Film Capacitors
Film capacitors or plastic film capacitors are non-polarized capacitors with an insulating plastic film as the dielectric. The dielectric films are drawn to a thin layer, provided with metallic electrodes and wound into a cylindrical winding. The electrodes of film capacitors may be metallized aluminum or zinc, applied on one or both sides of the plastic film, resulting in metallized film capacitors or a separate metallic foil overlying the film, called film/foil capacitors.
Metallized film capacitors offer self-healing properties. Dielectric breakdowns or shorts between the electrodes do not destroy the component. The metallized construction makes it possible to produce wound capacitors with larger capacitance values (up to 100 µF and larger) in smaller cases than within film/foil construction.
Film/foil capacitors or metal foil capacitors are use two plastic films as the dielectric. Each film is covered with a thin metal foil, mostly aluminium, to form the electrodes. The advantage of this construction is the ease of connecting the metal foil electrodes, along with an excellent current pulse strength.
A key advantage of every film capacitor's internal construction is direct contact to the electrodes on both ends of the winding. This contact keeps all current paths very short. The design behaves like a large number of individual capacitors connected in parallel, thus reducing the internal ohmic losses (ESR) and parasitic inductance (ESL). The inherent geometry of film capacitor structure results in low ohmic losses and a low parasitic inductance, which makes them suitable for applications with high surge currents (snubbers) and for AC power applications, or for applications at higher frequencies.
The plastic films used as the dielectric for film capacitors are Polypropylene (PP), Polyester (PET), Polyphenylene sulfide (PPS), Polyethylene naphthalate (PEN), and Polytetrafluoroethylene or Teflon (PTFE). Polypropylene film material with a market share of something about 50% and Polyester film with something about 40% are the most used film materials. The rest of something about 10% will be used by all other materials including PPS and paper with roughly 3%, each.
| Film material, abbreviated codes | |||||
|---|---|---|---|---|---|
| Film characteristics | PET | PEN | PPS | PP | |
| Relative permittivity at 1 kHz | 3.3 | 3.0 | 3.0 | 2.2 | |
| Minimum film thickness (µm) | 0.7–0.9 | 0.9–1.4 | 1.2 | 2.4–3.0 | |
| Moisture absorption (%) | low | 0.4 | 0.05 | <0.1 | |
| Dielectric strength (V/µm) | 580 | 500 | 470 | 650 | |
| Commercial realized voltage proof (V/µm) |
280 | 300 | 220 | 400 | |
| DC voltage range (V) | 50–1,000 | 16–250 | 16–100 | 40–2,000 | |
| Capacitance range | 100 pF–22 µF | 100 pF–1 µF | 100 pF–0.47 µF | 100 pF–10 µF | |
| Application temperature range (°C) | −55 to +125 /+150 | −55 to +150 | −55 to +150 | −55 to +105 | |
| ΔC/C versus temperature range (%) | ±5 | ±5 | ±1.5 | ±2.5 | |
| Dissipation factor (•10−4) | |||||
| at 1 kHz | 50–200 | 42–80 | 2–15 | 0.5–5 | |
| at 10 kHz | 110–150 | 54–150 | 2.5–25 | 2–8 | |
| at 100 kHz | 170–300 | 120–300 | 12–60 | 2–25 | |
| at 1 MHz | 200–350 | – | 18–70 | 4–40 | |
| Time constant RInsul•C (s) | at 25 °C | ≥10,000 | ≥10,000 | ≥10,000 | ≥100,000 |
| at 85 °C | 1,000 | 1,000 | 1,000 | 10,000 | |
| Dielectric absorption (%) | 0.2–0.5 | 1–1.2 | 0.05–0.1 | 0.01–0.1 | |
| Specific capacitance (nF•V/mm3) | 400 | 250 | 140 | 50 | |
Some film capacitors of special shapes and styles are used as capacitors for special applications, including RFI/EMI suppression capacitors for connection to the supply mains, also known as safety capacitors, Snubber capacitors for very high surge currents, Motor run capacitors, AC capacitors for motor-run applications
- High pulse current load is the most important feature of film capacitors so many of the available styles do have special terminations for high currents
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Radial style (single ended) for through-hole solder mounting on printed circuit boards
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SMD style for printed circuit board surface mounting, with metallized contacts on two opposite edges
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Radial style with heavy-duty solder terminals for snubber applications and high surge pulse loads
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Heavy-duty snubber capacitor with screw terminals
Read more about this topic: Types Of Capacitor, Types and Styles
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