Reliability is achieved by ensuring that all specifications are comfortably met and all links in the chain are strong. The performance and comfort in modern rolling stock is provided by many electrical systems, which all require power supplies and must be capable of operating to an exceptional level of reliability in challenging environmental conditions.
Electronic-equipment standards
Extremely wide temperature excursions occur in railway applications, together with high levels of vibration and electrical noise. To ensure the reliability of electronic equipment in trains, a number of national and international standards are applied. The most widely used standard in Europe is EN50155 (IEC571)1. In the UK, standard RIA12 includes a specific surge-withstand capability. These specifications include extremely tough requirements for input-voltage range, surge protection, power interruption and backup, operational temperature, shock and vibration. For power supplies, these requirements can only be met by both using modules specifically designed and manufactured for railway applications and applying the correct procedures for their mounting and protection in use. High-efficiency encapsulated DC/DC converters meet the demands of harsh industrial and transportation applications. InQor products use synchronous rectification to achieve a power-conversion efficiency of up to 93%, 10% more than conventional switching converters. In many cases, this can avoid the requirement for forced-air cooling. The use of forced-air cooling is to be avoided where possible because of the maintenance issues and the risk of ingesting dust or liquid pollutants into the power supply. The main requirements of EN50155 and how they may be met with DC/DC converters are considered below.
Operating input-voltage range
The electrical systems in trains operate from widely differing supply voltages, depending on the application, with some systems running as high as 110V. Table 1 lists the most commonly used supply voltages. With all systems, the supply input may vary widely and be subject to intermittent drops and surges. The DC input voltage may come from a single- or three-phase generator and be relatively unsmoothed, so a ripple of up to 15% of the nominal input voltage (VIN) must be expected.
Table 1: Input specifications for EN50155.
Nominal
Input range
Transients
InQor
Input (VIN)
0.7Vin -1.25Vin
Low (0.1s) 0.6Vin
High (1s) 1.4Vin
Available input ranges
24V
16.8-30V
14.4V
33.6V
9-36V
36V
25.2-45V
21.6V
50.4V
18-75V
18-135V
48V
33.6-60V
28.8V
67.2V
18-75V
18-135V
72V
50.4-90V
43.2V
100.8V
42-110V
18-135V
35-135V
110V
77-138V
66V
154V
66-160V
35-160V
Specification EN50155 lays down overall requirements for supply input variation for all voltages, with the widest extremes being applied for battery-powered equipment. In this case, the equipment must function normally with an input voltage as low as 0.7 and up to 1.25Vin. In addition, equipment must tolerate input-voltage drops of .6VIN for 100ms and overvoltage surges of 1.4Vin for 1s without damage. During supply fluctuations between 1.25 and 1.4Vin the equipment may temporarily shut down to protect both itself and associated systems. However, some classes of equipment must maintain continuous operation throughout the 0.6Vin/100ms break in supply voltage. The InQor range of products will operate without damage or shutdown over the entire voltage ranges mentioned above.
Surge protection
EN50155 requires electronic equipment to be capable of withstanding a direct fast transient of 1800V lasting 50μs. The impedance of the transient source is specified as 100Ohm with transient energy around 100mJ. This requirement may be met by connecting a TVS (transient voltage suppressor) able to withstand up to 1.5J directly across the converter.
RIA12 specifies in addition that equipment must withstand a surge voltage of 3.5 times Vin for a duration of 20ms. In this case, the energy of the surge would be too great to be absorbed by a TVS, and an active protection circuit must be employed. With this circuit, the operation of the converter is continuous and unaffected by both transients and supply surges.
Circuit description
At the circuit input, D1 clamps the fast high-voltage transients. The active part of the circuit made up of Q1 and the error amplifier limits the longer duration surges. Q1 must be selected for a safe operating area compatible with both the converter input power level and the maximum transient magnitude and duration. In normal operation, Q1 is kept fully saturated by the charge pump formed of C1, D5, D6 and the oscillator. When a surge occurs, the rising edge of the waveform causes an increase in voltage drop across R1 and R2. When the voltage across R1 exceeds that of the reference diode, the error amplifier pulls down the gate voltage of Q1. This limits conduction during the period of the surge, and the transient energy is dissipated in Q1. The soft-start circuit holds the gate of Q1 low during power up, limiting inrush current. This is important to avoid nuisance problems of tripped circuit breakers or blown fuses. When selecting component values for the zener diodes and resistors, full consideration should be given to the inherent transient-withstand capability of the converter. This will enable the most economic and effective choice of components to be made.
Power interruption and backup
EN51055 requires all equipment to withstand brief interruptions in the supply input without any functional failure. For equipment that may be supplied with power either from a battery or a stabilised source, operation must continue without interruption during supply changeover. Al equipment must withstand an interruption of the supply input for up to 10ms without causing any equipment failure. All equipment must withstand an interruption of the supply input for up to 10ms without causing any equipment failure. This requirement may be met by suitable choice of the value of C2 in figure 1. For equipment where the supply source may be switched, two classes of differing severity are specified, classes C1 and C2. Class C1 calls for a reduction in input voltage to 0.6Vin for 100ms. For InQor converters, with the exception of the IQ24 series, this requirement falls within the operational input-voltage range, and no additional components are required to achieve compliance. Class C2 is much more severe and calls for an interruption of 30ms in the supply input. For this requirement the value of capacitor C2 in Figure 1 must be increased so that the converter input remains above the minimum value at end of the interruption. For higher power levels this may be impractical. The power-supply, drive and braking control circuits for rolling stock are generally mounted in cabinets beneath the floor. They can be subjected to extreme cold and extreme heat when the motors and brakes are working at full capacity. EN50155 defines several classes of equipment with differing thermal requirements (Table 2).
Table 2: Thermal requirements for various temperature classes.
Operating- temperature classes
External ambient temperature
Internal cubicle temperature
Internal cubicle over-temperature
Air temp. around PCB
T1
-25 to +40ºC
-25 to +55ºC
+15ºC
-25 to +70ºC
T2
-40 to +35ºC
-40 to +55ºC
+15ºC
-40 to +70ºC
T3
-25 to +45ºC
-25 to +70ºC
+15ºC
-25 to +85ºC
TX
-40 to +50ºC
-40 to +70ºC
+15ºC
-40 to +85ºC
Minimising the use of fans and the consequent employment of conduction-only cooling in railway electronic systems puts a strong emphasis on using high-efficiency converters to reduce internal heat generation. InQor converters, with efficiencies of up to 93%, operate with significantly less heat dissipation. This enables greater power density to be employed and contributes to higher system reliability. Electronic systems in the railway environment must operate with exceptional reliability while subject to a daily torture of voltage surges, breaks in power, temperature extremes, shock and vibration. InQor rugged converters have been designed to perform in this environment, to help keep modern trains running economically, reliably and on time.
