What is the Thyristor?
The
name thyristor is derived from the combination of the first four letters of Thyratron and the last five letters of Transistor.
Thyratron
is an electrical switch that is used for high power applications. Construction
wise it is a gas filled tube.
Transistors
are low power and high switching speed electronic devices. Construction wise
these are formed by the combination of P-type semiconductor and N-type
semiconductor. Therefore, these are categorised as solid-state devices.
In
a similar context, Thyristor is a solid-state semiconductor device that has the
capabilities of both thyratron and transistor. Just like a thyratron, a
thyristor has high power withstand capabilities and from the construction point
of view it belongs to the transistor family. As it is formed by the combination
of alternating layers of P-type semiconductor and N-type
semiconductor, just like a transistor. Mainly it constitutes of four or
more alternating layers of P-type semiconductor and N-type semiconductor.
So,
a Thyristor can be defined as a three terminal (anode, cathode and gate) and
four layered i.e. p-n-p-n semiconductor switching device.
Thyristor Family
Thyristor
Family is the class of power semiconductor devices which has the properties of
both thyratron and transistor as stated above.
Some
members of thyristor family are listed as follows.
SCR
:- Silicon Controlled Rectifier
LASCR
:- Light Activated SCR
RCT
:- Reverse Conducting Thyristor
GTO :- Gate Turn Off Thyristor
TRIAC :- Triode
for AC
DIAC :- Diode
for AC
SCS
:- Silicon Controlled Switch
UJT :- Unijunction Transistor etc.
Silicon Controlled Rectifier (SCR)
SCR
stands for Silicon Control Rectifier. It is the very first and most popular
member of the thyristor family in Power Electronics and it is commonly known as
thyristor.
As
its name implies, it is a controlled switch or power semiconductor device which
is constructed by silicon semiconductor and it works as a rectifier in a
circuit, means it allows the conduction of current in one direction only that
means it is a unidirectional switch.
The
term, controlled switch drives from its controlling characteristics. In SCR we
can control its on-state by controlling the gate current (discussed in details
further in this article). As its on-state is controlled by the gate current, it
is categorised as a current controlled device.
A SCR has high power and least switching frequency among all the power semiconductor devices.
Construction of SCR
A SCR has three terminals named as Anode (positive terminal),
Cathode (negative terminal), and Gate (controlling terminal) and four
alternating layers of P-type and N-type semiconductor materials (Silicon is
used for its construction as its name indicates) i.e. (p-n-p-n) shown in the
given figure.
- The terminal connected to the outer p-region is called Anode.
- The terminal connected to the outer n-region is called Cathode.
- The terminal connected to the inner p-region is called Gate.
These
four alternating layers of P-type and N-type semiconductor materials formed
three junctions J1, J2 and J3, each of these
junctions itself referred to as a PN junction diode (This practice is only
theoretical, to get the better understanding about working of SCR) shown in the
given figure below.
SCR Working
The
working of SCR depends on the biasing in conjunction with triggering.
Biasing means
applying the voltage to the anode and cathode terminals of the SCR.
Triggering
of SCR means turning on the SCR. As we discussed above, SCR is a control
switch, in which we can control its on state or conduction state by controlling
the gate current (discussed in detail further in this article).
There
are two ways of biasing of the SCR or applying the voltage across the terminal
of SCR.
In
the first method i.e. forward biasing, the anode is connected to the positive
terminal of the supply, the cathode is connected to the negative terminal of
the supply and the gate terminal of the SCR is kept open. In this condition SCR
does not conduct current and is said to be in forward blocking mode.
In
the second method i.e. reverse biasing, the cathode is connected to the
positive terminal of the supply, the anode is connected to the negative
terminal of the supply and the gate terminal of the SCR is kept open. In this
condition SCR does not conduct current and is said to be in reverse blocking
mode.
There
is an another condition where the voltage applied to the SCR terminals is the
same as the first case i.e. forward biasing but with a triggering circuit
connected to the gate and cathode terminal of the SCR. In this condition, SCR
conducts current and is said to be in forward conduction mode.
Since
SCR does not conduct current in both forward biasing condition as well as
reverse biasing condition, it has bipolar voltage withstand capacity and
is categorized as a bipolar device.
