Powering Smart Rail Systems: DC/DC Converters for Modern Railway Applications

2020. 3. 9
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Around the world, rail networks are expanding, driven by increasing affluence, mobility, tourism, and asset upgrades for more sustainable and environmentally-friendly transport. China and India, in particular, are investing heavily in a market predicted to reach 180 billion euros by 2020 across 1.38 million route-kilometers [1]. Provision of new equipment is lagging demand, however, so there is also a program of upgrades to existing infrastructure and rolling stock to improve efficiency and keep transport moving.

Table of Contents

Smart Asset Utilization & Monitoring in Rail Systems

New and existing rolling stock can be utilised more efficiently with ‘smart’ systems, tracking idle and active stock to enable increased traffic density. Operational condition can also be monitored using Remote Condition Monitoring (RCM) and Condition Based Maintenance (CBM) techniques. These methods identify repair and maintenance needs just when required, saving costs and improving reliability and availability. Condition monitoring can also collect data such as occupancy rates and distance travelled, which can be analysed to achieve further efficiency gains. The rail sector is expected to invest around $27 billion in the next 12 years on this IoT technology [2].

Sensing Remote Condition in Rail Systems

Asset status and condition can be determined by sensors measuring axle count, bearing temperature, supply voltage fluctuations, acoustic noise, shock/vibration, door operation cycles, occupancy, air quality, light levels, and more. While a sensor can be as simple as a thermistor to measure temperature, increasingly intelligence is added, with components like a digital signal processor (DSP), data logger, and wired or wireless interface, possibly using long-range (LoRa) or WLAN radio. Rolling stock might also be tracked via GPS. With the sensor self-contained and communicating wirelessly, upgrades for legacy assets are easier, and in new stock, cabling is reduced and flexibility improved with the possibility of remote updates and customisation.

Powering Sensors with DC/DC Converters

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Fig. 1: A temperature sensor in a rail application
A sensor typically requires only a few watts to operate at low voltage, down-converted from the main system rail using a DC/DC converter. The system rail nominal is often 110VDC but can be as low as 24V. Power requirements may be higher in multi-channel sensor arrangements or when actuators must be driven, sometimes up to 40W.

Individual isolated DC/DC converters are normally employed for each sensor to maintain low output voltage drops and avoid ground loops that could cause EMC issues such as sensor cross-interference. The system supply rail is not ‘clean’. Standard EN 50155-2017 allows a variation of +25%/-30% in normal operation, with dips to 60% and surges to 140% of nominal for 100ms with no ‘deviation of function’ allowed, and 125% to 140% of nominal for one second with performance degradation. To cover these variations, DC/DC converters for 110V systems must typically operate from 43 to 160VDC.

Fast transient overvoltages are also present on the system rail as defined in the EN 61000-4-x series of standards; suppressors and simple LC filters can attenuate these. Complete loss of supply can also occur; EN 50155 defines interruptions in two classes, S1 and S2, with the worst being a 10ms loss of supply from nominal input with no performance degradation. For some equipment, the interruption can be 30ms on supply change-over (Class C2). To cover this, hold-up capacitance external to the converter is normally added after a series isolating diode, which can also provide reverse polarity protection – another requirement of the standard.

An example application is shown in Figure 1 where a fan is activated by a carriage temperature sensor, with temperature and status signalled via a WLAN connection. A RECOM 8W DC/DC converter is used with an input range of 43 to 160V, providing a regulated, isolated 5V power rail for the sensor circuitry. The DC/DC is very compact at just 32 x 20 x 10mm. Extended hold-up and reverse polarity protection are included, along with an optional EMI filter for high immunity to transients and low conducted emissions, although the RECOM DC/DC already meets high compliance levels with rolling stock EMC standard EN50121-3-2.

Compliance with RIA 12 for Railway Power

Specifications for rail DC/DC converters sometimes include compliance with the UK RIA 12 standard, which defines higher energy surges up to 385V for 20ms in 110V systems (Vin, nox x 3.5). However, with a source impedance of just 0.2 ohms, clamping the input with a transient voltage suppressor dissipates excessive power. A solution from RECOM is their RSPxx-168 pre-regulator (Figure 2), which reduces the surge voltage to a maximum within the input range of their DC/DC converters. Three different modules are offered for DC/DC converters with 20W, 150W, or 300W ratings.

RECOM surge protector operation for RIA 12 applications

Fig. 2: RECOM surge protector operation for RIA 12 applications

Conversion at Higher Power Levels

Centralised rolling stock systems and trackside installations also require power converters at higher power levels. ‘Brick’ format DC/DC converters are popular, such as the RPxxx-RW and RPxxxRUW series from RECOM, which include parts operating over a 12:1 input range with nominal values covering all rail standards from 24 to 110VDC, including surges and brownouts. The DIN rail format is also common, with an example being the 120W ruggedised SQ120 part with its 110VDC input, compliant with EN50155. The portfolio also includes rack-mounted AC/DC and DC/DC converters for rolling stock and trackside applications from 8W up to 4kW with DC, single-phase, or three-phase AC inputs. An EN50155-compliant 300W AC/AC inverter is also available with adjustable output frequency between 30 and 600Hz.

The Rail Environment Is Stressful

Standard EN 61373 defines the levels of shock, vibration, temperature, and humidity in the rail environment, dependent on installation category, ranging from most severe on the bogies to more benign areas such as body-mounted enclosures (Category 1, Class B). DC/DC converters for sensors are likely to be installed in this less stressful environment but still often require encapsulation and ruggedisation. Service life expectations are typically 20 years, so products such as those from RECOM are qualified with tests including full performance characterisation, highly accelerated life testing (HALT), high temperature soak, and thermal cycling.

Off-the-Shelf Solutions for Railway Power

RECOM has a wide portfolio of rugged EN50155-compliant DC/DC converter products and complete turnkey solutions for rail applications from low power (8W-240W) modules up to 10kW power supplies, with customisation available. The company has extensive experience in rail applications, offering comprehensive engineering support, detailed environmental compliance reports, and EMC evaluation. Design reference boards are available, including the necessary EMI filtering for EN50121-3-2 compliance for 24-48V or 72-110V DC nominal input voltage converters (R-REF04-RIA12-1 and R-REF04-RIA12-2 respectively).

Off-the-shelf DC/DC converters and railway power supplies qualified to railway standards from RECOM offer a cost-efficient and simple route to provide power for sensors and centralised systems from watts to kilowatts. RECOM parts are available through their distributor partner Arrow Electronics [3].
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