Modeling and control of modular multilevel DC-DC switched-capacitor power converter

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Wu, Liyao
Saeedifard, Maryam
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The Modular Multilevel Clamped Capacitor Converter (MMC3) is an attractive switched-capacitor DC-DC converter topology for applications with high output/input voltage conversion ratio (CR), featuring reduced voltage stress of power devices, high power density, bi-directional power flow capability and a modular structure. However, the voltage CR of the conventional MMC3 is fixed by the number of series-connected identical submodules (SMs), and an effective feedback control of output voltage is not yet available. In addition, large-amplitude current spikes may occur in the MMC3 due to the parallel connection of SM capacitors with imbalanced voltages, which lead to additional power losses and reliability issues. Furthermore, to expand the achievable output voltage range of the MMC3 and enhance its reliability based on its modular structure, a procedure to reliably bypass/insert SMs during operation is also required, such that the number of SMs can be varied. The purpose of this research is to address the aforementioned technical challenges associated with the design, operation and control of the MMC3. A detailed time-domain model and a small-signal state-space model have been derived for the MMC3. A closed-loop voltage control strategy for the MMC3 has been proposed based on the developed models, which regulates its output voltage by utilizing the impact of the Pulse Dropping Technique (PDT) and insertion/bypass of SMs. The developed models and proposed control strategy are validated by both simulation studies in MATLAB and LTSPICE environments as well as experimental studies on a compact 170-W Dickson converter prototype designed and built with GaN FETs, which achieves 93.8% efficiency.
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