AKKN ElectronicsA re-marked capacitor is not a benign substitution. The marking may match a 450 V, 100 µF, 105 °C part, but the internal foils, electrolyte, and pressure vent usually come from a lower-rated component. In a power stage that continuously stresses the capacitor, those differences surface as high ESR, elevated core temperature, and early failure. Incoming inspection for bulk aluminum electrolytic capacitors should therefore be derived from the electrical stress the part will see in the end application, not from a datasheet comparison alone.
Target Equipment and Electrical Stress
Consider a 250 W AC-DC power supply with active power factor correction. The bulk capacitor sits on the 390 V DC bus and sees three simultaneous stresses:
- Voltage stress. Nominal bus voltage is 385–390 V, with transient excursions above 420 V during load steps. The part must hold a 1.15× voltage margin, which sets the minimum rating at 450 V DC.
- Ripple current. Twice-line-frequency current (100/120 Hz) from the PFC stage combines with switching ripple from the downstream DC-DC converter. For a 250 W output at about 92% efficiency, the capacitor carries roughly 1.0–1.4 A RMS; the selected part must be rated above this value.
- Thermal stress. The enclosure ambient is typically 60 °C. The capacitor hot spot is the sum of ambient temperature, radiated heat from adjacent magnetics, and self-heating from ripple current (ESR × I²). At full load, case temperature rise should stay within 10–15 °C above ambient.
From these stresses, the required capacitor parameters become:
- Capacitance: 100 µF ±20% at 120 Hz, 20 °C
- Rated voltage: 450 V DC
- ESR: ≤ 0.7 Ω at 120 Hz, 20 °C (reference value)
- Ripple current rating: ≥ 1.3 A at 120 Hz, 105 °C
- Endurance: ≥ 3000 h at 105 °C with rated ripple applied
- Case: 18 × 35.5 mm with a top-mounted pressure relief vent
How Counterfeit Parts Fail Under These Conditions
Re-marked capacitors usually fall into one of the following categories:
- Voltage overreach. A 400 V or 350 V part is re-stamped as 450 V. It passes leakage testing at low voltage but shows a sharp rise in leakage current above 400 V.
- Capacitance code mismatch. A 47 µF or 68 µF part is re-marked as 100 µF. The part passes screening if the code alone is checked, but bus voltage ripple exceeds the design limit.
- ESR and electrolyte mismatch. A standard-grade part is re-marked for a high-ripple stage. ESR at 120 Hz may be 1.5–2× the genuine value, which doubles self-heating.
- Vent substitution. The correct case may carry a solid or undersized vent. This defeats the pressure-relief safety function; genuine parts are built to vent before the case ruptures, while low-cost shells often omit that feature.
Each failure mode changes a measurable electrical parameter, which is the basis for the incoming inspection plan below.
Incoming Inspection Plan
Visual and dimensional checks catch most re-marked parts:
- Compare sleeve print, logo, date-code format, and vent geometry against a known-good sample from the manufacturer or an authorized distributor.
- Check the orientation of the polarity marking relative to the case notch; re-marked parts often show rotated or double-struck printing.
- Measure case diameter and length with a calibrated caliper. For an 18 × 35.5 mm case, allow no more than ±1 mm deviation.
- Weigh each sample. Capacitor mass tracks foil area. Use a reference of ±10% of a verified sample; a 100 µF / 450 V part with the correct foil structure cannot be significantly lighter.
Electrical verification at 25 °C:
| Parameter | Method / condition | Acceptance criterion |
|---|---|---|
| Capacitance | 120 Hz, 1 V RMS bias, 25 °C | 80–120 µF for a 100 µF marking (±20%) |
| ESR | 120 Hz, 25 °C | ≤ 1.2× datasheet maximum (≤ 0.84 Ω for a 0.7 Ω reference) |
| Leakage current | Rated voltage applied for 2 min, 25 °C | ≤ 0.01 × C(µF) × V(V) µA (≤ 450 µA for 100 µF / 450 V) |
| Surge overreach | 1.1× rated voltage (495 V) for 30 s, 25 °C | Leakage stays ≤ 0.02 CV µA; capacitance returns to within 5% of initial value after 1 h |
A practical sample plan is five pieces per tube or reel, taken from at least three different positions in the lot. If any piece fails the electrical criteria, quarantine the lot and escalate to the sourcing team.
Installation and Test Notes
- Soldering recovery. Measure capacitance and ESR after the soldering process. Re-marked parts with lower-grade electrolyte show capacitance loss and ESR rise after soldering, while genuine parts recover within a few hours.
- Self-heating verification. In the initial assembled units, run the supply at full load and measure the capacitor case temperature. The measured temperature rise should match the vendor's thermal model within ±5 °C. A rise that is systematically higher across multiple units points to an ESR mismatch.
- Soak test. Run 100 h at 90% load at worst-case ambient (60 °C), and record bus voltage ripple every 24 h. Ripple that grows more than 20% over the soak period indicates capacitance loss or electrolyte drying that a genuine part at this voltage margin would not show.
- Record keeping. Store the measured ESR, capacitance, and mass per lot. This data becomes the baseline for future incoming lots and supports clear communication with the sourcing team when a lot is rejected.
Counterfeit detection at incoming inspection is achievable when the inspection criteria are tied to the stress the capacitor must sustain. Visual checks catch the careless re-mark; electrical checks at rated voltage and ripple-relevant frequency catch the deliberate one.
