Motor Control

6.9.2 Motor Control

A three-phase controlled bridge circuit used as a basic Another important application of thyristors is in motor control topology for many converter systems.

FIGURE 6.32

circuits. Historically thyristors have been used heavily in trac- tion, although most new designs are now based on IGBTs. Such motor control circuits broadly fall into four types: i) chopper

Starting from this basic configuration, it is possible to con- control of a dc motor from a dc supply; ii) single- or three- struct more complex circuits in order to obtain high-voltage phase converter control of a dc motor from an ac supply; iii) or high-current outputs, or just to reduce the output ripple by inverter control of an ac synchronous or induction machine constructing a multi-phase converter. One of the most impor- from a dc supply and iv) cycloconverter control of an ac tant systems using the topology shown in Fig. 6.32 as a basic machine from an ac supply. An example of a GTO chopper

circuit is the HVDC system represented in Fig. 6.33. This sys- is given in Fig. 6.35. L 1 ,R 1 ,D 1 , and C 1 are the turn-on snub- tem is made by two converters, a transmission line, and two ac ber; R 2 ,D 2 , and C 2 are the turn-off snubber; finally R 3 and systems. Each converter terminal is made of two poles. Each D 3 form the snubber for the freewheel diode D 3 . A thyristor pole is made by two six-pulse line-frequency converters con- cycloconverter is shown in Fig. 6.36; the waveforms show the nected through -Y and Y-Y transformers in order to obtain a fundamental component of the output voltage for one phase. twelve-pulse converter and a reduced output ripple. The filters Three double converters are used to produce a three-phase

Converter #1

Converter #2

12-pulse Converter for Positive Line

Filter Y

AC Power AC Power Grid #1

Grid #2

Filter

12-pulse Converter for Negative Line

FIGURE 6.33

A HVDC transmission system.

114 A. Bryant et al.

Interphase reactor

AC Grid

Load

AC Motor

3- φ

FIGURE 6.34 Parallel connection of two six-pulse converters for high

Line

current applications.

FIGURE 6.36 Cycloconverter for control of large ac machines.

C 3 use IGBTs. Low-power motor controllers generally use IGBT

Motor

inverters.

R 3 D 3 In motor control, thyristors are also used in CSI topologies. −

When the motor is controlled by a CSI, a controlled rectifier is also needed on the input side. Figure 6.38 shows a typical

FIGURE 6.35 GTO chopper for dc motor control. CSI inverter. The capacitors are needed to force the current in the thyristors to zero at each switching event. This is not needed when using GTOs. This inverter topology does not need any additional circuitry to provide the regenerative brak-

variable-frequency, variable voltage sinusoidal output for driv- ing (energy recovery when slowing the motor). Historically, ing ac motors. However, the limited frequency range (less than two back-to-back connected line-frequency thyristor convert-

a third of the line frequency) restricts the application to large, ers have been employed to allow bi-directional power flow, and low-speed machines at high powers.

thus regenerative braking. Use of anti-parallel GTOs with sym-

A single- or three-phase thyristor converter (controlled metric blocking capability, or the use of diodes in series with rectifier) may be used to provide a variable dc supply for con- each asymmetric GTO, reduces the number of power devices trolling a dc motor. Such a converter may also be used as the needed, but greatly increases the control complexity. front end of a three-phase induction motor drive. The variable voltage, variable frequency motor drive requires a dc supply, which is supplied by the thyristor converter. The drive may

6.9.3 VAR Compensators and Static Switching

use a square-wave or PWM voltage source inverter (VSI), or a

Systems

current source inverter (CSI). Figure 6.37 shows a square-wave or PWM VSI with a controlled rectifier on the input side. The Thyristors are also used to switch capacitors or inductors switch block inverter may be made of thyristors (usually GTOs in order to control the reactive power in the system. Such or IGCTs) for high power, although most new designs now arrangements may also be used in phase-balancing circuits for

6 Thyristors 115

AC 1- φ

AC Output,

or 3- φ,

Variable Voltage and

50 or

Variable Frequency

60 Hz

Filter

DC Induction

FIGURE 6.37 PWM or square-wave inverter with a controlled rectifier input.

L r i r the gate pulse of all thyristors in the circuit. The problem of this topology is the voltage across the capacitors at the thyristor turn-off. At turn-on the thyristor must be gated at the instant of the maximum ac voltage to avoid large over-currents. Many

recent SVCs have used GTOs.

