THYRISTOR

By Utsav Yagnik(Enrollment No. 150430707017)ME (Electrical), SSGEC, Bhavnagar

Subject : ADVANCED POWER ELECTRONICS

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 31 312

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.

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2. Structure (figure)

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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.

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

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3. Operation

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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

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

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

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.

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4. Static characteristics (graph)

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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.

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5. Transient characteristic during turning on

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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.)

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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.

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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.

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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.

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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.

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7. Turn off transients

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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.

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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.

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7. Remedies for limitations during turn off process

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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.

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8. Parameters in datasheet

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8. Parameters in datasheet

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8. Parameters in datasheet

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8. Parameters in datasheet

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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.

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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.

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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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THANK YOU

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