Low power DC/DC converters in the railway environment
2019. 8. 7
In the complex international railway market, efficient utilisation of rolling stock is key. Sensing and reporting on asset condition, status and position enables ‘smart’ decisions to be made improving rolling stock availability and lowering costs. However, multiple sensors and transmitters need power supplies which must be low cost, easily integrated and compliant with railway electrical and environmental specifications to enable existing rolling stock to be economically upgraded. This article describes the implementation of such DC/DC converters in typical railway sensing applications and introduces an off-the-shelf range which holds the necessary certifications.
Rail networks worldwide are expanding, predicted to reach 1.38 million route-kilometers round the world by 2020 with a market value of 180 billion euros [1]. Particular growth areas are in the Asia-Pacific region with both China and India investing heavily in railway infrastructure. Drivers for growth are the increase in affluence and mobility in emerging economies, rail-tourism and the push for more sustainable and environmentally-friendly transportation than road traffic. The current track network is rapidly becoming over-congested and older rolling stock is being pressed into service to meet the demand, with any failures leading to delays, network interruptions, or even catastrophic derailments. Many new lines are being developed and new rolling stock being ordered, but planning delays, troubles raising the necessary financing and cost overruns make expanding the existing rail network difficult.
Asset utilisation efficiency is key
A way to improve network capacity is to maximize utilisation efficiency of new and legacy stock by making it ‘smart’. This includes better tracking of the position of active and idle stock, so traffic density can be increased, and also incorporation of sensors to monitor the operational condition of the assets. Remote Condition Monitoring (RCM) is the technique for collecting data, and Condition Based Maintenance (CBM) is the process of analysing data to predict when maintenance or repair is necessary. These two techniques enhance reliability, improve availability and save costs by scheduling necessary repairs and avoiding unnecessary preventative maintenance.
‘Condition monitoring’ includes simple status information such as distance travelled and carriage occupancy rates which can be used for further passenger analysis and efficiency savings. The idea fits in with the wider trend towards the Internet of Things (IoT) - a report by Cisco [2] suggests that in the next 12 years, the rail sector will spend around $30 billion on IoT.
Remote Condition Monitoring needs sensors
Many parameters can be sensed to determine the condition and status of assets including axle counters, bearing temperature, shock/vibration, acoustic noise, supply voltage fluctuations, door operation cycles, occupancy, light levels, air quality and more. Although basic sensors can be passive components such as a thermistor for temperature, the trend is to add intelligence to the sensor, or ‘edge computing’, in the form of a digital signal processor (DSP), data logger and a wired or wireless interface, perhaps using long range (LoRa) or WLAN radio. GPS on rolling stock might also be included for detailed position tracking. Having a self-contained, communicating sensor means upgrades for older assets are easier and in new builds, cabling is reduced and features can be added or customised as needed.
Sensors need power
Most sensors need just a few watts at a low voltage to operate, derived from a DC/DC converter supplied by the main system power rail. This is typically 110VDC, although it can be other nominal values down to 24V (Figure 1). Power requirements can be higher in multi-channel sensor arrangements or when a local actuator is driven, up to perhaps 40W. Practically, the sensitive, low voltage supply for the sensor and its processor must be derived locally – distributing the voltage from a high-power central converter would cause unacceptable transient and noise pick-up issues. Additionally, galvanic isolation is needed in a DC/DC converter for each sensor to avoid ground loops and cross interference.
Figure 1. Possible rail supply nominals and their variations
A problem is that the 110V rail is far from ‘clean’; railway standard EN 50155-2017 states that voltage can vary +25%/-30% in normal operation with dips to 60% and surges to 140% of nominal for short periods – 100ms with ‘no deviation of function’ and for one second with some performance degradation. Practically, the connected DC/DC converter must operate over the full range with some margin, typically 43 – 160VDC. Figure 1 also shows the ‘standard’ 4:1 input ranges of DC/DC converters illustrating how some parts can cover at least some of the rail requirements, with an ‘ideal’ converter covering all variations with an input range of 10:1.
The DC/DC converter must often also withstand fast transient overvoltages as defined in the EN 61000-4-x series of standards, but simple LC filters and transient suppressors can attenuate these. EN 50155 additionally defines interruptions of supply in three classes S1, S2 and S3, the worst case being 20ms loss of supply from nominal input with no performance degradation. This normally requires hold-up capacitance external to the converter after an isolating series diode, which also provides reverse polarity protection.
