SCR- SILICON CONTROLLED RECTIFIER

Definition: When a pn junction is added to a junction transistor, the resulting three

pn junction device is called a silicon controlled rectifier. SCR can change alternating current into direct current and at the same time can

control the amount of power fed to the load. Thus SCR combines the features of a rectifier and a transistor.

1. Invented in 1957, an SCR can be used as a controlled switch to perform various functions

2. Such as rectification, inversion and regulation of power flow. 3. Handle currents upto several thousand amperes and voltages upto more than 1 kV.

4. a unidirectional power switch and is being extensively used in switching d.c. and a.c.,

rectifying a.c. to give controlled d.c. output, converting d.c. into a.c.

5. A silicon *controlled rectifier is a semiconductor **device that acts as a true electronic switch.

Symbol and equivalent circuit Construction

1. It is essentially an ordinary rectifier (pn) and a junction transistor (npn) combined in

one unit to form pnpn device. Three terminals are taken; one from the outer p-type

material called anode A,

2. Second from the outer n-type material called cathode K and the third from the base of transistor section and is called gate G.

3. In the normal operating conditions of SCR, anode is held at high positive potential w.r.t.

cathode and gate at small positive potential w.r.t. cathode.

4. The gate, anode and cathode of SCR correspond to the grid, plate and cathode of thyratron.

5. For this reason, SCR is sometimes called thyristor

Equivalent Circuit of SCR

Working

1. Load is connected in series with anode.

2. The anode is always kept at positive potential w.r.t. cathode.

3. The working of SCR can be studied under the following two heads When gate is open

1. No voltage applied to the gate.

2. Junction J2 is reverse biased while junctions J1 and J3 are forward biased.

3. Junctions J1 and J3 is just as in a npn transistor with base open

4. No current flows through the load RL and the SCR is cut off. 5. If the applied voltage is gradually increased, a stage is reached when reverse

biased junction J2 breaks down.

6. The SCR now conducts heavily and is said to be in the ON state.

7. The applied voltage at which SCR conducts heavily without gate voltage is called Breakover voltage.

When gate is positive w.r.t. cathode

1. The SCR can be made to conduct heavily at smaller applied voltage by applying a small positive potential to the gate .

2. Now junction J3 is forward biased and junction J2 is reverse biased.

3. The electrons from n-type material start mov-ing across junction J3 towards left whereas holes from p-type towards the right.

4. The electrons from junction J3 are attracted across junction J2 and gate current starts flowing. As soon as the gate current flows, anode current increases.

5. The increased anode current in turn makes more electrons available at junction J2.

6. This process continues and in an extremely small time, junction J2 breaks down and the SCR starts conducting heavily.

7. Once SCR starts conducting, the gate (the reason for this name is obvious) loses all

control. Even if gate voltage is removed, the anode current does not decrease at all. 8. The only way to stop conduction (i.e. bring SCR in off condition) is to reduce the applied

voltage to zero. Conclusion notes:

a. An SCR has two states i.e. either it does not conduct or it conducts heavily. There

is no state in between. Therefore, SCR behaves like a switch.

b. There are two ways to turn on the SCR. The first method is to keep the gate open

and make the supply voltage equal to the breakover voltage. The second method

is to operate SCR with supply voltage less than breakover voltage and then turn it

on by means of a small voltage ( typically 1.5 V, 30 mA) applied to the gate.

c. Applying small positive voltage to the gate is the normal way to close an SCR because the breakover voltage is usually much greater than supply voltage.

d. To open the SCR (i.e. to make it non-conducting ), reduce the supply voltage to

zero. Important Terms Breakover voltage

It is the minimum forward voltage, gate being open, at which SCR starts conducting heavily i.e. turned on

Peak reverse voltage (PRV) It is the maximum reverse voltage (cathode positive w.r.t. anode) that can be applied to

an SCR without conducting in the reverse direction. Holding current

It is the maximum anode current, gate being open, at which SCR is turned off from ON conditions. Forward current rating

It is the maximum anode current that an SCR is capable of passing without destruction.

Circuit fusing (I2t) rating

It is the product of square of forward surge current and the time of duration of the surge

i.e.,Circuit fusing rating = I2t

V-I Characteristics of SCR

It is the curve between anode-cathode voltage (V) and anode current (I) Of an SCR at constant gate voltage. i)Forward characteristics

1. When anode is positive w.r.t. cathode, the curve between V and I is called the forward characteristic.

2. In Fig. OABC is the forward characteristic of SCR at IG = 0. 3. If the supply voltage is increased from zero, a point is reached (point A) when the SCR

starts conduct-ing. 4. the voltage across SCR suddenly drops as shown by dotted curve AB and most of supply

voltage appears across the load resistance RL. 5. If proper gate current is made to flow, SCR can close at much smaller supply voltage.

