Capacitors are passive electronic components that store electrical energy in an electric field between conductors. When voltage is applied across the conductive plates of a capacitor, electric charge accumulates on the plates, getting stored in the electric field generated between the plates. Polypropylene capacitors harness the high dielectric strength and low dielectric losses of polypropylene polymer to construct the capacitor's dielectric.

Compared to other plastic film dielectrics like polyester and polystyrene, polypropylene has superior high-frequency characteristics. This makes polypropylene film capacitors well-suited for high-frequency applications such as radio frequency (RF) tuning circuits, bypass/coupling applications, snubbers, and timing circuits. They also see usage for filtering applications in power supplies due to their self-healing properties.

The structure of a polypropylene capacitor consists of a small sheet of PP film metalized with thin layers of aluminum, zinc, or alloy on both sides to serve as conductive plates. The metalized PP film is layered into a winding, inserted between external contacts, and encased in an insulating material to create the final capacitor component.

II. Properties and Characteristics of Polypropylene Capacitors

Here are some of the fundamental properties and characteristics of polypropylene capacitors:

Temperature Stability

Polypropylene capacitors exhibit excellent temperature stability from -55°C to +125°C for Class 1 dielectrics and from -55°C to +105°C for Class 2 dielectrics. This high working temperature range allows their use in demanding environments. The capacitance deviation over temperature is very low, from -5% to +5% over the rated temperature range.

High DC Voltage Capability

Polypropylene film has a high dielectric strength, allowing polyprop capacitors to operate at DC working voltages of up to 1,000V. Some specialized versions can work up to 30,000 volts DC. This makes them suitable for applications requiring high-voltage operation.

Low Leakage Current

Due to the highly insulating properties of polypropylene, these capacitors have very low leakage current levels, even at elevated voltages and temperatures. The typical leakage current is less than .01CV or 1μA, whichever is smaller.

Self-Healing Properties

Polypropylene capacitors exhibit a degree of self-healing. Defects such as minor internal shorts can often heal themselves, extending the capacitor's lifespan. This improves overall reliability.

High-Frequency Capability

Polyprop capacitors have low inductance and resistance properties, allowing them to perform well at high frequencies up to several hundred megahertz.

Long-Service Life

Properly applied polypropylene capacitors can achieve service lifetimes from 50,000 to 100,000 hours before failure. Their margin of safety allows prolonged exposure to applied working voltages and temperatures.

Low Dissipation Factor

Polypropylene has an inherently low dissipation factor, meaning it has low dielectric losses and higher efficiency. The dissipation factor of polypropylene film is below 0.001 at 1kHz at 25°C. This aids in low heat buildup.

Frequency Stability

Polyprop capacitors also exhibit good capacitance stability across a wide frequency range, varying less than 2% from 120Hz to 1MHz at room temperature. This makes them suitable for both high and low-frequency AC applications.

Long Shelf Life

Properly stored polypropylene film and foil capacitors can have a ten-year shelf life. They experience prolonged aging compared to other dielectrics. Their electrical parameters change over time in storage between uses.

Vibration Resistance

With their rugged rolled or stacked film and foil construction, these capacitors can handle vibration levels of up to 20G without damage or significant changes in electrical properties. They work well in mechanically demanding environments.

Pulse Handling Ability

Polyprop capacitors withstand high rates of voltage rise (dv/dt), making them appropriate for pulsed power and snubber applications. Certain high-current types handle peak pulse discharges into low impedances.

Safety

They contain no hazardous or toxic compounds like PCBs. Polypropylene dielectric material is biologically inert, meaning polyprop capacitors pose no health or environmental risks even if damaged.

Wide Capacitance Range

Available in capacitance values from a few picofarads all the way up to several microfarads. Unique designs go up to hundreds of microfarads. This allows circuit designers flexibility.

Moisture Resistance

The polypropylene dielectric provides excellent resistance to moisture and environmental contamination. Polyprop caps function well in high humidity conditions.

Small Size

Polypropylene's high dielectric strength allows the capacitors to achieve high capacitance values in a small package. This makes them useful for miniaturized modern circuit designs.

As such, polypropylene capacitors offer an optimal combination of electrical characteristics that make them versatile and reliable components for various electric circuit applications. Their properties satisfy the requirements of many demanding AC and DC uses across consumer, industrial, automotive, and specialized markets. Polyprop caps provide design engineers with a robust film dielectric capacitor option.

