DC/DC converters in transport applications

2019. 8. 21
DC/DC converters in transport applications Blog Post Image
Transport applications requiring DC/DC converters are unique in their requirement for wide input range and immunity to harsh environmental and EMI conditions making commercial or ‘standard’ industrial grade parts unlikely to be a reliable solution. This article gives an overview of the requirements in rail, automotive and industrial vehicles and introduces some off-the-shelf modular solutions which meet the specifications required.

Table of contents

DC/DC converters for electronic systems are found in just about all conceivable applications: consumer, IT, industrial, transport and more. Mostly they are embedded within systems, well protected from the outside world with its unpredictable power supply and environmental variations. Transport however, is different. DC/DCs are often powered directly from system batteries which can also feed other loads, generating spikes, surges and drop-outs.

The transport physical environment can also be harsh with condensation, shock, vibration and rapid temperature swings between wide extremes. Also, by its nature, transport moves between different radiated EMC environments with emissions present, sometimes at high power, from other vehicles and radio, television and cellphone masts. Modular DC/DCs have become commodity parts and even the lowest cost parts can boast high performance with safety certifications. Do these match transport requirements? An examination of the standards for performance in these markets shows that special designs are generally needed.

Transport applications such as rail can have different nominal voltages

Perhaps railways are seen as one of the most demanding applications for DC/DC converters with nominal system voltages that can vary between 24 and 110V DC. According to EN 50155-2017 ‘Railway applications – Electronic equipment used on rolling stock’, nominal voltages can vary +25%/-30% and dips can occur down to 60% of nominal while surges can reach 140%. Figure 1 summarises the range of voltages that could be seen. According to the standard, surges and dips to the specified levels up to 100ms duration should not cause ‘deviation of function’ although some degradation in performance is allowed for surges of up to one second duration. For power converters, it’s hard to suggest what degradation is acceptable to downstream equipment so practically, a DC/DC needs to function as normal, indefinitely, at the highest surge level of 140%.



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

Figure 1 also shows the ‘standard’ 4:1 input ranges of DC/DC converters illustrating how some parts can cover at least some of the rail requirements. However, an ‘ideal’ converter to cover all variations would have an input range of 10:1 or better as shown.

Some rail applications still require compliance with RIA 12, an old standard which includes immunity to surges up to 3.5 x nominal for 20ms. For 110V systems this means 385V peak and is practically very difficult to meet within a DC/DC input range or to absorb with a transient suppressor. The source impedance is just 0.2 ohms so if it were clamped to say 160V, the peak power dissipated in a transient suppressor would be an unmanageable 180kW. Various schemes have been devised to handle the surge, but an efficient way that is recommended by RECOM [1], is to ‘pre-regulate’ the supply with a series MOSFET, adding a timed switch-off so the dissipation in the MOSFET cannot exceed its rating if the surge persists. This solution is available as pre-built surge stopper modules handling up to 300W of continuous load or can be incorporated as discrete components (Figure 2).
Fig. 2: Outline ‘Surge stopper’ for RIA 12 applications (source RECOM)
DC/DC converters in rail applications will also often need to meet fast transient overvoltages as defined in the EN 61000-4-x series of standards. These are of relatively low energy so simple LC filters and transient suppressors are adequate. Complete interruptions of supply are also anticipated in EN 50155 with three classes S1, S2 and S3, the worst-case being loss of supply for 20ms from nominal input with no performance degradation allowed. This will normally require hold-up capacitance external to the DC/DC converter.

Rail equipment in rolling stock will also be subject to shock and vibration levels higher than typically found in most other applications. Standard EN 61373 defines the levels for different areas from category 3, axle-mounted to category 1, body mounted. Converter construction in all cases requires ruggedisation with conformal coating of open PCBs and often encapsulation to minimise mechanical stress and protect against humidity.

The range of DC/DC converters from RECOM and their new acquisition Power Control Systems [2] meets many of the rail requirements with EN 50155-certified products from 8W-rated SMD and DIP-24 parts all the way through to 240W ‘brick’ types with ultra-wide 12:1 inputs covering all of the standard nominals. The company also offers reference designs [3] which include filtering for the EN 50155 and RIA 12 requirements. AC input parts are also available up to 10kW 3-phase, typically for track-side applications.

Other industrial transport applications can have similar requirements to rail

The wide range of possible battery voltages can also apply in other applications such as in electric fork-lift trucks, hybrid vehicles or UPS systems. Nominal battery voltages can be from 12 to 48V but charging voltages and surges from heavy duty motor disconnects, literally ‘load dumps’, can raise the maximum voltage to 42V and higher in a 12V system (Figure 3). In 48V systems the voltages are correspondingly higher but as 60V is defined as a maximum ‘safe’ voltage, circuits like the surge stopper of Figure 2 may be used to 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 certification to higher agency-rated levels. Wide range DC/DCs are again a good solution for the lower input voltages seen under ‘cold-cranking’ conditions. The environment is less controlled than rail with the end use and location of the truck undefined so ruggedised parts are advisable for reliable operation. As electronics finds its way into agricultural and all types of heavy industrial vehicles the requirements for wide input range and robust construction are similar.



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

Automotive supply voltages are well defined

Fig. 4: Automotive 48V system voltage levels
In road vehicles, the supply voltages are quite well defined with LV124 a common specification for 12V systems, a standard established by German car manufacturers in 2013. The voltages shown in Figure 3 are typical, requiring converters than can operate over a wider than 4:1 input range. Although different manufacturers have their own interpretations and requirements, standard ISO 7637-2 can also apply for high voltage transients. Negative transients can be applied up to –150V for 2ms along with positive values, for example +150V for 150ns. Negative pulses result from parallel inductive loads being de-powered. The transients have relatively low energy and can be attenuated with LC filters and transient suppressors.

There is increasing interest in 48V DC systems in automotive for hybrid vehicles as a way to meet the imminent requirement for newly registered passenger vehicles from 2021 to emit less than 95 g CO2 per km. Conversion of the market to all-electric will not be achieved by then so hybrids are the only way forward for larger vehicles. A system with 12V and 48V system voltages allows betters gas consumption and emissions, with the 48V battery providing traction boost when necessary and power for peripherals such as oil and water pumps which can be more efficient when electrically driven.

48V systems have similar under- and over-voltage percentage ranges to 12V systems but an extra consideration is to keep the voltage under the safe 60V level to avoid costly extra insulation systems. Standard VDA 320 defines the levels (Figure 4).

If a modular DC/DC converter is used in these systems, perhaps in infotainment or navigation applications, a 4:1 (18 – 72V) input range may be sufficient but the part must still withstand the shock and vibration specifications and wide temperature variations between a parked vehicle in cold arctic conditions to the heat of tropical climates.

Modular DC/DCs have transport-compatible specifications

DC/DC requirements in transport can be conveniently met with off-the-shelf solutions from companies with long experience of the market such as RECOM and their sister company PCS. The wide range available includes parts with ultra-wide 12:1 inputs with EN 50155 certification for rail applications and ruggedised products for automotive and industrial vehicles. Products from RECOM for these demanding applications have guaranteed long life and optimum performance through thorough design verification and validation. Tests include full performance characterization, HALT, temperature cycling and high temperature soak.
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