Comparison of Metallized Film Capacitors and Film/Foil capacitors

Conventionally, capacitors are made with two layers of dielectric film (PP, PET, PC etc.) and metal foils. During 1980s, metallized film capacitors were introduced, which later became popular because of several advantages they offer. Film / foil capacitors continue their advantage in many places, and are very much in use today for several applications. It is of interest to understand the pros and cons of the two types of construction.

Plastic Film / Metal Foil Capacitors

Capacitor element is made by winding alternate layers of plastic film and metal foil (mostly aluminium. Plastic film could be PP, PET, PPS etc.). Plastic film has high dielectric strength and very low loss factor, but has discreet weak spots along its length due to dust, impurities, air bubbles etc. which can be minimized, but cannot be completely eliminated even with extreme care during their manufacture. One such weak spot can make the entire capacitor fail by creating a short circuit path across capacitor electrodes. Hence minimum of two layers are needed to make sound capacitor element. This makes the capacitor bulkier unavoidably.

PP is the most favoured dielectric due to its low dielectric loss, coupled with high electric strength, and is the cheapest material among plastic films. Its lower dielectric constant is not of much concern. PP has advantage that it can be impregnated with number of dielectric oils to remove air and moisture, which increase its effective strength and loss factor. 

Film foil construction is used mostly for small value capacitors. Advantage of this construction is ease of connection or contact with metal foil, and good pulse strength. To avoid possibility of failure due to weak spots, film of much higher thickness is used than theoretically possible. High voltage capacitors may muse more than two layers. Terminations of film / foil capacitors may be made brought out as extended foil construction, or may be brought through the winding by insertion of tabs for external connection. Extended foil gives much smaller loss factor and high pulse capacity, while tab insertion if preferred for large capacitors for large DC or AC capacitors. Some large power factor capacitors may also use extended foil construction for the same benefits.

Benefits of plastic film / foil capacitors

Foil has naturally much lower resistance (ESR) than a thin metal layer (as for metallized film), which give much lower loss factor.

• Very high electric strength
• Low tan delta (dielectric loss)
• Low series resistance (ESR)
• High insulation resistance
• Excellent pulse capacity (high dV/dt)
• High current carrying capacity
• Excellent heat dissipation
• High stability of capacitance throughout lifetime.
• Virtually no capacitance drift with applied voltage
• Good high frequency performance

Superior properties of this construction makes them ideal for high voltage applications. Film / foil capacitors are also ideal for high peak currents and high pulse applications.

Downside of film / foil construction

• Size of film / foil capacitors are much larger than metallized film capacitors.
• Minimum two layers of film necessary for winding the capacitor.
• High Inductance associated due to coiled winding. Extended foil is used to get low inductance capacitors.
• Life smaller than metallized film, as a single failure at a weak spot makes the entire capacitor fails.

Metallized Film Capacitors

The metal electrode foil on conventional capacitors is replaced by an extremely thin layer of metal deposited directly on plastic film trough a vacuum deposition process. This eliminates the thickness and volume occupied by metal electrode. The film metallization has following characteristics.

Metal deposition is extremely thin, and cannot be measured by conventional methods, and a method of resistivity measurement in ohm /unit area is adopted (which is characteristic of metal deposited). The deposit has very limited current carrying capacity compared to plain metal foil. It generates high heat in case of any local heating due to film breakdown.

Whenever a short circuit is developed at weak dielectric spot in film, either during manufacture or in service due to surges / impulses, the high current that flows through the fault is enough to evaporate the metal deposit at and around the spot almost instantly. The evaporated metal is thrown around and away from the fault location, and the spot gets isolated from metal layer.

With the weak spot thus isolated, capacitor starts working normally again, after having cleared the spot. This process of healing the capacitor during a fault is called ‘Self-healing’. This is the main benefit of metallized film capacitor. The area thus isolated during self-healing incident is so miniscule that capacitor value is practically unaffected.

For good clearing and evaporation of metal, sufficient energy must be available at the point of fault, and during the process of clearing, there is a small dip in voltage (in micro volts) across capacitor terminals.

In service, actual voltage across capacitor is much below rated voltage, and ambient temperatures are much below rated temperature most of the time. Hence, as a rule, metallized film capacitors should serve well in service for long time, and self-healing incidents in service are very occasional.

General construction of metallized film capacitor is as below.

Self-healing rectifies any defective spots during the manufacturing process of capacitors. This enables a single layer of plastic film to be used in winding (in place of minimum two in film/foil construction). This greatly reduces capacitor volume significantly.

Benefits of Metallized Film capacitors

1. Metal electrode thickness is eliminated.
2. Only one film layer can be used. So one layer of film also is eliminated.
3. Higher working dielectric stresses can be used in capacitor design.
4. Direct contact of film all along the length makes the capacitor non-inductive.
5. In case of undue impulse or overvoltage, if a fault develops, it gets cleared by self-healing, and capacitor is restored to its healthy condition.
6. In case of major fault, the metal layer near the end contacts also gets evaporated, and capacitor fails in open circuit mode. There is no danger of short circuit and corresponding hazard on this account.
7. As a result of above, capacitor life is substantially extended.
8. Size is much smaller than film/foil types, and costs of capacitor is much lower.

As a thumb rule, metallized capacitors reduce the size of capacitor by 20% to 60% depending on capacitor value for same voltage rating, even with similar voltage stress, mainly due to reduction in electrode volume.

Downside of Metallized construction

1. Resistance of metallized layer electrode is much higher than pure metal foil electrode. This increases loss factor compared to metal / foil capacitor.
2. Current carrying capacity of metallized layer is far lower than metal foil. This limits use of metallized capacitors where very high surge / impulse currents are to be faced frequently in service.
3. Every self-healing introduces a non-metallized area on film, reducing effective electrode area and reducing capacitance value, however small. Further, the isolated area has small amount oxidized surface due to metal clearing process, forming an area of lower insulation resistance, thereby adding to capacitor loss.
4. Over time, with successive self-healing incidents will cause reduction in capacitance value, and increase in loss factor. At some stage, these two will degrade the capacitor to a level as to be unacceptable, requiring capacitor replacement.
5. In case of uncontrolled excessive self-healing incidents due to some manufacturing fault, the self-healings can develop multiple low resistance paths, and cause burning of capacitor due to excessive eating.
6. In case, due to any reason, sufficient energy s not available at fault location for evaporation of metal layer, incomplete evaporation will degrade the capacitor, or even cause failure by burning.
7. Hugh frequency performance for AC does is much inferior to film / foil capacitor.

Sometimes, a separate film with double metallization (metallized on both sides) is used as electrode. Here the metallized film replaces metal foil in film / foil construction. This gives the benefit of metallized capacitor by way of self-healing and long life, and higher impulse and current rating of film / foil type.

In case of self-clearing of a defect, only one side of electrode (double metallization) gets evaporated, and the second side remains intact. Therefore, reduction in capacitance is just half of that observed in single side metallization. This gives increased capacitance stability.

The construction also allows thinner dielectric film to be used since the self-healing allows optimum film thickness to be used. This increases volumetric efficiency, makes size smaller than regular film / foil capacitor, and reduces the costs.

To summarize

Metallized film capacitors, using different types of film materials, are common in several applications because of their self-healing properties, small size, long life and economics. In low voltage small capacitors below 0.1 μF, there is not much economical or size advantage, and one has to choose depending on circuit requirements.

However, in cases where large currents are involved, particularly high pulse / surge current applications, as also many high frequency applications, metal foil / film construction capacitors are preferred. Foil / film capacitors have distinct advantage of lower loss factor (Tan delta).