Induction

motor

A similar application of thyristors is in solid-state fault cur-

i a rent limiters and circuit breakers. In normal operation, the thyristors are continuously gated. However, under fault condi- tions they are switched rapidly to increase the series impedance in the load and to limit the fault current. Key advantages are the flexibility of the current limiting, which is independent of

FIGURE 6.38 CSI on the output section of a motor drive system using the location of the fault and the change in load impedance, the

capacitors for power factor correction. reduction in fault level of the supply, and a smaller voltage sag during a short-circuit fault.

A less important application of thyristors is as a static trans- balancing the load fed from a three-phase supply. Examples fer switch, used to improve the reliability of uninterruptible of these circuits are shown in Fig. 6.39. These circuits act as power supplies (UPS) as shown in Fig. 6.40. There are two static VAR controllers. The topology represented on the left of modes of using the thyristors. The first leaves the load perma- Fig. 6.39 is called a thyristor-controlled inductor (TCI) and it nently connected to the UPS system and in case of emergency acts as a variable inductor where the inductive VAR supplied disconnects the load from the UPS and connects it directly to can be varied quickly. Because the system may require either the power line. The second mode is opposite to the first one. inductive or capacitive VAR compensation, it is possible to con- Under normal conditions the load is permanently connected nect a bank of capacitors in parallel with a TCI. The topology to the power line, and in event of a line outage, the load is shown on the right of Fig. 6.39 is called a thyristor-switched disconnected from the power line and connected to the UPS capacitor (TSC). Capacitors can be switched out by blocking system.

AC System

V AC L

FIGURE 6.39 Per phase TCI and TSC system.

116 A. Bryant et al.

AC Line In Load

Batteries Static Transfer

Rectifier

Inverter

Switch Pairs

FIGURE 6.40 Static transfer switch used in an UPS system.

Lamp 4. B. Beker, J.L. Hudgins, J. Coronati, B. Gillett, and S. Shekhawat, “Parasitic parameter extraction of PEBB module using VTB tech- nology,” IEEE IAS Ann. Mtg. Rec., pp. 467–471, Oct. 1997.

5. C.V. Godbold, V.A. Sankaran, and J.L. Hudgins, “Thermal analysis

Filter

MT2 Triac

of high power modules,” IEEE Trans. PEL, vol. 12, no. 1, pp. 3–11,

AC MT1

Jan. 1997.

Supply G 6. J.L. Hudgins and W.M. Portnoy, “High di/dt pulse switching of

Diac

thyristors,” IEEE Tran. PEL, vol. 2, pp. 143–148, April 1987. C 7. S.M. Sze, Physics of Semiconductor Devices, 2nd ed., New York, John Wiley and Sons, 1984, pp. 140–147.

8. V.A. Sankaran, J.L. Hudgins, and W.M. Portnoy, “Role of the amplify- FIGURE 6.41 Basic dimmer circuit used in lighting control.

ing gate in the turn-on process of involute structure thyristors,” IEEE Tran. PEL, vol. 5, no. 2, pp. 125–132, April 1990.

9. S. Menhart, J.L. Hudgins, and W.M. Portnoy, “The low temperature

6.9.4 Lighting Control Circuits

behavior of thyristors,” IEEE Tran. ED, vol. 39, pp. 1011–1013, April 1992.

An important circuit used in lighting control is the dimmer, 10. A. Herlet, “The forward characteristic of silicon power rectifiers based on a triac and shown in Fig. 6.41. The R–C network

at high current densities,” Solid-State Electron., vol. 11, no. 8, applies a phase shift to the gate voltage, delaying the triggering

pp. 717–742, 1968.

of the triac. Varying the resistance, controls the firing angle of 11. J.L. Hudgins, C.V. Godbold, W.M. Portnoy, and O.M. Mueller, “Tem- the triac and therefore the voltage across the load. The diac

perature effects on GTO characteristics,” IEEE IAS Annual Mtg. Rec., is used to provide symmetrical triggering for the positive and

pp. 1182–1186, Oct. 1994.

negative half-cycles, due to the non-symmetrical nature of the 12. P.R. Palmer and B.H. Stark, “A PSPICE model of the DG-EST based triac. This ensures symmetrical waveforms and elimination of

on the ambipolar diffusion equation,” IEEE PESC Rec., pp. 358–363, even harmonics. An L–C filter is often used to reduce any June 1999. remaining harmonics. 13. C.L. Tsay, R. Fischl, J. Schwartzenberg, H. Kan, and J. Barrow, “A high

power circuit model for the gate turn off thyristor,” IEEE IAS Annual Mtg. Rec., pp. 390–397, Oct. 1990.

Further Reading 14. K.J. Tseng and P.R. Palmer, “Mathematical model of gate-turn-off

thyristor for use in circuit simulations,” IEE Proc.-Electr. Power Appl., vol. 141, no. 6, pp. 284–292, Nov. 1994.

