1 Module -11 Thyristors 1. Introduction 2. Classification of Thyristors 3. Unidirectional Thyristors with Turn-On Capability 3.1. Phase-controlled thyristors (or SCRs) 3.2. Fast switching thyristors (or SCRs) 3.3. Asymmetrical Thyristor / Asymmetric Silicon Controlled Rectifier (ASCR) 3.4. Light activated silicon-controlled rectifiers (LASCRs) 3.5. FET–Controlled Thyristors (FET-CTHs) 3.6. Reverse Conducting Thyristors (RCTs) 4. Unidirectional Thyristors with Turn off capability 4.1. Gate Turn-Off Thyristors (GTOs) 4.2. MOS Turn-Off Thyristors (MTOs) 4.3. Emitter Turn-Off Thyristors (ETOs) 4.4. Integrated Gate-Commutated Thyristors (IGCTs) 4.5. MOS–Controlled Thyristors (MCTs) 4.6. Static Induction Thyristors (SITHs) 5. Bidirectional Control Thyristors 5.1. Bidirectional Triode Thyristors (TRIACs) 5.2. Bidirectional Phase-Controlled Thyristors (BCTs) 6. Summary Learning objectives 1. To get familiar with various types 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 application areas of different thyristors. 5. To make a comparative study on the basis of thyristor parameters, cost and applications etc. Power Electronics Electronic Science 11. Thyristors 2 1. Introduction Thyristors or silicon-controlled rectifiers (SCRs) have been used traditionally for power conversion and control in industry. The term ―thyristor‖ came from its gas tube equivalent, thyratron. Thyristor is a generic term for a bipolar semiconductor device which comprises four semiconductor layers and operates 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 applying gate signal. The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the holding current. Thyristors can be classified as standard or slow phase-control-type and fast-switching or inverter-type. A short duration gate pulse is sufficient to turn on the thyristor. The device with only turn on capability 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 capability. 2. Classification of Thyristors Depending on the physical construction, nature of i-v characteristics and turn-on and turn-off behavior, thyristors can be classified. The different types of thyristors are Thyristors with Turn on capability Thyristors with Turn off capability Bidirectional (Unidirectional control) (Unidirectional control) control Phase-controlled thyristors (or SCRs) Gate turn–off thyristors (GTOs) Bidirectional Amplifying Gate thyristors (or SCRs) MOS turn–off thyristors (MTO) triode thyristors Fast switching thyristors (or SCRs) Emitter turn-off thyristors (ETOs) (TRIACs) Asymmetrical thyristors (ASCRs) Integrated gate commutated Bidirectional Light activated silicon controlled thyristors (IGCTs) phase controlled rectifiers (LASCRs) MOS controlled thyristors (MCTs) thyristors (BCTs) FET–controlled thyristors (FET-CTHs) Static induction thyristors (SITHs) Reverse-conducting thyristors (RCTs) Among the above mentioned thyristors, some are conventional and some are having turn off capability. TRIAC and BCT can conduct in both directions with gate control. Power Electronics Electronic Science 11. Thyristors 3 3. Unidirectional Thyristors with Turn-On Capability Conventional thyristors are widely used and have only turn on capability. Those are used in line commutated converters (ac-dc, ac-ac, cycloconverters) as well as in dc choppers and inverters. For choppers and inverters, the main requirement is fast turn on and turns off. Unidirectional thyristors with turn-on capability are 1. Phase controlled thyristors (or SCRs) 2. Amplifying gate thyristors (or SCRs) 3. Fast switching thyristors (or SCRs) 4. Asymmetrical thyristors (ASCRs) 5. Light activated silicon-controlled rectifiers (LASCRs) 6. FET–controlled thyristors (FET-CTHs) 7. Reverse conducting thyristors (RCTs) 3.1 Phase Controlled Thyristors (PCTs or SCRs) PCTs generally operate at the line frequency. Natural communication is used to turn off. When a gate trigger current pulse is applied to gate-cathode, a thyristor starts conduction in a forward direction and rapidly latches into full conduction with a low forward voltage drop. When the anode current comes to zero, thyristor stop conducting in a few tens of microseconds and blocks the reverse voltage. The turn-off time tq is of the order of 50 to 100µs. PCTs are most suited for low speed switching applications and hence also known as a converter thyristors. The modern thyristors use an amplifying gate, in which an auxiliary thyristor TA is used with the main thyristor TM as shown in Figure 1. External trigger is applied to gate of TA turning it on. The amplified output of TA is applied as a gate