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Two transistor model of a thyristor(SCR)

Two transistor model of a thyristor(SCR)

 

Question: Explain two transistor model of a thyristor(SCR)?
or
Explain the construction and working principles of a thyristor?
or
Explain the positive regenerative feedback effect of a thyristor?

 

Solution:

The regenerative or latching action due to positive feedback can be demonstrated by using a two transistor model of a thyristor.

A thyristor can be considered as two complementary transistors, one PNP transistor Q1 and other NPN transistor Q2 as shown in figure 1(a).

The equivalent circuit model is shown in figure 1(b)

Two transistor model of a thyristorfigure1(a)                          figure 1(b)

 

In General, The collector current IC of a thyristor is related to the emitter current IE and the leakage current of the collector-base junction, ICBO as

\begin{array}{l}I_C=\alpha I_E+I_{CBO}............(i)\\\\and\;the\;common\;base\;current\;gain\;\alpha\simeq\frac{I_C}{I_E}\\\\For\;transistor\;Q_1\;equaton\;(i)\;becomes\\\\I_{C1}=\alpha_1I_A+I_{CBO1}............(ii)\;\;\;\;\;\lbrack I_E=I_A\rbrack\\\\For\;transistor\;Q_2\\\\I_{C2}=\alpha_2I_k+I_{CBO2}............(iii)\;\;\;\;\;\lbrack I_E=I_k\rbrack\\\\Combining\;(ii)\;and\;(iii)\;we\;get\\\\\;\;\;\;\;\;\;\;\;I_{A\;}=I_{C1}+I_{C2}\\\\\;\;\;\;\;\;\;\;\;I_{A\;}=\alpha_1I_A+I_{CBO1}+\alpha_2I_k+I_{CBO2}............(iv)\\\\For\;gatting\;current\;of\;I_G\\\\\;I_{K\;}=I_A+I_G........(v)\\\\putting\;this\;value\;of\;I_{k\;}\;in\;equation\;(iv)\;we\;get\\\\\;I_{A\;}=\alpha_1I_A+I_{CBO1}+\alpha_2(I_A+I_G)+I_{CBO2}\\\\\;I_{A\;}-\alpha_1I_A-\alpha_2I_A=I_{CBO1}+\alpha_2I_G+I_{CBO2}\\\\\;I_{A\;}=\frac{I_{CBO1}+\alpha_2I_G+I_{CBO2}}{1-(\alpha_1+\alpha_2)}..............(vi)\\\\\\;\;\;\;\;\;\;\;\\\end{array}

The current gain α1 varies with the emitter current IA=IE ; and α2 varies with Ik=IA+IG

From equation (vi) If the gate current IG is suddenly increased, the anode current IA also increases, which would further increase α1 and α2. Current gain α2 depends on IA and IG

Again the increase in the values of α1 and α2 further increases IA. Therefore, there is a regenerative or positive feedback effect.

If (α12) tends to be unity, the denominator of equation(vi) approaches zero, resulting in a large value of anode current IA, and the thyristor turns on with a small gate current.

সমীকরণ (vi)হতে দেখতে পাই যে, যদি গেট কারেন্ট (IG) হঠাৎ বৃদ্ধি পায়, তাহলে অ্যানোড কারেন্ট (IA)ও বৃদ্ধি পায়, যা α1 এবং α2 কে আরও বাড়াবে। কারেন্ট গেইন α2 ,IA এবং IG এর উপর নির্ভর করে.

আবার α1 এবং α2 এর মান বৃদ্ধির ফলে(IA)আবারো বৃদ্ধি পাবে। অতএব, এখানে একটি রিজেনেরেটিভ বা পজিটিভ প্রতিক্রিয়া প্রভাব আছে।

যদি (α12)এর মান উনিটির কাছাকাছি যায়,তাহলে সমীকরণ (vi) এর হর শূন্যের কাছাকাছি চলে যাবে, ফলে এনোড কারেন্ট IA এর মান অনেক বেশি হবে এবং সামান্য পরিমান গেট কারেন্ট দিয়েই থাইরিস্টর চালু হবে।

 

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