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Power Management


When picking a converter is no trivial task


Paul Lee looks at how to meet the tough requirements of Railway Standard EN 50155 and RIA12 when it comes to designing low power DC/DC converters


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N50155 and RIA12 are common standards for electronic equipment used on railway rolling stock. They define a wide range of nominal input voltages with dips, surges and interruptions for equipment powered by the vehicle


DC/DC power converters connected to the battery voltage or other low voltage source in railway rolling stock commonly have to comply with the requirements of European standard EN 50155:2007. This standard defines a range of nominal


fluctuations is therefore 14.4V to 154V (24V -40% to 110V +40%). Other national variations include US rail nominal battery voltages of 37V and 74V; higher surges of 1.5 x Un for one second and 3.5 x Un for 20ms found in the UK standard RIA12 and lower dips to 12V for 100ms found in the French standard NF F 01-510. Figure 1 summarises this. The ranges of voltages for the French standard NF F 01- 510 and for the USA are also shown for information. Additionally, according to EN 50155, complete interruptions of the


included. In both cases, the RIA12 3.5 x Un surge would need to be ‘pre-regulated’ down to a safe voltage for the converters. Let’s look at a realistic circuit arrangement for a 48V nominal system meeting the requirements of EN 50155 and RIA12 shown in Figure 2 using a standard 18-72V input DC/DC converter. The EMC filter according to EN 50121-


3-2 provides attenuation of DC/DC converter noise conducted back to the source and attenuation of the voltage transients and bursts from the source.


Figure 2: A typical 48V system using a standard 4:1 input converter Figure 1: Rolling stock battery supply voltage variations


battery and other low voltage sources. In this article we look at a new DC/DC converter from Murata Power Solutions which is able to operate continuously across all nominal inputs whilst providing ‘hold up’ through interruptions of any length with a small external user-supplied capacitor.


24 December 2012/January 2013


battery voltages that may be encountered with possible fluctuations and interruptions. Nominal voltages (Un) are 24V, 48V, 72V, 96V and 110VDC with tolerances of -30/+25%. Fluctuations can take the nominal voltages up +40% for one second and down -40% for 100ms. The possible total range of nominals and


Components in Electronics


supply can last for up to 10ms (Class S2) or 30ms during supply changeover (Class C2). EN 50155 also defines low energy surges, electrostatic discharge and transient bursts according to EN 50121-3-2. Picking a converter to operate from one


or more nominal voltages is not a trivial task. Standard 4:1 input converters available typically cover 9-36V or 18-72V and as shown in Figure 1, they would cover only the EN 50155 nominal 24V and 48V rails respectively with no margin at the top end if the RIA12 one-second surge is


These are at relatively low energy so the circuit can comprise of L-C filters and voltage clamps such as semiconductor transient voltage suppressors and varistors. The pre-regulator circuit must actively


drop the 3.5 x Un = 168V surge to a lower level through a series element such as a MOSFET. It is not practical to clamp the surge with a parallel device due to the energy involved. For example, clamping a 168V surge to a safe 70V from the RIA12 defined source impedance of 0.2 Ohms for 30 ms represents over 1000 Joules of


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