Let
us understand these three modes of operation of SCR in detail
Forward Blocking Mode of SCR
When the forward voltage is
applied to the SCR i.e. anode is made positive with respect to the cathode and
leaving the gate terminal open means the gate current is zero. In this biasing
condition, SCR does not conduct the current and it is said to be in forward
blocking mode (discussed previously).
In this condition, the
direction of current flows in such a way that it enters to the P side of the
junctions J1 and J3 therefore, these junctions are
forward biased and it enters to the n-side of the junction J2
therefore, it is reverse biased.
The direction of the current
flows through these junctions indicated by the blue arrow in the given figure.
These indications are only theoretical assumptions so that we can understand
these modes of operation of SCR easily.
As the junction J1 is forward biased, it allows the
conduction of current therefore, the current flows from the p region to n
region through junction J1. (refer working of p-n junction diode)
But
the junction J2 is reverse biased and due to this the width of the
depletion layer as junction J2 increases immediately after applying
the voltage across the SCR terminal (reverse voltage across junction J2).
Therefore, junction J2 offers a very high resistance (theoretically
infinite resistance) to the conduction of current, consequently the current
will not flow further through the junction J2. However, practically
a small amount of current called leakage current will flow from anode to
cathode.
Therefore, in this condition
SCR is said to be in off state or blocking state because it does not allow the
conduction of current.
Reverse Blocking Mode of SCR
In
this mode of operation, SCR is reverse biased, means anode is connected to the
negative terminal of the supply and cathode is connected to the positive
terminal of the supply, keeping the gate voltage zero.
In this condition the current enters to the n-side of the
junctions J1 and J3 therefore, these are reverse biased
and junction J2 is forward biased because the current enters to the
p-side of the junction J2.
As
junctions J1 and J3 are reverse biased, they do not allow
the conduction of current because the width of the depletion layer at junctions
J1 and J3 increases immediately after applying the
reverse voltage across the terminal of the SCR. Consequently, these junctions
offer very high resistance to conduction of current.
As
these junctions do not allow the conduction of current the SCR is said to be in
off state or blocking state and this mode of operation is called reverse
blocking mode of SCR because in this mode SCR is reverse biased and it does
not allow the conduction of current.
The
maximum amount of voltage up to which the SCR is said to be in reverse blocking
mode is called reverse breakdown voltage. Beyond this voltage breakdown of
junction J1 and J2 occurs, simultaneously breakdown of SCR occurs.
Forward Conduction Mode of SCR
In
this mode of operation, the voltage applied across the SCR terminals is the
same as that of the case of forward blocking mode, i.e. anode is made positive
with respect to cathode.
As
discussed previously, in this case junctions J1 and J3
are forward biased and they allow the conduction of current and junction J2
is reverse biased and it does not allow the conduction of current. Therefore,
initially the SCR is in forward blocking mode.
To
drive the SCR from forward blocking mode to forward conduction mode there are
two ways.
First
way is increasing the forward voltage applied across the SCR terminal beyond
the reverse breakdown voltage (VBR) of junction J2 while
keeping the gate terminal open or at zero potential. This will result in the
occurrence of avalanche breakdown at junction J2,
consequently the large amount of current will flow through the junction J2.
As
junctions J1 and J3 are forward biased and already
conducting current, and increasing the voltage will cause junction J2
conduct as well. Consequently, the SCR will conduct current.
Practically,
this method to turn on the SCR will not be preferred because the voltage
applied across SCR terminals cannot be increased beyond the safe rated value.
In the second method, SCR is brought from forward blocking mode to
forward conduction mode by applying the positive gate current pulses between
gate and cathode terminals. These gate current pulses initiate conduction near
the depletion region. Increasing the gate current pulse width will expand the
initial conduction area near the depletion region. This will result in the
current flowing through the depletion layer at junction J2.
As junctions J1 and J3 are forward biased
and already conducting current and by applying gate pulse between gate and
cathode terminals of the SCR junction J2 conducts as well.
Consequently, the SCR will start conducting current.
