Polypropylene vs polyester capacitor: a PCB engineer’s guide comparing dissipation factor, temperature stability, dielectric absorption, and the right application for each.

Walk into any PCB design review and bring up the topic of polypropylene vs polyester capacitor selection—you’ll quickly find out it’s one of those deceptively simple choices that actually has a lot riding on it. Get it right and the circuit does exactly what the simulation said it would. Get it wrong and you’re chasing phase errors, unexpected heating, or precision drift in the field.

Both are film capacitors. Both are non-polarized. Both look broadly similar on a BOM. But under the hood, polypropylene (PP) and polyester (PET, also called Mylar or CL) behave quite differently, and those differences map cleanly onto application requirements once you know what to look for.

This guide is written from the engineering bench outward—covering the dielectric physics, real performance data, and practical application guidance that actually moves the needle on a design decision.

Understanding the Two Dielectric Materials

Before going into numbers, it helps to understand what makes each plastic film behave the way it does at a molecular level—because the physics flows directly into the datasheet parameters.

Polypropylene (PP) Film

Polypropylene is a non-polar, semi-crystalline thermoplastic polymer from the polyolefin family. “Non-polar” here is the key electrical descriptor: because the molecular chains have no permanent dipole moments, the dielectric doesn’t try to reorient under an applied AC field. That means almost no energy loss from dipole rotation at any frequency. The result is polypropylene’s signature characteristic—an exceptionally low dissipation factor (tan δ) of roughly 0.0002 (0.02%), which holds stable from DC all the way well into the megahertz range.

PP film also absorbs virtually no moisture, which makes it suitable for open or “naked” designs without extra encapsulation. The main trade-off is a lower dielectric constant (εr ≈ 2.2) compared to polyester, meaning you need a physically larger piece of film to store the same charge at the same voltage.

Polyester (PET) Film

Polyester film—polyethylene terephthalate, shortened to PET and also known by the trade name Mylar—has a polar molecular structure. Its dielectric constant is significantly higher at εr ≈ 3.2 to 3.3, which directly translates to a smaller physical size for the same capacitance and voltage rating. That’s why PET dominates wherever board space is tight and the frequency is moderate.

The price of that higher dielectric constant is energy loss. PET’s polar chains rotate in an AC field, generating heat. Its dissipation factor is typically 0.01–0.015 (1.0–1.5%)—roughly 50 to 75 times higher than polypropylene. This matters a lot in AC and high-frequency circuits but very little in a simple DC blocking or bypass role.

PET is also available in thinner gauges than PP—down to about 0.7 µm versus about 1.9 µm minimum for polypropylene—enabling even smaller, higher-capacitance constructions.

Polypropylene vs Polyester Capacitor: Full Specification Comparison

Parameter	Polypropylene (PP / MKP / CBB)	Polyester (PET / MKT / CL)
Dielectric constant (εr)	~2.2	~3.2 – 3.3
Dissipation factor (tan δ)	0.0002 (0.02%)	0.010 – 0.015 (1.0 – 1.5%)
Dielectric absorption (DA)	Very low (<0.02%)	Moderate (~0.2%)
Temperature range	–55°C to +105°C	–55°C to +125°C
Capacitance vs. temperature	±2.5%, nearly linear	Up to ±5% (nonlinear)
Capacitance vs. frequency	Very stable to MHz range	–3% from 100 Hz to 100 kHz
Moisture absorption	Negligible	Some absorption
Insulation resistance (≤0.33µF)	>25,000 MΩ	>7,500 MΩ
Voltage rating (typical)	100 V – 2,000 V	50 V – 630 V
AC suitability	Excellent	Limited (high loss at frequency)
Physical size per µF	Larger	Smaller
Cost	Higher	Lower
Typical code/prefix	CBB, MKP, KP	CL, MKT, KS
Market share (film caps)	~50%	~40%

Key Performance Differences That Actually Matter on a Board

Dissipation Factor and Power Loss

This is the single most consequential difference between the two dielectric types for AC and high-frequency use. Dissipation factor—sometimes called loss tangent—tells you what fraction of the energy going into the capacitor each cycle converts to heat rather than getting stored and returned.

