AKKN ElectronicsA 48 V DC-link rail in a telecom rectifier showed 1.1 V of ripple during a load-step test, more than three times the specification limit. The board was returned to the repair station. The DC-link bank — three 1000 µF, 63 V capacitors in parallel — showed no visible damage: no bulge at the case top, no stained vent, no cracked sleeve. The rail voltage dropped by 400 mV under full load, and LCR readings pointed to one capacitor measuring 6 µF and 8 mΩ at 100 kHz. That ESR is below the range expected for a 1000 µF aluminium electrolytic, which suggests the installed part is a solid polymer type that failed short. The diagnostic path for the two families differs, and mixing them up leads to repeat failures.
First-Level Triage: What the Case Does and Does Not Tell You
Liquid electrolytic capacitors fail by drying out first. Electrolyte evaporates or decomposes over time at temperature; capacitance drifts downward, ESR climbs, and internal pressure eventually lifts the pressure vent. The visible signs are a bulged top or a stain below the vent. A solid polymer capacitor has no liquid to leak. Its case carries a scored pressure-relief groove, but a failing polymer cap rarely shows outward deformation; it fails abruptly short. The rail symptom is similar in both cases — excessive ripple — but the root cause is different: gradual degradation versus a sudden change.
- Measure rail ripple with an oscilloscope at the switching frequency and at the mains-related frequency. A shorted polymer cap sags the rail under load but leaves the ripple frequency content intact.
- Measure ESR at 100 kHz with a handheld LCR meter. An ESR below a few tens of milliohms on a 1000 µF part is normal for polymer, not a defect in itself.
- Identify the case type. A rubber vent plug on the case top indicates liquid electrolytic; a metal case with a scored groove indicates polymer.
Layered Root-Cause Checks
Once the capacitor type is confirmed, run the checks in this order.
Voltage stress. Liquid electrolytics are normally derated so continuous voltage stays below 80% of rated. Above that, oxide-layer wear and internal gas generation accelerate. Polymer types are rated for full voltage, but transients that approach or exceed the rating break down the dielectric layer, and the part degrades without visible warning. Capture the rail with a storage scope over one complete load cycle to record peak transient values.
Ripple current. Compute the RMS ripple current the capacitor sees from the load profile and compare it against the part's rated ripple current at the expected case temperature. A common rule is to keep RMS ripple below 70% of the rating at the operating temperature. The power loss is Irms² × ESR, so a liquid electrolytic with rising ESR runs hotter, which accelerates further ESR rise. A polymer cap has lower ESR, but it also loses margin as the dielectric degrades.
Thermal environment. Measure case temperature with a thermocouple after one hour at rated load. For liquid electrolytic, every 10 °C above the rated temperature reduces life by roughly half. For polymer, the failure mode is not dependent on electrolyte evaporation, but case temperature above the rated maximum still degrades the dielectric and accelerates the eventual short.
Measurement Method and Acceptance Values
Use an LCR meter at two settings. Measure capacitance at 120 Hz for liquid electrolytic; the datasheet specifies the initial value at that frequency. Measure ESR at 100 kHz for both types, or at 10 kHz for high-voltage liquid types if the datasheet specifies that frequency. For polymer, both capacitance and ESR are typically specified at 100 kHz.
| Parameter | Liquid electrolytic | Solid polymer |
|---|---|---|
| Capacitance check frequency | 120 Hz | 100 kHz |
| Capacitance acceptance | ≥80% of rated | ≥80% of rated |
| ESR check frequency | 100 kHz (or 10 kHz) | 100 kHz |
| ESR acceptance | ≤2× specified max | ≤1.5× specified max |
| Typical ESR, 1000 µF 63 V | 50–150 mΩ | 8–20 mΩ |
The values above are generic starting points; apply the thresholds from the datasheet of the installed part. On a liquid electrolytic, an ESR rise to twice the specification maximum means the part will lose capacitance at operating temperature within a few months. On a polymer cap, an ESR rise above 1.5× the maximum is a sign of dielectric degradation; a sudden drop in ESR toward zero indicates a hard short.
Prevention Checklist for the Next Repair Cycle
- Confirm the installed part type before ordering a replacement. A solid polymer is not a drop-in replacement for a liquid electrolytic on a rail whose control loop or transient behaviour was tuned around the higher ESR.
- Derate voltage with margin: at least 20% headroom for liquid electrolytic, at least 10% for polymer, with transients verified by measurement.
- Keep RMS ripple below 70% of the rated figure at worst-case ambient; re-check after any component change in the converter.
- Ensure the rail has short-circuit protection. A polymer cap fails short, so a fuse or current-limit path must clear the fault before the PCB trace is damaged.
- Sample-test each incoming batch for ESR and capacitance at the frequency stated in the datasheet; reject parts beyond the acceptance thresholds in the table above.
