When using thyristors in series and/or in parallel in power converters, when is current derating required?

Introduction / Context:
Thyristors are often connected in series (to increase voltage rating) and in parallel (to increase current rating). Practical devices never share perfectly due to parameter tolerances and dynamic effects, so derating ensures safe operation.

Given Data / Assumptions:

• Series strings face static and dynamic voltage imbalance.
• Parallel groups face static and dynamic current imbalance.
• Realistic spreads in VBO, tq, gate sensitivity, and thermal coefficients exist.

Concept / Approach:

Derating is applied to accommodate unequal sharing. In series, snubbers and grading networks help, but residual imbalance remains. In parallel, ballast inductors/resistors assist sharing, but positive temperature coefficients and switching transients still cause unequal currents. Hence derating is advisable in both cases.

Step-by-Step Solution:

Verification / Alternative check:

Industry application notes recommend derating plus equalizing components for both series and parallel configurations to maintain reliability under worst-case conditions.

Why Other Options Are Wrong:

• Series only / Parallel only: incomplete—both need safety margins.
• None: unsafe; ignores real-world mismatches.

Common Pitfalls:

• Assuming ideal matching eliminates sharing issues—manufacturing spreads persist.
• Neglecting temperature effects on on-state characteristics.

Final Answer:

series or parallel