THYRISTOR DETAILS CONSTRUCTION AND TURN ON-OFF PROCESS AND CHARACTERISTICS

1. Presentation on
Power Thyristers
/SCR
BY
Ashvani Shukla
MANAGER(C&I)
ME(I&C)
BGR ENERGY

2. SCR / Thyristor
Introduction
 Thyrister is a three
terminal device
having Gate ,Anode
and Cathode. Anode
is positive and
Cathode is negative
terminal
 Gate terminal for a
controlling input
signal
ANODE
GATE
CATHODE
E

3. SCR/ Thyristor
A thyristor is normally four layer three-terminal device. Four layers are
formed by alternating n – type and p – type semiconductor materials.
Consequently there are three p – n junctions formed in the device. It is a
bistable device. The three terminals of this device are called anode (A),
cathode (K) and gate (G) respectively. The gate (G) terminal is control
terminal of the device. That means, the current flowing through the device
is controlled by electrical signal applied to the gate (G) terminal. The
anode (A) and cathode (K) are the power terminals of the device handle
the large applied voltage and conduct the major current through the
thyristor. For example, when the device is connected in series with load
circuit, the load current will flow through the device from anode (A) to
cathode (K) but this load current will be controlled by the gate(G) signal
applied to the device externally. A tyristor is on – off switch which is used to
control output power of an electrical circuit by switching on and off the
load circuit periodically in a preset interval. The main difference of thyristors
with other digital and electronics switches is that, a thyristor can handle
large current and can withstand large voltage, whereas other digital and
electronic switches handle only tiny current and tiny voltage.

4. SCR / Thyristor
 Circuit Symbol
ANODE
CATHODE
GATE

5.  Basic Construction of Thyristor
 A high- resistive, n-base region, presents in every thyristor. As it is
seen in the figure, this n-base region is associated with junction, J2.
This must support the large applied forward voltages that occur
when the switch is in its off- or forward-blocking state (non-
conducting). This n-base region is typically doped with impurity
phosphorous atoms at a concentration of 1013
to 1014
per cube
centimeter. This region is typically made 10 to 100 micrometer
thick to support large voltages. High-voltage thyristors are
generally made by diffusing aluminum or gallium into both
surfaces to create p-doped regions forming deep junctions with
the n-base. The doping profile of the p-regions ranges from about
1015
to 1017
per cube centimeter. These p-regions can be up to tens
of micrometer thick. The cathode region (typically only a few
micrometer thick) is formed by using phosphorous atoms at a
doping density of 1017
to 1018
cube centimeter. For higher forward-
blocking voltage rating of thyristor, the n-base region is made
thicker. But thicker n - based high-resistive region slows down on
off operation of the device. This is because of more stored charge
during conduction. A device rated for forward blocking voltage
of 1 kV will operate much more slowly than the thyristor rated for
100 V. Thicker high-resistive region also causes larger forward
voltage drop during

6.  conduction. Impurity atoms, such as platinum or gold, or electron
irradiation are used to create charge-carrier recombination sites
in the thyristor. The large number of recombination sites reduces
the mean carrier lifetime (average time that an electron or hole
moves through the Si before recombining with its opposite
charge-carrier type). A reduced carrier lifetime shortens the
switching times (in particular the turn-off or recovery time) at the
expense of increasing the forward-conduction drop. There are
other effects associated with the relative thickness and layout of
the various regions that make up modern thyristors, but the major
trade off between forward-blocking voltage rating and switching
times and between forward-blocking voltage rating and forward-
voltage drop during conduction should be kept in mind. (In
signal-level electronics an analogous trade off appears as a
lowering of amplification (gain) to achieve higher operating
frequencies, and is often referred to as the gain-bandwidth
product.)

