The question remains, are aluminum electrolytic capacitors inherently unreliable? The short answer is no. The slightly longer answer is that capacitors have always had higher failure rates than other electronic components. Thus, relatively speaking, they have high failure rates, but in absolute terms, they are extremely reliable when correctly used. Aluminum electrolytic capacitors are ubiquitous, and are included in applications with the most stringent reliability requirements, e.g., Minuteman missiles. Also, the expected life numbers found on catalog pages assume the capacitor is being run at its maximum rated voltage and maximum rated temperature. Once these stress factors are reduced, the expected life increases dramatically.

For an objective estimate of reliability, the failure rates and methods of calculation contained within MIL-HDBK-217F are up-to-date, complete, and conservative. The failure rate for a given component is calculated by taking its base failure rate and multiplying that figure by various usage related factors. For aluminum electrolytic capacitors, the overall failure rate is given by:

In MIL-HDBK217F, failure rates are given in failures per 10^{6 }hours. For the capacitors in question the failure rate is:

The other factors are:

Some of the critical factors are calculated as follows:

For purposes of this calculation, it was assumed that commercial parts with no reliability screening would be used, along with a worst case ground-based environment That led to the factors used here being:

Using these values, the total failure rate per capacitor is:

Thus, if you are designing a circuit which uses ten aluminum electrolytic capacitors and you remain within the stress factors given, you would expect an average failure rate of 6.7 failures per million hours (or one failure approximately every 150,000 hours) based solely on capacitor failure rates. The overall failure rate will, of course, be higher once the rates for the other components are included. The failure rate of the capacitors is quite low and would certainly result in a driver with an expected life which exceeds the published L50 or L70 lumen maintainence figures given by the LED manufacturers.

It is also important to realize that not all of the capacitor failures noted during the so-called “capacitor plague” were catastrophic failures. Rather, some were characterized only by a reduction in capacitance. Whereas diminished capacitance would result increased ripple on the VCC supply in a computer could cause degraded operation, it would have much less impact on an LED driver.

This is encouraging in view of the fact that aluminum electrolytics have the highest available volumetric efficiency for energy storage in voltages over fifty volts. At lower voltages tantalum capacitors have an advantage in volumetric efficiency. The failure rate of tantalum is attractive at low stress factors, but increases drastically if the voltage stress exceeds 50%.

Peter R. Rahm

Chief Engineer

LEDdynamics, Inc.