Constant-current vs. constant-power protected rectifier as a DC UPS system's building block

Telecom systems have been powered by means of DC UPS systems for years. The emergence of digital exchange systems changed the nature of the load from a resistive one to a constant-power type. The basic active building block of a DC UPS system is a rectifier operating normally in parallel with several similar units as well as with a storage battery. The rectifier must be protected from overloading. All the overloading protection schemes are based on output current limiting. There are, in principle, three possible ways to implement the protection-constant-current, fold backing and constant-power. The constant-power type protection scheme seems to be the most ideal from the telecom power system's view point, but there are, anyway, several parameters, which affects the optimum. When they are all taken into account, the constant-current protection scheme is as an economical or even more economical solution than the constant-power scheme. This paper addresses the different parameters affecting the optimum and explains the reasoning behind the ideas the authors have come to.