The internal structure of film capacitors is mainly as follows: metal foil (or a foil obtained by metalizing plastic) is used as the electrode plate, and plastic is used as the dielectric. Obtained by winding or stacking process. The different arrangements of foils and films lead to a variety of construction methods. The figure below is a typical schematic diagram of thin-film capacitors.
In stacked-film production technology, large rings of metallized film are wound onto core wheels with diameters up to 60 cm. In this way the "master capacitors" are produced under well-defined and constant conditions.
The capacitor production lots obtained when the rings are sawed apart to produce the actual stacked-film capacitor bodies are especially homogeneous. The pulse handling capabilities of stacked-film capacitors are of a particular advantage. Each individual layer acts as a single capacitor of small capacitance, so any damage to the contacts due to overloading is restricted to the layers concerned, which might get disconnected. Therefore the total capacitance can decrease slightly, without further affecting the performance and reliability of the part. Additionally, uncoated stacked-film capacitors (SilverCapTM) offer the special advantage that they can be adapted to customer design requirements. A wide range of sizes can be offered without considering case size standards or provide special tools for new cases.
Stack film capacitor group consists of three different construction techniques. Wound film, Multilayer (PML), and Stacked film. Wound film includes polymers, paper and paper/plastic, with either metalized electrodes or discrete foil. A wound film Capacitor is just that, where two or more films/electrodes are spirally wound on a mandrel to a predetermined number of turns, length, or capacitance value. The mandrel is then ejected from the winding, which is then further processed depending on the style and/or application of that particular part.
Stacked film Capacitors or Mylar* Capacitors are one of the most widely used film capacitors. Stacked film capacitor grade Polyester film is available in a wide variety of thicknesses, ranging from 0.5 um up to 20 um. Combining the thin film with the relatively high dielectric constant (k) of 3.2, high energy densities are possible. However, Polyester is considered a high loss film and typically not used in applications that require high current or high frequency. Stacked film Capacitors are second in use only to Polyester. Capacitor grade Polypropylene is available in thicknesses ranging from 3 um to 15 um.
The electrical characteristics of stacked film exhibits many advantages, such as high break down strength, low moisture absorption, low loss factors, and capacitance stability throughout the temperature range. It is ideally suited for high frequency and/or high current applications. The disadvantages of Polypropylene are the relatively low dielectric constant (k) of 2.2 and low maximum temperature of 105 C. Polycarbonate Capacitors are typically found in high temperature low loss applications. Polycarbonate exhibits good electrical characteristics over the entire temperature range of -65 to +125 C.
The dielectric constant (k) of 2.7 is between Polyester and Polypropylene, so energy density is average. Many military specification capacitors require the use of polycarbonate as the dielectric. Polycarbonate has a questionable availability, so contact the factory with any requirements. Polystyrene Capacitors exhibit extremely low loss, and excellent electrical characteristics, specifically capacitance change over temperature, which is typically +-1% over the temperature range of -55 to 85 C. Polystyrene has a very low dielectric constant (k) of 2.1, so it is suited for low capacitance applications. Like Polycarbonate film, polystyrene also has a questionable availability, so contact the factor with any requirements. Polyimide or Kapton* Capacitors are typically used in extremely high temperature applications, up to 250 C. The electrical characteristics are average, and the dielectric constant (k) of 3.4 is close to that of Polyester. Typically, polyimide as a capacitor dielectric is typically combined with a discrete foil. Metalized Polyimide is possible, however it exhibits extremely poor self healing characteristics. The chemistry of the film is such that when a clearing occurs, traces of carbon remain in the area, thus causing tracing and poor electrical properties. PTFE or Teflon* Capacitors are also used in high temperature applications, up to 200 C. The electrical characteristics of PTFE are excellent, low loss, with high stability; however, cost is very prohibitive.
PPS is not an easily “clearable” film. When used with a metalized electrode, the self healing characteristic tends to leave small traces of carbon, which in turn, exhibits slightly higher insulation resistance than it’s polycarbonate counter part. Paper Capacitors are old technology, however are still widely used in high voltage and high power applications. Paper, as a dielectric, requires impregnation to enhance the electrical properties of the paper, thus making it suitable for high voltage capacitor applications. The oil suppresses the corona phenomenon, which if allowed to exist, will degrade the dielectric, leading to eventual failure. Paper can be combined with Polyester or Polypropylene to obtain different characteristics, such as lower loss, or high dielectric withstand. The dielectric constant (k) of paper is highly debated subject however, the actual K of paper is primarily dependant on the impregnants used during manufacturing. Capacitor Types and/or Styles can be reduced into two different categories, Non-hermetically Sealed and Hermetically Sealed. Each category can be further reduced into many sub-categories depending on specific application or costs associated with the capacitor. Non-Hermetically Sealed Capacitors are typically packaged in a nonmetallic housing or enclosure, such as tape wrap and epoxy endfill or a preformed epoxy case. This style offers basic environmental protection, however, costs are minimal.
The three primary types of non-hermetics are Wrap & Fills, preform case, and bare windings.
CAPACITOR APPLICATIONS Arizona Capacitors manufactures a broad spectrum of reliable wound film capacitors suitable for a wide variety of applications. Many of the catalog standards and/or custom capacitors can meet the most stringent requirement of the customer. Capacitance ranging from .0001 uf to 5000 uf, combined with possible voltages between 30 V to 150 KV. Many other electrical and environmental characteristics such as temperature range from -65 to 250 C are available. General applications include but not limited to: Blocking Coupling Bypass Filtering Timing Smoothing Pulse Coupling Snubber Energy Discharge Commutating Motor run Motor Start Decoupling EMI Suppression RFI Suppression Specific applications are too numerous to list or give details, however below is representative: Laser Audio Rangefinders Strobe Ignition Lighting Power supply Aircraft Lighting Speed Controls Avionics Electronic Filter AC Drives Inverters UPS Induction Heating.
