Power Management

Keeping the trains running

Mick Grahame-Dunn considers some practical approaches to protecting converters in demanding industrial applications

T

here are many applications in which DC-DC converters are subjected to surges and fast transients during their

service life. Taking the requirements of railway systems as an example, this article looks at a practical approach to the protection of converters in these and other demanding industrial applications. In railway systems, DC-DC converters will transform the 110V(dc) voltage usually employed in typical train batteries down to the 12 and 24V(dc) input power required in the rolling stock electrical equipment applications. Key to maintaining the operation of power converters, and end equipment, in harsh environments is not only the ruggedness and power handling capabilities of the converters, but also the specification of design requirements for protection against surges and fast transients. Important European standards used in the design requirement specifications for electrical and electronic equipment in railway rolling stock applications are EN50155 and RIA12.

Surge protection Specifically, the RIA12 standard specifies that electrical and electronic equipment must withstand a surge voltage of 3.5 times the nominal input voltage for 20 milliseconds (ms). If a TVS (Transient Voltage Suppressor) is used as a transient suppressor, then the approximate energy in the TVS is given by the following equation:

E = (3.5VIN – VTVS / RS) x VTVS x t

Putting values into the equation [VIN=110V, VTVS=160V, RS (Source Impedance)=0.2Ω, and t= 0.02] results in 3600 Joules. However, a typical SMB 160V TVS can only dissipate energy in the order of 10 Joules. Therefore a surge-isolation circuit is required with component values to be determined by the amplitude, duration and converter input-power level. Figure 1 shows a circuit that is designed

to protect the DC-DC converter from large transients. Each input voltage and transient

24 November 2011

Soft Start Turns Q1 on gradually at startup to lower surge current

Figure 2: Fast transient block diagram

OSC Provides gate drive to Q1 with C1, D5, D6 acting as a charge pump

C2 Decoupling and damping for input to the converter

Essentially, this surge-protection circuit is a linear regulator set for a voltage above any normal operating level, but lower than the maximum input voltage

Components in Electronics

Ruggedised converters In addition to surge and fast transient protection, the EN50155 and RIA12 standards are more exacting and stipulate further electrical-equipment requirements concerning: input voltage range; power interruption and backup (Class SI and S2); supply change-over (Class C1 and C2);

against the possibility of voltage surges and fast transients, well, then that’s one less thing for us to blame when our train still arrives late.

Luso Electronics | www.luso.co.uk

Mick Grahame-Dunn is Business Development Manager at Luso Electronics

www.cieonline.co.uk

specification will require different values of critical components. A control power source (VCC) is required to power this active circuit.

rating of the converter. During normal operation, the series FET transistor is saturated and dissipates very little power. When a transient occurs, the FET blocks the excess voltage and dissipates the transient energy while providing continuous power to the converter at a controlled voltage. The circuit can also function as a surge limiter on start-up, by holding the gate of Q1 switch at low, during the initial power-up time and then gradually allowing the circuit to drive it normally.

Fast transients Figure 1: Surge isolation block diagram D1 TVS is equal to maximum transient voltage

Q1 MOSFET switch blocking high voltages during transients and protecting the converter. It must have a safe operating area capability compatible with the converter input-power level, transient magnitude and duration

R1/R2 A voltage divider circuit to sense the voltage at the input of the converter Ref Control voltage reference supplied from Vcc

Error Amp Circuit to control the voltage at the input of the converter as set by R1/R2 and compared to a reference voltage

Rolling stock equipment must also be able to withstand a direct transient of 1800V lasting 50 microseconds. The impedance of the transient source is specified as 100Ω with transient energy of approximately 0.1 Joules. To protect the converter from such a transient, the energy must be diverted from the converter’s input and the peak voltage held at, or below, the transient input voltage specification on the converter’s datasheet. A TVS device is a good choice for this purpose. It should be connected across the converter’s input terminals to take best advantage of filtering impedances between the transient source and the converter (See Figure. 2). Any additional filtering inductance will help reduce the current in the TVS and limit its clamping voltage.

operating temperature; and shock and vibration.

As an example in meeting standards, the supply change-over specification asserts that equipment supplied with power alternatively from an accumulator battery and a stabilised source needs to operate to the following standards: Class C1 - at 0.6VIN during a period of 100ms (without interruptions); and Class C2 - during a supply break of only 30ms. Typical ruggedised converters will meet Class C1, but to meet Class C2, the capacitance at the output of the transient circuit will need to be increased, so the converter has at least the minimum voltage following the supply-line-break transient, and can maintain its output. Clearly, it is very important in tough

environments that the DC/DC converters selected are actually up to the job. One example is the ruggedised InQor DC/DC converter product line, which has been specifically designed for the harsh environments found in industrial applications and transportation industries such as railway rolling stock. These devices exceed the voltage and transient requirements in EN50155 (with the exception of the low transient and 0.7VIN requirement for 24V nominal input voltages).

So, as long as specifications have been