Thyristors are four-layer semiconductor devices that can efficiently control and convert large amounts of power in AC or DC systems. They have higher power handling capabilities than diodes and transistors. A thyristor is turned on by applying a voltage at its gate terminal and remains on until the current drops to zero. Thyristors are used in applications that require switching or controlling high currents like speed control, voltage regulation, and circuit protection. They can be connected in series and parallel configurations to achieve higher voltages and currents.

1. QUAID-E-AWAM UNIVERSITY
OF ENGINEERING, SCIENCE & TECHNOLOGY
NAWABSHAH, SINDH, PAKISTAN
Subject Faculty: Prof. Dr. Ghulam Mustafa Bhutto
M.E (Electrical Engineering) Chalmers University of Technology
Goteborg, Sweden,2010
PhD (Smart Grids) Aalborg University, Denmark, 2014
Presented By: Hafiz Saleem Ahmed (B.E 17EL)
Student of Electrical Engineering Department
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2. Contents:
Introduction of Thyristor
Power Semiconductor Switches
Why Thyristor are Used
Symbol & Construction of Thyristor
Thyristor family
Characteristics of Thyristor
V – I Characteristics of Thyristor
Four Modes in Thyristor Operation
Series & Parallel Operation of Thyristors
Merits & Demerits
Applications
Summary
References
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3. Introduction of Thyristor
• 4 – layer semiconductor device of alternating p- and n- material.
• The word Thyristor is coined from THYRatron and TransISTOR.
• Two – states: ON & OFF.
• Silicon Controlled Rectifier: SCR
• Trade Name of Thyristors commercialized by General Electric in
1957.
• 4-layered 3-terminal device.
• Have the highest power handling capability.
• Rating of 1200V / 1500A.
• Switching Frequency: 1KHz to 20KHz.
Definition: A thyristor is a four layer solid-state semiconductor device
with P and N type material.
Thyristor devices can convert and control large amounts of power in AC
or DC systems while using very low power for control.
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4. Power Semiconductor Switches
Power Diodes
2 layer device
Power
Transistors
3 layer Device
Thyristors
4 layer
Device
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5. Why Thyristor are Used
Diodes and Transistors are the tiny electronic components that
changed the world: you'll find them in everything
from calculators and computers to telephones, radios, and hearing
aids. They're amazingly versatile, but that doesn't mean they can
do everything. Although we can use them to switch tiny electrical
currents on and off (that's the basic principle behind computer
memory), and transform small currents into somewhat larger ones
(that's how an amplifier works), they're not very useful when it
comes to handling much bigger currents. Another drawback is that
they turn off altogether as soon as the switching current is removed,
which means they're not so useful in devices such as alarms where
you want a circuit to trigger and stay on indefinitely. For those sorts
of jobs, we can turn to a somewhat similar electronic component
called a thyristor
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6. Symbol & Construction
Construction: The thyristor consists of a four layer PNPN structure with
the outer layers are referred to as the anode (P-type) and cathode (N-type).
The control terminal of the thyristor is named the gate and it is connected
to the P-type layer located next to the cathode.
Alternately N-type or P-type material, for example P-N-P-N.
The control terminal, called the gate, is attached to p-type material near to
the cathode.
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7. 1- Silicon
controlled
Rectifier
(SCR)
2- Gate-
turnoff
thyristor
(GTO)
3- Triac
4- Diac
5- Silicon
controlled
switch
(SCS)
6- Mos-
controlled
switch
(MCT)
Thyristor family includes
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8. Characteristics of Thyristors
 When the anode is at a positive potential VAK with respect to the
cathode with no voltage applied at the gate, junctions J1 and J3 are
forward biased, while junction J2 is reverse biased. As J 2 is reverse
biased, no conduction takes place.
 Now if V AK is increased beyond the breakdown voltage VBO of the
thyristor, avalanche breakdown of J 2 takes place and the thyristor starts
conducting.
 If a positive potential VG is applied at the gate terminal with respect to
the cathode, the breakdown of the junction J2 occurs at a lower value of
VAK.By selecting an appropriate value of VG, the thyristor can be
switched into the on state suddenly.
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9. V-I Characteristics of SCR
A V-I Characteristic of SCR (Silicon Controlled Rectifier) is the
voltage current characteristics. The current through the SCR varies
as the Anode to Cathode terminal voltage and Gate to Cathode
terminal voltage is varied. The graphical representation of current
through the SCR and voltage across the anode to cathode terminal
is known as V-I Characteristics of SCR.
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10. Typical V-I Characteristics of SCR
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11. Four Modes in Thyristor Operation
• Reverse Blocking Mode: Thyristor Open Circuit
• Reverse Conduction Mode: Thyristor Closed Circuit
• Forward Blocking Mode: Thyristor Open Circuit
• Forward Conduction Mode: Thyristor Closed Circuit
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12. Series and Parallel SCR Connections
• SCRs are connected in series and parallel to extend voltage and
current ratings.
• For high-voltage, high-current applications, seriesparallel
combinations of SCRs are used.
SCRS IN SERIES
• Unequal distribution of voltage across two series SCRs.
• Two SCRs do not share the same supply voltage. Maximum
voltage that SCRs can block is V1+V2, not 2VBO.
SCRS IN PARALLEL
• Unequal current sharing between two SCRs
• Total rated current of parallel connection is I1+I2, not 2I2.
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13. Merits & Demerits of Thyristors
Merits of SCR:
1. SCRs with high voltage and current ratings are available.
2. On state losses in SCRs are reduced.
3. Very small amount of gate drive is required since SCR is a
regenerative device.
Demerits of SCR:
1. Gate has no control after the SCR is turned ON.
2. External circuits are required to turn OFF the SCR.
3. Operating frequencies are very low.
4. Snubber circuits are required for dv/dt protection.
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14. Applications:
• High Current High Voltage Applications.
• Controlling alternating currents, where the change of polarity
of the current causes the device to switch off automatically;
referred to as Zero Cross operation.
• Phase angle triggered controllers, also known as phase fired
controllers.
• “Circuit breaker" or “Crowbar" to prevent a failure in the
power supply from damaging downstream components, by
shorting the power supply output to ground.
• Load voltage regulated by thyristor phase control.
• Red trace: load voltage
• Blue trace: trigger signal.
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15. Summary:
The SCR is triggered on the positive cycle
and turns off on the negative cycle.
A circuit like this is useful for speed control
for fans or power tools and other related
applications.
The SCR can handle a large current, which
causes the fuse (or circuit breaker) to open.
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16. References:
• Introduction to Power Electronics - A Tutorial; Burak Ozpineci
• INTRODUCTION TO POWER ELECTRONICS SYSTEMS;
• Power Electronics and Drives (Version 3-2003). Dr. Zainal
Salam, UTM-JB
• Power Electronics: Circuits, Devices and Applications 3rd
Edition, Muhammad H.Rashid.
• Power Electronics, Dr. P. S. Bimbhra.
• Power Semiconductor Devices, Version 2, IIT – Kharagpur,
NPTEL
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