The Influence of EOS on LEDs and How to Make the Protection
WHAT IS EOS
Electrical overstress is simply exposing an LED to any current exceeding the maximum current specified in the LED’s data sheet. Depending on the duration and amplitude of the exposure, the effect on the LED variesin severity. what is certain is that any single EOS event has the potential to damage an LED. This damage might result in an immediate failure or in a gradual failure many hours after the event. Therefore all necessary precautions should be taken to avoid EOS events.
CAUSES OF ELECTRICAL OVERSTRESS
ESD is caused by the accumulation of electricity charge. The accumulation of the voltage can be as high as 10,000 volts, but the static power is not big, it will not threaten life. However, for some electronic devices can be fatal, resulting in device failure. LED devices with GaN-based, because it is wide bandgap semiconductor materials, it has high resistivity. For InGaN / GaN blue light, the thickness of the active layer of InGaN is generally only tens nanometers. Moreover, the distance between positive and negative electrodes is very small, if the static charge at both ends accumulated to a certain value, it may breakdown the PN to cause the failure of LED.
Transient over-current events
Transient over-current events subject one or more LEDs to current that is higher than the maximum rated current on
the LED data sheet, either directly through high current or indirectly through high voltage. These events are transient, meaning they happen for a short period of time – typically less than one second. They are sometimes referred to as surges or spikes, such as a “current spike” or “voltage spike.”An over-current event that occurs during the initial time period when the LeD turns on is commonly referred to as inrush current. This effect is typically caused when the capacitors in a driver circuit are initially energized. An overshoot can be seen by observing the output current to the LED.
Over-driving the LED
An LED is over?driven when the current applied to the LED exceeds the LED’s specified current rating.
EFFECTS Of EOS ON LEDs
Detecting damage to an LED subjected to EOS can be difficult. The damage is often not immediate and catastrophic and therefore goes undetected because the LED continues to produce light. However, changes to an LED’s electrical parameters can be detected by measuring forward and reverse bias currents of the suspect LED.
Catastrophic failures are usually easy to detect because the LED no longer produces light, although in a large array, a single failed LED may not be noticed. Two common symptoms, described below, are observed on XLamp LeDs that have catastrophically failed due to an EOS event. These can be seen by inspecting the LED under magnification. Other symptoms are not easily seen unless the LED package is de-processed and the bare chip is examined under advanced analysis techniques.
Damage near bond pads
When the EOS event is not severe, damage will be seen near the bond pads. Normally LEDs thathave failed in this manner will measure as a short circuit when tested with an ohmmeter or curve tracer. This is the most common EOS failure mode and the reason that nearly all EOS failures fail in the short circuit mode
ACTIVE PROTECTION CIRCUITS
1. Make sure that the driver is equipped with a transient voltage current suppressor.
2. Drive has overvoltage overcurrent protection.
3. Driver has AC power supply regulator.
4. Driver output voltage and current match LEDs, the maximum power supply voltage does not exceed the lamp output voltage of 1.3 times. The driver has a soft start.
5. Ensure that hot switching is not performed during the test.
6. Ensure that no peak / low frequency interference is present during the test.
7. Ensure to set the correct parameters ( not overvoltage ) during the test.
Some information are from CREE.com
Tranlator and Editor: Penny Peng