silicon-controlled rectifiers (scrs)

8/13/2019 Silicon-Controlled Rectifiers (SCRs)

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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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silicon-controlled rectifiers (scrs)

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