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POWER ELECTRONICS PRINCIPLES
C
C4
POWER SEMICONDUCTORS:
SILICON CONTROLLED RECTIFIER
=
C4.1 Thyristors
Thyristor is a generic (Family) name for a range of power semiconductor devices that have evolved since
the late 1950's.
1950 HOOK TRANSISTOR
Shockley
1956 SHOCKLEY DIODE
1957 SILICON CONTROLLED RECTIFIER
1958 SCR: 16 A : 25 V
General Electric: CR35
1960 SCR: 70 A: 500V
At this time Westinghouse used the name TRINISTOR.
As of the end of October 1970 the International Electrotechnical Commission (IEC) did not include the
term Thyristor in the International Electrotechnical Vocabulary. The British Standards Institution were
however preparing to include it in a British Standard Glossary containing terms and definitions relating
to semiconductors.
The draft definition proposed for Thyristor was
"A bistable semiconductor device comprising three or more junctions and having a trigger electrode
which initiates but does not control the current flowing between the principal terminals."
Since this was a power semiconductor with some transistor type action that would take the place of the
thyratron the name is most likely an acronym
THYRatron & transISTOR
As new devices evolved (see table C1-2, sec C1) the definition needed updating. The modern thyristor
family of devices are predominantly 3-terminal bistable bipolar devices (there is the MCT, moscontrolled SCR) that comprise three or more p-n junctions. The device may be triggered into conduction
and may also have turn-off initiated by a pulse that does not control the current flowing between the
principal terminals.
The device is normally in the HIGH IMPEDANCE OFF STATE with transfer to the second stable
LOW IMPEDANCE ON STATE in a few microseconds. The reverse transfer from ON-OFF
(Commutation) is slightly longer, but of the same order. Unlike the transistor which can operate either as
a switch or in the linear (active) region the thyristor family have no intermediate operational state and
are therefore switches.
Device structure together with p-n region size and doping determines the actual device name and its
parameters.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
1
Prof R T Kennedy
C4.2 SCR (Reverse Blocking Triode Thyristor): Structure
A simplified structure of the SCR showing comparative layer width and doping is given in Fig. C4-1
A
LAYER SIZE
Anode
ohmic contact
p
MODERATELY
THICK
DOPING LEVEL
p+
MODERATE
ANODE LAYER
ANODE
JUNCTION
nTHICK
LIGHT
BLOCKING LAYER
CONTROL /
BLOCKING
THIN
G
Gate
VERY THIN
p
CONTROL LAYER
JUNCTION
n+
JUNCTION
CATHODE LAYER
K
MODERATE
CATHODE
HEAVY
Cathode
FIGURE C4-1
The ability of a p-n junction to support voltage depends on the junction reverse leakage current. As a
result of the differing doping levels (impurity concentrations) either side of the junctions in Fig.C4-1 the
junctions with higher doping levels will have greater leakage current and will therefore support less
voltage.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
2
Prof R T Kennedy
C4.3 SCR (Reverse Blocking Triode Thyristor): Diode Analogy
There is a tendency to assume that the operation of devices comprising a number of diode junctions can
