Meeting requirements for mission critical stock power supplies

The railway industry is continuing to grow at least as fast as it has done in recent years, with brand new infrastructure projects rolling out all over the world. The market for new rolling stock in rail, suburban rail, metro and tram projects is also growing following high levels of investment in Europe, the Americas and Asia, with huge quantities of trains set to be built in Europe alone over the next few years. A considerable increase in passenger numbers is expected. Alongside brand new carriages, there is also a growing trend for adding retrofitted passenger comfort systems such as LED lighting, infotainment and connectivity systems to existing rolling stock. Both of these trends are resulting in increased demand for railway compatible power supplies.

All the electronics in rolling stock is subject to challenging environmental conditions, from high temperatures in confined spaces to the heavy vibration felt by body- or axle-mounted systems. Power supplies are of course no exception. Ordinary industrial or high reliability power supplies cannot be used in this heavily regulated sector; the demands are so challenging and so specific that specialist power supplies designed and built only for railway applications must be used. These power supplies are specially designed to comply with the relevant European or worldwide railway systems standards and, in most cases, traditional brands with long histories, traceability, and qualified production are used by railway equipment manufacturers.

Power supply applications in trains can broadly be split into mission-critical and non-mission critical. Mission-critical power systems power traction subsystems and braking subsystems. These power supplies require the highest possible reliability since there are heavy financial penalties for operators if trains break down, blocking the tracks, and obviously the safety implications of a brake failure are extremely severe. Non-mission-critical applications cover practically everything else on board a train carriage, from the passenger WiFi routers to the air conditioning and security cameras. These non-mission-critical applications can include sensitive electronics with complex power supply needs, such as CPUs and sensitive analogue circuitry.

Perhaps surprisingly, all rolling stock electronic equipment, from braking systems to LED lighting, must meet the same European quality standard, EN50155. Replacing older standards such as the UK’s BRB/RIA standards and the French NF F 01-510 standard, the European EN50155 is actually an umbrella, referring to various other standards where necessary to define the environmental conditions that power supplies must operate in as well as specifying electrical and mechanical performance parameters.

Rail power supplies experience a wide range in input conditions - amongst the most challenging for any industry - including brownout operation, transients and spikes. Power comes from batteries located at the front or rear of the train and travels long distances down cables, which make it susceptible to interference. Since the same batteries are also used to power high power loads such as starter motors, the supply can fluctuate surprisingly large amounts.

EN50155 defines nominal input voltages as 24, 48, 72, 96 and 110V; 110V is the voltage output from the train’s lead acid batteries, and the other figures represent common power bus voltages found in trains (there are railway power supplies on the market that can accept an input voltage range of up to 10:1, wide enough to cover all these nominal values, such as the Bel Power HP series). The continuous input voltage range required is between 0.7x to 1.25x nominal. Short-term fluctuations need to be allowable for a brownout level of 0.6x nominal for 100ms, and a transient level of 1.4x nominal for 1s. In practice, this means equipment should be ready to deal with the full 0.6 to 1.4x input voltage at all times as the regulations strictly prohibit any deviations in the output.

EN50155-compliant power supplies also have to be able to produce an output despite total interruptions of up to 10ms in the input voltage. This is typically achieved using bridging capacitors at the input. These capacitors also perform a smoothing function, helping the power supply to deal with the required DC ripple factor of 15% of nominal input voltage, though a diode is also required for reverse polarity protection, which also smooths the input.

EN50155 is also very strict on EMC, ESD and transient requirements, referring to EN50121-3-2. The latest version of this standard extends its radiated emissions scope up to 6GHz to avoid interfering with new short-range wireless standards in the 5GHz band. ESD discharge levels by direct contact or through the air are defined, as well as the limits for surges and fast transient bursts.

The environmental parts of the standard are just as stringent. There are four operating temperature classes that power supplies can conform to, with -40 to 70°C being the most stringent. They are also expected to withstand an extra 15°C overtemperature condition for ten minutes using only baseplate conduction/convection cooling. Humidity levels and shock and vibration requirements are also covered by referring to EN61373.

Since all power supplies, mission-critical and non-mission-critical, must conform to EN50155, a system designer working on, say, a 100W LED lighting retrofit system is faced with choosing a power supply with similar performance levels, reliability and longevity to the systems intended for powering the braking and control systems. These power supplies are manufactured to last 20 years minimum (MTBF in excess of 1 million hours) and they have a premium price tag to match. For a simple cabin lighting application, this would be overkill to say the least, but until recently rolling stock system designers had no other option.

To meet the needs of non-mission-critical applications like interior LED lighting in railway rolling stock, Bel Power has developed a series of new chassis mount power supplies under its well-known Melcher brand which is considerably more cost effective than a traditional railway power supply, while still fully complying with EN50155. Despite more cost effective construction and the use of components that wouldn’t be found in a traditional railway power supply such as electrolytic capacitors, the RCM series meets every part of EN50155 while maintaining a good DMTBF (demonstrated mean time between failures) of >300k hours.