Designed to perform in harmony with local electric utility conditions around the world, modern internal power supply units (PSUs) are more robust than ever. They operate normally over a wide range of input voltages and frequencies, have internal energy storage to ride through brief power interruptions, feature input power factor correction circuitry, and operate at a power factor close to unity. But in spite of their robust design, PSUs require additional protection from a range of power quality problems that are generated by the electric utility or arise within the facility. For this reason, IT equipment needs a consistent source of conditioned power that meets industry specifications to operate properly. Responsible for providing that consistent, conditioned power is the uninterruptible power system (UPS).

With a vast array of UPS options in the market today, which design will meet the needs of your IT equipment? The answer depends on a combination of factors, including industry trends, advances in technology, and the degree of protection required. It helps to look at the issue from an "end-user"point of view — the perspective of the PSU inside the IT equipment.

Following are five basic power quality requirements you should consider to meet the needs of an internal PSU.

Requirement #1 — input voltage within acceptable limits

Most equipment manufacturers use universal PSUs that support the various input voltages and frequencies found in different countries. That means the PSU in your IT equipment is likely to support the low, 100VAC voltage used in Japan as well as the high, 240VAC voltage used on most other continents. In North America, the PSU may have to accommodate single-phase sources of 120V and 240V and 3-phase sources with voltages of 120V, 208V, and 240V.

According to standards set forth by the Server System Infrastructure (SSI) Forum, a PSU rated for 120V to 127V should operate normally at voltages ranging from 90V to 140V. A PSU rated for 200V to 240V should operate normally on input voltage from 180V to 264V. Real design margins are somewhat broader, again because of the need to handle input voltages from any country around the world. The power output from the PSU may even be automatically limited by input voltage, to protect it and internal circuitry from damage if connected to the lower voltage range. The bottom line is today’s PSUs are more versatile, robust, and tolerant than they were even five years ago, with many supplies capable of handling input voltages anywhere between 90V and 264V.

This means the UPS must be able to supply voltage within the specified range required by the PSU, for all voltage variations found in the AC power sources (utility mains or generator). For example, for higher watt rated power supplies requiring an input voltage of 200V to 240V, the UPS must deliver power within the 180V to 264V range.

Requirement #2 — input frequency within allowable ranges

Once again, power supplies for IT equipment are typically designed for universal operations. That means a typical PSU can operate normally at frequencies from 47 Hz to 63 Hz (many supplies have a 45 Hz to 65 Hz window) to accept electric utility power at both 50 Hz and 60 Hz. For your specific uses, the UPS must be able to regulate output frequency to meet the PSU’s specification range of 47 Hz to 63 Hz for all frequency variations in the AC power source — whether that power is coming from utility mains or a generator.

Requirement #3 — sufficient input power to compensate for power factor

Circuits containing a mixture of reactive components (capacitors, inductors, switching devices) characteristically have what is known as a distortion power factor. The current drawn on the input is a mixture of the fundamental frequency as well as several harmonic frequencies (multiples of the base frequency). Power supplies used in IT equipment generally fit into this last category. Harmonic distortion and power factor are directly related. The higher the power factor of an IT power supply, the lower the harmonic distortion.

Poor power factor, due to high input total harmonic distortion (THD), has been known to cause failed neutral conductors, overheated transformers, and, in the worst cases, building fires. This was a concern with older switch-mode power supply designs used 10-plus years ago. These problems led to the creation of international design standards (EN61000-3-3, EN61000-3-2, IEC 1000-4-7) aimed at limiting allowable harmonic distortion on a power source.

Most modern power supplies above a 50W rating are designed to correct for poor power factor. They have an input power factor correction (PFC) circuit to raise power factor and lower input current distortion. In addition, most power supplies above 200W have an active PFC circuit that automatically adjusts the power factor based on the actual power required by the IT device. These capabilities enable such power supplies to drive power factor very close to unity; however, there still can be some issues with higher powered power supplies running at very low loads, with the power factor actually causing other issues for some UPS systems and generators.

In sizing a UPS, the power rating (kW) is actually more important than the kVA rating (apparent power) due to the high power factor of the IT equipment. When assessing output power and battery backup time, make sure to use the real power (kW) rating of the UPS. If the kW is not apparent in the UPS specifications, you can calculate it by multiplying the UPS kVA rating times the output power factor rating of the UPS.

PSUs used in IT equipment today have a power factor trending toward unity, because of the need to reduce harmonic current content in the AC source feeding the IT loads. As a result, in today’s power supply designs, a power factor of 0.9 would be considered acceptable, 0.95 would be typical, and a value of 0.99 would be excellent.

