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Capacitor Dielectrics & Descriptions
Here are few of the most commonly used dielectric materials for capacitors. A brief description and application examples are provide for many.
Air-Gap:
Low dielectric loss and offers good cooling. Large-valued tunable capacitors (like in old radios) are often made this way.
Aluminum:
Very high capacitance density (capacitance to volume). High dielectric leakage - prone to exploding.
Ceramic:
Ceramic dielectric types are differentiated by the temperature coefficient of capacitance, and the dielectric loss. Available in 1% tolerance for values up to about 1 ”F, typically made from Lead zirconate titanate (PZT) ferroelectric ceramic. Capacitance can change with applied voltage (piezoelectric effect)
C0G or NP0 (negative-positive-zero, ±0): Typically 4.7 pF to 0.047 ”F, 5%. High tolerance and temperature performance (flat over temp). Larger and more expensive. Lowest losses, used in filters, as timing elements, and for balancing crystal oscillators.
X7R: Typical 3300 pF to 0.33 ”F, 10%. Good for non-critical coupling, timing applications. Subject to microphonics.
Z5U or 2E6: Typical 0.01 ”F to 2.2 ”F, 20%. Good for bypass, coupling applications. Low price and small size. Subject to microphonics.
Combination Film:
Combination polyester (Mylar) and polypropylene. Extremely low temperature coefficient in the 0° C to 85° C temperature range. Volumetric efficiency similar to polycarbonate.
Glass:
Extremely stable & reliable
Kapton Film:
Electrical properties similar to Mylar with a much higher operating temperature going up to 250° C. A higher cost than Mylar.
KF (Polymer) Film:
Extremely high volumetric efficiency with about 4x the "K Factor" of Mylar, making it about 1/4 the size. Higher DF and lower IR are its disadvantages along with cost.
Mica:
Chemically inert nature means it does not change physically or chemically with age and it has good temperature stability. Plain mica can absorb moisture, but metallized mica and silver mica are more resistant to moisture.
Paper Film:
Paper or Kraft Paper is the oldest of the film capacitor dielectrics. The paper must be impregnated with Epoxy, Wax, Oil, or other suitable impregnate. It is still popular for high voltage and AC rated capacitors operating at lower frequencies. Paper is also wound with plastic dielectrics in combination dielectric capacitors. Its hydroscopic nature allows moisture to degrade performance over time.
Polyamide (plastic film):
Operating temperatures of up to 200șC. High insulation resistance, good stability and a low dissipation factor. High cost and large size.
Polycarbonate Film:
Lower DF, higher IR, better temperature coefficient and better stability than Mylar with a slightly lower volumetric efficiency. 2nd most popular dielectric. Polycarbonate capacitors have a 100% voltage rating from -55° C to +125° C.
Polyester (Mylar) Film:
A good general purpose plastic dielectric with relativity low cost and high volumetric efficiency. The most popular of the capacitor dielectrics.
Polypropylene Film:
Very good temperature coefficient high IR, and low DF make it suitable for AC operation. Usable to 105° C without derating. Popular for AC applications.
Polystyrene Film:
Very good electrical properties and excellent stability are its advantages. The big disadvantage is its operation is limited to below 85° C, and their large size.
Polysulfone Film:
Electrical properties similar to polycarbonate with a very good temperature coefficient and higher operating temperature. Very limited availability in the last few years has limited its use.
Tantalum:
Large capacitance to volume ratio, smaller size, good stability, wide operating temperature range, long reliable operating life. Widely used in miniaturized equipment and computers. Available in both polarized and unpolarized versions, so suitable for AC and DC. Solid tantalums have much better characteristics than wet slug (not permitted in any Mil-Spec equipment) versions.
Teflon Film:
The best electrical properties of all the dielectrics. Extremely high IR, low DF and operation to 250° C. Expensive and physically large.
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