When an enclosure is designed for a 70°C internal ambient, every capacitor inside operates with a thermal handicap. Electrolytic types fail by electrochemical evaporation of the electrolyte, while film types degrade through dielectric oxidation and mechanical stress. Both failures accelerate with temperature, yet the two families respond to derating very differently. This article sets out practical derating rules based on the core temperature budget — the hottest point inside the winding or foil stack — for applications running above 60°C ambient.
The Lifetime Model in Plain Terms
Electrolytic lifetime is approximated by an Arrhenius-based model, commonly reduced to the ten-degree rule: every 10°C reduction in core temperature doubles lifetime. A capacitor rated for 10,000 h at a 105°C core will reach roughly 20,000 h at 95°C and about 80,000 h at 75°C. At 65°C core — a plausible figure in a derated, fan-ventilated cabinet — the multiplier approaches 16×, which makes wear-out irrelevant compared with calendar aging and terminal oxidation.
Film capacitors behave differently. Their lifetime is usually expressed in the form of a voltage-versus-temperature derating curve rather than an hours rating. Above 85°C ambient, the recommended working voltage falls by about 1.5% per °C for common polyester and polypropylene dielectrics. Ripple-induced heating matters far less than for electrolytics, because the ESR of a film unit is typically one order of magnitude lower at the same capacitance and voltage class.
Ripple Current and Core Heating
Ripple current adds heat on top of the ambient. Core temperature can be approximated as:
Tc = Ta + Irms² × ESR × Rth,ca
where Rth,ca is the thermal resistance from core to ambient, usually given as a single value in the datasheet. For a typical 450 V electrolytic, ESR is roughly 30–80 mΩ at 100 kHz, but at 120 Hz it can be two to four times higher. The consequence is that ripple derating is frequency-dependent: a 10 A ripple at 120 Hz generates two to three times the internal heat of the same ripple at 100 kHz.
When ambient temperature is fixed, two levers keep the core within budget: cut ripple by spreading it across paralleled capacitors, or reduce ESR by choosing a larger case size. Every 0.5 W of dissipated heat removed from the core raises the expected life by about 10–20% under hot operating conditions.
Operating Conditions vs Expected Life
The table below summarizes a 10,000 h/105°C-rated electrolytic under different ambient and ripple loads, assuming a rated ripple rise of 20°C at full load and a 10°C rule multiplier.
| Ambient temp (°C) | Ripple load (% of rated) | Estimated core temp (°C) | Life multiplier | Estimated life (h) |
|---|---|---|---|---|
| 105 | 100 | 105 | 1× | 10,000 |
| 85 | 100 | 95 | 2× | 20,000 |
| 70 | 100 | 85 | 4× | 40,000 |
| 70 | 50 | 75 | 8× | 80,000 |
| 60 | 50 | 65 | 16× | 160,000 |
Note that halving the ripple load cuts self-heating to one quarter, because dissipation scales with the square of current. That single change effectively doubles the life when ambient stays the same.
Design Rules for High-Ambient Applications
- Set the core temperature budget before selecting parts. For long-service electrolytic use, target a 65–75°C core. For film capacitors, keep the body below 85°C if voltage derating is less than 10%.
- Derate ripple against the available thermal budget. A capacitor with a 105°C hot-spot limit at 90°C ambient has only 15°C of headroom. If rated ripple at 85°C ambient produces a 20°C rise, the new limit is Inew = Irated × √(15/20) ≈ 0.87 × Irated — a 13% reduction. This is usually enough to change the number of parallel capacitors required.
- Parallel capacitors split the ripple current evenly if the PCB layout uses symmetric traces. Two identical units in parallel halve per-unit current, reducing per-unit self-heating by roughly 75%.
- For film capacitors above 85°C ambient, derate voltage by 1.5% per °C and ignore ripple heating unless the switching frequency exceeds 100 kHz, where harmonic content can still raise core temperature.
- Use a thermocouple or infrared measurement during prototype validation. Keep a margin of at least 5°C between the measured core temperature and the hot-spot limit.
Derating is not a single static number; it is the outcome of a thermal budget. An electrolytic running at 70°C ambient with half-rated ripple will likely outlive the equipment it serves, while the same unit at its maximum rating may need replacement within two years. Building the budget early avoids most field failures and keeps replacement scheduling predictable.