Wet electrolytic capacitors in the DC link of switch-mode power supplies, variable-frequency drives, and uninterruptible power supplies operate under continuous ripple current, elevated ambient temperature, and occasional voltage transients. With time, those stresses produce one of three visible wear-out signatures: case bulging, electrolyte leakage, or open-circuit behavior. Reading the signature correctly narrows down the replacement specification before you cross-reference a part.
Failure Modes in Filter Capacitors
Bulging develops when internal pressure rises faster than the pressure vent can relieve it. Ripple current heats the capacitor element through I²R losses in the ESR; as the core temperature climbs above the rated value, the electrolyte generates gas. The aluminum case deforms, and the vent dome lifts. If pressure continues to rise, electrolyte weeps through the vent or the rubber seal, leaving a sticky residue at the base of the component. Both bulging and leakage indicate sustained thermal overstress, not a single overvoltage event.
Open-circuit failure follows a different mechanism. After thousands of thermal cycles, the internal tab-to-foil connection fatigues, or the electrolyte dries out until ESR rises sharply and the capacitor no longer carries current. The DC bus voltage shows excessive ripple, and the output collapses under load. An open capacitor is not shorted: measured capacitance drops to near zero, and ESR reads tens of ohms or higher, so it is easily distinguished from a shorted device with an ESR meter.
Replacement Specification Derived from the Application
For a bulged capacitor, the dominant stress was thermal, so the replacement must carry a ripple current rating at +105 °C that is at least 1.4× the measured operating ripple. ESR at 100 kHz should be equal to or lower than the original because ESR drives self-heating. For leakage after years of service, seal degradation and thermal cycling are the root causes; keep the rated lifetime at a comparable level (typically 2000–10000 hours at rated ripple and core temperature) and match the case size if the mounting clamp is fixed.
For an open circuit, prioritize a capacitor with a longer rated lifetime and lower ESR. Verify the vendor’s lifetime formula, usually expressed as L = L0 × 2^((T0 − Ta)/10), where L0 is the rated lifetime at the rating temperature, T0 is the rated core temperature, and Ta is the actual core temperature in the application. Derate the working voltage by 10–20% below rated voltage for continuous service; that derating reduces internal temperature and extends wear-out life.
Specification Comparison Table
| Parameter | Original requirement | Replacement check |
|---|---|---|
| Capacitance | Reference value from the circuit design | Measure at 1 kHz; ±20% tolerance is acceptable for electrolytic types |
| Rated voltage | Maximum DC bus voltage × 1.25 margin | Rated voltage ≥ maximum bus voltage under transient conditions |
| Ripple current | Operating ripple measured with a current probe | Rated ripple ≥ 1.4× measured value |
| ESR | Original datasheet value at 100 kHz | ESR ≤ original value; lower ESR reduces self-heating |
| Lifetime | Rated hours at rated ripple and core temperature | ≥ original rated hours; longer pref erable for high-ambient installs |
| Case size | Diameter × length from original | Match if clamp-mounted; larger acceptable if board space allows |
Incoming Inspection and Installation Notes
Before installing a replacement batch, measure capacitance at 1 kHz and ESR at 100 kHz on a sample of at least five units. Capacitance should fall within −10% to +30% of the marked value for electrolytic capacitors. ESR should be below 1.2× the vendor’s stated maximum. Visually inspect the pressure vent: it must be recessed or flush, never domed. A lifted vent on a new unit indicates storage damage or prior overstress, and the part should be rejected.
During installation, respect vent orientation. For screw-terminal capacitors, position the vent upward when horizontal mounting is unavoidable. Maintain at least 10 mm clearance between the capacitor body and adjacent power resistors or heatsinks. Torque screw terminals to the vendor’s specification, typically 2.0–3.0 N·m for M5 terminals, and use a flat washer to protect the plated surface.
After energizing, verify DC bus ripple at nominal load with an oscilloscope; ripple should remain within the design target. Measure case surface temperature with a thermocouple after one hour of continuous rated load: it should not exceed ambient by more than 10 °C. A larger delta indicates higher-than-expected ESR or insufficient airflow, and the part is operating outside its intended stress budget.