This invention relates to a turn-off control cirucit for a gate turn-off thyristor (hereinbelow abbreviated to GTO) and in particular to a turn-off control circuit of a GTO, with which an inductive load is connected on the cathode side.
A GTO is a thyristor, which is turned-off by making electric current flow-in through its P gate (which is in contact with its P base layer) or by making electric current flow-out through its N gate (which is in contact with its N base layer) similarly to a usual thyristor, but can be turned-off also by making electric current flow-out through the P gate, differently from a usual thyristor. A turn-on control circuit of a GTO is constructed similarly to a usual thyristor and a turn-off control circuit thereof is so constructed that a transistor is connected between the P gate and the cathode of the GTO, as described in JP-A-59-14355.
In this type of the turn-off control circuit there are two connection positions of the transistor, depending on the selection between the GTO and the load, as indicated in FIG. 1a and FIG. 1b. A turn-on control circuit is omitted in these circuits.
In FIG. 1a, a load 3 is connected between the positive electrode of a power source 70 for load and an anode 11 disposed on a P emitter layer P.sub.E of GTO 1 and a cathode 12 disposed on an N emitter layer N.sub.E of the GTO 1 and the negative electrode of the power source 70 for load are connected with a ground potential line 10. Further, the collector and the emitter of the turning off transistor Q.sub.30 are connected between the P gate 13 disposed on the P base layer P.sub.E of the GTO 1 and the ground potential line 10, in parallel with a gate resistor 5, and a turning-off power source 8 is connected between the base of the turning-off transistor Q.sub.30 and the ground potential line 10.
The passage of the GTO 1 from its on state to its off state is performed by turning-on the turning-off transistor Q.sub.30 disposed between the P gate 13 disposed on the P base layer P.sub.B and the cathode 12 by means of a pulse from the turning-off power source 8. If the collector-emitter voltage V.sub.CE in the on state of the turning-off transistor Q.sub.30 satisfies EQU V.sub.CE &lt;V.sub.GK .apprxeq.0.6 [V] (1)
with respect to the potential between the P gate 13 and the cathode 12 in the on state of the GTO 1, it is possible to make electric current flow-out through the P gate 13. For this reason it is necessary that the turning-off transistor Q.sub.30 operates at its saturation. On the other hand, electric current through the anode 11, when the GTO 1 is in the on state, being I.sub.A, the maximum gate current I.sub.G flowing-out through the P gate 13 being I.sub.G, the following relation is valid; EQU turn-off gain.ident.I.sub.A /I.sub.G .apprxeq.3 to 5 (2)
The value of the formula (2) varies, depending on the construction of the GTO and process parameters. Since the gate current I.sub.G determined by the formula (2) is the collector current I.sub.C of the turning-off transistor Q.sub.30 and the turning-off transistor Q.sub.30 operates at its saturation, the collector-emitter saturation voltage V.sub.CES of the turning-off transistor Q.sub.30, when the gate current I.sub.G given by the formula (2) flows therethrough, should satisfy the following relation EQU V.sub.CES &lt;0.6 [V] (1a)
In the case where the turning-off transistor Q.sub.30 is fed with a sufficient base current, the collector-emitter saturation voltage V.sub.CES of the turning-off transistor Q.sub.30 is determined almost wholly by the collector resistance R.sub.C. Since the collector resistance R.sub.C is inversely proportional to the geometrical dimension of the transistor, in the case where the GTO 1 and its turning-on or turning-off driving circuit are integrated on a silicon substrate (chip), enlargement of the turning-off transistor Q.sub.30 causes increase in size of the chip.
For this reason, as disclosed in JP-A-59-14355, the cathode potential V.sub.K may be raised by inserting a diode or a resistor between the cathode 12 and the ground potential. In this case, since the collector-emitter saturation voltage V.sub.CES of the turning off transistor Q.sub.30 is given by; EQU V.sub.CES &lt;V.sub.GK+ V.sub.K .apprxeq.0.6+V.sub.K ( 3)
the turning-off transistor Q.sub.30 can be made smaller, corresponding to the rise of V.sub.K, by disposing the diode or the resistor.
In the circuit indicated in FIG. 1b, the load 3 is inserted between the cathode 22 of the GTO 2 and the ground potential line 10 and the gate resistor 5' is connected between the P gate 23 and the cathode 22. When the GTO 2 is in the off state, the cathode potential V.sub.K is nearly equal to the ground potential and when the GTO 2 is in the on state, the cathode potential V.sub.K is nearly equal to the voltage V.sub.CC of the power source 70 for load. When the GTO 2 is switched-over from the on state to the off state, the turning-off transistor Q.sub.40 is turned-on. At this time, the emitter-collector voltage V.sub.EC of the turning-off transistor Q.sub.40 varies approximately from the voltage V.sub.CC of the power source 70 for load to the saturation voltage V.sub.CES and the operation of this turning-off transistor Q.sub.40 varies from the active state to the saturation state.
