SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium- to high-voltage AC power control applications, such as lamp dimming, power regulators and motor control.

1. `
Presentation on Basic Structure of Silicon
Controlled Rectifier (SCR)
Presented by: Submitted to:
1. Narayan Datta (EEE 013 06147) Ms. Rebeka Sultana
2. Md Fakhrul Islam (EEE 013 06172) Lecturer, Dept. Of EEE
3. Md Elias (EEE 013 06179) Port City International University
4. Md Yeasir Arafat (EEE 013 06171)
5. Dibbharaj Sarkar (EEE 013 06183)

2. Table of Contents
• 1) What is SCR?
• 2) SCR Application
• 3) Construction of Silicon Control rectifier
• 4) Working modes of operation of SCR
• 5) V-I characteristics curve explanation
• 6) Transistor Analogy of SCR
• 7) Methods of SCR
• 8) Using Control of SCR
• 9) Advantaged of SCR
• 10) Summary

3. What is SCR?
The Silicon Controlled Rectifier (SCR) is the most important and
mostly used member of the thyristor family. SCR can be used for
different applications like rectification, regulation of power and
inversion, etc.
Like a diode, SCR is a unidirectional device that allows the current in
one direction and opposes in another direction.
SCR is a three terminal device; anode, cathode and gate as shown
in figure. SCR has built in feature to turn ON or OFF and its
switching is controlled by biasing conditions and gate input terminal.
This results in varying the average power delivered at the load , by
varying the ON periods of the SCR. It can handle several thousands
of voltages and currents. SCR symbol and its terminals are shown in
figure.

4. SCR Applications:
• Use to load current, 0.8A to 10000A or more.
• Lighting control
• Speed ​​control of motor
• Rectifier circuits to convert alternating current to direct current.
• Or Convert from direct current to alternating current.
• Use instead of a switch or a relay to turn the load on and off.
• Switch circuits that require very high speeds.
• Need a spark-free switch that works.
• Used instead of a very high current diode. The advantage is that it is
controlled by small currents. Like transistor.
• And more.

5. Construction of Silicon Controlled Rectifier
• The SCR is a four layer and three terminal device. The four
layers made of P and N layers, are arranged alternately
such that they form three junctions J1, J2 and J3. These
junctions are either alloyed or diffused based on the type
of construction.
• The outer layers (P and N-layers) are heavily doped
whereas middle P and N-layers are lightly doped. The gate
terminal is taken at the middle P-layer, anode is from outer
P- layer and cathode is from N- layer terminals. The SCR is
made of silicon because compared to germanium leakage
current in silicon is very small.
• This construction is mainly used for high power Silicon
Controlled Rectifiers. To provide high mechanical strength,
the SCR is braced with plates made up of either
molybdenum or tungsten. And one of these plates is
soldered to a copper stud which is further threaded to
connect the heat sink.

6. Working or Modes of Operation of SCR
Depending on the biasing given to the SCR, the
operation of SCR is divided into three modes. They are
1. Forward blocking Mode
2. Forward Conduction Mode and
3. Reverse Blocking Mode

7. Forward Blocking Mode
• In this mode of operation, the Silicon Controlled
Rectifier is connected such that the anode
terminal is made positive with respect to
cathode while the gate terminal kept open. In
this state junctions J1 and J3 are forward biased
and the junction J2 reverse biased.
• Due to this, a small leakage current flows
through the SCR. Until the voltage applied
across the SCR is more than the break over
voltage of it, SCR offers a very high resistance to
the current flow. Therefore, the SCR acts as a
open switch in this mode by blocking forward
current flowing through the SCR as shown in the
VI characteristics curve of the SCR.

8. Forward Conduction Mode
• In this mode, SCR or thyristor comes into the conduction
mode from blocking mode. It can be done in two ways as
either by applying positive pulse to gate terminal or by
increasing the forward voltage (or voltage across the anode
and cathode) beyond the break over voltage of the SCR.
• Once any one of these methods is applied, the avalanche
breakdown occurs at junction J2. Therefore the SCR turns
into conduction mode and acts as a closed switch thereby
current starts flowing through it.
• It is also noted that if gate current is increasing, the voltage
required to turn ON the SCR is less if gate biasing is
preferred. The current at which the SCR turns into
conduction mode from blocking mode is called as latching
current (IL).
• And also when the forward current reaches to level at which
the SCR returns to blocking state is called as holding current
(IH). At this holding current level, depletion region starts to
develop around junction J2. Hence the holding current is
slightly less than the latching current.

