Power Electronics 11. Thyristors. Electronic Science. Module -11. Thyristors

Transcription

1 1 Module -11 Thyristors 1. Introduction 2. Classification of Thyristors 3. Unidirectional Thyristors with Turn-On Caability 3.1. Phase-controlled thyristors (or SCRs) 3.2. Fast switching thyristors (or SCRs) 3.3. symmetrical Thyristor / symmetric Silicon Controlled Rectifier (SCR) 3.4. Light activated silicon-controlled rectifiers (LSCRs) 3.5. FET Controlled Thyristors (FET-CTHs) 3.6. Reverse Conducting Thyristors (RCTs) 4. Unidirectional Thyristors with Turn off caability 4.1. ate Turn-Off Thyristors (TOs) 4.2. MOS Turn-Off Thyristors (MTOs) 4.3. Emitter Turn-Off Thyristors (ETOs) 4.4. Integrated ate-commutated Thyristors (ICTs) 4.5. MOS Controlled Thyristors (MCTs) 4.6. Static Induction Thyristors (SITHs) 5. Bidirectional Control Thyristors 5.1. Bidirectional Triode Thyristors (TRICs) 5.2. Bidirectional Phase-Controlled Thyristors (BCTs) 6. Summary Learning objectives 1. To get familiar with various tyes of thyristor. 2. To study the structure, v-i characteristics and equivalent circuit of various thyristors. 3. To understand turn on and turn off characteristics of thyristors. 4. To know the alication areas of different thyristors. 5. To make a comarative study on the basis of thyristor arameters, cost and alications etc.

2 2 1. Introduction Thyristors or silicon-controlled rectifiers (SCRs) have been used traditionally for ower conversion and control in industry. The term thyristor came from its gas tube equivalent, thyratron. Thyristor is a generic term for a biolar semiconductor device which comrises four semiconductor layers and oerates as a switch having a latched on-state and a stable off-state. Thyristors have three states: 1. Reverse blocking state 2. Forward blocking state 3. Forward conducting state Thyristors are manufactured by diffusion. Thyristors can be turned on by alying gate signal. The thyristor has been triggered into conduction and will remain conducting until the forward current dros below a threshold value known as the holding current. Thyristors can be classified as standard or slow hase-control-tye and fast-switching or inverter-tye. short duration gate ulse is sufficient to turn on the thyristor. The device with only turn on caability is referred to as conventional thyristor, or thyristor. Various gate structures are used to manufacture thyristors in order to control the di/dt, turn-on time, and turn-off time. There are several versions of thyristors with turn-off caability. 2. Classification of Thyristors Deending on the hysical construction, nature of i-v characteristics and turn-on and turn-off behavior, thyristors can be classified. The different tyes of thyristors are Thyristors with Turn on caability (Unidirectional control) Thyristors with Turn off caability (Unidirectional control) Bidirectional control Phase-controlled thyristors (or SCRs) mlifying ate thyristors (or SCRs) Fast switching thyristors (or SCRs) symmetrical thyristors (SCRs) Light activated silicon controlled rectifiers (LSCRs) FET controlled thyristors (FET-CTHs) Reverse-conducting thyristors (RCTs) ate turn off thyristors (TOs) MOS turn off thyristors (MTO) Emitter turn-off thyristors (ETOs) Integrated gate commutated thyristors (ICTs) MOS controlled thyristors (MCTs) Static induction thyristors (SITHs) Bidirectional triode thyristors (TRICs) Bidirectional hase controlled thyristors (BCTs) mong the above mentioned thyristors, some are conventional and some are having turn off caability. TRIC and BCT can conduct in both directions with gate control.

3 3 3. Unidirectional Thyristors with Turn-On Caability Conventional thyristors are widely used and have only turn on caability. Those are used in line commutated converters (ac-dc, ac-ac, cycloconverters) as well as in dc choers and inverters. For choers and inverters, the main requirement is fast turn on and turns off. Unidirectional thyristors with turn-on caability are 1. Phase controlled thyristors (or SCRs) 2. mlifying gate thyristors (or SCRs) 3. Fast switching thyristors (or SCRs) 4. symmetrical thyristors (SCRs) 5. Light activated silicon-controlled rectifiers (LSCRs) 6. FET controlled thyristors (FET-CTHs) 7. Reverse conducting thyristors (RCTs) 3.1 Phase Controlled Thyristors (PCTs or SCRs) PCTs generally oerate at the line frequency. Natural communication is used to turn off. When a gate trigger current ulse is alied to gate-cathode, a thyristor starts conduction in a forward direction and raidly latches into full conduction with a low forward voltage dro. When the anode current comes to zero, thyristor sto conducting in a few tens of microseconds and blocks the reverse voltage. The turn-off time t q is of the order of 50 to 100µs. PCTs are most suited for low seed switching alications and hence also known as a converter thyristors. The modern thyristors use an amlifying gate, in which an auxiliary thyristor T is used with the main thyristor T M as shown in Figure 1. External trigger is alied to gate of T turning it on. The amlified outut of T is alied as a gate signal to the main thyristor T M. The amlifying gate ermits high dynamic characteristics with tyical dv/dt of 1000 V/µs and di/dt of 500 /µs. It reduces the values of di/dt limiting inductor and dv/dt caacitor. The on-state voltage V T varies tyically from about 1.15V 2.5V deending on the current. The Thyristors are available u to 5-6 kv and maximum current 4-6 kv. Because of their low cost, high efficiency, ruggedness, and high voltage and current caability, these thyristors are extensively used in line commutated converters. They are used for almost all high-voltage dc (HVDC) transmission and high voltage DC drives and sulies.

