03 Thyristors by Lakshmi

8/3/2019 03 Thyristors by Lakshmi

http://slidepdf.com/reader/full/03-thyristors-by-lakshmi 1/51

1

Thyristors• Most important type of power semiconductor

device.

• Have the highest power handling

capability.they have a rating of 1200V / 1500A

with switching frequencies ranging from 1KHz

to 20KHz.



2

• Is inherently a slow switching device

compared to BJT or MOSFET.

• Used as a latching switch that can be turned on

by the control terminal but cannot be turned

off by the gate.



3

Different types of Thyristors• Silicon Controlled Rectifier (SCR).

•

TRIAC.• DIAC.

• Gate Turn-Off Thyristor (GTO).



4

SCR

Symbol of

Silicon Controlled Rectifier



5

Structure

Gate Cathode

J3

J2

J1

Anode

10 cm17 -3

10 -5 x 10 cm13 14 -3

10 cm17 -3

10 cm19 -3

10 cm19 -3

10 cm19 -3

n+

n+

p-

n–

p

p+

10 m

30-100 m

50-1000 m

30-50 m



6

Device Operation

Simplified model of a

thyristor



7

V-I Characteristics



8

Effects of gate current



9

Two Transistor Model of SCR



10



11

The general transistor equations are,

1

1

C B CBO

C E CBO

E C B

B E CBO

I I I

I I I

I I I

I I I



12

1 1

1 1

1 1

1

1

Considering PNP transistorof the equivalent circuit,

, , ,

,

1 1

E A C C

CBO CBO B B

B A CBO

I I I I

I I I I

I I I



13

2 2 2

2 2

2 2

2

2

Considering NPN transistor

of the equivalent circuit,

, ,

2

C C B B E K A G

C k CBO

C A G CBO

I I I I I I I I

I I I

I I I I



14

2 1

2 1 2

1 2

From the equivalent circuit,we see that

1

C B

g CBO CBO A

I I

I I I I



15

1 2

1 2

Case 1: When 0

1

g

CBO CBO

A

I

I I I

2 1 2

1 2

Case 2: When 0

1

G

g CBO CBO

A

I

I I I I



16

Turn-onCharacteristics

on d r t t t



17

Turn-offCharacteristics

Anode currentbegins todecrease

tCtq

t

t

Commutationdi

dt

Recovery Recombination

t1 t2 t3 t4 t5

trr tgr

tq

tc

VAK

IA

tq=device off time

tc=circuit off time



18

Methods of Thyristor Turn-on

• Thermal Turn-on.

•

Light.• High Voltage.

• Gate Current.

•dv/dt.



19

Thyristor RatingsFirst Subscript Second

Subscript

Third Subscript

D off state W working M Peak Value

T ON state R Repetitive

F Forward S Surge ornon-repetitive

R Reverse



20

Voltage Ratings

DWM DRM DSM

RWM RRM RSM

T

V V V

V V V

dvV dt



21

dv/dt Triggering

2 2 2

2 2 2

2

2

j

j j j

j dV j

j

dq d i C V dt dt

C dC V dt dt



22

2 2 2

2 2 22

2 j j j

j j j

j

dq d i C V

dt dt C dV dC

V dt dt



23

Current Ratings

Taverage TRMS L

H

I I I

di

I dt



24

Gate Specification

gt gt

gD RR

thjc

I V

V Q

R



25

Gate Triggering Methods- Efficient & reliable method for turning on SCR.

• Types

R - Triggering.

RC - Triggering.

UJT - Triggering.



