How to Select Power Supplies

Power supplies often are among the last elements of a system to be considered because including them in the design seems a simple matter of matching a unit's rated output to the application's need. However, use of power supplies involves many design and cost trade-offs, some obvious and others subtle.

09/01/1999


KEY WORDS

 

  • Control components

  • Networks & communications

  • Power supplies

  • Power quality

Sidebars:
Monitoring and Control Indicators

Power supplies often are among the last elements of a system to be considered because including them in the design seems a simple matter of matching a unit's rated output to the application's need. However, use of power supplies involves many design and cost trade-offs, some obvious and others subtle. Designers who consider these trade-offs early in a project are likely to see things go more smoothly, faster and at a lower cost than otherwise.

Power/current requirements

Power requirements under both turn-on and average operating conditions must be considered because motors, solenoids, and relay controls require higher levels of current when at startup.

While industrial power supplies can provide peak-power pulses beyond their ratings for limited time durations, the magnitude and duration of these pulses vary among models. Thus, an application may require a unit that carries a higher rating for average operating conditions, but this may bring size and cost trade-off.

Power-supply issues also arise from potential mechanical failure of equipment. For example, when an electric motor stalls or shorts it will draw a tremendous amount of current. Some applications may require a power supply that can safely operate indefinitely during overload or short-circuit conditions thus, protecting itself and the system to which it is connected. However, designers of critical systems or processes should be aware that some power supplies automatically shut down when there is a large voltage sag (e.g. "undervoltage lock-out"), so they won't draw damaging levels of current.

Input line disturbances

In most industrial environments input power is "dirty" because equipment connected to the line, like motors, can generate voltage sags or current spikes at turn-on. System designers can address input-line surges and sags with power supplies that have input surge/transient protection and a wide input range—typically 85-265 V ac.

To ensure the power supply can withstand high input spikes, it should meet VDE 0160 specifications (800 V for 3 msec). For protection from ESD, surges and EMI/RFI on the input line, the power supply should meet the input transient requirements of IEC 1000-4-(2,3,4,5). There are many clauses to the requirement, that is all dependent upon the design—maybe it can handle the power, and maybe not. However, some electrical environments are so noisy that a standard power supply alone can't perform adequately, and external filters may be required.

Where frequent voltage fluctuations and power disruptions occur, designers should specify power supplies with longer hold-up times, or the length of time the unit can maintain output ratings with insufficient input power. Typically, a power supply can do this for one line cycle or 16 msec, but some applications may require more.

Reliability is critical

Power supply reliability is critical in industrial environments both for safety and economic reasons. Many applications cannot tolerate any downtime, under threat of an explosion or a leak of hazardous materials.

Reliability is enhanced if power supplies made with high-quality and conservatively specified components are selected. Surface-mount technology also improves power supply reliability by reducing human errors in component placement. Power supply manufacturers should follow rigorous test procedures, including 100% burn-in to reduce infant mortality effects, and failure-mode and effect analysis (FMEA) to anticipate and safeguard against potential failures of individual components. Power supplies should also undergo accelerated life testing to uncover causes of potential failure.

Ambient temperature is a key reliability consideration. Most power supplies are rated for reliable operation at full power in ambient temperatures up to 50there is a linear decrease in reliable output wattage, to an ultimate 50% of the unit's rating.

Under these conditions system designers should carefully analyze whether the power supply will suffice even if it is derated at the higher temperature. Moving to a higher rated power supply would be larger, more expensive, and perhaps even require cooling.

Apart from good thermal design, one effective way to increase reliability is to combine power supplies in a current-sharing "N+1" redundant configuration, with "N" being the number of power supplies adequate to serve the load. Adding one additional power supply adds to system cost, but it also adds to reliability because each unit operates at a smaller percentage of its rated output and is under less stress. The additional unit also provides an instant spare should another fail.

Hot-swappable power supplies allow users to replace a failed unit with no interruption in power delivery.

Mounting considerations

Forethought about power supply mounting will avoid the problem—perhaps under time or budget constraints—with the problem of how to shoehorn a larger-than-expected unit or an unanticipated cooling system into a cabinet. Keep in mind that not all unit ratings are available in all form factors.

Generally, even high-density supplies are mounted in control cabinets with other control and electrical equipment such as PLCs, sensors, relays, signal conditioning devices, fuses, and wiring terminal blocks. This equipment is often mounted on a DIN rail, which gives designers a way to pack a number of devices into a small space. Some higher power industrial power supplies are too big for DIN-rail mounting and they are an integral wall-mount capability.

Most DIN-rail arrangements are convection-cooled, although power supplies providing greater than about 1,000 W need to be fan-cooled. Whether convection or fan-cooled, each power supply has an optimum orientation. Designers need to consider both the size of the unit's footprint and in what direction it faces. Also, heat dissipated from nearby components can affect the power supply. A designer who calculates necessary system airflow based on a 50


Author Information

Greg Laufman works in both the applications and engineering areas at Lambda Electronics, Melville, N.Y.

Ian Kolker is the Industrial Product Marketing Manager at Lambda Electronics, Melville, N.Y


Monitoring and Control Indicators

"DC Good"—tells whether the power supply is working

"AC Fail"—warns when power failure is imminent, giving a system's processors enough time to shut down in an organized way, preserve data, and send an alarm to the control system

"Overtemperature," "Over-voltage," and "Fan Fail" alarms warn of other faults



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