Compared to AC-DC converters based on high-voltage devices, it is easier for the multi-cell AC-DC converter to achieve high efficiency and power density, due to better switching properties of low-voltage devices. To maintain normal operation in multi-cell converters, dc-bus voltage balance and power balance among AC-DC cells should be achieved. Usually, complex voltage balance and power balance control with additional sensing circuits are required to achieve the balances. In some cases, although the balances can also be achieved by employing pulse-width modulation (PWM) DC-DC converters with common-duty-cycle control as the 2nd stage, their low switching frequency for high efficiency will lead to decrease in power density. In this paper, regulated dc transformer (RDCX) with simple common-duty-cycle control is introduced for high-efficiency high-density power conversion and simple control. Since the RDCX operates at a fixed frequency neighboring the resonant frequency, fully soft switching and transformer optimization can be easily achieved for high efficiency and power density. Moreover, due to its voltage regulation principle, dc-bus voltage balance and power balance can be realized by employing the common-duty-cycle control. A 6-cell 3 kW/220 Vac-48 Vdc AC-DC converter prototype with MHz RDCXs as the 2nd stage is established to verify the analysis.