Bench Talk

(Source: den-belitsky - stock.adobe.com)

Railway appliances are one of the main application areas for ruggedized high-power DC/DC converters, specifically "rolling stock". Rolling stock in the railroad industry refers to anything that uses steel wheels including rail cars, freight cars, flat cars, and locomotives. Traditionally, a power line for lighting and a signal connection would suffice, but as comfort expectations rose, the design and selection of power supplies became more challenging.

Power in rail rolling stock was originally all about traction, lighting, and rudimentary signaling with typically raw 110V_DC from the onboard batteries connected to everything. Switchgear on the DC bus did not have surge and transient limiters because they would degrade with time and couldn't be guaranteed to function continuously. Hence, individual equipment had to stand the full level of overvoltages, dropouts, and surges that resulted. It didn't matter, though, that the lights occasionally brightened or dimmed.

Since the batteries were 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.

Today it's very different—modern rail vehicles are packed with electronics for communications and control, with passengers also increasingly expecting cellular repeaters and Wi-Fi^® services with 'clean' AC mains and USB DC power available at each seat. Because of the environment and the diverse nature of connected equipment, power conversion is becoming increasingly distributed. Each converter that interfaces between the raw DC bus, the train electronics, and passengers' expensive portable devices must be designed to meet the bus voltage's electrical specification. This is essential due to the harsh physical environment of a rail application.

Where Is the Problem?

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 andcovers’ 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 24V, 48V, 72V, 96V, and 110V. 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 100ms) also being acceptable.

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. Different ports have different EMC requirements. There are no emissions limits for battery ports in the frequency of 9kHz~150kHz. The same EMC requirements apply to battery-referenced, signal and communication, process measurement, and control ports and 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. The train supplier can define these levels. Otherwise, they must meet the requirements of EN 61373 category 1, Class B (Table 1).

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).

1. Passenger and driver compartments are covered by classes OT1 and OT2 (with a standard reference temperature of +25°C).
2. Classes OT3 and OT4 cover equipment in technical cabinets (with a standard reference temperature of +45°C).
3. OT3 is the default class.

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:

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

Table 3: Isolation and withstand tests

Depending on the power level and application, DC/DC converters with various form factors are used in rolling stock. Chassis-mounted parts are typical at higher power with appropriate ruggedization to meet the shock and vibration requirements with any open PCBs lacquered. Up to around 600W, the Eurocassette style is very popular. This rack-mounting product uses the standard DIN 41612 H15 connector and is available at many power levels and with many options. Both chassis-mounted and cassette-style products will typically have 'bus' outputs of 12, 24, or 48V. They will often have options to include extra filtering, reverse polarity protection, and extended hold-up time. 'Hold-up' or 'ride through' can be pretty challenging for low nominal input voltages. For example, to hold up a 100W supply at 80 percent efficiency for 10ms for 'Class 2' applications with an input range of 16-36V around a 24V nominal would need a capacitor on the input of about 8,000µF rated at 40V—about 40 cm²- size. This is a significant size, cost, and major contributor to the overall reliability and lifetime calculations. EN 50155 describes interruptions as being caused by input short circuits. Hence, a series input diode is also necessary to isolate the hold-up capacitor, which causes additional power loss and voltage drop, requiring the input range of the converter to be extended. Other solutions for hold-up might be to internally boost the input to a higher voltage so a smaller capacitor can be used for the same energy storage, but in any case, the inrush current and charge rate into the capacitor have to be controlled and may limit the repetition rate of allowable dropouts to 1 in 10 seconds or so.

Some products can connect converter outputs in parallel for extra power or redundancy with associated signaling. The input voltage range of the converters will often be wide to encompass as many nominal battery voltages as possible with their dips and surges.

Board-mounted products are also used at up to around 100W giving tightly regulated low voltage outputs for digital and analog circuitry. These may also be powered directly from the DC bus, so often will need surge and transient as well as environmental protection. Isolation is generally required of DC/DC converters with levels defined by the application but is typically BASIC or REINFORCED.

EN 50155 includes a diagram identifying system EMC areas A, B, and C shown in abbreviated form in Figure 1 with the typical positioning of a DC-DC converter.

Figure 1: Rail systems EMC areas (Source:CUI)

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. These quarter-brick converters, which also come with EN 62368-1 certification, are offered in a range of power levels, including 50W and 75W. 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 94percent efficient, have an ultra-wide 4:1 input range, and offer overcurrent, over-voltage, and short-circuit protection.

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.

The quarter-brick converters of the 0RQB-50Y05x Series by Bel Power Solutions provide 50W of output power from a wide input range (24V, 48V, 72V, 96V, 110V typical). It delivers 5V/5mA auxiliary supply, and when a large hold-up capacitor is added, the unit can still work up to 12ms when the input supply is interrupted. Bel Power Solutions 0RQB-50Y05x Series Isolated DC/DC Converters feature remote on/off, input under-voltage protection, output over-voltage protection, over current, and short circuit protection.

Bel Power Solutions RQB-100Y 100W Isolated DC/DC Converters are high-performance devices specifically designed for railway applications and associated equipment. The RQB-100Y DC/DC Converters offer a 100W single output voltage, a wide 14V_DC to 160V_DC input voltage range, and a -40°C to +105°C operating temperature range. Protections include input undervoltage, output overvoltage, overcurrent, short circuit, and overtemperature. Bel Power Solutions RQB-100Y 100W Isolated DC-DC Converters also feature remote on/off control, remote sense compensation, programmable undervoltage lockout, hold-up function, and output voltage trim adjustment.

Designing for compliance with EN 50155 is not an easy task in electrical and environmental conditions that are just about as hostile as it gets. Commercial off-the-shelf solutions will most often simply not fit the specification, especially for long-term reliability. Specially designed converters from a company such as Bel Power Solutions are a safe solution with a long track record.

As Technical Content Specialist, Marcel is the internal contact person for technical questions in Mouser’s EMEA marketing team. Originally a physicist, he used to work as editor for special-interest magazines in electronics. In real life, he’s juggling two kids with too many chromosomes, a penchant for electronic gadgets and a fondness of books and beer. Until now, none has dropped.