Compared with other lights, LEDS are quite fragile. Hence, adverse environmental conditions can create high risks for surges and other sources of damage. As a result, it is important that LED strings receive the proper protection. Bharat Shenoy reports.
The future is looking bright for LEDs. Thanks in part to falling prices and global government mandates to reduce energy consumption, but also due to its benefits, the market share of light emitting diodes is constantly rising. However .
Key advantages of LED lighting include an extremely long lifespan, high efficiency, and a broad variety of different colors. Today, high-brightness LEDs (those that currently feature sapphire or SiC substrates, and AlN and GaN substrates in the future) are being used in a wide variety of different lighting applications like outdoor display boards, traffic signals and airport runway lighting.
Although they are known for their efficiency and reliability, LEDs are rather fragile devices: Thermo-mechanical stress on the wire bonds is known to be one of the main causes of an electrical open circuit in an LED.
Other causes include electrostatic discharge (ESD) events or surges induced by nearby lightning strikes. An open circuit or damaged LED inside a string can cause the entire string to go dark. And if the product is used in a mission-critical application, an airport runway light, for instance, the results could be catastrophic.
With that being the case, the benefits LEDs offer today can only be reaped if they are protected properly. Until recently, however, few solutions were available for protecting LEDs in harsh outdoor environments. To make matters even worse, engineers are often unaware of all of the considerations involved in specifying circuit protection for LED strings.
So, although a new group of devices known as open LED protectors are now available, they are not yet well understood, despite how important it is to provide circuit protection for the entire LED portion of a product design. The purpose of this article is to provide a few design tips and recommendations to incorporate techniques such as over-voltage suppression and open LED protectors in an LED design.
In an LED lighting application, three main areas require circuit protection: the AC portion of the circuit prior to DC rectification, the DC section which is also called the LED driver, and the LEDs themselves.
Use an AC line fuse
An LED lighting set-up usually has a switched-mode power supply (SMPS) on the front end. An SMPS is in even greater need of protection than linear power supplies, and proper sizing of the AC line fuse is a good place to start.
Exterior AC sources are highly susceptible to nearby environmental surges. Lightning surges are particularly problematic, and as a result the line fuses need to be sized appropriately so they do not nuisance trip during surge events.
Typical surge current can vary from 3KA up to 10KA for the highest exposure environments. The main criteria for selecting the line fuse for the AC input include the voltage, current, and the I2T rating. This last parameter is related to the amount of energy the fuse element can withstand instantaneously without opening.
Increasing the current or time delay rating for a fuse naturally increases I2T. For these high exposure outdoor environments, a high I2T fuse is required to withstand the lightning surge currents and keep the luminaire operational.
Another requirement to protecting a SMPS is through the use of a metal-oxide varistor (MOV). A MOV suppresses an overvoltage event as a shunt in the input circuit and is place in parallel with the power line or that particular mode that is being protected. A surge causes the MOV to lower its resistance and divert the surge away from the power supply.
The main difference between fuses, which are overcurrent devices and MOVs, which are overvoltage devices, is that fuses are specified as safety requirements, while a MOV is considered a reliability device.
Protect the LED driver
LEDs are usually connected in series and driven by a constant current source to drive them to full brightness, color, and intensity. A DC-DC converter provides the constant current, and it is a critical part of the overall LED circuit.
To protect the DC-DC converter, a high-voltage DC fuse is a critical component in the DC section. Its job is to open during overcurrent events. In addition, a transient voltage suppression diode (TVS diode) in the DC section then protects the oscillator in the DC-DC converter from surges. Even though there is a MOV on the front end of the AC/DC power supply, the response time of the MOV will still allow some surge current to pass thru into the rectifier and beyond. These circulating currents can cause damage to the LED driver or LEDs themselves. Therefore, it is prudent to consider a more fined tuned, faster, clamping device such as a TVS diode placed on the secondary side of the rectifier to reduce these surge currents.
Use a replaceable protection module on front end
It is becoming increasing popular to use an integrated, complete, replaceable module solution on the front end input to roadway, parking lot, and outdoor structure LED lighting. The main reason for this is ease of maintenance and long term reliability. The use of a protection module or SPD (Surge Protection Device) allows the integration of all three of the MOVs required for each mode of protection. The module may even have MOV thermal protection built-in to control end-of-life thermal runaway failure modes.
Typically, these modules are parallel connected to the input lines, but there are some newer modules that are series connected which remove power to the luminaire when thermal protection has been activated. This can be an effective method of indication that the protection module needs to be replaced.
Protect the LED
As low-voltage devices (forward voltage typically ranges from about 2.7 volts for red to approx. 4.1 volts for blue), high brightness LEDs are normally connected in series strings and fed by a constant-current supply. A string can contain anywhere from five to 20 LEDs. LEDs connected in series share a common current, which means more uniform LED-to-LED brightness, and it’s easier to control the brightness of the entire group.
Nevertheless, LEDs still have some inherent weaknesses: they can be damaged by temperature cycling, they are electrically fragile, and if one LED in a string fails open, that can cause the entire string to go dark. While this is only unsightly on a billboard, it can represent a serious safety hazard in a traffic light or airport runway marker.
Vibration, heat and aging can cause an LED wire bond to fail. As mentioned earlier, an open circuit in a single LED in a string can cause the entire string to go dark. In high-reliability applications, however, an open failure cannot be tolerated.
This problem can be avoided by placing an open-LED protection device in parallel with each LED on the string. This device is an electronic shunt that provides a current bypass in the case of an open circuit and saves the LED string from partial or complete failure. Such devices will turn on and conduct current around an open LED so that the remaining LEDs remain illuminated.
The PLED Series Open LED Protector from Littelfuse provides both overvoltage protection and continued string function if a single LED fails open. PLED devices are connected in parallel with each LED in a series string. If an LED should fail open-circuit, the PLED connected to it will turn on and carry the current that would have gone through the failed LED. This keeps the rest of the string operating, so that only a single LED goes dark rather than the entire string.
The PLED is a voltage-triggered switch with low leakage current on the order of microamps that becomes a low-impedance switch when it is triggered on to minimize power consumption. In the off state, a PLED draws only a few microamps, therefore it does not affect the circuit. Once an LED fails open, there is sufficient circuit voltage to trigger the protector to the on-state when it is placed in parallel with the LED. When triggered, it carries the full current of the string with a voltage drop of about 1.3 volts. Beyond open-LED protection, PLEDs contain reverse current diodes that protect the LED string if the power supply is connected with reverse polarity. Open-LED protection devices work well with various pulse width modulation (PWM) brightness control methods for LEDs.
PLED devices are typically connected across each LED in a string, however units are available to connect across two or even three LEDs. This reduces the cost of protection, but causes more than one LED to go dark if one fails.
Summary
As LEDs are put to use in harsh environments outside of their traditional role, circuit protection becomes increasingly important. In order to reap the benefits of LED lighting, designers need to pay much closer attention to safety and reliability. The first line of defense is a good circuit protection strategy that stretches all the way from the input power supply to the individual LEDs.
Bharat Shenoy is Director of Technical Marketing, Littelfuse Electronics Business Unit, Chicago, USA. www.littelfuse.com