signal to the main thyristor TM. The amplifying gate permits high dynamic characteristics with typical dv/dt of 1000 V/µs and di/dt of 500 A/µs. It reduces the values of di/dt limiting inductor and dv/dt capacitor. The on-state voltage VT varies typically from about 1.15V – 2.5V depending on the current. The Thyristors are available up to 5-6 kV and maximum current 4-6 kV. Because of their low cost, high efficiency, ruggedness, and high voltage and current capability, 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 supplies. Power Electronics Electronic Science 11. Thyristors 4 Anode R TA TM IG Gate Cathode Figure 1 Amplifying gate Thyristor Features of PCTs: Positive feedback — a latching device A minority carrier device Double injection leads to very low on-resistance, hence low forward voltage drops in very high voltage devices Cannot be actively turned off by gate control A voltage-bidirectional two-quadrant switch 5kV- 6kV, 1kA – 2 kA devices Applications: Line commutated converters DC motors drives AC/DC static switches SVC – static var compensator 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 depends on the voltage range. These are used in the high-speed switching applications with forced commutations, for example; DC choppers, forced commutated inverters and resonant inverters. Therefore, these thyristors are also known as an inverter thyristors. The on-state forward drop varies approximately as an inverse function of the turn-off time tq. These thyristors have high dv/dt of typically 1000 V/µs and di/dt of 1000 A/µs. The fast turn-off and high di/dt reduces the size and weight of commutating or reactive circuit components. The on-state voltage of a 1800-V, 2200-A thyristor is typically 1.7 V. Power Electronics Electronic Science 11. Thyristors 5 Applications: DC –DC converters for small power drives Converters for Resistive welding Forced commutated inverters Induction heating 3.3 Asymmetrical Thyristor / Asymmetric Silicon Controlled Rectifier (ASCR) ASCR is a modified version of thyristor. It is fast switching thyristor with a very limited reverse blocking capability, typically 10 V. A cross-sectional view and v-i characteristics of the ASCR is shown in Figure 2. The reverse blocking capacity is reduced in ASCR 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 ASCR is much shorter than SCR, typically 3 to 5 µs. Due to fast switching; ASCR 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 J2 from the p2 region to the n1 region. These holes have to disappear, mainly by recombination, before the junction J2, which is the junction responsible for blocking forward voltages, recovers its mocking ability. In normal thyristors, this recombination process takes a longer time because of the high purity level of the n1 region. In the asymmetrical thyristor, the presence of the higher impurity n+ region speeds up the recombination process and thus shortens the turn off time. (a) A (b) iT SCR ASCR p1 VBR + J n 1 n1 - VAK n VBO J2 p2 J3 n2 G K Figure 2 ASCR (a) structure and (b) I-Characteristics. Power Electronics Electronic Science 11. Thyristors 6 Applications: Frequency converters Induction heating Resistive welding, electrical heating DC motors control Forced commutated inverters Asynchronous drives Battery Charging equipment 3.4 Light activated silicon-controlled rectifiers (LASCRs) It is also called as light triggered thyristors (LTT). In LASCR, the light particles (photon) are made to strike the reverse biased junction, which causes an increase in the number of electron-hole pairs triggering the thyristor. For light triggered thyristors, a slot is made in the inner p layer. If the intensity of the light is greater than certain critical value, the thyristor will turn on. The gate terminal is also provided externally. For practical application the resistor is connected between gate and cathode to reduce the sensitivity of gate. It increases the dv/dt capability. An LASCR offers complete electrical isolation between the light-triggering source and the switching device of a power converter, which floats at a potential of as high as a few hundred kilovolts. Due to electrical isolation capability, LASCRs are used in high-voltage and high current applications, for example, HVDC transmission and static reactive power or VAR compensation. The voltage rating of an LASCR could be as high as 4 kV at 1500A with light triggering power of less than 100 mW. Because of this low turn-on energy, multiple cascaded amplifying gates are laterally integrated to achieve modest initial current rises limited to 300A/μs.