Industrial applications have employed a variety of control voltages over the years, ranging from 440 Vac to 110 Vac to 24 Vdc in more recent years. The move to 24 V was driven by safety concerns as well as the demand for this voltage by programmable logic controllers (PLCs). European industrial companies led the way in migrating to 24 V, so much so that the majority of industrial control applications in Europe are said to rely on 24 V. However, in the United States, industry is following suit and 24 V is now surpassing 110 V as the most popular control voltage in industrial settings.[1]
Over the last decade, DIN rail power supplies have emerged as a popular way of generating the needed 24-V supplies (see the figure). This type of supply takes advantage of the modularity afforded by DIN rail construction. In industrial applications, many electronic control panels have a removable backplate that allows a DIN rail to be mounted to it. The DIN rail serves as an easy mounting platform for a variety of components, such as contactors, relays, optos, terminal blocks and power supplies.
Products such as these are equipped with a mounting bracket that allows them to simply snap onto the rail. Although there are different-sized rails, the most popular is 35 mm. The emergence of DIN rail construction greatly reduced the effort and cost associated with assembling a control panel.
At a minimum, a DIN rail power supply is simply a power supply (typically a switch-mode supply) with a DIN rail clip. However, application requirements dictate that many of these supplies be able to withstand harsh environmental conditions. As a result, many DIN rail supplies feature industrial-grade performance. One benchmark of industrial performance is a supply's ability to operate over the -25°C to +60°C temperature range without derating. These same supplies may operate up to 70°C with a derated power output.
In contrast to other power supply applications, these industrial performance levels are normally achieved without any forced air (fan) cooling. In a typical application, the supply may be placed in a sealed NEMA 12 enclosure. According to Bruce Desmond of Phoenix Contact, “At least 80% of industrial-grade applications do not have cabinets with ventilation.”
The NEMA 12 enclosure is sealed to protect its contents against water, oil and dust. However, the lack of ventilation in the enclosure demands that the power supply mounted inside must be able to survive and operate at the elevated temperatures.
Even in the recent past, designers would accommodate the high temperatures by derating their supplies by 50%, but that imposes penalties in terms of cost and efficiency. The same derating also has been used to obtain a UL508 listing on a control panel when using a power supply without the UL508 listing. However, industrial-grade DIN rail power supplies can eliminate the need for derating by providing the needed performance over temperature along with the UL508 listing.
Product Development Trends
In manufacturing, a loss of power at the wrong time can result in reduced productivity and costly material losses. Many of the features associated with DIN rail supplies are dictated by customer concerns about power quality and reliability.
For instance, many DIN rail supplies, like other switchers, now offer a universal input, which allows operation from an 85-Vac to 264-Vac input. As with other power supply types, a universal input allows one unit to be used in different countries because it accommodates the range of ac line voltages encountered globally. However, a universal input also furthers power quality by maintaining a regulated output even in the presence of some voltage sags.
A feature that predates universal input is automatic switchover. Supplies with this feature sense the input voltage and automatically configure for operation in either the 90-Vac to 130-Vac range or the 180-Vac to 240-Vac range. In some (but not all) cases, these supplies are less costly than those with universal input. Nevertheless, because of the gap in the input voltage range, supplies with automatic switchover are more vulnerable to voltage sags and may lose regulation in cases where a supply with universal input would not.
Many DIN rail supplies with universal input also feature power factor correction (PFC), demand for which has been fueled by European regulatory requirements. Universal input and PFC go hand in hand, since the former is a by-product of the latter. In addition, supplies with universal input may also accommodate high-voltage dc input.
In addition, concerns over power quality have made hold up time a significant feature in DIN rail supplies. With a minimum hold up time of 20 ms, the supply can ride through a missing cycle of 60 Hz, 120-Vac input. Single-cycle disturbances, which may not produce noticeable effects on lighting and other loads, may be sufficient to disrupt a PLC and destroy data.
Furthermore, power quality demands drive the development of self-diagnostic features in DIN rail supplies. At the most basic level, self-diagnostics could take the form of LED status indicators. There are more sophisticated options as well, such as the maintenance forecast monitor offered in Omron's S8VS power supply. By monitoring temperature and runtime, this supply calculates the time expected for internal electrolytic capacitors to dry out. Based on these calculations, the supply displays its remaining operating life (in years), allowing operators to schedule replacement of the supply in a controlled manner.
However, status indicators and displays may not be helpful in applications where the supply is housed in a sealed enclosure. In those cases, a status output on the supply may be more useful. Some DIN rail supplies offer a self-powered transistor output capable of sinking or sourcing up to approximately 100 mA. Another option is a relay output, which can power an external device at currents up to 1 A.
In terms of reliability, MTBF ratings can be useful. But in this case, power supply users must read the fine print behind the rating. Some vendors base their MTBF specifications on a recognizable Mil spec or IEC standard.
Although power quality and reliability issues drive many DIN rail power-supply developments, these supplies are also being shaped by the usual concerns over power-supply performance that shape developments in other areas. For example, there are demands for higher power density; higher efficiency; different output voltage, current and power options; the ability to parallel multiple supplies; and various forms of fault protection. Naturally, power-supply cost influences most if not all product development. Table 1 lists some of the common features and performance specifications associated with existing DIN rail power supplies.
Compliance with Standards
DIN rail supplies are offered with a variety of regulatory approvals to support their use in different operating environments. The presence of these approvals can aid the customer in obtaining the desired approvals on the electronic control panel or other application.
For example, one of the more popular approvals on DIN rail supplies is the UL508 listing. It's possible to obtain a UL508 listing on a control panel without using a UL508-listed power supply. However, doing so could require either derating the supply or resorting to external fusing or other methods to meet UL508 requirements. In the end, populating the control panel with UL508 components may be the simplest approach to getting this listing on the panel. Consequently, UL508 has become a common specification on recently introduced DIN rail supplies.
Another common specification, UL1604, concerns the suitability of electrical equipment for use in hazardous environments. For example, UL1604 Class I, Division 2 requires that the power supply not have any exposed contacts that could ignite hazardous gases. Table 2 shows some of the commonly cited standards for DIN rail supplies.
Beyond Industrial Applications
Because DIN rail power supplies originated and were developed for use in industrial applications, many of the companies that produce these supplies are vendors of industrial automation equipment. Many of these companies are named in the list that appears at the end of this article. Often, the power supplies augment product lines consisting of PLCs, terminal blocks and other control-related DIN rail devices.
Nevertheless, several full-line power-supply vendors have begun offering DIN rail supplies in recent years. Although some of these supplies are aimed at industrial applications, some target other areas, such as telecom, transportation and aerospace, where ease of installation, performance features and cost make the DIN rail supplies attractive.
For example, in telecom applications outside the central office, lower levels of power density may be acceptable. So rather than using a “shoebox” switch-mode supply that demands forced air cooling, the system designer may opt for a larger, convection-cooled DIN rail supply that delivers more power over temperature. The fan-cooled shoebox supply might deliver full load to a 50°C ambient with 75% rated power at 60°C. In contrast, the DIN rail supply on its own could deliver its full power up to 60°C.
In spite of the requirement for convection-cooled operation, newer supplies are pushing efficiency and power density to higher levels. Today's DIN rail supplies can deliver 960 W while achieving about 4 W/in3.
Further details on recently developed DIN rail supplies will be presented in the next edition of “Analog Feedback.”
Thanks to Bob Williams of ABB Entrelec for supplying background on DIN rail-mounted power supplies and control panel construction.
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