Designing for Reliability for Outdoor LED Lighting in the Internet of Things

Growth in smart city initiatives is driving the current boom in upgrading outdoor lighting systems with LED-based luminaires. Intelligent city lighting schemes, covering streetlights, bridges, tunnels, parking areas, parks and other recreational facilities, as well as architectural outdoor lighting, are forming a significant part of the Internet of Things (IoT) movement. Interconnected via wired and wireless networks, individual nodes can be monitored and controlled centrally and in real-time to provide significant cost savings and improved quality of life.

Outdoor lighting systems are a major contributor to global energy use, making them a prime target for utilities and civic authorities aiming to meet targets for reducing energy consumption and CO₂ output. Case studies where street lighting has been upgraded to solid -state luminaires report additional benefits such as reduced crime rates and fewer accidents, and especially reduced maintenance costs.

However, these benefits are only possible through a fully networked system that provides visibility, monitoring and control. The IoT not only provides the necessary technology, but also facilitates integration with other smart city systems, such as security/CCTV, traffic monitoring and environmental monitoring. Projects such as installing mobile broadband access points on solid-state street lighting gantries, as recently announced through an Ericsson/Royal Philips partnership, illustrates the symbiotic growth that a lighting-centric IoT can stimulate.


Figure 1: Long-life LEDs make for reduced maintenance costs in outdoor lighting, but only if the ancillary components are designed for reliability (Courtesy RECOM Lighting).

Design for reliability

While low energy usage and CO₂ savings are important factors, detecting failures and predicting end of life through the IoT networks are aspects that need consideration at the design stage. Underlying this approach is designing in reliability and long life from the start.

While the LEDs themselves are renowned for dramatically longer lifetimes than the technologies they replace, ancillary components require careful consideration. High-performance, dedicated LED drivers can make a difference. Surge protection is another factor. Additionally, efficient thermal design can strongly influence the lifetime and consistency of the light output of LEDs.

High lumen output and tough environmental conditions can take their toll on optical, electronic and physical components. MTBF figures quoted by component manufacturers generally rely heavily on the operating temperature. This makes thermal management a critical design constraint for outdoor LED lighting products. While selecting an LED module specifically engineered for the application, where appropriate thermal design is paramount, designers should consider ancillary aspects, not only from a thermal management perspective but also for ease of maintenance. A modular design with field-replaceable components is ideal.

Selection of the LED driver for outdoor applications is another critical task. A number of useful application guides are available from world-class supplier of LED drivers, Thomas Research Products (TRP). The first, ‘LED Driver Application Information’¹ highlights a number of key design considerations, including determining the LED load and the advantages of a customized/dedicated driver rather than a programmable one.

Further sections cover LED driver basics and dimming issues. Thermal management is referenced throughout, with advice on how best to ensure heat is dissipated from the driver as well as the light engine. Mounting the driver on the same heatsink as the lighting fixture may not be a good idea, the guide explains. The final section covers flicker, its causes, such as a mismatch between driver and light engine, and solutions.

With the improved quality of LEDs today, and providing that LEDs designed for lighting applications have been selected, any problems with flicker, whether on dimmable luminaires or not, is likely due to the LED driver or power supply. The key is to source a driver/switched-mode power supply that avoids low-frequency harmonic currents and high-frequency noise/ripple. Large and/or inconsistent ripple current will give rise to flickering lights. In short, the LED driver should integrate effective power-conversion techniques.

A short primer on ‘Determining LED load’² explains how to calculate how much power is needed, depending on the LEDs on board, and whether they are connected in series or parallel. Forward voltage and drive current information is needed, along with dimming requirements, power factor, constant voltage or constant current, and protections such as shorts and transients. From this assessment, a designer can better match the appropriate LED driver to the actual load of the LED light engine.

In addition to light engines, lighting controls and other ancillary devices, TRP offers a wide range of LED drivers suitable for outdoor and other rugged applications. All of its drivers are designed for long life, and either IP66 or IP67 rated for outdoor use. Many feature ‘Black Magic Thermally Advantaged’ housings. Constant-current and constant-voltage versions are available in all series.

The standard range features models with power outputs ranging from 12 to 100 W. The LED25W-36-C0700-LE, for example, features total power of 25 W, has an input voltage range up to 277 V, an output of 18 to 36 V and a constant output current of 700 mA. PWM or 0-10 V dimming options are available. Other models from 350 mA to 2080 mA can be selected. Constant voltage versions range from 12 to 72 V. Typical efficiencies run from 81 to 86%. MTBF at 40°C ambient conditions is specified at 482,000 hours, and operating temperature range is -30°C to 60°C.

