Low Power DC/DC Converters in the Railway Environment

2019. 8. 7
Illustration of a modern train with power supplies from RECOM in the foreground
In the complex international railway market, efficient use of rolling stock is key. Sensing and reporting on railway equipment condition, status, and position enable smart decision-making, improving operational efficiency and lowering costs. However, multiple sensors and transmitters need cost-effective, easily integrated power solutions that comply with railway electrical and environmental specifications. This article describes the implementation of DC-DC converters in typical railway monitoring applications and introduces off-the-shelf solutions with the necessary certifications.

Table of contents

Rail networks worldwide are expanding, predicted to reach 1.38 million route kilometers globally by 2020 with a market value of €180 billion [1]. Significant growth is occurring in the Asia-Pacific region, with China and India investing heavily in railway infrastructure. Key growth drivers include increasing affluence, rail tourism, and a shift toward environmentally friendly transportation. The existing railway system is becoming congested, requiring older rolling stock to remain in operation. Failures in this aging infrastructure can cause delays, service disruptions, or derailments. While new lines and rolling stock are being developed, funding challenges and planning delays slow expansion efforts.

Asset Utilization Efficiency is Key

One way to improve network capacity is by maximizing the efficiency of both new and legacy stock through smart technology integration. This includes better tracking of rolling stock and incorporating sensors to monitor asset conditions. Remote Condition Monitoring (RCM) collects operational data, while Condition-Based Maintenance (CBM) analyzes it to predict when maintenance or repairs are needed. These techniques enhance reliability, improve availability, and reduce costs by preventing unnecessary maintenance and avoiding unexpected failures.

Condition monitoring includes status tracking, such as distance traveled, occupancy rates, and performance metrics, supporting efficiency improvements. This aligns with the broader Internet of Things (IoT) trend. According to Cisco [2], the rail sector is projected to spend $30 billion on IoT over the next 12 years.

Remote Condition Monitoring Needs Sensors

Various parameters can be monitored to assess asset status, including axle counters, bearing temperature, vibration, acoustic noise, voltage fluctuations, and door operations. While some sensors use passive components, the trend is toward edge computing, integrating digital signal processors (DSPs), data loggers, and wireless interfaces, such as LoRa or WLAN. GPS tracking on rolling stock may also be included. Self-contained sensors simplify upgrades for older assets while reducing cabling in new builds.

Sensors Need Power

Most railway sensors require only a few watts at low voltage, supplied by a DC-DC converter connected to the main system power rail. This is typically 110VDC, though other nominal values down to 24V are possible (Figure 1). Higher power levels—up to 40W—may be necessary for multi-channel sensors or local actuators. Localized voltage conversion is essential, as centralized distribution would lead to transient noise and ground loop issues. Galvanic isolation in each DC-DC converter prevents interference and cross-talk.

Possible rail supply nominals and their variations

Fig. 1: Possible rail supply nominals and their variations

The railway power supply is far from stable; railway standard EN 50155:2017 states that voltage can fluctuate from +25% to -30%, with dips to 60% and surges up to 140% of nominal. Converters must operate across a wide input range—typically 43–160VDC—to ensure reliability. Figure 1 illustrates standard 4:1 input ranges, with an ideal DC/DC converter covering a 10:1 input range.

Transient Protection and Compliance

A temperature sensor in a rail application
Fig. 2: A temperature sensor in a rail application
DC/DC converters must withstand fast transient overvoltages per EN 61000-4-x standards. LC filters and transient suppressors help mitigate these effects. EN 50155 also defines supply interruptions in three classes: S1, S2, and S3, with 20ms loss tolerance at nominal input requiring external hold-up capacitance and reverse polarity protection.

Figure 2 shows an example application where a carriage temperature sensor causes a fan to operate and signals temperature and status via a wireless connection. Here a very compact (32 x 20 x 10mm) RECOM 8W DC/DC converter with an input range of 43V to 160V provides an isolated, regulated low voltage power rail for the sensor circuitry. Reverse polarity protection and hold up is included. The DC/DC already has high levels of EMC compliance according to EN 50121-3-2, the standard for electromagnetic compatibility for rolling stock, but an additional EMI filter can be included for higher immunity to transients and still lower conducted emissions.

RIA 12 Compliance

In the UK, RIA 12 compliance requires converters to handle higher energy surges, up to 385V for 20ms in 110V systems. High source impedance prevents transient suppression using simple TVS diodes. A pre-regulator is a more effective solution, as shown in Figure 3. RECOM offers 20W, 150W, and 300W modules designed for these conditions.

Surge protector for RIA 12 applications

Fig. 3: Surge protector for RIA 12 applications

Environmental Stress

Railway DC/DC converters endure shock, vibration, extreme temperatures, and humidity, as defined by EN 61373. Most power supplies are installed in Category 1, Class B environments (body-mounted inside enclosures), but additional ruggedization may be necessary. With a 20-year service life expectancy, designs must undergo HALT testing, temperature cycling, and high-temperature soak testing. Since 110VDC is unsafe, reinforced insulation is mandatory to prevent electric shock.

Qualified Off-the-Shelf Solutions are Available

RECOM, along with its Italian acquisition Power Control Systems (PCS), offers rugged EN 50155-compliant DC/DC converters from 8W to 240W modules up to custom 2kW power supplies. A wide input voltage range covers all nominal railway values, including ultra-wide 16–160V inputs.

With extensive railway application experience, RECOM and PCS provide engineering support, EMC evaluation, and compliance reports. PCS specializes in cassettes, cards, and rack-mounted solutions, while RECOM focuses on low-power PCB-mount DC/DC modules. Their reference designs include EMI filtering for EN 50121-3-2 compliance, supporting 24-48V and 72-110V DC inputs (R-REF04-RIA12-1 and R-REF04-RIA12-2 respectively).

Railway-grade DC/DC converters enable cost-efficient IoT integration in existing rolling stock and new builds, enhancing monitoring and telemetry within railway networks.
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