In this example, too, owing to the fact that the turn-off gain given by Formula (2) remains same, when the GTO 2 begins to vary from the on state to the off state, since the turning-off transistor Q.sub.40 is in the active state, it is possible to take-out a sufficient gate current I.sub.G. However, as the GTO 2 approaches the off state, since the collector potential of the turning-off transistor Q.sub.40 approaches zero, the operation to take-out the gate current becomes same as that indicated in FIG. 1a. That is, the base bias current of the turning-off transistor Q.sub.40 being I.sub.B, the grounded emitter current amplification factor being h.sub.EF, the following inequality should be valid; EQU I.sub.G &lt;h.sub.EF .multidot.I.sub.E ( 5)
However, unless the load 3 is a constant current load, the anode current I.sub.A also decreases as the result of decrease of the potantial of the cathode 22 as the GTO 2 passes to the off state. Thus, it is not necessary that the collector internal resistance R.sub.C of the turning-off transistor Q.sub.40 is as high as that required in the case indicated in FIG. 1a and the geometrical size of the turning-off transistor Q.sub.40 can be made smaller correspondingly.
However, in the case where the load is an inductor, which is a winding of a motor, as indicated in FIG. 2, the GTOs 1 and 2 are connected with the winding L of the motor on both sides thereof. The winding L is connected with the anode 11 of the GTO 1 and a current detection resistor 7 is connected between the cathode 12 and the ground potential line 10. The collector and the emitter of the turning-off transistor Q.sub.30 are connected between the P gate 13 and the ground potential line 10 in parallel with a gate resistor 51 and a turning power source 81 is connected between the base and the ground potential line 10. On the other hand, the winding L is connected with the cathode 22 of the GTO 2 and a gate resistor 52 is connected between the P gate 23 and the cathode 22. The collector and the emitter of the turning-on transistor Q.sub.40 are connected between the P gate 23 and the ground potential line 10 and a turning off power source 82 is connected between the base and the ground potential line 10. The anode 21 of the GTO 2 is connected with the positive electrode of the power source 70 for load and the cathode 12 of the GTO 1 is connected with the negative electrode of the power source 70 for load through the ground potential line 10. A return diode 61 is connected between the cathode 22 of the GTO 2 and the ground potential line 10 and a return diode 62 is connected between the anode 11 of the GTO 1 and the positive electrode of the power source 70 for load through a Zener diode 63. The current detection resister 7 controls the pulse signal of the turning-off power source 82, the high voltage side of the current detection resistor 7 being connected with a chopper control circuit not shown in the figure. The turning-on control circuit is omitted in the figure.
In the construction described above the current control of the motor winding L is effected by on-off controlling the GTO 2, keeping the GTO 1 in the on state. At this time the turning-off power source 82 controls the GTO 2 through the turning-off transistor Q.sub.40, referring to the detection signal obtained by the current detection resistor 7, so that the current flowing through the motor winding L has a predetermined value.
When the GTO 2 is switched-off, current due to a voltage induced by the electro-magnetic energy stored in the motor winding L flows through a low voltage side terminal L-1 of the motor winding L.fwdarw.GTO 1.fwdarw.the current detection resistor 7.fwdarw.the ground potential line 10.fwdarw.the return diode 61.fwdarw.the high voltage side terminal L-2 of the motor winding L. When both the GTOs 1 and 2 are switched-off, the current due to the electro-magnetic energy stored in the motor winding L flows through the low voltage side terminal L-1 of the motor winding L.fwdarw.the return diode 62.fwdarw.the Zener diode 63.fwdarw.the power source 70.fwdarw.the ground potential line 10.fwdarw.the return diode 61.fwdarw.the high voltage side terminal of the motor winding L.
Now the behavior of the circuit, when the GTO 1 is in the on state and the GTO 2 passes from the on state to the off state, will be explained. At this time the turning-off power source 82 generates a turning-off pulse signal in order to keep the turning-off transistor Q.sub.40 in the on state and electric current is taken-out through the P gate 23 of the GTO 2. When the GTO 2 has been turned completely to the off state, the current due to the electro-magnetic energy stored in the motor winding L returns through the GTO 1. Thus, the current detection resister 7 and the return diode 61, the potential of the cathode 22 of the GTO 2 is -V.sub.BE with respect to the ground potential line 10, where V.sub.BE representes the forward voltage of the return diode 61. This voltage is transmitted to the collector of the turning-off transistor Q.sub.40 through two current paths, one being the gate resistor 52, the other being the P-N junction between the P gate 23 and the cathode 22 of the GTO 2. While a turning-off pulse signal is applied to the base of the turning-off transistor Q.sub.40, when the forward voltage -V.sub.BE described above is given to the collector of the turning-off transistor Q.sub.40, this works a as an inverted transistor and current flows from the emitter to the collector. Since this current flows in the direction to flow-in to the P gate 23 of the GTO 2, in this way the GTO 2 tends to be switched-over to the on state. However, when the GTO 2 has been switched-over to the on state, since the potential of the cathode 22 is raised in the positive direction and thus the turning-off transistor Q.sub.40 begins to take-out again current through the P gate 23, the GTO 2 cannot be switched-over to the on state. As the result, when the turning-off transistor Q.sub.40 works as an inverted transistor and current flows in the P gate 23 of the GTO 2, since the GTO 2 operates as an NPN transistor, whose collector is the N base layer N.sub.B, current flows through the motor winding L by this work of the GTO 2 as an NPN transistor, which produces unnecessary electric power consumption.
When the GTO 1 is switched-over to the off state, since such decrease in the cathode potential is not produced, there is not such inconvenience.
As explained above, in a prior art GTO driving circuit there was a problem that the operation of the turning-off transistor and the GTO is made unstable by influences of the voltage induced by the electromagnetic energy stored in the inductive load, when the GTO is turned-off by switching the turning-off transistor connected with the ground potential line and the P gate of the GTO, to which the inductive load is connected on the cathode side, to the on state, which causes unnecessary current flowing through the load.