AC System

figure 3.38 Per phase thyristor-controUed inductor (TCI) and thyristor-switched capacitor (TSC) system.

AC Line In

Batteries

Static Transfer Switch Pairs

Load

figure 3.39 Static transfer switch used in a UPS system

References

1. J. L. Hudgins, A review of modern power semiconductor electronic devices Microelectronics Journal 24: 41-54 (1993).

2. S. K. Ghandi, Semiconductor Power Devices - Physics of Operation and Fabrication Technology, John Wiley and Sons, New York, 1977, pp. 63-84.

3. B. J. Baliga, Power Semiconductor Devices, PWS Publishing, Boston, 1996, pp. 91-110.

4. B. Beker, J. L. Hudgins, J. Coronati, B. Gillett, and S. Shekhawat, Parasitic parameter extraction of PEBB module using VTB technology, , IEEE IAS Ann, Mtg. Rec, 467-471, Oct. 1997.

5. C. V. Godbold, V. A. Sankaran, and J. L. Hudgins, Thermal analysis of high power modules, IEEE Trans. PEI 12:1, 3-11 (1997).

6. J. L. Hudgins and W. M. Portnoy, High di/dt pulse switching of thyristors, IEEE Tran. PEI 2: 143-148 (1987).

7. S. M. Sze, Physics of Semiconductor Devices, 2nd ed., John Wiley and Sons, New York, 1984, pp. 140-147.

8. V. A. Sankaran, J. L. Hudgins, and W. M. Portnoy, Role of the amplif)Aing gate in the turn-on process of involute structure thyristors, IEEE Tran. PEI 5:2, 125-132 (1990).

9. S. Menhart, J. L. Hudgins, and W. M. Portnoy, The low temperature behavior of thyristors, IEEE Tran. ED 39: 1011-1013 (1992).

10. A. Herlet, The forward characteristic of silicon power rectifiers 12. P. R. Palmer and B. H. Stark, A PSPICE model of the DG-EST based on at high current densities Solid-State Electron. 11:8, 717-742 (1968). the ambipolar diffusion equation, lEEEPESCRec, 358-363, (1999).

11. J. L. Hudgins, C. V. Godbold, W. M. Portnoy, and O. M. Mueller, 13. C. L. Tsay, R. Fischl, J. Schwartzenberg, H. Kan, and J. Barrow, A Temperature effects on GTO characteristics, IEEE IAS Annual Mtg. high power circuit model for the gate turn off thyristor, IEEE IAS Rec, 1182-1186, Oct. 1994. Annual Mtg. Rec, 390-397, Oct. 1990.

Gate Turn-Off Thyristors

Muhammad H. Rashid, Ph.D.

UF/UWF Joint Program Electrical and Computer Engineering

University of West Florida 11000 University Parkway Pensacola, FL 32514-5754, USA

4.1 4.2 4.3 4.4

4.5 4.6 4.7

Introduction........................................................................................ 55

Basic Structure and Operation................................................................ 56

GTO Thyristor Models.......................................................................... 57

Static Characteristics............................................................................. 57

4.4.1 On-State Characteristics 4.4.2 Off-State Characteristics 4.4.3 Rate of Rise of Off-State Voltage {dvj/dt) 4AA Gate Triggering Characteristics

Switching Phases.................................................................................. 59

Spice GTO Model................................................................................ 60

Applications........................................................................................ 61

References........................................................................................... 61

4.1 Introduction

A gate turn-off thyristor (known as a GTO) is a three-terminal power semiconductor device that belongs to a thyristor family with a four-layer structure. They also belong to a group of power semiconductor devices that have the ability to fully control on and off states via the control terminal (gate). The design, development, and operation of the GTO is easier to understand if we compare it to the conventional thyristor. Like a conventional thyristor, applying a positive gate signal to its gate terminal can turn on a GTO. Unlike a standard thyristor, a GTO is designed to turn off by applying a negative gate signal.