be explained in terms of diode equivalent circuits. The following will not aid understanding SCR
operation but will show why a diode analogy does not work.
FORWARD BIAS
Vak > 0 : I g = 0
ON
A
(avalanche breakdown)
+I ak
OFF
p
s/c
forward
bias
n
G
reverse
bias
forward
bias
p
n
(leakage current)
=
o
-V ak
+V ak
-I ak
R.B.
s/c
(a)
K
REVERSE BIAS
V ak < 0
: I g= 0
A
+Iak
R.B.
p
R.B.
reverse
bias
n
G
p
n
=
=
forward
bias
reverse
bias
+Vak
-V ak
o
OFF
(leakage current)
s/c
-I ak
R.B.
ON
(avalanche breakdown)
(b)
K
FIGURE C4-2
As shown in Fig. C4-2 a diode 'only' approach produces a similar characteristic comprising an OFF state
and an unusable avalanche breakdown region in BOTH forward and reverse bias modes. Although the
reverse bias case is satisfactory the diode analogy alone is insufficient to describe SCR operation due to
the non-useable forward characteristic.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
3
Prof R T Kennedy
C4.4 SCR (Reverse Blocking Triode Thyristor): 2-Transistor Analogy
C4.4.i Forward Bias SCR : VAK > 0 : IG = 0
+
A
+
p
+A
A
A
+
p
V ce(sat)pnp
+I ak
ON
n
n
G
p
n
G
c
c
p
p
n
-K
(a)
V ce(sat)npn
-K
pnp b
n
=
e
G
b
-K
o
V bo
npn
+Vak
OFF
(reverse leakage current)
e
(c)
(b)
-K
(d)
FIGURE C4-3
The inadequacy of the diode analogy for the forward characteristic can be overcome by a transistor
approach as shown in Fig.C4-3 in which the 4-layer structure (a) is modified to two 3-layer interconnections (b) that can be represented by a 2-Transistor pnp / npn combination equivalent circuit (c).
An OFF-STATE when the SCR anode-cathode voltage is positive is due to the reverse biased 'blocking'
junction diode shown in Fig.C4-2, however as the forward bias is increased a voltage (Vb0) is reached at
which transistor action takes over resulting in transfer through a current controlled negative resistance
region (CCNR) to the ON-STATE that may be regarded as a saturated pnp device in series with a
saturated npn device as shown in Fig. C4-3(d)
The central junction being responsible for supporting forward voltage and thereby blocking forward
current flow gives rise to its name. Operation up to the forward breakover voltage is termed the
FORWARD BLOCKING MODE.
C4.4.ii Reverse Bias SCR : VAK < 0 : IG = 0
When the SCR anode-cathode voltage is negative the base emitter junctions of the pnp and npn
transistors are reverse biased and prevent transistor action hence the diode model of Fig. C4-2 (b)
applies.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
4
Prof R T Kennedy
C4.5 SCR Forward Breakover
C4.5.i Forward Current ; zero gate signal
A
+
Ia
e
pnp b
(1-p)Ia -Icbo(p)
 n)Ia+Icbo(n)
c
(  p)Ia + Icbo(p)
G
c
b
Ig = 0 (1- n)Ia -Icbo(n)
npn
e
Ik = Ia
K
FIGURE C4-4
When the SCR is in the forward bias mode the pnp and npn devices of the 2-Transistor analogy are in
the active region with currents as shown in Fig.C4-4. In order to include the effect of both devices in the
discussion some branch currents are shown in terms of both devices however care should be exercised to
use only ONE representation, NOT the sum.
Equating the active region npn collector and the pnp base currents
(1   p ) I a  I cbo, p

