worst-case operating condition for the device. For example, if an assembly of IGBT&#39;s were to generate 100 watts during a brief peak operation, a cooling system may be provided to convey away at least 100 watts from the assembly. This illustrative peak cooling capacity may be incorporated into a design of a motor controller or power converter, even if steady-state operation produces only a fraction of the peak heating. Such a design principle may not be problematic in ground-based applications of IGBT&#39;s. But when IGBT&#39;s are employed in aerospace applications, space and weight become paramount design considerations. 
         [0005]    In an aircraft or other aerospace vehicle, power converters and motor controllers may be subjected to widely varying operating conditions. Transient loads may greatly exceed steady-state loads. Prior-art cooling systems may be provided in anticipation of these high transient loads. Consequently, prior-art cooling systems may be space-consuming and may add undesirable weight to an aerospace vehicle. 
         [0006]    As can be seen, there is a need to provide a small and lightweight cooling system for power semiconductor devices. In particular, there is a need to provide such a cooling system that may be only as large as a steady-state cooling system while also having capability for transient condition cooling. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect of the present invention a power control apparatus comprises a switching unit having phase change material (PCM) for cooling at least one power semiconductor device. 
         [0008]    In another aspect of the present invention a switching unit comprises at least one insulated gate bipolar transistor (IGBT), at least one coolant passage, and at least one compartment containing PCM in thermal communication with the at least one cooling passage for cooling the at least one IGBT 
         [0009]    In still another aspect of the present invention a method for controlling electrical power comprises the steps of dissipating heat from IGBT&#39;s at a steady-state rate of heat production into a coolant, and dissipating heat from the IGBT into a PCM at a transient rate of heat production. 
         [0010]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram of a power control system in accordance with the invention; 
           [0012]      FIG. 2  is a perspective view of a switching unit in accordance with the invention; 
           [0013]      FIG. 3  is an exploded perspective view of the switching unit of  FIG. 2  in accordance with the present invention; 
           [0014]      FIG. 4  is a cross-sectional view of the switching unit of  FIGS. 2 and 3  in accordance with the invention; 
           [0015]      FIG. 5  is a longitudinal cross-sectional view of a switching module in accordance with the present invention; 
           [0016]      FIG. 6  is a perspective view of another embodiment of a switching unit in accordance with the invention; 
           [0017]      FIG. 7-1  is a perspective view of a switching module in accordance with the invention; 
           [0018]      FIG. 7-2  is a perspective view of another switching module in accordance with the invention; 
           [0019]      FIG. 7-3  is a perspective view of still another switching module in accordance with the invention; and 
           [0020]      FIG. 8  is a flow chart of a method for controlling power in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0022]    Broadly, the present invention may be useful in power converters and motor controllers which employ power semiconductor devices such as IGBT&#39;s. More particularly, the present invention may provide for cooling of IGBT&#39;s in these applications. The present invention may be particularly useful in aerospace vehicles such as aircraft which may produce widely varying load conditions on the IGBT&#39;s. In such vehicles, IGBT&#39;s may produce heat at a first rate when operating at a steady-state. In some transient modes of operation the IGBT&#39;s may produce heat at a second rate that is much higher than the steady-state rate. 
         [0023]    In contrast to prior-art power control systems, which may provide cooling systems designed and sized for worst-case heating of IGBT&#39;s, the present invention may perform cooling with a system that is only as large as one needed for steady-state cooling. The present invention, unlike the prior art, may utilize a phase change material (PCM) to temporarily absorb transient heat loads during peak operating conditions of the IGBT&#39;s. The PCM may absorb transient heat quickly and then release the heat gradually into a cooling system that is adapted and sized for steady-state operation. 
         [0024]    Referring now to  FIG. 1 , a power control system  10  may comprise a controller  12  and a power source  14 . The controller  12  may provide controlled power to a motor  16 . The controller  12  may comprise a switching unit  18  that may be comprised of one or more power semiconductor devices such as IGBT&#39;s. The switching unit  18  may be interconnected with a re-circulating reservoir  20  for a coolant  21 . 
