The article explains construction, application and features of film and foil organic dielectric capacitors:
- Paper MP capacitors
- Polyester PET /KT/MKT capacitors
- Polypropylene PP /KP/MKP capacitors
- Polycarbonate PC /KC/MKC capacitors
- Polystyrene PS, Polyphenylene sulfide PPS and other plastic film capacitors Teflon PTFE / Polysulfone PSU
Film capacitors are essential electrostatic capacitors suitable for medium, higher voltage and higher current circuits. Unlike most other dielectric systems, film capacitors feature low loss factor at very low temperature.
Dielectric constant is not big, but they feature very high dielectric strength. In combination with long life and self-healing aging capabilities it makes them ideal choice for high voltage, high power systems. While we focus on the most common dielectric types there is a wide list of organic dielectric materials with different features. Overview can be seen in the article: What is a Dielectric Constant of Plastic Materials ?
Polystyrene (PS) represents a non-polar dielectric material that just as polycarbonate to a great extent has been replaced by polypropylene. The availability is strongly limited due to ceased production.
Polyphenylene Sulfide or simply PPS has come to stay mainly because of its relatively high temperature resistance which has allowed SMD manufacture.
General Comments to PS
εr ≈ 2.4. Max. temp.+70°C (possibly +85°C). Smallest film thickness 4 μm (0.16 mils). Except for metal foils often double metallized plastic foils are used.
Advantages
- Very low losses; 0.01% at room ambient and 1 kHz.
- Very high IR; RC 200 000 s. Actually it’s more exterior factors that determines the IR than the material itself.
- Dielectric absorption 0.02%.
- Stability ΔC/C, max ± 0.5%.
Disadvantages
- Can not be metallized due to high carbon deposits and too little oxygen available at a self-healing breakdown.
- Chemically sensitive film.
- Availability limited due to ceased production.
Temperature and frequency dependencies for PS
General comments to PPS
Foil types exist but metallized design is a rule. If nothing else is said metallized film applies.
εr ≈3.0. Max. temp.+140 °C; most manufacturers recommend maximum +125 °C. Dielectric absorption ≈ 0.1%.
Smallest film thickness is 1.2 μm (0.05 mils) but former quality problems with the thinnest film may give cause for stronger derating of the lowest rated voltage. However, the supply of film is stabilized thus indicating an established demand..
The film is used in capacitors both for lead mount and SMD. Existing chip types are made in a stacked design.
Temperature and frequency dependencies for PPS
Other plastic organic film capacitors used in special designs:
Teflon (PTFE)
Teflon exists both in metallized and foil design. The greatest advantage of the film is its temperature resistance. The price, on the other hand, is high which gives cause for exclusive hermetic designs.
- Capacitance 470pF….4.3μF.
- Tolerance ±0.25….±20%.
- Temperature range -65/+200°C.
- TC -200…+50ppm/°C.
- Rated voltage 50…600 V DC.
- Tanδ , @ 1kHz, 20°C, ≤0.1%.
- IR, @ 20°C, foil ≥500GΩ/ ≥100 000s,
- IR, @ 20°C, metallized film 50GΩ/ 10 000s.
- Stability ΔC/C max -1%.
- εr ≈ 2.2.
- Dielectric absorption ≤0.02%.
- Recommended derating 0.6xVR.
Polysulfone (PSU)
Polysulfone capacitors belong to the same price group as the one of teflon. They are used only in special applications where high temperature capabilities together with excellent characteristics are of vital importance. The capacitors have been manufactured only in hermetic design and with foil electrodes.
- Capacitance 1nF….22μF.
- Tolerance ±0.25….±10%.
- Temperature range -65/+150°C.
- TC -200…+50ppm/°C.
- Rated voltage 50…400 V DC.
- Tanδ , @ 1kHz, 20°C, ≤0.15%.
- IR, @ 20°C, foil ≥1000GΩ
- εr ≈ 3.2.
- Dielectric absorption ≤0.2%.
- Recommended derating 0.6xVR.
