Selection criteria of capacitors for flying capacitor converters

Abstract

Multilevel converters have a number of capacitors as sources of multilevel voltages. The volume of the capacitors should be minimized to realize a high-power-density converter. In this paper, the selection criteria of the capacitors in a flying capacitor converter are investigated. In this type of converter, the required capacitance, consequently, volume of the capacitors are decreased when the PWMcarrier frequency is higher. However, there is a limitation on the reduction in the volume due to the temperature increase caused by the power losses in the capacitors when the volume becomes small during high-PWM-frequency operation. On the basis of an investigation including the temperature increase, it is clarified that a capacitor with a high capacitance density is not necessarily the best for the flying capacitor depending on the operating conditions.