The introduction of high-density power components brought unprecedented flexibility and creativity to power-system development. This provided designers with options to implement power architectures that were never before possible. Furthermore, using high-density power components simplified the design of distributed-power solutions.
The decision to use a centralized power-system design or a distributed-power design, however, often turned on cost considerations. More recently, though, increasing demands for higher reliability, higher performance, and higher system availability have introduced another factor into the cost equation. Respectively, that's the cost of diminished performance or downtime. Additionally, the decreasing cost per watt of high-density component power is making the distributed-power architecture (DPA) attractive for an increasingly broad range of applications. Today, component power can be applied to building distributed-power solutions more easily and at a more competitive cost.
DPAs offer a number of inherent advantages over a centralized system (Fig. 1). In the latter, the accurate voltage regulation that's necessary in some applications often proves difficult and costly to achieve. Inductance may also be an issue where loads are at varying distances from the source and/or they are dynamic. With distributed power, regulation takes place at or near the load. Output lead lengths are minimized, thereby reducing inductance and improving transient response.
In addition, a DPA system typically distributes higher voltages with significantly lower current levels through the system than does a centralized system. This architecture reduces power losses during distribution, which results in smaller and lower-cost power conductors. Thermal issues are managed better as well. By spreading power conversion around the system, heat is dispersed and hot spots are removed. Often, this reduces the need for heatsinking or fans.
Furthermore, with a DPA, the converters can be either on the pc boards they are powering, or close by, powering a group of boards. This provides a level of flexibility that's not available in centralized or scalable systems. Each board may be powered by its own converter, providing another level of flexibility. When power components are in use and a higher power rating is needed, it can usually be achieved by simply swapping one converter for another, assuming that the same pinout is used and the bus voltage and related external components are suitably rated. Extra filtering and other features can be easily added as they impact only one load, not the whole supply.
Several design issues need to be addressed concerning dc-dc converters in DPAs. First, the choice must be made regarding how the loads will be partitioned. Distributed systems have an unrestricted level of partitioning. It's possible to go from card-level conversion to a single module powering several cards.
High-density power components simplify partitioning and provide other advantages too. Most second-generation converters, for instance, can be trimmed or programmed from 10% to 110% of their output-voltage set point by using fixed resistors, potentiometers, or digital-to-analog converters (DACs). With such a wide range of output voltage, it's conceivable that not only to all of the converters in a distributed system have the same footprint and pinout, but also that they have the same nominal output voltage.
Generally, certain tasks—like thermal management, hot-swapping, and mechanical design—become simpler as the number of load partitions increases. Plus, if the load is partitioned to the card level, power-system assembly is easier, and the dc supply-bus design is simplified.
Power-system cost, on the other hand, tends to increase with the number of partitions (Fig. 2). One 200-W converter to power four 50-W circuit cards, for example, is less expensive than four 50-W converters. Additionally, overall aggregate power requirements may be higher because each card will need a converter that's able to supply the maximum power required for its function. This is true even though the system specification doesn't call for each card to function at maximum power. Each card, for instance, could have an average power consumption of 40 W and a peak of 60 W, with only two cards operating at peak at a given time. A single converter of 200 W could supply the needs of the system, but 240 W of total capacity would be needed if four converters were used.
Still, the designer should be cautioned not to consider the price of components in isolation. The short- and long-term savings resulting from more partitions—such as from easier thermal management, greater system flexibility, easier hot-swapping, and improved mechanical design—should also be remembered. Furthermore, power-system assembly is simplified with card-level partitioning. These usually have a significant offsetting influence on system cost.