Application Note Using Modular DC-DC Converters to Meet European Standards for Railway Applications In Europe, the performance of electrical and electronic equipment in railway applications is governed by two international standards. Electrical Requirements for EN50155 (IEC571) Operating Voltage Ranges. Table 1 lists the nominal input voltages (VN) provided by power sources used for railway applications. The one most frequently cited in design specifications is the document IEC571, “Electronic Equipment on Rail Vehicles,” also known as European Norm EN50155, “Electronic Equipment Used on Rolling Stock Equipment.” In the U.K. the standard that applies is RIA12, “General Specification for Protection of Traction and Rolling Stock Equipment from Transients and Surges in DC Control Systems,” developed by the Railway Industries Association (RIA). Equipment powered directly from batteries with no voltage stabilizing device must function properly with input voltages that range from 0.7(VN) to 1.25(VN) during normal operation. The equipment must also withstand input voltage drops of 0.6(VN) for 100ms and overvoltage surges of 1.4(VN) for one second that may occur during startup. While these two standards are similar in most respects, the RIA12 standard also requires a specific surge withstanding capability. Table 1 shows that, with some exceptions, standard Vicor DC-DC converters cover all of the specified input ranges and transients. The following discussion shows how to comply with these standards using modular Vicor DC-DC converters to facilitate the design of complex power supplies. Fast Transient Specification. Equipment must also be able to withstand a direct transient of 1800V lasting 50µs. The impedance of the transient source is specified as 100Ω, with transient energy around 100mJ. Table 1—Input Specifications for EN50155 vs. Vicor DC-DC Converters Nominal Inputs (VN) Input Ranges 0.7(VN)–1.25(VN) 24V 36V 48V 72V 96V 110V 17–30V 25–45V 34–60V 50–90V 67–120V 77–137V Transients Low (0.1s) High (1s) 0.6(VN) 1.4(VN) 14V 22V 29V 43V 58V 66V 34V 61V 67V 101V 135V 154V Vicor DC-DC Converter Input Ranges 18–36V1 21–56V 36–76V2 55–100V3 66–160V 66–160V To prevent damage to the DC-DC converter, the module must be connected to a suppression device, such as a transient voltage suppressor (TVS), directly across the input. A TVS able to withstand up to 1.5J would be suitable. The TVS’s clamping voltage must be selected so that it will not exceed the high line voltage or transient limits of the converter. RIA12 Surge Protection The RIA12 standard specifies that equipment must withstand a 20ms overvoltage surge 3.5 times greater than the nominal input voltage. To meet this standard, a suppression circuit must be used with the DC-DC converter. For each of the standard input voltages, the maximum input surges are as follows: VN 24V 36V 48V 72V 96V 110V 3.5(VN) 84V 126V 168V 252V 336V 385V Since the source impedance of this overvoltage pulse is 0.2Ω, a common clamping device, such as a TVS, will not provide a suitable solution because the energy dissipated would be: 3.5(VN) – VZener E = ––––––––––––– · VN · 0.02 0.2 Instead, an active circuit as shown in Figure 1 is necessary. 1 Low limit at 18V Low limit at 36V 3 Power derates to 75% of rated load at 45V 2 (continued) VICOR CORPORATION • 25 Frontage Road • Andover, MA 01810 • TEL: 800-735-6200 • FAX: 978-475-6715 • 4/96 Application Note Q1 + IN + OUT 1N4148 22k R1 R2 22k D1 1N4148 GATE OUT 33k D3 D2 470pF 500V P = RDS(on) · IN2 The diodes D2 and D3 can have the same breakdown voltage, in which case the output (clamped) voltage will equal D3 minus the gate source voltage. Q2 IRFD110 15V 10k – OUT – IN Figure 1—An active circuit designed to prevent fast transients and surges. Railway Standards (page 2) In this circuit, D1 clamps the fast, high voltage spikes, while the active part formed by D2, D3 and Q1, Q2 limits the surge. During normal operation, Q1 is kept in full conduction by the charge pump circuit made up of the 1N4148 diodes and the 470pF capacitor. The Gate Out signal from the converter drives the charge pump. When