Introduction

An aluminum electrolytic capacitor performs to its specification only when it is applied within its ripple and temperature limits. Two numbers decide the life of the part and the reliability of the design: the ripple current that flows through it and the temperature at which it runs. This application note explains the ripple and lifetime behavior of CapXon capacitors and gives the practical rules for keeping a design inside them.

Ripple Current and ESR

A capacitor that sits across a switching rail charges and discharges every cycle, so an alternating current flows through it. That ripple current heats the capacitor through its equivalent series resistance, the ESR, and the resulting temperature rise adds to the ambient to give the core temperature. The ripple rating in the datasheet is the current that produces the rated temperature rise at the rated temperature and frequency, so it is a thermal limit rather than an electrical one. A low-ESR capacitor heats less for the same ripple, which is why low-ESR parts are chosen at the output of a switching supply.

Frequency Matters

The ripple rating depends on the frequency, and a capacitor rated at 120 Hz allows a higher ripple at the switching frequency, so use the frequency multiplier in the datasheet rather than the headline number. At high frequency the ESR dominates the impedance, so the ripple and the heating depend on the ESR at that frequency more than on the capacitance.

Temperature and Lifetime

The life of an aluminum electrolytic capacitor is set by the evaporation of the electrolyte, which accelerates with temperature, so the endurance in the datasheet is quoted at a rated temperature and ripple. The practical rule is that the life roughly doubles for every 10 °C reduction in core temperature, and the core temperature is the ambient plus the ripple heating. A capacitor running at 65 °C instead of 105 °C lasts many times longer, which is why a cool layout extends the life far beyond the datasheet endurance.

Estimating the Core Temperature

Measure the case temperature at worst-case load and ambient with a thermocouple, then estimate the core temperature from the case temperature and the thermal resistance of the can, or use the ripple heating to compute the rise. Compare the result with the derating curve to confirm the margin, and remember that a sealed or hot enclosure raises the ambient the capacitor sees.

Long-Life Series

Where the service interval matters, choose a long-life series: the CapXon LE radial range is rated 12000 hours and the UJ snap-in range up to 5000 hours at 105 °C, against 2000 hours for a general-purpose part. The longer endurance gives a much longer service interval for continuous-duty equipment such as solar, industrial and lighting converters.

Practical Rules

Keep the ripple within the rating for the expected case temperature and frequency, space the capacitors for airflow and keep them away from the hottest components, and choose the larger can or the lower-ESR part where the ripple is high. Measure the case temperature at worst-case load and ambient and compare it with the derating curve, then estimate the lifetime and confirm the margin before you release the design. These steps turn a capacitor choice into a verified decision rather than a guess.

Verification

After integration, verify on the bench: measure the ripple current with a current probe, measure the case temperature at worst-case load and ambient, and compare both with the ratings. Our FAE team can review your measurements and help you interpret them, so the capacitor performs in the product as it does on the datasheet, and the design meets its life target with margin to spare.