Thermally-Compensated Modulation Strategies for Modular Power Converters
Power electronics converters are employed in various fields to take the advantages. Especially, a modular structure is getting attention, which exhibits a constitution of multiple basic cells rated for a lower power. Nevertheless, the reliability issue has been intensively argued due to the higher number of power semiconductor devices and capacitors. It has been revealed that the thermal stresses are highly responsible for the major failures of power semiconductor devices. The active thermal control methods have been considered, since they can be simply adopted by a software. However, the existing methods intend to increase the losses for compensating the temperature swing, which implies a limited lifetime improvement and a decreased efficiency. Considering the modular converter consisting of modular inverter and DC/DC converter, two active thermal control methods are proposed: multi-frequency and discontinuous modulations, which aims at controlling a loading power of each module. The DC/DC converters connected in parallel can have a different loading according to their remaining useful lifetime. Remarkably, the discontinuous modulation allows to improve the reliability of both DC/DC converters and inverter. Electrolytic capacitors are degraded due to the thermal stresses, which are determined with ESR and capacitor current. Therefore, the major task is to reduce the capacitor current. The optimal DC-link design approach has been introduced in literature. However, a clear relation between current frequency and thermal stresses has not been investigated. The power converter for deriving the relation is proposed, which injects a designated AC current and DC bias voltage to capacitor under test. The multi-level converter is most suitable with allowing lower THD and lower rated power devices. Also, the proposed control scheme simultaneously regulates two parameters. Finally, the correlation between the current frequency and the thermal stresses is experimentally derived.