9. Reverse Blocking Mode
• In this mode of operation, cathode is made positive
with respect to anode. Then the junctions J1 and J3
are reverse biased and J2 is forward biased. This
reverse voltage drives the SCR into reverse blocking
region results to flow a small leakage current
through it and acts as an open switch as shown in
figure.
• So, the device offers a high impedance in this mode
until the voltage applied is less than the reverse
breakdown voltage VBR of the SCR. If the reverse
applied voltage is increased beyond the VBR, then
avalanche breakdown occurs at junctions J1 and J3
which results to increase reverse current flow
through the SCR.
• This reverse current causes more losses in the SCR
and even to increase the heat of it. So there will be
a considerable damage to the SCR when the reverse
voltage applied more than VBR.

10. V-I Characteristics of SCR
The V-I characteristics of SCR is shown in the
below figure. The horizontal line in the below
figure across the SCR whereas the vertical
line represents the amount of current flows
in the SCR.
VA = Anode voltage, IA = Anode current, +VA =
Forward anode voltage, +IA = Forward
represents the amount of voltage applied
anode current, -VA = Reverse anode voltage,
+IA = Reverse anode current
The V-I characteristics of SCR is divided into
three regions:
•Forward blocking region
•Forward conduction region
•Reverse blocking region

11. Forward blocking region
• In this region, the positive voltage (+) is given to anode (+), negative
voltage (-) is given to cathode (-), and gate is open circuited. Due to
this the junction J1 and J3 become forward biased while J2 become
reverse biased. Therefore, a small leakage current flows from anode
to cathode terminals of the SCR. This small leakage current is known
as forward leakage current.
• The region OA of V-I characteristics is known as forward blocking
region in which the SCR does not conduct electric current.

12. Forward Conduction region
• If the forward bias voltage applied between anode and cathode is increased
beyond the breakdown voltage, the minority carriers (free electrons in anode and
holes in cathode) gains large amount of energy and accelerated to greater
velocities. This high speed minority carriers collides with other atoms and
generates more charge carriers. Likewise, many collisions happens with atoms.
Due to this, millions of charge carriers are generated. As a result depletion region
breakdown occurs at junction J2 and current starts flowing through the SCR. So
the SCR will be in On state. The current flow in the SCR increases rapidly after
junction breakdown occurs.
• The voltage at which the junction J2 gets broken when the gate is open is called
forward breakdown voltage (VBF).
• The region BC of the V-I characteristics is called conduction region. In this region,
the current flowing from anode to cathode increases rapidly. The region AB
indicates that as soon as the device becomes on, the voltage across the SCR drops
to some volts.

13. Reverse Blocking Region
• In this region, the negative voltage (-) is given to anode (+), positive voltage
(+) is given to cathode (-), and gate is open circuited. In this case, the
junction J1 and junction J3 are reverse biased whereas the junction J2
becomes forward biased.
• As the junctions J1 and junction J3 are reverse biased, no current flows
through the SCR circuit. But a small leakage current flows due to drift of
charge carriers in the forward biased junction J2. This small leakage current
is called reverse leakage current. This small leakage current is not sufficient
to turn on the SCR.
• If the reverse bias voltage applied between anode and cathode is increased
beyond the reverse breakdown voltage (VBR), an avalanche breakdown
occurs. As a result, the current increases rapidly. The region EF is called
reverse avalanche region. This rapid increase in current may damage the
SCR device.

14. Two Transistor Analogy of SCR
• The two transistor analogy or two transistor model of
SCR expresses the easiest way to understand the
working of SCR by visualizing it as a combination of two
transistors as shown in figure. The collector of each
transistor is connected to the base of the other
transistor.
• Assume that load resistance is connected between the
anode and cathode terminals and a small voltage is
applied at the gate and cathode terminals. When there
is no gate voltage, the transistor 2 is in cut-off mode
due to zero base current. Therefore, no current flows
through the collector and hence the base of transistor
T1. Hence, both transistors are open circuited and
thereby no current flows through the load.
• When a particular voltage is applied between the gate
and cathode, a small base current flows through the
base of the transistor 2 and thereby collector current
will increase. And hence the base current at the
transistor T1 drives the transistor into saturation mode
and thus load current will flow from anode to cathode.

