A Comparative Study of AC-link and DC-link Based Frequency Converters

Frequency converters are widely used in variable speed drives, aircraft power supplies and wind energy applications. Traditional frequency converters are based on the DC-link topology. Large electrolytic capacitors are needed to eliminate the ripples in the DC stage. The size, lifetime and reliability of those capacitors are detrimental to system power density and robustness. The hard-switching loss of DC-link based frequency converter is also a considerable disadvantage of this topology. Recent studies of frequency converters are focusing on a new topology based on the AC-link. This new topology utilizes an LC resonating pair between the rectifier and the inverter. By controlling the charging and discharging time of the LC resonating pair, the converter is capable of delivering power in different frequency and magnitude. In addition, soft-switching can be obtained in the AC-link based frequency converter, which increases the system efficiency. The elimination of electrolytic capacitors also reduces the size of the converter which improves system power density and reliability. For comparison, both topologies are simulated in Matlab and implemented in hardware. A 45 kW prototype is designed, assembled, and tested in the lab. Experimental results are presented at the end of this paper.