Specialized DC/DC Converters Meet the Unique Challenges of Railway Power Delivery

Modern railway systems have an increasing amount of electronics on board for functions such as passenger internet access, satellite links, intercoms and public-address (PA) systems, navigation subsystems, emergency radios, annunciator signboards, LED lighting, information systems, seat-located charging outlets, and other accessories. There are also battery-charging subsystems, as many of these functions must be powered during transient power gaps or extended periods of power loss. Each of these functions has unique voltage requirements, leading to the deployment of many DC/DC converters to convert higher voltage DC into multiple lower voltages.

However, designers specifying DC/DC converters for use in railways need to ensure those converters can perform reliably in confined spaces under difficult electrical, mechanical, and thermal stress conditions. They must also meet a long list of rigorous industry and regulatory requirements and be easily deployed to save time.

This article briefly examines the requirements of DC/DC power converters for railway applications. It then introduces DC/DC converters from TRACO Power and shows how they can be applied to meet those requirements.

Power distribution for railways

A typical power distribution path for an electric locomotive or trolley has many lower voltages derived from the primary DC overhead catenary source. As with any critical application, there are mandated industry standards that define performance requirements across multiple perspectives.

The dominant regulatory specification for rail electronics equipment is EN 50155, Railway applications – Rolling stock – Electronic Equipment. This defines environmental and service conditions, reliability expectations, safety, and design and construction methods. It also covers documentation and testing.

Other critical specifications include:

• EN 61373, Railway applications – Rolling stock equipment – Shock and Vibration Tests
• EN 61000-4, for electromagnetic compatibility (EMC)
• EN 45545-2, European railway standard for fire safety
• British Railway Industries Association standard RIA 12, General Specification for Protection of Traction and Rolling Stock Electronic Equipment from Transients and Surges in DC Control Systems

Meeting these regulatory mandates is a major design challenge, even if the do-it-yourself (DIY) power converter design works as intended during simulation and as a bench prototype. Fortunately, there is no need to take the DIY approach. Off-the-shelf, application specific, standard DC/DC converters that meet railway requirements are already available.

For example, the TEP 150UIR/TEP 200UIR families are two similar series of half-brick, board-mount converters rated at 150 and 200 watts, respectively. They have reinforced 3,000 volts AC (V_AC) input/output (I/O) isolation and built-in short circuit, overvoltage, and overtemperature protection.

All members of these two families have the same connection configuration and package size of 60 mm × 60 mm × 13 mm (Figure 1). Their efficiency is approximately 90%.

Figure 1: All members of the TEP 150UIR and TEP 200UIR families have the same enclosure size and form factor. (Image source: TRACO Power)

The TEP 150UIR series operates from an extremely wide input voltage range of 14 to 160 volts DC (V_DC), and is available in five output pairings ranging from 5 volts/30 amperes (A) to 48 volts/3.2 A (Figure 2).

Figure 2: The TEP 150UIR series is available with voltage/current ratings ranging from 5 volts/30 A to 48 volts/3.2 A. (Image source: TRACO Power)

The lowest voltage/highest current member of this family is the TEP 150-7211UIR, which can deliver up to 30 A at 5 volts.

The TEP 200UIR series has the same input and output voltage range, but higher currents, ranging from 5 volts/40 A to 48 volts/4.2 A (Figure 3).

Figure 3: The TEP 200UIR family offers 33% more power, with the same output-voltage values but higher output currents. (Image source: TRACO Power)

The highest voltage/lowest current member of this family is the TEP 200-7218UIR, which can deliver up to 4.2 A at 48 volts, compared to 3.2 A for its 150 watt counterpart at that voltage.

By maintaining a common size and footprint, users can easily upgrade a circuit to handle different needs or use different boards with minimal cabling and layout issues. They can also simplify inventory by stocking fewer unique models.

Three key features

The TEP 150UIR and TEP 200UIR units offer three standout features: a wide input voltage range, an extended holdup time, and active inrush current limitation.

1) Wide input voltage range: Typical industrial grade electronics might meet the general voltage/current requirements, but DC/DC power converters for this application must withstand much wider input voltage variations and a range of possible nominal values (Figure 4).

Figure 4: The DC input ranges for different rail applications span an extremely wide range, especially when allowable deviations from nominal values are factored into the analysis. (Image source: TRACO Power)

This includes the allowed variations in input voltage around each nominal value:

• Continuous range = 0.7 to 1.25, × V_NOM
• Brownout = 0.6 × V_NOM for 100 milliseconds (ms)
• Surge = 1.4 × V_NOM for one second

Designing a power converter that can ride through brownouts for 100 ms is difficult, while surges lasting one second have too much energy to clamp. Therefore, the converter must operate over the complete range shown in Figure 4 while including some safety margin. In practice, this means an input range of more than 2.33:1.

