Film Capacitors
1. What are Film Capacitors?
Film capacitors are capacitors made of plastic film (such as polypropylene, polyester, etc.) as dielectrics and metal foil or metalized film as electrodes. Its internal structure usually adopts a winding or stacking process, and the electrodes and dielectric layers are arranged alternately. The external structure is formed into a cylindrical or rectangular structure by epoxy resin or plastic encapsulation.
2. What are the Main Types of Film Capacitors?
Depending on the dielectric material, film capacitors can be divided into the following types:
Polyester (PET) Capacitors: also known as polyethylene capacitors, low cost and good temperature resistance.
Polypropylene (PP) Capacitors: excellent high-frequency characteristics, low loss, suitable for high-precision circuits.
Polystyrene (PS) Capacitors: high stability, often used in precision instruments. Metalized film capacitors: electrodes using a metallization process, with self-healing characteristics and higher reliability.
3. What are the Core Characteristics of Film Capacitors?
Non-polarity: no need to distinguish between positive and negative poles, suitable for AC and DC circuits.
High Frequency Response: wide frequency response range, suitable for high-frequency signal processing.
Low Loss: extremely low dielectric loss, which can reduce energy loss.
High Insulation Impedance: small leakage current and good long-term stability.
4. Where are Film Capacitors Used?
Signal Processing: signal coupling, filtering, and bypassing in analog circuits.
Power Supply System: power supply noise suppression, filtering, and energy storage, such as UPS and switching power supplies.
Industrial Electronics: motor control, inverter, new energy equipment (such as solar drive).
Consumer Electronics: used to improve sound quality in high-end audio equipment.
5. Structural Characteristics and Processes of Film Capacitors
Electrode Material: metal foil or metalized film (such as aluminum and zinc), the latter can reduce volume and improve self-healing ability.
Packaging Form: common epoxy encapsulation, plastic shell, or metal shell packaging to meet different environmental requirements.
Process Optimization: metalized polypropylene film capacitors (such as MKP type) achieve thinner dielectric layers and improve capacity density through the evaporation process.
6. Precautions for using Film Capacitors
Installation Requirements: Strictly follow the polarity (if any), voltage, and temperature parameters to avoid damage caused by overload.
Quality Assurance: High-quality film capacitors must pass to withstand voltage, temperature rise, and life tests to ensure long-term reliability.
Film Capacitors FAQs
1) What are the basic characteristics of film capacitors?
Film capacitors use plastic film as dielectric, and have characteristics such as non-polarity, high insulation impedance, and low loss. They are suitable for high-frequency circuits and scenarios with high stability requirements. Its structure is usually composed of alternating layers of metal electrodes and plastic films (such as polyester and polypropylene).
2) How do film capacitors store electrical energy?
Its principle is based on the ability of electric fields to store charges. When voltage is applied, the two plates accumulate equal amounts of opposite charges to form an electrostatic field. The amount of charge satisfies the formula Q = U×C (C is the capacitance value). For parallel plate structures, the capacitance value is proportional to the plate area and the dielectric constant of the dielectric, and inversely proportional to the distance between the plates.
3) Can film capacitors pass alternating current?
Yes. AC voltage generates an alternating electric field. According to Maxwell’s displacement current theory (ε(∂E/∂t)), the changing electric field is equivalent to current, making the capacitor “conduct” alternating current. Capacitive reactance decreases with increasing frequency (capacitive reactance = 1/ωC), so film capacitors are often used in high-frequency circuits such as filtering and coupling.
4) What is the current carrying capacity of film capacitors?
The current carrying capacity depends on the material and design. For example, some types of polyester film capacitors can withstand specific pulse currents or continuous AC currents, but the specific manufacturer’s rated parameters (such as voltage and temperature range) must be referred to avoid overload. The effects of equivalent series resistance (ESR) and inductance (ESL) must also be considered in high-frequency applications.
5) What non-ideal factors are included in the equivalent model of film capacitors?
The equivalent model of actual capacitance includes:
Leakage Resistance (Rleak): reflects the insulation performance of the dielectric;
Equivalent Series Resistance (ESR): caused by electrode and dielectric loss;
Equivalent Series Inductance (ESL): high-frequency impedance caused by leads and structure;
Hysteresis Effect: charge recovery phenomenon after rapid charging and discharging.
6) How to select film capacitors?
Key parameters include:
Rated Voltage: must be higher than the maximum operating voltage of the circuit;
Capacitance Value and Tolerance: such as ±5% or ±10%;
Temperature Characteristics: different materials (such as polypropylene has better high-temperature resistance);
Frequency Response: low ESL models are required for high-frequency scenarios.
7) What are the differences between film capacitors and electrolytic capacitors?
Polarity: film capacitors are non-polar, while electrolytic capacitors are polar;
Dielectric Material: film capacitors use plastic film, while electrolytic capacitors use oxide film;
Life and Stability: film capacitors have longer life and better voltage and temperature resistance.