Off-The-Shelf DC-DC Converters Simplify Challenges Of Ruggedized Applications

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Addresses the harsh specifications for DC-DC converters in rail equipment with a wide range of off-the-shelf products

Electronic systems in trains are powered by noisy DC sources. Voltage spikes, surges and drop-outs are common, along with radiated and conducted EMI from high-power transmission systems. Shock, vibration and extreme temperature swings are the norm. The safety of passengers and staff is also of utmost concern, so the flammability of materials used needs careful consideration.

Other markets face similar challenges: industrial mobile systems, such as construction and material-handling machinery and forklift trucks are exposed to harsh outdoors. With no clear standards, they typically turn to existing ruggedized solutions designed and built to railway standards. These are preferable to industrial-grade alternatives that are often simply commercial-grade solutions with support for extended temperature range with an expectation of ‘clean’ supply rails and low shock and vibration levels.

Ruggedized solutions leveraging standards for railway power electronics

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The rail environment, although harsh, has well-defined characteristics. The specification for rail electronics equipment, EN 50155, still allows extreme voltage swings. EN 61373 defines the shock and vibration environment for different train sections. EN 61000-4 deals with EMC. Compliance with British Railway Industries Association Standard RIA 12 is often required when encountering high-energy surges.

EN 50155

EN 50155 covers ruggedized DC-DC converters defining environmental and service conditions, reliability, safety, design and construction, documentation, and testing. Typical industrial-grade electronics might meet these requirements but DC-DC power converters must withstand much wider input voltage variations with several possible nominal ranges (Figure 1):

Fig. 1: DC input ranges for different rail applications
  • Continuous range = 0.7 – 1.25 x VNOM
  • Brownout = 0.6 x VNOM for 100ms
  • Surge = 1.4 x VNOM for one second

It is not practical to ride through brownouts of 100ms, and surges of one second have too much energy to clamp. Power converters must, therefore, operate over the complete range shown in Figure 1 with some safety margin, meaning an input range of more than 2.33:1. Nominal voltages of 48V and 96V are also possible with France and the U.S. having different minimums and maximums. DC-DC converter manufacturers meet these by offering a 4:1 input (typically 43 – 160V).

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Compliance with RIA 12

There may also be contractual requirements for different specifications, such as RIA 12, to withstand immunity to surges up to 1.5 x VNOM for 1s and 3.5 x VNOM for 20ms from a very low source impedance of 0.2Ω. For 110VDC systems this is a peak of 385VDC, which is outside the normal range of a converter, especially if it needs to work down to the 66VDC brownout minimum.

The energy from such a low-impedance source cannot be clamped by a TVS (Transient Voltage Suppressor). A pre-regulator may be required that switches off the input for the duration of the surge. Hold-up in the DC-DC converter is also required to maintain the output. Another requirement may be sustaining fast transients of up to 8.4kV from 100Ω sources with a trapezoidal waveform of 100ns. These can be clamped with a TVS or proprietary filters from DC-DC manufacturers.

EMI and electrostatic discharge requirements

EN-61000-4 is used when testing compliance with the EMC directive for AC power supplies, all of which need appropriate filtering. Industrial DC-DC converters are normally embedded and protected from conducted EMI. Since they normally do not include filtering suitable for rail DC supplies they require external filters.

EN 50121-3-2 includes requirements for conducted common-mode RF up to 80MHz and immunity to electrostatic discharge and electromagnetic fields at the enclosure port level. Conducted emissions follow the basic standard EN 55016-2-1. EN 50155 provides an example of physical EMC areas A, B and C in rail applications (Figure 2).

Fig. 2: Typical positioning of a DC/DC converter between EMC areas in a rail application

Tough railway shock and vibration requirements

Shock and vibration tests for rail applications are defined in EN 61373 specifying different categories of locations with increasing levels of test severity:

  • Category 1, Class A, Body mounted
  • Category 1 Class B, Body mounted
  • Category 2, Bogie Mounted
  • Category 3, Axle Mounted
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Low-mass items in Class B body-mounted equipment must withstand vibration accelerations of 5.72m/s2 in the vertical direction at test frequencies between 5Hz and 150Hz, with longitudinal shocks peaking at 50m/s2 (almost 5g). In the most severe axle-mounted environment equipment must withstand vibrations up to 144m/s2 (nearly 15g) and shocks of more than 100g in any direction. Using a vibration exciter (shaker) coupled with a stroboscope it is possible to evaluate individual components (figure 3) to detect potential mechanical weaknesses during the development process.

Fig. 3: Vibration test setup at the Traco Power Solutions laboratory in Wexford, Ireland

Functional conditions and stress levels are defined for different mounting orientations. At low power, SIP-style mounting of DC-DC converters does not provide sufficient support. Therefore the TMR 3WIR and similar products feature offset mounting pins that provide additional mechanical stability, as does encapsulation (figure 4).

Fig.4: Additional offset mounting pins on the TMR 3WIR offer mechanical stability

Larger devices, such as TEQ 300WIR, combat vibration via spring clamps for the cable connectors (Figure 5).

Fig 5. Combating vibration with spring clamps

Temperature shock, the operating environment and fire safety

Continuous Operating temperatures in rail applications are normally not very severe, with passenger and cab areas at 25 degrees Celsius nominal, rising up to 55 degrees Celsius. Equipment cubicles may be at 70 degrees Celsius. But equipment must survive an extra 15 degrees Celsius over 10 minutes. With its integrated heatsinks, the TEP 150WI and TEQ 300WIR fulfil these requirements (Figure 6).

Figure 6: TEP 150WI like the TEQ 300WIR includes a passive cooling system (heatsink)

At first glance, converters rated -40 degrees Celsius to +85 degrees Celsius seem adequate for most environmental challenges. Thermal shock, however, must be considered for a train entering and leaving a tunnel where a 40 degrees Celsius difference may be encountered at rates of change of 3 degrees Celsius per second. This sudden change may cause condensation. Power converters for such environments require encapsulation or conformal coating against damage from differential expansion rates. Nickel-plating, instead of gold, for the TEN 40WIR series of DC-DC converters, protects against corrosion from condensation.

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Off-the-shelf solutions

Traco Power addresses the harsh specifications for DC-DC converters in rail equipment with a wide range of off-the-shelf products (figure 7).

Figure 7: Products from Traco Power for rail and similar applications ranging from 3 to 300W

All offer a 4:1 input range including 43-160VDC for a 110VDC nominal, as well as 9-36VDC and 18-75VDC for 24 and 48VDC nominals. Packages include the metal-can SIP8 for 3W and 6W with additional mounting tabs for vibration-proofing, 8W in a DIP24 package, and industry-standard 1” x 1” and 2” x 1” types for 10W to 40W. Chassis-mounted versions are available from 20W to 300W. Depending on the variant, the converters include input filtering to EN 55032 Class B and feature ruggedized environmental sealing [2].

Summary

Swiss manufacturer Traco Power is known for its broad range of quality AC-DC power supplies and DC-DC converters. By using their ruggedized DC-DC converters, designers of critical systems for rail and similar industries, such as construction machinery and material handling operating in harsh conditions, can be sure that their power supply challenges are resolved.


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