Lecture 5: Controlled Thyristor Switches PDF

LECTURE NO. 5 CONTROLLED THYRISTOR SWITCHES CONTROLLED THYRISTOR SWITCHES This switches are low-power thyristors, but they can be switched both on and off through a gate input. Gate-controlled switch (GCS) Silicon-controlled switch (SCS) Silicon bilateral switch (SBS) Gate-Controlled Swi...

LECTURE NO. 5 CONTROLLED THYRISTOR SWITCHES CONTROLLED THYRISTOR SWITCHES This switches are low-power thyristors, but they can be switched both on and off through a gate input. Gate-controlled switch (GCS) Silicon-controlled switch (SCS) Silicon bilateral switch (SBS) Gate-Controlled Switch (GCS) Also known as Gate-Turnoff SCR (GTO) Unilateral devices Same turn-on characteristics of an SCR but can be turned off by applying negative potential to its gate Relatively large values of negative current are required Gate-Controlled Switch (GCS) For example: GTO rated with 2000A may need -500A current to switch off 𝐼𝐴 𝛼1 𝛽𝑜𝑓𝑓 = = 𝐼𝐺 𝛼1 + 𝛼2 − 1 Because of this typical turn off gain of GTO is low and is in range of 4 to 5 GTO are used in low power applications Gate-Controlled Switch (GCS) During conduction state, GTO behaves just like a thyristor GTO has a faster switching speed than other thyristor GTO has higher voltage and current ratings than the power transistors GTO Thyristor Principle of Operation Turn on GTO: similar to conventional thyristor Injection of holes(from gate) When G-K triggers on-state, n+ P (highly doped electrons from cathode) will inject more electrons N and p+(highly doped holes from anode) will inject more holes. GTO thyristor is at conduction state GTO Thyristor Principle of Operation To turn off conducting GTO: Reverse bias is applied at gate Holes from P base layer will be extracted through negative gate. P Injection of electrons from cathode will be suppress. N During the hole extraction, the anode current flows through remote areas forming high current density. No recombination will lead to GTO off-state GTO Thyristor VI Characteristics During the turn-on, GTO is similar to thyristor By application of positive gate signal triggers the GTO into conduction state Gate current can be removed if the anode current is above the holding current The GTO can be turned off by the application of reverse current GTO Thyristor Application Due to advantage of: Excellent switching No need for commutation circuit Maintenance-free operation It is used mainly on: AC drivers DC drivers or DC choppers AC stabilizing power supplies DC circuit breakers Induction heating Other low-power applications GTO Thyristor Application Automobile Ignition Circuit Uses transistor switch and a GCS to control the transfer of high voltage pulses to the spark plugs Transistor Q1 is either cutoff or in saturation Q1 controls the development of trigger pulses L stores energy in magnetic field C stores energy in electric field Silicon-Controlled Switch (SCS) Two-gate unilateral device AG stands for anode gate KG stands for cathode gate Same model with SCR but with addition of anode gate terminal SCS is similar to GCS, but the two gate inputs allow one of the gates to be biased while the other remains open Silicon-Controlled Switch (SCS) Switching pulses on the AG terminal are complementary to the pulses on the KG terminal Positive pulse on cathode gate would switch the SCS on Negative pulse on anode gate would switch the SCS on SCS Thyristor Working Operation Negative pulse on AG causes transistor Q1 to switch on Transistor Q1 base current to Q1 transistor Q2, and both transistors switch on Q2 SCS Thyristor Working Operation Positive pulse on KG causes Q2 to switch on. Transistor Q2 collector current to Q1 transistor Q1 base, and both transistors switch on Q2 To turn-off SCS thyristor o At KG, negative pulse o At AG, positive pulse SCS Thyristor VI Characteristic Positive pulse at KG or negative pulse at AG triggers SCS thyristor to on-state Negative pulse at KG or positive pulse at AG switch-off (current interruption) the SCS thyristor. SCS Thyristor Application Advantage of SCS Thyristor Fast turn-off Turned-off with positive or negative pulse at either gate Applications: Low power sensing circuits Timers, registers, and counters Digital logic circuits Pulse generators Oscillators SCS Thyristor Application DC Switching Circuit Positive Negative Silicon Bilateral Switch (SBS) Solid-state breakover device that produces a triggering pulse in both positive and negative direction With the gate terminal open, the operation of the SBS follows the VI characteristic curve, which is similar to DIAC Breakover voltages of lower than 8V Silicon Bilateral Switch (SBS) When SBS switches into conducting state, the voltage across its anode terminals drop almost to zero to about 1V. The VI characteristic of the SBS can be altered by using gate terminal SBS Equivalent Circuit (Open Gate) A1 positive voltage to A2 CR1 act as normal diode Q3 and Q4 remain off When voltage A1 reaches level equal to Q1’s base voltage plus the zener voltage of CR2, Q1 and CR1 are forward-biased, and CR2 operates in zener region. Collector current of Q1 flows into the base of Q2, causing Q2 to turn and the SBS break over and drops about 1V across the anode terminals. SBS Equivalent Circuit (Open Gate) A1 negative voltage to A2 Reference at A2 positive node CR2 act as normal diode Q1 and Q2 remain off When voltage A2 reaches level equal to Q4’s base voltage plus the zener voltage of CR1, Q4 and CR2 are forward-biased, and CR1 operates in zener region. Collector current of Q4 flows into the base of Q3, causing Q3 to switch on SBS Equivalent Circuit (Gate) A1 positive voltage to A2 Positive Gate Pulse: Triggers below the positive breakover voltage. Negative Gate Pulse: Turns off the SBS Thyristor A1 negative voltage to A2 Negative Gate Pulse: Triggers below the negative breakover voltage. Positive Gate Pulse: Turns off SBS Thyristor Application Advantage: Useful even without gate terminal Temperature stable than DIAC More symmetrical than DIAC Application: Use for triggering TRIACs Use for AC phase control by TRIACs CONTROLLED THYRISTOR SWITCHES Gate-controlled switch (GCS) Silicon-controlled switch (SCS) Silicon bilateral switch (SBS) ACTIVITY Create a short video explaining the working operation of the circuit below. Lamp Dimmer ACTIVITY Create a short video explaining the working operation of the circuit below. Zero-Point Switch Circuit