Figure 2 shows an example application where a carriage temperature sensor causes a fan to operate and signals temperature and status via a wireless connection. Here a very compact (32 x 20 x 10mm) RECOM 8W DC/DC converter with an input range of 43V to 160V provides an isolated, regulated low voltage power rail for the sensor circuitry. Reverse polarity protection and hold up is included. The DC/DC already has high levels of EMC compliance according to EN 50121-3-2, the standard for electromagnetic compatibility for rolling stock, but an additional EMI filter can be included for higher immunity to transients and still lower conducted emissions.
Figure 2: A temperature sensor in a rail application
RIA 12 compliance
There may also be a requirement to comply with the UK RIA 12 legacy specification which defines higher energy surges than EN 50155, up to 385V for 20ms in 110V systems. As the source impedance is just 0.2 ohms, simply clamping the input with a transient voltage suppressor would dissipate too much power and either blow the input fuse or damage the TVS. A solution is a pre-regulator (Figure 3) which regulates down the surge voltage to a preset maximum value. RECOM offers three different modules for DC/DC converters with 20W, 150W or 300W.
Figure 3: Surge protector for RIA 12 applications
Environmental stress
DC/DC converters in railway sensing applications can be subject to shock, vibration, temperature and humidity extremes. The level depends on the area where the equipment is installed, with standard EN 61373 defining the categories and levels. Most DC/DC power supplies will be mounted in the relatively benign Category 1, Class B environment (body mounted, inside an enclosure) but may still require additional ruggedisation and encapsulation. Equipment service life expectation might be 20 years, so designs have to be thoroughly verified and validated for reliability with a complete range of tests including full performance characterization, HALT, temperature cycling and high temperature soak. As 110VDC is an ‘unsafe’ voltage, converters must also have certified reinforced insulation to protect against electric shock where secondary connections are accessible.
Qualified off-the-shelf solutions are available
The Austrian company RECOM [3] and their new Italian acquisition Power Control Systems (PCS) [4] have many rugged EN 50155 compliant DC/DC converter products or complete turn-key solutions for railway applications from low power (8W-240W) modules up to 2kW custom power supplies, with a wide choice of input voltage ranges available covering all of the nominal railway values including ultra-wide 16 – 160V inputs.
With a long experience of rail applications, the companies also offer comprehensive engineering support, detailed EMC evaluation and environmental compliance reports. PCS supplies cassettes, cards and rack-mount products to meet individual customer needs, whereas RECOM concentrates on low power PCB-mount DC/DC modules and also offers reference designs which include all of the necessary EMI filtering for EN 50121-3-2 compliance for 24 - 48V or 72 - 110V DC nominal input voltage converters. (R-REF04-RIA12-1 and R-REF04-RIA12-2 respectively).
Railway grade DC/DC converters and power supplies provide an easy and cost efficient route to incorporate low power sensing and telemetry in existing rolling stock and new builds helping to enable IoT in the world of railways.
References
[1] Statista: https://www.statista.com/topics/1088/rail-industry/
[2] Cisco - The internet of Everything: Statista: PDF What is the internet of everything/
[3] RECOM: www.recom-power.com
[4] Power Control Systems: https://www.powercontrolsystems.com
RECOM: We Power your Products
Asset utilisation efficiency is key
A way to improve network capacity is to maximize utilisation efficiency of new and legacy stock by making it ‘smart’. This includes better tracking of the position of active and idle stock, so traffic density can be increased, and also incorporation of sensors to monitor the operational condition of the assets. Remote Condition Monitoring (RCM) is the technique for collecting data, and Condition Based Maintenance (CBM) is the process of analysing data to predict when maintenance or repair is necessary. These two techniques enhance reliability, improve availability and save costs by scheduling necessary repairs and avoiding unnecessary preventative maintenance.
‘Condition monitoring’ includes simple status information such as distance travelled and carriage occupancy rates which can be used for further passenger analysis and efficiency savings. The idea fits in with the wider trend towards the Internet of Things (IoT) - a report by Cisco [2] suggests that in the next 12 years, the rail sector will spend around $30 billion on IoT.
Remote Condition Monitoring needs sensors
Many parameters can be sensed to determine the condition and status of assets including axle counters, bearing temperature, shock/vibration, acoustic noise, supply voltage fluctuations, door operation cycles, occupancy, light levels, air quality and more. Although basic sensors can be passive components such as a thermistor for temperature, the trend is to add intelligence to the sensor, or ‘edge computing’, in the form of a digital signal processor (DSP), data logger and a wired or wireless interface, perhaps using long range (LoRa) or WLAN radio. GPS on rolling stock might also be included for detailed position tracking. Having a self-contained, communicating sensor means upgrades for older assets are easier and in new builds, cabling is reduced and features can be added or customised as needed.
