Series/parallel connection of thyristors (SCRs): When devices are connected in series and/or parallel for higher ratings, what derating is generally necessary to ensure safe operation?

Introduction / Context:
To achieve higher system ratings, SCRs are often connected in series (for voltage) and in parallel (for current). Due to inevitable parameter tolerances and dynamic effects, equal sharing is imperfect, so prudent derating is applied in design.

Given Data / Assumptions:

• Series strings require voltage sharing components (static and dynamic).
• Parallel groups require current sharing measures and thermal management.
• Device tolerances and transient behavior cannot be eliminated completely.

Concept / Approach:

In series connection, one device may see more voltage during transients → voltage derating. In parallel connection, one device may carry more current due to lower dynamic resistance or temperature effects → current derating. Therefore, both current and voltage derating are typically specified when combining devices for higher ratings.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer application notes prescribe derating factors and balancing networks for both series and parallel configurations.

Why Other Options Are Wrong:

• Only current or only voltage derating overlooks the other critical sharing problem.
• “None” ignores real-world tolerances and transient imbalances.

Common Pitfalls:

Assuming perfect sharing from idealized SPICE models; practical builds require margins and balancing networks.

Final Answer:

both current and voltage derating