This document provides an overview of thyristors, including: 1) Thyristors are four-layer semiconductor devices that can handle high currents and voltages with low control power. Common types include SCRs, triacs, and GTOs. 2) Compared to transistors, thyristors have lower conduction losses and higher power handling but worse switching performance. 3) Thyristors operate in forward blocking, forward conducting, or reverse blocking modes depending on voltage polarity. They can be turned on through various methods including gate triggering.

1. Thyristors
Introduction & Characteristics

2. Power Semiconductor Switches
Power Diodes Power Transistors Thyristors
2 layer device 3 layer Device 4 layer Device
• Thyristor devices can convert and control large amounts of power in AC or DC systems
while using very low power for control.
• Thyristor family includes
1- Silicon controlled switch (SCR)
2- Gate-turnoff thyristor (GTO)
3- Triac
4- Diac
5- Silicon controlled switch (SCS)
6- Mos-controlled switch (MCT)
2

3. • One of the most important type of power
semiconductor device.
• Compared to transistors, thyristors have lower on-
state conduction losses and higher power handling
capability.
• However, they have worse switching performances
than transistors.
Introduction

4. • Have the highest power handling capability.
• It has a rating of 1200V / 1500A with switching
frequencies ranging from 1KHz to 20KHz.
• This power can be controlled by a gate current
of about 1A only.
Cntd…

5. Thyristor Family Members
• SCR: Silicon Controlled Rectifier
• DIAC: Diode on Alternating Current
• TRIAC : Triode for Alternating Current
• SCS: Silicon Control Switch
• SUS: Silicon Unilateral Switch
• SBS: Silicon Bidirectional Switch
• SIS: Silicon Induction Switch
• LASCS: Light Activated Silicon Control Switch
• LASCR: Light Activated Silicon Control Rectifier
• SITh : Static Induction Thyristor
• RCT: Reverse Conducting Thyristor
• GTO : Gate Turn-Off thyristor
• MCT: MOSFET Controlled Thyristor
• ETOs: Emitter Turn ON thyristor

6. THYRISTOR
• Thyristor, a three terminal, four layers solid state
semiconductor device, each layer consisting of alternately N-
type or P-type material, i.e; P-N-P-N, that can handle high
currents and high voltages, with better switching speed and
improved breakdown voltage .
• Name ‘thyristor’, is derived by a combination of the capital
letters from THYRatron and transISTOR.
• Thyristor has characteristics similar to a thyratron tube which
is a type of gas filled tube used as a high energy electrical
switch and controlled rectifier.
• But from the construction view point, a thyristor (pnpn
device) belongs to transistor (pnp or npn device) family.
• This means that thyristor is a solid state device like a
transistor and has characteristics similar to that of a thyratron
tube.

7. THYRISTORS
• Thyristor (famous as Silicon Control Rectifier-SCR) can handle
high currents and high voltages.
• Thyristor a three terminal (Anode, Cathode and Gate), three
junctions and four layers solid-state semiconductor device,
with silicon doped alternate material with P-N-P-N structure.
• Thyristor act as bistable switches.
– It conducts when gate receives a current pulse, and
continue to conduct as long as forward biased (till device
voltage is not reversed).
– They stay ON once they are triggered, and will go OFF only
if current is too low or when triggered off.

8. Thyristor Schematic Representation
• .

9. Two-Transistor Model of Thyristors
• .

10. Thyristor- Operation Principle
• Thyristor has three p-n junctions (J1, J2, J3 from the anode).
• When anode is at a positive potential (VAK) w.r.t cathode with no
voltage applied at the gate, junctions J1 & J3 are forward biased,
while junction J2 is reverse biased.
– As J2 is reverse biased, no conduction takes place, so
thyristor is in forward blocking state (OFF state).
– Now if VAK (forward voltage) is increased w.r.t cathode,
forward leakage current will flow through the device.
– When this forward voltage reaches a value of breakdown
voltage (VBO) of the thyristor, forward leakage current will
reach saturation and reverse biased junction (J2) will have
avalanche breakdown and thyristor starts conducting (ON
state), known as forward conducting state .
• If Cathode is made more positive w.r.t anode, Junction J1 & J3
will be reverse biased and junction J2 will be forward biased.
• A small reverse leakage current flows, this state is known as
reverse blocking state.
• As cathode is made more and more positive, stage is reached
when both junctions A & C will be breakdown, this voltage is
referd as reverse breakdown voltage (OFF state), and device is in
reverse blocking state

11. Characteristics of Thyristors

12. Ideal Characteristic Of SCR
12

13. Thyristor Operating modes
Thyristors have three modes :
Forward blocking mode: Anode is positive w.r.t cathode, but the anode
voltage is less than the break over voltage (VBO) .
• only leakage current flows, so thyristor is not conducting .

14. .
Forward conducting mode: When anode voltage becomes greater than
VBO, thyristor switches from forward blocking to forward
conduction state, a large forward current flows.
• If the IG=IG1, thyristor can be turned ON even when anode voltage
is less than VBO.
– The current must be more than the latching current (IL).
– If the current reduced less than the holding current (IH), thyristor
switches back to forward blocking state.

15. .
Reverse blocking mode: When cathode is more positive than
anode , small reverse leakage current flows.
• However if cathode voltage is increased to reverse
breakdown voltage , Avalanche breakdown occurs and
large current flows.

16. Thyristor turn-ON methods
• Thyristor turning ON is also known as Triggering.
• With anode positive with respect to cathode, a thyristor
can be turned ON by any one of the following techniques :
– Forward voltage triggering
– Gate triggering
– dv/dt triggering
– Temperature triggering
– Light triggering

17. Forward Voltage Triggering
• When breakover voltage (VBO) across a thyristor is exceeded
than the rated maximum voltage of the device, thyristor
turns ON.
• At the breakover voltage the value of the thyristor anode
current is called the latching current (IL) .
• Breakover voltage triggering is not normally used as a
triggering method, and most circuit designs attempt to avoid
its occurrence.

