Abstract:
A current source inverter ( 1 ) comprises a bridge circuit ( 2 ) having a plurality of bridge paths ( 5 ), in each of which a controllable power semiconductor component ( 6 ) is arranged. 
     In the case of such an inverter, a loss in performance on account of inductance-dictated overvoltages during the turn-off of the power semiconductor components ( 6 ) is avoided by virtue of the fact that the controllable power semiconductor component ( 6 ) is avalanche-proof at least under loading in the forward direction.

Description:
FIELD ON THE INVENTION 
     The present invention concerns the field of power electronics. It relates to a reverse conducting GCT ( G ate  C ommutated  T hyristor) in accordance with the preamble of claim  1 , and also to an application of such a GCT. 
     BACKGROUND OF THE INVENTION 
     For applications in current source inverters, as are disclosed in U.S. Pat. No. 4,545,002, for example, reverse blocking power semiconductor components such as e.g. thyristors, GTOs or IGCTs ( I ntegrated  G ate Commutated  T hyristors) are used (with regard to an explanation of the function and the construction of IGCTs, reference is made e.g. to an article by Harold M. Stillman, “IGCTs—megawatt power switches for medium-voltage applications”, ABB Review 3 (1997)). In this case, it is possible to use both symmetrical components and a series circuit comprising an asymmetrical component and a diode. The problem in application is that a large overvoltage is induced by the commutation inductance during the turn-off into a positive voltage (in the case of GTOs and IGCTs). However, the commutation inductance can only be influenced to a limited extent since it is manifested by leakage inductances. 
     In the application, therefore, the overvoltage must either be controlled by the semiconductor component used, or be reduced by massive external circuitry. Both approaches lead to losses which limit the performance of the power converter. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention, therefore, to provide a current source inverter which does not have the abovementioned disadvantages of known current source inverters and, in particular, whose performance is not impaired by the overvoltages produced during turn-off, and also to specify a power semiconductor component for use in such a current source inverter. 
     The object is achieved by means of the totality of the features of claims  1  and  3 . The heart of the invention consists in using, in the bridge paths of the inverter, a power semiconductor component which is avalanche-proof at least under loading in the forward direction. This obviates, in particular, the need for providing additional circuit measures in the inverter. A series circuit comprising a reverse conducting GCT ( G ate  C ommutated  T hyristor) and a diode is preferably used. 
     In the reverse conducting GCT, in which a GCT section and a reverse-connected parallel diode section are integrated beside one another in a semiconductor substrate, the avalanche strength is achieved by virtue of the fact that the diode section is designed in such a way that its avalanche voltage is lower than the blocking voltage of the GCT section. 
     In accordance with a first preferred refinement of the GCT according to the invention, the diode section comprises, one above the other, a cathode emitter layer, a first base layer, and a first anode emitter layer, and the avalanche voltage or blocking voltage of the diode section is reduced by virtue of the cathode emitter layer being driven in a recessed manner into the semiconductor substrate or the first base layer. 
     Another preferred refinement is characterized in that the diode section comprises, one above the other, a cathode emitter layer, a first base layer, and a first anode emitter layer, and in that the avalanche voltage or blocking voltage of the diode section is reduced by virtue of the first anode emitter layer being driven in a recessed manner into the semiconductor substrate or the first base layer. 
     A further refinement of the GCT according to the invention is distinguished by the fact that the diode section comprises, one above the other, a cathode emitter layer, a first base layer, and a first anode emitter layer, and that the avalanche voltage or blocking voltage of the diode section is reduced by virtue of a reduction of the resistivity of the first base layer, the reduction of the resistivity of the first base layer preferably being effected by neutron irradiation. 
     Finally, it is also possible to effect the avalanche voltage or blocking voltage of the diode section by a reduction in the thickness of the semiconductor substrate in the region of the diode section. 
     Further embodiments emerge from the dependent claims. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention will be explained in more detail below using exemplary embodiments in connection with the drawing, in which 
     FIG. 1 shows the circuit diagram of a current source inverter with a series circuit comprising a GCT and a diode in the bridge paths; 
     FIG. 2 shows the section through a reverse conducting GCT according to the prior art; 
     FIG. 3 shows a first exemplary embodiment of a reverse conducting GCT according to the invention with a recessed cathode emitter layer in the diode section; 
     FIG. 4 shows a second exemplary embodiment of a reverse conducting GCT according to the invention with a recessed anode emitter layer in the diode section; 
     FIG. 5 shows a third exemplary embodiment of a reverse conducting GCT according to the invention with a reduced resistivity in the base layer of the diode section; and 
     FIG. 6 shows a fourth exemplary embodiment of a reverse conducting GCT according to the invention with a reduced thickness of the semiconductor substrate in the diode section. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A simplified circuit diagram of a current source inverter  1  is reproduced in FIG.  1 . The current source inverter  1  comprises a bridge circuit  2  (three-phase bridge circuit) having individual bridge paths  5 , in which controllable power semiconductor components are arranged, which are driven in a suitable manner by a controller (not illustrated) in such a way that an AC voltage of the desired frequency is present at the AC output  8  of the inverter. On the input side, the bridge circuit  2  receives a DC current from an intermediate circuit with corresponding intermediate circuit inductances  3 ,  4 . At the AC output, a plurality of capacitors  9  are arranged in the manner illustrated. 
