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Lecture Notes
Thyristors (SCRs)
OUTLINE
• SCR construction and I-V characteristics.
• Physical operation of SCRs.
• Switching behavior of SCRs
• dv/dt and di/dt limitations and methods of
improving them.
• SCR drive circuit considerations.
Copyright © by John Wiley & Sons 2002
SCRs -1
Thyristor (SCR) Geometry
Cathode
Gate
N+
J3
19
10 cm
-3
N+
17
10 cm
P
19
10 cm
10 
-3
-3
30100 
J 2
14
13
10 - 5x10 cm
N
501000
-3

J1
P
P
+
17
10 cm
-3
19
10 cm
-3
3050 µ
Anode
Gate and cathode metallization for
slow (phase control) thyristor.
cathode
gate
• Cross-sectional
view showing
vertical
orientation of
SCR.
• SCRs with
kiloamp ratings
have diameters
of 10 cm or
greater.
Gate and cathode metallization
for fast (inverter grade) SCR
wafer
distributed
gate
wafer
Copyright © by John Wiley & Sons 2002
cathode area
(metallization
not shown)
SCRs -2
Thyristor I-V Characteristics
i
• SCR triggerable from forward blocking
state to on-state by a gate current pulse.
A
forward
on-state
I
I
-V
i >0
G
H
• Thyristor latches on and gate cannot turn it
off. External circuit must force SCR off.
i =0
G
BO
RW
M
V
BO vAK
VH
forward blocking
state
Thyristor circuit symbol.
+
i
A
VA
K
• Current to several kiloamps for V(on) of 24 volts.
• Blocking voltages to 5-8 kilovolts.
• VBO = breakover voltage ; I BO =
breakover current
• VH = holding voltage I H = holding current
cathode
anode
iG
gate
Copyright © by John Wiley & Sons 2002
• Maximum junction temperature = 125 C limited by temperature dependence of
VBO.
SCRs -3
SCR Model and Equivalent Circuit
One dimensional SCR model.
A
• BJTs in equivalent circuit in active region.
P1
J
1
(N - )
N1
• Use Ebers-Moll equations for
J
2
P
2
G
J
3
(N +)
N2
BJTs
• IC1 = - 1 IE1 + I CO1 ; I C2 = - 2 IE2 + I CO
• IA = I E1 ; I K = -I E2 = I A + I G
K
• IC1 + I B1 + I E1 = 0
Two transistor equivalent circuit
A
• IA =
J
1
Q
1
J
2
IG + I CO1 + I CO2
11 - 2
• Blocking state
Q
2
J
1 + 2 << 1
G
3
K
Copyright © by John Wiley & Sons 2002
• At breakover 1 + 2 - 1
SCRs -4
Thyristor Turn-on Process
• In forward blocking state, both BJTs active.
• If 1 + 2 < 1, connect ion is stable.
• If VAK = VBO or if positive gate current pulse is applied
1 + 2 becomes equal to unity and ci rcuit connection beco mes
unstable and SCR switches on.
p
1
J1
n
+
J
+
-
2
+
J
3
Holes attracted
by ne gative
charge o f inje cted
ele ctro ns
+
1
+ +
+
p
2
-
J 2 d epletion
width - no gate
current
J 2 d epletion
width - with
gate current
n2
Electrons
injected in
resp onse to
gate current
f low
Copyright © by John Wiley & Sons 2002
• Negative charge of electrons swept into n 1
layer partially compensate positive charge
of ionized donors exposed by growth of
depletion of junction J2.
• Growth of depletion reduces width of
bases of Qnpn and Q pnp and thus
increases 1 and 2.
• Holes attracted by first wave of injected
elctrons attract additional electrons and so
on - regenerative action.
SCRs -5
Thyristor On-state Latchup
SCR with negative gate current
• Negative gate current causes lateral voltage
drops as indicated which lead to current
crowding in center of cathode.
K
Negative gate
current
N
G
+
N
+
-
P
N
-
-
+
+
• Conventional SCRs (phase control) have large
area cathodes - negative gate current cannot
remove stored charge from center of large
cathode area.
• SCR stays latched on in spite of negative gate
current.
P
A
Copyright © by John Wiley & Sons 2002
• External circuit must force anode current to
negative values in order that enough stored
charge be removed from SCR so that it can
turn off.
SCRs -6
Thyristor On-state Operation
G
N
2
K
P
2
N
1
P
1
A
x
total
carrier
density
N
N
A
1
1
D
2
N
A
2
N
D1
x
• On-state: all three junctions forward biased and BJTs
in equivalent circuit saturated.
