Abstract:
The present invention relates to power modules for controlling an input current to provide an output current, in particular, to such a power module ( 100 ) comprising at least one semiconductor switch for controlling the input current and a circuit substrate. The circuit substrate ( 108, 110 ) has at least one input terminal for being connected to an input voltage and at least one output terminal for outputting the output voltage. The input terminal and the output terminal are connected to each other via a main current path ( 104 ) that is controlled by the semiconductor switch. Said power module further comprises at least one transient current path ( 102 ) with extremely low inductivity for carrying a transient current during a switching operation of the semiconductor switch. The main path for the continuous current has a low ohmic resistance, but does not have to be designed to have a specifically low inductance.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to power modules for controlling an input current to provide an output current. In particular, the present invention relates to such a power module comprising at least one semiconductor switch for controlling the input current and a circuit substrate. The circuit substrate has at least one input terminal for being connected to an input voltage and at least one output terminal for outputting the output voltage. The input terminal and the output terminal are connected to each other via a main current path that is controlled by the semiconductor switch. 
         [0002]    Generally, parasitic inductances are a major problem with such power modules, in particular in fast switching applications. The parasitic inductance causes overvoltages and increases switch-off losses in the semiconductor. In particular, when using an insulated gate bipolar transistor (IGBT), the switching off of the semiconductor switch results in a current change and this causes an overvoltage spike by the current change in the parasitic inductances. The overvoltage spike can be calculated as follows: 
         [0000]        V   CE (Peak)= V   CE   +L·dl/dt.    
         [0003]    Here V CE  signifies the collector-emitter voltage, dl/dt is the current change with time, and L is the value of the parasitic inductivity. 
         [0004]    In order to avoid parasitic inductances in the power circuit, the lay-out of the circuit becomes important. As this is generally known, the parasitic inductance rises with the area encircled by the electric current. With an overlap of the tracks of positive and negative voltage the area becomes minimal. Capacitors are used to shorten these loops in order to reduce the encircled area. 
         [0005]    The power module is a key component regarding the parasitic inductance. Here the target in bridge configurations has to be to shortcut of the encircled area of the DC input and to reduce same as far as possible by means of a capacitor. The distance between the DC+ and the DC− contacts and the accessibility of these contacts will influence the system inductance and the switch-off losses. 
       SUMMARY OF THE INVENTION 
       [0006]    The principles according to the present invention can be applied in particular to power modules using insulated gate bipolar transistors, IGBT, for frequency control of three phase electromotors. Here the power module is structured as an IGBT half bridge with the possibility to combine three modules to form a three-phase inverter bridge. 
         [0007]    The problem underlying the present invention is to provide a power module which has a reduced cost and assembly complexity on the one hand and a reduced parasitic inductance enabling fast switching applications on the other hand. 
         [0008]    The present invention is based on the idea that by providing a low inductive transient current path for carrying the transient current during the switching operation of the semiconductor switch, over voltage spikes which are caused during the switching operation of the semiconductor switch are reduced and do not damage the semiconductor. The transient currents are deviated in a transient current path. Thus, the current flow in the power module can be separated into a path with extremely low inductivity for the transient current during the switching of the high voltage, and a main path for the continuous current which has a low ohmic resistance, but does not have to be designed to have a specifically low inductance. With a low inductive current path for the transient current overvoltage spikes are avoided or at least reduced. The overvoltage at switch-off at the semiconductor will be reduced. 
         [0009]    The transient current path having the low inductivity is active only during the short time wherein dl/dt is high during switching. On the other hand, the connections for this low-inductivity path will not face the thermal load of the high current because of the short duration of same. 
         [0010]    According to an advantageous embodiment of the present invention, the transient current path is at least partly arranged in a plane which is oriented across to the circuit substrate carrying the semiconductor switch. Thus, the low-inductivity current path can for instance be integrated into a connection element which is present for the continuous current inside the power module. 
         [0011]    For instance, the transient current path can at least partly be formed as a sandwich of at least two electrically conductive sheets, preferably fabricated from stamped and bent metal sheets. This is a particularly easy and cost efficient way of fabricating a contact bridge with integrated transient current path. 
