Power trends in transportation

Improvements in thermal management, novel electrical circuit solutions that enhance efficiency, combined with improved switching devices - especially lowered gate charge of power MOSFETs, have helped to deliver increased power densities. Faster switching capabilities make it possible to use smaller magnetics and reduced bulk capacitance giving more power output from industry standard form factors. Power density benefits are also emerging from recent research into 3D packaging techniques. 

One consequence of this interplay of trends is that where, in the past, a bulk supply might be used, now a point of load approach becomes practical.

Transport power standards
In order to standardise global requirements for power supplies in rail applications, EN50155 - ratified in 2007, established the rules. Alas, it does not make for easy reading. This is especially true given that it refers to many other existing standards, including, most importantly from a transient and EMC perspective, EN50121-3-2. However, EN50155 spells out the limits for environmental conditions; tests needed to assure long-term reliable operation related to the thermal environment and vibration sources.

Surprisingly, ambient operating temperature demands are not as arduous as might be anticipated; 85°C being the highest temperature specified. But this particular need has consequences for design, not least the need for a high and consistent efficiency to limit power dissipation and thus temperature rise. Managing temperature over all else, confers long service life for power supply solutions. In the transportation world, mean time between failures (MTBF) is expected to easily exceed 10 years.

Amongst the myriad demands, EN50155 calls out the need for a compliant supply to maintain the output rail for 10ms after input supply loss. This energy need must be bridged by a suitable input capacitor. The specification also demands that input ripple of 15% of nominal does not impact operation. Arguably though, the toughest aspect to fulfil aside from hitting efficiency goals is managing input transients. Ensuring full compliance can lead to a complex input filter structure and added expense. However, full compliance does mean that input transients can then be completely forgotten as a design consideration: when a supply integrates transient protection, considerable board real-estate savings result.

In the transportation world, other considerations met by some suppliers include the demands of EN45545-2, which dictates sustainable fire protection levels. Also, for outdoor operated supplies, EN60529 calls out Ingress Protection (IP) ratings to ensure moisture stays out of power enclosures (e.g. IP67).

Power regulation options (DC/DC)
For more than 40 years, the MELCHER line up of DC/DC cassette-style power converters from Bel Power Solutions (following the acquisition of Power One), with extremely robust electrical and mechanical designs, typified rugged performance for a diverse array of railway, transportation and industrial needs. Bel Power Solutions’ latest HR-series is destined to prolong the life of cassette type supplies, as it is one of the company’s latest portfolio innovations. This is one of only a few power supplies capable of operating over a 10:1 input range (from 15 to 168V). It features outstandingly high efficiency of 94.5% and supplies up to 300W of output power with no derating over the entire ambient temperature range of -40 to 70°C. Single or dual output variants are available and the three base models cover almost every conceivable input/output combination, offering considerable design flexibility.

With the arrival of the HR-series, just one ‘universal’ power supply covers all nominal battery terminal voltages from 24 to 120V. The static input range is 15.4 to 150V but dynamic transient deviations to 12V and 168V for two seconds are facilitated. The HR-series complies with EN50155, EN50121 and all common national and international railway standards. Standard features include current sharing - allowing multiple modules to be paralleled safely to increase current drive for large loads. Reverse polarity protection, inrush current limitation, over-voltage and programmable under-voltage lockout make this a fully self-protecting solution.

Point of load solutions
Recently, railway solutions have started to shift away from cassette-based packaging since practical, point of load designs have started to appear. For example, Bel’s 35W IMX35 family offers wide input operating range, with up to 4 independent outputs designed around a dual fly-back architecture switching at 240kHz. Two fly-back circuits feed four separate power trains. Switching is interleaved between the fly-back sections and current mode feedback control is implemented for fast transient performance. This board mountable portfolio is capable of delivering up to 90% efficiencies and is available with a wide range of output voltage options.

Continuing the move to smaller form factors, Delta’s H80SV series half-brick solution is an isolated 200W DC/DC converter, with 3000 VAC reinforced isolation. It follows on the heels of the company’s 100 and 120W HA1SV half-brick solution. The single output H80SV has a significant size advantage over previously mentioned cassette type solutions, albeit that it only offers a single output family of five parts (planned output options being 48, 24, 15, 12 & 5V with 1% tolerance). This family turns in a respectable 90% full load efficiency, irrespective of output variant, as well as half load efficiency figures just 1.5% shy of the full load performance. It comes with a host of industry-standard features: all models have an ultra-wide 9:1 input voltage range (16.8 to 154V). Specified to operate over -40 to 85°C ambient, no derating need be applied to the supply up to 100°C case temperature.

Artesyn ERM75

Dialling back power capabilities further; last year saw Artesyn  Embedded Technologies (formerly Emerson Network Power) introduce the ERM family, using the quarter-brick form factor (2.28x1.45x0.50"). Artesyn offers two power options: the 75W ERM75 and a lower power 50W ERM50 product. Each series includes eight models with 5, 12, 15, or 24V output options and input ranges of either 43 to 101V (72V nominal) or 66 to 160V (110V nominal). These DC/DC converters switch at 320kHz, deliver 92% efficiency and feature a wide operating temperature range of -40 to 85°C with derating.

Panel-Mount and Box type supplies
Meanwell, offers a series of general purpose Panel-Mount supplies, half-encapsulated transportation grade products meeting both EN50155 & EN45545-2. Its RSD100/200 ranges provide single DC outputs of 5, 12 and 15V at 100W and 12, 24 and 48V at 200W power levels respectively with 88% or better efficiency. These work over a more typical 2:1 input voltage range, provide 4kV dc isolation and offer a three-year warranty.

Murata’s MPD7K046S is a simple and small (330cm3), 200W unit for electric vehicle applications. The dual output 90% efficient design provides a 12V/10A (18A peak) primary output and a 5V/1A auxiliary rail generated from an input range between 36 and 60V. The device is packaged in an IP56 rated, dust resistant enclosure.

Finally, Delta’s B40SR-B62SR-B70SR series offers three, compact (640cm3), 12V single output, 300W Box Type options designed to work with the majority of DC secondary cell configurations. Three options work from 18 to 60 or 106V or from 36 to 106V. All options can even handle a 20% temporary overvoltage (for 10 minutes) and provides a full rated load at greater than 88% efficiency. It meets an IP67 moisture rating and can handle automotive transients as specified in ISO 7637-2. These supplies are ideally suited to electric transport applications including forklifts, golf buggies, and electric motorcycles. With 10% current matching accuracy it’s also possible to parallel multiple supplies to drive larger loads.

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