4 4 R node I T T M ate Cathode Figure 1 mlifying gate Thyristor Features of PCTs: Positive feedback a latching device minority carrier device Double injection leads to very low on-resistance, hence low forward voltage dros in very high voltage devices Cannot be actively turned off by gate control voltage-bidirectional two-quadrant switch 5kV- 6kV, 1k 2 k devices lications: Line commutated converters DC motors drives C/DC static switches SVC static var comensator 3.2 Fast Switching Thyristors (or SCRs) Turn-off time of these thyristors is small, generally in the range 5 to 50 µs. Turn off time deends on the voltage range. These are used in the high-seed switching alications with forced commutations, for examle; DC choers, forced commutated inverters and resonant inverters. Therefore, these thyristors are also known as an inverter thyristors. The on-state forward dro varies aroximately as an inverse function of the turn-off time t q. These thyristors have high dv/dt of tyically 1000 V/µs and di/dt of 1000 /µs. The fast turn-off and high di/dt reduces the size and weight of commutating or reactive circuit comonents. The on-state voltage of a 1800-V, thyristor is tyically 1.7 V.

5 5 lications: DC DC converters for small ower drives Converters for Resistive welding Forced commutated inverters Induction heating 3.3 symmetrical Thyristor / symmetric Silicon Controlled Rectifier (SCR) SCR is a modified version of thyristor. It is fast switching thyristor with a very limited reverse blocking caability, tyically 10 V. cross-sectional view and v-i characteristics of the SCR is shown in Figure 2. The reverse blocking caacity is reduced in SCR by making middle n layer thinner than that of SCR. The middle n layer consists of low resistivity region (n + ) and high resistivity region (n - ) as shown in structure. Turn off time of SCR is much shorter than SCR, tyically 3 to 5 µs. Due to fast switching; SCR is suitable in inverters and hence are also called as inverter thyristor. During the reverse recovery transient the flow of reverse current causes holes to be injected across the junction from the 2 region to the n 1 region. These holes have to disaear, mainly by recombination, before the junction, which is the junction resonsible for blocking forward voltages, recovers its mocking ability. In normal thyristors, this recombination rocess takes a longer time because of the high urity level of the n 1 region. In the asymmetrical thyristor, the resence of the higher imurity n + region seeds u the recombination rocess and thus shortens the turn off time. (a) (b) i T n1 1 n + n - J 1 V BR SCR SCR V BO V 2 n2 J 3 Figure 2 SCR (a) structure and (b) I-Characteristics.

6 6 lications: Frequency converters Induction heating Resistive welding, electrical heating DC motors control Forced commutated inverters synchronous drives Battery Charging equiment 3.4 Light activated silicon-controlled rectifiers (LSCRs) It is also called as light triggered thyristors (LTT). In LSCR, the light articles (hoton) are made to strike the reverse biased junction, which causes an increase in the number of electron-hole airs triggering the thyristor. For light triggered thyristors, a slot is made in the inner layer. If the intensity of the light is greater than certain critical value, the thyristor will turn on. The gate terminal is also rovided externally. For ractical alication the resistor is connected between gate and cathode to reduce the sensitivity of gate. It increases the dv/dt caability. n LSCR offers comlete electrical isolation between the light-triggering source and the switching device of a ower converter, which floats at a otential of as high as a few hundred kilovolts. Due to electrical isolation caability, LSCRs are used in high-voltage and high current alications, for examle, HVDC transmission and static reactive ower or VR comensation. The voltage rating of an LSCR could be as high as 4 kv at 1500 with light triggering ower of less than 100 mw. Because of this low turn-on energy, multile cascaded amlifying gates are laterally integrated to achieve modest initial current rises limited to 300/μs. The tyical di/dt is 250 /µs and the dv/dt could be as high as 2000 V/µs. lications: HVDC transmission equiment Reactive ower comensators High voltage drives High ower ulse generators

