Hybrid Symmetrizing Voltage-Clamp Design for an IGCT-Based Direct Current Transformer

Employing the three-level neutral point clamped (3L-NPC) topology for the LLC resonant DC-DC converter and utilizing the integrated gate commutated thyristor (IGCT) has enabled realization of a MW-level direct current transformer (DCT). Due to the use of three-level modulation only for soft-start and current limiting and two-level modulation in normal operation, design of static and dynamic voltage balancing snubbers is not trivial. While snubber capacitors in parallel with each IGCT ensure dynamic voltage balancing during two-level modulation, they inherently increase the switching transient duration and dead-time requirement, due to very low turn-off current, which is detrimental to the IGCT-based DCT operation. To overcome this limitation, this letter proposes a hybrid symmetrizing voltage-clamp design, by utilizing the symmetrizing resistor, in combination with neutral point diodes and a single (symmetrizing) snubber capacitor, rather than four. This achieves the required dynamic voltage balancing and reduces the dead-time duration despite low turn-off currents, further simplifying the topology and component count. A detailed analysis of the hybrid symmetrizing voltage-clamp scheme is provided, along with the experimental validation and comparison with the conventional dynamic voltage-balancing snubbers.