Once
SCR starts conduction no more gate pulses are required to keep it on. So, we
can safely remove the gate pulse to avoid the power losses at the gate
terminal. But we have to take care about the latching current of the SCR
while removing the gate pulses so that SCR cannot fails to turn on.
Latching Current in SCR
Latching
current is associated with the turn on process of the SCR. It is the minimum
amount of the anode to cathode current of the SCR below which if the gate
pulses are removed then SCR fails to turn on.
As
we discussed above the gate signal initiates the turn on process of the SCR.
Once the SCR starts the conduction gate loses control on it. Therefore, we can
remove the gate signal in order to avoid the continuous gate power
losses.
If
we remove the gate pulses when the anode to cathode current of the SCR is below
the latching current then the SCR will fail to turn on. Therefore, we have to
maintain the gate pulse width at least for a period until the anode to cathode
current reaches slightly greater than the latching current, to turn on the SCR.
Triggering Methods of SCR
Triggering
methods of SCR also known as turn on method or SCR. In the previous section on
working of SCR, we studied that both biasing and triggering are essential for
the proper operation of SCR.
There we
discussed that only applying the voltage across the terminal of the SCR is
insufficient to turn it on. To turn on the SCR, there we discussed two ways:
first is by increasing the forward voltage beyond the reverse breakdown
voltage of junction J2 and second is by applying the
positive gate voltage between gate and cathode terminals of the SCR.
Apart
from these two methods to turn on the SCR, there are three more methods to turn
on the SCR. So combinedly there are total five different turn on methods
of SCR
- Forward Voltage Triggering
- Gate Triggering
- dv/dt triggering
- Light Triggering
We discussed Forward Voltage Triggering and Gate Triggering in the above section. You can simply visit these sections by clicking their respective names.
dv/dt triggering of SCR
In the construction of SCR, we discussed that the four layers of the SCR make three junctions J1, J2 and J3 each of these junctions have their own junction capacitance. The current flowing through the SCR will pass through these junction capacitance and act as the charging current of these junction capacitance.
While
the anode current passes through the junction J2 that is reverse
biased the expression for the current.
So, from the above expression we can clearly see that if we increase dv/dt then the charging current (anode current Ia) across the junction capacitance Cj2 will increase.
So,
if we increase the dv/dt sufficiently high, so that the charging current will
be greater than the latching current of the SCR then SCR will turn on.
Light Triggering of SCR
When
a high intensity light is incident near the gate junction of the SCR then the
depletion layer absorbs the light and produces more electrons and holes. These
electrons and holes initiate the conduction in the depletion layer and
simultaneously support the conduction of SCR. The SCR which is turned on by
this method is known as Light activated Silicon Controlled Rectifier (LASCR).
VI Characteristics of SCR
The VI
characteristics of SCR illustrate the relationship between the voltage applied
across the anode and cathode terminals of the SCR (biasing voltage) and current
passing through it under different operating modes. It gives us the
understanding of different parameters of SCR, so that we utilize the SCR in any
circuit efficiently.
In above figure
- VBR is the Reverse Breakdown Voltage, it is the maximum amount of reverse biased voltage that the SCR can withstand in the reverse blocking state. Beyond this voltage breakdown of the junctions J1 and J2 of the SCR occurs, this will lead to the breakdown of the SCR (discussed previously in this article in Reverse Blocking Mode of SCR Section). This breakdown will result in the flow of a large amount of reverse current through the SCR. This flow of current is shown by the sharp decline line denoted by BC after the VBR
- VBO is the Forward Breakover Voltage, it is the maximum amount of forward biased voltage upto which SCR is in forward blocking state. Beyond this voltage, breakdown occurs at junction J2. This breakdown will results the flow of large amount of current through junction J2. This current supports the conduction of current near junction J2 as well as in the entire SCR.
- IL is the latching current of SCR discussed above in separate section
- IH is the Holding Current of the SCR.
Holding Current in SCR
Holding
current is associated with the turn off process of the SCR. It is the minimum
amount of the anode to cathode current below which SCR will turn off.
As
we have discussed above, SCR is a semi controlled or half-controlled device
that means gate signal only controls the turn on process of the SCR it has no
control on the turn off process of the SCR. SCR will turn off only when the
anode current of the SCR will become less then the holding current.
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