Analysis and Evaluation of DC-Link Capacitors for High-Power-Density Electric Vehicle Drive Systems

In electric vehicle (EV) inverter systems, direct-current-link capacitors, which are bulky, heavy, and susceptible to degradation from self heating, can become a critical obstacle to high power density. This paper presents a comprehensive method for the analysis and comparative evaluation of dc-link capacitor applications to minimize the volume, mass, and capacitance. Models of equivalent series resistance that are valid over a range of frequency and operating temperature are derived and experimentally validated. The root-mean-square values and frequency spectra of the capacitor current are analyzed with respect to three modulation strategies and various operating conditions over practical ranges of load power factor and modulation index in EV drive systems. The modeling and analysis also consider the self-heating process and resulting core temperature of the dc-link capacitors, which impacts their lifetimes. Based on an 80-kW permanent-magnet (PM) motor drive system, the application of electrolytic capacitors and film capacitors has been evaluated by both simulation and experimental tests. The inverter power density is improved from 2.99 kW/L to 13.3 kW/L, without sacrificing the system performance in terms of power loss, core temperature, and lifetime.