Cost-benefit approach to degradation of electrolytic capacitors

Abstract

Aluminum electrolytic capacitors are widely used as a filter or bulky capacitor after rectification stages of switching power supplies (SMPS). Fly-back, forward, and resonant converter topologies, which are widely used in consumer electronics products, computer power supplies, and various kind of adapters, require electrolytic capacitors (EL. CAP.) in primary stage and secondary stages for rectification to smooth non-DC current. Electrolytic capacitors must be used because of size and capacitance, unit price, and to withstand voltage. Overall reliability of an SMPS mainly depends on electrolytic capacitors used, because they have extremely short lifetime compared to other active and passive components. Failure modes of electrolytic capacitors are either catastrophic failures or degradation failures. Catastrophic failures generally occur because of fabrication processes. Degradation failures are seen during gradual aging of electrolytic capacitors. During degradation, electrochemical aging occurs between Al2O3 foils and electrolytic solvent, and this creates hydrogen gas inside cap. Degradation process also increases equivalent series resistance (ESR) and decreases capacitance. This degradation is mainly dependent on heat and is somewhat predictable, because main degradation agent is a heat-triggered accelerated electrochemical reaction. Previous works in the literature disclose the obvious fact that this degradation process due to heat can be modeled by a modified Arrhenius Equation, and therefore, it is an exponential curve [1][2]. All commercially available capacitors have their base rated life duration specified in their datasheets, given in hours, such as 1000 hrs, 2000 hrs, 5000 hrs, etc. Those values are measured according to the international standard IEC60384 (or Japanese equivalent JIS-C-5101) and are called “base life”. In an SMPS design, the real usage life of a capacitor is calculated based on the temperature that i- is used at. The acceleration factor is calculated with a modified approximation formula and multiplied with base life to calculate the predicted life. Low grade capacitors cause many capacitor plague issues and products of different manufacturers show completely different lifetime performance even their specifications and base lifetime are the same. In this paper, low and medium-grade capacitors from three different manufacturers and high-grade capacitors from two different Japanese manufacturers are selected and tested at their maximum allowable temperature limits, allowable max voltage and ripple current. Total of 100 samples are used during tests from each different manufacturer. Degradation of each capacitor brand, required life and unit cost are evaluated, and a cost/life optimization has been formulized between low-grade capacitors and high-grade capacitors. In the literature, there are a number of notable works carried out on degradation analysis of electrolytic capacitors; however, there is no comparative analysis of low cost capacitors used in consumer devices. The derived novel method is a useful tool to choose electrolytic capacitors for consumer products that use high voltage (≤450 V) on the primary side and low voltage (≤65 V) on the secondary side of SMPS. This cost-benefit approximation for capacitor selection process gives good opportunity to design engineers to do the selection based on reliability and overall cost. Results show that the optimized capacitor is not the cheapest, most expensive, or most reliable one; optimal capacitor is a modest Chinese “CAP. B”. It is the most desirable solution that minimizes overall cost inside the warranty duration.