Conformity Magazine - July 2008 - (Page 51)

Take An Integrated Approach Although the extremely high overall reliability that can be attained with redundancy is widely recognized, implementing redundancy is not always as straightforward as it may first appear to be. It’s not simply a matter of paralleling the outputs of two or more power supplies. It requires careful consideration of not only the power supplies to be used and the electrical configuration, but also of many mechanical details in order to have a redundant power system that will function as intended. First, consider the location and physical form of the system. Will it be a panel assembly mounted in an equipment rack, or will it be mounted on a wall, or something else? Although it hopefully will not occur for many years, obviously every power supply will fail at some point, so it’s necessary to allow for quick and easy replacement of a supply that’s failed or is in need of refurbishment. This requires that the supplies be openly accessible, without any need to remove cover panels or an adjacent assembly, or to disable anything else. For example, if the supplies are within an assembly mounted in an equipment rack, access within the rack may be limited. In such cases, it’s a good idea to equip the assembly with slides so that it can be extended out of the rack for access (and don’t forget to include handles on the front panel to pull it out!) Alternately, some power supply manufacturers make supplies that can be plugged into the front panel of an enclosure or rack adapter, permitting a supply to be, quite literally, changed in a matter of seconds. Another approach is to have the system’s controls and indicators located on a main control panel, but to mount the power supplies in a less crowded location a few feet away. Finally, the supplies should be mounted in such a way that they can be easily and quickly removed and replaced. One way of doing this is to use thumbscrews for mounting; this also permits removing a supply without having to search for a screwdriver first. Another approach is to use supplies that can be snapped onto a DIN rail. Similarly, it must be possible to do the actual connecting and disconnecting of the power supplies quickly and easily. Each power supply should have a separate AC input power switch, so that an inoperative supply’s wiring can be deenergized without affecting the other(s). If the supplies have screw terminals or lugs rather than connectors, then insulated connectors that can be easily pulled apart should be included in the wiring cable to each supply. Accounting for Diode Drop An isolation diode must be used in series with the output of each power supply, first, to avoid the possibility of the combined output being shorted if the output of one supply should short and, second, to prevent current from one supply flowing back into another and reverse biasing it (which could cause it to malfunction and possibly damage it). However, the use of diodes introduces a voltage drop in the output voltage from the supply as seen by the load. This is especially significant at low voltages. For example, a 5 volt output might drop to only 4 volts. Using Schottky diodes, which have a smaller voltage drop than those of commonly available rectifier diodes, can be a help. Figure 1 JULy 2008 Conformity 51 http://www.kikusui.us http://www.kikusui.us

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Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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