Introduction

An aluminum electrolytic capacitor ages with its core temperature, and the core temperature is set by the ambient, the case thermal resistance and the ripple heating. Because the capacitor is usually the first component to age in a power converter, understanding the ripple and the lifetime is central to building a reliable design. This application note explains the practical rules for ripple current, temperature and lifetime in AiSHi aluminum electrolytic capacitor banks.

Ripple Current

A converter's switching stage draws a pulsed current from its DC link, and the capacitor supplies that pulse, so the capacitor carries an AC ripple current in addition to the DC voltage. The ripple flows through the capacitor's equivalent series resistance and dissipates power, which raises the core temperature above the case temperature. The ripple rating in the datasheet is specified at a temperature and a frequency, so confirm that the operating ripple is within the rating at your conditions, and derate where the capacitor will run hot or at a different frequency.

Sharing Across a Bank

Paralleling capacitors shares the ripple, so the bank rating is the sum of the individual ratings, provided the current shares evenly. Keep the busbar impedance balanced so each capacitor carries its share, and verify the ripple on each part under load. Uneven sharing can overload one capacitor and shorten the life of the whole bank.

Temperature and Lifetime

The life of an aluminum electrolytic capacitor follows an Arrhenius-like rule: it roughly doubles for every 10 °C reduction in core temperature. Estimate the core temperature from the measured case temperature and the ripple heating, then use the datasheet endurance (for example 3000 hours at 105 °C or 5000 hours at 85 °C) and the rule to project the life at your conditions. Because the effect is exponential, keeping the capacitor cool is the single most effective way to extend its life.

The Case Temperature Is What You Measure

In practice you measure the case temperature with a thermocouple at worst-case load and ambient, then add the ripple heating to estimate the core. Keep the core well below the rated maximum, and choose a longer-life series such as the AiSHi LG range where the product must run for years with little servicing.

Layout and Thermal Design

Space the capacitors for airflow, keep them away from the transformer and the switching devices, and use a larger can where more thermal mass is needed. Avoid placing a capacitor in the shadow of a heat sink or next to a hot magnetics component, because the local ambient sets the case temperature. In a sealed enclosure, consider a thermal path to the chassis. These layout choices are a direct lever on the life of the converter.

Validation

After the design is built, measure the rail ripple, the capacitor case temperature and, over time, the capacitance, to confirm the lifetime estimate. A change from the initial value indicates the aging rate, and a case temperature above the estimate points to a layout problem. Our FAE team can review the measurements and help you interpret them, so the life you designed for is the life you ship.