Requirement #4 — transfer to backup power faster than PSU "hold-up"time

PSUs inside IT equipment have an energy storage device (usually a capacitor) that stores enough energy to keep the device running during very brief power interruptions. This is known as "hold-up"time and depends on the internal capacitance — consider it a very low-capacity battery — of the power supply and the output power rating. At higher output power, the energy is drawn from internal capacitance faster than at lower output power.

According to IT equipment standards set forth by the SSI Forum, minimum hold-up time at fully rated output power is one cycle. Because most IT equipment is designed for the global market, the minimum hold-up time is 20 msec (50 Hz AC cycle) — the time may be longer at lighter loads.

However, under pressures to reduce PSU size and cost, manufacturers are designing PSUs with smaller capacitors, which lead to shorter hold-up times. This effect is somewhat offset by the prevalence of redundant power supplies, because each power supply would be loaded to less than 50% of its capacity.
A related issue with respect to hold-up time is the peak inrush current required to charge up the capacitor that provides the ride-through capability. When first connected to an AC power source (or when powered up on an already connected source), the equipment temporarily draws a large inrush current that can last for 2 msec to 10 msec and be as much as 10 to 60 times the normal operating current.

Similar to the start-up inrush current, there is also a surge current drawn to recharge the capacitors after short interruptions in power. If the power interruption is less than 5 msec, surge currents will typically last for half a cycle (10 msec) and will be less than 300% of nominal current. For interruptions of 10 msec to 15 msec, the surge current could be 700% to more than 1,000% of nominal current and can last for 1.0 to 1.5 cycles (20 msec to 30 msec).

For performance requirements, the UPS must first ensure no interruption in its output that lasts longer than the hold-time of the IT equipment’s PSU. This means that the UPS must have an acceptable transfer time for all transitions between different modes of operation — such as from normal operating mode to battery mode and back again or between high-efficiency mode and double-conversion mode for new energy-saving UPSs.

Note that hold-up time will be different for single- or multi-corded servers, because the more PSUs on the IT equipment, the less power load on each PSU — and the longer the available hold-up time. Single-corded IT equipment will need a UPS with faster transition times to prevent unplanned shutdowns and reboots.

The UPS transfer time should actually be much faster than the maximum allowable hold-up time, because the longer the PSU goes without power, the larger the surge current it will draw when it receives power again. In cases where the PSU is without power for more than 5 msec to
10 msec, the inrush current required by the PSU could easily exceed the maximum current output capacity of the UPS inverter, forcing the UPS to shut down to protect its own inverter components.

Requirement #5 — protection from damaging power conditions

PSUs are designed to handle voltage that sags 10% below nominal specification or surges 10% above, without loss of function or performance. The PSU is also required to handle surges of 30% from the midpoint of nominal (286V for a 220V PSU) for 0.5 cycles (8 msec to 10 msec).

For fast AC line transients, the power supply is designed to meet the EN61000-4-5 directive and any additional requirements in IEC1000-4-5:1995/the Level 3 requirements for surge — and withstand capability without disruptions to normal operations.

These tolerances are well defined by the Information Technology Industry Council (ITIC) curve, published by ITIC Technical Committee 3 (TC3). The ITIC curve, which actually represents a stair step more than a curve, describes a voltage envelope that PSUs can typically tolerate without interrupting function.

UPS output voltage must be within the acceptable zone specified by the ITIC curve for all input AC line conditions. The UPS must be designed to ensure the voltage to the PSU is not in the prohibited range, because voltage in that range could damage the IT equipment. The UPS must also be designed to handle high-speed impulses, such as lightning strikes or surge currents of longer duration, even though most PSU are designed to handle some level of surge current without damage.

Final thoughts

Designed to serve global markets, modern PSUs have a wider range of capability than ever before. Despite their capabilities, PSUs still need vital UPS protection from power quality problems in the facility. However, with such a vast selection of UPS configurations available today, choosing the best fit for your needs can be challenging.

If you consider the "end-user"needs of the PSUs in your IT equipment, choose a UPS with the appropriate input voltage/frequency and sufficient input power to compensate power factor. In addition, by making sure the unit can transfer backup power faster than PSU "hold-up"time and provide adequate protection from damaging power conditions, you can meet the key power quality requirements set by international standards and provide consistent, conditioned power.

Loeffler is a data center applications manager for Eaton Corp., Raleigh, N.C. He can be reached at: ChrisALoeffler@Eaton.com.