Thyristor is a four-layered semiconductor device that can conduct current in one direction when a gate signal is applied. It has the property of latching where it stays in the forward conducting state even after the removal of the gate signal. The document discusses the structure of a thyristor including the doped layers, its operating characteristics like turn on, turn off and static characteristics. It describes the transient behaviors during turn on and turn off and limitations during these processes. The document also covers parameters mentioned in thyristor datasheets and different types of thyristors like SCR, DIAC and TRIAC.

1. THYRISTOR
By Utsav Yagnik(Enrollment No. 150430707017)
ME (Electrical), SSGEC, Bhavnagar
Subject : ADVANCED POWER ELECTRONICS

2. Table of contents
Sr. No. Topic name From
Slide
To Slide
1 Introduction 3 3
2 Structure 4 6
3 Operation 7 11
4 Static Characteristics 12 13
5 Transients during turn on 14 18
6 Safe Operating Area 19 19
7 Transients during turn off 20 24
8 Parameters in datasheet 25 28
9 Types of thyristor 29 30
10 References 31 31 2

3. 1. Introduction
• Oldest semiconductor device.(1957 General Electric Research
Laboratories)
• Also known as SCR which stands for Semi-conductor
Controlled Rectifier.
• Can handle currents above 100 A and voltages above 1 kV.
• Normally used at highest power levels for conditioning
circuits.
• Four layered, three terminal device.
• Has property of “Latching” i.e. Staying in forward conduction
even after removal of Gate signal.
3

4. 2. Structure (figure)
4

5. 2. Structure (theory)
• N-base region :
1. Doped with phosphorus.
2. It is the region which will withstand high voltages during
forward blocking or off state, so it is highly resistive.
• P-regions :
1. Aluminum or Gallium used to form p regions.
• Thicker N-base region – More blocking capacity but higher
time to turn on and off resulting in slower switching.
5

6. 2. Structure (with doping levels
and thickness of layers)
6

7. 3. Operation
7

8. 3. Operation
 Forward blocking mode
• The positive voltage applied to anode but it is not sufficient
enough to make thyristor in to forward conducting mode.
 Forward conducting mode
• Gate signal is given to the device
• Makes J3 forward biased i.e. Electrons travel from n-emitter
to p-base.
• Some of above electrons diffuse through p-base and get
collected in n-base.
• Collected charges changes bias condition of J1 which in turn
causes diffusion of holes from p-emitter to n-base.(contd.)
8

9. 3. Operation
• These holes diffuse through n-base and get collected in p-base
which acts same as gate current.
• The above process is regenerative and continues until J2 also
becomes forward biased and the thyristor is latched in to ON
state.
• Other methods to turn ON thyristor are:
1. By applying higher forward voltage.(Not suggested due to
possibility of damage.)
2. By increasing temperature.
3. By increasing rate of increase in the voltage.(Rate should be
controlled.)(contd.)
9

10. 3. Operation
• Once the thyristor has moved in to forward conduction, the
gate current is not needed to keep it in the same mode.
• Also it can not return to forward blocking mode by application
of gate current.
• To turn OFF, the anode current must be kept disconnected for
sufficient time to allow stored charge in the device to
recombine.
Reverse blocking mode
• Determined by J1 and J3 when voltage is applied in reverse
bias.
• Until the voltage reaches the reverse breakdown voltage, the
thyristor remains in revrse blocking mode.(contd.) 10

11. 3. Operation
• If symmetric thyristor is needed then it is achieved by
fabricating forward and reverse blocking junctions at the same
time with very long diffusion process at high temperatures.
• Asymmetric thyristors are made to optimize the forward
conduction and turn off properties. It is achieved by using
much more thinner n-base then symmetric thyristor.
11

12. 4. Static characteristics (graph)
12

13. 4. Static characteristics
(theory)
• Gate current : The current required to ON the thyristor
without risk of damage. More the gate current, less the
forward blocking voltage.
• Latching current : minimum current required t turn ON
thyristor.
• Holding current : minimum current required to keep thyristor
in ON state.
• Forward blocking voltage : maximum forward voltage up to
which thyristor remains OFF.
• Reverse blocking voltage : maximum revverse voltage up to
which thyristor remains in reverse blocking mode.
13

