High-power LED area lighting challenges and driver solutions

As people's awareness of energy conservation and environmental protection increases, the industry is paying more and more attention to the impact of energy consumption on the environment. According to statistics, among various energy consumption channels, up to 20% to 22% of electricity 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.

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, 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-ceiling lights, high-ceiling 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 lamps, safety issues that may arise when the light source fails, and a variety of extreme environmental conditions outdoors. In addition, it cannot be ignored that existing light sources 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 poor color rendering of high-pressure sodium lamps (CRI of about 22), high typical lamp loss of metal halide lamps (40%) and the time it takes from startup to full brightness may be as long as 10 minutes, linear fluorescent lamps have poor cold temperature performance, and compact fluorescent lamps have a slow startup speed.

On the other hand, as high-brightness white light emitting diodes (LEDs) continue to improve in 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 directional control, better color quality, environmental protection, and easier control of turning on and off, which facilitates 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 LED drivers is to limit current under various conditions, protect LEDs from surges and other fault conditions, and provide a certain level of safety to avoid vibration and fire (electrical and/or mechanical means). For area lighting applications, outdoor environments pose temperature challenges to LED drivers and 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 meet certain specifications for power factor or harmonic content. For example, the IEC61000-3-2 standard of the European Union's International Electrotechnical Commission (IEC) sets requirements for harmonic content for 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) generally require PF higher than 0.9 and THD lower than 20%.

Many area lighting applications are outdoors and may be subject to various stringent temperature conditions, which may affect the overall service life. The overall system design has a significant impact on the service life, so it is very important to use high-efficiency LED drivers with less internal heat and lower losses. In addition, the driver and LED heat source should be thermally isolated 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 driving solutions 1) Distributed/modular solutions suitable for linear lights, trough lights and other applications In high-power LED area lighting applications, a common power supply architecture is the 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 driving 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 lights and trough lights.

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

With this modular approach, one design can be expanded to multiple light output levels. And as LED light output performance increases, LED modules that provide the same light output level require better light strips. Each light strip 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 peripheral N-channel MOSFETs supporting high-voltage inputs: 30 W LED power at 100 V input voltage; 13 W LED power at 50 V input.

Figure 3: CAT4201 high-voltage LED driver configuration.

2) Integrated/single-stage solutions for applications such as wall washers and exterior wall lights

Not all area lighting applications require a distributed/modular solution. With the rapid improvement in the performance of white light LEDs, new LEDs can be used with 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 high currents of 1 A to 3 A. For example, ON Semiconductor's NCL30001 power factor corrected TRIAC dimmable LED driver can be used.

The 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.

Figure 4: 90 W LED driver demonstration board circuit based on the NCL30001 LED driver and NCS1002 controller.

ON Semiconductor has developed a single-stage high power factor LED driver demo board with a power of 90 W, a current of 0.7 A to 1.5 A, and a voltage below 60 V. The demo board uses the NCL30001 LED driver and the NCS1002 secondary-side constant voltage constant current (CVCC) controller with pulse width modulation (PWM) dimming function (see Figure 4), which is suitable for area lighting applications such as LED street lights.

This 90 W constant voltage constant current demo board accepts an extended universal input voltage of 90 to 265 Vac (supports 305 Vac under component replacement conditions), 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, the 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 energy efficiency of more than 87% under the conditions of 50 W output power, 1,000 mA output voltage/48 V forward voltage drop (see Table 1 for details), and a power factor of more than 0.9 under 50% to 100% load conditions, while meeting the IEC61000-3-2 Class C equipment harmonic content standard.

Table 1: 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, with the expectation of providing 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 takes 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 supply solution for street lighting based on NCP1607 and NCP1397.

Protection scheme to enhance LED string reliability

Multiple strings of LEDs are often used in area lighting applications. Although LEDs themselves 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 introduced the NUD4700 LED current bypass protector. This device is a shunt device. In the event that one LED in the LED string is open, it will provide current bypass to ensure 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.

Summary:

As the leading supplier of high-performance and energy-efficient silicon solutions for green electronic products, ON Semiconductor provides various solutions for LED area lighting applications, such as the CAT4201 buck LED driver for distributed/modular power architecture, the NCL30001 single-stage high power factor LED counterpart for integrated power architecture, and the NCP1607 PFC controller and NCP1397 resonant half-bridge LLC controller solutions for high-power LED area lighting to meet different application needs of customers. ON Semiconductor also provides networked LED street light control solutions based on products such as the AMIS-49587 power line communication (PLC) modem, as well as related MOSFETs, rectifiers, filters and protection products, etc., to provide users with a wealth of choices, helping them shorten the design cycle and accelerate product launch.