Assertion–Reason (SCR ratings): The surge current that a silicon-controlled rectifier (SCR) can withstand is much higher than its RMS on-state current; this is because the surge duration is very short. Evaluate the statements.

Difficulty: Easy

Both A and R are correct and R is correct explanation of A
Both A and R are correct but R is not correct explanation of A
A is correct but R is wrong
A is wrong but R is correct
Both A and R are wrong

Correct Answer: Both A and R are correct and R is correct explanation of A

Explanation:

Introduction / Context:
This item tests understanding of surge (non-repetitive) current capability of an SCR versus its continuous or RMS on-state current rating, and why data sheets allow a much higher one-cycle or few-cycle surge. Surge ratings are fundamental in power electronics for inrush and short-duration fault scenarios.

Given Data / Assumptions:

Device: SCR operating within manufacturer limits.
Comparison: non-repetitive surge current vs RMS on-state current.
Surge is of very short duration (typically one mains cycle or a few milliseconds).

Concept / Approach:
Semiconductor thermal mass and junction-to-case thermal impedance allow a brief energy pulse without exceeding maximum junction temperature. Hence, I_surge may be several times I_T(RMS). Since surge lasts a short time, average heating is limited even though instantaneous current is high.

Step-by-Step Solution:

Statement A: 'Surge current is much higher than RMS on-state current' → True; typical data sheets show 8–10× or more for one-cycle surge.Statement R: 'Duration of surge current is very small' → True; allowed only for short, non-repetitive intervals.Causality: Short duration limits junction temperature rise, explaining why larger current is permissible briefly → R explains A.

Verification / Alternative check:

Thermal equation: ΔT ≈ (∫i^2(t) dt) * R_th(j-x) / k; small time window keeps ΔT within safe bounds.

Why Other Options Are Wrong:

If R did not mention short duration, A would lack justification; here R directly explains A.

Common Pitfalls:

Confusing surge (non-repetitive) with repetitive overcurrent; ignoring thermal time constants and safe operating area.

Final Answer:

Both A and R are correct and R is correct explanation of A

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Assertion–Reason (SCR ratings): The surge current that a silicon-controlled rectifier (SCR) can withstand is much higher than its RMS on-state current; this is because the surge duration is very short. Evaluate the statements.

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