Derating has always been a questionable practice, but it had some legitimacy in older electronics because solid-state mechanisms typically took decades to evolve and induce any significant number of failures.
This focus on function instead of reliability puts today’s electronic components, like light emitting diodes (LEDs), at considerable risk since it brings power supply engineers no closer to identifying the maximum thermal temperature LEDs and optocouplers can withstand. However, there is hope for resolution by inserting thermal modeling results into predictive tools that are based on reliability physics instead of derating.
LEDs are typically incorporated into power supplies as indicator lights or as one half of optocoupler technology. LEDs have a natural lifespan that ends in a wear-out mechanism.
Defects within the LED active region can spur nucleation and dislocation growth and are particularly affected by temperature and current. However, indicator lights rarely experience “failure,” meaning a 50% reduction in brightness, because designers introduce a low duty cycle and a low forward current. This combination is more than sufficient to power high intensity LEDs in all but the brightest environments, and extend LED lifetime by several decades.
The real issue with LEDs comes with their use in optocouplers. Low voltage optocouplers are rated between 40 and 60A, although it’s not uncommon for designers to drive the LED at 1 to 10 mA. While the approach negates the thermal danger to optocouplers, it also upends LED reliability because the rated meantime to failure of 50,000 hours at maximum rated forward current at 25C is invalid.
This detrimental tradeoff would not be captured using the traditional derating approach to determine appropriate temperatures.
The LED lifespan example illustrates the need for reliability tools that accurately predict and analyze degradation behavior and power supply performance based on validated algorithms that use environmental, material and architectural information.
One such tool is DfR Solutions’ Sherlock Automated Design Analysis™ Software. Sherlock uses Physics of Failure (PoF) to help power supply engineers understand when thermal temperatures are too hot for electronics components, and find solutions based on real-life data inputs and complex calculations performed in comprehensive software databases. The results are tabulated in a matter of minutes, not days, and are available well before prototyping starts – saving money and time.
Find out more about reliability physics and Sherlock in our webinar, Introduction to Physics of Failure. Click the button below to download the slide presentation.