For a polypropylene capacitor running a power factor correction circuit at 50 Hz or a snubber cap across an IGBT at 20 kHz switching frequency, a tan δ of 0.02% means negligible heating even at significant reactive power levels. For a polyester cap in the same position, the 1.0–1.5% dissipation factor is 50× higher, which can cause meaningful temperature rise inside the capacitor body, accelerating aging and in worst cases causing thermal runaway failure.

For purely DC applications—bias bypass, supply rail decoupling at low frequency, coupling between stages at moderate signal levels—the dissipation factor difference is largely academic. Power loss in a DC circuit is determined by leakage current and ESR, not tan δ.

Temperature Stability of Capacitance

Polypropylene has a nearly linear, slightly negative temperature coefficient of capacitance (TCC) of about ±2.5% across its full temperature range. It’s predictable and well-behaved. Polyester’s TCC is larger and nonlinear—up to ±5% at the temperature extremes—and the capacitance actually changes by about 3% just moving from 100 Hz to 100 kHz signal frequency.

For timing circuits, resonant LC tanks, oscillators, or precision integrators, polyester’s drift is a problem. A 5% capacitance shift with temperature will shift resonant frequency, alter time constants, and cause calibration drift. Polypropylene holds its value much more consistently across temperature and frequency, which is why PP is the standard choice for precision and resonant applications.

Dielectric Absorption

Dielectric absorption (DA) is the tendency of a capacitor to partially “remember” a previous charge state—releasing a residual voltage after being discharged. It’s a small effect, but in sample-and-hold circuits, integrators, and D/A converters it causes a measurable error voltage that limits accuracy.

Polypropylene has extremely low DA—typically under 0.02%. Polyester’s DA is roughly 0.2%, an order of magnitude higher. In most coupling and bypass applications this doesn’t matter. In precision analog circuits it’s decisive.

Voltage Rating and dV/dt Capability

Polypropylene film capacitors are routinely available at 1000 V, 1500 V, and higher DC ratings. PET capacitors typically top out around 630 V for standard types. More importantly, PP’s higher breakdown strength and lower dissipation factor make it substantially better at handling rapid voltage transients (high dV/dt). This is the physical basis for why polypropylene is used in snubber circuits and PP is not interchangeable with PET there.

Physical Size

Polyester wins on compactness. Because PET’s dielectric constant is about 50% higher than PP’s, a PET capacitor can deliver the same capacitance in roughly two-thirds the volume of an equivalent PP type (at the same film thickness and voltage). For SMD designs or space-constrained PCBs where a small bypass or coupling cap is needed, PET is the practical choice.

Application Decision Guide: When to Use Each Type

Use Polypropylene Capacitors When

Snubber circuits across switching devices (IGBTs, MOSFETs, SCRs). The combination of low ESR, high dV/dt tolerance, low dissipation, and high voltage rating makes MKP/CBB polypropylene the standard choice for snubbers across any semiconductor switch. PET simply can’t survive the combination of high peak current, fast transients, and continuous duty in this position.

EMI/RFI suppression, X and Y capacitors. Safety-rated X2 and Y2 capacitors on AC mains lines must withstand continuous AC voltage with self-healing and tolerate transient overvoltages. Metallized polypropylene is the industry-standard dielectric for these applications. PET’s higher dissipation makes it unsuitable for sustained AC mains duty.

Power factor correction (PFC) capacitors. Reactive power capacitors for industrial PFC banks carry significant AC current continuously. PP’s low loss factor is essential to prevent overheating and deliver the long service life (20+ years) these installations demand.

DC-link capacitors in high-power inverters. Solar, wind, EV drive, and industrial servo inverters increasingly specify PP film for DC-link duty, particularly in high-reliability or maintenance-free applications where the self-healing and temperature stability of PP justify the higher cost.

Precision timing, resonant circuits, and oscillators. Wherever the capacitance value needs to stay accurate across temperature and frequency—oscillators, crystal filter circuits, active filter designs—polypropylene is the correct dielectric.

Audio signal coupling (where distortion matters). Polypropylene’s low DA and linear behavior makes it the preferred coupling capacitor in precision audio circuits, instrument amplifiers, and high-end signal path designs. PET’s higher DA introduces a subtle distortion mechanism that careful listeners and measurement engineers can detect.

