Why Use DIN Rail Mount Power Supplies?

DIN Rails are metal strips that provide a convenient means for mounting electric and electronic devices in a compact and neat manner. For example, DIN Rails are frequently used for mounting circuit breakers (Fig #4), terminal strips (Fig #3), power supplies (see photo above) and all sorts of industrial control equipment within racks/enclosures or attached to backboards. In this way, any combination of devices can be mounted next to each other to meet the system requirements.

Standard DIN rails are shaped as shown in Figure #1 (end view) and #2 (photo). They typically measure 35mm from edge to edge. The distance from the back to the rail of the front bends can be either 7.5mm or 15mm. These metal DIN Rail strips can be provided in any length to suit the application and multiple rows of rails can be used.

Figure #1 – End view of typical DIN Rail

Figure #2 – DIN Rail with slotted mounting holes


Figure #3- Terminals Strips mounted on DIN Rail

Figure #4- Circuit Breaker mounted on DIN Rail

The use of DIN Rail mounting systems saves installation time since all devices just snap onto the metal rails. A complete system can be quickly put together in an organized configuration that provides high density, flexibility, safety and design time savings. Associated devices can be mounted adjacent to each other, thus reducing the length of interconnect wiring.

The DIN Rail concept is widely used in industrial control, instrumentation and automation applications. Today, even DIN Rail mountable micro-computers are available and being used.
DIN rail mounted AC-DC power supplies provide a convenient means for powering DC operated devices including sensors, transmitters/receivers, analyzers, programmable controllers, motors, actuators, solenoids, relays, etc., to mention a few. Since these power supplies are convection cooled, no cooling fans are needed. Output voltages from these supplies range from 5V up to 56V with power ratings from 7.5W up to 480W. Many of these supplies can be connected in parallel for higher power applications.

In some cases, conventional power supplies can be utilized in DIN Rail systems by means of “DIN Rail Mounting Kits/Adapters”. See Figure #5 below and more details at this web site: http://www.us.tdk-lambda.com/lp/products/ldin-series.htm

Figure #5 - DIN Rail Mounting Adapter Kit for conventional power supplies

Detailed information about TDK-Lambda’s wide range of DIN Rail mount power supplies is available at this web link: http://www.us.tdk-lambda.com/lp/products/finder6.htm

Operating Power Supplies in Series

Although some users are nervous about operating power supplies in series, it is common practice in the industry. The benefit is that voltages greater than 60V can be obtained using off-the-shelf products.

It is possible to connect several power supplies in series, but please read the precautionary notes below:

• Connect back-biased diodes across the power supply terminals as shown below.

Rate these diodes at the same output current as the power supplies.

In the event both power supplies do not turn on at the same time, or if the load becomes a short circuit, then the diodes will protect the power supplies from any applied reverse voltage.

• Do not exceed the output to ground/chassis voltage rating. Inside most power supplies are noise filter capacitors connected from the output to ground. It is possible to exceed the operating voltage of those capacitors, particularly when configuring several units in series.

• Avoid using “fold-back style” current limited power supplies as these may lock up the power supply during initial switch on.

Maximizing the Life of Power Supply Fans

The vast majority of medium to high-power AC-DC power supplies have integral fans that are required to keep their internal components at safe operating temperatures. Since fans are electro-mechanical devices they are subject to wear out faster than any other component in the power supply.

The chart and diagram below illustrate this very well. As can be seen from the chart, if a power supply’s fan is operated with a high exhaust air temperature at perhaps 80°C (176°F) its life expectancy may be a short as 1.5 years. However, by reducing the exhaust air temperature (as measured 2-inches away) to perhaps 40°C (104°F) the fan’s life expectancy may now exceed 5 years.

Obviously the requirements of a specific application may require different operating temperatures. However, whenever possible, lowering the operating temperature of the power supply will increase the life of the fan as well as the components within the supply. Also, by derating a power supply below its maximum power rating will have a direct effect on its internally generated heat and, therefore, its exhausted air temperature, which will extend the life of its fan.

Positioning the power supply so cooler air is drawn in through the power supply from outside of the system will also help.

