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Interview: LED designing plays important role in its reliability and safety

LED designing plays important role in its reliability and safety

By BizLED Bureau

September 24, 2015: The unit costs of LEDs have been steadily declining by double-digit percentages, making LED lighting technology viable for commercial, industrial and outdoor lighting applications. In addition, large strides have been made in improving the efficiency, lumen output and performance of LED lights. Designing of LED lighting products plays an important role in the performance of the product.

In conversation with BizLED, Todd Phillips, Global Marketing Manager, Littelfuse, explains the importance of designing an LED product, the standards, and the factors that should be kept in mind for its reliability and safety.

What are the key factors that should be considered while designing an LED bulb?

Design engineers should first consider the applications and environment where the LED bulb will be used. This will lead to considerations for system voltage, electrical threats, which may be present, and other environmental factors that will affect the performance and lifespan of the LED bulb.

What safety and reliability issues should be kept in mind while designing an LED bulb?

Electrical hazards such as overcurrent and voltage surge events should be considered during the design of a bulb. LED bulbs are typically connected directly to AC mains of 120V, 220V or higher. These bulbs could easily be damaged by short circuit and overload conditions caused by component and/or circuit failures inside the bulb. Additionally, lightning surges or load switching transients originating outside the bulb can create voltage spikes that ultimately damage the components inside the bulb, rendering it dead.

How to use circuit protection devices to increase LED lighting reliability and safety?

Surge suppression components such as MOVs and TVS diodes are used to prevent internal components from being damaged by surges and transients. In the event of an internal component failure that leads to an overcurrent event, fuses are used to disconnect power and stop the overcurrent from overheating the wires providing power to the LED bulb. Overheating of the wire could lead to insulation breakdown and eventually to a fire.

What are the surge immunity requirements for consumer LED lighting/retrofit lamps?

IEC 61000-4-5 defines the following levels, and is the global standard for overvoltage transient surge testing, except for in the US. IEC 61000-4-5 uses a 1.2x50us / 8x20us combination wave of 10 strikes line-to-line, 5 strikes at a phase angle of 900, and 5 strikes at a phase angle of 2700. The voltage and current used depends on the power rating of the lamp. For lamps and luminaires of less than 25W, 500V and 250A are used. For lamps and luminaires of greater than 25W, 1kV and 500A are used.

What, according to you, are the main global standards that an LED bulb should be compliant to?

Most countries have their own standards, but fortunately, all of them outside the US reference IEC/EN 61000-4-5.

What are the design considerations in MOV selection?

MOV?s are very good at suppressing high levels of surge transients. Overtime, however, the suppression of these surges will cause the MOV to degrade and eventually fail as an electrical short. Proper thermal protection must be considered to prevent this from causing a fire.

How an LED bulb can be protected against temporary overvoltage and limited current abnormal overvoltage?

Littelfuse invented the thermally protected MOV, also commonly called the TMOV. This device includes a thermal link, which disconnects the MOV from the circuit when it senses the MOV is degrading, before the MOV enters into thermal run-away.

Is the process same for streetlights and LED tube lights as well?

Yes. Littelfuse offers thermally protected MOV solutions for streetlights and LED tube lights. These operate under the same principals as the TMOV.

What are the design considerations in fuse selection to withstand surges?

Each fuse has a rating for the amount of energy required to melt its internal fusing element. This rating is the I2t, or amps-squared-seconds. As mentioned previously, surges are characterized by both voltage and current over a given time. Using the current characteristic of the surge allows us to calculate an I2t rating for the surge. When picking a fuse, it is critical to select one with an I2t, which is greater, by a large safety margin, than the I2t rating of the surge, which will be applied during immunity testing.

What are the potential problems with alternative LED lamp overcurrent protection technologies?

Alternate overcurrent protection technologies are not held to the same high standards as fuses nor are they designed with all of the same considerations as fuses. Often it takes a much greater level of heat to cause the other technologies to activate, which can lead to a fire.

What innovative LED lighting design solutions does Littelfuse offer?

We have fuses for overcurrent protection and MOV?s, surge protection modules, and TVS diodes for overvoltage (surge) protection of various levels and for various applications. Selecting each type of circuit protection technology for LED lighting presents its own challenges. This is the key reason we created the LED Lighting design guide. It provides background and considerations for selecting the right part and can be downloaded from our website, www.littelfuse.com/industries/led-lighting


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