Parallel thyristors – causes of unequal current sharing When thyristors are connected in parallel to share current, which effects can make the current distribution non-uniform?

Introduction / Context:
High-current applications often use parallel thyristors to reduce per-device stress. However, unequal current sharing can cause thermal runaway and premature failure. Understanding parasitic effects helps in designing equalizing networks and proper layouts.

Given Data / Assumptions:

• Two or more thyristors in parallel.
• Interconnects introduce parasitic inductance and capacitance.
• Devices have tolerances in V–I characteristics and thermal behavior.

Concept / Approach:

Dynamic sharing during turn-on/turn-off is influenced by stray inductances (affecting di/dt and current division) and capacitances (affecting dv/dt and initial current spikes). Static sharing is influenced by unequal on-state voltage drops and temperature coefficients. Equalizing resistors (static) and saturable/series inductors or RC snubbers (dynamic) mitigate these issues.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer application notes recommend both static and dynamic equalizing elements specifically because both inductive and capacitive parasitics disturb current balance.

Why Other Options Are Wrong:

• Inductive only or capacitive only (options a, b) understate the problem; both phenomena are relevant.
• None of the above (option d) ignores widely documented parasitic effects.

Common Pitfalls:

Relying solely on matched devices without proper layout or equalization. Long busbars can add significant inductance and imbalance.

Final Answer:

Both inductive and capacitive effects