Comparing Film Capacitor Specifications: Dielectric, ESR, and Lifetime Trade-offs
Film capacitor selection goes far beyond capacitance and voltage rating. This article compares polypropylene, polyester, and PPS dielectrics across critical specifications—ESR, dissipation factor, temperature stability, self-healing, and lifetime—so you can choose the right capacitor for high-frequency, high-ripple, and long-life designs.
Why Film Capacitor Selection Hinges on More Than Capacitance and Voltage
Wide-bandgap semiconductors—SiC MOSFETs and GaN HEMTs—have pushed switching frequencies into the hundreds of kilohertz and beyond. In these fast-switching power stages, a film capacitor that looks perfect on a BOM (correct capacitance, ample DC voltage margin) can still fail prematurely. The reason: subtle parameters like equivalent series resistance (ESR), equivalent series inductance (ESL), and thermal resistance become dominant loss and stress factors at high dV/dt and high ripple current.
Consider a DC-link capacitor in a 100 kW SiC inverter. The capacitance value is chosen to limit voltage ripple, and the 900 V rating comfortably exceeds the 800 V bus. Yet after 500 hours, the capacitor’s hotspot reaches 105 °C and the capacitance has dropped 3%. The root cause is not voltage breakdown but excessive ESR-driven self-heating under a 20 A RMS ripple current at 50 kHz. The datasheet’s 1 kHz dissipation factor gave no hint of the actual tan δ at the operating frequency, and the thermal resistance from winding to case was ignored.
Engineers who select film capacitors solely by capacitance and voltage risk two costly outcomes. Under-specifying leads to thermal runaway, accelerated aging, and field failures. Over-specifying—choosing an unnecessarily large or exotic dielectric—inflates cost and board area. The sweet spot lies in comparing the specifications that matter for the actual circuit: dielectric material, ESR vs. frequency, ripple current capability, temperature and voltage derating, and long-term stability. This article walks through those trade-offs, using real part series and datasheet parameters to help you make an informed choice.
At Misemicon, we see procurement teams and design engineers wrestling with these decisions daily. Our film capacitor catalog spans multiple dielectrics and constructions, and the following sections reflect the questions we answer most often.
How Dielectric Material, Metallization, and Construction Shape Key Specifications
Film capacitor performance starts with the dielectric film. The four most common materials—polypropylene (PP), polyester (PET), polyethylene naphthalate (PEN), and polyphenylene sulfide (PPS)—each impose a distinct set of electrical, thermal, and cost boundaries. The electrode system (foil vs. metallized) and the internal geometry (wound vs. stacked) then modulate those inherent properties to produce the final component specifications.
Polypropylene dominates power electronics because of its exceptionally low dissipation factor (tan δ < 0.001 at 1 kHz, often staying below 0.001 up to 100 kHz) and high dielectric strength. Its dielectric constant of 2.2, however, yields the lowest capacitance density, so PP capacitors are physically larger per microfarad than PET or PPS. Polyester offers roughly 50% higher capacitance per volume (εr ≈ 3.3) and is cheaper, but its tan δ is an order of magnitude higher (0.005–0.01 at 1 kHz) and increases sharply with frequency and temperature. PPS bridges the gap: εr ≈ 3.0, tan δ around 0.002, and excellent capacitance stability from –55 °C to +125 °C (or even 150 °C with special grades). PEN pushes the temperature ceiling to 150 °C while retaining moderate losses, making it attractive for automotive under-hood applications.
Electrode construction further refines the specifications. Foil electrodes carry higher peak currents and exhibit lower ESR but cannot self-heal, so a single dielectric defect causes a short-circuit failure. Metallized electrodes—a thin aluminum or zinc layer deposited directly on the film—enable self-healing: when a weak spot breaks down, the metallization around the puncture evaporates, isolating the fault. This self-healing comes at the cost of slightly higher ESR and a gradual capacitance loss over life (typically 1–2%). Stacked flat windings or true stacked-plate construction reduce ESL dramatically compared to conventional cylindrical winding, often below 1 nH, which is critical for high-speed decoupling and snubber applications.
