Series-connected thyristors: the capacitor value in a dynamic equalizing (voltage-sharing) network is primarily selected based on which device characteristic?
-
AForward conduction characteristics only
-
BReverse-recovery characteristics
-
CBoth forward and reverse-recovery characteristics
-
DGate trigger characteristics
-
ENone of the above
Answer
Correct Answer: Reverse-recovery characteristics
Explanation
Introduction / Context:When SCRs are placed in series for high-voltage applications, differences in device dynamics during turn-off can cause unequal voltage sharing. Dynamic equalizing RC networks mitigate this problem; choosing C correctly is critical.
Given Data / Assumptions:
- Multiple SCRs in series across a high voltage.
- Each device exhibits slightly different reverse-recovery charge and tail current.
- Equalizing network: typically a capacitor (with or without a resistor) across each SCR.
Concept / Approach:During commutation or reverse recovery, some devices cease conduction earlier and start blocking sooner, momentarily taking higher voltage. The capacitor provides a transient current path so that device voltages share more evenly. Therefore, C is chosen based on worst-case reverse-recovery behavior (charge, time, di/dt) and expected dv/dt across devices, not on steady forward characteristics.
Step-by-Step Solution:
Identify worst-case reverse-recovery differences between series devices.Select C so that i_C = C * dv/dt can accommodate transient imbalances.Choose accompanying resistor to damp oscillations and equalize static division if required.Verification / Alternative check:
Manufacturers provide guidance: C sized from expected dv/dt and Q_rr spread to keep transient V across each SCR within limits.Why Other Options Are Wrong:
Forward conduction traits do not govern turn-off imbalance; gate trigger spread affects timing, but the dominant stress is during reverse recovery.Common Pitfalls:
Confusing static equalization (resistors) with dynamic equalization (capacitors) and overlooking reverse-recovery spread.Final Answer:
Reverse-recovery characteristics