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Railway Applications Demand DC-DC Converters That Handle a Diversity of Input Voltages

Modern railways rely on a host of electronic systems in order to operate safely and provide passenger comfort and convenience. A typical railcar distributes either 72V or 110V DC of battery power to these various systems.

Each of these systems has its own DC voltage which is often lower than the central battery voltage. Railway distributed power system designs incorporate DC-DC converters to convert the main battery voltage into the input voltage of the downstream electronic system, and DC-DC converters with a wide input voltage range can accommodate a greater number of power systems. In addition, the main battery power may also fluctuate when feeding higher power systems onboard the train, so a DC-DC converter with a wider input range is better suited to handle those wide variations in voltage.

Wide input range DC-DC converters are especially desirable for railway applications because:

  • They can accommodate a broad range of common battery and bus distribution voltages.
  • Designs can account for wide voltage tolerances and fluctuations.

DC-DC Converters Satisfy a Diversity of Railway Power Requirements

Polytron Devices offers DC-DC converters that address the diverse needs of railway power systems. For example, the OFQC60 Series 60-watt quarter-brick DC-DC converter features a wide 12:1 input voltage range covering potential distribution voltages of 9-75 V DC or 14V to 160V DC, according to the EN50155 standard. The wide input range allows the OFQC60 to accommodate many different power systems within the railway environment. Additional features and characteristics include:

  • Output voltages of 5, 12, 15, 24, 28, 48 and 53V DC.
  • Isolation voltage up to 3000V AC.
  • Efficiencies up to 92 percent.
  • EN55032 compliance.
  • Protection against over current, over voltage, under voltage, short circuits and over temperature.
  • RoHS and REACH compliance with CE Mark pending.

A Wide Input Voltage Range Means More Design Flexibility

As railways add more power systems to enable train operation and provide passenger amenities, DC-DC converters must accommodate a greater number of input voltages. Polytron Devices’ OFQC60 Series of 60-W DC-DC converters boasts an extremely wide 12:1 input range so designers can use them in a variety of power systems for modern railways.

For more information about OFQC60 Series DC-DC converters, visit


EN50155 Certification Ensures DC-DC Converters Handle Multiple Railway Challenges

Railway applications present many potential uses for DC-DC converters. Electronic systems onboard modern trains for both critical systems or passenger comfort and convenience need DC-DC converters to lower the main DC battery voltage to their individual input requirements. But, all these systems in close proximity create unwanted signals that affect the safe operation of the train. They also encounter harsh environmental conditions aboard the train including shock, vibration and temperature and humidity extremes.

What is EN50155?

EN50155 is an international standard pertaining to the conditions that can impact railway operations and safety such as harsh environments or the interacting signals between multiple electrical/electronic systems. It encompasses several existing standards, such as EN50121-3-2 for EMC (electromagnetic compatibility) and EN61373 for shock and vibration. In order to attain EN50155 approval, power supplies undergo stringent testing for a host of challenging conditions. Some of the most important parameters pertaining to power supplies include:

  • Input voltage. Since trains present a diversity of bus voltages, EN50155 outlines nominal input voltages of 24, 48, 72, 96 and 110V. EN50155 sets limits for fluctuations of the rated voltage and establishes short-term deviations. The supplies must also provide an output during power interruptions.
  • EMC. With so many electronic systems populating railway vehicles, EN50121-3-2 sets strict limits to radiated emissions and ESD levels, including events like surges and fast transient bursts.
  • Shock and vibration. Not surprisingly, components inside moving rail stock will encounter frequent vibration and mechanical stresses. EN50155 establishes minimum vibration and shock requirements depending on where the device is mounted in the railway vehicle: body mounted, bogie or "truck" mounted or axle mounted.
  • Temperature and humidity. The standard provides different classes of operating temperatures for power supplies, depending on the environment. They must operate at 85 percent efficiency to reduce dissipated power that can otherwise be lost as heat, and they are required to withstand a 15-degree overtemperature at startup for 10 minutes. EN50155 also establishes humidity requirements.

Look for EN50155 Compliance

Designers that choose a DC-DC converter with EN50155 approval can be sure it will perform reliably aboard rolling stock. One example of a DC-DC converter designed especially for railway applications is Polytron Devices’ HWB Series DC-DC converters. These half-brick power supplies operate from 9-36V, 8.5-36V, 16.5-75V and 43-160V DC with no minimum load requirement, and they boast a high efficiency of up to 91 percent. The series’ EN 50155 approval also means it satisfies EN50121-3-2 (with external filter) and EN61373 for shock and vibration. Additional approvals include the EN61000-4-2, -3, -4, -5 and -6 EMC standards and UL/EN/IEC 60950-1 for safety. HWB Series converter safeguards include overvoltage, overtemperature and short-circuit protections plus heat sink options.


