Power Supplies for Railway Data Communications

Due to the variety of these use cases, railway networks use multiple wired and wireless standards. Wired standards include CAN bus—developed for networks within automobiles—and derivations of industrial fieldbus standards; for instance, ProfiNet and Industrial Ethernet (IE) networks (e.g., Cat 5e, Cat 6, and Cat 7) are becoming common, and Power over Ethernet (PoE) technology is increasingly being adopted.

Wireless standards include commercial cellular systems (3G/4G/LTE/5G) or custom systems such as the GSM-R standard used for the European Rail Traffic Management System (ERTMS). For medium and short-range communication, low-power radios such as LoRaWAN or WiFi (IEEE802.11a/b/g/n/ac) are commonly employed.

These communication networks play a critical role in the operation of a train and the quality of the passengers’ experience. As rail equipment has a typical 20-year service life, communication systems must be highly reliable and easily updateable.

However, this is a major challenge. The railway environment is harsh, with extreme temperature variations (-40°C to +85°C), long-term shock and vibration, and exposure to uncontrolled EMC environments. Thus, power supplies intended for railway data communication systems must meet strict performance standards such as the following:

• EN 50155 Railways Applications—Electronic equipment used on rolling stock
• EN 61373 Rolling Stock Equipment—Shock and vibration tests
• EN 50121-3-2 Electromagnetic Compatibility—Rolling stock—Apparatus
• EN 50124-1 Isolation Coordination—Rolling stock—Apparatus
• EN 50125-1 Ambient Conditions—Rolling stock—Apparatus
• EN 45545-2 Fire Protection on Railway Vehicles

For example, standard EN 61373 defines the levels of shock, vibration, and temperature in the rail environment, depending on the installation category. These levels range from most severe on the bogies to more benign in areas such as body-mounted inside enclosures (Category 1, Class B). Standard EN 50125-1 covers high-temperature soak, salt mist, and thermal cycling. This standard conforms to highly accelerated life testing (HALT), a combination of shock/vibration and ambient temperature (not mandatory).

To meet these demanding standards, products from RECOM are qualified with tests including full performance characterization with in-house vibration test equipment.

DC/DC converters are a critical part of the communication system’s power distribution system. Railway rolling stock employs a DC power distribution system so that batteries can be used to maintain electrical power in the event of a generator or pantograph failure.

The latest version of EN 50155 defines seven different standardized DC voltages, although 110 VDC is the most common on passenger trains. Nonetheless, the 24V through 72VDC voltages are still widely used, particularly in light railways, trams, and trolleybuses. To meet these requirements for nominal input voltages, railway application developers opt for 4:1 converters such as the RP20-FR and RP40-FR series from RECOM, as they cover the entire nominal input voltage range with three different converter models only.

The railway DC supply, however, is far from a clean voltage source. Standard EN 50155 allows a variation of +25%/-30% in normal operations, with dips to 60% for 100ms with no “deviation of function” allowed and 125% to 140% of nominal for one second without performance degradation. To cover these variations, DC/DC converters for 110V systems must typically operate from 66 to 154VDC. Furthermore, fast transient over-voltages are also present on the system rail as defined in the EN 61000-4-x series of standards, requiring the power supply to handle additional voltages of up to 160 to 200V.

Additionally, complete loss of supply can also occur; in this regard, EN 50155 defines interruptions in the two following classes: the common 10ms (S2) and the worst case of 20ms input short circuit with no degradation in performance (S3). For some equipment, the interruption can be 30 ms during supply change-over (Class C2). Specifications for rail DC/DC converters sometimes include compliance with the UK RIA 12 standard, which defines higher energy surges up to 385 V for 20 ms in 110 V systems.

A power supply for data communications can range from a low-power DC/DC converter supplying a module that contains both sensors and a WLAN node to a high-power trackside installation that must handle the needs of a depot rolling stock system. A low-power WLAN system as described above can be supplied by a DC/DC converter such as the RP08-AW series, which can supply regulated power of up to 8 W. This EN 50155-certified family features a 4:1 input voltage range and 1.6kVDC isolation. Furthermore, it demonstrates an efficiency of up to 88%, plus overload protection (OLP), over-voltage protection (OVP), and short-circuit protection (SCP).

“Brick” format DC/DC converters are popular for higher-power railway applications. Examples include the RPxxx-RW and RPxxx-RUW series, which support a 12:1 input range with nominal values covering all the rail standards from 24 to 110VDC, including surges and brownouts. RECOM supports this with its design recommendations for external circuits. It is important to calculate the power budget based on the expected use case. The typical power consumption of a PoE node such as an IP camera is about 7–8W. For a carriage with six such nodes, a total PoE output budget of 60W is indicated.

Be sure to choose proper DC/DC converters to provide sufficient output power for normal operations while factoring in system power requirements and output power budget. Remember to factor in the output power derating of the DC/DC converters for high operating temperature conditions. Additionally, the system voltage also differs depending on the network chosen. For instance, the CAN-bus is a 5-volt system; accordingly, the IEEE 802.3at PoE standard can use from 42.5 to 57VDC.

RECOM has an extensive portfolio of rugged EN 50155-compliant DC/DC converter products and provide turn-key solutions for rail applications from low-power (8–240W) modules up to 10kW power supplies, with customization available. The chassis mounting format is also common; in this regard, RECOM provides Plug & Play solutions from 40W up to 1kW.

Another example is the (40/75 W) 150–300W ruggedized RMD series with its 12:1 range 24V–110VDC input, fully compliant with EN 50155 and all related standards. These “ready-to-use” units do not need any external circuit and are equipped with features such as ultra-wide range input 14.4–154V/170V, boost power, EMC/disturbance protection, active inrush current limitation, active input reverse polarity protection, hold-up time (S2), reinforced isolation based on OVC3 and PD2, trimmable output, and output ORing diode.

For customized designs, RECOM offers comprehensive engineering support, environmental compliance reports, and EMC evaluation. Reference designs include EMI filtering needed for EN 50121-3-2 compliance with 24–48V or 72–110VDC nominal input voltage converters.