Abstract:
A semiconductor power device, e.g., an Insulated Gate Bi-polar Transistor (IGBT) or a Metal-Oxide Field Effect Transistor (MOSFET) may be constructed in a reusable and repairable cost-effective sealed shell. The switch may be provided with direct-pressure-contact caps which may perform as electrical conductors for a semiconductor die of the switch and also as thermal heat-sink contacts for the device. The switch may be provided with internal self-powered gate driving control and PHM incorporated in sealed shell. Embodiments of the switch may be constructed with no external gating/PHM connection pin penetrations through the shell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/917,287 filed May 10, 2007 and U.S. Provisional Application No. 60/917,289 filed May 10, 2007, the disclosures of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is in the field of solid-state switching devices and, more particularly, packaging and thermal management of solid-state switches. 
     In some applications, a solid-state switch (e.g., an insulated gate bipolar transistor (IGBT) or a metal-oxide field-effect transistor (MOSFET)) may perform high frequency switching at high power levels. As a consequence, the switch may produce heat. Multiple switches may be assembled in series or in parallel combinations in order to service some high voltage and/or high power applications such as motor control systems. In this context it may be necessary to provide cooling systems for the switches. 
     Various cooling and heat-sinking systems for switches have been employed in the prior art. A typical prior-art cooling system may employ electrically non-conductive heat transfer elements in contact with electrodes of a semiconductor die or switch (e.g. a MOSFET or IGBT). The heat transfer elements may convey heat into a solid or fluid-filled heat sink. See for example, U.S. Patent Application 2004/0207968. In such prior-art arrangements, heat generated within the die must be conducted through the electrically non-conductive heat transfer element before the heat may be removed from the switch. Consequently, undesirable thermal gradients may develop within the die. 
     Some prior-art high-power solid-state devices or switches are assembled by soldering multiple semiconductor dice to a substrate and then using wire bonds to electrically connect emitters, collectors and gates of the dice together in parallel/series arrangement to obtain a device with higher current or higher voltage blocking capability. The substrate may be electrically isolated and placed on a heatsink. In high power/high-frequency applications, connection points for these wire-bonds may be a source of undesirable stray inductance, voltage drop, power dissipation and variation in propagation delays (to power and/or control signals) due to presence of different paths with varying lengths dictated by layout geometry. 
     In such prior art assemblies, reliability may be compromised because of disconnection of bond wires due to corrosion of wires, bond lift-offs, heel cracks, and reconstruction of Al-metallization on the chips are identified as significant limiting factors for device reliability (see for example, “Selected failure mechanisms of modern power modules” by Mauro Ciappa, Microelectronics Reliability 42 (2002) 653-667). 
     Additionally, in these prior-art assemblies only one-side is typically available and used for cooling of the two-sided dice. This is because one of the two sides is used for internal wire-bond connection of dice. In this regard, an equivalent thermal-resistance from junction to base-plate may be undesirably large due to the existence of a stack-up of multiple-layers of different materials including but not limited to base-plate, direct copper bonded ceramic substrate, the semiconductor die itself, metallization layers and various attachment materials such as solder and/or glue). This large equivalent thermal resistance may dictate de-rating of the device current significantly and higher thermal management may also be required which in turn may drive-up the overall cost, weight, volume of a power conditioning system in which the devices may be employed. 
     Presence of high thermal gradients within prior-art packaged solid-state switches may diminish reliability of the switches. One particular failure mechanism may occur when the switches are employed in applications with varying atmospheric pressures such as in airborne applications. In these cases, moisture may enter a switch package at entrance points for connection pins (power and/or gating/PHM). Various attempts have been made to provide sealed packages with special sealing around the connection pins to prevent moisture intrusion resulting from altitude changes. So-called “press pack” assemblies may be examples of such prior-art devices. These prior-art sealed packages are expensive and difficult to produce. 
