Approximately 25 percent of global energy consumption goes to lighting applications, so making lighting more energy-efficient could have a dramatic impact on overall energy usage or make more power available for other uses. Legislation designed to discourage the use of incandescent lamps has been a significant factor in the growth in demand for LED lighting equipment. At the same time, both consumers and industrial users are increasingly interested in energy-efficient lighting options, further stimulating the demand for LED lighting.
Technical innovations that impact LED efficiency (more lumens per watt), secondary optics (better lenses/reflectors), and thermal dissipation are increasingly allowing LED lighting to replace legacy light sources like mercury vapor, metal halide, and sodium vapor lights in outdoor applications. However, outdoor LED lighting can be very expensive to install; payback must be determined based on lower wattage demands, lower maintenance costs, and a longer operating life. To prevent outdoor LED lighting from experiencing failures within an investment payback period of about five years, high durability and reliability are essential. Transient surge events in AC power lines represent a serious threat to outdoor LED lighting fixtures.
Indirect Lightning-Induced Surges
Whenever electrical devices are switched on or off, overvoltage transient surges can affect nearby AC power lines. Similarly, lightning strikes (Figure 1) can generate transient surges in AC power lines, especially in outdoor environments.
Indirect lightning energy can affect outdoor LED lighting installations adversely. Transient voltage protection is crucial to eliminate field failures driven by the electrical environment. Luminaires are vulnerable to damage both in the differential and common modes:
- Differential mode: High voltage/current transient between the L-N or L-L terminals of a luminaire could damage components in the power supply unit or LED module board.
- Common mode:High voltage/current transient between the L-G (earth) or N-G (earth) of the luminaire could break over safety insulation in the power supply unit or LED module board, including the LED to heat-sink insulation.
LED lighting equipment manufacturers rely on carefully chosen fuses, metal oxide varistors (MOVs) and transient voltage suppression (TVS) diodes to meet important regulatory and safety standards related to overvoltage transients. The United States is leading the way in establishing uniform performance and safety standards for both indoor commercial lighting and outdoor roadway, parking lot and garage illumination.
Overvoltage transient surge testing per IEC 61000-4-5 is a global requirement for LED lighting assemblies, except in the United States, which has its own set of standards. In addition, part of IEC61547, “Equipment for General Lighting Purposes,” requires electromagnetic compatibility (EMC) immunity testing. Figure 2 shows two waveforms that define rise time and duration of the test voltage and current. The test waveform is a combination 1.2×50μs open circuit voltage and 8×20μs short circuit current waveform. To conduct this test, the specified peak current is calibrated on the surge generator by shorting the output to ground prior to connecting to the luminaire.
To prevent damage caused by surge energy, enhance reliability, minimize maintenance, and extend the useful life of an outdoor lighting installation, a robust surge suppression circuit is essential. Figure 3 illustrates the various elements often incorporated into a street light surge protection circuit.
Thermally Protected Metal Oxide Varistors (MOVs)
MOV technology is an affordable, highly effective method for suppressing transients in power supplies and other applications, such as the SPD modules often located in front of an LED driver.
MOVs are designed to clamp overvoltage transients within microseconds. However, when built into SPD modules, MOVs can be subject to temporary overvoltage conditions caused by loss of neutral or by faulty installation wiring. These conditions can severely stress an MOV and cause it to experience thermal runaway, resulting in smoke, overheating, and possibly fire. North American safety standards for SPDs (including UL 1449) define atypical conditions under which devices must be tested to ensure SPD safety. Robust SPD designs feature thermal disconnects to protect the MOVs from thermal runaway.