Reliability-Oriented Routing of Internal Current Stress in the Two-Stage SST Submodule

Power routing has emerged as a promising technique for balancing the remaining useful lifetime of modular converters, e.g., solid-state transformers (SSTs). However, this approach does not address the internal stress distribution within the individual two-stage submodules. In this article, a method to balance the internal stress by redistributing the second harmonic current between the dc-link capacitors and the devices in dc/dc stage is presented. This method introduces an adjustable ripple to the basic phase-shift modulation, directing a portion of the second harmonic current to the rear-end dc/dc stage in accordance with the lifetime differences within submodules. It requires minimal control effort and provides sufficient thermal balance capability within the submodule. Notably, it focuses exclusively on the second harmonic current, with negligible impact on the output power. Experimental results demonstrate its effectiveness in achieving thermal regulation and lifetime balance within the submodules.