Transient surge events in AC power lines can easily cause damage to outdoor LED lighting equipment. Installing a surge protection component on the front of the LED power supply is an affordable and effective solution to suppress overvoltage transients in microseconds and avoid overheating, thereby improving the reliability and longevity of LED lighting equipment. .
About 25% of the world's energy consumption is generated by lighting applications. Therefore, making lighting more energy-efficient will have a major impact on overall energy use and save more energy for other applications. Legislation that discourages people from using incandescent lamps has become an important factor in the growth of demand for LED lighting equipment. At the same time, consumer and industrial users are increasingly interested in energy-efficient lighting solutions, further stimulating the market demand for LED lighting.
LED efficiency (higher lumens per watt), secondary optics (better lenses/mirrors), and thermal innovations have led LED lighting to gradually replace traditional light sources such as mercury lamps for outdoor applications and metal halide lamps. sodium lamp. However, outdoor LED lighting can be very expensive to install, and the return on investment must be based on lower wattage requirements, lower maintenance costs, and longer life. In order to prevent outdoor LEDs from failing during the investment return period of about 5 years, high durability and reliability are essential. Transient surge events in AC power lines pose a serious threat to outdoor LED lighting.
Suppressing surge events to extend lighting life
Overvoltage transients can affect nearby AC power lines whenever electrical equipment is turned on or off. Similarly, lightning strikes can also cause transient surges in AC power lines (Figure 1), especially in outdoor environments. Indirect lightning strike energy can adversely affect outdoor LED lighting. Transient voltage protection is the key to eliminating field failures that are affected by the electrical environment. Whether it is differential or common mode, the luminaire is extremely vulnerable.
Figure 1 Impact of indirect lightning strikes on AC power lines
. Differential mode
High voltage/current transients between the LN or LL terminals of the luminaire can cause damage to components in the power module or LED module board.
. Common mode
High voltage/current transients between the LG (ground) or NG (ground) terminals of the luminaire can damage the safety insulation in the power module or LED module board, including the LED to heat sink insulation.
To meet important regulatory and safety standards related to overvoltage transients, LED lighting manufacturers rely on carefully selected fuses, metal oxide varistors (MOVs), and transient voltage suppression (TVS) diodes. The United States is taking the lead in establishing uniform performance and safety standards for some indoor commercial lighting, as well as outdoor roads, parking lots and garage lighting.
In addition to the United States has its own set of standards, IEC 61000-4-5 overvoltage transient surge test is a global requirement for LED lighting components. In addition, the "General Lighting Equipment" IEC 61547 also requires electromagnetic compatibility (EMC) immunity testing. Figure 2 shows two waveform definitions for test voltage and current rise time and duration. The test waveform is a combination of a 1.2x50μs open circuit voltage waveform and an 8x20μs short circuit current waveform. To perform this test, the specified peak current is calibrated on the surge generator by shorting the output to ground before connecting to the luminaire.
Figure 2 IEC 61000-4-5 surge immunity test waveform: 1.2 × 50μs open circuit voltage and 8 × 20μs short circuit current.
In order to prevent damage caused by surge energy, improve the reliability of outdoor lighting devices, reduce maintenance and extend service life, a powerful surge suppression circuit is essential. Figure 3 shows the various components that are typically incorporated into the streetlight surge protection circuit.
Figure 3 LED street light protection scheme
MOV helps SPD suppress transients
Metal oxide varistor technology is both affordable and extremely effective in suppressing transients in power supplies and other applications such as Surge Protection Device (SPD) modules that are typically placed at the front end of LED drivers. Methods. The MOV is designed to clamp overvoltage transients in microseconds. However, built into the SPD module, the MOV is affected by temporary overvoltage conditions caused by loss of neutral points or incorrect mounting wiring. These conditions can put severe stress on the MOV, causing it to experience thermal runaway, causing smoke, overheating and possible fires. North American safety standards for SPD (including UL1449) define typical test conditions to ensure SPD safety. Overheat trip protection is a feature of a reliable SPD design designed to protect the MOV from thermal runaway.
After exposure to a large surge or multiple small surges, the performance of the MOV tends to decrease gradually, resulting in an increase in MOV leakage current. Even under normal conditions (such as 120VAC/240VAC operating voltage), this degradation increases the temperature of the MOV. As for a thermal disconnect device next to the MOV, it can be used to sense the MOV due to the continued degradation of its temperature. When the MOV has reached the end of its useful life, the thermal disconnect device will open the circuit, remove the degraded MOV from the circuit, and prevent catastrophic failure.
SPD joins end of life/replacement indication mechanism
Once the MOV is disconnected from the circuit, the SPD no longer provides surge suppression protection. In order to prevent damage to the luminaire caused by subsequent surges, the circuit designer should adopt a method that can alert the maintenance personnel that the SPD needs to be replaced. Lighting designers have two main SPD module configuration types to choose from, depending on their maintenance and warranty strategy: parallel and series surge protection components.
. in parallel
The SPD module is connected in parallel with the load (Figure 4). Here the SPD module that reaches the end of life condition is disconnected from the Power Supply while the AC/DC power supply remains energized. The lighting unit is still running, but the power supply and LED modules are no longer protected against the next surge. Today, the SPD module has a small LED that acts as a replacement indicator. For example, a green LED indicates that the SPD module is online, and a red LED indicates that the SPD module is offline.
Figure 4 Parallel SPD Module
You can also remotely indicate whether the SPD module needs to be replaced by using a lighting management center. In this management center, the SPD module end of life indicator line connects to a networked intelligent lighting system without the need to install an indicator light on each luminaire.
. Tandem
In this configuration, the SPD module is in series with the load (Figure 5). The SPD module is disconnected from the power source at the end of its useful life, thereby extinguishing the light, indicating that a service call is required. The disconnected SPD module not only extinguishes the light, but also isolates the AC/DC power supply from subsequent surge surges. This configuration is becoming more popular because it protects the lighting device investment while the SPD module is waiting to be replaced. It is also more economical to replace a series of SPD modules than to replace the entire lighting unit with a parallel SPD module configuration.
Figure 5 series SPD module
The SPD module is installed at the front of the LED power module to provide effective protection for the lighting system. In these SPD modules, the installation of thermal disconnect devices improves their overall safety and enables them to successfully pass UL1449 certification. In order for the initial investment in LED luminaires to be rewarded, designers must incorporate mechanisms for SPD module replacement instructions in their designs.
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