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SSL extends lifespan of LED bulb

Posted: 10 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Light-emitting diode? solid-state lighting? SSL? A19? luminaire?

Light-emitting diode (LED) bulbs offer the potential of 50,000 hours of operating lifetime, which is almost 25 years of typical usage. This is a 50x improvement over incandescent-equivalent technology. With demand for LED lighting growing rapidly, a key issue that could hold the industry back is if solid-state lighting (SSL) bulbs do not achieve the promise of long operating life. The obvious design considerations for solid-state lighting are efficiency and cost. But, thermal management is just as vital as any other design criteria, because too much heat can impact operating life, not to mention bulb safety. The energy savings of solid-state lighting gains the most over the full operating lifetime potential of the actual luminaire. While the LEDs offer the promise of this long lifetime, additional components required in the LED driver circuit can dramatically decrease luminaire operating life if intelligent thermal management is not implemented.

It is easy to overlook some important thermal aspects of LED designissues that can result in potentially catastrophic luminaire failures. An LED bulb can be used in an enclosed lighting fixture or a fixture that is open to normal air circulation. The thermal conditions in these two cases are radically different, but the bulb in both instances has the same physical and electrical design. The temperature inside a closed lighting fixture can rise quickly to levels above 60C, which subsequently causes the temperature inside the light bulb to exceed 90C. In open-air fixtures, the temperature inside the bulb itself can be as much as 30C lower than its closed fixture counterpart.

An LED-based bulb with no thermal protection whatsoever used under conditions where there is near zero air flow could result in a thermal runaway condition. Figure 1 shows the construction of a typical A19 retrofit LED bulb and the confined space in which the driver circuit needs to operate. This tight space exacerbates the temperature issues. Early examples of poorly designed LED luminaires include devices that failed after 1,000 hours, just like the incandescent bulbs they were intended to replace, and even a design where the bulb itself experienced thermal runaway, melting the casing and posing a potential fire risk. The end result was a costly recall of a large number of bulbs. These early models did not take into account the importance of thermal design on the overall quality of the LED bulb. A simple solution is to integrate a basic thermal shutdown circuit, something that is already very common in IC technology.

Figure 1: Typical construction of an LED-based solid-state lighting luminaire.

Most LED drivers used in solid-state lighting contain a straightforward thermal protection circuit. Most power-management ICs employ a simple thermal shutdown function where the output of the regulator shuts down to protect itself when a maximum temperature is reached. This does protect the main IC, but when applied to an LED lighting circuit, it presents two critical problems. First, the output of the LED driver shuts down completely, eliminating the light. The output doesn't turn on again until the thermal event clears and the temperature of the IC drops below the hysteresis point in the thermal shutdown circuit.

Next issue
The second issue is less obvious, but much more crucial to the lifetime of the luminaire. At elevated temperatures, the passive components in the LED driver, including electrolytic capacitors, will see reduced operating lifetimes.

Aluminium electrolytic capacitors offer an optimal combination of size, capacitance and cost for applications such as power supplies and LED drivers. Solid-state lighting applications require cost-effective components that can handle rugged lighting operating environments. When they gained popularity, electrolytic capacitors were mainly used in open-air power supplies where their operating temperatures normally did not exceed 60C. When encapsulated power supplies gained popularity, the electrolytic capacitor manufacturers created high-temperature-rated devices, capable of operating up to 105C. But, the guaranteed lifetime at 105C was only on the order of 2,000 hours. For some power-supply applications, this is fine, but for solid state lighting, with the promise of nearly 50,000 hours of operating life, this falls way short. However, with careful thermal management, 50,000 hours can be achieved.

Figure 2 shows a typical lifetime curve based the operating temperature of a high-temperature-rated electrolytic capacitor. The relationship between temperature and operating life is non-linear, where for every 10C reduction in temperature, the lifetime of the capacitor doubles. An average expected lifetime of 5,000 hours at 105C ambient temperature for a typical electrolytic capacitor will increase to 40,000 hours at 75C. The solid state lighting market needs to maintain the ambient temperature of these components down to a level where the capacitor can operate within the maximum expected lifetime of the overall bulb.

Figure 2: Temperature characteristics of a typical electrolytic capacitor vs. ambient temperature.

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