1, film capacitor

A film capacitor is a capacitor whose core part is composed of a metal foil (as an electrode) and one or more layers of extremely thin plastic film (as a dielectric) which are alternately stacked or wound. The electrode is closely fitted with the dielectric film and maintains insulation between each other to form a large number of parallel plate capacitor units. These films can be polyester (PET, such as polyethyl ester), polypropylene (PP), polystyrene (PS), polycarbonate (PC), or other high-performance polymer materials. The overall shape of the capacitor is usually cylindrical or flat laminated, packaged in a suitable housing for easy installation and protection of the internal components. Widely used in electronics, home appliances, communications, electric power, electrified railway, hybrid electric vehicles, wind power, solar power and other industries.

Performance characteristics

• High stability: the dielectric material performance of the film capacitor is stable, the temperature coefficient is small, the impact of the environment is small, and the capacitance value changes little after long-term use.

• Excellent frequency response: The thin film capacitor has a low loss factor and a high self-resonant frequency, which is suitable for filtering, coupling and tuning applications in high-frequency circuits.

• Strong voltage resistance: the film capacitor can withstand high DC bias, and there is no polarity limit, suitable for high voltage pulse or inverter and other occasions.

• Long life: Since there is no electrolyte, the film capacitor has almost no aging phenomenon, and the service life is long.

Working principle

The working principle of thin film capacitors is based on the charge separation principle, that is, when a voltage is applied, the charge forms opposite charge accumulation on the surface of the two electrodes, forming an electric field, thereby storing energy. Because of its structural characteristics, film capacitors are usually packaged by winding or lamination, which has extremely high accuracy and flatness, making the capacitance value stable and the temperature coefficient small.

The role of film capacitors

Filtering and decoupling:

Power filter: At the power input, thin film capacitors can eliminate high-frequency noise and pulse interference on the power line, providing a cleaner DC voltage. They are used in EMI (Electromagnetic interference) filtering circuits to reduce the impact of grid noise on electronic devices, while also reducing the electromagnetic interference of devices to the external environment.

Signal filtering: In signal processing circuits, thin film capacitors are used to filter out unwanted high-frequency components, smooth signal waveforms, and maintain signal integrity. They often appear in anti-aliasing filters, low-pass filters and high-frequency attenuation networks of audio, video and data communication systems.

Energy storage and transient response:

Storage buffer: Thin film capacitors can store a large amount of charge in a short period of time, providing instantaneous energy to the circuit to help cope with load mutations or transient power events, such as in switching power supplies, pulse power systems as energy storage components.

Power factor correction (PFC) : In high-power electronics, thin film capacitors are used to improve power factor, reduce grid reactive power requirements, and increase energy efficiency.

Resonance and oscillation:

Resonant circuit: Thin film capacitors and inductors are combined to form LC resonant circuits, which are used to generate, select or eliminate signals of a specific frequency. In RF (radio frequency) circuits, LC filters, tuned circuits, oscillators, wireless communication devices, the high Q value (quality factor) and stable frequency characteristics of thin film capacitors make them ideal resonant components.

Timing and control: In the clock generator and oscillator circuit, the film capacitor and the crystal or ceramic resonator jointly determine the oscillation frequency of the circuit to provide a stable clock signal for the system.

Signal connectivity and isolation:

Coupling and decoupling: In the analog signal path, the film capacitor is used to couple the signal, transfer the AC component, while preventing the transmission of the DC component, and realize the isolation between the circuits. This helps prevent DC bias interactions between different circuits and ensures clear signal transmission.

High frequency signal transmission: Thin film capacitors have good high frequency response and are used in high frequency signal links to ensure that signals are transmitted quickly, accurately and without distortion between different modules.

Protection and stability:

Overvoltage protection: In some circuits, thin film capacitors act as surge absorbing elements, absorbing and dissipating transient overvoltages to protect sensitive electronic components from voltage spikes.

Voltage stability: In the voltage regulator circuit, the film capacitor is paired with the voltage regulator to provide transient current support and help maintain the stability of the output voltage.

Resistance-capacitance buck circuit:

In some low-cost, simple power supply circuits, film capacitors (such as CBB series capacitors) are connected in series with resistors to form a resistance-capacitance step-down circuit, which reduces the AC mains voltage to a suitable DC voltage level for driving small power loads, such as LED lighting and small controllers.