7 7 3.5 FET Controlled Thyristors (FET-CTHs) In FET-CTH device contains inbuilt an n-channel enhancement MOSFET across thyristor in as shown in Figure 3. It is turned on by alying sufficient voltage, tyically 3 V, across gate cathode. Triggering current of main thyristor is generated internally. Since it is voltage controlled device drive requirement is less that of SCR. This thyristor cannot be turned off by gate control. It has a high switching seed, high di/dt, and high dv/dt. Because of variety of turn off thyristors develoed in two decades, this device could not become oular. node ate M 1 T 1 R Cathode Figure 3 Equivalent circuit of FET-Controlled Thyristor. 3.6 Reverse Conducting Thyristors (RCTs) In many converters and inverter circuits, an anti-arallel diode is connected across an SCR to allow a reverse current flow due to inductive load and to imrove the turn-off requirement of commutation circuit. The diode clams the reverse blocking voltage of the SCR to 1 or 2 V under steadystate conditions. However, under transient conditions, the reverse voltage may rise to 30V due to induced voltage in the circuit stray inductance within the device. n RCT is a comromise between the device characteristics and circuit requirement as mentioned above. It has asymmetric unch through (PT) structure with an integrated anti-arallel diode. The reason for integrating the SCR and diode is to minimize the interconnecting lead inductance. The circuit symbol, and cross sectional wafer view, are shown in Figure 4. Since no reverse voltage will be alied to the RCT there is only the cathode side dee -diffused layer. This and the SCR n-region tye field stoer result in low forward voltage characteristics. s in the SCR case, the highly n-tye doed anode end of the wide n-region also allows higher forward

8 8 voltages to be blocked. n RCT is also called as SCR. The forward blocking voltage varies from 400 to 2000 V and the current rating goes u to 500. The reverse blocking voltage is tyically 30 to 40V. (a) (b) + n n - n + n + n - n T 1 D n + Thyristor Section + Diode Section + Figure 4 Reverse Conducting Thyristor (a) structure and (b) equivalent circuit. dvantages: Comactness of a converter is obtained due to inbuilt diode. Undesired loo inductance effect gets eliminated. Unwanted reverse voltage transients gets reduced which results in the better commutation. lications: DC drives for traction alications High ower choers and inverters. 4. Unidirectional Thyristors with Turn-Off Caability The disadvantage of the conventional thyristor is no turn off caability. Hence, forced commutation circuitry is required if used in inverters, choers. Forced commutation circuitry is bulky and heavy. Commutating chokes also roduces the acoustic noise. To avoid this, thyristors with turn off caability are referred. Many thyristors with turn off caabilty are develoed in last two decades. Some of those which are used widely for different alications are 1. ate turn off thyristors (TOs)

9 9 2. MOS turn off thyristors (MTO) 3. Emitter turn-of (control) thyristors (ETOs) 4. Integrated gate commutated thyristors (ICTs) 5. MOS controlled thyristors (MCTs) 6. Static induction thyristors (SITHs) 4.1 ate Turn-Off Thyristors (TOs) TO is like an SCR, but it is fully controllable switch with turn on as well as turn-off caability using gate signal. Turn on is accomlished by a ositive gate current ulse between the gate and cathode terminals. Turn off is accomlished by a negative current ulse between the gate and cathode terminals reverse biasing the gate junction. Practically, TO is turned on by alying short ositive ulse and turned off by a short negative ulse to its gate. The negative gate current required is higher than the ositive current. TOs are used at very high ower levels, and they require secial gate control circuitry. TO is a non-latching device. static characteristic of TO is similar to the conventional SCR. The symbol, basic structure and equivalent circuit of TO are shown in Figure 5. Comared to SCR, there is an additional n + -layer near the anode that forms a turn-off circuit between the gate and the cathode in arallel with the turn-on gate. The equivalent circuit that is shown in Figure 5(c) is similar to SCR, excet for its internal turn off mechanism. Turn-on: The TO has a highly interdigited gate structure with no regenerative gate. s a consequence, a large initial gate trigger ulse is required to turn on. The turn on stages after alying gate ulse is shown in Figure 6.

10 10 (a) (b) NODE (c) n + n n + Turn on Turn off CTHODE TE Figure 5 ate turn-off thyristor (TO) (a) symbols (b) structure and (b) equivalent circuit. If a turn on current ulse is alied to gate as shown in Figure 6, nn transistor Q 1 turns on which turns on the base-emitter junction of the n transistor Q 2. Because of that Q 2 turns on which further drives Q1 and thyristor conducts. J 1 J 1 J 1 J 3 J 3 J 3 Figure 6 Turn on sequence of TO. On-state: Once the TO is turned on, forward gate current must be continued further, to insure the device remains in conduction. Otherwise, the device cannot remains in conduction during the on-state eriod. fter turning on the TO, the current required to kee TO in conduction is much less than the initial ulse magnitude, generally > 1% of the eak gate current. It ensures that the gate does not unlatch. This is illustrated with the hel of turn on characteristics as shown in Figure 7. tyical turn on gate ulse and its effect on thyristor current and voltage are deicted in the timing diagram. Minimum and maximum values of I M and di g /dt are given in the device data sheet. The rate of rise of gate current di g /dt

