When an aluminum electrolytic capacitor fails in a sealed enclosure, the trigger is normally heat accumulation, not an electrical overload. Ambient temperature determines the core temperature, and the core temperature controls electrolyte evaporation and the part’s end-of-life point. For a sourcing engineer replacing capacitors in industrial power supplies, the practical task is to convert a datasheet lifetime at a reference temperature into an expected service life at the real operating temperature. This article presents a simple derating method with numerical limits, and a set of design rules for selecting replacement parts for continuous duty.
The Lifetime Model in Plain Terms
The lifetime of a standard aluminum electrolytic follows an Arrhenius-type relationship: for each 10 °C reduction in core temperature, expected life roughly doubles. A part specified for 10,000 hours at 105 °C core, operated at a core of 95 °C, behaves like a 20,000-hour part in practice.
Core temperature is not the same as ambient temperature. Ripple current flows through the capacitor’s ESR and produces I²R losses that raise the core above the surrounding air. For a medium snap-in capacitor with ESR around 30 mΩ carrying 2 A of ripple, the dissipation is 0.12 W; with a typical thermal resistance of 15–20 °C/W, that adds roughly 2–3 °C. Larger ripple, such as 3 A through an 80 mΩ ESR, dissipates about 0.7 W and can shift the core 10 °C or more above ambient. The derating task therefore has two steps: control the ambient contribution, then verify the ripple contribution.
Ripple and Voltage Derating with Numbers
Most datasheets list a maximum ripple current at the rated operating temperature, for example 105 °C. When the ambient inside the equipment is close to that rating, the allowable ripple shrinks quickly. A conservative rule is to keep ripple below 60 % of the rated value when ambient exceeds 85 °C, below 45 % when it exceeds 95 °C, and below 30 % at 105 °C. For a part rated at 2.5 A at 105 °C, that means roughly 1.5 A at 85 °C ambient, 1.1 A at 95 °C ambient, and 0.75 A at 105 °C ambient.
Voltage derating follows a separate line. Running at or near rated voltage accelerates wear on the dielectric oxide layer. At ambient temperatures up to 85 °C, keeping the applied voltage at 80 % or less of the rated value preserves life. At ambient above 85 °C, dropping to 70 % of rated voltage adds safety margin without a meaningful performance loss.
Combining both rules: a 450 V rated capacitor on a 400 V DC bus is already at 89 % of rating — marginal for continuous duty above 85 °C. Selecting a 500 V rated part brings the loading to 80 %, which is a safer starting point for sealed enclosures.
Operating Conditions vs. Expected Life
The table below shows estimated life for a typical 105 °C-rated electrolytic with a nominal life of 10,000 hours at rated ripple and rated core temperature. Figures are conservative estimates for continuous duty; actual values vary by series and manufacturer.
| Ambient (°C) | Suggested ripple limit (% of rated) | Voltage loading (% of rated) | Estimated life (hours) | Equivalent field years (24/7) |
|---|---|---|---|---|
| 65 | 100 | 80 | 80,000–160,000 | 9–18 |
| 75 | 80 | 80 | 40,000–80,000 | 4.5–9 |
| 85 | 60 | 75 | 20,000–40,000 | 2.3–4.5 |
| 95 | 45 | 70 | 10,000–20,000 | 1.1–2.3 |
| 105 | 30 | 70 | 5,000–10,000 | 0.6–1.1 |
Interpretation: for an industrial drive running 24/7 in an 85 °C cabinet, a mid-range part at the ripple limits above gives roughly 2–4 years of service. If the maintenance plan expects 5 years, the viable options are a higher-rated series with the same capacitance and voltage but a longer rated life, a larger case size to reduce thermal resistance, or active airflow to lower the ambient by 10 °C.
Design Rules When Sourcing
- Ask the distributor for a lifetime-versus-ripple curve, not a single datasheet point. A supplier stocking multiple series can show the same capacitance value at different ripple ratings and thermal resistances.
- Check ESR at the actual ripple frequency. ESR at 100 kHz can be roughly one-third of the 120 Hz value; if the converter sees switching ripple around 50–100 kHz, use the higher-frequency ESR in the I²R calculation.
- Keep core temperature below 95 °C when continuous duty exceeds 3 years. That typically means limiting ambient to 85 °C and ripple to 60 % of rated.
- Verify RoHS and REACH declarations on every lot. Cross-border procurement should include a certificate of conformity, especially when parts are destined for EU or North American assembly.
- For sealed enclosures with no airflow, add 5–10 °C to the measured ambient before applying the table. Take the reference temperature with a thermocouple at the mounting point, not in free air.