     As can be seen, there is a need to provide cost-effective and easy-to-manufacture thermal management and packaging systems to overcome these problems. In particular, there is a need to provide such switches in a configuration that may be effectively cooled. Additionally there is a need to provide such switches in a package that may reliably prevent moisture intrusion. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention an apparatus for switching electrical current comprises a semiconductor device and at least one electrical conductor for providing electrical power to and from the switch. At least one electrical conductor serves as a heat sink for extracting heat from the switch. 
     In another aspect of the present invention an electrical switch module comprises at least two switches, each of which comprise a semiconductor die and a sealed shell enclosing the die and at least one electrical conductor for providing electrical power to the module. At least one electrical conductor is a heat sink for extracting heat from the dies. 
     In still another aspect of the present invention a method for cooling a switching electrical power supply comprises the steps of providing electrical power to a switch on a first conductor, extracting heat from the switch on the first conductor and employing the first conductor as a heat sink to dissipate the heat extracted by the first conductor. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     The accompanying figures are simplified and not drawn to scale. In the figures, each identical or substantially similar component that is illustrated in various figures may be represented by a single item or notation. For purposes of clarity, not every component is labeled in every figure. Nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a switch assembly in accordance with the invention; 
         FIG. 2A  is a schematic cross-sectional view of a switch in accordance with the invention; 
         FIG. 2B  is a simplified schematic plan view of an embodiment of the switch of  FIG. 2A  in accordance with the invention; 
         FIG. 2C  is an electrical schematic diagram of the embodiment of the switch of  FIG. 2A ; 
         FIG. 2D  is a schematic cross-sectional view of the switch of  FIG. 2  showing a contact enhancement system in accordance with the invention; 
         FIG. 2E  is an exploded view of the switch assembly in accordance with one embodiment of the present invention; 
         FIG. 3  is a schematic cross-sectional view of another embodiment of a switch in accordance with the present invention; 
         FIG. 4  is a partial schematic cross-sectional view of a portion of another embodiment of a switch in accordance with the invention; 
         FIG. 5  is a perspective view of a switch module in accordance with the invention; 
         FIG. 6  is cross sectional view of the switch module of  FIG. 5  in accordance with the invention; 
         FIG. 7  is an example of an array configuration of the switches of  FIG. 2  in accordance with the present invention; and 
         FIG. 8  is a flow chart of a method for performing switching in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Broadly, the present invention may be useful in electrical control systems which provide power sequencing and/or control for flow of power through solid-state switches. More particularly, the present invention may provide improved reliability for high power, high efficiency solid-state switches that may be operated at high frequencies. The present invention may be useful in land/air/sea traction applications and particularly aerospace applications which may require sealed solid-state switches for primary and secondary power devices and electric motor control systems for more electric aircraft (MEA). In such controlled electric power systems the present invention may provide for reliable, efficient and effective cooling of the switches. 
     In contrast to prior-art power control systems, among other things, the present invention may provide simultaneous cooling of multiple surfaces of a semiconductor die of a switch. The present invention, instead of wire-bonded construction, may employ a connection system in which electrical contact elements also perform a thermal conduction role for cooling. A switch package design may be constructed without external gating/PHM pins and may thus provide a reliable sealed switch at low cost. 
     Referring now to  FIG. 1 , an illustrative embodiment of the present invention comprises a switch assembly  10  that may be configured to perform a direct current (DC) to alternating current (AC) conversion. In that regard the illustrative assembly  10  may be referred to as a converter  10  which is capable of operation as a rectifier for AC-DC or inverter for DC-AC power conversion. The converter  10  may comprise solid-state switches  12 , a positive DC bus  14 , a negative DC bus  16  and phase conductors  18 . Each of the switches  12  may be in electrical and thermal contact with at least one of the DC buses  14  or  16  and one of the phase conductors  18 . 
     In operation, the switches  12  may be controlled to electrically interconnect one of the DC buses  14  or  16  to one of the phase conductors  18 . The switches may be operated at a predetermined frequency and sequence to provide three-phase AC power on the phase conductors  18 . It may be noted that various configurations of the switches  12  may be assembled and employed in various well-known electrical applications such as, in primary and/or secondary solid-state power controllers (SSPC) for AC and DC applications replacing mechanical contactors or in aerospace More Electric Aircraft motor controls or general industry. In this regard, the switches  12  may be incorporated into assemblies that may require low frequency on-off command for control of flow of power for the first application and high-frequency and/or high power operation of the switches  12  for PWM control of the latter application. 
