MSF 1433

Semiconductor Devices (Dr. Mohamad Alam Saeed)

Prepared by Azwin Hamdan (MS111039)

Thyristors

Content

What is thyristor?General ideaCharacteristics How it works?Advantages Applications

What is Thyristors?

Thyristors

SCR

Diac

GTO

MCT

IGCT

Triac

*Family of semiconductor devices*SCR, IGCT,MCT etc

The oldest member of this family is Silicon Controlled Rectifier (SCR).

Because it widely used, the word thyristor has become synonymous with it.

Thyristors ≈ SCR

Definition : A controlled electronic switch (can only be turned on/off electronically)

Consists of 4 layers of differently doped Silicon rather than the 3 layers of the conventional bipolar transistor.

Thyristor FamilyDevice Name

Description

Feature Function

SCR Rectifier with control gate

One way switch

Phase control switching

Triac Two inverse parallel SCRs

Two way AC switch

Phase control switching

Diac Two way triggering device

Low voltage VBO trigger

Switching

Quadrac Triac and Diac in one package

Phase control switching

General Idea

• Consider this circuit and observe the switch.

• Thyristor is just like a switch before the gate signal is applied.

When a triggering signal is applied, the switch turns ON and current starts to flow.

Characteristics

SymbolHas anode and cathode

terminals like diodeBut, there is an additional

terminal = gate (control terminal and triggering signal are applied

here

I-V Characteristics

I-V Characteristics

SCR conducts in first quadrant When gate signal is applied, SCR

made sufficiently forward biased to cross holding current limit, it starts to conduct.

Once in conduction state, it continues to conduct even the gate is removed.

Special technique ~ commutation

2 methods

- Anode current interruption method (turn OFF the switch)

- Forced commutation method(reverse polarity battery is connected, so current through SCR is decreased and turn off)

I-V Characteristics

How It Works?

• has 4 layers of pn junction device•Without any application voltage, it has 3 diffusion region (J1, J2,J3)

J1

J2

J3

How It Works? If we apply +ve at

anode with respect to cathode, J1 and J3 become forward biased while J2 in reverse biased.

This will remain until a +ve signal is applied at the gate terminal.

How It Works? So, when +ve

signal is applied at the gate, J2 will turns to forward biased state and current starts to flow.

Thyristor as Two Transistor Model

Revealed that it actually made up of PNP and NPN transistor.

Collector is connected to the base of 2nd. Gate is connected to the base of NPN

transistor.

Thyristor as Two Transistor Model On application of

signal, the NPN transistor conducts sending a signal to the base of PNP transistor which in turn conducts and send another signal to base of NPN.

The process continues.

-

+

+

Why Thyristor/ SCR?

Simple to use Cheap to buy Easy to build (thyristor circuits) Able to handle the voltage and

current required for high power applications

Has good thermal properties

Applications

Series Static SwitchBattery Charging Regulator

Temperature ControllerEmergency Lighting System

Series Static Switch

• If the switch is closed as shown in the Fig. 20.11b, a gate current will flow during the positive portion of the input signal, turning the SCR on.•A half-wave series static switch is shown in Fig. 20.11a.

Series Static Switch

Resistor R1 limits the magnitude of the gate current. When the SCR turns on, the anode-to cathode

voltage (VF) will drop to the gate circuitry. For the negative region of the input signal, the SCR

will turn off since the anode is negative with respect to the cathode.

The diode D1 is included to prevent a reversal in the gate current.

Battery Charging Regulator

• D1 and D2 establish a full-wave-rectifier signal across SCR1 and the 12-V battery to be charged.

• At low battery voltages, SCR2 is in the “off” state.

• With SCR2 open, the SCR1 controlling circuit is exactly the same as the series static switch control.

Battery Charging Regulator

• When the full-wave-rectifier input is sufficiently large to produce the required turn-on gate current (controlled by R1), SCR1 will turn on and charging of the battery will commence.

• At the start of charging, the low battery voltage will result in a low voltage VR as determined by the single voltage-driver circuit.

Battery Charging Regulator

• Voltage VR is turn too small to cause 11.0-V Zener conduction.

• In the “off” state, the Zener is effectively an open circuit, maintaining SCR2 in the “off” state since the gate current is zero.

• The capacitor C1 is included to prevent any voltage transients in the circuit from accidental turn on the SCR2.

Battery Charging Regulator

• As charging continues, the battery voltage rises to a point where VR is sufficiently high to both turn on the 11.0-V Zener and fire SCR2.

• Once, SCR2 has fired, the short-circuit representation for SCR2 will result in a voltage-divider circuit determined by R1 and R2 that will maintain V2 at a level too small to turn SCR1 on.

Battery Charging Regulator

• When this occurs, the battery is fully charged and the open circuit state of SCR1 will cut off the charging current.

• Thus, the regulator recharges the battery whenever the voltage drops and prevents overcharging when fully charged.

Temperature Controller• SCR serves as a

current amplifier in a load-switching element.

• It is not an amplifier in the sense that it magnifies the current level of the thermostat. Rather it is advice whose higher current level is controlled by the behavior of the thermostats.

Emergency Lighting System

• Charging will only take place when the anode of D1 is more positive than its cathode.

• The dc level of the full-wave-rectified signal will ensure that the bulb is lit when the power is on.

Emergency Lighting System

If the power should fail, the capacitor C1, will discharge through D1, R1 and R3, until the cathode of SCR1 is less positive than the anode. At the same time the junction of R2 and R3 will become positive and establish sufficient gate-to-cathode voltage to trigger the SCR.

END

References

http://www.electronics-radio.com/articles/electronic_components/scr/what-is-a-thyristor.php

http://www.circuitstoday.com/scr-principle-of-operation

Muhammad Usman Yusuf’s Slide (youtube – Thyristor)