AKKN ElectronicsIn an aluminum electrolytic capacitor, wear-out is governed by electrolyte evaporation through the vent and seals. The evaporation rate follows an Arrhenius-type relationship, so the internal hot-spot temperature — not the case temperature, and not the ambient temperature alone — is the variable that determines service life. Every derating decision in a high-ambient application has one practical goal: keep the hot-spot temperature low enough that the capacitor reaches the required lifetime.
How the Lifetime Model Works in Plain Terms
For mainstream 105 °C-rated electrolytics, a useful planning approximation is that service life doubles for every 10 °C reduction in hot-spot temperature:
L = Lr × 2^((Tr − Th)/10)
where Lr is the rated life at the rated hot-spot temperature Tr (typically 105 °C) and Th is the actual hot-spot temperature. This is an estimation rule, not a guarantee. Datasheet life figures are tied to specific conditions: rated ripple current, a defined ambient temperature, and an end-of-life criterion such as a 20 % capacitance drop or a doubling of tan δ.
The hot-spot temperature is obtained by adding ripple-induced self-heating to the ambient temperature:
Th = Ta + ΔTr × (Irms / Irated)²
Here Ta is the ambient temperature at the capacitor surface, Irms is the applied ripple current, Irated is the rated ripple current, and ΔTr is the internal temperature rise produced by rated ripple current — typically 5 °C to 10 °C for radial and snap-in aluminum electrolytics. Because ripple enters the equation as a square, halving the ripple current cuts self-heating to one quarter.
Two effects make the real operating point less stable than the formula suggests. First, ESR falls as temperature rises, so self-heating in a hot ambient is initially moderate. Second, ESR increases as electrolyte dries out, which raises self-heating and accelerates further evaporation. The model represents the capacitor in a healthy state; it does not capture end-of-life escalation.
Temperature and Ripple Derating: Numbers That Matter
Derating for high ambient temperature has three levers: voltage, ripple current, and worst-case ambient margin.
Voltage derating provides a modest but finite benefit. Operating a 105 °C electrolytic at 80 % of rated voltage instead of 100 % extends life by roughly 5–10 % at the same temperature, and it adds margin against transient overvoltage. Derating below about 50–70 % of rated voltage yields little additional lifetime gain.
Ripple current derating is where the meaningful gains are found. Consider a 105 °C-rated unit with a base life of 5,000 h at rated ripple and a rated core rise of 10 °C. At an 85 °C ambient with 70 % of rated ripple, self-heating adds about 4.9 °C, and the expected life is approximately 14,000 h. Dropping the ripple to 40 % of rated removes most of the self-heating and raises the estimate to roughly 17,900 h. Moving the same ripple load to a 105 °C ambient with 70 % ripple gives an estimated life of about 3,600 h — a factor of four difference between these two operating points.
Reference Table: Operating Conditions vs Expected Life
| Ambient temp | Ripple load (% of rated) | Estimated hot-spot temp | Estimated life (h) |
|---|---|---|---|
| 85 °C | 40 % | ≈ 87 °C | ≈ 17,900 |
| 85 °C | 70 % | ≈ 90 °C | ≈ 14,000 |
| 85 °C | 100 % | ≈ 95 °C | 10,000 |
| 105 °C | 40 % | ≈ 107 °C | ≈ 4,500 |
| 105 °C | 70 % | ≈ 110 °C | ≈ 3,600 |
| 105 °C | 100 % | ≈ 115 °C | 2,500 |
The table values are model estimates for a generic 105 °C-rated capacitor with a 5,000 h rated life and a 10 °C core rise at rated ripple. When evaluating a specific part, replace the table inputs with the values from that datasheet.
Design Rules for High-Ambient Capacitor Banks
- Define the worst-case ambient, not the average. In sealed enclosures, the temperature at the capacitor surface can exceed the measured air temperature by 5–10 °C.
- Budget ripple at the line-frequency component (100/120 Hz) and the switching-frequency component separately. Output-stage electrolytics often see ripple at twice the mains frequency plus a high-frequency component from the converter; both contribute to total heating.
- Sum the external ambient rise and the internal rise. If the hot-spot estimate exceeds the rated temperature, the practical options, in order, are a larger can size, parallel capacitors, or a longer-life series.
- Parallel units split the ripple. Two identical capacitors share the current, so each dissipates one quarter of the power of a single unit, not one half.
- Verify case temperature on the prototype. An infrared reading of the can is not the hot-spot temperature; add the internal gradient at the operating ripple, which corresponds to the ΔTr term in the model.
- Where the ambient temperature cannot be lowered, specify a longer-life grade rather than accepting a core temperature above the rated value.
For film and MLCC capacitors, the same thermal discipline applies even though the failure mechanisms differ. Film capacitors gain reliability when voltage is derated to 70–80 % of rated in hot environments. MLCC effective capacitance drops with DC bias more severely at elevated temperature, so re-verify the effective capacitance at the maximum operating temperature before finalizing a capacitor bank layout.
