Dual active bridge converter with PWM control for solid state transformer application

For the single-phase, dual active bridge converter (DAB) based three-stage solid state transformer (SST), the instantaneous load power seen by the DAB converter varies from zero to twice the load average power at double-line frequency. Traditionally with phase-shift modulation (PSM) control, large DC link capacitors are needed to handle the instantaneous load power variation while DAB converter only processes the constant power resulting in better soft-switching range and consequently high efficiency. However, the large electrolytic capacitors required in the DC link adversely affect the power density and the reliability of SST. In this paper, a PWM control is applied to minimize the DC link capacitor for the DAB based three-stage SST. The experimental results prove that DAB converter with PWM control has extended ZVS range down to nearly zero load and therefore improved efficiency at light load. Consequently, DAB can achieve comparable high efficiency while the DC link capacitors of SST can be reduced to a value low enough for film capacitor implementation.