Page 70 - Rashid, Power Electronics Handbook

4

Gate Turn-Off Thyristors

Muhammad H. Rashid, Ph.D. 4.1 Introduction........................................................................................ 55
UF=UWF Joint Program 4.2 Basic Structure and Operation................................................................ 56
Electrical and Computer
Engineering 4.3 GTO Thyristor Models.......................................................................... 57
University of West Florida 4.4 Static Characteristics............................................................................. 57
11000 University Parkway 4.4.1 On-State Characteristics 4.4.2 Off-State Characteristics 4.4.3 Rate of Rise of
Pensacola, FL 32514-5754, Off-State Voltage (dv T =dt) 4.4.4 Gate Triggering Characteristics
USA
4.5 Switching Phases .................................................................................. 59
4.6 Spice GTO Model ................................................................................ 60
4.7 Applications ........................................................................................ 61
References ........................................................................................... 61

4.1 Introduction neously, otherwise the current may be concentrated into fewer
®ngers, with damage due to overheating more likely.
A gate turn-off thyristor (known as a GTO) is a three-terminal The high level of gate interdigitation also results in a fast
power semiconductor device that belongs to a thyristor family turn-on speed and high di=dt performance of GTOs. The most
with a four-layer structure. They also belong to a group of power remote part of a cathode region is no more than 0.16 mm from
semiconductor devices that have the ability to fully control on a gate edge and hence the entire GTO can conduct within
and off states via the control terminal (gate). The design, 5 ms with suf®cient gate drive and the turn-on losses can be
development, and operation of the GTO is easier to understand reduced. However, interdigitation reduces the available emitter
if we compare it to the conventional thyristor. Like a conven- area and therefore the low-frequency average current rating is
tional thyristor, applying a positive gate signal to its gate less than for a standard thyristor with an equivalent diameter.
terminal can turn on a GTO. Unlike a standard thyristor, a The basic structure of a GTO, a four-layer p-n-p-n semi-
GTO is designed to turn off by applying a negative gate signal. conductor device, is very similar in construction to a thyristor.
There are two types of GTOs: asymmetrical and symme- It has several design features that allow it to be turned on and
trical. The asymmetrical GTOs are the most common type on off by reversing the polarity of the gate signal. The most
the market. This type is normally used with an antiparallel important differences are that the GTO has long narrow
diode and hence high reverse-blocking capability is not avail- emitter ®ngers surrounded by gate electrodes and no cathode
able. Reverse conducting is accomplished with an antiparallel shorts.
diode integrated onto the same silicon wafer. The symmetrical The turn-on mode is similar to that of a standard thyristor.
GTOs have equal forward- and reverse-blocking capability. The injection of the hole current from the gate forward biases
the cathode p-base junction, causing electron emission from
the cathode. These electrons ¯ow to the anode and induce hole
4.2 Basic Structure and Operation injection by the anode emitter. The injection of holes and
electrons into the base regions continues until charge multi-
The symbol for a GTO is shown in Fig. 4.1a. A high degree of plication effects bring the GTO into conduction. This is shown
interdigitation is required in GTOs in order to achieve ef®cient in Fig. 4.2a. As with a conventional thyristor, only the area of
turn-off. The most common design employs the cathode area cathode adjacent to the gate electrode is turned on initially and
separated into multiple segments (cathode ®ngers) and the remaining area is brought into conduction by plasma
arranged in concentric rings around the device center. The spreading. However, unlike the thyristor, the GTO consists of
internal structure is shown in Fig. 4.1b. A common contact many narrow cathode elements, heavily interdigitated with the
disk pressed against the cathode ®ngers connects the ®ngers gate electrode, and therefore the initial turned-on area is very
together. It is important that all the ®ngers turn off simulta- large and the time required for plasma spreading is small.

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