Use Polyester (PET) Capacitors When

General purpose DC bypass and decoupling. For bypassing a supply rail, decoupling a digital IC, or bulk filtering where frequency is low and precision is not required, PET delivers more capacitance per cubic millimeter at lower cost. This covers a very large fraction of film capacitor usage on everyday PCBs.

Signal coupling at moderate frequencies with space constraints. When a 10 nF to 100 nF coupling capacitor needs to fit into a dense SMD layout and the circuit has modest performance requirements, MKT/CL polyester is the practical answer.

Cost-sensitive, high-volume consumer applications. LED driver boards, power adapters, consumer audio, white goods electronics—anywhere BOM cost is scrutinized heavily and performance margins are comfortable—PET is the dominant choice. Per unit of capacitance, PET is noticeably cheaper than PP.

SMD film capacitor positions. PET is more compatible with SMD reflow soldering processes than PP, because polyester’s higher melting point survives the thermal profile better. SMD polypropylene film capacitors exist but are limited in range. If you need an SMD film cap, PET is almost certainly what’s in the catalog.

General noise suppression and bypassing on signal lines. For EMI bypassing on low-frequency signal lines (audio, low-speed control) where safety ratings are not required and the cap sees essentially DC with noise riding on it, PET handles the job at attractive cost.

Polypropylene vs Polyester Capacitor Application Table

Application	Recommended Dielectric	Why
IGBT / MOSFET snubber	Polypropylene (PP)	High dV/dt, low loss, high voltage
X2 / Y2 mains EMI cap	Polypropylene (PP)	AC rating, self-healing, safety
Industrial PFC bank	Polypropylene (PP)	Low loss, long life under AC
DC-link, HV inverter	Polypropylene (PP)	Overvoltage tolerance, stable C
Resonant / LC tank circuit	Polypropylene (PP)	Stable C over temp & frequency
Precision integrator / S&H	Polypropylene (PP)	Very low dielectric absorption
Audio signal coupling (hi-fi)	Polypropylene (PP)	Low DA, low distortion
General bypass, DC decoupling	Polyester (PET)	Small size, low cost
Signal coupling, moderate freq	Polyester (PET)	Compact, sufficient performance
SMD film capacitor position	Polyester (PET)	Reflow compatible, available
Consumer electronics coupling	Polyester (PET)	Cost-effective for the performance level
Timer / clock circuit (tolerant)	Polyester (PET)	Acceptable where ±5% drift is OK

Understanding the Role of a Capacitor on Your PCB

Choosing the right capacitor for a given PCB position requires understanding the electrical stress the component will actually see in operation—AC voltage amplitude and frequency, peak current, temperature rise, duty cycle, and required precision. Polypropylene and polyester each carve out a large and well-defined space in that parameter landscape. The mistake most often seen in design reviews is defaulting to PET across the board for cost savings, then discovering that a snubber or PFC cap is running hot in thermal testing—at which point the redesign costs far more than the premium PP part would have.

Marking Codes: How to Identify Each Type in the Field

In Chinese and Asian manufacturer coding, polypropylene film capacitors often start with CBB (e.g., CBB21, CBB22), while polyester film capacitors start with CL (e.g., CL11, CL21). In European IEC coding, polypropylene types use the MKP prefix (Metallized Kunststoff Polypropylen) and polyester types use MKT (Metallized Kunststoff Terephthalate). WIMA, one of the most respected film capacitor manufacturers, uses MKP for polypropylene and MKS for polyester (stacked) and MKT for wound polyester types.

A reliable field test for telling them apart is a hot-air gun and a capacitance meter: measure cold capacitance, apply gentle heat, measure again. PET capacitors show a significant capacitance shift; PP capacitors change very little. That behavioral difference is the same one that matters in production circuits under thermal cycling.

Useful Resources for Film Capacitor Selection

These are the reference materials worth bookmarking for serious capacitor selection work:

KEMET Film Capacitor Selection Guide – kemet.com/film-capacitors — Parametric selector covering MKP, MKT, PPS, and power film types with full datasheets.

Vishay Film Capacitor Technical Reference – vishay.com/docs/26033/gentechinfofilm.pdf — Detailed dielectric comparison, construction explanation, and application derating guidance.

WIMA Technical Capacitor Information – wima.com/en/technical-information — One of the most respected film capacitor makers; application guides and material comparisons.

