Power Management
Constructing a redundant
power system W
Redundant power supplies are designed to prevent system downtime, so what do you need to consider when designing backup supplies to ensure an uninterrupted system service? CIE talks to TDK-Lambda UK
hen designing systems or products that must have a minimum downtime, the
system’s power source must be carefully selected. As a general rule, the longer the power supply’s design life, the longer the power supply and the end product or system is likely to operate without a failure.
Power supplies include large electrolytic capacitors that over time can dry out and eventually fail. High-quality industrial- grade power supplies use higher-grade and more expensive electrolytic capacitors that resist drying out and therefore ensure the longest power-supply life possible. Also, many medium- to high-power supplies use fans to keep them at the correct operating temperature. Since fans are electro-mechanical devices, they often have the shortest lifespan of any component in a power supply and will eventually fail.
In cases where an OEM product or an end user’s system cannot tolerate any downtime, a redundant power-supply configuration should be considered. Figure 1 shows a schematic for a redundant power system with three power supplies connected in parallel via internal isolation (ORing) diodes to the system load. The isolation diodes’ function is to become back-biased (as an open-switch) if one of the power supplies fails. If this situation should occur, the failed supply is isolated from the load, its fault indicator turns from green to red, and the system load continues to receive full power from the remaining two redundant power supplies.
The failed supply is designed to be
replaced, or “hot-swapped,” while the system input and output power remains on. Therefore, it can be replaced at the user’s convenience without impeding the operation of the system.
Redundant reasoning One of the main reasons for using redundant power supplies that are connected in parallel with isolation diodes is to construct a fault-tolerant power system (Figure 2). This means that even if one of the paralleled supplies should fail, the system will continue to provide full power to its power bus - this is sometimes referred to as 100% power availability. In redundant power systems, each supply must include a circuit that automatically disconnects its output from the others should it malfunction. Typically this automatic disconnect is accomplished by having isolation diodes or MOSFETs placed in series with the output of each paralleled supply.
If one of the supplies develops a short
Figure 1: In this typical redundant power system, three power supplies interface in parallel to the system load via internal ORing diodes
36 September 2012 Components in Electronics
circuit on its output (a worst-case scenario) or shuts down for any reason, the isolation diodes would become back- biased or the isolation-MOSFET switch would be turned off (high impedance state). This would prevent the output current from the other supplies from flowing into the shorted or defective output of the failed supply.
In addition to having this automatic output disconnect feature, each supply must include a signal and visual indicator that can be used to alert the user or the monitoring external system that a specific redundant power supply has failed, so it can be replaced and repaired in the future.
N+1 and N+2 redundancy There are several ways to construct a redundant or fault-tolerant power system. The most common method is to have at least one supply with sufficient output power to fully satisfy the system’s power requirements. Then, a second power supply with the exact same ratings is provided as a backup in case one of the two supplies fails.
This forms a basic N+1 redundant and fault-tolerant power system (1+1 system) where N equals the number of supplies required to fully power the system and +1 equals one backup or redundant supply that will take over for a failed supply. Alternatively, N could consist of two power supplies, each providing 50% of the total load power with the +1 supply having the same power rating as the others.
www.cieonline.co.uk
Figure 2: This HFE1600 series 19-in. rack-mount redundant power system houses five 1.6-kW hot-swap power supplies. If fewer supplies are required, blanking plates can be installed in the unused slots
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44