Transient surge events in ac power lines can easily cause damage to outdoor LED lighting equipment.
The installation of surge protection module at the front end of the LED power supply is an affordable and effective solution, which can suppress overvoltage transient in microsecond time and avoid overheating, so as to improve the reliability of LED lighting equipment and extend the service life.
About 25 per cent of global energy consumption is generated by lighting applications, so making lighting more energy-efficient would have a significant impact on overall energy use and save more available energy for other applications.
Legislation to discourage the use of incandescent bulbs has become an important factor in the growing demand for LED lighting.
At the same time, consumers and industrial users are increasingly interested in energy-saving lighting solutions, further stimulating demand for LED lighting.
LED efficiency (higher lumens per watt), secondary optical components (better lenses/reflectors), and innovations in heat dissipation allow LED lighting to gradually replace traditional sources of light, such as mercury, metal halide, and sodium.
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 service life.
High durability and reliability are essential to prevent outdoor leds from malfunctioning during the investment return period of about five years.
Transient surge events in ac power lines pose a serious threat to outdoor LED lighting.
Curb surge events to extend lighting life
When the electrical equipment is turned on or off, the transient surge will affect the nearby ac power line.
Similarly, lightning strikes can cause transient surges in ac power lines (figure 1), especially in outdoor environments.
Indirect lightning energy can adversely affect outdoor LED lighting.
Transient voltage protection is the key to eliminate field fault affected by electrical environment.
No matter be difference model or common model, lamps and lanterns suffers damage extremely easily.
FIG. 1 influence of indirect lightning strike on ac power line
The high voltage/current transient between the l-n or l-l terminals of the lamp will cause damage to the components in the power module or LED module board.
The high voltage/current transient between the l-g (ground) or n-g (ground) terminals of the lamp can damage the safety insulation in the power module or LED module board, including the insulation from LED to the radiator.
To meet key regulations and safety standards related to overvoltage transient, LED lighting manufacturers rely on carefully selected fuses, metal oxide varistor (MOV), 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 for outdoor roads, parking lots and garages.
In addition to the us having its own standards, the IEC 61000-4-5 transient surge test is a global requirement for LED lighting components.
In addition, electromagnetic compatibility (EMC) immunity test is required in IEC 61547.
Figure 2 shows two waveform definitions for test voltage and current rise time and duration.
The test waveform is a combination of a 1.2x50 s open circuit voltage waveform and an 8x20 s short circuit current waveform.
For this test, calibrate the specified peak current on the surge generator by short-circuiting the output to the ground before connecting to the lamp.
Fig.2 test waveform of IEC 61000-4-5 wave resistance: 1.1mm open circuit voltage and 8mm short circuit current.
In order to prevent damage caused by surge energy, improve the reliability of outdoor lighting equipment, reduce maintenance and extend service life, a strong surge suppression circuit is essential.
Figure 3 shows the various components normally incorporated into the street lamp surge protection circuit.
FIG. 3 LED street lamp protection scheme
MOV helps the SPD suppress transient
Among other applications of the power supply and Surge Protection Device (SPD) modules, which are often placed at the front of LED drives, metal oxide pressure-sensitive resistance technology is an inexpensive and extremely effective way to suppress transient changes.
MOV is designed to clamp overvoltage transients in microseconds.
However, if embedded in the SPD module, MOV will be affected by the temporary overvoltage condition caused by the loss of or incorrect installation wiring of the neutral point.
These conditions can put severe pressure on MOV, causing it to experience runaway heat, resulting in smoke, overheating and possible fires.
North American safety standards for the SPD (including UL1449) define typical test conditions to ensure the SPD's safety.
Overheat disconnection protection is a reliable feature of SPD design to protect MOV from heat loss.
After being exposed to one big surge or several small surges, the performance of MOV tends to decline gradually, which leads to the increase of MOV leakage current.
Even under normal conditions (such as 120VAC/240VAC operating voltage), this degradation increases the temperature of MOV.
As for an overheat disconnection device next to MOV, it can be used to sense MOV and increase the temperature due to its continuous degradation.
When the MOV service life ends, the overheat disconnection device will open the circuit, removing the degraded MOV from the circuit and preventing catastrophic failure.
SPDS add life termination/replacement indication mechanism
SPD no longer provides surge suppression protection once MOV interrupts the connection from the circuit.
In order to prevent damage to the lamp from the surge, the circuit designer should adopt a method to remind the maintenance personnel that the SPD should be replaced.
Lighting designers have two main SPD module configurations to choose from, depending on their maintenance and warranty policies: parallel and series surge protection components.
The SPD module is in parallel with the load (figure 4).
Here the SPD module which meets the service life termination condition is disconnected from the power supply, while the AC/DC power supply keeps the power on.
The lighting is still running, but the power supply and LED modules are no longer protected against the next surge.
Today, the SPD module comes with small leds that act as replacement indicators, such as green leds that indicate the SPD module is online and red leds that indicate the SPD module is offline.
Figure 4 parallel SPD module
The SPD module can also be remotely prompted for replacement by using a lighting management center.
In this management center, the SPD module service life termination indicator line is connected to the networked intelligent lighting system, instead of having to install the indicator light on each lamp.
In this configuration, the SPD module is in series with the load (figure 5).
The SPD module is disconnected from the power supply at the end of its service life, thus extinguishing the lights, indicating the need to make maintenance calls.
The disconnected SPD module not only turns out the lights, but also isolates the AC/DC power supply from the subsequent surge shock.
This is increasingly popular because it protects investment in lighting while SPD modules are waiting to be replaced.
Replacing an SPD module in series is also more economical than replacing the entire lighting unit in parallel SPD module configurations.
Figure 5 serial SPD module
The SPD module is installed at the front end of the LED power module, which provides effective protection for the lighting system.
In these SPD modules, the installation of overheating disconnect devices improves their overall safety and enables them to successfully pass the UL1449 certification.
In order for the initial investment of LED lamps to pay off, the designers must include an SPD module replacement indicator mechanism in their design.