How to Design Drivers for Long Life

In order to understand how to design LED drivers for long life, one first has to understand what factors can limit driver life span. The most basic potential cause for premature failure exists in the components that are used in the design of the driver. When do different components typically wear out and what is considered their “normal use”? 

Different types of components wear out in different ways. This concept seems simple, but it really needs to be understood. For example, semiconductors have a very long life when operated at a constant temperature within specifications. However, if the temperature is cycled, for example, by the driver being turned ON and OFF, then the thermal expansion and contraction of the semiconductor can cause mechanical failures in the connections between the semiconductor die and the outside world.  This failure mechanism can be mitigated by operating the semiconductors at temperatures significantly lower than their maximum rated temperature. For example, most MOSFET’s are rated to operate at a junction temperature of 150C. However, the maximum junction temperature is usually limited to 100-110C or lower at the maximum operating temperature, to limit the damaging effects of thermal cycling.

 Another “wear-out mechanism” for drivers is related to the limitations of electrolytic capacitors. These typically have a life rating of 5,000 to 10,000 hours for high grade capacitors at their rated maximum temperature. The life of an electrolytic capacitor increases by a factor of two for every ten degrees the operating temperature is lowered.  Since a year is about 8,000 hours, it is easy to see that the electrolytic capacitors must be operated at a lower temperature than their maximum rated temperature to achieve a “lifetime” that would exceed one year.  Other components such as film capacitors and magnetics also have maximum temperature ratings that must be observed to achieve long life.

One key life-span measurement addresses how long the driver will continue to function in a controlled laboratory environment with elevated temperature or with temperature cycling.  This type of test discounts other factors that can also limit the life or damage a driver in the field.  One such factor includes extreme voltages on the input, outside the normal specified input voltage operating range. Lightning strikes are very short, high voltage (several thousand volts) impulses applied to the input of the driver that can cause damage if input surge suppression components are not used in the design. The IEEE has recommended levels of lightning protection depending on the location that drivers are applied.  Europe has a general requirement for lightning immunity that is commonly met by U.S. products.  However, the levels and frequency of lightning that can happen in certain parts of the U.S. (such as the Southeast) are much higher than the European standards.  Moreover, normal power line voltage variations (due to load switching and power outages) can also produce voltages on the input that can damage components as well as limit the life of the driver. 

Designing drivers for long life can only be achieved by understanding the limitations of all the components that are used in creating them. Operating the components at levels well below their maximum ratings and understanding “real world” occurrences that could affect a driver’s longevity goes a long way in improving the long-term effectiveness of a driver design.

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