Also, a power supply fan’s life will naturally be extended if the supply is turned off when not needed. Some of the newer fans are thermally controlled so they turn on and off automatically. There are also variable-speed fans that increase or decrease the fan’s speed depending upon the sensed ambient temperature or the load required of the power supply. These have the advantage of extending the fan’s life as well as reducing the audible noise when the load current is low.

Another important factor for fan life maximization is to keep the area around the power supply (inlet and outlet) as free of dust and dirt as possible. Dust, metal and chemical particulates can sometimes kill a fan quicker than high temperatures.

If a fan starts making squeaking sounds, it’s a good indication that it should be replaced very soon, before it freezes up. Fan replacements should only be done by qualified electronic technicians who are familiar with the high voltages that can exist within power supplies even after the AC power is removed.

Why pay more for a power supply with a longer warranty?

Since all power supplies contain similar electronic components such as capacitors, semiconductors, resistors, transformers, inductors, etc., why pay more for one with a longer warranty period? In today’s cost sensitive world, questions like this come up all the time. It’s easy to get caught up in the idea of buying a power supply with the lowest price rather than its warranty time-span.

It’s interesting to note that over 50% of TDK-Lambda’s standard power supplies that are sold each year carry a five-year or longer warranty. Is it that these customers have lots of money to fritter away on this luxury, or do they realize some hidden benefits?

One of the major cost drivers in power supplies is, not surprisingly, the component costs. For example, all power supplies use electrolytic capacitors, which are available with various capacitance, voltage and operating temperature ratings.

Electrolytic capacitors contain a paste-like electrolyte which will eventually dry out and cause the capacitor to fail. How quickly this process occurs depends heavily upon what materials are used to make these capacitors and how close to their maximum ratings these components are utilized.

Electrolytic capacitors used in industrial-rated power supplies are more costly than those used in light commercial applications, but they are made to last for many, many years without failing. It’s like comparing a professional mechanic’s tools to those sold in variety stores. You get what you pay for when it comes to high quality tools; the same holds true when buying power supplies.

Furthermore, the power supply designer can choose to operate the capacitors at or near their maximum ratings, which will result in a low-cost product, but with a shorter life. Or, if a longer field life is a consideration, the designer will “derate” the capacitors, which means he will make sure the capacitors are running at a lower voltage and operating at temperatures that are well below its maximum. In this way the designer can achieve a much more reliable and longer life design at a somewhat higher cost. The same trade-offs in design are made for the semiconductors, resistors and other components that comprise the power supply.

In addition to the above, the life span of a power supply depends a great deal on the operating environment. In an industrial environment where a manufacturing plant is running multiple shifts, the power supply may be operating 24 hours a day, 360 days a year, with an ambient temperature within the equipment of perhaps +50°C (+122°F) or higher. Compare this to an office or medical environment where the ambient temperature might be typically +30°C (+86°F) and the equipment is running 8 hours/day, 5 days a week. Obviously, in the industrial application a more robust and higher quality power supply would be required to handle the rigors of these applications.

Power supply manufacturers want to avoid paying the high costs associated with repairing a failed unit within its warranty period. Therefore, based on their predicted life calculations and field return data, they set the warranty period such that the power supply will, in the vast majority of cases, not fail within the warranty period. And, they usually ensure that their supplies have a buffer life-time of 6-months to a year or so beyond their warranty period. So, it turns out that the warranty period is a fairly good indicator of how long you can expect the power supply in your equipment to run without failing. If you purchase a low cost commercial power supply with perhaps a one year warranty and install it in your industrial equipment that may carry a 3 year warranty, that would be a big mistake. Your low-cost power supply would quickly lose its cost advantage when it fails prior to your OEM warranty expiring.

So, we now come to the answer of our headline question:
Why pay more for a power supply with a longer warranty?

Answer: Because it’s the most cost effective way for the OEM to avoid premature field failures, trouble calls, unhappy customers, and high field service/product repair costs.

HWS Series power supplies from TDK-Lambda come with a Limited Lifetime Warranty -- an industry first

Power Supply Considerations for Industrial Applications

Although power supplies are among the most important components of any industrial application, they seldom receive any significant attention. Engineers often do not fully understand all of the variables that go into choosing the correct power supply, and may select a product that is insufficient or more costly than what is needed.

When considering a power supply for an industrial application, it's helpful if a designer has an understanding of the steady state output parameters of the product, as well as the electrical and physical environment that the equipment will operate in. Here are some critical considerations.

