Overview of Film Capacitors

A film capacitor is a type of capacitor that uses a thin plastic film as the dielectric material. These capacitors are widely used in various electronic circuits due to their stability, reliability, and cost-effectiveness. Film capacitors are available in a variety of types, and they are often used for applications that require low to moderate capacitance values and relatively high voltages. The construction of a film capacitor consists of two conductive layers (often metal) separated by a dielectric film. This construction offers several advantages, including low leakage current and excellent insulation properties.

What are film capacitors?

Film capacitors are capacitors that utilize a thin plastic film as the dielectric. This film is crafted to be extremely thin through an advanced film drawing technique. After the film is produced, it can be metallized or left untreated based on the desired characteristics of the capacitor. Electrodes are subsequently attached, and the entire assembly is housed in a casing that shields it from environmental influences. They are widely used in various applications due to their stability, low inductance, and affordability. There are numerous types of film capacitors, such as polyester film, metallized film, polypropylene film, PTFE film, and polystyrene film. The primary distinction among these capacitor types lies in the dielectric material used, and the appropriate dielectric must be selected based on the specific application.

For instance, PTFE film capacitors are heat-resistant and are employed in aerospace and military applications, whereas metallized polyester film capacitors are utilized in scenarios that demand long-term stability at a relatively low cost. If cost is a more significant consideration than performance, cheaper plastics are used.

Panasonic Surface Mount Film Capacitors

Panasonic Surface Mount Film Capacitors are designed to offer stable capacitance and temperature characteristics. They have tight tolerances, low ESR, low dissipation factors, and are free from shock noise, piezoelectric effects, and audible noise. These characteristics make them ideal for applications like DC blocking and filtering, PLL circuits, audio equipment, DC/DC converters, and backlighting.

In addition to the surface-mount options, Panasonic also offers a wide range of leaded film capacitors. These include Metallized Polyester, Metallized Polypropylene, PPS Film, AC Capacitors, and X Caps. Leaded Film Capacitors are commonly used as X Caps in AC/DC circuits and are suitable for various general electronic applications.

Film Capacitors

Characteristics

Film capacitors are highly popular due to their exceptional features. These capacitors are non-polarized, making them ideal for AC signal and power applications. Film capacitors can be manufactured with extremely precise capacitance values, and they maintain these values over a longer period compared to other types of capacitors. This implies that the aging process is generally slower than in other capacitor types, such as electrolytic capacitors. Film capacitors boast a long shelf and service life, and they are highly reliable, with a very low average failure rate. They have low ESR (Equivalent Series Resistance), low self-inductance (ESL), and consequently very low dissipation factors. They can be designed to withstand voltages in the kilovolt range and can deliver very high surge current pulses. A specialized category of film capacitors, known as power film capacitors, is available. This class of capacitors can handle reactive power exceeding 200 volt-amperes. These capacitors may feature special screw-type terminals capable of withstanding high currents. Screw-type terminals replace soldered joints because power film capacitors sometimes need to be replaced in the field. Unfortunately, their superior electrical properties and stability come at a cost. Film capacitors are bulkier than their electrolytic counterparts, which means that limited SMT (Surface-Mount Technology) packages are available. They can also burst into flames if overloaded, a characteristic somewhat common among different capacitor types.

Construction and Properties

Film capacitors are constructed using a thin dielectric film, which may or may not be metallized on one side. The film is incredibly thin, with a thickness of less than 1 µm. After the film is drawn to the desired thickness, it is cut into ribbons. The width of the ribbons depends on the capacitance of the capacitor being produced. Two ribbons of film are wound together into a roll, which is often pressed into an oval shape to fit into a rectangular case. This is important because rectangular components save valuable space on the printed circuit board. Electrodes are added by connecting each of the two electrodes to one of the films. A voltage is applied to eliminate any imperfections using the self-healing property of film capacitors. The case is then sealed using silicon oil to protect the film roll from moisture and dipped in plastic to create a hermetic seal.

Typical film capacitors have capacitances ranging from below 1 nF to 30 µF. They can be manufactured with voltage ratings as low as 50 V, up to above 2 kV. They can be produced for use in high-vibration automotive environments, high-temperature environments, and high-power applications. Film capacitors offer low losses and high efficiency while providing a long service life.

