Quasi-Two-Level Bridgeless PFC Rectifier for Cascaded Unidirectional Solid State Transformer

Abstract

With the advancements in wide-bandgap power devices, the solid state transformer (SST) becomes attractive to industry due to its merits of high efficiency and power density, modularity, and scalability. However, the complexity and cost remain obstacles to widespread adoption. This article presents a unidirectional SST with a cascaded three-level bridgeless power factor correction rectifier stage and a series-half-bridge (SHB) <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> dc–dc stage, which is simpler and more cost effective than its bidirectional counterpart. The quasi-two-level (Q2L) modulation method is proposed for the three-level bridgeless PFC rectifier. In contrast with the three-level modulation, the Q2L modulation almost eliminates the neutral-point (NP) voltage ripple at line frequency, resulting in much less dc-link capacitor volume. Further, the only safe switching sequence to implement the Q2L modulation is identified and analyzed. The NP voltage shall be balanced by the SHB <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> dc–dc stage for proper operation of the system. A phase-shift-based three-level operation mode is proposed for the SHB <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converter to balance the NP voltage. The proposed methods are experimentally verified on a 10 kW silicon carbide-based three-level converter cell and a single-phase multicell SST prototype.