Assertion–Reason: Inverters and choppers employ fast-switching thyristors because these devices have low turn-off time (tq). Evaluate the statements.
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ABoth A and R are correct and R is correct explanation of A
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BBoth A and R correct but R is not correct explanation of A
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CA is correct but R is wrong
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DA is wrong but R is correct
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EBoth A and R are wrong
Answer
Correct Answer: Both A and R are correct and R is correct explanation of A
Explanation
Introduction / Context:High-frequency switching converters (inverters and choppers) demand devices that can turn on and, crucially, turn off rapidly to minimize switching losses and allow higher operating frequencies. Fast-switching thyristors are engineered for low reverse recovery and short turn-off times.
Given Data / Assumptions:
- Assertion (A): Inverters and choppers use fast-switching SCRs/thyristors.
- Reason (R): Fast-switching SCRs have low turn-off time tq.
- Focus is on commutation speed and loss reduction.
Concept / Approach:
Turn-off time tq determines the minimum interval after current extinction before a thyristor can block forward voltage again. Lower tq enables higher switching frequency and reduces overlap/recovery losses—key requirements in PWM inverters and dc choppers.
Step-by-Step Evaluation:
Check A: True—fast devices are chosen to meet switching frequency targets.Check R: True—low tq is the defining characteristic of fast thyristors.Causality: The reason directly explains why such thyristors are preferred in high-frequency converters.Verification / Alternative check:
Datasheets specify tq and trr (recovery) for inverter-grade SCRs; application notes tie allowable PWM frequency to tq margins.
Why Other Options Are Wrong:
- Stating R does not explain A contradicts the direct link between low tq and converter suitability.
- Any claim that A or R is wrong conflicts with standard converter design practice.
Common Pitfalls:
- Confusing di/dt ratings with tq; both matter, but tq governs maximum switching frequency.
- Overlooking that modern designs often prefer IGBTs/MOSFETs for even faster switching.
Final Answer:
Both A and R are correct and R is correct explanation of A