Abstract:
A device includes at least one semiconductor switching circuit connected to a power source and a load and at least one breaker switch integrated with the at least one semiconductor switching circuit. The breaker circuit may be connected in series with the at least one semiconductor switching circuit and the at least one breaker switch is configured to create an open circuit in less than about twenty microseconds of receipt of a predetermined threshold of semiconductor switch current to thereby prevent damage to the at least one semiconductor switching circuit or housing. A method of preventing damage to a semiconductor switching circuit or device is also presented.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates in general to power electronic circuits and, more particularly, to semiconductor switches used in power electronic circuits. 
         [0003]    2. Related Art 
         [0004]    Inverters used in variable speed drive systems and power converters typically use semiconductor switches such as insulated gate bipolar transistors (IGBT) packaged into IGBT modules. 
         [0005]    A problem arises with the use of the IGBT modules where the semiconductor switch looses its voltage blocking capability causing a short circuit. One possible reason for this type of failure is a break down of the semiconductor induced by, e.g., radiant energy. This short circuit of the semiconductor switch, leads, in turn, to a short of the connected equipment which may be a multiphase power grid, a large rotating electrical machine or internal energy storage as for example the DC link capacitor. 
         [0006]    Failures such as these can have significant consequences, for example, damage to the IGBT module or to the equipment located physically near the IGBT module. 
         [0007]    Referring now to  FIG. 1 , a typical IGBT module is shown generally at  1 . The IGBT module  1  comprises a base plate  2  comprising, e.g., copper, a housing  3  comprising, e.g., a plastic and an insulating ceramics layer  4  comprising, e.g., aluminum oxide layer laminated with a conductive material such as copper. The aluminiumoxide layer may be laminated with copper or other metal in a special process like e.g. direct copper bonding. A solder joint  5  may be employed to affix the base plate  2  to the insulating ceramics layer  4  and power terminals (emitter and collector)  6  are connected via wire bonding to a topside layer (not numbered) of the insulating ceramics layer. The topside layer of the insulating ceramics layer  4  may be etched to provide the IGBT module internal circuit connections. An IGBT chip  7  and a diode chip  8  may be connected by bond wires  9  to the topside layer of the insulating ceramics layer  4 . A soft gel  10  may be provided within the housing  3  for encapsulation of the chips  7  and  8 . 
         [0008]    In case of a failure, a very high current amplitude may be reached within the IGBT module  1 . This high current amplitude leads to a thermal overload of the bond wires  9 . The melt down of the bond wires  9  initiates an arc that builds up and excessively heats the surrounding insulation soft gel  10 . With the arc heating up the module  1  internal structure, the pressure within the housing  3  rises until the housing  1  ruptures. This is usually referred to as an explosion of an IGBT module. 
         [0009]    Referring now also to  FIG. 2 , a circuit diagram of a typical IGBT module including a six pack, three phase, bridge configuration with short circuit is shown generally at  20 . Phase connections L 1 , L 2  and L 3  from a power source such as the power grid or a large rotating synchronous machine or the like (not shown) are connected to a plurality of IGBT devices  22 ,  24 ,  26 ,  28 ,  30 ,  32  such as IGBTs and corresponding diodes and a direct current (dc) link capacitor  34 . A short circuit current path  36  is shown where a either the IGBT or the diode of the semiconductor device  24  has lost its blocking capability. 
         [0010]    In this case, one or more of the diodes of the remaining functioning bridge devices are forward biased depending on the actual phase voltage. Where the short circuit current path  36  is established, the current is only limited by the grid or load impedance (not shown). The current will flow as long as the power circuit is connected to the grid or load. Typical protection equipment like circuit breakers will need several cycles of the ac frequency to disconnect the failed circuit. During this time span the amount of energy dissipated inside the failed IGBT module  1  will lead to a rupture of the module housing  3 . The explosion of the IGBT module will take place within the time span that is typically needed by fuses or circuit breakers to clear the fault. 
         [0011]    Standard protection equipment cannot protect the IGBT module from these types of failures. The melting integral of the IGBTs bond wires is usually an order of magnitude lower than the corresponding value of a fuse. Even fast acting fuses, so called semiconductor fuses, which are well positioned to protect thyristor type devices, are not acting fast enough to protect an IGBT module. The explosion cannot be avoided, so common design practice for power converters is to use mechanical separation of modules. One possibility is to use so called Blast Shield to protect neighbor modules. Another solution used within power electronics is to minimize fuse energy let through rating to limit damage within the system. Measures to protect the switch board, like blow off values are used. 