Sensors need power
Most sensors need just a few watts at a low voltage to operate, derived from a DC/DC converter supplied by the main system power rail. This is typically 110VDC, although it can be other nominal values down to 24V (Figure 1). Power requirements can be higher in multi-channel sensor arrangements or when a local actuator is driven, up to perhaps 40W. Practically, the sensitive, low voltage supply for the sensor and its processor must be derived locally – distributing the voltage from a high-power central converter would cause unacceptable transient and noise pick-up issues. Additionally, galvanic isolation is needed in a DC/DC converter for each sensor to avoid ground loops and cross interference.
Figure 1. Possible rail supply nominals and their variations
A problem is that the 110V rail is far from ‘clean’; railway standard EN 50155-2017 states that voltage can vary +25%/-30% in normal operation with dips to 60% and surges to 140% of nominal for short periods – 100ms with ‘no deviation of function’ and for one second with some performance degradation. Practically, the connected DC/DC converter must operate over the full range with some margin, typically 43 – 160VDC. Figure 1 also shows the ‘standard’ 4:1 input ranges of DC/DC converters illustrating how some parts can cover at least some of the rail requirements, with an ‘ideal’ converter covering all variations with an input range of 10:1.
The DC/DC converter must often also withstand fast transient overvoltages as defined in the EN 61000-4-x series of standards, but simple LC filters and transient suppressors can attenuate these. EN 50155 additionally defines interruptions of supply in three classes S1, S2 and S3, the worst case being 20ms loss of supply from nominal input with no performance degradation. This normally requires hold-up capacitance external to the converter after an isolating series diode, which also provides reverse polarity protection.
Figure 2 shows an example application where a carriage temperature sensor causes a fan to operate and signals temperature and status via a wireless connection. Here a very compact (32 x 20 x 10mm) RECOM 8W DC/DC converter with an input range of 43V to 160V provides an isolated, regulated low voltage power rail for the sensor circuitry. Reverse polarity protection and hold up is included. The DC/DC already has high levels of EMC compliance according to EN 50121-3-2, the standard for electromagnetic compatibility for rolling stock, but an additional EMI filter can be included for higher immunity to transients and still lower conducted emissions.
Figure 2: A temperature sensor in a rail application
RIA 12 compliance
There may also be a requirement to comply with the UK RIA 12 legacy specification which defines higher energy surges than EN 50155, up to 385V for 20ms in 110V systems. As the source impedance is just 0.2 ohms, simply clamping the input with a transient voltage suppressor would dissipate too much power and either blow the input fuse or damage the TVS. A solution is a pre-regulator (Figure 3) which regulates down the surge voltage to a preset maximum value. RECOM offers three different modules for DC/DC converters with 20W, 150W or 300W.
Figure 3: Surge protector for RIA 12 applications
Environmental stress
DC/DC converters in railway sensing applications can be subject to shock, vibration, temperature and humidity extremes. The level depends on the area where the equipment is installed, with standard EN 61373 defining the categories and levels. Most DC/DC power supplies will be mounted in the relatively benign Category 1, Class B environment (body mounted, inside an enclosure) but may still require additional ruggedisation and encapsulation. Equipment service life expectation might be 20 years, so designs have to be thoroughly verified and validated for reliability with a complete range of tests including full performance characterization, HALT, temperature cycling and high temperature soak. As 110VDC is an ‘unsafe’ voltage, converters must also have certified reinforced insulation to protect against electric shock where secondary connections are accessible.
Qualified off-the-shelf solutions are available
The Austrian company RECOM [3] and their new Italian acquisition Power Control Systems (PCS) [4] have many rugged EN 50155 compliant DC/DC converter products or complete turn-key solutions for railway applications from low power (8W-240W) modules up to 2kW custom power supplies, with a wide choice of input voltage ranges available covering all of the nominal railway values including ultra-wide 16 – 160V inputs.
With a long experience of rail applications, the companies also offer comprehensive engineering support, detailed EMC evaluation and environmental compliance reports. PCS supplies cassettes, cards and rack-mount products to meet individual customer needs, whereas RECOM concentrates on low power PCB-mount DC/DC modules and also offers reference designs which include all of the necessary EMI filtering for EN 50121-3-2 compliance for 24 - 48V or 72 - 110V DC nominal input voltage converters. (R-REF04-RIA12-1 and R-REF04-RIA12-2 respectively).
Railway grade DC/DC converters and power supplies provide an easy and cost efficient route to incorporate low power sensing and telemetry in existing rolling stock and new builds helping to enable IoT in the world of railways.
References
[1] Statista: https://www.statista.com/topics/1088/rail-industry/
[2] Cisco - The internet of Everything: Statista: PDF What is the internet of everything/
[3] RECOM: www.recom-power.com
[4] Power Control Systems: https://www.powercontrolsystems.com
RECOM: We Power your Products
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