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Silicon-Controlled
Rectifier
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INTRODUCTIONThe thyristor is applied to family of multiplayer
semiconductor devices which exhibit bistable
switching action. Though the integrated
circuits have captured the lead-in
semiconductor sales, yet thyristor and other
solid-state power devices have a veryimportant role.
Basically, by thyristor we mean solid-state
devices with two or more junction. A thyristormay be switched from ON state to OFF state
between two conducting layers or vice versa.
These are capable of handling large currents,
even up to hundreds of amperes.
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Silicon-Controlled Rectifier (SCR)
It is semiconductor device which acts
an electronic switch. A silicon-controlledrectifier can change an alternating
current into direct one and also it can
control the amount of power fed to theload. Thus in a sense it combines the
features of both rectifier and transistor.
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Construction
If a P-N junction is added to a junction transistor ,
then the resulting P-N junction device is termed as
a silicon-controlled rectifier. The construction ofthe SCR is shown in fig. 10.1 (a) while fig. 101.1
(b) shows its symbolic representation. It is a
combination of a rectifier (P-N) and a junction
transistor ( N-P-N) in one unit to from a P-N-P-Ndevice. There are three terminals: one from the
router P-type material is called anode (A), the
second from the router N-type material is called
the cathode (K) and the third from the base oftransistor section is the gate (G). The anode is
kept at high positive with respect to cathode while
the gate is held at small positive potential with
respect to cathode.
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The SCR is a solid state equivalent of thyratron.
The anode, gate and cathode of SCR
correspond to the plate, gird and cathode of
thyratron and that is why SCR is also called as
thyristor.
Fig. 10.1
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SCR as a Switch
The silicon-controlled rectifier has two states
only: (i) ON state and (ii) OFF state. If anappropriate value of the gate current is
passed, the SCR begins to conduct heavily
and remains in the position for an indefinite
period even if the gate voltage is removed.This is the ON state of the SCR. But if the
anode current is reduced to the holding
current, the SCR is turned OFF. Thus itbehaves as a switch, being an electronic
device may be termed as an electronic
switch.
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SCR in Normal Operation
To operate a silicon-controlled rectifier in normal
operation, some points are kept in view as
stated below:
a) Its general, the supply voltage is much less
than the break over voltage.
b) If the gate current is increased above the
required value, the SCR will close at much
reduced supply voltage.c) By passing an appropriate amount of current,
a few amp say, the SCR is turned on and
not by break over voltage.
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d) To turn OFF the SCR from the ON state, the
anode current should be reduced to holding
current.
e) If the SCR is operated by using an a.c supply,
then care should be taken so that the peak
reverse voltage which arises during negativehalf-cycle does not exceed the reverse
breakdown voltage.
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Operation
The load in an SCR is connected in series with
the anode which is always kept at a positivepotential with respect to the cathode. The
operation of the SCR can be explained by
considering the following two cases:
Fig. 10.2
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(i)When the gate is open
The circuit diagram with gate open, i.e., when no
voltage is applied to the gate is shown in Fig. 10.2.
In this case the junction J2is reversed biased butother two junctions J1and J3 are forward biased.
Thus the situation in J1and J3becomes similar to
N-P-N transistor with based open. As a result
there will be a flow of current through the load ofcurrent RLand so the SCR is cut off. If the applied
voltage is increased gradually, a stage is attained
when the reverse biased junction J2breaks down.
As a matter of fact, SCR begins to conduct heavilyand is said to be in the ON state. The voltage
applied at that instant due to which the SCR
conducts heavily without gate voltage is known as
breakdown voltage.
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Fig. 10.3
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(ii) When the gate is positive
As shown in fig.10.3. The SCR can be made to
conduct heavily by applying a small positive
voltage to the gate. In this case, the junction J3 isforward biased but junction J2is reverse biased.
From N-type material electrons move across
junction J3are attracted across junction J2 and the
gate current starts to flow. With the flow of thegate current the anode current increase which in
turn makes more electrons available at the
junction J2.This process continues and within a
very small time junction J2breaks down and theSCR starts to conduct heavily. Once the SCR
begins to conduct, the gate loses all control. The
conduction stops only when the applied voltage is
reduced to zero.
