Versatile DIN-Rail Supplies Fuel Industrial Automation Applications

The muted appearance that typifies DIN-rail mounting industrial equipment often belies the complexity, variety, and versatility of the electronics that lie within. Power supplies are no exception, with Digi-Key currently stocking more than 200 AC-DC and DC-DC models. It follows that designers are bound to be able to specify an off-the-shelf solution to powering DIN-rail equipment, with the attendant advantages that a ready-made module offers compared with designing your own supply – notably in meeting the EMC and safety qualifications that cost so many of us massive amounts of time and frustration.

To accommodate the widest range of equipment and ease installation issues, it is essential to select a mounting system that is standardized. Today’s standards for DIN-rail assemblies have their roots within the Deutsches Institut für Normung (the German national institute for standardization) and the pioneering DIN 46277 series of specifications that lent the name to this generic format. The European standard EN 50022:1977, based upon DIN 46277-3, defines two types of “top hat” rail (also known as O or Omega rails) that equipment snaps onto for mounting purposes. Both versions are 35 mm wide, with one – the most popular version which is most often called TS35 - being 7.5 mm high, while a taller 15 mm variant (TS35D) suits heavier assemblies and/or fewer fixing points. There is also a miniature 15x5.5 mm rail that EN 50045 introduced. Each of these profiles employs a very simple cross-section, but others exist such as the popular G-section profile (TS32) from EN 50035 (usefully, “universal mounting” enclosures are available that fit both TS32 and TS35 rails).

International standards for rail mounts

The international standards IEC 60715:1981 and the European derivative EN 60715:2001 prescribe a set of dimensions for low-voltage control gear and switchgear, and notably include specifications for the most popular rail formats. These comprise the three top hat rails and the G-section rail, as well as a mechanically-stable C-section design that originates from Australia’s AS 2756-1997 standard. Figure 1 shows these examples.

Figure 1: Examples of popular DIN-rail mounting formats.

Mechanical issues apart, the relative current-handling capacity of these rails differs considerably due to the materials that are used to suit the target environment - normally aluminum, copper, or steel, but plastics and polycarbonate may be necessary in corrosive locations - and the rail’s cross-sectional area. This can be a consideration in applications that might need to accommodate large fault currents flowing to safety Earth via the rail, or simply require a solid ground for attached equipment. For instance, a corrosion-resistant steel version of the industry-standard TS35 rail tolerates a short-circuit current of 1.92 kA for 1 second, while a copper version typically used for grounding purposes tolerates three times that level.

It is worth recognizing at this point that the DIN-rail format may compromise the performance of some equipment classes, such as high-accuracy data-acquisition and RF communications systems that require very careful grounding. In any case, the likely proximity of power relays that switch other inductive loads, such as solenoids and motors, requires designers to exercise great care with grounding practices.

Digi-Key offers a range of rails and equipment mounting accessories from suppliers including American Electrical, Omron, Phoenix Contact, TE Connectivity, and Weidmuller that cover the most popular TS35 and TS35D formats.

Considerations for enclosures and accessories

The dimensions of the housings that fit the respective rail formats are obviously equally important and – being a modular concept – must scale in a manner that accommodates commonplace industrial circuitry, from simple relay modules to embedded PCs. Because there are some dimensional differences between the regional and international standards, designers wishing to package their own electronics will be wise to select components from suppliers such as BUD Industries and Phoenix Contact that offer a range of compatible enclosures, terminal blocks, and mounting accessories.

Within a particular series, the depth and height of the enclosure will normally be constant, while the width varies in increments of units whose dimensions vary according to any standard that may apply, such as the popular DIN 43880 format, frequently appearing within IEC/EN 60715-compliant systems. Other considerations for enclosures and any associated parts include the materials that are used in their construction, which must be fire-retardant and are often rated in accordance with UL-94 qualifications. Similarly, the degree of environmental protection that the enclosure provides can be crucial, and this factor is normally rated on the IP scale of relative protection. Any or all of these considerations can equally apply to off-the-shelf assemblies including power supplies.

Reject assumptions when selecting a DIN-rail power supply

Having evolved over many years, today’s variety of DIN-rail-mount power supplies is still surprising. For instance, Digi-Key’s AC-DC stock range addresses output power levels from 5 to 1,000 W, while its DC-DC converter offerings suit applications that require isolated power within the range 24 - 240 W. Yet for busy systems designers who wish to specify a functional assembly to fulfill a seemingly mundane task, it can be tempting to select the solution that most readily presents itself. Despite datasheet promises, experienced designers know that the real-world performances of apparently similar supplies often vary considerably, hence it is essential to evaluate potential solutions within the target system. Because the supply most often represents a potential single point of system failure, it is similarly essential to check mean-time-to-failure qualifications and to ensure adequate environmental derating.

