Electrical Protection
Over-Current and Over-voltage Protection
Like all semiconductor components LEDs are susceptible to damage from over-voltage. However, LEDs using InGaN and GaN technologies are particularly sensitive to over-voltage. These technologies are used in Blue and some Green LEDs, also White LEDs that use the InGaN / GaN Blue base colour.
An over-voltage threat can come from many sources, but these can generally be split into four types: -
• Lighting strikes both direct and secondary
• Inductive spikes
• NEMP (Nuclear Electro-Magnetic Pulse)
• ESD Electro-Static Discharge (Human Body Model)
The threat to the LED component from over-voltage can occur: -
1. Prior to, or during installation of the LED on the PCB
2. In normal operation of the final product
Electronic components that have been exposed to ESD hazard do not always fail immediately, but in LEDs ESD damage can manifest itself as reduced light output and increased field failure rate.
ESD Protection during Production: - Protecting LEDs prior to and during the production process requires the adoption of ESD handling procedures: -
1. The LED manufacturer must adopt ESD procedures in their production process Note: - all YEG suppliers have stringent on-going quality procedures to ensure compliance with ESD protection schemes .
2. The InGaN and GaN LED components should be transported in Anti-Static packaging.
3. Production operators must wear antistatic wrist bands and ensure connection to grounded points provided.
4. Staff should wear antistatic suits when entering work areas with conductive machines
5. All test instruments and production equipment to be grounded in line with manufacturers recommendations
6. Minimise friction between the products and surroundings to avoid the build up of static charge.
7. A minimum humidity level of 50% should be maintained in production areas
8. All ESD protection areas and antistatic materials should be checked on a routine basis to ensure correct operation.
9. All workstations that handle ESD-sensitive devices must maintain an electrostatic condition of 150V or less.
Over-Current Protection: - The reliability and operating life of an LED is directly proportional to the junction temperature (Tj) of the die and this is closely related to the amount of current flowing (If) through the device. In addition, the ambient temperature and the implementation of the cooling mechanism around the LED can impact Tj thus directly increasing or reducing product life
Similar to a signal diode or power rectifier, when working in Forward Bias an LED has very low impedance i.e. small changes in voltage will result in large changes in current. This requires that the LED be protected by a current limiting resistor or driven by a constant current source. This subject is covered more fully in the LED Driving section of this site, Click to go to LED Driving section
Over-voltage Protection: - The over-voltage threat(s) needs to be assessed before selecting the most suitable protection scheme. It is possible that the system as a whole is protected and that individual components within the system do not require specific protection. Looking at some examples of overvoltage protection; -
a) In equipment where there are exposed terminals (USB, Ethernet ports etc.) the connections will require protection from an ESD (Human Body) strike. In addition an Ethernet port that is connected over a land-line may need protection from a secondary lightning strike.
b) Equipment connected to heavy electrical motors will need protection from potential inductive load dump i.e. under fault condition where the energy stored in a coil / inductor is pushed back into the supply rails.
c) Equipment connected to an AC mains source will need to be protected against secondary lighting strikes and inductive spikes caused in the generation process.
In general, the Over-voltage protection device should be placed across the LED to divert the voltage away from the LED: -
In the above diagram, a TVS is used to provide a very low impedance path to the over voltage spike, holding the voltage at the intrinsic Clamping Voltage (Vcl) of the individual TVS device. TVS devices are manufactured with a range of preset Vcl values. Vcl should be selected to match the operating requirements and over-voltage threat.
There are many different technologies for over-voltage protection device including: -
• Silicon Avalanche Diodes (TVS)
• Gas Discharge Tubes
• Metal Oxide Varistor (MOV)
• Thyristors
All are designed to protect from over-voltage threats, but there is not a ‘one size fits all’ solution each of the technologies will have strengths and weaknesses that make them more / less suitable for certain applications. Device selection can be a complex process, but there are some key parameters that should be assessed when selecting an over voltage protection device: -
• Current handling capabilities
• Input capacitance
• Speed of operation
• Wear out rate i.e. number of repeated operations before degradation
• Operating mode (Crowbar / Clamp)
Determination of the over-voltage threat and selection of the correct protection scheme is complex topic and beyond the scope of this article. However, the technical team at YEG can help direct you to sources of information on this topic.