LED area lighting drive architecture and typical design

From the perspective of application areas, lighting covers different categories such as residential lighting, industrial lighting, street lighting, and restaurant, retail and service lighting. From the perspective of power level, in addition to low-power lighting, it also includes high-power area lighting, with typical applications such as column lights, wall washers, exterior wall lights, tunnel lighting, street lights, parking lots and public safety lighting, industrial and retail lighting and other outdoor lighting, as well as low-top lights, high-top lights, freezers/refrigerators and parking garages and other indoor lighting.

　　There are many challenges in high-power area lighting, such as the difficulty of approaching the lamps, the potential safety issues when the light source fails, and the multiple extreme environmental conditions in the outdoors. In addition, it cannot be ignored that the existing light sources used for high-power area lighting (such as metal halide lamps, high-pressure sodium lamps, linear fluorescent lamps and compact fluorescent lamps) have many limitations, such as the poor color rendering of high-pressure sodium lamps (CRI is about 22), the high typical lamp loss of metal halide lamps (40%) and the time from start to full brightness can be as long as 10 minutes, the poor cold temperature performance of linear fluorescent lamps, and the slow start speed of compact fluorescent lamps.

　　On the other hand, as high-brightness white light emitting diodes ( LEDs ) continue to improve in terms of performance and cost, they are increasingly used for high-power area lighting and provide advantages that traditional light sources do not have, such as less power consumption per lumen of light, better directional control, better color quality, environmental protection, and easier control of turning them on and off, allowing for automatic detection of ambient light to change brightness; in addition, LEDs are more reliable, which helps reduce maintenance costs and total cost of ownership.

　　1. Application requirements of LED area lighting

　　The main function of the LED driver is to limit the current under various conditions, protect the LED from surge and other fault conditions, and provide a certain level of safety to avoid vibration and fire (electrical and/or mechanical). For area lighting applications, the outdoor environment will bring temperature challenges to the LED driver, and it may need to withstand higher AC input voltages than standard voltages such as 277 Vac, 347 Vac or even 480 Vac.

　　LED drivers for area lighting applications may also need to comply with certain standards regarding power factor or harmonic content. For example, the IEC61000-3-2 standard of the European Union's International Electrotechnical Commission (IEC) sets requirements for the harmonic content of lighting equipment with a power of more than 25 W (Class C), which is equivalent to a total harmonic distortion (THD) of less than 35%; but meeting the IEC61000-3-2 Class C harmonic content requirements does not necessarily mean that the power factor (PF) is higher than 0.9. Some markets (such as the United States) usually require PF higher than 0.9 and THD lower than 20%.

　　Many area lighting applications are outdoors and may be subject to a variety of severe temperature conditions, which may affect the overall service life. The overall system design has an important impact on the service life, so it is very important to use energy-efficient LED drivers with less internal heat and lower losses, and to thermally isolate the driver from the LED heat source in the design to enhance system reliability.

　　LED lighting control can also become more intelligent. Traditional street lights are autonomously controlled by timers or ambient light sensors. Using power line communication (PLC) or wireless control technology, highly flexible LED area lighting control can be provided, such as centralized control of light output levels based on time, light level control based on traffic flow sensors, and regulating city center lighting based on the detection of human and vehicle activities, taking into account pedestrian and street lighting. LED intelligent control technology saves energy without compromising safety. Typical applications include intelligent dual brightness level lighting, such as parks, gas station roofs, parking spaces, stairs and refrigerator cabinet lighting, which all support lighting with brightness levels adjusted according to needs. LEDs can be turned on and off instantly, and can easily adjust the lighting level according to actions or activities in these applications, such as providing 20%-40% brightness level when no activity is detected, and 100% brightness lighting when activity is detected. This helps save a lot of additional power consumption.

　　2. LED area lighting power supply architecture and typical LED drive solutions

　　1) Distributed/modular solutions for linear lights, trough lights, etc.

　　In high-power LED area lighting applications, a common power supply architecture is a three-stage architecture of "power factor correction (PFC) + constant voltage (CV) + constant current (CC)". In this architecture, the AC input power is converted by power factor correction and isolated DC-DC (DC-DC) to output a fixed voltage of 24 to 80 Vdc, which is provided to the constant current LED module with a built-in DC-DC buck conversion circuit at the back (see Figure 2). The design of this architecture provides a modular approach that can be upgraded on site. The number of LED light strips can be flexibly changed according to actual needs, thereby increasing or decreasing light output to meet specific area lighting application requirements. In this architecture, the AC-DC conversion and LED drive circuit are not integrated together, but a distributed configuration is adopted, which simplifies safety considerations and enhances system flexibility. It is also called a distributed solution. Typical applications include linear lamps and trough lamps.

