A Thyristor is basically an on-off switch to control the output power of an electrical circuit by switching on and off the load circuit in intervals of time. In this post, we will try to understand what is it, How it works, its Voltage Ampere (VI) characteristics, modes of operation, applications, advantages and disadvantages.

Introduction to Thyristor

A Thyristor is a unidirectional semiconductor solid state device with four layers of alternating P and N type material. It consist of three electrodes i.e. Anode, Cathode and a Gate. Anode is the positive terminal and Cathode is the negative terminal.

The Gate controls the flow of current between anode and cathode. It is used in electronic devices and equipment to control the electric power or current. It acts as a rectifier and can only transmits current in one direction.

The first Thyristor was produced in the year 1956. The most common type of Thyristor is silicon Controlled Rectifier (SCR).

Symbol of Thyristor

Fig.1 – Symbol of Thyristor

How Thyristor Works

A Thyristor acts like a diode. It has two layers of semiconductors namely p-type and n-type sandwiched together to form a junction. The anode is connected to the outer p-layer, cathode to the outer n-layer and gate to the internal p-layer. It has 3 junctions namely J1, J2, J3 as shown in the Figure 2 below.

When the anode is at positive potential with respect to cathode, no voltage is applied to the gate. The junctions J1, J3 is forward biased and J2 is reverse biased. So no conduction takes place here.

Layer Diagram of Thyristor

Fig. 2 – Layer Diagram of Thyristor

Now, when the positive potential is increased beyond the breakdown voltage, breakdown of junction J2 takes place and it starts conducting. Once the breakdown has occurred, it continues to conduct irrespective of the gate voltage, until the potential at the anode is removed or current through the device is made less than the holding current.

Now when a positive potential is applied at the gate terminal with respect to cathode, the breakdown of junction J2 takes place. To switch on the Thyristor quickly, an appropriate potential value has to be selected.

The gate acts as a controlling electrode. When a small voltage known as gate pulse is applied to its gate , the device is triggered into conduction state .This continues until the voltage across the device is reversed or removed.

The gate trigger current varies inversely with the gate voltage and a minimum gate charge is required to trigger it. Thus the switching of Thyristors can be controlled through its gate pulse.

Two Transistor Analogy of Thyristor

The collector current from the NPN transistor is fed directly to the base of PNP transistor, while the collector current of PNP transistor is fed to the base of NPN. These interconnected transistors rely on each other for conduction.

So for one of the transistors to conduct, a base current is required. When the Thyristor’s anode terminal is negative with respect to cathode, the NP junction becomes forward biased and the PN junction becomes reverse biased.

Two Transistor Analogy of Thyristor

Fig. 3 – Two transistor Analogy of Thyristor

Here, the flow of reverse current is blocked until a breakdown voltage is applied. After breakdown voltage, it starts to conduct without the application of gate signal. This is one of the negative characteristics of Thyristors as it triggers into conduction by a reverse break over voltage.

When the anode terminal is made positive with respect to cathode, the outer junctions are forward biased and the centre NP junction is reverse biased and blocks the forward current. So to trigger it into conduction, a positive current is applied to the base of transistors.

The two transistors are connected in a regenerative loop and this force the transistor to conduct to saturation. Thus, it can be said that a Thyristors block current in both the direction of an AC supply in its OFF state and can be turned ON by the application of positive current to the base of transistor.

Voltage Ampere (VI) Characteristics of Thyristor

Thyristors can either be forward biased or reverse biased. We will see how it works in both states.

Thyristors in Forward Biased State

When anode is made positive, the PN junctions at the ends are forward biased and center junction (NP) becomes reverse biased. It will stay in blocked (OFF) mode (also known as Forward Blocking Stage) till the time it is triggered by Gate current pulse or the applied voltage reaches the forward breakover voltage.

Triggering by Gate Current Pulse – When it is triggered by the gate current pulse, it starts conducting and will act as a close switch. The Thyristors remains in the ON-state, i.e. it remains in the latched state. Here the gate loses its control to turn off the device.

Triggering by Forward Breakover Voltage – When a forward voltage is applied, a leakage current starts to flow through the blocking (J2) in the middle junction of Thyristors. When voltage exceeds the forward break over voltage or critical limit, then J2 breaks down and it reaches to the ON state.

When the Gate current (Ig) is increased, it reduces the blocking area and so the forward break over voltage is reduced. It will turn ON when a minimum current called latching current is maintained.

When the gate current Ig=0 and anode current falls below a certain value called holding current during the ON state, it again reaches to its forward blocking state.

