Quick answer: For inverter and DC-Link duty, choose between three technologies — electrolytic (highest capacitance density, low voltage, high losses), film-foil (highest current capability), and metallized film (high field stress with self-healing, the modern default). Then match the dielectric (polypropylene for low losses, PET for volumetric efficiency, PEN/PPS for high temperature), size it against your mission profile using temperature, voltage, and humidity acceleration factors, and prefer short, wide elements for high ripple current.
The wrong capacitor choice rarely fails immediately — it fails after thousands of hours, in the field, at the worst possible moment. This guide walks through the selection process the way experienced power electronics engineers do it.
Step 1: Choose the Capacitor Technology
TechnologyStrengthsWeaknessesBest forElectrolyticVery high capacitance per volumeLow rated voltage, high losses (ionic conductivity); the higher the capacitance density, the lower the rated voltageBulk energy storage at low voltageFilm-foil (plain aluminium foils + dielectric film)Sustains very high currentsLarger size for given energyHigh-current pulse and snubber dutyMetallized film (metal-coated dielectric film)Much higher electric-field stress thanks to self-healing; compact; reliableMetallization design is criticalDC-Link, AC filtering, inverter DC bus
Metallized film is the default choice for modern DC-Link applications because of self-healing: when a breakdown punctures the dielectric, the current density around the defect evaporates the ~100 nm metal layer and isolates the fault. The capacitor loses a trace of capacitance and keeps working — a fail-safe behaviour no electrolytic can match.
Note the design trade-off: a thicker metallization lowers ESR and Joule losses but releases more energy per self-healing event; a thinner layer heals gently but carries more resistance. The electrode sheet resistance (Ω/square) must match your field stress and ripple current.
Step 2: Choose the Dielectric Film
DielectricMax temperatureDielectric constantCharacterPolypropylene (PP)Standard range2.2Very low loss factor — the power electronics standardPET~+10 °C above PP3.350% more capacitance per volume, better mechanical strength (higher self-healing endurance), thinner films possible — but 10× higher loss factor, i.e., roughly 10× the temperature rise at the same rated powerPENup to 125 °C—High-temperature special applicationsPPSup to 150 °C—Highest-temperature film option
Rule of thumb: choose PP whenever ripple current and losses matter (DC-Link, snubbers, resonant circuits). Choose PET when volume is the binding constraint and losses are acceptable. Reserve PEN/PPS for genuinely hot environments.
Historical note: paper (pure or mixed with PP) was used in film-foil technology but has been displaced by polymer films.
Step 3: Check the Construction
Impregnation: capacitive elements are dried to remove moisture (residual water accelerates aging and raises losses), then impregnated with vegetable oil or gas (SF₆, N₂). Oil impregnation supports higher field stress; dry/gas types demand superior winding quality.
Winding quality: the space factor — dielectric thickness vs. total electrode spacing — determines the gas gap between films. Large gaps invite corona discharges that rapidly erode the metallization. Space factor is hardest to control in the curved sections of flat windings, which is why serious manufacturers use precision tension-controlled winding machines, round windings, or stacked elements (film wound on a large wheel, then cut into stacks).
Housing: plastic cases are not fully moisture-tight. For humid environments, demand proven sealing, impregnation, or hermetic solutions — otherwise electrode corrosion and corona demetallization will shorten life.
Step 4: Size for Ripple Current and Geometry
Current capability is specified through ESR (R_s) and the dissipation factor:
tan δ = R_s · ωC (high-frequency domain)
Excess ripple current → self-heating → reduced breakdown voltage → accelerated aging, or in severe cases melting. For high-power applications:
Choose short elements with many parallel layers — shorter current path, less heat.
Verify RMS current at your actual switching frequency, not a nominal datasheet point.
Step 5: Size for Lifetime Against the Mission Profile
This is where most designs go wrong. Lifetime follows a Weibull statistical model scaled by three acceleration factors:
Temperature (Arrhenius): with Eₐ/k_B = 7000 K, going from 40 °C to 70 °C ages the capacitor 7.1× faster.
Voltage (inverse power law, exponent n ≈ 3.5): running at U_n instead of U_n/2 ages it 5–10× faster.