Applications for Film Capacitors

Power film capacitors are utilized in power electronics devices, phase shifters, X-ray flashes, and pulsed lasers, while the low-power variants serve as decoupling capacitors, filters, and in A/D converters. Other notable applications include safety capacitors, electromagnetic interference suppression, fluorescent light ballasts, and snubber capacitors.

Lighting ballasts are essential for the proper starting and operation of fluorescent lights. When a ballast is faulty, the light will flicker or fail to start properly. Older ballasts used only an inductor, a solution that provided a poor power factor. New designs use a switched power supply that relies on film capacitors for power factor correction.

Snubber capacitors are protective devices that dampen or “snub” inductive kickback voltage spikes. These circuits often use film capacitors because of their low self-inductance, high peak current, and low ESR, which are all critical factors in a snubber design. Polypropylene film capacitors are most often used in this type of circuit. Snubbers are used in many areas of electronics, especially power electronics in devices such as flyback DC-DC converters and others.

Film capacitors can also be used in a more conventional manner as voltage smoothing capacitors, in filters, audio crossovers. They can be used to store energy and release it in a high-current pulse when needed. High-current electrical pulses are used to power pulsed lasers or generate lighting discharges.

Features of Film Capacitors

Film capacitors are popular for their many advantageous features:

High Insulation Resistance

Due to the properties of the dielectric material (plastic film), film capacitors typically have high insulation resistance, which makes them less prone to leakage current. This feature is essential for ensuring long-term performance and reliability.

Stable Performance

Film capacitors are known for their stability over time. Unlike electrolytic capacitors, which can degrade due to the chemical nature of their electrolyte, film capacitors maintain consistent performance throughout their lifespan, making them suitable for critical applications where reliability is essential.

Low Losses

Film capacitors exhibit low losses, meaning they have minimal power dissipation, which results in improved energy efficiency, particularly in high-frequency circuits.

High Voltage Capability

Depending on the type and size of the film capacitor, they can withstand higher voltages compared to other capacitors, such as ceramic or electrolytic types. This makes them ideal for use in power electronic applications.

Non-polarized

Unlike electrolytic capacitors, film capacitors are generally non-polarized, meaning they can be connected in either direction in a circuit. This adds flexibility in design and application.

Temperature and Voltage Stability

Film capacitors are less affected by temperature and voltage fluctuations, contributing to their reliability in demanding environments.

Types of Film Capacitors

Film capacitors are categorized based on the material used for the dielectric. Some common types include:

Polyester (PET) Film Capacitors

These are the most commonly used type of film capacitor. They are known for their affordability and reasonable performance, particularly in low-voltage applications. PET capacitors are often used in consumer electronics, audio equipment, and other general-purpose circuits.

Polypropylene (PP) Film Capacitors

These capacitors offer better performance than polyester capacitors, with lower losses and higher voltage capabilities. Polypropylene capacitors are widely used in power supplies, audio circuits, and other applications where high performance and stability are required.

Polystyrene (PS) Film Capacitors

Known for their excellent accuracy and stability, polystyrene film capacitors are often used in high-precision applications such as RF (radio frequency) circuits, oscillators, and signal processing circuits. However, they are less common in modern designs due to their relatively larger size and sensitivity to temperature.

Polycarbonate (PC) Film Capacitors

These capacitors offer good electrical performance and stability. They are often used in high-temperature environments and industrial applications, although they have been largely replaced by polypropylene capacitors in many applications.

PVC (Polyvinyl Chloride) Film Capacitors

PVC film capacitors are generally used in applications requiring low capacitance values and good environmental resistance. They are suitable for use in power supplies, industrial machines, and automotive applications.

Paper-Film Capacitors

While less commonly used today, paper-film capacitors were historically used in older electronic devices. They offer decent capacitance but have lower stability and durability compared to other types of film capacitors.

Applications of Film Capacitors

Due to their excellent performance characteristics, film capacitors are used in a wide variety of applications. Below are some common uses:

Power Supplies

Film capacitors are often used in power supply circuits, including both AC and DC filtering, voltage smoothing, and energy storage. Their ability to handle high voltages and provide low ESR (Equivalent Series Resistance) makes them ideal for use in these applications.

