Supercapacitor-based DC-DC converter technique for DC-microgrids with UPS capability

Abstract

When DC loads are powered by a DC-microgrid (DCµG) based on a renewable source such as a PV system, energy storage becomes mandatory due to fluctuating nature of the source. Localized DC-energy storage within DC-DC converters could address this requirement. A variation of supercapacitorassisted low-dropout regulators (SCALDO) could provide localized energy storage with low-noise and fast transient response. For a generalized SCALDO configuration, end-to-end efficiency is given by ( 1 + k) (v;.eg/Vp) where k equals n or 1 / n, and n is the number of supercapacitors required to step-down an input voltage of v;, to an output regulated voltage of v;.eg· For SCALDO-based converters, (1 + k) is the efficiency improvement factor of a Vp-to-v;.eg standard linear regulator. I. INTRODUCTIO N The rapid development and uptake of electronic appliances requiring DC power has stimulated interest in DC-based power generators [1]. In the last few decades appliances that use renewable DC-energy sources have expanded from a simple solar-powered calculator to an industry-level equipment such as servers in a data center [2]. The fluctuating nature of solar PV and similar DC-energy generation systems motivates use of batteries and supercapacitors for storage [3]-[5].Fig. 1 illustrates a small-scale grid-a DC-microgrid (DCµG) with various DC-components connected via DC-DC converters [2] and a point (point of common coupling (PCC))connects with another microgrid. Due to the proliferation of renewable energy sources with direct DC-outputs, and appliances with electronic power converters, it is both practical and more energy efficient to use DC-DC converters rather than DC-AC and AC-DC converters with their inherent inefficiencies. µGrid dc--dc converter DC power generation system dc--dc converter dc--dc converter DC energy storage ' ' '---Fig. I. DCµG components and its connection with DC-DC convertersThere are three common DC-DC converter design approaches in practical electronic systems: (i) linear regulators, (ii) switched-mode converters with magnetic components (inductors), and (iii) switched-mode converters without magnetic components (switched-capacitors). (i) Linear regulators are simple, low noise and have fast transient response to load and line fluctuation, but they are rather inefficient due to their large dropout voltages (> 1 V). To address this low efficiency issue, a new family of low-dropout regulators (LDO) were introduced in late 1980s; in LDO design the input-to-output differential voltage can be kept very low, typically rv200 m V, to achieve high efficiency. However, the end-to-end efficiency (ETEE) of series linear regulators (Y;egfVp) in wide input-tooutput voltage applications is very low. (ii) Switched-mode regulators have high efficiency, with a theoretical performance of 100% ETEE. But in practice the achieved ETEE is only about 75-90% in general; in addition they emit very high level of RFUEMI and have limited transient response. (iii) Switchedcapacitor converters (SW-CAP) are high-frequency convertors which u...

Reduced‐switch SCALDO: an extra low‐frequency DC–DC converter technique for VRM applications

“…Localised DC‐energy storage within DC–DC converters can address this requirement. A SCALDO variation with DC‐UPS capability is discussed in [29].…”