A Comparative Study of AC-Link and DC-Link Based AC-AC Frequency Converter

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 can deliver 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, a 45 kW lab prototype is designed, built, and tested. It can operate in both DC-link and AC-link mode. The output frequency range can be adjusted between 400 Hz and 800 Hz. The efficiency when operating in AC-link mode is 5.2% higher than that in DC-link mode. Other factors, such as power density, system robustness, IGBT total cost, control complexity and EMI requirements should also be considered when choosing between DC-link and AC-link frequency converters.