A High Step-Down Isolated Three-Phase AC-DC Converter

In this paper, a single-stage three-phase isolated ac-dc converter topology utilizing SiC MOSFETs is proposed for power rectification with a stepped-down output voltage. Unlike the conventional two-stage [front-end power factor correction (PFC) stage and isolated dc-dc stage] ac-dc converters, the full/half bridge structure in dc-dc stage is eliminated in this structure. The high-frequency pulsating voltage is obtained directly from the PFC stage and is applied across the high-frequency transformer, leading to a more compact design. In addition, there is an advantage of zero voltage switching (ZVS) in four PFC MOSFETs connected to the high-frequency tank, which is not achievable in the case of a conventional two-staged ac-dc converter. A sine-pulse width modulation (PWM)-based control scheme is applied with the common-mode duty ratio injection method to minimize the current harmonics without affecting the power factor. An LC filter is used after the PFC semistage to suppress the line-frequency voltage ripple. Furthermore, the intermediate dc-link capacitor value can be greatly reduced through no additional ripple constraints. Experimental and simulation results are included for a laboratory prototype, which converts 115-V, 400-Hz three-phase input voltage to 28-V dc output voltage. The experimental results demonstrate a power factor of 0.993 with a conversion efficiency of 95.4%, and total harmonic distortion (THD) as low as 3.5% at 2.1-kW load condition.