Polyester Film Capacitors

Among all the film dielectrics, the most common is polyester. This has the highest dielectric constant and so is capable of the highest capacitance per unit volume. It approaches multilayer ceramic capacitors for volumetric efficiency and in fact can be used in most of the same applications: decoupling, coupling and by-pass, where the stability and loss factor of the capacitor are not too important. This capacitor uses thin polyester film as the dielectric. They are not high tolerance, but they are cheap and handy. Their tolerance is about ±5% to ±10%.

Polyester has a non-linear and comparatively high temperature coefficient. Its dissipation factor is also high, of the order of 8 x 10-3 at 1kHz and 20OC and varies markedly with temperature and operating frequency. These factors make it less useful for critical circuits where a stable, low-loss component is needed.

Polycarbonate Film Capacitors

For the applications where a stable, low-loss component is needed, polycarbonate film capacitor is best suited. This has a near flat temperature – capacitance characteristic at room temperature, with a decrease of about 1% at the operating extremes. It also has a lower dissipation factor, typically less than 2 x 10-3 at 20 OC, 1 kHz.

Polycarbonate would normally be specified for frequency-sensitive circuits such as filters and timing functions. It is also a good general-purpose dielectric for higher power use, but in these applications polypropylene comes into its own.

Polystyrene Film Capacitors

In these devices, polystyrene film is used as the dielectric. This type of capacitor is not for use in high frequency circuits, because they are constructed like a coil inside. They are used well in filter circuits or timing circuits which run at several hundred KHz or less.

Polypropylene Film Capacitors

This capacitor is used when a higher tolerance is necessary than polyester capacitors offer. Polypropylene film is used for the dielectric. It is said that there is almost no change of capacitance in these devices if they are used with frequencies of 100KHz or less.

Polypropylene has a lower dielectric constant than the others and does not metalize so easily, and so gives a larger component for a given CV product. Also it exhibits a fairly constant negative temperature coefficient of-200ppm/OC which restricts its use in frequency-critical circuits, although a defined tempco can be useful for temperature compensation in some instances. Its main advantage is its very low dissipation factor of around 3 x 10-4 at 20 OC and 1kHz, almost constant with temperature. This allows it to handle much higher powers at higher frequencies than the other types, so it is suitable for switch-mode power supplies, TV line deflection circuits and other high-power pulse applications.

Polypropylene capacitors can also be made to close tolerances and this makes them competitors to polystyrene in many tuned circuit and timing applications. Additionally, both polypropylene and polystyrene have similar temperature coefficients and loss factors (polystyrene tempco-125ppm/ OC, dissipation factor typically 5 x 10-4). They also both show a better dielectric absorption performance (0.02% to 0.03%) than the other film types, which makes them best suited to sample-and-hold circuits. Both types suffer from a reduced high temperature rating, generally limited to 85 OC though some polystyrene types are restricted to 70 OC and some polypropylene types are extended to 100 OC Suppliers of polystyrene types are scarce and it is rarely used.