Unique Load Requirements
Motors, solenoids and relay controls require higher levels of current when they are turned on than they do for continuous operation. It is necessary for the designer to examine the magnitude of the pulses and either specify a power supply that is capable of providing the surge currents continuously, or use a product that can provide peak power for a limited time. Certain models, for example, can deliver up to 200% of the nominal rated current for up to 30 seconds. This enables the user to purchase a 240W unit to meet a 480W surge load, saving both money and space.

The designer should also anticipate potential mechanical failure of factory equipment. If a motor stalls or a relay "sticks", the current draw can rise dramatically. Using a power supply that is capable of protecting itself in overload conditions will both protect the unit and the system.

Input Line Disturbances
In most industrial environments the AC line is far from clean. This is because the same line that feeds a power supply is also being used to drive larger equipment. Large disturbances such as power sags and surges are commonplace.

High spikes on the AC line can damage a power supply in a similar way that ESD can damage semiconductors. On the surface, the unit can pass bench testing but long-term damage may occur to capacitors and power semiconductors, which leads to failure after just a few months of operation in the field. Industrial power supplies should meet EN61000-4 standards for immunity to line transients, and for extremely dirty AC line conditions the designer should consider using an external AC line EMI filter with high voltage pulse attenuation specs.

To prevent loss of DC power during sags, which is typical when a large piece of equipment is switched on that is in close proximity to our designer's system, it will help to specify a power supply that has a wide AC input range. If the AC line is 208VAC nominal, and sags down to 140VAC occur, utilizing a product that has an input range of 85 – 264VAC will allow DC power to be supplied without interruption. Even a short dip in the DC output can cause microcontrollers to reset and the host equipment to run through a reboot sequence.

Mounting Considerations
Most power supplies typically use electrolytic capacitors for filtering and energy storage. The higher the operating temperature of these capacitors, the shorter the life. As these parts age, the output ripple of the power supply increases, causing functional problems with the load equipment.

When mounting the power supply, ensure that adequate space is provided around the product to allow air to circulate. Do not block off heatsink fins with mounting brackets, restrict air inlet or exit from fan cooled units (1.5 to 2" clear space is a good rule of thumb), or mount the supply in a plane other than its standard-mounting orientations without consulting the installation manual.

In the event that other fans are in the enclosure, take note of the general system airflow direction, and be aware of any potential backpressure issues that may occur.

Operating Temperature and Life Effects
In addition to mounting considerations, the operating ambient temperature also plays a key part in the life of the power supply. The life of an electrolytic capacitor doubles for every 10°C reduction in temperature. The designer should be aware of the derating characteristics of the proposed power supply. Most AC/DC power supplies start to derate from 40°C or 50°C, and can only operate at 50% of its rated load at 70°C.

The derating calculations may indicate that a higher power unit is needed. Using a manufacturer with a broad base of products and a large number of models within a series will simplify this choice.

As a note, the ambient temperature is specified at the inlet of the fan or close proximity to the power supply. Designers should take into account any internal temperature rises in their system when considering potential derating.

To make an "apples-to-apples" comparison on competing products, also consider the warranty of the power supply. A product with a five-year warranty will have greater component deratings and higher quality components (use of 105°C rather than 85°C rated capacitors) for a longer field life than a product with a one-year warranty.

Operating Environment
Vibration and shock will also heavily influence the life of a power supply. A more rugged power supply will meet more stringent MIL-STD specifications. When considering the specifications, remember that how the power supply is mounted can cause mechanical resonance in the system. When the entire system is subjected to shock and vibration, a power shelf containing one or two supplies may start to vibrate at amplitudes greater than the system itself.

Think Ahead
While it is true that the power supply is only a small fraction of the size, complexity, and cost of industrial equipment, it is a key component that can have a disproportionate impact when the role in the system is not carefully considered. Because of the power supply's high unit cost compared to other electrical and electronic components, it is often targeted as an item for cost reduction. In the world of power supplies, you truly get what you pay for. Bargain-priced power supplies are not a bargain when the costs of field-failures, customer complaints, warranty repairs and potential damage to your company’s brand name are included in the equation.

Designers who consider their power applications carefully and early in the project are more likely to see their project go more smoothly, faster and most importantly protect their company's name and reputation with greater field reliability.

