Decoupling Control of an LLC-Quad-Active-Bridge Cascaded Power Electronic Transformer Based on Accurate Small-Signal Modeling

In modular ac–dc power electronic transformers (PETs) based on cascaded H-bridges, usually, a bulky module capacitor is required to store the low-frequency ripple powers introduced from ac phases, which degrades the power density of the system. A considerable amount of power devices arises a lot of switching loss, thus compromising the system efficiency. In this article, an
LLC
-type quad-active-bridge-based power channel PET (
LLC
-QAB-based PC-PET) topology is first presented. It is intended for improving both the system power density and efficiency. The
LLC
-QAB is featured by the ripple power counteracting channels as well as
LLC
resonant tanks. By using the frequency control combined with ripple power decoupling control, this topology can perfectly prevent ripple power spread and realize soft switching for power devices as well. In order to enhance the dynamic performance and make the control parameter tuning easy, a detailed small-signal model is derived based on extended describing function (EDF). Simulation and experimental results have verified the proposed topology, linearized model, and control strategy. Their effectiveness on soft-switching capability, system efficiency promotion, as well as power density improvement are also analyzed.