There are two types of GTOs: asymmetrical and symmetrical. The asymmetrical GTOs are the most common type on the market. This type is normally used with an antiparallel diode and hence high reverse-bioeking capability is not available. Reverse conducting is accomplished with an antiparallel diode integrated onto the same silicon wafer. The symmetrical GTOs have equal forward- and reverse-blocking capability.

4.2 Basic Structure and Operation

The symbol for a GTO is shown in Fig. 4.1a. A high degree of interdigitation is required in GTOs in order to achieve efficient turn-off. The most common design employs the cathode area separated into multiple segments (cathode fingers) and arranged in concentric rings around the device center. The internal structure is shown in Fig. 4.1b. A common contact disk pressed against the cathode fingers connects the fingers together. It is important that all the fingers turn off simulta-

neously, otherwise the current may be concentrated into fewer fingers, with damage due to overheating more likely.

The high level of gate interdigitation also results in a fast turn-on speed and high di/dt performance of GTOs. The most remote part of a cathode region is no more than 0.16 mm from a gate edge and hence the entire GTO can conduct within 5 is with sufficient gate drive and the turn-on losses can be reduced. Fiowever, interdigitation reduces the available emitter area and therefore the low-frequency average current rating is less than for a standard thyristor with an equivalent diameter.

The basic structure of a GTO, a four-layer p-n-p-n semiconductor device, is very similar in construction to a thyristor. It has several design features that allow it to be turned on and off by reversing the polarity of the gate signal. The most important differences are that the GTO has long narrow emitter fingers surrounded by gate electrodes and no cathode shorts.

The turn-on mode is similar to that of a standard thyristor. The injection of the hole current from the gate forward biases the cathode p-base junction, causing electron emission from the cathode. These electrons flow to the anode and induce hole injection by the anode emitter. The injection of holes and electrons into the base regions continues until charge multiplication effects bring the GTO into conduction. This is shown in Fig. 4.2a. As with a conventional thyristor, only the area of cathode adjacent to the gate electrode is turned on initially and the remaining area is brought into conduction by plasma spreading. Fiowever, unlike the thyristor, the GTO consists of many narrow cathode elements, heavily interdigitated with the gate electrode, and therefore the initial turned-on area is very large and the time required for plasma spreading is small.

Copyright © 2001 by Academic Press.

All rights of reproduction in any form reserved.

Cathode

Cathode emitter

Anode

(a) (b)

FIGURE 4.1 GTO structure: (a) GTO symbol; (b) GTO structure.

Therefore, the GTO is brought into conduction very rapidly and can withstand a high turn-on di/dt.

In order to turn off a GTO, the gate is reversed-biased with respect to the cathode and holes from the anode are extracted from the p-base. This is shown in Fig. 4.2b. As a result, a voltage drop is developed in the p-base region, which eventually reverse biases the gate cathode junction and cuts off the injection of electrons. As the hole extraction continues, the p-base is further depleted, thereby squeezing the remaining conduction area. The anode current then flows through the areas most remote from the gate contacts, forming high current density filaments. This is the most crucial phase of the turn-off process in GTOs because high-density filaments lead to localized heating, which can cause device failure unless

these filaments are extinguished quickly. An application of higher negative gate voltage may aid in extinguishing the filaments rapidly. However, the breakdown voltage of the gate-cathode junction hmits this method.

When the excess carrier concentration is low enough for carrier multiplication to cease and the device reverts to the forward blocking condition. Although the cathode current has stopped flowing at this point, anode-to-gate current supplied by the carriers from an n-base region-stored charge continues to flow. This is observed as a tail current that decays exponentially as the remaining charge concentration is reduced by a recombination process. The presence of this tail current with the combination of high GTO off-state voltage produces substantial power losses. During this transition period, the

TURN-ON

Electron; Holes

TURN-OFF

Electron; Holes

(a) (b)

FIGURE 4.2 (a) Turn-on; and (b) turn-off of GTOs.

electric field in the n-base region is grossly distorted due to the presence of the charge carriers and may result in premature avalanche breakdown. The resulting impact ionization can cause device failure. This phenomenon is known as dynamic avalanche. The device regains its steady-state blocking characteristics when the tail current diminishes to leakage current level.