When selecting DC-DC converters for railway applications, look for devices that carry EN50155 certification. HWB Series DC-DC converters have been tested and approved to EN50155 so you can be sure they can both manage EMC and perform in the challenging conditions aboard rolling stock. Polytron Devices can furnish the applicable EN50155 test reports upon request.

For more information about Polytron Devices’ HWB Series DC-DC converters for railway applications, visit

Remote ON/OFF Control: Remember These Basics Before Buying DC/DC Converters

PR45-24S300 v1Many DC/DC converters come with remote ON/OFF functionality to give designers a means to control the unit externally. This elementary function ordinarily appears as a bullet point on manufacturer product pages, but it helps to review this feature before checking off this requirement and moving on. Here’s a quick overview of what you should know about remote ON/OFF as you make your selection.

Some designs may not require remote control of the DC/DC converter. However, many designs require the converter to be enabled or disabled intermittently, such as in devices that only run on standby. Or, the function can turn the converter off to protect against damaging inrush currents, to name just two examples.

The Highs and Lows of Logic

When ordering, you’ll need to know the type of logic required.

  • Positive logic. Control pin logic 1 (a “high” signal) turns the converter ON, and a 0 (“low” signal) turns it OFF. If the DC/DC converter does not need outside control, a converter with positive logic uses the high signal to operate as default.
  • Negative Logic. Control pin logic is 0 (a “low” signal) turns the converter ON, and 1 (a “high” signal), turns the converter OFF. If the DC/DC converter does not need outside control, a converter with positive logic disables the converter by default.

The way you implement remote ON/OFF will vary depending on the converter and its logic. For both isolated and non-isolated converters with positive logic, the converter will operate by default with an open control pin or if the pin is connected to a high signal. If negative logic, the converter is enabled if the control pin is connected to a low level or to the negative input.

Things You’ll See On a Datasheet

Datasheets typically indicate whether remote ON/OFF control is available under the “Input” section along with the control pin’s orientation. Polytron Devices will state whether positive or negative turn-on is available, either as standard or as an option. Depending on the converter, Polytron may also show how and where ON/OFF control is referenced (for example, which secondary pin is used and if it is referenced to GND, or whether the control voltage reference is TTL- or CMOS-compatible). Other information on the datasheet may include the voltage required to enable or disable the converter as well as the input current the converter is rated to when ON and OFF. Keep in mind that some DC/DC converter implementations may need an external component to provide isolation, so be sure to check with the manufacturer when ordering.

For more information about Polytron Devices DC/DC converters, visit

​Internal Capacitors May Decide Your Power Supply’s Reliability

Internal-Capacitors-transDesigners commonly look at the MTBF (mean time between failure) ratings of power supplies to make sure they will operate reliably in an intended application. Although it can be a useful indicator, MTBF doesn’t give you the entire story about a supply’s reliability.

For instance, MTBF does not predict the power supply’s lifetime. It is the total functional life divided by the number of failures. But that expected time between failures can be longer than the life expectancy of the power supply’s internal components.

Electrolytic capacitors are usually the first components to break down inside a power supply. If the capacitor cannot reliably store energy as needed, the power supply’s reliability suffers. Several conditions can cause a capacitor to fail, so consult with your power supply manufacturer to determine how the capacitor will handle the rigors of the application environment.

Why Capacitors Fail

Some of the common reasons why capacitors fail or wear out include:

  • Voltage rating. Applying a higher voltage than the capacitor’s voltage rating can cause catastrophic failure.
  • Ripple currents. Extreme ripple currents can heat up the capacitor and dry out the electrolyte.
  • Heat. Hot operating conditions shorten the capacitor’s life. Or, the circuit board can transmit heat to cause the electrolyte to vaporize.
  • Short or open circuits. Short circuits can occur between the electrodes, and mounting errors can cause open circuits.
  • ESR. A capacitor with a higher equivalent series resistance (ESR) is less able to handle high ripple currents. High temperatures can raise a capacitor’s ESR.
  • Reduced capacitance. A capacitor’s performance can decline over time. As ESR increases, the capacitor heats up and dries the dielectric.
  • Storage life. If you expect your power supply to be inactive for long periods, remember that electrolytic capacitors have a limited storage life.
  • Other. Chemical leakage (which leads to corrosion), high leakage current, cold temperatures, capacitor size and many other factors affect the life of an electrolytic capacitor.