The table below summarizes the typical specification ranges for the four dielectrics. These values are drawn from manufacturer datasheets and application notes, including WIMA technical information and TDK film capacitor resources.
| Parameter | Polypropylene (PP) | Polyester (PET) | PPS | PEN |
|---|---|---|---|---|
| Dielectric constant (εr) | 2.2 | 3.3 | 3.0 | 3.0 |
| Dissipation factor (tan δ) at 1 kHz | 0.0002–0.001 | 0.005–0.01 | 0.002–0.004 | 0.004–0.008 |
| Temperature coefficient (ppm/°C) | –200 (typical) | +400 ±200 | ±50 | +200 |
| Max. continuous operating temp. | 105 °C (derate above 85 °C) | 125 °C | 125–150 °C | 150 °C |
| Voltage derating at 100 kHz (vs. DC rating) | Minimal derating (often 80–100% VDC) | Significant derating (50–70% VDC) | Moderate derating (70–85% VDC) | Moderate derating (70–85% VDC) |
| Relative cost (1 µF, 630 V) | 1.0 (baseline) | 0.6–0.8 | 1.5–2.0 | 1.2–1.6 |
Tip: When comparing datasheets, always check the test frequency for tan δ. A single 1 kHz value can be misleading if your circuit operates at 100 kHz. Look for a graph of dissipation factor vs. frequency and temperature; many manufacturers provide this in the detailed technical notes rather than the summary page.
Polypropylene vs. Polyester vs. PPS: A Side-by-Side Specification Comparison for Real-World Circuits
Choosing among PP, PET, and PPS is the most common decision point for power conversion and precision analog circuits. The following comparison matrix uses actual part series—WIMA MKP (PP) and MKS (PET), Panasonic ECW-F (PP) and ECQ (PET), and KEMET PPS series—to ground the specifications in real components.
| Comparison Metric | Polypropylene (e.g., WIMA MKP10, Panasonic ECW-F) | Polyester (e.g., WIMA MKS4, Panasonic ECQ) | PPS (e.g., Panasonic ECH-U, KEMET PPS) | Selection Criteria & Failure Boundary |
|---|---|---|---|---|
| Capacitance stability over –40 to +85 °C | ±2.5% (stable, negative temp coefficient) | +5/–10% (large positive drift) | ±1.5% (flat, near-zero TC) | Use PPS for precision timing or sample-and-hold; PET drift can cause control loop instability. |
| ESR at 100 kHz (typical 1 µF, 630 V) | 0.003–0.008 Ω | 0.02–0.05 Ω | 0.005–0.015 Ω | PP wins for lowest losses; PET ESR causes excessive heating above 20 kHz. |
| Ripple current capability (10 kHz, same case size) | High (low self-heating) | Medium (derate by 40–50% vs. PP) | Medium-high | For resonant converters, PP handles 2× the ripple of PET in the same footprint. |
| Self-healing behavior | Excellent, very low capacitance loss | Good, but higher loss per event | Good, stable over life | PP is preferred where end-of-life capacitance tolerance is tight (±2%). |
| Humidity sensitivity (85 °C/85% RH) | Low moisture absorption (<0.1%) | Moderate (0.4% moisture absorption) | Low (<0.05%) | PPS excels in humid environments; PET may require conformal coating. |
| Size (volume per µF, 630 V) | Largest (baseline 1.0) | ~40% smaller | ~25% smaller than PP | Space-constrained designs may force PET or PPS, but check thermal limits. |
The data reveals clear trade-offs. Polypropylene is the low-loss champion and the safest choice for high-ripple, high-frequency power stages, but it demands more board space. Polyester shrinks the footprint and cost but introduces a temperature coefficient that can shift a filter’s corner frequency by several percent over the industrial temperature range. PPS delivers the best temperature stability and humidity resistance, making it ideal for precision analog circuits and harsh environments, though at a premium price.
Key Takeaway: Don’t select a dielectric by habit. Map your circuit’s actual operating frequency, ambient temperature, and acceptable capacitance drift to the specification rows above. A 100 kHz LLC converter will punish polyester’s ESR, while a 60 Hz power supply with a 10% tolerance may never notice the difference.