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Considering the working of the SCR one may come to
the following important conclusions:
i. An SCR either conducts heavily or its does not
conduct. It has thus two states and there is no statein-between. Hence it behaves like a switch.
ii. There are two different ways to turn on the SCR. In
the first method, the gate is kept open and the supply
voltage is made equal to the break over voltage. Inthe second method, the supply voltage is applied less
than the break over voltage and then it is turned on
by a small voltage applied to the gate.
iii. It is the general way, to apply a small positive voltageto the gate for closing an SCR. This is because the
break over voltage is usually greater than the supply
voltage.
iv. To make an SCR non-conducting, the supply voltage
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Equivalent Circuit of SCR
The SCR in Fig.10.4 (a) shows its structure. Itis a four-layer semiconductor device which can
be separated into two transistors as shown in
fig.10.4 (b). Thus the equivalent circuit of the
SCR can be treated as constituted of twotransistors, P-N-P transistor and an N-P-N
transistor connected as shown in fig. 10.5. It is
seen from the figure that the collector of each
transistor is coupled to the base of the other
and thus making a positive feedback loop.
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Fig.10.4
Fig.10.5
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The equivalent electrical circuit of an SCR is
shown fig. 10.6. Let at the first the gate
terminal G is not connected to any external
circuit and the gate current Ig is zero. Also asupply voltage Vss is connected with series
resistor Rl between the anode A and cathode
K. With this biasing arrangement the junctionsJ1 and J3are forward biased while the junction
J2, gets reverse biased. In this way, the three
junction J1 , J2 and J3 are properly biased for
the operation of the transistors T1 and T2. Thecollector current of the transistors T1 and T2 are
then respectively given by,
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Ic1= 1I + ICO1 (10.1)
and Ic1= 2I + ICO2 ...(10.2)
Where 1 and 2 are for transistors T1 and T2respectively. Hence the total current Ientering
the anode terminal given by,
I = Ic1+Ic2
= (1 + 2) I+ ICO1 + ICO2
or, I[1( 1 + 2)] = ICO1 + ICO2ICO1+ICO2
I= -------------
1-(1+ 2)
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SCR is so designed that its (1 + 2) becomes
slightly less than unity. In order to satisfy the
condition that (1+ 2)
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The current-voltage characteristics of an SCR
is shown in fig. 10.7 for different values of
gate current Ig. Lets us consider the curve for
Ig= 0. In the low current region OA. (1+ 2)
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Once SCR gets fired and comes to ON condition,
can be stopped only by reducing the anode
voltage to reduce the anode current below IH.When the current comes down below IH, theconduction ceases and the operating points
shifts from C to O.
To explain the effect of the gate current on theworking to the SCR, we consider that the supply
voltage Vss is less than that needed to fire the
SCR. When Ig is applied a2 increase so that (
1+ 2) becomes unity and the SCR fres evenwith low supply voltage. It is also clear from the
figure that if the value of Igis greater, the applied
voltage Vbecomes lower.
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Fig.10.7
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SCR HALF-wave Rectifier
Fig. 10.8 (a) reveals the circuit diagram of an
SCR half-wave rectifier. In the figure, T is atransformer, RL the load resistance connected
in series with the anode while Rh is variable
resistance inserted in the gate circuit for
controlling the gate current.
Operation: The a.c. supply is applied to the
primary of the transformer. Let the peak
inverse voltage (PIV) across the secondary isless than the reverse breakdown voltage of the
SCR which ensures that the SCR will not break
down during the negative half-cycle of the a.c.
supply.
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Fig.10.8
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If suitable gate current is allowed to flow, the
SCR will conduct during the positive half-
cycle. The greater the gate current, the
smaller is the supply voltage at which the
SCR is turned on. The gate current can be
changed by means of the variable resistance
Rh.us now assume that the gate current is
adjusted to such a value that SCR closes at
positive voltage V1 which is smaller than thepeak value of the voltage Vm. from fig. 10.8
(b) it is seen that the SCR will conduct when
secondary a.c. voltage becomes zero when it
is turned OFF.
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It is also clear from the figure that the firing
angle is a and the conducting angle is
[=180- ].
Let = Vm sin be the alternating voltage
appears across the secondary of the
transformer and a be the firing angle.During the positive half-cycle the rectifier
will conduct from to 180.
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