4.3 GTO Thyristor Models

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:

regenerative action commences, but the device does not latch on (remain on when the gate current is removed) until

(4.2)

This process takes only a short time for the current and the 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 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:

ap p(composite) = a.

l-Vr

Sanode

(4.3)

(4.1)

where /4 is the anode current and Iq the gate current at turn-off, and a p and (Xpp are the common-base current gains in the n-p-n and p-n-p transistor sections of the device. For a nonshorted device, the charge is drawn from the anode and

Anode

Gate

Cathode

figure 4.3 Two-transistor model representing the GTO thyristor.

where V = emitter base voltage (generally 0.6 V for injection of carriers) and Rg is the anode-short resistance. The anode emitter injects when the voltage around it exceeds 0.06 V, and therefore the collector current of the n-p-n transistor flowing through the anode shorts influences turn-on. The GTO remains in a transistor state if the load circuit limits the current through the shorts.

4.4 Static Characteristics 4.4.1 On-State Characteristics

In the on-state, the GTO operates in a similar manner to the thyristor. If the anode current remains above the holding current level then positive gate drive may be reduced to zero and the GTO will remain in conduction. Fiowever, 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 addition, the cathode of the GTO thyristor is subdivided into small finger elements to assist turn-off. Thus, if the GTO thyristor anode current transiently dips below the holding current level, localized regions of the device may turn off.

SYMMETRICAL GTO STRUCTURE

P- base

<

>

ASYMMETRICAL GTO STRUCTURE

P- base

P N+ P

Anode shorted area

figure 4.4 Two-transistor models of GTO structures.

thus forcing a high anode current back into the GTO at a high rate of rise of anode current after this partial turn-off. This situation could be potentially destructive. Therefore, it is recommended that the positive gate drive not be removed during conduction but held at a value /g(on) where /g(on) is greater than the maximum critical trigger current (Iq) over the expected operating temperature range of the GTO thyristor.

Figure 4.5 shows the typical on-state i;-/characteristics for a 4000-A, 4500-V GTO from the Dynex range of GTOs [1] at junction temperatures of 25 and 125 °C. The curves can be approximated to a straight line of the form:

V =Vo + IRo

(4.4)

where Vq = voltage intercept and it models the voltage across the cathode and anode forward-biased junctions, and Rq =on-state resistance. When average and RMS values of on-state current (/tav trms) known, then the on-state power dissipation Pqn be determined using Vq o- That is.

Pqn - VqItav + RqItrms

(4.5)

figure 4.6 GTO blocking voltage vs Rq (see the data sheet in Reference 1). GTO gate characteristic information reproduced by kind permission of Dynex Semiconductor.

14.4.2 Off-State Characteristics

Unlike the standard thyristor, the GTO does not include cathode emitter shorts to prevent nongated turn-on effects due to dv/dt-induced forward-biased leakage current. In the off-state of the GTO, steps should therefore be taken to prevent such potentially dangerous triggering. This can be accomphshed by either connecting the recommended value of resistance between gate and cathode (Rq) or maintaining a small reverse bias on the gate contact (Vr = -2 V). This will prevent the cathode emitter from becoming forward-biased and will therefore sustain the GTO thyristor in the off state.

4000

Measured under pulse conditions. Igioni = lOA Half sine wave 10 ms

3000

Tj= 25°C

The peak off-state voltage is a function of resistance Rq. This is shown in Fig. 4.6. Under ordinary operating conditions, GTOs are biased with a negative gate voltage of -15 V supplied from the gate drive unit during the off-state interval. Nevertheless, provision of be a desirable design practice in the event the gate-drive failure for any reason {Rq < 1.5Q is recommended for a large GTO). Here Rq dissipates energy and hence adds to the system losses.

