Voltage allocation (VA) is analysed for a multi-module power conversion system using a parallel-input and series-output (PISO) connection with different power ratings. In PISO system, the multi-module converters with different power ratings cannot regulate total output voltage when one of the series module is operated in its voltage limited mode. Therefore an automatic master loop regulation (AMLR) control scheme is proposed to allocate output voltage of individual modules, as well as to regulate total output voltage and achieve power management. Presented also is a green-mode control (AMLR+backup module) that could shield backup module in light-load to improve light-load efficiency. Finally, by using green-mode control, the experimental prototypes of three PISO forward modules with different power ratings are implemented in order to achieve VA mechanism. The efficiency of the multi-module PISO system with the proposed control scheme in the light-load is improved by over 12+, as compared to the conventional control scheme.