How to Compare Datasheet Specifications for Long-Life and High-Ripple Film Capacitor Designs
Datasheets for film capacitors contain the information needed to predict lifetime and thermal behavior, but the critical parameters are often buried in graphs or footnotes. The following checklist and table will help you extract the right numbers and avoid the most common pitfalls.
1. Estimate hotspot temperature, not just ambient. The capacitor’s internal temperature rise is ΔT = IRMS² × ESR × Rth, where Rth is the thermal resistance from the hotspot to the case (and from case to ambient). Many datasheets provide Rth or a permissible ripple current vs. temperature chart. Always calculate the hotspot temperature at worst-case ripple current and maximum ambient. Even a 10 °C rise can halve the expected lifetime according to the Arrhenius law (10 °C rule).
2. Verify ripple current ratings at the actual operating frequency. A capacitor rated for 10 A RMS at 10 kHz may only handle 4 A at 100 kHz due to increased ESR and dielectric losses. Look for frequency derating curves. If none is provided, use the tan δ vs. frequency graph to estimate the additional loss.
3. Examine lifetime curves versus voltage and temperature. Reputable manufacturers like Vishay and Cornell Dubilier publish expected life graphs that show hours to failure as a function of applied voltage (as a percentage of rated) and hotspot temperature. For long-life designs, target a point on the curve that gives at least 60,000 hours at maximum stress.
4. Check dissipation factor change with frequency. A PP capacitor with tan δ = 0.0005 at 1 kHz may still be 0.001 at 100 kHz—still excellent. A PET capacitor that reads 0.008 at 1 kHz can climb to 0.02 at 100 kHz, tripling the heat generation. Always request the full tan δ vs. frequency sweep if the datasheet only shows a single point.
5. Watch for AC voltage derating above 10 kHz. The AC voltage capability of a film capacitor drops as frequency increases due to dielectric losses and corona effects. A 630 VDC PP capacitor might handle 400 VAC at 50 Hz but only 150 VAC at 20 kHz. The derating factor is dielectric-specific: PP retains a high percentage, while PET and PEN derate more aggressively. The table below summarizes typical values from manufacturer application notes.
| Parameter to Check | What to Look For | Typical PP Value | Typical PET Value | Typical PPS Value |
|---|---|---|---|---|
| ESR at 100 kHz (1 µF, 630 V) | Directly impacts I²R heating | 0.003–0.008 Ω | 0.02–0.05 Ω | 0.005–0.015 Ω |
| Thermal resistance Rth (hotspot to case) | Used to calculate ΔT | 10–20 K/W | 12–25 K/W | 10–18 K/W |
| Max. ripple current at 10 kHz (same case size) | Check frequency derating curve | 8–15 A | 4–8 A | 6–12 A |
| AC voltage derating factor at 20 kHz (vs. 50 Hz) | Prevents overvoltage at high frequency | 0.7–0.9 | 0.4–0.6 | 0.6–0.8 |
| Lifetime at rated V and 85 °C (hours) | From manufacturer life curve | 100,000+ | 30,000–60,000 | 60,000–100,000 |
| Capacitance drift end-of-life | Self-healing loss over life | –1 to –2% | –2 to –5% | –1 to –2% |
Common Pitfall: Assuming the DC voltage rating applies at high frequency. A 630 VDC polypropylene capacitor can typically withstand 250 VAC at 10 kHz, but a polyester capacitor of the same DC rating may only tolerate 120 VAC. If your snubber sees a 200 VAC, 50 kHz ringing waveform, the polyester part will fail even though the DC bus is only 400 V. Always cross-check the manufacturer’s AC voltage vs. frequency graph.
When sourcing film capacitors for long-life designs, also verify that the manufacturer provides an endurance test report (typically 1,000–2,000 hours at rated temperature and voltage) and that the part is from a controlled production lot. At Misemicon, we help customers match these datasheet parameters to their application profiles, ensuring that the selected capacitor meets both electrical and lifetime requirements without overpaying for unnecessary performance.
Film Capacitor Specification Questions Engineers Ask Before Final BOM Selection
Q: What is the practical difference between polypropylene and polyester film capacitors for DC-link snubber applications?
