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What are the important considerations in designing an LED driver to match the lifetime of the LED die?

13 Apr 2011

LStacey

United Kingdom

What are the important considerations in designing an LED driver to match the lifetime of the LED die?

Replies

iainmosely

United Kingdom
1 year ago

Reliability prediction of products generally focuses on specifying an MTBF figure and expected product lifetime. MTBF is used to estimate field failure rates during the normal life of a product and lifetime gives the length of time a product can be expected to perform its normal function under nominal operating conditions. MTBF and lifetime are often mistakenly interchanged but they are not the same. 

The lifetime of an LED die/housing is generally quoted as the length of time for which a certain lumen output can be maintained under nominal operating conditions. For example, one LED manufacturer datasheet states:-

“A Lumen maintenance of greater than 70% after 50,000 hours”

In order to design a driver to match the lifetime of the LED itself, it is important to understand which component in the driver dominates its lifetime. Often, electrolytic capacitors are used in converter electronics since they offer higher energy density than other technologies and are relatively low cost. If electrolytics are used, they are likely to dominate the life of the driver electronics.

Over time, the electrolyte used inside the electrolytic capacitor tends to leak away which leads to a reduction in effective capacitance and a rise in capacitor ESR. Increasing the operating temperature of the capacitor will speed up the process of electrolyte leakage too. To achieve a certain capacitor lifetime it is important to operate within the specified ripple current of the part and to ensure the ambient temperature does not exceed a certain level.

A useful calculation can be performed using the ‘Arrhenius Equation’ which allows capacitor lifetimes to be estimated. This equation states that every 10 Deg C drop in operating temperature will tend to double a components lifetime. If we take the example of a 105 deg C, 2000 hour capacitor, it will have lifetimes of:-

Ambient of 105 deg C   2000 hours

Ambient of 95 deg C    4000 hours

Ambient of 85 deg C    8000 hours      etc

For the 105 deg C, 2000 hour cap to operate for 50,000 hours, it should be operated within its rated ripple current capability in an ambient of no higher than about 58 deg C. Choosing longer life or higher temperature electrolytics will allow for operation in a higher ambient condition but these are often more expensive.

Lastly, remember that the ambient temperature is that measured right next to the capacitor. This is likely to be much higher than that in a general room environment. Be careful placing electrolytics near to hot heatsinks as this is a sure way to reduce the capacitor operational life.