Developing smart LED-based lighting systems
The different types of LED-based lighting applications and their requirements vary widely (Table 1). To enable the complex control functionality (e.g. specialised colour mixing, balancing and dimming, adaptive lighting control, remote connectivity and self-diagnostics) and consistent light quality required for more advanced lighting applications, simple fixed-function LED drivers may become insufficient. Although fixed-function LED drivers have many advantageous qualities (low-cost, good power efficiency, no software programming, and ease of use) and are appropriate for many lighting systems, their capability for intelligent lighting systems is limited. Similarly, with relatively fixed function operation, more flexible solutions become advantageous when looking at design reuse across a product line. Significant cost reductions can be realised in design and purchasing when varying power and feature set requirements can be implemented without significant hardware modification.
Table 1. LED-based lighting applications and requirements
Smart LED-based lighting systems require digital control using programmable microcontroller-based architectures that enable increased levels of intelligence and design flexibility, as well as efficient and cost-effective control of the luminaire power supply. Such architectures are able to support varying LED types and string lengths/numbers, and unique power stage requirements, generally with no major hardware changes. Most importantly, programmability also enables more advanced lighting control (sensing functions, remote connectivity, timing schedules, etc.), and likewise, this flexibility afforded by digital control means that a single controller can support a wider range of products to help lower overall system costs.
Smart LED-based lighting systems
By employing a flexible digital approach, it is possible for a single microcontroller, with sufficient performance, optimised power-control peripherals and robust communications ports, to provide a programmable platform that controls the three main stages of a smart LED-based lighting system; i.e. power conversion (precise/flexible control of AC rectification, power factor correction [PFC] and DC/DC conversion), LED control, and communications (Figure 1). This avoids the need for separate controllers for each stage as is necessary when using a fixed-function, analogue approach. This high level of integration reduces system complexity, and the component cost of a lighting unit power supply.
Figure 1: The main stages of a smart LED-based lighting system
Digital power control can enable greater conversion efficiency for dynamic systems. Thus, when adjusting the light (i.e. dimming, colour changes etc.) greater efficiency is possible in the power stages of an LED-based lighting system. In addition, the ability to use more advanced power stage design potentially increases operating efficiency under fixed lighting conditions. Such efficiency gains can result in substantial cost savings for end-users (see street lighting example, Figure 2), and help differentiate between similar LED-based systems.
Consistency of light quality
An intelligent digital controller can compensate for the main factors (LED ageing, temperature variations, LED manufacturing variations) that influence LED performance and light quality consistency i.e. colour/intensity (Table 2).
Table 2. Factors influencing LED performance and light quality consistency
With an intelligent microcontroller it is possible to compensate for:
LED ageing and correct the colour profile to ensure the light quality is consistent over the LED lifespan Changes in ambient temperature via a temperature sensor (low overhead function) and adjusting the LED drive to maintain light colour/intensity. Also, the microcontroller can lower the light intensity or shut down LED strings if the temperature threshold is surpassed: the system monitors safe operation (and remotely communicates any problems) to ensure the lifespan of the LED is maximised Any variations in LED manufacturing to ensure consistency between products
Improved safety and efficiency
The intelligent digital controller can also improve safety and efficiency, particularly when combined with remote connectivity and sensors. Specifically, it can control when individual lights are turned off or dimmed depending on how the lights are being used. In a warehouse for example, occupancy sensors can be used to ensure that only the areas that are currently being used are illuminated, resulting in considerable energy savings. Similarly, intelligent street lighting can respond to the ambient light; turning on early in overcast conditions or remaining off for longer in bright conditions to save power.
Motion sensors can be used with a communication system to monitor evening/night traffic levels so that the street lights can be dynamically turned on and off in response to the traffic conditions. When the traffic is heavy the street lighting can be at full illumination, and when the traffic is light the street lighting illumination can be reduced, resulting in considerable cost savings. Taking the example in Figure 2, if the lights can be shut down for 25% of the time, this will lead to 25% energy savings ($68,218). Combining the savings of this intelligent operation with power supply efficiency savings, there is a substantial total annual operational saving of $101,844 (33% reduction).
An important feature of intelligent lighting systems is remote control. This facilitates significant improvements in quality and efficiency (and cost savings) via the automatic management of certain operations. Various remote operations, including dimming, shutoff and emergency control, are made possible by networking components and coordinating operations across the entire system. To maximise functionality, the lighting system must be able to communicate with a centralised controller. This will, for example, avoid having to adjust individual lights and allow the light intensity of the entire lighting installation to be altered from a central location.
In order to perform simple self-diagnostics, a two-way communication between each unit and the central operator is necessary. There are many established communications standards including DALI, DMX512, and KNX. This communication allows technicians to check the system remotely, and for maintenance to be initiated only when a problem has been identified by the system (i.e. a burnt out or poorly performing LED), reducing the need for scheduled maintenance. Technical problems can be identified immediately, ensuring increased operational safety. This can lead to significant savings in maintenance costs.
Several advanced features that improve efficiency and reduce operating costs are possible with remote connectivity. These include:
Dynamic control of multiple lighting installations from a single distant location Accurate tracking of power consumption Data logging of actual usage
Dynamic light control allows operators to make changes remotely to lighting schedules rather than sending a technician to the location e.g. making street light adjustments for daylight saving times. Unexpected or unscheduled lighting changes can also be made remotely. For example during a seasonal busy period in a factory the time the lights remain on can be extended temporarily, or roads can remain illuminated for longer in certain circumstances i.e. after a late-night music concert or sporting event. Direct control of street lighting can also improve safety during emergency situations.
