A Modular Multilevel DC-DC Converter with Auxiliary Inductor Circuits for Cell Voltage Balancing and Fast Output Response

Battery-equipped apparatuses, such as electric vehicles (EVs), renewable energy generations, and mobile devices, are widely used and still being developed. For the design and evaluation of these devices, there is a growing demand for dc power supplies to test the charge-discharge characteristics and lifetimes of batteries. As the voltage and performance of the batteries increase, the power supplies for the battery test must be able to handle higher voltages and have a faster output transient response. To handle a high voltage, multilevel converters that use multiple power devices with lower voltage ratings have often been used in recent years, in addition to using a single power device with high-voltage ratings. Among various circuit topologies, modular multilevel converters (MMCs) have been actively studied and developed. However, unlike dc-ac and ac-dc conversions, dc-dc conversion in MMCs has a problem in that the cell capacitor voltages cannot be balanced in principle because of less redundancy of operation states. Therefore, most MMC-based dc-dc converters achieve voltage balancing control by combining a dc-ac conversion, isolation transformer or inductor, and ac-dc conversion. However, these circuit configurations tend to have a large equivalent inductance and are unsuitable for achieving a fast output response. In this study, we analyze the dc-dc operation and its problems in the MMC circuit. Based on the analysis, a circuit topology using auxiliary inductor circuits and a control method suitable for a dc-dc converter are proposed. A prototype of a 6-cell MMC-based dc-dc converter using SiC-MOSFETs was designed and implemented, and it was demonstrated that cell voltage balancing control and fast output response could be realized.