Power Supplies within Train Subsystems
It is estimated that currently up to a third of the cost of a train is accounted for by the on-board electronic systems, equipment and components. As the demand for improved performance, safety and reliability of railway rolling stock continues, even more applications for electronics are likely to be found.
It is clear that the uninterrupted operation of electronic equipment is crucial to the running of the train and in some cases may be safety-critical. A secure and convenient source of power for the electronics is available from the train’s battery, and this is nearly always the preferred arrangement where the power demand of the equipment is relatively low – typically below 1KW. The battery output voltage will depend on the design of the train, being as low as 24V in the case of DMUs, but more often 72V or 110V.
The actual output voltage delivered by the battery varies widely according to its state of charge and loading. In addition, by the time this voltage has passed along cables and through connectors it will be affected by large surges and transients superimposed on the original supply. These variations are detailed in specification EN50155 which applies to new rolling stock, and RIA12 for pre-1998 stock.
The electronic circuitry used in the train’s systems and equipment will include low-power analogue and digital processors, input and output devices, and power stages requiring a variety of supply voltage levels, commonly three, five, 12 and 24 but sometimes 48 or higher. These voltages must be provided from the train’s battery by a DC-DC converter which can deliver uninterrupted and stable outputs when subjected to the extremes of voltage coming from the battery. Large dips in the battery voltage may require the converter to include energy storage to provide continuous output power to avoid reset or malfunction of the equipment it is supplying.
The DC-DC converter will be subjected to the environmental requirements of the position where it is installed, which can include extremes of temperature, or high vibration. One application required a 400W Powertron converter to be installed under the roof of the car to provide power for low-voltage halogen lights. This area could vary from a low associated with continental winter night times to highs due to direct full summer sun. The use of fans was avoided by providing additional heatsinking, and conduction to the mounting rails.
A more typical application for Powertron converters is in a Passenger Information Display (PID). This will commonly have an array of LEDs as the display and an overall power requirement of typically 100W. However, as the LEDs are switched on and off to control the display, the load on the converter can change very quickly by up to 100%. Powertron had to develop techniques to cope with fast load transients and short-term peak overloads.
The lowest power level required from a converter connected to the train’s battery is around 2W. For this, and up to about 20W, our DC-DC converters are usually encapsulated modules with either pins for PCB mounting or flying leads for panel mount. This gives excellent environmental, vibration and sealing performance. At higher power levels converters may be of open, enclosed, modular or rack mounting design depending on the application or host equipment. Environmental protection can be to IP54 or even IP65 if required.
The most demanding technical challenge in the design of converters for rolling stock equipment is posed by the input voltage variation. For a nominal input from the battery of say 110V, the converter must run from a minimum of 66V to 154V. The converter must survive a variety of transients and surges with increases in this voltage as high as 8.5KV. At the lower voltages, these surges and transients can supply a potentially destructive amount of energy. Powertron has developed an active filter which is incorporated in most of the designs. The transient voltage is allowed to develop across this filter, which is also designed to limit the current.
Large transient voltages will not only appear at the input to the converter, but can also be coupled onto the wiring connected to the output. These transients may appear in series mode between positive and negative output connections, in common mode between outputs and ground, or between the common input supply and output. Again, EN50155 specifies the levels and waveforms which must be applied to ensure that the converter is immune to damage or malfunction. This requirement is met by suitably adapting the PCB layout and heat exchange design and the use of voltage clamping devices.
Standards for electronic equipment on rolling stock are covered by European norms, and these have been accepted as guidance in many countries outside Europe. However, Powertron’s experience is that these requirements are often extended or overlaid with extra demands. This often relates to upper temperature limits where operating temperatures can be as high as 85°C. This requires the use of special components, and may point to the exclusion of some types (e.g. electrolytic capacitors). There are also demands for very high MTBF. This involves reducing the number of, and the stress on, certain components, particularly capacitors and opto-isolators.
There are a number of common features which are required of DC-DC converters for use on railway rolling stock. They must comply with the European norms as a minimum. There is also emphasis on the highest possible electrical efficiency to minimise heat generation. Alove all, they must be reliable, lightweight and compact. Several ranges of standard products have been developed by Powertron, but constraints imposed by a particular system or equipment often mean that some customisation must be considered, very often involving the mechanical or connector interface. For example, a customer in Canada has designed a special aluminium extrusion as a housing for his equipment. The converter has to slide into this same extrusion with a PCB connector which mates with the customer’s board. The converter assembly is pressed against the back of the extrusion to facilitate heat transfer without the use of screws. Examples of electrical customisation have involved unusual or additional output voltage rails, extra wide input voltage range or special monitoring or control of outputs.
The rail industry imposes very individual demands on electronic equipment, and on the DC-DC converters required by all such equipment. The challenge is to develop elegant and cost-effective solutions for an industry which is at the core of modern rail transport.