Vertical Graphene Arrays for AC Line-Filtering Capacitors

High-frequency responsive electrochemical capacitor (EC), which can convert alternating current (AC) in the circuit to direct current (DC), is an ideal filtering capacitor with lightweight superiority to replace the bulky aluminum electrolytic capacitor (AEC). However, current electrodes are difficult to achieve high energy density and high-frequency response properties simultaneously, primarily due to the electrode structure dilemmas of maximizing the electrode area or accelerating the ion transport. Herein, strictly vertical graphene arrays (SVGAs) directly prepared by electric-field-assisted plasma enhanced chemical vapor deposition have been successfully designed as the main electrode material of ECs to ensure the ions rapidly adsorb/desorb within the richly available surface spaces. The SVGAs exhibit an excellent specific areal capacitance of 1.72 mF‧cm− 2 at Φ120 = 80.6° even after 500,000 cycles in the aqueous ECs, which is far better than that of most quasi-vertical electrodes and carbon-related materials. Impressively, the output voltage could also be improved to 2.5 V when using the organic electrolyte, and an ultra-high energy density of 4.75 mF‧V2‧cm− 2 at Φ120 = 80.6° can also be handily achieved. Moreover, both aqueous and organic ECs-SVGAs can well smooth arbitrary AC waveforms into DC signals, indicating that ECs-SVGAs have colossal potentials to replace outmoded AECs.

ESR Estimation Schemes of Output Capacitor for Buck Converter from Capacitor Perspective

The aluminum electrolytic capacitor (AEC) is one of the most vulnerable parts in power electronic converters and its reliability is crucial to the whole system. With the growth of service time, the equivalent series resistance (ESR) increases and the capacitance (C) decreases due to the loss of electrolytes, which will result in extra power loss and even damage to transistors. To prevent significant damages, the AEC must be replaced at an optimal period and online health monitoring is indispensable. Through the analysis of degradation parameters (ESR and C), ESR is proved to be a better health indicator and therefore is determined as the monitoring parameter for AEC. From the capacitor perspective, ESR estimation schemes of output capacitors for a Buck converter are studied. Based on the voltage–current characteristics, two ESR calculation models are proposed, which are applicable for both continuous conduction mode (CCM) and discontinuous conduction mode (DCM). From the point of implementation view, the advantages and disadvantages of the two estimation schemes are pointed out, respectively. A Buck prototype is built and tested, and simulation and experimental results are provided to validate the proposed ESR estimation schemes.