GATE EE 2023 Set 1 — Question 55
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Power Electronics → Power Semiconductor Devices → Thyristor Commutation
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Question
The circuit shown in the figure has reached steady state with thyristor ‘T’ in OFF condition. Assume that the latching and holding currents of the thyristor are zero. The thyristor is turned ON at sec. The duration in microseconds for which the thyristor would conduct, before it turns off, is _____ (Round off to 2 decimal places).
Correct answer
7.1 to 7.5
Solution
Circuit and initial conditionThe 100 V source feeds node , the anode of the thyristor T. The series combination of F and H connects node to node , the cathode of T, and the resistor joins node to the negative terminal of the source.With T OFF no current flows anywhere, so there is no drop across the and none across :After turn-on at T shorts to , so the resistor now sees the whole source voltage and carries the constant currentAt the same time , and T form a loop containing no resistance, so the charged capacitor rings with the inductor atThis resonant current leaves the positive plate of at node and returns through the thyristor, i.e. it flows through T in the same direction as the resistor current, soTurn-off instantSince the holding current is zero, T turns off at the first instant at which reaches zero:Answer:
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