LED Technology
Protect the DC-DC converter LEDs are usually connected in series and driven by a constant current source to drive them to full brightness, colour, 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 through 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 fine 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 It is becoming increasing popular to use an integrated, complete, replaceable module solution on the front end
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. One solution is the PLED Series Open LED Protector
from Littelfuse which 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-
TMOV 25S thermally-protected, 25mm, high-surge varistor
input to roadway, parking lot, and outdoor structure LED lighting. The main reason for this is the 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.
Don’t forget the LED string 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.
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impedance switch when it is triggered on to minimise 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. LEDs offer many advantages over other lighting solutions. However, especially as LEDs are more and more put to use in harsh outdoor environments, circuit protection is set to become increasingly important. To reap the benefits of LED lighting systems, designers need to pay much closer attention to safety and reliability. The first line of defence is a good circuit protection strategy that stretches all the way from the input power supply to the individual LEDs.
Littelfuse |
www.littelfuse.com
Bharat Shenoy is Director of Technical Marketing at Littelfuse Electronics Business Unit
Components in Electronics November 2013 27
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