GATE EE 2024 Set 1 — Question 35
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Power Electronics → DC-DC Converters → Buck Converter
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Question
A forced commutated thyristorized step-down chopper is shown in the figure. Neglect the ON-state drop across the power devices. Assume that the capacitor is initially charged to 50 V with the polarity shown in the figure. The load current () can be assumed to be constant at 10 A. Initially, is ON and is OFF. The turn-off time available to in microseconds, when is triggered, is_________ (rounded off to the nearest integer).
Correct answer
50 to 50
Solution
When is triggered: connects the positive supply rail (the anode of ) to the lower () plate of the capacitor. The upper () plate sits on the cathode of . So the capacitor voltage is placed directly across with the polarity reversed:'s current transfers to the capacitor path and is reverse biased.Capacitor discharge: The constant load current A now flows from the supply through and the capacitor into the load. Because the current is constant, falls linearly: stays reverse biased only while . So the circuit turn-off time is the time for to fall from V to :Turn-off time available to :
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