         [0012]    For electrically insulating the conductive sheets from each other, an electrically insulating foil can be provided. These foils, for instance fabricated from polyimide, have the advantage that they can be assembled easily and do not require much space. 
         [0013]    Alternatively, the transient current path can also be partly formed as a separate multi-layer printed circuit board. In this case, a plurality of conductive layers can be provided in a particularly space-saving way. 
         [0014]    According to an advantageous embodiment of the present invention, the transient current path comprises at least one capacitor for storing the charge of the transient current during the switching operation of the semiconductor switch. In this case, the energy is absorbed in the capacitor which can be mounted in a way that it can be effectively cooled, as this is known in the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a better understanding of the present invention same will be explained in the following based on the embodiments shown in the figures. Corresponding parts are given corresponding reference numerals and terms. Furthermore, those features or combinations of features which show or described different embodiments may form separate inventive solutions in themselves. The invention will now be described by way of example with reference to the drawings, wherein: 
           [0016]      FIG. 1  is a circuit diagram of a power module according to a first advantageous embodiment; 
           [0017]      FIG. 2  is a circuit diagram of a power module according to a second advantageous embodiment; 
           [0018]      FIG. 3  is a schematic perspective view of a power module with a transient current path according to a first embodiment; 
           [0019]      FIG. 4  is a circuit diagram of a power module system comprising two half bridges according to a first embodiment; 
           [0020]      FIG. 5  is a circuit diagram of a power module system comprising two half bridges according to a second embodiment; 
           [0021]      FIG. 6  is a schematic perspective view of a transient current path according to a second advantageous embodiment; 
           [0022]      FIG. 7  is a perspective view of a power module system comprising three power modules and forming a three-phase inverter bridge. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows a schematic circuit diagram of a power module  100  according to the present invention in a first embodiment. The power module  100  is structured as a half bridge comprising two insulated gate bipolar transistors, IGBT, connecting a DC input to a load L L . According to the present invention, the current flow in the power module  100  is separated into a transient path  102  with extremely low inductivity for the transient current during switching the high voltage and a main path  104  having a low resistance for the continuous current. The discrete elements L 1 , R 1  and L 2 , R 2  represent the distributed parasitic inductivities and resistances of the respective current paths. As shown in  FIG. 1 , the main current path  104  does not have a significant ohmic resistance, but only a measurable inductivity L 1  and L 2 . 
         [0024]    On the other hand, the inventive transient current path  102  exhibits a measurable ohmic resistance R 1 , R 2 , but is designed in a way that it is extremely low in inductivity. Although due to the ohmic resistance the transient current path would not be able to carry the thermal load of the continuous current, it is well capable of carrying the short term current of the switching transient current. 
         [0025]    The transient current path  102  having the low inductivity is only active during the short time when dl/dt is high at switch on and off of IGBT 1  and IGBT 2 , respectively. The connections for this low inductive transient current path therefore will not face the thermal load of the high current because of the short duration of current flow. 
         [0026]    As indicated by the connections  106 , the transient current paths  102  and the main current paths  104  can optionally be connected to each other. In case that the connections  106  are not present, a capacitor CT has to be provided for absorbing the energy of the transient current. 
         [0027]      FIG. 2  shows a further embodiment of the power module  100  wherein two diodes D 1  and D 2  are provided in the transient current path  102  for assigning only the switch-off current to the transient current path with low inductivity  102 . 
         [0028]      FIG. 3  shows in a schematic perspective view a first possibility of mechanically realizing the inventive transient current path  102 . 
         [0029]    As this is known to a person skilled in the art, power modules  100  are often designed with several DCB (direct copper bonding) substrates  108 ,  110 . These first and second substrates  108 ,  110  have to be connected to each other electrically by means of a conductive bridge. Normally, these bridges  112  represent an additional source of parasitic inductance. According to the present invention, however, the bridge  112  is formed in a way that it has minimized parasitic inductivity. According to the embodiment of  FIG. 3 , it is formed by two stamped and bent metal sheets. The two metal sheets  114 ,  116  are bent in a way that a first part of the sheet is connected to the first circuit substrate  108  and a second part is bent to be connected to the second substrate  110 . 
