DC/DC & AC/DC converters
Onboard power converters on railway rolling stock supply the low-voltage DC networks that power electronic control, communications, passenger information, lighting, and safety systems independently of the primary traction circuit.
Railway rolling stock carries two distinct electrical systems: the high-voltage traction circuit, which may operate at catenary voltage or intermediate DC link voltage, and the low-voltage auxiliary circuit, which distributes power to all non-traction electrical loads.
Auxiliary power architecture
The auxiliary circuit typically runs at 24 V, 72 V, or 110 V DC — the nominal battery voltages defined in EN 50155 — and is maintained by a battery system that provides power during interruptions in traction supply, including at stations and during system faults. The DC/DC converter sits between the battery bus and the electronic subsystems, stepping the battery voltage down to the 5 V, 12 V, or 24 V levels required by control electronics, communications equipment, and display systems.
The AC/DC converter — also called the auxiliary converter or hotel power converter — takes a supply from the traction circuit or an onboard auxiliary generator and produces the stabilised DC output that charges the battery and feeds the auxiliary bus.
On AC-fed trains, the auxiliary winding of the traction transformer provides the AC input; on DC-fed trains, a dedicated DC/AC inverter stage precedes the rectification.
EN 50155 requirements
EN 50155 — Railway applications: Rolling stock — Electronic equipment — is the governing standard for all electronic equipment installed in rail vehicles, including power converters.
It defines operating temperature classes, vibration and shock categories referenced to EN 61373, electromagnetic compatibility requirements, and the permissible voltage range for battery-sourced DC inputs, which EN 50155 specifies may vary between 0.7 and 1.25 times nominal under normal conditions and between 0.6 and 1.4 times nominal for transients up to 100 ms.
Railway DC/DC converters must provide galvanic isolation — typically achieved with a high-frequency transformer inside the module — and must maintain output regulation across the full input range. Reinforced insulation to 3 kV AC is a common specification for converters intended for the railway battery interface.
Application range
DC/DC converters in rail applications span power levels from a few watts for field bus node power to several kilowatts for air conditioning control and door system drives.
Modular designs allow multiple units to be paralleled for higher power requirements or configured with redundancy.
The supply chain for EN 50155-compliant converters is served by specialist manufacturers including Bel Melcher, RECOM, TDK-Lambda, and others producing standard brick-format and PCB-mounted modules in the relevant input voltage ranges.