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A Family of Transformerless Stacked Active Bridge Converters

This paper describes a family of transformerless stacked active bridge (TSAB) converters with N-to-1 nominal conversion ratios and continuous voltage regulation capabilities. An N-to-1 TSAB is a hybrid converter derived from the N-to-1 Dickson switched-capacitor (SC) converter, with small ac inductors inserted in the flying-capacitor branches to achieve lossless ("soft") capacitor charging and discharging. In operation, a TSAB converter resembles the isolated Dual Active Bridge (DAB) converter but without the need for an isolation transformer. The TSAB family has several favorable characteristics, which lead to very high efficiency around nominal conversion ratio, including small ac inductors with near-trapezoidal low RMS currents, low voltage stresses, and zero-voltage-switching (ZVS) over wide load range. Moreover, the operation is robust with respect to component tolerances, and continuous voltage regulation can be achieved through simple phase shift control. Experimental results are presented for a 120 W, 48V-to-12V TSAB prototype, demonstrating a flat efficiency characteristic with 98.6% peak efficiency, and 98.0% full-load efficiency.