Again, taking the street lighting example, where cities often pay a set fee for the power used to operate them, irrespective of usage, the ability to accurately track power consumption represents a major benefit and potential cost saving. An intelligent controller can measure the amount of power being consumed and communicate the data remotely to a central location. This ensures that only the actual amount of power used is paid for, which can lead to substantial savings.
The future planning of operating costs, maintenance resources, and investment is essential. Data logging of actual usage makes this task much easier and enables more sophisticated predictive diagnostics to be employed. Operators can be alerted to, and quickly solve, potential issues e.g. increased energy consumption or a rise in the number of replacement bulbs, therefore minimising operating and maintenance expenditure.
Exploiting power line communications
Power Line Communication (PLC) uses existing power lines to send and receive data. Electrical power companies use PLC to transmit operational data relating to the power grid over many miles of electrical cable to a central location. PLC can be exploited by engineers to provide a communication link for a LED-based lighting system by connecting it over the same lines that power the application. This eliminates the need for a dedicated cable to act as a communication link.
Rather than implement the full PLC standard, developers can employ a low data rate PLC implementation such as Texas Instruments’ PLC-lite, a simple, flexible standard that is particularly applicable for low-cost applications requiring a robust communications link e.g. simple light bulbs or wall switches within a home network.
Compared with more complex PLC types (i.e. G3 or PRIME), there is a considerably lower cost per link, owing to the lower data rate and reduced protocol overhead. The flexibility of PLC-Lite allows developers to implement specific channel characteristics that improve link robustness in situations where line interference may be a problem.
Radio frequency (RF) technology can also be utilised to wirelessly connect devices. The modular architecture allows the connectivity technology to be employed that is most applicable to the end user. Data is transmitted to the microcontroller over a standard I2C or SPI port for both PLC- or Wi-Fi-based links.
Intelligent digital solution – the Piccolo™ microcontroller platform
Texas Instruments’ C2000™ Piccolo platform of microcontrollers (Figure 3) is designed to support a wide range of lighting applications. Piccolo microcontrollers have a high-performance, highly integrated architecture, providing flexible digital power control to support a variety of power topologies. Integrated I2C, SPI, UART, USB and CAN peripherals are available with production-ready firmware drivers to meet the connectivity needs of all applications. The advanced PWM generation (Table 3) and 12-bit high-resolution analog-to-digital converter (ADC) modules (with fast sampling and conversion speeds; 4.6 Mega samples/second) enable the creation of a tight feedback loop to react rapidly to shifting operating conditions.
Figure 3. The Piccolo platform of microcontrollers
The Piccolo platform of microcontrollers can support a range of applications from entry-level devices to complex, multi-string systems with PLC. The 32-bit TMS320C28x™ core can implement power stage calculations, LED string control and lighting protocols such as DMX512. The Piccolo microcontrollers possess programmable flexibility, optimised math operation, and real-time control (via a responsive, interrupt-driven architecture).
Piccolo microcontrollers even offer dual core architecture configurations. Alongside the CX28x core, Piccolo F2803x devices have a separate independently running Control Law Accelerator (CLA) core that provides parallel processing. The lighting system functions and communications can be separately partitioned between the C28x core (digital power conversion and LED string control) and the CLA core (PLC algorithms). For advanced/higher bandwidth PLC applications, Piccolo F2806x microcontrollers are available with an additional integrated Viterbi Complex Math Unit (VCU) to speed up PLC processing.
Piccolo microcontrollers are available to support a wide range of intelligent lighting systems:
Piccolo F2802x microcontrollers for low-cost systems: sufficient performance to reduce system component count, real-time digital power technology, supports DALI, DMX512, and KNX Piccolo F2803x microcontrollers for entry-level PLC remote connectivity: supports PLC-Lite, with additional LED channels and performance than the F2802x Piccolo F2806x microcontrollers for high-performance systems: supports advanced PLC and USB, with additional LED channels and processing capabilities.
In some lighting systems, however, the presence of high and low voltages dictates that an isolation boundary is needed to separate PFC and DC/DC conversion. In this case, it may be simpler to use two Piccolo microcontrollers that communicate via an I2C or SPI interface, owing to the difficulty crossing the boundary (Figure 4). It can, however, be more cost effective to implement PFC and DC/DC conversion on a single microcontroller if the design is non-isolated.
Figure 4. LED-based lighting system with isolation boundary
In order to help support LED-based lighting design engineers create new systems, a range of development hardware and software is available for all applications from low voltage to remotely connected full-AC mains powered systems. Tools offered by Texas Instruments include the:
TMS320C2000 AC LED Lighting and Communications Developer’s Kit for AC mains-powered, intelligent lighting products (Figure 5) DC/DC LED Lighting Developer’s Kit Multi-DC/DC Colour LED Kit Power Line Communications (PLC) Add-on Kit LED BoosterPack™ with the C2000 LaunchPad™ or MSP430™ LaunchPad, supported by the C2000 controlSUITE software.
Figure 5: The TMS320C2000 AC LED Lighting and Communications Developer’s Kit with high operating efficiency (around 90%), remote connectivity and lighting communication protocol support (e.g. DALI, DMX512, KNX and PLC)
For more information, visit https://www.ti.com/ww/en/lighting/products.htm.
Patrick Carner – biography
Patrick Carner is the C2000™ microcontrollers and lighting applications marketing manager at Texas Instruments. He is responsible for product definition and positioning, customer design engagements, business development, and customer support for the C2000 product line. Carner received a B.S. in EECS from the University of California at Berkeley.