Furthermore, commonplace assumptions may prove non-optimal or even costly, and the de-facto expectation for isolation in a DC-DC converter is a case in point. In an AC-DC converter, it is essential to isolate users who may easily come into contact with live parts, such as terminal blocks from hazardous AC-line voltages and currents. The international standard IEC/EN 60950-1 “Information technology equipment – Safety” applies to virtually all equipment that runs from utility or battery supplies of up to 600 V, and describes a two-layer approach to safeguarding people and property from hazards that include electric shock and fire. The key ingredient is isolation between the primary (AC-line) circuit and the secondary DC circuit that powers the end-user’s equipment. Most often, the AC-DC supply employs double-insulation that – providing the output voltage is ≤ 60 VDC – allows the secondary circuit a SELV (safety extra-low voltage) rating that is safe to the touch. It is nevertheless important to consider a connection to safety Earth that prevents the secondary circuit from floating to arbitrary potentials and prevents the possibility of current flowing through the operator. Be sure to see IEC/EN 60950-1 for guidance.

In a great many DC-DC converter applications, where designers presume the need for similar isolation that the industry has standardized upon at 1500 V, the input negative and output negative terminals connect at some point before returning to safety Earth – galvanically short-circuiting the relatively expensive and inefficient isolation barrier within the converter. The isolation barrier may still have an important role in, for instance, separating ground-return loops from externally-induced transient voltages that may damage the DC-DC converter. Because manufacturers have to meet market expectations and producing multiple lines of similar products is not cost-effective, non-isolated DIN-rail DC-DC supplies are hard to find. However, the key point is that every designer will benefit from closely examining what is truly necessary for the power supply specification, and from viewing this component as the bedrock of any application.

Think beyond traditional power-converter metrics

As ever, the three predominant power-supply metrics - conversion efficiency, electrical performance, and power density - characterize the relative merit of any supply and we will examine some examples later in this article. Yet no one can afford to ignore the imperative of preserving energy, and a sort on the “Efficiency” column in Digi-Key’s power supply listings reveals a large spread of values. For instance, representative AC-DC DIN-rail models vary from around 68 to 94 percent, while CUI chooses to qualify its V-Infinity supplies to Energy Star Level 4 for EPS (external power supplies).

As Figure 2 shows, extracts from the underlying Energy Star for EPS V2.0 specification, which the US Environmental Protection Agency surprisingly terminated on December 31st,2010 but has direct parallels in other regions including Europe - reveal that low-power AC-DC supplies are likely to appear relatively poor in efficiency terms. This is because in any conventional design, a greater proportion of operating current goes into powering the converter’s core. Also notice the tight requirements for no-load power consumption for all supplies up to the 250 W level that the specification targets. For an overview of the regulations that apply to external power supplies, reference 1 links to a training module on Digi-Key’s website.

Figure 2: Extracts from Energy Star® external power supply specification V2.0.

Features that every user routinely expects include comprehensive protection mechanisms, but less well-studied considerations include accommodating the temperature range over which the converter will work satisfactorily. This can often be crucial in industrial applications where large temperature variations are commonplace. A design needs to take into account the power output derating that will almost certainly apply as ambient temperatures rise. Most supplies require significant derating in ambient temperatures above 50 - 70°C, and the rate at which the output power rating falls can be truly dramatic.

Similarly, be sure to account for the latent power dissipation of the converter that will raise the ambient temperature, especially in small form-factor devices or confined spaces. Simply put, if a converter that consumes 100 W is 80 percent efficient, 20 W will be dissipated into the local environment. This is one often-overlooked reason for selecting highly-efficient supplies. While they waste less energy and are therefore self-evidently cheaper to run than less efficient alternatives, the heat that is dissipated during the power-conversion process must be managed, and if its level becomes too high, expensive and energy-hungry cooling systems may be necessary.

Other environmental factors that industrial applications may highlight include the supply’s tolerance to humidity, vibration, and any unusually high levels of chemical contamination. While these terms are application-specific, it is always good to ensure that the supply meets the widest possible range of international safety and EMC standards. If it then becomes necessary to validate the system as a whole, ensuring that each component is properly qualified should obviate the need for any further costly and time-consuming checks on those elements. Also watch for the need for external filtering to meet conducted emissions standards. This area looms large in every power-supply designer’s consciousness, and you may avoid nightmare scenarios by specifying supplies that handle any necessary filtering internally.

Some representative examples

By far, the majority of DIN-rail supplies are AC-DC converters that operate from utility supplies, with many offering universal-input operation from around 90 to 260 VAC at 50 or 60 Hz, as well as equivalent DC voltage levels to allow a single design to service global markets. DIN-rail mounting DC-DC converters are also available. Almost without exception, any such supplies are single-output devices. Digi-Key’s major automation-industry power-supply makers include CR Magnetics, CUI, Delta Product Groups, Emerson Network Power, Omron Industrial Automation, Phoenix Contact, TDK-Lambda, and Weidmuller.

Capable of sourcing just 200 mA at 24 V DC (~5 W), the CRPS24VDC-120/240 from CR Magnetics offers the lowest power output of any DIN-rail AC-DC converter from Digi-Key. These are specialty low-noise supplies (≤10 mV of output ripple) intended to power the company’s sensitive range of analog and digital transducers, notably the DIN-rail mounting voltage and current sensors that Digi-Key stocks as key elements in a wide range of control and automation applications.

