Circuit arrangement for reducing the recovery time of a thyristor comprising R-C-D network between auxiliary and main emitters

Circuit arrangement for reducing recovery time of a thyristor formed with a semiconductor body having at least one emitter zone contacted by an emitter electrode and having at least one auxiliary emitter zone and a base zone contacted by an auxiliary emitter electrode, includes a shunt between the emitter zone and the base zone, a capacitor disposed in a line mutually connecting the emitter electrode and the auxiliary emitter electrode, the emitter electrode being the cathode and being disposed on the surface of the semiconductor body and electrically connected to the emitter zone, the shunt between the emitter zone and the base zone being formed through the capacitor, a series connection of a resistance and a diode being connected in parallel with the capacitor, the diode having a lower threshold voltage than that of an imputed diode formed by the cathode-sided emitter zone and the base zone of the thyristor, the diode of the series connection being poled with respect to the cathodic emitter electrode in the same sense as is the imputed diode of the thyristor.

The invention relates to a circuit arrangement for reducing recovery time 
of a thyristor formed with a semiconductor body having at least one 
emitter zone contacted by an emitter electrode and having at least one 
auxiliary emitter zone and a base zone contacted by an auxiliary emitter 
electrode, with a shunt between the emitter zone and the base zone, as 
well as with a capacitor connected between the emitter electrode and the 
auxiliary emitter electrode. 
By the recovery time of a thyristor, there is meant the time between the 
zero crossing of the load current and the restoration of the blocking 
ability of the thyristor. The recovery time can be reduced considerably by 
the incorporation of recombination centers. Excessive doping with 
recombination centers, however, causes a great increase in forward losses. 
In the hereinaforementioned circuit, the recovery time is shortened as a 
result of the capacitor being charged negatively relative to the cathode 
by the reverse current flowing in the thyristor when the latter is 
switched off. Upon the recurrence of the voltage appearing in the blocking 
direction of the thyristor, the current produced by the charge carriers 
that remain stored in the semiconductor body thereby flows off through the 
auxiliary emitter electrode. This results in a reduction of the recovery 
time. The effectiveness of the heretofore known circuit arrangements of 
this general type is limited due to the fact that the capacitor is 
previously discharged through the base-emitter shunts of the thyristor 
before the blocking voltage returns. 
It is accordingly an object of the invention of the instant application to 
provide an improved circuit arrangement of the foregoing general type 
wherein the capacitor can discharge only when the blocking voltage 
returns. 
With the foregoing and other objects in view, there is provided, in 
accordance with the invention, circuit arrangement for reducing recovery 
time of a thyristor formed with semiconductor body and having at least one 
emitter zone contacted by an emitter electrode and having at least one 
auxiliary emitter zone and a base zone contacted by an auxiliary emitter 
electrode, comprising a shunt between the emitter zone and the base zone, 
a capacitor disposed in a line mutually connecting the emitter electrode 
and the auxiliary emitter electrode, the emitter electrode being the 
cathode and being disposed on the surface of the semiconductor body and 
electrically connected to the emitter zone, the shunt between the emitter 
zone and the base zone being formed through the capacitor, a series 
connection of a resistance and a diode being connected in parallel with 
the capacitor, the diode having a lower threshold voltage than that of an 
imputed diode formed by the cathode-sided emitter zone and the base zone 
of the thyristor, the diode of the series connection being poled with 
respect to the cathodic emitter electrode in the same sense as is the 
imputed diode of the thyristor. 
In accordance with another feature of the invention, the diode of the 
series connection is a Schottky diode. 
In accordance with a further feature of the invention the circuit 
arrangement includes at least one Zener diode connected in parallel with 
the capacitor, the Zener diode being poled with respect to the cathodic 
emitter electrode in the same sense as is the diode of the series 
connection. 
In accordance with an added feature of the invention the circuit 
arrangement includes a primary winding of a current transformer with a 
saturation characteristic design in the anode-cathode circuit of the 
thyristor, the current transformer having a secondary winding connected 
through another diode to the capacitor, the othe diode being poled in a 
manner that the capacitor is negatively charged by a reverse current 
flowing through the thyristor. 
In accordance with a concomitant feature of the invention the circuit 
arrangement includes a R-C component connected in parallel with the 
thyristor, a primary winding of a current transformer with a saturation 
characteristic connected in series with the R-C component, the current 
transformer having a secondary winding connected through another diode to 
the capacitor, the other diode being poled in a manner that the capacitor 
is negatively charged by a reverse current flowing through the thyristor. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
circuit arrangement for reducing the recovery time of a thyristor, it is 
nevertheless not intended to be limited to the details shown, since 
various modifications and structural changes may be made therein without 
departing from the spirit of the invention and within the scope and range 
of equivalents of the claims.