11 11 affects the device turn-on losses. longer eriod is required if the anode current di/dt is low such that I M is maintained until a sufficient level of anode current is established. i, v di /dt I M i I v 0 t i, v i V D v I TQ 0 V T Figure 7 Tyical turn on characteristics of TO. Turn off: If a large ulse current is assed from the anode to the gate, it takes away sufficient charge carriers from the cathode, that is, from the nn transistor Q 1. Thus, n transistor Q 2, can be drawn out of the generative action. s transistor Q 1 turns off, transistor Q 2 is left with an oen base, and the TO returns to the non-conducting state. The turn off rocess is illustrated by turn off sequence as shown in Figure 8. The turn off erformance of a TO is greatly influenced by the characteristics of the gate turn-off circuit. Thus, the characteristics of the turn off circuit must match the device requirement. TO has low gain during turn-off, tyically 4-6. Turn off gain is requires a relatively high negative current ulse to turn off. Turn off gain is given as I m I nn nn n 1

12 12 J 1 J 1 J 1 I T J 3 3/4 I T 1/4 I T J 3 m = 4 J 3 Figure 8 Turn off sequence of TO. The turn-off rocess involves the extraction of the gate charge, the gate avalanche eriod, and the anode current decay. The amount of the charge extraction is a device arameter and its value is not significantly affected by the external circuit conditions. The initial eak turn-off current and turn-off time, which are imortant arameters of the turning-off rocess, deends on the external circuit comonents. tyical anode current versus the turn-off ulse is shown in Figure 9. The TO has a long turn-off, tail-off current at the end of the turn-off which limits the high frequency oeration. The next turn-on must wait until the residual charge on the anode side is dissiated through the recombination rocess. i, v I i 0 t I QM di /dt v i, v I TQ i V DM V D V T Tail current v 0 Figure 9 Tyical turn off characteristics of TO.

13 13 The TO gate drive has to fulfill the four following functions: 1. Turn the TO on by means of a high current ulse (I M ) 2. Maintain conduction through rovision of a continuous gate ulse during the on-state (I also known as the back-orch current ) 3. Turn the TO off with a high negative gate current ulse (I QM ) 4. Reinforce the blocking caability of the off-state device, by negative gate voltage or, at least, by a low imedance resistor. There are different aroaches to TO gate drive design. Figure 10 deicts a simle gate drive circuit with turn on and turn off caability. It is suitable for both industrial and traction alications. To turn on the TO, ositive ulse to the gate transistor of T 1 turns on the transistor. The high current ulse (with eak current I M ) gets alied to gate of TO via C 1, R 1, R 2, C 2 and T 1. fter some time C 2 gets charged and low current (I ) gets alied through high resistor R 1. Thus, R 1 determines amlitude of the continuous gate current, whereas R 2 and C 2 shae the initial gate ulse (I M ). Transistor T 2 and C 3 constitute the turn-off channel, and rovide negative gate voltage during the TO s blocking eriod. Resistor R guarantees minimum blocking caability for the TO in case the gate unit ower suly fails. Transistor T 2 consists of several transistors in arallel, deending on the required eak negative gate current I QM. +V 1 R 1 R 2 C 2 ON T 1 C 1 V 2 OFF T 2 R (-20V) 0 C 3 Figure 10 Circuit Diagram of TO gate driver with turn on and turn off caability.

14 14 Comarison of TOs over SCRs 1. Elimination of commutating comonents (chokes and caacitors) in forced commutation, results in reduction in cost, weight and volume. 2. Due to the elimination of commutation chokes, acoustic and electromagnetic noise is reduced. 3. Faster turn-off, ermits high-switching frequencies and imroves efficiency of converters. 4. It has higher on-state voltage than that of SCRs. dvantages: 1. High current voltage caability 2. Low conduction loss, but higher than SCR 3. Low cost due to turn off caability which eliminates forced commutation circuitry Disadvantages: 1. Non-uniform turn-off oor RBSO and dv/dt snubber required 2. Non-uniform turn-on di/dt snubber required 3. Current control high gating ower 4. Long switching time due to turn off tail, hence high switching loss long storage time, minimum ontime and off-time requirements 5. No current limitation caability limits FBSO In voltage-source converters, a fast recovery anti-arallel diode is required across each TO. In such cases, asymmetric TOs are used. This is achieved by introducing a heavily doed n + -layer (buffer layer) at the end of the n-layer. symmetric TOs have lower on state voltage dro and higher voltage and current ratings. lications: Motor drives Static VR comensators (SVCs) C/DC ower sulies with high ower ratings Force-commutated voltage-fed thyristor inverters.