     The physical gating/control/PHM pins of the present invention can be eliminated and such signals may be communicated by high frequency transformer coupling (transfer of both power and gating/PHM signal as is known in the art) or RF transmission or PLC. Control signals could be in the form of sending speed, voltage, or current command as a reference value in a wireless mode to be further processed internally by an embedded central controller doing PWM/PHM control of one or more integrated package as described in this invention. As per below references, those skilled in the art appreciate how wireless communication/transfer of command and control signals can be used for on-off gating control of SSPCs or PWM gating/PHM of controlled devices of a power electronic equipment. As can be seen from below technical literature, transfer of gating/control/PHM information from a controller to a controlled device (or plurality of devices integrated in one packages) or vice versa can be easily implemented and will not be further discussed here. The following references are herein incorporated by reference in their entirety: S. K. Mazumder, M. Tahir, S. L. Kamisetty, “Wireless PWM control of a parallel dc-dc buck converter”, provisionally accepted,  IEEE Transactions on Power Electronics,  2004. S. K. Mazumder, K. Acharya, and M. Tahir, “Wireless control of spatially distributed power electronics,” in  Proc.  20 th    Annu. IEEE Applied Power Electronics Conf. Expo , vol. 1, March 2005, pp. 75-81. M. Tahir, S. K. Mazumder, “Markov-chain-model-based Performance Analysis of Transmitter Power Control in Wireless MAC Protocol: Towards Delay Minimization in Power-network Control,” Accepted: Proceedings of IEEE International Conference on Advanced Information Networking and Applications, 2007. M. Tahir, S. K. Mazumder, “Improving Throughput-delay Performance by Merged Packet Routing in Wirelessly Controlled Interactive Power Networks,” Accepted: Proceedings of IEEE International Conference on Advanced Information Networking and Applications, 2007. “HomePlug AV White Paper”, Document version number: HPAVWP-050818, Homeplug Powerline Alliance, Inc., 2005. Ahola, Jero, “Applicability of Power-Line Communications to Data Transfer of On-line Condition Monitoring of Electrical Drives”, Thesis for the degree of Doctor of Science, Lappeenranta University of Technology, 2003 
     Referring now to  FIG. 2A , an illustrative embodiment of one of the switches  12  is shown in accordance with the present invention. The switch  12  may be comprised of electrically conductive contact caps  102 . The contact caps  102  may be jar-type contact caps, wherein a lid can be tightened by rotation of the lid on a threaded shell. Alternatively, the contact caps  102  may be plate-like lids on each side of the switch  12  fastened by screws onto the upper and lower surface of the shell. The contact caps  102  may be threaded and screwed on a threaded shell  104  which may be comprised of an electrically non-conductive material such as plastic or ceramic. The contact caps  102  may be in contact with a semiconductor die  106  such as an IGBT or MOSFET through solder bumps or other methods known in prior art (e.g., conducting vertical springs). In  FIG. 2 , for purposes of simplicity, the die  106  is illustrated symbolically as a single device. The die  106  is shown with a collector or source region  106 - 1  in contact with a first one of the contact caps  102  and with an emitter or drain region  106 - 2  in contact with a second one of the contact caps  102 . A gate region  106 - 3  may be interposed between the regions  106 - 1  and  106 - 2  and the gate region  106 - 3  may be electrically isolated from the contact caps  102 . 
     It may be noted that many types of devices may be configured in accordance with the present invention. For example, one or more of the switches may comprise a Si or SiC diode, a symmetrical blocking thyristor, an asymmetrical thyristor or a gate turn-off device (GTO). Additionally, it may be noted that the switch  12  may be constructed with a plurality of the dies  106  connected together electrically in parallel. For example, the switches may comprise a plurality of MOSFETs connected together in parallel by being in contact with the contact caps  102  In such an arrangement, the switch  12  may provide power capacity that may be associated with multiple MOSFETs without a need to provide prior-art wire-bond connections between the multiple MOSFETs. Elimination of prior-art wire bonds improves reliability, provides a MOSFET, or other semiconductor device, based switch with particularly low resistive losses and may significantly reduce stray inductances within the device package. 