15. SCR Turn ON Methods
From the obtained equation the conditions to turn the SCR into turn ON are
• 1. The leakage current through the SCR will increase when the temperature
of the device is very high. This turns the SCR into conduction.
• 2. When the current flowing through the device is extremely small then α1
and α2 are very small. The conditions for break over voltage are the larger
values of electron multiplication factor Mn and hole multiplication factor
Mp near the junction J2. Therefore the by increasing the voltage across the
device to break over voltage VBO causes the junction J2 breakdown and
thereby the SCR is turned ON.
• 3. And also by increasing α1 and α2 break over condition is achieved. The
current gains of the transistors depend on the value of Ig so by increasing
Ig, SCR can be turned ON.

16. SCR Turn OFF Methods
• An SCR cannot be turned OFF through the gate terminal like turning ON process.
To turn OFF the SCR, anode current must be reduced to a level below the holding
current level of the SCR. The process of turning OFF the SCR is called as
commutation. Two major types of commutating the SCR are,
1. Natural Commutation and
2. Forced Commutation
Forced commutation is again classified into several types such as
• Class A Commutation
• Class B Commutation
• Class C Commutation
• Class D Commutation
• Class E Commutation

17. DC Motor Control Using SCR
• by controlling the voltage applied to the
armature, the speed of the dc motor is controlled.
• the ac mains supply is connected to transformer
primary and to the secondary winding , two scrs
are connected in parallel as shown in figure. the
output from these scrs drives the dc motor. the
field winding is connected through the diodes
which gives uncontrollable dc power to the field
winding. during the positive half cycle of the
input, scr1 is forward biased and when the
triggering pulse is given to the gate, scr1 starts
conducting. so the load current flows to the dc
motor through scr1. during the negative half cycle
of the input, scr 2 is forward biased and scr 1 is
reverse biased and hence scr1 is turned off.
• when the gate triggering is given to scr2 , it starts
conducting. by varying the trigger input to the
respective scrs the average output to the dc
motor is varied and hence its speed is controlled.

18. AC Motor Control Using SCR
• An AC induction motor speed is controlled
by varying the stator voltage applied to it.
The below figure shows the connection of
SCR for varying the voltage applied to the
stator of induction motor.
• Each phase consists of two anti-parallel
SCRs, one for positive peak and another
for negative peak. Therefore, total six SCR
configurations are used for producing the
variable power.
• The input three phase AC supply is given
to the three phase induction motor via
these set of thyristors. When these SCRs
are triggered with delayed pulses, the
average voltage applied to the induction
motor is get varied and hence the speed.

19. Advantages of Silicon Controlled Rectifier
1. As compared with electromechanical or mechanical switch, SCR has
no moving parts. Hence, with a high efficiency it can deliver noiseless
operation.
2.The switching speed is very high as it can perform 1 nano operations
per second.
3.These can be operated at high voltage and current ratings with a
small gate current.
4.More suitable for AC operations because at every zero position of the
AC cycle the SCR will automatically switch OFF.
5. Small in size, hence easy to mount and trouble free service.

20. Summary
1. The Silicon Controlled Rectifier behaves like a switch with two states that is
either non-conducting or conducting.
2. There are three modes in which SCR operates. Those are forward blocking,
forward conduction mode and reverse blocking mode.
3. There are mainly two ways to turn ON the SCR that means either by
increasing the voltage across the SCR beyond the break over voltage of the
SCR or by applying a small voltage to the gate. The typical value of the gate
is 1.5 V, 30 mA . If the gate current is increased the SCR will turn ON at
much reduced supply voltage.
4. The SCR cannot be turned OFF through the gate so to open the SCR,
applied voltage must reduced to zero.
5. Silicon Controlled Rectifier can be used for both AC and DC switching
applications.