4.4.3 Rate of Rise of Off-State Voltage {dvp/dt)

The rate of rise of off-state voltage {dvj/dt) depends on the resistance Rq connected between the gate and the cathode and the reverse bias applied between the gate and the cathode. This relationship is shown in Fig. 4.7.

О

1 2000

О

S 1000

0

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Instantaneous on-state voltage Yjy -(V)

figure 4.5 V-I Characteristics of GTO (see the data sheet in Reference 1). GTO gate characteristic information reproduced by kind permission of Dynex Semiconductor.

1.4.4 Gate Triggering Characteristics

The gate trigger current (Iqj) and the gate trigger voltage (Vqj) are both dependent on junction temperature Tj as shown in Fig. 4.8. During the conduction state of the GTO a certain value of gate current must be supplied and this value should be larger than the Iq at the lowest junction temperature at which the GTO operates. In dynamic conditions the specified Iqj is not sufficient to trigger the GTO switching from higher voltage and high di/ dt. In practice, a much higher peak gate current (on the order of 10 times Iqj) at Tj min is recommended to obtain good turn-on performance.

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

figure 4.7 dv/dt vs Rq (see the data sheet in Reference 1). GTO gate characteristic information reproduced by kind permission of Dynex Semiconductor.

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 parameters are shown in Fig. 4.9. Minimum and maximum values of can be derived from the device data sheet. A value of dig/dt positioned against turn-on time is given under the device characteristics found on the data sheet [2]. The rate of rise of gate current di/dt will affect the device turn-on losses. The duration of the pulse should not be less than 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 /(3m maintained until a sufficient level of anode current is established.

figure 4.8

ence 1).

GTO trigger characteristics (see the data sheet in Refer-

4.5 Switching Phases

The switching process of a GTO thyristor goes through four operating phases: (a) turn-on; (b) on-state; (c) turn-off; and (d) off-state.

On-state: Once the GTO is turned on, forward gate current must be continued for the entire conduction period. Otherwise, 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 Iq may be required. Fiowever, much lower values of Iq are required when the device has heated up.

Turn-off: The turn-off performance of a GTO is greatly influenced 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 significantly by the external circuit conditions. The initial peak turn-off current and turn-off time, which are important parameters of the turning-off process, depend on the external circuit components. The device data sheet gives typical values for Iqq.

The turn-off circuit arrangement of a GTO is shown in Fig. 4.11. The turn-off current gain of a GTO is low, typically 6 to 15. Thus, for a GTO with a turn-off gain of 10, it will require a turn-off gate current of 10 A to turn-off an on-state

figure 4.10 Anode and gate currents during turn-off (see the data sheet in Reference 2). Courtesy of Westcode.

figure 4.12 Gate-cathode resistance, Rq (see the data sheet in Reference 2). Courtesy of Westcode.

of 100 A. A charged capacitor С is normally used to provide the required turn-off gate current. Inductor L limits the turn-off di/dt of the gate current through the circuit formed by Rl, R2, SWi, and L. The gate circuit supply voltage

should be selected to give the required value of Vqq. values of Ri and R2 should also be minimized.

Off-state period: During the off-state period, which begins after the fall of the tail current to zero, the gate should ideally remain reverse-biased. This reverse bias ensures maximum blocking capability and dv/dt rejection. The reverse bias can be obtained either by keeping SW closed during the whole off-state period or via a higher impedance circuit SW2 and R provided a minimum negative voltage exits. This higher impedance circuit SW2 and R must sink the gate leakage current.