     Referring now to  FIGS. 2B and 2C , in an illustrative embodiment, a plurality of devices such as MOSFETs  106  may be arranged around a gate driver area  106 - 2  in the switch  12 .  FIG. 2C  presents an electrically equivalent schematic of the arrangement of the MOSFETs  106  of  FIG. 2B   
     Referring back now to  FIG. 2A , the contact caps  102  may comprise an internally threaded retention portion  102 - 1  and a contact portion  102 - 2 . The contact portion  102 - 2  may be connected to the retention portion  102 - 1  with a flexible connection portion  102 - 3 . The contact cap  102  is illustratively shown in  FIG. 2  as an integral structure that may be formed from a single piece of material. In this illustrative integral configuration, the connection portion  102 - 3  may comprise a portion of the material with a reduced thickness. It may be noted that, within the scope of the present invention, the contact cap  102  may be constructed from multiple elements of material attached together by conventional brazing or welding. 
     As the contact cap  102  is assembled onto the shell  104 , the contact portion  102 - 2  may engage with the die  106  before the retention portion  102 - 1  reaches a limiting position on the shell  104 . In other words, the contact portion  102 - 2  may engage with the die  106  while there is still a space  110  between the retention portion  102 - 1  and the shell  104 . The retention portion  102 - 1  may be further threadably advanced onto the shell  104  after the contact portion  102 - 2  is engaged with the die  106 . Such further advancement of the retention portion  102 - 1  may result in a relative displacement between the contact portion  102 - 2  and the retention portion  102 - 1 . This may result in a flexure of the flexible portion  102 - 3  with a resultant compressive force between the die  106  and the contact portion  102 - 2 . 
     Referring now to  FIG. 2D , a schematic illustrative embodiment of a contact enhancement system is shown. Compressible metal elements such as solder bumps  107  may be interposed between the die  106  and the contact caps  102  to assure sound electrical contact. The solder bumps  107  may be compressed when the contact caps  102  are pressed onto the die  106 . In a case of multiple dies such as the MOSFETs  106 - 1  of  FIG. 2D , the solder bumps  107  may provide accommodation for varying thicknesses of the MOSFETs  106 - 1 . In an illustrative embodiment, the solder bumps  107  may be printed on surfaces of the dies or the contact caps  102  with methods such as silk-screen printing. 
     In a simplified conceptual electrical diagram  FIG. 2C , it may be seen that a common gate signal  220  may control simultaneous turn-on and turn-off of all the parallel MOSFETs  106 - 1 . The gate signal may be brought to the controlled MOSFETs  106 - 1  through a) traditional physical gating pins (not shown); or b) through Power Line Carrier (PLC) in which “on” and “off” signal patterns are modulated on a high frequency low voltage signal and superimposed on the power input and output lines (in this case, drain line  221  and source line  222 ) and demodulated internally through conventional prior art PLC methods to obtain a desired gating pattern; or c) through fiber optics lines; or, d) through use of wireless technology in which one or more remote master transmitter(Tx)/receiver(Rx) modules may send encrypted control signal and receive PHM data from one or more controlled systems through a defined communication protocol established for seamless operation of all controlled nodes with minimal delay. At each distributed controlled node (e.g., one of the MOSFETs  106 - 1  shown in  FIG. 2B ) the gate driver in case d) may include Rx &amp; Tx and de-encryption to extract the sent command/control data and transfer the gating signals  220  to the gates  106 - 3  through gate resistor  223 . 
     Sealing members  112  may be positioned in channels  104 - 1  formed in the shell  104 . As the retention portions  102 - 1  are advanced into position on the shell  104 , the sealing members  112  may be compressed to form a seal for the switch  12 . The sealing member  112  may comprise an O-ring. 
