Bench Talk

For several reasons, the communications industry has made 1U high-power-supply racks the standard for powering data centers and other datacom-related equipment for networking activities. As defined by the Electronic Industries Association’s (EIA’s) EIA-310-D standard, the height of rack-mounted equipment is specified in multiples of “U,” where 1U is defined as 44.5mm (1.752in).

Designed for easy use in high-density environments, the rack—or shelf as it is sometimes referred to—is slightly lower in height to allow easy insertion and removal without binding on adjacent shelves. They are also designed with redundant architecture in mind so that if one fails, it can be replaced without disturbing equipment operations. The amount of power that is deliverable from a 1U rack can be 10 to 14kW with four plug-in power supplies.

With such benefits, 1U high-power-supply racks seem ideal for use in industrial environments, yet they have not found a great deal of popularity in this segment. Although they are used in some industrial environments where space is limited, uncontaminated, and fan noise is not a factor, 1U high-power-supply racks tend to be less suitable for industrial environments.

Cooling a 2 to 3kW 1U power supply requires fan cooling by way of two high-speed, 40mm fans. These emit very high audible-noise levels at frequencies quite annoying to the human ear. As data centers rarely have human operators present for long periods of time, this is not an issue. For industrial use—where operators are constantly monitoring and using equipment, like test and measurement analyzers—such levels of noise are not considered acceptable.

60mm fans can operate at lower speeds to produce the same airflow volume and therefore can dramatically reduce fan noise. For example, the TDK-Lambda’s multiple output QM7 series, which uses 60mm fans, has an acoustic-noise level of just 45.3dBA. A similar product with 40mm fans measured 58.9dBA. (Note that a 10dBA increase equates to a doubling of the perceived loudness.)

Additionally, industrial environments often contain airborne contaminants that affect fan life. The faster a fan rotates, the quicker the life of the fan reduces due to mechanical bearing wear.

The higher the airflow speed, the more contaminants (such as dust and dirt) will be drawn into the power supply. Eventually, this will block airflow and/or cause product failure from circuit shorts if the contaminant is conductive. Here again, the lower-speed fans of the 2U and 3U high-power supplies are more suitable for industrial environments.

A redundant configuration of the power supply is usually a mandate for datacom equipment, as the loss of a data center connection or cell phone service can cause severe and widespread disruption. In redundant mode, power supplies actively share the amount of current they provide, and they normally do not run at a 100 percent load level.

For example, in a 3+1, 2500W power-supply configuration, where the maximum load drawn is 7500W, each of the four power supplies provides 1875W and runs at a 75 percent capacity. The power supplies operate at a full load level only when a unit fails and while awaiting its replacement. Such derating of power supplies reduces internal temperatures, particularly those of the electrolytic capacitors, thus improving the operating life of the supplies. Considering these power supplies are very cost sensitive, product designs should take into account that the supplies do not operate for long periods of time at a 100 percent load level.

The operation of power supplies in a redundant mode can be very important to some industrial users to minimize production downtime. Modern factories operate on a just-in-time basis with multiple, smaller production cells feeding a main assembly line. If one machine is down on a critical path, it can affect an entire plant. Therefore, machine builders will often fit two power supplies, like TDK-Lambda’s industrial RWS-1500B series, in a configuration—where the load current is not balanced and one unit may be at zero load, though both units can operate indefinitely at a 100 percent load level.  For higher power requirements, TDK-Lambda’s industrial 3000W, 84mm high TPS series features the same current share and reverse current protection circuitry as a 1U data-center power supply.

Usually, two dedicated feeds act as input supplies for a data center (for redundancy): That is, a switchgear serves as protection against alternating current (AC) line transients and uninterruptible power supplies serve as backups. Industrial equipment incurs input transients when large neighboring inductive equipment switches on and off. Datacom equipment power supply loads are usually well defined and relatively static, with few severe load changes. Industrial power supplies can drive inductive direct current (DC) motors, relays, and capacitive loads, all of which can stress a power supply.

Industrial power supply designs must withstand large, repetitive input and output surges. The traces on the printed circuit boards (PCBs) have additional space for isolation clearance to accommodate higher voltages, and the input filter is larger to withstand the higher-voltage transients. Zero to 100 percent, pulsed, capacitive and inductive loads on an output can often exceed the output-current rate of a supply for short periods of time, which the over-current protection circuitry of the power supply is the only limitation. To handle these large, peak currents, the semiconductors and electrolytic capacitors must receive parallel ratings. This involves the use of larger, taller components that only come inside a power supply that is taller than 1U.

Technologies in the communications industry are constantly evolving, and this trend includes the 1U high-power-supply architecture. Datacom systems initially required 48V output power supplies but then migrated to 12V to drive non-isolated DC/DC converters. Now data centers are run using high-voltage 380VDC outputs. The vast amount of electricity that these centers consume warrants an increase in efficiency by way of major changes to the power architecture, both in output voltage and size. However, such adjustments can result in early power-supply obsolescence because of high volume production from these products.

Industrial equipment, including the power supply, must be in service for 10 or more years. Having to rework a system when a spare power supply is not available can lead to long downtimes. Unlike the high volume 1U high-power supplies developed for data centers, industrial power supplies are often produced with lower-volume, 15 to 20-year lifecycles in mind.

1U high-power-supply racks are widely used in data centers because they are compact, easy to use, and have a benign operating environment. These power-supply racks, however, are less ideal for industrial environments. Instead, racks containing 2U and 3U high-power supplies can accommodate lower cooling-fan requirements, increased fan life, and redundancy as well as withstand transient power-supply loading needs. Likewise, 2U and 3U high-power supplies, like those from TDK-Lambda, align with industrial product needs including long lifecycles, availability, and maintenance-free operations.