         [0012]    Accordingly, to date, no suitable device or method of protecting semiconductor switches from damage during a short circuit failure condition as described above is available. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0013]    In accordance with an embodiment of the present invention, a device comprises at least one semiconductor switching circuit connected to a power source and load and at least one breaker switch integrated with the at least one semiconductor switching circuit. The breaker circuit may be connected in series with the at least one semiconductor switching circuit and the at least one breaker switch is configured to create an open circuit within less than about twenty microseconds of receipt of a predetermined threshold of current from the power source to thereby prevent damage to the at least one semiconductor switching circuit. 
         [0014]    In accordance with another aspect of the invention a method of preventing damage to a semiconductor switching circuit comprises connecting at least one semiconductor switching circuit to a power source and load; integrating at least one breaker switch with the at least one semiconductor switching circuit; connecting the at least one breaker switch in series with the at least one semiconductor switching circuit; and configuring the at least one breaker switch to create an open circuit in less than about twenty microseconds of receipt of a predetermined threshold of current from the power source to thereby prevent damage to the at least one semiconductor switching circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The following detailed description is made with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a sectional diagram showing a typical configuration of an IGBT module; 
           [0017]      FIG. 2  is a circuit diagram showing a three phase bridge circuit including a failed bridge device and a short circuit current path; 
           [0018]      FIG. 3  is a circuit diagram showing a three phase bridge circuit including breaker switches in accordance with one embodiment of the present invention; 
           [0019]      FIG. 3A  is a sectional diagram showing a portion of an integrated IGBT module in accordance with another aspect of the present invention; 
           [0020]      FIG. 4  is a circuit diagram showing a half bridge IGBT module and a breaker switch in accordance with another embodiment of the present invention; 
           [0021]      FIG. 5  is a graph showing current versus time flowing through a failed IGBT module. 
           [0022]      FIG. 6  is a circuit diagram showing a half bridge IGBT module and a breaker switch in accordance with another embodiment of the present invention; 
           [0023]      FIG. 7  is a block diagram illustrating an example switching system, usable with the embodiment of  FIG. 6 ; 
           [0024]      FIG. 8  is a circuit diagram showing an IGBT module and a breaker switch in accordance with another embodiment of the present invention; 
           [0025]      FIG. 9  is a circuit diagram showing a half bridge IGBT module and a breaker switch configuration in accordance with another embodiment of the present invention; 
           [0026]      FIG. 10  is a circuit diagram showing a half bridge IGBT module and a breaker switch configuration in accordance with another embodiment of the present invention; and 
           [0027]      FIG. 11  is a circuit diagram showing a single switch IGBT module and a breaker switch configuration in accordance with a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]    One embodiment of the present invention concerns a device and a method for preventing damage to one or more semiconductor switches by providing a breaker switch capable of creating an open circuit condition in less than about twenty microseconds of determination of an over current condition in a circuit carrying current to the semiconductor switch(es). In one particular embodiment, a microelectromechanical system (MEMS) breaker switch may be integrated into an insulated gate bipolar transistor (IGBT) module. In case a failure is detected by the main inverter control system, the MEMS breaker switch is commanded to disconnect the IGBT from all loads or power sources. Use of the MEMS breaker switch allows the current to the IGBT to be interrupted in less than about twenty microseconds (μs), especially within the IGBTs rated short circuit withstand time. By this method an explosion of the IGBT module housing is avoided. The risk for damaging neighbor devices is minimized. Bulky external protection devices, such as circuit breakers or fuses can be avoided. Auxiliary elements employed with a MEMS breaker switch also may be integrated into an IGBT module. 
         [0029]    It will be appreciated that a MEMS based switch is a fast acting switch but it cannot break high currents. Accordingly, a MEMS switch may be connected in a circuit, e.g., configured such that the MEMS switch switches when a current level is nearly zero or, e.g., configured to comprise an external circuit configured to reduce a current level in the MEMS switch to zero. One exemplary document describing use of an external circuit appropriately configured using MEMS technology is found in US Patent Publication No. 20070139829A1 which is assigned to the General Electric Company. 
