And the next train stop is...

Climb aboard and take a look at the EN50155 power supply's specifications and some of the design techniques used to meet these requirements.

Travelling by train today is like flying very close to the ground. More electronic equipment is being used than ever before, making our journey more pleasant and keeping us better informed. Perhaps some of the most frequently used information is the announcement of the next train stop. Although broadcasting this ubiquitous audio information may seem like a routine task, the equipment that provides the power for this application must function in an environment that's anything but benign. It's also got to meet the rigorous standards defined in EN50155, titled "Railway Applications—Electronic Equipment Used On Rolling Stock."

Electrical loads that are being powered in the real-world light-rail-system application described in this article contrast the remainder of the operating environment. That's because they are relatively benign. The 20 public-address speakers being powered each require 500mA, including inrush, for a total of 10A. To provide N+1 redundancy, two 10A power supplies (Power-One part number EWN2660-6) are configured for parallel operation (Fig. 1).

The operational goal of EN50155 is for electronics in rail cars to operate 24 hours a day for 20 years, or approximately 175,000 hours (Fig. 2). This is accomplished by defining requirements for practically every environmental aspect, including input-voltage fluctuations and transients, ambient temperature ranges, shock and vibration, as well as fog and salt spray.

The following is a summary of the EN50155 specifications that most affect power supplies, and some of the design techniques that were used to meet these requirements. Let's start with a discussion of general requirements for all electronics equipment.

• Humidity: EN50155 gives general guidelines, and they are an average of 75% relative humidity, with 30 consecutive days at 95% relative humidity. For most electronic assemblies, this means either a waterproof enclosure or conformal-coated PCB assemblies.

• Ambient temperatures: Ambient temperature ranges in EN50155 vary from -25/+40°C to -40/+85°C. There's also a 3°C/second thermal shock requirement for trains entering or leaving tunnels. As a result of these temperature extremes, thermal ramp rates, and humidity, components and PCBs must also withstand the condensation that often accompanies recovery from sub-zero temperatures.

• Atmospheric pollutants: Depending on the equipment location, there could be airborne pollution such as conductive dust, oil mist, salt spray, and/or sulphur dioxide. Conformal coatings and enclosures with IP ratings help to control the negative effects of these pollutants. The EWN2660-6 product used in this application utilises double conformal-coated PCBs and IP20 enclosures.

• Cooling: In general, forced-air cooling should be avoided because of potential maintenance issues related to fans and the possibility of ingesting pollutants into the power supply. Conduction-only-cooling operation necessitates using high-efficiency, power-conversion topologies to reduce internal heat generation. In addition, advanced thermal-management techniques are employed to improve heat dissipation.

• EMC: The rolling-stock environment is laden with a diverse array of potential EMC issues as addressed in EN50155. The ability to withstand transient bursts is called up from EN50121-3-2 and EN61000-4-4. Radio-frequency susceptibility is referred to EN50121-3-2. Radio-frequency emissions are also from EN50121-3-2.

• Shock and vibration: EN50155 utilises the test methods and limits that are defined in EN61373. Although the application being described utilised a DIN-Rail mounting bracket, steel chassis-mount brackets are also available for high-vibration environments. The inherent 50g shock rating of Power-One's EWN2660-6 drops to 1g when DIN-Rail mounted with no additional support.

The need to meet all of the general specifications described above and the power-specific requirements summarised below rules out using most commercially available power supplies on trains. These stringent requirements also preclude the adoption of prior designs; power supplies must be designed from the ground up with EN50155 in mind. The requirements are:

• Input voltage range: For a nominal 110Vdc system, the input voltage can vary from 77 to 137.5Vdc without any time limit. The input voltage can also vary to the extreme values of 66 to 154Vdc for a period of 0.1 second. This unique input-range requirement forces the power supply to be designed specifically for railway applications.

• Input voltage ripple: The dc input voltage may originate from a single or three-phase generator and be relatively unsmoothed. It may contain a significant ripple, up to 15% of the average value, which must not resonate within the power-supply input filters. In addition, the power-supply control loops should be fast enough to regulate out the input variations, so that they're not present at the output.

• Input surges: EN50155 defines surge voltage, time, and source-impedance relationships with higher voltages having shorter durations, and higher source impedances. For example, the 110V, 1.4X surge test (154Vdc) lasts for 1 second and has a source impedance of 1Ω. The 1800V surge lasts for 50 µs and has a source impedance of 100Ω. The 8400V surge has the same 100Ω source impedance, with only a 100ns duration. Typically, these surges are met by multi-stage filtering and nonlinear components at the input stage of the power supply.

Although not specified by EN50155, ease of installation and maintenance were also important application considerations. Front-mounted cage-clamp terminals with screw terminals facilitated the use of the customer's pre-assembled harnesses. Extremely low inrush currents reduce nuisance maintenance calls for tripped breakers and fuses. Reliability was also a key concern as the selected product was based on a design with a ground-mobile 50°C MTBF of 68,000 hours, as calculated in accordance with MIL-HDBK-217E, notice 2.

As is the case with most other types of high-availability infrastructure equipment, railway public-address power systems require 24/7 uptime in rugged environments. And they're supported by an N+1 redundant architecture. The next time that you hear routine destination information, or potentially life-saving emergency instructions, being announced, think of them as being broadcasted courtesy of EN50155.

Steve Underwood is director of Technical Sales-Europe, Power-One Inc.

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