How to measure the film capacitance

Measuring the quality of thin film capacitors usually involves a series of steps and methods to assess their electrical properties, physical state and compliance with nominal parameters. The following is the detailed operation process:

Step 1: Check appearance

1. Check the external package: Observe the capacitor housing for cracks, deformation, leakage, corrosion, burn marks and other abnormal conditions, which may be signs of internal failure or improper use.

2. Label information verification: Confirm that the rated voltage, capacity, temperature rating, withstand voltage value and other parameters marked on the capacitor are clearly visible and consistent with the expected use requirements.

Step 2: Resistance method for preliminary detection

Use the resistance gear of the multimeter (such as high resistance gear, such as R×10kΩ or higher, depending on the capacitor capacity) to make a preliminary judgment:

1. Short circuit or open circuit check:

   • Set the multimeter at the appropriate resistance gear and short connect the pen to confirm that the meter is functioning properly.

   • Contact the two pins of the capacitor to see if the watch hands swing quickly and return infinity (∞). If the pointer does not move or always maintains a fixed value, it may indicate a short circuit or open circuit of the capacitor.

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2. Leakage check:

   • If the watch hand is not completely returned to zero after swinging, but stays in a non-zero position, it means that there is leakage phenomenon in the capacitor, and the larger the swing, the more serious the leakage degree.

Step 3: Capacitance measurement

Use the capacitance measurement file of a capacitance meter or multimeter (if available) for accurate measurements:

1. Set the measuring gear: Select the appropriate measuring range according to the nominal capacity of the capacitor to avoid the loss of accuracy due to too large range or too small range resulting in reading overflow.

2. Connect the capacitor: Follow the directions of the capacitor meter or multimeter to connect the pin of the capacitor correctly.

3. Read value: Record the displayed capacitance value and compare it with the rated capacity marked on the capacitor housing. Under normal circumstances, the measured value should be within a certain tolerance of the nominal value (such as ±20% or the tolerance specified by the manufacturer).

Step 4: Withstand pressure test (optional)

For capacitors in critical applications or where a voltage resistance problem is suspected, a dedicated capacitor voltage tester can be used to test:

1. Set the test voltage: According to the rated voltage of the capacitor, set the appropriate test voltage, usually 1.5 times the rated voltage or more (but not more than the maximum withstand voltage of the capacitor).

2. Perform the test: Apply the voltage step by step and observe the current or voltage reading according to the instrument operating procedures. If the current suddenly increases or the voltage drops sharply, it may indicate that the capacitor has a breakdown or serious leakage.

Step 5: ESR (Equivalent Series Resistance) measurement (optional)

For applications requiring low losses, ESR meters or devices with ESR measurement capabilities can be used to detect:

1. Set the measurement mode: Select the appropriate ESR measurement gear or frequency (usually 100kHz or the manufacturer's recommended frequency).

2. Connect capacitors: Connect capacitors according to instrument instructions.

3. Read the ESR value: Record the displayed ESR value and compare it to the specifications or expected requirements provided by the manufacturer. An excessively high ESR may indicate that the capacitor has degraded performance or is unsuitable for use in circuits requiring low impedance.

Step 6: Temperature Coefficient (TC) or frequency response test (Advanced application)

For capacitors that need to maintain stable performance over a specific temperature range or wide band, more specialized testing may be required to verify their temperature coefficient (TC) or frequency response characteristics, which is usually done in a professional laboratory environment.

2, electrolytic capacitor

An electrolytic capacitor is a capacitor whose positive electrode is usually made of a metal foil such as aluminum or tantalum, and an insulating oxide film (such as alumina or tantalum pentoxide) is formed on its surface as a medium. The negative electrode consists of a conductive material, an electrolyte (which can be liquid or solid), and other auxiliary materials. The electrolyte generates ion migration under the action of electric field, forming a double electric layer structure to store charge. Electrolytic capacitors have obvious polarity, and must be connected strictly in accordance with their positive and negative electrode identification when used, and cannot be reversed, otherwise it may lead to capacitor damage, leakage or even explosion. Its characteristics include large capacitance and relatively low cost, and it is widely used in a variety of electronic circuits requiring large capacity capacitors, such as power filter, signal coupling, energy storage and transient response.

Electrolytic capacitor symbol

Electrolytic capacitive action

Electrolytic capacitors play a variety of important roles in electronic circuits, and their main roles include:

Power filter:

• In the power supply circuit, the electrolytic capacitor is mainly used to filter out the AC component (ripple) in the rectified fluctuating DC voltage and convert it into a relatively smooth DC voltage. By absorbing voltage fluctuations and noise, electrolytic capacitors help to improve the quality of the power supply, ensuring a stable, pure power supply for subsequent circuits.