What Size Fan Do I Need?

There are many AC-DC power supplies and DC-DC converters with output power ratings that can vary dependent upon the type of air cooling provided. “Convection air cooling” usually refers to situations where a power supply or converter is cooled by the prevailing ambient air temperature, adjacent to the power device, without forced-air-flow from fans or blowers. If the power device has two output power ratings, the “convection cooled” (still-air) power rating is lower than the “forced-air convection cooled” rating.

The power supply pictured above is an open frame switchmode supply with two output power ratings. For “convection cooled” applications, this supply can provide up to 151W of output power. However, with “forced-air-cooling” it can provide up to 201W of output power. The datasheet for this power supply indicates that for “forced-air-cooled” applications, 1.5 m/s (Meters per Second) must be provided by the user. 1.5 m/s equals 295 LFM (Linear Feet per Minute). Refer to conversion factors shown below.

Most fans are rated in CFM or Cubic Feet per Minute of air “Volume” flow. So, what size fan do you need to provide 295 LFM of air “Velocity” flow for the above application?

Most times the power supply is cooled by directing the air flow along its longest dimension; for example, from the input connector end to the output connector end. However, always read the power supply’s instruction manual to determine the manufacturer’s recommended axis for the cooling air-flow. The usual method for determining the required fan size is to first determine the height and width for the opening or port through which the air will flow around and through the power supply. In this instance the power supply is 3.15” wide and 1.46” high (and 8.2” long). We can consider the supply’s width times its height as the minimum area of the inlet port for forced air cooling of the supply. Then, we need to convert these dimensions from inches to feet by dividing by 12”. 3.15” = 0.26’ and 1.46” = 0.12’. So, the minimum “Area” of the port through which the air must flow to cool the power supply is 0.26’ x 0.12’ = 0.0312 square feet. The formula for determining the CFM (volume) rating of the fan when the required LFM (velocity) is known is as follows:

CFM = LFM x Area (in square feet)

Therefore, in this example:

CFM = 295 LFM x 0.0312 ft² = 9.2 CFM (min. fan rating)

Fans are rated in CFM based upon the expected free-flow of the air coming from them, without obstructions, which cause back-pressure. Of course, real world applications always include some obstructions. To ensure the least amount of back-pressure, it is best to have the exit ports in the enclosure about 1.5 times the area of the minimum entry port. In most applications there are other heat loads and components that can obstruct the path or free flow of the cooling air. It is therefore wise to select a fan with a higher rating than is calculated. Perhaps a 10 CFM or larger fan should be used in this application.

Tip: The use of a larger fan running at a slower speed can deliver the same airflow as a smaller fan running at a higher speed, but the larger fan will be much quieter.

Since most fans have round air outlets and square mounting patterns, the air-flow from the fan may require ducting within the end-product’s enclosure to direct the cooling air to the high power devices including the power supply.

The same process would be used to determine the correct fan rating for AC-DC power modules or DC-DC converters, with or without heatsinks that require forced-air-cooling. When heatsinks are used (see photo below), always direct the air flow in the same direction as the slots between the fins of the heatsink.
In all situations, the system must be tested with the selected fan and all other devices in-place to confirm that the power supply or converter and the load it drives do not exceed their maximum operating temperature, under worst case conditions (maximum ambient inlet air temperature, 100% power load, etc.). If problems are observed, a higher CFM rated fan or dual fans may be required.

In the metric world, fans are sometimes rated in “m³/hr” (Cubic Meters per Hour) and the air velocity is rated in “m/s” (Meters per Second). The following Metric to English conversion factors may be useful.

1 m³/hr = 36 ft³/hr ÷ 60 min. = 0.60 CFM (cubic feet per minute)
1 m/s = 3.28 ft/sec x 60 sec = 196.85 LFM (linear feet per minute)

Some fans and power supplies have dimensions in mm (millimeters).
Just remember that 1 inch = 25.4 mm, and 1 mm = 0.04”

There are a number of very good online calculators to assist you in determining the fan size and ratings required for various forced-air-cooling applications. Here are a few of those websites:

http://www.airperformancetech.com/conversion-tools.htm
http://www.aavidthermalloy.com/technical/airflow.shtml
http://www.calculatoredge.com/optical%20engg/air%20flow.htm