     According to the invention, reverse conducting GCTs  6  are now provided as controllable power semiconductor components in the bridge paths  5 , said GCTs each being connected in series with a diode  7  and being designed in such a way that they are avalancheproof under loading in the forward direction. In this case, the component defined by its internal structure is generally designated as GCT, while the IGCT described in the introduction is a GCT with gate driving integrated in a special way. 
     The internal structure of a conventional reverse conducting GCT is illustrated in section in FIG. 2, in which case—as in the further FIGS. 3 to  6  as well—hatching of the different regions and layers has largely been dispensed with. The known GCT  10  comprises a GCT section GT and a diode section DT, which are integrated beside one another in a semiconductor substrate  11 . The GCT section GT concentrically surrounds the diode section DT arranged in the center and has, at its outer edge, a peripheral edge termination  22  characterized by an edge bevel  18 . It is also possible for the diode section DT to surround the GCT section. The diode section DT constitutes a reverse-connected parallel diode which is reverse-biased during operation of the GCT in the forward direction. The diode section DT or the diode has, in the reverse direction, a specific avalanche voltage or blocking voltage in the case of which, when said voltage is exceeded, the avalanche breakdown commences. The GCT section GT or the GCT likewise has a blocking voltage which corresponds to the blocking voltage of the edge termination  22 . 
     The GCT section GT and the diode section DT each have a specific sequence of layers or regions of different conductivity and charge carrier concentration which have been produced for example by mask indiffusion of impurities into the semiconductor substrate  11 . In the diode section DT, the following are arranged (from the cathode to the anode) one above the other: an n + -doped cathode emitter layer  16 , an n-doped first base layer  19 ′ and a p-doped first anode emitter layer  13 . In the GCT section GT, the following are arranged (from the anode to the cathode) one above the other: a p + -doped second anode emitter layer  15 , an n-doped stop layer  14 , an n − -doped second base layer  19 , a p-doped third base layer  12  and, distributed thereon, individual n + -doped cathode emitters  17 . In this case, the second base layer  19  and the first base layer  19 ′ are part of the same n + -doped base material of the semiconductor substrate  11 . The third base layer  12  of the GCT section GT and the anode emitter layer  13  of the diode section DT are indiffused into the semiconductor substrate  11  to approximately the same depth. The stop layer  14  of the GCT section GT also extends through the diode section DT and covers the cathode emitter layer  16  therein. 
     Proceeding from this known structure of the reverse conducting GCT, according to the invention the avalanche voltage of the integrated diode (in the diode section DT) is now reduced by means of suitable measures to an extent such that it lies below the blocking voltage of the edge termination  22  of the GCT. This can specifically be done in different ways, as will be explained below with reference to FIGS. 3 to  6 : 
     In the case of the reverse conducting GCT  20  in accordance with FIG. 3, the reduction of the avalanche voltage or blocking voltage of the diode is achieved by virtue of the fact that in the diode section, the cathode emitter layer  16 ′ is driven more deeply, in particular distinctly beyond the stop layer  14 , into the semiconductor substrate  11  or the first base layer  19 ′. 
     In the case of the reverse conducting GCT  30  in accordance with FIG. 4, the reduction of the avalanche voltage or blocking voltage of the diode is achieved by virtue of the fact that the first anode emitter layer  13 ′ in the diode section is driven more deeply than the third base layer  12  of the GCT into the semiconductor substrate  11  or the first base layer  19 ′. 
     In the case of the reverse conducting GCT  40  in accordance with FIG. 5, the reduction of the avalanche voltage or blocking voltage of the diode is achieved by virtue of the fact that the resistivity is reduced in the first base layer  19 ′ in the diode section. This is preferably achieved by a radiation region  21  being produced through subsequent neutron irradiation in this layer. 
     Finally, in the case of the reverse conducting GCT  50  in accordance with FIG. 6, the reduction of the avalanche voltage or blocking voltage of the diode is achieved by virtue of the fact that in the diode section, the thickness of the semiconductor substrate  11  is reduced to a reduced thickness D r  relative to the GCT section. 
     LIST OF REFERENCE SYMBOLS 
       1  Current source inverter 
       2  Bridge circuit 
       3 , 4  Intermediate circuit inductance 
       5  Bridge path 
       6  GCT 
       7  Diode 
       8  Ac output 
       9  capacitor 
       10 , 20 , 30 , 40 , 50  gct (reverse conducting) 
       11  semiconductor substrate 
       12  base layer (p-doped) 
       13 , 13 ′ anode emitter layer (diode; p-doped) 
       14  stop layer (n-doped) 
       15  anode emitter layer (gct; p + -doped) 
       16 , 16 ′ cathode emitter layer (diode; n + -doped) 
       17  cathode emitter (gct; n + -doped) 
       18  edge bevel 
       19 , 19 ′ base layer (n − -doped) 
       21  irradiation region 
       22  edge termination 
     gct section 
     DE diode section