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How to Select a DC-DC Converter for Rail Applications

June 21, 2022 by Ron Stull - 7 Minute Read

How to Select a DC-DC Converter for Rail Applications

Introduction

Historically, the 110 V battery located in the main locomotive was the source of dc power for most electrical rail applications. This meant providing light to the carriages required running cables along the full train length.

Since the battery was also connected to switchgear, relays and other heavy electrical loads like starter motors, the cable voltage was subject to dropouts and transient spikes regularly as well as electromagnetic and radio frequency interference (EMI/RFI). Passengers remained largely oblivious to these effects, apart from experiencing occasional shifts in cabin lighting levels.

Modern trains are much more technically advanced than their predecessors and now include many advanced safety features that use multiple low voltage sensors and actuators. Additionally, contemporary rail passengers have come to expect their travel experience to include comforts like air conditioning, wireless communication, and sophisticated infotainment features. Ensuring safety and comfort would not be possible without the availability of multiple power supplies to provide constant dc voltage levels, uncontaminated by external sources of electrical noise. This blog post reviews some of the specific requirements for using dc-dc converters in rail applications.

Railway Power Standards

The EN 50155 - Railway applications - Rolling stock - Electronic equipment standard applies to electrical equipment supplied to the railway industry and has been adopted by most railway equipment manufacturers. It relates to battery-powered systems and low voltage power supplies that are directly (or indirectly) connected to the contact system and covers control, adjustment, protection, and supply applications. Dc-dc converters in power in-vehicle electronic equipment must adhere to EN 50155 in each of the following ways:

Input voltage range

The battery voltages most used in rail applications are 24 V, 48 V, 72 V, 96 V, and 100 V. The inevitable dips and surges caused by demand from multiple onboard systems mean these levels vary widely. EN 50155 specifies that these are allowed to range from 0.7 to 1.25 times their nominal value, with more significant transient variations of 0.6 to 1.4 nominal (for durations up to 100 ms) also being acceptable.

EN50155 Voltage Ranges
Figure 1: EN50155 Voltage Ranges

Electromagnetic compatibility

The Electromagnetic Compatibility (EMC) requirement for electrical equipment is defined by how it interfaces with the external environment. These interfaces are known as ports (Figure 1). Different ports have different EMC requirements.

Locomotive Electrical Interfaces (ports)
Figure 2: Locomotive Electrical Interfaces (ports)

For battery ports, there are no conducted emissions limits in the frequency of 9 kHz~150 kHz. The same EMC requirements apply to battery-referenced, signal and communication, process measurement and control ports and are specified in EN 50121-3-2. Immunity test requirements are specified in EN 50121-3-2.

Mechanical shock and vibration

Electronic equipment must be able to withstand the levels of shock and vibration experienced by a train in regular service without degradation in performance. These levels can be defined by the train supplier. Otherwise, they must meet the requirements of EN 61373 category 1, Class B (Table 1).

Performance EN50155 / IEC61373
Vibration Category < 0.3kg 5-150Hz 5g
Shock Long./Trans./Vert. axis 5g/3g/3g 30mS/30mS/30mS
Table 1: Shock and vibration tests

Temperature and humidity

Electronic equipment must be designed to operate to its full specification based on a range of different temperature classes (Table 2).

  • Passenger and driver compartments are covered by classes OT1 and OT2 (with a standard reference temperature of +25°C).
  • Classes OT3 and OT4 cover equipment in technical cabinets (with a standard reference temperature of +45°C).
  • OT3 is the default class.
Class Operating Temperature Range (°C)
OT1 -25 to 55
OT2 -40 to 55
OT3 -25 to 70
OT4 -40 to 70
OT5 -25 to 85
OT6 -40 to 85
Table 2: Temperature tests

Isolation voltage

These specifications ensure that conductors within the power supplies have sufficient electrical insulation and physical spacing such that leakage currents and electrical arcing are not issues. This test consists of two parts:

  1. An insulation resistance measurement that is performed at 500 Vdc. The minimum level of insulation resistance required is 20 Megaohms.

  2. The second part is a voltage withstand test (Table 3). Voltage levels are increased slowly (typically over 10-second intervals) up to the maximum value. The voltage is maintained for ten seconds or one minute, depending on the purpose of the test.
Vehicle Battery (V) Isolation Requirement VAC(50Hz) / DC
24 500/750
48 500/750
72 - 125 1000/1500
125 - 315 1500/2200
Table 3: Isolation and withstand tests

Certified power solutions

The PRQE series of isolated dc-dc converters by CUI Inc. fully meet the EMC testing requirements for EN 50121-3-2 and are designed to enable systems that comply with EN 50155 in rail applications.

EN50155 Voltage Ranges
Figure 3: PRQE Series 50 W and 75 W quarter brick dc-dc converters

These quarter-brick converters, which also come with EN 62368-1 certification, are offered in a range of power levels, including 50 W and 75 W. These devices are housed in a fully encapsulated aluminum alloy case with a black plastic bottom and have a UL 94V-0 flammability rating. They are up to 94% efficient, have an ultra-wide 4:1 input range and offer overcurrent, over-voltage, and short-circuit protections.

PRQ series converters are available in a DIP package with or without a heatsink or baseplate. Apart from rail applications, they are ideal for data, telecom, robotics, and industrial applications – anywhere you need a large amount of power from a device in a compact package.

Categories: Product Selection , Safety & Compliance


Have comments regarding this post or topics that you would like to see us cover in the future?
Send us an email at powerblog@cui.com

Ron Stull

Ron Stull

Power Systems Engineer

Ron Stull has gathered a range of knowledge and experience in the areas of analog and digital power as well as ac-dc and dc-dc power conversion since joining CUI in 2009. He has played a key role on CUI’s Engineering team with responsibilities including application support, test and validation, and design. Outside of power engineering Ron can be found playing guitar, running, and touring the outdoors with his wife, where their goal is to visit all of the U.S. National Parks.

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