Energy storage and transient response:

• Due to their large capacity characteristics, electrolytic capacitors are often used as energy storage elements, storing electrical energy and releasing it quickly when needed. In switching power supplies, inverters, motor drives and other systems, electrolytic capacitors can provide instantaneous power support and improve the dynamic response of the system, especially to maintain the output voltage stability when the load changes.

Coupling and decoupling:

• In the signal processing circuit, the electrolytic capacitor can be used as a coupling capacitor to transmit the AC signal and prevent the transmission of the DC component, so as to achieve effective isolation between different circuit modules. This helps to prevent the DC operating points of each part of the circuit from influencing each other and ensure clear signal transmission. At the same time, electrolytic capacitors are also used for decoupling to reduce the interference of power supply noise on sensitive circuits and ensure signal quality.

Low-pass filtering and anti-interference:

• Because electrolytic capacitors exhibit a large impedance to high-frequency signals, they are often used as part of a low-pass filter, allowing low-frequency signals to pass through while suppressing high-frequency noise. In audio, communication, control and other systems, electrolytic capacitors help to filter out unnecessary high-frequency interference and maintain signal integrity.

Time constant setting and delay:

• In some circuits, the electrolytic capacitor and the resistor form the RC (resistance-capacitance) time constant circuit, which is used to set a specific delay or control the signal rise/fall rate, such as in flip-flop, timer, oscillator, etc.

Peak current limitation and Surge absorption:

• When the power supply starts at the moment or the circuit has a transient high voltage, the electrolytic capacitor can absorb and temporarily store excess electrical energy, limit the peak current flowing into the subsequent circuit, and prevent the overcurrent from causing damage to the components.

Pulse power supply and pulse load support:

• For occasions that need to provide short, high-intensity current pulses, such as flash circuits, pulsed power amplifiers, etc., electrolytic capacitors can store electrical energy in advance and then release it quickly in a short period of time to provide the required pulse current.

Power decoupling and bypass:

• Near the integrated circuit (IC) power supply pins, electrolytic capacitors are used for power supply decoupling to reduce the influence of high-frequency noise on the power line on the internal operation of the IC. At the same time, they are also used in bypass capacitors to provide a low-impedance charge source to the IC nearby, ensuring the transient current requirements during high-speed switching.

3, the difference between film capacitors and electrolytic capacitors

A. Structure and medium: The film capacitor is made of metal film or semiconductor film and insulating film (such as polyester, polypropylene, etc.) alternately laminated or wound, and the medium is a solid film; The electrolytic capacitor is composed of a metal foil (oxide film as a medium) and an electrolyte (liquid or solid).

b. Polarity: the film capacitor is non-polar and can apply voltage bidirectionally; Electrolytic capacitors have polarity, and must be correctly connected according to the positive and negative electrode labels.

c. Capacitance: under the same volume, the capacitance of electrolytic capacitors is usually much larger than that of thin film capacitors.

d. Stability: thin film capacitor capacitance value is high stability, good temperature characteristics, low ESR; Electrolytic capacitor Capacitance value is greatly affected by temperature and aging, and ESR is relatively high.

e. Operating temperature and life: film capacitors adapt to a wider operating temperature range, long life; Electrolytic capacitors (especially liquid electrolyte types) are limited by temperature, and the electrolyte may evaporate at high temperatures for a long time, affecting the life.

f. Application: Thin film capacitor is suitable for high frequency, high stability, high precision circuits, such as filtering, timing, oscillation, power decoupling, etc. Electrolytic capacitors are often used in power supply filtering, energy storage, low-frequency coupling, decoupling, etc., especially for occasions where large capacitance values are required.

4. Summary of differences

Thin film capacitor and electrolytic capacitor, as the two main forces in the capacitor family, have their own unique structure, working principle, performance characteristics and application scenarios. Thin film capacitors are widely used in high frequency electronic equipment, precision instruments and high voltage power systems for their excellent stability, high frequency response and long life. Electrolytic capacitors, with their advantages of large capacity, low cost and low self-inductance, have become the first choice in the fields of power filtering, audio equipment and power decoupling. In the actual engineering design and selection, the advantages and disadvantages of the two should be rationally weighed according to the specific application needs, cost budget, working environment and other factors to make the most suitable capacitor choice.