14. 5. Transient characteristic during
turning on
14

15. 5. Transient characteristic during
turning on (theory)
• Gate current is given of fixed magnitude for fixed duration.
• The anode current increases by a fixed rate which is
determined by the stray inductance or an external circuit.
• Turn on delay time: during which, thyristor appears to be in
blocking state.
• Gate current continuously adds charge carriers to J2 until the
device reaches the anode current begins to increase.
• Rise time: during which excess-carrier density in the device
increases which in turn gives rise to the anode current until it
reaches its steady on state value.(contd.)
15

16. 5. Transient characteristic during
turning on (theory)
• The increase in the current value decreases the voltage across
anode and cathode simultaneously.
• The rate of rise of current should be kept below the specified
value in the datasheet otherwise it can damage the device.
• Spreading time : during which the current remains constant
until any attempt or any phenomenon of turning off the
thyristor is not observed.
16

17. 5. Limitations during turn on
process
• If the rate of rise in current is above specified then it will
damage the device.
• If the rate of rise in current is large then it will result in less
area for current conduction in device.
• This will not allow the voltage to drop sufficiently and it will be
near to blocking state voltage.
• This will result in higher power dissipation in device which
may not be within limits of power handling capacity of device.
17

18. 5. Remedies for limitations
during turn on process
• If faster turning on is required then initially large gate current is
given to maximize the initial turned on areas and then it is
decreased to a smaller value.
18

19. 6. Safe Operating Area
• The graph here shows SOA(Safe
Operating Area) of the Power
MOSFET.
• The solid lines shows the
bounding area for the DC
operation.
• When the device is being used
for shorter duration, the power
dissipation is less then DC
operation.
• So, the limits of current can be
extended. 19

20. 7. Turn off transients
20

21. 7. Turn off transients (theory)
• Achieved by reverse biasing the device for sufficient time.
• The current becomes negative at t1 and voltage goes negative
after the J1 or J3 has been reverse biased.
• The current attains its negative peak value and then decays
back to zero.
• The voltage also attains negative peak value and retains it until
current decays to zero.
• The value of reverse voltage decides how quickly the current
will attain zero. It is governed by inductance of circuit.
21

22. 7. Limitations during turn off
process
• During recovery of current from its negative value to zero, the
current might attain some reverse recovery value which might
be large enough to accidently turn on the device due to
remaining excess carriers unintentionally.
• Such pulse of current should be contained.
22

23. 7. Remedies for limitations
during turn off process
23

24. 7. Remedies for limitations
during turn off process
• The reverse blocking mode should be continued until t3.
• The rate of change of voltage should be controlled so that
reverse recovery current is small enough.
• Both of the above things limits are mentioned by the
manufacturer in datasheet.
24

25. 8. Parameters in datasheet
25

26. 8. Parameters in datasheet
26

27. 8. Parameters in datasheet
27

28. 8. Parameters in datasheet
28

29. 9. Types of thyristor
• SCRs
• It has same construction to thyristor device that has been
discussed in the ppt.
• Used in power control applications unlike thyristor.
• DIACs
• Can be imagined as two back to back thyristor connected
without gate.
• Can flow current in both direction.
• TRIACs
• DIAC with non interchangeable anodes and gate junction.
29

30. 9. Types of thyristor
• Silicon Controlled Switch(SCS)
• It is similar to SCR but with two gates so that turning on can be
done by positive pulse on Cathode and turning off can be
done by positive pulse on Anode.
• Unijunction Transistor(UJT)
• Has a block of lightly doped n-material with p-material grown
on its side.
• Often used in triggering of SCRs and Triacs.
• Programmable UJT(PUT)
• Main aim to design it is to use SCRs and Triacs as UJT.
• Can be used in relaxation oscillators.
30

31. 10. References
1) Muhammad Rashid, “Power Electronics Handbook”,
Butterworth-Heinemann, Third Edition, 2010.
2) Ned Mohan, Tore M Undeland, William P Robbins, “Power
Electronics-Converter, Applications and Design, John Wiley
& Sons, 2003.
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32. THANK YOU
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