In case of a failure of the auxiliary supphes for the gate turn-off circuit, the gate may be in reverse-biased condition and the GTO may not be able to block the voltage. To ensure that the blocking voltage of the device is maintained, a minimum gate-cathode resistance (icK) should be applied as shown in Fig. 4.12. The value of Rq for a given Ипе voltage can be derived from the data sheet.

figure 4.11 Turn-off circuit (see the data sheet in Reference 2). Courtesy of Westcode.

4.6 SPICE GTO Model

A GTO may be modeled with two transistors as shown in Fig. 4.3. However, a GTO model [3] consisting of two thyristors, which are connected in parallel, yield improved on-state, turn-on and turn-off characteristics. This is shown in Fig. 4.13 with four transistors.

When the anode to cathode voltage Уд is positive and there is no gate voltage, the GTO model will be in the off state like a standard thyristor. If a positive voltage (V) is apphed to the anode with respect to the cathode and no gate pulse is apphed, I =/2 = 0 and, therefore, /i = C2 = - Thus, no anode current will flow and 1 = 1. = 0.

When a small voltage is applied to the gate, then 12 is nonzero and, therefore, both /i = C2 = nonzero. Thus the internal circuit will conduct and there will be a current flow from the anode to the cathode.

When a negative gate pulse is applied to the GTO model, the p-n-p junction near to the cathode will behave as a diode.

Anode

R2 10 Q

Cathode

figure 4.13 Four-transistor GTO model. Courtesy of Westcode.

The diode will be reverse biased because the gate voltage is 4.7 Applications

negative to the cathode. Therefore, the GTO will stop conduc- -

tion.

When the anode-to-cathode voltage is negative, that is, the anode voltage is negative with respect to the cathode, the GTO model will act like a reverse-biased diode. This is because the p-n-p transistor will see a negative voltage at the emitter and the n-p-n transistor will see a positive voltage at the emitter. Therefore both transistors will be in the off state and hence the GTO will not conduct. The SPICE subcircuit description of the GTO model will be as follows:

Gate turn-off thyristors have many applications, including motor drives, induction heating [4], distribution lines [5], pulsed power [6], and flexible ac transmission systems [7].

Acknowledgment

The author would like to thank Mr Dinesh Chamund, Principal Engineer for Applications, Dynex Semiconductor, Doddington Road, Lincoln LN6 3LF, United Kingdom, who was invited to write this chapter and could not complete it due to his work assignment

References

1. Dynex Semiconductor: Data GTO data-sheets: web-site: http: www.dynexsemi.com/products/proddata.htm

2. Westcode Semiconductor: Data GTO data-sheets: web-site: http: www.westcode.com/ws-gto.html

3. El-Amin, I. M. A GTO PSPICE model and its apphcations, The Fourth Saudi Engineering Conference, November 1995, Vol. Ill, pp. 271-77.

4. Busatto, G., lannuzzo, R, and FrateUi, L. PSPICE model for GTOs,

Proceedings of Symposium on Power Electronics Electrical Drives. Advanced Machine Power Quality. SPEEDAM Conference. Sorrento, Italy; 3-5 June 1998, Vol. 1, pp. 2/5-10.

5. Malesani, L., and Tenti, P. Medium-frequency GTO inverter for induction heating apphcations. Second European Conference on Power Electronics and Applications: EPE. Proceedings, Grenoble, France; 22-24 Sept. 1987, Vol. 1, pp. 271-276.

6. Souza, L. R W., Watanabe, E. H., and Aredes, M. A. GTO controlled series capacitor for distribution lines. International Conference on large High Voltage Electric Systems. CIGRE98, 1998.

7. Chamund, D. J. Characterisation of 3.3 kV asymmetrical thyristor for pulsed power application, IEEE Symposium Pulsed Power 2000 (Digest No. 00/053) pp. 35/1-4, London, UK, 3-4 May 2000.

8. Moore, P. and Ashmole, P. Flexible AC transmission systems: 4. Advanced FACTS controllers. Power Engineering lournal 12:2, 95-100, 1998.