Voltage Ampere (VI) Characteristics of Thyristor

Fig. 4 – Voltage Ampere (VI) Characteristics of Thyristor

Thyristors in Reverse Biased State

If the anode is negative with respect to cathode, i.e., with the application of reverse voltage, both PN junctions at the end i.e. J1 and J3 become reverse biased and the centre junction J2 becomes forward biased. Only a small leakage current flows through it. This is the reverse voltage blocking mode or OFF state of Thyristor.

When the reverse voltage is increased further, then at a certain voltage, avalanche breakdown of J1 and J2 occurs and it starts conducting in the reverse direction. The maximum reverse voltage at which a thyristor starts conducting is known as Reverse Breakdown Voltage.

Summary

  • Thyristor blocks voltage in both forward and reverse direction and thus a symmetric blocking is formed.
  • A Thyristor turns ON by the application of positive gate current and turns OFF when the anode voltage drops to zero.
  • A small current from gate to cathode can trigger the Thyristor by changing it from open circuit to short circuit.

Modes of Operation of Thyristor

A Thyristor has three operating modes. They are: –

  • Forward Blocking
  • Reverse Blocking
  • Forward Conducting

Forward Blocking

In this state or mode, the forward current conduction is blocked .The upper diode and lower diode are forward biased and the junction in the center is reverse biased. Thus the Thyristor does not turn on as the gate is not fired and no current flows through it.

Reverse Blocking

In this mode, the connection of anode and cathode is reversed and still no current flows through it. Thyristors can conduct current only in one direction and it blocks in the reverse direction and so the flow of current is blocked.

Forward Conducting

When current is applied to the gate, the Thyristor is triggered and it will start conducting. This stays on until the forward current drops below the threshold value and that can be achieved by switching off the circuit.

Types of Thyristors

Based on the turning on and off capabilities and the physical structure, Thyristor are classified as:

  • Silicon Controlled Thyristors (SCR)
  • Emitter turn off Thyristors (ETO)
  • Fast Switching Thyristors (SCR)
  • Light Activated Silicon Controlled Rectifiers (LASCR)
  • Gate Turn Off Thyristors (GTO)
  • Reverse Conducting Thyristors (RCT)
  • FET Controlled Thyristors (FET-CTH)
  • MOS Turn Off Thyristors (MTO)
  • Bidirectional Phase Controlled Thyristors (BCT)

Applications of Thyristor

Thyristor is used in various applications such as:

  • Mainly used in variable speed motor drives.
  • Used in controlling high power electrical application.
  • Used mainly in AC motors, lights, welding machines etc.
  • Used in fault current limiter and circuit breaker.
  • Fast switching speed and low conduction is possible in ETO thyristor.
  • Used as light dimmers in television, movie theatres.
  • Used in photography for flashes.
  • Can be used in burglar alarms.
  • Used in electric fan speed control.
  • Used in car ignition switches.

Advantages of Thyristor

The advantages of Thyristor includes:

  • Low cost.
  • Can be protected with the help of fuse.
  • Can handle large voltage/ current.
  • Able to control AC power.
  • Very easy to control.
  • Easy to turn on.
  • GTO or Gate Turnoff Thyristor has high efficiency.
  • Takes less time to operate.
  • Thyristor switches can operate with large frequency.
  • Requires less space when compared to mechanical switches.
  • Can be used for robust operations.
  • Maintenance cost of Thyristor is very less.
  • Very easy to use for sophisticated controlling.
  • Power handling capacity is very good.
  • Can be used as an oscillator in digital circuits.
  • Can be connected in parallel and in series to provide electronic control at high power levels.
  • Thyristors conduct current only in one direction.
  • It can be used as a protection device, like a fuse in a power line.

Disadvantages of Thyristor

  • The disadvantages of Thyristor includes:
  • Cannot be used for higher frequencies.
  • In AC circuit, Thyristor needs to be turned on each cycle.
  • SCR takes time to turn on and off. This causes delay or damage in the load.
  • It can stop the motor when connected, but cannot hold it stationary.
  • The response rate of Thyristor is very low.
  • Not much use in DC circuits, as the Thyristor cannot be cutoff just by removing the gate drive.
  • Low Efficiency.
  • Latching and Holding current is more in GTO Thyristor.
  • Reverse blocking capability of voltage is less than forward blocking capability.
  • Reliability of TRIAC thyristor is less than SCR.
  • TRIACs have lower dv/dt rating when compared to SCR.
Author: Sajana M, Academics

 

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