An Investigation on Development of Novel Switched Capacitor Converter Topologies with Reduced Ratings for DC-AC & DC-DC Power Conversion

Abstract

Switched Mode Power Converters (SMPCs), which utilize switches and passive components, are the standard for energy conversion in medium and high-power applications. These converters can be classified as isolated or non-isolated based on galvanic isolation. Common non-isolated topologies include buck, boost, buck-boost, and Cuk, while forward, flyback, and H-bridge topologies are examples of isolated converters. All these well-established topologies rely on inductors, coupled inductors, and transformers to function. However, these inductors become bulky, heavy, and expensive at higher power levels. Additionally, inductors generate ’di/dt’ voltage spikes leading to EMI noise and have temperature constraints as they tend to saturate at high temperatures. The leakage inductance of transformers causes large voltage spikes and high switching losses in associated power devices, necessitating high power-rated devices or complex combinations of lower voltage-rated devices.

With the rapid growth in renewable energy resources, transportation electrification, and space applications, there is a rising demand for space-efficient, lightweight, and reliable power converter topologies. Eliminating bulky inductors would be highly beneficial in such applications, and switched capacitor converters (SCCs) offer an ideal solution by utilizing only capacitors for energy transfer. Capacitors have higher power and energy density, are compact, and can operate at higher temperatures than inductors. SCCs have historically been used for low-power on-chip applications where space and cost are critical constraints. However, reliable topologies with reduced ratings of capacitors and power devices for medium and high-power applications remain underexplored. This research aims to develop SCC topologies for both DC-AC and DC-DC power conversion with minimized ratings of passive and active components and a reduced device count.

The first work proposes a current-fed DC-AC SCC converter. This converter offers advantages such as a reduced count of switched capacitors and power devices, elimination of load-side filtering elements, reduced switching ripple in output voltage due to inherent interleaving, reduced voltage and current Total Harmonic Distortion (THD), and lower ratings of the switched capacitors. An adaptive hysteresis control scheme tracks the voltage across the switched capacitors to a desired sinusoidal reference. The overall control architecture is straightforward to implement, and the switching architecture uses overlapping logic to prevent interrupting the input current. The work includes a complete analysis and design procedure. A 500 W laboratory prototype of the proposed converter is built, with the control architecture implemented using the TMS320F28379D microcontroller. Simulation results are verified with experimental outcomes.

The second work establishes a Resonant Switched Capacitor Converter (RSCC) topology for DC-DC power conversion. In this converter, all switched capacitors and approximately fifty percent of the switching power devices are rated explicitly for the input voltage, making it suitable for high-power applications. A compact inductor is used for resonant operation, significantly reducing the switching frequency, leading to reduced switching losses and improved efficiency. Zero Current Switching (ZCS) during the turn-on and turn-off of switching devices is achieved due to the resonant inductor current. The proposed converter eliminates the bulky load-side bypass capacitor by reducing switching ripple through inherent interleaving action. The work extensively covers steady-state analysis and the effects of non-idealities during resonant operation. Discussions on detailed topology design and component selection are presented. The converter is validated with a 200 W experimental prototype achieving 95.83% efficiency.

The third work proposes step-up and step-down RSCC topology with discrete variable gain control voltage regulation. Continuous gain control of the proposed SCC using Fast Switching Limit (FSL) and Slow Switching Limit (SSL) impedance modulation leads to undesirable high current stress on switching devices and capacitors. This work introduces a multiple gain resonant SCC topology capable of step-up and step-down operations by varying the converter’s switching patterns. Resonant operation ensures ZCS turn-on and turn-off of switches. The converter achieves a high gain of six, with significantly lowered passive components’ size and switching frequency, ensuring reduced losses and improved efficiency. A comprehensive steady-state analysis is performed, and a 200 W hardware prototype is built for experimental validation, achieving a peak efficiency of 96.48%.