Eight basic points for capacitor selection
A capacitor is an electronic component that stores electrical energy by charging it. This article will provide a detailed introduction to the basic principle, classification, selection points, and purchase precautions of capacitors, helping everyone better understand and use capacitors.
The basic principle of capacitance is based on an electric field. It consists of two parallel plates separated by a layer of insulation material. When a voltage is applied between two boards, the insulating material in the middle generates an electric field, thereby storing electrical energy between the two boards. This process is the charging process of the capacitor. When the voltage is removed, the capacitor continues to maintain its charge through the electric field, and this process is called the discharge process of the capacitor.
Capacitors are classified according to their uses:
1. High frequency bypass: ceramic capacitors, mica capacitors, glass film capacitors, polyester capacitors, glass glazed capacitors.
2. Low frequency bypass: paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polyester capacitors.
3. Filtering: aluminum electrolytic capacitors, paper dielectric capacitors, composite paper dielectric capacitors, liquid tantalum capacitors.
4. Tune: ceramic capacitors, mica capacitors, glass film capacitors, polystyrene capacitors.
5. Low coupling: paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polyester capacitors, solid tantalum capacitors.
6. Small capacitors: metallized paper dielectric capacitors, ceramic capacitors, aluminum electrolytic capacitors, polystyrene capacitors, solid tantalum capacitors, glass glazed capacitors, metallized polyester capacitors, polypropylene capacitors, mica capacitors.
2. Different capacitors and characteristics
1. CBB capacitor (polypropylene)
The insulation resistance, loss, and frequency stability of CBB capacitors are better than those of polyester film capacitors, second only to polystyrene capacitors, but their heat resistance is better than theirs. Polypropylene film capacitors have the characteristics of low loss and high insulation, making them particularly suitable for use in direct current and pulsating circuits with alternating current components in color televisions. They have a withstand voltage higher than 400V, making them suitable for certain high and medium voltage circuits in color television circuits. The capacitance of this type of capacitor decreases with increasing temperature (i.e. negative capacity temperature coefficient), which is exactly the opposite of polyester capacitors.
Solid state capacitor is a type of capacitor without electrolyte, which uses a solid electrolyte as the insulating material in the middle. Its characteristics are large capacity, good stability, but high cost, suitable for high-frequency circuits. Solid state capacitors are generally used for filtering, oscillation, bypass, and other applications.
6. Ceramic capacitors
Ceramic capacitors are small capacitors that use ceramic materials as the insulation material in the middle. Its characteristics are small capacity, good stability, high temperature resistance, but high cost, suitable for high-frequency and high-temperature circuits. The capacity range of ceramic capacitors is 0.5pF to 100 μ F. The crystallization of unique materials and thin film technology caters to today's design philosophy of "lighter, thinner, and more energy-efficient". Ceramic capacitors are generally used in filtering, oscillation, timing, and other applications.
7. Supercapacitor
Supercapacitors have a capacity range of 0.022F to 70F, with extremely high capacitance values, hence they are also known as "gold capacitors" or "Faraday capacitors". The main characteristics are: ultra-high capacitance, good charging/discharging characteristics, suitable for energy storage and power backup. The disadvantage is low voltage resistance and narrow operating temperature range.
8. Monolithic capacitor
Monolithic capacitors are multilayer stacked ultra small capacitors made mainly of barium titanate ceramic material and sintered. The advantages are reliable performance, high temperature resistance, moisture resistance, and large capacity (with a capacity range of 1pF~1) μ F) Advantages such as low leakage current, but disadvantages include low operating voltage (withstand voltage below 100V). Widely used in resonance, bypass, coupling, filtering, etc. Commonly used ones include CT4 (low frequency) and CT42 (low frequency); CC4 (high frequency), CC42 (high frequency) and other series.
9. Polyester capacitor
Polyestercapacitorsarenonpolar capacitors made with polar polyester film as the medium and have a positive temperature coefficient (i.e., as the temperature increases, the capacitance increases). High temperature resistance, high pressure resistance, moisture resistance, and low price. Generally used in medium and low-frequency circuits. Commonly used models include the CL11, CL21, and other series.
