As trains become more sophisticated, the electrical distribution infrastructure is getting more complex. Not only does it need to deliver power to the traction systems used for train propulsion, but also a variety of other systems throughout the train. These include the essential lighting, door opening/closing, HVAC and braking systems, as well as infotainment and passenger communication equipment, as well as USB charging points that will help to make the journey more enjoyable and/or productive for passengers. Consequently, non-traction distribution rails need to be capable of supporting power levels at hundreds of Watts going down to ones meant for just a few Watts.
The voltage levels available on rolling stock vary greatly around the world and, in order to avoid the need to have multiple designs, train manufacturers operating globally have to address this challenge with a flexible approach. Selecting DC/DC converters that are capable of supporting a wide input voltage range will make them adaptable to different train models and regional variations. To ensure power systems are implemented that have minimal energy consumption (and are therefore more environmentally friendly), DC/DC converters that deliver elevated efficiency figures are also vital.
Relevant standards compliance
Rail applications will prove to be a harsh working environment for all electronic systems, including DC/DC converters. Compliance with stringent international standards as well as many regional/operator specific requirements is therefore paramount. In order to meet the isolation requirements that are associated with rail applications, and cope with exposure to potential voltage spikes, the selected DC/DC converters will need to conform to the EN50155 standard.
Another standard that is important in rail applications is EN50121-3-2, which refers to the electro-magnetic compatibility (EMC) requirements. This deals with restricting the amount of radiated and conducted electro-magnetic interference (EMI) that is produced by electronic equipment, as well as ensuring that the equipment is immune to EMI from other nearby sources. As with all standards, it is important to make sure that the DC/DC converter selected complies with the most up to date version, as some older products may not.
Other operational factors
Next, there is the mechanical robustness to consider. Any devices mounted in or on rolling stock will have shocks, vibrations, elevated humidity and extreme temperatures to contend with. DC/DC converters are generally situated within the carriage or the locomotive itself, but in some cases they will be positioned in places where the operating conditions are even more difficult (such as close to the braking systems on a train’s undercarriage). Here greater ruggedness will be needed. There will also be the prospect of thermal shocks (when coming out of a tunnel into a significantly colder external environment, for instance, or vice versa). The EN61373 standard specifically addresses the mechanical robustness aspects within rail applications and EN50155 covers this area too, as well as temperature and humidity requirements. Even with devices that comply with these standards, there will be additional considerations for designers – including how the device is mechanically fixed to the PCB and the form factor (with low profile devices tending to be less susceptible to mechanical issues).
Key system design concerns
When designing for rail applications, there are a number of other areas for designers to consider. Among the most prominent of these are:
- Reliability is of course a very high priority, as the consequences of failure need to be avoided. These could range from a failed HVAC or infotainment system (that is either uncomfortable or inconvenient for passengers) through to a stranded train (with all of the associated costs of disruption and recovery, as well as reputational damage, etc.).
- Efficiency of the power conversion device is equally important. Within high power systems such as the traction drive, efficiency will play an essential role in managing operating expenses, especially as energy costs skyrocket. However, even in lower power systems this will be a major consideration as electronic designs are increasingly compact – the smaller the electronics cabinet on a train, the more revenue-generating passengers can be carried. All waste heat has to be dealt with to allow systems to operate reliably. Heatsinks are simple to use but add engineering effort, bulk and cost – and often involve customization, especially in tight spaces. Fans are effective but they sap efficiency and are generally the least reliable component of a system. They also require the enclosure to be open – and that can allow dust and dirt ingress, which will often lead to premature failure.
- Voltage fluctuations are simply a fact of life in rail applications, with spikes being introduced into supplies by the heavy switching elements (such as traction motors). Additional circuitry to protect the input stages of DC/DC converters is thus required. Ideally, this will be incorporated directly into the DC/DC unit to avoid the need to design external circuitry – not to mention the additional space required. There are also situations where the supply voltage may dip out of specification for a period of time, commonly referred to as a ‘brown out’. The DC/DC must be able to withstand this, in order to avoid an intermittent output which would interrupt services on the train.
- Furthermore, to protect both the DC/DC converter and surrounding circuitry, input under-voltage, short circuit, output over-current, over-voltage and over-temperature protection mechanisms should all be incorporated. Fortunately, many DC/DC converters intended for railway applications have these included.
- One of the challenges in designing for global rail applications is the huge range of different potential supply voltages. While DC/DC converters are available for each of these, it means that there needs to be different versions of a design for use in different countries/regions. While this is manageable, it is inefficient and leads to increased inventory, as well as maintaining multiple drawings, etc. Selecting a wide-range input DC/DC converter will significantly reduce the number of variant designs needed, allowing companies to benefit from greater economies-of-scale.
Although there is increasing electrification of rail networks and a shift away from diesel powered trains, there will be a need for hybrid diesel/electric systems for electric trains when they are travelling through areas where the overhead lines are not yet installed. This will also mean that diesel trains can switch over to electric propulsion when they are arriving at stations – so as to reduce air pollution when close to people. In both cases, 48V battery packs are now starting to be integrated into locomotives to enable this interchanging.
Similarly, some cities are now using electric trams that can run off batteries. This approach avoids having to deploy unsightly overhead infrastructure, as well as the associated maintenance costs and weather-related issues (especially with ice build-up). Top-up charging can be carried out at each stop, allowing trams to run on battery power for longer periods.
There is, in addition, increasing scope for DC/DC converters in trackside applications. These include the back-up supplies needed to run the signalling – providing the necessary redundancy to life-critical functions, where any failure could otherwise have disastrous outcomes. While the requirements for trackside DC/DC units are less stringent than those relating to rolling stock, the hardware involved will still need to operate in harsh environments (with extreme temperatures and vibrations from passing trains).
Sourcing advanced DC/DC converters
Thanks to its strong relationship with RECOM, TME supplies a number of DC/DC converter products that are optimized for modern rail applications. These are qualified in accordance with the standards already outlined.
The 24V and 110VDC input versions of the RECOM 8W-rated RP08-AW have been especially designed for rail-based application scenarios and are certified to EN50155. They offer a wide (4:1) input range, 16kVDC of isolation and efficiency levels of 88%. Output protection (including overload, over-voltage and short circuit) is fitted as standard. The DIP24 package is available in both pinned and SMD case styles and meets military (and rail) standards for thermal shock and vibration tolerance.
For higher power applications, RECOM’s half-brick RPA100H series of DC/DC converters has a 10:1 input voltage range to cover all input voltages from nominal 24VDC up to 110VDC. They provide isolated and regulated 12V, 15V, 24V or 48VDC outputs. These converters have a consistently high efficiency over their entire input voltage range, and come with metal baseplates to permit a wide operating temperature range, spanning from -40°C to +97°C (when baseplate cooled), to be supported. Their cases are fitted with threaded inserts to allow secure mounting to the PCB or bulkhead, giving them greater ability to withstand high shock and vibrational environments.