7.  Basic Operating Principle of Thyristor
 Although there are different types of thyristors but basic
operating principle of all thyristor more or less same. The
figure below represents a conceptual view of a typical
thyristor. There are three p–n junctions J1, J2 and J3. There
are also three terminals anode (A), cathode (K) and gate
(G) as levelled in the figure. When the anode (A) is in
higher potential with respect to the cathode, the junctions
J1 and J3 are forward biased and J2 is reverse biased and
the thyristor is in the forward blocking mode. A thyristor can
be considered as back to back connected two bipolar
transistors. A p-n-p-n structure of thyristor can be
represented by the p-n-p and n-p-n transistors, as shown in
the figure. Here in this device, the collector current of one
transistor is used as base current of other transistor. When
the device is in forward blocking mode if a hole current is
injected through the gate (G) terminal, the device is
triggered on.

8.  When potential is applied in reverse direction, the thyristor
behaves as a reverse biased diode. That means it blocks
current to flow in revere direction. Considering ICO to be the
leakage current of each transistor in cut-off condition, the
anode current can be expressed in terms of gate current.
Where α is the common base current gain of the transistor (α =
IC/IE). The anode current becomes arbitrarily large as (α1 + α2)
approaches unity. As the anode–cathode voltage increases,
the depletion region expands and reduces the neutral base
width of the n1 and p2 regions. This causes a corresponding
increase in the α of the two transistors. If a positive gate
current of sufficient magnitude is applied to the thyristor, a
significant amount of electrons will be injected across the
forward-biased junction, J3, into the base of the n1p2n2 transistor.
The resulting collector current provides base current to the
p1n1p2 transistor. The combination of the positive feedback
connection of the npn and pnp BJTs and the current-
dependent base transport factors eventually turn the thyristor
on by regenerative action. Among the power semiconductor
devices known, the thyristor shows the lowest forward voltage
drop at large current densities. The large current flow between
the anode and cathode maintains both transistors in
saturation region, and gate control is lost once the thyristor
latches on.

9.  Transient Operation of Thyristor
 A thyristor is not turned on as soon as the gate current is injected,
there is one minimum time delay is required for regenerative action.
After this time delay, the anode current starts rising rapidly to on-
state value. The rate of rising of anode current can only be limited
by external current elements. The gate signal can only turn on the
thyristor but it cannot turn off the device. It is turned off naturally
when the anode current tends to flow in reverse direction during the
reverse cycle of the alternating current. A thyristor exhibits turn-off
reverse recovery characteristics just like a diode. Excess charge is
removed once the current crosses zero and attains a negative
value at a rate determined by external circuit elements. The reverse
recovery peak is reached when either junction J1 or J3 becomes
reverse biased. The reverse recovery current starts decaying, and
the anode–cathode voltage rapidly attains its off-state value.
Because of the finite time required for spreading or collecting the
charge plasma during turn-on or turn-off stage, the maximum dI/dt
and dV/dt that may be imposed across the device are limited in
magnitude. Further, device manufacturers specify a circuit-
commutated recovery time, for the thyristor, which represents the
minimum time for which the thyristor must remain in its reverse
blocking mode before forward voltage is reapplied.

10.  A thyristor is a four layer 3 junction p-n-p-n semiconductor device
consisting of at least three p-n junctions, functioning as an electrical
switch for high power operations. It has three basic terminals, namely
the anode, cathode and the gate mounted on the semiconductor
layers of the device. The symbolic diagram and the basic circuit
diagram for determining the characteristics of thyristor is shown in the
figure below.
 Reverse Blocking Mode of Thyristor
 Initially for the reverse blocking mode of the thyristor, the cathode is
made positive with respect to anode by supplying voltage E and the
gate to cathode supply voltage Es is detached initially by keeping
switch S open. For understanding this mode we should look into the
fourth quadrant where the thyristor is reverse biased. Here Junctions J1
and J3 are reverse biased whereas the junction J2 is forward biased.
The behavior of the thyristor here is similar to that of two diodes are
connected in series with reverse voltage applied across them. As a
result only a small leakage current of the order of a few μAmps flows.
This is the reverse blocking mode or the off-state, of the thyristor. If the
reverse voltage is now increased, then at a particular voltage, known
as the critical breakdown voltage VBR, an avalanche occurs at J1 and
J3 and the reverse current increases rapidly.