overvoltage occurs, the diode D2 conducts, with current limited by R1, To select Q1 it is necessary to consider the values of 3.5(VN) and the operating current (IN) as well as the S.O.A. limits for a single 20ms pulse. The RDS(on) must also be taken into account, since the power dissipation under normal operating conditions is calculated as: and turns Q2 on. With Q2 in conduction, the gate voltage of Q1 is held to the clamping voltage of D3. The output voltage equals the clamping voltage of D3 minus the gate source voltage of Q1. When overvoltage ceases, the system resumes normal operation. Component Selection. The components in the Figure 1 circuit remain essentially the same for all input voltage ranges. Only the main MOSFET Q1 and the clamping diodes D1, D2 and D3 are selected according to nominal operating voltage and power. The RIA12 standard prescribes an input voltage variation of up to 1.5(VN) for one second. For converters with an input voltage range that allows the module to withstand this, the breakdown voltage for D2 and D3 must be greater than 1.5(VN) to prevent the protection circuit from operating under this condition, but less than the converter’s high line voltage. The diode D1 clamps high voltage spikes that have low energy but amplitudes that may exceed 1000V. Therefore, D1 must be a TVS with a clamping voltage higher than 3.5(VN) to prevent damage by this transient. (continued) 385V Surge 168V Surge Module Input Module Input 48V. 110V. Gnd. Gnd. Figure 2—For a 48V input, a 20ms input transient with output voltage clamped to the converter’s high line limit. VICOR CORPORATION Figure 3—For a 110V input, a 20ms input transient with output voltage clamped to the converter’s high line limit. • 25 Frontage Road • Andover, MA 01810 • TEL: 800-735-6200 • FAX: 978-475-6715 • 4/96 Application Note Railway Standards (page 3) Test Results. This circuit has been tested in the lab for 48V and 110V inputs—the two most common in railway applications—supplying a 150W DC-DC converter. The table below lists the components used. Converter 48VN 110VN D1 1.5KE200A 1.5KE440A D2 68V/0.5W 160V/0.5W D3 68V/0.5W 160V/0.5W Q1 IRFP250 IRFP450 Figures 2 and 3 show the 20ms input transient and the output voltage clamped to the converter’s high line limit. Physical Requirements Operating temperature range. Operating temperatures are divided into four classes according to the severity of the environment, as shown in Table 2. When designing the power supply, it is necessary to consider overtemperature during start-up, indicated in the third column. In some railway applications, the equipment is bulkhead mounted on the wagon walls. This setup can take advantage of the Vicor converters’ planar baseplate and use the cabinet as a large heat sink. Operating Temp. Classes T1 T2 T3 TX Vibration and Shock. EN50155 specifies that electronic equipment mounted on boards and boxes fixed to the vehicle frame must be able to withstand vibration on all three axes at the levels below: • Frequency range: 5–150Hz • Cross-over frequency: 8.2Hz • Displacement amplitude (below cross-over frequency): 7.5mm • Acceleration amplitude (above crossover frequency): 20m/s2 The equipment also must be able to withstand a 50ms, semi-sinusoidal shock of 50m/s2 amplitude. Because they are fully encapsulated in epoxy resin, Vicor modules can easily sustain these mechanical stresses. However, close attention must be paid to the PCB design and wiring to prevent relative movements that can stress the modules’ pins. • • • The Vicor Applications Manual contains complete information about how to use Vicor component power products. To address application conditions outside the range of standard Vicor DC-DC converters, contact Vicor Applications Engineering for alternative solutions. Table 2 — Operating Temperature Ranges Internal Internal Cubicle Air Temp. Cubicle Overtemp. Around Temp. (10 min.) PCB -25/+55ºC +15ºC -25/+70ºC -40/+55ºC +15ºC -40/+70ºC -25/+70ºC +15ºC -25/+85ºC -40/+70ºC +15ºC -40/+85ºC VICOR CORPORATION • 25 Frontage Road • Andover, MA 01810 • TEL: 800-735-6200 • FAX: 978-475-6715 • 4/96