Auxiliary commutation classification Which thyristor commutation class specifically employs an auxiliary SCR to assist in turning off the main SCR?

Introduction / Context:
In forced-commutated converters, SCRs must be turned off by creating a reverse-bias or a current-zero using external networks. Classification by commutation method helps select suitable topologies for choppers, inverters, and DC drives.

Given Data / Assumptions:

• Commutation classes A, B, C, D (classical nomenclature).
• Auxiliary SCR availability for commutation tasks.

Concept / Approach:

Class D, called auxiliary commutation, uses an auxiliary thyristor and a commutating capacitor/inductor to divert current and apply reverse voltage across the main SCR, forcing it to turn off. Class C (complementary) typically uses a second main SCR (in another leg) to commutate the first without a separate auxiliary device. Classes A and B rely on load or resonant pulse phenomena rather than an auxiliary device.

Step-by-Step Solution:

Verification / Alternative check:

Classic commutation schematics label the helper device as “aux SCR” in Class D circuits, timing its firing to discharge a precharged capacitor across the main SCR.

Why Other Options Are Wrong:

Class A/B do not need an auxiliary SCR; Class C uses complementary main devices; Class E is not a standard SCR commutation class in the classic scheme.

Common Pitfalls:

Confusing complementary (C) with auxiliary (D); both use additional paths but only D explicitly uses a distinct auxiliary SCR for commutation.

Final Answer:

Class D (auxiliary commutation)