Analysis and Experimental Verification of Current Sharing among Parallel-Connected DC-Link Capacitors with Stray Inductances

DC-link capacitors in power converters have a shorter lifetime than the other devices, which is an obstacle to improving reliabilityand downsizing of the converters. The lifetime of the capacitor depends on the amount of its ripple current because a larger amount of current results in greater increase in internal temperature. This paper presents a theoretical analysis and experimental verification of current sharing among parallel-connected DC-link capacitors with stray inductances. The analysis is obtained by the current transfer function of each capacitor, revealing that resonance caused by the capacitors and stray inductances drastically increases the root-mean-square (RMS) value of the capacitor current. This also implies that the sum of the RMS currents of all the capacitors becomes much larger than the capacitor current flowing out of a converter. The results of the analysis show that the capacitors closest to and farthest from the converter have larger currents than the middle capacitors, regardless of the number of capacitors connected in parallel. Chopper circuits rated at 450 V and 20 A with SiC MOSFETs were designed and constructed to test capacitors with a switching frequency of 10-200 kHz, which confirmed the validity of the theoretical analysis.