Humidity (power law): moisture ingress shortens life in plastic-cased parts.
Combined:
t(T,U,RH) = t_ref · exp[(Eₐ/k_B)(1/T − 1/T_n)] · (U/U_n)^(−n) · (RH_n/RH)^m
The correct procedure has two steps:
Calculate the required dielectric thickness at the reference temperature for your voltage distribution.
Adapt the thickness to your temperature profile.
Convert every phase of the mission profile to reference-condition aging weight and sum the contributions. Example — a solar inverter spending 50% of time at 20 °C/no voltage, 30% at 0.8 × U_n/60 °C, and 20% at full voltage/90 °C: that final 20% of time consumes ~70% of the aging budget. Size for the peaks.
Worked example: a capacitor specified at 50 FIT (40 °C, U_n/2, exponential model) offers ~2.1×10⁶ h lifetime expectancy. At 70 °C and full rated voltage, expect ≈ 60,000 h (~7 years). The same part — chosen with derating — can outlast the product; chosen at the margin, it becomes the weakest link.
Step 6: Demand Qualification Evidence
For high-reliability applications, lifetime claims must be test-backed:
Accelerated testing per IEC 61071 (capacitors for power electronics).
Statistical qualification: testing n units with zero failures demonstrates confidence CL that proportion p of the population survives, via n = log(1−CL)/log(p). Example: 16 units × 2,000 h at 70 °C/1.4 × U_n, zero failures → 80% confidence that ≥90% of units retain >90% capacitance at 60,000 h at U_n/40 °C.
Field-proven designs: metallized film technology is validated in the harshest duty — TGV high-speed trains have run 1800 V d.c. / 8 mF oil-impregnated film capacitor banks at 200–240 V/µm field stress with 20-year lifetime requirements.
Selection Checklist
Technology: metallized film for DC-Link; film-foil for extreme pulse current
Dielectric: PP default; PET for compactness; PEN/PPS above 105–125 °C
Voltage derating: target 0.7–0.8 × U_n at worst-case bus voltage
Ripple current verified at actual frequency, short/wide element geometry
Mission-profile lifetime calculation, not average-condition sizing
Humidity strategy: impregnation, sealing, conformal environment control
Certifications: VDE, UL, CQC as the application market requires
Supplier qualification data: Weibull parameters, accelerated test results
Frequently Asked Questions
What is the difference between metallized film and film-foil capacitors? Film-foil capacitors use plain aluminium foils and withstand very high currents. Metallized film capacitors use a thin metal coating on the dielectric, allowing much higher electric-field stress because defects self-heal by locally evaporating the metallization.
Why is polypropylene the standard dielectric for power electronics? PP combines a very low loss factor with adequate dielectric constant (2.2). PET packs 50% more capacitance per volume (ε = 3.3) and tolerates thinner films and +10 °C, but its tenfold higher loss factor causes far more self-heating at the same power.
How much voltage derating should I apply to a DC-Link capacitor? Aging accelerates roughly with (U/U_n)^3.5, so operating at 0.7–0.8 × U_n multiplies lifetime severalfold compared with running at the rating. Combined with thermal margin, derating is the cheapest lifetime insurance available.
Which capacitor is best for a solar inverter DC link? A metallized polypropylene DC-Link capacitor, sized against the mission profile: night/day voltage and temperature swings must be converted to reference-condition aging and summed. Short, wide elements with high ripple-current capability are preferred.
What certifications should a power capacitor supplier hold? ISO 9001 quality management plus product certifications relevant to your market — typically VDE, UL, and CQC — together with documented accelerated-life test data.
Source the Right Capacitor from JGS Technology
JGS Technology & Manufacturing is one of Türkiye’s leading suppliers of film capacitors (DC-Link, snubber, AC filter), ceramic capacitors, electrolytic capacitors, and electronic manufacturing raw materials. With 25+ years of industry experience, a factory-direct supply model, ISO 9001-compliant production, and VDE, UL, and CQC certifications, we support OEM manufacturing, supply-chain management, and R&D — from component selection to qualified series supply.
Not sure which capacitor fits your mission profile? Send our engineers your voltage, ripple, and temperature profile — we will recommend and size the right component with you.