1. J.L. Hudgins, “A review of modern power semiconductor electronic 15. X. Wang, A. Caiafa, J. Hudgins, and E. Santi, “Temperature devices,” Microelectronics Journal, vol. 24, pp. 41–54, Jan. 1993.

effects on IGCT performance,” IEEE IAS Annual Mtg. Rec., 2. S.K. Ghandi, Semiconductor Power Devices – Physics of Operation and

Oct. 2003.

Fabrication Technology, New York, John Wiley and Sons, 1977, pp. 63– 16. X. Wang, A. Caiafa, J.L. Hudgins, E. Santi, and P.R. Palmer, “Imple- 84.

mentation and validation of a physics-based circuit model for IGCT 3. B.J. Baliga, Power Semiconductor Devices, Boston, PWS Publishing,

with full temperature dependencies,” IEEE PESC Rec., pp. 597–603, 1996, pp. 91–110.

June 2004.

Gate Turn-off Thyristors

Muhammad H.

7.1 Introduction .......................................................................................... 117

Rashid, Ph.D.

7.2 Basic Structure and Operation................................................................... 117

Electrical and Computer

7.3 GTO Thyristor Models............................................................................. 118

Engineering, University of West Florida, 11000 University

7.4 Static Characteristics ............................................................................... 119

Parkway, Pensacola, 7.4.1 On-state Characteristics • 7.4.2 Off-state Characteristics • 7.4.3 Rate of Rise of Off-state Florida 32514-5754, USA

Voltage (dv T /dt ) • 7.4.4 Gate Triggering Characteristics

7.5 Switching Phases..................................................................................... 120

7.6 SPICE GTO Model.................................................................................. 122

7.7 Applications ........................................................................................... 123 References ............................................................................................. 123

7.1 Introduction

separated into multiple segments (cathode fingers) arranged in concentric rings around the device center. The internal

A gate turn-off thyristor (known as a GTO) is a three terminal structure is shown in Fig. 7.1b. A common contact disc pressed power semiconductor device. GTOs belong to a thyristor against the cathode fingers connects the fingers together. family having a four-layer structure. GTOs also belong to a It is important that all the fingers turns off simultaneously, group of power semiconductor devices that have the ability for otherwise the current may be concentrated into a fewer fingers full control of on- and off-states via the control terminal (gate). which are likely to be damaged due to over heating. To fully understand the design, development and operation of

The high level of gate interdigitation also results in a fast the GTO, it is easier to compare with the conventional thyris- turn-on speed and a high di/dt performance of the GTOs. tor. Like a conventional thyristor, applying a positive gate signal The most remote part of a cathode region is not more than to its gate terminal can turn-on to a GTO. Unlike a standard

0.16 mm from a gate edge and hence the whole GTO can thyristor, a GTO is designed to turn-off by applying a negative conduct within about 5 µs with sufficient gate drive and the gate signal.

turn-on losses can be reduced. However, the interdigitation GTOs are of two types: asymmetrical and symmetrical. The reduces the available emitter area so the low frequency aver- asymmetrical GTOs are the most common type on the market. age current rating is less than for a standard thyristor with an This type of GTOs is normally used with a anti-parallel diode equivalent diameter. and hence high reverse blocking capability is not available.

The basic structure of a GTO consists of a four-layer-PNPN The reverse conducting is accomplished with an anti-parallel semiconductor device, which is very similar in construction to diode integrated onto the same silicon wafer. The symmetri-

a thyristor. It has several design features which allow it to be cal type of GTOs has an equal forward and reverse blocking turned on and off by reversing the polarity of the gate signal. capability.

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 a standard thyristor. The

7.2 Basic Structure and Operation

injection of the hole current from the gate forward biases the cathode p-base junction causing electron emission from

The symbol of a GTO is shown in Fig. 7.1a. A high degree of the cathode. These electrons flow to the anode and induce interdigitation is required in GTOs in order to achieve efficient hole injection by the anode emitter. The injection of holes turn-off. The most common design employs the cathode area and electrons into the base regions continues until charge

118 M. H. Rashid

Cathode

Cathode emitter

most remote areas from the gate contacts, forming high current

Gate

A n+

density filaments. This is the most crucial phase of the turn- off process in GTOs, because high density filaments leads to

p-base

localized heating which can cause device failure unless these filaments are extinguished quickly. An application of higher

n-base

negative gate voltage may aid in extinguishing the filaments

p-emitter

rapidly. However, the breakdown voltage of the gate-cathode junction limits this method.

