dv/dt triggering of a thyristor: assertion–reason Assertion (A): A high dv/dt across a thyristor can cause unintended triggering. Reason (R): A high dv/dt drives a displacement (charging) current through the device junctions and capacitances.
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ABoth A and R are correct and R is the correct explanation of A
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BBoth A and R are correct but R is not the 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 the correct explanation of A
Explanation
Introduction / Context:Thyristors contain junction capacitances. Rapid changes of voltage (high dv/dt) can force displacement currents that mimic gate-trigger current, inadvertently turning the device on. Protection against dv/dt is a fundamental aspect of gate and snubber design.
Given Data / Assumptions:
- SCR in forward-blocking state.
- Non-zero junction capacitances are present.
- No intentional gate drive applied.
Concept / Approach:
Displacement current through a capacitance is i = C * dv/dt. If this current flows through the gate-cathode or internal junction path, it can exceed the latching threshold and cause turn-on even without a gate pulse.
Step-by-Step Solution:
High dv/dt → high iC = C * dv/dt through internal capacitances.This current can be equivalent to gate current → unintended triggering.Therefore, both the assertion and the reason are correct and causally linked.Verification / Alternative check:
Datasheets specify maximum dv/dt ratings and recommend RC snubbers to limit dv/dt and avoid false turn-on.
Why Other Options Are Wrong:
It is not unrelated; the reason precisely explains the mechanism. Claims that the assertion is false contradict well-known thyristor behavior.
Common Pitfalls:
Neglecting layout-induced stray capacitances; assuming that only gate drive controls turn-on without recognizing dv/dt effects.
Final Answer:
Both A and R are correct and R is the correct explanation of A