Small-Signal Models for Constant-Current Regulated Converters

The telecom DC uninterruptible power supply (UPS) systems include a storage battery to ensure the uninterrupted operation of the system during the power outages in the commercial grid system. The storage battery behaves as an ideal DC voltage source with very low internal impedance but needs to be recharged after discharge of its energy. Therefore, the telecom rectifiers taking care of the recharging have to be provided with output overload limiting for protecting them from damage. Typical overload-protection schemes are modified-constant-power and constant-current methods. It has been observed that the crossover frequency of the output-current loop tends to increase even one decade, when the storage battery is connected as a load. In this paper, the dynamical properties of the current-output converter are investigated, and the set of transfers representing the dynamics is developed based both on the state-space averaging method and on the two-port model of the corresponding voltage-output converter. According to the investigations, the observed tendency to increasing crossover frequency can be addressed to the internal dynamics of the converter, which the storage battery invokes into effect. This means that the origin of the problem is an erroneous control design.