Polypropylene offers a dissipation factor of approximately 0.001 versus 0.01 for polyester, meaning ten times lower dielectric losses. In a snubber that absorbs high dV/dt transients, this directly reduces self-heating. PP also has higher insulation resistance and better high-frequency performance, making it the preferred choice for low-loss snubbers in SiC and GaN converters. Polyester is more compact and cheaper but exhibits higher dielectric absorption and a temperature coefficient of +400 ppm/°C, which can shift the snubber’s resonant frequency. If your ambient stays below 85 °C and you can tolerate a 5% capacitance drift, polyester may be acceptable; otherwise, polypropylene is the safer bet.
Q: How should I interpret dissipation factor (tan δ) when comparing film capacitors for a 100 kHz resonant converter?
Tan δ directly adds to total losses. For a 1 µF capacitor with 10 A RMS at 100 kHz, a tan δ of 0.001 (PP) generates about 0.63 W of dielectric loss, while a tan δ of 0.02 (PET) generates 12.6 W—a 20× increase. Always check the datasheet curve for tan δ versus frequency, not just a single 1 kHz value. Polypropylene maintains tan δ below 0.001 up to 100 kHz; polyester rises to 0.01–0.02, causing significant heating. In a resonant tank, excess loss degrades Q factor and can lead to thermal runaway. If the datasheet lacks a frequency sweep, request it from the manufacturer or choose a PP capacitor by default for frequencies above 20 kHz.
Q: What derating guidelines apply to film capacitors in automotive under-hood environments with 125 °C ambient?
Use PPS or PEN film rated for 125 °C or 150 °C continuous operation. Derate the voltage by at least 20% for continuous operation at maximum temperature; for example, a 100 V rated PPS capacitor should not see more than 80 V DC at 125 °C. Polypropylene is limited to 105 °C and requires severe voltage derating above 85 °C—typically 50% or more at 105 °C. If you must use PP, consider forced cooling and verify the lifetime curve at worst-case voltage and temperature. Many automotive-qualified film capacitors (AEC-Q200) include a 1,000-hour endurance test at rated temperature and voltage; use this data to confirm that your derated operating point meets the required 10,000–15,000 hour service life.
Q: How does self-healing affect long-term capacitance drift and reliability in metallized film capacitors?
Self-healing clears dielectric weak spots by vaporizing the thin metallization around a breakdown site. Each event removes a tiny electrode area, causing a cumulative capacitance drop. Typical end-of-life capacitance tolerance for metallized PP capacitors is –1 to –2% after 100,000 hours at rated conditions. Polyester may drift –2 to –5% due to more frequent, larger-area clearing events. When comparing datasheets, look for the guaranteed capacitance at end of life and the number of self-healing events the design can tolerate before reaching minimum capacitance. In safety-critical circuits, foil electrode capacitors avoid self-healing altogether but sacrifice fault tolerance.
Q: Are stacked film capacitors always better than wound for low ESL in high-speed decoupling?
Stacked construction yields inherently lower ESL—often below 1 nH—because the flat internal connections minimize loop area. Wound capacitors can achieve moderate ESL (2–10 nH) with extended foil or double-metallized designs, but they are generally not as low. For decoupling above 10 MHz, stacked film or MLCCs are preferred. However, stacked film capacitors typically have lower capacitance per volume and higher cost. If your decoupling requirement is below 5 MHz and you can tolerate a few nanohenries, a wound PP capacitor with extended foil may suffice. Always compare the self-resonant frequency (SRF) on the datasheet; a higher SRF indicates lower ESL.
Q: When is it worth paying extra for PPS film over polyester in a cost-sensitive power supply?
PPS is justified when you need stable capacitance over temperature (–55 to +125 °C with ±1.5%), low dielectric absorption for sample-and-hold or precision timing circuits, or consistent ESR up to 150 °C. If the application stays below 85 °C and tolerates a 5% capacitance drift, polyester is sufficient and costs 40–60% less. For a cost-sensitive 100 W flyback supply, polyester may be perfectly adequate for the output filter. But in a 48 V telecom rectifier where the control loop relies on a precise compensation capacitor, PPS prevents drift-induced instability and avoids field returns—making the extra cost a wise investment.