Introductory lecture on Thyristor family.

1. Thyristor Family
Rajesh B. Lohani
Dept. of E & TC
Goa College of Engineering
1. SCR,
2. Diac,
3. Triac,
4. SCS,
5. GTO,
6. Light activated SCR.
Constructional Features, Operating Principle,
Characteristics and Specification
Contents:
References
R. Boylestad, L. Nashelsky; Electronic Devices and Circuits, PHI.
P. S. Bimbhra; Power Electronics – Khanna Publishers.
Mohd. Rasheed; Power Electronic Circuits, Devices and Applications; Pearson Education.
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2. Dedicated to my loving parents
Late. Janaki B. Lohani and Mr. Basant B. Lohani
(Retd. Principal)
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3. 1) The concept and features
Power electronic devices: are the electronic devices
that can be directly used in the power processing circuits
to convert or control electric power.
Very often: Power electronic devices= Power semiconductor
Major material used in power semiconductor devices——
In broad sense
Power electronic devices
Vacuum devices: Mercury arc
rectifier thyratron, etc. . seldom
in use today
Semiconductor devices:
major power electronic devices
An introductory overview of power electronic devices
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4. a) The electric power that power electronic device deals with is usually much larger
than that the information electronic device does.
b) Usually working in switching states to reduce power losses
c)Need to be controlled by information electronic circuits.
d)Very often, drive circuits are necessary to interface between information circuits and
power circuits.
e)Dissipated power loss usually larger than information electronic devices —special
packaging and heat sink are necessary.
On-state Voltage across the device is 0 p=vi=0
V=0
Off-state Current through the device is 0 p=vi=0
i=0
Features of power devices
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5. 1. SCR
Thyristor or Silicon Controlled Rectifier (SCR)
Silicon Controlled Rectifier (SCR) is a unidirectional
semiconductor device made of silicon. This device is the solid
state equivalent of thyratron and hence it is also referred to as
thyristor or thyroid transistor. In fact, SCR (Silicon Controlled
Rectifier) is a trade name given to the thyristor by General
Electric Company.
Basically SCR is a three-terminal, four-layer
semiconductor device consisting of alternate layers of p-type
and n-type material. Hence it has three p-n junctions J1, J2 and
J3. The figure below shows an SCR with the layers p-n-p-n. The
device has terminals Anode(A), Cathode(K) and the Gate(G).
The Gate terminal(G) is attached to the p-layer nearer to the
Cathode(K) terminal.
https://www.electrical4u.com/thyristor-silicon-controlled-rectifier-scr/
The symbol of SCR or Thyristor is shown in figure above.
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6. An SCR can be considered as two inter-connected transistors as
shown below.
It is seen that a single SCR is the combination of one
pnp transistor (Q1) and one npn transistor (Q2). Here, the
emitter of Q1 acts as the anode terminal of the SCR while the
emitter of Q2 is its cathode. Further, the base of Q1 is
connected to the collector of Q2 and the collector of Q1 is
connected to the base of Q2. The gate terminal of the SCR is
connected to the base of Q2, too.
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7. The working of SCR can be understood by analyzing its
behaviour in the following modes:
In this mode, the SCR is reverse biased by
connecting its anode terminal (A) to negative end and
the cathode terminal (K) to the positive end of the
battery. This leads to the reverse biasing of the
junctions J1 and J3, which in turn prohibits the flow of
current through the device, in spite of the fact that
the junction J2 remains in forward biased condition.
In this state, the SCR behaves as a typical
diode. In this reverse biased condition, only reverse
saturation current flows through the device as in the
case of the reverse biased diode which is shown in the
characteristic curve by blue line. The device also exhibits
the reverse breakdown phenomenon beyond a reverse
safe voltage limit just like a diode.
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8. Forward Blocking Mode of SCR
Here a positive bias is applied to
the SCR by connecting anode terminal (A) to
the positive and cathode terminal (K) to the
negative terminal of the battery, as shown in
the figure below. Under this condition, the
junction J1 and J3 get forward biased while
junction J2 gets reverse biased.
Here also current cannot pass through the
thyristor except the tiny current flowing as saturation
current as shown by the blue curve in the
characteristics curve below.
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9. Forward Conduction Mode of SCR
The SCR can be made to conduct either
(i) By increasing the positive voltage applied at anode terminal (A) beyond the Break Over Voltage, VB or
(ii) By applying positive voltage at the gate terminal (G) as shown in the figure below.
In the first case, the increase in the applied bias
causes the initially reverse biased junction J2 to break down at
the point corresponding to forward Break Over Voltage, VB.
This results in the sudden increase in the current flowing
through the SCR as shown by the pink curve in the
characteristic curve, although the gate terminal of the SCR
remains unbiased.
However, SCR can also be turned on at a much
smaller voltage level by proving small positive voltage at the
gate terminal.
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10. SCR can also be turned on at a much
smaller voltage level by proving small positive
voltage at the gate terminal. The reason behind
this can be better understood by considering
the transistor equivalent circuit of the SCR
shown in the figure below.
Here it is seen that on applying a positive
voltage at the gate terminal, transistor Q2 switches
ON and its collector current flows into the base of
transistor Q1. This causes Q1 to turn ON which in turn
results in the flow of its collector current into the base
of Q2. This causes either transistor to get saturated at
a very rapid rate and the action cannot be stopped
even by removing the bias applied at the gate
terminal, provided the current through the SCR is
greater than that of the Latching current. Here the
latching current is defined as the minimum current
required to maintain the SCR in conducting state even
after the gate pulse is removed.
In such state, the SCR is said to be latched
and there will be no means to limit the current
through the device, unless by using an external
impedance in the circuit. This necessitates one to
resort for different techniques like Natural
Commutation, Forced Commutation or Reverse Bias
Turn Off and Gate Turn-Off to switch OFF a conducting
SCR. Holding current is defined as the minimum
current to maintain the SCR in its conducting mode.10

