A Compact Auxiliary Power Supply Design for a Medium Frequency Solid State Transformer

The design of auxiliary power supply (APS) in solid-state transformers (SSTs) is quite challenging. To minimize the isolation requirement, this APS is usually powered by dc-link capacitors. However, it poses voltage unbalance issues when the main converter is not in operation. To address this problem, a compact and cost-effective APS design is proposed, which utilizes the isolation structure of main medium-frequency transformers (MFTs). The proposed APS system also provides half-duplex communication, which replaces the costly optical fiber system. The interference between main and auxiliary power transfer links has been significantly reduced by the decoupled transformer design and the dual-band scheme for inductor-inductor-capacitor (LLC) and inductor-capacitor-inductor-capacitor (LCLC) converters. In addition, a new startup strategy with a low standby power is applied to this APS system. A 3.2-kW SST prototype with a peak efficiency of 97.9% has been built, whose APS is 6 W. The experimental results verify the feasibility and performance of the proposed techniques.