III. Advantages of Using Polypropylene Capacitors

Polypropylene capacitors, also known as PP capacitors, are a popular type of capacitor used in many electronic devices and circuits. They have several key advantages that make them suitable choices in various applications.

Durability

One of the main advantages of polypropylene capacitors is their durability. The plastic polypropylene film used as the dielectric in these capacitors is powerful and flexible. PP capacitors can withstand mechanical stress, vibrations, and extreme temperatures without damage or significant degradation. They can last for many years, even in demanding conditions.

Temperature Stability

Polypropylene capacitors maintain their electrical properties over a wide temperature range. Their capacitance varies from -55°C to +125°C, making them an exceptionally stable choice for circuits operating in hot or cold environments. The dielectric material has a very high melting point, further improving thermal endurance.

Low Leakage Current

Polypropylene film has excellent insulating properties and exhibits very low conductivity. This results in a shallow leakage current in PP capacitors that use this film. Leakage current contributes to energy loss and self-heating, so a low value here improves efficiency and longevity.

Self-Healing Properties

Minor defects or thin spots in the dielectric film can occasionally occur in capacitors over time or due to damage. PP capacitors display a degree of self-healing - when a localized breakdown occurs, the materials redistribute to isolate the fault. This prevents catastrophic failures and extends the usable lifespan.

High-Frequency Response

With low dielectric losses even at high frequencies, narrow tolerances, and low parasitic inductance, polypropylene capacitors maintain impedance and perform well into the hundreds of kilohertz or low megahertz range. This makes them suitable for filtering, tuning, and matching circuits.

Moisture Resistance

Polypropylene film is hydrophobic, meaning it repels water. This gives PP capacitors excellent moisture and humidity resistance. They can be used in wet environments or outdoor applications while maintaining stable electrical properties, reducing the risk of short circuits due to condensation or water ingress.

Low Equivalent Series Resistance

A capacitor's equivalent series resistance (ESR) includes all internal losses and impedances. A low ESR ensures an efficient capacitor. Polypropylene's very low dielectric losses, good conductivity electrodes, and benign temperature coefficient of resistance result in excellent ESR performance.

RoHS Compliance

Polypropylene and other materials used in PP capacitors do not contain hazardous substances banned under the Restriction of Hazardous Substances (RoHS) directive. This makes them suitable for new electronic devices and "green" applications.

IV. Applications of Polypropylene Capacitors

Some of the most common applications of polypropylene capacitors are:

Power Supplies

One of the most common uses of polypropylene capacitors is in power supply units for electronic devices. They are often used as filtering capacitors to convert the pulsating DC output from rectifiers into smooth DC. Polypropylene offers low dielectric losses even at high frequencies, making them well-suited for switch mode power supplies and other high current applications. Their self-healing property allows them to fix small faults caused by momentary voltage surges.

Timing/Snubber Circuits

The low dielectric absorption and high allowable frequency range of polypropylene capacitors allow them to perform well in timing circuits. They can also be used in snubber applications to suppress voltage transients in inductive loads and improve electromagnetic compatibility in electronic devices. Their stability over temperature fluctuations makes them reliable in these applications.

Audio Circuits

Polypropylene capacitors are valued for their musicality, transparency, and neutral sound in audio applications like crossover networks and noise filters. The low dielectric losses, high purity, and stability of polypropylene film make it well-suited for high-fidelity audio circuits that demand minimal signal coloration.

Medical Devices

Reliability and stability over long service life are critical for applications like defibrillators and patient monitoring equipment. Polypropylene capacitors meet the safety requirements for medical devices, given their self-healing ability, flame-retardant polypropylene film, and extended lifetime at temperature extremes.

High-Temperature Applications

With their ability to operate at temperatures up to 105°C, polypropylene capacitors can reliably perform in high-temperature environments found in automotive under-hood electronics and geophysical equipment used for well-logging. The low moisture absorption of the polypropylene film also makes it suitable for these humid conditions.

V. Conclusion

A polypropylene capacitor is a film capacitor that utilizes polypropylene film as the dielectric between its conductive plates. Polypropylene capacitors have several desirable characteristics, including high-temperature tolerance, low dielectric losses, exceptionally low leakage current, high insulation resistance, and high capability to withstand voltage transients, which make them well-suited for applications requiring stable and reliable capacitance across a broad range of operating temperatures and frequencies. Their self-healing property allows them to operate reliably even after experiencing dielectric breakdown events. Polypropylene’s inherent properties result in capacitors with excellent performance parameters that suit them in demanding electrical and electronic applications.