11. There are many variations of SCR devices viz.,
Reverse Conducting Thyristor (RCT), Gate Turn-Off Thyristor
(GTO), Gate Assisted Turn-Off Thyristor (GATT), Asymmetric
Thyristor, Static Induction Thyristors (SITH), MOS Controlled
Thyristors (MCT), Light Activated Thyristors (LASCR) etc.
Normally SCRs have high switching speed and can handle
heavy current flow.
Applications:
Power switching circuits (for both AC and DC)
Zero-voltage switching circuits
Over voltage protection circuits
Controlled Rectifiers
Inverters
AC Power Control (including lights, motors, etc.)
Pulse Circuits
Battery Charging Regulator
Latching Relays
Computer Logic Circuits
Remote Switching Units
Phase Angle Triggered Controllers
Timing Circuits
IC Triggering Circuits
Welding Machine Control
Temperature Control Systems
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12. Diac2.
https://www.electrical4u.com/diac/
Diac is a device which has two
electrodes. It is a member of the thyristor
family. It is mainly used in triggering of
thyristor. The advantage of using this device is
that it can be turned on or off simply by
reducing the voltage level below its avalanche
breakdown voltage.
Construction of Diac
It is a device which consists of four layers and
two terminals. The construction is almost same as that
of the transistor. But there are certain points which
deviate from the construction from the transistor. The
differentiating points are-
1. There is no base terminal in the diac.
2. The three regions have almost same level of
doping.
3. It gives symmetrical switching characteristics for
either polarity of voltages.
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13. Operation of Diac
From the figure, we see that it has two p-type material and
three n-type materials. Also it does not have any gate terminal in it.
The diac can be turned on for both the polarity of voltages.
When A2 is more positive with respect to A1 then the current does not
flows through the corresponding N-layer but flows from P2-N2-P1-N1.
When A1 is more positive A2 then the current flows through P1-N2-P2-
N3.
The construction resembles the diode connected in series.
When applied voltage is small in either polarity, a very small current
flows which is known as leakage current because of drift of electrons
and holes in the depletion region. Although a small current flows, but
it is not sufficient enough to produce avalanche breakdown so the
device remains in the non conducting state.
When the applied voltage in either polarity exceeds the
breakdown voltage, diac current rises and the device conducts in
accordance with its V-I characteristics.
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14. VI Characteristics of Diac The V-I characteristics resembles the english word
Z. The diac acts as open circuit when the voltage is less than
its avalanche breakdown voltage. When the device has to be
turned off, the voltage must be reduced below its avalanche
breakdown voltage.
Application of Diac
It can be used mainly in the triac triggering circuit.
The diac is connected in the gate terminal of the triac. When
the voltage across the gate decreases below a predetermined
value, the gate voltage will be zero and hence the triac will
be turned off.
The main applications are-
1.It can be used in the lamp dimmer circuit.
2.It is used in the heat control circuit.
3.It is used in the speed control of a universal motor.
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15. 3. Triac
Triac is a three terminal AC switch which is
different from the other silicon controlled rectifiers in the
sense that it can conduct in both the directions that is
whether the applied gate signal is positive or negative, it will
conduct. Thus, this device can be used for AC systems as a
switch.
This is a three terminal, four layer, bi-directional
semiconductor device that controls AC power. The triac of
maximum rating of 16 kw
Figure shows the symbol of triac, which has two
main terminals MT1 and MT2 connected in inverse parallel
and a gate terminal.
https://www.electrical4u.com/triac/
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16. Construction of Triac
Two SCRs are connected in inverse parallel with gate
terminal as common. Gate terminals is connected to both the N
and P regions due to which gate signal may be applied which is
irrespective of the polarity of the signal. Here, we do not have
anode and cathode since it works for both the polarities which
means that device is bilateral. It consists of three terminals
namely, main terminal 1(MT1), main terminal 2(MT2), and gate
terminal G.
Figure shows the construction of a triac. There are
two main terminals namely MT1 and MT2 and the remaining
terminal is gate terminal.
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17. Operation of Triac
The triac can be turned on by applying the gate voltage higher than break over voltage. However,
without making the voltage high, it can be turned on by applying the gate pulse of 35 micro seconds to turn it on.
When the voltage applied is less than the break over voltage, then generally we tend to use the gate triggering
approach to turn it on.
There are four different modes of operations, they are-
1.When MT2 and Gate being Positive with Respect to MT1
When this happens, current flows through the path P1-N1-P2-N2. Here, P1-N1 and P2-N2 are forward biased
but N1-P2 is reverse biased. The triac is said to be operated in positively biased region. Positive gate with respect
to MT1 forward biases P2-N2 and breakdown occurs.
2.When MT2 is Positive but Gate is Negative with Respect to MT1
The current flows through the path P1-N1-P2-N2. But P2-N3 is forward biased and current carriers injected
into P2 on the triac.
3.When MT2 and Gate are Negative with Respect to MT1
Current flows through the path P2-N1-P1-N4. Two junctions P2-N1 and P1-N4 are forward biased but the
junction N1-P1 is reverse biased. The triac is said to be in the negatively biased region.
4.When MT2 is Negative but Gate is Positive with Respect to MT1
P2-N2 is forward biased at that condition. Current carriers are injected so the triac turns on. This mode of
operation has a disadvantage that it should not be used for high (di/dt) circuits. Sensitivity of triggering in mode 2
and 3 is high and if marginal triggering capability is required, negative gate pulses should be used. Triggering in
mode 1 is more sensitive than mode 2 and mode 3. 17

