03 thyristors by lakshmi
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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.
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• 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.
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Different types of Thyristors• Silicon Controlled Rectifier (SCR).
•
TRIAC.• DIAC.
• Gate Turn-Off Thyristor (GTO).
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SCR
Symbol of
Silicon Controlled Rectifier
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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
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Device Operation
Simplified model of a
thyristor
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V-I Characteristics
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Effects of gate current
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Two Transistor Model of SCR
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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
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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
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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
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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
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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
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Turn-onCharacteristics
on d r t t t
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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
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Methods of Thyristor Turn-on
• Thermal Turn-on.
•
Light.• High Voltage.
• Gate Current.
•dv/dt.
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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
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Voltage Ratings
DWM DRM DSM
RWM RRM RSM
T
V V V
V V V
dvV dt
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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
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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
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Current Ratings
Taverage TRMS L
H
I I I
di
I dt
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Gate Specification
gt gt
gD RR
thjc
I V
V Q
R
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Gate Triggering Methods- Efficient & reliable method for turning on SCR.
• Types
R - Triggering.
RC - Triggering.
UJT - Triggering.
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R-Triggering
Resistance firing circuit
LOAD
vO
a b
i R1
R2
D
R Vg
VT
v =V sin tS m
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2With 0 R
11
,m m
gmgm
V V I R
R I
1 2
mgm V RV
R R R
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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
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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
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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
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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
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RC Triggering
RC half-wave trigger circuit
LOAD
vO
R
C
VT
v =V sin tS m
D2
VC
+
-D1
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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
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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
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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
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RC full-wave trigger circuit
vO
R
C
VT
v =V sin tS m
+
-
LOAD+
-
D1 D3
D4 D2
vd
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2
T RC
s gt
gt
v V
R I
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vs
vd
vo
vT
t
t
t
V sin tm
vd
vc vc vcvgt
Waveforms of
RC full-wavetriggering with R
at high value
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V sin tm vs
vd
vo
vT
t
t
t
vgt
Waveforms of
RC full-wavetriggering with R
at low value
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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
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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
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UJTCharacteristics
Ve
VBBR load line
Vp
Vv
IeIvIp0
Peak Point
Cutoff region
Negative ResistanceRegion
Saturationregion
Valley Point
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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
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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
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T
RC P BB V BBV V V V e
ln BB V
BB P
V V T RC V V
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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
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1ln1
T RC
V D << V BB
V P = V BB
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Design
BB P P
V I R V
BB P
P
V V R I
.
,
.
.
.
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BB V
V
V V R I
1g Bt R C 4
2 10 B
BB
RV
BB V V V I R V
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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
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1 2 1 2 1 2
PulseVoltage
vc,vdc
vc vc vc
VdcVZ
VZt
t