         [0030]    Referring now to  FIG. 3 , a circuit comprising a breaker switch, in accordance with a first embodiment of the present invention, is illustrated generally at  300 . In this embodiment, the circuit  300  comprises phase connections L 1 , L 2  and L 3 , semiconductor switching circuits such as IGBT devices  302  through  312 , a direct current (DC) link capacitor  314  and breaker switches  316 ,  318  and  320 . 
         [0031]    The phase connections L 1 , L 2  and L 3  may be in circuit with a power source such as the public grid or a rotating synchronous machine (not shown). The three phase bridge is using IGBT devices  302  through  312  and parallel connected diodes  334  through  344  in all switch positions. 
         [0032]    Each breaker switch  316 ,  318  and  320  comprises a MEMS switch that is respectively integrated into each phase connection L 1 , L 2  and L 3 . Integrating of the breaker switches  316 ,  318  and  320  comprises the formation of the breaker switches and the IGBT in a single module or housing. It is intended that the term “integrated”, as used in this document, means an assembly of one or more IGBT and/or one or more diode chips into one module housing.  FIG. 3A  illustrates a portion of an exemplary integrated IGBT module  100  in accordance with an embodiment of the present invention. As shown therein, the IGBT module  100  may be similar to the IGBT module  1  described above in connection with  FIG. 1  and as such similar elements are labeled similarly accepting that a one hundred prefixes each reference number. A difference is in that a breaker switch  111 , such as a MEMS switch, is connected by wire bonds  109  to an IGBT chip  107 . In this way, the breaker switch  111  is integrated with the IGBT chip in the IGBT module  100 . 
         [0033]    Referring now to  FIGS. 3 and 5  and in the example of a failure of one of the diodes  334  through  344 , a typical current waveform  500  for a short circuit is shown in  FIG. 5 . The waveform  500  features natural zero current phases  502  that can be identified easily by an appropriately configured control circuit  336  ( FIG. 3 ). Referring again also to  FIG. 3 , the control circuit  336  is configured to switch the breaker switches  316 ,  318  and  320  upon sensing the zero current phases  502  of the waveform  500 . It will be understood that the control circuit  336  may comprise a motor controller circuit. In another embodiment control circuit  336  may be integrated with the inverter control circuit or the inverter protection circuit. Upon receiving a command from the control circuit  336 , each breaker switch  316 ,  318  and  320  will switch open within twenty microseconds to prevent damage to the IGBT devices  302  through  312  or the diodes  334  through  344 . 
         [0034]    Another embodiment of a circuit comprising a breaker switch in accordance with the present invention is shown generally at  400  in  FIG. 4 . The circuit  400  comprises a pair of IGBT devices  402  and  404  connected in a half bridge configuration. The conductors  406  and  408  are representing the DC link connection. A breaker switch  410  may be connected in series to the AC terminal of the half bridge  412  and may comprise a MEMS switch. The breaker switch  410  may be controlled by a control circuit (not shown) in a similar manner to that described above and illustrated in  FIG. 3 . 
         [0035]    In another embodiment, a circuit that is configured to reduce a current through a breaker switch to almost zero is shown generally at  600  in  FIG. 6 . The circuit  600  comprises a pair of IGBT devices  602  and  604  connected in a half bridge configuration. The conductors  606  and  608  are representing the DC link connection. A breaker switch  610  may be connected in series with the AC terminal of the half bridge  612  and may comprise a MEMS switch. The breaker switch  610  may be controlled by a control circuit (not shown) in a similar manner to that described above and illustrated in  FIG. 3 . In this embodiment an additional switching circuit  614  may be provided which provides a separate path for the current to pass instead of through the breaker switch  610 . By this means the MEMS current is brought to almost zero to support the switches opening. 
         [0036]      FIG. 7  is a block diagram representation of the switching circuit  614  that connects in a parallel circuit with the breaker switch  610  that comprises a MEMS-based switch. The switching circuit  614  may comprises a solid-state switching circuitry  616 , an over-current protection circuitry  618  and a controller  620 . In another embodiment of the invention the over-current protection circuit and/or the control circuit  620  may be integrated with the inverters control or protection circuitry. 
         [0037]    The controller  620  may be coupled to the breaker switch  610 , the solid-state switching circuitry  616  and the over-current protection circuitry  618 . To reduce the current through the breaker switch  610 , the controller  620  may be configured to selectively transfer current back and forth between the breaker switch and the solid state switching circuitry by performing a control strategy configured to determine when to actuate over-current protection circuitry  618 , and also when to open and close each respective switching circuitry, such as may be performed in response to load current conditions appropriate to the current-carrying capabilities of a respective one of the switching circuitries and/or during fault conditions that may affect the switching system. It is noted that in such a control strategy it is desirable to be prepared to perform fault current limiting while transferring current back and forth between the respective switching circuitries  610  and  616 , as well as performing current limiting and load de-energization whenever the load current approaches the maximum current handling capacity of either switching circuitry. 