18. Characteristics of a Triac
First Quadrant Operation of Triac
Voltage at terminal MT2 is positive with respect to terminal
MT1 and gate voltage is also positive with respect to first
terminal.
Second Quadrant Operation of Triac
Voltage at terminal 2 is positive with respect to terminal 1
and gate voltage is negative with respect to terminal 1.
Third Quadrant Operation of Triac
Voltage of terminal 1 is positive with respect to terminal 2
and the gate voltage is negative.
Fourth Quadrant Operation of Triac
Voltage of terminal 2 is negative with respect to terminal 1
and gate voltage is positive.
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19. Advantages of Triac
1.It can be triggered with positive or negative polarity of gate pulses.
2.It requires single heat sink of slightly larger size, whereas for SCR, two heat sinks should be required of smaller size.
3.It requires single fuse for protection.
4.A safe breakdown in either direction is possible but for SCR protection should be given with parallel diode.
Disadvantages of Triac
1.They are not much reliable compared to SCR.
2.It has (dv/dt) rating lower than SCR.
3.Lower ratings are available compared to SCR.
4.We need to be careful about the triggering circuit as it can be triggered in either direction.
Uses of Triac
1.They are used in control circuits.
2.It is used in High power lamp switching.
3.It is used in AC power control.
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20. Silicon-Controlled Switch (SCS)4.
Silicon controlled switch (SCS),
like the SCR, is a unilateral, four layer three
junction P-N-P-N silicon device with four
electrodes namely cathode C, cathode gate
Gx, anode gate G2 and the anode A, as
shown in figure.
The SCS is a low power device
compared with the SCR. It handles currents
in milli amperes rather than amperes. SCS
differs from an SCR in the following
aspects. It has an additional gate the
anode gate. It is physically smaller than
SCR. It has smaller leakage and holding
currents than SCR. It needs small triggering
signals. It gives more uniform triggering
characteristics from sample to sample.
The basic structure and schematic symbol of
SCS are shown in the figures. It may be fabricated by
using either the grown junction technique or the planar
technique.
http://www.circuitstoday.com/scs-silicon-controlled-switch
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21. Operation of a Silicon Controlled Switch
The easiest way to understand how it operates is to
realize it to be formed of two transistors Q1 and Q2 placed
back-to-back, as shown in figure.
In a two-transistor equivalent circuit shown in figure,
it is seen that a negative pulse at the anode gate G2 causes
transistor Q1 to switch on. Transistor Q1 supplies base current
to transistor Q2, and both transistors switch-on. Similarly, a
positive pulse at the cathÂode gate G1 can switch the device
on. Since only small currents are involved, the SCS may be
switched off by an appropriate polarity pulse at one of the
gates. At the cathode gate a negative pulse is required for
switching-off while at the anode gate a positive pulse is
needed.
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22. Volt-Ampere Characteristic of SCS
The volt-ampere characteristic of an SCS is similar to
that of an SCR and is shown in figure. With the increase in
applied voltage, the current first increases slowly upto point A
and then rapidly in the region AB, as shown in the figure. At
point B, the product β1β2 exceeds unity and the device is
suddenly switched on. In the on-state, the current increases
enormously and is limited by the external series resistor. SCS
also exhibits negative differential resistance in the on region
similar to SCR. SCS gets switched on accidentally if the anode
voltage gets applied suddenly. This is known as rate effect,
which is caused by inter-electrode capacitance between elec-
trodes G1 and G2, known as transition capacitance.
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23. Advantages and Applications of SCS
An advantage of SCS over an SCR is the reduced turn-
off time, typically within the range of 1 to 10 micro seconds for
the SCS and 5 to 30 micro seconds for the SCR. Other
advantages of the SCS over SCR are increased control and
triggering sensitivity and a more predictable firing situation.
However, the SCS is limited to low power, current, and voltage
ratings (typical maximum anode currents range from 100 mA to
300 mA with dissipation rating of 100 to 500 mW).
A few of the more common areas of application of
SCS include a variety of computer circuits (such as counters,
registers, and timing circuits) voltage sensors, pulse generators,
oscillators etc.
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24. 6. Gate Turn-off Transistor
A Gate Turn off Thyristor or GTO is a three terminal,
bipolar (current controlled minority carrier) semiconductor
switching device. Similar to conventional thyristor, the
terminals are anode, cathode and gate as shown in figure
below. As the name indicates, it has gate turn off capability.
These are capable not only to turn ON the main
current with a gate drive circuit, but also to turn it OFF. A small
positive gate current triggers the GTO into conduction mode
and also by a negative pulse on the gate, it is capable of being
turned off. Observe in below figure that the gate has double
arrows on it which distinguish the GTO from normal thyristor.
This indicates the bidirectional current flow through the gate
terminal.
On the other hand, during the conduction state GTO behaves just like a thyristor with a small ON state
voltage drop. The GTO has faster switching speed than the thyristor and has higher voltage and current ratings than
the power transistors. 24