         [0025]    Referring now to  FIGS. 2 and 3 , an illustrative embodiment of the switching unit  18  is shown. The switching unit  18  may comprise a body  22  with a cooling passage  24 . The body  22  may comprise sides  22 - 1  on which power semiconductor devices such as IGBT&#39;s  30  may be assembled. Protective covering caps  32  maybe provided to cover to IGBT&#39;s  30 . 
         [0026]    In the embodiment of  FIGS. 2 and 3 , the body  22  may have four sides  22 - 1 . One or more of the IGBT&#39;s  30  may be attached to each of the sides  22 - 1 . The body  22  may be formed from a thermally conductive material which may provide a conductive path for heat produced by the IGBT&#39;s  30 . Heat may be conducted into the passage  24 . The passage  24  may accommodate recirculating flow of coolant from the coolant reservoir  20  of  FIG. 1 . 
         [0027]    Referring now to  FIG. 4 , a cross-sectional view of the switching unit  18  may show an illustrative configuration of the body  22 . It may be seen that the passage  24  may be formed as a tube with a coolant retaining wall or tube wall  24 - 1 . The passage  24  may also be provided with heat transfer fins  24 - 2  attached to the wall  24 - 1 . Heat transfer members  22 - 2  may be interposed between the sides  22 - 1  of the body  22  and the tube wall  24 - 1 . Consequently the sides  22 - 1  may be in thermal communication with the passage  24 . It may be seen that compartments  40  may be provided inside the body  22 . The compartment  40  may be thermally connected with the sides  22 - 1  of the body and the tube wall  24 - 1 . However, the compartments  40  may be separated from the passage  24 . In other words, material or fluid which may be in the compartments  40  may not co-mingle with material or fluid that may be in the passage  24 . Nevertheless, the compartments  40  may be in thermal communication with the passage  24   
         [0028]    In the present invention, the compartments  40  may be filled with a phase change material (PCM)  42 . In operation, the switching unit  18  may produce a steady-state amount of heat that may be conveyed away with the coolant  21  circulating through the passage  24 . In the event of transient loading of the switching unit  18 , the temperature of the switching unit  18  may rise to a temperature Tm above a steady-state temperature Ts. In that case, the PCM  42  may begin to melt, i.e., change phase. The PCM  42  may continue to melt and absorb its latent heat of fusion at the temperature Tm. When the transient loading condition ceases, the temperature of the switching unit may reduce to the temperature Ts and the PCM  42  may begin to solidify. In that case, the latent heat of fusion of the PCM  42  may be released into the circulating coolant  21 . 
         [0029]    In an illustrative embodiment of the present invention the switching unit  18  may comprise twelve of the IGBT&#39;s  30 . Each of the IGBT&#39;s  30  may dissipate about 5 watts in steady-state operation. Thus, the illustrative switching unit  18  may produce heat at a steady state rate of about 60 watts. The switching unit  18  may be configured so that the coolant  21  may be re-circulated at a rate that absorbs about 60 watts to maintain a temperature Ts of about 100° C. The compartments  40  may be filled with a PCM that has a melting temperature Tm that is about 1° C. to about 5° C. above Ts. Methyle fumarate, for example, has a melting temperature of about 102° C. and, as such, this material may be suitable for use in one of the switching units  18  that may be designed to operate at a Ts of about 100° C. It may be seen that when transient loading of the switching unit  18  exceeds steady-state loading, heat dissipation may rise substantially above the steady-state level of about 60 watts. For example transient loading may produce heat at 100 watts. Temperature of the switching unit  18  may begin to rise above the temperature Ts. As the temperature rises to a temperature Tm of 102° C., the PCM  42  may begin to melt. As melting proceeds, the temperature Tm of 102° C. may be maintained in the switching unit  18 . 
