Voltage Balancing of a Split-Capacitor IGCT 3L-NPC Leg for the Resonant DC Transformer

Abstract:
This work investigates the static voltage balancing challenges in an IGCT-based LLC resonant converter with split-capacitor 3L-NPC power stages. It focuses on its operation in a two-level mode with a 50% duty cycle, especially under conditions of medium frequency switching and extremely low turn-off currents. The paper conducts a comparative analysis of two static balancing strategies: one using the parallel balancing resistors for each IGCT, and another employing a single symmetrizing resistor across the two inner IGCTs. These methods are assessed in terms of performance and losses. Additionally, it investigates how the symmetrizing resistor influences the commutation process behavior under these specific conditions. Experimental results are provided to validate the effectiveness and trade-offs of each approach.