15 MOS Turn-Off Thyristors (MTOs) The MTO was develoed by Silicon Power Comany (SPCO). Figure 11 shows the symbol, structure, and equivalent circuit of the MTO. It has two control terminals turn on gate and turn off gate. It is a combination of a TO and an MOSFET, which overcome the limitations of the TO turn-off ability. TOs require a high ulse-current drive for the low imedance gate due to low turn off gain, tyically 3-5. The gate circuit must rovide high gate turn-off current whose tyical eak amlitude is 20-35% of the current to be controlled i.e. anode current. This drawback is overcome using MTO, in which the signal voltage is necessary to turn MOS transistor on and off. Its structure is similar to that of a TO. MTOs are available with high voltage u to 10 kv and high current u to 4 k. MTOs can be used in high-ower alications ranging from 1 to 20 MV. (a) (b) (c) NODE Turn-on ate FET Turn-off ate Turn-on gate n + n n + CTHODE FET Turn-off gate Turn-off Turn-on Turn on Turn off Figure 11 MOS Turn Off Thyristor (MTO) (a) Symbol. (b) Structure and (c) Equivalent circuits. Turn-on: It is similar to TO. The MTO is turned on by alying gate current ulse to the turn-on gate. Turn-on ulse turns on the nn transistor Q 1, which then turns on the n-transistor Q 2 latching the MTO. Turn-off: To turn-off the MTO, a voltage ulse is alied to the MOSFET gate. Turning on the MOSFETs, shorts the emitter and base of the Q 1, thereby stoing the latching rocess. In contrast, a TO is turned off by sweeing enough current out of the emitter base of nn transistor with a large negative ulse to sto the regenerative latching action. s a result, the MTO turns off much faster than a TO (tyically 1-2 µs). Therefore the losses associated with the storage time are almost eliminated. MTO has a higher dv/dt and hence requires much smaller snubber comonents. Similar to TO, the MTO has a long turn-off tail of current at the end of the turn-off which limits the oerating frequency of MTO.

16 16 dvantages: High ower rating (u to 10 kv and 4 k) Fast switching seed (u to 5 khz) higher dv/dt and hence requires much smaller snubber comonents Cost and gate drive ower requirement is low as comared with other turn off devices. lications: High voltage alications uto 20 MV Voltage source inverters for high ower Flexible C line Transmissions (FCTs) Motor drives 4.3 Emitter Turn-Off Thyristors (ETOs) The ETOs is MOS-TO hybrid device that combines the advantages of both the TO and the MOSFET. ETO was invented at Virgina Center in collaboration with SPCO. Figure 12 shows the ETO symbol, its equivalent circuit, and the nn structure. ETO has two gates, turn on gate and turn off gate. Turn-on gate is similar to that of TO. Turn-off gate is the gate of MOSFET in series with TO structure. High ower ETOs with a current rating of u to 4 k and a voltage rating of u to 6 kv are available. (a) (b) (c) Turn-on Turn-off Turn-off M 2 N-MOS M 1 P-MOS Turn-on ate 1 n n ate 2 M 1 P-MOS M 2 N-MOS Figure 12 Emitter turn-off thyristor (a) symbol, (b) equivalent circuit and (c) structure.

17 17 Turn On: n ETO is turned on by alying ositive voltages gate1 and gate2. ositive voltage to gate 2 turns on NMOS and turns off PMOS. n injection current into the TO gate (through ate1) turns on the ETO. Turn off: When a turn-off negative voltage signal is alied to the gate 2 i.e. gate of NMOS M 2, it turns off and transfers all the current away from the cathode (n emitter of the nn-transistor of the TO) into the base via gate of PMOS M 1. This stos the regenerative latching rocess and results in a fast turn-off. It is imortant to note that both MOSFETs are not subjected to high-voltage stress, no matter how high the voltage is on the ETO. This is due to the internal structure of the TO s gate-cathode is a PNjunction. Series MOSFET, M 2 has to carry the main TO current which increases the total voltage dro by about 0.3 to 0.5 V and corresonding ower dissiation. Similar to a TO, the ETO has a long turn-off tail of current at the end of the turn-off which limits the high frequency oeration. dvantages: High-ower rating (u to 4 k and 6 kv) Fast switching seed (u to 5 khz) Cost and gate drive ower requirement is low as comared with other turn off devices. wide reverse biased safe oeration area (RBSO) Snubberless turn-off caability Simlicity in over-current rotection Caable of arallel and series oeration lications: Voltage source inverters for high ower Flexible C line Transmissions (FCTs) Motor drives Static Synchronous Comensator (STTCOM) 4.4 Integrated ate-commutated Thyristors (ICTs) The integrated gate-commutated thyristor (ICT) was introduced by BB in The ICT integrates a gate commutated thyristor (CT) with a multilayered rinted circuit board gate drive. Basically, it is a high-voltage, high-ower, hard-driven, asymmetric blocking TO with unity turn-off current gain. This means that a 4500V ICT with a controllable anode current of 3000 requires turnoff

