Film capacitors can be broadly categorised into plastic film capacitors, metallized plastic film capacitors, and composite film capacitors. The same dielectric materials are used in the construction of these different types of film capacitors. In composite film capacitors, a combination of dielectric materials is used in the construction of the component.
Film capacitors have characteristics that make them suitable for a broad spectrum of applications including filtering, decoupling, bypassing, EMI suppression, pulse coupling, blocking, and smoothing. Plastic film capacitors deliver high reliability under extreme environmental conditions. Compared to other capacitor types, plastic film devices have lower equivalent series resistance and dissipation factor.
Dielectrics used in plastic film capacitors
Plastic film capacitors offer high stability, long shelf life, low equivalent series resistance, low self-inductance, and a high ability to absorb power surges. The dielectric material used in a film capacitor greatly determines the properties of a component. Some of the key properties of a dielectric material that determine the performance of a capacitor include dielectric constant, dissipation factor, volume resistivity, maximum application temperature, dielectric strength, and dielectric absorption. These parameters vary from one dielectric material to another.
The most common dielectric materials used in the construction of plastic film capacitors are polypropylene and polyester. Other dielectrics used in the construction of film capacitors include polycarbonate, polystyrene, polytetrafluoroethylene (PTFE), polyethylene naphthalate (PEN), polyphenylene sulphide (PPS), polyimide, and paper.
Polyester
Polyester has a high dielectric constant compared to polypropylene, and is one of the most widely used dielectric materials. This high dielectric constant allows construction of capacitors with small physical sizes. Polyester capacitors, also known as mylar capacitors, have good self-healing properties and are relatively cheap.
At high temperatures, polyester capacitors dissipate more power. This characteristic makes these capacitors unsuitable for high frequency and high current AC applications. Furthermore, polyester exhibits a significant capacitance change, up to 5%, as temperature approaches low or high-temperature limits. Due to this characteristic, polyester is an unsuitable material for constructing precision capacitors. Polyester capacitors are mostly used in general purpose board level applications such as blocking, bypassing, decoupling, and some noise suppression circuits.
Polypropylene
Polypropylene is commonly used in the construction of capacitors for high frequency AC applications. This dielectric material has a low dissipation factor, high breakdown strength, low dielectric absorption, high insulation resistance, and is readily available. These properties make polypropylene a dielectric material of choice for a wide range of applications including snubber circuits, high frequency AC systems, high voltage DC & AC systems, and high current DC applications.