Energy-efficient LED area lighting

The world is paying more and more attention to energy-saving high-efficiency lighting. According to the International Energy Agency (IEA) report, global electric lighting accounts for about 19% of the total power generation capacity. Recent news reports have gradually reminded consumers of the low energy efficiency of incandescent lamps and strengthened everyone's awareness of compact energy-saving lamps. In addition, the US Energy Star (ENERGY STAR?) also plans to set standards for home and commercial lighting. However, in these news, it is overlooked that about 70% of lighting energy consumption is outside the home, including stores, factories, schools, hospitals and regional lighting occasions; regional lighting covers applications in public spaces such as street lighting, parking lots and parks.

Regional lighting not only provides lighting functions, but also needs to address public safety, operating environment, exterior design and aesthetics, energy saving, system reliability and maintenance costs. The most common regional lighting sources are high-voltage discharge (HID) types, such as high-pressure sodium lamps, metal halide lamps, low-pressure sodium lamps and high-pressure mercury lamps. Advanced HID lamps, such as metal halide lamps, have high energy efficiency of more than 80 lm/W and a reasonable life of 10,000 to 15,000 hours. Calculated at 4,000 hours of use per year, the life is about 2 to 4 years. However, due to the difficulty of replacement, maintenance time or labor costs are quite expensive, which is a big problem in tunnels or bridges. In addition, most HID lamps contain mercury, so they must be handled with care after replacement to avoid causing environmental problems.

The technology of high-brightness white light emitting diodes (LEDs) continues to advance, bringing great potential solutions for energy-efficient lighting applications. The most important reason for the popularity of high-brightness white light LEDs is their ultra-long life and luminous efficiency. When its brightness is maintained at 70%, the life span can reach 50,000 hours, and outdoor area lighting can reach 12 years of operation, greatly reducing subsequent maintenance and replacement costs. The currently commercialized high-power white LED can achieve a luminous output of 80 to 120 lumens per lamp, with an efficiency of about 80 lm/W. It is estimated that the overall performance will be doubled in 2009, reaching 200 lumens per lamp, and the luminous efficiency is expected to reach 160 lm/W. For reference, a 100 W metal halide HID lamp can produce about 8,000 lumens output. The luminous output of HID lamps is omnidirectional, so there will be a lot of loss in the light projection path. However, LEDs do not have this problem because they are directional. It takes about 110 LEDs with 80 lm per package to replace the lumen output of a 100 W HID lamp. If the 2009 goal is achieved, this number will be halved.

LED lighting can be composed of one or more LED arrays with a control circuit that converts AC power into the current used by the LEDs. Therefore, a modular design that includes a string of LEDs and a driver circuit can be considered. The advantage of this concept is that the same circuit design can be reused by adding more arrays for different lighting needs. In addition, since multiple arrays are used in the lighting source, if a single LED has a problem, only a portion of the LEDs will stop working, and the entire lighting source can still provide a lower brightness lighting output. LED lighting sources must also comply with industry and international harmonic standards. In the European Union, such products fall under the IEC61000-3-2 and other specifications for power line harmonic distortion (power factor). Although this standard is not used in the United States and other places, power companies also require a minimum power factor of 0.9 for unmetered area lighting.

Another consideration is the safety of isolation. For area lighting sources that are not easily replaced, non-isolated designs are actually quite common. The main advantage of non-isolated designs is that they replace bulky transformers with lower-cost inductors. The actual needs of driving LEDs are also very important. Although LEDs require constant current drive, this current does not necessarily need to be pure DC, so they can also be driven with a pulsed DC waveform as long as the average and maximum current values ​​meet the current specifications specified by the LED itself. Therefore, we can use ON Semiconductor's NCP1216 control chip with a high-power MOSFET, an inductor, and a few external passive components to implement a simple, high-efficiency PFC and constant current converter in a single power stage circuit. Since the output usually does not need to filter out the frequency component of the 100/120 Hz main power supply, it is not necessary to use large electrolytic capacitors in the circuit, which can not only reduce the circuit size, but also improve the reliability of the overall power supply. The following is the circuit diagram of the relevant circuit.

Figure 1: Circuit design for a 115 Vac, 350 mA configuration.

This circuit diagram shows the most basic implementation of a non-isolated converter circuit, which is a step-down converter circuit that takes the rectified AC power from D1 to D4 and passes it through inductor L1, MOSFET-Q1, output capacitor C4 and controller. In this particular circuit with a 90 to 135 Vac input, a simple feedback network consisting of a parallel current sensing resistor R4, an integrator circuit R6 and C6, and an optocoupler allows the circuit to operate in a constant current output mode. The optocoupler is not usually required in non-isolated designs, but it is used here to shift the current sensing signal at the top of the LED string. The special implementation of this circuit allows it to provide high power factor and constant current output. The step-down input capacitor C2, commonly called the buck capacitor, must have a high impedance to the 120 Hz full-wave rectified waveform presented to the input bridge rectifier circuit, otherwise the power factor will be greatly degraded as with the capacitive input filter. The typical value of this capacitor is about 0.1 μF to 0.47 μF, mainly depending on the target output power of the circuit. The inductance of L1 should be low enough to allow the buck converter to operate in discontinuous conduction mode, which is very important for the high power factor of the circuit. In discontinuous conduction mode, the value of C4 can also be quite small, about 1 to 5 μF, because it is only used to filter out the high-frequency switching components in the current waveform, and a low-ESR polypropylene film capacitor should be used.

Figure 2: Input voltage and current waveforms.

Vin = 115 Vac, Vf = 31 V, Iout = 350 nom

Figure 3: Power factor and current regulation versus LED forward voltage.

Vin = 115 Vac

As can be seen above, ON Semiconductor has developed an optimized high power factor single-stage LED driver circuit that can be used to drive LED arrays. Although the operating voltage of the circuit in the example is 115 Vac, we can change the value of the device to meet the application of 230 Vac, and the number of driven LEDs can be doubled.

The use of LED arrays in area lighting applications has attracted global attention, and multiple tests and small-scale introduction plans have been launched in various places. Many government organizations also understand the advantages of LED technology, as can be seen from the widespread introduction of traffic signs. They also understand that long-life products do not require the same maintenance costs. For example, the city of Raleigh, North Carolina, replaced 140 120 W high-pressure sodium lamps in the parking lot with LED lighting devices, reducing energy consumption by 40% and having almost no annual maintenance costs. In the next few years, we will see LEDs used in more area lighting, especially lower-power lighting applications that require only a small number of LEDs and designs where the light output can be varied according to the specific needs of the application. For these applications, optimizing the driver circuit to meet power and regulatory requirements while also being flexible to meet the changing LED configuration can achieve the economic cost goal of widespread adoption.