18 18 negative gate current of very fast and large gate current ulse of full rated current draws out all the current from the cathode into the gate in about 1μs to ensure a fast turn-off. The cross section of an ICT and equivalent circuit of a CT are similar to that of a TO shown in Figure 13. ctually ICT is the close integration of TO and the gate drive circuit with multile MOSFETs in arallel roviding the high gate currents. n ICT may also have an integrated reverse diode, as shown by the n + n - junction on the right side of Figure 13. Similar to a TO, an MTO and an ETO, the n-buffer layer evens out the voltage stress across the n - -layer, decreases the on-state conduction loss, and makes the devices asymmetric. The anode -layer is made thin and lightly doed to allow faster removal of charges from the anode-side during turn-off. (a) (b) n + (c) n - n + n + Turn on Turn off Figure 13 ICT (a) symbols, (b) structure and (c) equivalent circuit. Turn-on: Similar to a TO, the ICT is turned on by alying the turn-on current to its gate. With high gate current, turn-on is initially by nn BJT, not SCR regeneration. The n transistor is inoerative since the carriers in the n-base are initially ineffective since they require a finite time to transit the wide n-base. Turn-off: ICT is nothing but imroved TO with unity gain turn-off drive i.e. the gate current equal to the anode current. Because of that ositive feedback loo of thyristor is broken and nn transistor turns off first. Therefore, thyristor turns off in oen-base n transistor mode. The turn-off sequence from on state is deicted using Figure 14.

19 19 J 1 J 1 J 1 J 3 J 3 J 3 Figure 14 Turn off sequence of TO. The ICT is turned off by a multilayered gate-driver circuit board. Driver circuit can suly a fast rising turn-off ulse, for examle, a gate current of 4 k/μs with a gate-cathode voltage of 20V only. With this rate of gate current, the nn-transistor is totally turned off within about 1 μs and the anode-side n transistor is effectively left with an oen base and it is turned-off almost immediately. Due to a very short duration ulse, the gate-drive energy is greatly reduced and the gate drive energy consumtion is minimized. The gate-drive ower requirement is decreased by a factor of five comared with that of the TO. To aly a fast-rising and high-gate current, the ICT incororates a secial effort to reduce the inductance of the gate circuitry as low as ossible. This feature is also necessary for gate-drive circuits of the MTO and ETO. The high reverse gate current results in a very short saturation delay time, enabling accurate turn-off synchronization necessary for devices to be series connected. ate Driver: The key to achieve a hard-driven or unity-gain turn-off condition lies in the gate current commutation rate. rate as high as 6 k/μs is required for 4-k turn-off of TO to achieve turn off. The gate drive circuit is built-in on the device module. It is necessary to hold the gate loo inductance low enough (L 3 nh) so that a required dc gate (18 to 22 V) voltage less than the breakdown voltage of the gate cathode junction can generate a slew rate of 6 k/μs. High cost associated with the low-inductance housing design for the TO. Tyical TO gate drive configuration with a small gate inductance L is shown in Figure 15. The SCR on-state regenerative mechanism is avoided at both turn-off and turn-on switching transitions thereby yielding a device more robust than the TO. s with the TO, an inductive series turn-on snubber is still required to coe with the initial high di/dt current. The CT switch is thermally limited, rather than frequency limited as with the conventional TO. Multile ICTs can be connected in series or in arallel for higher ower alications.

20 20 L 3nH Drive Signal T 20V Figure 15 Tyical TO gate drive configuration with a small gate inductance L. dvantages of ICTs over TOs Low conduction dro High Small minority carrier storage time, turn on di/dt, and turn-off dv/dt Low gate driver loss Faster switching of the device ermits snubberless oeration Higher switching frequency Disadvantage: High cost associated with the low-inductance housing design for the TO High cost associated with low inductance and high current design for the gate driver lications: High-ower converters in excess of 100MV Static vol-amere reactive (VR) comensators Converters for distributed generation such as wind ower

21 MOS Controlled Thyristors (MCTs) n MCT is a regenerative four layer thyristor with a MOS gate structure. n MCT is an imrovement over a thyristor with a air of MOSFETs to turn on and turn off. In general, there are two tyes of MCT, -channel MCT (-MCT) and n-channel MCT (n-mct). The -MCT is widely used because of low on state voltage as comared with n-mct. symbol, structure and equivalent circuit of a -MCT cell is shown in Figure 16. The nn structure may be reresented by an nn transistor Q 1 and a n-transistor Q 2. The MOS-gate structure can be reresented by a -channel MOSFET (M 1 ) and an n-channel MOSFET (M 2 ). Due to an nn structure, instead of the nn structure of SCR, the anode serves as the reference terminal. The gate signals are alied with reference to anode. Since gate signal of the -MCT is alied with resect to the anode instead of the cathode, it is sometimes referred to as comlementary MCT (C-MCT). In case of n-mct the cathode serves as the reference terminal. The gate signals are alied with reference to cathode. It is turned on by a negative voltage ulse at the gate with resect to the anode and is turned off by a ositive voltage ulse. Negative gate-anode voltage turns PMOS (M 1 ) on, latching both transistors Q 1 and Q 2. Positive gate-anode voltage turns NMOS (M 2 ) on, reverse biasing the base-emitter junction of Q 2 and turning off the device. Maximum current that can be interruted is limited by the on-resistance of NMOS (M 2 ). The device has a microcell construction. In a ractical MCT, about 100,000 cells similar to the one shown in Figure 16 are aralleled to achieve the desired current rating. Each cell contains a wide-base nn-transistor and a narrow-base n-transistor. Each n-transistor in a cell is rovided with an NMOS across emitter and base to rovide higher current for turn off. But, a small ercentage (around 4%) of ntransistors is rovided with PMOS across its emitter and collector to rovide sufficient current to turn on. Turn on: When a -channel MCT is in the forward blocking state, it can be turned on by alying a negative ulse to its gate with resect to the anode. n MCT remains in the on-state until the device current is reversed or a turn-off ulse is alied to its gate.