• On-state stable because saturated BJTs have
1 + 2 << 1.
• On-state voltage V AK(on) = Vj1 - V j2 + Vj3 + Vn
Copyright © by John Wiley & Sons 2002
SCRs -7
Thyristor Turn-on Behavior
i (t)
G
T
A
t
di
v
A
TB
i (t)
A
vB
T
C
v
t
C
d(on)
Io
F
dt
Io
t
tr
t ps
v (t)
AK
control
t
v
A
vB
v
• td(on) = turn-on delay time; time required for charge
injection by gate current to make 1 + 2 = 1.
C
t
• Time intervals that T
i
• tr = time required for anode current to reach on-state
value. Anode current rate-of-rise diF/dt limited by
external inductance.
can be on
A
• tps = time required for plasma to spread over whole
cathode area from cathode periphery near gate.
G
Delay or trigger angle
Copyright © by John Wiley & Sons 2002
t
• VAK does not attain on-state value until complete area
of cathode is conducting.
SCRs -8
Thyristor Turn-off Behavior
diR
t
dt
iA ( t )
t2
I
t1
3
t
IR
R
4
Turn-off waveforms
dv F
dt
V
REV
v (t)
AK
recovery time t > t q
t
3
• SCR turn-off quite similar to power diode turn-off.
• Anode current rate-of-fall controlled by external inductance.
• Reverse voltage overshoot caused by external inductance.
• Junction J1 is blocking junction in reverse bias. J 3 has low
breakdown voltage (20-40 volts) because of the heavy doping on
both sides of the junction.
Copyright © by John Wiley & Sons 2002
SCRs -9
Thyristor di/dt Limit at Turn-on
i
G
K
G
N2
N2
P2
i (t)
G
N
1
P
1
i
A
t
A
• SCR first turns on at cathode pe riphe ry n earest gate.
• Current constricted to small areas during initial pha ses of turnon, td(on) and tr.
• Use shaped gate current pulse for
rapid turn-on.
• If anode cu rrent rate-of-rise, di F/dt, not kept le ss than some
specified maximum, current density in constricted area will be
too large.
• Localized pow er dissipation too high and thermal runaway
likely.
Copyright © by John Wiley & Sons 2002
SCRs -10
Thyristor Re-applied dv/dt Limits
dv F
dt
v (t)
AK
• Removal of all stored charge in SCR requires a
minimum time tq.
VF
• Application of positive dVF/dt larger than a specified
value before tq results in a pulse of positive anode
current which may produce unintentioned turn-on of
the SCR.
t
V REV
forward
recover
y
current
iA(t)
t
• Avoidance of unintentioned turn-on requires
dVF/dt < dVF,max/dt and remaining in reverse bias
for a minimum time tq.
Rate effect
Copyright © by John Wiley & Sons 2002
SCRs -11
Methods of Improving Thyristor di/dt Rating
• Interdigitated gate-cathode structure used to greatly
increase gate-cathode periphery.
• Distance from periphery to center of any cathode region
significantly shortened.
• Ability of negative gate current to break latching
condition in on-state increased.
• Combination of pilot thyristor, diode, and iterdigitated
gate-cathode geometry tgermed a gate-assisted turn-off
thyristor or GATT
• Use of pilot thyristor to increase turn-on gate
current to main thyristor.
• Larger gate current increases amount of initial
conducting area of cathode and thus improves
diF/dt capabiities.
• Diode allowes negative gate current to flow from
main SCR.
Copyright © by John Wiley & Sons 2002
SCRs -12
Improvement in dv/dt Rating Via Cathode Shorts
• Current thru Cj2 indistinguishable from positive gate current with
respect to turn-on of SCR.
• If current thru Cj2 bypasses junction J 3, then SCR will not be
turned on by the large displacement currents.
• Cathode shorts provide this desirable bypa ss. Most effective with
interdigitaated gate-cathode geometry.
dVF
• dt 
significantly increased.
max
Copyright © by John Wiley & Sons 2002
SCRs -13
Thyristor Gate Trigger Requirements
V
G
K
V
G
G
trigger
circuit load
line
maximum gate
power
diss ipation
Equivalent circuit of
minimum
temperature
SCR drive circuit
RG
maximum
temperature
I
minimum
trigger
current
I
G
2
G
1
V
G
R
GG
V +
G
G
IG
i (t)
G
Gate current must be on for a specified
minimum time interval (few tens of
microseconds) to guarantee SCR turn-on
Copyright © by John Wiley & Sons 2002
SCRs -14