(ii) Reverse characteristics.

1. When anode is negative w.r.t. cathode, the curve between V and I is known as reverse characteristic.

2. The reverse voltage does come across SCR when it is operated with a.c. supply.

3. If the reverse voltage is gradually increased, at first the anode current remains small (i.e. leakage current) and at some reverse voltage, avalanche breakdown occurs and the SCR starts con-ducting heavily in the reverse direction as shown by the curve DE.

4. This maximum reverse voltage at which SCR starts conducting heavily is known as

reverse breakdown voltage. Applications of SCR

1. static contactor 2. Power control. 3. Speed control of d.c. shunt motor. 4. Overlight detector 5.

SCR as a Switch 1. SCR turn-on methods.

the gate voltage VG is increased upto a minimum value to initiate triggering. This minimum value of gate voltage at which SCR is turned ON is called gate triggering voltage VGT.

The resulting gate current is called gate triggering current IGT. Thus to turn on an SCR all that we have to do is to apply positive gate voltage equal to

VGT or pass a gate current equal to IGT.

(i) D.C. gate trigger circuit.

1. When the switch is closed, the gate receives sufficient positive voltage (=

VGT) to turn the SCR on. 2. The resistance R1 connected in the circuit provides noise suppression

and improves the turn-on time. 3. The higher the gate-triggered current, the shorter the turn-on time

A.C. trigger circuit. 1. An SCR can also be turned on with positive cycle of a.c. gate current.

2. Fig. (ii) shows During the positive half-cycle of the gate current, at some point IG IGT, the device is turned on.

SCR turn-off methods.

The SCR turn-off poses more problems than SCR turn-on. It is because once the device is ON, the gate loses all control. There are many methods

of SCR turn-off.

i) Anode current interruption. When the anode current is reduced below a minimum value called holding current,

the SCR turns off. The simple way to turn off the SCR is to open the line switch S (ii) Forced commutation

The method of discharging a capacitor in parallel with an SCR to turn off the SCR is called forced commutation. SCR Half-Wave Rectifier

1. One important application of an SCR is the controlled half-wave rectification. 2. Fig (i) shows the circuit of an SCR half-wave rectifier. 3. The a.c. supply to be rectified is supplied through the transformer.

4. The load resistance RL is connected in series with the anode. 5. A variable resistance r is inserted in the gate circuit to control the gate current.

1. The a.c. supply to be converted into d.c. supply is applied to the primary of the

transformer. Suppose the peak reverse voltage appearing across secondary is less than

the reverse breakdown voltage of the SCR. 2. This condition ensures that SCR will not break down during negative half-cycles of a.c.

supply. 3. During the negative half-cycles of a.c. voltage appearing across secondary, 4. The SCR does not conduct regardless of the gate voltage. 5. It is because in this condition, anode is negative w.r.t. cathode and also PRV is less than

the reverse breakdown voltage. 6. The SCR will conduct during the positive half-cycles provided proper gate current is

made to flow. 7. The greater the gate current, the lesser the supply voltage at which SCR is turned ON. 8. The gate current can be changed by the variable resistance r. 9. Suppose that gate current is adjusted to such a value that SCR closes at a positive voltage

V1 which is less than the peak voltage Vm.

10. SCR will start conducting when secondary a.c. voltage becomes V1 in the positive half-

cycle. 11. Beyond this, the SCR will continue to conduct till voltage becomes zero at which point it

is turned OFF. 12. Again at the start of the next positive half-cycle, SCR will start conducting when

secondary voltage becomes V1. 13. Firing angle is α i.e. at this angle in the positive half-cycle, SCR starts conduction. The

conduction angle is φ (= 180° − α ). 14. It is worthwhile to distinguish between an ordinary half-wave rectifier and SCR half-

wave rectifier.

15. Whereas an ordinary half-wave rectifier will conduct full positive half-cycle, an SCR

half-wave rectifier can be made to conduct full or part of a positive half-cycle by proper

adjustment of gate current.

16. Therefore, an SCR can control power fed to the load and hence the name controlled rectifier.

Unijunction Transistor (UJT)

It is a three-terminal semiconductor switching device.

This device has a unique characteristic that when it is triggered, the emitter current increases regeneratively until it is limited by emitter power supply

Construction

1. consists of an n-type silicon bar with an electrical connection on each end. The leads to these connections are called base leads base-one B1 and base two B2

Structure and symbol 1.The emitter is heavily doped having many holes.

2.The n region, is lightly doped. For this reason, the resistance between the base terminals is very high ( 5 to 10 kΩ) when emitter lead is open.