25. Construction of a Gate Turn-Off Thyristor
Consider the below structure of GTO, which is almost
similar to the thyristor. It is also a four layer, three junction P-N-
P-N device like a standard thyristor. In this, the n+ layer at the
cathode end is highly doped to obtain high emitter efficiency.
This result the breakdown voltage of the junction J3 is low
which is typically in the range of 20 to 40 volts.
The doping level of the p type gate is highly graded
because the doping level should be low to maintain high
emitter efficiency, whereas for having a good turn OFF
properties, doping of this region should be high. In addition,
gate and cathodes should be highly interdigited with various
geometric forms to optimize the current turn off capability.
The junction between the P+ anode and N base is called anode junction. A heavily doped P+ anode region is
required to obtain the higher efficiency anode junction so that a good turn ON properties is achieved. However, the
turn OFF capabilities are affected with such GTOs.
25

26. Gate Turn-Off Thyristor Operation Principles
The turn ON operation of GTO is similar to a conventional
thyristor. When the anode terminal is made positive with respect to
cathode by applying a positive gate current, the hole current injection
from gate forward bias the cathode p-base junction.
This results in the emission of electrons from the cathode
towards the anode terminal. This induces the hole injection from the
anode terminal into the base region. This injection of holes and
electrons continuous till the GTO comes into the conduction state.
In case of thyristor, the conduction starts initially by turning
ON the area of cathode adjacent to the gate terminal. And thus, by
plasma spreading the remaining area comes into the conduction.
Unlike a thyristor, GTO consists of narrow cathode elements
which are heavily interdigitated with gate terminal, thereby initial
turned ON area is very large and plasma spreading is small. Hence the
GTO comes into the conduction state very quickly.
26

