This presentation was originally given by Andrew Smith, product manager, LED Lighting at Power Integrations for EDN’s May 2011 live LED event in San Jose. The next free LED event will be part of DesignWest on March 27, 2012. Register now at this link 4shared .)
Consumers often see LED lamp failures. If you look at the growing number traffic lights using LEDs, for example, you might see that many individual LED segments are no longer working. Early deployments, such as LED street lights in Asia, have experienced many failures. An examination of these failures shows that the LEDs themselves have not failed; rather, the power supplies that provide the power to the LED circuit have failed. We need to understand why LED lamps are failing and what it is about the power supply that is limiting the lifetime of the deployed circuits so that we can develop a better solution to improve LED lamp lifetimes.
One major issue is that LED lighting environments are very stressful for the power supply. LED lamps will typically run at full load for their entire operating period and they operated in an extremely high ambient environment.
An LED lamp will generate a large amount of heat, like any other lighting circuit (about 80% of the output from the power supply is lost as heat). The problem is that the LED lamp is located very close to the power supply. Therefore, the power supply itself sees the equivalent of its full rated output power (its own dissipation plus 80% of the output power) being dissipated as heat in its proximity.
With that introduction, let’s begin by looking at what causes LED power supplies to fail. There are two basic causes: heat and time. As the temperature rises, the likelihood of LED lamp failure increases and, as we have noted, high temperatures (often in excess of 90°C) are the norm for LED lamps.
When we examine the effect of heat, we find that the optocouplers and aluminum electrolytic capacitors are the most vulnerable components in the power supply. We are not going to cover the optocouplers today since that subject would comprise an entire paper. We are going to concentrate on what happens to an electrolytic capacitor in the circuit, how heat affects it over time, and what happens when the electrolytic capacitor gets to the end of its life. We will then look at how to fix the problem by removing electrolytic capacitors from key circuit locations.
First, let’s talk about the lifetime of an LED lamp. ENERGY STAR is specifying a lifetime requirement of more than 25,000 hours for residential applications and more than 35,000 hours for commercial applications. They describe the L70* characteristic for the lamp. This means that the relative light output must not fall below 70% of its initial brightness value in less than the rated lifetime of the system. (Lumen depreciation is the decrease in lumen output that occurs. L70 means that lumen depreciation to 70% of initial lumen output; stated conversely, it indicates 70% lumen maintenance.)
Lifetime of the circuit is only as long as that of the element in the circuit with the shortest lifetime. As seen in Fig. 1, the typical high-brightness LED circuit could last perhaps 45,000 hours before it will get to the L70 point. However, lifetime for a typical power supply may be only 20,000 hours. So what effectively happens is that it doesn’t really matter how strong your LED is if the power supply behind it can only last 20,000 hours 4shared .