A replacement bulk capacitor on a 2.2 kW variable-frequency drive DC link passed the capacitance check at the bench, yet the drive returned with a vented can within six weeks. The incoming inspector had measured capacitance, which is rarely the discriminator between a genuine part and a re-marked or counterfeit product. The failure modes that matter in service are ESR rise, leakage current increase, and a can whose printed rating exceeds the capability of the internal element.
This article covers a practical incoming inspection routine for aluminum electrolytic capacitors destined for switch-mode power supplies (SMPS) and drive DC links. The procedures are inexpensive, repeatable, and do not require an X-ray station or a laboratory SEM.
Electrical stress in DC-link and bulk-buffer service
Bulk electrolytics in SMPS and VFD inverters experience a combination of high RMS ripple current, elevated ambient temperature inside sealed enclosures, and unsuppressed voltage transients from the switching stage. A typical 560 V DC link in a 400 V-class drive uses two 400 V or 450 V capacitors in series, each carrying a ripple component of 2 to 4 A RMS at 8 to 20 kHz switching frequency. In a sealed enclosure with ambient temperatures of 55 to 70 °C, the capacitor surface temperature can reach 85 to 95 °C.
The consequence is that the capacitor’s real limiting parameters—ESR at the switching frequency, rated ripple current at 105 °C, and endurance hours at rated ripple—are the ones that keep the part alive. A re-marked capacitor with a legitimate-looking capacitance value but a higher ESR dissipates more heat for the same ripple, raises internal hot-spot temperature, and accelerates electrolyte dry-out. This is the mechanism behind early venting.
Component requirements derived from service
For replacement sourcing, the part must meet or exceed the original specifications. The critical parameters are:
- Rated voltage: ensure the working DC-link voltage does not exceed 80 % of the rated voltage (a 450 V capacitor for a 350–380 V DC link, or two 400 V parts in series for 560 V links).
- Ripple current at 105 °C, 100 kHz: must be at least equal to the application’s RMS ripple plus a 20 % margin.
- ESR at 100 kHz, 20 °C: below the value stated in the datasheet for the nominal capacitance; re-marked parts frequently show ESR 2–3× above spec.
- Endurance rating: 2000 to 3000 h at 105 °C rated ripple is common for general-purpose series; long-life series specify 5000–10000 h.
- Can dimensions: diameter, height, and terminal pitch must match the PCB footprint of the original.
Where the application operates in a warm sealed enclosure, also require a temperature rating of at least 105 °C. A 85 °C-rated part may pass a capacitance test at 20 °C and still fail thermally in service.
Typical acceptance thresholds
| Parameter | Test condition | Acceptance threshold |
|---|---|---|
| Capacitance | 120 Hz, 20 °C | Within ±20 % of printed value (tighter for low-ESR series: ±10 %) |
| ESR | 100 kHz, 20 °C | Below manufacturer’s datasheet limit; reject if more than 1.5× datasheet max |
| Leakage current | Rated voltage applied for 5 min at 20 °C | Below 0.01 CV (µA) for most general-purpose types, or 0.03 CV for large-can types |
| Weight | As received, at 20 °C | Compare against a known-good sample; a deviation greater than 10 % is suspicious |
| Can height / diameter | Vernier caliper | Within ±1 mm of datasheet profile |
| Date code | Visual | Within 24 months of manufacture for general-purpose, 12 months for long-life series |
Incoming inspection procedures
Perform the following checks in the listed order. The routine adds roughly three minutes per component and catches most re-marked parts before they enter assembly.
1. Visual and dimensional check. Examine the printing: genuine parts from established manufacturers have consistent ink density, a clear lot code, and a sharp trademark. Re-marked parts often show a second imprint under oblique light, or a scratched surface where the original marking was removed. Compare the can diameter, height, and terminal pitch against the datasheet values. A 450 V 470 µF, 30×40 mm can weighs approximately 55 to 65 g; a deviation of more than 10 % suggests a smaller internal element or filler material.
2. Capacitance and ESR measurement. Use an LCR meter with a bias option. Measure at 120 Hz for capacitance and at 100 kHz for ESR, both at 20 °C. A genuine 470 µF capacitor reads between 376 and 564 µF at 120 Hz. The ESR at 100 kHz should be below the datasheet maximum; a low-ESR series is typically 30 to 60 mΩ at this rating. If the ESR is more than 1.5× the datasheet value, reject the lot even if the capacitance matches.
3. Leakage current test. Charge the capacitor through a 1 kΩ series resistor to the rated voltage. After five minutes, measure the current through the resistor. Exceeding 0.01 CV in microamperes (with C in farads and V in volts) indicates an aged or re-formed element. For a 450 V 470 µF part this limit is approximately 2.1 mA—a genuine new part typically settles below 0.5 mA.
4. Ripple-temperature spot check. For a batch of more than five pieces, apply a ripple current equal to 70–80 % of the rated value at 100 kHz and 105 °C ambient in a small thermal chamber. Measure the case temperature after one hour. A genuine part should show a temperature rise of 5 to 15 °C above ambient; a counterfeit with elevated ESR will rise 20 °C or more. This test is time-consuming, so apply it to a sample rather than the full lot.
Installation and shelf-life notes
Capacitors that pass the above checks still require correct handling. Maintain a 24-month inventory rotation for general-purpose aluminum electrolytics. When soldering, limit the soak time at peak temperature to the manufacturer’s profile; excessive preheat can weaken the internal pressure valve seal. For VFD DC-link applications, verify that the working voltage stays at or below 80 % of the capacitor’s rated voltage, and confirm the vent orientation faces away from adjacent wiring before mounting.
A final caution: a part that passes a single capacitance reading is still risky when installed in a high-ripple circuit. Combine capacitance, ESR, leakage, and weight checks in the inspection procedure. These four measurements are sufficient to separate a genuine part from a re-marked unit in the majority of cases encountered in procurement. Where the result remains ambiguous, retain a sample and request the distributor’s traceability documentation before committing a batch to production.