Referring now to the drawing and first, particularly, to FIG. 1, thereof, 
part of the semiconductor body of a thyristor 1 is diagrammatically shown. 
It has an emitter zone 2 on the cathode side, an auxiliary emitter zone 3, 
and a base zone 4 also on the cathode side. Other zones of the thyristor 
body 1 have been omitted in the interest of greater viewing clarity. The 
emitter zone 2 is connected to an emitter electrode 5. The emitter 
electrode 5 contacts the surface of the semiconductor body only at and 
within the confines of the emitter zone 2 thereof. The p-n junction 8 
between the emitter zone 2 and the base zone 400 is therefore not 
short-circuited by the emitter electrode 5 at the surface of the 
semiconductor body. The auxiliary emitter zone 3 is contacted by an 
auxiliary emitter electrode 6, 15, which is also electrically connected to 
the base zone. The base zone 4 is provided with a control electrode 7 
directly in contact with the surface thereof. Control current is fed to 
the control electrode 7 through a transformer 9 having a secondary winding 
which is connected to the auxiliary emitter electrode 6 and the control 
electrode 7. A capacitor 10 is connected between and to the emitter 
electrode 5 and the auxiliary emitter electrode 6 and is shunted by a 
resistor 11 in series with a Schottky diode 12. Instead of the Schottky 
diode, a different diode can also be used, provided it has a threshold 
voltage which is lower than that of the diode formed by the zones 2 and 4 
of the thyristor. A germanium diode can be used, for example, instead of a 
Schottky diode if the thyristor 1 is formed of silicon. The diode 12 is 
poled relative to the cathode in the same sense as the diode formed by the 
zones 2 and 4 of the thyristor 1. One or more Zener diodes 13 and 14 can 
be connected in parallel with the capacitor 10. The sum of the threshold 
voltages of the Zener diodes 13 and 14 must be greater than the threshold 
voltage of the diode formed by the zones 2 and 4 of the thyristor 1. The 
Zener voltage thereof must be lower than the breakdown voltage of the p-n 
junction 8, i.e. lower than 10 V, for example. 
To explain the operation of the circuit arrangement of the invention 
according to the embodiment of FIG. 1, reference is made to the plot 
diagrams of FIGS. 2 and 3. These, respectively, show the cathode current i 
and the anode-cathode voltage U, on the other hand, and the auxiliary 
terminal current i.sub.H and the voltage U.sub.c at the capacitor 10, on 
the other hand. It will be assumed, first, that the thyristor is in 
conducting state. Thus, a current i determined by the driving voltage and 
the resistance in the load circuit flows from the anode to the cathode. A 
small positive forward voltage U exists across the thyristor. When the 
current i is commutated away at the end of a half-wave, that current i, as 
well as the voltage U, goes through zero and assumes negative values, as 
shown at A in FIG. 3. The negative current (reverse current) results from 
the charge carriers stored in the semiconductor body. Since transition of 
the emitter p-n junction 8 on the cathode side occurs into the cut-off 
state, the current flows through the auxiliary emitter electrode 6, 15 as 
indicated by the broken-line arrows and charges the capacitor 10 
negatively relative to the emitter electrode 5, i.e. relative to the 
cathode. Since the p-n junction 8 is not short-circuited by the emitter 
electrode 5, the reverse current initially flows exclusively into the 
capacitor 10 until the break-down voltage of the p-n junction under the 
emitter 2 is reached. The capacitor 10 is charged negatively with respect 
to the cathode. The maximal capacitor voltage is determined by the Zener 
diodes 13, 14. Discharge of the capacitor 10 to a value below the Zener 
voltage is prevented by the p-n junction 8 which is biased in cut-off 
direction. 
When positive voltage (blocking voltage) sets in, the current is likewise 
reversed. This is characterized by a positive current path B after a zero 
crossing of the voltage, as shown in FIG. 3. The current path is 
represented in FIG. 1 by the aforementioned broken line arrows. This 
current stems from as yet nonrecombined charge carriers that are stored in 
the semiconductor body. This current could fire the thyristor if it did 
not flow into the capacitor 10. The thyristor does not fire as long as the 
voltage at the p-n junction 8 remains below 0.3 . . . 0.5 V, depending 
upon the temperature. This voltage is determined by the shunt resistivity 
of the base zone 4, the width of the emitter zone 2 and the current 
i.sub.H. The more negatively the capacitor is charged, the more the 
recovery time of the thyristor is shortened, since a larger current can 
accordingly flow before firing occurs. 
