Finding ‘Ready to Use’ DC/DCs for Mobility and Rail Applications

Of all the environments for electronic equipment, mobile ones are perhaps the most severe. But it is a growing market across all sectors, from railways to materials handling equipment, EVs to e-scooters, and much more. In rail rolling stock applications particularly, automation is being increasingly incorporated for safety and efficiency savings, along with convenience features such as data connectivity and intelligent signage. All of this must operate reliably in the typical rail environment of high pollution, shock, and vibration with power most often from the traditional DC rail of 110VDC nominal, with some global variations down to 24VDC. The standard that typically applies is EN 50155, currently version July 2021, which defines the electrical and physical environment and technical construction features, reliability, maintenance, service life, documentation, and testing.

Rail supply voltages vary widely

As well as meeting environmental, isolation, and EMC specifications, the power supply modules that convert the DC supply to a clean rail for the sensitive electronics must cope with wide variations in input voltage, which also has superimposed surges, dips, and drop-outs. Figure 1 shows the levels defined in EN 50155 for no impact on performance.



The supply can drop to zero for periods as shown in Figure 1, which are defined in classes S1, S2, and S3 (Figure 2). S2 is the default requirement and as noted, interruptions up to 10ms should meet performance criteria A and above 10ms, criteria C.

Performance criteria A’ means there is no impact, criteria B means performance degrades during the test but returns to normal afterward and criteria C means loss of function is allowed, although the equipment must be capable of reset, automatically or manually. The effects of interruptions are tested from the supply nominal voltage.

There are various locations where equipment can be fitted in a rail application, from relatively benign passenger compartments to hot equipment cabinets. EN 50155 specifies operating temperature classes to reflect this, OT1 to OT6 (Figure 3) with OT3 the default if not otherwise specified. Classes OT5 and 6 are only used in exceptional conditions.

These conditions are defined in three classes of ‘switch-on’ temperature ST0, ST1, and ST2 (Figure 4), ST1 being the default and ST1 and ST2 not applicable to OT5 and OT6.

The standard also recognizes that equipment may need to switch on at a higher temperature than ‘normal’ before natural or forced cooling comes into play. The timing and rates of temperature change for each class are defined in the standard section 13.4.5 as Dry Heat Thermal Cycle tests A, B, and C respectively.

Searching for a DC/DC converter ‘ready to use for rail applications

It is no surprise that finding a DC/DC converter meeting these specifications and ‘ready to use’ for a rail application is not trivial. While a general-purpose DC/DC converter with a wide input range may cover the ‘normal’ railway input voltage variations, in practice, to meet the full requirements of EN 50155, considerable extra external circuitry may be needed. For example, reverse input polarity protection and inrush current limitation is required, along with at least 10ms hold-up time from external capacitors to meet the supply interruption specification. A typical ‘standard’ DC/DC converter has little or no internal hold-up energy storage, so when provided externally, the required capacitor must be rated for the highest input voltage and with a capacitance to give the necessary hold-up from the lowest nominal input at rated output power. This can make the component large and inconvenient to fit with the potential for large uncontrolled inrush current. Most DC/DCs also have limited isolation voltage ratings, ‘Functional’ or at best ‘Basic’, but in the railway application ‘Reinforced’ with 3kVAC isolation voltage is typically specified by system designers.

If a general-purpose open frame DC/DCs could be found to meet the electrical specifications, it might be compact for high output power but require forced air to achieve this. In the rail application, fans are not favored due to maintenance, noise, and lifetime issues so convection cooling is preferred with a baseplate to take advantage of any ‘cold wall’ available. In practice, DC/DCs in baseplate-cooled brick formats sink all heat into the cold wall but then require the extensive external circuitry and interconnections mentioned.

A solution

A solution is a DC/DC explicitly designed to meet the requirements of EN 50155 with a wide input range to cover multiple nominals, surges, dips, and 10ms drop-outs. As an example, the ‘ready to use’ RMD range (Figure 5) from RECOM is available rated at 150W, 300W, and 500W and is in an open-frame baseplate-cooled format for fan-less operation up to the OT4 and ST2 class of -40°C to +85°C ambient. Connections are via screw terminals for convenient installation.

The input range for the 150W and 300W models is 16.8VDC to 137.5VDC (14.4VDC to 154VDC for 100ms and 1 second respectively). This 12:1 range covers the extremes of 1.4 x Vnom at 110VDC nominal input and 0.6 x Vnom at 24VDC input while the 500W model operates over 50.4VDC to 137.5VDC (170VDC/3 seconds) for 72VDC and 110VDC nominal inputs. All models have a single 24VDC output with OR-ing diodes for redundant parallel operation, input reverse polarity protection, 10ms hold-up time, remote control, and inrush current limitation. The products are also compliant with the railway EMC specification EN 50121.

Other mobile applications such as disability scooters, golf buggies, and more can also benefit from the ‘plug-and-play’ feature of products such as the RMD series, catering to a wide range of possible supply voltages and the harsh environments encountered. In these applications, the high efficiency of the RMD series also rounds off the benefits, helping to achieve extended battery run times.