A design scheme for high-power LED driver power supply

According to statistics, up to 20% to 22% of electrical energy is used for lighting. Improving the energy efficiency of lighting applications and even further reducing their energy consumption will help reduce carbon dioxide emissions and create a greener and more environmentally friendly world. Therefore, high-efficiency lighting is becoming a focus of competition in the industry.

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, and environmental protection. In addition, their on and off can be more easily controlled, 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.

LED area lighting application requirements

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.

Figure 1: Example of intelligent dual brightness level LED street lighting.

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.

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 the light output to meet the requirements of specific area lighting applications. 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.

With this modular approach, one design can be scaled for multiple light output levels. And as LED light output performance increases, the LED modules will need to provide the same light output level with better light bars. And each light bar has a dedicated DC-DC.

Figure 2: Schematic diagram of a typical modular LED area lighting power supply architecture.