11.  A large current associated with VBR gives rise to more losses in
the SCR, which results in heating. This may lead to thyristor
damage as the junction temperature may exceed its
permissible temperature rise. It should, therefore, be ensured
that maximum working reverse voltage across a thyristor does
not exceed VBR. When reverse voltage applied across a
thyristor is less than VBR, the device offers very high impedance
in the reverse direction. The SCR in the reverse blocking mode
may therefore be treated as open circuit.

12. P
NN
P
NN
J1
J2
J3
ANODE
CATHODE
GATE
Reverse blocking mode

13.  Forward Blocking Mode
 Now considering the anode is positive with respect to the cathode,
with gate kept in open condition. The thyristor is now said to be
forward biased as shown the figure below.
 As we can see the junctions J1 and J3arenow forward biased but
junction J2goes into reverse biased condition. In this particular mode, a
small current, called forward leakage current is allowed to flow initially
as shown in the diagram for characteristics of thyristor. Now, if we keep
on increasing the forward biased anode to cathode voltage.
 In this particular mode, the thyristor conducts currents from anode to
cathode with a very small voltage drop across it. A thyristor is brought
from forward blocking mode to forward conduction mode by turning it
on by exceeding the forward break over voltage or by applying a
gate pulse between gate and cathode. In this mode, thyristor is in on-
state and behaves like a closed switch. Voltage drop across thyristor in
the on state is of the order of 1 to 2 V depending beyond a certain
point, then the reverse biased junction J2 will have an avalanche
breakdown at a voltage called forward break over voltage VB0 of the
thyristor. But, if we keep the forward voltage less than VBO, we can see
from the characteristics of thyristor, that the device offers a high
impedance. Thus even here the thyristor operates as an open switch
during the forward blocking mode.

14. P
NN
P
NN
J1
J2
J3
ANODE
CATHODE
GATE
Forward blocking mode

15.  Forward Conduction Mode
 When the anode to cathode forward voltage is increased,
with gate circuit open, the reverse junction J2 will have an
avalanche breakdown at forward break over voltage VBO
leading to thyristor turn on. Once the thyristor is turned on we
can see from the diagram for characteristics of thyristor, that
the point M at once shifts toward N and then anywhere
between N and K. Here NK represents the forward
conduction mode of the thyristor. In this mode of operation,
the thyristor conducts maximum current with minimum
voltage drop, this is known as the forward conduction
forward conduction or the turn on mode of the thyristor.

16. V/I Characteristics
Vbo
Ibo
Vh
Ih
v
Forward
blocking
voltage
Ia

17.  When positive potential applied to the anode with respect to
the cathode, ideally no current will flow through the device
and this condition is called forward – blocking state but when
appropriate gate signal is applied, a large forward anode
current starts flowing, with a small anode–cathode potential
drop and the device becomes in forward-conduction state.
Although after removing the gate signal, the device will
remain in its forward-conduction mode until the polarity of
the load reverses. Some thyristors are also controllable in
switching from forward-conduction back to a forward-
blocking state.

18.  Turn ON Time of SCR
 A forward biased thyristor can be turned on by
applying a positive voltage between gate and
cathode terminal. But it takes some transition time to
go from forward blocking mode to forward
conduction mode. This transition time is called turn on
time of SCR and it can be subdivided into three small
intervals as delay time (td) rise time(tr), spread time(ts).
 Delay Time of SCR
 After application of gate current, the thyristor will start
conducting over a very tiny region. Delay time of SCR
can be defined as the time taken by the gate current
to increase from 90% to 100% of its final value Ig. From
another point of view, delay time is the interval in
which anode current rises from forward leakage
current to 10% of its final value and at the same time
anode voltage will fall from 100% to 90% of its initial
value Va. Rise Time of SCR