         [0030]    After the switching  18  reverts to steady-state operation, heat transfer balance in the switching unit  18  may revert to steady-state conditions. In other words, heat may be produced at a rate of about 60 watts. The coolant  21  may carry away heat at this rate of 60 watts. Some of the latent heat of fusion of the PCM  42  may then transfer into the coolant  21 . Consequently, the PCM  42  may begin to revert to its original phase. After all of the PCM  42  has returned to its original phase condition, the temperature of the switching unit  18  may again revert to about 100° C. 
         [0031]    In general terms, the PCM  42  may be considered to function as a buffer. It may absorb excessive heat under high-load transient conditions and then may allow the absorbed heat to be dissipated when steady-state operating conditions are restored. To continue with the above example, it may be seen that the switching unit  18  may be only to be configured to accommodate about 60 watts of heat dissipation. This may be compared with prior-art systems which may be configured with a capability to continuously dissipate heat at the high loading rates associated with transient operating conditions. These transient operating conditions may cause heat dissipation to be three to five times as great as steady-state heat dissipation. Thus it may be seen that the present invention facilitates construction of switching units that may be three to five times smaller and lighter than prior-art switching units. 
         [0032]    Referring now to  FIG. 5  and back to  FIG. 2 , it may be seen how the switching units  18  may be readily joined with one another. The switching unit  18  may be provided with a female threaded connector member  46  at one end and a male threaded connector member  48  at an opposite end. The member  48  may be threadably inserted into the member  46  to produce a switching module  50 . The switching module  50  may comprise any number of the switching units  18  which may be connected in series or in parallel. The switching module may be constructed to meet various capacity requirements of the control system  10  of  FIG. 1 . 
         [0033]    Referring now to  FIG. 6 , a switching module  60  is shown in a planar configuration. The switching module  60  may comprises a mounting board  62  for the IGBT&#39;s  30 . Coolant channels  64  may be positioned on opposite sides of the mounting board  62 . In this regard, the switching module  60  may provided for cooling on two sides of the IGBT&#39;s  30 . A PCM tank  64  may be positioned in contact with the coolant channels  64 . As described above with respect to the switching unit  18  in  FIG. 4 , the PCM  42  may act as a heat absorbing buffer. The coolant channels  64  may be configured to accommodate steady-state heat dissipation. The PCM tank  66  may function to absorb increases of heat dissipation that may result from transient loading of the switching module  60 . 
         [0034]    Referring now to  FIGS. 7-1 ,  7 - 2  and  7 - 3  various switching modules are illustrated. As illustrated in  FIG. 7-1 , four of the switching units  60  may be configured to produce a vertically-stacked switching module  71 . As illustrated in  FIG. 7-2 , four of the switching units  60  may be configured to produce a horizontally-stacked switching module  72 . As illustrated in  FIG. 7-3 , four of the switching units  60  may be configured to produce a composite switching module  73 . 
         [0035]    In one embodiment of the present invention, a method is provided for controlling electrical power with semiconductor devices such as IGBT&#39;s. In that regard the method may be understood by referring to  FIG. 8 . In  FIG. 8 , a flow chart portrays various aspects of an inventive method  800 . In a step  802 , a heat may be produced in an IGBT at a steady-state rate (e.g., the control system  10  may place a steady-state load on the switching unit  18 ). In a step  804 , the heat produced in step  802  may be dissipated (e.g., heat from the IGBT&#39;s  30  may be conducted into the coolant  21 ). 
         [0036]    In a step  806 , heat may be produced at a transient rate in the IGBT (e.g., the control system  10  may respond to high transient load on the motor  16  and the IGBT&#39;s  30  may produce heat at a rate greater than the steady-state rate). In a step  808 , the transient heat may melt a phase change material (e.g., temperature of the switching unit  18  may rise to Tm and PCM  42  may begin melting to maintain temperature at Tm). In a step  810 , transient production of heat of step  806  may end. In a step  812 , the phase change material may solidify (e.g., the PCM  42  may solidify and release latent heat of fusion into the coolant  21 ). In a step  814 , heat may be conveyed away in a re-circulating coolant (e.g., heat produced in either/or steps  802  and  806  or heat released in step  812  may be conveyed away in the coolant and through the reservoir  20 ). 
         [0037]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.