"High efficiency and high power-density DC-DC converters are a key requirement in modern Power Electronics systems. Solar photovoltaic (PV) systems, EV/HEV powertrains and Data-centers are a few key examples. Traditional bridge-based DC-DC converters are commonly used in most systems that need a high voltage-gain. The presence of multiple stages in them affects the overall efficiency of the converter. In comparison, Dickson type switched capacitor DC-DC converters (SCCs) offer a single stage solution that lead to key advantages such as high efficiency, high power density, ease of control and elimination of central magnetic components in the system. However, their application in power conversion is limited due to the following reasons: 1. Conventional SCCs have two fixed operating states. Operating the converter between these states leads to a fixed voltage gain in boost mode and fixed step-down ratio in buck mode operation. The output voltage cannot be regulated for varying inputs and results in larger output capacitor ratings and larger voltage stress on downstream converters. 2. As the required voltage-gain increases, it leads to increased switch count and larger volume of required passives. Due to the presence of multiple floating switches, it also increases the complexity of gate drive and auxiliary power stages. This leads to lower efficiency and power density at high gains. The key contributions of this dissertation to the field of switched capacitor-based power conversion are as follows: 1. The concept of utilizing additional transition mode operating states for Dickson type SCCs is introduced. This enables the possibility of achieving real-time gain variation using SCCs without adding additional stages or passive components down-stream. These states are designed to not affect the steady-state efficiency of the converter. In comparison to traditional SCCs, it results in a 30% reduction in the voltage rating of output capacitors and down-stream converter switches when used for a PV system application. A peak efficiency of 96+% is achieved for a 1-kW prototype. 2. During gain transition, transient current at the input side of the SCCs relies (to some extent) on the trace inductance present in individual switched capacitor cells. A capacitive energy re-distribution approach for switched capacitor cells is proposed to minimize the input current transitions seen at the input of the converter. This enables 'smooth' gain transitions in the SCCs. Experimental results on a 1-kW prototype converter illustrate that peak current transient is limited to well-within 1.5 times the rated current through the converter. 3. A resonant Dickson type SCC configuration using GaN HEMTs that enables ultra-high efficiency and power density at high voltage step-down ratios is proposed. Some of the key aspects of the proposed configuration are the ability to maintain minimum number of switches, a distributed inductive-capacitive approach to power transfer in each individual SCC cell, a highly-efficient integrated gate drive stage and optimally designed passive components. The proposed configuration is designed to meet specifications of a 270 V/28 V (nominal) power converter in a more-electric aircraft. A peak efficiency of 99.1% and a power density of over 5 kW/L is achieved for a 300 W converter prototype."--Abstract.
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