G Anode

When the excess carrier concentration is low enough for carrier multiplication to cease, the device reverts to the forward blocking condition. At this point although the cathode current

C has stopped flowing, anode-to-gate current continues to flow

supplied by the carriers from n-base region stored charge. This (a) GTO symbol

is observed as a tail current that decays exponentially as the FIGURE 7.1 GTO structure.

(b) GTO structure

remaining charge concentration is reduced by recombination process. The presence of this tail current with the combina- tion of high GTO off-state voltage produces substantial power

multiplication effects bring the GTO into conduction. This is losses. During this transition period, the electric field in the shown in Fig. 7.2a. As with a conventional thyristor only n-base region is grossly distorted due to the presence of the the area of cathode adjacent to the gate electrode is turned charge carriers and may result in premature avalanche break- on initially, and the remaining area is brought into con- down. The resulting impact ionization can cause device failure.

This phenomenon is known as “dynamic avalanche.” The duction by plasma spreading. However, unlike the thyristor, device regains its steady-state blocking characteristics when the GTO consists of many narrow cathode elements, heavily the tail current diminishes to leakage current level. interdigitated with the gate electrode, and therefore the ini-

tial turned-on area is very large and the time required for plasma spreading is small. The GTO, therefore, is brought into conduction very rapidly and can withstand a high turn-on di/dt.

7.3 GTO Thyristor Models

In order to turn-off a GTO, the gate is reversed biased with respect to the cathode and holes from the anode are extracted One-dimensional two-transistor model of GTOs is shown in from the p-base. This is shown in Fig. 7.2b. As a result a Fig. 7.3. The device is expected to yield the turn-off gain g voltage drop is developed in the p-base region, which even- given by tually reverse biases the gate cathode junction cutting off the

I A α npn injection of electrons. As the hole extraction continues, the

(7.1) p-base is further depleted, thereby squeezing the remaining

I G α pnp +α npn −1 conduction area. The anode current then flows through the

TURN-ON

TURN-OFF

Electrons Holes

(a) Turn-on

(b) Turn-off

FIGURE 7.2 Turn-on and turn-off of GTOs.

7 Gate Turn-off Thyristors 119 Anode

A A The GTO remains in a transistor state if the load circuit limits the current through the shorts.

α pnp

7.4 Static Characteristics

Gate P

G α npn

7.4.1 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

C current level then positive gate drive may be reduced to zero

C and the GTO will remain in conduction. However, as a result FIGURE 7.3 Two-transistor model representing the GTO thyristor.

Cathode

of the turn-off ability of the GTO, it does posses a higher hold- ing current level than the standard thyristor, and in addition, the cathode of the GTO thyristor is sub-divided into small

where I A is the anode current and I G the gate current at finger elements to assist turn-off. Thus, if the GTO thyristor turn-off, and α npn and α pnp are the common-base current anode current transiently dips below the holding current level, gains in the NPN and PNP transistors sections of the device. localized regions of the device may turn-off, thus forcing a For a non-shorted device, the charge is drawn from the anode high anode current back into the GTO at a high rate of rise of and regenerative action commences, but the device does not anode current after this partial turn-off. This situation could latch on (remain on when the gate current is removed) until

be potentially destructive. It is recommended, therefore, that α

the positive gate drive is not removed during conduction but is npn +α pnp ≥1

held at a value I G(ON ) , where I G(ON ) is greater than the maxi-

This process takes a short period while the current and mum critical trigger current (I GT ) over the expected operating the current gains increase until they satisfy Eq. (7.2). For temperature range of the GTO thyristor. anode-shorted devices, the mechanism is similar but the anode

Figure 7.5 shows the typical on-state V–I characteristics for short impairs the turn-on process by providing a base–emitter

a 4000 A, 4500 V GTO from Dynex range of GTOs [1] at junction temperatures of 25 ◦

C. The curves can short thus reducing the PNP transistor gain, which is shown

C and 125 ◦

be approximated to a straight line of the form in Fig. 7.4. The composite PNP gain of the emitter-shorted

structure is given as follows

V TM =V 0 + IR 0 (7.4) −V α be

pnp (composite) =α pnp

(7.3) where V 0

= voltage intercept, models the voltage across the cathode and anode forward biased junctions and R 0 = on state

R Sanode

where V be = emitter base voltage (generally 0.6 V for injec- resistance. When average and RMS values of on-state current tion of carriers), and R S is the anode short resistance. The (I TAV ,I TRMS ) are known, then the on-state power dissipation anode emitter injects when the voltage around it exceed P ON can be determined using V 0 and R 0 . That is,

0.06 V, and therefore the collector current of the NPN tran- sistor flowing through the anode shorts influences turn-on.

P ON =V 0 I TAV

0 I +R 2 TRMS (7.5)