19.  Rise time of SCR in the time taken by the anode
current to rise from 10% to 90% of its final value. At the
same time anode voltage will fall from 90% to 10% of
its initial value Va. The phenomenon of decreasing
anode voltage and increasing anode current is
entirely dependent upon the type of the load. For
example if we connect a inductive load, voltage will
fall in a faster rate than the current increasing. This is
happened because induction does not allow initially
high voltage change through it. On the other hand if
we connect a capacitive load it does not allow initial
high voltage change through it, hence current
increasing rate will be faster than the voltage falling
rate.

20.  High increasing rate of dia/dt can create local hot spot
in the device which is not suitable for proper
operation. So, it is advisable to use a inductor in series
with the device to tackle high dia/dt. Usually value of
maximum allowable di/dt is in the range of 20 to 200 A
per microsecond.
 Spread Time of SCR
 It is the time taken by the anode current to rise from
90% to 100% of its final value. At the same time the
anode voltage decreases from 10% of its initial value
to smallest possible value. In this interval of time
conduction spreads all over the area of cathode and
the SCR will go to fully ON State. Spread time of SCR
depends upon the cross-sectional area of cathode.

21. Vg Gate Pulse
Va
Ig
0.9Ia

22.  Turn OFF Time of SCR
 Once the thyristor is switched on or in other point of
view, the anode current is above latching current, the
gate losses control over it. That means gate circuit
cannot turn off the device. For turning off the SCR
anode current must fall below the holding current.
After anode current fall to zero we cannot apply
forward voltage across the device due to presence of
carrier charges into the four layers. So we must sweep
out or recombine these charges to proper turn off of
SCR. So turn off time of SCR can be defined as the
interval between anode current falls to zero and
device regains its forward blocking mode. On the
basis of removing carrier charges from the four layers,
turn off time of SCR can be divided into two time
regions, Reverse Recovery Time.
 Gate Recovery Time

23.  Reverse Recovery Time
 It is the interval in which change carriers remove from
J1, and J3 junction. At time t1, anode current falls to zero
and it will continue to increase in reverse direction with
same slope (di/dt) of the forward decreasing current.
This negative current will help to sweep out the carrier
charges from junction J1 and J3. At the time t2 carrier
charge density is not sufficient to maintain the reverse
current hence after t2 this negative current will start to
decrease. The value of current at t2 is called reverse
recovery current. Due to rapid decreasing of anode
current, a reverse spike of voltage may appear across
the SCR. Total recovery time t3 - t1 is called reverse
recovery time. After that, device will start to follow the
applied reverse voltage and it gains the property to
block the forward voltage.

24.  Gate Recovery Time
 After sweeping out the carrier charges from junction J1
and J3 during reverse recovery time, there still remain
trapped charges in J2 junction which prevent the SCR
from blocking the forward voltage. These trapped
charge can be removed by recombination only and
the interval in which this recombination is done, called
gate recovery time.

25.  Application of Thyristor
 As we already said that a thyristor is designed to handle large
current and voltage, it is used mainly in electrical power circuit
with system voltage more than 1 kV or currents more than 100
A. The main advantage of using thyristors as power control
device is that as the power is controlled by periodic on – off
switching operation hence (ideally) there is no internal power
loss in the device for controlling power in output circuit.
Thyristors are commonly used in some alternating power
circuits to control alternating output power of the circuit to
optimize internal power loss at the expense of switching
speed.
 In this case thyristors are turned from forward-blocking into
forward-conducting state at some predetermined phase
angle of the input sinusoidal anode–cathode voltage
waveform. Thyristors are also very popularly used in inverter for
converting direct power to alternating power of specified
frequency. These are also used in converter to convert an
alternating power into alternating power of different
amplitude and frequency.This is the most common application
of thyristor.