When the thyristor is fired through the control electrode 7, the load 
current of the auxiliary thyristor containing the auxiliary emitter zone 3 
flows initially into the capacitor 10 until a voltage is reached at the 
auxiliary emitter electrode 6 which is higher than the threshold voltage 
of the diode formed by zones 2 and 4 of the thyristor. If this voltage at 
the auxiliary emitter electrode 6 is, for example, about 0.5 V, than the 
capacitor 10 is charged to about 0.5 V relative to the cathode i.e. the 
emitter electrode 5. In the conduction state of the thyristor, no current 
flows any longer into the capacitor 10. Current flows to the cathode only 
through the resistor 11 and the diode 12. This current can be set or 
adjusted by appropriate choice of the resistor 11. The current through the 
resistor 11 and the diode 12 can be equal to or also smaller than that 
current which, in the case of thyristors with conventional emitter-base 
shunts, would flow operationally through the shunts. 
Beside recovery time, breakover resistance and du/dt limit are further 
characteristics of a thyristor. For the action of the circuit arrangement 
with respect to the breakover resistance, it is assumed that the capacitor 
10 is charged positively relative to the emitter electrode 5. The cut-off 
current of the thyristor flows to the cathode through the resistor 11 and 
the diode 12. The voltage drop across the resistor 11 and the diode 12 
must then remain smaller than the threshold voltage of the thyristor diode 
2, 4, or, for example, 0.3 V. A current of 100 mA, for example, can flow 
without any emission by the emitter zone 2. In that case, the thyristor 
does not, therefore, fire. The function of conventional emitter short 
circuits is fundamentally assumed by the resistor 11 and the diode 12. 
To explain the du/dt limit, a starting situation will be assumed wherein 
the capacitor 10 is charged negatively relative to the emitter electrode 5 
or is discharged. The pulse-shaped displacement current which occurs if a 
voltage with a steep slope is applied, flows into the capacitor 10 and 
through the resistor 11 and the diode 12 to the cathode. The elements must 
be of such dimensions and construction, also in this case, that no voltage 
drop larger than about 0.3 V occurs. This can also be aided by making the 
emitter zone 2 narrow. In addition, the auxiliary emitter electrode 6, 15, 
can be subdivided and a part 15 thereof can be disposed at a side 3. Part 
of the current then flows through the part 15 of the auxiliary emitter 
electrode 6, 15, so that the voltage drop below the emitter zone 2 is 
reduced. In thyristors with a subdivided emitter zone, part of the 
auxiliary emitter electrode is then advantageously situated on both sides 
of all emitter zone parts and, thus, also at the outer edge of the 
semiconductor body. The function of the conventional emitter short 
circuits is, in this case, assumed fundamentally by the capacitor. 
In the circuit arrangement according to FIG. 4, the capacitor 10 is charged 
by the reverse current through the secondary winding of a current 
transformer 18 with a saturation characteristic. The primary winding of 
the current transformer 18 is connected into the anode-cathode circuit of 
the thyristor 1. The turns ratio can be 1:1, for example. Between the 
secondary winding of the current transformer 18 and the capacitor 10, 
there is disposed a diode 19 which prevents the capacitor 10 from being 
discharged through the secondary winding. A diode 22 is connected in 
parallel with the secondary winding of the current transformer 18 and is 
traversed by current when the thyristor carries current in forward 
direction. If this diode 22 is not provided, then the diode 19 must have a 
high cut-off capability. The primary winding of the transformer 18 could 
also be connected into the anode lead 16 instead of into the cathode lead 
17. The lead for the control electrode and the lead for the auxiliary 
emitter are, respectively, identified by the reference numerals 20 and 21 
of the emitter zone 2 which faces away from the auxiliary emitter 
component of the circuit arrangement shown in FIG. 4, are identified by 
the same reference characters as those of the corresponding components in 
FIG. 1. 
In the circuit arrangement according to FIG. 5, the primary winding of the 
current transformer 18 is connected in series with an R-C component 23, 
24. The thus-formed series circuit is shunted across the anode-cathode 
path of the thyristor 1. This manner of coupling the primary winding is 
advantageous in complex systems, wherein the anode-cathode circuit is not 
readily accessible. 
With the hereinaforedescribed circuit arrangement, a reduction in the 
recovery time can be achieved. The formation of a shunt by means of 
elements or components located outside the thyristor results in the 
emitter per se having no shunt at all. Thus, the firing process can spread 
unimpededly over the emitter area. 
For a diameter of the semiconductor body of 50 mm, for example, the 
elements or components of the circuit arrangement according to the 
invention, can have the following dimensions: 
EQU Capacitor 10=40 .mu.F; resistor 11=1 ohm. 
The size of both of the latter components should be matched somewhat 
proportionally to the wafer areas. 
The Schottky diode 12 should have a forward voltage of .ltoreq.0.2 V for a 
currrent intensity of 1 A. The areal resistance or resistivity below the 
emitter zone 2 can be between 50 and 500 ohms per square. The width of the 
emitter zone 2 should be between 1 and 5 mm i.e. the emitter can be 
constructed, for example, as a finger structure.