10. Mica capacitor
Mica capacitors are made by using mica as the medium, spraying a layer of metal film (silver) on the surface of mica as the electrode, laminating according to the required capacity, and then impregnating and pressing it into a bakelite shell (or ceramic or plastic shell). The advantages include good stability, small distributed inductance, high accuracy, low loss, high insulation resistance, good temperature and frequency characteristics, and high operating voltage (50V~7kV). Generally used for signal coupling, bypass, tuning, and other purposes in high-frequency circuits. Commonly used series include CY, CYZ, CYRX, etc.
11. Paper capacitor
Paper capacitors are made by using thin capacitor specific paper as the medium, aluminum foil or lead foil as the electrode, winding and forming, impregnating, and packaging. Large capacitance (100pF~100) μ F) The working voltage range is wide, and the maximum withstand voltage value can reach 6.3 kV. Large volume, low capacity accuracy, high loss, and poor stability. Common series include CZ11, CZ30, CZ31, CZ32, CZ40, CZ80, etc.
12. Air variable capacitor
The electrodes of an air variable capacitor are composed of two sets of metal sheets. One group is fixed pieces, and the other group is moving pieces. Air is used as the medium between the moving pieces and the fixed pieces. When all the moving pieces are rotated into the fixed piece, their capacitance is maximum. Conversely, when all the moving pieces are rotated out of the fixed piece, their capacitance is minimum. There are two types of air variable capacitors: single connected and double connected. The advantages are easy adjustment, stable performance, and less prone to wear and tear. The disadvantage is its large volume. Applied to radios, electronic instruments, high-frequency signal generators, and communication electronic devices
Made by spraying a special mixture with a concentration suitable for spraying into a thin film, the medium is then sintered with a silver layer electrode to form a "single stone" structure. Its performance is comparable to that of mica capacitors, and it can withstand various climatic environments. Generally, it can work at 200 ℃ or higher temperatures, with a rated working voltage of up to 500V and a loss of tg δ 0.0005-0.008.
14. Ceramic capacitors
A ceramic dielectric capacitor with a through or pillar structure, one of its electrodes is a mounting screw. The lead inductance is extremely small, with good frequency characteristics and low dielectric loss. It cannot be made into a large capacity due to temperature compensation. Vibration can cause capacity changes, making it particularly suitable for high-frequency bypass.
1. Coupling:
The capacitor used in a coupling circuit is called a coupling capacitor, which is widely used in resistance capacitance coupling amplifiers and other capacitance coupling circuits to isolate direct current and facilitate communication.
2. Filtering:
The capacitor used in the filtering circuit is called a filtering capacitor, which is used in power filtering and various filtering circuits. The filtering capacitor removes signals within a certain frequency band from the total signal.
3. Decoupling:
The capacitor used in the decoupling circuit is called a decoupling capacitor, which is used in the DC voltage supply circuit of a multi-stage amplifier. The decoupling capacitor eliminates harmful low-frequency interconnections between each stage of the amplifier.
4. High frequency vibration damping:
The capacitor used in high-frequency damping circuits is called a high-frequency damping capacitor. In audio negative feedback amplifiers, this type of capacitor circuit is used to eliminate the high-frequency self-excitation that may occur in the amplifier.
5. Resonance:
The capacitor used in LC resonant circuits is called a resonant capacitor, which is required in both LC parallel and series resonant circuits.
6. Bypass:
The capacitor used in a bypass circuit is called a bypass capacitor. If a certain frequency band signal needs to be removed from the signal in the circuit, a bypass capacitor circuit can be used. Depending on the frequency of the removed signal, there are full frequency domain (all AC signals) bypass capacitor circuits and high-frequency bypass capacitor circuits.
7. Neutrality:
The capacitor used in a neutralizing circuit is called a neutralizing capacitor. In radio high-frequency and intermediate frequency amplifiers, as well as television high-frequency amplifiers, this neutralizing capacitor circuit is used to eliminate self-excitation.
