Silicon Controlled Rectifiers

8/12/2019 Silicon Controlled Rectifiers

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A Three Terminal Semiconductor Device

Invented in 1957 Used as a controlled switch to perform

rectification, inversion and regulation of powerflow.

Can handle currents up to 1000 A and voltagesmore than 1 KV

Converts AC into DC & also controls theamount of power fed to the load.

Hence, combines the features of a rectifier anda transistor


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Leakage current in Si is very small when

compared to Ge

Device is used as switch minimum leakagecurrent.


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PN Junction + junction transistor = SCR pnpndevice

3 terminal device

Anode from outer p region(A)Cathodefrom outer n region(K)Gate base of transistor section

Also called THYRISTOR as it is the solid

state equivalent of a thyratron.


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In SCR, the load is connected in series withthe anode

Anode is always kept at a positive potentialw.r.t cathode

The working can be understood under 2conditions:

1. When Gate is open

2. When gate is positive w.r.t cathode


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No voltage is applied to the gate . J2 is FB andJ1 & J3 are RB.

No current flow through SCR because J2 is RBSCR is cut off


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When the supply voltage V exceeds a certainthreshold, the junction J2 breaks down.

The SCR now conducts heavily and is in theO state.

Breakover Voltage the voltage at which theSCR conducts heavily without gate voltage.


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The device can be made to conduct heavily byapplying a small positive potential at the gate


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J3 is FB and J2 is RB

Electrons from the n type material move across J3

towards left Holes from p type move towards right

Electrons from J3 are attracted across J2 gatecurrent

As J2 is now conducting, anode current flowsthrough SCR

J2 breaks down SCR is ON anode currentdoesnt decrease even when gate voltage is

removed To make SCR OFF reduce the applied voltage to

zero.


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1. SCR has two states - either it does not conduct orit conducts heavily. There is no intermediatestate. Hence SCR behaves like a switch

2. There are 2 ways to turn ON the SCR

(i) with Gate open and increasing the supplyvoltage,

(ii) with a positive Gate trigger. (gate triggershould be >10mA)

3. The SCR is turned OFF only by reducing thesupply voltage to zero, thereby reducing theanode current


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The curve between V & I when anode is +vew.r.t cathode

When supply voltage is increased from zero,suddenly the SCR starts conducting => breakover voltage

Voltage drops at this point suddenly asshown by the dotted line.

If proper gate current is made to flow, thenSCR can close at smaller supply voltage.


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The curve between V & I when anode is -ve w.r.t

cathode

If reverse voltage is increased gradually, at first,

the anode current remains small (leakage current)

Beyond a particular reverse voltage, the SCR starts

massive conduction (avalanche) => Reverse

breakdown voltage.


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1. Supply voltage < Break over voltage2. SCR is turned on by passing an appropriate

gate current (few mA) and not by break overvoltage

3. Peak Reverse Voltage should not exceed thereverse breakdown voltage

4. When SCR needs to be turned OFF , the

anode current should be reduced to theholding current


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Switch

Controlled Rectification

Power control of load

Speed Control of dc shunt motor

Over light detector


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Documents

Silicon Controlled Rectifiers