Figure 1 Schematic diagram of a typical modular LED area lighting power supply architecture　　In this modular approach, a design can be expanded for multiple light output levels. And as LED light output performance increases, the LED modules need to provide the same light output level, and the light bar needs to be better. Each light bar has a dedicated DC-DC  LED driver , such as the CAT4201 high-efficiency buck LED driver from ON Semiconductor. The CAT4201 is optimized for driving high-current LEDs and uses a patented switch control algorithm to provide high efficiency and accurate LED current regulation (up to 350 mA). The CAT4201 can be powered by a supply voltage of up to 36 V and is compatible with 12 V and 24 V standard lighting systems. Figure 3 shows the high-voltage LED driver configuration of the CAT4201, with the peripheral N-channel MOSFET supporting high-voltage input: 30 W LED power at 100 V input voltage; 13 W LED power at 50 V input.

Figure 2 CAT4201 high voltage LED driver configuration

　　2) Integral/single-stage solutions suitable for wall washers, exterior wall lights, etc.

　　Not all area lighting applications require a distributed/modular solution. With the rapid improvement in the performance of white light LEDs, new LEDs have been able to adapt to new LED driver design methods. Leading LED manufacturers have introduced new LEDs that support higher currents and have higher luminous performance, such as Cree 's XP-G series LEDs (forward voltage drop of 3.3 V) that can provide 330 lumens of light output at 1 A current, and Seoul Semiconductor's P7 series LEDs (forward voltage drop of 3.3 V) that can provide 400 lumens of light output at 1.4 A. Under these conditions, novel LED drivers can be configured to directly drive high currents of 1 A to 3 A. For example, the NCL30001 power factor corrected TRIAC dimmable LED driver from ON Semiconductor can be used.

　　NCL30001 is an integrated/single-stage LED driver solution that integrates PFC and isolated DC-DC conversion circuits and provides constant current to directly drive LEDs. This solution is equivalent to integrating the AC-DC conversion and LED drive circuits together, both of which are located in the lighting fixture, saving the linear or DC-DC converter integrated in the LED light strip. This integrated solution has fewer power conversion stages, reduces the number of components used (such as optical components, LEDs, electronic components and printed circuit boards), reduces system costs, and supports higher overall LED power efficiency. Of course, this solution has a higher power density and may not be suitable for all area lighting applications. Its optical pattern may be more suitable for lower-power LEDs. Typical applications include LED street lights, exterior wall lights, wall washers and refrigerator cabinet lighting.

Figure 3. 90 W LED driver demonstration board circuit based on NCL30001 LED driver and NCS1002 controller

　　This 90 W constant voltage and constant current demo board accepts an extended universal input voltage of 90 to 265 Vac (supports 305 Vac with component replacement), provides a constant current output range of 0.7 A to 1.5 A (selectable by slightly adjusting the resistor) and a constant output voltage range of 30 V to 55 V (selectable by a resistor divider), with a maximum output power of 90 W, supports 50 to 1,000 Hz dimming control, and includes a 6-pin interface that can be connected to an optional dimming card for intelligent dimming applications such as analog current regulation/dual brightness level digital dimming. In addition, this demo board also provides a variety of protection features such as short circuit protection, open circuit protection, over-temperature protection, over-current protection, and over-voltage protection. Tests show that the demo board has an efficiency of more than 87% at 50 W output power, 1,000 mA output voltage/48 V forward voltage drop (see Table 1 for details), a power factor of more than 0.9 at 50% to 100% load conditions, and complies with the IEC61000-3-2 Class C equipment harmonic content standard.

Figure 4 Energy efficiency test results of a 90 W LED driver demonstration board based on NCL30001 and NCS1002　　3) High-efficiency LLC topology driver for higher power area lighting applications

　　In recent years, the industry has shown a growing interest in ultra-high-efficiency LED lighting topologies, hoping to provide high efficiency (e.g., higher than 90%) in higher-power 50 W to 250 W LED area lighting applications. To provide such high efficiency, it is necessary to adopt a high-efficiency power supply topology, such as a resonant half-bridge dual-inductor plus single-capacitor (LLC) topology, to give full play to the advantages of zero voltage switching (ZVS).

　　In this type of higher power LED area lighting application that requires ultra-high energy efficiency, ON Semiconductor's NCP1607 PFC controller and NCP1397 dual inductor plus single capacitor (LLC) half-bridge resonant controller can be combined for high-efficiency LED street lighting applications in the power range of 50 to 300 W. The NCP1397 is the latest high-performance resonant mode LLC controller with an integrated 600 V high-voltage floating driver, supporting high-frequency operation from 50 to 500 kHz, built-in high-side and low-side drivers, supporting adjustable and accurate minimum frequency, providing extremely high energy efficiency, and with a variety of fault protection features.

Figure 5 High-efficiency LED power supply solution for street lighting based on NCP1607 and NCP1397

　　3. Protection scheme to enhance LED string reliability

　　Multiple strings of LEDs are often used in area lighting applications. Although LEDs are highly reliable, if one LED in the LED string is open, the entire string of LEDs may shut down, and this situation should be avoided in applications such as street lighting to reduce subsequent maintenance costs. ON Semiconductor has launched the NUD4700 LED current bypass protector. This device is a shunt device that provides current bypass in the event that one LED in the LED string is open, ensuring that the entire string of LEDs will not shut down if one LED fails; and if heat dissipation is properly handled, it can also support large currents greater than 1 A.