18. Gate Triggering
• Turning ON of thyristors by gate triggering is simple and efficient
method of firing the forward biased SCRs.
• In Gate Triggering, thyristor with forward breakover voltage (VBO),
higher than the normal working voltage is chosen.
– This means that thyristor will remain in forward blocking state
with normal working voltage across anode and cathode with
gate open.
• Whenever thyristor’s turn-ON is required, a positive gate voltage
b/w gate and cathode is applied.
• With gate current established, charges are injected into the inner p
layer and voltage at which forward breakover occurs is reduced.

19. • Forward voltage at which device switches to on-state
depends upon the magnitude of gate current.
- Higher the gate current, lower is the forward breakover
voltage.
• When positive gate current is applied, gate P layer is
flooded with electrons from cathode, as cathode N layer is
heavily doped as compared to gate P layer.
• As the thyristor is forward biased, some of these electrons
reach junction J2.
• As a result, width of depletion layer around junction J2 is
reduced.
- This causes junction J2 to breakdown at an applied
voltage lower than forward breakover voltage VB0.
• If magnitude of gate current is increased, more electrons
will reach junction J2, thus thyristor will get turned ON at a
much lower forward applied voltage.

20. 20
SCR Turnoff (Commutation) Circuits

21. What is Commutation?
The process of turning off an SCR is called
commutation.
It is achieved by
1. Reducing anode current below holding current
2. Make anode negative with respect to cathode
 Types of commutation are:
1. Natural or line commutation
2. Forced commutation
21

22. ~
T
+

vo
vs
R
 
Natural Commutation
•Natural commutation Occurs in AC circuits

23. t
t
t
t
Supply voltage vs
Sinusoidal
Voltage across SCR
Load voltage vo
Turn off
occurs here
0
0


2
2
3
3

tc
Gate Pulse

 

24. SCR Turnoff Methods
1. Diverting the anode current to an alternate path
2. Shorting the SCR from anode to cathode
3. Applying a reverse voltage (by making the cathode positive with
respect to the anode) across the SCR
4. Forcing the anode current to zero for a brief period
5. Opening the external path from its anode supply voltage
6. Momentarily reducing supply voltage to zero
24

27. AC Thyristor Circuit

28. The thyristor firing circuit shown in previous slide is similar in
design to the DC SCR circuit except for the omission of an
additional “OFF” switch and the inclusion of diode D1 which
prevents reverse bias being applied to the Gate.
During the positive half-cycle of the sinusoidal waveform, the
device is forward biased but with switch S1 open, zero gate
current is applied to the thyristor and it remains “OFF”.
On the negative half-cycle, the device is reverse biased and
will remain “OFF” regardless of the condition of switch S1.
If switch S1 is closed, at the beginning of each positive half-
cycle the thyristor is fully “OFF” but shortly after there will be
sufficient positive trigger voltage and therefore current
present at the Gate to turn the thyristor and the lamp “ON”.

29. The thyristor is now latched-“ON” for the duration of the
positive half-cycle and will automatically turn “OFF” again
when the positive half-cycle ends and the Anode current falls
below the holding current value.
During the next negative half-cycle the device is fully “OFF”
anyway until the following positive half-cycle when the
process repeats itself and the thyristor conducts again as long
as the switch is closed.
Then in this condition the lamp will receive only half of the
available power from the AC source as the thyristor acts like a
rectifying diode, and conducts current only during the positive
half-cycles when it is forward biased. The thyristor continues
to supply half power to the lamp until the switch is opened.

30. If it were possible to rapidly turn switch S1 ON and OFF, so that
the thyristor received its Gate signal at the “peak” (90o) point of
each positive half-cycle, the device would only conduct for one
half of the positive half-cycle.
In other words, conduction would only take place during one-
half of one-half of a sine wave and this condition would cause
the lamp to receive “one-fourth” or a quarter of the total power
available from the AC source.
By accurately varying the timing relationship between the Gate
pulse and the positive half-cycle, the Thyristor could be made
to supply any percentage of power desired to the load, between
0% and 50%. Obviously, using this circuit configuration it
cannot supply more than 50% power to the lamp, because it
cannot conduct during the negative half-cycles when it is
reverse biased.

31. Silicon Controlled Rectifier
• Industrially SCRs are applied to produce DC voltages for
motors from AC line voltage
• Rectifier
– Half-wave rectifier, full-wave rectifier

32. Half-wave rectifier

33. Half-wave rectifier

34. Half-wave rectifier

35. Half Wave Phase Control

36. Phase control is the most common form of thyristor AC power
control and a basic AC phase-control circuit can be constructed
as shown above. Here the thyristors Gate voltage is derived
from the RC charging circuit via the trigger diode, D1.
During the positive half-cycle when the thyristor is forward
biased, capacitor, C charges up via resistor R1 following the AC
supply voltage. The Gate is activated only when the voltage at
point A has risen enough to cause the trigger diode D1, to
conduct and the capacitor discharges into the Gate of the
thyristor turning it “ON”.
The time duration in the positive half of the cycle at which
conduction starts is controlled by RC time constant set by the
variable resistor, R1.

37. Increasing the value of R1 has the effect of delaying the
triggering voltage and current supplied to the thyristors Gate
which in turn causes a lag in the devices conduction time.
As a result, the fraction of the half-cycle over which the
device conducts can be controlled between 0 and 180o, which
means that the average power dissipated by the lamp can be
adjusted.
However, the thyristor is a unidirectional device so only a
maximum of 50% power can be supplied during each positive
half-cycle.