The Japanese Experiment Module–Extreme Universe Space Observatory (JEM-EUSO) is a wide-field (+/-~30°of aperture) 2.5m refractor telescope to be installed in the International Space Station (ISS). The instrument looks downward from its orbit, into Earth’s atmosphere, with the main objective of observing ultra-violet (UV) fluorescence light generated by Ultra-High Energy Cosmic Rays (UHECR) extensive air showers (EAS). It is a frontier particle-physics experiment, the first of its kind. The validation of the technical readiness level of such a complex and unique instrument requires prototypes at several levels of integration. At the highest level, the EUSO-Balloon instrument has been conceived, through French space agency (CNES). At a smaller scale and in suborbital flight, EUSO-Balloon integrates all the sub-systems of the full space JEM-EUSO telescope, allowing end-to-end testing of hardware and interfaces, and to probing the global detection chain and strategy, while improving at the same time our knowledge of atmospheric and terrestrial UV background. EUSO-Balloon will be flown by CNES for the first time from Timmins, Canada; on spring 2014.This article presents the low-voltage power supply (LVPS) subsystem development for the EUSO-Balloon instrument. This LVPS is the fully operational prototype for the space instrument JEM-EUSO. Besides design and construction, all performance tests and integration results with the other involved subsystems are shown.