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Power supplies and the rail industry

Power supplies are one of the single most expensive components in an electronics assembly yet they are often selected against vague specifications or with little evaluation. It is essential for the potential user to develop a thorough critique for the assessment of potential products. This article sets out some of the key criteria.

1) Power density -

Consider very carefully if the manufacturer’s claims for power density are realistic given the overall size, mechanical construction and environment into which the power supply is going. Since reliability is highly dependent on temperature, factors such as derating, heat sinking, airflow requirements and specified temperature ratings need to be carefully considered. Consider very carefully the number of thermal barriers between dissipating components and the environment.

2) Reliability

While much can be determined from the product on offer, a considerable amount more can be done to cross check relia¬bility potential. Where possible, establish the manufacturer’s record on returns and what is judged to be an acceptable failure rate. Also, determine the manufacturer’s development and production practices (eg type testing, perfor¬mance checks, failure mode and effects analysis (FMEA), burn-in programme, component screening, component evaluation).

A copy of any mean time between failure (MTBF ) calculations showing parts stress analysis according to MIL HDBK 217F (or similar) should also be considered, alongside actual field reliability studies to determine the real MTBF of the product. The latter should include enough detailed information to confirm that any assumptions made in determining the MTBF figure are valid.

3) Specifications

The manufacturer should have detailed data sheets and supporting documentation for the power supply product under consideration. While quantity does not necessarily mean quality, the actual amount of data provided may be a good indication of the level of testing that the manufacturer has performed before the product is made available to the market. If the amount of information provided is limited or omits key performance criteria, these should be thoroughly investigated before proceeding. Also, while specifications are important, they should not be allowed to drive the evaluation. It is very common for evaluations to centre on manufacturers’ data sheets rather than what is required for a given application. It is good practice to ask the manufacturer to demonstrate the performance of their products and allow plenty of time to cover all points.

4) Behind the Specification

For all railway applications, meeting the requirements of EN50155, 50121 (in the case of older rolling stock RIA12 / 13)is a given, but there are certain other factors, not necessarily listed in the product specification, which can have a major impact on the suitability of the product. Below are a few factors worth checking about your chosen supplier:

a) burn-in – find out more about their capabilities. Are products subjected to elevated temperatures, varying load conditions and continuous monitoring?

b) feedback – optical feedback circuits suffer from ageing effects and hence lower reliability figures when compared to magnetic feedback

c) mechanical suitability – look beyond the test data and examine the internal construction of the power supply. Are key components thoroughly secured and are units protected against high humidity and conductive particulates?

Selecting a power supply for any application is one of the key factors in establishing a reliable design for the product overall. This is even more critical given the harsh environments seen on most rolling stock. It is therefore essential that the technical sales staff have a thorough understanding of the requirements on the industry. John Stone and Jonathan Smith have worked both for, and alongside Power-one / Melcher for the past 17 years. Now with Relec Electronics, one of the UK’s fastest growing power supply specialists, they are able to give unparalleled advice on a wide range of power converters for both trainborne and trackside applications.

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