27. V-I Characteristics of Gate Turn-Off Thyristor
During the turn ON, GTO is similar to thyristor in its
operates.So the first quadrant characteristics are similar to the
thyristor. When the anode is made positive with respect to
cathode, the device operates in forward blocking mode. By the
application of positive gate signal triggers the GTO into
conduction state.
The latching current and forward leakage currents are
considerably higher in GTO compared to the thyristor as shown
in figure. The gate drive can be removed if the anode current is
above the holding current level.
But it is recommended not to remove the positive
gate drive during conduction and to hold at value more than
the maximum critical gate current. This is because the cathode
is subdivided into small finger elements as discussed above to
assist the turn OFF process.
This causes the anode current dips below the holding
current level transiently, which forces a high anode current at a
high rate back into the GTO. This can be potentially destructive.
Therefore, some manufacturers recommend the continuous
gate signal during the conduction state.https://www.electronicshub.org/gate-turn-off-thyristor/ 27

28. Gate Turn-Off Thyristor Applications
Due to the advantages like excellent switching
characteristics, no need of commutation circuit, maintenance-
free operation, etc makes the GTO usage predominant over
thyristor in many applications. It is used as a main control
device in choppers and inverters. Some of these applications
are
•AC drives
•DC drives or DC choppers
•AC stabilizing power supplies
•DC circuit breakers
•Induction heating
•And other low power applications
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29. What are the disadvantages of GTO?
Compared to a conventional SCR, the device has the following disadvantages
•Magnitude of latching, holding currents is more. The latching current of the GTO is several times more as
compared to conventional thyristors of the same rating.
•On state voltage drop and the associated loss is more.
•Due to multi-cathode structure of GTO, triggering gate current is higher than that required for normal SCR.
•Gate drive circuit losses are more. Its reverse voltage blocking capability is less than the forward voltage
blocking capability.
What are the advantages of GTO?
The prime design goal of GTO devices are to achieve fast turn off time and high current turn off capability
and to enhance the safe operating area during turn off. The GTO’s turn off occurs by removal of excess holes
in the cathode base region by reversing the current through the gate terminal. Compare to BJT the GTO has
the following advantages:
• High blocking voltage capabilities
• High over current capabilities
•exhibits low gate currents
•fast and efficient turn off http://www.completepowerelectronics.com/gate-turn-off-thyristor-gto/ 29

30. 7. Light activated SCR.
LASCR or light activated SCR is a semiconductor
device which turns ON when it is exposed to light. The
constituent element of SCR is silicon, and it works like a
rectifier, and thus, it is termed as Silicon Controlled Rectifier.
The LASCR is a type of thyristor which is triggered by photons
present in the light rays.
It is a three terminal device, consists of cathode,
anode and gate terminal. The gate terminal is used when the
electrical triggering is supplied to the LASCR. The advantage of
using triggering of the thyristor by light is prevention from
electrical noise disturbances. Thus, LASCR is considered to be
one of the best devices.
30

31. Construction of LASCR
The LASCR is made up of silicon material, and
the glass lens in the LASCR is used to focus the light from
the external source on the semiconductor material. The
silicon pellet is used in the bottom of the device, and the
light intensity dislodges electrons in the semiconductor
crystal and contributes to conduction.
The constructional architecture of the LASCR is
described below in the diagram.
https://electronicscoach.com/light-activated-scr.html
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32. Working of LASCR
The LASCR works on the principle of photoconduction that is conduction
due to photon striking the semiconductor surface. The LASCR is basically
a thyristor; it is made up of semiconductor material. The light rays falling
on the device are focused at one place to intensify it.
The more the intensity of light, the more will be the current through the
LASCR. The internal architecture of LASCR consists of two transistors in
such a way that the collector of one transistor is connected to the base
of another transistor.
The light falling on the light activated SCR generates the electron from
the valence band, and these electrons will enter conduction band. The
electrons will move from collector of one region to base of another
region, and then the cascading effect can be seen.
The best thing about Light Activated SCR is that they do not turn off
even when the supply of external light is ceased. If you want to turn off
the SCR, then you need to reverse the properties of electrodes.
32

33. Applications of the Light Activated SCR
1.Low Power Applications: The Light activated SCR are generally used for the application
which requires low power to operate. This is because power generated by SCR is low in
magnitude.
2.Motor Control: The Light Activated SCR finds applications in the working of Motor
Control.
3.Computer Applications: The components used in the computer system also require
LASCR for meeting power requirements.
4.Optical light Controls: The optical light control use the principle of photoconduction
for generating the control signals. Therefore, the LASCR finds extensive application in
Optical light control.
5.Solid State Relay: In solid state relays, two LASCR are connected in reverse parallel so
that they can generate power in both the half cycle of AC.
33

34. Acknowledgement
I would like to thank Dr. Samarth Borkar and Mr. Pushpshil
Satardekar for their technical support and constructive criticism.
34