26

R-Triggering

Resistance firing circuit

LOAD

vO

a b

i R1

R2

D

R Vg

VT

v =V sin tS m



27

2With 0 R

11

,m m

gmgm

V V I R

R I

1 2

mgm V RV

R R R



28

1 2

1 2

1 2

gm m

gm m gm

gm

m gm

V R R R V R

V R R R V V

V R R RV V



29

Waveforms of R-triggering with R2

at large value

VS

23 4

t

V sin tm

Vg Vgt

t

t

t

t

Vo

io

VT

VgpVgtVgp

R Large2

gp gt V V



30

t

t

t

t

t

23 4

VS

Vg

Vo

io

VT

V =Vgp gt

2700

23 4

900 =90

0

Waveforms of R-triggering with R2

at optimum value

gp gt V V



31

t

t

t

t

t

23 4

VS

Vg

Vo

io

VT

<900

V >Vgp gt Waveforms of R-

triggering with R2at small value

gp gt V V



32

RC Triggering

RC half-wave trigger circuit

LOAD

vO

R

C

VT

v =V sin tS m

D2

VC

+

-D1



33

1.3

2

T RC

1C gt d V V V

1s gt C gt gt d v I R V I R V V

1s gt d

gt

v V V R I



34

vs

0

V sin tm

0 t

t

avc

- /2

a

vc

Vgt

vo

vT

Vm

-Vm

t Waveforms of

RC half-wavetriggering with R

at high value



35

t

vs

0

V sin tm

0t

avc

- /2

a

vc

Vgt

0

0

vo

vT

Vm Vm

-Vm

(2 + )

t Waveforms of

RC half-wavetriggering with R

at low value



36

RC full-wave trigger circuit

vO

R

C

VT

v =V sin tS m

+

-

LOAD+

-

D1 D3

D4 D2

vd



37

50

2

T RC

s gt

gt

v V

R I



38

vs

vd

vo

vT

t

t

t

V sin tm

vd

vc vc vcvgt

Waveforms of

RC full-wavetriggering with R

at high value



39

V sin tm vs

vd

vo

vT

t

t

t

vgt

Waveforms of

RC full-wavetriggering with R

at low value



40

Uni-Junction Transistor

E

B2

B1B1

A

B2

E

RB2

RB1n-type

p-type

Eta-point

Basic Structure Symbol

1 2 BB B B R R R



41

11

1 2

B

BB

B B

BB

P BB D

RV V

R R

V

V V V

RB2

VBB

+

-

E

B1

RB1 VBB

A+

-

Ve Ie

B2

Eta-point

V1

VD

Equivalent Circuit of UJT



42

UJTCharacteristics

Ve

VBBR load line

Vp

Vv

IeIvIp0

Peak Point

Cutoff region

Negative ResistanceRegion

Saturationregion

Valley Point



43

UJT Relaxation Oscillator

Circuit Diagram Waveforms

R R2

VBB

R1C

E B2

B1Ve vo

Ve

Vp

VV

Vo

t

t

Capacitorcharging

1=RC

T

V +VBB

VP

2 1=R C

Capacitordischarging

Vv



44

Expression for period of

oscillation T

Voltage of capacitor

t RC C final initial finalV V V V e

,, ,C P initial V final BB

At t T V V V V V V



45

T

RC P BB V BBV V V V e

ln BB V

BB P

V V T RC V V



46

if ,

1ln ln

1

V BB

BB

P BB P

BB

V V

V T RC RC V V V

V

P BB D But V V V



47

1ln1

T RC

V D << V BB

V P = V BB



48

Design

BB P P

V I R V

BB P

P

V V R I

.

,

.

.

.



49

BB V

V

V V R I

1g Bt R C 4

2 10 B

BB

RV

BB V V V I R V



50

SYNCHRONIZEDUJT OSCILLATOR

R

C

+

-

D1 D3

D4 D2

Vdc

R1

VZ

+

-

Z

i1

vc

+

-

R2

G1

C1

G2

C2

Pulse TransformerE

B2

B1

To SCRGates



51

1 2 1 2 1 2

PulseVoltage

vc,vdc

vc vc vc

VdcVZ

VZt

t

03 Thyristors by Lakshmi

Documents

Transcript of 03 Thyristors by Lakshmi