An electrolytic capacitor in the cold
Symptom
Section titled “Symptom”At the cold end of its temperature range a converter’s output ripple grows several times over, while everything behaves at room temperature.
The electrolyte’s resistivity rises in the cold, and an aluminum electrolytic capacitor’s ESR at its lowest rated temperature is often 10 to 100 times its room-temperature ESR [Cornell Dubilier Electronics, Aluminum Electrolytic Capacitor Application Guide]. The ripple current through the capacitor makes a ripple voltage across the ESR in proportion, which adds to the capacitive ripple (ESR ripple). In the cold example, 50 mΩ of ESR under 2 A of ripple current, peak to peak, gives 100 mV peak to peak; ten times the ESR, the low end of that range, 500 mΩ, gives 1 V.
How to confirm
Section titled “How to confirm”Measure the output ripple, or the capacitor’s ESR at the switching frequency, with the capacitor at the lowest temperature of the specification, in a cold chamber, and compare it with the value at room temperature.
Size the capacitor with its ESR at the lowest temperature, from its data sheet, so that the ripple meets the limit there. Capacitors in parallel share the ripple current and divide the ESR, so more of them lower the ripple as well.
References
Section titled “References”- [Cornell Dubilier Electronics, Aluminum Electrolytic Capacitor Application Guide]: an aluminum electrolytic capacitor’s ESR at its lowest rated temperature, often 10 to 100 times its room-temperature ESR.
- Low temperature: the ESR ripple, worked through, and a bench exercise.
The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.