Neutral-Point Voltage Balancing Methods of Series-Half-Bridge LLC Converter for Solid State Transformer

Abstract

With the advancements in silicon carbide (SiC) power devices, the solid state transformer (SST)-based power delivery architecture shows advantages over traditional line-frequency transformer-based solution in terms of efficiency, power density, modularity, and scalability. This article presents a three-level converter cell for a modular SST, which consists of a neutral-point-clamped (NPC) H-bridge ac-dc stage and a series-half-bridge (SHB) LLC dc-dc stage. Compared with two-level cell-based SST, the total cell number is reduced to half, which means less system complexity and cost. The neutral-point (NP) voltage within each cell must be balanced for proper operation of the system. A simple and effective balancing method is proposed by adding a phase-shift angle to the modulation of the SHB LLC converter with zero-voltage switching. This idea is applicable to both two-level and three-level modulation modes for wide-range operation. Further, a novel NP voltage balancing method for burst-mode operation under light-load condition is proposed by short-circuiting the transformer secondary side during burst-off period and utilizing the resonant tank energy to balance the NP voltage without disturbing the load or the grid. The proposed method is experimentally verified on the SiC-based three-level converter cell operated around 200 kHz for SST.