Introduction / Context:
Choppers (DC–DC converters) using inductive loads rely on a freewheeling path to maintain current continuity when the main switch turns off. This question evaluates understanding of inductor current behavior and the role of the freewheeling diode in protecting components and smoothing current.
Given Data / Assumptions:
- Load contains an inductor (L) and possibly a back EMF (E) such as a motor.
- Main switch is a thyristor or transistor that alternately applies and removes source voltage.
- Diode is placed to provide a path for inductor current during off intervals.
Concept / Approach:
Inductor current resists abrupt changes. When the main device turns off, the inductor maintains current; without an alternate path, voltage spikes would occur. The freewheeling diode becomes forward-biased and conducts, letting the current circulate through the load and diode, preventing overvoltage and smoothing current decay.
Step-by-Step Solution:
Main device ON: Source supplies the load; diode is reverse-biased.Main device OFF: Inductor attempts to keep current flowing; diode forward-biases, providing a closed loop.Therefore, the freewheeling diode conducts during the OFF interval of the thyristor.
Verification / Alternative check:
Observe current waveforms in chopper textbooks: current ripple continues through the diode path in OFF time, confirming conduction then.
Why Other Options Are Wrong:
When the thyristor is on: Diode is typically reverse-biased by source voltage.Partly on and off: Standard operation is OFF-time conduction.Never conducts / Only at start-up: Incorrect; conduction happens every cycle during OFF-time.
Common Pitfalls:
Forgetting that inductors demand a path for current continuity; misidentifying diode polarity.
Final Answer:
When the thyristor is off
Discussion & Comments