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Acopian Power Supplies TechTips 
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Acopian Power Supplies TechTips

By Alex 

Acopian Power Supplies TechTips

Acopian Power Supplies
TechTips May, 2006

TechTip 1: Considerations When Paralleling Power Supply Outputs for Redundancy

Although the extremely high overall reliability that can be attained with redundancy is widely recognized, implementing redundancy is not always as easy as it may first appear to be. In particular, it takes more than simply paralleling the outputs of two or more power supplies to have a redundant power system that will function as intended.

An isolation diode must be used in series with the output of each power supply, for two reasons - to avoid the possibility of the combined output being shorted by a shorted supply, and to prevent current from one supply flowing back into another and reverse biasing it (which could cause it to malfunction). However, the use of diodes introduces a significant voltage drop in the output voltage from the supply; for example, a 5 volt output might drop to only 4 volts. Using Schottky diodes can in some cases mitigate the diode drop, but it must still be considered. And keep in mind that the supply must be capable of providing a voltage equal to the sum of the voltage required across the load, the diode drop AND the drops in the wiring. A typical power supply can compensate up to a volt or so of drops in the wiring, but may not be capable of compensating BOTH the wiring and the diode drops. And if you’re using remote sensing to regulate the voltage across the load, you might not be able to solve this problem by simply stepping up to a supply with a higher nominal output voltage (for example, going from a 5 volt supply to a 6 volt supply), because then the sense lines of that supply would try to maintain a nominal 6 volts across the load! In summary, be sure to use a supply that can put out a voltage high enough to compensate both the diode and wiring drops under worst-case conditions (usually, low line and maximum load current), and also has the desired load voltage within its adjustment range.

Power supplies don’t always go low when they fail; the voltage can instead go high – by 50% or more in some cases – and fry the load. Therefore, it’s vitally important that power supplies used in redundant applications should always be equipped with fast-acting overvoltage protection to assure that the output voltage can’t go much higher than the nominal.

Replacing an inoperative power supply while the system remains in operation requires that each power supply have a separate AC input switch, so that the inoperative supply’s wiring can be deactivated without affecting the other(s). Similarly, it must be possible to disconnect the inoperative supply and connect the replacement quickly and easily, so insulated connectors that can be easily pulled apart should be used in the wiring to each supply. And 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 eliminates the need for tools.

All Acopian’s Redundant Power Packages and Modular Redundant Power Systems (www.acopian.com) meet all of these requirements, and have contacts that can be used to control a remote alarm. The Redundant Power Packages also have metering as a standard feature, and audible alarms are available as an option.

Visit www.acopian.com for instant, easy access to thousands of individual power modules. Choose Acopian - the power supplies of choice for the world’s leading engineers.

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