Configuration for reducing the turn-off time of a thyristor

The present invention is directed to a thyristor having a configuration for educing turn-off time, said thyristor comprises a main thyristor portion having a main emitter electrode and a control electrode and an auxiliary thyristor portion having an auxiliary emitter electrode and adapted for amplifying the control current, the arrangement includes a voltage source connected to the auxiliary thyristor, and supplying the auxiliary emitter electrode with a current the direction of which is opposite to the direction of the control current. PAC BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of discrete semiconductor devices and is particularly directed to discrete thyristors. 2. Description of the Prior Art The "turn-off time" of a thyristor is the time between the zero passage of a load current and the moment when the thyristor has regained its blocking capability. It is known that the turn-off time of a thyristor can be decreased considerably by provision of recombination centers. However, excessive doping to provide the recombination centers results in considerable increase in power dissipation in the forward direction. It is also known that the turn-off time of a thyristor can be reduced by supplying the control electrode with a current opposite in direction to the control current, thereby upon recovery of the positive potential, causing part of the charge carriers stored in the semiconductor body to be drawn off and thus made unavailable for effecting an undesirable firing. However, the known techniques for reducing turn-off time of a thyristor require a considerable number of electronic components because both control impluses of one polarity and impulses of the opposite polarity must be produced by means of one and the same impulse source. PAC SUMMARY OF THE INVENTION The present invention comprises a thyristor having a reduced turn-off time comprising a main thyristor having a main emitter electrode and a control electrode and an auxiliary thyristor, said auxiliary thyristor being adapted to amplify a control current, said auxiliary thyristor having an auxiliary emitter electrode, a voltage source electrically connected to the auxiliary thyristor to supply through said auxiliary emitter of said auxiliary thyristor a current opposite in direction to the control current .

DESCRIPTION OF PREFERRED EMBODIMENTS 
The known method for reducing the turn-off time of a thyristor consists in 
principle of a voltage source which can provide impulses of both 
polarities. 
With reference to FIG. 1 which shows a thyristor for the sake of clarity 
the power source is not shown. The power source is connected, through an 
electrode 8, to p-base zone 2 of a thyristor body consisting of zones 1, 
2, 3, and 4, and, on the other hand, is connected to auxiliary emitter 5 
through a diode 10 and an auxiliary emitter electrode 7. The n-emitter 1 
and the p-emitter 4 are provided with an emitter electrode 6 and with an 
electrode 9, respectively. 
For the purpose of explaining the operation, let it be assumed that charge 
carriers are stored in the semiconductor body from the previous conductive 
mode and that a steeply rising positive potential is applied to the 
thyristor. For example, let the emitter electrode 6 be at zero potential, 
and the electrode 9 at a positive potential with respect to emitter 1. 
Under these conditions, a part of the charge carriers stored in the 
semiconductor body will flow to the emitter electrode 6 in a direction 
parallel to the pn-junction between emitter 1 and base 2. This current is 
indicated by the boken-line arrows. If, at any point along the 
pn-junction, the firing potential of about 0.5 V is reached, the main 
thyristor comprising the zones 1, 2, 3, 4, can fire even without a control 
impulse applied to control electrode 8. This means that the turn-off time 
of the thyristor is too low for the operating frequency for which it is 
used. If a negative impulse is applied to the auxiliary emitter electrode 
7 through the diode 10, a large portion of the charge carriers stored in 
the semiconductor body will flow to the auxiliary emitter electrode 7, and 
only a correspondingly smaller portion will fow under the pn-junction 
between zones 1 and 2 to the main emitter electrode 6. This current flow 
pattern is illustrated by the solid lines. At the moment when the negative 
impulse on the auxiliary emitter electrode 7 is strong enough, the current 
provided by the stored charge carriers flowing to the main emitter 
electrode 6 will not, in this case be sufficient to fire the thyristor. 
A first embodiment of the invention is illustrated in FIG. 2, in which for 
the sake of clarity, the larger portion of the semiconductor body is not 
shown. Components of the thyristor which correspond to components shown in 
FIG. 1 are provided with the same reference symbols as are corresponding 
components in FIGS. 3 to 5. The current source for reducing the turn-off 
time is connected between the main emitter electrode 6 and the auxiliary 
emitter electrode 7. It consists of a condenser C, and a resistance 
R.sub.1 having a diode D.sub.1 connected in parallel therewith. The 
polarity of the diode D.sub.1 is such that the condenser is charged by a 
reverse current which flows in the blocking direction upon commutation of 
the thyristor. The diode D.sub.1 should have a low forward voltage drop. 