         [0030]    The two sheets  114 ,  116  establish in this way two main current paths  104 , which are leading in two directions. According to the present invention, via printed circuit board (PCB)  118  a further transient current path  102  is formed. The two metal sheets  114 ,  116  are arranged in parallel and are insulated from each other by means of a dielectric material  120 , so that the inductivity of the transient current path  102  is minimal. As shown schematically in  FIG. 3 , the dielectric material  120  for instance can be a foil preferably made from polyimide, which is wrapped around the metal sheet  116 . 
         [0031]    According to the present invention as shown in  FIG. 3 , the printed circuit board carries a transient current capacitor CT for absorbing the energy of the transient voltage spike in the transient current path  102 . This way of assembly allows an easy and efficient cooling of the capacitor CT. The connection to the printed circuit board  118  is formed by means of protrusions, in the following called “contact fingers”,  122  formed at the first and second metal sheets  114 ,  116 . 
         [0032]    According to this embodiment, the transient current  102  on the substrates  108 ,  110  is routed in a low inductive way over the printed circuit board  118  to the capacitor CT. The continuous current  104  is assigned to the external connection. The parasitic inductivity of the internal connection will not influence the transient current. 
         [0033]    The advantage of the present invention can be seen in the possibility to design modules for fast-switching, high-power applications without the effort of complex construction of the external connections. In high-power modules the external connection often is a screw connector with massive copper bars. A low-inductive connection here is expensive and with the inventive idea and the improvement of the anyway necessary bridges  112  between the substrate, a very cost-effective solution to the problem can be provided. Consequently, due to the lower switch-off losses the efficiency is increased and a higher DC voltage can be switched due to the reduced voltage overshot at switch off. 
         [0034]    The inventive solution can be used for power modules with screw connections, particularly with power modules having more than one substrate, which can be used in motor drives, uninterruptible power supplies (UPS), or solar inverters. 
         [0035]    For some applications, for instance zero voltage switching (ZVS), the same low inductive path right parallel to the switches is needed. For three level inverters for instance, three signals with low inductivity regarding to each other are necessary. Therefore, also a third conductive sheet (not shown in the figures) with a second insulation layer there between, can be provided. Again, in order to achieve a reliable electrically insulation between the conductive sheets the insulation foil can be wrapped around the metal sheets. 
         [0036]    The inventive concept which was explained above for a power module comprising two insulated gate bipolar transistors can also be extended to more complex power module systems which use a combination of two or more power modules  100 . 
         [0037]      FIG. 4  shows a first arrangement of two power modules, one comprising IGBT 1  and IGBT 2 , and the other comprising IGBT 3  and IGBT 4 . Again, transient current paths  102 , which possess a low inductivity and a measurable ohmic resistance are provided for carrying the transient current during switch-off. The inductivities represent the parasitic inductivities of the main current paths  104 . In  FIG. 4 , the inductivities L designate the parasitic inductances, whereas R signifies the parasitic resistances. 
         [0038]    With the power module system of  FIG. 4  a higher power can be switched by distributing same onto two modules comprising IGBT 1  and IGBT 2 , and IGBT 3  and IGBT 4 , respectively. 
         [0039]    Still another system constituted by two power modules according to the present invention is shown in  FIG. 5 . Here, the two power modules are connected in parallel to each other. Again, the inductivities and resistances designate the parasitic inductivities and the reference numerals  1 ,  2 ,  3  and  4  designate the screw connections of a four screw module. 
         [0040]      FIG. 6  shows in a perspective view an alternative embodiment of a connecting bridge  112 . According to this embodiment, the bridge element  112  is formed by a printed circuit board. The advantage of this arrangement can be seen in the fact that when for instance using a four-layer printed circuit board, relevant potentials can be carried layered above each other. For contacting the substrates  108 ,  110 , smaller metal bridges  124  are provided. Contact fingers  122  are formed on the bridge element  112  for contacting the printed circuit board  118 . These fingers  122  lead the respective potential on the outside and the corresponding voltage on the inner layers. In the remaining area of the bridge element  112  positive and negative voltages are layered above each other in an alternating way. For instance, the layers of a bridge element having four metal layers can be arranged as follows. 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Layers of the printed 
               