     It may be noted that in an alternative embodiment, the contact caps may be secured with screws. 
     Referring to  FIGS. 2A-2E , the switch  12  may comprise a control block  114  that may provide various controlling functions for the die  106 . For example, the control block  114  may include circuitry that performs various functions such as gate driver functions, protection and prognostics health monitoring (PHM) functions, isolated self-powered internal positive voltage (e.g., +15V) power supply for turn-on and negative supply (e.g., −5V) for keeping the device in the off-position. Furthermore, passive or active snubbers can be internally included in the device to alleviate stress on the switching device, as required. The concept, as is known in the art, of passing power and signal (both gating and PHM) can be used in this invention. Self-powered power supply can be obtained by using an isolated converter which takes power from the voltage across the device when it is in off position, or reuse some the energy recovered during commutation and stored for internal use. Detailed implementation of these concepts is well understood to those skilled in the art and will not be covered here. This novel inclusion of the control block  114  within the shell  104  of the switch  12  may provide numerous advantages. 
     The control block  114  may be positioned close to the die  106 . Consequently interconnections  118  between the control block  114  and the die  106  may produce only minimal losses and time delays. For example a snubber may be incorporated within the control block  114  and may provide for re-generation of energy during operation of the switch  12 . Interconnections  118  between the control block  114  and the die  106  may be made internally within the sealed switch  12 . Internalization of the interconnections  118  may allow for elimination of external connection pins which may otherwise be required for operation of prior-art switches. 
     While internalization of the interconnections  118  is generally desirable, there are some disadvantages to such internalization. Inclusion of the control block  114  in the switch  12  may result in a lower manufacturing yield for the switches  12 . The control block  114  may comprise numerous interacting devices and connections which may be subject to failure during manufacturing. If the switch  12  were produced with conventional prior-art “press-pack” techniques, reduced yields resulting from internalization of the control block  114  may be intolerable. In another embodiment of the present invention, the contact caps  102  are removable. Because of this novel feature, one of the switches  12  may be opened for repair in the event that a manufacturing defect arises. Performing a repair may preclude a need to discard an assembly of expensive components because of a failure of only one of the internal components, a failure of a connection or any other internal failure or defect. 
     In the embodiment of the present invention shown in  FIG. 2A , a signal input member  120  may be provided to interconnect a signaling block  122  with the control block  114 . The signal input member may pass through the shell  104  in a sealed passageway  104 - 2 . The switch  12  shown in  FIG. 2  may provide a cost-effective sealed/air-tight press-pack-style package. One of the main advantages of this type of package is the fact that many such switches can be put in series/parallel connection because of the ease with which they can be connected electrically and mechanically in an assembly. In addition, in a failed state, where one of the switches  12  may fail short, it may be possible to ensure overall combined functionality by putting more of the switches  12  in series. When one switches  12  fails, it may go short. Thus by employing two or more levels of redundancy, even if one or two switches  12  fail, voltage blocking capability may be guaranteed by the incorporated level of redundancy. Dynamic and static voltage sharing may be provided with a combination of Resistors and/or RC snubbers placed across each series switch. 
     Referring specifically to  FIG. 2E , the shell  104  and caps  102  may create a cavity  104   a  reserved for placing semiconductors, such as dies  106 , in direct contact with the caps  102 . The present invention may provide the concept of double-sided cooling with one integrated path on each side for both electrical conduction and thermal management. 
     In another embodiment of the present invention shown in  FIG. 3 , a switch  300  may comprise a control block  302  which may be provided with remote signaling. A remote signaling block  304  may produce signals  306  which may be transmitted remotely or wirelessly to the control block  302 . For example, the signals  306  may be transmitted as radio frequency (RF) signals (e.g. with transmitters/Receivers on the sending end and receiving end, as described above) or as power line carrier signals (e.g., through use of power lines for carrying modulated high frequency information as described above). 