TDK/EPCOS Film Capacitor Portfolio – product.tdk.com — Parametric search with full data across B3xxxx series PP and PET types.

Passive Components Industry Guide (passive-components.eu) – passive-components.eu/film-and-foil-organic-dielectric-capacitors — Thorough technical overview including dielectric comparison tables and construction details.

Wikipedia: Film Capacitor – en.wikipedia.org/wiki/Film_capacitor — Solid reference for historical context, dielectric material properties table, and construction types.

Digi-Key Film Capacitor Parametric Search – digikey.com/film-capacitors — Real-time stock, pricing, and datasheets across all major PP and PET film families.

Frequently Asked Questions

Can I replace a polyester capacitor with a polypropylene one?

In most cases, yes—and it usually improves performance. Polypropylene capacitors have lower dissipation, lower dielectric absorption, and better temperature stability. The practical considerations are physical size (PP is larger for the same capacitance and voltage) and cost (PP is more expensive). Verify that the replacement fits the PCB footprint and that the circuit doesn’t rely on any temperature coefficient matching. For coupling, bypass, and snubber positions, swapping PET for PP is almost always a neutral or positive change.

Why is polypropylene preferred for audio circuits over polyester?

Two main reasons. First, polypropylene’s dielectric absorption (DA) is roughly ten times lower than polyester’s (~0.02% vs ~0.2%). In circuits with a DC bias component, DA causes the capacitor to release a residual voltage that acts as an error signal—audible as a subtle smearing or overhang in transient response. Second, PP’s dissipation factor is far lower and more consistent across frequency, meaning it introduces less distortion into the signal path. This is why high-quality audio amplifiers, tube amps, and precision instruments consistently specify PP film types in signal-path positions.

Is polyester (PET) suitable for AC mains circuits?

Not for direct AC mains duty as a safety capacitor. PET’s relatively high dissipation factor causes excessive heating in sustained AC applications, especially at higher voltages. More critically, standard PET capacitors are not safety-rated for the transient overvoltages present on AC mains. X2 and Y2 safety capacitors for EMI filtering must be metallized polypropylene with the appropriate safety certification (IEC 60384-14, UL, etc.). Using a PET cap in a safety-capacitor position is both a performance risk and a regulatory compliance violation.

What is the difference between MKP and MKT capacitors?

MKP stands for Metallized Kunststoff (plastic) Polypropylen—it’s a metallized polypropylene film capacitor. MKT stands for Metallized Kunststoff Terephthalate—a metallized polyester (PET) film capacitor. Both are metallized wound constructions with self-healing properties, but MKP offers lower dissipation, higher voltage ratings, better AC performance, and superior temperature stability. MKT is smaller and cheaper for the same capacitance. When you see these designations on a BOM or manufacturer catalog, the dielectric material is the key differentiator driving all the downstream performance differences discussed in this article.

Why does polypropylene have a lower maximum temperature than polyester?

Polypropylene’s maximum rated operating temperature is typically 105°C, while polyester can go to 125°C. This comes down to the melting and softening behavior of the polymer. Polypropylene has a lower melting point (~165°C) compared to PET (~260°C), and the practical rated temperature is set conservatively well below the melting point to ensure the dielectric maintains its mechanical and electrical integrity over the component’s service life. For most PCB environments this isn’t a limiting factor, but in high-ambient-temperature positions—near heat sinks, power devices, or in automotive under-hood locations—polyester’s higher temperature capability can be the deciding factor in its favor.

Summary

The polypropylene vs polyester capacitor decision comes down to a clear performance versus size-and-cost trade-off. Polypropylene is the premium dielectric: lower loss, lower dielectric absorption, better AC performance, higher voltage ratings, and excellent temperature and frequency stability. It belongs in snubbers, PFC circuits, EMI X/Y caps, DC-link positions, precision analog, and high-quality audio signal paths. Polyester is the practical workhorse: more capacitance per unit volume, lower cost, SMD-compatible, perfectly adequate for general DC bypass, coupling, and decoupling where precision and high frequency are not the design constraints.

Most production PCBs will carry both types—polyester in the dozens of general-purpose positions and polypropylene in the few critical performance positions where it genuinely earns its cost premium. Knowing which is which, and why, is the difference between a design that passes validation on the first spin and one that needs an unexpected second revision.