         [0038]    A system embodying the foregoing example circuitry may be controlled such that the surge current is not carried by the breaker switch  610  comprising a relatively low level current rated MEMS based switching circuitry and such a current is instead carried by solid-state switching circuitry  616 . The steady-state current would be carried by breaker switch  610 , and over-current and/or fault protection would be available during system operation through over-current protection circuit  618 . 
         [0039]    In accordance with another embodiment of the present invention a circuit  800  comprises phase connections L 1 , L 2  and L 3 , switches  802  through  812  each comprising a parallel connection of one or more IGBT devices and diode devices, a DC link capacitor  814  and a breaker switch  816 . A switching circuit  820  similar to the switching circuit  614  may be provided. 
         [0040]    The phase connections L 1 , L 2  and L 3  may be in circuit with a power source such as the power grid or a large rotating synchronous machine or the like (not shown) that may have a very high short circuit current rating. Each IGBT and corresponding anti parallel diode, in the bridge  818 , are shown as switches only. 
         [0041]    The breaker switch  816  comprises a MEMS switch and is used to disconnect the bridge circuit  818  from the DC link capacitor  814  to avoid discharging of its energy into the failed module. In this embodiment, an advantage of using a MEMS switch integrated into the module is that stray inductance is minimized due to the small size of the unit and its tight integration with the IGBT module. 
         [0042]    Referring now to  FIG. 9 , another embodiment of a circuit in accordance with the present invention is shown generally at  900 . Circuit  900  represents a half bridge configuration comprising a pair of IGBT devices  902  and  904  and parallel connected diodes  902  and  904 . Two breaker switches  910  and  912 , each comprising a MEMS switch, are connected between the IGBT devices  902  and  904 . A junction  914  is located between the breaker switches  902  and  904  that is connected with AC current. Such an arrangement, it will be appreciated, provides more opportunity to switch off the breaker switches  910  and  912  during natural zero current conditions of a failure waveform ( FIG. 5 ). This would speed up the current interruption. By means of this the use of a switching circuit (such as  614  or  820 , described above) in parallel to the breaker switches  910  and  912  may be omitted. 
         [0043]    In a further embodiment shown in  FIG. 10 , a circuit  1000  may be similar to the circuit  900  of  FIG. 9 , although, the breaker switches  1010  and  1012  are connected into different position. 
         [0044]    In a further embodiment (not shown) the breaker switches are series connected to the IGBT emitters. 
         [0045]    In a further embodiment (not shown) the breaker switches are series connected to the IGBT collectors. 
         [0046]    A circuit in accordance with a further embodiment is shown generally at  1100  in  FIG. 11 . The circuit  1100  comprises a single switch IGBT module comprising a multiple of parallel connected IGBT device  1102  through  1106  and a multiple of diodes  1108  through  1112 . This type of IGBT module is especially suited for relatively high power/current ratings. In this embodiment one breaker switch is series connected to each single internal branch ( 1102  and  1108 ) of the module. Using a breaker switch  1118 ,  1120  and  1122 , each of which may comprise a MEMS switch, in each of the parallel branches enables, e.g., a failed IGBT chip  1102  or diode  1108  to be disconnected while operating the remaining IGBTs  1104  and  1106  and diodes  1110  and  1112  at a reduced current rating. Accordingly, a design featuring n+1 redundancy is also a viable option. 
         [0047]    In an further embodiment similar to  FIG. 11  the breaker switches  1118  through  1122  are series connected to the IGBT emitters. 
         [0048]    The above described principle of individual breaker switches may also be applied to IGBT modules in half bridge configuration or IGBT modules in six-pack configuration. 
         [0049]    It will be appreciated that each of the circuits described above may also be applied for IGBT modules in single switch configuration, in half bridge configuration, six-pack configurations or combinations thereof. Also the above described principals may be applied to other module configurations like copper modules or the like. 
         [0050]    While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to these herein disclosed embodiments. Rather, the present invention is intended to cover all of the various modifications and equivalent arrangements included within the spirit and scope of the appended claims.