Power Management
Figure 3: This unit, a SWS1000L series power supply, delivers up to 1 kW of output power and features alarm signals and active current sharing. Two or more can be connected in parallel with external isolation diodes to form a redundant and fault tolerant power system
One advantage of the 2+1 power system is that under normal operating conditions each of the three paralleled supplies only provides 33.3% of the total system power, reducing the thermal stress on each supply and improving its mean- time-to-repair (MTTR).
Some mission-critical applications may need an N+2 redundant power system. For applications like air control, data backup, or life support systems, two backup supplies provide much better fault tolerance than one. However, there is the added expense to achieve this improved degree of power availability and fault- tolerance.
Hot swapping It is best if all the redundant and fault- tolerant supplies have the type of interface circuits and input/output connectors that allow the supplies to be replaced while the system’s ac input and dc output power is still in the on state. The ability to do this, called hot- swapping, is an important feature to have. Maintenance personnel then can replace the faulty power supply without interrupting the system’s operation.
Active current sharing To properly parallel two or more power supplies, the supplies should include an active-current-share or master/slave feature. This function forces each of the paralleled supplies to contribute its share of current to the load. For example, two paralleled supplies would each provide 50% of the total load current, three supplies would each provide 33.3% of the total current, and so on. These current share connections must be made between all the paralleled supplies. Without active current share, one of the paralleled supply’s outputs could drift higher than the others and draw most of the load current, which can be dangerous and can lead to the premature failure of the power supply, especially in high-power applications.
Scalable power
Economic savings can be achieved where a system requires a specific power level when it is originally built and shipped to the end user, but can be upgraded in the
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field to add more power as the system grows. For example, for data centre servers the original power requirements for the system may require two or more power supplies in parallel. But as more data-handling capacity is added, more power will be required. The ability to add or reduce the total power provided to a system is known as scalable power. With a rack-mounted redundant power system, the original configuration can consist of some number of power supplies with blanking plates covering the unused slots. As the system needs more power, one or more blanking plates then can be removed and additional hot-swap power supplies can be added to supply additional power.
Enclosures
A rack-mount enclosure provides a convenient means for paralleling supplies for expansion (scalability) and/or to form a redundant, hot-swap configuration. However, many power supplies have the necessary features for being connected in parallel with active current sharing and failure alarm signals (Figure. 3). By adding external isolation diodes, these supplies can be configured to form a redundant power system.
Always check your power supply’s instruction manual to be sure that your supplies can be connected in parallel. The only feature that paralleling individual supplies will lack compared to the rack- mounted systems, previously described, is the special mechanical aspects that allow them to be hot-swapped while the input and output power is active. So, in non- hot-swap applications, a scheduled maintenance shut-down period is required to allow time for replacing the faulty power supply. We have discussed when and how to consider using a redundant and fault- tolerant power system plus the economics of scalable power and the features required of the power supplies to construct a redundant power system with the greatest amount of reliability and ease of system maintenance. Now all you have to do is choose your supplies wisely.
TDK-Lambda UK |
www.uk.tdk-lambda.com
Components in Electronics September 2012 37
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