22 22 (a) 14V V (b) NODE 0-7V ON OFF ON TE S 2 D 2 M 2 NMOS Q 2 M 1 PMOS S 1 (c) ate Cathode D 1 Q 1 SiO 2 SiO 2 n + n + + n CTHODE n + node Figure 16 MCT (a) Symbol, (b) equivalent circuit and (c) structure. Turn off: When a -channel MCT is in the on-state, it can be turned off by alying a ositive ulse to its gate with resect to the anode. When an n-channel MCT is in the on-state, it can be turned off by alying a negative ulse to its gate with resect to the cathode. ttemting to turn off the MCT at currents higher than its rated eak controllable current may result in destroying the device. For higher values of current, the MCT has to be commutated off like a SCR. The gate ulse widths are not critical for smaller device currents. For larger currents, the width of the turn-off ulse should be larger. The gate draws a eak current during turn-off.

23 23 The MOS structure is sread across the entire surface of the device resulting fast turn-on and turn-off with low-switching losses. The ower or energy required for the turn-on and turn-off is very small, and the delay time due to the charge storage is also very small. n MCT has a low on state voltage around 1V like SCR, hence low conduction loss a fast turn-on time, tyically 0.4 s, and a fast turn-off time, tyically 1.25 s for an MCT of 500 V, 300 Low switching losses a low reverse voltage blocking caability high gate inut imedance, which greatly simlifies the drive circuit a limited safe oerating area (SO), and therefore a snubber circuit is mandatory a asymmetric voltage-blocking caability The device has a limited safe oerating area; therefore, a snubber circuit is mandatory in an MCT converter. lso, it has comlex geometry. These disadvantages have hamered its alication, and the MCT has not gained widesread accetance in the ower electronics community. 4.6 Static Induction Thyristors (SITHs) The SITH is also known as field-controlled diode (FCD) or field controlled thyristor (FCTh). It contains containing a gate structure that can shut down anode current flow. This device was first introduced by Teszner in the 1960s. It is minority carrier device, a JFET structure with an additional injecting layer. Since it is a minority carrier device, SITH has low on-state resistance and therefore low voltage dro. The cross section of a half SITH cell structure is shown in Figure 17. Its symbol and equivalent circuit is also shown in Figure 17. The device is essentially a in diode with a gate structure that can inch-off anode current flow. Large area devices are generally the buried-gate tye because larger cathode areas and, hence, larger current densities are ossible. SITH devices can have high voltage ratings u to 2.5 kv, but low current ratings are limited to 500. Turn-on: SITH is normally turned on by alying ositive gate voltage with resect to the cathode. Providing sufficient ositive gate current and voltage, the gate cathode -i-n diode turns on and injects electrons into the channel (anode drift region) resulting in large conductivity modulation. ortion of hole current flows through the + gate and the channel toward the cathode directly. The remaining hole

24 current flows through the + gate to the channel as the gate current of the Biolar Mode JFET (BMFET). Consequently, there is small on state resistance resulting low on state voltage, even at large currents. 24 (a) (b) (c) + Q2 J 1 J 3 J 4 + n-base n + Q 1 B 1 Figure 17 SITH (a) symbols, (b) structure and (c) equivalent circuit. Turn-off: n SITH is normally turned off by alying a large reverse bias across gate and cathode. Because of large reverse bias of gate cathode junction, the deletion region of the gate junction grows and inches off the channel connecting anode and cathode reventing current flow. Because of addition of the n junction at anode SITH also block high reverse voltage. The device does not have regenerative turn on and turn off i.e. it does not latch on or off. If the device is on, the removal of gate drive will cause device to turn-off. ey features: It is a fast-switching device. The switching time is 1 to 6 μs. High dv/dt and di/dt caabilities. The voltage rating can go u to 2500 V. The current rating is limited to 300. This device is highly rocess sensitive. Small erturbations in the manufacturing rocess would roduce major changes in the device characteristics.

