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|Can Electrolytic Capacitors Meet the Demands of High Reliability Applications?|
|Keywords: electrolytic capacitors, reliability, life expectancy|
|Electrolytic capacitors have long been identified as a weak link for long term high reliability applications. However, capacitor manufacturers have made significant improvements to the materials and manufacturing processes to enhance their reliability. This paper will discuss those changes, provide insight into the various failure mechanisms for electrolytic capacitors and describe appropriate accelerated tests to validate performance. We will take a deeper dive into the methodologies utilized to improve capacitor performance, e.g. foil purity and electrolyte volume. We will also discuss, from a reliability perspective, the impact of changing to a higher temperature electrolyte (from ethylene glycol to DMF, DMA and GBL) and also changes in the bung material (from butyl to EPDM). There are several environmental factors involved in the aging of electrolytic capacitors. Electrolyte loss due to drying out and leakage current due to oxide degradation are thermally related as is the self-heating associated with ripple current. The impact of the applied voltage level is also a driver as it can cause leakage current increases as well. All of these issues result in a capacitance decrease, an increase in ESR, and a change to the dissipation factor. Many other failure mechanisms associated with manufacturing will also be discussed. Aluminum electrolytic capacitor manufacturers state the endurance lifetime of their products in their datasheets, supported by testing that applies rated voltage and ripple current at the maximum rated temperature to the capacitor. In service, electrolytic capacitors are rarely run at their rated values, so the endurance lifetime is not directly applicable. The industry uses a rule of thumb approach of lifetime doubling for every 10°C decrease below rated temperature to estimate lifetime at lower temperatures. However, there is no explicit approach to estimate lifetime at lower applied ripple currents, which is also known to increase lifetime. So, how can we examine the different suppliers to ascertain their differences in reliability? Evaporation prediction has been based on a widely held standard aging relationship that ties back to the 10°C previously noted. However, there are differences in the way that the manufacturers make this determination. The paper will discuss these differences as the ripple current, voltage, and can size play a large part. This paper will then identify which parameters have a distinct impact on capacitor lifetime and how a Reliability Physics approach offers the best methodology for ascertaining the most reliable capacitor for your application. The process of determining the capacitor’s ESR, then calculating its core temperature, leading to the calculation of its vapor pressure and finally calculating the loss of electrolyte will be presented as will an approach to shorten test time when validating electrolytic capacitors. Examples of calculations for life expectancy will be shown to demonstrate the effects of applied voltage, rated temperature, ripple current, and the endurance factor coupled with the application usage profile. Finally, a best in class qualification methodology will be presented.|
|Greg Caswell, Sr. Member of the Technical Staff