8. Timing:
The capacitor used in a timing circuit is called a timing capacitor. In circuits that require time control through capacitor charging and discharging, a timing capacitor circuit is used, and the capacitor plays a role in controlling the magnitude of the time constant.
9. Points:
The capacitor used in an integrating circuit is called an integrating capacitor. In the synchronous separation circuit of potential field scanning, this integral capacitor circuit can be used to extract the field synchronization signal from the field composite synchronization signal.
10. Differential:
The capacitor used in differential circuits is called a differential capacitor. In trigger circuits, this differential capacitor circuit is used to obtain sharp pulse trigger signals from various types (mainly rectangular pulses) of signals.
11. Compensation:
The capacitor used in the compensation circuit is called a compensation capacitor. In the low-frequency compensation circuit of the socket, this low-frequency compensation capacitor circuit is used to enhance the low-frequency signal in the playback signal. In addition, there is a high-frequency compensation capacitor circuit.
12. Bootstrap:
The capacitor used in a bootstrap circuit is called a bootstrap capacitor. The commonly used OTL power amplifier output stage circuit uses this bootstrap capacitor circuit to slightly increase the positive half cycle amplitude of the signal through positive feedback.
13. Frequency division:
The capacitor in the divider circuit is called a divider capacitor. In the speaker divider circuit of a speaker, a divider capacitor circuit is used to make the high-frequency speaker work in the high frequency range, the mid-range speaker work in the mid frequency range, and the low-frequency speaker work in the low frequency range.
14. Temperature compensation:
Compensate for the insufficient adaptability of other components to temperature and improve the stability of the circuit.
15. Rectification:
Open or close the semi closed conductor switching element at the scheduled time.
16. Energy storage:
Store electrical energy for release when necessary. For example, camera flash, heating equipment, and so on. Nowadays, the energy storage level of some capacitors is close to that of lithium batteries, and the energy stored in one capacitor can be used for a phone for a day.
Choosing the right capacitor is a crucial step in circuit design. How should we choose a suitable capacitor for the circuit? Firstly, consider the following parameters:
1) Electrostatic capacity;
2) Rated withstand voltage;
3) Tolerance error;
4) The change in capacitance under DC bias voltage;
5) Noise level;
6) The type of capacitor;
7) Specifications of capacitors.
Secondly, the following key points should be followed during the selection process, for your reference only:
1. The frequency of use
If the operating frequency of a certain circuit is very high, exceeding MHz, and the signal strength of the circuit is weak, then stacked ceramic capacitors are the best choice. Although they are all filtering and energy storage charging and discharging, when the operating frequency is constant, it is necessary to consider whether the frequency characteristics of different types of capacitors match the operating frequency of the circuit, because different types of capacitors have their own suitable frequency range, and all capacitors increase with the increase of operating or testing frequency. The capacitance value will gradually decrease, and the loss will also gradually increase. The frequency response of a capacitor refers to its capacity and loss at different frequencies. Capacitors with appropriate frequency response should be selected based on the actual needs of the circuit.Forexample,ifthereishighfrequency signal processing in the circuit, it is necessary to choose capacitors with good frequency response to maintain the quality of the signal.
If the operating frequency is below the mid frequency range and there is a high requirement for consistency in the parameter values of capacitors at different temperatures, then choosing a solid tantalum capacitor may be more appropriate. Sometimes, you have to make trade-offs about their performance characteristics. First, you must understand which aspect of a certain capacitor is the reason you must choose it.
2. Requirements for environmental temperature changes
At present, solid tantalum capacitors have the best temperature characteristics among capacitors. Some high-voltage solid tantalum capacitors can achieve a capacity change rate of within -3-+5% in the temperature range of -55-+125 degrees. For aviation and aerospace circuits, capacitors must have excellent temperature characteristics to meet usage requirements. The temperature coefficient refers to the percentage change in capacitance with temperature. Capacitors with appropriate temperature coefficients should be selected based on the actual needs of the circuit. For example, if the working temperature of the circuit changes significantly, it is necessary to choose a capacitor with a smaller temperature coefficient to maintain stable performance of the circuit.