The company’s PLED premium range, with power outputs from 75 to 200 W, can endure 320 V_AC overvoltage for 48 hours and 350 V_AC for 2 hours. With a high power factor of better than 0.90 at 75 to 100% load over the full input voltage range of 100 to 277 V AC, these switch-mode LED drivers are specified with an MTBF of 474,000 hours at full load and 40°C ambient conditions.

A typical example in the range is the PLED75W-166-C0450-D, with a power output of 75 W. It features a constant current output of 450 mA and voltage output range of 56 to 166 V. Dimming options can be specified. Operating temperature range is -30°C to 65°C. Overvoltage, overcurrent and short-circuit with auto-recovery output protection is provided as standard.

Modular design

RECOM Lighting, as a division of specialist power conversion company RECOM Power, also emphasizes the importance of carefully matching the driver/power supply to the LED load. Its IP67 sealed power supplies are widely used by streetlight manufacturers. The modular design approach not only provides scalability in the light output and distribution of the lamps themselves, but is also applied to the ancillary components. The AC/DC power supply with the temperature-critical electrolytic capacitor can, for example, be housed in the pole of the streetlight, where natural airflow helps cool the power supply, thereby improving reliability and lifetime. The supply drives a low-voltage LED driver in the lamp head, which does not increase the temperature of the lamp head.

The RCD-48 series constant-current drivers are built using ceramic capacitors, and can operate at temperatures up to 85oC. Power outputs range from 20 to 67 W, with current outputs at 350 mA up to 1.2 A, depending on model. Minimum voltage input is 9 V, with 2 to 56 V output range. Additional features include buck-converter topology, analog and PWM dimming options, and 96% efficiency. A typical device in the range is the 28 W output power model, RCD-48-0.50, with 500 mA constant-current output.

The LED modules can be used at full power, or optionally, controlled by a microcontroller using the PWM input to the RCD-48 driver. This enables remote switching and dimming, allowing the streetlights to be effectively networked and linked to the IoT in a smart city environment.


Figure 2: The RACD100/150 series LED driver from RECOM Lighting is IP67 rated for outdoor lighting applications.

RECOM’s extensive range of AC LED drivers are dual mode for either constant-current or constant-voltage operation, making them suitable either for driving LEDs directly, or, as referenced above, via a local constant-current DC/DC module. The RACD100 and RACD150 ranges can run at input voltages up to 277 V_AC, ideal for streetlights and outdoor area lighting. Contained in IP67 sealed metal cases, these power supplies are rated for ambient temperatures of -30 to 70°C. They feature full-load efficiency of better than 90% and are designed with active power-factor correction, delivering power factors exceeding 0.98. Analog and PWM dimming is supported. The devices are protected against short-circuit, open-circuit, overload, and over-temperature conditions.

Typical examples in the range are the Lightline RACD100-700A and RACD100-1400A, constant-current (700 mA and 1.4 A respectively), 100 W power output, AC dimmable LED drivers. Output voltages range from 100 to 142 V (700 mA model) and 50 to 71 V (1.4 A model).

Summary

Outdoor lighting, especially street lighting, not only benefits from the networking, monitoring and control functionality of being connected to the Internet of Things, but is likely to stimulate greater uptake of the IoT through hosting additional wireless sensor networks.

Designing solid-state streetlights for long life and reliability requires more than simply selecting the appropriate LED modules. Consideration must be given to electrical, thermal and mechanical factors associated with ancillary components, especially the LED driver. Understanding the LED load requirements of the application and selecting the most appropriate driver with superior power-conversion techniques and power factor is essential.

Surging ahead

Outdoor lighting, such as road lighting and parking lots are particularly susceptible to spikes on the mains power line, making surge protection an important factor in system durability. From a maintenance standpoint, it is essential to know when the surge protector needs to be replaced. The latest models from Thomas Research Products incorporate an LED indicator that lights up when the capabilities of the surge protector have been exceeded. In-line fusing shuts down the luminaire it is protecting, thereby signaling to maintenance teams that there is a failure, and also the reason for the failure.

The EOL3-277-20KA is the standard unit with a local LED indicator, while the EOL3-277-20KA RMT version allows a remote indicator to be mounted elsewhere on the luminaire. Protecting luminaires from surges up to 20 kA, the components are UL and CE certified, and suitable for street lighting, parking garages, transportation facilities and warehouses.

References:

1. Thomas Research Products ‘LED Driver Application Information’
2. Thomas Research Products ‘LED Driver Essentials’
3. Thomas Research Products ‘Determining LED Load’