It may be a Schottky diode for example. The resistance R.sub.1 is selected 
to be smaller than the resistance between the auxiliary emitter electrode 
7 and the emitter electrode 6 in zone 2 of the semiconductor body, which 
last mentioned resistance is indicated in broken lines at R.sub.2 within 
the semiconductor body. 
The circuit must fulfill several conditions: 
1. The resistance R.sub.1 must have a value to ensure that there will not 
be too much current flow into the discharged condenser C when the 
auxiliary thyristor is turned on. 
2. The discharge time constant (R.sub.1 + R.sub.2) C must be large enough 
to ensure that there will still be a potential on the condenser at the end 
of the turn-off time. On the other hand, the arrangement is more 
effective, with regard to reducing the turn-off time, the smaller the 
resistance R.sub.1. The resistance R.sub.2 is determined by the component 
and may be 2 Ohms for example. Resistance R.sub.1 should be smaller than 
R.sub.2 and may have a value of 0.5 Ohm, for example. With a charge Q 
flowing upon commutation into the condenser, the potential on the 
condenser is U.sub.o = Q/C. Towards the end of the turn-off time, the 
residual voltage should be as high as possible. This is attained if, for a 
turn-off time t.sub.q, the resistance R.sub.1 and the capacitance C are 
selected such that (R.sub.1 + R.sub.2)C = t.sub.q. If the thyristor has a 
turn-off time of 10.mu.s, the condenser should have a value of 4.mu.F in 
this example. 
With a load applied in the blocking direction, the current indicated by 
solid line arrows will flow from the semiconductor body for the most part 
through the auxiliary emitter electrode 7 and the diode D.sub.1. The 
condenser C is charged by the reverse current i.sub.R to have the polarity 
indicated. A small part of the current will flow also to the emitter 
electrode 6. 
In the embodiment of FIG. 2, the control circuit is not connected between 
the control electrode and the main emitter electrode, but is connected 
between the control electrode and the auxiliary emitter electrode is 
interconnected through the secondary winding of a transformer 110. This is 
necessary to prevent the condenser C from discharging through the control 
circuit. In general the auxiliary thyristor requires no negative bias 
voltage because in most cases it remains turned on for a considerably 
shorter period than the main thyristor so that the stored carriers have 
more time for recombination. 
A modification of the embodiment of FIG. 2 is illustrated in FIG. 3. The 
two arrangements differ from each other essentially only in that the 
control circuit in FIG. 3 is connected to the main emitter electrode. A 
discharge of the condenser C through the control circuit is prevented in 
this case by means of the diodes D.sub.2 which will pass the positive 
control pulse. The number of diodes D.sub.2 must be such that the sum of 
the threshold potentials (about 0.5 V per diode) exceeds the maximum 
potential on the condenser C. 
FIG. 4 shows an arrangement in which the current source between the 
auxiliary emitter electrode and the main emitter electrode is formed by a 
battery 11 and a resistance R.sub.1 connected electrically in series 
relationship. In this embodiment the auxiliary thyristor is constantly 
biased, the bias being a function of the power loss which may occur in the 
main thyristor, as indicated by the resistance R.sub.2. The maximum 
voltage of the battery 11 is determined by the blocking potential of the 
pn-junction between the emitter 1 and the base 2, which lies at about 20 V 
in the usual silicon thyristors. The effectiveness of the device is the 
greater the smaller the sum of the resistance R.sub.1 and the cross 
resistance R.sub.2 under the n-emitter 1. For example, the battery may 
have a voltage of -5 V, and the value of the resistance R.sub.1 may be 0.5 
Ohm. 
FIG. 5 shows an arrangement in which the auxiliary thyristor is also 
provided with a constant negative bias. In addition to the battery 11, a 
further source of constant potential comprising a battery 12 is connected 
between the main emitter electrode 6 and the control electrode 8. In this 
case, the voltage of battery 12 must be higher than that of battery 11, a 
potential difference of 0.5 V being sufficient. In this case, for example, 
battery 11 may provide -5 V, and the battery 12 may provide -5.5 V. In 
general the resistance R.sub.4 in the semiconductor body between the 
electrodes 6 and 8 is considerably higher than R.sub.2. Therefore, it will 
be possible in most cases to omit the current limiting resistance R.sub.3. 
With the arrangements of FIGS. 2 and 3, in the given parameters the 
turn-off time is reduced about 20%. The other circuit configurations, with 
the given voltages and sizes, will reduce the turn-off time over 30%. This 
will suffice in many cases in which the use of such electronically complex 
arrangements as herebefore employed for this purpose of reducing turn-off 
time would not be deemed justified. 
The circuit configurations in accordance with the invention preferably will 
be used wherever the simplicity of the circuit is considered more 
important than the maximum possible decrease of turn-off time.