Multiple Inputs Power Supply Architecture

July 8, 2007

Some electronic systems, such as field communication shelters or portable emergency equipment, must be able to operate from a variety of power sources, for example worldwide AC (85–264 Vrms), vehicle battery power, or portable generator sets, in both the 12 V or 24 V standards. In addition, these systems may also require backup power from auxiliary batteries in case the primary source is interrupted.

An example of this power supply architecture is shown in the figure below (click for larger image). For AC line use, a front-end module accepts a universal AC input and provides power factor correction. A high power DC-DC converter feeds a regulated 28 V output to the point of load converters. The 28 V converter can be trimmed to 30 V, so that no power will be delivered by the backup battery, as long as the AC main is active.

The point of load converters have an input range of 18–36 V, which allows them to accept 30 V from the AC line or 24 V from the backup battery or the vehicle source.

A peculiarity of this structure is the way the 12 V source is adapted to supply the 24 V input modules. Instead of converting the 12 V source directly to 24 V, the 12 V input feeds a 12 V output, 225 W converter array, that provides half the power needed. The output of this array is placed in series with the 12 V source to obtain a total of 24 V, at the required power level. This approach has the advantage of halving the number of modules required, which reduces costs, simplifies layout and increases reliability.

Overall efficiency is also increased, since half of the output power is provided by the battery, which is effectively 100% efficient.

To prevent back driving, the outputs of the intermediate converters and the 24 V source are diode OR’ed. Diodes should be selected to handle maximum load current and voltage.

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