DC/DC Converters in Transport Applications

Aug 21, 2019
Various transport vehicles
Transport applications requiring DC/DC converters are unique in their need for a wide input range and immunity to harsh environmental and EMI conditions, making commercial or standard industrial-grade parts unlikely to provide reliable performance. This article provides an overview of requirements in rail, automotive, and industrial vehicles and introduces off-the-shelf modular DC/DC solutions that meet these specifications.

Table of Contents

DC/DC converters for electronic systems are used in almost every application: consumer, IT, industrial, transport, and more. Typically, they are embedded within systems and well protected from external power fluctuations and environmental variations. Transport applications, however, are different. DC/DC converters are often powered directly from system batteries that also supply other loads, generating spikes, surges, and drop-outs.

The transport physical environment can be harsh, with condensation, shock, vibration, and rapid temperature swings across wide extremes. Additionally, transport systems operate in varying radiated EMC environments, with emissions sometimes at high power from other vehicles, radio, television, and cellular masts. Modular DC/DC converters have become commodity parts, and even low-cost options can offer high performance with safety certifications. Do these meet transport requirements? An examination of market performance standards shows that special designs are generally required.

Nominal Voltage Variations in Rail and Transport Applications

Railways are often considered among the most demanding applications for DC/DC converters, with nominal system voltages ranging from 24 to 110VDC. According to EN 50155-2017 ‘Railway applications – Electronic equipment used on rolling stock,’ nominal voltages can vary +25%/-30%, with dips down to 60% of nominal and surges reaching 140%. Figure 1 summarizes the voltage ranges that may be encountered. The standard specifies that surges and dips up to 100ms should not cause functional deviations, although some performance degradation is allowed for surges lasting up to one second. For power converters, it is challenging to define acceptable degradation for downstream equipment, so a DC/DC must operate normally at the maximum 140% surge level.

Range of supply voltages for different nominals

Fig. 1: Range of supply voltages for different nominals according to EN 50155-2017 for rail applications

Figure 1 also shows the typical 4:1 input ranges of DC/DC converters, illustrating that some parts can meet at least part of the rail requirements. An ideal converter covering all variations would require an input range of 10:1 or more.

Some rail applications still require compliance with RIA 12, an older standard including immunity to surges up to 3.5 times nominal for 20ms. For 110V systems, this translates to a 385V peak, which is challenging to accommodate within a DC/DC input range or absorb with a transient suppressor. With a source impedance of only 0.2 ohms, clamping to 160V would result in peak power dissipation in a transient suppressor of 180kW, which is unmanageable. Various solutions exist to handle surges, but an efficient approach recommended by RECOM is to pre-regulate the supply with a series MOSFET and a timed switch-off, ensuring the MOSFET does not exceed its dissipation rating if the surge persists. This solution is available in pre-built surge stopper modules handling up to 300W continuous load or can be implemented with discrete components (Figure 2).
Outline ‘Surge stopper’ for RIA 12 applications
Fig. 2: Outline ‘Surge stopper’ for RIA 12 applications
DC/DC converters in rail applications often need to meet fast transient overvoltages defined in the EN 61000-4-x series. These transients are of relatively low energy, so simple LC filters and transient suppressors are sufficient. Complete supply interruptions are also addressed in EN 50155 with three classes S1, S2, and S3, the worst-case being a 20ms loss of supply from nominal input with no performance degradation allowed. This typically requires hold-up capacitance external to the DC/DC converter.

Rail equipment on rolling stock is also exposed to shock and vibration levels higher than in most other applications. Standard EN 61373 defines levels for various areas, from category 3 axle-mounted to category 1 body-mounted. Converter construction requires ruggedization with conformal coating of open PCBs and often encapsulation to minimize mechanical stress and protect against humidity.

The RECOM range of DC/DC converters meets many rail requirements with EN 50155-certified products from 8W-rated SMD and DIP24 parts up to 240W ‘brick’ types with ultra-wide 12:1 inputs covering all standard nominals. RECOM also provides reference designs including filtering for EN 50155 and RIA 12 requirements. AC input parts are available up to 10kW 3-phase, typically for track-side applications.

Industrial Transport Applications Share Rail-Level Requirements

The wide range of battery voltages also applies in applications such as electric forklifts, hybrid vehicles, or UPS systems. Nominal voltages range from 12 to 48V, but charging voltages and surges from heavy-duty motor disconnects, or ‘load dumps,’ can raise the maximum voltage to 42V or higher in a 12V system (Figure 3). In 48V systems, voltages are proportionally higher, but with 60V defined as a maximum safe voltage, circuits like the Figure 2 surge stopper can clamp the DC/DC input to 60V maximum.

At this safe extra-low voltage, DC/DC insulation systems can be rated as ‘functional’ rather than requiring higher agency certifications. Wide-range DC/DC converters remain a good solution for low input voltages under cold-cranking conditions. The environment is less controlled than rail, with vehicle end-use and location undefined, so ruggedized parts are advisable for reliable operation. As electronics enter agricultural and heavy industrial vehicles, requirements for wide input range and robust construction are similar.

Surge voltages seen in 12V vehicle applications (from LV124)

Fig. 3: Surge voltages seen in 12V vehicle applications (from LV124)

Automotive Supply Voltages Are Clearly Defined

Automotive 48V system voltage levels
Fig. 4: Automotive 48V system voltage levels
In road vehicles, supply voltages are well defined, with LV124 a common specification for 12V systems, established by German manufacturers in 2013. The voltages shown in Figure 3 are typical, requiring converters capable of operating over wider than 4:1 input ranges. While manufacturers have different interpretations, ISO 7637-2 also applies for high-voltage transients. Negative transients can reach –150V for 2ms, with positive transients such as +150V for 150ns. Negative pulses result from de-energized parallel inductive loads. These transients are relatively low energy and can be attenuated with LC filters and transient suppressors.

Interest is growing in 48VDC systems in automotive for hybrid vehicles to meet the upcoming 2021 requirement for newly registered passenger vehicles to emit less than 95 g CO2/km. Full market conversion to all-electric is unlikely by then, so hybrids remain the path for larger vehicles. Dual 12V and 48V systems improve fuel consumption and emissions, with the 48V battery providing traction boost and power for peripherals such as oil and water pumps, which are more efficient when electrically driven.

48V systems share similar under- and over-voltage percentage ranges with 12V systems, with the added requirement to remain under the safe 60V limit to avoid costly additional insulation. Standard VDA 320 defines these levels (Figure 4).

Using a modular DC/DC converter in these systems, for infotainment or navigation applications, a 4:1 (18–72V) input range may suffice, but the unit must still withstand shock, vibration, and wide temperature variations from cold arctic conditions to tropical heat.

Modular DC/DC Converters Meet Transport Application Standards

DC/DC requirements in transport can be met with off-the-shelf solutions from experienced suppliers such as RECOM and RECOM Power Systems. The wide range includes ultra-wide 12:1 input parts with EN 50155 certification for rail and ruggedized products for automotive and industrial vehicles. RECOM products for demanding applications guarantee long life and optimal performance through thorough design verification and validation, including full performance characterization, HALT, temperature cycling, and high-temperature soak.
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