Designing and maintaining electronic systems used in the rail environment is never going to be easy and yet trains are increasingly using electronic systems to enhance safety, performance and the passenger experience. CCTV systems provide additional security for staff and passengers, touch-screen displays deliver more comprehensive information to the driver, and Wi-Fi backbones and USB charging sockets help passengers to stay connected throughout their journey.
All of these systems, and the components inside them, have a common challenge: they must offer reliable operation despite the presence of spikes and transients, contaminants such as dust and dirt, and the mechanical stresses caused by shock and vibration.
However, the majority of companies in the electronics industry are focussed on the high-volume, fast-changing world of consumer electronics and not the low volumes and long lifetimes of the rail industry. This leaves a few specialist component manufacturers and distributors who have consistently focussed their resources on providing rugged components and have the technical expertise to support legacy and next-generation electronics systems for the rail industry.
On-board power supplies
All of the electronic systems installed on rolling stock must draw power from the sources that are available on the train. The difficulty is that each of these systems may need to operate on a different voltage level which typically means using an external power supply. These power- supply modules are used to convert the input down to a lower DC voltage or, in some cases, to invert the voltage to an AC mains supply.
Whilst approval to the relevant rail safety standards provides baseline assurance that a converter meets the appropriate performance levels, other features can help to increase overall system performance.
For example, the PCMD/S series of converters from MTM Power are approved to EN50155 for temperature, humidity, shock and vibration and to EN45545 for fire and toxicity. In addition, they offer a wide input range which enables them to handle brown-outs and transients. This helps to ensure a smooth, regulated supply without the need for any additional suppression components. Built-in safety features, such as active current-limiting, eliminate the need for ground-loading whilst protecting the system and the converter from damage caused by spikes and surges. The converters also eliminate potential hotspots by using a patented thermo-selective vacuum encapsulation process. This provides an even thermal path for heat dissipation via a metal baseplate or a finned heat-sink.
Whilst the PCMD and PCMDS would be suitable for low- to mid-power systems rated up to 600W, MTM Power’s MPG series can power systems up to 2000W and includes units rated to IP65 under-body mounting.
Discrete components for power supply designs
For engineers developing a power supply from discrete components, making the right component choices is extremely important. This will help to ensure that the system will be able to withstand the rigors of the railway environment, meet lifetime expectations and avoid costly delays or resource availability issues for the operator.
A standard off-the-shelf inductor or transformer may meet the specifications for EMC filtering, main conversion transformers or output inductors in a switched-mode power supply or linear transformer, but a custom device will sometimes provide a better fit for the design. Custom inductors and transformers for AC-DC and DC-DC converter topologies can be based on standard cores or on Microspire’s innovative SESI or CCM series. These are designed specifically for environments where size, shock, vibration and temperature are critical issues.
Selecting the right capacitor is also key. The voltage, current-handling capability and temperature rating are important criteria whether the capacitor is used as a
filter or for bulk-energy storage. Aluminium electrolytic capacitors, such as Sic-Safco’s FELSIC range, are designed to withstand high vibration and shock. This series includes the high-capacitance CAPAX series, with values up to 2.2 Farads, or the I-PULS family which offers high repetitive peak current of up to 7,700A at 40oC, and 3,200A at 85oC, not exceeding 80Arms.
In space-constrained or high-temperature applications with a voltage rating of 150V or less, a tantalum capacitor may be a better choice. One of the highest energy densities available in a wet tantalum capacitor is achieved by Firadec’s WT83 series. This provides 10,000 microFarads in a standard D case measuring 34mm in length with a 10.1mm diameter. For high-temperature applications the CT79-HT200 series, also from Firadec, is rated for up to 200oC.