Internal Capacitors May Decide Your Power Supply’s Reliability

MTBF is not the only reliability metric to look at when selecting your power supply. Internal components like electrolytic capacitors have limited lifetimes, so be sure to take them into account. Capacitor lifetimes can be determined by a host of both operating and environmental factors, especially when it comes to thermal conditions.

Although it is desirable to look for built-in capacitors that offer long lifetimes in a wide range of operating environments, not all capacitors are equal: Better-quality capacitors use better-quality electrolytes. Also, be sure to get as much information as possible about the built-in electrolytic capacitor from your power supply manufacturer in order to determine whether it is appropriate for the intended application. Polytron Devices can provide the technical information, expertise and testing pertaining to internal electrolytic capacitors to help you select a power supply that will meet your expectations in the field.

For more information about Polytron power supplies, visit

MTBF and MTTF: Fundamentals for Power Supply Selection

When evaluating power supplies, you’ll come across two important parameters: mean time between failures (MTBF) and mean time to failure (MTTF). Although these different but related terms typically appear in the reliability section of a power supply manufacturer’s datasheet, they should be used judiciously.

For instance, MTBF is often misunderstood as being an indicator of how long a power supply will last. However, this metric is actually based on consecutive failures, derived from field data, over the device’s functional lifetime. It is also dependent on the failure rates of the supply’s internal components and environmental stressors. MTBF is useful for inferring the supply’s overall reliability, not as a predictor of a device’s lifetime.

Individual manufacturers present MTBF and MTTF figures differently on their datasheets and predicate that information using various standards and test methodologies. When determining whether a power supply will perform reliably in its intended application, buyers should have a basic understanding of MTBF and MTTF and the tests vendors use to establish these metrics. Here is a quick overview:

  • Mean-time-between-failures: A statistical average of the amount of time between failures for a device in the field. Prediction guides exist to help power supply manufacturers calculate MTBF. MIL-HDBK-217F and Telcordia SR/TR-322 (Bellcore) are the most accepted guides among those summarized below:
  • MIL-HDBK-217F: The Reliability Prediction of Electronic Equipment in the U.S. Military Handbook. MIL-HDBK-217F — also common in commercial areas — provides failure rate and stress factors for components used in electronic systems as well as application-specific stresses.
  • Telcordia SR/TR-332 (Bellcore): Bellcore took MIL-HDBK-217 and modified it for commercial applications, emphasizing parts count, lab test, field test and burn-in test data to predict reliability.
  • IEC 61709:2017: This guide emphasizes environmental factors to forecast reliability.
  • 217Plus: 2015: Based on MIL-HDBK-217, Quanterion Solutions developed the methodology using “enhanced approaches to account for environments, for quality, and for cycling effects on reliability”* for government and industry.
  • Others: 299C (Chinese standard), RCR-9102 (Japanese standard).

These guides and methodologies place different emphases on various stress and environmental factors, so be sure to ask the power supply manufacturer how it calculates MTBF. Knowing which prediction method was used can influence your confidence in a supply’s MTBF figure.

  • Mean-time-to-failure: An average amount of time that the device is expected to perform in the field. It applies to non-repairable devices, so consider the power supply’s end product. If you expect it to have a short service life or operate a limited amount of times before replacement, MTTF may be a useful reference. It may also be suitable for critical applications in which failure is not an option.

Anyone interested in power supply reliability should have an understanding of MTBF and MTTF. Polytron Devices publishes MTBF information in its datasheets (under “Physical Specifications”) as well as product pages on Since military testing is more stringent, we typically base our MTBF figures on MIL-HDBK-217F, but other MIL standards or guides may be used depending on the product. The datasheet may also include the test conditions such as ambient temperature and whether the device was tested under full load. Our technical staff can answer any concerns you may have about how our power products are tested for reliability.

About Internal Components

Engineers should also ask the power supply vendor for reliability information pertinent to the unit’s internal components. Note the electrolytic capacitors because they are often the first internal component to fail. Finally, keep the anticipated thermal conditions in mind, too.

When selecting your power supply, consider MTBF or MTTF an initial clue to its reliability. Find out how the manufacturer calculated the statistic, based its prediction methodology, and tested the unit and under what conditions. Also, learn as much as you can about the reliability of the internal components. The more information you obtain about these and other factors, the better you’ll be able to decide which power supply suits your application.

For more information about Polytron power supplies, visit

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