Reliability Evaluation of Liquid and Polymer Aluminum Electrolytic Capacitors
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Liquid aluminum electrolytic capacitors are known for their reliability problems. They are considered as the weakest link in the power electronics system. The liquid electrolyte of these capacitors is the single most important component which affects the reliability of these capacitors. The principal ingredients of the liquid electrolyte are solvent, water, solute and additives such as corrosion inhibitors and hydrogen absorbers. Usually, the primary solvent used in liquid electrolyte of aluminum electrolytic capacitors is ethylene glycol or γ-butyrolactone. The effect of liquid electrolyte solvent on the failure mechanisms observed in liquid aluminum electrolytic capacitors is missing. Effect of ripple current on the observed failure mechanisms is unknown. Polymer aluminum (PA) capacitors were introduced as the polymer electrolyte is conductive and solid therefore, it does not evaporate and the equivalent series resistance (ESR) of the PA capacitors is low. Manufacturers advise not to use PA capacitors in elevated temperature-humidity environments. But, the Failure modes and mechanisms of polymer aluminum electrolytic capacitors in elevated temperature-humidity are unknown. In this study, life testing of liquid aluminum electrolytic capacitors chosen based on primary solvent of the electrolyte was performed. For γ-butyrolactone solvent based capacitors, the failure mechanisms observed causing decrease in capacitance were evaporation of electrolyte and decrease in surface area of the aluminum oxide dielectric layer. The observed ESR increase was due to evaporation of electrolyte. For ethylene glycol solvent based capacitors, ESR increase was observed due to ester and amide formation, along with decrease in concentration of the carboxylic acid salts in the electrolyte and evaporation of electrolyte. The failure mechanisms observed in life tests with and without ripple current were the same. PA capacitors were tested at elevated temperature-humidity of 85ºC, 85%RH and Highly Accelerated Stress Test (HAST) condition of 110ºC, 85%RH. PA capacitors failed due to increase in ESR and increase in leakage current. Iron particles in dielectric layer from the manufacturing process of PA capacitors caused the high leakage current failure. This is a new failure mechanism which has not been reported in the literature. Failure modes observed in 85ºC, 85%RH and HAST tests were same therefore, HAST tests can be used as rapid assessment test for PA capacitors in elevated temperature-humidity environment.