               
                   
                 circuit board (outside, 
               
               
                   
                 inside, inside, outside) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Finger 1: 
                 + − − + 
               
               
                   
                 Finger 2: 
                 − + + − 
               
               
                   
                 Rest of PCB: 
                 + − + − 
               
               
                   
                   
               
               
                   
                 (contact pads to the metal bridge 124 in correspondence to the fingers.) 
               
             
          
         
       
     
         [0041]    Consequently, the voltage potentials overlap almost completely. In the region of the contact fingers  122  the current carrying regions do not completely overlap in order to comply with the necessary security distances. Because always the respectively other voltage potential is opposite, a capacitor effect is generated and thus a transformer which is short-circuited at the output, wherein the inductivity is mostly compensated by means of added currents. 
         [0042]    The bridge element  112  can be formed by any known circuit carrier technique, such as printed circuit board, ceramic circuit carrier etc. The metal bridge  124  is preferably formed as a lead frame. Furthermore, the embodiment using a circuit carrier as the bridge element  112  has the advantage that additional capacitors  126  can be mounted thereon. 
         [0043]      FIG. 7  shows a perspective view of a three-level inverter using three power modules according to the present invention. In the embodiment of  FIG. 7  three modules are combined with one common printed circuit board  118 . The coupling of several modules generates the following advantages: Firstly, the residue inductivity from the screw connecting point to the external capacitor is to be divided by the number of terminals. In this case the already low inductivity is further reduced to one third. Secondly, the capacitors which are assembled on the printed circuit board  118  can be used by all three modules and consequently the performance is improved and the expenditure is reduced. 
         [0044]    Optionally, the printed circuit board can also be connected to the screw connections  128  so that all screw connections are commonly screwed and generate a low inductive path to the capacitors. In this case, the capacitors are of additional advantage but not compulsory for the functioning of the circuit. Coupled module systems like the one shown in  FIG. 7  can either be formed by coupling three half bridges with one common circuit board and one bridge element  112  each, wherein each module has the structure of  FIG. 1  or  FIG. 2 . Alternatively, also three NPC power modules, as shown in  FIGS. 4 and 5 , can be coupled as one power module system. 
         [0045]    One power module contains each the complete circuit of  FIG. 4  or  5 . The combination of these three modules is coupled via a common circuit board  118 . Such neutral point clamped (NPC) or three leveled topologies have the additional characteristic that in addition to the screw terminals of the three basic modules the terminal of the neutral point has to be led to the outside by means of one or more screwed clamps on the printed circuit board. Because these clamps are also connected with external capacitors, these terminals are led out on the same side as the positive and negative DC terminals. In  FIGS. 4 and 5  the neutral point is connected on the left-hand side, DC plus on the above side, DC minus on the lower side and AC output on the right-hand side with respect to the figure. 
         [0046]    Power module systems according to the present invention comprising at least two power modules can be used advantageously in a great variety of designs and configurations. According to a first advantageous embodiment, the transient current path of each power module ( 100 ) is connected with a common printed circuit board, PCB, or the transient current paths are connected in parallel. 
         [0047]    Furthermore, the system can be a three-phase inverter, wherein a three-phase three-level topology is used. As shown in  FIG. 5 , the low inductive path can be provided also between the output ( 3 / 4 ) and the neutral point (centre point between the capacitors). This design will improve also reactive switching. 
         [0048]    According to a still further advantageous embodiment, the common PCB is connected with the main path e.g. via an overlap of the PCB with the screw contacts ( 128 ).