The ranking of temperature characteristics of capacitors from good to bad is roughly as follows: tantalum capacitors ≥ NPO type ceramic capacitors ≥ solid aluminum capacitors ≥ liquid tantalum capacitors ≥ mica capacitors ≥ stacked ceramic capacitors [MLCC] ≥ liquid aluminum capacitors.
3. Input power and output power size
When using mobile electronic products with very low power consumption and high operating frequency, even if the leakage current is too high and the ESR is low, it is generally difficult for the product to have quality problems. Unless the capacitor itself is scrap. When used in circuits with high input and output power, such as power filtering and discharge circuits, capacitors not only need to have lower ESR, but also must have very low leakage current, otherwise it will increase the probability of breakdown and the output power waveform cannot meet the requirements. Due to the varying volume and electrical capacity of different types of capacitors, it is necessary to select capacitors with sufficient capacity and withstand voltage based on output power requirements during design. The loss of capacitance refers to the loss of electrical energy generated when passing through a capacitor. Capacitors with smaller losses should be selected based on the actual needs of the circuit. For example, if there is high-frequency signal transmission in the circuit, it is necessary to choose capacitors with lower losses to reduce signal loss.
When used in filtering circuits, capacitors must withstand the thermal shock caused by AC voltage and current of a certain frequency and amplitude. At the same time, capacitors must withstand inevitable DC high voltage and high current surges at the moment of switching. When using capacitors in this circuit, it is necessary to choose capacitors with appropriate specifications and types. If you only consider that the DC withstand voltage is sufficient, it is far from enough. At the same time, you must consider that different capacitors have different ripple resistance capabilities. The ranking of the ripple resistance of capacitors is as follows:
MLCC ≥ coiled polyester capacitor ≥ chip niobium oxide capacitor ≥ polymer chip tantalum capacitor ≥ polymer solid chip aluminum capacitor ≥ chip tantalum capacitor with manganese dioxide as cathode ≥ liquid aluminum capacitor ≥ liquid tantalum battery.
5. Voltage high and low
For circuits with high voltage, once breakdown occurs, it may lead to serious consequences, so safety is the first factor to consider. Voltage withstand is the maximum voltage that a capacitor can withstand. To ensure the safe operation of the circuit, it is necessary to select capacitors with appropriate voltage resistance based on the actual voltage in the circuit. Generally speaking, the withstand voltage of a capacitor should be higher than the highest voltage that may occur in the circuit. The safety of high-voltage ceramic capacitors is the highest, followed by polyester capacitors and high-voltage aluminum capacitors. The polymer capacitors and niobium oxide capacitors have the lowest voltage resistance.
6. Price
Sometimes, capacitors are also widely used in civilian disposable consumer goods with lower sales prices. When price has a significant impact on profits, safety is more important. We must not choose capacitors with insufficient reliability by sacrificing safety. In fact, practice often proves that excessive consideration of costs leads to counterproductive results.
7. Comprehensive factors
Many times, the characteristics of circuit signals are very complex, and in this case, the reasonable selection of capacitors must be based on your thorough understanding of the performance characteristics of various capacitors. Due to the deeper knowledge of capacitors possessed by capacitor manufacturers, users can request more detailed technical services from capacitor manufacturers. It can even be required that a certain type of capacitor provided by the manufacturer must pass a special experiment.
8. Packaging method and shape
Choose appropriate packaging methods and shapes based on circuit layout and space requirements. Different packaging methods and shapes have different characteristics and applicable scenarios, which need to be selected according to the actual situation.
Capacitors are the most prone electronic components to problems among all electronic equipment. According to statistics, more than 70% of faults in power circuits are directly related to capacitors, with a higher proportion in high-power equipment. Therefore, a reasonable selection of capacitors can avoid many quality issues and circuit signal problems. Sometimes, the correct selection is even more important than a reasonable circuit design. Selection has become a part of complex circuit system manufacturing engineering, and must be jointly valued by capacitor manufacturers and circuit designers. This step is very necessary for both parties. At the same time, when purchasing capacitors, it is also necessary to pay attention to some matters, such as choosing a legitimate supplier, checking product certification, etc.