Over 99.7% Efficient GaN-based 6-Level Capacitive-Load Power Converter

This work presents a highly-efficient 6-level multilevel converter, which uses two back-to-back connected 650V GaN HEMTs as bidirectional blocking switch for each of the four inner voltage levels and one unidirectional HEMT for each of the the highest and lowest voltage level. The converter is applied to cyclical charging and discharging of a 40 muF capacitive load between 0 and 435V at 5.5 Hz. External power is provided only to the highest voltage level; the inner levels are operated sufficiently self-balanced for the low-loss, linear and almost ideal capacitive load. A power-stage efficiency as high as 99.75% is experimentally achieved by zero-voltage-switching with an analog hysteretic current control after optimization of the dead times and the peak and valley current setpoints. A low-loss gate signal and power isolation is realized, which reduced the system efficiency only slightly to 99.71%. The high number of 10 discrete unidirectional GaN HEMTs used can be replaced with just 4 monolithic bidirectional switches and 2 unidirectional switches in future. Such highly efficient converters for capacitive loads enable a high system coefficient of performance for emerging emission-free, solid-state (electrocaloric) heat-pumps.