Complicating the situation, the nominal voltage can be anywhere from 24 V_DC to 110 V_DC. Many DC/DC converter manufacturers meet these requirements by offering converters with a wider 4:1 input range (typically 43 to 160 volts) to cover most applications, but a single converter has typically not been able to meet all of them.

To address this, the TRACO units support an ultra-wide 12:1 input of 14 to 160 V_DC. This range enables the system application engineer to target an array of nominal system voltages with a single power supply.

2) Extended holdup time: The DC line is subject to fast transients of ±2 kilovolts (kV) with rise times of 5 nanoseconds (ns), fall times of 50 ns, and a repetition rate of 5 kilohertz (kHz). There are also surges of ±2 kV line-to-ground and ±1 kV line-to-line with rise times of 1.2 microseconds (μs) and fall times of 50 μs from a defined, AC-coupled source impedance.

Some requirements go beyond EN 50155 and demand immunity to surges of up to 1.5 x V_NOM for one second and 3.5 × V_NOM for 20 ms from an extremely low source impedance of 0.2 ohms (Ω). For a system at 110 V_DC (nominal), this corresponds to a peak value of 385 V_DC, which is outside the normal range of a converter, especially if it needs to work down to the 66 V_DC brownout minimum.

The energy available from such a low-impedance source means that the voltage cannot be clamped by a transient voltage suppressor (TVS). Depending on the power level, a pre-regulator on the supply input or a circuit that switches the input off for the duration of the surge is required. A holdup function is necessary in the DC/DC converter to maintain the output during this time.

To resolve this problem, the TRACO units come with an important feature in the form of a BUS pin output. This output provides a fixed voltage to charge a capacitor, enabling the capacitor to provide the energy needed for a longer holdup time (Figure 5). These capacitors are significantly smaller and less expensive than those used in the conventional front-end capacitor holdup scheme.

Figure 5: This is the recommended input circuit to be used with the bus capacitor C_BUS to simplify the implementation of an extended holdup time. (Image source: TRACO Power)

Note that a series diode does not need to be added to the input circuit, as these converters have an integrated diode to avoid a short circuit and keep the energy from the capacitor flowing into the power supply.

When a supply voltage interrupt occurs, the input voltage will drop to the BUS voltage before the capacitors start discharging to provide energy to the power module. Due to their relatively high power density, the TEP 150UIR series and TEP 200UIR series can provide a fixed BUS voltage at up to 80 volts input voltage. On higher input voltages, the BUS voltage increases linearly with the actual input voltage (Figure 6).

Figure 6: The converters provide a fixed BUS voltage at up to 80 volts input voltage; on higher input voltages, the BUS voltage increases linearly with the actual input voltage. (Image source: TRACO Power)

3) Active inrush current limitation: This addresses a common problem with power converters: when the input voltage begins to ramp up, the holdup capacitors at the input terminal will cause a high inrush current. This can blow a fuse or trip a circuit and cause errors and faults in connected devices.

To avoid this, a Pulse pin from both the TEP 150UIR and TEP 200UIR series provides a 12 volt, 1 kHz square wave signal that can be used on the inrush current limit circuit (Figure 7).

Figure 7: The TEP 150UIR and TEP 200UIR series offer a simple way to limit inrush current at start-up using a Pulse pin with a square wave signal. (Image source: TRACO Power)

By connecting the Active Inrush Current Limitation circuitry to the Pulse pin, the inrush current is effectively limited (Figure 8). Without limiting, the inrush current is approximately 120 A (left), while with limiting it drops to about 24.5 A (right).

Figure 8: Driving the active inrush current limitation circuitry of the converters with the pulse pin cuts the inrush current by a factor of five. Shown is an example with a V_in of 72 volts. The horizontal scale on the left is 50 volts/division and the right is 10 volts/division, with a transducer scale factor of 1 volt = 1 A. (Image source: TRACO Power)

Conclusion

DC/DC converters for lower voltage railroad applications must do more than provide reliable, consistent power performance. They must be compact, easy to manage and deploy, accommodating to a diverse range of applications, capable of operating in harsh environments, and able to meet a lengthy list of challenging electrical, thermal, and mechanical regulatory standards and mandates. As shown, the TRACO Power TEP 150UIR and TEP 200UIR families are up to the task with features that include a wide 12:1 input voltage range of 14 to 160 V_DC, a holdup pin to charge capacitors to supply energy during voltage dropouts, the ability to withstand surges, and numerous output voltage/current pairings, all in a single form factor.