Transformerless Stacked Active Bridge Converters: Analysis, Properties, and Synthesis

This article presents analysis, properties, and systematic synthesis of a new class of hybrid dc-dc converters named transformerless stacked active bridge (TSAB) converters. The TSAB converters, which are obtained from switched-capacitor (SC) converters by insertion of small ac inductors, inherit advantages of parent SC converters together with conversion and control characteristics similar to transformer-isolated dual-active-bridge (DAB) dc-dc converters. Features of TSAB converters include soft charging and discharging of capacitors, zero voltage switching, low peak and rms current stresses, low energy storage requirement of magnetic components, and regulation capabilities using simple phase-shift control. Based on a network-theoretic approach, an algebraic representation of TSAB converters yields general results for dc characteristics and component stresses. Furthermore, a systematic synthesis approach is formulated allowing construction of TSAB converter topologies starting from various two-phase SC converters. Synthesis results are presented for TSAB converters with two inductors obtained from well-known SC parent topologies, including Dickson, ladder, stacked-ladder, doubler, and Fibonacci. Experimental results are summarized for 48-to-12 V doubler, 48-to-12 V Dickson, and 36-to-12 V ladder TSAB prototypes. It is shown how the TSAB prototypes use small ac inductors (tens to hundreds of nanohenries) while operating at relatively low switching frequencies (150-200 kHz) and have measured efficiency above 98% over wide load range.