Audio Equipment

In audio applications, film capacitors are preferred for coupling and decoupling signals. They help in maintaining signal integrity without introducing significant distortion. Polypropylene film capacitors, in particular, are popular for high-fidelity audio circuits due to their low loss and stable performance.

Motor Starters

Film capacitors are widely used in motor starter circuits for applications such as HVAC systems, pumps, and other industrial machines. Their ability to withstand high voltage and operate at high temperatures makes them suitable for these demanding environments.

Signal Processing

In communication equipment, RF circuits, and other signal processing systems, film capacitors are used for filtering, decoupling, and signal coupling applications. Their accuracy and stability are particularly beneficial in high-frequency applications.

Lighting Circuits

Film capacitors are often found in fluorescent lighting ballasts and in circuits for controlling lighting systems. They are used for filtering noise and providing energy-efficient operation.

Switching Power Supplies

In DC-DC converters and AC-DC power supplies, film capacitors provide energy storage, smoothing, and filtering functions. They help to ensure the stability of the power supply and improve the overall performance and efficiency of the system.

Automotive and Industrial Electronics

In automotive electronics and industrial control systems, film capacitors are used in applications such as power conditioning, noise filtering, and signal smoothing. Their durability and resistance to temperature fluctuations make them well-suited for such demanding environments.

High-Frequency Applications

Polypropylene and polystyrene film capacitors are commonly used in high-frequency applications, such as oscillators, filters, and tuning circuits in communication devices, due to their minimal losses and high precision.

Film Capacitors Use in Panasonic

AC Motor Use

Panasonic offers a product lineup suited for various applications, from low-voltage to high-voltage ranges, in both dry-type and oil-filled types.

Automotive, Industrial, and Infrastructure Use

These film capacitors are used in high-voltage and high-current applications. They are commonly employed as smoothers, filters, and snubbers in HEV/EV and industrial inverters.

Electronic Equipment Use

Panasonic Film Capacitors for electronic equipment offer low ESR, no shock or audible noise, and no piezoelectric effects. They provide stable capacitance and temperature characteristics with tight tolerances and low dissipation factors. Available in Surface-Mount (SMT) and radial leaded types, these include Metallized Polypropylene Class X2 Capacitors. These capacitors are reliable even in high-temperature environments and ensure high performance without generating shock noise, piezoelectric effects, or audible noise.

Film Capacitors for MLCC Replacement

Panasonic also provides many alternatives to MLCCs in their Film Capacitor product line. Some of the suggested series include ECW-U(C), ECW-U(X), ECH-U(X), ECH-U(C), ECW-U(V16), and ECP-U(A). These alternatives offer similar or improved performance characteristics compared to traditional MLCCs.

Conclusion

Film capacitors are essential components in modern electronics, offering a blend of stability, reliability, and versatility across a wide range of applications. Whether in power supplies, audio equipment, motor control, or high-frequency circuits, film capacitors continue to play a vital role in ensuring efficient and reliable performance in a variety of electronic systems. Understanding their features, types, and applications helps in selecting the right capacitor for the right job, ultimately improving the performance and longevity of electronic devices.

Frequently Ask Questions

What is a film capacitor used for?

PP film capacitors are used in high-frequency, high-power applications, like induction heating. They're also used in pulsed power energy discharge applications and as AC capacitors for electrical distribution. These capacitors can handle AC voltages up to 400 kV.

Are film capacitors good for audio?

A lot of people recommend using film capacitors for audio because they’re more stable when it comes to applied DC voltage. This makes them more accurate for things like filtering. After that, COG/NP0 capacitors are often a good choice because they maintain stable capacitance with temperature changes. But what about COG/NP0 capacitors with applied DC voltage?

What are the disadvantages of film capacitors?

One downside of film capacitors is that they're big and expensive. So, they're typically used in situations where ceramic capacitors can’t do the job, or in high-performance, high-accuracy applications.

What is the lifespan of a film capacitor?

For example, the MKP1848 film capacitor is rated for 100,000 hours at 70°C. On the other hand, a Rubycon BXC series wet electrolytic capacitor lasts about 12,000 hours at 105°C.

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