A member of CUI’s V-Infinity series, whose clean lines demonstrate attention to product design, the VDRS-10-xx is a representative AC-DC converter that sources up to 10 W of single-output 12, 15, or 24 VDC as indicated by the xx suffix. It accepts input voltages from 88 to 264 VAC from 47 to 63 Hz or DC levels from 127 to 364 VDC, with full output power available from 100 VAC upwards, as indicated in Figure 3. Notice also the rapid drop in output power capability from 50°C upwards, which is by no means unusual. The 81 percent typical efficiency that CUI claims for each model is good for a device of this output power capability.

Figure 3: CUI’s V-Infinity series benefits from sleek packaging design.

Line and load regulation are tight at ±1 percent and the output voltage is adjustable within ±10 percent via a trim pot. A separate “DC OK” output is available to drive external circuitry such as an LED or relay. Output ripple and noise is within 100 to 120 mV peak-to-peak in a 20 MHz bandwidth under full load conditions, and includes a 47 μF bulk capacitor in parallel with a 0.1 μF ceramic at the converter output. Notice these qualifications typify measurement practices for this type of equipment. The converter provides at least 16 msec of output hold-up time at 115 VAC and 32 msec at 230 VAC, allowing it to continue powering through short brown-outs. The overvoltage protection circuitry latches the output off between 115 and 150 percent of nominal output voltage, while the overload and short-circuit protection mechanisms auto-restart when the fault condition is removed.

Capable of withstanding 3,000 VAC between input and output for one minute, CUI’s low-power converter benefits from an array of safety approvals, including the key IEC/EN 60950-1 safety standard, in addition to EMC compliance to EN 55022:2006 and relevant parts of IEC/EN 61000. Despite the supposed prevalence of international standards, several regions continue to favor qualifications from their traditional agencies, notably UL in the United States and TUV in Germany. If these market-specific expectations are important for your sales, be sure to select products that are suitably qualified, as the cost of such an exercise will frighten any project manager. Other models in CUI’s V-Infinity family source power levels of up to 40, 60, and 100 W.

Elsewhere, Delta, Omron, Phoenix Contact, TDK-Lambda, and Weidmuller offer ranges of DIN-mounting supplies that vary hugely in output power capability. For instance, at 1,152 W, Weidmuller’s model 8951380000 from its PRO-M series boasts the highest apparent level courtesy of a power-boost capability that will source 48 A of 24 VDC for 1 minute, with 40 A continuously available. Do not ignore other features that may prove crucial, such as internal surge suppression, carefully-controlled inrush current levels, and the ability to work in parallel with other converters to source very high currents and/or to support redundant operation.

Appreciate application-specific features

With eleven models that start at around 80 W and similarly offer up to 40 A of 24 VDC power, Digi-Key’s recent addition of the Quint series from Phoenix Contact is particularly worth examination. Figure 4 shows the 12 VDC / 20 A model. Primarily intended for 320 to 575 VAC three-phase supplies, the top-of-the-range model 2866802 also accepts input levels from 450 to 800 VDC to continuously source around 1,000 W with >94 percent efficiency “at 400 VAC and nominal values”. As ever, it is important to understand the significance of such qualifications, which for banner efficiency metrics invariably reflect the converter’s optimal operating conditions. While input voltages will be set to nominal values, power-converter designers typically characterize their circuitry to work best at between 50 and 70 percent of full load, as this is traditionally the level that users continuously apply.

Figure 4: Phoenix Contact’s Quint family is a recent addition to Digi-Key’s range of DIN-rail mounting power supplies.

However, any power converter’s efficiency drops drastically as the no-load condition approaches, and few are capable of decent performance much below 10 percent of full load. Keep in mind these points if your load levels experience large variations – for instance, in demand-driven systems that power down circuit blocks to save energy – when you may consider alternative strategies, such as paralleling supplies to accommodate normal loads while making it possible to turn off a supply under light-load conditions and save quiescent current.

With best-case efficiency of 94 percent or better, the Quint converter is impressive and points to circuit design that has not been hampered by its DIN-rail packaging format. Space constraints are one of the largest challenges for power-supply designers, and if, for instance, this Quint product constantly operated at 94 percent efficiency while delivering 1,000 W, it still needs to manage 60 W of dissipation within its 96x130x176 mm package at ambient temperatures of up to 60°C. That is not easy.

Notable features include a wide output voltage setting range of 18 to 29.5 VDC and a power-boost function that allows it to source 45 A continuously at ambient temperatures of up to 40°C, or as much as 215 A for 12 msec to cater for loads that have large inrush currents. Internal surge protection helps assure reliable operation in the presence of the switching transients that frequently accompany industrial applications, and the converter will operate faultlessly in the event of a single AC-line phase failure. Its mean-time-to-failure metric of 500,000 hours, according to IEC 61709, complements a wider-than-normal array of safety and EMC compliance certification, while its ±1 percent regulation and < 40 mV peak-to-peak output ripple assure excellent electrical performance. Furthermore, it is possible to parallel units for greater power capability or to provide redundancy without the need for lossy external OR-ing diodes or MOSFETs.

Reference

1. Worldwide External Power Adapter Efficiency Standards Product Training Module