EE-321 N

Lecture-3Thyristor Characteristics Contd

&Thyristor Turn ON Methods

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Supplying Gate Current

• The forward break over voltage can bereduced if a current is supplied through thegate terminal

• Higher the magnitude, lower will be theforward break over voltage

• This is illustrated next

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Effect of Gate current on forward BO voltage

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> >

Reverse Blocking Mode

• Anode –ve w. r. t. cathode. Junctions J1 & J3are reverse biased while J2 is forward biased

• Only a small leakage current flows (~µA)• Exactly similar to RB mode of a pn junction

diode• Device is said to be in OFF state• Avalanche BD of J1 & J3 occurs at a voltage

known as reverse breakdown voltage VBR

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SCR switching SummaryTurn ON Conditions1. VAK > 02. IDevice > IL

3. IG > 0Turn OFF Conditions1. IDevice < IH

2. Reverse bias onlyadds to reliability

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Device will continue to conduct even if IG is just one pulse

Thyristor turns OFF and remains OFF until turning ON conditions are satisfied

Two Transistor Model & Turn ON Mechanism of SCR

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• The collector current IC of a thyristor is related tothe emitter current IE and the leakage current ofthe collector-base junction, ICBO.

• The common-base current gain is defined as:

CBOEC III +=α

E

C

II

=α

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Therefore,

and

(Using KCL at T1)

I I IC A CBO1 1 1= +α

I I IC K CBO2 2 2= +α

)1(221121 CBOKCBOACCA IIIIIII +++=+= αα

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But,

(Using KCL at T2)

Substituting (2) into (1) and solving for IA:

)2(GAK III +=

)(1 21

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ααα

+−++

= CBOCBOGA

IIII

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•On application of small magnitude of IG, IKincreases. Since IK = IE2, α2 also increases which inturn increases IC2.

•Thus IB1 and hence IE1 increases leading toincrease in α1. Ultimately, α1 + α2 → 1 and IA →∞

•Each transistor drives another one into saturationand device reaches conduction state.

• IA is practically limited by external circuitimpedance and the voltage drop is approx. that ofa single pn junction

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