     In another embodiment of the present invention, signals may be provided to a switch  400  through transformer coupling, as is known in the art.  FIG. 4  shows a configuration of a shell  404  of the switch  400 . The shell  404  may be provided with a transformer adapter  404 - 1 . The adapter  404 - 1  may comprise a slot  404 - 11  for a transformer primary winding  406  and a slot  404 - 13  for a transformer secondary winding  408 . A transformer core  404 - 14  may be incorporated into the transformer adapter  404 - 1 . Signaling produced in a signaling block  410  may be transmitted a control block  412  through the transformer windings  406  and  408 . 
     It may be seen that the embodiment of the invention shown in  FIGS. 3 and 4  may be operated without external connection pins. Reliable sealing may be readily achieved when there are no external-connection penetrations through the shell  104  or  404 . 
     Referring back now to  FIGS. 1 and 2 , it may be seen how the inventive configuration of the switch  12  may be effectively cooled during its operation. The switches  12  may be positioned so that their respective contact caps  102  may be in contact with one of the DC buses  14  or  16  or one of the phase conductors  18 . The DC buses  14  and  16  and the phase conductors  18  may be constructed from material that is both electrically and thermally conductive (i.e., integrated thermal management may be included into conducting bus-bars). Furthermore, the buses  14  and  16  and the conductors  18  may be constructed from heavy gauge materials with a large thermal mass. Indeed the buses  14  and  16  may be hollow so that cooling fluid may be circulated through the buses. Thus the buses  14  and  16  and the conductors  18  may function in a dual role within the switch assembly  10 . They may perform an electrical conduction role and in addition, they may perform a thermal conduction role and a heat dissipation role as heat sinks. 
     Heat may be transferred from the die  106  from both the collector region  106 - 1  and the emitter region  106 - 2  (IGBT example, also applicable to other types of devices). As a consequence, heat generated within the die  106  may be readily transferred out of the die  106  across two of its surfaces thereby reducing a potential for formation of thermal gradients within the die  106 . Furthermore, because the switch  12  may be sealed, the switch assembly  10  may be immersed in a liquid coolant in order to further enhance cooling. 
     Referring now to  FIGS. 5 and 6A  and  6 B, it may be seen how switches may be assembled into a module  500  with internal cooling. Two switches  502  may be assembled back-to-back on a fluid-cooled electrical conductor  504 . The switches may be similar in construction to the switches  12  except that the switches  502  may be provided with only one contact cap  502 - 1 . The switches  502  may be positioned in contact with the fluid-cooled conductor  504 . The fluid-cooled conductor may be provided with a fluid passage  504 - 1  through which cooling fluid  506  may flow. In a cross-sectional  FIG. 6B , it may be seen that heat sink fins  504 - 2  may be incorporated into the fluid-cooled electrical conductor  504 . 
     Referring now to  FIG. 7 , it may be seen the inventive switches  12  and their various embodiments may be combined in a modular array.  FIG. 7  illustrates but one example of an array that may be achieved with the inventive modular switches  12 . As can be appreciated by one skilled in the present art, many possible arrays of parallel and series combinations may be produced. 
     In one embodiment of the present invention, a method is provided for operating and cooling a solid-state switch. 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 , electrical power may be provided to one side of a switch (e.g. power from the bus  14  may be provided the contact cap  102  of the switch  12 ). In a step  804 , a gating signal may be produced remotely from the switch (e.g., the signaling block  122 ,  304  or  410  may produce a command signal for the control block  114 ,  302  or  412  respectively). In a step  806 , the switch may be operated with internal gating control (e.g., the gating region  106 - 3  of the die  106  may be driven by the control block  114  which is internal to the switch  12 ). 
     In a step  808 , electrical power may be extracted from a second side of the switch (e.g., electrical power from one of the contact caps  102  may be extracted by one of the phase buses  18 ). In a step  810 , heat may be extracted from the first side of the switch (e.g., the DC bus  14  may extract heat through one of the contacts caps  102 ). In a step  812 , heat may be extracted from the second side of the switch (e.g., the phase conductor  18  may extract heat through one of the contacts caps  102 ). In a step  814 , electrical power may be delivered and cooling may be performed on the same conductors (e.g., the phase conductors  18  may perform power delivery and cooling functions). 
     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.