Page 74 - Rashid, Power Electronics Handbook

4 Gate Turn-Off Thyristors 59

FIGURE 4.9 A typical turn-on gate pulse (see the data sheet in
Reference 2). Courtesy of Westcode.

Turn-on: A GTO has a highly interdigited gate structure
with no regenerative gate. Thus it requires a large initial gate
trigger pulse. A typical turn-on gate pulse and its important
FIGURE 4.7 dv D =dt vs R GK (see the data sheet in Reference 1). GTO parameters are shown in Fig. 4.9. Minimum and maximum
gate characteristic information reproduced by kind permission of Dynex values of I can be derived from the device data sheet. A
GM
Semiconductor. value of di =dt positioned against turn-on time is given under
g
the device characteristics found on the data sheet [2]. The rate
of rise of gate current di =dt will affect the device turn-on
g
losses. The duration of the I pulse should not be less than
GM
half the minimum for time given in data sheet ratings. A
longer period will be required if the anode current di=dt is low
such that I GM is maintained until a suf®cient level of anode
current is established.
On-state: Once the GTO is turned on, forward gate current
must be continued for the entire conduction period. Other-
wise, the device will not remain in conduction during the on-
state period. If large negative di=dt or anode current reversal
occurs in the circuit during the on-state, then higher values of
I may be required. However, much lower values of I are
G
G
required when the device has heated up.
Turn-off: The turn-off performance of a GTO is greatly
in¯uenced by the characteristics of the gate turn-off circuit.
Thus the characteristics of the turn-off circuit must match
with the de-ice requirements. Fig. 4.10 shows the typical anode
and gate currents during the turn-off. The gate turn-off
process involves the extraction of the gate charge, the gate
avalanche period, and the anode current decay. The amount of
charge extraction is a device parameter and its value is not
affected signi®cantly by the external circuit conditions. The
FIGURE 4.8 GTO trigger characteristics (see the data sheet in Refer- initial peak turn-off current and turn-off time, which are
ence 1).
important parameters of the turning-off process, depend on
the external circuit components. The device data sheet gives
4.5 Switching Phases typical values for I GQ .
The turn-off circuit arrangement of a GTO is shown in
The switching process of a GTO thyristor goes through four Fig. 4.11. The turn-off current gain of a GTO is low, typically 6
operating phases: (a) turn-on; (b) on-state; (c) turn-off; and to 15. Thus, for a GTO with a turn-off gain of 10, it will
(d) off-state. require a turn-off gate current of 10 A to turn-off an on-state