Page 72 - Rashid, Power Electronics Handbook

4 Gate Turn-Off Thyristors 57

electric ®eld in the n-base region is grossly distorted due to the regenerative action commences, but the device does not latch
presence of the charge carriers and may result in premature on (remain on when the gate current is removed) until
avalanche breakdown. The resulting impact ionization can
cause device failure. This phenomenon is known as ‘‘dynamic a npn þ a pnp 1 ð4:2Þ
avalanche.'' The device regains its steady-state blocking char-
acteristics when the tail current diminishes to leakage current
This process takes only a short time for the current and the
level.
current gains to increase enough to satisfy Eq. (4.2). For
anode-shorted devices, the mechanism is similar but the
anode short impairs the turn-on process by providing a
4.3 GTO Thyristor Models base–emitter short, thus reducing the p-n-p transistor gain,
which is shown in Fig. 4.4. The composite p-n-p gain of the
emitter-shorted structure is given as follows:
A one-dimensional two-transistor GTO model is shown in
Fig. 4.3. The device is expected to yield the turn-off gain g
given by: a ðcompositeÞ¼ a 1 ÿ V be ð4:3Þ
pnp
pnp
R Sanode
I A a npn where V ¼ emitter base voltage (generally 0.6 V for injection
A ¼ ¼ ð4:1Þ be
g
I G a pnp þ a npn ÿ 1 of carriers) and R is the anode-short resistance. The anode
S
emitter injects when the voltage around it exceeds 0.06 V, and
therefore the collector current of the n-p-n transistor ¯owing
where I is the anode current and I the gate current at turn- through the anode shorts in¯uences turn-on. The GTO
G
A
off, and a npn and a pnp are the common-base current gains in remains in a transistor state if the load circuit limits the
the n-p-n and p-n-p transistor sections of the device. For a current through the shorts.
nonshorted device, the charge is drawn from the anode and
4.4 Static Characteristics
Anode A A
4.4.1 On-State Characteristics
In the on-state, the GTO operates in a similar manner to the
apnp
thyristor. If the anode current remains above the holding
current level then positive gate drive may be reduced to zero
Gate G G
anpn and the GTO will remain in conduction. However, as a result
of the turn-off ability of the GTO, it does possess a higher
holding current level than the standard thyristor and, in
C addition, the cathode of the GTO thyristor is subdivided
Cathode C into small ®nger elements to assist turn-off. Thus, if the
GTO thyristor anode current transiently dips below the hold-
FIGURE 4.3 Two-transistor model representing the GTO thyristor. ing current level, localized regions of the device may turn off,

FIGURE 4.4 Two-transistor models of GTO structures.