Analysis of LED area lighting driver power supply solutions

To replace HID with LEDs in street and area lighting applications and produce equivalent light output, large arrays of LEDs are required. To drive these large arrays of LEDs, designers can choose different solutions. In addition, the brightness of different LED strings needs to be consistent. If an LED fails in an open circuit, it may cause the entire string of LEDs to shut down, so solutions to provide protection for the LED strings need to be considered. High-intensity gas discharge lamps (HID) such as high-pressure sodium lamps (HPS) have the characteristics of high light intensity and long life, and are widely used in area lighting applications in public places such as street lighting, parking lots and parks. On the other hand, high-brightness white light-emitting diodes (LEDs) continue to improve in terms of performance and cost, and are very suitable for area lighting applications. They also provide some advantages that HIDs do not have, such as better directionality, better color quality, environmental protection, and their opening and closing can be more easily controlled, which is convenient for automatic detection of ambient light to change brightness; in addition, LEDs are more reliable, which helps to reduce maintenance costs and total cost of ownership.

Building a networked intelligent LED street light control system also requires the use of appropriate communication and line drive solutions. The following will discuss these issues and the corresponding ON Semiconductor solutions.

LED Street Light and Area Lighting Driver Selection

The power of area lighting applications is generally higher than 40W. Depending on the application conditions or requirements, different power solutions can be used to drive the LED array. As the leading high-performance, energy-efficient silicon solution supplier for green electronic products, ON Semiconductor provides different power solutions for LED street lights and area lighting to meet the different needs of customers.

1) Current adjustable constant current power factor corrected area lighting LED power supply based on NCL30001

Some area lighting applications require an isolated, regulated output voltage with input power factor correction. These applications typically use a two-stage power conversion architecture, where a boost power factor correction (PFC) converts and pre-regulates the AC input line voltage to 400V DC, which is then fed to a conventional DC-DC (DC-DC) converter of any suitable topology (usually a flyback converter for applications up to 150W).

We can use a simpler approach to improve this traditional two-stage conversion architecture into a single-stage architecture that integrates power factor correction and the main converter (i.e., DC-DC converter). This single-stage architecture provides significant application advantages because it does not require the use of large-sized boost inductors, high-voltage MOSFETs, power rectifiers, and large capacitors. Of course, this single-stage architecture will bring some performance trade-offs in certain aspects, but it is an energy-efficient and cost-effective solution suitable for applications with relatively constant loads, such as LED area lighting.

The NCL30001 is a single-stage controller that integrates power factor correction and isolated step-down DC-DC conversion circuits to help reduce component count, reduce system costs, and support higher overall energy efficiency of LED power supplies. The NCL30001 provides a constant current to directly drive the LED, eliminating the need for linear or DC-DC conversion integrated in the LED light bar (see Figure 1). ON Semiconductor's design note AND8427 describes an LED power supply based on the NCL30001 single-stage continuous current mode (CCM) PFC controller and the NCS1002 secondary-side constant voltage constant current (CVCC) controller, which is suitable for area lighting applications such as street lights.

Figure 1: NCL30001-based LED driver solution is suitable for 40 to 125W area lighting applications

Tests show that under 90 to 264Vac input voltage conditions, the LED power supply based on NCL30001 provides high energy efficiency of more than 85%, power factor is also higher than 0.9, and provides short-circuit and overvoltage protection. Designers can adjust the regulated current and regulated voltage by slightly adjusting the resistance in the secondary control circuit to support the specific application requirements of the terminal product. It is also worth mentioning that although this power supply is designed to provide a fixed current with tight current regulation, it can also work in constant voltage mode because its current and voltage are regulated according to the tightly regulated 2.5V reference voltage integrated in NCS1002 to regulate current and voltage.

2) Ultra-high energy efficiency high power LED street light power supply solution based on NCP1607 and NCP1397

In recent years, the industry has shown a growing interest in ultra-high-efficiency LED lighting topologies, expecting to provide more than 90% efficiency at relatively low power levels (<50W), an efficiency target that is even higher than the Energy Star 2.0 external power supply efficiency requirements (more than 87% efficiency at power levels not exceeding 49W). To achieve such high efficiency, new topologies need to be adopted, such as switching from a flyback topology to a resonant half-bridge topology, thereby fully leveraging the advantages of zero voltage switching (ZVS). Fortunately, ON Semiconductor has already begun developing high-efficiency half-bridge solutions that can be used for LED driver power supplies, such as the NCP1396 and its upgraded version, the NCP1397 high-performance resonant mode controller. The NCP1397 has built-in high-end and low-end drivers, supports adjustable and precise minimum frequencies, provides extremely high efficiency, and has a variety of fault protection features. The LED power supply solution based on the NCP1607 PFC controller and the NCP1397 dual-inductor plus single-capacitor (LLC) half-bridge resonant controller (see Figure 2) is very suitable for high-efficiency LED street lighting applications in the power range of 50 to 300W.

Figure 2: High-efficiency LED power supply solution for street lighting based on NCP1607 and NCP1397

In addition to these solutions, designers can also choose other LED power solutions from ON Semiconductor according to design requirements, such as NCP1607PFC controller + NCP1377 quasi-resonant controller, or NCP1607 + NCP1392/3, etc.

Protection scheme to enhance LED string reliability

As mentioned earlier, multiple strings of LEDs are often used in area lighting applications. Although the LED itself is highly reliable, if an LED in the LED string fails to open circuit, 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. In view of this, ON Semiconductor has launched the NUD4700LED current bypass protector (see Figure 3). This device is a shunt device. In case an LED in the LED string is open circuit, 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 1A.

Figure 3: LED shunt protection

Building a networked LED street light intelligent control system

For LED street lighting, two-way communication can be applied between the centralized control center and the street lights to build a fully networked intelligent street lighting system. In this way, government agencies, power companies or business groups can remotely adjust the light output of LED street lights, thereby reducing the overall energy consumption of the street lighting network and reducing expenses. In addition, ambient light sensors can be integrated into the system to automatically adjust the light output of the LED according to the brightness of the ambient light, thereby helping to save energy.

Fortunately, designers can easily build a networked LED street light intelligent control system based on power lines using ON Semiconductor's AMIS-49587 power line carrier (PLC) modem, PLC line driver NCS5650 and ambient light sensors such as NOA1302 (see Figure 4). Among them, ON Semiconductor's ambient light sensors include output types, such as NOA1302 and NOA1305 provide digital output, while NOA1211 is analog output. These light sensors have extremely low operating currents of only 58μA, 550μA and 115μA respectively at 100 lumens of light output, so the impact on system energy efficiency is minimal.

Figure 4: Networked LED street light intelligent control system based on AMIS-49587 and other devices

As a leading supplier of high-performance, energy-efficient silicon solutions for green electronics, ON Semiconductor provides complete solutions for LED area lighting applications, such as current-adjustable constant-current power factor correction solutions based on NCL30001 and ultra-high-efficiency high-power solutions based on NCP1607 and NCP1397, LED string protection solutions based on NUD4700, networked LED street light control solutions based on AMIS-49587, NCS5650 and NOA1302, as well as related MOSFETs, rectifiers, filters and thermal protection products, etc., providing users with a wealth of choices to help them shorten the design cycle and speed up the time to market.