Ia 
 n I a  I cbo, n

I cbo, n  I cbo, p
1  ( n   p )
(C4.1)
The SCR terminal current as given by equation C4.1 is a function of the relatively small reverse biased
collector-base junction leakage currents and the SUM of the transistor alphas.
At low applied forward voltage the SCR is in the OFF-STATE or FORWARD BLOCKING MODE as
the alphas of both transistors are small and the terminal current is effectively the sum of the leakage
currents. Increase of forward voltage (or temperature) results in increased leakage current which in turn
forces  n   p  1 resulting in forward breakover to the low impedance ON-STATE, and the need to
limit the SCR current which normally would be performed by the external load.
The 'FORWARD BREAKOVER' terminology is carefully chosen as this is not a destructive
breakdown condition like reverse breakdown provided that the SCR forward current is limited within the
device current rating.
To minimise turn-on by applied voltage, which is the same as requiring a high forward voltage
capability,  n   p must be low.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
5
Prof R T Kennedy
C4.5.ii Alpha Selection
1.0

n+ p

n
p
0.5
current density
FIGURE C4-5
Selection of device alphas at the design stage is crucial to the operation and a typical selection is shown
in Fig.C4-5. At low current density,  n   p  1 increasing towards unity with current density increase.
The information in Fig.C4-6 is not surprising when considered in conjunction with the layer structure of
Fig. C4-1 in which the narrower p-base region of the npn device will have a higher alpha than the wider
n-base region of the pnp transistor. The question does however arise as to why this choice was made.
C4.5.iii Reverse Breakdown Voltage
When the anode-cathode voltage is negative the reverse voltage is supported by the anode and cathode
junctions as shown in Fig. C4-2(a). The doping levels and layer thickness of Fig. C4-1 indicate that the
majority of the reverse voltage is supported by the anode junction.
The reverse breakdown voltage of the anode junction (VRBD) given by equation C4.2 (ref. sec. C4.5.ii).
is determined from the collector-base junction breakdown voltage of the open-base pnp transistor
VRBD  Vcbo( BD)  (1   p
1
m
)
(C4.2)
To maximise the SCR reverse voltage capability the pnp transistor alpha ( p ) must be low as shown in
equation C4.2.
C4.5.iv Forward Breakover Voltage
Forward voltage breakover is determined by the avalanche breakdown of the central blocking junction as
shown in Fig. C4-3(b) and is given by equation C4.3 (ref. sec. C4.5.ii )

VFBO  Vcbo( BD)  1  ( n   p )
m
1
(C4.3)
Equations C4.2 and C4.3 show that the forward breakover voltage is lower than the reverse breakdown
voltage (VFBO  VRBD )
Comparable forward and reverse voltage capability requires  n   p
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
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Prof R T Kennedy
C4.5.v Forward Current ; Positive gate signal
C4.5.v(a) SCR Current
A
+
Ia
e
pnp b
(1- p)Ia -Icbo(p)
c
 p)Ia + Icbo(p)
G
b
Ig
Ia
 n)Ik+Icbo(n)
c
G
A
npn
x (1- n)Ik -Icbo(n)
Ig
Ik
e
Ik
K
(a)
K
(b)
FIGURE C4-6
Applying Kirchoff's current law at point 'x' in Fig. C4-6 (a)
I b,n  I c , p  I g
(1   n ) I k  I cbo,n   p I a  I cbo, p  I g
(C4.4)
The SCR external currents of Fig. C4-6(b) gives
Ik  Ia  I g
(C4.5)
Combining equations C4.4 and C4.5
Ia 
I cbo,n  I cbo, p   n I g
1  ( n   p )
(C4.6)
Injection of a gate current effectively increases the alphas by transistor action thereby controlling the
SCR turn-on to the low impedance on-state at relatively low forward voltage. Whilst there is an increase
in the SCR current due to the presence of IG in the numerator of equation C4.6 the main function of Ig is
the increase in alphas. Once the device is ON the gate current is no longer effective and may be removed
with no effect on the conducting state provided the anode-cathode current exceeds a minimum value
termed the LATCHING CURRENT (IL). Data sheets tend to provide gate pulse width versus gate pulse
amplitude curves.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
7
Prof R T Kennedy
C4.5.iv(b) Regenerative Action
A
F.B.
LOOP
e
=
pnp b
pnp
=
c
c
G
b
npn
npn
e
(a)
(b)
(c)
FIGURE C4-7
The operation of the SCR from the OFF-ON states is a regenerative (positive feedback) effect as shown
by the 'small signal' equivalent circuit of Fig.C4-7(c).
I g  I b,n  I c ,n  I b, p  I c , p  I b,n
FIGURE C4-8
Regenerative action accounts for the SCR remaining in the on-state after the gate signal is removed as
shown in Fig C4-8.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
8
Prof R T Kennedy
C4.6 Forward Bias; Negative Gate Current
C4.6.i SCR Gate Turn-Off
A
+
Ia
e
pnp b
(1- p)Ia -Icbo(p)
b
Ig(rev)
Ia
 n)Ik+Icbo(n)
c
c
(  p)Ia + Icbo(p)
G
A
G
npn
(1-  n)Ik -Icbo(n)
Ig(rev)
Ik
e
Ik
-
K
K
FIGURE C4-9
Bipolar transistor operation is continuously under the control of the base current with turn-off initiated
by the removal of the base current and turn-off speed increased (turn-off time reduced) by the
application of negative (reverse) base current. Removal of SCR gate current has already been shown to
have no influence on the SCR turn-off mechanism. What then of negative gate current as shown in
Fig.C4.9?
To retain the regenerative effect a minimum (sustaining) npn base current is required. Turn-off will
occur when the actual base current falls below the sustaining level as given by equation C4.7.
SCR gate controlled turn-off
actual base current < sustaining base current
 p I a  I cbo, p  I g