Equivalent Circuit of a UJT

1. RB2 is the resistance of silicon bar between B2 and the point at which the emitter

junction lies.

2. RB1 is the resistance of the bar between B1 and emitter junction. This resistance is

shown variable because its value depends upon the bias voltage across the pn

junction.

(i) With no voltage applied to the UJT, the inter-base resistance is given by R

BB =

RB1

+ RB2

The value of RBB generally lies between 4 kΩ and 10 kΩ. (ii) If a voltage VBB is applied between the bases with emitter open, the voltage will divide up

across RB1 and RB2.

The ratio V1/VBB is called intrinsic stand-off ratio and is represented by Greek letter η.

The value of η usually lies between 0.51 and 0.82.

Voltage across RB1 = η VBB

The voltage η VBB appearing across RB1 reverse biases the diode. Therefore, the emitter current is zero. (iii) positive voltage is applied to the emitter, the diode will become forward biased when input voltage exceeds η VBB by VD, the forward voltage drop across the silicon diode i.e.

VP = η VBB + VD

Where VP = ‘peak point voltage’ forward voltage drop across silicon diode

VD = (j 0.7 V)

.

Operation

1. When the diode D starts conducting, holes are injected from p-type material to the n-

type bar. These holes are swept down towards the terminal B1.

2. This decreases the resistance between emitter and B1 (indicated by variable resistance

symbol for R B1) and hence the internal drop from emitter to B1.

3. The emitter current now increases regeneratively until it is limited by the emitter

power supply The device has normally B2 positive w.r.t. B1.

4. If voltage VBB is applied between B2 and B 1 with emitter open a voltage gradient is

established along the n-type bar.

5. Since the emitter is located nearer to B2, more than **half of VBB appears between

the emitter and B1.

6. The voltage V1 between emitter and B1 establishes a reverse bias on the pn junction

and the emitter current is cut off.

7. A small leakage current flows from B2 to emitter due to minority carriers.

8. If a positive voltage is applied at the emitter the pn junction will remain reverse

biased so long as the input voltage is less than V1.

9. If the input voltage to the emitter exceeds V1, the pn junction becomes *forward

biased.

10. Under these conditions, holes are injected from p-type material into the n-type bar.

These holes are repelled by positive B2 terminal and they are attracted towards B1

terminal of the bar.

11. This accumulation of holes in the emitter to B1 region results in the decrease of

resistance in this section of the bar.

12. The result is that internal voltage drop from emitter to B1 is decreased and hence the

emitter current I E increases.

13. As more holes are injected, a condition of saturation will eventually be reached.

14. At this point, the emitter current is limited by emitter power supply only. The device

is now in the ON state.

15. If a negative pulse is applied to the emitter, the pn junction is reverse biased and the emitter current is cut off. The device is then said to be in the OFF state

Characteristics of UJT

1. Initially, in the cut-off region, as VE increases from zero, slight leakage current flows from terminal B2 to the emitter.

This current is due to the minority carriers in the reverse biased diode. 2. Above a certain value of VE, forward IE begins to flow, increasing until the peak

voltage VP and current IP are reached at point P

3. After the peak point P, an attempt to increase in VE suddenly increase in emitter current IE

with a corresponding decrease in VE.

4. a negative resistance portion with increase inIE, VE decreases has a reliable quality.

5. The negative portion of the curve lasts until the valley point V is reached with

valley-point volt-age VV and valley-point current IV.

6. After the valley point, the device is driven to saturation.

21.14 Advantages of UJT (i) It is a low cost device. (ii) It has excellent characteristics. (iii) It is a low-power absorbing device under normal operating conditions

Due to above reasons, this device is being used in a variety of applications. A few

include oscillators, trigger circuits, saw-tooth generators, bistable network. Applications of UJT

i) UJT relaxation oscillator

(ii) Overvoltage detector

UJT relaxation oscillator- UJT relaxation oscillator where the discharging of a capacitor through UJT can develop a saw-tooth output.

1. When battery VBB is turned on, the capacitor C charges through resistor R1.

2. During the charging period, the voltage across the capacitor rises in an exponential manner until it reaches the peak - point voltage.

3. At this instant of time, the UJT switches to its low resistance conducting mode and the

capacitor is discharged between E and B1.

4. As the capacitor voltage flys back to zero, the emitter ceases to conduct and the UJT is switched off.

5. The next cycle then begins, allowing the capacitor C to charge again.

6. The frequency of the output saw-tooth wave can be varied by changing the value of R1

since this controls the time constant R1C of the capacitor charging circuit.

7. The time period and hence the frequency of the saw-tooth wave can be calculated as

follows. Assuming that the capacitor is initially uncharged, the voltage VC across the

capacitor prior to break-down is given by :