Designers of LED-based systems may be glossing over one of the most important components, leaving their products susceptible to catastrophic failures, a circuit protection manufacturer said recently. "When you're designing LEDs, a lot of thought goes into driver efficiencies and power factors, and very little thought goes into the surge immunity standards you have to meet," Usha Patel, director of Latin American sales and segment marketing for Littelfuse Inc., a circuit protection manufacturer, said in a recent interview. "Circuit protection is like the lost stepchild. But if you do it wrong, it can kill your entire application."
Ironically, insufficient circuit protection may compromise the very reason for using light-emitting diodes (LEDs) in the first place. Many engineers specify LEDs because they offer long life and low maintenance. But both of those advantages are lost if designers fail to specify the correct protection against over-current and over-voltage situations. "LED luminaire replacements are two to three times the cost of incumbent technologies," Patel told us. "So you want it to last a minimum of five years without having to service it."
Patel breaks down circuit protection into two broad categories: over-current protection, which typically involves safety concerns; and surge immunity, or over-voltage protection, which often involves protection against lightning. "Over-voltage focuses on equipment reliability," she said. "Over-current involves the risk of shock and fire."
Both, however, can ruin an LED-based application. Voltage spikes from lightning or so-called "line swells" can ruin sensitive electronics downstream from power supplies. Too often, though, designers are unaware of the standards involved, especially with relatively new technologies, such as LEDs. Patel said:
LED lighting is a very fragmented market. There are many small companies that are trying to do the entire design, but they don't know what the standards are, or how to meet them. For many of the lighting designers, this is something new.
To know what the surge level will be, Patel recommends that designers first consider the location of the application. Indoor applications differ from covered outdoor applications, such as parking garages, and from uncovered applications, such as streetlights. "A lot of designers will buy the power supply, LEDs, heat sinks and thermal parts, then try to put it all together," Patel told us. "But the driver may only be rated at 2 kV or 4 kV, whereas an outdoor application needs to meet 10 kV."
Engineers should start considering such issues after they've picked their power supply and LED driver. That way they'll know how much current will be driven through the system and what the driver's rating will be. They can then add protection in front of the driver or as an external module. Protection is critical, not only because products can be ruined, but because unprotected systems can have safety concerns. "You need to have short-circuit protection," Patel said. "Excessive current causes heat, which leads to fire."
To deal with over-voltage situations, Patel recommends customers thoroughly understand their LED drivers, and in some cases, use transient voltage suppression (TVS) diodes. Such solutions aren't always obvious, however, especially for those who are exploring new technical territories. Patel said:
We are bailing out customers all day long. Their electronics are blowing, and they say, 'but we put in AC protection.' For some reason, though, they still have too much energy running through their circuits. It happens all the time.
This is indeed an issue with many electronic systems. There are good CAE tools that would allow one to model these situations and to plan proper mitigation. The software is expensive, though, and working with suppliers, like Littlefuse, can help avoid some of those costs.
When I was designing LED lighting fixtures for a major ceiling fan supplier, I would immediately draw out a system block diagram including the power supply. Within this subcircuit block, I included all interconnects, transient suppression devices, and electromechanical devices. A detailed product specification will be written describing the functionality of the subcircuit blocks and their power and electrical requirements (Undervoltage, Overcurrent, etc) of the system block diagram. This engineering approach prevented significant damage to the prototype during testing because of the stringent transient electrical specifications spelled out in the design document.
@mrdon--I like your approach to this type of circuit design; it accentuates the positive from the requirements and minimalizes the negative effects of other design facets. Well done, sir.
notarboca, Thank you for the kind response. After seeing blue smoke let out of LED prototypes, there's a point where you start to think logical and document the exact function(s) required for the system/product under development.
LEDs are typically always the INDICATORS of failure, but not always the source of failure. Improper circuit protection against abnormal conditions can just as easily kill a drive component as it would an LED, but the LED will almost always turn off due to any failure. (message to Quality guys...if it doesn't light up, it's not necessarily the LED's fault!)
The failure that I see mostly is when the LED system design does not properly manage the heat generated within the LED and/or drive electronics. Good, worst case analysis should be done to ensure that the LED will always operate in a 'warm and fuzzy' region thus ensuring long and reliable life. (note: I don't have to design against lighting strikes! That sounds tough!)
General rule of thumb; Lumen maintenance failures result from overdriving the LED (high current density), but catastrophic LED failure results from exceeding the Junction Temperature rating.
Very good points! Your absolutely correct regarding thermal manaagement concerns with LEDs and their driver circuits. Lumen maintenance failures caused by overdriving LEDs was the biggest challenge I faced when developing my lighting fixture prototypes at Hunter Fan. Although the LED continued emitting light, the opto-device was overstress by driving the component extremely hard with max current (Ifd). The naked eye couldn't see it put measuring its luminous flux showed the LED being degraded over time. In addition to protecting the overall product's system circuits from high voltage transients, thermal management must be high on the design priority list as well.
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