AKKN ElectronicsA mid-sized industrial drive returned from the field with a visibly bulged snap-in capacitor after roughly eighteen months of service. The drive sat inside a sealed enclosure where the internal air temperature, measured during a service visit, reached 92°C on a warm summer day. The failure was reproduced in a second unit three weeks later. Initial assumptions pointed to a sourcing defect, but the measured ambient temperature and the applied ripple current told a different story: the capacitor had never been derated for its actual working environment. This article lays out the layered checks for such failures, the measurement method used to confirm the root cause, and the derating rules that apply when bulk electrolytics run near their temperature ceiling.
Starting Symptom: Bulged Can and Shorted Element
The dominant field symptom for this failure mode is a swollen can top, sometimes accompanied by a weeping vent or a complete short-circuit in the element. The bulging results from internal gas generation triggered by electrolyte vaporization at sustained high core temperature. The capacitor in question was rated for 105°C with a 450 VDC maximum. Under the enclosure conditions it operated at an ambient of roughly 92°C with continuous ripple current close to the rated value. The combination left almost no thermal headroom, pushing the core temperature above 105°C for a significant portion of each duty cycle.
Before blaming the component, confirm the symptom pattern. Bulging that appears only in hot months, or only in units mounted near heat-producing components such as braking resistors or heatsinks, is a strong indication of a thermal derating problem rather than a material defect.
Layered Root-Cause Checks
Work through the following checks in order. Each one narrows the root cause and provides an auditable record for the sourcing decision.
- Measure the real ambient. Place a thermocouple 10 mm from the capacitor body inside the enclosure, not outside it. Log the temperature over a full 24-hour period under worst-case load. Accept a margin of at least 10°C between the maximum recorded ambient and the capacitor’s rated temperature.
- Measure the ripple current. Use a clamp-on current probe on the capacitor’s positive lead. Record the RMS value at full load. This ripple current, multiplied through the capacitor’s ESR, sets the internal self-heating and is often the missing variable in the calculation.
- Verify the ESR at 100 kHz. Remove the capacitor from the circuit and measure with an LCR meter at room temperature. Compare against the datasheet maximum ESR. A reading above the datasheet limit indicates ageing; a reading within limit indicates the capacitor was healthy when installed and the failure was thermal.
- Check thermal coupling. Inspect the layout for sources of radiant or conducted heat. A nearby heatsink or power resistor can raise the capacitor’s effective ambient by 10–15°C beyond the measured enclosure air temperature.
Measurement Method and Acceptance Values
For a quantitative pass/fail decision, standardize the measurement procedure. Attach a 0.5 mm thermocouple bead to the can top near the vent with thermally conductive adhesive. Run the unit at full load for 30 minutes, allowing the temperature to stabilize. Record the can-top temperature and the enclosure ambient simultaneously.
The acceptance criteria for a healthy installation are straightforward:
- Can-top temperature rise over ambient under full load: ≤ 10°C.
- If the rise exceeds 15°C, the ripple current contribution is too high and the capacitor must be derated or the ripple reduced.
- ESR measured at 100 kHz, room temperature: within +20% of the datasheet maximum value.
- Maximum steady-state can-top temperature for a capacitor rated at 105°C: ≤ 105°C including both ambient and self-heating.
The table below gives a practical derating matrix for bulk electrolytics rated at 105°C. These factors are general guidance, not a substitute for manufacturer-specific lifetime curves.
| Enclosure ambient | Voltage derating (fraction of rated VDC) | Ripple current derating (fraction of rated at 105°C) | Expected lifetime multiplier |
|---|---|---|---|
| ≤ 85°C | 0.8 | 0.7 | ≥ 2× rated life |
| 85–95°C | 0.7 | 0.6 | 0.5–1× rated life |
| 95–105°C | 0.6 | 0.5 | 0.25–0.5× rated life |
A frequently cited rule for aluminum electrolytics is that lifetime halves for every 10°C increase in core temperature. Applying the derating factors above keeps the core temperature within the rated window and provides the expected lifetime multiplier shown.
Prevention Checklist for Sourcing and Design
Use the following checklist when specifying or cross-referencing capacitors for high-ambient applications.
- Determine the maximum internal enclosure temperature from measurement, not from datasheet assumptions. Add 10°C margin for airflow obstruction.
- Specify a capacitor with a rated temperature at least 10°C above the measured ambient. If the ambient is 95°C, a 105°C rated part is the minimum acceptable choice.
- Calculate ripple-current derating from the table, and confirm the design ripple is below the derated value. For continuous duty, target 80% or less of the derated ripple.
- Apply voltage derating as a lifetime extension measure. Operating a 450 VDC part at 360 VDC or below in a 85°C ambient yields a lifetime multiplier well above the rated value.
- Request datasheet-level temperature-rise curves from the supplier during cross-referencing. A supplier that cannot provide ripple-current versus ambient-temperature curves should not be selected for high-temperature duty.
- During incoming inspection, verify ESR and capacitance against the datasheet maximum values, and reject any part with ESR above the limit before it enters production.
The bulged capacitor in the motor drive was ultimately traced to a derating gap: the sourcing engineer matched the rated voltage and capacitance but not the thermal budget. Measuring the ambient and applying the derating matrix above would have flagged the problem at the design stage, avoiding an entire season of field returns.
