AKKN ElectronicsAn aluminum electrolytic capacitor that fails in service rarely announces the true cause at first inspection. A bulged can, a damp residue at the seal, or a high ESR reading on a capacitance meter can each point toward a different chain of electrical and thermal events. For a distributor or a design engineer evaluating a field return, the failure mode is the starting point, not the conclusion. The visible defect only becomes useful when it is traced back to the stress that produced it.
Target Equipment and the Electrical Stress That Produces the Failure
Consider a 240 W AC-DC power supply used in continuous-duty industrial control. The capacitor immediately after the bridge rectifier sits in the worst position on the board: it filters a waveform carrying a strong 100 or 120 Hz ripple component and several harmonics of the switching frequency, while operating near a transformer and power semiconductors that raise the local ambient temperature. When the load is a DC motor or a group of solenoid valves, the current draw is far from steady, and the bus voltage sags and recovers repeatedly over every operating cycle.
Three electrical stresses dominate the failure mechanism. First, ripple current: each ampere of RMS ripple flowing through the capacitor's ESR generates heat in the interior. If the ripple exceeds the published rating, the electrolyte temperature rises well above the can temperature and accelerates evaporation. Second, DC voltage plus line transients: a capacitor rated close to the steady-state bus voltage leaves no margin for a surge or a distorted mains waveform. Third, operating temperature: for each 10 °C increase of core temperature above the rated value, the expected life is roughly halved. These three stresses combine to produce the three visible failure modes. Bulging follows internal gas generation or sustained overpressure as the electrolyte decomposes; leakage appears as electrolyte seeping past the seal after repeated thermal cycling; open circuit is often the end stage of electrolyte dry-out, when ESR rises so far that the part no longer passes ripple current at the operating frequency.
Component Requirements Derived from the Load Profile
From this stress picture, the replacement sourcing requirements follow directly. The DC voltage rating must hold at least 10% margin over the maximum steady-state bus voltage; for a 400 V bus, a 450 V rated part is a reasonable starting point. The ripple current rating must be checked at the actual operating frequency, because the allowed ripple is frequency-dependent. At 100 kHz, a general-purpose part permits considerably less ripple than at 120 Hz, while a low-ESR part is the opposite. The core temperature, calculated as ambient plus self-heating from ripple, decides the lifetime. For a long-life application, a 105 °C-rated part with a 10,000 h declared lifetime at rated ripple provides a dependable baseline and then derates according to the Arrhenius model.
A common error in cross-referencing is to match capacitance and voltage while ignoring the ESR-versus-frequency envelope. Two parts in the same case size and the same nominal rating can differ by 40% in ESR, which changes the internal temperature and, in turn, the expected field life. The physical fit matters before the electrical fit: terminal pitch and case diameter must be verified, because a close electrical equivalent that does not mount correctly is not a useful substitute.
Sourcing Parameter Table
The table summarizes the parameters that a cross-reference check should hold, with representative values for a 400 V bus capacitor in the 150–470 µF range.
| Parameter | Representative value | Selection note |
|---|---|---|
| Capacitance at 120 Hz | 150–470 µF | Sized for hold-up time at the specified minimum output voltage during a brief mains dropout |
| DC voltage rating | 450 V | Minimum 10% margin over the 400 V steady-state bus, allowing for line tolerance and transients |
| Ripple current at 100 kHz, 105 °C | 1.2–2.5 A RMS | Must exceed the calculated RMS ripple at the actual operating frequency |
| ESR at 20 °C, 100 kHz | 40–80 mΩ | Lower ESR limits self-heating; compare at the same frequency as the original part |
| Lifetime at 105 °C, rated ripple | 5,000–10,000 h | Halve the lifetime for each 10 °C increase in core temperature |
| Operating temperature range | -40 to +105 °C | Ambient plus self-heating must remain below the rated core temperature |
| Terminal configuration | 10 mm pitch, snap-in | Verify case diameter and pitch before electrical evaluation |
Installation and Test Notes
At goods receiving or during failure analysis, three measurements plus a visual check separate an aging part from a genuinely defective one.
A vent that has visibly opened, or a can with a permanent bulge under the vent, records overpressure. A damp or crystallized residue at the terminal seal indicates electrolyte leakage, which can follow from overpressure, overtemperature, or a seal damaged by excessive tightening torque on screw terminals. Electrolyte on the board is a secondary failure source because it attacks solder joints and copper traces over time.
An ESR measurement at 100 kHz defines the state of the electrolyte. A field-returned part reading three or more times its datasheet maximum ESR is effectively open circuit at ripple frequencies, even if the capacitance at 120 Hz still reads within tolerance. Capacitance should fall within ±20% of nominal; a low capacitance reading combined with a bulged can points to dry-out accompanied by gas generation. Leakage current is measured after charging at rated voltage for two minutes and should remain below the maximum stated in the datasheet. An elevated leakage current in an otherwise dry part often indicates a damaged oxide layer from overvoltage or reverse polarity.
When a capacitor fails in a multi-capacitor bank, replace the full bank or match the new parts to the remaining ones. The surviving capacitors have experienced the same ripple and thermal history, and their ESR has drifted upward. Mixing a fresh low-ESR part with aged ones shifts the ripple current split and can overload the new part within a few thousand hours of operation.