Passives for signal conditioning
To protect against overvoltage spikes, it is sometimes necessary to dump excess power into a resistive load. Many of these circuits will typically use a power resistor such as the axial, through-hole W series from TT Electronics, Welwyn. Although these vitreous-enamelled glass wire-wound resistors are not new products,
they have earned the reputation as a ‘fit and forget’ component. Rated at up to 14W and 750V limiting element voltage they combine high reliability with the capability to dissipate surges in rolling stock or trackside signalling equipment.
For dissipating energy from short-term over-voltage conditions, the 1600 and 1900 series provide a larger tubular package with terminal options such as pigtails or screw tabs for secure connection. Rated at up to 208W, with a 3kV limiting element voltage, the hollow design allows these resistors to be mounted vertically to dissipate energy through convection cooling. This feature can also be used to provide an active source of heat within trackside equipment, to remove condensation and prevent icing.
Pulse transformer or opto-isolator
In high-current traction applications pulse transformers can be used to provide isolation in the power semiconductor control circuit. Replacing the conventional choice of a GTO thyristor with a modern IGBT power switch can provide switching times which are up to four times faster. Of course, the control-circuit then needs to be upgraded to support the different circuit drive characteristics. Pulse transformers can be ordered as standard off-the-shelf devices or manufactured to meet a custom specification based on standard cores or Microspire’s own SESI or CCM technology.
As an alternative to pulse transformers, opto-isolators can be used to control the switching of the power device. Whilst the pulse transformer may offer a lower unit cost, the overall circuit design and performance of a pulse transformer does not match that of an opto-isolator. Problems with waveform fidelity are avoided by using a high-speed opto-isolator, which typically also eliminates the need for additional compensation circuitry. This provides a lower Bill of Materials (BoM) and therefore can provide a lower total solution cost.
An opto-isolator such as the new OPI1268S from TT Electronics Optek combines 20kV isolation and 30kV/ micro-second dv/dt immunity with a fast 2Mbit/second transfer rate. In addition to being sealed to IP65 to protect against contamination by dirt or water, the OPI1268S is also approved for use in explosive environments.
For higher-voltage isolation there is a choice of devices and packaging options which includes the OPI155 with 50kV isolation and a signal rate of over 5MBd.
Circuit protection & HVAC
Circuit breakers and thermostats are used throughout rail electronics to provide vital protection for components and systems. Approval to EN45545 will ensure the safety of the materials in which the components are manufactured but, again, additional features can significantly improve overall system performance.
With the amount of electronic systems being added to rolling stock, size is a key factor in the selection of a circuit breaker. Other factors which will influence the decision will include the number of circuits to be protected, current ratings, type of load, trip response time, and the need for auxiliary contacts or shunt trips. A choice of mounting options can provide the flexibility to save a few additional millimetres which can be useful in rolling stock applications. Sensata’s Airpax range of circuit breakers features rail-approved breakers for front-panel mounting, with the IAL, IAG and JAE series, and for DIN-rail mounting with the IELR series.
Space is also a factor for the thermostats and pressure sensors used in the Heating, Ventilation and Air- Conditioning (HVAC) systems used in rolling stock. The capability to handle large currents up to 48A enables Sensata’s Klixon 3/4 inch bi-metal disc thermostats to eliminate the need for additional contactors, saving both space and cost. The product range also includes pressure sensors which offer improved control of HVAC systems compared to pressure switches.
With many trains now remaining in service for much longer than the original lifespan forecasts, reliability and efficient Maintenance, Repair and Overhaul (MRO) procedures are becoming increasingly important.
It is always worth checking for a manufacturer warranty before specifying a component, in addition to considering the testing and burn-in procedures which can ensure higher reliability. Some power converters also offer maintenance-free operation which can make a significant contribution to reducing MRO costs over the lifetime of the converter.
The combination of ‘made for rail’ components and specialist technical support for design-in, maintenance, repair and overhaul can help engineers to overcome many of the challenges of using sensitive electronic components in the harsh rail environment.
Thanks to Chris Leek, Jeff Gurr and Roger Tall of Charcroft Electronics for their help in preparing this article.