(1   n ) I k  I cbo,n
Ik

Ia  I g
 p I a  I cbo, p  I g
Ig 


( n   p  1) I a  I cbo,n  I cbo, p
n
(1   n )( I a  I g )  I cbo,n
(C4.7)
Equation C4.7 shows that a turn -off condition by gate current reversal is possible. In practice turn-on
and turn-off should be achieved by low level gate control currents which requires that  n  high. This
criterion is incompatible with the need for  n to be low to avoid forward voltage breakover hence the
reverse gate current required to turn-off the SCR is too large to be practicable.
Turn-off which cannot therefore be gate controlled requires the SCR current to be either interrupted or
reduced to a level below which the regenerative action cannot be sustained.
C4.6.ii SCR Holding Current (IH )
The minimum anode-cathode current required to maintain the SCR in the on-state. Holding current may
vary from a few tens of mA's to several hundred mA's.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
9
Prof R T Kennedy
C4.7 SCR Turn-ON
Undesirable SCR turn-on is as possible as the desirable controlled turn-on as shown in Fig. C4-12
SCR TURN-ON METHODS
UNDESIRED
DESIRED
(iii) EXCEED V FBO
(iv) INCREASED TEMPERATURE
(i) GATE SIGNAL
(ii) INCIDENT LIGHT
(v) RATE OF RISE OF FORWARD VOLTAGE
FIGURE C4-10
(i)
(ii)
(iii)
(iv)
(v)
Application of a gate signal that induces transistor action is the natural and most common approach.
Incident light and the resulting release of electrons is the basis of the light activated SCR (LASCR)
Forward voltage breakdown (sec. C4.6.iv)
Increased temperature and the resulting increase in leakage current that in turn increases 
Rate of change of voltage applied to a p-n junction capacitance provides a gate current effect.
C4.7.i
Reapplied Forward Voltage Turn-On
A
A
+
p
e
pnp b
n-
c
G
C
c
ak
b
G
p
e
n+
(a)
V
t
K
(b)
K
(c)
FIGURE C4-11
During forward blocking the reverse biased 'blocking junction' depletion region, extending deeper into
the more lightly doped n- region (Fig.C4-11(a)), can be regarded as a non linear capacitor as shown in
Fig.C4-11 (b). A displacement current Idis due to the injection of minority carriers into bases flows via
the anode and cathode junctions to charge C and is given by equation C4.8
I dis

dQdep
I dis

d
(CV )
dt
I dis
 C
dV
dC
V
dt
dt
dt
(C4.8)
The displacement current which is in the same direction as normal gate current increases the emitter
efficiency of the npn transistor and if the rate of reapplied forward voltage is large enough turn-on will
occur when the displacement current and the junction leakage current exceed the latching level
,generally approximated by equation C4.9
(C4.9)
dV
C
 I latching
dt
Turn-on by reapplied forward voltage will create a circuit malfunction but is non destructive provided
that the device forward current is limited.
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
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Prof R T Kennedy
C4.7.ii
Gate Current Turn-On
Instantaneous response to the application of a gate signal does not occur. Carriers injected by the gate
initiate carrier multiplication at the blocking junction however there is a finite time required for the
electrons to diffuse across the npn transistor p-base layer followed by hole diffusion across the pnp
transistor n-base layer. The times involved are the base transit times and are base width, carrier lifetime
and gate drive dependent.
FIGURE C4-14
Anode current
SCR turn-on switching performance is usually specified based on resistive loads as current delay is more
likely to be due to the SCR switching time. Current rise time in inductive circuits will invariably be more
load than SCR dependent. The main points are shown in Fig. C4-14
Forward on
state
Forward breakdown voltage (VFB) vs.
gate current
Reverse
breakdown
voltage
ig4
ig3
ig2
ig1
ig0 = 0
Anode-Cathode voltage
Reverse bias
Forward bias
FIGURE C4-15
In the presence of gate drive the forward characteristic is modified as shown in Fig C4-15
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
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Prof R T Kennedy
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
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Prof R T Kennedy
FIGURE C4-10
EET 307 POWER ELECTRONICS
Silicon Controlled Rectifier (SCR)
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Prof R T Kennedy