Thyristor

A silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solidstate

current-controlling device. The name "silicon controlled rectifier" is General Electric's trade
name for a type of Thyristor.
SCRs are mainly used in electronic devices that require control of high voltage and power. This
makes them applicable in medium and high AC power operations such as motor control function.
An SCR conducts when a gate pulse is applied to it, just like a diode. It has four layers of
semiconductors that form two structures namely; NPNP or PNPN. In addition, it has three
junctions labeled as J1, J2 and J3 and three terminals anode, cathode and a gate. An
SCR is diagrammatically represented as shown below.

The anode connects to the P-type, cathode to the N-type and the gate to the P-type as shown
below.

In an SCR, the intrinsic semiconductor is silicon to which the required dopants are infused.
However, doping a PNPN junction is dependent on the SCR application.
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.

To manufacture the SCR, three types of constructions are used, namely the planar type, Mesa
type and Press pack type. For low power SCRs, planar construction is used where all the
junctions in an SCR are diffused. In mesa type construction, junction J2 is formed by diffusion
method and thereby outer layers are alloyed to it.

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

Modes of Operation in SCR

 OFF state forward blocking mode − Here the anode is assigned a positive voltage,
the gate is assigned a zero voltage disconnected and the cathode is assigned a negative
voltage. As a result, Junctions J1 and J3 are in forward bias while J2 is in reverse bias. J2
 reaches its breakdown avalanche value and starts to conduct. Below this value, the
resistance of J1 is significantly high and is thus said to be in the off state.
 ON state conducting mode − An SCR is brought to this state either by increasing the
potential difference between the anode and cathode above the avalanche voltage or by
applying a positive signal at the gate. Immediately the SCR starts to conduct, gate voltage
is no longer needed to maintain the ON state and is, therefore, switched off by −
o Decreasing the current flow through it to the lowest value called holding current
o Using a transistor placed across the junction.
 Reverse blocking − This compensates the drop in forward voltage. This is due to the fact
that a low doped region in P1 is needed. It is important to note that the voltage ratings of
forward and reverse blocking are equal.

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.

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.
Note that in the VI characteristic figure, if the gate current value is high, the minimum will be the
time to come in conduction mode as Ig3 > Ig2 > Ig1. In this mode, maximum current flows
through the SCR and its value depends on the load resistance or impedance.
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.
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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.

1) Forward Blocking Mode (Off State)

In this mode of operation, the positive voltage (+) is given to anode A (+), negative voltage (-) is
given to cathode K (-), and gate G is open circuited as shown in the below figure. In this case,
the junction J1 and junction J3 are forward biased whereas the junction J2 becomes reverse
biased. Due to the reverse bias voltage, the width of depletion region increases at junction J 2.
This depletion region at junction J2 acts as a wall or obstacle between the junction J1 and junction
J3. It blocks the current flowing between junction J1 and junction J3. Therefore, the majority of
the current does not flow between junction J1 and junction J3. However, a small amount of
leakage current flows between junction J1 and junction J3.

When the voltage applied to the SCR reaches a breakdown value, the high energy minority
carriers causes avalanche breakdown. At this breakdown voltage, current starts flowing through
the SCR. But below this breakdown voltage, the SCR offers very high resistance to the current
and so it will be in off state.

In this mode of operation, SCR is forward biased but still current does flows through it. Hence, it
is named as Forward Blocking Mode.

2) Forward Conducting Mode (On State)

The Silicon Controlled Rectifier can be made to conduct in two ways:

i. By increasing the forward bias voltage applied between anode and cathode beyond the
breakdown voltage
ii. By applying positive voltage at gate terminal.

In the first case, 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 other 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.

In the second case, a small positive voltage VG is applied to the gate terminal. As we know that,
in forward blocking mode, current does not flows through the circuit because of the wide
depletion region present at the junction J2. This depletion region was formed because of the
reverse biased gate terminal. So this problem can be easily solved by applying a small positive
voltage at the Gate terminal. When a small positive voltage is applied to the gate terminal, it will
become forward biased. So the depletion region width at junction J2 becomes very narrow. Under
this condition, applying a small forward bias voltage between anode and cathode is enough for
electric current to penetrate through this narrow depletion region. Therefore, electric current
starts flowing through the SCR circuit.

In second case, we no need to apply large voltage between anode and cathode. A small voltage
between anode and cathode, and positive voltage to gate terminal is enough to brought SCR from
blocking mode to conducting mode.

In this mode of operation, SCR is forward biased and current flows through it. Hence, it is named
as Forward Conducting Mode.

3) Reverse Blocking Mode (On State)

In this mode of operation, the negative voltage (-) is given to anode (+), positive voltage (+) is
given to cathode (-), and gate is open circuited as shown in the below figure. 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 not enough to turn on the SCR. So the SCR will be in Off state.

V-I Characteristics of SCR

The V-I characteristics of SCR is shown in the below figure. The horizontal line in the below
figure represents the amount of voltage applied 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 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

 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.
  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.

 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.

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.

From the above figure the base current of transistor T2 becomes the collector current of
transistor T1 and vice-versa.

SCR Turn ON Methods

From the above equation, if (α1 + α2) is equal to one then Ia becomes infinite. That means anode
current suddenly rises to a high value and latches into conduction mode from non-conductive
state. This is called regenerative action of SCR. So for triggering of SCR the gate current value
(α1 + α2) must approach to unity. 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.
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

DC Motor Control Using SCR

Consider the below figure in which SCRs are used to control the speed of the DC motor. As we
that DC motor consists of a field and armature windings. 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.

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.

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.

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.