25 25 5. Bidirectional Thyristors Most of the thyristors are unidirectional and therefore widely used on controlled rectifiers, dc- dc converters and inverters. In case of ac voltage control, thyristors are used, but it requires two thyristors connected in anti-arallel. It requires two searate control circuits. Hence, it requires more electrical wire connections. To reduce number of electrical connections and for easy control bidirectional thyristors are used. The most widely used bidirectional thyristor are a) Bidirectional triode thyristors (TRICs) b) Bidirectional hase-controlled thyristors (BCTs) 5.1 Bidirectional Triode Thyristors (TRICs) Bidirectional Triode Thyristors are known as TRIC. It is acronym of TRIode for lternating Current. TRIC can conduct in both directions. Therefore is used in ac voltage controllers (ac-ac line commutated converters) for low ower alications. Figure 18 shows symbol, structure, and equivalent circuit of TRIC. TRIC can be considered as two SCRs connected in anti-arallel with a common gate connection. The main terminal I-V characteristics showing four trigger modes is shown in Figure 19. (a) MT 2 (b) MT2 (c) MT2 n n n n MT 1 MT1 Figure 18 TRIC (a) Symbol, (b) Structure, (c) equivalent circuit MT1 Because a TRIC is a bidirectional device, its terminal cannot be designated as anode and cathode. If terminal MT2 ositive with resect to terminal MT1, the TRIC can be turned on by alying ositive gate signal between gate and terminal MT1. If terminal MT2 is negative with resect to terminal MT1, it is turned on by alying a negative gate signal between gate and terminal MT1. It is not necessary to have both olarities of gate signal, and a TRIC can be turned on with either a ositive or a negative gate signal. Turn-on mechanism for each mode is as follows.

26 26 Mode I MT2 ositive, Ig ositive Mode II MT2 ositive, Ig negative Mode III MTl ositive, Ig negative Mode IV MTl ositive, Ig ositive Mode I and Mode III has high gate sensitivity. Hence these modes are referred over other modes. Once the Triac is in the ON state, the gate signal can be removed and the Triac will remain ON until the main current falls below the holding current (I H ) value. I I 1 st quadrant + V Break over Voltage - 3 rd quadrant I Rated Minimum Blocking Voltage V DRM - V 0 V Minimum Holding Current I H Figure 19 Tyical TRIC V-I characteristics. V Integrated construction of TRIC has some disadvantages. Because of the integration, the triac has oor realied dv/dt, oor gate current sensitivity at turn-on, and longer turn-off time. It is due to the minority carrier storage effect. Poor realied dv/dt rating makes it difficult to use with inductive load. well-designed RC snubber is essential for a Triac circuit I Minimum Holding Current I H Rated Minimum Blocking Voltage V DRM Break over Voltage Rated Current (I L ) TRICS are used for the various alications at 50/60 Hz suly frequency. Triac is widely used to control the seed of single hase induction motors. It is also used in domestic lam dimmers and heat control circuits, solid state C relays and full wave C regulators.

27 27 lications: Lighting technology (light dimming) Heating equiment (temerature control) Electric motors (velocity control) Solid state C relays Low ower C regulators 5.2 Bidirectional Phase-Controlled Thyristors (BCTs) The BCT is a new concet for high ower hase control develoed by BB Semiconductors. It combines two anti-arallel high ower thyristor with onto a single silicon wafer. ddition of this new feature enables comact equiment design, simlifies the cooling system, reduces the cost of the end roduct and increases the system reliability. They are suitable for alications as static var comensators, static switches, soft starters, and motor drives. Its symbol, equivalent circuit and (c) schematic view are shown in Figure 20. The BCT wafer has anode and cathode region regions on each face. BCT has two gates. The and B thyristor are identified on the wafer by letters and B resectively. Figure 21 shows the crosssection of a BCT wafer showing and B thyristor halves. (a) (b) (c) Searation Region v D(B) 1 I T() B B V D(B) V D() v D(B) 2 ( formerly conducting) B side side (formerly conducting) Figure 20 BCT (a) symbol, (b) equivalent circuit and (c) schematic view of wafer. major challenge in the integration of two thyristors halves is to avoid harmful crosstalk between the two halves under all relevant oerating conditions. The device must show very high uniformity between the two halves in device arameters such as reverse recovery change and on-state voltage dro. Region 1 and 2 shown in Figure 20 are the most sensitive with resect to surge current having realied reverse voltage and the t q caability of a BCT.

28 28 Thyristor Half B Searation Region Thyristor Half node B ate V B (t) Cathode Shallow P base Dee P base n base Dee P base Shallow P base V B (t) Cathode B ate B (not visible) node Figure 21 Cross section of a BCT wafer showing and B thyristor halves and defining the two forward voltage directions V (t) and V B (t). The maximum voltage rating of BCTs can be as high as 6.5 kv at 1.8 k and the maximum current rating can be as high as 3 k at 1.8 kv. Turn-on and off: BCT has two gates: one for the turning on the forward current and one for the reverse current. This thyristor is turned on with a ulse current to one of its gates. It is turned off, if the anode current falls below the holding current due to the natural behavior of the voltage or the current. dvantages: Imroved volume consumtion and reduced art count in the magnitude of 25% comared with equally rated anti-arallel thyristors Reduction in cost for high ower alications High reliability lications: Static var comensators Static switches three hase systems Soft starters for asynchronous machines Motor drives 4 quadrant DC drive

29 29 6 Summary The thyristors may be classified by considering different ways such such as current direction. Turn on and turn off caability. In chooers thyristors with turn off caability are used whereas in ac-dc convertes unidirectional thyristor are used. In ac regulators, biidirectional thyristor are referred. In high voltage alications for electrical isolation LSCR are used,