Analyze the driving architecture and typical design solutions of LED area lighting

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, outdoor lighting such as industrial and retail lighting, and indoor lighting such as low-ceiling lights, high-ceiling lights, freezers/refrigerators and parking garages.

There are many challenges in high-power area lighting, such as the difficulty of approaching lamps, the potential safety issues when the light source fails, and the presence of various extreme environmental conditions 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 it takes from starting to emitting light to full brightness may be as long as 10 minutes, the poor cold temperature performance of linear fluorescent lamps, and the slow start-up 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 in high-power area lighting and provide advantages that traditional light sources do not have, such as less power consumption per lumen, better direction control, better color quality, environmental protection, and easier control of its on and off, which is convenient for automatic detection of ambient light to change brightness; in addition, LED reliability is also better, which helps to reduce maintenance costs and total cost of ownership.

LED area lighting applications require that

the main function of the LED driver is to limit current under various conditions, protect the LED from surges and other fault conditions, and provide a certain level of safety to avoid vibration (electrical and/or mechanical) and fire. For area lighting applications, the outdoor environment will bring temperature challenges to the LED driver, and it may be necessary 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 meet certain regulatory standards regarding power factor or harmonic content. For example, the IEC61000-3-2 standard of the European Union's International Electrotechnical Union (IEC) has set requirements for the harmonic content of lighting equipment (Class C) with a power of more than 25 W, 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 to be higher than 0.9 and THD to be lower than 20%.

Many regional lighting applications are outdoors and may be subject to various strict temperature conditions, which will affect the overall service life. The overall system design has an important impact on the service life, so it is very important to use high-efficiency LED drivers with less internal heat and lower losses, and in the design, the driver should be thermally isolated from the LED heat source to enhance system reliability.

The control of LED lighting can also become more intelligent. Traditional street lights are autonomously controlled by timers or ambient light sensors. The use of power line communication (PLC) or wireless control technology can provide highly flexible LED area lighting control, such as centralized control of light output levels based on time, luminous 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 lots, stairs and refrigerator cabinet lighting, which all support lighting with brightness levels adjusted as needed. LEDs can be turned on and off instantly, and can easily adjust lighting levels according to actions or activities in these applications, such as providing 20%-40% brightness levels when no activity is detected, and 100% brightness lighting when activity is detected. This helps save a lot of additional power consumption.

LED area lighting power supply architecture and typical LED drive solutions

1) Distributed/modular solutions suitable for linear lamps, trough lamps and other applications

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. According to actual needs, the number of LED light strips can be flexibly changed to increase or decrease 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

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

2) Monolithic/single-stage solution for applications such as wall washers and exterior wall lights

Not all area lighting applications require a distributed/modular solution. With the rapid improvement of white light LED performance, new LEDs have been able to match 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 such conditions, novel LED drivers can be configured to directly drive large currents of 1 A to 3 A. For example, ON Semiconductor's NCL30001 power factor corrected TRIAC dimmable LED driver 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 energy efficiency of LED power supplies. 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. 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., greater than 90%) in higher power 50 W to 250 W LED area lighting applications. To provide such high efficiency, a high-efficiency power supply topology is required, such as a resonant half-bridge dual inductor plus single capacitor (LLC) topology, which can take full advantage of zero voltage switching (ZVS).

In such higher power LED area lighting applications that require ultra-high 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 50 to 300 W range. NCP1397 is the latest high-performance resonant mode LLC controller with integrated 600 V high-voltage floating driver, supporting high-frequency operation from 50 to 500 kHz, built-in high-end and low-end drivers, supporting adjustable and precise minimum frequency, providing extremely high energy efficiency, and having multiple fault protection features.

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

Protection solution to enhance LED string reliability

Multiple strings of LEDs are usually used in regional lighting applications. Although the LED itself is highly reliable, if one LED in the LED string is open, the entire string of LEDs may be turned off, 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. In case one LED in the LED string is open, it will provide current bypass to ensure that the entire string of LEDs will not be turned off under the condition of a certain LED failure; and if the heat dissipation is properly handled, it can also support large currents greater than 1 A.